Surface modification of Coal Fly Ash by Sodium Lauryl Sulphate

Surface modification of Coal Fly Ash by Sodium Lauryl Sulphate
Surface modification of Coal Fly Ash by Sodium Lauryl Sulphate
By
Confidence Lethabo Mathebula
Submitted in partial fulfilment of the degree
MAGISTER SCIENTIAE
In the Faculty of Natural and Agricultural Sciences
UNIVERSITY OF PRETORIA
PRETORIA
Supervisor: Dr E.M van der Merwe
February 2013
© University of Pretoria
DECLARATION
I Confidence Lethabo Mathebula declare that the dissertation, which I hereby submit for the
degree of Magister Scientiae at the University of Pretoria, is my own work and has not
previously been submitted by me for a degree at another university.
…….………………..…………..…..
SIGNATURE
…….………………..…………..…..
DATE
i
ACKNOWLEDGEMENTS
Dr E.M van der Merwe (Supervisor) – For being more than just a supervisor but a
friend as well and for allowing me to be part of your life. Thank you for the best 3 years
of post-graduate studies
Dr L.C Prinsloo (Co-Supervisor) – For your helpful input and smile every time I came
to ask for your help.
Ma, Naidah Mathebula – I know that when you finally read this you will be speechless,
Thank you for being my rock. Love you so much.
Grandpa, Elson Mathebula – This journey started with you, I‟m thankful that you are
still in it with me.
Prof van Rooyen and Prof Vleggar – Thank you for the big round bottom flasks and the
heating mantle
Mr Allan Hall and Mr C. van der Merwe from the Department of Microscopy – For
their availability and help with SEM and TEM
Mr Bernard Reeksting from Polymer Science at CSIR - For his valued and appreciated
help with filler and polymer science
Miss Nontete– For her help with TGA-FTIR analysis and interpretation, thank you dear
Dr RA Kruger – For his expertise in fly ash
Impala Platinum (Bursaries and Processing Laboratories) – For allowing me an extra
year to continue with my studies, thank you.
MTV (L) [Mboni, Thandi, Van and Me] – You are the craziest bunch that I know and
that is the just what I needed and I appreciate each of you. What‟s next ladies
My Family ELIM Full Gospel Church – Thank you for accepting and loving me as I
am, and for all your support and prayers. I miss you all.
Friend and Family – Thank you for being there and for all the support, much
appreciated
This project is supported financially by Ash Resources Pty Ltd and the South African National
Research Foundation, NRF THRIP Grant number TP2009062900014. Any opinions, findings
and recommendations expressed in this material are those of the authors and therefore the NRF
does not accept any liability in regard thereto.
ii
Let no one despise you for your youth,
but set the believers an example in
speech, in conduct, in love, in faith, in
purity
1 Timothy 4:12
Rejoice always, pray continually, give
thanks in all circumstances; for this is
God's will for you in Christ Jesus
1 Thessalonians 5:16-18
iii
SUMMARY
Thirty million tons of coal fly ash are produced each year in South Africa of which
approximately 5% is utilised beneficially. With the growing concern about pollution and
increasing landfill costs, the study of the utilisation and application of coal fly ash has
increased worldwide.
The morphology and particle size of fly ash make it suitable for application as filler in
polymers, but its application is hindered by the lack of compatibility between the inorganic
surface of the ash and the organic matrix of the polymer. Another concern is the agglomeration
between fly ash particles. For this reasons, surface treatment is usually performed on mineral
fillers to enhance workability and compatibility between the polymer and filler.
This study involved the surface modification of South African coal fly ash with an anionic
surfactant, sodium lauryl sulphate (SLS), under different treatment conditions. Surface and
physical properties of the untreated and treated fly ash were studied systematically by scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) in order to determine
the extent of interaction between the SLS and the fly ash surface. Other analytical techniques
applied include Thermogravimetric analysis (TGA-FTIR), Particle size distribution, X-ray
diffraction (XRD) and X-ray fluorescence spectroscopy (XRF), Raman spectroscopy and
Fourier Transform Infrared spectroscopy (FTIR).
Although the overall chemical composition of the SLS modified coal fly ash investigated in
this study was not altered extensively, significant changes could be observed in its physical
properties. The hydrophilic surface of untreated fly ash was rendered hydrophobic after SLS
treatment. SEM results indicated a decrease in agglomeration between fly ash spheres upon
surfactant treatment, while results obtained from TEM have shown agglomerates on the surface
of most of the fly ash spheres. There is a distinct difference between the morphology of
agglomerates on the untreated and SLS modified fly ash, and also between samples treated
under different conditions. Not all SLS modified fly ash particles were covered with
agglomerates to the same degree.
Results obtained from FTIR and TGA-FTIR studies were promising in the sense that
hydrocarbon fractions could be observed in the TGA-FTIR decomposition products. The
possibility of interactions between fly ash and SLS could be deduced from the FTIR results of
the solid samples, due to a small shift in peak positions of the S-O stretch vibration, which may
iv
be indicative of electrostatic interactions rather than bonding interactions between SLS and fly
ash. The presence of SLS could not be confirmed by Raman spectroscopy, but rendered
information about the spatial distribution of the various phases in the fly ash.
Feasibility tests were performed on the application of fly ash samples as filler in PVC. These
results indicate that SLS treated fly ash can successfully replace CaCO3 as filler in PVC under
conditions of low filler loadings
v
TABLE OF CONTENTS
DECLARATION ..................................................................................................................... i
ACKNOWLEDGEMENTS ....................................................................................................ii
SUMMARY ........................................................................................................................... iv
LIST OF ABBREVIATIONS ................................................................................................ xi
LIST OF FIGURES ..............................................................................................................xii
LIST OF TABLES ............................................................................................................... xvi
CHAPTER 1 ........................................................................................................................... 1
INTRODUCTION .................................................................................................................. 1
1.1
Background ............................................................................................................................... 1
1.2
Fly ash ....................................................................................................................................... 2
1.2.1
Utilisation of fly ash ............................................................................................................. 3
a.
Cement and concrete................................................................................................................. 4
b.
Soil ameliorant .......................................................................................................................... 5
c.
Wastewater Treatment .............................................................................................................. 5
d.
Toxic waste Treatment.............................................................................................................. 6
e.
Mine Backfill ............................................................................................................................ 6
f.
Mullite and glass ceramics production ..................................................................................... 6
g.
Inorganic Filler into Polymers .................................................................................................. 7
1.3
Surfactants ................................................................................................................................ 8
1.3.1
Cationic surfactants .............................................................................................................. 9
1.3.2
Anionic surfactants ............................................................................................................. 10
1.3.3
Critical Micelle Concentration (c.m.c) ............................................................................... 11
1.3.4
Mechanism of surface surfactant adsorption ...................................................................... 12
1.3.5
Counter ion effect on surfactant aggregation ..................................................................... 14
1.4
Study Objective....................................................................................................................... 15
1.5
Reference ................................................................................................................................ 16
vi
CHAPTER 2 ......................................................................................................................... 20
THEORETICAL DESCRIPTION OF ANALYTICAL TECHNIQUES ............................. 20
2.1
X-ray Powder Diffraction (XRD) ........................................................................................... 20
2.2
X-ray Fluorescence (XRF)...................................................................................................... 21
2.3
Raman and Infrared spectroscopy........................................................................................... 22
2.3.1
Infrared (IR) Spectroscopy ................................................................................................. 23
2.3.2
Raman Spectroscopy .......................................................................................................... 24
2.4
Thermogravimetric analysis (TGA-FTIR) .............................................................................. 25
2.5
Scanning Electron Microscopy (SEM) ................................................................................... 26
2.6
Transition Electron Microscopy (TEM) ................................................................................. 28
2.7
Particle Size distribution ......................................................................................................... 29
2.8
References............................................................................................................................... 30
CHAPTER 3 ......................................................................................................................... 33
EXPERIMENTAL ................................................................................................................ 33
3.1
Materials ................................................................................................................................. 33
3.2
Coal Fly Ash modification ...................................................................................................... 33
3.3
Characterisation techniques .................................................................................................... 34
3.3.1
X-ray Diffraction (XRD) .................................................................................................... 34
3.3.2
X-ray Fluorescence (XRF) ................................................................................................. 35
3.3.3
FTIR spectroscopy.............................................................................................................. 35
3.3.4
Raman spectroscopy ........................................................................................................... 36
3.3.5
Thermogravimetric analysis (TGA-FTIR) ......................................................................... 37
3.3.6
Scanning Electron Microscopy (SEM) ............................................................................... 37
3.3.7
Transmission Electron Microscopy (TEM) ........................................................................ 38
3.3.8
Particle size......................................................................................................................... 39
3.4
References............................................................................................................................... 40
vii
CHAPTER 4 ......................................................................................................................... 41
CHARACTERISATION OF UNTREATED FLY ASH ...................................................... 41
4.1
Introduction............................................................................................................................. 41
4.2
Qualitative and quantitative analysis of the chemical composition of Lethabo coal fly ash
using XRD and XRF ............................................................................................................................ 41
4.3
Morphological and topographic characterisation of the untreated coal fly ash sample .......... 44
4.4
FTIR and Raman spectroscopic analysis of untreated Lethabo fly ash .................................. 46
4.4.1
FTIR spectroscopy.............................................................................................................. 46
4.4.2
Raman spectroscopy ........................................................................................................... 49
4.5
Thermogravimetric analysis of untreated Lethabo fly ash...................................................... 52
4.6
Particle size distribution.......................................................................................................... 53
4.7
Conclusion .............................................................................................................................. 54
4.8
Reference ................................................................................................................................ 55
CHAPTER 5 ......................................................................................................................... 57
SURFACTANT TREATMENT OF FLY ASH SAMPLES ................................................ 57
5.1
Introduction............................................................................................................................. 57
5.2
Sodium Lauryl Sulphate (SLS) ............................................................................................... 58
5.3
Results and Discussion ........................................................................................................... 58
5.3.1
XRD and XRF results ......................................................................................................... 58
5.3.2
Scanning Electron Microscopy........................................................................................... 62
5.3.3
Effect of SLS treatment on the particle size distribution of fly ash.................................... 65
5.3.4
FTIR and Raman spectroscopic analysis of the treated fly ash samples ............................ 67
5.3.5
Topography of the SLS modified fly ash ........................................................................... 75
5.3.6
Thermogravimetric Analysis of SLS modified fly ash ....................................................... 77
5.3.7
Contact angle measurements .............................................................................................. 80
5.4
Conclusion .............................................................................................................................. 81
5.5
References............................................................................................................................... 82
viii
CHAPTER 6 ......................................................................................................................... 83
COUNTER ION EFFECT ON SURFACTANT AGGREGATION .................................... 83
6.1
Introduction............................................................................................................................. 83
6.2
Experimental ........................................................................................................................... 83
6.3
Results and Discussion ........................................................................................................... 84
6.3.1
XRF .................................................................................................................................... 84
6.3.2
Scanning Electron Microscopy........................................................................................... 84
6.3.3
Transmission Electron Microscopy .................................................................................... 86
6.3.4
FTIR and Raman spectroscopic analysis ............................................................................ 88
6.4
Conclusion .............................................................................................................................. 91
6.5
References............................................................................................................................... 92
CHAPTER 7 ......................................................................................................................... 93
REFLUX TREATMENT OF FLY ASH .............................................................................. 93
7.1
Introduction............................................................................................................................. 93
7.2
Experimental ........................................................................................................................... 93
7.3
Results and Discussion ........................................................................................................... 94
7.3.1
SEM and TEM.................................................................................................................... 94
7.3.2
FTIR and Raman Spectroscopic analysis ........................................................................... 96
7.3.3
TGA-FTIR .......................................................................................................................... 97
7.3.4
Contact angle measurements .............................................................................................. 98
7.4
Conclusion .............................................................................................................................. 99
CHAPTER 8 ....................................................................................................................... 100
FLY ASH – POLY VINYL CHLORIDE ........................................................................... 100
8.1
Introduction........................................................................................................................... 100
8.2
Experimental ......................................................................................................................... 102
8.3
Results and discussion .......................................................................................................... 103
ix
8.3.1
Observations ..................................................................................................................... 103
8.3.2
Relative densities .............................................................................................................. 103
8.3.3
Mechanical Performance .................................................................................................. 104
8.4
Conclusion ............................................................................................................................ 110
8.5
References............................................................................................................................. 111
CHAPTER 9 ....................................................................................................................... 112
CONCLUSION AND RECOMMENDATIONS ............................................................... 112
9.1
Conclusion ............................................................................................................................ 112
9.2
Recommendations................................................................................................................. 115
x
LIST OF ABBREVIATIONS
c.m.c
CaCO3
CaO
cm-1
Cu
FA
FTIR
g/cm3
HCl
hrs
IR
KB
KBr
KC
KIO3
M
MD
MPa
NaCl
NaOH
Pb
Phr
PSD
PVA
PVC
SEM
SLS
TEM
TFA
TGA
TV
XRD
XRF
Critical micelle concentration
Calcium Carbonate
Calcium Oxide
Wavenumber
Copper
Fly Ash
Fourier Transform Infrared
grams per cubic centimetre
Hydrochloric Acid
Hours
Infrared
Kulucote
Potassium Bromide
Kulubrite
Potassium Iodate
Molar
Milling Direction
Megapascal
Sodium Chloride
Sodium Hydroxide
Lead
Parts per hundred resin
Particle Size Distribution
Polyvinyl Alcohol
Polyvinyl Chloride
Scanning Electron Microscopy
Sodium Lauryl Sulphate
Transmission Electron Microscopy
Treated Fly Ash
Thermogravimetric analysis
Transverse Direction
X-ray Diffraction
X-ray Fluorescence
xi
LIST OF FIGURES
Figure 1. 1: Sample of raw fly ash ................................................................................................ 2
Figure 1. 2: Spherical nature of fly ash particles observed under a scanning electron microscope
....................................................................................................................................................... 7
Figure 1. 3: Surfactant classification according to the charge of their head group ...................... 8
Figure 1. 4: Example of a Cationic Surfactant, Trimethylhexadecyl Ammonium Chloride ...... 10
Figure 1. 5: Example of an Anionic Surfactant, Sodium dodecylbenzenesulphonate ................ 10
Figure 1. 6: Representation of a spherical micelle [Dominguez, 1997] ..................................... 11
Figure 1. 7:The four region model of surfactant adsorption as from Atkin 2003 (a) ................. 13
Figure 1. 8: The two-step model of surfactant adsorption as taken from Atkin 2003 (a) The
regions presented in the figure are (I) a low surface excess region, (II) the first plateau region,
(III) a hydrophobic interaction region and (IV) the second plateau. .......................................... 14
Figure 2. 1: Diffraction of X-rays by a crystalline substance ..................................................... 21
Figure 2. 2:Principles of X-ray Fluorescence ............................................................................. 22
Figure 2. 3: Example of stretching and bending vibrations of a water molecule ....................... 24
Figure 2. 4: Illustration of fluorescence, infrared absorption and Raman scattering (Rayleigh,
Stokes and anti-Stokes) ............................................................................................................... 25
Figure 3. 1: PANalyticalX‟Pert Pro powder diffractometer ....................................................... 34
Figure 3. 2: ARL 9400XP ........................................................................................................... 35
Figure 3. 3: The Bruker 70v Fourier Transform Infrared (FTIR) spectrometer with microscope
attachment (left) and the TX6400 Raman spectrometer (right). ................................................. 36
Figure 3. 4: View of fly ash particles under the Raman microscope with 50x (left)
magnification and 100x magnification (right) ............................................................................ 36
Figure 3. 5: Perkin Elmer TGA 4000 FTIR ............................................................................... 37
Figure 3. 6: JEOL JSM 840 Scanning Electron Microscope (SEM) .......................................... 38
Figure 3. 7:JEOL JEM 2100F TEM ........................................................................................... 38
Figure 3. 8: Malvern Mastersizer 2000 ....................................................................................... 39
xii
Figure 4. 1: XRD spectrum of untreated fly ash ......................................................................... 42
Figure 4. 2: The silica-oxygen framework of a glass.................................................................. 43
Figure 4. 3: SEM micrographs of untreated Lethabo coal fly ash .............................................. 45
Figure 4. 4:TEM micrograph of untreated Lethabo coal fly ash ................................................ 45
Figure 4. 5: FTIR transmission spectrum of untreated fly ash ................................................... 47
Figure 4. 6: FTIR spectra of glass (bottom) and quartz (top). .................................................... 48
Figure 4. 7: FTIR reflectance spectrum of untreated fly ash ...................................................... 49
Figure 4. 8:Raman spectra of untreated fly ash at different analysis points ............................... 50
Figure 4. 9:Raman spectra of untreated fly ash at different analysis points ............................... 52
Figure 4. 10: TGA-FTIR results obtained for the untreated Lethabo fly ash ............................. 53
Figure 4. 11: Particle size distribution of untreated Lethabo fly ash .......................................... 54
Figure 5. 1: Chemical and structural formula of Sodium Lauryl Sulphate ................................. 58
Figure 5.2: XRD spectrum of 2.0% SLS treated fly ash sample treated for 18 hours at 80 °C .. 60
Figure 5. 3: SEM monograms of untreated and SLS treated fly ash .......................................... 62
Figure 5. 4: SLS treated fly ash samples for 6 hours at 80 °C .................................................... 63
Figure 5. 5: 2.0% SLS treated fly ash samples at 80 °C and different exposure periods ........... 64
Figure 5. 6: 2.0% SLS treated fly ash samples at 50 and 80 °C for 6 hours ............................... 65
Figure 5. 7: Comparison of the lognormal size distributions of untreated fly ash, and samples
treated in distilled water and SLS for 6 hours at 80 °C .............................................................. 65
Figure 5. 8: Comparison of the lognormal size distributions of untreated fly ash, and samples
treated in distilled water and SLS for 66 hours at 80 °C ............................................................ 66
Figure 5. 9: FTIR spectrum of Sodium Lauryl Sulphate ............................................................ 68
Figure 5. 10: Raman spectrum of Pure SLS taken at two wavenumber ranges .......................... 68
Figure 5. 11: FTIR spectra of treated FA samples at 50 °C for 18 hours, untreated FA and pure
SLS .............................................................................................................................................. 69
Figure 5. 12: FTIR spectra of treated FA samples at 80 °C for 18 hours, untreated FA and pure
SLS .............................................................................................................................................. 70
xiii
Figure 5. 13: FTIR spectra of treated FA samples at 50 °C for 6 hours, untreated FA and pure
SLS .............................................................................................................................................. 71
Figure 5. 14: FTIR spectra of treated FA samples at 80 °C for 6 hours, untreated FA and pure
SLS .............................................................................................................................................. 72
Figure 5. 15: Comparison of FTIR spectra of pure SLS, untreated fly ash, and 2.0% SLS treated
fly ash for 6 hours at 80 °C ......................................................................................................... 73
Figure 5. 16: The different Raman spectra of 2.0% SLS treated fly ash at 80 °C for 6 hours
obtained during 2D scanning of a small area of flyash ............................................................... 74
Figure 5. 17:Raman spectra of 2.0% SLS treated fly ash at 80 °C for 6 hours using different
analysis sites shown .................................................................................................................... 74
Figure 5. 18:Comparison of Raman spectra of pure SLS and 2.0% SLS treated fly ash for 6
hours at 80 °C ............................................................................................................................. 75
Figure 5. 19: TEM image of fly ash treated for 18 hours in distilled water at 50 °C ................. 76
Figure 5. 20: TEM images of treated Lethabo fly ash for 6 hours at 80 °C ............................... 76
Figure 5. 21: TGA-FTIR results obtained for SLS Powder ........................................................ 78
Figure 5. 22: TGA-FTIR results obtained for the 2.0 % SLS treated fly ash, at 80 °C for 6h ... 79
Figure 5. 23: Photographs of water droplets deposited on surfaces of a fly ash powder bed,
taken 1 s after deposition ............................................................................................................ 80
Figure 6. 1: SEM images of fly ash samples treated at 80 °C for 6 hours .................................. 85
Figure 6. 2: SEM images of fly ash samples treated at 80 °C for 6 hours .................................. 85
Figure 6. 3: SEM images of fly ash samples treated at 80°C for 6 hours ................................... 86
Figure 6. 4: TEM images of fly ash samples treated at 80 °C for 6 hours KIO3-Surfactant
solution........................................................................................................................................ 87
Figure 6. 5: TEM images of fly ash samples treated at 80 °C for 6 hours with NaCl-Surfactant
solution........................................................................................................................................ 88
Figure 6. 6: FTIR spectra of fly ash samples treated at 80 °C for 6 hours with KIO3-SLS........ 89
Figure 6. 7: Raman spectra of 2.0% SLS-0.1 M KIO3 treated fly ash at 80 °C for 6 hours at
different analysis sites shown ..................................................................................................... 89
Figure 6. 8: FTIR spectra of fly ash samples treated at 80 °C for 6 hours with NaCl-SLS........ 90
Figure 6. 9: Raman spectra of 2.0% SLS-0.1 M NaCl treated fly ash at 80 °C for 6 hours at
different analysis sites shown ..................................................................................................... 90
xiv
Figure 7. 1: Reflux system for surfactant treatment on fly ash ................................................... 93
Figure 7. 2:SEM images of refluxed fly ash samples treated for 6 hours ................................... 94
Figure 7. 3: : SEM images of SLS refluxed fly ash samples at higher magnifications, showing
surface coating ............................................................................................................................ 95
Figure 7. 4: TEM images of 2.0% SLS refluxed fly ash samples .............................................. 95
Figure 7. 5: TEM images of 4.0% SLS refluxed fly ash samples .............................................. 96
Figure 7. 6: FTIR spectra of refluxed fly ash samples treated for 6 hours ................................. 96
Figure 7. 7: Raman spectra of 4.0% SLS reflux treated fly ash for 6 hours taken at different
analysis sites shown .................................................................................................................... 97
Figure 7. 8: TGA-FTIR results obtained for the 4.0 % SLS refluxed fly ash treated for 6hrs ... 98
Figure 7. 9: Photographs of water droplets deposited on surfaces of a fly ash powder bed, taken
1 s after deposition ...................................................................................................................... 99
Figure 8. 1: Comparison of the elongation at break of untreated fly ash (FA), treated fly ash
(TFA), kulubrite 2 (KB) and kulucote 2 (KC) filled PVC formulations along milling direction
................................................................................................................................................... 105
Figure 8. 2: Comparison of the tensile strength of untreated fly ash (FA), treated fly ash (TFA),
kulubrite 2 (KB) and kulucote 2 (KC) filled PVC composites along milling direction ........... 105
Figure 8. 3: Comparison of the elongation at break of untreated fly ash (FA), treated fly ash
(TFA), kulubrite 2 (KB) and kulucote 2 (KC) filled PVC composites in transverse direction 107
Figure 8. 4: Comparison of the tensile strength of untreated fly ash (FA), treated fly ash (TFA),
kulubrite 2 (KB) and kulucote 2 (KC) filled PVC composites in transverse direction ............ 107
Figure 8. 5: SEM micrographs of the fracture surface of untreated fly ash samples and PVC
formulation for the different amounts of filler added ............................................................... 108
Figure 8. 6: SEM micrographs of the fracture surface of SLS treated fly ash samples and PVC
formulation for the different amounts of filler added ............................................................... 109
Figure 8. 7:SEM micrographs of the CaCO3 and PVC formulation for 30 phr ........................ 109
xv
LIST OF TABLES
Table 4. 1: Quantitative XRD results of untreated Lethabo fly ash............................................ 43
Table 4. 2: XRF chemical composition of Lethabo fly ash (Class F) ......................................... 44
Table 4. 3: The FTIR wavenumber assignment of silicate species as well as other compounds
that were found in the analysed fly ash sample. ......................................................................... 46
Table 4. 4: Main peaks in Raman spectra of components found in fly ash [Voll, 2002: and
Schneider, 2005] ......................................................................................................................... 49
Table 5. 1: XRD quantitative results of untreated fly ash (FA) and 2.0% SLS treated fly ash
sample at 80 °C for 18 hours (18hrs SLS) values are reported as weight percentage ................ 59
Table 5. 2: XRF chemical composition of untreated fly ash (FA) and of 2.0% SLS treated fly
ash sample at 80 °C for 6 hours (6 h SLS), values are reported as weight percentage. .............. 61
Table 5. 3: Assignment of FTIR and Raman Frequencies for SLS [Larkin, 2011] .................... 67
Table 6. 1: XRF chemical composition of untreated fly ash (FA), KIO3-SLS and NaCl-SLS
treated fly ash for 6 hours; values are reported as weight percentages. ...................................... 84
Table 8. 1: Materials used for the formation of PVC formulation........................................... 102
Table 8. 2: Relative densities of filled PVC composites .......................................................... 103
Table 8. 3: Mechanical properties of untreated and treated fly ash filled PVC composites along
milling direction (MD) .............................................................................................................. 104
Table 8. 4: Mechanical properties of KB and KC filled PVC composites along milling direction
(MD) ......................................................................................................................................... 104
Table 8. 5: Mechanical properties of untreated and treated fly ash filled PVC composites in
transverse direction (TV) .......................................................................................................... 106
Table 8. 6: Mechanical properties of KB and KC filled PVC composites in transverse direction
(TV)........................................................................................................................................... 106
xvi
CHAPTER 1
INTRODUCTION
1.1
Background
Globally, the most important application of the beneficiation of coal fly ash is in its partial
replacement for Portland cement in the cement and concrete industry. The application of fly ash
as an engineering material primarily originates from its pozzolanic nature, spherical shape, and
relative uniformity.
Fillers are generally used in polymers and rubber to reduce their production costs and to
improve certain physical characteristics of these products. The physical properties of coal fly
ash make it a suitable filler for polymers. In particular, the sphericity of coal fly ash particles
facilitates dispersity and fluidity within polymeric materials, while the reduced density and cost
of fly ash adds to the list of advantages when compared to conventional fillers. However, the
application of coal fly ash in these fields is not common yet. The main reason is the lack of
interaction between the fly ash and polymer or rubber, resulting in undesirable properties in the
final products. Another concern is the agglomeration between fly ash particles, which has an
undesirable effect upon its application as filler.
In order to meet some of the demands of the coal fly ash, polymer and rubber industries; the
chemical and physical properties of the coal fly ash surface need to be chemically modified in
order to add functionality to its surface, before its application as filler may be profitable. Very
little work has been done in this field, and the surface properties of fly ash are little understood.
Alkan et al [1995] studied the incorporation of fly ash into polyethylene and Ma et al [2001]
studied changes in the properties of fly ash - polypropylene systems after coupling agents were
added. In South Africa, fly ash is currently being used as filler in some polymers [Kruger,
1999] but its application remains limited.
In this study, the surface properties of untreated South African fly ash are investigated; and
subsequently, the feasibility of altering its characteristics and surface reactivity are tested using
1
a surfactant. No other references could be found where the surface of South African coal fly
ash was chemically modified. Furthermore, the feasibility of application of the untreated and
surfactant treated fly ash as inorganic fillers in PVC are tested.
1.2
Fly ash
Fly ash (FA) is a residue that is generated from the combustion of coal in power stations. It
contains small amounts of residual carbon that was not completely combusted, which gives the
fly ash its characteristic grey colour, Figure 1.1. The composition of fly ash varies according to
the source, method of combustion and the composition of the coal [Vassilev, 1996]. It can be
classified into a variety of particle size fractions, some characterised by hollow silica-alumina
glass spheres, called cenospheres [Potgieter-Vermaak, 2005].
Figure 1. 1: Sample of raw fly ash
Fly ash consists of an inorganic crystalline skeleton, covered with a glass phase of varying
compositions [Kruger, 1997]. Its main constituents are silicon dioxide, (SiO2), found in both
amorphous and crystalline phases, aluminium oxide (Al2O3), iron oxide (Fe2O3) and calcium
oxide (CaO). These phases are also the main building blocks of many coal bearing rock
divisions.
Since fly ash is obtained from coal combustion, it is useful to have some insight into the
chemistry and geology of the coal from which a specific ash was obtained. There are different
types or ranks of coal namely: anthracite, bituminous, sub-bituminous, lignite and others. The
2
chemical content of the burned coal has great influence on the chemical properties of the
produced fly ash [Schmidt, 2008].
Fly ash is generally classified into two classes namely Class F and Class C. The grouping
depends on the kind of coal combusted to generate the fly ash and the amount of oxides present
in the ash. Fly ash that contains less than 10 percent of CaO is classified as Class F while a
Class C fly ash contains more than 10 percent of CaO [Vempati, 1994].
Class F fly ash is generated from the combustion of hard and older coals, i.e. anthracite or
bituminous coal, and the obtained ash is pozzolanic in nature with at least 70% of the
percentage weight of the ash consisting of SiO2, Al2O3, and Fe2O3. Class C fly ash is obtained
from the combustion of younger coals, i.e. lignite or sub-bituminous coal, and is both
pozzalonic and cementitious in nature with at least 50% of the percentage weight of the ash
consisting of SiO2, Al2O3, and Fe2O3 [Kruger, 1997 and Landman, 2003]. South African fly ash
is classified as Class F.
The main source of power generation in South Africa is coal-firedpower thermal stations which
currently produce about 30 million tons of fly ash annually. Of the produced ash approximately
1.4 million tons (≈ 5%) are employed beneficially. The highest percentage of the used ash is for
the production of cement extensions and concrete [Potgieter-Vermaak 2005].
The great amounts that are not used pose significant environmental and economical challenges
and therefore there is a great need of environmentally safe and economically affordable ways
of disposal and handling.There has been global interest to increase the utilisation of coal fly
ash. Benefits for doing so include a reduction in the usage of non-renewable natural resources
and the substitution of materials that may be energy intensive to manufacture.
1.2.1
Utilisation of fly ash
Recently, Ahmaruzzaman [2010] and Blisset [2012] published a review article on the
utilisation of fly ash. In these papers they discussed current and potential applications of coal
fly ash, including its utilisation in cement and concrete, as an adsorbent for the removal of
organic compounds, waste water treatment, light weight aggregates, zeolite synthesis, mine
3
back fill and road construction. However, no reference is given to the application of fly ash as
mineral filler in the polymer manufacturing industry. Some of these applications will be
discussed briefly.
a.
Cement and concrete
The low density and morphology of fly ash; in particular its sphericity, makes it an
advantageous additive to cement and concrete [Copeland, 2011]. Fly ash has been reported to
reduce the water requirement per amount of cement used. Improvement on the overall density
of the concrete is observed when fly ash is introduced because of the decrease in voids and
capillary pores in the cement, therefore increasing the flow and workability of the concrete
[Caires, 2011; Siddique, 2001].
Fly ash also converts free lime found in cement or concrete into calcium silicate hydrate (CSH)
which acts as a binder for the cement. CSH greatly contributes to the concrete paste‟s strength
and durability [Ramachandran, 2001]. The pozzolanic reaction of fly ash with free lime then
increases fly ash‟s cementious nature and therefore it can be deduced that fly ash increases the
strength and durability of concrete [Siddique, 2001]. Studies have shown that even though the
strength of the concrete prepared from fly ash might be lower than that of pure cement concrete
before 28 days, it is equivalent at 28 days and after a period of a year or longer it is
significantly higher [Obla, 2008].
The utilisation of fly ash in cement also increases its ability to withstand attacks from chlorides
and sulphates. By reacting with free lime and reducing the quantity of free aluminates, fly ash
prevents sulphates and chlorides from attacking the concrete and causing fractures [Tikalsky,
2003; Siddique, 2001].
4
b. Soil ameliorant
Fly ash can be used as an ameliorant for the improvement of the physical, chemical and
biological properties of damaged soils such as mine soils [Kruger, 2009]. This improvement
can lead to elevated soil efficiency due to increased nutrition retention and better leaching
capabilities.
Nutrient and pH imbalances in problem soils can be corrected with the use of fly ash. The trace
element content of fly ash in conjunction with the organic nature of the soil itself is believed to
have beneficial effects on plant growth [Jala, 2004]. Fly ash has also been reported to improve
the water retention capacity of soils [lyer, 2001].
A combination mixture of fly ash with sewage sludge and lime (forming SLASH- Sludge, Lime
and fly Ash) was used to develop an artificial soil. The SLASH soil was investigated on sandy,
loam and acidic soil and the authors reported enhanced initial plant growth [Reynolds, 1999;
and Truter, 2000].
c.
Wastewater Treatment
The use of fly ash as an adsorbent of heavy metals in wastewaters was found to reduce its
toxicity by the removal of toxic substances; phosphates, nitrates, copper (Cu) and lead (Pb)
[Gupta and Torres, 1998]. Bada and Potgieter-Vermaak [2008] studied the properties of fly ash
as an adsorbent for organic compounds in wastewater. They treated fly ash with a strong acid
(HCl) which resulted in an increase in the specific surface area and resultant changes to the
surface properties of the ash, leading to an improvement in the adsorption capacity of the ash.
Acid mine wastewaters contain heavy metals which are highly acidic and can be problematic if
they leach into the ground or end up in the water main streams. Gitari et al [2008] reported on
the use of fly ash for mine wastewater treatment as an alternative for limestone, lime and
sodium hydroxide. At controlled pH levels, the concentration of heavy metals was reduced
significantly, lowering the acidity of the wastewater.
The adsorption of lead onto NaOH treated fly ash was reported by Woolard et al [2000]. The
treated fly ash samples have shown an increase in adsorption capacity compared to the
5
untreated ash. Madzivre et al [2009] studied the treatment of mine waste water with fly ash for
the removal of sulphates instead of chemical or biological treatments. This was reported to be
cost effective because fly ash itself is a waste product and can be accessed with ease from coal
power stations for use in the treatment of mine waste waters
d. Toxic waste Treatment
Reynolds et al [1999] reported on the use of fly ash in neutralising toxic sewage sludge. It was
found that the sewage sludge can be recycled and applied to agriculture through treatment of
the substance with fly ash. Eye and Basu [1970] studied the conditioning of sewage sludge with
fly ash and found that it can be useful in the recycling of wastewater.
e.
Mine Backfill
The use of fly ash as a backfill in mines has found much interest of lately. Knowing that fly ash
can be used as a soil ameliorant, an adsorbent for organic compounds and toxic materials and
that it is cementious, adds value to its suitability for use in backfilling [Ward, 2006]. Ward
mentioned a number of advantages of using fly ash in mine back filling, including void filling,
stabilisation of soil cover, improvement of water retention and many more.
f.
Mullite and glass ceramics production
The high content of SiO2 and Al2O3 in fly ash makes it a suitable material for the development
of mullite and glass materials [Suriyanarayanan, 2009]. It is also considered to be an affordable
resource for the glass and ceramics industry acting as a replacement for kaolinite [Blissett,
2012]. In their studies, Suriyanarayanan [2009] and Iyer [2001] both observed that an equal
ratio (1:1) of coal ash and alumina was required to develop a homogenous mullite composite.
Tan [2011] studied the development of mullite whiskers from coal ash and reported good
diameters for the products.
Glass and ceramics produced from coal fly ash have been reported to have comparable physical
and mechanical properties to available literature values for these materials [Blissett, 2012].
6
g.
Inorganic Filler into Polymers
Fillers are generally used in polymeric materials to reduce their production costs and to
improve selected characteristics of the final product. The physical properties of fly ash, in
particular its spherical shape, Figure 1.2, facilitate dispersion and fluidity making it suitable for
application as filler. Furthermore, compared to conventional fillers, for example CaCO3 or
SiO2, the low density and cost of fly ash adds to the list of its advantages.
Figure 1. 2: Spherical nature of fly ash particles
observed under a scanning electron microscope
Research in the field of utilizing fly ash as filler in different polymeric materials is growing
exponentially. Examples of these studies includes the work by Alkan [1995] who studied the
incorporation of fly ash into polyethylene; while Ma [2001] investigated changes in the
properties of fly ash–polypropylene systems after coupling agents were added.
Yang et al [2006] studied the surface modification of purified coal fly ash, with subsequent
application in polypropylene. One of the principle problems that occur when fly ash is used as
filler in polymers is that there are no binding interactions between the polymer and fly ash, with
the consequence that the quality of the product is unsatisfactory. If the surface properties of fly
ash can be modified in such a way that this problem is conquered, a new market for the
utilisation of fly ash can materialize.
Nath [2010a and 2010b] addressed this problem by studying the effect of surfactant modified
fly ash on the properties of composite films fabricated with polyvinyl alcohol (PVA). Their
results showed an enhancement in the physical properties of the polymer, which they attributed
7
to the elimination of particle-particle interaction, and a better distribution of fly ash within the
polymer.
1.3
Surfactants
Surfactants are surface active agents. They are amphiphilic substances consisting of a lyophilic
(or hydrophilic) part which is a polar-group, and a lyophobic (or hydrophobic) part which is
generally a hydrocarbon chain [Tadros, 2005]. Due to their amphiphilic nature, surfactants tend
to concentrate at interfaces and assemble into aggregates, such as micelles and vesicles, in bulk
solution.
Surfactants can be classified into non-ionics and ionics, where the ionics can be further divided
into three groups namely; cationics, anionics, and zwitterionic, as shown in Figure 1.3.
Figure 1. 3: Surfactant classification according to the charge of their head group
8
Non-ionic surfactants can be defined as surfactants that have no charge in the predominant
working pH range. These surfactants dissociate into two neutral parts resulting in a final charge
of zero [Porter, 1994]. The non-polar part of the nonionic surfactants is mainly characterised by
a polyether or polyhydroxyl unit with an oxyethylene unit as a polar part [Malik, 2011].
Ionic surfactants are amphiphilic substances that have an ionic group that is attached directly or
through intermediates to a hydrocarbon chain. The ionic group of the surfactant acts as the
surface active entity [Malik, 2011].
Surfactants can be used to alter the physical properties of certain materials. Treatment of a
charged surface with an ionic surfactant can change its hydrophobic or hydrophilic nature. For
example, for a negatively charged material; treatment with a cationic surfactant may cause it to
become hydrophobic while an anionic surfactant may render the material hydrophilic and vice
versa [Rosen, 2012].
The interaction of a nonionic surfactant with a surface depends on the physical properties of the
material, and may lead to formation of a hydrophobic or hydrophilic surface. For a polar
surface, for example, the hydrophilic part of the surfactant will interact with the surface and
render it more hydrophobic [Malik, 2011 and Rosen, 2012].
Zwitterionic surfactants, however, tend not to significantly alter the surface charge of materials
or their physical properties. Having both positive and negative charges, they can interact with a
surface material with both head groups resulting in a neutralized effect [Rosen, 2012].
1.3.1
Cationic surfactants
Cationic surfactants are classified as such if their molecules dissociate in solution to form a
surface active cationic entity and a normal anion [Tadros, 2005]. Cationic surfactants are
subdivided into four main classes according to their chemical structure; alkyl amines,
ethoxylated amines, alkyl imidozolines and quaternaries.
Figure 1.4 gives an example of a cationic surfactant with an alkyl amine chemical structure.
Their applications include hydrophobisation, where the surfactants act as corrosion inhibitors,
9
anti-caking agents in fertilisers, floating agents, adhesion promoters, and as dispersants for
mineral fillers.
Figure 1. 4: Example of a Cationic Surfactant, Trimethylhexadecyl Ammonium Chloride
1.3.2
Anionic surfactants
Anionic surfactants (detergents) are amphiphilic substances that have an anionic group attached
to a long hydrocarbon chain. The anion is the surface active entity and the length of the carbon
chain increases their detergency strength. Surfactants with a hydrocarbon chain of 12-16 carbon
atoms have been reported to be strong detergents [Tadros, 2005]. The counterions of anionic
surfactants are mainly sodium, potassium, ammonium, magnesium and calcium. Sodium and
potassium are used because they give the surfactant increased water solubility while
magnesium and calcium are suitable for oil solubility. Amine counterions cause the surfactant
to be both water and oil soluble [Malik, 2011].
There are three important anionic groups that are mainly used, namely: the sulphates (-OSO3-),
sulphonate (-SO3-) and carboxylates (-CO2-) [Lange, 1999]. An example of sulphonate anionic
surfactant is shown in Figure 1.5. Anionic surfactants are characterised by high dispersing and
foaming properties.
Figure 1. 5: Example of an Anionic Surfactant, Sodium dodecylbenzenesulphonate
10
They are highly stable but are sensitive to hard water (i.e. the presence of metal ions) and
changes in pH. Anionic surfactants are relatively basic and tend to protonate under conditions
of low pH values and precipitate in the presence of metal ions [Tadros, 2005]. An advantage of
using anionic surfactants is that they are affordable and easily accessible.
1.3.3
Critical Micelle Concentration (c.m.c)
Micelles can be defined as small colloid particles composed of molecules of a surface active
substance. The formation of these micelles on a solid surface by surfactants is driven by the
amphiphilic nature of the surfactants [Dominguez, 1997].
For surfactants, there exists a concentration above which the surfactant begins to form
aggregates (micelles) spontaneously and any increase in the concentration leads to the
formation of micelles [Lange, 1999]. This effect is shown in Figure 1.6. The concentration at
which micelles are formed is known as the critical micelle concentration (c.m.c). The c.m.c can
also be defined as a measure of surfactant efficiency; the lower the c.m.c the less surfactant is
required to saturate the system and induce the formation of micelles. [Rosen, 2012]
Figure 1. 6: Representation of a spherical micelle [Dominguez, 1997]
The surface of a solid material exposed to a surfactant solution changes rapidly with an
increase in the concentration of the surfactant. This occurs before the c.m.c. is reached and
surface tension remains fairly constant above the c.m.c. [Floriano, 1998].
11
Micellisation is dependent on temperature conditions, the length of the surfactant alkyl chain as
well as the concentration of the surfactant. Temperature effects are mostly prominent for ionics
and zwitterionics and the c.m.c decreases with increasing temperature. The hydrophobic part of
the surfactants tends to change with varying temperature conditions [Kim and Lim, 2003]. The
length of the alkyl chain influences both the c.m.c and the hydrophobicity of surfactants. An
increase in chain length will increase both the c.m.c and the hydrophobicity [Atkin, 2003b].
1.3.4
Mechanism of surface surfactant adsorption
Surfactant surface adsorption at different interfaces; liquid-liquid or solid-liquid, has been
reported to be essential in many industrial process applications, including metallic surface
interactions, detergency and many more [Schniepp, 2007; Sammalkorpi, 2008]. Understanding
surfactant adsorption on solid surfaces in an aqueous surfactant solution has received increased
attention in recent years as seen by the number of different studies on different substrates that
include fly ash, graphite, silica, fiber, gold and many more [Goloub, 1996; Sammalkorpi,
2008].
The aggregation of both ionic and non-ionic surfactants has been investigated either on
hydrophilic
(silica)
[Goulob, 1996;
Moglianetti,
2009]
or hydrophobic (graphite)
[Sammalkorpi, 2008] substrates. Many reported that surfactant adsorption is dependent on the
nature of the substrate as well as the concentration of the surfactant.
Electrostatic attractions (van der Waals interactions) are considered to be the main driving
force for interactions between ionic surfactants and oppositely charged hydrophilic surfaces.
The interaction of hydrophobic substrates with ionic surfactants is reported to be primarily due
to the hydrophobic interaction between the surfactant‟s tail and the substrate [Manne, 1994;
Atkin, 2003a].
The aspects of surfactant adsorption have been accepted to some extent but there is some room
for further development of the existing theories. The interaction of surfactants and surfaces can
be explained by two different models; the three or four region model and the two-step model
[Atkin, 2003a; Manne, 1994]. Each region in these models indicates the progression of
12
surfactant aggregation and surface coating/coverage. The two-step model is mainly associated
with aqueous ionic surfactants [Manne, 1994].
The four region model is shown in Figure 1.7. The x-axis, C, of the isotherm represents the
residual surfactant concentration while the y-axis, Γ, denotes the adsorption density of the
surfactant onto a surface. The four region model can then be explained as follows: region I
represents electrostatic adsorption of surfactant monomers with head groups interacting with
the substrate, region II is the beginning of surface coverage due to interaction between
adsorbed surfactant monomers, hemimicelle formation, the hydrocarbon tail forms hydrophobic
areas on the surface, region III represents the growth of the hemimicelles formed in region II
with the same total coverage and the hydrophilic parts are formed with the head-groups in
solutions, and region IV is where the c.m.c is reached and full micelles are formed
spontaneously [Atkin, 2003a]. Atkin et al [2003a] defined hemicelles as “a spherical structure
with surfactant head-groups facing both towards the substrate and into the solution”
Figure 1. 7:The four region model of surfactant adsorption as from Atkin 2003 (a)
The two-step model consists of two plateau regions, as represented in Figure 1.8. The pre-hmc
(hemimicelle concentration) plateau occurs at low surfactant concentrations and the saturation
level plateau occurring above the c.m.c. with a high increase of surface coverage observed
above the c.m.c. The pre-hmc plateau is characterised with two parts, the electrostatic
adsorption of surfactant on the substrate and the neutralization of the surface charge with
increased surfactant monomer adsorption approaching the hmc.
13
The saturation plateau level also consist of two sections, the occurrence of hydrophobic
interactions between adsorbed monomers leading to the formation of hemicelles and the
formation of micelles with high surface coverage [Atkin, 2003a].
Figure 1. 8: The two-step model of surfactant adsorption as taken from Atkin 2003 (a)
The regions presented in the figure are (I) a low surface excess region, (II) the first
plateau region, (III) a hydrophobic interaction region and (IV) the second plateau.
1.3.5
Counter ion effect on surfactant aggregation
Columbic forces have been attributed as the primary forces that influence the formation of
ionic surfactant-surface aggregates [Bitting and Harwell, 1987]. Electrolytes have been found
to have a great influence on the aggregation patterns of ionic surfactants in relation with the
counter ion and ionic micelle interaction that can occur during micellisation [Umlong and
Ismail; 2006].
14
Bitting and Harwell [1987] reported on the importance of counter ions in surfactant-surface
adsorption. They found that adding NaCl to the surfactant solution increased the adsorption
plateau, on an alumina surface, with increasing concentration of the salt. Wanless et al [1997]
also observed the same trend on a graphite surface and concluded that aggregation can form on
the surface at concentrations below the c.m.c in the presence of NaCl and other counter ionic
salts. Silbert et al [2010] reported that the hydrophobicity and hydrophobic interactions of
surfactant aggregates formed on substrates can be affected by the counterions of the surfactant.
Atkin et al [2003b] observed an increase in the rate of adsorption of different surfactants,
investigated in his study, in the presence of electrolytes added to the surfactant aqueous
solution in the form of salts.
Counter ions and additional electrolytes in ionic surfactant solutions tend to decrease the c.m.c
of the surfactant and therefore influence the formation of surfactant aggregates on a surface
interface. Counter ions also decrease the repulsive forces that may exist between the ionic head
groups of the surfactant and a surface and therefore allowing for better interaction and therefore
increased chances of micellisation [Umlong and Ismail; 2006; and Paria, 2004].
1.4
Study Objective
The purpose of this study is to report on the characterization of the surface of a typical South
African coal fly ash, modified by Sodium Lauryl Sulphate (SLS), and to elaborate on the
problems and challenges experienced in characterization of the product.
The effect of water on the untreated fly ash sample was also investigated by comparing the
results under similar conditions to samples exposed to different concentrations of the surfactant
solution. Untreated and SLS modified fly ash samples were used as filler for polyvinyl chloride
(PVC) and their mechanical properties were investigated in comparison to two different
calcium carbonate fillers.
15
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19
CHAPTER 2
THEORETICAL DESCRIPTION OF ANALYTICAL
TECHNIQUES
2.1
X-ray Powder Diffraction (XRD)
X-ray powder diffraction is a non-destructive analytical technique used for the characterisation
of crystallographic structure, grain size, and preferred orientation in powdered solid samples. It
can also be used to determine relative abundance of crystalline substances in solid materials.
XRD is based on two phenomena: (a) the scattering of X-rays by the electron cloud of each
individual atom, and (b) the interference of the scattered X-rays [Guiner, 1963]. In 1912,
M. von Laue discovered the nature of interaction between X-rays and crystalline substances.
He suggested that the scattered X-rays can interfere with one another constructively, resulting
in the wavelength of the diffracted X-rays being similar to the interatomic spacing of a crystal
lattice [Moore, 1997].
The interference pattern of the scattered X-rays is explained by Bragg‟s law n
2d sin . If
the wavelength, λ, of the diffracted X-rays and the angle, θ, at which they were produced, the
interatomic distance, d, of a crystal can be determined, with n being an integer [Moore, 1997].
During measurements of an XRD pattern, a sample is radiated with a monochromatic X-ray
beam from a cathode tube. The incident rays interact with the sample to produce diffracted rays
according to Bragg‟s law, as shown in Figure 2.1. The intensity of the diffracted X-rays is
detected, with all the possible diffraction patterns of the sample attained. The diffraction peaks
lead to the identification of crystalline phases using the interatomic spacing of the crystalline
phase. Each phase in a heterogeneous sample has a unique interatomic space and thus the
characterisation of an unknown sample is possible [Dinnebier, 2008].
20
Figure 2. 1: Diffraction of X-rays by a crystalline
substance
The XRD technique is quick in identifying unknown inorganic materials; it provides
unambiguous mineral identification, while sample preparation and data interpretation is fairly
easy.
2.2
X-ray Fluorescence (XRF)
XRF is an analysis technique used for elemental analysis of solids with detection limits that are
in the range of 1 ppm [Wirth, 2011]. It is a non-destructive analysis method, requiring a
significant amount (approximately 1 g) of a finely ground analyte. Sample preparation for XRF
analysis is relatively easy and cost effective.
XRF is dependent on the absorption of radiated X-rays. The absorbed X-rays excite the
sample‟s atoms leading to ionisation (Figure 2.2) where an inner electron (lower energy orbital)
of the atom in a sample is displaced, making the atom unstable [Birkholz, 2006]. An outer
electron (higher energy level) then replaces the missing inner electron and energy is released.
The released energy represents the energy difference of the binding energies of the higher and
lower energy levels [Jenkins, 1999].
Characteristic X-rays are emitted during the energy release, representing the atoms present in
the sample. The emitted X-rays depend on the chemistry of each atom present in the sample
and are emitted at different wavelengths corresponding to a certain atom. The wavelengths at
which X-rays are emitted are plotted on a spectrum and give information about the identity of
the elements in the sample [Williams, 1987].
21
Figure 2. 2:Principles of X-ray Fluorescence
The concentration of a species in an analyte is determined by measuring the intensity of the
emitted X-ray. The relationship between the concentration of elements in a sample and the
characteristics X-ray intensities was first established in the mid-1950s, and has become the
basis of the “fundamental parameters” approach of the calibration of XRF [Thomsen, 2007].
XRF is said to be a qualitative and quantitative analysis technique because it can identify
elements as well as quantify them. Its applications extend over a variety of disciplines
including chemistry, geology, environmental sciences, industry and many more.
2.3
Raman and Infrared spectroscopy
Raman and infrared spectroscopy are the main analytical techniques used to study the
vibrations of atoms in molecules. The energy of vibrations are measured with infrared
spectroscopy after absorption of infrared light from a polychromatic source, while Raman
spectroscopy measures the energy of scattered light after irradiation with a strong
monochromatic light (laser) [Larkin, 2011].
22
Raman and infrared spectra can be described as a fingerprint of a material and can therefore be
used for identification purposes. The chemical and physical structure of materials can be
studied with the two techniques [Smith, 2005]. Infrared spectroscopy can also be used for
quantitative analysis, while Raman spectroscopy can also be used for semi-quantitative
analysis.
Both techniques can be applied to a wide range of samples, including gases, liquids and solids,
with little or no sample preparation. Although useful in a wide range of applications, Raman
spectroscopy sometimes has a problem with sample degradation under the strong laser light and
fluorescence might interfere with recording of the Raman spectrum. Thus Raman spectroscopy
is less widely used in comparison to infrared spectroscopy [Smith, 2005].
2.3.1
Infrared (IR) Spectroscopy
IR spectroscopy was first commercialised during the 1940‟s. Prisms were used as dispersive
elements until the mid-1950s when diffraction gratings were implemented [Stuart, 2004]. A
great improvement to IR spectroscopy was the understanding of Fourier-transformation
mathematics, which improved the quality of IR spectra and minimised the time required to
acquire data – from there the term FTIR (Fourier-transform infrared spectroscopy) was
introduced [Christy, 2001].
A sample irradiated with infrared electromagnetic energy will either absorb or transmit the
energy. For a vibration of a molecule to be active in infrared spectroscopy, its electric dipole
moment must change during vibration. Vibrations are broadly classified according to the type
of movement, namely stretching and bending [Stuart, 2004]. These are explained in Figure 2.3.
Each molecule is characterised by a certain number of vibrational degrees of freedom
according to the number of atoms forming the molecule. For a linear molecule there are 3N-5
degrees of freedom while a non-linear molecule has 3N-6 degrees of freedom (N = number of
atoms in a molecule). Vibrational modes, both stretching and bending, occur at different
frequencies according to the type of bonds that exist in a molecule and the masses of the
individual atoms involved in a vibrational mode [Smith, 1996].
23
Bending
Symmetric stretching
Asymmetric stretching
Figure 2. 3: Example of stretching and bending vibrations of a water molecule
2.3.2
Raman Spectroscopy
The Raman effect was first discovered in 1928 by C.V. Raman, who won a Nobel Prize in
Physics in 1930 for this breakthrough. He conducted his studies on liquid samples and a few
years later G. Landsberg conducted the same study on crystal structures [Larkin, 2011].
Compared to IR spectroscopy, Raman spectroscopy was not widely used in laboratories as the
effect was very small and difficult to measure. Only with the availability of lasers as strong
monochromatic sources and the use of computers to enhance the signal electronically, did
Raman spectroscopy become a technique that is today available in most analytical laboratories
[Smith, 2005].
In a Raman experiment a sample is irradiated with strong monochromatic light (laser), which
excites the molecules to a higher virtual energy state. Once the molecules return to a normal
energy level, light is scattered into all directions and three possibilities exist. These are
illustrated in Figure 2.4. Firstly, most of the light is elastically scattered (Rayleigh scattering)
where the energy of the incident light is equal to that of the scattered light, Figure 2.4a
[Wartewig, 2003].
24
Figure 2. 4: Illustration of fluorescence, infrared absorption and Raman scattering
(Rayleigh, Stokes and anti-Stokes)
Secondly, molecules in the ground vibrational state fall back to a higher energy level in which
case the scattered light is at a lower energy level than the incident light, namely Stokes Raman
spectroscopy, Figure2.4b. And thirdly, in Anti-Stokes Raman shown in Figure 2.4c, molecules
that are at a higher energy level fall back to the ground state after excitation and the energy of
the incident light is lower than that of the scattered light [Wartewig, 2003].
As most molecules are in their ground state at room temperature, the Stokes scattering is much
stronger than the Anti-Stokes and are most commonly used [Wartewig, 2003]. In some
instances the strong laser light might also excite an electron to the next energy level, which is
the cause of fluorescence which can interfere with a Raman spectrum (see Figure 2.4).
2.4
Thermogravimetric analysis (TGA-FTIR)
Thermogravimetric analysis (TGA) is one of many primary thermal analysis techniques.
Thermal analysis is defined as “the analysis of a change in a property of a sample, which is
related to an imposed temperature” [Brown, 2001]. TGA measures the change in the weight of
a sample as a function of temperature or time in a controlled environment. It can be conducted
in three different ways namely; under dynamic measurement that is at a constant heating rate,
isothermal measurement that is constant temperature, and controlled TGA which involves nonlinear temperature programs [Gabbott, 2008].
25
TGA is mainly used for the determination of the purity, composition and thermal stability of
sample materials. It uses the properties of the analysed material for characterisation, by
measuring the change in weight due to volatility, oxidation, dehydration or decomposition
[Haines, 2002]. The obtained thermogravimetric curves (weight versus temperature or time) are
characteristic to the material being analysed. The curves give an indication of weight loss or
gain due to a certain process at a particular temperature or time [Gabbott, 2008].
The integration of TGA with other instruments has been found to be greatly helpful in
determining the composition of the analysed material. TGA-FTIR (integration of TGA with
FTIR spectroscopy) allows for the identification of evolved gases from the analysed material
during a TGA analysis [Brown, 2001]. The obtained IR spectra can be used together with the
TGA curve for complete analysis of the material by comparing the process that occurred during
temperature changes and the chemical composition of the evolved gas.
TGA-FTIR can be used for the analysis of materials such as polymers, plastics, composites,
organic and inorganic substances. However its application is limited to substances that are
subjected to mass loss and gas formation [Brown, 2001].
2.5
Scanning Electron Microscopy (SEM)
In 1938 M. Von Ardenne worked and published the principles of the scanning electron
microscopy. The first instrument was built by Sir C. Oatleyand it was commercialised in 1965
[Voutou, 2008].
The SEM is primarily designed for imaging rather than chemical analysis. SEM images are
produced by the scanning of a sample with a high-energy beam of electrons in a raster scan
pattern to cover the whole sample site [Egerton, 2005]. The interaction of the sample‟s atoms
with the electrons results in the production of signals that have information concerning the
sample‟s surface topography (texture), morphology (shape and size of particles) chemical
composition, and crystallographic information (atom arrangement); depending on the type of
image signals used [Reed, 1996].
Secondary electron (SE) images, which show topographic features, together with the
backscattered electrons (BSE) images, which give the compositional variations in a sample, are
the most commonly used image signals [Reed, 1996]. The output of the SEM can therefore be
26
specified to be either topographic or compositional (compositional referring to the mean atomic
number of the sample). SEM cannot distinguish between individual elements.
The main components of a SEM instrument are as follows: the electron source, focussing
lenses (electromagnetic), sample chamber, detector, and display system. The most commonly
used electron source is an electron triode gun fitted with a hot tungsten filament of about
0.1 mm in diameter. The electron gun is heated by a current of approximately 2.5 A to a point
where “thermionic” emission takes place; thus giving the electrons enough thermal energy to
overcome the potential barrier at the surface [Reed, 1996].
The electron gun and the electromagnetic lenses (focussing lenses) are housed inside the
electron column. The gun emits electrons and accelerates them to energy levels in the range of
0.1–30 keV; the lenses influence the travelling path of the electrons while focussing them into a
fine beam. The electron beam then leaves the electron column through the final lens into the
sample chamber, where sample-electron beam interaction occurs. The interaction occurs at a
depth of field of about 1 μm, generating signals that then form an image of the analysed
sample.
The generated signals are collected from the beam-sample interaction. These interactions differ
from one point on the sample to another. The collected signals are then converted to point by
point intensity changes to produce an image using SE and BSE image signals according to the
type of analysis. The generated images are digitalised and displayed for analysis.
SEM is advantageous over other electron microscopes because it can focus on large areas of
analysis sites of a sample at a time due to its large depth of field [Standländer, 2007]. It has
high image resolution with strong feature specificity. Sample preparation is fairly easy.
A number of limitations have been reported concerning scanning electron microscopy. The
most important are that the nature of sample analysis, which occurs in vacuum, presents
problems for powder material analysis, and the generated images do not show the true colour of
the analysed material.
27
2.6
Transition Electron Microscopy (TEM)
Transmission Electron Microscopy was the first kind of Electron Microscopy to be developed.
The first TEM image was obtained by Ernst Ruska and Max Knoll in Germany in 1931
[Voutou, 2008].
TEM is a microscopy analysis technique that uses a beam of electrons transmitted through a
thin sample, approximately 5 nm to 0.5 μm in thickness. It uses an electromagnetic lens system
to focus the thin electron beam onto the sample. The electron energy of the electron beams
ranges between 60 and 150 keV [Egerton, 2005].
TEM can use three different electron beam-specimen interactions for sample analysis, namely;
unscattered electrons from the transmitted beam, the elastically scattered electron from the
diffracted beam and the inelastically scattered electrons. [Voutou, 2008]
Transmitted unscattered electrons are responsible for the visibility of the images obtained
during analysis. There exist an inversely proportional relation between the transmission of
unscattered electrons and sample thickness, thus the primary requirement for thin samples for
analysis. Thick samples cause the images to be dark due to diminished transmitted unscattered
electrons.
Sample orientation and atomic arrangement are achieved from scattered electrons obtained
from the interaction of electrons and the atoms present in the sample. The use of inelastic
interaction between the electrons and atoms in the analysed sample allows for instrument
integration for further analysis, for example the Electron Energy Loss Spectroscopy,
For optimum results certain conditions are required for TEM, which are sometimes also
considered as limitations to the instrument. Due to the high vacuum requirements during
analysis, the thin sample specimen needs to be absolutely dry, without any traces of water.
Sample stability is also important, especially when exposed to the electron beams [Bouchet and
Gaillard, 2005].
28
2.7
Particle Size distribution
Particle Size Distribution (PSD) analysis is used to measure the size and range of particles in a
particular substance [Stanley-Wood, 1992]. Particles are 3-dimensional objects with three
parameters (length, breadth and height) which are necessary for their full description
[McDonagh, 2010]. Many particle sizing techniques assume that the material being measured is
spherical. The advantage of this assumption originates from the single parameter (diameter)
which is a description of the sphere. The assumption simplifies particle size distribution
presentation but it can produce inconsistent results when non-spherical particles are analysed
[Cooper, 1998].
Laser diffraction has become a favoured analysis method for determining particle size
distribution of aerosols, suspensions emulsion and solid samples. It is a robust particle
measurement technique that can be used for both dry and wet samples. The size analysis range
of laser diffraction lies between 0.02 to 2000 microns [McDonagh, 2010].
Laser diffraction based particle size analysis is based on the scattered light generated when a
laser beam is applied onto particles. The interaction of laser light with the particles produces
diffraction of the laser light. The Fraunhofer and Mie theory are mathematical theories that can
be used to describe the interaction between the laser light and the particles. The theories state
that for a single spherical particle, the diffraction pattern shows a typical ring structure
[Cooper, 1998]. The Mie theory has been adopted for application in laser diffraction.
Light scattering occurs at an angle that is directly proportional to the size of the interacting
particle. The scattering angle increases logarithmically with decreasing particle size. Scattering
is dependent on the particle sizes; large particles scatter light at narrow angles with high
intensity whereas small particles scatter at wider angles with low intensities [Allen 1997].
29
2.8 References
1.
Allen T, 1997, Particle Size Measurement: Surface area and pore size determination,
5th ed., Chapman and Hall, London.
2.
Birkholz M, Fewster P and Genzel C, 2006, Thin Film Analysis by X-Ray Scattering,
Wiley-VCH, Weinberg.
3.
Bouchet B and Gaillard C, 2005, Principles of transmission electron microscopy, INRA
Nantes – Plateform BIBS – Microscopy.Viewed: 16 January 2012
<http://www.angers-nantes.inra.fr/content/download/1758/.../TEM-principle.pdf>.
4.
Brown ME, 2001, Introduction to thermal analysis: techniques and applications, Kluwer
Academic Publishers, Boston, pp. 1-45.
5.
Christy AA, Ozaki Y and Gregoriou VG, 2001. Modern Fourier Transform Infrared
Spectroscopy, Elsevier, Amsterdam.
6.
Cooper J, 1998, Materials World, vol. 6, no. 1, Particle Size Analysis – The Laser
Diffraction Technique, pp. 5-7.
7.
Dinnebier RE and Billinge S, 2008, Powder Diffraction: Theory and Practice, Royal
Society of Chemistry, Cambridge, pp. 1-19 and 58-63.
8.
Egerton RF, 2005, Physical principles of electron microscopy: an introduction to TEM,
SEM, and AEM. Springer.
9.
Gabbott P, 2008, Principles and applications of thermal analysis. Blackwell Pub., Oxford,
pp. 87-118.
10.
Guinier A, 1963, X-ray Diffraction in Crystals, Imperfect Crystals, and Amorphous
Bodies, General Publishing Company, Canada, pp. 1-51.
11.
Haines PJ, 2002, Principles of Thermal Analysis and Calorimetry, The Royal Society of
Chemistry, Cambridge, pp. 1-53 and 166-187.
12.
Jenkins R, 1999, X-ray fluorescence spectrometry, 2nd Ed., Wiley, New York.
30
13.
Larkin PJ, 2011, Infrared and Raman Spectroscopy: Principles and Spectral
Interpretation, Elsevier, USA, pp. 1-54 and 73-177.
14.
McDonagh B, 2010, An Overview of the Different Particle Size Measurement
Techniques. Viewed 28 October 2011
<http://www.atascientific.com.au/blog/2010/10/08/overview-particle-size-measurementtechniques>.
15.
Moore DM and Reynolds RC Jr., 1997, X-Ray diffraction and the identification and
analysis of clay minerals, 2nd Ed., Oxford University Press, New York.
16.
Reed SJB, 1996, Electron Microscope Analysis and Scanning Electron Microscopy in
Geology, Cambridge University Press, pp. 1-5 and 65-94.
17.
Smith BC, 1996, Fundamentals of Fourier Transform Infrared Spectroscopy, CRC Press,
Boca Raton.
18.
Smith WE and Dent G, 2005, Modern Raman spectroscopy: a practical approach, Wiley,
Chichester.
19.
Standländer CTKH, 2007, Modern Research and Educational Topics in Microscopy,
pp. 121–131.
20.
Stanley-Wood NG and Lines RW, 1992, Particle size analysis, Royal Society of
Chemistry, Cambridge, pp. 67-81 and 108-143.
21.
Stuart BH, 2004, Infrared spectroscopy: fundamentals and applications, Wiley, England.
22.
Thomsen
V,
2007,
Basic
Fundamental
Parameters
in
X-Ray
Fluorescence.
Viewed 23 November 2011
<http://spectroscopyonline.findanalytichem.com/spectroscopy/article/articleDetail.jsp?id
=428075>
23.
Voutou B, and Stefanaki E-C, 2008, Electron Microscopy: The Basics, Physics of
Advanced
Materials
Winter
School.
Viewed
23
November
2011
<http://www.mansic.eu/documents/PAM1/Giannakopoulos1.pdf>.
24.
Wartewig S, 2003, IR and Raman spectroscopy: fundamental processing, Wiley-VCH
Weinheim.
25.
Williams KL, 1987, An Introduction to X-ray Spectrometry: X-ray Fluorescence and
Electron Microprobe Analysis, Allen & Unwin, Boston.
31
26.
Wirth K and Barth A, 2011, Geochemical Instrumentation and Analysis. Viewed 23
November 2011
<http://serc.carleton.edu/research_education/geochemsheets/techniques/XRF.html>.
32
CHAPTER 3
EXPERIMENTAL
3.1
Materials
The analysed coal fly ash in this study is SuperPozz® ash with 95% of its particles having a
diameter of less than 5 μm. It was obtained from the Ash Resources (Pty) Ltd Ash beneficiation
site at the Lethabo Thermal Power station located in the Free State, South Africa.
Sodium Lauryl Sulphate (SLS) with a purity of 98% was obtained from Merck, and was used
with no further purification.
3.2
Coal Fly Ash modification
A quantity of 10 or 20 g of the dry fly ash sample was weighed and treated with the surfactant
solution, sodium lauryl sulphate (SLS), in a 1:10 solid:liquid ratio. Different concentrations of
the surfactant solution (0.1%, 0.5%, and 2.0% by weight) were studied at different temperature
conditions (30, 50, and 80 °C) as well as exposure times (6, 18, and 66 hours). The effect of
water on the properties of the fly ash sample was investigated by comparing the results to
samples where only distilled water was used under similar treatment conditions.
The fly ash-surfactant mixtures were placed in a WiseBath® WSB digital precise shaking water
bath, obtained from Daihan Scientific, which was controlled at the relevant temperature with
continuous shaking at 130 revolutions per minute.
For the reflux experiments, a reflux system consisting of a heating mantle, round bottom flask
and condenser was assembled. The mixtures were refluxed for 6 hours using water and SLS
solutions with concentrations of 0.5%, 2.0%, and 4.0% by weight.
After the respective treatments, the samples were washed with distilled water numerous times
under vacuum filtration. They were then dried in a laboratory oven at 50 °C for 2 days.
33
It is important to note that a fresh surfactant solution was prepared every day an experiment
was conducted, this was done in order to minimise chances of decomposition of SLS that may
occur during storage. Sugàr et al [1999] investigated the stability of SLS according to
temperature, concentration, time, and material used for storage. They reported that the
concentration of SLS can be influenced by time of storage especially if it is at low quantities.
Bacterial growth was also observed for low concentrated SLS solutions, which could lead to
contaminations.
3.3
Characterisation techniques
3.3.1
X-ray Diffraction (XRD)
X-ray powder diffraction analyses were performed on a PANalyticalX‟Pert Pro powder
diffractometer, Figure 3.1, with an X‟Celerator detector and variable divergence- and receiving
slits with Fe filtered Cu-Kα radiation. The phases were identified using X‟PertHighscore plus
software. The relative phase amounts were estimated using the Rietveld method (Autoquan
Program). 20% of Si (Aldrich 99% pure) was added to the sample for the determination of the
amorphous content. Each sample was milled in a McCrone micronizing mill and prepared for
XRD analysis using a back loading preparation method.
Figure 3. 1: PANalyticalX’Pert Pro powder diffractometer
34
3.3.2
X-ray Fluorescence (XRF)
X-ray fluorescence analyses were carried out on an ARL9400XP + spectrometer (Thermo
Fischer Scientific, Switzerland), Figure 3.2. The samples were ground to <75μm in a tungsten
carbide milling vessel and roasted at 1000 °C to determine the loss on ignition. A mixture of 1
g of the sample and 6 g of Li2B4O7 was then fused into a glass slide. Major element analyses
were executed on the fused bead.
Figure 3. 2: ARL 9400XP
3.3.3 FTIR spectroscopy
Mid-infrared spectra were recorded with a Bruker 70v Fourier transform infrared (FTIR)
spectrometer, Figure 3.3, by placing the finely grounded samples in a diamond ATR
(attenuated total reflection) cell. The sample compartment was evacuated during the
acquisitions and eliminated any contributions from CO2 and water vapour in the atmosphere.
The resolution was 2 cm-1 and 64 scans were signal-averaged in each interferogram.
Reflectance spectra were recorded with the Hyperion microscope attached to the same
instrument. The spectra recorded with the ATR attachment are transmission spectra and
represent the composition of the whole sample, while the reflectance spectra highlight the
composition of the surface of the sample. Both techniques measured the bulk sample, that is the
powder as whole and not individual particles.
35
Figure 3. 3: The Bruker 70v Fourier Transform Infrared (FTIR) spectrometer with
microscope attachment (left) and the TX6400 Raman spectrometer (right).
3.3.4
Raman spectroscopy
Micro-Raman spectroscopy was performed with a T64000 micro-Raman spectrometer from
HORIBA Scientific, JobinYvon Technology (Villeneuve d‟Ascq, France). The Raman spectra
were excited with the 514.5 nm line of an Innova 70v argon ion laser from Coherent and either
the 50x or 100x objective of an Olympus microscope was used to focus the laser beam (spot
size ~10 µm) on individual spheres (Figure 3.4) of the fly ash samples, and also collected the
backscattered Raman signal.
An integrated triple spectrometer was used in the double subtractive mode to reject Rayleigh
scattering and dispersed the light onto a liquid nitrogen cooled Symphony CCD detector. The
laser power at the sample varied between 6 - 10 mW and most of the spectra were recorded
with a 150 s acquisition time and 2 accumulations, but in some cases the accumulations were
increased to result in smoother spectra.
Figure 3. 4: View of fly ash particles under the Raman microscope with 50x (left) magnification
and 100x magnification (right)
36
Raman 2D mapping images were recorded with a WiTec Alpha 300AR instrument equipped
with: an UHTS spectrometer, an EM-CCD camera operated in conventional mode, a 488 nm
excitation laser and a Nikon glass corrected at 60x (NA = 0.8) air objective.
3.3.5
Thermogravimetric analysis (TGA-FTIR)
Thermal stability and evolved gas analyses were performed on a Perkin Elmer TGA 4000
Thermogravimetric Analyzer coupled to a Perkin Elmer Spectrum 100 FTIR spectrometer,
Figure 3.5. Approximately 20 mg of the solid sample was placed in an alumina pan and heated
in air at 10 ºC/min from room temperature up to 400 ºC
Figure 3. 5: Perkin Elmer TGA 4000 FTIR
3.3.6
Scanning Electron Microscopy (SEM)
Ash samples were mounted on a double sided carbon tape by dipping carbon stubs into the
samples. Excess sample was removed from the carbon stubs by gentle blowing with
compressed nitrogen. The samples were then coated with gold using a Sputter-coater
(Emitech K550X, Ashford, England). The samples were viewed on a JEOL JSM 840 Scanning
Electron Microscope (SEM), Figure 3.6, operated at 5 kV. Images were collected with the aid
of a flame-grabber (Orion-version 6)
37
Figure 3. 6: JEOL JSM 840 Scanning Electron Microscope (SEM)
3.3.7
Transmission Electron Microscopy (TEM)
A JEOL JEM 2100F TEM, Figure 3.7, operated at 200 kV, was used to examine the structure
and composition of the fly ash samples at high resolution. The fly ash sample studied was
dispersed in 100% ethanol with the aid of sonication; after which a drop of the diluted
suspension was poured onto a copper grid. The sample was left to dry before it was injected
into the sample holder for analysis.
Figure 3. 7:JEOL JEM 2100F TEM
38
3.3.8
Particle size
Particle size distribution (PSD) of untreated and SLS-treated fly ash particles was obtained by
laser diffraction using a Malvern Mastersizer 2000 fitted with a Hydro 2000G dispersion unit
(Figure 3.8) obtained from Malvern Instruments Ltd. Worcester, UK. This was done in order to
examine the effect of SLS treatment on the size distribution.
Scattered light data were recorded from 2000 to 5000 snapshots of 10 μs each. A polydisperse
mode of analysis and a refractive index of 1.533 with an adsorption of 0.1 were chosen. Size
data collection was performed at constant obscuration in the range 10 – 20%. Samples were run
in duplicate with three runs per duplicate.
Figure 3. 8: Malvern Mastersizer 2000
3.3.9
Contact angle
Contact angle measurements were used to obtain rapid information about the change in surface
properties of the fly ash samples. These measurements were performed on an OCA-20 Contact
Angle-meter (Data Physics Instruments), using the sessile drop method with water as wetting
liquid. To perform these measurements, a small drop of water is deposited on the surface of a
fly ash powder bed which was fixed to a microscope slide by double-sided tape. A picture of
the profile of the drop is taken about 1 second after contact with the powder surface.
39
3.4
References
1. Sugàr M, Schnetz E and Fartash M, 1999, Contact Dermatitis, no. 40, pp. 146–149.
40
CHAPTER 4
CHARACTERISATION OF UNTREATED FLY ASH
4.1
Introduction
Fly ash is a complex and heterogeneous substance. Several characterisation techniques have
been used and reported in literature to characterise its surface, chemical and physical
properties. Guedes et al [2008] and Potgieter-Vermaak et al [2005] employed micro Raman
spectroscopic analysis for the identification and characterisation of the different inorganic and
organic substances that are present in fly ash.
Sarbak et al [2004] studied the surface properties of various fly ashes using numerous
techniques including IR spectroscopy, XRD, thermal analysis and scanning electron
microscopy. TGA-FTIR studies of fly ash were reported by Fermo et al [2000]. The
decomposition of fly ash was observed according to temperature and time. Mahlaba et al
[2011] reported on the particle size distribution of two different South African fly ash samples.
Similar techniques were used to characterise the Lethabo fly ash used in this study. XRD
analysis was conducted to determine its mineralogical composition and phase identification.
Elemental compositions were determined by the use of XRF analysis. Morphological and
topographic characterisation was done with the aid of a SEM and TEM respectively. TGAFTIR analysis was used to study the thermal stability while FTIR and Raman spectroscopy was
used to identify crystalline phases as well as organic and amorphous material.
4.2
Qualitative and quantitative analysis of the chemical composition of Lethabo
coal fly ash using XRD and XRF
Table 4.1 and Figure 4.1 give a presentation of the qualitative and quantitative results obtained
from XRD analysis. The XRD spectrum shows the quantity of crystalline and amorphous
materials present in the sample. Silicon was added to the sample as an internal standard.
41
Counts
10000
5000
0
Lethabo_FA-exp + Si
10
Peak List
20
30
01-079-1458; Mullite, syn; Al4.56 Si1.44 O9.72
00-027-1402; Silicon, syn; Si
01-078-1252; Quartz low, syn; Si O2
40
50
Position [°2Theta] (Cobalt (Co))
60
Figure 4. 1: XRD spectrum of untreated fly ash
70
80
42
The Lethabo coal fly ash sample consists of an amorphous phase (glass) making up 62.1 % of
the total sample weight, with mullite (3Al2O3.2SiO2) [Schneider, 2005] and quartz (SiO2) as the
main crystalline phases.
Table 4. 1: Quantitative XRD results of untreated
Lethabo fly ash
Phase
Amorphous
Weight %
62.1
Mullite
31.8
Quartz
6.2
The quantitative XRD analysis of the untreated fly ash (Table 4.1) shows that 62% of the fly
ash consists of an amorphous phase which in this instance is an alumina silica glass, consisting
of a silicon-oxygen framework in which each silicon atom is surrounded by four oxygen atoms
forming a tetrahedron (Figure 4.2) [Atkin, 2003a].
Cations such as Ca2+, Na+ or K+ inserted into the polymeric structure breaks some of the
connecting bonds, thus lowering the melting temperature of the glass. This results in silicon
atoms that are connected to either 4, 3, 2, 1 or even no oxygen atoms [Tournie, 2008]. The Al3+
atoms can be four-fold or six-fold coordinated. Due to this factor Al3+ can be either a network
forming or network-modifying cation in aluminosilicate melts.
Figure 4. 2: The silica-oxygen framework of a glass
.Figure 1
43
The elemental analysis of Lethabo coal fly ash obtained from the XRF analysis is summarised
in Table 4.2. The sample was found to contain a very low percentage of moisture, carbonates
and hydroxides, thus the low loss on ignition value. The percentage composition of CaO in the
ash sample was found to be approximately 6%, confirming its Class F classification.
The elemental analysis is consistent with the different phases obtained from the XRD data and
the low amount of fluxing ions (Na+, K+, Ca2+, etc.) present indicates that the glass (amorphous
phase) consists of a high percentage of silica.
Table 4. 2: XRF chemical composition of
Lethabo fly ash (Class F)
Compound
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
SO3
P2O5
Cr2O3
NiO
V2O5
ZrO2
Loss on Ignition
TOTAL
4.3
Weight Percentage
49.3
2.01
34.0
5.78
0.05
0.99
5.06
<0.01
0.87
0.24
0.59
0.07
0.05
0.04
0.08
0.52
99.63
Morphological and topographic characterisation of the untreated coal fly ash
sample
The morphology of the analysed coal fly ash sample, observed from SEM, is shown in
Figure 4.3. The particle shape and distribution of the fly ash is reported to be spherical or “ballbearing” with some of the particles seen to be tightly attached to each other, forming
agglomerates. These characteristics are clearly visible in the micrograph.
44
Figure 4. 3: SEM micrographs of untreated Lethabo
coal fly ash
The spherical nature of the particles was also observed on the TEM micrographs (Figure 4.4)
clearly showing the agglomerates formed by the particles.
Figure 4. 4:TEM micrograph of untreated Lethabo coal
fly ash
45
4.4
FTIR and Raman spectroscopic analysis of untreated Lethabo fly ash
4.4.1
FTIR spectroscopy
A summary of FTIR wavenumber assignments relevant to this study is presented in Table 4.3.
Table 4. 3: The FTIR wavenumber assignment of silicate species as well as
other compounds that were found in the analysed fly ash sample.
Wavenumber (cm-1)
~1055
902
828
773, 795
670
Assignment [Voll, 2002]
Si-O asymmetric stretch, mullite, glass, quartz
Al-O stretch, out-of –plane, mullite
Al-O stretch, in-plane, mullite
Asymmetric bend (Si-O-Si), quartz,
Al-O-Al bend , mullite
584
462
Al-O stretch, mullite
Symmetric bend (Si-O-Si, Al-O-Al)
The mid-infrared transmission spectrum, shown in Figure 4.5, recorded in an ATR diamond
cell is characterised by a broad band between 500-1100 cm-1. The spectrum is a
superimposition of the FTIR spectra of silica glass, mullite and a small contribution of -quartz
(see flyash composition, Table 4.1). In the insert, the 500-1100 cm-1 region is enlarged in order,
to distinguish between the small bands that are superimposed on the broad band.
46
Figure 4. 5: FTIR transmission spectrum of untreated fly ash
The FTIR spetra of -quartz and silica glass are shown in Figure 4.6 for comparison purposes
and it is clear that both the spectra exhibit broad peaks around 1000 cm-1, peaks in the region
600-800 cm-1 and a prominet peak at ~460 cm-1. The exact position of the peaks depends on the
degree of crystallinity and percentage of aluminium present. The FTIR spectrum of mullite has
peaks at 1168, 1131, 988, 909, 828, 737, 602, 578 and 482 cm-1.It is clear that many of the
peaks in the spectrum of fly ash overlap with that of glass and quartz.
The strongest peak in the fly ash spectrum observed around 1000 cm-1 is attributed Si-O-Si
asymmetric stretching vibrations and has contributions of glass, mullite and quartz. The band at
902 cm-1 is attributed to Al-O symmetric stretching vibrations from mullite [Voll, 2002 and
Fernańdez - Jimeńez, 2005].
Al-O-Si symmetrical stretching vibrations occur at about 770 cm-1[Vempati, 1994;
Chindaprasirt, 2009] and overlaps of the very characteristic doublet of quartz at 778 and
792 cm-1, which can be distinguished in the insert. The peak at 670 cm-1 has contributions from
47
quartz and Al-O-Al bending vibrations from mullite. The 584 cm-1 peak belongs to Al-O
stretching vibrations (mullite) and the 462 cm-1 peak to symmetric bending vibrations of Si-OSi and Al-O-Al.
Figure 4. 6: FTIR spectra of glass (bottom) and quartz (top).
A reflectance spectrum, recorded with the Hyperion microscope attached to the FTIR
instrument, of the untreated fly ash (Figure 4.7) was also recorded as in reflectance mode the
vibrations on the surface of a sample are enhanced. The spectra were recorded between
850 and 4000 cm-1 as the KBr windows of the microscope cut off infrared radiation below
850 cm-1. The broad band representing Si-O stretch vibrations is clearly visible between
900 and 1400 cm-1 and two extra features can be seen. The broad peak between 3000-3500 cm-1
belongs to water (probably adsorbed on the surface) and the two sharp peaks (2860, 2845 cm -1)
to carbon dioxide and originates from CO2 from the atmosphere.
48
Figure 4. 7: FTIR reflectance spectrum of untreated fly ash
4.4.2
Raman spectroscopy
Raman spectroscopy was used to identify compounds, as well as to the study changes in
chemical bonding that might have taken place during treatments. In order to obtain a picture of
the spatial distribution of the samples 2D mapping of the samples was undertaken.
Table 4. 4: Main peaks in Raman spectra of components found in fly
ash [Voll, 2002: and Schneider, 2005]
Component
-quartz
Mullite
Rutile (high temperature
Alumina
glass
phase TiOsilica
2)
CaSO4
CaPO4
Wavenumber /cm-1 positions
462 (vs), 358 (sm), 206 (m)
1104, 965 (vs), 408, 304,
613 (s), 430 (vs), 258 (s), 153 (sm)
~480 (br), ~1000 (br)
1002 (vs)
961 (vs)
49
The Raman mapping of the fly ash samples were performed with a WiTec Alpha 300 RA
Raman spectrometer. From the 2D array of approximately 10 000 Raman spectra recorded
from the sample, five different spectra could be identified. These spectra are presented in
Figure 4.8 and wavenumber assignments for each component given in Table 4.4. For easier
viewing of the spectra, an offset was added to each spectrum. The images in Figure 4.8
reflect the distribution of the various materials within the analysed sample area. Arrows from
the spectrum to the image indicate which spectrum corresponds to which image.
Figure 4. 8:Raman spectra of untreated fly ash at different analysis points
Spectrum 1 (red) is of α-quartz crystallites with the strongest peak at 462 cm-1. In spectrum 2
(purple) the quartz peak at 462 cm-1 is also visible, with a triplet appearing at 860, 955 and
1008 cm-1. These peaks can be attributed to phosphates and sulphates, but the absence of any
crystalline phases of these materials in the XRD data makes it more likely to be an
intermediate phase of mullite. It has been shown that the mullite Raman spectrum can vary
50
considerably depending on the temperature of formation and other materials such as TiO 2
present in the sample [Shoval, 2001].
The bands in spectrum 3 (green) are broad and not very intense which is typical for a glass.
The Raman spectrum of mullite has its strongest peaks around 304, 408 and 965 cm-1
[Shoval, 2001]. The 408 cm-1 peak overlaps with the first peak of an alumina/silicate glass
representing Si-O bending vibrations at approximately 480 cm-1, the spectrum is therefore a
superimposition of various amorphous phases of mullite and silica glass.
In contrast the very strong bands of rutile, the high temperature phase of TiO2, can be
observed in spectrum 4 (aqua). The Raman cross-section of rutile is very large and therefore a
strong Raman signal is obtained, even though rutile is only present in small quantities
(see Table 4.2). The last spectrum (blue) with a strong band at 965 cm-1 and smaller peaks at
304 and 408 cm-1 can be attributed to mullite. These results are summarised in Table 4.4.
The images were then coloured in the colours of the spectra and overlaid into one colour coded
image as shown in Figure 4.9. Mixed phases appear as mixed colours in the Raman image. The
spectra were normalized to the maximum Raman peak for each component. It is clear from the
image that the main phase of the fly ash is amorphous silica glass and amorphous mullite
(green).
51
Figure 4. 9:Raman spectra of untreated fly ash at different analysis points
It also shows that crystalline quartz (red) is embedded in the spheres, but as separate
crystallites. Rutile (aqua) is also not homogenously distributed, but interestingly enough the
image is spherically shaped and probably formed a layer around one of the glass spheres.
Crystalline mullite (blue) is also sparse. It has been observed that nano-mullite crystals are not
easily observed with Raman spectroscopy and only larger particles can be detected.
4.5
Thermogravimetric analysis of untreated Lethabo fly ash
The results obtained from TGA-IR analyses of the untreated fly ash sample are presented in
Figure 4.10.The temperature range studied was chosen in consideration of the decomposition
temperature of pure sodium lauryl sulphate (SLS). This will be discussed in more detail in the
next chapter.
A very small weight loss percentage (0.1%) was observed for the untreated fly ash over a
temperature range of 25 to 400 °C. The weight loss can be ascribed to the loss of moisture and
52
the onset of oxidation of residual coal present in low percentages in the fly ash. This was
confirmed by the occurrence of a CO2 band in the FTIR of the decomposition gas, taken at
288 °C. However, most of the residual coal trapped in the glass matrix will oxidise at
temperatures exceeding 400 °C and this peak probably originates from carbon close to the
surface of the glass spheres. The low weight loss observed from the TGA-FTIR is in agreement
with the low loss of ignition that was observed on the XRF analysis.
Figure 4. 10: TGA-FTIR results obtained for the untreated Lethabo fly ash
FTIR spectra of the decomposition products were acquired at regular time intervals throughout
the thermal measurement performed in the TGA. No other decomposition products were
observed within the detection limits of the instrument.
4.6
Particle size distribution
Particle size distribution data for the untreated fly ash sample is presented in Figure 4.11. The
lognormal distribution curve indicates that most of the particles in the fly ash sample have a
particle size of approximately 10 microns. The fly ash that was analysed was industrially
classified as over 90% of the material having a particle diameter of less than 15 microns; hence
the particle size distribution indicates the existence of agglomeration of the particles which
leads to the great number of large particles.
53
9
Untreated fly ash
8
7
Volume%
6
5
4
3
2
1
0
0.01
0.10
1.00
10.00
100.00
1000.00
Particle Size (microns)
Figure 4. 11: Particle size distribution of untreated Lethabo fly ash
4.7 Conclusion
The phase characteristics of untreated South African Lethabo fly ash were obtained by using a
number of analytical techniques. XRD analysis has shown that 62% of the fly ash is in the
amorphous glass phase, and that the main crystalline phases are mullite and quartz. The
chemical composition as determined by XRF analysis has confirmed that this is a Class F fly
ash, with low CaO content.
The spherical morphology and small particle size of the fly ash may enhance its applicability as
filler in polymers. TEM analysis has indicated that this sample has a relatively smooth surface
topography, with few agglomerates on its surface.
The phases detected in the Raman and FTIR spectra are in line with the XRD data, and the
Raman image that was generated with the Raman spectra gives a good spatial representation of
the distribution of the different phases in the sample.
54
The sample showed a very low weight loss for temperatures between 25 and 400 °C. This can
be an added advantage when using fly ash as filler in polymers because the filler will be
thermally stable with low volatility at general polymer processing temperatures.
4.8 Reference
1.
Atkin R, Craig VSJ, Wanless EJ, and Biggs S, 2003b, Journal of Colloid and Interface
Science, no. 266, pp. 236–244.
2.
Chindaprasirt P, Jaturapitakkul C, Chalee W and Rattanasak U, 2009, Waste
Management, no. 29, pp. 539–543.
3.
Fermo P, Cariati F, Santacesaria S, Bruni S, Lasagni M, Tettamanti M, Collina E and
Pitea D, 2000, Environ. Sci. Technol., no. 34, pp. 4370-4375.
4.
Fernańdez-Jimeńez A and Palomo A, 2005, Microporous and Mesoporous Materials
no. 86, pp. 207–214.
5.
Guedes A, Valentim B, Prieto AC, Sanz A, Flores D and Noronha F, 2008,International
Journal of Coal Geology, no. 73, pp. 359–370.
6.
Mahlaba JS, Kearsleya EP, Kruger RA and Pretorius PC, 2011, Minerals Engineering
vol. 24, no. 10, pp. 1077–1081.
7.
Petrik L and Mavundla S, 2004, Characteristics of classified and unclassified ash, Ash
Resources (Pty) Ltd., University of Western Cape, pp. 1-19.
8.
Potgieter-Vermaak SS, Potgieter JH, Kruger RA, Spolnika Z and van Grieken R, 2005,
Fuel, no. 84 , pp. 2295-2300.
9.
Sarbak Z, Stan´czyk A and Kramer-Wachowiak M, 2004, Powder Technology, no. 145,
pp. 82-87.
10. Schneider H and Komarneni S, 2005, Mullite, Weinheim, Wiley-VHC Verleg and Co.,
pp. 27-126 and 378-381.
11.
Shoval S, Boudeulle M, Yariv S, Lapides I and Panczer G, 2001, Optical Materials,
no. 16, pp. 319-327.
12.
Tournie A, Ricciardi P and Colomban PH, 2008, Solid State Ionics, no. 179, pp. 21422154.
55
13.
Vempati RK, Rao A, Hess T R, Cocke DL and Lauer HV Jr., 1994, Cement and Concrete
Research, vol. 24, no. 6, pp. 1153-1164.
14.
Voll D, Angerer P, Beran A and Schneider H, 2002, Vibrational Spectroscopy, no. 30, pp.
237-243.
56
CHAPTER 5
SURFACTANT TREATMENT OF FLY ASH SAMPLES
5.1
Introduction
Surface treatment is a principal method applied to change the wetting behaviour of mineral or
inorganic fillers. Surfactants are generally used as surface modifiers to increase the
hydrophobicity of the surface of hydrophilic inorganic fillers, which consequently increases
compatibility between the polar inorganic and non-polar organic matrices.
Nath [2010a and b] have shown how the surface of Australian fly ash can be modified by
sodium lauryl sulphate (SLS), and the resulting fly ash was used as filler in composite films
with polyvinyl alcohol (PVA). They then compared the properties of the unmodified and
modified fly ash using a range of analytical methods. The PVA composite films reinforced with
SLS modified fly ash showed an increase in strength compared to those of unmodified fly ash
filled films. The enhancement of tensile strength was attributed to increased physical bonding
between SLS–FA and PVA surfaces
Ma (2001) modified silica nanoparticles by using a cationic surfactant-CTAB. Optimal
conditions were discussed, and the combined results of FTIR, TGA and BET confirmed that
there exist interactions between the cationic surfactant and anionic surface of the silica. They
have shown that the agglomeration in the silica nanoparticles was reduced upon treatment, and
that the better dispersal state of CTAB-modified silica nanoparticles will be advantageous
when used as a filler in polymeric materials.
In this chapter, the effect of an anionic surfactant (sodium lauryl sulphate) on the surface and
physical properties of South African coal fly ash is investigated, and the resultant products are
then characterised.
Fly ash was initially treated for 18 and 66 hours and after considering the results the time was
reduced to 6 hours under the conditions described in Chapter 3. Two temperature conditions
were investigated for each period of exposure of the fly ash sample to the surfactant.
57
5.2
Sodium Lauryl Sulphate (SLS)
Sodium Lauryl Sulphate (SLS) also known as Sodium dodecyl Sulphate (SDS) is an anionic
surfactant that consists of a 12-carbon chain and a sulphonate head group. Its chemical and
structural formula are given in Figure 5.1. The sulphonate group of the SLS is negatively
charged and polar and acts as the hydrophilic part of the surfactant while the 12-carbon chain is
non-polar and acts as the hydrophilic part.
C12H25SO4Na
Figure 5. 1: Chemical and structural formula of Sodium Lauryl Sulphate
SLS is mainly used in cosmetics and industrial chemicals as a as a detergent which is supported
by its anionic nature. It is a widely used, affordable and easily accessible surfactant, rendering
it suitable for application in the surface treatment of fly ash.
5.3
Results and Discussion
5.3.1
XRD and XRF results
The phase composition of the 2.0 % SLS treated fly ash sample, reported in Figure 5.2 and
Table 5.1, do not differ significantly from those of the untreated fly ash sample, Figure 4.1 and
Table 4.1. Mineral phases were found to be similar for the two fly ash samples. The bulk phase
composition of the treated fly ash was not affected by the SLS treatment.
58
The same observations were made for the fly ash treated at 50 °C for 6, 18 and 66 hours as well
as for the 80 °C 6 and 66 hours treatment conditions. The concentration of the SLS, considering
different treatment time and temperature, did not affect the chemical composition of the
obtained fly ash products.
Table 5. 1: XRD quantitative results of untreated fly ash (FA) and
2.0% SLS treated fly ash sample at 80 °C for 18 hours (18hrs SLS)
values are reported as weight percentage
Phase
Amorphous
Mullite
Quartz
FA
.
62.1
31.8
6.2
59
18hrs SLS
62.8
31.1
6.1
Counts
10000
5000
0
Lethabo_FA-Sls + Si
10
Peak List
20
30
01-079-1457; Mullite, syn; Al4.52 Si1.48 O9.74
01-078-1252; Quartz low, syn; Si O2
00-027-1402; Silicon, syn; Si
40
50
Position [°2Theta] (Cobalt (Co))
60
70
80
Figure 5.2: XRD spectrum of 2.0% SLS treated fly ash sample treated for 18 hours at 80 °C
60
The elemental analyses of the fly ash sample treated with 2% SLS at 6 hours obtained from the
XRF analyses are summarised in Table 5.2. The sample was found to contain higher moisture
content, observed from the increased loss on ignition value in comparison to that of the
untreated fly ash sample.
The percentage composition of CaO in the treated ash sample was found to be slightly lower
than that of the untreated ash. The CaO content of fly ash influences its hydration, and a
decrease in CaO content may also decrease the hydrophilicity of the ash [Ćojbašić, 2005]. A
significant decrease was observed in the iron content after the aqueous SLS treatment,
indicating that both calcium and iron leached into solution.
Table 5. 2: XRF chemical composition of untreated fly ash (FA)
and of 2.0% SLS treated fly ash sample at 80 °C for 6 hours
(6 h SLS), values are reported as weight percentage.
Compound
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P 2 O5
Cr2O3
NiO
V2O5
ZrO2
Loss on Ignition
TOTAL
FA
49.3
2.01
34.0
5.78
0.05
0.99
5.06
<0.01
0.87
0.59
0.07
0.05
0.04
0.08
0.52
99.39
61
6h SLS
52.1
1.71
34.1
3.08
0.03
1.31
4.30
0.11
0.92
0.68
0.03
0.01
0.03
0.04
2.23
100.64
5.3.2
Scanning Electron Microscopy
It seems that the particle distribution of the SLS treated fly ash samples changed in comparison
to that of the analysed raw fly ash sample. Untreated fly ash particles were observed to be
closely packed together forming a considerable amount of agglomerates, as explained in
Chapter 4. The particles of the treated fly ash (Figure 5.3) are spread out, and many of the large
agglomerates were broken down by the interaction of the SLS with the fly ash sample.
Untreated fly ash sample
SLS treated fly ash sample
Figure 5. 3: SEM monograms of untreated and SLS treated fly ash
Better particle separation was observed for all temperature conditions. The degree of
agglomeration is reduced in the treated fly ash resulting in well separated particles. This occurs
mostly for the 0.5 % and the 2.0% SLS treated FA samples. The 0.1% SLS treated sample also
shows a reduction in agglomeration as well particle separation, although to a lesser extent.
The water treated samples also have a slight reduction in agglomeration which could be due to
mechanical action. The samples were subjected to vigorous shaking during treatment.
Figure 5.4 shows the same trend of reduction of agglomeration and increase in particle
separation.
62
Water
0.1% SLS
0.5% SLS
2.0% SLS
Figure 5. 4: SLS treated fly ash samples for 6 hours at 80 °C
Comparing the 2.0% SLS treated samples for 6, 18 and 66 hours at 80 °C respectively
(Figure 5.5) the difference in particle separation can be clearly observed. The micrographs
show that for the same concentration of SLS and the same temperature the exposure period is
important. The 6 hours treated samples show a higher degree of particle separation and smaller
degree of agglomeration when compared to the 18 and 66 hours treated samples.
This effect can possibly be explained by any of the following reasons, but further investigations
are necessary for clarification. Wolff [1992] reported that the presence of silanol groups on the
surface of silica, used as filler in rubber, will cause strong filler-filler interactions leading to
agglomeration of the filler in a rubber matrix. Prolonged exposure of the fly ash to water will
cause hydroxylation of the fly ash surface, which may lead to increased agglomeration of fly
ash particles.
63
6 hours
18 hours
66 hours
Figure 5. 5: 2.0% SLS treated fly ash samples at 80 °C and different exposure periods
On the other hand, the stability of SLS solutions has been studied by Sugàr et al [1999], who
reported that these solutions will degrade with time, and that fresh solutions need to be
prepared on the day of use. It is possible that the SLS solutions decayed during treatment
leading to a decrease in separation of agglomerates.
When considering temperature conditions, the extent of particle separation was seen to be
greater for the samples treated at 80 °C than those treated at 50 °C. FTIR results, still to be
discussed, indicate a significant change in characteristics of the fly ash samples after treatment.
64
50 °C
80 °C
Figure 5. 6: 2.0% SLS treated fly ash samples at 50 and 80 °C for 6 hours
5.3.3
Effect of SLS treatment on the particle size distribution of fly ash
Particle size distribution data was obtained for the untreated and SLS treated fly ash samples.
The results obtained for all treated samples were similar, irrespective of the treatment
conditions. A representation of the plotted data is given in Figure 5.7 for the samples treated
with SLS for 6 hours at 80 °C in comparison to the plotted data of untreated fly ash
Figure 5. 7: Comparison of the lognormal size distributions of untreated fly ash, and samples
treated in distilled water and SLS for 6 hours at 80 °C
65
The median particle size decreased upon treatment, with a corresponding reduction in the
frequency (volume %). The shape of the lognormal distribution curves are wider towards
smaller particle size fractions for the treated samples compared to that of the untreated fly ash,
indicating that the amount of smaller particles have increased upon treatment. This supports the
results obtained from the SEM, indicating smaller particles and less agglomerates for the
treated samples.
Figure 5. 8: Comparison of the lognormal size distributions of untreated fly ash, and samples
treated in distilled water and SLS for 66 hours at 80 °C
Although the shape of the lognormal distribution curves were similar, for the 66 hours
treatment, as seen in Figure 5.8, a small fraction of particles with a size above 100 microns was
observed, indicating increased agglomeration for the 0.5 and 2.0% SLS treated samples. This
effect was not observed for the fly ash treated in a 0.1% SLS solution. These results confirm
the SEM results, shown in Figure 5.5, where a higher degree of agglomeration for the 66 hours
treated samples was observed.
66
5.3.4
FTIR and Raman spectroscopic analysis of the treated fly ash samples
FTIR and Raman spectra of pure SLS, (CH3(CH2)11OSO3Na), were acquired and are presented
in Figures 5.9 and 5.10. The FTIR spectrum shows the presence of water in the sample through
the OH stretching bands at approximately 3500 cm-1 and bending vibrations at 1645 cm-1.
Table 5.3 lists the vibrational frequencies of the functional groups (long aliphatic chain and
SO4) group present in SLS.
Table 5. 3: Assignment of FTIR and Raman Frequencies for SLS [Larkin, 2011]
Wavenumber (cm-1)
2936, 2848
2914, 2845
~3600 (br)
1600
1440
1380
1050
1080-1180 and 1370-1420
600-1300
1000-1300
1000-1300
Assignment
CH3 asymmetric and symmetricstretching
CH2 asymmetric and symmetricstretching
OH stretching, water
OH bending, water
bending CH2
CH3 bending
C-C stretching
Asymmetric and symmetric stretch of SO3
C-C aliphatic chain
C-O Ester Stretch
C-O Ester Stretch
The CH asymmetric stretching bands is observed at 2936 cm-1 (CH3), 2914 cm-1 (CH2) and the
symmetric stretch vibrations at 2848 cm-1 (CH3) and 2845 cm-1 (CH2). The 1466 cm-1 peak
represents the bending vibrations of the CH2 chain The skeletal structure (C-C) of the
surfactant is observed at 1215, 1237 and at 630 cm-1. SLS is characterised with organic
sulphates at 1087 cm-1.
67
Figure 5. 9: FTIR spectrum of Sodium Lauryl Sulphate
Pure SLS
2500
Intensity
2000
1500
1000
500
0
50
550
1050
1550
2050
Wavenumber (cm-1)
2550
3050
3550
Figure 5. 10: Raman spectrum of Pure SLS taken at two wavenumber ranges
68
4050
The most intense bands in the SLS Raman spectrum belong to CH stretching vibrations, similar
as in the FTIR spectrum. The CH2 stretching band was observed at approximately 2882 cm-1
with the CH2 band at 2847 cm-1.
The treated fly ash samples of all treatment conditions were analysed with FTIR and some with
Raman spectroscopy.
Figure 5. 11: FTIR spectra of treated FA samples at 50 °C for 18 hours, untreated FA and pure SLS
For the 18 hours treated fly ash samples, SLS corresponding peaks were observed between
1200 and 1400 cm-1 on the 0.1 %, 0.5% and 2.0 % SLS treated sample, shown in Figures 5.11
and 5.12. CH bands were observed for samples treated at both 50 and 80 °C. The same peaks
also occurred in spectra of the 66 hours treatment sample at both treatment temperatures.
69
Figure 5. 12: FTIR spectra of treated FA samples at 80 °C for 18 hours, untreated FA and pure SLS
Extra SLS corresponding peaks, indicated by an arrow on the spectra in Figure 5.13, were
observed for the 6 hours treated samples compared to the 18 and 66 hours treated samples.
These peaks were observed for the 2.0% SLS treated samples for both temperature treatment
conditions, and were found to be more prominent for the 80 °C treatment temperature as shown
below in the respective temperature FTIR spectra, Figures 5.13 and 5.14. The pure SLS as well
as the raw fly ash spectra were included for comparison.
70
Figure 5. 13: FTIR spectra of treated FA samples at 50 °C for 6 hours, untreated FA and pure
SLS
The spectrum of the fly ash treated with the 2.0% SLS solution show peaks on the spectrum
that correspond to those present in the FTIR spectrum for pure SLS. The SLS spectrum is
characterised by peaks that represent a CH2 bending band at 1468 cm-1 and CH stretching
vibration bands at 2917 and 2850 cm-1. The corresponding peaks on the 2.0% SLS treated fly
ash sample were observed at 1476, 2933 and 2836 cm-1 respectively, Figure 5.14.
Comparing the FTIR spectra obtained at 50 °C to those obtained at 80 °C, not much difference
was observed. However, the peaks on the 2.0% SLS treated fly ash sample were more
prominent and could easily be correlated to those of the SLS sample. The peaks on the 2.0%
SLS treated fly ash sample spectrum were observed at 1476, 2921 and 2855cm-1 respectively.
Two more peaks were observed and were characteristic to the skeletal vibration of the SLS.
These were found to be positioned at 1237 and 1215 cm-1 for pure SLS while for the 2.0% SLS
treated fly ash sample they were positioned at 1224 and 1192 cm-1 respectively.
71
Figure 5. 14: FTIR spectra of treated FA samples at 80 °C for 6 hours, untreated FA and pure SLS
The observed peaks on the treated fly ash sample were found to be relatively at the same
positions for the different treatment temperatures, but the intensities of the peaks differed with
those at 80 °C being more prominent. This could indicate that there isn‟t much difference in
bonding patterns that might exist between the fly ash particles and the SLS.
In Figure 5.15, small sharp peaks at the exact positions of the SLS bands are clearly visible in
the spectrum of the 2.0% SLS modified fly ash. A closer look (see insert) shows that the peaks
at 1248 and 1216 cm-1, assigned to S-O stretch vibrations, have shifted slightly towards lower
wavenumbers, which is an indication that there might be interaction between the SLS and the
fly ash surface. As the shift is quite small this could be attributed to electrostatic interaction.
As the peaks in the C-H stretch region do not display a shift, it points to interaction through the
sulphate anion implying that the hydrocarbon chain is aligned outwards from the fly ash kernel.
This would explain the hydrophobic behaviour observed in the contact angle experiments, still
to be discussed, and is also in line with the shape of the agglomerates observed in the TEM
monographs.
72
It should be noted that this was not observed in all of the spectra recorded for this sample
which suggests that the particles are not evenly coated with SLS, which is in accordance with
the results obtained from the TEM micrographs. In some of the spectra recorded on other
modified samples the same peaks were observed, but with lower intensities.
Figure 5. 15: Comparison of FTIR spectra of pure SLS, untreated fly ash, and 2.0% SLS treated fly ash for
6 hours at 80 °C
Figures 5.16 and 5.17 represent the Raman spectra of the 2.0% SLS treated Fly ash sample at
80 °C for 6 hours. For better viewing and band assignment Figure 5.17 was adapted from
Figure 5.16, with a magnification of the section that contains bands. The spectra were acquired
on different sites of the sample as shown in Figure 5.17. The composition of fly ash is
heterogeneous and that can be seen in the treated sample as well. For the different spectra,
different peaks were observed.
73
2.0% SLS 6 hours 80°C
(a)
1100
(b)
(c)
(d)
Intensity
900
700
500
300
100
-100
400
900
1400
1900
2400
Wavenumber (cm-1)
2900
3400
3900
Figure 5. 16: The different Raman spectra of 2.0% SLS treated fly ash at 80 °C for 6 hours
obtained during 2D scanning of a small area of flyash
Figure 5. 17:Raman spectra of 2.0% SLS treated fly ash at 80 °C for 6 hours using different
analysis sites shown
74
Using the table in Chapter 4, spectrum (a) is of silica-aluminium glass, spectrum (b) belongs to
α-quartz, spectrum (c) to TiO2 (anatase phase) and spectrum (d) to mullite [Schneider, 2005 ].
As from the untreated fly ash the glass component is the main phase present. Quartz, mullite
and the titanium dioxide phases are randomly spaced and in most instances adsorbed on the
surface of spherical glass particles.
2.0% SLS 6 hours 80°C
1200
(b)
(c)
(d)
Pure SLS
1000
Intensity
(a)
800
600
400
200
0
-150
350
850
1350
1850
2350
Wavenumber (cm-1)
2850
3350
3850
4350
4850
Figure 5. 18:Comparison of Raman spectra of pure SLS and 2.0% SLS treated fly ash for 6 hours at
80 °C
5.3.5
Topography of the SLS modified fly ash
The TEM images, Figures 5.19 and 5.20, show agglomerates on the surface of the treated fly
ash spheres. There is a distinct difference between the morphology of the agglomerates on the
untreated and SLS treated fly ash samples. The agglomerates on the untreated fly ash spheres
are considerably less than those on the SLS treated fly ash, and have a less ordered spherical
structure. Another interesting observation, which has been confirmed by the results obtained
from the FTIR and Raman measurements, is that not all SLS modified fly ash particles were
covered with agglomerates to the same degree
75
1µm
Figure 5. 19: TEM image of fly ash treated for 18 hours
in distilled water at 50 °C
0.5% SLS
2.0% SLS
Figure 5. 20: TEM images of treated Lethabo fly ash for 6 hours at 80 °C
Some fly ash particles had a low degree of coverage, while others were covered extensively as
was observed on the SEM monographs in earlier figures. Also, the needle-like shape of the
agglomerates on the SLS treated fly ash was different from that of the agglomerates observed
on the untreated and distilled water treated fly ash sample‟s surfaces, which were more rounded
in shape.
Both Chen et al [2005] and Hower et al [2008] have described the occurrence of carbonaceous
agglomerates on different types of fly ash particles. Chen performed a TEM study on ultrafine
fly ash, and described the morphologies of soot aggregates on the fly ash surface to have
branching chain-like structures. The typical particle size of a these aggregates was in the region
76
of 20-50 nm. The micro textures of these soot particles were described as consisting of
concentrically stacked graphitic layers. These results were confirmed by Hower [2008], who
described the soot particles to have a “fullerene-like nanocarbon with concentric ringstructure”.
However, the morphology of the agglomerates described by these authors was distinctly
different from the needle-like agglomerates observed on the surface of the SLS modified fly
ash in this study.
5.3.6
Thermogravimetric Analysis of SLS modified fly ash
The TGA curve of pure SLS (Figure 5.21) shows a small weight loss corresponding to the loss
of water between 100 and 200 °C. This confirms the FTIR results for pure SLS, discussed
earlier in the chapter, where a broad band corresponding to the occurrence of water was
observed in the FTIR spectrum.
Between 200 and 300 °C, a significant weight loss occurred with a corresponding evolution of
CO2, sulphates and hydrocarbons from the sample. The FTIR spectrum of the gaseous
decomposition product, taken at 288 °C is shown in Figure 5.21. Characteristic peaks of the
aliphatic chain of SLS are observed at approximately 1438 and 2930 cm-1 for bending and
stretching CH2 bands respectively. The CO2 band was prominent at 2346 cm-1with that of the
inorganic sulphate noted at approximately 1165 cm-1.
The occurrence of CO2 as decomposition product of pure SLS is possibly due to oxidation of
the alkyl chain. These measurements were performed in an air atmosphere. Sreedhar [2006] has
also reported the occurrence of CO2 upon thermal decomposition of SLS-doped polyaniline
between 260 – 600 °C, as studied by TGA-MS.
77
Figure 5. 21: TGA-FTIR results obtained for SLS Powder
The results obtained from TGA-FTIR analyses of the SLS modified fly ash sample are
presented in Figure 5.22. Since SLS decomposes in the region 200-300 °C, TGA-FTIR
measurements of the treated fly ash samples were performed only up to 400 °C. FTIR spectra
of the decomposition products were acquired at regular time intervals throughout the thermal
measurement performed in the TGA.
Thermal analysis of SLS modified fly ash sample was different from that of the untreated fly
ash, which was reported in Chapter 4. A significant weight loss was observed over the
temperature range 200 to 250 °C, and two different decomposition products were observed
from the FTIR data.
Firstly, C-H stretching vibrations was observed and was found to be strongest at a
corresponding TGA temperature of 236 °C, which is consistent with the 0.4% weight loss
observed at this temperature. This decomposition product can be ascribed to the presence of
hydrocarbons originating from the SLS in the modified fly ash sample.
78
Figure 5. 22: TGA-FTIR results obtained for the 2.0 % SLS treated fly ash, at 80 °C for 6h
Secondly, the vibrational bands of CO2 were also observed over a wide temperature range, and
were found to be strongest at 319 °C. This can be ascribed to the onset of oxidation of residual
coal within the fly ash as well as the decomposition product of the SLS that might be on the
surface of the fly ash particles.
Sicard [2001] studied the thermal decomposition of SLS from mesoporous alumina by sample
controlled thermal analysis. His study has shown that thermal decomposition of pure SLS
commences at 200 °C, and that the alkyl chain will be removed first. The sulphate head group
is removed thereafter. The author noticed that SLS interacting with the alumina will be
thermally more stable due to strong interactions between the sulphate head group and the
alumina surface, causing an increase in the decomposition temperature of the sulphate group
from the alumina surface. The temperature at which the alkyl chain is lost from the alumina
surface decreased in comparison to that of the pure SLS. Due to the small amount of SLS
adsorbed on the surface of the fly ash, this could not be confirmed in this study.
79
The TGA-FTIR results confirmed the presence of SLS on the surface of the treated samples
which correspond to the TEM results. The TGA-FTIR results also confirm the FTIR results that
were obtained for the treated samples showing an occurrence of SLS corresponding vibrational
bands as well as a shift in the position of the CH bands.
5.3.7 Contact angle measurements
The hydrophobicity of a solid material can be determined by contact angle measurements
[Lazghab, 2005]. Contact angle can be defined as the measure of the ability of a liquid to
disperse on a surface. Water gives an excellent indication of hydrophobicity of a solid material
if used as the testing liquid. A contact angle of less than 90° indicates a hydrophilic surface while
that greater than 90° indicates a hydrophobic surface.
The contact angle of untreated fly ash and the 2.0% SLS treated fly ash at 80 °C for 6 hours
was measured using water as the testing liquid. Untreated fly ash was found to be hydrophilic
with an initial contact angle of approximately 32°. The treated fly ash has shown an initial
contact angle of 137°, indicating a very hydrophobic surface. Figure 5.23 shows the ability of a
water droplet to disperse on the two fly ash samples.
Figure 5. 23: Photographs of water droplets deposited on surfaces of a fly ash powder bed, taken
1 s after deposition
80
5.4
Conclusion
Although the phase and chemical composition of the SLS modified coal fly ash samples was
not altered extensively, significant changes could be observed in its physical properties.An
increased loss on ignition value in comparison to that of the untreated fly ash sample was noted
from the XRF result of the 2.0% SLS treated ash sample.
Particle distribution of the SLS treated fly ash samples changed in comparison to that of the
analysed raw fly ash sample, implying better particle separation for the treated samples. The
median particle size of the fly ash samples decreased upon treatment.
The occurrence of agglomerates on the surface of the SLS modified fly ash seen on the SEM
was confirmed by TEM. The TGA showed a significant weight loss over the temperature range
of 230 to 250 °C, and two different decomposition products were observed from the FTIR data,
CH2 and CO2. The decomposition products can be ascribed to the presence of hydrocarbons
originating from the SLS in the modified fly ash sample and the onset of oxidation of residual
coal within the fly ash.
The possibility of interactions between fly ash and SLS could be deduced from the FTIR
results, where a small shift in peak positions of the S-O stretch was observed. This may be
indicative of electrostatic interactions rather than bonding interactions between SLS and fly
ash. The peaks corresponding to C-H groups of SLS, observed on the treated samples, did not
display a shift. This could point to interaction of the SLS to the surface of the fly ash being
through the sulphate anion, implying that the hydrocarbon chain is aligned outwards from the
fly ash kernel. In contrast to FTIR, the presence of SLS could not be observed with Raman
spectroscopy.
In comparison to the untreated fly ash, the 2.0% SLS treated fly ash was found to be
hydrophobic with an initial contact angle of 137°.
81
5.5 References
1. Chen Y, Shah N, Huggins FE, and Huffman GP, 2005, Environmental Science and
Technology, no. 39, pp. 1177-1151.
2. Ćojbašić LJ, Stefanović G, Sekulić Ž, and Heckmann S, 2005, Mechanical Engineering,
vol. 3, no. 1, pp. 117-125.
3. Hower JC, Dozier A, Tseng MT, and Khatri RA, 2008, Environmental Science and
Technology, no.42, pp. 8471-8477.
4. Larkin PJ, 2011, Infrared and Raman Spectroscopy: Principles and Spectral Interpretation.
Elsevier, USA.
5. Lazghab M, Saleh K, Pezron I, Guigon P and Komunjer L, 2005, Powder Technology,
no. 157, pp. 79-91.
6. Ma J, 2001, Fly Ash Compr. Util., no. 4, p.17 (in Chinese).
7. Nath DCD, Bandyopadhyay S, Gupta S, Yua A, Blackburn D and White C, 2010a, Applied
Surface Science, no. 256, pp. 2759–2763.
8. Nath DCD, Bandyopadhyay S, Gupta S, Yua A, Blackburn D and White C, 2010b, Applied
Surface Science, no. 257, pp. 1216–1221.
9. Schneider H and Komarneni S, 2005, Mullite, Weinheim, Wiley-VHC Verleg and Co.
10. Sicard L, Llewellyn PL, Patarin J and Kolenda F, 2001, Microporous and Mesoporous
Materials, no. 44-45, pp. 195-201.
11. Sreedhar B, Sairam M, Chattopadhyay DK, Mitra PP, and Mohan Rao DV, 2006, Journal
of Applied Polymer Science, vol. 101, pp. 499–508.
12. Sugàr M, Schnetz E and Fartash M, 1999, Contact Dermatitis, no. 40, pp. 146–149.
13. Wolff S and Wang M.J, 1992, “Filler-elastomer interactions. Part IV. The effect of the
surface energies of fillers on elastomer reinforcement” Rubber Chem. Technol. 65 (1992),
pp. 329-342.
82
CHAPTER 6
COUNTER ION EFFECT ON SURFACTANT AGGREGATION
6.1 Introduction
Counter ions have been reported to have an influence on the interaction of surfactants with a
variety of surfaces [Bitting, 1985]. The presence of a counter ion in a surfactant solution
increases the chances of surfactant aggregate formation. Surfactant adsorption can also be
enhanced below the stated c.m.c of the surfactant [Atkin, 2003b].
Surfactant counter ions can alter the hydrophilicity of a surface as well as it hydrophobic
interactions with aggregates that form on the surface [Silbert, 2010]. Enhanced surfactant
adsorption can lead to a decrease in the hydrophilic nature of a surface due to the change of its
morphology after surfactant adsorption [Atkin, 2003b]. Ion binding in a surfactant solution is
increased by the presence of a counter ion, influencing the extent of surfactant adsorption and
micelle formation [Umlong, 2006].
6.2 Experimental
Solutions of NaCl and KIO3 with a concentration of 0.1 M were prepared separately. The
solutions were used to prepare a salt-surfactant solution. SLS was dissolved in the salt solution
to obtain concentrations of 0.1, 0.5 and 2.0% of the SLS. Salt solutions without any added
surfactant were used as controls to determine their effect on the fly ash surface. The saltsurfactant solution was then used to treat fly ash samples as mentioned in Chapter 3. All
samples were treated at 80 °C for 6 hours.
83
6.3 Results and Discussion
6.3.1
XRF
No significant changes were observed on the chemical composition of the treated fly ash
samples, as presented in Table 6.1
Table 6. 1: XRF chemical composition of untreated fly ash (FA), KIO3-SLS and
NaCl-SLS treated fly ash for 6 hours; values are reported as weight percentages.
Compound
FA
KIO3
NaCl
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
Cr2O3
NiO
V2O5
ZrO2
CuO
Loss on Ignition
TOTAL
49.3
2.01
34.0
5.78
0.05
0.99
5.06
<0.01
0.87
0.59
0.07
0.05
0.04
0.08
<0.01
0.52
99.4
51.0
1.7
33.3
3.02
0.03
1.27
4.20
0.15
1.1
0.66
0.03
0.01
0.03
0.04
<0.01
3.66
100.2
52.2
1.71
34.2
3.08
0.03
1.32
4.28
0.14
0.93
0.68
0.04
0.01
0.03
0.04
<0.01
1.70
98.9
6.3.2
Scanning Electron Microscopy
SEM analysis of the treated samples, Figure 6.1, showed that the particles treated with
0.1 M KIO3 solution seem to have formed more agglomerates with each other. When compared
to the particles of the untreated fly ash, the 0.1 M KIO3 treated fly ash particles were not
significantly different. As the concentration of the SLS increases particle separation seems to
be more prominent and particle coating is observed on the surface of some of the particles
(Figure 6.2).
84
0.1 M KIO3
0.1% SLS – 0.1 M KIO3
0.5% SLS – 0.1 M KIO3
2.0% SLS – 0.1 M KIO3
Figure 6. 1: SEM images of fly ash samples treated at 80 °C for 6 hours
Figure 6.2 shows the coating observed on some of the fly ash particles treated with KIO 3–0.5%
SLS and KIO3–2.0% SLS solutions. Only a few particles were found to be coated (shown with
red arrows) in the KIO3–0.5% SLS treated sample. More particles were found to have been
coated for the KIO3–2.0% SLS treated sample, with more than one coated particle found in the
same area of analysis.
0.5% SLS –0.1 M KIO3
2.0% SLS – 0.1 M KIO3
Figure 6. 2: SEM images of fly ash samples treated at 80 °C for 6 hours
85
The particles of fly ash treated with 0.1 M NaCl solution still show the occurrence of
agglomerates, yet smaller in comparison to those of untreated fly ash shown in Figure 4.2.
Particle separation also increased with increasing concentration of SLS in the salt-surfactant
treatment combination.
0.1 M NaCl
0.1% SLS – 0.1 M NaCl
0.5% SLS – 0.1 M NaCl
2.0% SLS – 0.1 M NaCl
Figure 6. 3: SEM images of fly ash samples treated at 80°C for 6 hours
6.3.3
Transmission Electron Microscopy
The coating observed on the SEM micrographs was confirmed by the TEM micrographs,
Figures 6.4 and 6.5. Needle-like shaped aggregates on the surface of the fly ash particles were
observed on both salt-surfactant solution treatments for SLS concentrations of 0.5% and 2.0%.
The needle-like shaped aggregates were seen to be more prominent for the 2.0% SLS
treatment. Most of the fly ash particles were observed to have aggregates on their surface, but
not all of them were completely coated.
86
0.5% SLS – 0.1 M KIO3
0.5% SLS – 0.1 M KIO3 (magnified)
2.0% SLS – 0.1 M KIO3
2.0% SLS – 0.1 M KIO3 (magnified)
Figure 6. 4: TEM images of fly ash samples treated at 80 °C for 6 hours KIO3-Surfactant
solution
The extent of coating observed for the SLS–0.1 M KIO3 treated sample differed for the 0.5%
and 2.0% SLS-salt solution treated sample. Less coating was observed on the 0.1% SLS-salt
solution treated sample. The coating observed on the 0.5% SLS–0.1 M KIO3 treated sample is
defined needle-like structures while those on the 2.0% SLS–0.1 M KIO3 is a network of needle
like structures (Figure 6.4). The increased concentration of SLS led to increased coverage and
build-up of SLS agglomerated on the fly ash surface.
For the SLS–0.1 M NaCl sample treatment, well defined needle like structures were observed
for both the 0.5% and 2.0% SLS–0.1 M NaCl treated samples.
87
0.5% SLS – 0.1 M NaCl
0.5% SLS – 0.1 M NaCl(magnified)
2.0% SLS – 0.1 M NaCl
2.0% SLS – 0.1 M NaCl (magnified)
Figure 6. 5: TEM images of fly ash samples treated at 80 °C for 6 hours with NaClSurfactant solution
6.3.4
FTIR and Raman spectroscopic analysis
SLS characteristic bands were observed on the treated samples, especially for the 0.5% and
2.0% SLS–0.1 M salts, KIO3 and NaCl, (Figures 6.6 and 6.8). SLS characteristic bands were
mainly observed on the 0.5% SLS-0.1 M NaCl treated sample with the 2.0% SLS-0.1 M NaCl
having only a few bands, Figure 6.8.
The bands that were observed for this treatment solutions correspond to those observed for the
SLS solution treatment, as discussed in Chapter 5, Figure 5.11. A CH2 bending band was
observed at 1479 cm-1 and two stretching CH2bands at 2906 and 2841 cm-1. Two bands at 1233
and 1206 cm-1 representing organic sulphate peaks were also present.
88
Figure 6. 6: FTIR spectra of fly ash samples treated at 80 °C for 6 hours with KIO3-SLS
Figure 6. 7: Raman spectra of 2.0% SLS-0.1 M KIO3 treated fly ash at 80 °C for 6 hours at
different analysis sites shown
89
Figure 6. 8: FTIR spectra of fly ash samples treated at 80 °C for 6 hours with NaCl-SLS
Figure 6. 9: Raman spectra of 2.0% SLS-0.1 M NaCl treated fly ash at 80 °C for 6 hours at
different analysis sites shown
90
The Raman spectra, represented in Figures 6.7 and 6.9 are very similar to that of the untreated
samples. The two broad bands in the green specrtum, (a), represent silica alumina glass and is
the main phase in flyash (see Chapter 4). Mullite peaks (spectrum d) are seen on this spectrum
at approximately 920 cm-1 with a peak at 408 cm-1 that overlaps with silicate glass peaks (green
spectrum). The red spectrum, (b), with the strongest peak observed at 462 cm-1 is of crystalline
α-quartz.
Amorphous carbon was observed on the 2.0% SLS-0.1 M KIO3 treated sample through the two
characteristic peaks at ~1350 and ~1600 cm-1 in the yellow spectrum (c). The yellow spectrum
(c) with peaks at 613, 430 and 258 is rutile, the high temperature phase of TiO2 (see Table 4.4,
Chapter 4)
The presence of SLS was not observed using Raman spectroscopy and is below the detection
limits of the Raman instrument used. However, the Raman mapping data rendered interesting
information about the spatial distribution of the various phases in the fly ash. The spherical
shape of rutile, anatase, mullite and in some instances quartz phases is an indication that these
phases form on the surface of the spherical glass particles.
6.4 Conclusion
Treatment of fly ash with a salt-surfactant solution did not alter the chemical composition of
the ash. The introduction of a counter ion in the surfactant solution increased the aggregation of
the surfactant on the surface of the fly ash. The increase of surfactant adsorption led to the
decrease in the extent of agglomeration of fly ash particles.
FTIR spectra confirm the presence of surfactant aggregation on the fly ash particles, showing
the different phases in the sample as well as sulphates that can be attributed to SLS micelles.
Raman spectroscopy was not sensitive enough to detect the organic phase of SLS on the
samples.
91
6.5 References
1. Atkin R, Craig VSJ, Wanless EJ and Biggs S, 2003b, Journal of Colloid and Interface
Science, no. 266, pp. 236-244.
2. Bitting D, and Harwell JH, 1987, Langmuir, vol. 3, no. 4, pp. 500-511.
3. Silbert G, Klein J and Perkin S, 2010, The Royal Society of Chemistry, Faraday Discuss,
no. 146, pp. 309-324.
4. Umlong IM and Ismail K, 2006, J Surface Sci. Technol., vol. 22, no. 1-2, pp. 101-117.
92
CHAPTER 7
REFLUX TREATMENT OF FLY ASH
7.1
Introduction
At a concentration of 2.0% of SLS, the surface coverage of fly ash with the surfactant was
found to be at its optimum at 80 °C and 6 hours, as reported in Chapters 5 and 6. The objective
of this chapter is to investigate the effect of a further increase in temperature and surfactant
concentration on the surface coverage onto fly ash.
The reflux treatment introduced an increase in temperature approaching the boiling point of the
aqueous surfactant solution. An increase in the temperature of ionic surfactants tends to
decrease the c.m.c of a surfactant, leading to an increase in micellisation [Kim, 2003]. This
effect is expected to result in increased coverage of the surfactant on the fly ash surface.
7.2
Experimental
A reflux system, as shown in Figure 7.1, was set up for the reflux treatment of fly ash. The
aqueous concentrations of SLS used were 0.5%, 2.0% and 4.0% by weight. Distilled water was
used as a control for the treatment. The fly ash-SLS mixtures were refluxed at the boiling
temperature of the SLS-water solution for 6 hours.
Figure 7. 1: Reflux system for surfactant treatment on fly ash
93
7.3 Results and Discussion
7.3.1
SEM and TEM
The SEM micrographs of reflux samples, shown in Figure 7.2, show agglomeration of the fly
ash particles mainly for the sample treated in distilled water. The particles of the 2.0% SLS
treatment formed a network of agglomerates. This phenomenon cannot be explained.
Distilled water
0.5% SLS
2.0% SLS
4.0% SLS
Figure 7. 2:SEM images of refluxed fly ash samples treated for 6 hours
Particle coating was observed for all the SLS reflux treated samples with a higher degree of
surfactant-coverage on the surface of the 2.0% and 4.0% SLS treated fly ash samples compared
to the 0.5% SLS treated sample. A great number of particles of these treated samples were
found to be coated. The SEM images of the SLS refluxed fly ash samples, taken at higher
magnification, are shown in Figure 7.3. These images show an even distribution of the coating
of particles as well as the density of the coating, indicating that most of the fly ash particles
were coated with the surfactant.
94
0.5%
2.0%
4.0%
Figure 7. 3: : SEM images of SLS refluxed fly ash samples at higher magnifications, showing
surface
4.0%coating
SLS
Figure 7. 4: TEM images of 2.0% SLS refluxed fly ash samples
The morphology of the coating on reflux treated fly ash samples was seen to be different from
those seen on the 2.0% SLS treated sample at 80 °C for 6 hours. The agglomerates are tighter
and thicker and the observation was confirmed by the TEM micrographs shown in
Figures 7.4 and 7.5. Figure 7.5 shows the extensive coverage of a particle observed for the
4.0% SLS refluxed sample.
95
Figure 7. 5: TEM images of 4.0% SLS refluxed fly ash samples
7.3.2
FTIR and Raman Spectroscopic analysis
The reflux treated samples were characterised with broad OH stretching and H2O bending
bands at approximately 3300 and 1600 cm-1 respectively, Figure 7.6. The presence of SLS
corresponding peaks was notable on the spectra, with CH2 stretching bands at approximately
2840 and 2932 cm-1, CH2 bending bands at 1460 cm-1 and an organic sulphate band which was
observed on the 2.0% SLS treated sample at 1141 cm-1.
Figure 7. 6: FTIR spectra of refluxed fly ash samples treated for 6 hours
96
The Raman spectra (Figure 7.7) shows different components present in the treated sample. The
red spectrum is characteristic of α-quartz at 462 cm-1, the blue spectrum represents mullite with
bands at approximately 310, and 945 cm-1, with glass being represented by the green spectrum.
Sulphates were observed in the red spectrum at approximately 1100 cm-1., as well as
amorphous carbon.
Figure 7. 7: Raman spectra of 4.0% SLS reflux treated fly ash for 6 hours taken at different analysis
sites shown
7.3.3
TGA-FTIR
TGA-FTIR analyses were run for the 4.0% SLS reflux treated sample and the results are
represented in Figure 7.8. A continuous weight loss was observed over the studied temperature
range, with CH and CO2 being observed on the FTIR spectra of the gaseous decomposition
products.
97
The FTIR results of the decomposition products, Figure 7.8, indicate that the weight loss can
initially be ascribed to the decomposition of the SLS from the fly ash surface, followed by
oxidation of carbon contained in the fly ash and in the SLS that might on the surface of the fly
ash particles thereafter. The vibrational band of CO2 was observed at approximately 2350 cm-1
and that of the CH stretch band at approximately 2930 cm-1.
Figure 7. 8: TGA-FTIR results obtained for the 4.0 % SLS refluxed fly ash treated for 6hrs
7.3.4
Contact angle measurements
The coating on the 4.0% SLS refluxed fly ash sample, as observed on the TEM, changes the
surface of the fly ash from hydrophilic to hydrophobic, thus the inability of the water droplet to
disperse on the surface, Figure 7.9. The 4.0% SLS refluxed fly ash samples has shown a
contact angle of 140°.
98
Figure 7. 9: Photographs of water droplets deposited on surfaces of a fly ash powder bed, taken
1 s after deposition
7.4 Conclusion
An increase in the temperature treatment of fly ash samples with aqueous surfactant solution
resulted in increased surface coverage of the fly ash samples treated with 0.5%, 2.0% and 4.0%
surfactant solutions. Kim et al [2003] mentioned that an increase in temperature leads to
increased micellisation of a surfactant, which can explain why more extensive particle coverage
of the surfactant onto the fly ash surface is observed.
The particle coverage of fly ash with SLS changes its surface from hydrophilic to hydrophobic
indicated by the measured contact angles. FTIR spectra show characteristic SLS bands on the
treated fly ash samples. A very low weight loss was observed from the TGA-FTIR result for
the 4.0% SLS treated samples but could clearly indicate the presence of SLS in the gas phase
decomposition product.
7.5
1.
References
Kim H and Lim K, 2003, Bull. Korean Chem Soc., vol. 24, no. 10, pp. 1449-1454.
99
CHAPTER 8
FLY ASH – POLY VINYL CHLORIDE
8.1 Introduction
Poly-Vinyl Chloride (PVC) is a polymer made by the catalytic polymerisation of vinyl
chloride, with 57% chlorine and 43% carbon [Wiebking, 1998]. It is a strong thermoplastic
resin that can be either rigid or flexible and can be softened by heating. The properties of PVC
are modified by the addition of plasticisers and fillers and its flexibility can be influenced by
the kind of compounds added to it [Bryant, 2002]. Plasticisers have been found to increase the
flexibility of PVC products, as well as reduce the difficulty of processing.
PVC is the second most widely used polymer due to its strength, ease of blending and
processing, and its fire preventing properties [Yang, 1999]. The colour of PVC products can be
altered with pigments according to the desired application. It is supplied in powder form. The
density of a rigid PVC sheet ranges from 1.3-1.45 g/cm3 and that of flexible sheets ranges
between 1.1 and 1.35 g/cm3 [Chauffoureaux, 1979].
Additives that can be found in PVC include fillers, stabilisers, plasticisers, pigments and many
more. Fillers can be added to the polymer to improve processing and mechanical properties,
reduce costs and to add bulk to the plastic [Wiebking, 1998]. Fillers are mainly inexpensive
inert materials that are generally inorganic and polar. The chemical composition of fillers has a
great influence on polymer-filler compatibility and interaction [Wiebking, 1998]. The particle
size, shape and distribution as well as surface chemistry of fillers differ and is responsible for
its characteristic strength or weakness when used in polymers [Bryant, 2002]. The amount of
filler added to a polymer depends on the physical requirements of the product.
Calcium carbonates are widely used as functional fillers in rubber and plastic because of their
ability to improve impact strength, replace expensive plastic resin and act as a processing aid
[Xanthos, 2005 and Wiebking, 1998]. Their optical properties are also important to their
functionality as filler. The particle size of a filler is important in polymer processing, since it
can affect the cohesiveness of the produced material [Wiebking, 1998].
100
Fly ash has similar chemical and physical properties as calcium carbonate, however some
differences do exist [Schut, 1999]. Its characteristics are highly comparable to several
commercial fillers [Huang, 2003]. The spherical nature of fly ash has been reported to add
rigidity and compressive strength to plastic compounds and improve throughput but can lead to
increased frail points on plastic formulations [Schut, 1999]. The varying particle size and
colour of fly ash have been reported to be unfavourable characteristics of the fly ash as a filler
[Bryant, 2002].
Fly ash is a cheaper filler in comparison to calcium carbonate due to its low density. The
reported density of fly ash is approximately 2.22 g/cm3 with that of calcium carbonate being
2.70 g/cm3. This implies that a smaller amount of fly ash can be used in a polymer formulation
to give the same physical properties as using a higher amount of calcium carbonate [Schut,
1999].
Fly ash and cenospheres have been studied as filler for different polymers and rubbers and the
research in this field is growing exponentially. Yang et al. [1999] reported that an additive of
10% of untreated fly ash to PVC reduced the wear when compared to PVC without the
additive. The effects of fly ash on polymers and rubbers have been found to improve when the
fly ash samples used had undergone some kind of treatment. Yang et al. [2006] studied the
surface modification of purified coal fly ash, with subsequent application in polypropylene and
reported higher mechanical performance for the modified fly ash.
Deepthi et al [2010] reported that the addition of surface modified cenospheres to polyethylene
increased the tensile strength values. He also mentioned that the spherical nature of the
cenospheres impacted negatively on the values obtained for the elongation at break tests, and
that the samples did not undergo sufficient elongation.
Li et al [1998] reported that fly ash added to post-consumer PET plastic waste did not only act
as a filler but also as a heat conductor, decomposition inhibitor as well as a lubricating agent.
Fly ash can be used in polymers to enhance more than one of its important functions.
101
8.2 Experimental
Different amounts of untreated and treated fly ash samples were tested as filler. Flexible PVC
sheets were made with CaZn as a stabiliser and Di-Octyl phthalate as the plasticiser. Three
different ratios of filler to resin namely, 30, 40, and 70 phr with 100 g of PVC powder, 3 g of
stabiliser and 40 g of plasticiser per sample as indicated in Table 8.1.
Table 8. 1: Materials used for the formation of PVC
Materials
Parts per hundred resin
formulation
Stabiliser (Ca/Zn stearate)
(phr)
100
30
40
70
3.0
Plasticiser (Di Octyl phthalate)
40.0
PVC S68
Fly ash
(Treated or untreated)
Two CaCO3 fillers, Kulubrite 2 and Kulucote 2 (2% stearic acid coated CaCO3), namely were
also tested as standard filler for comparison purposes. A filler to resin ratio of 30 phr was tested
for the two CaCO3 fillers. The dry samples, which include the PVC resin, the filler, and the
stabiliser, were mixed together in a high speed mixer for 3 minutes before the liquid plasticiser
was added to the mix. After adding the plasticiser, the contents were mixed together in the
mixer for a further 5 minutes. The sample was collected and milled on a two roll bridge mill at
168 ºC.
Tensile strength tests were done according to the method and provisions of ISO 527 part 1, on
each of the samples, testing along the direction of milling as well as the transverse direction.
The samples were cut with a die cutter, 5 specimens from each direction. The samples were
then placed in the mechanical laboratory for 36 hours to condition at 23 °C and a humidity of
43%.
Tensile testing was done at a constant speed of 50 mm/min with a gauge distance of 50 mm
according to ISO 527 for tensile strength and elongation analysis on flexible sheets. The
relative densities of the different PVC composites were measured on a Micromeritics
AccuPyc II, 1340 Gas Pycnometer instrument, at 25 °C under a pressure of 1 atm.
102
8.3
Results and discussion
8.3.1
Observations
The time for the formulation of 4.0 % SLS refluxed fly ash and PVC to form a homogenous
band of material on the mill averaged at 6 minutes, while for CaCO3 and PVC it averaged at 5
minutes for the 30 parts per resin formulation. The untreated fly ash and PVC formulation took
longer to flux, averaging at approximately 8 minutes for 30 and 40 phr and 14 minutes for
70 phr respectively.
In comparison, the PVC filled with the SLS treated fly ash had a shorter flux time than the PVC
filled with untreated fly ash. By increasing the filler loading to 70 phr a considerable increase
in the mass of the composite is expected. This can explain the increased fluxing time of the
70 phr composite, resulting in a certain difficulty in processing.
8.3.2
Relative densities
The densities of the different formulations as given in Table 8.2 were found to be between 1.37
to 1.47 g/cm3 indicating values typical of a rigid PVC [Chauffoureaux, 1979].
Table 8. 2: Relative densities of filled PVC composites
Sample Name
Relative Density
30 phrkulubrite 2 (KB)
30 phrkulucote 2 (KC)
30 phr untreated fly ash
30 phr treated fly ash
40 phr untreated fly ash
40 phr treated fly ash
70 phr untreated fly ash
70 phr treated fly ash
1.3982
1.3982
1.3763
1.3745
1.3988
1.4034
1.4691
1.4772
Comparing the fillers at 30 phr, CaCO3 was found to be heavier which is in agreement with
previous results reported by Schut et al [1999]. With the same amount of filler different
densities were observed, confirming the lower density of fly ash.
103
8.3.3
Mechanical Performance
The tensile strength and elongation at break, reported in Table 8.3, for the treated fly ash PVC
formulation is marginally higher compared to that of the untreated fly ash PVC formulation.
Elongation was also observed to be significantly elevated for the TFA formulation for the 30
and 40 phr, but not for the 70 phr as seen in Figure 8.1.
Table 8. 3: Mechanical properties of untreated and treated fly ash filled PVC composites along
milling direction (MD)
FA
TFA
Tensile Strength
Elongation at
Tensile Strength
Elongation at
30
(MPa)
14.3±0.4
break (%)
199.9±9.8
(MPa)
15.2±0.1
break (%)
265.5±4.8
40
10.1±0.3
209.1±7.5
13.4±0.2
221.0±11.1
70
9.0±0.2
185.1±4.9
9.4±0.6
160.1±1.9
Phr
The values obtained for the elongation at break for the two CaCO3 fillers were included
(Table 8.4) to compare the elongation at break of the different fillers. Treated fly ash still
showed a higher resistance to pulling force in comparison to the other fillers at 30 phr, with
kulubrite 2 showing a slightly higher elongation at break between the two CaCO3 fillers. The
untreated fly ash was found to have the lowest resistance to pulling force.
Table 8. 4: Mechanical properties of KB and KC filled PVC composites
along milling direction (MD)
KB
KC
Tensile Strength (MPa)
15.6±0.9
15.6±0.7
Elongation at break (%)
238.5±21.3
231.6±20.4
104
Elongation at break (%)
Elongation MD
280
FA
TFA
250
KC
KB
220
190
160
130
100
25
30
35
40
45
50
55
60
65
70
75
parts per hundred resin
Figure 8. 1: Comparison of the elongation at break of untreated fly ash (FA), treated fly ash
(TFA), kulubrite 2 (KB) and kulucote 2 (KC) filled PVC formulations along milling direction
Even though the tensile strength (Figure 8.2) of the formulation of the PVC containing TFA
was found to be slightly higher than that containing the untreated FA; in comparison to the KB
and KC filled PVC composites no significant difference was observed. Approximately the
same tensile strength was obtained for the PVC containing treated fly ash and the two CaCO3
fillers at a loading of 30 phr.
Tensile Strength MD
18
Tensile Strength (MPa)
17
16
15
FA
TFA
KC
KB
14
13
12
11
10
9
8
25
30
35
40
45
50
55
60
65
70
75
parts per hundred resin (phr)
Figure 8. 2: Comparison of the tensile strength of untreated fly ash (FA), treated fly ash (TFA),
kulubrite 2 (KB) and kulucote 2 (KC) filled PVC composites along milling direction
105
Similar results were obtained for the elongation at break tests performed on the resin
formulations which were cut along the milling direction, compared to samples taken from the
transverse direction. The elongation at break of the TFA and PVC formulation (Table 8.5 and
Figure 8.3) was found to be significantly greater than that of the untreated FA. KB and KC
elongation at break (Table 8.6 and Figure 8.3) was found to be slightly greater than that of the
treated fly ash filler with KB being the highest.
Table 8. 5: Mechanical properties of untreated and treated fly ash filled PVC composites in
transverse direction (TV)
phr
FA
TFA
Tensile
Elongation at break
Tensile Strength
Elongation at break
30
Strength
9.8±0.4
(MPa)
(to the nearest 5%)
147.6±24.2
(MPa)
13.7±0.2
(to the nearest 5%)
210±10
40
9.3±0.5
143.2±18.9
10.5±0.3
150±10
70
8.7±0.3
111.2±13.2
9.7±0.4
145±5
The tensile strength of the samples taken in the transverse direction, Table 8.5 and 8.6 and
Figure 8.4, decreased in comparison to those taken in the milling direction; however the same
trend was seen for all the fillers. The tensile strengths obtained for the PVC resins containing
SLS treated fly ash were higher than those containing untreated fly ash for all filler loadings. At
a loading of 30 phr, the tensile strength of resins containing TFA and KC were equivalent.
Table 8. 6: Mechanical properties of KB and KC filled PVC composites in
transverse direction (TV)
KB
KC
Tensile Strength (MPa)
13.7±0.3
13.0±0.4
Elongation at break (%)
231.3±8.7
214.7±12.0
106
Elongation TV
Elongation at break (%)
250
220
FA
TFA
KC
KB
190
160
130
100
25
30
35
40
45
50
55
60
65
70
75
parts per hundred resin
Figure 8. 3: Comparison of the elongation at break of untreated fly ash (FA), treated fly ash
(TFA), kulubrite 2 (KB) and kulucote 2 (KC) filled PVC composites in transverse direction
Tensile Strength TV
Tensile Strength (MPa)
16
15
FA
TFA
14
KC
KB
13
12
11
10
9
8
25
30
35
40
45
50
55
60
65
70
75
parts per hundred resin
Figure 8. 4: Comparison of the tensile strength of untreated fly ash (FA), treated fly ash (TFA),
kulubrite 2 (KB) and kulucote 2 (KC) filled PVC composites in transverse direction
Alkan (1995) reported that the tensile strength of a resin is expected to decrease with increasing
fly ash content, which was confirmed by the tensile strength observed for the 70 phr
formulation in this study. The 30 phr filler loading was found to be the resin formulation that
107
performed best compared to other filler loadings investigated in this study. It was observed to
have a greater tensile strength and elongation at break in comparison to the other ratios. This
indicates the ability of the formulation to withstand pressure and has shown increased
durability.
The increased volume of fly ash particles in the resin formulation influences the elongation at
break as well as the tensile strength. Figure 8.5 represents the SEM micrographs of the treated
fly ash samples and PVC formulation for the different amounts of filler added. The
micrographs show the volume of the fly ash particles in formulation and the breaking points
which occur at the contact point of the PVC and fly ash.
The greater the volume of fly ash particles the greater the points of possible breaks hence the
observed low values of elongation at break with increased amounts of filler. The spherical
nature of the fly ash as seen in the micrographs can also influence the breaking patterns
[Deepthi, 2010]. Since their stability prevents them from breaking, or disintegrating, the break
point is observed at the point of contact between the spherical fly ash particles and the PVC
resin.
30 phr FA and PVC formulation
40 phr FA and PVC formulation
70 phr FA and PVC formulation
Figure 8. 5: SEM micrographs of the fracture surface of untreated fly ash samples and PVC
formulation for the different amounts of filler added
108
The fly ash particles are seen to be mixed homogeneously with the PVC resin with a certain
level of even distribution allowing for good interactions between the two materials, Figures 8.5.
30 phr TFA and PVC formulation
40 phr TFA and PVC formulation
70 phr TFA and PVC formulation
Figure 8. 6: SEM micrographs of the fracture surface of SLS treated fly ash samples and PVC
formulation for the different amounts of filler added
Figure 8.7 indicates the fracture surface of the CaCO3 filler and shows its irregular shape. The
particles are spread throughout the formulation and the fracture points do not only occur at
filler–resin interaction as seen with fly ash.
Kulubrite 2
Kulucote 2
Figure 8. 7:SEM micrographs of the CaCO3 and PVC formulation for 30 phr
109
8.4
Conclusion
The tensile strength of the SLS treated fly ash-PVC formulation was found to be greater than
the formulation containing untreated fly ash and at a loading of 30 phr, the resin containing
SLS treated fly ash showed maximum strength. The tensile strength of the PVC resin with
treated fly ash, kulubrite 2 and kulucote 2 used as fillers are comparable at 30 phr, however the
elongation at break of the composites containing the CaCO3 fillers was found to be greater.
Greater amounts of filler (FA or TFA) in the formulation resulted in a decrease in the tensile
strength and decreased elongation at break.
Treatment of fly ash with SLS was found to be favourable, since an increase in tensile strength
and elongation at break was observed for the TFA filled PVC, when compared to the PVC
containing untreated fly ash at similar loadings. This can possibly be ascribed to the fact that
the treated fly ash has better particle separation and can therefore have enhanced particle–
particle interaction with the resin.
When considering only the strength and elongation tests, these results indicate that SLS treated
fly ash can successfully replace CaCO3 as filler in PVC when a low loading of filler is used.
However, these tests are preliminary and further research work need to be done; this may
include optimisation of the filler-PVC formulation. The fly ash containing a lower degree of
surfactant coating may also be tested as possible filler. Due to the grey colour of the fly ash, the
poor colour characteristics of the obtained fly ash-PVC resins also remain a concern.
110
8.5
1.
References
Alkan C, Arslan M, Cici M, Kaya M, and Aksoy M, 1995,Resources, Conservation and
Recycling, no. 13, pp. 147-154.
2.
Bryant WS, and Wiebking HE, 2002, ANTEC conference proceedings, no. 3, The effect
of Calcium Carbonate size and loading level on the impact performance of rigid PVC
compounds containing varying amounts of acrylic impact modifiers.
3.
Chauffoureaux JC, 1979, Pure & Appi. Chern., 51, pp.1123-1147.
4.
Deepthi MV, Sharma M, Sailaja RRN, Anantha P, Sampathkumaran P, and
Seetharamu S,2010, Materials and Design, no. 31, pp. 2051-2060.
5.
Huang X, Hwang JY, and Gillis JM, 2003, Journal of Minerals & Materials
Characterization & Engineering, vol. 2, no. 1, pp. 11-31.
6.
Li Y, White DJ and Peyton RL, 1998, Resources, Conservation and Recycling, no. 24,
pp. 87-93.
7.
Schut JH, 1999,
Fly-Ash Filler Stages
a
Comeback;
Plastics Technology.
Viewed: 10 November 2011
<http://www.ptonline.com/articles/fly-ash-filler-stages-a-comeback>.
8.
Wiebking H, 1998, Fillers in PVC: A Review of the Basics, Specialty Minerals Inc.1998.
Viewed: 10 November 2011
<http://www.specialtyminerals.com/fileadmin/user_upload/mti/DataSheets/S-PM-AT177%20pvc%20fillers%20bro.pdf.>.
9.
Xanthos M, 2005, Functional Fillers for Plastics, Wiley-VCH, Weinhem, pp. 1-17 and
43-59.
10.
Yang F, and Hlavacek V, 1999, Powder Technology, no. 103, pp. 182–188.
11.
Yang Yu-F, GaiGuo-S, Cai Zhen-F, and Chen Qing-R, 2006, Journal of Hazardous
Materials, B133, pp. 276-282.
111
CHAPTER 9
CONCLUSION AND RECOMMENDATIONS
9.1
Conclusion
This study, in the first place characterised some of the chemical, physical and surface
properties of untreated South African fly ash using a number of analytical and physical
techniques. Secondly, the surface properties of fly ash were modified using the surfactant
sodium lauryl sulphate (SLS) under a range of different experimental conditions. Thirdly, the
chemical and physical properties of the surfactant treated fly ash samples were compared to
that of the untreated fly ash. Lastly, the validity of the experimental results was tested by
performing a feasibility study to investigate if the SLS treatment of fly ash samples improved
the workability of fly ash as filler in PVC.
A number of conclusions can be drawn from the results obtained from each technique used
during the study namely:
XRD analysis showed that 62% of the untreated fly ash sample used in this study
consists of an amorphous glass phase with the main crystalline phases mullite and quartz. The chemical composition, as determined by XRF analysis, confirmed that the
studied fly ash is a Class F fly ash with low CaO content.The introduction of aqueous
surfactant solutions or additional electrolytes in the form of salts to the surfactant
treatment solutions did not alter the phase or chemical composition of the ash. Although
the phase and chemical composition of the SLS modified coal fly ash samples was not
altered extensively, significant changes could be observed in its physical properties.
The spherical morphology and small particle size of the fly ash may enhance its
applicability as filler in polymers. The results obtained from SEM show significant
agglomeration between the untreated fly ash particles. TEM analysis indicated that the
untreated fly ash sample has a relatively smooth surface topography, with few
agglomerates on its surface.
112
Compared to that of the untreated fly ash, the SEM micrographs show a change in the
particle distribution observed on the SLS modified fly ash samples, implying better
particle separation for the treated samples. The median particle size of the fly ash
samples decreased upon treatment, confirming breakdown of agglomerates. Nath et al
[2010a] also reported a reduction in particle-particle interaction upon surfactant
treatment of Australian coal fly ash, but found an increase in the particle size of the size
fraction below 25 microns. The author attributed this to the coating of surfactant on the
surface of the fly ash. In this study, the occurrence of needle-like surfactant
agglomerates on the surface of the SLS modified fly ash seen on the SEM images was
confirmed by TEM analysis. An increase in the median particle size of the fly ash was
however not observed. The morphology of the agglomerates on the untreated fly ash
was distinctively different from those on the SLS modified fly ash and between samples
treated under different conditions. The modified fly ash particles were not all covered
with surfactant agglomerates to the same degree.
The introduction of a counter ion in the surfactant solution increased the aggregation of
the surfactant onto the surface of the fly ash, which led to a decrease in the extent of
agglomeration between fly ash particles.
The phases detected in the Raman and FTIR spectra for the untreated fly ash agrees
well with the results obtained from XRD. Raman 2D mapping provided a good spatial
representation of the distribution of the different phases in the samples. Interactions
between fly ash particles and SLS could be deduced from the FTIR results, where a
small shift in peak positions of the S-O stretch was observed. This may be indicative of
electrostatic interactions rather than bonding interactions between SLS and the fly ash
surface. The peaks of the C-H stretch region did not display a shift, pointing that the
interaction between SLS and fly ash may take place through the sulphate anion. This
implies that the hydrocarbon chain is aligned outwards from the fly ash kernel, which
would explain the hydrophobic behaviour observed in the contact angle experiments.
The needle-like shape of the agglomerates observed in the TEM images supports these
observations.
113
TGA-FTIR measurements of the untreated fly ash sample showed a very low weight
loss up to 400 °C. The thermal stability of fly ash up to this temperature can be
considered an added advantage when using it as filler in polymers, since general
polymer processing temperatures are below this temperature, and low filler volatility is
advantageous.
The results obtained from TGA measurements showed a significant weight loss over the
temperature range of 230 to 250 °C for the surfactant treated fly ash samples, and CH2
and CO2 could be identified as decomposition products in the gas phase FTIR data. The
occurrence of CH2 as a decomposition product can be ascribed to the presence of
hydrocarbons originating from the SLS in the modified fly ash sample, while CO2
points to the onset of oxidation of residual coal within the fly ash. These results confirm
the FTIR results obtained for the treated fly ash samples.
Contact angle measurements were performed to investigate changes upon wettability of
the fly ash samples. The surfactant coating on the modified fly ash samples, as observed
on the TEM, changes the surface of the fly ash from hydrophilic to hydrophobic, thus
the inability of the water droplet to disperse on the surface.
The use of modified fly ash as filler to PVC was found to be favourable, since an
increase in tensile strength and elongation at break, in comparison to untreated fly ash
filled PVC, was observed. Nath et al [2010b] reported an enhanced strength of
approximately 33% for SLS-FA filled PVA when compared to FA filled PVA. This can
possibly be ascribed to the fact that the treated fly ash has better particle separation and
therefore enhanced particle–particle interaction within the resin. Changing the
wettability of fly ash from hydrophilic to hydrophobic enhanced compatibility between
the fly ash filler and PVC matrix
Results from the feasibility study proved that SLS treated fly ash can replace CaCO3 in
PVC at low filler loading. However the grey colour of fly ash is a disadvantage when
compared to white CaCO3.
114
9.2
Recommendations
Future work will include an investigation into the stability of the surfactant agglomerates on the
fly ash surface, and possible interaction of the surface modified fly ash with different types of
polymers. The reactivity of the fly ash towards other types of surfactants will also be
investigated, with the aim of enhancing interaction between the surfactant and polymer
From the results obtained, it is evident that further investigations need to be performed in order
to fully understand the interactions between SLS and the fly ash surface. The occurrence of
agglomerates on the surface of the SLS modified fly ash was confirmed by TEM and SEM
measurements, but its exact structure and composition is not known yet.
9.3
1.
References
Nath DCD, Bandyopadhyay S, Gupta S, Yua A, Blackburn D and White C, 2010a,
Applied Surface Science, no. 256, pp. 2759–2763.
2.
Nath DCD, Bandyopadhyay S, Gupta S, Yua A, Blackburn D and White C, 2010b,
Applied Surface Science, no. 257, pp. 1216–1221
115
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