Preparation, Flame Retardancy, and Photodegradation

Sang et al. Nanoscale Research Letters (2017) 12:441
DOI 10.1186/s11671-017-2211-9
NANO EXPRESS
Open Access
Titanate Nanotubes Decorated Graphene
Oxide Nanocomposites: Preparation, Flame
Retardancy, and Photodegradation
Bin Sang1,2, Zhi-wei Li1,2*, Xiao-hong Li1,2*, Lai-gui Yu1,2 and Zhi-jun Zhang1,2
Abstract
Most polymers exhibit high flammability and poor degradability, which restrict their applications and causes serious
environmental problem like “white pollution.” Thus, titanate nanotubes (TNTs) were adopted to decorate graphene
oxide (GO) by a facile solution method to afford TNTs/GO nanocomposites with potential in improving the flame
retardancy and photodegradability of flexible polyvinyl chloride (PVC). Results show that the as-prepared TNTs/GO
can effectively improve the thermal stability and flame retardancy than TNTs and GO, especially, the peak heat
release rate and total heat release were reduced by 20 and 29% with only 2.5 wt.% loading. And more, the TNTs/
GO also improve the photodegradability of PVC compared with the neat PVC. The reasons can be attributed to
synergistic flame-retardant and photocatalytic effects between TNTs and GO. The present research could contribute
to paving a feasible pathway to constructing polymer-matrix composites with desired flame retardancy and
photodegradability, thereby adding to the elimination of white pollution caused by polymers.
Keywords: Titanate nanotube, Graphene oxide, Flame retardant, Photodegradation
Background
Polymer-based materials are widely used in our daily
lives and many industrial fields, due to their good properties such as low weight to strength ratio, relatively low
cost, and good physical and chemical stability. However,
most polymers are flammable and could cause potential
hazard to human’s life and property, owing to their organic nature [1–4]. In the meantime, they usually exhibit
chemical inertness and non-biodegradability, thereby
producing severe white pollution to contaminate soil
and water [5–8]. To deal with these issues, many researchers have made efforts to construct novel flame retardants in order to improve the flame retardancy and
reduce the waste pollution of polymers.
For overcoming the flammability of polymer, researchers
have explored a variety of strategies in the past decades
[9]. It has been found that the introduction of nano-fillers
is effective in improving the flame retardancy of polymer
* Correspondence: zhiweili@henu.edu.cn; xiaohongli12345@aliyun.com
1
National & Local Joint Engineering Research Center for Applied Technology
of Hybrid Nanomaterials, Henan University, Kaifeng 475004, People’s Republic
of China
Full list of author information is available at the end of the article
matrix, and non-toxic and environmentally friendly flameretardant additives are of special significance in responding to people’s environmental concern. Among a variety of
non-toxic and environmentally friendly additives,
graphene-based materials are potentially attractive, because graphene and graphene oxide (GO) with layered
structure and high specific surface area can act as barriers
to inhibit heat release and prevent combustion gases from
contact with flame [10–12]. Particularly, graphene or GO
as a significant adjuvant can be combined with inorganic
nanomaterials to afford promising candidates of flame retardants [13–15]. This is ascribed to the fact that the combination of two or more components can often present a
synergism or integrate different flame retarding models,
thereby offering an unexpected enhancement in the properties of composites. For example, inorganic nano-fillers,
metals or metal derivative-based nanomaterials, such as
Ce-MnO2 [16], TiO2 [17, 18], MoS2 [19], layered double
hydroxide [20], and ZnSn(OH)6 [21] can be readily combined with graphene to provide graphene-based flame
retardants.
The abovementioned synergistic strategy makes sense
in improving the flame retardancy of polymer. However,
© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.
Sang et al. Nanoscale Research Letters (2017) 12:441
it would still be infeasible in engineering unless the
white pollution of polymer is simultaneously reduced or
even eliminated. Currently available routes to dealing
with the white pollution of polymer cover landfill and
incineration. Landfill and incineration, nevertheless, can
often cause a serious secondary pollution, such as contamination of soil and water by landfill as well as the release of toxic gas during incineration. This bottleneck,
fortunately, could be overcome by applying sunlight to
photodegrade waste polymer in an efficient and environmentally acceptable mode [5]. For example, TiO2, an important solid-phase photocatalyst, can be incorporated
in polystyrene to afford polystyrene-TiO2 nanocomposite
film that can be efficiently photocatalytically degraded
under ultraviolet (UV) illumination in air [22, 23]. Vitamin C (VC)-modified TiO2 can endow photodegradable
polystyrene-TiO2 nanocomposite films with a high
photodegradation efficiency, which is attributed to the
formation of a TiIV–VC charge-transfer complex with
five-member chelate ring structure that can prolong the
separation of rapidly photogenerated charge [24].
In the present research, therefore, we try to combine
proper flame-retardant additive with phtodegradation
additive in order to simultaneously improve the flame
retardancy and photodegradability of flexible polyvinyl
chloride (PVC), a thermoplastic widely used in the fields
of electronic industry, household electrical appliances,
and building materials. We pay special attention to one
dimensional titanate nanotubes (TNTs) rather than titania nanoparticles with a relatively small specific surface
area, because TNTs combined with GO could have desired flame-retardant properties and photocatalytic activity towards polymer [17, 24]. Such a combination
strategy might be feasible, because TNTs could catalyze
charring and form a net-work structure which acts as an
effective barrier to resist the release of flammable gases
and change degradation pathway [25, 26]. In the meantime, TNTs with radical adsorption effect exhibit excellent smoke suppression ability as well as excellent
photocatalytic activity towards Rhodamine B or waste
water treatment. This article reports the preparation of
TNTs decorated graphene oxide nanocomposites
(TNTs/GO) by a facile solution reaction route. It also
deals with the flame retardancy and photodegradation of
TNTs/GO-PVC composites, with the emphasis being
placed on the strategy to simultaneously improve the
flame retardancy and reduce the white pollution of
polymer.
Methods
Materials
PVC (for injection molding) was purchased from Tianjin
Botian Chemical Company Limited (Tianjin, China).
Commercial sodium titanate nanotubes (NaTA) were
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supplied by Engineering Technology Research Center
for Nanomaterials (Jiyuan, China). Graphite powder
(spectrally pure) was purchased from Sinopharm Chemical Reagent Company Limited (Shanghai, China). Ethanol (C2H5OH) was purchased from Anhui Ante Food
Company Limited (Suzhou, China). Reagent grade concentrated sulfuric acid (98%), 30% H2O2 solution, hydrochloric acid, and 1, 2-ethanediamine (C2H4(NH2)2) were
provided by Tianjin Kermel Chemical Reagent Company
(Tianjin, China). Deionized water was prepared at our
laboratory. All reagents were used as received without
further purification.
Preparation of GO Nanosheets and TNTs/GO Nano-filler
GO nanosheets were prepared from purified natural
graphite through the method reported by Hummers and
Offeman [27, 28]. TNTs/GO nano-fillers were prepared
by a simple and practical solution method. In a typical
procedure, 1.5 g of NaTA was added to 150 mL of H2O
under mild stirring with the assistance of sonication, and
the pH of the solution was adjusted to 1.6 with hydrochloric acid. After 30 min of stirring, 0.1 g of the asprepared GO was added to the solution and sonicated for
1 h to afford a uniform suspension. The suspension was
transferred into a 250-mL flask and maintained at 70 °C
for 5 h. Upon completion of reaction, the precipitate was
collected by filtration and washed several times with distilled water and ethyl alcohol to remove remnant impurities. The as-obtained precipitate was dried at 60 °C for
18 h to provide the TNTs/GO nano-filler.
Preparation of TNTs/GO-PVC Composites
TNTs/GO-PVC composites filled with different contents
of TNTs/GO nano-fillers were prepared with the
method reported in our previous research [29]. A series
of PVC composites denoted as PVC 0.5, PVC 1.5, PVC
2.5, and PVC 3.5 (mass fraction; the same hereafter except for explanation) were prepared in the same manners except that different dosages of TNTs/GO were
incorporated. In addition, PVC composites with 2.5% of
TNTs and GO (TNTs-PVC and GO-PVC) were also
prepared under the same condition for comparative
studies.
Preparation of TNTs/GO-PVC Film
PVC powder (39 g) was suspended in 30 mL of tetrahydrofuran under 2 h of ultrasonic vibration; then,
TNTs/GO (1 g) was dissolved in the suspension under
24 h of continuous vigorous stirring. Upon completion
of stirring, the mixture was spread on a glass plate and
dried for 72 h in an airtight vacuum vessel to afford the
TNTs/GO-PVC film.
Sang et al. Nanoscale Research Letters (2017) 12:441
Characterization
X-ray powder diffraction (XRD) patterns were collected
with an X′ Pert Pro diffractometer (Cu Kα radiation; λ
= 0.15418 nm, operation voltage 40 kV, current 40 mA).
A JEM-2010 transmission electron microscope (TEM)
was performed to observe the morphology and microstructure of various products. X-ray photoelectron spectroscope (XPS) analysis was performed on an Axis Ultra
multifunctional X-ray photoelectron spectrometer, using
Al Kα excitation radiation (hv = 1486.6 eV). Raman spectra were recorder on a Renishaw inVia spectrometer,
laser excitation light at 532 nm. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were
conducted with a DSC6200 thermal analyzer at the scanning rate of 10 °C/min. A JF-3 oxygen index meter was
employed to measure the LOI values of the specimens
with dimensions of 100 × 6.5 × 3 mm3. A WDW-10D
microcomputer control electronic universal testing machine (Jinan Test Machine Company Limited; Jinan,
China) was performed to determine the tensile strength
of PVC-matrix composites. Cone calorimeter (Fire Testing Technology, UK) tests were conducted following the
procedures described in ISO5660. Each specimen with
the dimensions of 100 × 100 × 3 mm3 was exposed to
35 kW/m2 heat flux. The dispersion state of the additives in PVC matrix and the topography of residue chars
were observed with a Nova Nano SEM 450 scanning
electron microscope (SEM). An UV accelerated weathering tester (UV-II, Shanghai Pushen Chemical Machinery
Co. Ltd.; Shanghai, China) was run to evaluate the
photocatalytic degradation behavior of the rectangular
PVC-matrix nanocomposite film (size 7.5 cm × 15 cm)
under 45 °C and 40% humidity.
Results and Discussion
Microstructure of TNTs/GO Nano-filler
Figure 1a shows the XRD patterns of GO, NaTA, and
TNTs/GO nano-filler. GO has a major (001) diffraction
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peak at 2θ = 10.3°, and it corresponds to an interplanar
spacing of 0.84 nm [30]. This means that graphite has
been oxidized and completely exfoliated into sheets.
The diffraction peaks of NaTA can be well indexed to
Na2 − xHxTi2O5·H2O, as reported elsewhere [31]. The
diffraction patterns of TNTs/GO nanocomposite are
similar to those of NaTA. However, the peak intensity
of the TNTs/GO nanocomposite at 28° is lower than
that of NaTA, which could be ascribed to the gradual
transformation in the crystalline structure of NaTA
yielding TNTs during the fabrication of the nanocomposite [32]. Namely, the replacement of Na+ with H+
leads to the decrease in the Na:H ratio of the titanate
in this progress [31]. Moreover, the TNTs/GO nanocomposites exhibit no signals of any other phase of
GO, which is possibly because TNTs are inserted into
the GO layers to cause enhanced exfoliation of GO
[33]. These XRD data demonstrate that the attachment of TNTs to the GO nanosheets contributes to
preventing the aggregation and restacking of the assynthesized GO.
Raman scattering spectra were recorded to investigate
the changes in the structure of the as-prepared carbonaceous materials. Figure 1b shows the Raman spectra of
GO, NaTA, and TNTs/GO nanocomposite. GO and
TNTs/GO nanocomposite exhibit two typical peaks of
GO at about 1588 cm−1 (G band; derived from the inplane vibration of sp2-bonded carbon atoms) and a peak
at 1338 cm−1 (D band; associated with the vibrations of
carbon atoms with sp3 electronic configuration of disordered graphene.) [34]. Besides, the peak intensity ratio of
the D band to G band (ID/IG) is 1.2 for GO but 1.6 for
TNTs/GO nanocomposite, which also proves that TNTs
are successfully incorporated into GO nanosheets to
provide TNTs/GO nanocomposite through the formation of Ti–C and Ti–O–C bonds and the reduction of
GO, and they are will observed in FTIR data (see Additional file 1: Figure S1) [35, 36]. Moreover, aside from
Fig. 1 XRD patterns a and Raman patterns b of GO, NaTA, and TNTs/GO nanocomposites. The XRD patterns displayed that GO was exfoliated
into sheets. The Raman patterns further verified the attendance of GO and proved that TNTs are successfully incorporated into GO nanosheets to
provide TNTs/GO nanocomposite
Sang et al. Nanoscale Research Letters (2017) 12:441
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Fig. 2 TEM images of a GO and b TNTs/GO nanocomposites. GO nanosheets exhibit a layered structure and a typical crumpled morphology,
while the surface of TNTs/GO nanocomposite is relatively smooth and contains a small amount of unconspicuous and slight wrinkle, implying
that the incorporation of TNTs can well prevent the aggregation and restacking of the GO nanosheets
the predominant Raman peaks of GO, the TNTs/GO
nanocomposite shows the characteristic peaks of NaTA,
and these further indicates that TNTs have been successfully incorporated into GO nanosheets, which well
conforms to relevant XRD.
Figure 2 shows the TEM morphology and microstructure of GO and TNTs/GO nanocomposite. GO nanosheets exhibit a layered structure and a typical crumpled
morphology (Fig. 2a), due to their high specific area and
surface energy. And the GO sheets are about one–two
layers stack (as displayed in Additional file 1: Figure S2).
The surface of TNTs/GO nanocomposite, however, is
relatively smooth and contains a small amount of
unconspicuous and slight wrinkle, and the aggregation
and restacking of GO seem to be effectively prevented as
compared with neat GO (Fig. 2b). This implies that the
incorporation of TNTs can well prevent the aggregation
and restacking of the GO nanosheets.
Fig. 3 a–d XPS spectra of GO and TNTs/GO nanocomposites. Ti–C bonds are formed between GO and TNTs to provide TNTs/GO nanocomposite
through a stable chemical attachment rather than a physical absorption
Sang et al. Nanoscale Research Letters (2017) 12:441
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Table 1 Peak area (A) ratios of the oxygen-containing bonds to
the total carbon bonds (obtained by XPS)
Sample
Peak area ratio
ACC/A
ACO/A
AC(O)/A
ATiC/A
GO
0.4625
0.4527
0.0848
–
TNTs/GO
0.6313
0.1114
0.0931
0.1594
In order to further elucidate the interaction between
GO and TNTs in TNTs/GO nanocomposites, we performed XPS measurements. As shown in Fig. 3, the
C1s XPS spectrum of GO is fitted into three peaks
attributed to sp2-bonded carbons (C–C, C=C,
284.7 eV), epoxyl/hydroxyl (C–O, 286.9 eV), and
carboxyl (C(O)O, 288.5 eV), respectively [37]. The
C1s peak at 284.7 eV proves the attendance of 2D
carbon structure, and the C1s peaks at 286.9 eV and
288.5 eV indicate a high percentage of oxygencontaining functional groups. It can be observed that
the oxygen-containing groups of GO and TNTs/GO
have changed during the synthetic process. In order
to quantitatively confirm and compare the change of
the oxygen-containing groups, we calculated the peak
area ratios of oxygen-containing groups to the total
carbon bonds. As listed in Table 1 (notes: A = ACC +
ACO + AC(O) + ATiC), the percentage of C–O bond of
TNTs/GO is remarkably lower than that of GO, and
the percentage of sp2-bonded carbon increases from
46.25% of GO to 63.13% of the TNTs/GO nanocomposite. This indicates that GO is partly reduced and
most of the oxygen-containing groups are removed
from GO during the formation of TNTs/GO nanocomposite, which is also supported by relevant Raman
data (the ID/IG ratio of GO is smaller than that of
the TNTs/GO nanocomposite). The reason might lie
in that TNTs can reduce GO into graphene under
UV or visible light photocatalytic process [38, 39].
Moreover, TNTs/GO nanocomposite shows a weak
C1s peak at 283.2 eV, and this peak is assigned to
Ti–C bond (corresponding Ti(2p3/2) and Ti(2p1/3)
peaks emerge at 460.4 and 465.9 eV) [36].
The possible formation process of Ti–C bonds can be
described as following. TNTs have a scroll-type nanotube structure, and their (100) facets are of a stepped
surface structure consisting of Ti and exposed O atoms
[40]. In an acidic solution of pH = 1.6, the walls of TNTs
will undergo dehydration and structure transformation
to afford defects [32, 41]. As a result, Ti–C bonds are
formed between GO and TNTs to provide TNTs/GO
nanocomposite through a stable chemical attachment rather than a physical absorption. Since Ti–C bonds can
facilitate the interfacial charge transfer between TiO2
and graphene [42], the high proportion of Ti–C bonds
could be of special significance for the application of
TNTs/GO nanocomposite in the photodegradation
catalysis.
Thermal Stability and Mechanical Properties of Flexible
PVC Composites
The TGA and DTG curves of PVC and PVC-matrix
composites filled with various contents of TNTs/GO are
displayed in Fig. 4. Corresponding thermogravimetric
data are summarized in Table 2, where the temperatures
at which 5% (T5%), 50% (T50%), and maximum (Tmax)
mass loss occur are described as the initial degradation
temperature, half degradation temperature, and maximum degradation temperature, respectively. It can be
seen that the T5%, T50%, and Tmax of TNTs/GO-filled
PVC composites are a little bit higher than those of pure
PVC; and in particular, the PVC-matrix nanocomposite
with 2.5% of TNTs/GO nano-filer has a quite higher
Tmax than the virgin PVC. These data indicate that
TNTs/GO nano-filler can enhance the thermal stability
of PVC. This could be attributed to the good dispersion
state of TNTs/GO in PVC matrix (see Additional file 1:
Figure S3), the good interfacial interaction between
TNTs/GO and PVC molecules, and the synergistic effects between TNTs and GO. Namely, the good
Fig. 4 TGA a and DTG b curves of flexible PVC and PVC-matrix composites in air atmosphere. The thermal stability of the PVC-matrix
nanocomposite was improved, which could be ascribed to the synergistic effects between TNTs and GO
Sang et al. Nanoscale Research Letters (2017) 12:441
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Table 2 Thermogravimetric data of flexible PVC and PVC-matrix
composites in air atmosphere
T5%/°C
T50%/°C
Tmax1/°C
Tmax2/°C
PVC 0
228
300
296
524
PVC 0.5
239
308
310
544
PVC 1.5
230
306
308
548
PVC 2.5
232
308
309
554
PVC 3.5
230
309
311
551
Sample
dispersion of TNTs/GO can promote the crosslinking of
PVC chains; the GO nanosheets can act as physical barriers to inhibit the transport of heat and mass under the
help of TNTs; the TNTs can catalyze charring and anchor in the char to enhance the stability of residue char.
As a result, the underlying PVC matrix is protected, and
the thermal stability of the PVC-matrix nanocomposite
is improved.
The tensile strength at break of pure PVC and its
composites with different contents of TNTs/GO nanofiller is presented in Fig. 5a, and that of the PVC filled
with TNTs alone or GO alone is presented in Fig. 5b for
a comparison. It can be seen that the incorporation of
TNTs or GO causes a decrease in the tensile strength of
PVC matrix, which is because the inorganic fillers exhibit poor compatibility and weak interaction with the
PVC matrix. To our surprise, although the elongations
at break of the PVC-matrix composites tends to decrease
with increasing content of TNTs/GO nano-filler, the
tensile strength of the PVC-matrix composites is always
higher than that of neat PVC whether the content of
TNTs/GO nano-filler is high or low. This could be because the TNTs/GO nano-filler exhibits good exfoliation
and dispersion as well as enhanced interfacial adhesion
with the PVC matrix and can transfer stress effectively.
Flame Retardancy of Flexible PVC Composites
Limiting oxygen index (LOI) is a criterion to screen inflammable materials. The LOI data of PVC and PVCmatrix composites are displayed in Fig. 6. The LOI value
of the neat PVC is 25.8, corresponding to its inherent
flammability. The LOI values of PVC filled with GO
alone or TNTs alone are 26.2 and 26.0, respectively,
which indicates that GO and TNTs can separately improve the flame retardancy of PVC to some extent. This
is because GO exhibits a barrier effect, while TNTs can
catalyze the formation of char and exhibits adsorption
effect and radical adsorption effect. As to TNTs/GOfilled PVC composites, their LOI values tend to increase
with increasing content of the nano-filler up to a mass
fraction of 2.5%. Particularly, the PVC-matrix nanocomposites containing 2.5% of TNTs/GO nano-filler exhibits
a maximum LOI of 27.4, higher than that of the PVC
filled with GO alone or TNTs alone. This demonstrates
that there is some kind of synergistic flame-retardant effect between TNTs and GO.
Peak heat release rate (pHRR), total heat release
(THR), total smoke release (TSR), and average specific
mass rate (AMLR) are important parameters to evaluate
the flammability of various materials under real-world
Fig. 5 Tensile strength and elongation at break of flexible PVC and PVC-matrix composites with various content of TNTs/GO (a, c) and with
2.5 wt.% of GO or of TNTs and TNTs/GO (b, d). The incorporated TNTs/GO to PVC could enhance the tensile strength of the PVC-matrix composites, because the TNTs/GO nano-filler exhibits good exfoliation and dispersion that enhance the interfacial adhesion with the PVC matrix
Sang et al. Nanoscale Research Letters (2017) 12:441
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Fig. 6 LOI values of flexible PVC and PVC-matrix composites with various content of TNTs/GO (a) and with 2.5 wt.% of GO or of TNTs and TNTs/
GO (b). The PVC-matrix nanocomposites containing 2.5% of TNTs/GO nano-filler exhibits a maximum LOI of 27.4, higher than that of PVC filled
with GO alone or TNTs alone. This demonstrates that there is the synergistic flame-retardant effect between TNTs and GO
fire conditions, and they can be obtained from cone calorimetry tests. Corresponding test results are shown in
Fig. 7, and the data are summarized in Table 3. It can be
seen that neat PVC has a sharp HRR peak with a pHRR
value of 355.4 kW m−2 (Fig. 7a). Incorporating 2.5%
TNTs into PVC drops the pHRR value down to
233.7 kW m−2, and such a reduction by 34.2% is attributed to the fast charring on the surface of filled PVC
under the catalytic action of TNTs. However, the HRR
curve of TNTs-filled PVC contains a second peak
around 230 s, which means that the char is unstable and
can be destroyed easily. In contrast, the PVC nanocomposite filled with 2.5% of TNTs/GO has a pHRR value of
282.4 kW m−2, a reduction by 20.5% in comparison with
that of neat PVC. Moreover, the pHRR of TNTs/GOfilled PVC nanocomposite declines rapidly to a low value
at 130 s and maintains steady with extending duration.
This indicates that the char on the surface of the TNTs/
GO-filled PVC composites is very stable and can act as a
physical barrier to hinder the transmission of heat,
Fig. 7 HRR a, THR b, SPR c, and TSR d versus time curves of pure flexible PVC and PVC-matrix composites obtained from cone calorimetry test at
35 kW m−2. The HRR curve of TNTs-filled PVC contains a second peak around 230 s, which means that the char is unstable and can be destroyed
easily. The TNTs/GO inhibited the second heat release peak, and with 2.5% content, the pHRR value is 282.4 kW m−2, a reduction by 20.5% in
comparison with that of neat PVC. This indicates that the char on the surface of the TNTs/GO-filled PVC composites is very stable and can act as
a physical barrier to hinder the transmission of heat
Sang et al. Nanoscale Research Letters (2017) 12:441
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Table 3 Cone calorimetric data of pure PVC and its composites
pHRR
(kW m−2)
THR
(MJ m−2)
TSR
(m2 m−2)
AMLR
(g s−1)
Pure PVC
355.4
65.3
3936.8
20.4
TNTs-PVC
233.7
51.9
3670.9
14.6
TNTs/GO-PVC
282.4
46.3
3322.7
14.2
Sample
thereby leading to greatly lowered THR, TSR, and
AMLR.
To further confirm the flame-retardant mechanism
of PVC-matrix composites, we conducted SEM analyses of the residual chars. As shown in Fig. 8a–c, all
of the exterior chars have holes with different sizes
and distributions. This means that the holes on pure
PVC occupy a large area and are deep enough to
penetrate the bulk. The exterior chars of TNTs-PVC
are similar to those of neat PVC, but the holes of the
former are much bigger. These holes can act as transport channels for heat and mass. Therefore, neat PVC
and TNTs-PVC allow the heat to easily transfer from
combustion surface to polymeric matrix and flammable organic volatiles to escape from underlying
matrix to combustion zone. On the contrary, the exterior chars of TNTs/GO-PVC contain fewer holes
which are mostly impotent. This means that the
channels for heat and mass transport are cut off. The
interior chars of TNTs/GO-PVC (Fig. 8f ) are compact
and continuous at the surface and contain titanium
dioxide anchored inside, while those of neat PVC and
TNTs-PVC contain many small cracks or holes. This,
in association with the much smaller ID/IG of TNTs/
GO-PVC (1.1) as compared with that of neat PVC
(1.6; see Additional file 1: Figure S4), suggests that
TNTs/GO can transform carbon sources into char,
thereby adding to the flame retardancy of the PVCmatrix composites. In one word, the stable, compact,
and continuous char layers of TNTs/GO-PVC composites can act as good physical barriers to reduce
the pHRR, THR, SPR, and TSR and resist thermal
shock as well.
The synergistic flame-retardant effect between TNTs
and GO is schematically illustrated in Fig. 9. Firstly,
TNTs decorated on the surface of GO nanosheets suppress the re-stack of GO and promote the uniform dispersion of TNTs/GO in the PVC matrix, thereby
enhancing the thermal stability of PVC. Secondly, TNTs
and GO with large surface areas make contributions to
absorbing pyrolysis gas during combustion, prolonging
the diffusion way of volatile gases, and increasing the
catalyst residence time, thereby allowing easy transformation of the pyrolysis gases into carbonaceous char
under the catalytic action of TNTs. Thirdly, the GO
skeleton can act as a template for the carbonaceous char
and promote the formation of multiple char under the
help of TNTs, thereby affording a continuous and compact char layer. Finally, titanium dioxide transformed
from TNTs during combustion (see Additional file 1:
Figure S4 for more details) can be anchored in the char
layers to enhance the thermal stability of the char layers,
which makes it feasible for the char layers to more effectively resist the thermal shock at the second peak,
prevent the heat, and reduce smoke release. These multiple factors jointly function and account for the enhanced flame retardancy of PVC-matrix composites.
Fig. 8 SEM images of exterior and interior morphology of the residue chars of PVC (a, d) TNTs-PVC (b, e) and TNTs/GO-PVC (c, f) composites. The
char of TNTs/GO-PVC was more compact and continuous than that of the neat PVC or TNTs-PVC
Sang et al. Nanoscale Research Letters (2017) 12:441
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Fig. 9 Schematic illustration of flame-retardant and photocatalytic degradation mechanism of TNTs/GO
Photodegradation of TNTs/GO-filled PVC Film
As mentioned previously, the TNTs/GO nano-filler
could act as a photocatalyst for PVC degradation. Figure 10 presents the weight loss of TNTs-PVC film and
TNTs/GO-PVC film under UV illumination. The weight
loss of TNTs/GO-PVC film decreases gradually with extending irradiation time and undergoes a reduction of
4.6% in 216 h, while the PVC film undergoes only 2.5%
of weight loss under identical experimental condition.
Besides, the weight loss rate of TNTs/GO-PVC film is
higher than those of the PVC film and TNTs-PVC film.
This could be attributed to two aspects. On the one
hand, reduced graphene oxide nanosheets have a large
surface area, providing a large interfacial contact surface
area and strong interaction with TNT that helps to
Fig. 10 Weight loss of pure PVC and PVC-matrix nanocomposite
films under UV-light irradiation (40% humidity). Incorporated TNTs/
GO to PVC could accelerate the photo degradation of PVC. The large
interfacial contact surface area and strong interaction between TNT
and GO could help to reduce the charge resistance and the charge
recombination rate, and the Ti–C bond could also promote the
interfacial charge transfer, thereby accelerating the degradation of
TNTs/GO-PVC nanocomposite films under UV irradiation
reduce the charge resistance and the charge recombination rate, which makes it feasible for reduced graphene
oxide nanosheets to act as excellent electron acceptors
and well transport charge [43–46]. On the other hand,
the Ti–C bond can also promote the interfacial charge
transfer, thereby accelerating the degradation of TNTs/
GO-PVC nanocomposite films under UV irradiation
[42]. The catalytic efficiency of the TNTs/GO nano-filler,
however, is fairly low. This is possibly because the incompletely reduced GO nanosheets have a large amount
of oxygen-containing functional groups and defects that
hinder the charge transport while the dehydration of
TNTs at pH = 1.6 also causes defects in the nanotubes to
facilitate charge recombination. Therefore, the oxygencontaining functional groups on the surface of graphene
nanosheets are simultaneously beneficial to the flame resistance and the photodegradation of PVC-matrix
composites.
Figure 11 shows the surface morphologies of PVC film
and PVC-matrix nanocomposite film before and after
216 h of UV irradiation. It can be seen that the two
kinds of PVC films are smooth before UV irradiation.
After UV irradiation, many holes are formed on the surface of PVC film (Fig. 11c, d), which indicates that the
film undergoes obvious decomposition under UV irradiation. In the meantime, more and large cavities are
formed on the surface of TNTs/GO-PVC film after UV
irradiation (Fig. 11d), which indicates that the TNTs/GO
nano-filler does promote the photocatalytic degradation
of PVC under UV irradiation.
The functional groups in the PVC-matrix nanocomposite films were also monitored by FTIR analysis. As
shown in Fig. 12a, the Raman peak of carbonyl (C=O)
groups at 1680–1800 cm−1 proves that the TNTs/GOPVC nanocomposite film does undergo degradation reaction during UV irradiation. Moreover, the intensity of
the C=O absorption peak increases continually with increasing irradiation time; and the increase in the intensity of the C=O absorption peak is more pronounced for
Sang et al. Nanoscale Research Letters (2017) 12:441
Page 10 of 12
Fig. 11 SEM images of the morphology of PVC (a, c) and TNTs/GO-PVC (b, d) films before and after 72 h of UV irradiation. More and large
cavities were formed on the surface of TNTs/GO-PVC film after UV irradiation, which indicates that the TNTs/GO could promote the photocatalytic
degradation of PVC under UV irradiation
TNTs/GO-PVC film than for neat PVC film and for
TNTs-PVC film (Fig. 12b). This further demonstrates
that the TNTs/GO nano-filler can indeed promote the
photo-oxidation reaction of the PVC matrix.
Conclusions
In summary, TNTs/GO nanocomposites were prepared
through a facile solution method. The as-prepared
TNTs/GO nano-filler can simultaneously improve the
flame retardancy and photodegradability of PVC, which
could be attributed to the synergistic effects between
TNTs and GO. On the one hand, TNTs can suppress
the re-stack of GO and promote the uniform dispersion
of TNTs/GO in the PVC matrix; GO nanosheets can act
as electron acceptors to reduce the charge resistance
and charge recombination rate, and the GO skeleton can
also act as a template for the carbonaceous char and
promote the formation of multiple char under the help
of TNTs. On the other hand, titanium dioxide transformed from TNTs during combustion can be anchored
in the char layers to enhance the thermal stability of the
char layers and accelerate the photodegradation of PVC
Fig. 12 FTIR spectra of the carbonyl (C=O) groups of TNTs/GO-PVC film versus UV irradiation time (a) and before and after photodegradation (b).
The intensity of the C=O absorption peak of TNTs/GO-PVC film increases continually with increasing irradiation time; it is more pronounced than
that for neat PVC film and for TNTs-PVC film. This further demonstrates that the TNTs/GO nano-filler can indeed promote the photo-oxidation
reaction of the PVC matrix
Sang et al. Nanoscale Research Letters (2017) 12:441
matrix under UV irradiation. As a result, TNTs/GOPVC composites exhibit enhanced flame retardancy and
photodegradability than TNTs-PVC and GO-PVC counterparts. The present research, hopefully, would help to
provide a promising strategy for constructing polymermatrix composites with simultaneously improved flame
retardancy and photodegradability, thereby shedding light
on dealing with the white pollution of commonly used
polymers. Further researches are to be conducted concerning the enhancement in the flame-retardant and
photodegradation efficiencies of the TNTs/GO nano-filler.
Page 11 of 12
6.
7.
8.
9.
10.
11.
Additional file
Additional file 1: Figure S1. AFM images of GO sheets, the thickness
of the GO fragment is ca. 1.72 nm. Figure S2. FTIR spectra of GO, TNTs,
and TNTs/GO nanocomposites. Figure S3. SEM images of the fracture
surface of flexible PVC (a), and GO-PVC (b), TNTs-PVC (c), and TNTs/GO
(d). The inset image of d is Ti-element EDA mapping of TNTs/GO
composites. Figure S4. XRD patterns (a) and Raman patterns (b) of
char of PVC and TNTs/GO-PVC.
12.
13.
14.
15.
Acknowledgements
This work was supported by the Ministry of Science and Technology of
China (project of “973” Plan; grant no. 2015CB654703), the Scientific
Innovation Talent of Henan Province (grant no. 164200510005), the Science
and Technology Research Program of Henan Educational Committee (grant
no. 16A430001), and the Program for Innovative Research Team from the
University of Henan Province (grant no. 17IRTSTHN004).
Authors’ Contributions
ZL, XL, and ZZ conceived and designed the experiments. BS performed the
experiments and analyzed the data. LY contributed the analysis tools. BS and
ZL wrote the paper. All authors read and approved the final manuscript.
16.
17.
18.
19.
Competing Interests
The authors declare that they have no competing interests.
20.
Publisher’s Note
21.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
Author details
1
National & Local Joint Engineering Research Center for Applied Technology
of Hybrid Nanomaterials, Henan University, Kaifeng 475004, People’s Republic
of China. 2Collaborative Innovation Center of Nano Functional Materials and
Applications of Henan Province, Henan University, Kaifeng 475004, People’s
Republic of China.
22.
23.
24.
Received: 21 April 2017 Accepted: 22 June 2017
25.
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