Polythiourethane microcapsules as novel self

Polym. Bull.
DOI 10.1007/s00289-017-2021-3
ORIGINAL PAPER
Polythiourethane microcapsules as novel self-healing
systems for epoxy coatings
Tomasz Szmechtyk1 • Natalia Sienkiewicz1
Krzysztof Strzelec1
•
Received: 2 February 2017 / Revised: 29 March 2017 / Accepted: 8 April 2017
Ó The Author(s) 2017. This article is an open access publication
Abstract A novel group of microcapsule-based self-healing systems for epoxy
coatings was developed. Microcapsules with polythiourethane shell wall were
synthesized via interfacial polymerization from selected diisocyanates and thiols
and dispersed in epoxy matrix. The obtained composites were tested for their selfhealing efficiency using three-point bending test (TPBT) and scratch test. Two sets
of each composite samples as well as reference (neat diglycidyl ether of bisphenol A
epoxy with polyamine adduct hardener) were tested with TPBT using two methods.
Standard method according to ISO 178 was applied for the first set and custom
method with pre-bending with 20 N force and standard TPBT after 24 h of selfhealing—for the second set. Pre-bending was applied to obtain microcracks
(without sample cracking) for internal self-healing process occurrence. Scratch test
allowed to evaluate self-healing efficiency at composite surface and chemical
resistance of samples. FT-IR spectroscopy was conducted to confirm occurrence of
self-healing process based on polyurethane secondary network forming.
Keywords Self-healing Microcapsules Epoxy coating Polythiourethanes Polyurethanes
Introduction
Crack propagation and material damage during service are common problems for
coatings made of epoxy resins. Solutions which have to meet these limitations are
self-healing systems (S-HS). The simplest and the least expensive S-HS are
microcontainers (microcapsules and hollow fibers) with healing agent inside,
& Tomasz Szmechtyk
tomaszszmechtyk@gmail.com
1
Faculty of Chemistry, Institute of Polymer and Dye Technology, Lodz University of
Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland
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Polym. Bull.
dispersed in polymer matrix. Microcapsules are also more versatile option, because
of wider spectrum and ease of synthesis methods. In both cases, self-healing process
occurs when microcracks are propagating and shell wall of microcontainer ruptures.
Consequently, liquid healing agent is released to polymer matrix and polymerizes
forming secondary polymer matrix, which binds separated surfaces of microcracks.
First application of microcontainers as S-HS was in 1996—hollow glass fibers
were filled with two-component epoxy glue [1]. Another solution was proposed by
White et al. [2]. Urea–formaldehyde microcapsules containing dicyclopentadiene
(DCPD) were dispersed in polymer matrix with Grubbs catalyst, which enabled
ring-opening metathesis polymerization of DCPD released during crack propagation. Current, second generation of microcapsule-based S-HS focused on one-part
and catalyst-free solutions, which were able to work in harsh environment, like
water-reactive diisocyanates in polyurethane microcapsules [3]. The concept of this
system was similar to first-generation self-healing mechanism. The difference is that
self-healing after crack propagation is caused by isocyanate healing agent reactivity
without the presence of catalyst (Fig. 1).
Reactivity of the healing agents is also a drawback, which could result in
undesirable release and polymerization without crack healing. To avoid it,
appropriate microcapsule shell wall is needed. Selection of shell wall with good
mechanical properties is also important, because of particle dispersion in polymer
matrix without significant loss of healing agent [4]. Wider spectrum of healing
chemistries forced development of different microcapsule shell walls and methods
of their synthesis [5–7]. Most common materials used for shell wall are polyureas
[8–10], polyurethanes [3, 11, 12], melamine–formaldehyde [13–15] and urea–
formaldehyde [2, 16, 17]. They can be obtained using different methods like in situ
polymerization [2, 13–19], interfacial polymerization [3, 8–12, 20, 21], Pickering
emulsion [22–24], microemulsion polymerization [25, 26], solvent evaporation/
Fig. 1 Schematic of second-generation self-healing process: (1) coating with microcapsules before
microcrack propagation, (2) microcracks occur, (3) microcapsules rupture and healing agent fills crack
area, (4) healing agent reacts with water from environment and functional groups from matrix
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Polym. Bull.
extraction [27, 28] or sol–gel reaction [29, 30]. This variety of solutions entails
much more self-healing mechanisms, which do not result only from aforementioned
parameters. Current research focuses on modification of microcapsules to improve
self-healing process. Di Credico et al. [31] tuned thickness of shell wall through
diverse ingredients and process parameters. Other studies explored potential of
multilayer microcapsules [23]: two healing agents in matrix [8, 9] or both solutions
[22].
In this study, we present preliminary work on new S-HS for epoxy coatings and
focus on influence of shell wall structure on self-healing mechanism. We selected
polythiourethane (PTUR) as external shell material, because of thiourethane group
reactivity with epoxy ring, which was reported in previous research [32, 33] and
may improve adhesion between microcapsules and epoxy matrix. According to our
knowledge, the only report that describes PTUR as shell material shows
microcapsules obtained using Pickering emulsion [22] although, external shell
layer of poly(glycidyl methacrylate) particles separates internal PTUR layer from
epoxy matrix and there is no interaction between them.
Experimental
Materials
Isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), chlorobenzene,
3,6-dioxa-1,8-octanedithiol (DODT), trimethylolpropane tris(3-mercaptopropionate) (TTMP), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP),
dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), potassium hydroxide (KOH),
sulfuric acid (H2SO4) and 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) were
obtained from Sigma-Aldrich. Epidian 5 epoxy resin (diglycidyl ether of bisphenol
A with epoxide number of 0.487 and viscosity 23.9 Ns/m2 at 25 °C.) with ET
hardener (triethylenetetramine adduct with amine number 700–900 mg KOH/g,
viscosity 200–300 mPa s and density 1.02–1.05 g/cm3 [34]) were obtained from
Organika-Sarzyna Inc. (Poland).
Synthesis of thiourethane prepolymer with reactive isocyanate groups
Thiourethane prepolymer (p-TUR) with reactive isocyanate groups was synthesized
through reaction of DODT with HDI. Molar ratio [NCO]/[SH] = 2. 20.17 g
(0.12 mol) of HDI was slowly added to 50 ml flask with stirring 11.2 g (0.06 mol)
of DODT. The mixture was heated to 130 °C in oil bath and stirred for 1 h. Then the
mixture was stirred for additional 3 h without heating. Obtained p-TUR was white
and viscous paste, which was stored in 4 °C to avoid further polymerization.
Synthesis of IPDI filled polythiourethane microcapsules
Polythiourethane microcapsules (PTURmcaps) were prepared by two methods: with
p-TUR (method 1) and without it (method 2). Microencapsulation via interfacial
123
Polym. Bull.
polymerization based on Yang et al. recipe [3] was used in both methods. Lack of
p-TUR in method 2 can cause formation of polyurea (PU) internal sub-layer.
Method 1 4.50 g of gum arabic was dissolved in 30 ml of deionized water and
stirred for 3 h. 1 g of p-TUR was dissolved in 1.33 g (0.012 mol) of chlorobenzene
at 50 °C. After obtaining clear solution of p-TUR 3.15 g (0.014 mol) of isophorone
diisocyanate was added dropwise with stirring. The mixture was slowly added to
10 ml of gum arabic solution, heated to 50 °C and stirred (1000 rpm). Appropriate
amount of thiol chain extender [DODT—2.13 g (0.012 mol); TTMP—3.15 g
(0.008 mol); PETMP—2.82 g (0.006 mol)] was added and temperature of mixture
was increased to 60 °C. After 1 h mixture was stirred without heating for additional
1 h to avoid agglomeration of microcapsules. Suspension of microcapsules was
centrifuged and dried at 40 °C for 24 h to remove water.
Method 2 7.66 g (0.034 mol) of isophorone diisocyanate was mixed with 2.99 g
(0.027 mol) of chlorobenzene at ambient temperature. Obtained mixture was slowly
added to 15 ml of the gum arabic solution, heated to 50 °C and stirred with rotation
rate of 1000 rpm. Appropriate amount of thiol chain extender [TTMP—3.15 g
(0.008 mol); PETMP—2.82 g (0.006 mol)] was added. After 1 h mixture was
stirred without heating for additional 1 h to avoid agglomeration of microcapsules.
Suspension of microcapsules was centrifuged and dried at 40 °C for 24 h to remove
water.
Preparation of epoxy composites
Obtained PTURmcaps were gently dispersed for 15 min with two-bladed glass
propeller (powered by an electric motor) in Epidian 5 epoxy resin heated up to
40 °C. Composition was cured with ET hardener at room temperature. DMP-30 was
used in one of the compositions as catalyst improving reactions between
thiourethane and epoxy groups. In Table 1 seven various composites with five
types of PTUR shell walls, different amount of microcapsules and presence of
DMP-30 are listed.
Table 1 List of tested composites
No.
Composite name (type of shell wall/amount of
microcapsules/additives)
Amount of components (phr)
Epidian
5
ET
PTURmcaps
DMP30
1
p-TUR-DODT/18
100
18
18
0
2
p-TUR-TTMP/18
100
18
18
0
3
p-TUR-PETMP/18
100
18
18
0
4
IPDI-TTMP/18
100
18
18
0
5
IPDI-PETMP/18
100
18
18
0
6
IPDI-PETMP/30
100
18
30
0
7
IPDI-PETMP/30/DMP
100
18
30
1
phr parts by weight per 100 parts of epoxy resin
123
Polym. Bull.
Optical microscopy
Optical microscopy was performed on a Leica MZ6 microscope. Obtained p-TURDODT microcapsules after centrifugation were placed on tefloned fabric and
observed to investigate their morphology.
Atomic force microscopy (AFM)
AFM images were taken using Metrology Series 2000 (Molecular Imaging, USA)
with tapping frequency 340 kHz. Selected images were also transformed into 3D
surface maps.
Three-point bending test
Three-point bending test was conducted using Zwick/Roell 1435 universal testing
machine (Germany) at room temperature, according to PN-EN ISO 178:2010 [35].
Unnotched rectangular specimens (80 9 10 9 4 mm3) were bent with testing speed
2 mm/min. First set of all composites and hardened neat Epidian 5 as reference
sample were tested sixfold using standard method. Obtained flexural stress at break
(rfB) results and flexural modulus (Ef) for each composite were expressed as a mean
value. Second set of all composites and reference were also tested sixfold using
custom method (based on method proposed by Prajer et al. [36]) with pre-bending
with 20 N force and standard test after 24 h. Pre-bending was applied to obtain
microcracks (without sample cracking) and induce internal self-healing, which
occurred at room temperature. Obtained flexural stress at break (rfB) and flexural
modulus (Ef) results were also expressed as a mean value and compared to standard
method results to quantify self-healing efficiency.
Fourier transform infrared (FT-IR) analysis
The FT-IR spectra were obtained by using Nicolet 6700 FT-IR spectrophotometer
(Thermo Scientific, USA) equipped with a diamond crystal in an air atmosphere, at
room temperature in a range of 400–4000 cm-1. Spectra of each composite were
taken before and after three-point bending test using ATR-mode with 32-scan
signal. Spectra of virgin samples (before TPBT) and samples with self-healing
effect (after TPBT) were taken three times, each from different region of sample to
ensure self-healing evidence. The bands scales were normalized with OMNIC
Spectra tool. Self-healing process was also confirmed by examination of
3360–3390 cm-1 shift.
Scratch test
Scratch test (according to method proposed by Huang et al. [9, 12]) was conducted to
evaluate effect of microcapsules on chemical resistance and self-healing ability of
composites (also in the presence of solvents and aqueous solutions). Samples were
scratched (width of edge 0.58 ± 0.01 mm) and treated with selected solvents and
123
Polym. Bull.
Table 2 Scratch test scales
Scale 1: self-healing efficiency
Point
Scale 2: chemical resistance
Point
No self-healing observed
0
No changes on surface
A
Low efficiency
1
Low level of changes
B
Average efficiency
2
Average level of changes
C
Good efficiency
3
High level of changes
D
Excellent efficiency
4
Severe damage on sample
E
solutions: water, 20% KOH aq., 20% H2SO4 aq., THF and DMSO. Also reference
samples without treating were tested. All samples were covered with Petri dish to avoid
evaporation of solvents and exclude the effect of atmospheric moisture. Optical
micrographs were taken before treating and after 24 h exposure. Self-healing process
occurred at room temperature. Changes on the surface and scratch area were evaluated
visually (examples in Fig. 5) and scored on two five-point scales (Table 2).
Results and discussion
Synthesis and characterization of prepolymer and microcapsules
The synthesis of thiourethane prepolymer was achieved by reaction between
isocyanate groups of HDI and thiol groups of DODT. HDI/DODT molar ratio 2:1
allowed to obtain prepolymer with isocyanate groups at chain ends (Scheme 1).
Prepolymer was used in method 1 to synthesize PTUR shell wall of
microcapsules with DODT, TTMP and PETMP as chain extenders. Ratio of thiol
groups to isocyanate groups from p-TUR was 1:1 with theoretical excess of
isocyanate groups from IPDI for microcapsule content. Gum arabic worked as
stabilizer together with p-TUR and both formed interfacial protecting layer to avoid
NCO
OCN
+
O
HS
HDI
DODT
O
OCN
RHDI
N
H
SH
O
O
S
RDODT
S
O
O
N
H
RHDI
N
H
S
n
RDODT
S
N
RHDI
H
tiourethane prepolymer
Scheme 1 Synthesis of thiourethane prepolymer (RHDI HDI radical, RDODT DODT radical)
123
NCO
Polym. Bull.
H
OCN
NCO
H
H
N C N
O
NCO
+
H
N C N
NCO
O
H2O
n
IPDI
urea prepolymer
Scheme 2 Urea prepolymer formation from reaction of IPDI and water
1)
OCN R1 NCO
+
thiourethane
prepolymer
HS
O
O
R1
DODT
2)
O
HS
O
O
HS
O
CH3
O
OCN R1 NCO
O
SH
HS
O
O
SH
O
4)
NCO
O
HS
O
CH3
IPDI
TTMP
N
R1
N
O
O
SH
HS
O
O
SH
O
PETMP
S
S
N
H
n
O
C
S
S
N
N
S
S
RIPDI C
H O
RTTMP
N
S
RIPDI C
H
n
O
N
H
O
HS
O
C
H
H O
RPETMP
N
S
S
N
R1
C
H
O
O
O
O
+
H
n
H
SH
+
O
NCO
O
O
NCO
5)
6)
7)
O
PETMP
HS
IPDI
N
O
H
O
NCO
R1
O
O
S
RDODT
O
+ HS
thiourethane
prepolymer
S
H O
RTTMP
N
S
R1
C
TTMP
O
3)
C
H
SH
+
thiourethane
prepolymer
N
O
O
OCN R1 NCO
O
H
SH
O
S
N
N
S
RIPDI C
H
H O
RPETMP
N
S
S
N
RIPDI C
H
O
O
n
n
Scheme 3 Synthesis of all PTUR shell walls (R1 p-TUR radical, RDODT DODT radical, RTTMP TTMP
radical, RPETMP PETMP radical, RIPDI IPDI radical)
123
Polym. Bull.
contact of reactive IPDI with water [31, 37]. In method 2 IPDI replaced thiourethane
prepolymer as shell wall substrate. Consequently, lower stabilizing potential of pure
gum arabic layer might cause initial reaction of IPDI with water and urea
prepolymer (p-Urea) formation (Scheme 2).
Presence of p-Urea with gum arabic allowed to obtain interfacial protecting layer
with stabilizing potential similar to p-TUR/gum arabic system and avoid further
IPDI/water reaction [38]. The obtained microcapsules had polyurea internal sublayer and external polythiourethane shell wall from TTMP or PETMP. Reactions of
all PTUR shell walls are presented in Scheme 3.
Self-healing mechanism characterization
Microcrack propagation causes release of IPDI to epoxy matrix with free secondary
hydroxyl groups. Presence of tertiary amines in cured epoxy matrix catalyze
reaction between these hydroxyl groups and isocyanate groups from IPDI
(Scheme 4). Also DMP-30 contains tertiary amine groups which support catalyst
effect in composite 7 (highest self-healing efficiency in TPBT).
Optical microscopy
Optical microscopy images (Fig. 2) showed that PTURmcaps had spherical,
ellipsoidal or quasi-spherical shape and their diameters varied from less than 0.1 to
H
NCO
N C O R2
tertiary amines
NCO
IPDI
+
HO R2
Epoxy matrix with secondary
hydroxyl groups
O
H
N C O R2
O
Polyurethane secondary matrix
Scheme 4 Polyurethane secondary matrix forming Turing self-healing process (R2 epoxy matrix)
Fig. 2 Optical micrographs of p-TUR-DODT microcapsules
123
Polym. Bull.
0.9 mm. Surface texture of external PTUR shell wall was uneven and rough.
Microcapsules also showed tendency to form small clusters. Other types of
microcapsules, even without prepolymer, looked very similar to presented one.
AFM
AFM images and 3D surface maps of IPDI-PETMP/30 composite (Fig. 3) confirm
the presence of microcapsules at composite surface and small range of their
diameters (2–20 lm). Also clustering effect is less pronounced in epoxy composite
than between untreated microcapsules.
Three-point bending test
Mean values for both methods were compared to each other and calculated selfhealing efficiencies after microcracking were presented in Table 3. Self-healing
efficiency was calculated according to formula (1) proposed by Wool and O’Connor
[39]:
ESH ¼
PH
100%;
PV
ð1Þ
where ESH is self-healing efficiency, PH is property of healed composite and PV is
property of virgin composite.
Three composites (p-TUR-TTMP/18, p-TUR-PETMP/18, IPDI-TTMP/18) did
not reach 100% self-healing efficiency based on flexural stress at break, but this
property was relatively higher in comparison with composites with IPDI-PETMP
microcapsules. Similar tendency was observed for self-healing efficiencies based on
flexural modulus. However, the presence of microcapsules reduced flexural rigidity
of epoxy matrix, which resulted in lower flexural stress and modulus in all samples
with PTURmcaps. As it might be expected, higher amount of PTURmcaps also
resulted in better self-healing efficiency. Type of synthesis method also played role
in self-healing efficiency. Composites with microcapsules containing p-TUR in
shell wall provided lower efficiency than their counterparts without prepolymer. The
best self-healing efficiency is shown by three systems with IPDI-PETMP
microcapsules. IPDI-PETMP/18 composite comprises both high self-healing
efficiency and satisfactory virgin properties.
FT-IR analysis
FT-IR spectra of reference sample and all composites with PTURmcaps (before and
after three-point bending test) allowed to investigate changes in composite after
self-healing. Spectra of reference sample and p-TUR-DODT/18 were presented in
Fig. 4. Significant peaks of other composites with PTURmcaps were presented in
Table 4 because of their resemblance to p-TUR-DODT/18.
All spectra of composites before self-healing have N–H and O–H stretching
vibration broad band with highest peak around 3360 cm-1, while all spectra of
123
Polym. Bull.
Fig. 3 AFM images (left) and 3D surface maps (right) of IPDI-PETMP/30 composite
composites after self-healing have this peak around 3390 cm-1. This shift results
from reduction of hydrogen bonding [40, 41] during self-healing process, which
provides better cross-linking and aforementioned reduction. The higher intensity of
123
Polym. Bull.
Table 3 Microcracks’ self-healing efficiencies based on flexural stress at break and flexural modulus,
measured using three-point bending test
No.
Composite name
Flexural stress at break
(rfB) (MPa)
Flexural modulus (Ef)
(MPa)
Microcracks
self-healing
efficiency
(%)
Standard
method
(virgin)
Custom method
(after 20 N prebending)
(healed)
Standard
method
(virgin)
Custom method
(after 20 N prebending)
(healed)
rfB
Ef
–
0
Reference sample
43.7
–
2580
–
–
1
p-TUR-DODT/18
16.7
17.3
1980
2010
103.6
101.5
2
p-TUR-TTMP/18
22.0
14.3
1680
1580
65.0
94.0
3
p-TUR-PETMP/18
31.4
24.7
2280
2050
78.7
89.9
4
IPDI-TTMP/18
24.2
17.8
1840
1940
73.6
105.4
5
IPDI-PETMP/18
19.7
25.6
1630
2510
129.9
154.0
6
IPDI-PETMP/30
12.7
19.7
1290
1720
155.1
133.3
7
IPDI-PETMP/30/
DMP
9.6
17.0
1230
2060
177.1
167.5
Fig. 4 FT-IR spectra of reference sample and p-TUR-DODT/18 (before and after self-healing): 1 N–H
and O–H stretching; 2 C=O stretching (*aromatic ring overtone); 3 N–H bending; 4 NH–CO–S bending;
5 C–N axial stretching; 6 C–H bending (IPDI); 7 CH2–S bending (*C–O stretching in alcohols); 8 C–S
stretching
3390 cm-1 peaks after self-healing compared to 3360 cm-1 peaks is also effect of
better cross-linking, more precisely polyurethane secondary network forming,
which results from the conversion of isocyanate groups into urethane bonding. The
lack of absorption band at 2600–2540 cm-1 (S–H stretching, thiol group) proved
successful formation of PTUR shell wall with all amount of thiol chain extenders.
Also isocyanate absorption bands around 2270–2260 cm-1 are not detected, even
for composites before self-healing. This is justifiable for low microcapsule content
and presence of PTUR shell wall around IPDI liquid core. The second reason for
123
123
553
1030
–CH2–S– bending [33]; C–O stretching (alcohols)*
1028*
1361
C–S stretching
1383
C–H bending in geminal dimethyl group (IPDI)
1039*
1644
1579
C–N axial stretching (isocyanate group [42])
N–H bending (amines, thiourethanes and urethanes)
556
1034
1362
1386
1508
1579
1651
1648
1652
3388
1656
3360
1A
1656
1506
1580
C=O stretching, (amide groups: ureas, thiourethanes
and urethanes); aromatic ring overtone*
1
–NH–CO–S– bending (thiourethanes); triazine
compounds (isocyanurates)
3396
1654*
N–H and O–H stretching (alcohols, amines, amides)
0
Composite number ?
Approximate assignment ;
552
1030
1361
1384
1505
1580
1653
3359
2
547
1037
1361
1388
1510
1575
1650
3393
2A
554
1031
1361
1384
1506
1580
1641
1652
3360
3
546
559
1029
1362
1386
1507
1574
1580
1645
1656
3393
3A
553
1031
1361
1383
1506
1579
1644
1652
3360
4
552
1027
1361
1383
1506
1581
1648
3397
4A
Table 4 Significant FT-IR absorption peaks and their approximate assignments for all composites (A—after self-healing)
547
553
1032
1390
1508
1556
1560
1577
1652
3360
5
546
1038
1362
1388
1509
1544
1578
1648
3397
5A
552
1026
1361
1382
1507
1580
1644
3360
6
547
1038
1362
1387
1508
1543
1574
1651
3396
6A
553
1031
1361
1383
1507
1556
1581
1651
3361
7
552
1028
1362
1386
1507
1574
1650
1657
3390
7A
Polym. Bull.
Polym. Bull.
this absence is limited ATR penetration depth—only approximately 0.9 lm for a
diamond crystal, which also shows strong absorption in wavenumber range between
1800 and 2650 cm-1. Another characteristic peak at 1650 cm-1 is associated with
C=O stretching vibration of secondary amide group in thiourethane and urethane
moieties. This peak is also characteristic for C=O stretching vibration of tertiary
amide group of urea. Significant increase of this peak is observed in all composites
after self-healing, what confirms polyurethane secondary network forming. The
presence of polythiourethanes and isocyanurates (as triazine derivatives) is
confirmed by 1510–1500 cm-1 absorption band. Characteristic vibrations (C–N
axial stretching) of isocyanate group are also observed at 1390–1380 cm-1 [42].
The presence of C–H bending vibration in geminal dimethyl group at 1360 cm-1
gives the proof of IPDI presence. Peaks around 1030 cm-1 (increased in all
composites in comparison to reference sample alcohol C–O stretching) and around
550 cm-1 represent –CH2–S– bending [33] and C–S stretching in PTUR shell,
respectively.
Scratch test
The obtained results (examples in Fig. 5) show show that the microcapsules content
does not significantly affect the chemical resistance of composites to solvents and
aqueous solutions (Table 5). Only chemical resistance to aqueous solutions of KOH
and H2SO4 is lower in case of microcapsule-filled composites. However, all
composites are still vulnerable to organic solvents, especially DMSO. The most
effective self-healing process was obtained for p-TUR-TTMP/18 and p-TURPETMP/18 in the presence of water. Better self-healing efficiency is observed for
Fig. 5 Micrographs of samples before (above description) and after self-healing (below description)
with all points observed on both scales. Description shows: classification on both scales, composite
number and selected solvent/solution, respectively
123
Polym. Bull.
Table 5 Evaluation of self-healing efficiency and chemical resistance after 24 h
No.
Composite name
Solvent/solution
–
Water
20% KOH aq.
20% H2SO4 aq.
THF
DMSO
0
Reference sample
0.A
0.A
0.A
0.A
0.B
0.C
1
p-TUR-DODT/18
1.A
2.A
2-3.A
3.B-C
3.B
3.B
2
p-TUR-TTMP/18
3.A
3-4.A
3.A-B
2.A-B
2-3.A-B
2.A-B
3
p-TUR-PETMP/18
2.A
3-4.A
2-3.A
2-3.A-B
2-3.B
2-3.Da
4
IPDI-TTMP/18
0-1.A
0-1.A
1.B
2.C
2.A-B
1-2.B
5
IPDI-PETMP/18
0-1.A
0-1.A
1.C-D
1.A
2.B
2.B-C
6
IPDI-PETMP/30
1-2.A
2.A
2-3.A
1-2.B
2-3.A-B
2.B-C
7
IPDI-PETMP/30/DMP
1-2.A
1-2.A
1.B-C
1-2.B
2.A-B
2.C
Bold values show best results
a
High level of surface destruction impedes S–H efficiency evaluation
composites with prepolymer microcapsules, while content of microcapsules without
p-TUR is not so effective.
Conclusions
Five types of microcapsules with PTUR shell wall were synthesized and applied in
seven composites as self-healing systems. The obtained composites were tested for
their surface self-healing efficiency (scratch test) and internal self-healing efficiency
(three-point bending test). Also chemical resistance of the composites to selected
solvents and solutions was examined. Overall, higher surface self-healing efficiency
reveals composites with prepolymer microcapsules, contrary to internal self-healing
efficiency, which is better provided by microcapsules without p-TUR. Healing
process of microcracks in composites 1, 5, 6 and 7 furthermore strengthened these
composites (over 100% self-healing efficiency). Comparison with other microcapsule-based self-healing systems for epoxy matrix shows that our self-healing system
is competitive. Thiol-epoxy healing system in melamine–formaldehyde (MF)
microcapsules from Yan et al. reached similar self-healing efficiency of 80–105% in
the presence of amine catalyst [15]. In another system from Zhang and co-workers,
MF microcapsules with functional glycidyl methacrylate achieved 75–90% ESH
without catalyst [14]. IPDI filled polyurethane and polyurea–formaldehyde microcapsules proposed by Di Credico et al. show more than 50% recovery (optical
microscope evaluation) [31]. However, our goal was not only to obtain competitive
self-healing system. Selection of similar systems, differing only in the structure of
microcapsule PTUR shell wall allowed to investigate influence of these variables.
As mentioned above, microcapsules with shell made of linear p-TUR oligomer
worked better during scratch test, which examined self-healing ability of surface
area. It probably results from oligomer chain flexibility, which provides better
performance to scratch damage, but is less effective during internal crack
123
Polym. Bull.
propagation. The only exception is composite with p-TUR-DODT microcapsules
which performs well in both tests, probably due to the absence of crosslinking
components—p-TUR and DODT are bifunctional and more organized structure
results from chain entanglement. On the other hand, IPDI-based microcapsules with
PU sub-layer work better during internal microcrack propagation, but are less
effective when scratch damage occurs. It is presumably the result of their rigidity
and greater brittleness, which helps IPDI-based microcapsules remain intact during
primary epoxy curing and work properly, when microcrack propagation occurs.
FT-IR spectroscopy allowed to confirm self-healing process based on
polyurethane secondary network forming. In conclusion, self-healing systems
obtained in our preliminary work are suitable for epoxy coatings, especially in water
and moisture environment. Future research will let us improve these self-healing
systems’ performance and investigate more thoroughly correlation between
microcapsule structure and self-healing efficiency.
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.
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