Preparation and characterization of microencapsulated polythiol
Yan Chao Yuana, Min Zhi Rongb, Ming Qiu Zhangb,*
aKey Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, OFCM Institute,
School of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275, PR China
bMaterials Science Institute, Zhongshan University, Guangzhou 510275, PR China
a r t i c l e i n f o
Received 7 December 2007
Received in revised form 13 March 2008
Accepted 28 March 2008
Available online 3 April 2008
a b s t r a c t
Microcapsules containing curing agent for epoxy were successfully prepared by in situ polymerization
with poly(melamine–formaldehyde) (PMF) as the shell material and high-activity polythiol (pentaery-
thritol tetrakis (3-mercaptopropionate), PETMP) as the core substance. Having been encapsulated, the
core material PETMP had the same activity as its raw version. The synthesis approach was so improved
that the consumption of polythiol was reduced to a low level. By carefully analyzing the influencing
factors including catalyst concentration, reaction time, reaction temperature, feeding weight ratio of
core/shell monomers, dispersion rate and emulsifier content, the optimum synthetic conditions were
found out. The results indicated that not only core content and size of the microcapsules but also
thickness and strength of the shell wall can be readily adjusted by the proposed technical route. The
relatively thin shell wall (w0.2 mm) assured sufficient core content even if the microcapsules were very
small (1–10 mm). The polythiol-loaded microcapsules proved to be qualified for acting as the mate of
epoxy in making two-part microencapsulated healing agent of self-healing composites.
? 2008 Elsevier Ltd. All rights reserved.
In recent years, self-healing of thermosetting polymer (mostly
epoxy) based composites has attracted increasing attention be-
cause they represent an important class of structural materials that
require long-term durability and reliability [1–10]. For healing
agent aided self-mending, the agent should be liquid at least at the
healing temperature. It is generally encapsulated by fragile-walled
containers and embedded into the composites’ matrix. As soon as
the cracks destroy the containers, the healing agent would be
released into the crack planes due to capillary effect and bind the
cracks as a result of polymerization of the released healing agent.
Dry et al. filled glass pipette tubes with two-part epoxy adhe-
sives consisting of epoxy and amine hardener, respectively, and
embedded them into epoxy matrix [1,2]. To eliminate the possi-
bility that the thick hollow glass capillaries might act as initiation
for composites failure, Bleay et al. employed hollow glass fibers
possessing nearly the same diameter as the reinforcements and
applied epoxy-hardener pair as the repair agent . However, fill-
ing of repair species into such fine tubes is very difficult. Jung et al.
used polyoxymethyleneurea (PMU)-walled microspheres to store
an epoxide monomer to be released into cracks and rebond the
cracked faces in a polyester matrix . Solidification of the epoxy
resin (i.e. the repair action) was triggered by the excessive amine in
the composites. White et al. indicated that the method was not
feasible as the amine groups did not retain sufficient activity .
Zako and Takano proposed an intelligent material system using
40% volume fraction unmodified epoxy particles to repair micro-
cracks and delamination damage in a glass/epoxy composite
laminate . By heating to 120?C, the embedded epoxy particles
(w50 mm) would melt, flow to the crack faces and repair the
damage with the help of the excessive amine in the composite. Our
group also reported a self-healing epoxy composite containing
epoxy-loaded poly(urea–formaldehyde) (PUF) microcapsules .
The complex of CuBr2and 2-methylimidazole (CuBr2(2-MeIm)4)
served as latent hardener and was pre-dissolved in the matrix
during composites’ manufacturing. Self-healing of cracks can be
conducted at 130?C as a result of the curing of the released epoxy
initiated by CuBr2(2-MeIm)4.
By analyzing the available approaches, it is known that self-
healing based on microencapsulated healing agents offers tremen-
dous potential for practical applications [8–10]. This is particularly
true when mass production, long shelf life and self-healing free of
manual intervention are concerned. However, microencapsulation
of hardener for epoxy healing agent is difficult, despite that
microcapsules containing epoxy resin are easy to be synthesized by
in situ polymerization, complex coacervation and interfacial co-
polymerization [7,11–14]. The conventional amine-type hardeners
for curing epoxy at room temperature are amphoteric and highly
active, and hence hard to be encapsulated in water or solvent by
* Corresponding author.
E-mail address: email@example.com (M.Q. Zhang).
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Polymer 49 (2008) 2531–2541
0032-3861/$ – see front matter ? 2008 Elsevier Ltd. All rights reserved.
under acidic condition. Although physical extrusion art was used to
produce some hardener-loaded capsules [15,16], such as the cap-
sules containing mixture of diethylenetriamine and nonyl phenol
with alginate wall and the capsules containing diethylamine with
thermoplastic wall, they were not suitable for fabricating self-
healing composites. The shell wall of the former capsules might
suffer from bacterial degradation, while that of the latter was too
thick (w100 mm) to be sensitive to propagating cracks. Moreover,
special equipments had to be involved.
Besides amine, polythiols can also used for curing epoxy. Epoxy
resin with polythiol as the hardener and strong base as the hard-
ening accelerator is well known for its low-temperature fast cur-
ability . Accordingly, microencapsulated epoxy and polythiol
might constitute a group of self-healing agent that is able to take
effect at or below room temperature. In this context, self-healing
can be done free of manual intervention (i.e. heating). To the au-
thors’ knowledge, however, microencapsulation of polythiol has
not yet been reported. In fact, it is also very difficult to microen-
capsulate polythiol by the existing techniques like in situ poly-
merization of urea–formaldehyde or interface polymerization due
to the very high activity of hydrosulfide group of polythiol. Liquid
polythiol can be cured by a wide variety of compounds including
aldehyde, ketone, peroxides, epoxy resins, isocyanates, acrylic and
anhydrides through a number of reaction paths such as oxidation,
addition and substitution reactions of hydrosulfide terminal groups
[18–25]. In most cases, hollow spheres with very thick wall rather
than the desired capsules have to be obtained.
To solve the problem, the authors of the present work planed to
try another method via in situ polymerization of melamine–form-
aldehyde in an oil-in-water emulsion. Compared to urea–formal-
dehyde, polymerization of melamine–formaldehyde proceeds
faster and poly(melamine–formaldehyde) (PMF)-walled micro-
capsules are superior to PUF-walled microcapsules in mechanical
properties, stability and chemical resistance [26–29]. In this con-
text, there is a possibility that the consumption of polythiol might
be substantially reduced. In situ polymerization with PMF or
methanol etherified PMF has been applied to encapsulate liquids
such as dye solution , dicyclopentadiene , lauryl alcohol
, fragrant oil [31,32] and methylparathion . To obtain
microcapsules with sufficient strength, the encapsulation process
was usually conducted at higher pH value of 3.5–6.5 and higher
temperature of 60–80?C or longer reaction time of 3–8 h. Although
these rigorous processing parameters had little effect on the
aforesaid inert core materials, they may lead to failure of micro-
encapsulation or excessive consumption of the active core material,
e.g., polythiol in the present work. Therefore, it is necessary to
explore suitable reaction conditions (like catalyst concentration,
reaction time and temperature, feeding weight ratio of core/shell
monomers, dispersion rate and emulsifier content) and illuminate
the mechanism of in situ polymerization for microencapsulation of
polythiol. It is hoped that our method would greatly reduce con-
sumption of polythiol during encapsulation and help to control the
thickness and strength of shell wall.
In this paper, the feasibility of the proposed approach and
structure and properties of the resultant microcapsules were
examined, so as to provide preparation parameters for making
self-healing composites in the subsequent works.
2.1. Raw materials
PETMP, boiling point¼275?C at 1 mmHg, density¼1.28 gml?1at
25?C and hydrosulfide group content¼26.55%) used as the core
material to be encapsulated was purchased from Fluka Chemie AG
(Buchs, Switzerland). Melamine (M), analytically pure agent, was
provided by Shanghai First Chemical Co., China. Formaldehyde (F),
analytically pure agent, 37 wt.%, was supplied by Guangzhou
Chemical Co., China. Triethanolamine and citric acid used for con-
trolling pH value of solution were provided by Shanghai Medical
Group Reagent Co., China.
Styrene, maleic anhydride, and the initiator dibenzoyl peroxide
(BPO) were purchased from Shanghai Medical Group Reagent Co.,
China. Diglycidyl ether of bisphenol A (DGEBA, EPON 828) was
supplied by Shell Co. as the composite’s matrix resin. Diglycidyl
tetrahydro-o-phthalate (DTP) was provided by Tianjin Jindong
Chemical Plant, China as the polymerizable component of the
healing agent. Diethylenetriamine (DETA) was supplied by Shang-
hai Medical Group Reagent Co., China working for DGEBA. The
catalyst benzyl dimethylamine (BDMA) with boiling point 183.5?C
was purchased from Shanghai Medical Group Reagent Co., China.
2.2. Preparation of the emulsifier poly(styrene–maleic sodium)
BPO-initiated copolymerization of styrene and maleic anhydride
at 1:1 molar ratio was carried out in toluene solution at 75?C for
3 h. The average molecular weight and molecular weight distri-
bution of poly(styrene–maleic anhydride) were determined by gel
permeation chromatography (GPC) measurements on a Waters
Breeze relative to polystyrene standards. That is, Mn¼8.9?103and
Mw/Mn¼2.3. According to the results of element analysis, the
contents of styrene and maleic anhydride were found to be 50.6
and 49.4 wt.%, respectively. The 10 wt.% neutral hydrolyzed
solution was prepared by dissolving the copolymer in sodium
hydroxide solution, and stirred at 80?C for 5 h.
2.3. Preparation of the microcapsules
PETMP was added into an aqueous solution of PSMS (150 ml).
The mixture was vigorously dispersed by a homogenizer for 5 min
at a selected rate and then two drops of 1-octanol were added to
eliminate surface bubbles of the emulsion. The pre-condensate of
M (0.04 mol) and F (0.12 mol) was prepared at 70?C for 30 min and
the pH value of the solution was kept at about 9–10 by adding
triethanolamine (Fig. 1(a)). Subsequently, the pre-condensate
solution was added to the above emulsion with 450 rpm continu-
ous mechanical agitation by a two-bladed stirring paddle while the
pH value of the solution was kept at about 2.7–3.5 by adding citric
acid. Eventually, the reaction mixture was cooled to room tem-
perature. The resultant slurry was neutralized by sodium carbonate
solution, and then diluted with deionized water. The deposit of
CH2OH + HOH2C
Step I :
Step II :
CH2OH + H
Fig. 1. Reaction schemes of M–F resins. (a) Formation of M–F pre-condensate.
(b) Formation of PMF.
Y.C. Yuan et al. / Polymer 49 (2008) 2531–25412532
microcapsules was separated through a Buchner funnel and rinsed
with deionized water and acetone, and then vacuum dried. Fig.1(b)
shows the condensation reaction scheme of M–Fresins. Table 1 lists
the influences of the processing parameters on the resultant
microcapsules as well as the sample IDs.
A comparison experiment was conducted using PUF to encap-
sulate PETMP. The preparation processes were the same as those
applied for making PMF-walled microcapsules, except that the
content of urea was 0.06 mol.
2.4. Preparation of epoxy composite containing
the polythiol-loaded microcapsules
The unfilled epoxy specimens were produced through mixing
100 parts EPON 828 epoxy resin with 12 parts curing agent DETA.
The filled epoxy composites were prepared by uniformly mixing
10 wt.% of the PETMP-loaded microcapsules together with the
aforesaid mixture of EPON 828 and DETA. To obtain cured version,
the unfilled epoxy and the filled version were degassed and poured
into closed silicone rubber molds and cured for 24 h at room
temperature, followed by 24 h at 40?C.
EQUINOX 55 FTIR spectrometer to identify the chemical structure of
the specimens, which were prepared by grinding the samples with
potassium bromide (KBr) or by attaching the samples to a KBr disc.
Average diameter, size distribution and specific surface area of
the resultant microcapsules were determined by a Malvern Mas-
terSizer 2000 Particle Size Analyzer.
Surface morphology and shell thickness of the microcapsules
were monitored by scanning electron microscopy (FEI/OXFORD/
HKL Quanta 400FEG and Philips XL30-FEG). The microcapsules’
residue produced by grinding or acetone extraction was mounted
on a conductive stage to facilitate membrane thickness measure-
ment prior to observation. The samples were sputtered with a thin
layer (w10 nm) of gold–palladium to prevent charging under the
electron beam. The appearance of the microcapsules was also
observed by optical microscope (Orthoplan Pol, Leitz).
Micro-Raman measurements were carried out using a Renishaw
inVia (UK) spectrometer equipped with a Leica microscope system.
Raman spectrawereexcited bya 785 nm laser line at a resolution of
1 cm?1, and the laser was focused by a 20? objective to a spot size
of about 1 mm. These parameters were kept constant for all the
samples. The spectra were calibrated using the 520.0 cm?1line of
a silicon wafer.
Thermogravimetric analyses (TGA) of the microcapsules were
carried out using Netzsch TG-209. The samples were heated
from 25 to 600?C at a rate of 10?Cmin?1under the protection of
Both isothermal and non-isothermal curing kinetics were
studied by differential scanning calorimetry (DSC) with a TA DSC
Q10 calorimeter under the protection of nitrogen flow. The heating
rate for the non-isothermal measurements was 5?Cmin?1from
?20 to 100?C. Calibration of the calorimeter with regard to
temperature and energy was achieved by measurement of the
temperature and enthalpy of melting of indium as standard
Storage stability of the microcapsules was characterized by their
weight loss exposed to room temperature at periodical intervals
and heat treatment at different temperatures, respectively.
Yields of the preparationweredefined bythe ratio of the mass of
the collected microcapsules tothe total feeding mass of PETMP core
and shell monomers.
2.6. Determination of core content and loss factor
The microcapsule’score contentandlossfactor were
determined by elemental analysis and extraction method. Using
acetone as extraction solvent, the microcapsule samples were
extracted by Soxhlet apparatus for 240 h to remove the core
material. The contents of N and S elements of the samples were
examined using a carbon-hydrogen-nitrogen-sulfur (CHNS) ana-
lyzer (Vario EL, Germany Elementar Inc.). The initial N and S con-
tents of the intact microcapsules are denoted by WN(i)and WS(i),
while the N and S contents of the residual shell wall are denoted by
WN(r)and WS(r). As N element in either the microcapsule or the
residual shell wall should completely come from melamine rings in
the microcapsulewall materialanditsamount remained
Descriptions of the microcapsules prepared using different processing parameters
System pHFeeding weight
ratio of core/shell
Y.C. Yuan et al. / Polymer 49 (2008) 2531–25412533
unchanged before and after the extraction, the shell wall content
(Wwall) and core content (Wcore) can thus be calculated from:
? 100% (1)
? 100% (2)
In the course of microencapsulation, some PETMP was con-
sumed form the shell wall. The ratio of the losing part to PETMP in
the microcapsules was defined as the loss factor (LF):
? 100% (3)
3. Results and discussion
3.1. Microencapsulation of polythiol
Formaldehyde mostly presents itself as methylene glycol in for-
malin and has two active functional groups, while melamine is
a weakly basic reagent with six functional groups. Therefore, they
can react with each other forming melamine–formaldehyde poly-
mer. In general, the reaction consists of two stages as described in
the following (Fig. 1). (i) The nucleophilic addition stage in basic
solution proceeds at about 70?C, when melamine reversibly and
successively reacts with formaldehyde to give nine different meth-
ylol melamines ranging from mono- to hexamethylol melamines (a
typical reaction with trimethylol melamines is shown in Fig. 1(a)).
(ii) The condensation stage proceeds in acidic solution at about
50?C, during which two mechanisms are involved: formation of
ether bridges via the reaction between two methylol groups and
in Fig. 1(b)) [34–37]. The condensation stage overlaps in time with
the in situ polymerization or the microencapsulation process.
During the in situ polymerization, M–F monomers and the pre-
condensate reacted with each other in acidic water phase to form
low molecular weight oligomers. These pre-condensate and olig-
omers possess surface activity like surfactants because their
molecular structure contains both hydrophilic and hydrophobic
groups [38,39]. Consequently, the M–F pre-condensate and oligo-
mers tended to gather together at the phase boundary between the
dispersed water insoluble core substance PETMP droplets and the
aqueous volume phase. The driving force for the enrichment at
the surface of the dispersed phase was the surfactant character of
the M–F pre-condensate and oligomers. The higher concentration
of the reactive resin molecules (i.e. the M–F pre-condensate and
oligomers) in the boundary layer must favor the condensation
reaction. In other words, the resin condensation proceeded much
faster at the interface than in the volume phase. As a result, gel-like
structures were built up at the beginning owing to liquid–liquid
phase separation, which was further hardened to give the capsule
walls . Subsequently, integrity and mechanical strength of the
capsules’ wall were enhanced owing to the increased thickness and
cross-linking degree of PMF. These mean that the condensation
reaction kinetics of M–F must play a key role in the course of
In fact, the condensation reaction rate and degree are
determined by reaction time, reaction temperature and catalyst Hþ
concentration . These factors are investigated in detail herein
after trying to find out the optimum conditions. As illustrated by
Figs. 2 and 3, the microcapsules prepared at excessively short
reaction time (20 min, for example, see Fig. 2(a)), or excessively
high pH value (3.5, for example, see Fig. 2(h)), or excessively low
reaction temperature (40?C, for example, see Fig. 3(a)) cannot
maintain their integrity after drying because of the weak shell
walls, in spite of holding spherical shape in water. When the
reaction time exceeds 40 min, microcapsules’ shape, yield and loss
factor do not show significant change with the reaction (refer to
Fig. 2(b)–(d) and the samples 1-2 to 1-4 in Table 1).
Comparatively, the influence of pH value on reaction kinetics is
quite complex due to interference of the parallel reactions taking
place at the condensation stage. When the pH value of solution is
reduced, the yield slightly increases (refer to the samples 3-1 to 3-4
inTable 1). At pH¼2.9, both loss factor and core content reach their
minimum and maximum values of 5.8 and 81.7%, respectively. This
implies that higher catalyst concentration may accelerate the con-
densation reaction, so that less time is left for the reaction between
the polythiol and M–F pre-condensate and oligomers. Meanwhile,
the stronger walls generated at lower pH values facilitate formation
of spherical microcapsules, as shown in Fig. 2(e)–(g).
It is worth noting that some hollow microcapsules resulting
from air bubbles appear in the case of low pH value (Fig. 2(e) and
(f)). The amount of the hollow microcapsules seems to increase
Fig. 2. Optical microscopic images of the PMF-walled microcapsule prepared under different conditions: (a) 20 min, 50?C, pH ¼3.2; (b) 40 min, 50?C, pH ¼3.2; (c) 60 min, 50?C,
pH¼ 3.2; (d) 80 min, 50?C, pH¼ 3.2; (e) pH¼ 2.7, 60 min, 50?C; (f) pH¼ 2.9, 60 min, 50?C; (g) pH¼ 3.2, 60 min, 50?C; (h) pH ¼3.5, 60 min, 50?C. The attached scale bars
represent 20 mm in length.
Y.C. Yuan et al. / Polymer 49 (2008) 2531–25412534
with decreasing the pH value. It is attributed to the fact that a faster
condensation reaction increases the probability of wall formation
on the surface of air bubbles. Increased number of hollow micro-
capsules would consume much wall materials and might reduce
the quality of microcapsules containing PETMP. Consequently, an
appropriate catalyst concentration (pH¼2.9–3.2) is required to
balance the loss factor, core content, as well as the amount of
On the other hand, Fig. 4(a) shows that a few powder-like PMF
nanoparticles adhere to the microcapsules. They should be formed
by the M–F oligomers through successive polymerization, liquid–
along with ceaseless decrease of their solubility [32,40]. Compared
with UF microcapsules, the surfaces of the M–F microcapsules are
smoother [7,11,41]. In addition, the relative higher yields of M–F
microcapsules indicate that the outgrowth of PMF nanoparticles is
not significant. Yield of microencapsulation is defined as the ratio of
the mass of the collected microcapsules to the total feeding mass
of PETMP core and shell monomers. Table 1 shows that the yields of
and 4-1. In fact, the amounts of hollow microcapsules and PMF
nanoparticles greatly affect the yield. In the diluted resultant slurry,
the microcapsules containing PETMP gradually deposited because
theirdensity(1.24 gcm?3) ishigherthan thatofwater.However,the
hollow microcapsules and some PMF nanoparticles used to be
thrown away as they were suspended in the solution.
is neutral at pH w3. Its molecules tended to have such directional
arrangement with hydrophobic groups oriented into the oil droplet
and hydrophilic groups out of the oil droplets, establishing a steric
allowed the M–F pre-condensate and oligomers to pass through
itself and to accumulate at the interface because of their surface
activity. When the M–F pre-condensate and oligomers were grad-
ually polymerized and hardened, the newly formed walls began to
protect the core materials . Actually, no PSMS molecules can be
found on the microcapsules’ wall by FTIR and NMR analysis, im-
plying that the external emulsifier has been washed away.
In short, in situ polymerization of melamine–formaldehyde can
be successfully used to prepare microcapsules containing polythiol.
Under certain conditions, satisfactory microcapsules with high
yielding and core content but lower loss factor can be obtained.
Surprisingly, thelossfactor in most cases is about 10% orevenlower.
This demonstrates that our idea proposed in Section 1 works, i.e.
Fig. 3. SEM micrographs of PMF-walled microcapsules prepared at (a) 40?C, (b) 50?C, (c) 60?C, and (d) 70?C. Conditions of synthesis: 60 min, pH¼3.2.
Fig. 4. SEM micrographs of (a) PMF-walled microcapsules, (b) PMF nanoparticles, and (c) broken PMF-walled microcapsules (that had been extracted with acetone) showing the
shell wall section. Conditions of synthesis: (a, b) 60 min, 50?C, pH ¼3.2; and (c) 60 min, 60?C, pH ¼3.2.
Y.C. Yuan et al. / Polymer 49 (2008) 2531–2541 2535
would help to reduce polythiol consumption. Besides, the polythiol
has four –SH groups and acts as a curing agent to form cross-linking
network whenever it reacts with M–F pre-condensate and oligo-
mers. Thus, the inner layer of polythiol core is hard to contact the
outside M–F pre-condensate and oligomers because the cross-link-
ing film hinders diffusion of the components from both sides.
If PMF were replaced by PUF for the microencapsulation, no
intact microcapsules would be obtained because the slower
reaction rate led to weak shell. Even though the shell strength of
PUF-walled microcapsules can be enhanced by increasing reaction
time, and/or reaction temperature and/or catalyst concentration,
the core content had to be very low (<20%) and many hollow
spheres with very thick wall were produced. Excessive consump-
tion of the active core material and different microencapsulation
mechanism of U–F resins should take the responsibility.
3.2. Morphology of the microcapsules and shell thickness
Morphology inspection is important for verifying the resultant
of the microencapsulation. Optical microscopic observation of PMF-
walled microcapsules showed appearance of diffraction rings.
According to the theory of optics, when core and shell in a micro-
capsule have different refractive indexes, diffraction rings would be
perceived at the interface between them . Therefore, it is known
that PMF has successfully microencapsulated PETMP with single
To have a general view of the microcapsules, SEM micrographs
were taken. Fig. 3(b) indicates that the microcapsules are spherical
with diameter of about 10 mm, and their outer surfaces are smooth
and compact. Some PMF nanoparticles and their aggregates are
attached to the microcapsules’ surface. A magnified image of the
PMF nanoparticles (i.e. sample 4-1 in Table 1, in which no core
material is encapsulated) show that their diameters are about
150–350 nm (Fig. 4(b)), which are nearly independent on the
The wall thickness can be estimated from Fig. 4(c). That is about
210 nm for sample 2-3. The inner surface of the microcapsule is also
smooth and compact. Having examined all the microcapsules, we
found that there is nearly no variation in shell wall thickness within
the same microcapsule, while the microcapsules prepared under
different conditions have slightly different wall thickness (ranging
from 150 to 350 nm).
The morphological observation implies that the formation rate
and the cross-linking degree of the shell wall must be sufficiently
high, and the nanoparticles separated from the aqueous phase
didn’t react with the wall to yield polynuclear surface structure like
the case of PUF-walled microcapsules [7,27,30,41]. In fact, the
present microcapsules (w10 mm in diameter) with thin shell wall
(w0.2 mm) offer enough interior space for storing the core
substance, which guarantees high healing efficiency when the
microcapsules serve as self-healing agent.
To assess whether the microcapsules having such shell thick-
ness are robust enough to survive handling during manufacturing
self-healing composites, simulative experiments were made.
Fig. 5(a) showed that in the mixture with uncured epoxy matrix,
the microcapsules are not damaged by the prior processing (i.e.
mechanical stirring, ultrasonication and vacuum degasification).
Moreover, after curing of the epoxy resin, the embedded micro-
capsules still maintain their integrity and are well adhered to the
matrix (Fig. 5(b)). Evidently,
requirement of practical application in terms of their stability with
3.3. Chemical structure of the microcapsules and activity
of the core material
Besides morphological characterization, chemical structure of
the resultant microcapsules should be understood (Fig. 6). For PMF,
Fig. 5. Optical microscopic and SEM photos of (a) PMF-walled microcapsules dispersed epoxy diluted by acetone and (b) fractured surface of cured epoxy filled with PMF-walled
microcapsules, in which the PETMP core was rinsed away by acetone prior to the observation. Conditions of synthesis of the PMF-walled microcapsules: 60 min, 50?C, pH¼3.2.
Extracted shell wall
Fig. 6. FTIR spectrum of PMF-walled microcapsules in comparison with those of PMF,
extracted shell wall and core material. Conditions of synthesis of the PMF-walled
microcapsules: 60 min, 50?C, pH ¼3.2. When the mixture of PMF-walled microcap-
sules and KBr was ground during sample preparation for FTIR measurement, the
microcapsules were broken.
Y.C. Yuan et al. / Polymer 49 (2008) 2531–25412536
its FTIR spectrum exhibits board stretching vibration of N–H and
O–H at 3390 cm?1, stretching of C]N at 1565 cm?1and charac-
teristic stretching peak of triazine ring at 814 cm?1. Meanwhile, the
spectrum of PETMP reveals S–H stretching mode at 2568 cm?1and
C]O stretching at 1737 cm?1, respectively. Clearly, in the spectrum
of the broken microcapsules, all the characteristic peaks of both
PMF and PETMP appear, which confirms that PETMP has been
encapsulated by PMF.
Owing to the high activity of hydrosulfide group, polythiol is
able to react with aldehyde or ketone via addition reaction .
M–F pre-condensate or oligomers also possess high activity, and
their self-condensation or co-condensation with poly(ols) would
easily take place because of the active methylol groups and
formaldehyde [34,36]. The reversible addition of thiols to alde-
hydes and ketones can be conducted with acidic or basic catalyst
, and polycondensation reaction of bis-thiols with formalde-
hyde and acetone is allowed to proceed in alkaline medium .
Besides, hydrogen sulfide might react with formaldehyde in acid
medium to form a hydrosulfide adduct . It means that the core
material PETMP might probably react with formaldehyde and/or
methylol groups of M–F pre-condensate or oligomers during the
Fig. 7 shows the dependence of C, N and S contents, core content
and loss factor of the ground microcapsules on acetone extraction
time. It is seen that within the time range 144–240 h, the elements’
contents, core content and loss factorare almost the same, meaning
a further increase in the extraction time is unnecessary. Although
the core material must have been fully extracted, there is still
9.45 wt.% S element in the residual shell wall of the microcapsules
after extraction (Fig. 7). In other words, 10.37 wt.% PETMP was
consumed in the course of microencapsulation for constructing the
shell wall, and hydrosulfide groups in the consumed PETMP took
part in the cross-linking or condensation reaction with M–F pre-
of the related materials are also able to evidence the above analysis
from another angle (Fig. 6). By using peak area of carbonyl group as
H/C]O peak area ratio of raw PETMP is estimated to be 4.33?10?2,
while that of the broken microcapsules is about 2.83 ?10?2. The
reduction in S–H/C]O peak area ratio suggests that hydrosulfide
groups of PETMP must have been partly consumed when the
microcapsules wereproduced. Besides, thecharacteristicstretching
peak of C]O rather than S–H appears in the spectrum of residual
shell wall of the microcapsules after extraction. It is indicative of
deactivated polythiol in the shell wall. These again reflect that the
reaction between polythiol and M–F oligomers is restricted due to
the formation of cross-linking network.
Since some of the core material PETMP has been involved in the
synthesis of the shell wall, it should be known whether the rest
portion keeps the chemical activity. Fig. 8 shows the micro-Raman
spectra of a raw PETMP droplet on cured epoxy resin substrate and
the PETMP flowing out from the broken PMF-walled microcapsules
embedded in cured epoxy. The stretching peak of S–H appears at
2573 cm?1and that of C]O at 1738 cm?1, which conform to the
characteristic adsorptions in the FTIR spectra (Fig. 6). When peak
area of carbonyl group serves as the internal standard, the S–H/
C]O peak area ratio of raw PETMP and the core material from the
broken microcapsules are estimated to be 1.72 and 1.62, re-
spectively. The values are very close, demonstrating that PETMP
remains unchanged after being encapsulated. On the other hand,
the accompanying microphoto of Fig. 8 suggests that the micro-
capsules embedded in the epoxy matrix are able to rupture and
release their core material at the damaged sites, which meets the
needs of self-healing composites.
The reactivity of the core material can be further evaluated by
DSC measurement (Fig. 9). The heating DSCscans indicate that with
a rise in temperature no reaction occurs in either the mixture of
0 50 100 150 200 250
Extraction time [h]
Fig. 7. Effect of acetone extraction time on elements’ contents, core content and loss
factor of the extraction residue of the ground PMF-walled microcapsules. Conditions of
synthesis of the PMF-walled microcapsules: 60 min, 50?C, pH ¼3.2. For ensuring
complete extraction, both ground and intact microcapsule samples had been mea-
sured, respectively. It was found that the results are hardly different, so that the data in
this figure were only collected from the ground microcapsules.
Raman shift [cm-1]
Fig. 8. Micro-Raman spectra of (a) cured epoxy; (b) a raw PETMP droplet on cured
epoxy resin substrate and (c) the PETMP flowing out from the broken PMF-walled
microcapsules embedded in cured epoxy. The accompanying microphoto shows the
fractured surface of cured epoxy filled with PMF-walled microcapsules. The center of
the cross indicates the position where the spectrum (c) was collected.
Y.C. Yuan et al. / Polymer 49 (2008) 2531–2541 2537
epoxy (DTP) and the catalyst BDMA or the mixture of epoxy (DTP)
andPETMP. Forthemixture ofepoxy(DTP)/PETMP/BDMA, however,
an obviousexothermicreactionis detected. The peak temperature is
56.4?C and the heat of reaction is 466.3 J/g. When PETMP was
replaced by the ground microcapsules containing PETMP, similar
exothermic peak with a heat of reaction of 212.6 J/g appears at
59.9?C. The difference might result from the fact that (i) grinding of
the microcapsules cannot ensure 100% breakage and the quantity
inert and do not take part in the curing reaction.
3.4. Core content and loss factor of the microcapsules
In the course of microencapsulation of PETMP, as discussed in
the last section, some PETMP was consumed to build up the shell
wall. Considering that excessive reduction in the core content
would lower the repair efficiency of the future self-healing com-
posites, loss factor of the encapsulated material and core content
should be studied. It is worth noting that although all the pro-
cessing parameters exert more or less influence on the core content
and loss factor, the effects of reaction temperature and feeding
weight ratio of core to shell monomers seem to be much more
As illustrated in Fig. 10, with increasing reaction temperature,
the core content decreases while the loss factor increases.
Accordingly, morphologies of the microcapsules prepared at dif-
ferent temperatures are different (Fig. 3). Moreover, the amount of
PMF nanoparticles also increases and their aggregation extent is
gradually enhanced with temperature. A careful survey of the
micrographs in Fig. 3 indicates that except for the microcapsules
prepared at 50?C (Fig. 3(b)), the rest microcapsules have collapsed
and shrunk to different extents. Because the reaction rate slows
down at lower reaction temperature (40?C, for instance), the shell
wall of the microcapsules has to be thinner, the cross-linking
degree of the shell wall material becomes lower, and the deposition
quantity of PMF nanoparticles is reduced. Consequently, the core
content of the microcapsules is higher and the loss factor is smaller.
The weak shell wall would easily cause collapse and shrinkage of
the capsules. On the other hand, as higher reaction temperature (60
or 70?C, for instance) accelerates the reaction, the shell wall of the
microcapsules would be thicker, coupled with higher cross-linking
degree and larger deposition quantity of PMF nanoparticles.
Eventually, the core content is lower and the loss factor is higher.
Fig. 3(c) and (d) shows that some of the microcapsules prepared
under the very conditions also fail to keep their integrity, probably
because (i) substantial core material was consumed to induce
shrinkage of the microcapsules and (ii) more drastic collision
among the microcapsules in the lower-viscosity solution at ele-
vated temperature leads to collapse of the capsules.
To view the problem from another angle, the effect of feeding
weight ratio of core/shell monomers is exhibited in Figs. 11 and
12. As the feeding weight ratio of core/shell monomers increases,
the core content increases and loss factor decreases. The amount
of PMF nanoparticles and their aggregation extent also decrease.
In particular, when the feeding weight ratio of core/shell mono-
mers is 1.2, the core content reaches the lowest value while loss
factor the highest. About half of the core material was consumed
during microencapsulation as overmuch M–F pre-condensate or
oligomers have reacted with PETMP. In the case of feeding weight
ratio of core/shell monomers of 2.9, although the core content
becomes the highest and loss factor the lowest, the weak shell
wall has to result in a great number of collapsed and shrunk
Besides reaction temperature and feeding weight ratio of core/
shell monomers, dispersion rate and emulsifier content also in-
fluence the core content and loss factor bychanging microcapsules’
diameter and specific surface area, which will be discussed in the
next section. The smaller capsules with larger specific surface area
certainly impart a higher loss factor. The pH value of solution is
another parameter that should not be neglected as it controls the
-200 204060 80 100
Fig. 9. DSC heating traces of (a) epoxy (DTP)/PETMP/BDMA¼ 10.9/8.7/1, (b) epoxy
BDMA ¼10.9/1, and (d) epoxy (DTP)/PETMP ¼10.9/8.7. The compositions are ex-
pressed in terms of weight ratios.
Core content [wt.%]
Loss factor [wt.%]
Fig. 10. Effect of reaction temperature on core content and loss factor of PMF-walled
microcapsules. Conditions of synthesis of the PMF-walled microcapsules: 60 min,
1.0 1.5 2.02.5 3.0
Core content [wt.%]
Feeding weight ratio of core/shell monomers
Loss factor [wt.%]
Fig. 11. Effect of feeding weight ratio of core/shell monomers on core content and loss
factor of PMF-walled microcapsules. Conditions of synthesis: 60 min, 50?C, pH¼3.2.
Y.C. Yuan et al. / Polymer 49 (2008) 2531–25412538
reaction rate of M–F pre-condensate or oligomers. The faster the
condensation reaction, the lower the loss factor.
3.5. Size and size distribution of the microcapsules
Before being encapsulated, PETMP had to experience emulsifi-
cation. Since the shell wall thickness of the microcapsules varies
within the narrow limits of 150–350 nm, the size and size distri-
bution of the microcapsules have to be mainly determined by the
geometry of the core material droplets formed in the emulsification
stage. In this context, dispersion rate of the homogenizer for
making the emulsion is critical, and emulsifier content is also
important for keeping the body of PETMP droplets in shape.
Fig. 13 shows that higher dispersion rate yields finer emulsion.
As a result, with increasing the dispersion rate, the microcapsules
average diameter decreases and their size distribution becomes
narrow. It is worth noting that there is a small shoulder peak at
about 1 mm on each size distribution curve (Fig. 13). It might
represent the aggregates of PMF nanoparticles. The inset of Fig. 13
gives the plots of average diameter and specific surface area of the
microcapsules against dispersion rate on log–log scale. Linear
relationships are observed, which coincides with the results of
other researchers (e.g., PUF-walled microcapsules containing
diluted epoxy , PMF-walled microcapsules containing dicyclo-
pentadiene  and PUF-walled microcapsules containing dicy-
clopentadiene ). The influence of emulsifier content is shown in
Fig.14. When the emulsifier content is increased, a fineremulsion is
obtained. Accordingly, the microcapsule’s average diameter de-
creases and their size distribution is narrowed down.
Fig. 12. SEM micrographs of PMF-walled microcapsules with different feeding weight ratios of core/shell monomers: (a) 1.2, (b) 1.7, (c) 2.3, and (d) 2.9. Conditions of synthesis:
60 min, 50?C, pH ¼3.2.
0 20 4060 80100
Dispersion rate [x103rpm]
10 x 103rpm
15 x 103rpm
20 x 103rpm
Size [ m]
Fig. 13. Effect of dispersion rate on size distribution of PMF-walled microcapsules at
a constant emulsifier content of 2%. Conditions of synthesis: 60 min, 50?C, pH ¼3.2.
0 204060 80 100 120
Size [ m]
Emulsifier content [%]
Fig. 14. Effect of emulsifier content on size distribution of PMF-walled microcapsules
at a constant dispersion rate of 15 ?103rpm. Conditions of synthesis: 60 min, 50?C,
Y.C. Yuan et al. / Polymer 49 (2008) 2531–2541 2539
3.6. Stability of the microcapsules
When self-healing composites are thermally cured, the em-
bedded microcapsules have to experience heat impact. Therefore,
their thermal stability should be known first. From the pyrolytic
behaviors shown in Fig. 15, it is seen that there are three main
phases of weight loss for the shell wall PMF within the temperature
range of interests. The first phase with w2% weight loss from 25 to
100?C should be due to evaporation of the adsorbed water. The
second phase with w10% weight loss in the range of 100–270?C is
attributed to evaporation of formaldehyde from the deformalde-
hyde reaction of PMF at elevated temperature . The third one
with w61% weight loss in the range of 270–600?C originates from
decomposition of a portion of melamine and thermal degradation
of PMF .
In contrast to PMF, PETMP is quite stable below 250?C. It also
exhibits three stages of pyrolysis from 250 to 600?C corresponding
to its evaporation and decomposition processes. The detailed
mechanisms involved need further investigation. Nevertheless,
thermal degradation of the PMF-walled microcapsules containing
PETMP is a combination of the contributions made by PMF and
PETMP. Fig.15 indicates that the peak pyrolytic temperatures of the
microcapsules are not exactly the same as those of the individual
components. Thermally induced reaction might occur between the
shell and core materials during decomposition.
In addition to thermal stability, the microcapsules in service
should have long-term stability. Fig. 16 shows effects of storage
time and temperature on weight loss of the microcapsules.
Evidently, weight loss of the microcapsules exposed to room
temperature for six months is only about 0.14 wt.%. When the time
is increased to one year, the microcapsules’ weight loss is still low
(i.e. 0.31 wt.%). Visual inspection does not find any leakage from the
microcapsules and their appearance also remains unchanged after
long time storage. As the microcapsules were kept in a desiccator
and the saturated vapor pressure of polythiol is very low
(w6.5?10?11Torr at 25?C) , it can be deduced that the above
weight loss should result from evaporation of the physically
When the storage temperature is increased, weight loss of the
microcapsules becomes more and more significant. Staying at
higher temperature for longer time would lead to substantial
weight loss. It suggests that the microcapsules couldn’t be time-
lessly exposed to heat. At elevated temperature, evaporation of
adsorbed water, release of formaldehyde, diffusion of the core
material out of shell wall and volatilization of some core material
would happen [49,50]. Compared tothe microcapsules using PUFas
the wall material, the current PMF-walled microcapsules have
obviously higher storage stability and heat resistance because of
the difference in their chemical structures .
Microcapsules containing highly active polythiol were success-
fully prepared by in situ polymerization of melamine–formalde-
hyde in an oil-in-water emulsion. Surface activity of M–F
pre-condensate and oligomers, faster reaction rate, and suitable
reaction conditions were found to be the key factors. The mecha-
nism of the microcapsules’ formation followed the enrichment and
fast curing of M–F oligomers at the surface of polythiol droplets,
which ensured lower loss factor.
The synthesis method was improved by optimizing the
processing parameters. The appropriate reaction time was about
40–60 min. Further extension of time had little influence on the
products. The reaction should proceed at about 50?C, otherwise
the microcapsules might collapse and shrink. The pH value of about
2.9–3.2 was appropriate, which had complex effect on the prop-
erties of the products. The feeding weight ratio of core/shell
monomers should be set at about 2.3. Size and size distribution of
the microcapsules were mainly determined by dispersion rate and
emulsifier content. When the microencapsulation was carried out
under the suitable conditions mentioned above, loss of core
material was greatly reduced, while thickness and strength of shell
wall were balanced. Although a small part of the core material
PETMP was inevitablyconsumed for constructing the shell wall, the
majority remained unchanged, possessing the same activity as the
The resultant microcapsules’ shell wall was relatively thin
(w0.2 mm), which assured sufficient core content even if the mi-
crocapsules were very small (1–10 mm). The microcapsules proved
to be strong enough to survive handling during manufacturing
self-healing composites. Having been pre-embedded in cured
epoxy, the microcapsules had satisfied affinity to the matrix, and
can be readily ruptured releasing PETMP fluid as expected upon
damaging of the composite.
The PETMP-loaded microcapsules had high stability. They can
be kept at room temperature for long time and could bear the
100200300 400500 600
a, d: PETMP
b, e: PMF
c, f: PMF-walled microcapsules
Rate of weight loss [arb. unit]
Fig.15. TG and DTG curves of PMF-walled microcapsules in comparison to those of the
shell wall and core material. Conditions of synthesis of the PMF-walled microcapsules:
60 min, 50?C, pH ¼3.2.
90 120150 180 210240 270
Weight loss [%]
Weight loss [%]
Storage time [month]
Fig. 16. Effects of storage time and temperature on stability of PMF-walled micro-
capsules: (a) weight loss of the microcapsules as a function of time at room temper-
ature, and (b) weight loss of the microcapsules as a function of temperatures at
constant storage times.
Y.C. Yuan et al. / Polymer 49 (2008) 2531–25412540
moderate temperature or high temperature applied for curing Download full-text
The present work provided a new approach for producing
microcapsules containing hardener for epoxy, which can be used to
manufacture self-healing composites or other self-curing stuffs like
The authors are grateful to the support of the Natural Science
Foundation of China (Grants: 50573093, U0634001).
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