CHAPTER 3 Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 75 CHAPTER 3 SYNTHESIS OF HYBRID EPOXY RESIN EMULSION FOR INDUSTRIAL COATING APPLICATIONS 3.1. INTRODUCTION The field of polymer chemistry since last few decades is approaching towards new eco-friendly route to develop polymers, in order to minimize or eliminate the utilization of toxic chemicals, particularly organic solvents, which are hazardous to health and the environment [321]. Scientists and technologists trying to innovate green technologies like powder coatings, UV cured coatings, solvent-less coatings, and waterborne coatings. Water-based coatings have become more widely used in the past several decades because, they are environment friendly and offer easy clean up also their properties and application performance characteristics have been improved. Efforts are being made to develop industrially viable water-based coating systems [322]. Several waterborne coatings have been developed, which are showing superior properties than those of solvent based systems. Waterborne coatings exhibit good weather stability, better durability as well as good physicomechanical properties [323]. Epoxy resins have functional epoxy groups and have subsequent excellent characteristics, such as heat resistance, high strength, good corrosion resistance and good adhesion [324]. However; they have poor or low fracture energy, high shrinkage, and show brittle behavior. Acrylic latexes have hydrolytic, light, and oxidative stability [325]. The purpose of this work was to examine the feasibility of polymerizing acrylic monomers in the presence of epoxy resins to determine if this hybrid system could offer the advantages of epoxy properties in a water- based acrylic coating. Hybrid polymer emulsion is basically defined as, the system in which each particle contains at least two distinct polymers [326, 327]. Mostly the hybrid polymers are prepared using three general routes, (i) hybrids from mini-emulsion polymerization of a solution of polycondensate in acrylic monomers, (ii) hybrids from polycondensation polymers prepared in mini-emulsion [228], (iii) Hybrids from a modified polycondensates used as seed for emulsion polymerization of acrylic (or other) monomers [229, 230]. The miniemulsion technique is used for polymerizing acrylates in the presence of resins or grafting polyacrylates on the backbone of
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CHAPTER 3
Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 75
CHAPTER 3 SYNTHESIS OF HYBRID EPOXY RESIN EMULSION FOR
INDUSTRIAL COATING APPLICATIONS
3.1. INTRODUCTION
The field of polymer chemistry since last few decades is approaching towards new eco-friendly
route to develop polymers, in order to minimize or eliminate the utilization of toxic chemicals,
particularly organic solvents, which are hazardous to health and the environment [321].
Scientists and technologists trying to innovate green technologies like powder coatings, UV
cured coatings, solvent-less coatings, and waterborne coatings. Water-based coatings have
become more widely used in the past several decades because, they are environment friendly and
offer easy clean up also their properties and application performance characteristics have been
improved. Efforts are being made to develop industrially viable water-based coating systems
[322]. Several waterborne coatings have been developed, which are showing superior properties
than those of solvent based systems. Waterborne coatings exhibit good weather stability, better
durability as well as good physicomechanical properties [323].
Epoxy resins have functional epoxy groups and have subsequent excellent characteristics,
such as heat resistance, high strength, good corrosion resistance and good adhesion [324].
However; they have poor or low fracture energy, high shrinkage, and show brittle behavior.
Acrylic latexes have hydrolytic, light, and oxidative stability [325]. The purpose of this work
was to examine the feasibility of polymerizing acrylic monomers in the presence of epoxy resins
to determine if this hybrid system could offer the advantages of epoxy properties in a water-
based acrylic coating. Hybrid polymer emulsion is basically defined as, the system in which each
particle contains at least two distinct polymers [326, 327]. Mostly the hybrid polymers are
prepared using three general routes, (i) hybrids from mini-emulsion polymerization of a solution
of polycondensate in acrylic monomers, (ii) hybrids from polycondensation polymers prepared in
mini-emulsion [228], (iii) Hybrids from a modified polycondensates used as seed for emulsion
polymerization of acrylic (or other) monomers [229, 230]. The miniemulsion technique is used
for polymerizing acrylates in the presence of resins or grafting polyacrylates on the backbone of
CHAPTER 3
Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 76
resins [331-334]. Hydrophilic acrylic molecules can be incorporated into the chemical structure
of the epoxy to make it water dispersible which is highly patented work, Grafting was
extensively used to produce water-reducible epoxy graft copolymers [335].
Miniemulsion polymerization is widely used for synthesis of hybrid systems but the
conventional polymerization technique is more simple and economical. This research discloses
the polymerization of acrylates in the presence of epoxy resin was carried out via hybrid
macroemulsion polymerisation to get the hybrid Epoxy Acrylate emulsion. The combination of
anionic and nonionic surfactants was used for the emulsification and stabilization of emulsion
against coalescence. Hybrid Ep-Ac macroemulsion was synthesized with increasing percentage
of epoxy resin. Hybrid Ep-Ac polymer coating was analyzed using FT-IR, GPC and DSC to
evaluate the structural orientation.
3.2. OBJECTIVES
This chapter discloses the synthesis of hybrid epoxy resin emulsion with conventional emulsion
polymerization technique. The main objective of the work is to investigate the emulsion
polymerization of acrylate monomers in the presence of epoxy resin and to study whether this
hybrid gives superior properties then both participating systems. The following studies were
planned to attain the objectives:
1. Synthesis and characterization of acrylate copolymer emulsion for coating applications.
2. Synthesis of hybrid emulsion with incorporation of epoxy resin into acrylate emulsion.
3. Synthesis of hybrid epoxy-acrylate emulsion with increasing resin content and
optimization with respect to shelf life and corrosion resistance.
4. Investigation of emulsion polymerization mechanism for the hybrid epoxy resin system
to understand the structural configuration.
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3.3. EXPERIMENTAL METHODOLOGY
3.3.1. MATERIALS
The chemicals used for experimental work are summarized in Table 3.1.
methacrylate are around 31, 11.1, 39 and 37 mN/m, respectively [336], so the critical surface
tension of the acrylic copolymer should be between 11 and 37 mN/m, which is lower than that of
the epoxy resin, which is around 44 mN/m.
Thus, during the process of casting and drying the hybrid films, the acrylic-copolymer segments
tried to segregate near the air-facing layer and the epoxy segments moved to the mold-facing
side to minimize the surface energy. This migration is very beneficial in the application of
coatings, because epoxy resins have excellent adhesion to substrates while acrylic copolymers
remaining on the air-facing side have very good weatherability and appearance.
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3.5.4. Glass-Transition Temperature and Degree of Grafting
For the hybrid polymerization system expected to compose of ungrafted epoxy, epoxy-acrylate
graft copolymer and ungrafted acrylic copolymer. To confirm this, the polymer samples were
analyzed by DSC. Figures 3.11-3.14 shows a typical heat flowchart for hybrid coatings. From
the chart for hybrid H6, three glass-transition temperatures (Tg) can be identified, indicating
three distinct types of polymer. The first Tg is at about -30 to -25°C. This is thought to be
ungrafted Epoxy .The second Tg is at -5 to -9°C. This peak corresponds to poly (acrylate-graft-
epoxy). The third Tg is at 0 to 10°C and results from polyacrylate copolymer. The Tg of a
copolymer can be estimated by the following equation:
1푇푔(copolymer) =
푊푖(푇푔푖)homopolymer
Here, Wi and Tgi refer to the weight fraction and Tg of the i th homopolymer, respectively. The
measured and calculated glass-transition temperatures for all samples are given in Table 3.9.
Table 3.9: Glass transition temperature of hybrid polymers
Hybrid Tg Tested oC
I II III
Tg Calculated oC
IIa IIIb
H1 N/A N/A 20.38 N/A 19.39
H2 -28.88 -3.82 21.35 2.54 19.39
H3 -25.98 -8.10 19.85 -4.91 19.39
H4 -25.43 -7.98 18.56 -7.74 19.39
H5 -30.12 -9.03 22.17 -11.76 19.39
H6 -28.93 -7.07 19.25 -16.19 19.39
H7 -27.76 -8.45 18.39 -19.98 19.39 a: tg calculated on the basis of epoxy and acrylate monomers in recipes, b: tg calculated on the basis total acrylate monomers in recipes
The higher Tgs correlate with the Tgs of polyacrylate copolymer and poly (acrylate-graft-epoxy)
copolymer. Therefore, the polymer resulting from hybrid macroemulsion polymerization appears
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to be of two types: polyacrylates and poly (acrylate-graft-epoxy). The relative proportions of
these polymers are important to the properties of emulsion. For instance, the presence of poly
(acrylate-graft-epoxy) serves to compatibilize free epoxy and polyacrylate during film formation.
Figure 3.11: DSC chart for H1 hybrid coating
Figure 3.12: DSC chart for H3 hybrid coating
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Figure 3.13: DSC chart for H6 hybrid coating
The relative proportions of grafted and pure polyacrylates were determined by solvent extraction.
Grafted epoxy and free epoxy are known to be soluble in THF and diethyl ether; polyacrylates
are not soluble in ethyl ether but do dissolve in THF. Highly crosslinked polymer does not
dissolve in any solvent. All of the samples analyzed here dissolved in THF. This indicates feebly
crosslinked polymer existed in the samples. Extraction was performed in steps as described in
characterization section. Figure 3.14 FTIR spectra of Polymer Before and after extraction.
Spectra indicate lowering of carbonyl peak intensity after extraction. The fraction of grafted
epoxy resin was calculated as:
Degree of Grafting = Weight of polyacrylate grafted to epoxy/ Weight of total acrylate
monomers × 100
As shown in Table 3.10, the degree of grafting decreases as the resin content of the emulsion
increases. The polyacrylate chains are grafted to epoxy to form poly acrylate-graft-epoxy. As
resin content of the emulsion increases percentage of acrylate decreases which will lower the
final percentage of acrylate grafted to epoxy resin. Also due to its high hydrophobicity, epoxy is
unlikely to diffuse into an aqueous phase therefore; poly (acrylate-graft-epoxy) should be formed
mainly in nucleated droplets.
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Table 3.10: Degree of Grafting of Epoxy Resin
Sample Ungrafted
Epoxy Grafted Epoxy
Poly (MMA) Poly (MMA-co-BA) Poly (BA)
H1 N.A. N. A. 35.51 31.72 30.12
H2 11.47 8.53 28.16 25.31 11.12
H3 17.48 12.52 22.82 18.95 11.03
H4 23.88 16..12 13.18 8.02 9.56
H5 31.98 18.02 9.78 2.15 8.23
H6 38.57 21.43 5.33 1.49 3.22
H7 50.63 19.37 11.64 8.53 3.01
Figure 3.14: FTIR Spectra of H5 Before and after soxhlet extraction
3.5.5. Particle size distribution
Particle size is the most important factor in emulsion polymerization. It affects reaction rate and
mechanism, emulsion stability over period of time, the formation of coagulation and other forms
4000.0 3000 2000 1500 1000 400.0
cm-1
%T
3447.8
2960.3
2875.9
2368.2 2345.4
1734.7
1458.8
1258.9 1146.6
1033.0
990.9
828.4
753.5
558.1 474.6
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of agglomeration, polymer solubility, film formation mechanism and film properties such as
gloss or opacity. The particle size in the Hybrid Ep-Ac emulsion system with increasing resin to
acrylate monomer ratio was analyzed and it was found that with increase in amount of epoxy
resin the particle size of the emulsion increases (Table 3.11), particle size distribution graphs for
all hybrids are indicated in Figure 3.15-3.20. This will affect the stability of the emulsion as with
increase in the particle size stability of the emulsion decreases. Increase in particle size will also
affect the final coating properties of hybrid Ep-Ac.
Table 3.11: Particle size with different Epoxy/acrylate percentage
Formulation Particle size mean (µm) Size distribution (µm)
d (0.1) d (0.5) d (0.9)
H1 0.167 0.109 0.161 0.235
H2 0.176 0.134 0.177 0.225
H3 0.184 0.132 0.177 0.247
H4 0.187 0.134 0.180 0.251
H5 0.189 0.137 0.186 0.256
H6 0.191 0.139 0.193 0.261
H7 1.306 1.30 0.198 3.270
Figure 3.15: Particle size distribution of Hybrid H1
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Figure 3.16: Particle size distribution of Hybrid H2
Figure 3.17: Particle size distribution of Hybrid H3
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Figure 3.18: Particle size distribution of Hybrid H4
Figure 3.19: Particle size distribution of Hybrid H5
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Figure 3.20: Particle size distribution of Hybrid H6
3.5.6. Emulsion stability
Stable emulsions are essential in order to achieve successful emulsion products. An accelerated
storage stability test was carried out for the hybrid products. The samples were kept in an oven at
50ºC for 7 days. Stable behavior was confirmed with all samples. The mechanical storage
stability was evaluated by the centrifugation method to speed up the potential destabilization
processes. The samples were centrifuged at ambient temperature with centrifuge at 6000 rpm
speed for 1 hr period of time. The samples were evaluated for any kind of coagulation or
precipitate and confirmed to be stable.
Table 3.12: pH of the emulsions after storage
Hybrid Fresh pH after 24 hr pH after 6 months
H1 9.0 9.0 8.5
H2 9.5 9.5 8.5
H3 9.0 9.0 8.0
H4 10 9.0 8.5
H5 9.5 9.0 8.5
H6 10.0 10.0 9.5
H7 10.0 9.5 8.0
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The pH of the emulsion plays vital role in stability and properties of emulsion so that effect of
storage time on pH of the hybrids was examined (Table 3.12). pH of the hybrids was measured
within 24 hr after preparation (fresh) and after approximately 6 months of storage at ambient
temperature. The insignificant decrease in pH value was expected and also observed.
3.5.6.1. Electrolytic stability
The Hybrid EP-Ac emulsion was tested for electrolytic stability using 5% alum solution. 100 g
of emulsion required 25 ml of alum, for complete coagulation. Daninol 25P and H-301 is the
anionic emulsifier which promotes electrostatic stabilization of the emulsion. On adding Alum
electrolyte, decrease of the double layer thickness takes place. This result decrease in the stability
of the latex particle and coagulation takes place.
3.5.6.2. Freeze-thaw stability
Emulsion can freeze during storage of transportation and therefore resistance to freeze-thaw
cycles are very important for commercialization. The hybrid emulsion were tested for freeze-
thaw stability by being subjected to cycles where the sample was frozen at -17°C for 12 hr and
then allowed to thaw at room temperature for 12 hr. Emulsion showed excellent stability to three
cycles. When freezing occurs, ice crystals separate from the unfrozen emulsion, reducing the
volume of the continuous phase and increasing the ionic concentration of this phase. Therefore,
the stability of the emulsion is reduced and the emulsion, which is subjected to high pressure,
coagulates. On the other hand, non-ionic surfactants with long ethoxy chains can reduce
coagulation during the freeze-thaw process. The neoigen DK X 405 nonionic surfactant may be
playing the role of ethoxy chains, reducing coagulation during the freeze-thaw process and
increasing the stability.
3.5.7. Molecular Weight
Figures 3.26, is representative of the GPC measurements for the hybrid epoxy-acrylate emulsion
using the RI detector. In the RI curve the first peak corresponds to a molecular weight of more
than 700,000, which can be taken as the GPC chromatogram of the copolymer, including the
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Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 121
epoxy-acrylic graft copolymer and the ungrafted acrylic copolymer. GPC chromatograph for all
the hybrids represented from Figure 3.21-3.27.
Table 3.13: Molecular weight distribution of hybrid polymers
Formulation Molecular weight Polydispersity
Mn Mw
H1 905437 928021 1.009
H2 778620 796574 1.013
H3 748721 766417 1.025
H4 615632 630491 1.024
H5 679854 680538 1.0314
H6 776543 789793 1.196
H7 689641 697865 1.212
Figure 3.21: GPC chromatograms for Hybrid H1
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Figure 3.22: GPC chromatograms for Hybrid H2
Figure 3.23: GPC chromatograms for Hybrid H3
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Figure 3.24: GPC chromatograms for Hybrid H4
Figure 3.25: GPC chromatograms for Hybrid H5
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Figure 3.26: GPC chromatograms for Hybrid H6
Figure 3.27: GPC chromatograms for Hybrid H7
The second one corresponds to a molecular weight of less than 1000, which is obviously the
GPC chromatogram of the ungrafted epoxy resin. Table 3.13 summarizes the weight-average
(Mw) and number average (Mn) molecular weight for all hybrid polymers. The increase in the
epoxy resin concentration corresponds to the decrease in the acrylic monomer concentration, so
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the weight-average molecular weight decreases. The number-average molecular weight does not
have an obvious change because it is only sensitive to those species with small molecular weight
and is dominated by epoxy resin molecules. The increase in the epoxy resin concentration causes
an increase in the numbers of active hydrogen atoms, but the increase in the epoxy resin
concentration decreases the concentration of acrylic monomers concurrently because the total
solid content was maintained constant in our experiments. As a result, graft branches decrease in
other words; the graft ratio has a decreasing trend with the augmentation of the epoxy resin
concentration.
3.5.8. Chemical resistance
The acid and alkali resistance (Table 3.14) of acrylate and hybrid epoxy acrylate emulsions was
evaluated; hybrid epoxy system reveals a slight decrease in performance in acid-resistance test
compared to acrylate emulsion. This may be due to the slightly inferior performance of epoxies
in an acidic environment. The alkali resistance was found to be excellent in all the experimental
compositions based on Ac as well as hybrid Ep-Ac.
3.5.9. Water and salt-water resistance
Table 3.14: Chemical Resistance of Epoxy-Acrylate
Formulation H2O (24 hr) NaOH(2%)(48 hr) HCl (2%) (12 hr) Salt water (96 hr)
H1 C a C e
H2 C a D d
H3 B a E d
H4 A a E c
H5 A a E c
H6 A a E c
H7 E d E e a: not affected; b:films partially removed, c:partial blistering/rust spot, d:complete film lift-off, e:complete corrosion
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salt-water resistance of the hybrid Ep-Ac by immersion test was quite satisfactory. The results in
Table 3.14 reveal a somewhat poor performance of the H1 emulsions, which can be attributed to
the hydrophilic nature of the acrylate copolymers; it forms the active centre on the surface of the
film through which the attack by the polar moieties, such as salt water or other corrosive
materials, is facilitated.
3.5.10. Impact resistance
The results of impact resistance (Table 3.8) of the dried films followed the same trend as that of
the flexibility and adhesion. This can be attributed to tough films resulting from the hybrid epoxy
emulsions. Toughness is one of the inherent characteristics of epoxies. The improved
performance hybrid epoxy emulsions can also be attributed to the high molecular weight of the
polymer (as identified by GPC).
3.5.11. Water and alkali absorption
Samples of the latex films were cut into 4cm x 4cm squares which were left to soak in either
deionized water, or a 0.05N NaOH solution (both at 29°C) for 48 hr.
Figure 3.28: Water and alkali absorption of hybrid Epoxy-Acrylate.
050
100150200250300350400450
0 20 40 60 80
% A
lkal
i A
bsor
banc
e
% Epoxy Resin
0
50
100
150
200
250
300
350
0 20 40 60 80
% W
ater
Abs
orbn
ce
% Epoxy Resin
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The weight gain was Measured and reported in terms of a percentage of the original sample
mass. Both the water and alkali absorption shows same trend (Figure 3.28) as the percentage of
resin increases the value of absorption decreases. This was attributed to hydrophobic nature of
the epoxy resin. An acrylate film has the hydrophilic groups present on the backbone of the
polymer due to which value of absorption was high for the acrylate films without epoxy resin.
3.5.12. Corrosion Resistance
The corrosion resistance of the hybrid Ep-Ac coating was tested with salt spray (ASTM-117)
method for 500hr.analysis reveals that, with increasing percentage of epoxy resin the corrosion
resistance of the coating improves. Hybrid Ep-Ac coatings shows far superior corrosion
resistance properties compared to acrylate emulsion. The improvement in the corrosion
resistance of the hybrid coating with the content of the epoxy resin was attributed to the
hydrophobic nature of resin. There is increasing trend in corrosion resistance properties up to H6
but for H7 formulation the corrosion resistance is not satisfactory. The poor corrosion resistance
of H7 is attributed to the presence of unreacted epoxy resin. The larger particle size of the
emulsion affects the distribution of polymer coating on steel surface.
H1 H2 H3
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H4 H5 H6
Figure 3.29.: Metal specimen coated with hybrid coatings
H1 H2 H3
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H4 H5 H6
Figure 3.30: Metal specimen after exposure to salt spray for 500 Hr
Figure 3.30, shows metal panels after 500 hr of exposure to salt spray chamber. The corrosion
resistance of hybrid coatings shows improvement with increases in resin content.
3.5.13. Film formation and surface topography
Film formation of the hybrid was evaluated. The results show that a hybrid gives a very smooth
surface of the film. The binder formed a uniform and crack-free film and has good gloss. The
film of epoxy–acrylic hybrid was dried at room temperature (29-300C) to give clear and
transparent polymer film. The formation of a continuous film is dependent on the rate of drying
and the minimum film formation temperature (MFFT) of the polymer, which in turn is dependent
on the elastic modulus of the polymer [337]. MFFT is tending to be close to Tg of the polymer as
the both are influenced by the same molecular features. The glass transition temperature of the
hybrid was measured by DSC and it was found to be below the room temperature (300C) that’s
why the hybrid gives the good film formation at the room temperature.
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3.5.14. Mechanical properties of coatings
Hardness of coatings was determined by the Persoz pendulum method by which the ability of
coatings to the damping of oscillations was measured. Flexibility of coating applied on Mild
steel panel was tested using cylindrical mandrel bent test for both these properties hybrid coating
shows good results indicated in Table 3.8.
3.5.15. Accelerated QUV weathering
The hybrid Ep-Ac emulsion coating was exposed to UV radiation and the behavior of coatings in
QUV cabinet. These tests were chosen because their results strongly depended on the resin
structure. Gloss of coatings before and after 500 hr of exposition to UV radiation was measured,
and the results are shown in Table 3.15. Clear differences in the resistance of tested coatings to
UV were observed. Coatings based on conventional resins were more resistant to UV radiation
than coatings based on resins with increased branching. Similarly evident differences of
resistance properties of coatings were observed during tests in QUV cabinet, although
unexpected results were obtained.
Table 3.15: Gloss analysis for UV radiation exposure
Formulation At 00 hrs After 500 hrs Gloss retention (%)
H1 85.5 81.4 95.2
H2 82.4 54.3 65.8
H3 83.3 47.3 56.7
H4 81.0 52.8 65.1
H5 80.9 50.9 62.9
H6 82.5 46.2 56
H7 65.2 36.4 55.8
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3.5.16. GRAFTING MECHANISM IN HYBRID POLYMERIZATION
In order to understand the pathway of reaction hybrid macroemulsions of MMA/epoxy,
BA/epoxy, MMA/ BA/epoxy, MMA/ BA/AA/epoxy and AA/MMA/ BA/HEMA/epoxy were
carried out. KPS, a water-soluble initiator was chosen due to its common use in emulsion
systems; AIBN chosen as suitable oil-soluble initiators and their effect on grafting has been well
documented. The 13C NMR spectroscopy was used to monitor grafting in hybrid emulsion