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CHAPTER 4
Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 147
CHAPTER 4 EFFECT OF DIFFERENT PARAMETERS ON HYBRID
EMULSION POLYMERIZATION
4.1. INTRODUCTION
Evolution in the field of water-based epoxy coatings has resulted in the development of so called
transitional products, so named because they signify the transition of the traditional solvent based
coatings to coatings dispersed in water. Emulsion polymerization is the major industrial process
for the production of waterborne polymeric dispersions that are used for coatings, paints, paper
coating, adhesives, and carpet backing among other applications [350, 351]. Emulsion
polymerization is carried out by polymerizing an oil-in-water emulsion stabilized by surfactant
molecules and using a suitable initiator system. Substantial effort has been made in the
development of water-based epoxy systems because of the environmental concern to reduce
volatile organic compounds (VOC), and some interesting methods have been proposed [352-
355]. One method is to produce water-based Ep-Ac copolymers via grafting polymerization
[356-358]. Epoxy coatings commonly are used because of the functional epoxy groups in them
which have subsequent excellent characteristics, such as heat resistance and good adhesion
[324]. The Acrylic latexes possess hydrolytic, light, and oxidative stability so we combine epoxy
resin with acrylic latexes to achieve the advantage of both in hybrid using emulsion
polymerization. The word hybrid is basically defined as the system in which each particle
contains at least two distinct polymers [327].
We term emulsion polymerizations in which the polymerization of acrylate monomer is carried
out in the presence of another resin for the purpose of forming graft copolymers and acrylate
copolymer hybrid macroemulsion polymerizations. The technique used to carry out the
polymerization is simple macroemulsion polymerization in which epoxy resin is grafted with
acrylate monomers using water soluble thermal initiator.
In commercial processes, the production rate is often limited by the heat removal rate that is
proportional to the difference in temperature between the reaction mixture and the cooling fluid
in the reactor jacket. Therefore, emulsion polymerization is carried out at 75–90°C, namely,
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trying to maximize the production rate without reaching the water boiling point. In this
temperature range, thermal initiators are often used because they provide enough radical
generation rate during the emulsion polymerization process when the monomer concentration is
relatively high. However, their radical generation rate is not fast enough to reduce the
concentration of the residual monomer to the low value required for commercialization. In these
cases, redox initiators, which present a much higher radical generation rate, are used.
From preperative standpoint there are two classes of initiating system.
1. A thermal initiator system in which use is made of water soluble material which produce
free radicals. most commonly used initiator is potassium persulfate.
2. The activated or redox initiation system. Since these systems depend on the generation of
free radicals by oxidation –reduction reaction of water soluble compounds. initiation near
room temperature is possible, in fact redox system operating below room temperature are
available. The typical redox system is potassium persulfate and sodium metabisulfite.
Trace of iron salt catalyst may be supplied in the form of ferrous ammonium sulfate
Polymer microstructure is strongly affected by polymerization temperature [359-361]. Therefore,
to enlarge the envelope of achievable polymer micro-structures, it is interesting to expand the
range of polymerization temperatures. Expansion to higher temperatures can be performed by
using thermal initiators, but its application is limited by the fact that most commercial reactors
cannot withstand pressure. Therefore, the expansion of the temperature range to lower values is
the most practical alternative to enlarge the properties envelope. At low temperatures, redox
initiators should be used because the thermal initiators do not generate enough radicals. When
low temperatures are required to obtain the desired polymer architectures, lower production rates
are achieved under isothermal conditions. However, nonisothermal polymerization strategies
allow obtaining polymer architecture characteristics of low temperature polymerization at high
production rates [362].
Peroxide in combination with reducing agents are a common source of radicals.
H2O2 + Fe2+ HO- + HO* + Fe3+
ROOR + Fe2+ RO- + RO* + Fe3+
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Other reductunt such as Cr2+, V2+, Ti3+, Cu2+, can be employed
The conditions of polymerization had an effect on the stability and performance of hybrid
system. With each parameter of the system, properties of emulsion changes, making it necessary
to study and fix the parameters of the system.
The main aim of this chapter is to highlight the effects that various factors on stability of hybrid
Ep-Ac emulsions, in particular the overall conversion of monomers. Some of the above
mentioned factors include resin content, polymerization temperature, initiator type and its
concentration used for polymerization, and the amount of surfactant used for stabilizing
emulsion.
4.2. OBJECTIVES
The hybrid epoxy resin was synthesized with conventional emulsion polymerization technique.
The shelf life of emulsion polymers depends on many parameters contributing to its stability.
The effect of each parameter on emulsion polymerization where study. The objectives of this
study can be summarized as:
1. To study the effect of initiator type and concentration on hybrid epoxy polymer. Water
soluble and oil soluble initiator system were used for emulsion polymerization.
2. The redox initiator system was used for emulsion polymerization and compared with the
radical system method.
3. The effect of surfactant concentration on particle size of hybrid emulsion polymer where
study. Also the shelf life of emulsion polymer with combination of different of
surfactants were analyzed.
4. The total solid content of emulsion polymerization affects the conversion and rate of
reaction. The ratio of organic to aqueous phase was increased and properties of emulsion
polymerization where studied.
5. The polymerization temperature affects the degree of grafting and rate of monomer
conversion. The polymerization temperature was varied and effect on the emulsion
polymerization where study.
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4.3. EXPERIMENTAL METHODOLOGY
4.3.1. MATERIALS
4.3.1.1. Specification of Thermal initiator
1. Potassium persulfate
Structure:
- O S
O
O
S
O
O
O-O O K+K+
CAS No: [7727-21-1]
Molecular formula: K2S2O8
Molecular weight: 270.32
Appearance: white crystalline solid
Density: 2.477 g/cm3
Melting point: < 100 ºC (decomposes)
Specific gravity: 2.44
Solubility (in water): 1.75 g/100 mL
2. Hydrogen peroxide
Structure:
OHHO CAS No: [7722-84-1]
Molecular formula: H2O2
Molecular weight: 34
Appearance: Coloureless liquid
Melting point: -52 ºC
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Boiling point: 114 ºC
Refractive index: 1.366
Specific gravity : 1.18
3. Azobisisobuteronitril (AIBN)
Structure:
H3C CN
CH3
N N CH3
CH3
CN
CAS No: [78-67-1]
Molecular formula: C8H12N4
Molecular weight: 164.21
Appearance: white solid
Melting point: 99 ºC
Refractive index: 1.366
4.3.1.2. Specification of Redox initiator System
1. Sodium Bisulfite
Structure:
Na+
O-
S
O
HO
CAS No: [7631-90-5]
Appearance: White solid
Molecular formula: NaHSO3
Molecular weight: 104.06
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Density: 1.48 g/cm3
Melting point: 150 ºC
Refactive index: 1.526
Solubility (in water): 42g/100ml
2. Sodium metabisulfite
Structure:
OS
OS
O
O
O2Na
2-
CAS No: [7681-57-4]
Molecular formula: Na2S2O5
Molecular weight: 190.10
Appearance: white powder
Density: 1.48 g/cm3
Melting point: >170ºC
Solubility (in water): 54g/100ml
3. Sodium thiosulfate
Structure:
S S
O
O
O2Na
2-
CAS No: [7772-98-7]
Molecular formula: Na2S2O3
Molecular weight: 158.11
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Appearance: white crystals
Density: 1.66 g/cm3
Melting point: 48.3 ºC
Refractive index: 1.489
Solubility (in water): 70.1 g/100 ml
4. Ferrous sulfate
Structure:
Fe++
S
O
O
-O O-
CAS No: [7720-78-7]
Molecular formula: FeSO4
Molecular weight: 151.9
Appearance: blue/green crystals
Density: 2.84 g/cm3
Melting point: 70 ºC
Refractive index: 1.536
Solubility (in water): 25.6 g/100 ml
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4.3.2. EXPERIMENTAL PROCEDURE 4.3.2.1. Effect of initiator type and concentration
The effect of initiator type and concentration were studied with the use of three different free
radical and redox initiator systems.
Table 4.1: Free radical initiator concentrations for hybrid epoxy emulsion polymerization
Experiment Initiator concentration w.r.t. monomer (%)
Potassium persulfate Hydrogen Peroxide Azobisisobuteronitril
E01 0.1 - -
E02 0.2 - -
E03 0.4 - -
E04 0.6 - -
E05 0.8 - -
E06 0.5
E07 - 1.0 -
E08 - 1.2 -
E09 - 1.5 -
E10 - 2.0 -
E11 - - 0.2
E12 - - 0.4
E13 - - 0.6
E14 - - 0.8
E15 - - 1.0
Table 4.1 represents, the concentration of free radical initiators used for hybrid emulsion
polymerization. The potassium persulfate and hydrogen peroxide are water soluble initiators
while, Azobisisobuteronitril is an oil soluble initiator used for synthesis. The effect of different
initiators is studied with respect to monomer conversion and emulsion stability.
The hybrid emulsion polymerization was also performed by the redox initiation method with
different oxidant/reductent pair. Table 4.2 represents, concentration of different redox systems
applied for emulsion polymerization. The emulsion polymerization with redox initiator was
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performed at a lower temperature than the dissociation temperature of potassium persulfate in
thermal method. Table 4.2 also reports the temperature of hybrid emulsion polymerization.
Table 4.2: Redox initiator system for hybrid epoxy emulsion polymerization
Experiment Redox initiator system (mole) Temperature
Potassium persulfate Sodium Bisulfite Ferrous sulfate
R01 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 30 ºC
R02 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 40 ºC
R03 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 50 ºC
Potassium persulfate Sodium metabisulfite Ferrous sulfate
R04 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 30 ºC
R05 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 40 ºC
R06 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 50 ºC
Potassium persulfate Sodium thiosulfate Ferrous sulfate
R07 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 30 ºC
R08 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 40 ºC
R09 9.2 × 10-4 9.2 × 10-4 6.5 × 10-4 50 ºC
4.3.2.2. Effect epoxy resin concentration
The effect of epoxy resin content on properties of hybrid emulsion was analyzed, Table 4.3
represents the percentage of resin w. r. t. acrylate monomers. Other parameters of emulsion
polymerization were kept constant indicated in Table 3.3. the experimental procedure for hybrid
synthesis were reported in 3.3.3.2.
Table 4.3: Effect of epoxy resin concentration on hybrid emulsion polymerization
Experiment Epoxy resin (%)
B01 55
B02 60
B03 65
B04 70
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4.3.2.3. Effect of surfactant type and concentration
The surfactants acts in two ways during emulsion polymerization. Surfactants gives rise to
micelle formation where polymerization takes place and they also stabilize the emulsion polymer
from coagulation. The concentration of surfactants used were above their critical micelle
concentration (CMC). A critical issue in commercial latex manufacture is their stability during and after production. As
mentioned, surfactants have an effect on overall emulsion stability. Thus, the appropriate
surfactant selection is an important consideration when designing a formulation. Table 4.4,
shows the combinations of different surfactants used for hybrid emulsion polymerization. The
experiments were performed with procedure indicated in 3.3.3.2. Anionic and nonionic are the
most effective and widely used surfactants in emulsion polymerization. While anionic surfactants
prevent coagulation due to electrostatic repulsions, nonionic surfactants prevent coagulation due
to steric stabilization.
Table 4.4: Effect of surfactant concentration on emulsion polymerization
Experiment Aqueous phase (%) Organic phase (%)
Neoigen DK X 405 Daninol 25P Triton X 100 H-301
S01 5.27 8.10 - -
S02 - - 1.62 0.20
S03 5.27 - 1.62 -
S04 5.27 - - 0.20
S05 - 8.10 1.62 -
S06 - 8.10 - 0.20
4.3.2.4. Effect of speed of agitation
The stability and size of polymer particles are greatly affected by strring speed used for emulsion
polymerization. The emulsion polymerization was performed at different agitation speed (Table
4.5). The synthesis of hybrid was carried out in a 500 ml four necked reaction vessel equipped
with a reflux condenser, nitrogen gas inlet, mechanical stirrer, addition funnels, thermometer
placed in a water bath. Stirrer with button to hold Teflon (shaft diameter 8 mm and length 400
mm) was used in this process. The formulation used for experiments were reported in Table 3.3.
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Table 4.5: Effect of agitation speed on emulsion polymerization
Experiment Stirring speed (RPM)
P01 50
P02 200
P03 500
P04 700
4.3.2.5. Effect of polymerization temperature
The emulsion polymerization was carried out at different polymerization temperatures with
potassium persulfate initiator. The primary aim of this work was to study the effect of increasing
temperature on monomer conversion and emulsion stability. The experiment were performed
according to formulation in Table 3.3 and procedure reported in 3.3.3.2.
Table 4.6: Effect of polymerization temperature
Experiment Polymerization temperature (ºC)
T01 60
T02 65
T03 70
T04 75
4.3.2.6. Effect of total solid content
The emulsion polymerization of acrylate monomers in the presence of epoxy resin was
performed in aqueous phase. The effect of ratio of organic phase to aqueous phase was studied
keeping the ratio of epoxy to acrylate constant.The experimental procedure followed were
indicated in 3.3.3.2.
Table 4.7: Variation of Aqueous to Organic phase ratio
Experiment Aqueous Phase (%) Organic Phase (%)
C01 70 30
C02 60 40
C03 55 45
C04 50 50
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4.4. CHRECTERIZATION
4.4.1. Physicochemical characterization of Emulsion
The progress of the reaction was determined gravimetrically (ASTM D 2834). Samples of the
emulsion were taken at regular intervals and analyzed for solid content and monomer conversion.
4.4.1.1. Sample Handling
Samples for solid content analysis were withdrawn from the reactor during runs with a 1 mL
syringe at regular intervals. All samples were quickly added to a vile containing a solution of
hydroquinone and acetic acid. The role of reason for the acetic acid was to adjust the pH for the
hydroquinone to be effective as an inhibitor. Directly after addition of the withdrawn sample to
the quenching solution, the vial was cooled down in an ice bath.
The pH values of the hybrid emulsions were measured by means of a digital pH meter (Mettler
Toledo). Specific gravity (ASTM D 1475) was also reported. The viscosity of the emulsion was
recorded using (ASTM D2196) Brookfield Viscometer using spindle no. 3 at 30°C. Electrolytic
stability of emulsion was tested using 5% alum solution prepared in D.I. Water. The amount of
electrolyte required for coagulation to take place was taken as a measure of electrolytic stability.
All other analysis reported in Table 4.8 was performed according to ASTM.
4.4.2. Particle size distribution
Particle size and distribution of emulsion were measured by dynamic light scattering malvern
mastersizer. Samples were diluted to low concentrations (5mL/1000mL) with deionized water
and then subjected for the particle size and particle size distribution analysis.
4.4.3. Thermal analysis
DTGA (METTLER TA 4000 SYSTEM) was carried out in a nitrogen atmosphere at a heating
rate of 10ºC min-1 to study the thermal stability of the cured films. Hybrid coating was first dry at
50ºC in a vacuum oven and used for analysis.
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4.5. RESULTS AND DISSCUSSION
4.5.1. Analysis of hybrid Ep-Ac Emulsion
Hybrid emulsion synthesized with conventional polymerization and analyzed for its properties
and stability, Table 4.8 shows, the properties of emulsion synthesized using standard recipe.
Table 4.8: Properties of Hybrid Ep-Ac emulsion
Properties Observation Methods
Appearance Milky white ASTM D 2244
Solid content % 45 ASTM D 2834
Emulsion pH 9 -
Specific gravity 1.024 ASTM D 1475
Viscosity cps 45 ASTM D-1200
Electrolytic stability a 47 -
Freeze-thaw cycles 3 ASTM D 2243-95
Shelf stability (months) >6 ASTM D 869
Accelerated Stability (Days) >7 ASTM D 3707
Drying Timeb (min) 5 ASTM D 1640
Gloss 600 82.5 ASTM D 523
Impact (150 lb/in.) Pass ASTM D 2794
Adhesion (%) 100 ASTM D 3359
Pencil hardness HB ASTM D 3363
Pendulum hardness(cycles) 102 ASTM D 4366 a: 5 % Alum soln mL/100 g of emulsion, b: Touch to dry
The stability of hybrid polymer emulsion was depended on many parameters of polymerization,
variation in any of such parameter alters the properties of hybrid emulsion. In order to study the
effect of such parameters on hybrid emulsion polymerization, one parameter was selected at the
time keeping other content.
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4.5.2. Free radical initiator concentrations for hybrid epoxy emulsion polymerization
The hybrid emulsion polymerization was performed with water soluble and oil soluble free
radical initiators. Dissociation temperature and dissociation constant for different initiators used
for emulsion polymerization are reported in the Table 4.9 (Brandrup, J. 1975).
Table 4.9: Dissociation constant
Initiator Temperature °C Dissociation constant (Kd sec-1) Ea (KJ/mol)
Potassium persulfate 70 9.23×10-6 121.5
Hydrogen peroxide 70 2.4×10-12 124.1
AIBN 70 3.17×10-5 128
The effect of potassium persulfate and hydrogen peroxide initiators on monomer conversion was
studied by the variation in concentration. The ratio of resin to the monomer and other
polymerization parameters were kept constant. Both initiator shows the same trend for monomer
conversion with increasing concentration. Figure 4.1, represents a plot of conversion against
time for potassium persulfate initiator.
Figure 4.1: Effect of initiator concentration on monomer conversion
The Table 4.10 summarized properties of hybrid emulsion synthesized with different free radical
initiators.
0102030405060708090
100
0 50 100 150 200 250 300
Con
vers
ion
Time (min)
0.1% KPS
0.2% KPS
0.4% KPS
0.6% KPS
0.8% AIBN
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Table 4.10: Properties of hybrid emulsion synthesized with free radical initiator
Experiment E02 E10 E14
Specific gravity 1.024 1.022 1.0
Viscosity cps 45 40 25
Solid content @ 110ºC (%) 44.5 39 20.4
Electrolytic stability a 47 16 20
Freeze-thaw cycles 3 2 0
Shelf stability (months) >6 <6 <6
Accelerated Stability (Days) >7 <7 <7
Mechanical stability Pass Pass Fail
The monomer conversion for hybrid emulsion with increasing concentration of KPS were found
to increase with maximum value at 0.2% concentration. We speculate that, the higher percentage
of KPS enhances primary radical termination in the small emulsion droplets/particles and thus
reduce the initiator efficiency. The presence of SO-4 anion in polymer chain was found to
increase the electrolytic and shelf stability of hybrid emulsion.
For emulsion polymerization performed with H2O2 initiator lower monomer conversion was
observed w.r.t. Potassium persulfate initiator attributed to lower free radical yield of hydrogen
peroxide. The shelf life stability of hybrid emulsion synthesized with H2O2 initiator was found to
be lower with the formation of coagulation during storage.
The polymerization was also carried out with oil soluble initiator AIBN with other ingredient
constant in order to study its effect on monomer conversion. Total monomer conversion
observed was around 20% which is far less than water soluble initiator. The lower conversion
attributed to more radical termination in the organic phase and acrylate monomer such as acrylic
acid, methyl methacrylate are hydrophilic which will retard the propagation rate of
polymerization.
4.5.3. Redox initiator system for hybrid epoxy emulsion polymerization
The hybrid epoxy resin polymer was synthesized with the redox initiator system, potassium
persulfate in combination of different reductant were used for synthesis. The emulsion
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polymerization was carried out at different temperature and properties of hybrid emulsions were
studied. Table 4.11 represents properties of hybrid emulsion synthesized with redox initiator.
Table 4.11: Properties of hybrid with redox initiator
Experiments R01 R02 R03 R04 R05 R06 R07 R08 R09
Appearance Milky
white
Milky
white
Milky
white
Milky
white
Milky
white
Milky
white
Milky
white
Milky
white
Milky
white
Solid content % 32.1 38.7 43.2 38.9 40.4 42.8 37.5 40.3 44.1
Emulsion pH 10 10 10 10 10 10 10 10 10
Specific gravity 1.01 1.01 1.02 1.01 1.02 1.02 1.01 1.02 1.02
Viscosity cps 38 40 45 38 40 42 35 40 40
Electrolytic stability 19 25 21 24 20 26 21 20 28
Freeze-thaw 0 1 1 1 1 0 1 0 1
Shelf stability (month) 0 1 1 0 0 1 0 1 1
The emulsion polymerization was performed with the redox initiator system at different
temperatures. All redox systems show higher solid content at 50°C, however compared to free
radical initiators level of coagulation was higher. The hybrid emulsion synthesized with the
redox initiator system has lower shelf stability then compared to free radical initiator. The lower
shelf stability of hybrid emulsion attributed to phase separation between polymer and aqueous
phases. The presence of epoxy resin increases hydrobhobicity of an emulsion system which gives
hybrid emulsion with lower stability.
In this strategy, the polymerization starts at low temperature, and the polymerization heat is used
to increase the reactor temperature. Therefore, a large part of the polymer is produced at low
temperatures. Starting the emulsion polymerization at low temperature under industrial like
conditions in a consistent way is challenging, often due to the presence of an induction period at
the beginning of the process. The induction period is due to the use of technical grade monomers,
which contained a substantial amount of inhibitor and to the modest rate of radical generation.
The low propagation rate constant (at this low temperature) may also contribute to the observed
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induction periods. A way of improving consistency in the production is to reduce/eliminate the
induction period. This depends on the choice of the initiator system.
4.5.4. Effect of epoxy resin concentration on hybrid emulsion polymerization
4.5.4.1. Effect of epoxy resin concentration on monomer conversion
The Figure 4.2 shows the total monomer conversion as a function of time for a standard
emulsion recipe with increasing ratio of the epoxy resin to the monomer. The monomer mixture
in these runs was a mixture of BA, MMA, HEMA and AA in the ratio 48:47:2.5:2.5 by weight.
The initiator concentration was 9.2 × 10-4 in each run and reaction temperature was 75ºC. The
Figure 4.2 shows that as resin-to-monomer ratio was increased the reaction rate and monomer
conversion decreases.
Figure 4.2: Effect of epoxy resin content on monomer conversion
The rate of conversion for the reaction was found to be decreased with resin content, the
presence of resin at the polymerization site can be seen as a steric obstacle that each monomer
unit must bypass to be able to polymerize on to the growing polymer chain. The study shows
that, increased resin concentration creates larger impending effect on monomer unit transport to
the growing polymer chain. This explains why at low to moderate monomer conversions a
decrease in polymerization rate is observed. Polymerization progressed to approximately 94%
monomer conversion (defined as acrylic conversion, on an epoxy free basis) in around 2.5 hours,
then remained fairly constant. Higher level of resin results in lower monomer conversion.
0102030405060708090
100
0 50 100 150 200 250 300
Conv
ersi
on(%
)
Time(min)
E01
E02
E03
E04
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4.5.4.2. Effect of epoxy resin concentration on particle size
Resin concentration also had effects on the particle size of the emulsion; Figure 4.3 shows the
variation of particle size of hybrid emulsion with increasing ratio of epoxy to acrylate monomers.
Figure 4.3: Effect of epoxy resin content on particle size
The results indicate that, particle size of emulsion increases with resin percentage at constant
surfactant and initiator concentration. The increasing particle size was attributed to increasing the
hydrophobic epoxy resin percentage in micelle as the numbers of micelle remain constant.
4.5.4.3. Effect of epoxy resin concentration on thermal stability
The epoxy resin percentage also affects the thermal stability of the emulsion polymer. Thermal
study was carried with DTGA thermograms of hybrid Ep-Ac with increasing resin percentage
are given in Figure 4.4. The TGA thermogram for hybrid with 60 % epoxy resin content shows
an initial lethargic degradation rate up to 381.9ºC. Beyond this, a faster rate of degradation
occurs, which extends up to 413.04ºC with 93.5 wt % loss. Table 4.12 shows, the results of
DTGA analysis of hybrid system with increasing resin content.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
50 55 60 65 70 75 80
Part
icle
Siz
e µm
Resin %
55
60
65
70
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Table 4.12: Results of DTGA analysis
Epoxy Resin % Onset point ºC End set point ºC Weight loss%
55 358.6 420.8 93.5
60 370.1 421.3 92.5
65 381.9 413.04 88..1
Figure 4.4: Overlay of DTGA analysis of hybrid epoxy coatings
With increasing resin content thermal decomposition temperature for hybrid rises, which shows
that they possess higher thermal stability.
It is also well known that, an increase in crosslink density increases the thermal stability of resin.
Improvment in thermal stability of hybrid with incresing resin content attributed to the presence
of highly crosslinked network.
4.5.5. Effect of surfactant concentration on emulsion polymerization
The combination of anionic and nonionic surfactants were used for preparation and stabilization
of hybrid emulsion. Anionic surfactants prevent coagulation by electrostatic repulsions
originated from the anionic charges adsorbed on the polymer particles and their associated
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double layers. And nonionic surfactants, especially polyethoxylates, prevent coagulation by
spatial or steric stabilization. The effect of surfactant concentration on emulsion polymerization,
stability and coating properties were reported in Table 4.13.
Table 4.13: Effect of surfactant concentration on emulsion polymerization
Experiment S01 S02 S03 S04 S05 S06
Appearance Milky
white
Milky
white
Milky
white
Milky
white
Milky
white
Milky
white
Solid content % 30.2 34.1 41.2 42.0 40.8 37.9
Coagulum (%) 12.78 10.32 3.21 2.59 3.12 6.43
Emulsion pH 10 10 10 10 10 10
Specific gravity 1.01 1.01 1.02 1.02 1.02 1.01
Viscosity cps 38 40 45 45 45 40
Electrolytic stability 32 38 45 40 27 31
Freeze-thaw cycles 2 3 3 1 2 2
Shelf life (month) 4 4 3 4 3 2
The hybrid emulsion synthesized with a combination of nonionic and anionic surfactant system
in both the aqueous as well as organic phase. The hybrid emulsion was synthesized with different
combinations of surfactants and it is observed that, the elimination of any of the surfactant will
affect the stability of emulsions.
Table 4.14: Particle size (in µm) of hybrid emulsion polymer
Experiment S01 S02 S03 S04 S05 S06
At 00 day 0.281 0.269 1.102 0.241 0.219 0.201
At 30 day 1.121 1.030 2.345 1.019 1.102 1.171
From the particle size study it is observed that, the emulsion with only nonionic surfactants gives
larger particle sizes, mainly due to their lack of charge. However, a small percentage of an
anionic surfactant used in combination with a nonionic surfactant is sufficient to reduce the
particle size. This can be observed from particle size of experiments with anionic surfactants.
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In order to achieve a desirable balance of properties, most commercial emulsion polymers are
made using appropriate combinations of anionic surfactants for particle size control and
electrostatic stabilization, and nonionic surfactants to enhance mechanical, electrolyte, freeze
thaw and thermal stabilities.
4.5.6. Effect of agitation speed on emulsion polymerization
The hybrid emulsion polymerization was carried out with varying agitation speed. Properties of
hybrid emulsion are indicated in Table 4.15.
Table 4.15: Characteristics of hybrid emulsions
Formulation P01 P02 P03 P04
Coagulum (%) 0.51 0.98 5.70 12.59
Particle size (µm) 0.235 0.251 0.168 3.270
Specific gravity 1.01 1.024 1.022 1.01
Viscosity cps 35 45 45 30
Solid content @ 110ºC (%) 44.5 44.1 39.6 31.23
Electrolytic stability a 40 65 60 39
Freeze-thaw cycles 2 4 3 1
a: 5 % alum soln ml/100 gm of emulsion, b: touch to dry (min)
The stirring speed for hybrid synthesis affects the particle size, solid content and the percentage
of coagulum formation during polymerization. At higher speed of agitation the percent coagulum
formation is higher due to destabilization of micelles. Hybrid emulsions synthesized at higher
agitation level has lower stability.
CHAPTER 4
Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 168
4.5.7. Effect of polymerization temperature
The variation of polymerization temperature has marked effect on conversion time and total
monomer conversion. Figure 4.5, shows the effect of temperature; all conversion-time curves at
the higher temperature are above the equivalent curves (equal resin and solids) at the lower
temperature. In fact, all 75°C curves are above all 60 °C curves.
Figure 4.5: Effect of polymerization temperature on monomer conversion.
This means that, not surprisingly temperature had a more significant effect on the reaction rate
than the effect of any of the other variables studied. Increased temperature has marked effect on
the production of free radicals. Table 4.16 indicates that, rate of free radical generation from
potassium persulfate can be increased hundred fold by raising temperature from 50°C to 90°C
[336].
Table 4.16: Variation of dissociation constant of potassium persulfate with temperature
Temperature (°C) Dissociation constant (Kd/S2)
50 9.5×10-7
70 2.3×10-5
90 3.5×10-4
0102030405060708090
100
0 50 100 150 200 250 300
Con
vers
ion
(%)
Time(min)
75706560
CHAPTER 4
Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 169
Also with increase in reaction temperature, the inner viscosity of emulsion particles decrease,
which promote the diffusion rate of acrylate monomers and KPS in the polymer particles. Thus,
the conversion of monomer increased with temperature.
4.5.8. Variation of Aqueous to Organic phase ratio
The effect of solid content on the reaction rate is shown in Figure 4.6. The ratio of organic phase
to aqueous phase was varied at constant resin to monomer ratio. The examination of the
conversion graph indicates, with increasing solid content of system rate of conversion decreases.
Figure 4.6: Effect of total solid content on monomer conversion
The lower rates of conversion at higher solids are attributable to many factors like, higher levels
of epoxy to the initiator. At higher resin content system is more hydrobhobic which retards the
rate of acrylate monomer conversion. Also at higher monomer concentration particle size of
emulsion droplet will increase which lowers total surface area of emulsion. Thus, the rate was
decreased with increasing solid of emulsion. All the experiments are carried out with the same
amount of surfactants are obviously not enough to stabilize the polymer particles and overall
viscosity of the system increase which retards the conversion rate. The hybrid emulsion
synthesized with higher solid content has lower stability and shelf life below six months.
0102030405060708090
100
0 50 100 150 200 250 300
Conv
ersi
on(%
)
Time(min)
C03
C01
C02
C04
CHAPTER 4
Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 170
4.6. CONCLUSION Hybrid epoxy acrylate emulsion was synthesized using macroemulsion polymerization technique
and studied for the effect of different reaction parameters on the system. It was observed that
with increasing resin content overall monomer conversion and rate of polymerization decreases.
Particle size of hybrid emulsion also gets affected by resin percentage, with an increase in epoxy
to monomer ratio particle size of the emulsion increases. Thermal stability of the hybrid polymer
was studied using DTGA analysis and it was found that with increasing resin content the thermal
stability of hybrid films increases which will attribute to higher crosslinking percentage in the
cured film.
The effect of initiator level on hybrid system was studied using water soluble and oil soluble
initiator. The monomer conversion was elevated in case of KPS compared to AIBN; In case of
water soluble initiator conversion was optimized at level of 0.2%. Emulsion polymerization was
also synthesized and optimized with the redox initiator system. However, this system gives
emulsions with lower shelf life stability.
Effect of a combination of different surfactant combination and concentration was also observed
in the emulsion polymerization process. Temperature of polymerization had pronounced effect
on the rate and conversion. In order to study the effect of solid content on reaction rate the ratio
of organic phase to aqueous phase was increased with constant resin to monomer ratio and other
parameters. It was concluded that with increasing solid content rate of polymerization and
conversion decreases which was due to the higher viscosity of the system.
It is essential to balance many parameters to obtain a stable hybrid emulsion with good
performance properties. Variation in any parameter can alter the properties of hybrid emulsion.