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CHAPTER 4

Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications 47

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,

CHAPTER 4

Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications Page 148

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+

CHAPTER 4


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.

CHAPTER 4


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

CHAPTER 4


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


Appearance: white solid

Melting point: 99 ºC


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


CHAPTER 4


Density: 1.48 g/cm3


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


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


CHAPTER 4


Appearance: white crystals

Density: 1.66 g/cm3

Melting point: 48.3 ºC


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


Appearance: blue/green crystals

Density: 2.84 g/cm3



Solubility (in water): 25.6 g/100 ml

CHAPTER 4


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

CHAPTER 4


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.

CHAPTER 4


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

CHAPTER 4


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.

CHAPTER 4


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.

CHAPTER 4


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

CHAPTER 4


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

CHAPTER 4

Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications 2

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

CHAPTER 4


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

CHAPTER 4


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

CHAPTER 4


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

CHAPTER 4


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.

CHAPTER 4


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


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

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100

0 50 100 150 200 250 300

Con

vers

ion

(%)

Time(min)

75706560

CHAPTER 4


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


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.

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