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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


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.

CHAPTER 3


3.3. EXPERIMENTAL METHODOLOGY

3.3.1. MATERIALS

The chemicals used for experimental work are summarized in Table 3.1.

Table 3.1: Chemicals used for experiments

Chemical Purity Supplier Function

Epoxy resin (NPEL-128s) Industrial grade Resin & Plastic Ltd. Binder

Butyl acrylate Industrial grade Indofile Monomer

Methyl methacrylate Industrial grade Indofile Monomer

2-hydroxyl

ethylmethylacrylate

Industrial grade Indofile Monomer

Acrylic acid Industrial grade Indofile Monomer

Potassium persulfate Laboratory grade S. D. Fine Initiator

Sodium bicarbonate Laboratory grade S. D. Fine Buffer

Ethylene Diamine Laboratory grade S. D. Fine Neutralizing

agent

Neoigen DK X 405 Industrial grade Dai-Ichi Karkaria Nonionic

surfactant

Daninol 25P Industrial grade Dai-Ichi Karkaria Anionic

surfactant

EOPO copolymer Industrial grade Venus Ethoxyethers pvt. Ltd Nonionic

surfactant

H-301 Industrial grade Venus Ethoxyethers pvt. Ltd Anionic

surfactant

The deionized water was used for synthesis of hybrid epoxy emulsion polymerization. All other

chemicals used for analysis were procured from S. D. Fine chemicals.

CHAPTER 3


3.3.2. SPECIFICATIONS RAW MATERIALS

3.3.2.1. Specifications of DGEBA epoxy resin

Structure:

H2C

O

CHH2C O

OH

CH2CH

H2CO

CH3

C

CH3 O

OH2CC

H

H2COC

CH3

CH3

n

Appearance: Colourless liquid

Colour (Gardner scale): ≤1

Solid content: 98.66%

Epoxy equivalent weight (g/Eq.): 185

Epoxy content (eq /Kg): 3.48

Hydroxyl content (mg KOH/gm): 384

Repeating units (n): 0.17

Surface tension: 47 dynes/cm

Chloride content (%): 0.015

Diglycol content: 1.30 mol/100gm

FTIR Spectrum

4 00 0.0 3000 2000 1 50 0 1 00 0 550.01 5.0

16

18

20

22

24

26

28

30

32

34

36

3 7.3

cm-1

%T

3899.88

3499.30

3054.45

2966.212926.69

2873.66

2758.35

2489.45

2341.702063.89

1888.07

1747.88

1605.73

1501.69

1360.87

1295.96

1246.58

1184.38

1133.051085.32

1035.34

971.39

915.38

830.64

773.32

667.27575.81555.90

CHAPTER 3


3.3.2.2. Specifications of Acrylate monomers

1. Acrylic acid:

Structure:

H2C CH

C

OH

O

CAS No: [79-10-7]

Molecular formula: C3H4O2

Molecular weight: 72.06

Appearance: Colorless liquid

Melting point: 14 ºC

Boiling point: 141ºC

Specific gravity: 1.051

Solubility (in water): Completely Soluble

Surface tension: 28.5 dynes/cm

FTIR Specrtum

4000.0 3 000 2 000 1500 1000 400.0

0 .0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 041.9

cm-1

%T

3115.89

2661.69

2366.13

2156.83

1953.21

1715.481636.021618.21

1415.401298.20

1192.351060.94

986.01

928.80

812.88

649.11

629.93

448.38

CHAPTER 3


2. Methylmethacrylate:

Structure:

H2C

C

OCH3

CH3

O

CAS No: [80-62-6]



Appearance: colorless liquid

Melting point: - 48 ºC

Boiling point: 101 ºC


Solubility (in water): 1.5 g/100 mL


FTIR Spectrum

4 00 0.0 3 00 0 2000 1500 1000 500.0

1 0.0

15

20

25

30

35

40

45

4 7.3

cm-1

%T

3428.54

3106.312984.72

2954.63

2930.722848.71

2355.032339.32

2002.271892.41

1725.37

1634.20

1440.08

1378.06

1325.041301.49

1198.78

1163.31

1019.70

941.41

815.71

652.08599.90

CHAPTER 3


3. Butyl acrylate:

Structure:

H2C CH

C

OC4H9

O

CAS No: [141-32-2]




Melting point: -65 ºC





FTIR Spectrum

4 00 0.0 3 00 0 2000 1500 1 00 0 400.0

5.0

10

15

20

25

30

35

40

4 6.5

cm-1

%T

3435.33

3105.083037.58

2961.81

2936.88

2875.60

2354.562339.66

1937.43

1728.09

1635.521621.51

1463.691456.01

1434.88

1408.66

1385.91

1296.26

1274.76

1238.05

1192.08

1119.85

1065.36

1021.04

985.37968.05

910.09859.40

810.92

738.87

667.20

610.83

454.73

CHAPTER 3

Synthesis of Hybrid Epoxy resin Emulsions for Industrial Coating Applications 2

4. 2-hydroxylethylmethacrylate:

Structure:

H2C

C

OC2H4OH

CH3

O

CAS No: [868-77-9]







Solubility (in water): completely soluble


FTIR spectrum

400 0.0 300 0 200 0 150 0 100 0 400.0

0 .0

5

10

15

20

25

30

35

41.4

cm-1

%T

3418.29

2958.852931.86

2887.71

2544.322488.91

2352.15

2336.28

2141.542005.97

1894.01

1713.91

1634.971455.41

1405.271379.48

1322.631300.54

1177.171081.07

1032.37949.01

903.60

867.93

816.63

656.04

604.35

454.85

CHAPTER 3


3.3.2.3. Specification of initiator (Potassium persulfate)

Structure:

- O S

O

O

S

O

O

O-O O K+K+

CAS No: [7727-21-1]

Molecular formula: K2S2O8


Appearance: white crystalline solid

Melting point: < 100 ºC



3.3.2.4. Specification of buffer (sodium bicarbonate)

Structure:

-O C OH

O

Na+

CAS No: [144-55-8]

Molecular formula: NaHCO3


Appearance: white crystalline solid




CHAPTER 3


3.3.2.5. Specification of surfactants

1. Neoigen DK X 405 Nonionic surfactant

Chemical name: Nonyl phenoxy poly(ethylene oxy) ethanol with 40 moles of

ethylene

Structure:

OOH40

CAS No: [9016-45-9]

Appearance: Clear oily liquid

Molecular formula: C9H19C6H4(OCH2CH2)40OH

Molecular weight: 1996

Viscosity: 240 cps @ 25ºC


pH: 6.3

Hydrophobic lyophobic balance (HLB): 17.5

Critical micelle concentration @ 25ºC: 2.3 ×10-2 gm/L or 230 ppm

Surface tension @ 25ºC: 50 dyne/cm

Enthalpy of micellization: 1.67 Kcal/mole

Entropy of micellization: 31.0 cal./mol.K

2. Daninol 25P Anionic surfactant:

Chemical name: Sodium dodecyl benzene sulfonate

Structure:

S

O

O

O

Na

CAS No: [25155-30-0]

CHAPTER 3



Molecular formula: C18H29SO3Na





Critical micelle concentration @ 25ºC: 1201.3 ppm (Brandrup)




3. Triton X 100 Nonionic surfactant:

Chemical name : Nonyl phenoxy poly(ethylene oxy) ethanol with 12 moles of

ethylene

Structure:

OO

OH

12

CAS No: [9016-45-9]


Molecular formula: C9H19C6H4(OCH2CH2)12OH

Molecular weight: 1728




Critical micelle concentration @ 25ºC: 4.7× 10-5 gm/L or 85 ppm



CHAPTER 3



4. H-301 Anionic surfactant:

Chemical name: Sodium di decyl sulfo succinate

Structure: O

OO

O

S

O

O

O Na

CAS No: [2673-22-5]


Molecular formula: C30H57SO7Na




Critical micelle concentration @ 25ºC: 9.3×10-5 gm/L or 10 ppm


Enthalpy of micellization: - 0.05 Kcal/mole


CHAPTER 3


3.3.3. EXPERIMENTAL PROCEDURE

The experimental work was performed in steps, first the acrylate polymer emulsion was

synthesized where after the epoxy resin was incorporated in order to get hybrid emulsion. The

conventional emulsion polymerization technique was used for synthesis.

3.3.3.1. SYNTHESIS OF WATERBORNE ACRYLIC COPOLYMER EMULSION

COATING APPLICATIOS:

The synthesis of acrylate polymer emulsion was carried out with conventional emulsion

polymerization technique. The acrylate monomers present in organic phase were polymerized

using free radical initiator. Combination of anionic and nonionic surfactants was used for

stabilization of emulsion. Optimized formulation for synthesis of acrylate emulsion is indicated

in Table 3.2.

Table 3.2: Typical recipe for Synthesis of Acrylate emulsion

Components Concentration

In gm In % In mole

Aqueous Phase

D.I. water 38 30.11 2.11

Neoigen DK X 405 6.5 6.3 3.2 × 10-3

Daninol 25P 10 9.8 2.8 × 10-2

SBC (buffer) 0.05 0.04 5.9 × 10-4

Organic Phase

BA 18.4 15.8 0.14

MMA 25.9 25.4 0.25

HEMA 0.89 0.87 6.8 × 10-3

AA 0.89 0.87 0.012

Triton X 100 0.8 0.87 4.6 × 10-4

H-301 0.25 0.24 4.2 × 10-4

K2S2O8 0.25 0.24 9.2 × 10-4

CHAPTER 3


3.3.3.2. SYNTHESIS OF HYBRID EPOXY RESIN EMULSION FOR INDUSTRIAL

COATING APPLICATION

The hybrid epoxy-acrylate polymer emulsion were obtained by polymerizing ethylenically

unsaturated acrylate monomers in the presence of epoxy resin. The combination of anionic and

nonionic surfactants were used for stabilizing emulsion polymer. The conventional emulsion

polymerization technique was used for the synthesis of hybrid emulsion. The synthesis of hybrid

epoxy-acrylate emulsion was performed in two steps.

Part A: Preparation of Epoxy-Acrylate organic Phase

The organic phase was prepared by dissolving epoxy resin into acrylate monomers. The

optimized recipe used in the preparation of organic phase for emulsion polymerization is

indicated in Table 3.3. The epoxy resin was added to acrylate mixture to prepare solution for

polymerization. Stirrer with button to hold teflon (shaft diameter 8 mm and length 400 mm) was

used in this process. The combination of anionic and nonionic surfactants was used for organic

phase preparation.

Part B: Macroemulsion polymerization of acrylate monomers

1) The aqueous phase was prepared in the reaction vessel with the addition of surfactants to

deionized water followed by buffer. The mixture was agitated for 15 min at 250 RPM.

2) The aqueous phase was agitation at rpm of 250 with heating to attain a temperature 60-

65ºC.

3) The N2 gas was purged for 20 min to remove air oxygen present in the reaction vessel.

4) The potassium persulfate initiator was used for emulsion polymerization. Initiation was

performed with the addition of about 1/3 part of total initiator followed by the addition of

22.48% of total organic phase within 15-20 min.

5) The temperature of reaction mass was raised to 73- 75ºC after complete addition of

organic phase.

6) After an initiation period temperature was maintained to73- 75ºC for 40 min and then

raised to 77- 80ºC and further held it for 15 min.

CHAPTER 3


7) Then the remaining part of organic phase was added in 2 hr while maintaining the

temperature between 77 – 80ºC. Also the remaining initiator was added simultaneously.

8) After complete addition of organic phase the temperature of reaction mass was raised to

85ºC and maintain for 1 hr to remove residual monomer.

9) Finally the reaction mass was cooled to room temperature and filter through 200 mesh

filtration cloth to remove any coagulation if present.

10) The pH of final emulsion was adjusted to 10 by the addition of 33 % ethylene diamine

solution.

Table 3.3: Typical recipe for Synthesis of Hybrid Epoxy-Acrylic emulsion

Components Concentration

In gm In % In mole

Aqueous Phase

Deionized water 38 30.81 2.11

Neoigen DK X 405 6.5 5.27 3.2 × 10-3

Daninol 25P 10 8.10 2.8 × 10-2

SBC (buffer) 0.05 0.04 5.9 × 10-4

Organic Phase

Epoxy resin 30.62 24.83 0.082

BA 9.83 7.97 0.076

MMA 9.64 7.81 0.096

HEMA 0.46 0.37 0.0035

AA 0.46 0.37 0.0063

EOPO copolymer 2.0 1.62 0.0011

H-301 0.25 0.20 4.2 × 10-4

K2S2O8 0.25 0.20 9.2 × 10-4

CHAPTER 3


3.3.3.3. OPTIMIZATION OF HYBRID EPOXY ACRYLATE EMULSION WITH

RESPECT TO MONOMERS

The effect of monomer concentration on properties of hybrid emulsion were analyzed, Table 3.4

indicates variation in formulation considering only acrylate monomers used for experimental

procedures. The concentration of monomers was varied keeping the ratio of organic to aqueous

phase constant. Also the ratio of total acrylate monomers to epoxy resin was kept constant

(40:60%). All the other parameters for experiment were kept constant indicated in Table 3.3.

The experimental procedure for hybrid emulsion synthesis is indicated in 3.3.3.2.

Table 3.4: Variation of monomer concentration

Experiment BA% MMA% 2-HEMA% AA%

R1 0 93.31 3.21 3.21

R2 10 84.44 2.75 2.75

R3 20 74.34 2.71 2.71

R4 30 65.36 2.32 2.32

R5 35 60.76 2.06 2.06

R6 38 57.94 1.99 1.99

R7 40 56.02 1.93 1.93

R8 45 51.43 1.73 1.73

3.3.3.4. OPTIMIZATION OF HYBRID EPOXY ACRYLATE EMULSION WITH

RESPECT TO EPOXY RESIN

The main aim of this study was to synthesized hybrid epoxy resin emulsion,therefore the hybrid

emulsions was synthesized with higher level of epoxy resin. Jn this study hybrid emulsion was

synthesized with increasing percentage of epoxy resin. The formulation used for the experiments

are indicated in Table 3.5. The ratio of organic to aqueous phase was kept constant. The

emulsion polymerization was carried out with same experimental procedure and all other

parameters.

CHAPTER 3


Table 3.5: Formulations with increasing epoxy resin content

Formulation (%) H1 H2 H3 H4 H5 H6 H7

Epoxy resin 0 20 30 40 50 60 70

BA 56.20 40.37 33.69 28.83 24.06 19.24 14.44

MMA 39.93 36.07 33.02 28.26 23.57 18.87 14.15

HEMA 1.93 1.79 1.60 1.37 1.15 0.90 0.68

AA 1.93 1.79 1.60 1.37 1.15 0.90 0.68

3.3.3.5. STUDY OF GRAFTING MECHANISM IN HYBRID POLYMERIZATION

In order to understand the pathway of reaction it is necessary to evaluate the mechanism of

grafting in hybrid polymer. The grafting mechanism of hybrid Ep-Ac polymer grafting in

different system was studied. Emulsion polymerization was carried out with each acrylate

monomer in combination of epoxy resin. The measure of the degree of grafting can give the

significant information about the pathway of reaction. The main formulations used to understand

the mechanism of grafting were indicated in Table 3.6. The experimental procedure used for

experiments were reported in 3.3.3.2. All the other parameters for experiment were kept constant

indicated in Table 3.3.

Table 3.6: Formulation of different systems studied to understand grafting mechanism

System Initiator (%) Epoxy% BA% MMA% HEMA% AA%

MMA/Epoxy KPS (0.20) 49.65 - 49.65 - -

MMA/Epoxy AIBN (0.20) 49.65 - 49.65 - -

MMA/Epoxy AIBN+NaNO2

(0.20+0.12)

49.65 - 49.65 - -

BA/Epoxy KPS (0.20) 49.65 49.65 - - -

BA/ MMA/Epoxy KPS (0.20) 33.10 33.10 33.10 - -

BA/MMA/AA/HEMA/Epoxy KPS (0.20) 60 19.24 18.87 0.90 0.90

BA/ MMA/AA/Epoxy KPS 24.82 24.82 24.82 - 24.82

CHAPTER 3


3.4. CHRECTERIZATION 3.4.1. Spectral Analysis

3.4.1.1. FT-IR spectroscopy

FTIR analysis was performed on Perkin Elmer FTIR system spectrum BX. The hybrid emulsion

was cast on clean glass and dried to prepare the films for FTIR analysis.

FT-IR ATR was performed on Magna-IR 550 FTIR spectrometer (Nicolet Instruments, Madison,

WI). The scanning was repeated at least 200 times before the spectra were recorded at a

resolution of 2 cm-1. The hybrid emulsion was cast on clean glass and dried to prepare the films

for analysis.

3.4.1.2. NMR measurements

1H NMR spectra of the polymer were taken on a Bruker 300-MHz spectrometer with CDCl3 as a

solvent and tetramethylsilane was used as an internal standard.

13C NMR Spectrum was obtained by Bruker/advance AV 500WB spectrophotometer (Bruker

Biospin, Switzerland) operating at 500-MHz solid state NMR Spectroscopy. De-emulsified

emulsion sample was dried at 50 ºC for 24 hrs and then ground to get fine powder for the

analysis.

3.4.2. Thermal analysis

3.4.2.1. Differential Scanning Calorimetry

The hybrid polymer emulsion was de-emulsified washed, dried, and then analyzed by DSC

Q100, TA calorimeter instruments for the thermal behavior analysis. About 10 mg of sample was

used for the analysis. The DSC scanning was performed from 100 to -50°C at a heating rate of

10°C/min under a nitrogen atmosphere.

CHAPTER 3


3.4.2.2. Thermal Gravimetric 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.

3.4.3. Measurement of Degree of Grafting

The degree of grafting of epoxy resin with polyacrylate was determined by the solvent extraction

method. The extraction procedure involved about 1 g of dried emulsion wrapped with filter paper

and put into the extractor, after extraction with ethyl ether for 24 hr the sample was dried and

weighed. The selection of solvent for the extraction of any specific polymer was based on hansan

solubility parameters namely dispersion (δd), polar (δp), hydrogen bonding (δh) component. The

Hansen parameters for some of the polymers are represented in Table 3.7.

Extraction was performed with selected solvent to separate particular component from system,

steps involved during extraction are as follows:

Step I: Extraction with Diethyl ether

Free and grafted epoxy were extracted with the diethyl ether; that which remained was pure

polyacrylate, crosslinked material, or both. This residue was then dry for 24 hr at 40 ºC in oven,

after which further extraction process was performed.

Step II: Extraction with Ethyl acetate

The mixture of ungrafted and grafted epoxy resin polymer was extracted with ethyl acetate as a

solvent. The unreacted epoxy resin is known to be soluble in ethyl acetate, so that the extraction

process separates these components.

Step III: Extraction with Ethanol

The ethanol was selected as solvent for extraction of homo polymer of methyl methacrylate

based on Hansen parameters. The poly(MMA) was separated from other polyacrylats using an

extraction technique.

CHAPTER 3


Table 3.7: Hansen solubility parameters for different polymers

Polymer δd δp δh Solvents

Epoxy resin 8.5 5.5 5.5 Cyclohexanol, M-cresol, Ethyl lactate, Butyl

lactate, Diethylene glycol, Dipropylene glycol,

2-butoxy ethanol, Methyl dioxitol, Cellosolve

(oxitol), Diacetone alcohol, Cellosolve acetate,

Methyl cellosolve, Furan, Dioxane, Methylal,

Dimethyl sulphoxide, Propylene carbonate,

gama-butyrolactone, Acetone, MEK, MIBK,

Methyl isoamyl ketone, Isophorone,

Acetophenone, Cyclohexanone, THF, Mesityl

oxide, Ethyl acetate, Butyl acetate, Isoamyl

Actate, Acetonitrile, Butronitrile.

Poly (Butyl acrylate) 8.21 12.6 12.5 Benzene, Chloroform, THF, Dichloromethane,

Tetrachloromethane, Toluene, Butanol,

Dimethyltetrahydrofuran, Glycolic ester ether,

Ketone ester, chlorinated hydrocarbon

Poly (Methyl methacrylate) 18.1 10.5 5.0 Granular starch, Benzene, Dichloroethane,

Toluene, MEK, Acetone, Ethyl benzene, 3,3-

dimethyl-2-butanone, THF, Chloroform, 2-

butanone, Cyclohexanone, Dichloromethane,

1,2-dichloroethane, p-xylene, Mesitylene,

Methyl acetate, Tetrachloromethane, Isobutanol,

Cyclohexanone, Acetic acid, Butyl lactate

Poly (BA-co-MMA) 15.6 9.2 10.1 Chloroform, THF, DMF, Toluene, Diethylene

glycol, p-xylene, Dioxane

Poly (HEMA) 7.82 6.45 6.55 n-butyl acetate, Ethanolamine, Carbon

tetrachloride, THF, Diethyl amine,

Chlorobenzene, Ethylene chloride,

Nitrobenzene, Pyridine, DMF, Ethanol

CHAPTER 3


Poly (Acrylic acid) 15.6 8.7 14. Water, THF, Ethanol, Methanol, DMF,

Formamide, Acetic acid, Dioxane, Alkali

solution

Step IV: Extraction with Dichloromethane

The extraction of Poly (BA) was performed with Dichloromethane as solvent. Extraction was

performed for 24 hr in solvent extractor. Polyacrylate polymers and homo butyl acrylate get

separated into two phases.

Step V: Extraction with Dimethyl formamide

The poly(BA-co-MMA) in hybrid system was extracted with DMF solvent. Extraction of

samples was performed for 8 hr in solvent extractor.

Step VI: Extraction with Tetrahydrofuran

The measurement of crosslinking percentage was performed with THF as solvent for 12 hr. After

extraction with THF solvents, the remaining material was the crosslinked polymer.

3.4.4. Physicochemical characterization of Emulsion

3.4.4.1. Solid content:

The progress of the reaction was determined gravimetrically. For each experiment, three parallel

samples were dried and solid content was determined as an average value of the three samples

according to ASTM D 2834. The sample weight about 2 gm was used for analysis, solid content

was determined at 110ºC in oven for 1 hr. The solid content of the emulsion was calculated with

the formula:

Solid content (%) = ×100

Where,

CHAPTER 3


W0= weight of empty Petri dish

W1= weight of petri dish with emulsion before heating

W2= weight of petri dish with emulsion after heating

3.4.4.2. pH:

The pH values of the hybrid emulsions were measured by means of a digital pH meter (Mettler

Toledo).The pH electrode was calibrated at 25ºC with standard buffer solutions at pH 7.00 and

9.21, or at pH 7.00 and 4.01 depending on the pH of the sample.

3.4.4.3. Specific gravity:

The specific gravity of emulsion (ASTM D 1475) was also reported. The specific gravity of

emulsion was measured using pycnometer. The value was reported by comparing with D. I.

Water density.

3.4.4.4. Viscosity:

The viscosity of the emulsion was recorded using (ASTM D2196) Brookfield Viscometer using

spindle no.3 at 30 ºC. The viscosity of the emulsion was calculated by multiplication of dial

reading with factor for that RPM of spindle. The viscosity of the emulsion was reported in

centipoises (cps).

3.4.4.5. Hydrophobicity of Polymer Films

The Hydroscopicity of copolymer’s film was tested according to ASTMD 570-8. The weighted

Films were dipped in distilled water at 25 °C for 48 hr. Then, the free water on surface of films

was cleared by filter paper, and the film was weighted again. The water absorption ratio of films

was calculated by following equation:

CHAPTER 3


Water Absorption Ratio (Wt %) = ( – )×100

Where W0 is the weight of dry film, and W1 is the weight of the film absorbing water,

respectively.

3.4.4.6. Emulsion stability

The shelf-life of waterborne emulsions is a very important characteristic, which determines their

safe storage period. Shelf life of the emulsion was analyzed by (ASTM D 869) storage of

emulsion under room temperature (30 ºC) for six months and Accelerated storage stability test

was performed by (ASTM D 3707) evaluating the stability of the sample after storage at 50 ºC in

oven for period of 7 days. Any kind of phase separation or coagulation in emulsion was noticed.

Mechanical storage stability was evaluated by centrifuging the sample at ambient temperature at

6000 rpm speed for a 1hr period of time.

Freeze thaw stability was measured according to ASTM D 2243-95 by subjecting samples to

cycles of the frozen environment followed by ambient one. Capped vials containing samples

were put in the freezer at -17 ºC for 12 hr and then put on the shelf for the same time. This cycle

was repeated until coagulation or separation occurred.

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.

3.4.4.7. Particle size distribution

Particle size and distribution is correlated with number of emulsion properties and coating

performance. Particle size of hybrid emulsion was measured using 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.

CHAPTER 3


3.4.4.8. Molecular Weight

Gel Permeable Chromatography was used for determination of molecular weight, molecular

weight distribution, and the graft ratio of the hybrid polymer. The hybrid epoxy-acrylate

emulsion was de-emulsified with methanol, washed 5 times with deionized water, followed by

drying at 50°C under a vacuum oven for 24 hr to obtain the hybrid copolymers. The molecular

weight was obtained by running 0.8 wt % polymer in tetrahydrofuran (THF) through a Waters

Liquid Chromatograph at 30°C. This system consisted of a Waters 150 pump, refractive index

(RI) detector, and two Ultrastyragel columns. THF was used as the eluent phase. The elution

volumes were converted to apparent molecular weights using narrow distribution polystyrene

standards.

3.4.5. Physicochemical properties of Coatings

Analysis of the prepared hybrid emulsion coatings was performed by application of emulsion on

the mild steel panels with dimensions 600 × 200 ×0.5 mm size for chemical resistance and of

1000 × 700 × 0.5 mm size for physicomechanical properties. The coating was cured at 30 ºC and

analyzed for touch to dry and hard to dry time by ASTM D1640-03. Dry film thickness of the

film was measured using a dry film thickness meter.

3.4.5.1. Gloss

The gloss is a measure of ability of the coated surface to reflect light at a particular angle without

scattering. The gloss was determined according to ASTM D523-67. Gloss of the cured sample

was measured 60° of reflectance using a digital mini gloss meter calibrated against internal

standard i.e. refractive index (Komal Scientific Co. Mumbai, India) and the results are reported

in terms of gloss unit (GU).

3.4.5.2. Cross-hatch adhesion

This test carried out as per ASTM D3359-83. Cross cut adhesion tape test was used to assess the

adhesion of coating films to metallic substrates. Cuts were made on the coating in one steady

motion with sufficient pressure on the cutting tool having a cutting edge angle between 15° and

30°. After making two such cuts at 90°, the grid area was brushed and a 2.5 cm wide semi-

CHAPTER 3


transparent pressure-sensitive tape was placed over the grid. After 30 seconds of application, the

tape was removed rapidly and the grid inspected according to the ASTM standards. The amount

of coated area retained under the tape corresponds to the adhesion efficiency of the coating.

More the coated material removed by the tape, the poorer the adhesion of the coating to the

substrate.

3.4.5.3. Pencil hardness

Pencil hardness property of coating was determined as per ASTM D 3363 using pencil hardness

tester with a calibrated set of drawing leads ranging from 2B (the softest) to 6H (the hardest).

The process was started with softest pencil to end with hardest pencil. The scratch resistance is

equal to the hardness value of the pencil, which makes a scratch and a gauge hardness value of

the films is equal to the hardness value of the pencil that removes the film.

3.4.5.4. Impact Resistance

A tubular impact resistance test was conducted as per ASTM D2794 to predict the ability of the

coating to resist cracking caused by rapid deformation. It is reported as number of inch-pound

(height-load). A tubular impact resistance test was carried out using an indenter with

hemispherical heads of diameter 0.625 inch and 2lb load.

3.4.5.5. Flexibility

The Mandrel Conical Bend Test of the coatings was done as per ASTM D522. The Mandrel

Bend Test is a measure of flexibility of the coating. As applied coat was dried enough the

prepared panels were kept between the mandrel and draw bar. The lever bar was drawn down at

uniform velocity to bend the specimen approximately 135°. The bent surface was examined for

crack or other surface defects.

3.4.5.6. Pendulum Hardness

The Pendulum hardness was measured according to ASTM D 4368 using SP0500 pendulum

hardness tester with Persoz Pendulum. The number of cycles is reported as hardness of the

coating.

CHAPTER 3


3.4.5.7. Chemical resistance

Acid and alkali resistance of the cured coatings was carried out by immersion test as per ASTM

D 2248-01a. 5% HCl and 5% NaOH solutions were used for immersion of the coated panels.

The panels were removed for examination after every 6 hours and observed for loss of adhesion,

blistering and any other deterioration of the film.

3.4.5.8. Solvent resistance

Coated panels were tested for solvent resistance according to ASTM D 870-02. Distilled water

and salt water (5% NaCl) were used as solvents for testing. The immersed panels were

maintained at constant temperature. The panels were removed for examination after 6, 12, 18 and

24 hours from the start of the test and observed loss of adhesion, blistering, or any other

deterioration of the film.

3.4.5.9. Corrosion resistance

Salt spray test (ASTM D 117-94) was performed to study the corrosion resistance of hybrid

coatings. 5% NaCl salt water solution was used as corrosive environment; test was performed in

a period of 500 hours. Coated panels were examined periodically after 100 hours for blistering or

sign of corrosion.

3.4.5.10. QUV resistance

QUV accelerated weathering tester, Q-panel lab products, Premier color scan instruments Pvt

Ltd, Mumbai was used for weather ability testing.. The panels were kept in the weathering tester

to check the weathering resistance. This test is conducted according to ASTM G 154. The

samples were tested for the optical properties to observe the change in gloss and color.

CHAPTER 3


3.5. RESULTS AND DISCUSSION

3.5.1. Analysis of hybrid emulsion:

The hybrid epoxy resin emulsions were synthesized and optimized with increasing ratio of epoxy

to acrylate monomer. The stable hybrid emulsions were obtained with shelf life more than six

months. Hybrid emulsion containing 60% of epoxy resin shows superior properties compared to

all other formulations. However, hybrid emulsion with epoxy more than 70% shows detoriatation

in properties. The properties of hybrid emulsion with increasing resin content were reported in

Table 3.8.

Table 3.8: Physicochemical and Physicomechanical Characteristics of hybrid emulsions

Formulation H1 H2 H3 H4 H5 H6 H7

Specific gravity 1.021 1.024 1.022 1.024 1.022 1.024 1.023

Viscosity cps 35 40 45 40 45 45 40

pH

Before neutralization 5.5 5.0 5.0 5.0 5.0 5.5 5.0

After neutralization 9.0 9.5 9.0 10 9.5 10 10

Solid content @ 110ºC (%) 44.5 39.5 41.6 39.4 40.4 39.8 35.2

Electrolytic stability a 60 32 25 41 46 47 12

Freeze-thaw cycles 4 5 4 3 4 3 1

Shelf stability (months) >6 >6 >6 >6 >6 >6 <48 hr

Accelerated Stability (Days) >7 >7 >7 >7 >7 >7 <1

Mechanical stability Pass Pass Pass Pass Pass Pass Fail

Drying Timeb 5 5 7 10 3 5 5

CHAPTER 3


Wet film thickness (µm) 50 55 50 50 50 50 55

Dry film thickness (µm) 46 40 42 32 34 35 30

Gloss 600 85.5 82.4 83.3 81.0 80.9 82.5 65.2

Pendulum hardness (Persoz) 90.1 91.3 95.5 97.9 99.3 102.3 -

Impact (150 lb/in.) Pass Pass Pass Pass Pass Pass N.A

Flexibility Pass Pass Pass Pass Pass Pass N.A

Adhesion (%) 20 25 50 70 100 100 0

Salt spray rating 0 2 3. 4 5 5 5

a: 5 % alum soln ml/100 gm of emulsion, b: touch to dry (min), 5: unaffected, 4: change in color and loss in gloss, 3: blistering of the film, 2:

softening of the film, 1: partial removal of film, 0: complete corrosion of the panel

3.5.2. Spectral analysis

3.5.2.1. Pure epoxy (FTIR, cm-1)

The peak at 3499.30 cm-1 is due to O-H stretching, C-O-C asymmetric stretching of aryl alkyl

ether appears at 1295.96, 1133.05 cm-1 and 1085.32 cm-1. Methyl group (-CH3 stretch) attached

to quaternary carbon shows peak at 2966.21 cm-1 and 1360.87 cm-1. Aromatic ring present in

epoxy resin backbone shows characteristic peaks in region of 3054.45 cm-1, 1501.69 cm-1and

773.32 cm-1 . The peaks at 2926.69 cm-1and 2873.66 cm-1 are due to -CH2 symmetrical and

asymmetrical stretch respectively. Oxirane ring present in resin shows characteristic peaks in

region 971.39 – 915.38 cm-1. This analysis confirms the structure of DGEBA epoxy resin.

CHAPTER 3


.

Figure 3.1: FTIR spectra of DGEBA epoxy resin

3.5.2.2. Pure epoxy (1H NMR, ppm)

The signal at1.6s is attributed to (CH3)2-C hydrogen atom of epoxy resin. Similarly signals at

2.8δ and 2.3ρ are due to the presence of CH2 (O) CH and CH2 (O) CH respectively. The CH-

CH2-O-group hydrogen shows signals at 4.19δ. The signals at 4.18δ, 4.23ρ, and 2.72s indicate

presence of -O-CH2-CH-, CH-OH, and CH-OH groups respectively.

O CH2 CH

HO

CH2 OCH2 CH CH2 O

O

C

CH3

CH3

CH2O CH2CH

O

C

CH3

CH3n

a

a

b c d e f

g

e

a

a

d c b

Figure 3.2: Structure of DGEBA Epoxy resin

4000.0 3000 2000 1500 1000 550.015.0

16

18

20

22

24

26

28

30

32

34

36

37.3

cm-1

%T

3899.88

3499.30

3054.45

2966.212926.69

2873.66

2758.35

2489.45

2341.702063.89

1888.07

1747.88

1605.73

1501.69

1360.87

1295.96

1246.58

1184.38

1133.051085.32

1035.34

971.39

915.38

830.64

773.32

667.27575.81555.90

CHAPTER 3


Figure 3.3: 1H NMR Spectra of DGEBA Epoxy Resin

3.5.2.3. Pure epoxy (13C NMR, ppm)

The 13C NMR spectra for DGEBA epoxy resin contains signals at 31.1ppm due to presence of –

CH3 group. The signal at 41.7ppm indicates –C (CH3)2 group,while –CH2 and –CH carbon of

oxirane shows signals at 44.5ppm and 50.2ppm respectivley. The CH–OH carbone of epoxy

resin gives signals at 63.7ppm, and CH2–O–Ar at 68.8ppm. the signals at 114.1ppm, 127.8ppm,

143.6ppm and 156.4ppm are due to aromatic ring carbons.

CHAPTER 3


Figure 3.4: 13C NMR Spectra of DGEBA Epoxy Resin

3.5.2.4. Acrylate copolymer (FTIR-ATR, cm-1)

Acrylate polymer FT-IR spectra show a broad peak at 3417.84 cm-1 which confirms the presence

of -OH group. Methyl group (-CH3 str.) and methylene (-CH2 asym. str) shows the vibrational

frequency at 2920.65 cm-1 and 2854.01 cm-1respectivly. The peak at 1614.25 cm-1 was due to the

carbonyl group (>C=O) of acrylate. Characteristic peak at 1416.74 cm-1 due to acid C-O-H in

plane bending vibration which confirms the presence of acidic group. Peak at 1120.95 cm-1 in

FT-IR represents C-O-C str vibrational frequency.

CHAPTER 3


Figure 3.5: FTIR-ATR Spectra of H1acrylate

3.5.2.5. Hybrid Epoxy-Acrylate (FTIR-ATR, cm-1)

The peak at 3438 cm-1 was due to -OH stretching, peak at 2922 cm-1 indicates -CH2, sym str

vibration. The carbonyl group (>C=O) peak at 1729.28 cm-1 confirms presence of acrylate

polymer. The peak at 1038.79 cm-1 was due to aryl alkyl ether, sym str present in epoxy resin

(Figure 3.6). Secondary hydroxyl groups present in the epoxy backbone show medium intensity

band at 1171 cm-1. Fig: 3.6 shows presence of a characteristic broad and pronounced band at

3438 cm-1 for hydroxyl and band at 1606.46 cm-1 for carboxylic group in Ep-Ac after soxhlet

extraction which confirms the chemical reaction between epoxy and acrylate monomers. Fig.3.6

Shows the FT-IR spectra of the hybrid H6 film after neutralizing emulsion with EDA at room

temperature. The comparison of the two spectra shows that, the intensity of the peak at 917cm-1

due to the oxirane ring of the epoxy resin was decreased after addition of EDA. The lowering of

800100012001400160018002000240028003200360040001/cm

93

94

95

96

97

98

99

100

%T

EXP - 21- 2

CHAPTER 3


the oxirane ring peak intensity was expected due to the ring opening and crosslinking between

oxirane of epoxy resin and –NH2 group of the EDA. The absence of two sharp N–H stretching

bands near 3335 cm-1 and broadening of 2922 cm-1peak intensity due to tertiary amine N-CH2

stretching shows that the four hydrogen atoms present in the EDA has reacted with four oxirane

ring to give tertiary amine in crosslinked structure.

Figure 3.6: FTIR Spectra of H6 Before and after curing

4 00 0.0 3 00 0 2000 1500 1000 400.0cm-1

%T 3408.312919.93

2852.25

2543.552352.36

2337.182071.38

1887.53

1729.211611.28

1453.97

838.29

760.44

738.00

3438.98

2922.14

2596.19

2352.032328.02

2062.70

1729.891606.46

1455.07 1171.471038.79

917.15837.38

758.63

CHAPTER 3


Figure 3.7: FTIR-ATR spectra H6 Air Facing

Figure 3.8: FTIR-ATR spectra H6 Metal Facing

800100012001400160018002000240028003200360040001/cm

84

85.5

87

88.5

90

91.5

93

94.5

%T

EXP - 39 - 2

800100012001400160018002000240028003200360040001/cm

70

72.5

75

77.5

80

82.5

85

87.5

90

%T

EXP - 39

CHAPTER 3


3.5.2.6. Hybrid Epoxy-Acrylate (13C NMR, ppm)

Structural study of hybrid Ep-Ac polymer was performed with 13C NMR analysis (Figure 3.9).

The signal at 14.18 ppm and 19.71 ppm represents α-CH3 of 2-hydroxyl ethylmethacrylate and

α-CH2 of methyl methacrylate carbon respectivley. The -CH2- carbon of butyl acrylate appares at

chemical shift 39.68 ppm. The quaternary carbon of methyl methacrylate shows signal at 45.00

ppm. The methyl methacrylate used in recipe give signal at 52.1 ppm for -OCH3 group. The

carbon (-CH2OH) of HEMA shows signal at 64.9 ppm. The carbonyl carbon of acrylates gives

signal at 176.7 ppm, the presence of signals for acrylate confirms their presence in hybrid

polymer. Signal for -CH3 group of epoxy resin appears at 31.29 while for –C (CH3)2 at 45 ppm.

The quaternary C–OH of epoxy resin after grafting shows peak at 71.15 ppm. Signals at 114.2

ppm, 127.7 ppm, 144.6 ppm and 157.5 ppm are due to aromatic ring carbons of epoxy resin.

Figure 3.9: Solid state13C NMR hybrid Epoxy-Acrylate

CHAPTER 3


C

C

CH3

O

O

CH3

H2C

n

52.13

176.7

19.77

44.5 44.00

Poly (MethylMethacrylate)

H2C

HC

C

OH

O

n41.4 35.6

176.7

Poly(Acrylic Acid)

**

C

HC

H2C

O

O

CH2

CH2

CH2

H2C

OH

**39.68 34.9

176.7

6.46

30.6

19.1

13.6

n

Poly(ButylAcrylate)

C

C

O

O

CH3

CH2

H2C

OH

H2C **

n43 54

176.7

64.9

64.9

16

Poly (2-HydroxylethylMethacrylate)

C

CH3

O

O

CH3

H2C*

C

HC

H2C

O

O

CH2

CH2

CH2

CH2

OH

*

177.942.8

22.7

52.13

32.1

173.1

33.750.3

6.46

30.6

19.1

13.6

n

Poly (MMA-co-BA)

Figure 3.10: 13C NMR shifts for homopolymer and copolymers

CHAPTER 3


The presence of homopolymers in hybrid emulsion was analyzed with 13C NMR spectroscopy,

chemical shift value for homopolymers were represented in the Figure 3.10. The signals at 45.00

ppm and 39.68 ppm indicates presence of poly(Methyl methacrylate) and poly (Butyl acrylate)

respectively in hybrid polymer.

3.5.3. Structure and Properties of Hybrid Epoxy-Acrylate emulsion

The hybrid latex with different levels of epoxy resin were cast on clean glass and dried to

prepare the films for FTIR-ATR analyses. Figure 3.7 demonstrates the representative FTIR

spectra for the air-facing side and Figure 3.8 metal-facing side of the hybrid H6 containing 60

wt % epoxy resin. Both spectra display similar absorbing bands at the same wavelength,

suggesting that they are analogous in structure. If the peak at 1508 cm-1 for stretching of

paraphenyl could be indicated as the concentration for the epoxy resin and the peak at 1729 cm-1

for the absorption of carbonyl used as an index for the concentration of the acrylic-copolymer,

then the relative concentration of the epoxy distributed in the film can be judged by the

absorbance area ratio of the peak at 1508 cm-1 to the peak at 1729 cm-1, as shown in Figures. The

absorbance area of 1508 cm-1 peak at the metal facing side is higher than those at the air-facing

side, suggesting that the epoxy resin part in the emulsion tends to move to the metal facing side.

The driving force for this movement could be attributed to the difference in the surface free

energy between the epoxy resin and the acrylic copolymer. The critical surface tensions of the

poly butyl acrylate, polyacrylic acid, poly methyl methacrylate and poly 2-hydoxyl ethyl

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.

CHAPTER 3


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

CHAPTER 3


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


CHAPTER 3



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.

CHAPTER 3


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

CHAPTER 3


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

CHAPTER 3




CHAPTER 3



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

CHAPTER 3


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

CHAPTER 3


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

CHAPTER 3




CHAPTER 3




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

CHAPTER 3


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

CHAPTER 3


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

CHAPTER 3


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

CHAPTER 3


H4 H5 H6

Figure 3.29.: Metal specimen coated with hybrid coatings

H1 H2 H3

CHAPTER 3


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.

CHAPTER 3


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

CHAPTER 3


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

polymerization.

3.5.16.1. Methyl methacrylate/Butyl acrylate/Epoxy resin

The MMA/epoxy and BA/epoxy systems discussed behaved quite differently in terms of

grafting. It is only natural then to study the behavior of an acrylic copolymer of the two in the

presence of epoxy. For simplicity, the ratio of MMA to BA in the monomeric portion of the

recipe was kept at 50:50 (wt:wt). Total monomer concentration (by weight) was also kept 50:50

(wt:wt) with the concentration of epoxy in the recipe. As one might expect, grafting resulting

from this copolymer/epoxy system were found to be more similar to those of the MMA/epoxy

systems than to the BA/ epoxy ones (Figure 3.33 & 3.34). Referring to Table 3.16, the

difference between the degree of grafting of MMA/epoxy and MMA/BA/epoxy systems is much

less pronounced than that of BA/epoxy and MMA/BA/epoxy. This is not unexpected.

Considering only the free radical copolymerization of MMA and BA, one would expect to find

an alternating copolymer since the reactivity ratios of MMA and BA have been reported to be

1.74 and 0.20, respectively. Another study [338] reported 1.8 ± 0.1 and 0.37 ±0.1; in either case,

the multiplication of r2 by r2 results in a value much less than unity (0.35 or 0.67), indicating

alternating copolymer. Thus, after every other monomer unit adds to the growing polymer chain,

the radical center will be on a BA unit. With an acrylate radical at the head of the chain, attack of

the epoxy can easily occur at tertiary carbon. Thus, the MMA/BA/epoxy copolymer system

exhibits a degree of grafting more similar to the MMA/ epoxy system than to the BA/epoxy

system. The absence of grafting in the BA / epoxy system can be attributed to the hydrophobic

nature of the BA which will rule out free radical attack of water soluble persulfate radical.

Insignificant grafting in the MMA / epoxy system was attributed to the tendency for MMA to be

involved in homogeneous nucleation. The effective concentration of MMA in the organic phase

is less in this system, due to the hydrophilic nature of MMA reducing the propensity of MMA to

CHAPTER 3


partition into the organic phase. Nonetheless, there is a possibility of some portion of

homogeneous nucleation to occur; leading to PMMA homopolymer. The MMA/epoxy system

generates a hybrid polymer of intermediate flexibility due to the dramatically divergent Tgs of

the individual components (105°C and -30°C). The BA/epoxy hybrid Tg does not differ much

from those of the individual comprising compounds since both pure component transitions are

similar and very low (-30°C and -54°C). The hybrid polymer of MMA/BA/epoxy results in an

intermediate Tg, of approximately -4°C.

3.5.16.2. Acrylic acid/Methyl methacrylate/Butyl acrylate/2-Hydroxyl ethyl methacrylate

/Epoxy resin

In contrast to MMA/epoxy, and BA/epoxy systems AA/MMA/BA/HEMA/epoxy grafts

significantly to epoxy when initiated by KPS. Extraction data in Table 3.16 show that, the

degree of grafting is nearly 18.05%. Those two initiators were chosen due to their different

tendencies in relation to primary radical attack. When the structure of butyl acrylate is compared

to that of methyl methacrylate, it can be seen that the butyl acrylate involves far fewer steric

hindrances (Figure 3.38). Other than offering the ability to add to a double bond, it also

increases reactivity toward hydrogen abstraction. This is seen in its larger chain transfer

coefficient compared to that of MMA. (At 70°C, ktr, BA = 4.04 L mol-1 s-1 and ktr, MMA = 0.11

L mol-1 s-1) At first glance, one might try to correlate this higher transfer coefficient to give the

higher degrees of grafting in BA versus MMA. Such a correlation is not correct, however, when

propagation is considered. The propagation rate coefficient for BA is at least an order of

magnitude higher than that of MMA.

On this basis, that value is quite similar for both systems. (At 70°C, ktr/kp, MMA = 6.95 E-5 and

ktr/kp, BA = 9.37 E-5.) In light of grafting in the AA/MMA/BA/epoxy hybrid system, and a

fairly low ktr/kp value, this strongly suggests the mode of attack for AA/MMA/ BA/epoxy is

because of hydrogen abstraction grafting accompanied by the formation of homopolymer. The

individual glass transition temperatures for BA and epoxy are so close to each other (-54°C, -

30°C) that isolating a transition specific to the hybrid graft copolymer is quite difficult. One

should also consider that butyl acrylate has very little solubility in water (0.0062 mol/L) [338].

This translates to a relative lack of homogeneous nucleation and thus less chance of

homopolymer resulting from homogeneous nucleation. Although polymerization of BA often

CHAPTER 3


leads to some degree of self-grafting and crosslinking through backbiting reactions, the hybrid

poly (epoxy-g-BA) polymer was fully soluble in THF. This suggests that, a negligible amount of

PBA crosslinking occurs in this type of system.

Figure 3.31: 13C NMR for MMA/BA/HEMA/AA/Epoxy system.

Figure 3.32: 13C NMR for MMA/BA/AA/Epoxy system.

CHAPTER 3


Figure 3.33: 13C NMR for MMA/EP system

Figure 3.34: 13C NMR for BA/ Epoxy system

CHAPTER 3


Figure 3.35: 13C NMR for MMA/BA/AA/HEMA/EP system

3.5.16.3.Methyl methacrylate /epoxy resin

In Table 3.16, the degree of grafting for MMA/epoxy initiated with KPS is given at monomer

conversions of 39%. This hybrid system leads to both homopolymer of PMMA and poly (epoxy-

g-MMA). MMA has some tendency to partition into the aqueous phase [339]. Although the

solubility of MMA in water is only around 1.5 wt % [338], it is enough to promote some

homogeneous nucleation, especially since the initiator radicals (KPS) are derived in the

continuous phase. Once some degree of homopolymer is formed in aqueous phase, the tendency

for more monomer to transport to those sites is also increased, whether it from hybrid particles or

from unnucleated droplets that will essentially act as reservoirs. Second, the methyl group of the

MMA vinyl bond renders it sterically more difficult for the radical center of MMA to be

involved in grafting reactions. The radical is on a tertiary carbon that aids radical stability since

the alkyl groups are electron-donating [340]. The carbonyl group will draw electrons, but when

compared to similar monomers, such as butyl acrylate, the effect cancels out. Butyl acrylate has a

higher radical reactivity [341]. Primarily from its lack of steric hindrances surrounding the

radical center. For these reasons, MMA (or PMMA) is more likely to abstract an hydrogen on the

CHAPTER 3


epoxy backbone forming short chain homopolymer that may or may not graft through

termination by combination with that generated site. Low levels of grafting have been

documented in several MMA/resin systems and have often been explained by the propensity for

a methacrylate radical to favor hydrogen abstraction. Huang and Sundberg [340] report that, in

systems where grafting can occur through either abstraction or addition, the latter mechanism is

responsible for the majority of grafting. This is explained by the relative inactivity of radicals

created by hydrogen abstraction. This inactivity is then conjectured to be a major influence on

the retardation of the overall polymerization rate. Mayo et al. [338], have documented rate

coefficients for several monomers dictating their reactivity. AIBN was chosen for its relative

inability to attack the epoxy backbone as a primary radical. Bevington [342] reported that, the

inability of AIBN to abstract an methylenic hydrogen is likely to do with the resonance stability

of the (CH3)2C˙(CN) radical formed upon AIBN decomposition. The reason Huang and

Sundberg [340] wanted an initiator that would not promote primary radical attack was to isolate

and study a system dominated by macroradical attack (chain transfer to polymer). Although these

authors were studying grafting by macroradical attack in a solution polymerization environment,

the study of a macroemulsion initiated by KPS actually is quite similar. In the case of KPS, the

initiator first dissociates in the continuous phase only to enter a organic phase after it has gained

a certain number of monomeric units (the z-mer value specific to monomer) [338]. For this

reason, the primary radical attack is a moot point, making it necessary for grafting to occur from

some form of macroradical whether it be oligomeric or polymeric.

It was postulated that, the use of an oil-phase initiator such as AIBN in place of KPS would

increase levels of grafting due to its oil solubility and thus the possibility for primary radical

attack, as well as the suppression of homogeneous nucleation. However, Table 3.16 indicates

that, the use of AIBN over KPS did not impart noticeable changes in grafting. This can be

explained from two perspectives. First, as discussed earlier, AIBN has been reported not to

participate in the primary radical attack [343] on this basis alone, the similar results of the KPS

and AIBN initiated systems would then be expected. Second, there is some evidence that the

AIBN radicals involved in initiation of macroemulsion systems are actually those generated in

the aqueous phase. AIBN partitioned in the droplets dissociates and can immediately recombine,

effectively creating very few live radicals from within the oil phase. Radicals derived from the

AIBN partitioned in the aqueous phase can add several monomer units and enter a droplet, If

CHAPTER 3


this, scenario is true, then there is no reason to suspect that the grafting results of the KPS or

AIBN systems should be any different. In a macroemulsion, neither would have a chance at

primary radical attack, even if they were prone to such an event.

A series of hybrid MMA/epoxy polymerizations were performed to test this hypothesis. Figure

3.36 shows the conversion time curves for the hybrid system. The decrease in the MMA

polymerization rate from the KPS to the AIBN initiated system can be explained by the

reduction in AIBN initiator concentration due to a large percentage of oil-phase initiator being

consumed in radical–radical termination reactions within the droplets. The effective initiator

concentration used in propagation would then be, closer to the percentage of AIBN partitioned in

the aqueous phase. This is also compounded by the longer half-life of AIBN when compared to

KPS, which translates to a slower decomposition rate. The third kinetic profile is of an AIBN-

initiated MMA/epoxy macroemulsion polymerization with sodium nitrite (NaNO2) present in the

continuous phase. Sodium nitrite is known to scavenge aqueous-phase radicals. If the conversion

time curves were identical with and without aqueous-phase inhibitor, the theory of AIBN

radicals entering from the aqueous phase would be unsupported.

Table 3.16: Degree of grafting for different systems extracted with diethyl ether

System Monomer

conversion

Degree of

grafting

Conditions

MMA/Epoxy 39.2% 11.57% Initiated with KPS

MMA/Epoxy 19.1% 1.29% Initiated with AIBN

MMA/Epoxy 9.4% 0.08% Initiated with AIBN NaNO2

BA/Epoxy 11.3% 0.15% Initiated with KPS

BA/ MMA/Epoxy 79% 17.82% Initiated with KPS

BA/ MMA/AA/Epoxy 65.8% 18.01% Initiated with KPS

BA/ MMA/AA/HEMA/Epoxy 94% 18.05% Initiated with KPS

Figure 3.36, however, shows a dramatic reduction in rate and conversion indicating in the least

that a significant percentage of initiating radicals in fact do come from the aqueous phase. The

CHAPTER 3


fact that some polymerization still occurs indicates that some initiation does take place in the

particles/droplets. It should be noted that, there will also be some partition of NaNO2 in the oil

phase, but this contribution to termination is assumed to be negligible. So, for the MMA/epoxy

system, initiation by KPS or AIBN does not introduce markedly different results in a degree of

grafting. Grafting occurs, but the final polymer is still a distributed mixture of hybrid graft

copolymer, homo-PMMA, and unused epoxy (Table 3.16).

Figure 3.36: Comparison of monomer conversion for hybrid system

3.5.17. PROPOSED GENERALIZED GRAFTING MECHANISM

In order to understand the pathway of reaction the following mechanism is proposed for hybrid

macroemulsion polymerization. None of these reactions are new to the literature; however, we

list them all here for the sake of completeness, and in order to make some comments on their

relative importance based on our investigations.

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300

BA/MMA/AA/HEMA/ER

MMA/ER/KPS

MMA/ER/AIBN

MMA/ER/AIBN+NaNO2

CHAPTER 3


Aqueous-phase reactions:

I 2 R*

R* + M M1*

M1* + M2 Me*

[1]

[2]

[3]

fkd

Where, e is the z-mer value for the particular monomer (e.g., ZBA = 2-3, ZMMA = 5+) [344].

Mn* + Mm* Mn+m [4]

Organic phase

Me* + M Mn* [5]

Mn* + M M*n+1 [6]

Mn* + GC-H G* + Mn*H [7]

G* + M GM*

Ktr,p

Ka1 [8]

GMn* + Mm GMn+m*Ka2 [9]

G* + Mn* GMnKt,p

[10]

GMn* + Mm * GMn+m

GMn* + Mm*

[11]

GMnH + Mm [12]

Mn* + Mm* Mn+m [13]

In the above equations,

I = Initiator,

R*= Initiator derived radical (primary radical),

CHAPTER 3


M = monomer (MMA, BA, AA, HEMA), and

G= grafting point on epoxy resin.

The above mechanism does not account for oil phase derived initiator radicals, nor does it

address the primary radical attack. These two possible pathways were omitted from the

mechanism in light of their negligible influence on systems of this study. In the mechanism, eq.

(1) represents dissociation of free radical initiator potassium per sulfate in aqueous phase. Eqs.

(2) & (3) illustrate that radical fist attack more hydrophilic monomers which undergoes

homogeneous nucleation and acquired a certain oligomeric chain length before reacting with

hydrophobic monomers. For more hydrophilic monomers such as AA and HEMA, this required

chain length is longer than for highly water insoluble monomers such as BA [338]. These rules

out both primary radical attack and monomeric radical attack on the epoxy resin. Also the epoxy

resin present in organic phase has higher surface tension value so that it will be surrounded by

acrylate monomers with lower surface tension values, this fact also inhibit the primary radical

attack. Equations (4) and (5) describe termination in the aqueous phase. These are important

because they can lead to homo and copolymer in homogeneously nucleated particles. Copolymer

particles derived in the aqueous phase are observed in the AA/MMA/epoxy system. Mn+m, from

eq. (4), become dead copolymer. This of course, is relevant to eqs. (13). Equations (7) and (8)

refer to oligoradical or macroradical attack on the epoxy resin by hydrogen abstraction and direct

addition of monomer units, respectively. As described earlier, eq. (5) is primarily relevant for

MMA and eq. (6) is heavily favored over eq. (5) for BA. The resultant G* radical on the epoxy

after is quite stable from conjugation with the adjacent electron withdrawing hydroxyl group and

thus the rate coefficient for eq. (8), ka1, is projected to be quite low. This inactivity causes the

reduced rate of polymerization .In eq. (9), the resultant GMn+m radical is less stable and more

reactive (ka2 >> ka1). Since this equation is really only relevant to hydrophobic acrylate

monomers, such as BA, which explain the grafting in the BA/MMA/epoxy system. AA, MMA

and HEMA rely mainly on the combination of eqs. (7) And (10) to graft with epoxy, whereas BA

can create grafts through the same mechanism or through any combination of Eqs. (8), (9), and

(10).

Considering the high percentage of homopolymer created in the AA/epoxy and HEMA/epoxy

system eqs. (13) Should be considered. Earlier, the production of homopolymer was attributed to

CHAPTER 3


homogeneous nucleation in the aqueous phase, derived from the high water solubility of these

monomers. Equations (4) describe copolymer derived from the aqueous phase. Homo

termination by eqs. (13) is also a prospect for production of copolymer within organic phase.

Homopolymer derived from the transferred hydrogen in eq. (7) is likely the dominant mechanism

for the creation of that species. For the BA/MMA/epoxy system, grafting has been shown to be

highly probable from the combination of abstraction and termination events. Both transfer and

termination are then somewhat unusual in that they involve two relatively large species. There is

a possibility of an oligomer/polymer (short-polyacrylic/ epoxy) interaction, but from the rates of

propagation versus transfer, the interaction is more likely one of polymer–polymer

(polyacrylic/epoxy). If, after moderate conversion in this hybrid BA/MMA/epoxy system, the

radical-containing end groups of the epoxy and copolymer of acrylate are in close proximity,

propagation will eventually lead to termination creating a graft.

The selectivity of the grafting reaction can be study using simple rate theory. This states that, the

rate of a given reaction is governed by the Arrhenius equation, k =A exp (Ea/RT). For the

reactions under consideration, the pre-exponential factor A would be expected to be nearly

constant. Hence the relative rates of reaction would be controlled by the energy of activation, Ea.

This is primarily a function of the relative strengths of the bonds being broken and the influence

of polarity in the transition state. The reaction between the electronegative free radical and epoxy

resin assume to be exothermic as per the calculated partial charges on the hydrogen atom of

epoxy and free radical.

Table 3.17: Partial charges on carbon of epoxy resin

Epoxy resin carbon Partial charges

ΦOCH2-CH(OH)-CH2Φ + 0.10

ΦOCH2-CH(OH)-CH2Φ -0.12

For a reaction series where the abstracting species are constant, the bond dissociation energies

are a relative measure of radical stability [345]. The stability can be rationalized by considering

the partial atomic charges for the carbon atoms. The secondary carbons of epoxy backbone have

a slight partial negative charge due to the electron-donating character of the adjacent phenyl

ether group. The tertiary carbon, however, has a slight partial positive charge due to the electron-

CHAPTER 3


withdrawing effect of the bonded hydroxyl group. The free radical is calculated to have a partial

charge of -0.18. Formation of the transition state is influenced by the polarity factor; radical

formation is slightly favored on the electropositive carbon. Also the tertiary radical formed has a

higher dipole moment than a secondary radical. This radical would be stabilized in polar solvent

water. There is empirical relationships between the ionization potential and enthalpy of the

transition state for hydrogen-abstraction reactions [346]. It has been suggested that, the

ionization potential of the radical is a measure of the Swain-Lupton resonance parameter [347]

which corresponds to the stabilization of the energy of the singly occupied orbital due to

resonance. According to these calculations, this resonance favors the formation of the tertiary

centered radical. N. L. Allinger et al conducted molecular mechanics calculations [348] which

indicate that formation of a radical centered on the secondary carbon atoms releases more steric

compression energy than the formation of a tertiary radical [349].

However, this is not expected to influence the reaction rate for epoxy grafting reactions, due to

the early transition state for such reactions [339]. Epoxy resin has three active site i.e. two

secondary and one tertiary carbon atom where free radical can attack and give rise to site for

grafting. Out of these three sites the attack of free radical to carbon hydrogen bond of tertiary

carbon atom will give rise to most stable radical due to directly attached electron withdrawing

hydroxyl group. 13C spectra shows a peak at 71.1 ppm for grafted tertiary carbon atom. The free

radical formed by abstraction of hydrogen from epoxy resin gives free radical center where

polymerization can propagate to give grafting of acrylate monomers on the backbone.

Reaction between Epoxy and acrylate can be represented as in Figure 3.37 and Figure 3.38.

CHAPTER 3


OHH OH

OHOH

1. macroradical attacks tertiary carbon hydrogen bond on epoxy

2. Hydrogen is Abstracted

3. New macroradical approaches 4. Propagation leads to grafting on epoxy

Figure 3.37: Grafting reaction between epoxy and acrylate

CHAPTER 3


H2CHC

O

H2C O

CH3

CH3

OH2C C

HO

CH3

CH3

OH2C

HC CH2

O

OH

H2C

n

+ H2C CH

C

OC4H9

H2C

C

OCH3

CH3 H2C

C

OC2H4OH

CH3 H2C CH

C

OH

OO O O

++ +

Butyl acrylate Methylmethacrylate 2-hydroxyl ethylmethacrylate Acrylic acid

KPS

75oC

H2CHC

O

H2C O

CH3

CH3

O CH

C O

CH3

CH3

OH2C

HC CH2

O

OH

CH

nCH2

CH C OC4H9

H2C

CH3CO CH3

H2C

C OC2H4OHH3C

H2C

CH2CHO

O

O

O

O

Diglycidyl Ethers Bisphenol A Epoxy Resin

Figure 3.38: Grafting reaction between epoxy resin and acrylate monomers

Also due to presence of hydroxyl group on the backbone of epoxy there will be the effect of

branching on graft site selectivity. Epoxy resin can undergo self condensation reaction to give

crosslinking shown in Figure 3.39. Oxirane opening by reaction with hydroxyl group will give

β-hydroxyl ether slightly reduce the partial charges on the tertiary carbon and secondary carbons,

which predicts that, a branched epoxy resin will be slightly less likely to undergo hydrogen

abstraction and form an acrylic graft copolymer.

CHAPTER 3


O

CH3

C

CH3

OH2CC

H

H2COC

CH3

CH3

H2C CH C

H2O

OH2C

OH

CH

H2C O

Figure 3.39: Self condensation of Epoxy resin

CH3

C

C O

OCH3

"MMA"

H

C

C O

O(CH2)3CH3

"BA"

C

C O

OH

"AA"

CH3

C

C O

O(CH2)2OH

"HEMA"

CH3

Figure 3.40: Comparison of Acrylate monomer structures.

CHAPTER 3


3.6. CONCLUSION Hybrid Epoxy Acrylate emulsion stable for more than six months were synthesized using a

conventional emulsion polymerization technique. FTIR-ATR study shows that the grafted epoxy

segments tend to move towards metal facing side while polyacrylate polymer moves toward air

facing side of the coating. Because of this movement epoxy resins gives excellent adhesion to

substrates while acrylic copolymers on the air-facing side give very good weatherability and

appearance to coating.

Hybrid Ep-Ac emulsion was synthesized with varying ratios (0 to 70%) of resin and analyzed for

storage, mechanical stability and found to be stable for more than six months. Alkali-water

absorption and molecular weight of the emulsion decreases with increase in epoxy resin content.

Corrosion resistance of hybrid coating was found to be increased with resin content, however the

stability of emulsion suffers after the epoxy resin content of more than 60%.

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