1

CHAPTER I

INTRODUCTION

1.1 INTRODUCTION

Epoxy, also known as

"resin" with "curing agent". Epoxy resins

Epoxy/oxirane group (I).

They have the following characteristics

1. Good thermal & electric property

2. Excellent mechanical property & cohesiveness to variety of substrates

3. Chemical & corrosion resistance

4. Good Processability & electrical conductivity

The epoxy resins has a wide variety of applications including conventional house

systems as binders for floor coatings. Typical applications are :

1. General Purpose adhesive

2. Anti-skid & Industrial Coatings

3. Non-flexible foams

4. Oil-Drilling Surface solidification

5. Potting & Encapsulation

6. Thermoplastics fibre

But the conventionally available epoxy

field of advanced materials such as flexibility, flammability resistance etc.

The oxirane ring in epoxy is capable of reacting with a variety of chemical agents. On

curing the epoxy rsins become rigid the

2

1.1 INTRODUCTION

, also known as poly-epoxide, is a polymer formed from reaction of an

"resin" with "curing agent". Epoxy resins consist of a 3-membered ring termed as

(I).

(I)

They have the following characteristics

Good thermal & electric property

xcellent mechanical property & cohesiveness to variety of substrates

Chemical & corrosion resistance

Good Processability & electrical conductivity

The epoxy resins has a wide variety of applications including conventional house

floor coatings. Typical applications are :

General Purpose adhesive

skid & Industrial Coatings

flexible foams

Drilling Surface solidification

Potting & Encapsulation

Thermoplastics fibre-reinforced

But the conventionally available epoxy resins don’t meet the expectations requisite in the

field of advanced materials such as flexibility, flammability resistance etc.

The oxirane ring in epoxy is capable of reacting with a variety of chemical agents. On

curing the epoxy rsins become rigid thermosets. Variety of chemical compounds such as

formed from reaction of an epoxide

membered ring termed as

xcellent mechanical property & cohesiveness to variety of substrates

The epoxy resins has a wide variety of applications including conventional house-hold

resins don’t meet the expectations requisite in the

field of advanced materials such as flexibility, flammability resistance etc.

The oxirane ring in epoxy is capable of reacting with a variety of chemical agents. On

rmosets. Variety of chemical compounds such as

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diluents, coated fillers, ground fillers, curing agent accelerator are added to improve the

resin system properties. The main factors which decide the cure process are

1. Type of Epoxy resin

2. Curing Agent Chemistry & Curing condition

When the chemical structure of the cured epoxy system contains heterocyclic or aromatic

rings or both, its thermal resistivity is higher than a system with flexible chains6. Also, if

the curing agent’s chemical composition contains carboxylic anhydride, imide11, or

hydroxyl-terminated imides12, 13 & imide-acid14-16, the thermal resistance is improved .

Although epoxy resin curing has been extensively researched by different curing agents

of variating structures, however, few reports are available on the application of imide

amines, anhydrides, & benzoxazines. Further no report is available on the use of these

hardeners in carrying out curing in DGBT, (N,N'-

diglycidylbenzophenontetracarboxydiimide), a type of epoxy resin. Therefore, it is of

interest to investigate systematically the effect of the structure of curing agents on the

curing & thermal behaviour of DGBT.

1.2 EPOXY RESINS: A QUICK SNAP

Epoxy Resins wer first synthesized by Dr. Pierre Castan of Switzerl& & Dr. S.O.

Greenlee of the United States in 1936. This technology was sold off to many major

players like Ciba, Huntsman, Momentive Specialty Chemicals etc to name a few.

Epoxy resins are essentially thermosets because after cure they don’t flow on heating

because of permanent locking of molecular chains.

Typical advantages of epoxy resin systems are

• Low cure shrinkage

• Excellent chemical & moisture resistance

• Increased Fatigue & impact resistance

• Good electrical conductivity & longer shelf stability

4

The curing of epoxy resins is carried out by addition of hardener. Most common hardeners

are amine based. Typical resin to hardener ratio is 2: 1 or 1:1.

Thus, based on the reactivity of curing agents & the curing conditions, cured epoxy resins

show versatile property, including excellent heat & chemical resistance, high strength,

good impact resistance, high hardness & electrical insulation17,18

1.3 CLASSIFICATION OF EPOXY RESINS

1.3.1 Liquid DGEBA Resins:

The liquid epoxy resins are of low molecular weight & are characterized by a repeating

(Scheme 1.1) structures having secondary hydroxyl group. The degree of polymerization,

is ranging from 0 to 0.5 & two terminal epoxy groups.

A liquid epoxy resin is produced by gradually feeding an aqueous solution of alkali metal

hydroxide into a solution of bisphenol-A in epichlorohydrin, while maintaining the

reaction medium at boiling point, distilling off water in the form of an azeotrope with

epichlorohydrin & recycling the latter, & maintaining a water content of from 0.1 to 0.7

wt.% & a pH value between 7 & 9 in said reaction medium. The liquid epoxy resins thus

obtained have low values of epoxy equivalent & chlorine content (, here n is nearly zero

(0.2)).

epichlorohydrin (large excess)

bisphenol A

HOCH2-CHCH2Cl

O

CH2CHCH2

O

O OCH2CHCH2

OHnO CH2CHCH2

O

O

NaCl

OH

NaOH

Scheme 1.1

The epichlorohydrin monomer in DGEBA synthesis behaves as bi-functional monomer,

because of its ease of alkali dehydro-halogenation.

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1.3.2 Solid DGEBA Resins

This class of epoxy resin also contains a repeating unit (Scheme 1.1) having a secondary

hydroxyl group. The degree of polymerization is ranging from 0 to 0.5 with two terminal

epoxy groups (II).

Bisphenol-A & Epichlorohydrin are added to a reactor in theoretical molar proportion

with a little excess of Epichlorohydrin. Now, during this process, aqueous caustic soda is

added such that it is well mixed into the system. On carrying the reaction for an hour, a

viscous mass is obtained. After that, Phase separation is carried out by adding an inert

solvent. The Brine solution is removed & the resin solution is then thoroughly washed

with water to remove traces of salts. The solid resin is then obtained by removing the

solvent by vacuum distillation.

O OCH2CHCH2

CH3

CH3

O OCH2CHCH2

CH3

CH3 OO n

CH2CHCH2

OH

(II)

1.3.3 Phenoxy Resins

Phenoxy resins contain no epoxy groups & are of higher molecular weights, & also true

thermoplastics. They are prepared by reacting epichlorohydrin with bisphenol A &

caustic soda in dimethyl sulfoxide. However, the presence of many free hydroxyl groups

permits cross-linking with various curing agents like anhydrides, triazines, isocyanates &

melamine. The phenoxy resins are also classified as poly (hydroxy ethers) (III):

O CH2CHCH3

OH

On

(III)

These resins are compatible with many polymers, & are efficient flexibilizers & have also

shown utility in compatibilizing blends of diverse plastic materials. Phenoxy Resin has a

solubility parameter of 10.68, implying excellent compatibility with polar plastic

materials such as Nylon, PU etc. but is incompatible with acrylics, olefins, & vinyls..

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They have excellent vapor barrier properties (water vapor, oxygen, carbon dioxide) & is

compliant with 21CFR175.300 for indirect & direct food/beverage container coatings, as

well as other regulations pertinent to adhesives in multilayer packaging & plastic

components for containers. Certain grades of Phenoxy (PKHW-series) resins made by

grafting onto the aliphatic carbon segments allow formulators significant advantage in

designing VOC compliant coatings & adhesives.

1.3.4 Halogenated Epoxy Resins

They have been developed to meet customized requirements like, Chlorinated &

brominated epoxies were used for significant flame retardency properties. The best

combination of cost & performance is obtained with brominated epoxy resins.

Halogenated Epoxy resins are listed as below:

(A) Chlorinated & Brominated Bisphenol A Based Epoxy Resins

A common method of imparting ignition resistance is the incorporation of

tetrabromobisphenol A (TBBA) or tetrachlorobisphenol A (TCBA). The diglycidyl ether

of TBBA (IV) or TCBA (V) is produced by conventional liquid epoxy resin synthesis.

The diglycidyl ether of TBBA or TCBA is used where high flame retardency is required

like for electrical or electronic encapsulation.

Br

Br

Br

Br

OO CH2CHCH2H2CHCH2C

OHHO

(IV)

Cl

Cl

Cl

Cl

OO CH2CHCH2H2CHCH2C

OHHO

7

(V)

(B) Fluorinated Epoxy Resins

Incorporation of fluorinated substituents into a polymer structure (VI) greatly improves

the electrical insulation performance of a polymer. Because of their small dipolemoment

& the low polarizability of the C–F bond, the dielectric constant of polymers is

decreased. Due to the non-polar nature of fluorocarbon groups, they will improve the

resins durability in moist environment & lower the moisture absorption. Studies have

reported that curing of DGEBA by perfluorobutenyloxyphthalic anhydride reduces the

water absorption by 75% & the dielectric constant decreased to 2.7–2.8.

CF3

CF3

CC

OO

OOOO

(VI)

1.3.5 Modified Epoxy resin

The esterification of epoxy resins with commercially available fatty acids is a very useful

process employed by industries for air-dried, protective & decorative coatings. Scheme

1.2

8

CH--CH2

OR C

O

OH CH--CH2

OH

RCO

O

CH--CH2

OH

O O R C

O

OH-H2O

CH--CH2

O

O O

C

O

R

Scheme 1.2

Various fatty acid modifiers, include linseed oil, dehydrogenated castor oil, tall in fatty

acid, etc. The chemical resistance of the epoxy esters is generally lower than unmodified

epoxy resins.

In recent years, to improve the toughness, the modification of epoxy resins by amine or

carboxy terminated liquid poly (butadiene-co-acrylonitrile) has also been investigated 19.

Triphenylphosphine or alkyl phosphonium salts are used as catalyst in the synthesis of

adducts of epoxy resins & carboxylated butadiene-acrylonitrile copolymers (CTBN).

Main types of modified epoxy resins are:

(A) Glycidyl Esters

Glycidyl Esters are synthesized by the reaction of epichlorohydrin with cyclo-aliphatic

dicarboxylic acids followed by dehydro-halogenation with brine (Scheme 1.3).

Glycidyl ester derivatives are prepared by reacting intermediate diimide acids20,

synthesized by treating benzophenone tetracarboxylic acid dianhydride (BTDA) with

amino acids, with epichlorohydrin (using quaternary ammonium halide as catalyst) (VII).

These resins were had processing characteristics comparable to epoxies with improved

thermal stability because of imide groups. They are soluble in highly polar organic

solvents

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C

O

OH

hexahydrophthalic acid

C

O

OH

CH2CHCH2Cl

O

2

C

O

O

C

O

O

CH2CHCH2Cl

OH

CH2CHCH2Cl

OH

NaOH

C

O

O

C

O

O

CH2CHCH2

O

CH2CHCH2

O

Scheme 1.3

O

CH2-CH-CH2-O-C-R'-N N-R'-C-O-CH2-CH-CH2

O

O

O

O

O

OO

O

(R'= AMINO ACID GROUPS)

(VII)

Saito et al 21-22 synthesized heat resistant aromatic imide epoxy esters (VIII) using

trimellitic acid anhydride & 3, 3’- diaminodiphenyl sulfone.

O

O

CH2-CH-CH2-O-CN

O

O

SO2 N

O

OO

C O-CH2

O

CH-CH2

(VIII)

Epoxy-terminated imide resins were synthesized by reacting methyl-trimellitimide with

aliphatic diols which on reaction with epichlorohydrin gave N, N’- diglycidyl imide

derivatives (IX).

10

O

O

CH2-CH-CH2-O-CN

O

O

R' N

O

OO

C O-CH2

O

CH-CH2

O2Swhere R'=

(IX)

Starting with BTDA & allyl amine, N, N’-diglycidyl benzophenone tetracarboxydiimide,

DGBT (X) was synthesized. The peroxidation of this diallyl intermediate gave epoxy end

capping23.

O O

CH2-CH-CH2-N N-CH2-CH-CH2

O

O O

OO

(X)

Laminates moulded from these novel resins demonstrated flexural strength retention at

260oC in the order of 70% based on short-term exposure.

(B) Cycloaliphatic Epoxy Resins & Epoxidized Oils

Cyclo-aliphatic epoxy resin synthesis based on the epoxidation of cyclo-olefins with

peracids24-30 (Scheme 1.4) viz., peracetic acids. Cycloaliphatic Epoxy Resin with the

following chemical structure are already commercially available (XI-XIII).

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3'-cyclohexenylmethyl3-cyclohexenecarboxylate

COCH2

O

CH3COOH

O

COCH2

O

OO

3',4'-epoxycyclohexenylmethyl3,4-cyclohexenecarboxylate

Scheme 1.4

OCH2O

O

C (CH2)4 OCH2

O

CO

(XI)

OO

(XII)

(XIII)

The secondary reaction viz., acid-catalysed opening of the epoxide groups, is minimized

at low temperatures & strongly depends on the chemical constituents & the reaction

media.

1.3.6 Multifunctional Epoxy Resins

O O

O

O

12

The multifunctionality of these resins provides higher cross-linking density, leading to

improved thermal & chemical resistance properties over bisphenol-A epoxies.

(A) Epoxy Novlac resin / Epoxy Cresol Novolac Resin

Two resins which have attained commercial significance in industry is epoxy phenol

novalac resins (EPN) (XIV) & epoxy cresol novalac resins (ECN) (XV) 30-36. EPN is

synthesized from the phenol-formaldehyde condensates (novalacs) obtained from acid

catalysed resinification of phenol/ or cresol & formaldehyde37,38 by the process of

glycidylation which produces r&om para & ortho-methylene bridges (Scheme 1.5).

HOR

(n + 2) (n + 1) CH2OH+

OHR

CH2

OHR

CH2

OHR

n

(n + 1) H2OCH2CHCH2Cl

O

OR

CH2

OR

CH2

OR

n

(n + 1) H2O

CH2CHCH2

O

CH2CHCH2

O

CH2CHCH2

O

(XIV) EPN, R = H(XV) ECN, R = CH3

Scheme 1.5

The functionality of resins increases if the molecular weight of novalac increases 39.

Branching can be prevented by epoxidation with an excess of epichlorohydrin thus

minimizing the reaction of phenolic OH with glycidylated phenol groups40. The high

functionality of phenol novalac resins (compared to st&ard DGEB-A based resins) results

in increased cross-link density & improved chemical & thermal resistance.

A polyfunctional resin based on phenol & glyoxal forms the basis of a speciality epoxy

resin41-45 (Scheme 1.6).

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OH

4

O O

CHHC

HO OH

HO CH OH

CH

CH2 CHCH2Cl

O

tetrakis(4-hydroxyphenyl)ethane

O O

O CH O

CH

CHCH2H2C

O

CH2 CHCH2

O

CH CH2

O

CH2

CH CH2

O

CH2

Tetraglycidyl ether of tetrakis(4-hydroxyphenyl)ethane

Scheme 1.6

(B) Aromatic Glycidyl Amine Resins

Very few multifunctional epoxy resins with an aromatic amine backbone, have

commercial significance46-49. Under carefully controlled conditions, they are synthesized

by Glycidylation of p-aminophenol or 4, 4’-diaminodiphenyl methane with a large excess

of epichlorohydrin. This is because such multifunctional resins exhibits limited thermal

stability & polymerizes vigorously under the influence of a tertiary amine50. The structure

of the resins based on p-aminophenol &/or 4, 4’-diaminodiphenyl methane is shown

below (XVI & XVII) & they are available commercially.

N

O

CH2CHCH2

O

CH2CHCH2

O

CH2CHCH2

O

(XVI)

14

CH2

N N O

O

O

O

(XVII)

1.3.7 Specialty Epoxy Resins

(A) Crystalline Epoxy Resins Development

Japanese resin producers have produced a no. of new epoxy resins used in epoxy

molding compounds (EMC) to satisfy the increased performance requirements of the

semiconductor industry51-58. Most researched is the commercialization of crystalline

epoxies based on bisphenol by Yuka-Shell59. Novel semiconductor manufacturing

processes such as Surface Mount Technology60 (SMT) based on the concept that high

filler loading reduces the coefficient of thermal expansion (CET) & helps manage

thermal shock & moisture & crack resistance of molding compounds. But the cured

thermoset compounds derived from these crystalline resins do not retain crystallinity.

Liquid crystal thermoplastics & thermosets based on this novel chemistry showed

excellent combinations of thermal, mechanical, & chemical properties, unachievable with

traditional epoxies.

(B) Weatherable Epoxy Resins

Poor weatherability is one of the major drawbacks of the aromatic epoxies. This is

because of the aromatic ether segment of the backbone. The aromatic ether of bisphenol

A absorbs UV lights up to about 310 nm, undergoes photocleavage directly producing

free radicals that lead to oxidative degradation of Bisphenol A epoxies, resulting in

chalking. Numerous researches have been carried out to address this issue, resulting in a

number of weatherable epoxy products61-65. However, because of higher resin costs &

the end users prefer to use topcoat epoxy primers with weatherable coatings based on

polyesters, polyurethanes, or acrylics etc, their commercial success has been limited.

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The following epoxy products when formulated with appropriate reactants can provide

certain outdoor weatherability:

(a) Hydrogenated DGEBA

(b) Heterocyclic Glycidyl Imides & Amides

1.4 CURING AGENTS

Curing Agents or Hardeners on reaction with epoxy resin monomers forms epoxy

products66-68. There are several categories of curing agents & are usually liquids.

Examples include:

o Anhydrides such as phthalic anhydride & nadic methyl anhydride (NMA);

o Aliphatic amines such as triethylenetetramine (TETA) & diethylenetriamine

(DETA);

o Aromatic amines, including diaminodiphenyl sulfone (DDS) & dimethylaniline

(DMA);

o Amine/phenol formaldehydes such as urea formaldehyde &

melamineformaldehyde;

o Catalytic curing agents such as tertiary amines & boron trifluoride complexes.

The choice of resin & curing agent depends on the

1. application & product h&ling characteristics such as viscosity, pot life,

flow rate , gel time;

2. curing temperature & time;

3. Usage properties (mechanical, chemical, & thermal, electrical);

4. Toxicological & environmental limitations & cost.

Curing agent selection plays an vital role in determining the final cured epoxy properties

such as pot life, dry time, penetration & wettability etc. Universally, Amine based curing

agents are considered more chemically resistant & durable than based on amide but they

have a tendency to blush in humid conditions. Most of the free amines are carcinogenic in

nature. Whereas amides are more surfaces tolerant & less troubled by moisture69.

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A description of various amines, anhydrides, & catalytic curing agents is tabulated below

(Table 1.1):

Table 1.1: Curing agents For Epoxy resins

Type Disadvantages Advantages Applications

Aliphatic amines

& adducts

Short pot life;

rapid heat evolution;

critical mix ratio;

moderately toxic;

high moisture

absorption

Low viscosity;

little colour

Flooring, civil

engineering, marine &

industrial coatings;

adhesive

Cycloaliphatic

amines

slower reactivity; high

costs

good color; low

toxicity; good

electrical, mechanical,

flooring; paving;

aggregate; industrial

coatings; adhesives.

Aromatic amines long cure cycles at

high temperature;

toxicity

Good chemical

resistance, low

moisture absorption

filament winding;

electrical

encapsulation

Amido-amines Poor performance at

high temperature

low viscosity;

decreased volatility;

better pot life;

ambient curing

temperature;

convenient mix ratios;

good toughness

Construction

adhesives; sealants;

flooring; concrete

bonding.

Polyamides Low temperature

performance; high

viscosity; poor colour

Good mix ratios, pot

life, flexibility,

toughness, &

corrosion resistance;

ambient cure

temperature; low

toxicity

Maintenance coatings;

castings; trade sale

paints; adhesives;

marine coatings

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Anhydrides long cure cycles at

high temperature

(200 _C)

low exotherm; good

thermal (high

Tg), mechanical,

electrical properties;

composites; castings;

potting;

encapsulation

Catalytic Long cure cycles at

high temperature;

brittle

High temperature

resistance; very long

pot lives

Powder coatings;

adhesives; electrical

encapsulation

Dicyanodiamide incompatibility with

epoxy resins

good electrical

properties; high

temperature resistance

electrical laminates;

powder

coatings;

Isocyanates moisture-sensitive;

toxic

fast cure at low

temperature; good

flexibility

powder coatings;

maintenance

coatings

1.4.1 Polyamines & Polyamides

With DGEBA-type resins, Primary & secondary polyamines have good room

temperature cures Aliphatic amines react with cycloaliphatic resins only at high

temperatures & in presence of accelerators such as tertiary amines or bisphenol-A.

Chemical modification to yield epoxy adducts by reaction with epoxy groups creates

products with better h&ling functions. For example, in the presence of water

diethylenetriamine (DETA) readily reacts with ethylene oxide to produce a mixture of

mono- & dihydroxyethyl diethylenetriamine with a longer shelf life & less

dermatitic/skin effects than free DETA. (Scheme 1.7)

CH2NH2CH2CH2NHCH2CH2NH2 +

O

NH2CH2CH2NHCH2CH2NH2CH2 CH2CH2OH

+ HOCH2CH2 NHCH2CH2NHCH2CH2NH2 CH2CH2OH

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Scheme 1.7

Resinous adducts are synthesized by reaction of excess diamine with epoxy resin.

(Scheme 1.8) Those adduct which have a lower vapour pressure than the diamine,

reduces the odour.

CH2

O

CH2 R CH2

O

CH2 NH2R'NH2(excess)+ NH2R'NH CH2CHRCHCH2 NHR'NH2

OHOH

Scheme 1.8

An adduct having higher molecular weight yields a more desirable ratio of resin to curing

agent & lower water absorption on curing. The processability of an epoxy-composite

system is improved in case of adduct. For example, the solubility of diaminodiphenyl

sulfone (DDS) is very poor in multifunctional epoxy resins. But DDS epoxy adduct70 has

improved solubility of hardener in the epoxy resin & thus reducing processing

difficulties. The adduct functions as a solubilising agent for the free amine, although the

epoxy adduct contains 20-35% DDS,

Ketimines can also be considered as be considered blocked amines or latent hardeners,

produced by the reaction products of ketones & primary aliphatic amines. (Scheme 1.9)

In the absence of reactive hydrogens, they do not react with epoxy resins but are readily

hydrolysed.

2 R RC

O

NH2 NH2R' CR

RR

R

R' NN C

Scheme 1.9

Cycloaliphatic amines use as epoxy curing agents is well established. Unmodified

cycloaliphatic amines cure quickly & have excellent colour stability, low viscosity, &

good chemical resistance but require elevated temperatures & are more expensive than

other types of curing agents. Isophorone diamine (IPD), N-aminoethylpiperazine (AEP),

& 1, 2-diaminocyclohexane (1, 2-DAC), are the major commercially available

cycloaliphatic polyamine curing agents.

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Polyamides as curing agents are versatile, inexpensive, have little colour, & can be mixed

in any ratio & cure under mild conditions. They exhibit readily workable pot lives &

provide good mechanical properties,. They are primarily used in coating formulations.

Amidoamines are similar to polyamides, but are of lower viscosity. They are synthesized

by the reaction of tall-oil fatty acid with a mono functional-amine such as DETA,

forming an imidazoline structure. (Scheme 1.10)

RCOOH NH2CH2CH2NHCH2CH2NH2 RC

O

NHCH2CH2NHCH2CH2NH2

NH2CH2CH2 N

R C N

CH2

CH2

H2O

Scheme 1.10

Aromatic amines react slowly with epoxy resins at RT, and need elevated curing

temperatures. Aromatic amines provide better chemical- & thermal- resistance properties

than aliphatic amines. 4, 4’-Diaminodiphenylsulfone (DDS), 4, 4’-

Diaminodiphenylmethane (DDM), & m-phenylenediamine, are the principal

commercially available aromatic amines. To improve on the formers solubility, liquid

eutectic blends of diaminodiphenylmethane & m-phenylenediamine are synthesized

commercially. The commercially availablepolyamine curing agents are given in Table

1.2.

Table 1.2: Commercial Amine Curing Agents

Structure Name

Aliphatic

NH2CH2CH2NHCH2CH2NH2

Diethylenetriamine (DETA)

NH2CH2CH2NHCH2CH2NHCH2CH2NH2

Cycloaliphatic

Triethylenetetramine (TETA)

1, 2-diamino cyclohexane

20

H2N

H2N

N NH

H2N

(DAC)

N-aminoethylpiperazine

(AEP)

Aromatic

S

O

O

NH2H2N

4,4’-diaminodiphenylsulfone

(DDS)

NH2H2N

m-phenylene diamine

The stoichiometric ratio of Resin to polyamine required to cure is a function of the

o molecular weight,

o the number of active hydrogens of the polyamine, &

o the equivalent weight of epoxy resins.

1.4.2 Anhydrides

Various type of structurally different anhydrides are used as epoxy curing agents. The

most significant commercially available anhydrides are based on a cycloaliphatic

structure (Table 1.3).

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Table 1.3: Commercial Anhydride Curing Agents

Name Structure

Phthalic anhydride

O

O

O

Tetrahydrophthalic anhydride

O

O

O

Hexahydrophthalic anhydride

Nadic methyl anhydride

O

O

O

CH3O

O

O

Optimum properties are achieved by curing at higher temperatures. The most effective

catalysts for reducing the curing time are tertiary amines such as benzyldimethylamine,

dimethylaminophenol, tris(dimethylaminomethyl) phenol, boron trihalide complexes, &

substituted imidazoles.

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Epoxy-anhydride systems have low exothermic heats of reaction, low viscosity, longer

pot life, & little shrinkage when cured at higher temperatures. Good mechanical &

electrical properties are shown by the cured system & are used in electrical-casting &

filament-wound epoxy pipe applications. They have imrpoved thermal stabilities

compared to similar amine-cured systems. Anhydrides are the principal hardeners for

cycloaliphatic & epoxidized olefin resins.

1.4.3 Polyimide

Aromatic polyimides71-73 find use for a myriad of applications due to their outstanding

mechanical, electrical & thermal properties. However, hindrances exist in the use of

polyimides as tougheners, as they are not miscible with epoxy resin. Copolyimides have

also been developed to improve solubility & macroscopic properties. More particularly,

the invention is of significance to those epoxy resin compositions, which are curable at

ambient temperatures. Polyimides when based on all aromatic ring structures provides

improved high temperature resistance as well as increased chemical & solvent resistance,

to cured epoxy resin systems. Unfortunately, the only drawback is that they are generally

high melting solids which are not soluble to any appreciable extent in common solvents

or epoxy resins & thus are difficult to incorporate in epoxy resins except at higher curing

temperatures and curing cycles.

1.4.4 Isocyanate Curing Agents

An oxazolidone structure or with a hydroxyl group yielding an urethane linkage is

produced when isocyanates react with epoxy resins via the epoxy group. This urethane

linkage provides improved impact, flexibility, & abrasion resistance. They have been

successfully commercialized in high temperature resistance coating applications.

Although their toxicity has limited the use, blocked isocyanates are used as cross-linkers

for epoxy in PPG’s CED coatings & can also be used to cure epoxies in some powder

coatings.

1.5 CURING MECHANISMS

The choice of curing agent depends on curing conditions, processing methods & the

physical & chemical properties desired.

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Hardeners are either catalytic or co-reactive. A catalytic hardener functions as an initiator

for epoxy resin homo-polymerization, whereas the co-reactive hardener acts as a co-

monomer in the polymerization process (Scheme 1.11 & 1.12). The hardeners can react

with the epoxy & pendant hydroxyl groups on the resin backbone by way of either an

anionic or cationic mechanism.

The functional groups adjacent to the epoxy resin also affect the curing process74, 75.

Electron-withdrawing groups surrounding the epoxy ring often enhance the reactivity of

epoxy resin to nucleophilic reagents, retarding its reactivity toward electrophilic

reagents74, 76,77.

The epoxide ring is susceptible to attack from chemicals with various structures,

epecially, those co-reactive curing agents with active hydrogen atoms, mainly, alcohols,

thiols, phenols, primary & secondary amines, & carboxylic acids.

1.5.1 Catalytic Cure

In homo-polymerization systems, catalytic curing agents are used as a supplement curing

agent to polyamides or polyamines, or as accelerators for an anhydride-cured system.

Catalytic cures are initiated by Lewis acids, e.g. boron trihalides, & Lewis bases (tertiary

amines).

(n + 1) CH CH2

O

R3NR3N

+CH2CH OCH2CH O-

nCatalytic:

Scheme 1.11

(i) Lewis Bases

Lewis bases contain an unshared pair of electrons in outer orbitals and react with areas of

low electron density. When epoxy resins are cured with primary or secondary amines,

tertiary amines are formed, which then acts as a catalyst for homopolymerization as

shown in reaction scheme 1.7.

24

Glycidyl amines such as triglycidyl-p-aminophenol (XVI) & tetraglycidyl methylene

dianiline (XVII) contain built-in tertiary amines in the resin backbone. Compared to

nitrogen-free multifunctional epoxy resins78, these epoxy resin systems are less thermally

stable.

The curing rate of epoxy resins with tertiary amines is dependant upon the sterical

hindrance of nitrogen. The homopolymerization reaction depends on the

o curing temperature as well as

o the concentration & type of tertiary amine.

Tertiary amines are primarily used as accelerators for other curing agents, e.g.,

benzyldimethyl amine & 2,4,6-tris(dimethylaminoethyl) phenol in the curing of

anhydride- & dicyanamide-epoxy based systems.

Imidazoles also have both a secondary & a tertiary amine functional group & are mainly

used as both curing agent & accelerator79.

(ii) Lewis Acids.

Contrary to Lewis bases, Lewis acids, have an empty outer orbital & look to react with

areas of high electron density. For example, Boron trifluoride, BF3, a corrosive gas, reacts

very easily with epoxy resins, causing gelation within few minutes. But complexing

boron trihalides with amines enhances the curing action.

Various mechanisms have been proposed for hardening epoxy resins with BF3 complexes

or salts80-82. Thermal dissociation of BF3.amine complex may result a proton that further

reacts with the epoxy group to initiate the curing process81. A mechanism, more

consistent with the available data, assumes an amine adduct or salt is solvated by the

epoxy groups, forming an oxonium ion82. The curing reaction is initiated & propagated

by attack of other epoxy groups on the oxonium ion.

1.5.2 Co-reactive Cure

25

Coreactive: CH CH2

O

NH2RNH2 CHCH2

OH

NHRNH CH2CH

OH

CH CH2

O

CHCH2

OH

NRN CH2CH

OH

CH2

CH

CH2

OH

OH

CH

Scheme 1.12

(i) Primary & Secondary Amines.

The most widely used agents for epoxy resins as shown in the reaction scheme 1.12 are

Primary & secondary amines83

Reaction of epoxy group with a primary amine initially produces a secondary alcohol & a

secondary amine. Initially curing with a secondary amine affords a tertiary amine & a

secondary alcohol. No competitive reaction is detectable between a secondary hydroxy

group in the backbone & an epoxy group to afford an ether84, 85, provided a stoichiometric

equivalent or excess amine is maintained. However, the secondary hydroxyl groups

formed gradually add to the epoxide groups86, with excess epoxy resin (Scheme 1.13). It

has been researched that primary amines react twice as fast as secondary amines87.

R CHCH2N + CH2 CH

O

R CHCH2N

R CH2 CH OHOH

Scheme 1.13

The rate of amine curing is accelerated by hydroxyl compounds. The proposed

mechanism83 is that the hydrogen atom of hydroxyl group partially protonates the oxygen

atom on the epoxy group (Scheme 1.14), making the methylene group more susceptible

to attack by the nucleophilic amine. However, unsaturated monofunctional aliphatic

26

alcohols are poor accelerators. Reactivity of the system is proportional to hydroxyl

functionality. The best results are achieved with poly-functional alcohols

RCH2 CH

O

NHR

R'

N

H

CH2 CH

O+

HOR"

R

R'

N CH2 CH HOR"

OHR'

Scheme 1.14

The basicity of Aromatic amines is lower than aliphatic amines. For example, the latter

cures epoxy resins at room temperature without accelerators, whereas aromatic amines

require higher hardening temperature. However, with the help of accelerators, the

hardening rates of aromatic amines can approach those of aliphatic amines. On the other

hand, because of the higher acidity of aromatic amines, they react faster than the aliphatic

amines with cyclo-aliphatic epoxy resins. The overall rate of reaction of an amine with

epoxy resin is influenced by the steric hindrance & the electron-withdrawing or electron-

donating groups surrounding the amine. The rate of reaction is lower; if larger the

groups’ are surrounding the amine. This is because electron-withdrawing groups diminish

the nucleophilic character of the amine & lower the reaction rate.

Dicyanamide (DICY) is a solid state latent hardener used in prepreg laminating,

adhesives, & powder-coating applications. The latency is because of its insolubility in

epoxy resins at room temperature. With monofunctional epoxides, DICY88,89 acts as a

latent cyanamide donor, producing dialkyl cyanamides & substituted 2-amino-2-

oxazolidines or 2-imino-2-oxazolidines.

(ii) Mercaptans

Especially at low temperatures, the epoxy-mercaptan reaction is faster than the epoxy-

amine reaction,. It is accelerated by primary & secondary amines. The reaction rate

increases on Increasing the basic strength of the amine. (Scheme 1.15)

RSH CH2 CH2

O

RS CH2 CH2

OH

Scheme 1.15

(iii) Isocyanates

27

Isocyanates react with the epoxy resin (Scheme 1.16) to form an oxa-zoldone structure

(by reacting with epoxy group) or with a hydroxyl group (Scheme 1.17) to yield a

urethane linkage. A cross-linked system is achieved from a di- or polyfunctional

isocyanate & a polyepoxide90.

R N OC CH2 CH

O

CH

CH2

O

CO

N R

Scheme 1.16

R N OC CH2 CH

OH

CH2 CH

O

O

C NHR

Scheme 1.17

(iv) Melamine-, Urea, & Phenol-Formaldehyde Resins

Between the two chemically reactive functional groups in an epoxy resin viz., epoxy &

hydroxyl groups; as the molecular weight of DGEBA resins increases, the epoxy content

decreases, while the hydroxyl content increases. Urea-formaldehyde, Melamine-

formaldehyde, & phenol-formaldehyde resins form cross-linked networks by reacting

with hydroxyl groups of high molecular weight epoxy resins.

Novolacs are Phenol-formaldehyde resins prepared from the acid-catalysed condensation

of phenol & formaldehyde. At higher temperatures, novalac resins with poly-phenolic

functionality react with epoxy resins to form highly cross-linked polymers. (Scheme

1.18)

28

OH

CH2

OH

CH2

n

OH

CH2 CH

O

OH

CH2

O

CH2

n

O CH2 CH

OH

CH2 CH

OH

Scheme 1.18

(v) Carboxylic Acids

Poly-carboxylic polyesters were known as curing agents for epoxy powder coatings from

1970’s, but have not found widespread use. The chemical reaction between a carboxylic

acid & an epoxy resin is depicted below (Scheme 1.19):

RCOOH CH2 CH

O

CH2 RCOO CH2 CH

OH

CH2

RCOOH RCOO CH2 CH

OH

CH2 RCOO CH2 CH

OOCR

CH2 H2O

RCOO CH2 CH

OH

CH2 CH2 CH

O

CH2RCOO CH2 CH

OCH2CHCH2

CH2

OH

CH2 CH

O

CH2 H2O HOCH2CHCH2

OH

Scheme 1.19

29

The first reaction produces a beta-hydroxypropyl ester, which reacts with a second mole

of carboxylic acid to form a diester. The hydroxypropyl esters also undergo

polymerization by reacting with secondary hydroxyl group of epoxy.

(vi) Acid dianhydrides

Both esterification & etherification occur during the uncatalyzed reaction of epoxy resins

with an acid dianhydrides which occurs slowly even at 200oC91, 92; the chemical reaction

is depicted below. Secondary alcohols present in epoxy backbone react with anhydride to

form a half ester, which reacts with epoxy group to give diester. A side-reaction is that

with a secondary alcohol, either on the resin back-bone or formed during the

esterification, resulting in a β-hydroxy ether. Basic catalysts favour esterification

(Scheme 1.20).

CH2

CHOH

O

O

O

R

R

O

O

OH

R

R

O

CH2

CH

half ester

O

O

OH

R

R

O

CH2

CH

H2C CH

O

O

O

OCH2CH

R

R

O

CH2

CH

OH

CH2

CH OH H2C CH

O CH2

CHO CH2 CH

OH

Scheme 1.20

1.6 PROPERTIES OF EPOXY RESIN

1.6.1 Uncured Epoxy Resin

The characterization of liquid Epoxy resins is mainly verified by epoxy content, color,

density, viscosity, hydrolysable chloride & volatility. The less significantly analyzed are

30

glycol content, total chloride content, ionic chloride, & sodium. Whereas, Solid epoxy

resins are characterized by epoxy content, melting point, solution viscosity, color, &

volatility. The Less significantly quoted are phenolic hydroxyl content, ionic chloride,

sodium, hydrolysable chloride, & esterification equivalent.

The epoxy content of liquid resins is expressed as epoxide equivalent weight (EEW) or

weight per epoxide (WPE), and defined as the weight (grams) that contains 1 g

equivalent of epoxide. The analysis of epoxy content of liquid resins & solid resins is

commonly carried out by titration of the epoxide ring by hydrogen bromide in acetic

acid93.

High viscosity liquid epoxies prevent good mixing with curing agents, resulting in non-

homogeneous mixtures, incomplete network formation, & poor performance. On the

other hand, too low viscosity would affect application characteristics such as coverage &

appearance. Viscosities of liquid resins are typically measured with a Cannon–Fenske

capillary viscometer at 25oC, or a Brookfield viscometer. The viscosity of the epoxies is a

function of the temperature & Hydrolysable Chloride (HyCl) content of liquid.

1.6.2 Cured Epoxy resin

The cured epoxy performance is affected by the epoxy curing process. Thus, to obtain

optimum network structure & performance, it is imperative to understand the curing

process & kinetics to design an optimal cure schedule. It is very important to understand

the reactivity of different curing agents towards the epoxy structure to develop a proper

curing process,.

Cured epoxy thermosets are more difficult for analysis than thermoplastics since they are

insoluble & generally intractable. So such systems must be evaluated by considering all

variables affecting performance. However, the properties are influenced by factors at the

molecular level, such as

o epoxy resin backbone structure & curing agent structure;

o nature of the covalent bond developed between the epoxy resin & the curing agent

during cross-linking; &

o degree of cure or density/ or extent of cross-linking

31

The di-functional DGEBA resins are available commercially in a wide range of

molecular weights. As the resin molecular weight increases, the cross-link density of

difunctional resin cured by way of epoxy group decreases. For high molecular weight

resins, they are frequently cured via the secondary hydroxyl group.

The degree of cure is measured by the extent of cross-linking & the most favoured

properties are obtained by highest cross-linking. The ultimate cross-link density is

strongly influenced by the curing temperature and the post-cure property influenced is the

increase in chemical resistance.

The transformation from sol to gel to glass has been measured by a variety of

techniques94, such as viscosity95, calorimetry93, dielectric96, 97 & mechanical relaxations98,

99, dilatometry99 & ultrasonic measurements100. Several spectroscopic methods including

infrared101, 102 & Raman spectroscopy103, 104, nuclear magnetic resonance105, electron

paramagnetic resonance106, fluorescence107, Brillouin scattering108 & photon correlation

spectroscopy100 have also been used. Amongst all the techniques described above, DSC

has been used widely to study kinetics & mechanism of curing epoxy resins.

The reactivity of epoxy resin-curing agent systems is determined by Differential

Scanning Calorimetry (DSC) 109-121.According to Arrhenius equation, the reaction rate is

a function of the temperature. This dependency is measured by the activation energy Ea,

which is influenced not by the chemical structures of the resins & hardeners but by the

type of chemical reaction. Fairly low activation energy of 50-58.5 kJ/mol (12-14

kcal/mol) is required when curing with phenols or aromatic & aliphatic amines. But when

epoxy compounds of low hydroxyl content are cured in the presence of accelerators or

with DICY, activation energies are higher. This is primarily due to the low solubility of

the hardener in the resin.

Wet chemical or physical analysis methods such as solvent swell122, titration of

functional groups123-125 are used to monitor the curing process. To measure the

disappearance of epoxy groups, Fourier transform infrared spectroscopy (FT-IR)114-119,

126, 127, & high pressure liquid chromatography (HPLC)119,120,126-130 is reliable technique

for epoxy resin analysis. The thermal properties reflect the degree of cure & thermal

analysis is used in studies of epoxy resins131-133.

32

The structure & reactivity of the curing agent plays an important role in controlling the

curing reaction of epoxy resin. The curing exotherm onset is primarily dependant on the

nucleophilicity of the amino group. Aromatic amines having electron donor substituents

start the epoxy resin curing at lower temperatures & also have lower activation energy 134.

The relative reactivities135-138 of diamines were as follows: DDS < 3, 3’- DDS < DDM

This is because of due to the presence of sulfone group which is electron withdrawing,

the nucleophilic character of amino group is decreased, resulting in delay of curing

exotherm. The electron transfer due to resonance is more predominant for DDS 136

Whereas, DDM is more reactive than DDS because the methylene group acts an electron

donor.

Other factors which affect the cure evolution & final properties are

o steric hindrances to the epoxy amine addition reaction,

o physical interaction between various functional groups of the constituent

components, &

o cure extension

The structure of the hardener greatly affects the thermal stability. As measured by

thermogravimetric analysis (TGA), the heat resistance of aliphatic amines is low,

whereas, Anhydride systems, at temperatures well below its main decomposition point of

392oC, tend to separate off the anhydride backbone.

The resin to hardener ratio has a very strong impact on the structure of the cured resin &

its properties139. A wide variety of products is obtained by changing the ratios. The

products range from an amine-epoxy adduct with excess amine to an epoxy-amine adduct

with excess epoxy. Theoretically, when equal molar quantities of resin & hardener are

combined, a cross-linked thermoset polymer structure is achieved.

1.7 THERMAL STABILITY OF CURED RESINS

Depending on the chemical structure, thermal decomposition of cured epoxy resin

proceeds in two or more steps. The first step of degradation of the epoxy network is the

thermal decomposition of secondary alcohol groups (generated during curing) with

33

elimination of water (dehydration) preceded by chain scission. The source of water is

Dehydration and is the major gas evolved on heating epoxy formulations140-144. Further

Epoxy decomposition depends on the nature of the dehydrated structure. The presence of

such unsaturations is indicated by appearance of 1650 cm-1 band in IR145-148. They are

responsible for weakening the aliphatic C-O or C-N bonds in the β position. The

calculated energy of these allylic bonds is is lower than the energy of other bonds in the

cured epoxy network, approx 290 & 270 kJ/mol, respectively. By thermal decomposition

of the weak C-O bonds, phenolic chain ends are formed, whereas from the scission of C-

N bonds, secondary amine terminal functions result. Cyclic chain structures, may also be

formed simultaneously, and are favoured due to the reduced mobility of macroradicals in

the solid matrix. If scission of C-O &/or C-N bonds occurs before dehydration of

secondary alcohols, organic products such as acetone are also formed. The volatilization

of DGEBA in these reactions is a result of the fact that DGEBA is difunctional, therefore

repetition of the reactions on the same units leads to loss of corresponding DGEBA units

from the residue149.

1.8 EFFECTS OF EPOXY RESIN SYSTEM ON HUMAN BODY

The chemicals in epoxy resin systems can impact your well being if they interactwith

skin, or when they evaporate or form a mist or dust in the air you breathe. The key

aftereffects of overexposure are irritation of the eyes, nose, throat, & skin, skin allergies,

& asthma. The solvent additives could cause other effects such as for instance headaches,

dizziness, & confusion.

(A)Lungs: Vapors & spray mists of all epoxy resin system chemicals can irritate lungs.

Some individuals develop asthma from the curing agents. ApparentSymptoms of asthma

include chest tightness, shortness of breath, wheezing, & coughing. These symptoms may

occur after work or at night. Once an individual becomes allergic to curing agents, even

the dusts from sanding or grinding the hardened plastics can cause an asthma attack.

(B) Skin: Epoxy resins can cause skin irritation. Symptoms include redness, swelling,

shedding, & itching on the hands, experience, or other regions of contact. Some

individuals create a skin allergy or sensitivity to epoxy resins or mists. Skin allergies may

possibly build following only a few days of contact or following many years of exposure

34

to epoxies. Sensitized skin may become red, inflamed, blistered, & itchy actually from

brief connection with epoxy resins.

(C) Eyes, Nose, & Throat: Most epoxy resin system compounds & their vapors

(especially the hardeners & solvents) may worsen eyes, nose, & throat. Some individuals

build headaches consequently of the irritation. If the fluids are splashed in to eys they'll

sting, & they can seriously injury the eye. In case there is vision contact, immediately

wash the eyes with water. Keep on rinsing for fifteen minutes & then find medical

attention.

(D) Nervous System: Solvents consumed or absorbed through your skin can affect main

nervous program exactly the same way consuming alcohol does. Apparent symptoms of

solvent overexposure contain head-aches, vomiting, dizziness, slurred presentation,

distress, & loss in consciousness.

(E) Reproductive System: Epoxy resins & hardeners themselves possibly don’t affect

pregnancy & reproductive cycle in humans. However, a number of the diluents &

solvents in epoxy resin methods might affect reproduction. Two solvents sometimes

within epoxy resin systems (2-ethoxyethanol & 2-methoxyethanol) cause birth problems

in laboratory animals & decreased sperm counts in men. Some glycidyl ethers also harm

the testes & cause birth problems in test animals. It is unknown whether they've the same

results in humans. Other solvent ingredients have not been adequately tested to find out

when they affect reproduction. However, we do understand that solvents inhaled by a

woman can reach a developing foetus & might contaminate the breast milk. They could

affect the child only as they affect the mother. We recommend that pregnant & nursing

women reduce their exposure to solvents, just like they should reduce their exposure to

liquor150.

1.9 APPLICATIONS OF EPOXY RESINS

The applications for epoxy-based materials are extensive & include coatings, adhesives &

composite materials such as for instance those using carbon fibre & fibreglass

reinforcements (although polyester, vinyl ester, & other thermosetting resins are also

used for glass-reinforced plastic). The chemistry of epoxies & the range of commercially

available variations allow cure polymers to be produced with a really broad range of

35

properties. Generally, epoxies are noted for their excellent adhesion, chemical & heat

resistance, good-to-excellent mechanical properties & very good electrical insulating

properties. Many properties of epoxies can be modified (for example silver-filled epoxies

with good electrical conductivity are available, although epoxies are generally electrically

insulating). Variations offering high thermal insulation, or thermal conductivity coupled

with high electrical resistance for electronics applications, are available151,152.

1.9.1 Adhesives

Epoxy adhesives are a major area of the class of adhesives called "structural adhesives"

or "engineering adhesives" (that includes polyurethane, acrylic, cyanoacrylate, & other

chemistries.) These high-performance adhesives are utilized in the construction of

aircraft, automobiles, bicycles, boats, golf clubs, skis, snowboards, & other applications

where high strength bonds are needed153,154. Epoxy adhesives could be developed to

match nearly every application. They may be used as adhesives for wood, metal, glass,

stone, & some plastics. They could be made flexible or rigid, transparent or

opaque/colored, fast setting or slow setting. Epoxy adhesives are better in heat &

chemical resistance than other common adhesives. Generally speaking, epoxy adhesives

cured with heat could be more heat- & chemical-resistant than those cured at room

temperature. The potency of epoxy adhesives is degraded at temperatures above 350 °F

(177 °C). Some epoxies are cured by exposure to ultraviolet light.Such epoxies are

commonly utilized in optics, fibre optics, & optoelectronics.

1.9.2 Paints & Coatings

Coatings Application Technologies includes below:

(A) Low Solids Solvent borne Coatings

(B) High Solids Solvent borne Coatings

(C) Solvent-Free Coatings (100%Solids)

(D) Waterborne Coatings

(E) Powder Coatings

(F) Radiation-Curable Coatings

36

Two part epoxy coatings were developed for high quality service on metal substrates &

use less energy than heat-cured powder coatings. These systems make use of a 4:1 by

volume mixing ratio, & dry quickly providing a hardcore, protective coating with

excellent hardness. Their low volatility & water cleanup makes them helpful for factory

cast iron, cast steel, cast aluminium applications & reduces exposure & flammability

issues associated with solvent-borne coatings. They're usually utilized in industrial &

automotive applications being that they are more heat resistant than latex-based & alkyd-

based paints. Epoxy paints often deteriorate, called chalk out, as a result of UV exposure.

Polyester epoxies are employed as powder coatings for washers, driers & other "white

goods ".Fusion Bonded Epoxy Powder Coatings (FBE) are extensively employed for

corrosion protection of steel pipes & fittings utilized in the oil & gas industry, potable

water transmission pipelines (steel), concrete reinforcing rebar, etc. Epoxy coatings will

also be popular as primers to boost the adhesion of automotive & marine paints especially

on metal surfaces where corrosion (rusting) resistance is important. Metal cans &

containers tend to be coated with epoxy to stop rusting, specifically for foods like

tomatoes which can be acidic. Epoxy resins will also be employed for high performance

& decorative flooring applications especially terrazzo flooring, chip flooring & colored

aggregate flooring155,156.

1.9.3 Composites

Epoxies may also be used in producing fibre-reinforced or composite parts. They are

more costly than polyester resins & vinyl ester resins, but usually produce stronger &

more temperature-resistant composite parts. Epoxy resins are suitable as a fibre-

reinforcing material since they exhibit excellent adhesion to reinforcement, cure with low

shrinkage & provide good mechanical, electrical & thermal-, chemical-, fatigue-, &

moisture-resistant properties.

The processes for making composites encompass the whole range of epoxy resin

technology, i.e., laminating, filament winding, pultrusion, casting, & moulding. For their

excellent adhesion, good mechanical properties, & resistance to humidity & chemicals,

epoxy resins are employed in combination with glass, graphite, & boron & Kevlar fibers.

The orientation of the fibers is important in establishing the properties of the laminate.

Unidirectional, bidirectional, & random orientation are possible. The characteristics of

37

the cured resin system are really important because it must transmit the applied stresses to

each other. The critical point in a composite may be the resin-fibre interface. The

adhesive properties of epoxy resins cause them to become especially suited to composite

applications.

Filament-wound glass-reinforced pipe is used in oil-field applications, chemical plants, as

electrical conduits & in water-distribution networks. Low viscosity liquid DGEBA cured

with liquid anhydride or aromatic diamine hardeners are the systems of choice. Filament-

wound epoxy components are employed for rocket-motor casings, pressure vessels, &

tanks157,158.

1.9.4 Industrial Tooling & Casting

From the mid-1950s, electrical-equipment manufacturers have looked at the design

freedom afforded by epoxy casting techniques to create switchgear components,

transformers, insulators, high voltage cable accessories, & similar devices. Epoxy

systems are utilized in industrial tooling applications to create molds, master models,

laminates, castings, fixtures, & other industrial production aids. This "plastic tooling"

replaces metal, wood & other traditional materials, & generally improves the efficiency

& either lowers the entire cost or shortens the lead-time for a lot of industrial processes.

In casting, a resin-curing agent system is charged right into a specially designed mould

containing the electrical element of be insulated. After cure, the insulated part retains the

design of mould. In encapsulation, an attached electronic component like a transistor or

semiconductor in a mould is encased in an epoxy resin-based system. Coil windings,

laminates, lead wires, etc, are impregnated with the epoxy system. Both DGEBA &

cycloaliphatic epoxy resins are utilized in casting systems. The cycloaliphatic resin

systems exhibit good tracking properties153 & better resistance than DGEBA resins to

UV radiations, that causes crazing & surface breakdown. Amine curing agents are

utilized in small castings & anhydrides in large castings. Transfer moulding can be used

to encapsulate the solid-state devices such as for example diodes, transistors & integrated

circuits in epoxy moulding powders. In the manufacture of tools, epoxy casting resins are

utilized as prototype & master models for product design, drilling & welding jigs,

checking fixtures, vacuum forming & injection moulding, foundry patterns & stretch

38

blocks. They're less expensive than metals & can be modified quickly & cheaply. They

provide high dimensional stability, low shrinkage, & good mechanical properties. Simple

casting & hand layup laminating techniques are employed159,160.

1.9.5 Consumer & Marine Applications

Epoxies can be purchased in hardware stores, typically as a bunch containing separate

resin & hardener, which must be mixed immediately before use. They are also sold in

boat shops as repair resins for marine applications. Epoxies typically aren't used in the

outer layer of a boat because they deteriorate by experience of UV light. They are often

used during boat repair & assembly, & then over-coated with conventional or two-part

polyurethane paint or marine-varnishes offering UV protection.

There are two main regions of marine use. Because of the better mechanical properties in

accordance with the more common polyester resins, epoxies are useful for commercial

manufacture of components where a high strength/weight ratio is required. The next area

is that their strength, gap filling properties & excellent adhesion to numerous materials

including timber have created a boom in amateur building projects including aircraft &

boats. Normal gelcoat formulated for use with polyester resins & vinylester resins doesn't

stick to epoxy surfaces, though epoxy adheres very well if applied to polyester resin

surfaces. "Flocoat" that is normally used to coat the inside of polyester fibreglass yachts

can also be suitable for epoxies161-163.

Epoxy materials tend to harden somewhat more gradually, while polyester materials tend

to harden quickly, especially if lots of catalyst is used. The chemical reactions in both

cases are exothermic. Large quantities of mix will create their own heat & greatly speed

the reaction, so it is usual to mix small amounts which can be used quickly.

While it is common to associate polyester resins & epoxy resins, their properties are

sufficiently different that they're properly treated as distinct materials. Polyester resins

are typically low strength unless used in combination with a reinforcing material like

glass fibre, are relatively brittle unless reinforced, & have low adhesion. Epoxies, in

comparison, are inherently strong, somewhat flexible & have excellent adhesion.

However, polyester resins are much cheaper. Epoxy resins typically require an exact mix

of two components which form a third chemical. With regards to the properties required,

39

the ratio might be anything from 1:1 or higher 10:1, in every case they should be mixed

exactly. The final product is then the precise thermo-setting plastic. Until they are mixed

both elements are relatively inert, even though the'hardeners'are generally more

chemically active & should really be protected from the atmosphere & moisture. The rate

of the reaction can be changed by using different hardeners, which may change the type

of the last product, or by controlling the temperature. By comparison, polyester resins

usually are made available in a'promoted'form, in a way that the progress of previously-

mixed resins from liquid to solid is underway, albeit very slowly. The only variable

offered to the user is to alter the rate of this method employing a catalyst, often Methyl-

Ethyl-Ketone-Peroxide (MEKP), which is very toxic. The clear presence of the catalyst in

the last product actually detracts from the desirable properties, in order that small

amounts of catalyst are preferable, so long as the hardening proceeds at a satisfactory

pace. The rate of cure of polyesters can therefore be controlled by the amount & kind of

catalyst along with by the temperature.

As adhesives, epoxies bond in three ways: a) Mechanically, since the bonding surfaces

are roughened; b) By proximity, since the cured resins are physically so near the bonding

surfaces that they're hard to separate your lives; c) Ionically, since the epoxy resins form

ionic bonds at an atomic level with the bonding surfaces. This last is substantially the

strongest of the three. By comparison, polyester resins can only bond utilising the first

two of those, which greatly reduces their utility as adhesives & in marine repair.

Epoxy adhesives really are a major the main class of adhesives called "structural

adhesives" or "engineering adhesives" (that includes polyurethane, acrylic, cyanoacrylate,

& other chemistries.) These high-performance adhesives are found in the construction of

aircraft, automobiles, bicycles, boats, golf clubs, skis, snowboards, & other applications

where high strength bonds are required. Epoxy adhesives may be developed to match

nearly every application. They may be used as adhesives for wood, metal, glass, stone, &

some plastics. They may be made flexible or rigid, transparent or opaque/colored, fast

setting or slow setting. Epoxy adhesives are better in heat & chemical resistance than

other common adhesives. Generally speaking, epoxy adhesives cured with heat may well

be more heat- & chemical-resistant than those cured at room temperature. The strength of

epoxy adhesives is degraded at temperatures above 350 °F (177 °C).

40

Some epoxies are cured by exposure to ultraviolet light.Such epoxies are commonly

found in optics, fibre optics, & optoelectronics164,165.

1.9.6 Aerospace Applications

In the aerospace industry, epoxy is employed as a structural matrix material which can be

then reinforced by fibre. Typical fibre reinforcements include glass, carbon, Kevlar, &

boron. Epoxies may also be used as structural glue. Materials like wood, & others which

are 'low-tech' are glued with epoxy resin166,167.

1.9.7 Electrical Systems & Electronics

Epoxy resin formulations are essential in the electronics industry, & are employed in

motors, generators, transformers, switchgear, bushings, & insulators. They are excellent

electrical insulators & also protect from dust & moisture. In the electronics industry

epoxy resins are the principal resin used in over molding integrated circuits, transistors &

hybrid circuits, & making printed circuit boards. The largest type of circuit board "FR-4

board" is a sandwich of layers of glass cloth bonded in to a composite by an epoxy resin.

Epoxy resins are acustomed to bond copper foil to circuit board substrates, & really are a

component of the solder mask on many circuit boards.

Flexible epoxy resins are used for potting transformers & inductors. By utilizing vacuum

impregnation on uncured epoxy, winding-to-winding, winding-to-core, & winding-to-

insulator air voids are eliminated. The cured epoxy is an electrical insulator & a better

conductor of heat than air. Transformer & inductor hot spots are greatly reduced, giving

the component a well balanced & longer life than un-potted product168,169.

1.9.8 Inks & Resists

Inks & resists comprise a relatively small but high value & growing market for epoxies &

epoxy derivatives. In 2001, there clearly was an estimated of 6800 MT of epoxies &

epoxy derivatives found in this market to create ink & resist formulations worth almost

$400 million in the U.S. market. Epoxies are often used with other resins such as for

example polyester acrylates & urethane acrylates in these formulations. The biggest

applications are lithographic & flexographic ink resist technology is widely found in the

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electronics industry to manufacture printed circuits. The resist (a coating or ink) is

applied over a conducting substrate such as for example copper in a structure to safeguard

its surface during etching, plating, or soldering. Cure is either by radiation or heat. The

uncured coating (or ink) is removed later by solvents. Solder masks perform similar

functions in the manufacturing of printed circuit boards170,171.

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1.10 SCOPE OF THE PRESENT STUDIES

Unmodified epoxies have poor chemical, mechanical & thermal properties. Hence

modification of epoxy resin has been the main topic of research interest since its

discovery. A substantial work has been carried out to boost the properties of epoxy resins.

The effect of structure of cyclic compounds as curing agents on the curing of epoxy resin

has been adequately described in the literature. Since aromatic cyclic compounds are

noted for their high thermal stability, it has created a pursuit to investigate the curing &

thermal behaviour of epoxy resin using amines, anhydrides & benzoxazines having cyclic

aromatic ring.