I poxy resins are generally recognized as workhorse products among l Z \the category of thermosetting polymers due to their outstanding

mechanical properties and good handling characteristic^.'^ The use of

epoxy resins in industry extends back over fifty years since their

introduction commerc:ially and they find an extremely wide range of

applications as coatings, adhesives and matrix resins in different fields G

like electronics, defense, aerospace industry etc. Though epoxy resins

were first synthesiz,ed in 18915, commercial epoxy resins were marketed

only in 1940~. The earliest epoxy resins marketed were the reaction

products of bisphenol-A and epichlorohydrin and this is still the major

route for the manufacture of most of the epoxy resins marketed today7.

The main advantages of epoxy resins are their good mechanical

properties, minimum shrinkage after cure and suitable weather, chemical

and electrical resistace.

The term epoxy resin is applied to both the prepolymers and to the

cured resins. Epa~xy resin in the unreacted form contains an

epoxide, oxirane or ethoxylene group which is a three membered ring

consisting of an oxygen atom attached to two connected carbon atoms

i.e., / \d'c/ \ . The epoxide function is usually a 1,2- or a-epoxide that /4

appears in the form, ,ax,--c-xt--mi , called the glycidyl group, which

is attached to the remainder of the molecule by an oxygen, nitrogen or

carboxyl linkage and are termed as glycidylether, glycidylamine or

glycidylester respectively.

1.1 Synthesis of different types of epoxy resins

Two basic processes are used in the manufacture of epoxy resins: (i) the

reaction of epichlorohydrin with compounds containing reactive hydrogen

atoms, such as phenols or amines, and (ii) the peracid epoxidation of

olefms.

1.1.1 Epoxidation using epichlorohydrin

Diglycidylether of bisphenol-A (DGEBA) and its higher

homologues synthesized by reacting bisphenol-A with epichlorohydrin in

presence of aqueous caustic soda (Scheme 1.1.) constitute the major

portion of commercially used epoxy resins.' The reaction is always

carried out with an excess of epichlorohydrin so that the resulting resin

has terminal epoxy groups. By varying the manufacturing conditions and

amount of epichlorohydrin, resins of low, intermediate or high molecular

weight can be As the value of 'n' in DGEBA increases, the

resin progresses from a viscous liquid to a solid having high softening

point

Besides DGEBA resins novolac epoxy resins are also widely used

in industq. These resins are polyglycidyl ethers of novolac resins and

vary from the standard bisphenol-A-based resin in their

mu~tifunctionalit~.~ Novolac epoxy resins are prepared by reacting

novolac resins with epichlorohydrin'O (Scheme 1.2). The base novolac

2

where x = 0

and n =-13-12

Scheme I . 1. Synthesis of DGEBA

resin is a reaction product of phenol and formaldehyde in an acidic

environment. The complete conversion of all phenolic hydroxyl groups

to epoxides does not clccur due to stearic hindrance. Novolac epoxy resins

with functionalities ranging from 2.5 to 6.0 are commercially available.

where n = - 0.5- 4

Scheme 1.2. Synthesis of novolac epoxy resin.

3

Because of their multifi~nctionality, the novolac epoxy resins when cured

with any of the conventional epoxy curing agents produce a tightly

crosslinked system with better elevated temperature performance,

chemical resistance and adhesion than those obtained from bisphenol A-

based resins.

1.1.2. Epoxidation of olefins

The second most important method for manufacturing epoxy resin

is by the epoxidation of olefms. There are three important routes for

producing epoxides from olefms6~": (i) catalytic epoxidation in which the

olefins are directly omdized in the vapour phase in the presence of a

catalyst such as silver, (ii) epoxidation by organic peroxides and their

esters and (iii) epoxidation by inorganic peroxides and inorganic peroxy

acids." Of the above methods the most commonly used one is the

epoxidation by peracetic acid in aqueous or non-aqueous media.

Epoxidation of olefms by peracetic acid is shown in Scheme 1.3.

Scht:me 1.3. Epoxidation of olefin

Latha et al.I2 synthesized epoxidized hydroxyl-terminated

polybutadiene (EHTPB:) by reacting hydroxyl-terminated polybutadiene

(HTPB) with perfonnic acid formed in situ by the reaction of hydrogen

peroxide with formic acid. A major advantage of peracid route for the

synthesis of epoxy intermediates is that the resins obtained do not contain

hydrolysable chlorine as no species containing chlorine is involved in this

synthesis. These resins have low ash and ionic content and hence, have

better weathering and ageing properties than conventional epoxy resins.

Cycloaliphatic epoxy resins are manufactured by the epoxidation !

of aliphatic unsaturated compounds with peraceticacid. These compounds

do not contain aromatic compounds and hence, are more stable to UV

exposure than the bisphenol-A derived epoxy resins6. 1 3 .

Scheme 1.4. Synthesis of cycloaliphuiic epoxy resin

1.2 Cure reaction and curatives of epoxy resins

In order to ccnvert epoxy resins to hard infusible thennoset

networks it is necessary to use crosslinking agents. These crosslinkers,

hardeners, curing agents or curatives as they are widely known, promote

cross-linking or curing, of epoxy resins. The curing of the epoxy resin

takes place either between the epoxide molecules themselves or by the

reaction between the epoxy group and the ~urat ive~.~. The former is

known as homopolymerization which may take place with the help of a

catalyst and the latter is an addition curing reaction which can take place

with or without the help of a catalyst. Both the reactions result in coupling

as well as crosslmking.

Curing agents or curatives have two or more reactive groups in a

molecule, which can react with epoxy The properties of cured

resins are determined by the epoxy prepolymer and the curative. Due to

the presence of a strained three-membered ring structure, an epoxy group

can react with many nucleophilic and electrophilic reagents. Compounds

5

k..

with active hydrogen atoms such as amines, phenols, alcohols, thiols and \

carboxylic acids, and acid anhydrides are widely used as curatives due to

their workability and availability. Except for acid anhydrides, the

conventional curatives of epoxy resins leave pendant hydroxyl groups in

the cured resins. The curing of epoxy resins with different types of

curatives are shown in Scheme 1.5.

+ -R2-NH, - amine

alcohol

R + E l i C H - C H , + -R2-SH - 'd thiol

-1- -R~--COOH

acid WVO

/O\ Rl-CHZ-CB- I

+ O=c\dGO - F anhydride

-c-

Scheme 1.5. C u ~ e reaction of epoxy resin with d~fferent curatives

1.3. Modified epox:y resins

In addition to the two main ingredients of an epoxy formulation,

viz., epoxy resin and curative, numerous other formulatory materials are

available and have been frequently used to improvelmodify the properties

of epoxy resins. Modification of epoxy resins by incorporating other

components is an important technique for tailoring the properties to meet

end-use requirements. Widely used modifiers are diluents, fillers, rubber

toughening agents, thennoplastics etc14. Usually diluents are used as a

means of reducxng viscosity of the epoxy resin in order to improve

6

handling characte~istics'~. Diluents improve the performance of room

temperature property of the cured epoxy resin, but chemical resistances

and thermal propelties are usually substantially reduced. Fillers are used

with epoxy resin system to reduce the cost of epoxy resins and to improve 14,15 the mechanical, electrical and thermal properties. A major advantage

of the addition of fillers to epoxy resin is that an improvement in modulus

can be achieved wthout much sacrifice in other properties. The inherent

brittleness of epoxy resins can be reduced by improving the fracture

toughness of epoxy systems which in turn prevents crack propagation and

premature failure.16 This can be achieved by the addition of rubber

toughening agentslthennoplastic modifiers to epoxy resins. This section

presents a brief account of such modifications.

1 . 1 Epoxy resins modified with reactive rubbers

When rubbery domains in the micrometer range are randomly

dispersed in the epoxy matrix, the fracture energy can be greatly

increased. Addition of reactive liquid elastomers to epoxy resins improves

mechanical properties of epoxy resin^.'"'^ The primary objective of

rubber modification is the improvement of fracture properties with a

minimal decrease in the stiffness and mechanical strength. Telechelic

butadienelacryloni~ile co-polymers are widely used as toughening agents

for epoxy resins." Structures of carboxyl-terminated butadiene-

acrylonitrile copolymer (CTBN) and amine-terminated butadiene-

aclylonitrile copolymer (ATBN) are shown in Fig. 1.1.

The term toughness is a measure of material's resistance to failure.

Unlike flexibilising,, it does not affect the other properties, for example 17-18 T,, strength and hardness. Toughness is usually measured as either the

stress or the energy required to fail a specimen under a specific

Amme-te~minated butadiene-acryhitfk copolymer ( A m

Fig:. 1 . 1 . Structures of CTBN and ATBN

loading condition. Common toughening mechanisms are matrix shear

yielding, particle cavitation, and rubber bridging.' There are two main

procedures for dispersing rubbery particles in the epoxy matrix? phase

separation during polymerization of an initial homogeneous solution

(reaction induced phase separation), and a two-phase initial formulation

by dispersing elastomeric particles in the mixture of monomers. Reflected

optical microscopy and scanning electron microscopy (SEM) are widely

used for studying both morphology and failure mechanisms in the damage

zone of the rubber-modified epoxies.'' These microscopic techniques,

however, only provide information relating to the fracture surface.

Transmitted optical microscopy (TOM) or transmission electron

microscopy (TEM) provides detailed information about the sub surface

fracture surface zone.

McGany et al.21-23 toughened a DGEBA-type resin by adding low

molecular weight CTBN copolymers. Rubbery domains which

precipitated in situ during cure imparted toughness to epoxy materials.

Preparing rubber-modified epoxies from reaction induced phase

separation has the advantage of processing initial homogeneous solution

with low viscosity. The morphologies generated and the resultant

properties depend on the initial amount of modifier, particle size, cure

cycle and the presenc'e of other additives in the formulation.

Studies on the biaxial yield behaviour of epoxies containing large

(1-22 pm) and small (< 0. lpm) rubber particles reveal that larger particles

promote microcavitation while smaller particles enhance the shear

yielding process.23 Bascom et al.24 used a combination of liquid and solid

CTBN rubbers to produce a dual particle size distribution of 0.5 pm and

1-2 p respectively. It was observed that smaller particles deformed

principally by voiding and induced local shear yielding, and larger

particles produced localized ylelding in the surrounding matrix which was

facilitated by the presence of smaller particles. Lee et al." reported that

considerable improvement in toughening is obtained with small amount

of polymethacrylate-natural rubber (PMMA-NR) graft copolymer content

in epoxy than with higher content of CTBN rubber. This is attributed to

the bimodal particle size distribution of the graft copolymer modified

system with large particles of 0.1-3 pm and small particles of less than

0.1 p.

Manzione et a1.26 reported that the compatibility of the rubber and

epoxy could be controlled by the acrylonitrile content of the rubber

modifier as well as the cure conditions. The higher the acrylonitrile

content, better is the compatibility of CTBN with epoxy in terms of

solubility parameter arid slower is the precipitation of rubber phase in the

epoxy matrix. A wide range of morphology of phase separated and

dissolved rubber can be obtained in rubber-modified epoxies through

control of rubber-epoxy compatibility and cure conditi~ns.~' More

compatible formulations result in smaller rubber domains. Moreover,

greater acrylonitrile: content of CTBN copolymer and higher cure

temperature promote dissolution of rubber in the epoxy phase and hence,

a distinct damping peak associated with phase separated rubber is absent

in the spectrum of dynamic mechanical analysis. This indicates either

complete blending or absence of particles above a small critical size.

Dissolution of rubber is reflected in the mechanical properties, especially

in the impact strenbd. of epoxy matrix.

Liao and R ~ ~ I I ~ ~ ~ studied the effect of CTBN on epoxy-graphite

composites and observed an increase in the fracture toughness for 10-15% 29 . addition of CTBN. Sohn et al. investigated the interfacial tension

between CTBN and epoxy resin (EPON 828) as a function of temperature

and copolymer composition, using digital image processing techniques.

The interfacial tension is found to correlate qualitatively with the

morphology of pre-reacted epoxy systems. With the increase in

acrylonitrile content of the copolymer, a decrease in the interfacial tension

and a corresponding decrease in the domain phase in the epoxy matrix is

observed.

The influence of silica on rubber phase separation during cure of a

CTBN-modified epoxy resin was examined by Klung et al.30 Phase

growth rates were depressed by silica resulting in a lower percentage of

toughening domains. Hence, the morphology and the fracture toughness

of the silica containing toughened epoxy are different from that of the

modified epoxy without silica. The effect of CTBN content on adhesive

lap shear strength and T-peel strength of an epoxy at room temperature

and at 120°C was reported by Achary et a ~ . ~ ' Maximum adhesive strength

was obtained when LO phr CTBN was used. Addition of CTBN also

increased the bulk te:nsile strength and impact energy. Cure kinetics of

epoxy-amine system modified with CTBN and ATBN was examined by

Wise et a1.32 The rate of epoxy-mine reaction increases with the addition

of CTBN and decreases with the addition of ATBN. Preformed

dispersions of epoxy insoluble rubbers are also used for toughening of

epoxy-based adhesives and composites."

10

Yee et al." modified a DGEBA-type resin with methacrylate-

butadiene-styrene (MBS) rubber and investigated its fracture toughness.

The effect of testing rate and temperature on the fracture toughness of

unmodified and rubber modified DGEBA was studied. They observed

that the fracture toughness of unmodified epoxy does not dependant on

the testing conditions while that of the rubber-modified samples increases

with a decrease in testing rate or an increase in temperature.

Latha et al." reported the toughening effect of epoxidized

hydroxyl-terminated polybutadiene (EHTPB) on epoxy resins cured with

an amine. Lap shear strength and T-peel strength were found to increase

with the increase in EHTPB content up to 10 phr. This is attributed to the

higher toughness produced by the dispersed rubber particles. At higher

EHTPB content, the rubber phase became continuous, and flexibilization

effect predominated over toughening effect of EHTPB.

1.3.2 Epoxy resins modified with silolanes

The main objective of rubber modification of epoxy resins is to

improve the fracture properties with a minimum sacrifice in their

mechanical properties. The CTBN and ATBN copolymers have done

much towards accomplishment of this objective as discussed in the

section 1.3.1. But their high T, limits their low temperature flexibility and

their unsaturated stnicture makes them unsuitable for use at elevated

temperatures.35 Siloxane modifiers present an attractive alternative to the

butadiene-acrylonitrile copolymers due to their superior thermo-oxidative

stability, high flexibility, good weatherability and low T,. In addition to

this, because of their low surface energy and non-polar structure,

siloxaner tend to migate to the

hydrophobic surface for the v . I,

modifiers used to modify epoxy resins are 3 - , !

11

~ 2 ~ a 8([p;!m&**2 n

Fig. 1.2. D@rent types of siloxane modifiers for epoxy resins

Riffle et al.35 synthesized functionally terminated polydimethyl-

siloxanes and utiliz,ed them to modify epoxy resins. DGEBA resin (Epon

828) was mixed with siloxane modifier and an amine curative and cured

at 160°C for 2h. The behavior of the resultant network is found to depend

on the nature of the end-functional group of the modifier. Network

prepared with secondary amhe terminated system, e.g., piperazine

functionality were able to produce homogeneous cured network. Electron

spectroscopy for c;hemical analysis (ESCA) revealed that piperazine

capped oligomers significantly enriched the surface with siloxane

structures.

Yorgitis et a~.~%ynthesized siloxane-modified epoxy prepolymers

by reacting DGEBA-type resin with piperazine-terminated

polydimethylsiloxane (Scheme 1.6) and its statistical copolymers with

either methyltrifluoropropylsiloxane or diphenylsiioxane.

12

I O H b H

Crosslinked netwok

Scheme 1.6. Synthesis of siloxane-modified epoxy resin

These prepolymers were cured with cycloaliphatic diamines.

Increasing the percentage of methyl trifluoropropyl or diphenyl units

relative to the dimethylsiloxane content of the oligomers enhanced the

compatibility with epoxy resins. This enhancement produced smaller

rubber particles and altered particle morphology. Improved fiacture

toughness relative to the cycloaliphatic diamine-cured control resin was

achieved in resins modified with polysiloxane copolymers containing

40% or more of methyl trifluoropropyl units 01. 20% diphenylsiloxane

units.

Lin and ~ u a n ~ " synthesized siloxane-modified epoxy resin by

reacting dangling hydroxyl group with methoxy or silanol-terminated

polydimethylsiloxane in presence of a tetraisopropyl titanate. Siloxane-

epoxy resin was cured with 2,4,6-tris-(dimethyl aminomethy1)phenol.

TGA studies showed that the siloxane incorporated epoxy resins provide

enhanced thermal :stability over the unmodified ones. Morphological

studies suggest that siloxane segment acts as a toughening agent for the

epoxy network contributing to the impact strength improvement of the

copolymer.

As a variation of the above approach, Lin and ~ u a n ~ ~ * introduced

sulfone groups into the epoxy resin by reacting with bisparaphenol

sulfones before carrying out the siloxane modification. This approach

helps in securing the structural rigidity in maintaining the original T, or

better T, of the epoxy resin after modifying with siloxane. Studies on cure

kinetics of siloxane: modified and conventional epoxy resins suggest that

both followed a similar curing pattern under the same curing process and

same curing agent.39 Higher activation energy is observed for siloxane

modified epoxy than that for unmodified epoxy resin and this has been

attributed to the stearic hindrance of the bulky phenyl group of the

siloxane oligomer in the epoxy resin.

The thermal degradation study made by Lin and ~ u a n ~ ~ ' on

siloxane modified Epikote 1001 (DGEBA type resin) suggested that the

thermal degradation is being affected by structure or the content of

siloxane moiety in the copolymer. The study reveals that siloxane

modified epoxy copolymer with phenyl enriched siloxane oligomer

provides higher thermal stability than the dimethyl siloxane modified

copolymer.

Lee et al.4' modified tetrafunctional epoxy resins, obtained by

epoxidation of the condensation products of dialdehydes and 2,6-

dimethylphenol, with amine-terminated polydimethylsiloxane to obtain

resins suitable for semiconductor encapsulation application. The above

resins were cured with 4,4'-dlaminodiphenyl sulfone to obtain crosslinked

matrices. Dispersed silicone rubber effectively reduced the stress of the

crosslinked matrices by reducing the flexural modulus and the coefficient

of thermal expansion,,

Lin et synthesized a vinyl functionalized epoxy resin by

reacting a trifunctional epoxy resin with ally1 phenol. The vinyl group

present in the epoxy resin was hydrosilylated with polysiloxanes

containing Si-H groups to obtain siloxane-modified epoxy resin. The

modified and unmodified resins were cured with phenol formaldehyde

novolac resin in the presence of tiphenyl phosphine catalyst. The

investigation on tl~elrnal and mechanical properties, and the flexural

behavior of the modified epoxy resins reveals a decrease in the Young's

modulus and a slight decrease in its T,. This may be due to the presence

of dispersed silicon rubber.

Epoxy/anhydride-terminated oligomers containing different

amounts of trialkoxysilane groups were synthesized by Mauri et al.43 from

phenyl glycidyl ether,, 3-glycidyloxypropyl trimethoxysilane and methyl

tetrahydrophthalic anhydride (MTHPA) using benzyldimethylamine

(BDMA) as an initiator. These oligomers were hydrolyzed in the presence

of dilute formic acid and added to a DGEBA resin. By curing with a

stoichiometric amount of MTHPA, in the presence of BDMA, plasticized

epoxy/anhydride networks were obtained without any evidence of phase

separation. These materials exhibit better abrasion resistance and

adhesion than the unmodified epoxy resin.

Hou et synthesized siloxane-epoxy resin which has pendant

epoxy rings on the side chain of the polysiloxane backbone through

hydrosilylation reaction of poly(methylhydrosi1oxane) with ally1 glycidyl

ether. This siloxane-epoxy resin was blended with a commercial

DGEBA-type resin at various ratios, and cured with dicyandiamide

(DICY). DSC studies show that the initial curing temperature and the

peak curing temperature were increased by the addition of siloxane-epoxy

resin to the commercial epoxy resins. SEM studies suggest that siloxane-

rich and DGEBA-rich domains are present in the cured system. This is

due to the difference in the reactivity of siloxane-epoxy resin and

DGEBA resin toward the curing agent.

Wang et al.45 synthesized a new epoxy monomer,

tiglycidyloxyphenyl silane and mixed it with DGEBA resin (Epon 828)

and a curing agenf 4,4'-diaminodiphenylmethane. Triglycidyloxyphenyl-

silane is compatible with DGEBA resin in all proportions. The thermal

stability is higher for triglycidyloxyphenylsilane-modified epoxy resins

than that of the unmodified epoxy resins.

Since both silicone rubbers and epoxy resins are incompatible,

complete separation of silicone rubber from epoxy occurs before curing

which in turn affects the final properties of siloxane-modified epoxy

resins. Therefore, it is necessary to increase the dispersibility of the

silicone rubber in epoxy resins for effective toughening. This can be

aclueved through the use of compatibiliers. Ochi et al.46 used aramid-

siloxane block copolymers as compatibilizers for modification of epoxy

resins with amine-terminated siloxane oligomer. TEM studies of the

modified resins cured with 4,4'-diaminodiphenylmethane reveal that the

siloxane-dispersed phase is covered with the block copolymer. The

toughness of the siloxane-modified system increases considerably with

the increase in dispersibility of the silicone rubber. Improvement in

16

toughness is obtain,ed. for the modified system when compared to the

unmodified systems and this has been attributed to the increase in the area

of the damage zone caused by the formation of the fine silicone phase

facilitated by the aramid-siloxane block copolymers.

Ochi and ~ h i r n a o k a ~ ~ used siloxane-methylmethacrylate copolymer

for dispersing RTV-silicone elastomer as fine particles in a DGEBA resin.

The molecular weight of the siloxane segment and the MMA segment

between the siloxane branches in the graft copolymer strongly affected

the effectiveness of the compatibilizer. Morphological studies suggested

that the graft copolymer was more highly concentrated at the interface

between the silicone-dispersed phase and the epoxy resin. The dimension

of the dispersed phase decreased with a decrease in the interfacial tension,

which was brought about by the addition of the graft copolymer. The

fracture toughness of the modified resins increases with a decrease in the

diameter of the silicone phase.

Multicomponent graft interpenetrating network elastomers

composed of polyurethane, epoxy resin and polysiloxane were prepared

by Sung et a14' by using a sequential technique. Phase separation and the

compatibility could be improved si@cantly by varying

polysiloxanelPU ratio. Broad and high tan 6 were observed from DMA,

suggesting that the multicomponent graft-IPN is an effective absorbing

material,

1.3.3 Epoxy resins modified with thermoplastics

The toughness of cured epoxy resins can be improved by blending

with reactive rubbers such as CTBN and ATBN, and with siloxane

modifiers as discussed in the preceding section. Addition of rubber or

siloxane modifier offers toughening to epoxy resins with less crosslink

density. However, these elastomers are not always effective modifiers for 49.50 improving toughness of highly crosslinked epoxy matrices . Recently,

engineering thermoplastics have gained the attention of researchers as

epoxy modifiers as; they toughen more effectively the highly crosslinked

epoxy resins than the low crosslink density resin^.^^-^* Moreover,

thermoplastic tougheners can maintain the mechanical and thermal

properties of unmodified epoxy resins. Various types of ductile

thermoplastics have been used as alternatives to reactive rubbers for

improving toughness of epoxy resins. Fig.l.3 shows representative

thermoplastic mod:ifiers for the epoxy resins.

Poly(ether sulfone) Poly(pheny1ene oxide) Polycarbonate

Poly(ether ketone) Poly(ether ether ketone)

Fig. 1.3. Thermoplastic rnodljiers for epoxy resins

Polyether sulfones were studied as tougheners for epoxy resins by 51-57 several authors . Raghava 51.52 studied the compatibility of

polyethersulfones, Victrex lOOP (low molecular weight) and Victrex

300P (high molecular weight) and a tetrafunctional epoxy resin (MY-720)

cured with an aromatic anhydride. SEM studies revealed that Victrex

300P-modified epoxy resin gave a higher percentage of large round

particles (1-5 )~m) as compared with agglomerated particles varying in

size from 1 km to 5 pm for Victrex 100P-modified epoxy. Large particles

present in a blend of Victrex 300P and MY-720 exhibited crack arrest

between large precipitated particles. Crack arrest &om Victrex

100P-MY-720 blend was negligible. It was observed that Victrex lOOP

and 300P form solid solutions or molecular entanglements with MY-720

epoxy resin of the continuous phase.

Bucknall and ~ a r t d r i ~ e ~ ~ studied the phase separation behavior of

polyether sulfones dissolved in trifunctional and tetrafunctional epoxy

resins during curing with 4,4'-diaminodiphenyhethane and

dicyandiamide. Despite the varieties of morphologies obtained in

mixtures of polyether sulfones with different hardeners and resins,

modulus and fracture toughness showed little dependence upon

composition. Wang et a1.54 reported structure-property correlation of

polyether sulfone-modified epoxy adhesives. Modified epoxy exhibits

excellent mechanical properties. SEM studies showed a two-phase

structure with a polyether sulfone dispersed phase and an epoxy

continuous phase for lower polyether sulfone content, and two continuous

phases for higher polyether sulfone content.

MacKinnon et al.55 reported the effect of varying the proportion of

polyether sulfone on the cure and the mechanical, dielectric, thermal,

rheological and morphological properties of an aromatic diamine cured

trifunctional aromatic epoxy system. The phase separation of polyether

sulfone from epoxy matrix upon curing is shown to have a pronounced

effect on the mechanical properties of these materials. The microstructure

of two epoxy resins, viz., triglycidyl aminophenol and a diglycidyl ether

of bisphenol-F (DGEBF) cured with 4,4'-diaminodiphenylsulfone and

modified with a reactive group terminated polyether suifone has been

studied by Kinloch et a156. At low concentration of polyether sulfone, a

single-phase microstructure is observed which changes to a particulate

microstructure of the thermoplastic-rich phase in epoxy-rich matrix then

to a co-continuous structure and then to phase inverted form with the

19

increase in the concentration of the added polyether sulfone. Kim et alS7

used hydroxyl-terminated, mine-terminated and non-reactive polyether

sulfones as modifiers for triglycidyl p-aminophenoY4,4'-diarninodiphenyl

sulfone system and studied the morphology development using SEM. In

contrast to the spherical domain structure in a non-reactive polyether

sulfone system, a bicontinuous two-phase structure is obtained when

reactive polyether sulfone is used. This may be due to suppressed

spinodal decomposition (SD) induced by the cure reaction and also due to

the delayed coarsening by the in situ formation of polyether sulfone-

epoxy block copolymer.

End-functionalized polysulfones (PSF) are more effective than 58-59 commercial grades of polyethersulfones. It is observed that when a

DGEBA-4,4'-diaminodiphenylsulfone system was modified with

hydroxyl-terminated polysulfone, the toughness factor increases with the

increase in the molecular weight and the content of polysulfone. SEM

studies of the polysulfone modified epoxy resins showed a particulate

sbucture with PSF-rich particles well dispersed in the epoxy matrix.

Toughening is attained due to the ductile tearing of PSF and the plastic

deformation of the matrix. Amine-terminated PSF is found to be more

effective than hydroxyl-terminated ones.49 Recently, amine-terminated

PSFS~' have been reported as effective modifiers for the Epon 828/4,4'-

DDS system and toughening of this system is attained based on the phase

inverted structure of the modified resins. Tetraglycidyl-terminated PSFS~'

having oxyethylene units was prepared and used as a modifier for the

DGEBA/4,4'-DDS system, but it is less effective than non-reactive PSF,

due to its higher compatibility with the epoxy resin.

lnoue et a1.6'! examined the relation between the curing conditions

and the microstrucl.we of the cured resins obtained from DGEBA/4,4'-

DDSI polyethersulfone blend. It was observed that the morphology of the

20

cured resins depends on the curing conditions, as the phase separation

competes with gelation or vitrification. Polyethersulfone-modified

epoxies have particulate, co-continous, or phase inverted morphologies,

depending on both. the resin compositions and curing conditions.

Triglycidyl aminophenol-4,4'-DDS system was modified with W e -

terminated and epoxy terminated P S F . ~ ~

Polyether ether ketones (PEEK) are also used as modifiers for

epoxy resins but their poor compatibility with epoxy resins demands

solvents llke dichloromethane for proper mixing.49 Fracture toughness

factor kc of Epon 828/4,4'-diaminodiphenyl sulfone system increases

with the increase in molecular weight of PEEK." Recently both

bisphenol-A type and t-butyl-hydroquinone-type PEEKS with terminal

atnine groups were used as modifiers without ~olvents.~' Imidazole unit

containing PEEK also was used as modifiers for liquid tetraglycidyl-4,4'-

diaminodiphenylrnethane (TGDDM)/4,4'-DDS system.66

Polyetherimide [PEI] has been extensively studied as a modifier

for epoxy resin by several authors 49, SO, 67-71 and this is dealt in the epoxy-

imide section (section 1.4).

Apart from. polyetherimide, polysulfone, polyether sulfone,

polyether ether ketone and other thermoplastic modifiers have also been

used with varying degrees of success.49 Polyphenylene oxide (PPO) has

been used by Pearson and yee7' as a modifier for DGEBA resin while

Kim and Robertson 73s74 toughened a DGEBAIDDS system with

poly@utylene terephthalate), nylon 6 and polyvinyledene fluoride.

Polycarbonate was used by Don and ~ e 1 1 ~ ' to toughen DGEBA resin.

Aromatic polyesters having phthalate units were used as modifiers for

DGEBAImethyl hexahydrophthalic anhydride system without any

so~vents'~. Polyethylene phthalate (PEP) and polybutylene phenolate

(PBP) were also used as effective modifiers for epoxy resins.77

21

Though epoxy resins are widely used as adhesives, matrix resins

for composites etc., they often do not retain the properties at elevated

temperatures. Retention of room temperature properties at higher

temperatures is required for certain field applications. Modification of

epoxy resins with thermally stable groups is expected to improve their

heat resistance as well as their mechanical properties at elevated

temperatures. Aromatic polyimides are well known for their thermal

stability. However, they suffer from poor processability. It is expected

that a combination of' polyimides and epoxy resins would give polymers

which combine the easy processability of epoxy resins and high

temperature properties of polyimides. In view of this, in recent years,

researchers have focused their attention on the development of novel

epoxy-imide resins. 67-71, '79-121

Epoxy-imide resins can be obtained by the following three

different methods:- (i) by curing conventional epoxy resins with imide

group containing curatives, (ii) by curing imide group containing epoxies

either with conventional epoxy curatives or with imide group containing

curatives and (iii) by blending epoxy resins with thermoplastic

polyimides or with functionalized polyimides. Epoxy-imide resins

combine the easy processability, low shrinkage upon cure, chemical

resistance and versatility of epoxy resins, and the possible improvement

in high temperature properties due to the presence of thermally stable

imide groups. The obvious advantages of epoxy-imide resins are the use

of cheaper raw materials and easy synthetic procedures. Though the

epoxy-imide resins are somewhat inferior to high temperature polymers

such as polyimides, polybenzimidazole, polybenoxazole etc., in terms of

the high temperature properties, obvious advantages of epoxy resins such

as good adhesion, ease of synthesis and solvent free work-up may

outweigh the sacrifice made in high temperature properties. Epoxy-imides

which are obtained by the three different methods mentioned above are

discussed in some detail in this section.

1.4.1 Epoxy-imide resins from conventional epoxy resins and imide group containing curatives

Ichino and ~ a s u d a ~ ~ synthesized bis(hydroxyphthalimide)s

(Fig. 1.4) from 4-hydroxyphthalic anhydride and diamines such as

4,4'-diaminodlphenylsulphone (DDS), 4,4'-diaminodiphenylether (DDE),

4,4'-diaminodiphenylmethane (DDM) and hexamethylene diamine

(HMD) and used them as curatives for DGEBA resin (Epikote 828). It

was observed that triethylamine promoted the cure reaction. Adhesive

strength of 32 MPa was obtained at room temperature for DGEBA-BHPI

system based on DDS and 47% of the room temperature adhesive

strength was retained at 175OC.

Where R = f CE& Or *+ ; X=S02 ,c f t2 ,0

Fig. 1.4. Structures of bis(hydroxyphtha1imide)s

Adhinarayanan et al.," reported the synthesis of different types of

bis(carboxyphtha1imide)s [BCPIs], (Fig. 1.5.) and their use as curatives

for difunctional epoxy (Araldite GY 250; DGEBA) and polyfunctional

epoxy (Araldite EPN 1138; novolac epoxy) resins. Epoxy-imide resins so

obtained were used as adhesives for bondmg stainless steel substrate. It

was observed that BCPI-EPN 1138 systems gave adhesive strength of 14-

18 MPa at room temperature and retain about 84-100% of the adhesive

strength at 150°C. BCPI-GY 250 systems gave room temperature

adhesive strength of 18-19 MPa. About 7644% of the room temperature

adhesive strength was retained at 150°C. The retention of adhesive

strength at elevated temperature was found to increase with the increase

in imide content. ,411 epoxy-imides were stable upto 370-380°C in

nitrogen atmosphere.

Fig. 1.5. Structure of bis(carboxyphtha1imide)s

Epoxy-imide resins in aqueous emulsions for lamination and

electrodeposition was synthesized by reacting a water dispersed

difunctional epoxy resin with a water soluble salt of an imide compound

containing at least one carboxyl group, in the presence of a crosslinking

agent."

Shau and china2 prepared a novel epoxy-imide polymer by

reacting DGEBA (Epon 828) with monoaminophthalimides [MAPIs].

MAPIs were synthesized from phthalic anhydride and 4,4'-diamino-

diphenylsulfone, 4,4'-diaminodiphenylmethane and 4,4'-diamino-

diphenylether. Epoxy-imide polymers were synthesized by melt curing of

DGEBA with MAPls at 160°C for 2 h and at 200°C for 1.5 h. All these

epoxy-imides were soluble in polar solvents like DMF, DMSO etc.

Thermogravimetric studies showed that the epoxy-imide polymers were

stable up to 360°C. 83-85 Gounder and Jeary reported the synthesis of imide amines and

imide anhydrides which serve as curatives for room temperature curing of

epoxy resins. ~ ~ ~ 8 6 . 8 7 and, Park and ~ang8' reported the synthesis of addition

curable epoxy-hide: resins. Addition-type polyimides, due to their high

crosslink density, are often brittle, resulting in low impact and fracture

toughness. Introduction of a long, flexible epoxy chain in the backbone of

the end-capped imides is expected to reduce crosslink density and also to

improve the fracture toughness by dissipating the impact energy along its

entire molecular chain. Addition-type epoxy-imides were synthesized

through the reaction of N-(3- or 4-carboxypheny1)maleimide or N-(3- or

4-hydroxyphenylmaleimidej or N-(4-carboxypheny1)methyInadimide

with DGEBA resin. These end-capped imides showed excellent

processability while retaining good thermal stability. 89-92 Pate1 and Shah synthesized amine-terminated oligoimides

through the reaction of 4,4'-diaminodiphenylmethane with bismaleimides

synthesized from 1,,4-phenylenediamine, 4,4'-diaminodiphenylmethane,

benzidine and ethylene diamine. These oligoimides were used as

curatives for epoxy resins. Glass fiber composites made from these

epoxy-imides gave flexural strength in the range of 165-308 MPa and

impact strength in the range of 200-2 10 MPa.

1.4.2 Epoxy-imide resins obtained by the modification of backbone of epoxy resin with imide groups

Modification of epoxy backbone by incorporating imide groups is

the second synthetic route for the synthesis of epoxy-imides. Although,

the literature concerning glycidylester compounds obtained by the

condensation of carboxylic acid with epichlorohydrin is quite 93-98 substantial, glycidyl derivatives containing imide group have been

reported only in a few patents99-'0' until Cadiz et aI.'oz published their

work in 1985.

Usually, epoxy resins are synthesized by reacting a

hydroxyl/carboxyl-terminated monomer with epichlorohydrin in the

presence of sodium hydroxide.8'9"' This method is not desirable for

synthesizing imide goup containing epoxies as sodium hydroxide may

open up the imide ring. Cadiz et synthesized diglycidyl ester

derivatives from pyromellitirnide containing diacids in two ways:

(i) using imide carboxylic acid with epichlorohydrin in presence of a

catalyst (quaternary ammonium chloride) and (ii) through the reaction of

epichlorohydnn with sodium salts of imide carboxylic acids previously

obtained in DMF. However, it was observed that the yields obtained by

the second method were better only in a few cases owing to the

insolubility of salts1o2. The reaction scheme for the synthesis of diepoxy

containing pyromellitimide units is given below (Scheme 1.7.):

Scheme. 1. 7. ,Synthesis of diimide-diepoxy resin containing pyromellitimide unit

Following a similar procedure Cadiz et al. 103,104 and other

researchers 10s-107 synthesized a variety of diimide-diglycidylester resins

(Fig. 1.6.).

Where It = -C&-, <a& , 7

Fig. 1.6. Structures of dtfferent diimide-diepoxy resins

These diimide-diglycidyl ester resins can be cured with 108-1 11 conventional epoxy curatives or irnide group containing

curatives 112-1 14 (Schemel.8.) to obtain epoxy-hide cured resins.

CIIr CH- C H r C H - CHI

'0' \

0 + 0 'd

CH- I

OH

Scheme 1.8. Synthesis of epoxy-imide resins from irnide group containing epoxy resin and imide group containing curative

Shau et al. 115-117 used phosphorus containing diamine, triamine and

diacid curing agents (Fig. 1.7.) to improve the flame and thermal

resistance of cured diimide-diepoxy resins.

Bis(3-aminopheny1)methylphosphine 10-phenylphenoxyaphosphine-3,8- dicarbaxylic acid 10-oxide

F I ~ . 1.7. Structures ofphosphorus containing curing agents for epoxy resins

Bikiaris and Karayannidis '06s107 used diimide-glycidyl esters as

chain extenders for poly(ethyleneterephtha1ate) and

poly(buty1eneterephthalate).

Cadiz et a1.1'8 reported the synthesis of diimide-diepoxy resin in

which the glycidyl group is directly attached to N of the imide group

(Fig. 1.8.). Methy ltrimellitimide prepared from trimellitimide and

methanol was reacted with different diols in presence of PbO catalyst to

obtain diimide-diesters which were further reacted with epichlorohydrin

to obtain diimide-diepoxy resins.

Fig. 1.8. Structure oj'diimide-diepoxy resin in which glycidyl group is directly attached to nitrogen

Cadiz et al.Il9 also reported the synthesis of glycidyl derivatives of

nadiimides. Homopolymerization of these monomers was camed out to

obtain polyethers having nadiirnide derivatives as pendant groups

(Fig. 1.9).

Fig. 1.9. Structure rfpolye thers containing nadiimide pendant groups

C!E2 n I

F Where R =

-~00-(Ca&

-coo-(a&

1.4.3. Epoxy-imide resins obtained by blending epoxy resins with polyimides

Modification with thermoplastic polyimides: Addition of a

thermoplastic to a highly crosslinked thermoset often increases the

toughness without sacrificing the inherent properties of the latter.

Bucknall et al.lZ reported that polyetherimide (PEI) can be used as a

toughening agent for tetraglycidyl-4,4'-diaminodiphenylmethane

(TGDDM) cured with diaminodiphenylsulphone (DDS). This PEI forms a

separate phase with dynamic loss peak in the temperature range from 200

to 212OC. Different types of blends were prepared by Hourston and

~ a n e " " ~ adding PEI in varying proportions to a trifunctional epoxy

resin viz., triglycidyl-p-aminophenol cured with DDS. Dynamic

mechanical analysis (DMA) and SEM studies of the blends showed a

two-phase morphology. Addition of PEI has improved the fracture

properties of the epoxy resin.

Li et observed that the molecular weight of PEI plays an

important role in the morphology of the modified system. With the

increase in PEI molecular weight, the morphology of the modified system

changes from a PEI spherical domain dispersed in the epoxy matrix to a

phase inverted epoxy domain dispersed in the PEI matix. In order to

study the mechanism of formation of morphology, Li et al.70a studied the

morphology transformation on curing of epoxy-PEI blend. Two different

molecular weight phenyl-terminated PEIs having inherent viscosity 0.39

dL/g and 0.61 dL/g and 4.4'-DDS curing agent were used for the study.

The phase morphology of pure epoxy, epoxy1PEI (EIP) blend (0.39) and

E/P blend (0.61) was studied by SEM as a function of time at 150°C.

Noticeable feature was not observed in the pure epoxy resin irrespective

of the cure time whereas for E/P (0.39), the phase separation started after

15 minutes of curing at 150°C and at the end of curing, the PEI rich

domain clearly exrsted as fine dispersed particles surrounded by the

continuous epoxy phase. For E/P (0.61) system, phase separation took

place before 2 minute of curing. SEM of E/P (0.61) showed that after 30

minute curing, PEI rich domain (large continuous domain) is full of

epoxy particles (smooth and darker region) surrounded by a continuous

PEI domain. The phase separation of EIP (0.39) is in accordance with the

spinodal decomposition (SD) mechanism whereas for E/P (0.61) the large

co-continuous phases are not formed via SD mechanism.

The solubility of thermoplastic polyimide in epoxy resins is crucial

to obtain a homogeneous mixture before curing. The structure

dependence of these polyimides on solubility, thermal and mechanical

properties was also investigated by Li et aL7'

Sautereau et a1.68 reported the toughening of an epoxy resin using a

non-fimctionalized polyetherimide. It was observed that the pre-curing

temperature had a strong effect on the morphology of the epoxy, because

of the viscosity change at the cloud point and the extent of the separation

process. DMA showed two peaks for blends which indicates the phase

separation of epoxy-rich phase and polyetherimide-rich phase. The ratio

of the height of loss peaks corresponding to each peak is found to be an

important parameter in predicting the final morphology of the system.

Fracture toughness of blends was found to increase significantly only

when bicontinuous or inverted structure was generated.

Biolley et al.Iz4 prepared a hot melt processable thermoset by

blending an epoxy resin viz., tetraglycidyl-4,4'-diaminodiphenyl-

methane/4,4'-DDS with a high T, thermoplastic polylmide synthesized

from 3,3',4,4'-benzophenone tetracarboxylic dianhydride and 4,4'-(9H-

fluoren-9-y1idene)bisphenylamine. The addition

crosslinking density of the system. No phase sep K-'- ' M . the cured epoxy system. Only one Tg was obsewJ&k this blen3, whchj, i \: *,:,\.-

3 1 , . - .- .. x.. . ... - .--

confirms the complete miscibility of polyimide and epoxy resins. Blend

exhibited a slight improvement in their stress at rupture and strain-energy

release rate when compared to those of unmodified epoxy matrix. A

difunctional epoxy resin, MY 750 (DGEBA) cured with DDS was

toughened with polyimides derived from bisaniline~'~~. Toughening

efficiency was different for different polyimides. Greater toughening

effect was observed for polylmides derived from rigid dianhydrides than

from flexlble dianhydride.

Modification with functionalized thermoplastic polyimide: The

desired morphology for the toughening of thermosetting polymers with

thermoplastics is a two-phase morphology with strong interactions at the

interface. In thls incompatible blend, the interaction between the matrix

and the occluded phases could be strengthened by appropriate chemical

modification of the thermoplastic polyimide. The effect of functional

soups of polylmide toughener on the fracture toughness of epoxy resins 67,126 has been investigated by Shin and Jang and by Chen et al.Iz7

PEI having amine functional groups was used as a toughening

agent6' for N, N, N', N'-tetraglycidyl-a,a'-bis(4-aminophenyl)-p-diiso-

propylbenzenela,a'-bis(4-aminophenyl)p-diisopropylbenzene system.

The toughness of the epoxy resinslthermoplastic blend strongly depends

on the phase separation of the cured epoxy resin. In PEI-toughened

epoxy, finely dispersed PEI domains in epoxy make the crack propagation

path more complex which in turn increases the fracture toughness of the

system. But in the case of aminated PEI-epoxy, along with the crack

propagation of PEI domain, the ductile tearing of PEI facilitated by the

strengthened-domain interfacial adhesion also played a role in increasing

the fracture toughness. T, is higher for aminated PEI-epoxy when

compared to that of PEl-epoxy.

Improved interfacial adhesion of the modified epoxy resin was

obtained through the hydrolysis of ~ ~ 1 . l ~ ~ At a concentration of 5 wt%

PEI, hydrolyzed PEUepoxy resin showed an improvement in fracture

toughness by 40% when compared to the unhydrolyzed PEUepoxy resin.

Chen et al.'27chemically modified a commercially available

polyetherimide to a nitrated polyetherimide (NI-PEI). NI-PEI and

NI-PEVPEI mixtures were blended with an epoxy resin (MY 05 10) which

was cured with 3,3'-diaminodiphenyl sulfone. DMA studies and SEM

analysis suggest that a two-phase morphology exists with substantial

toughening when 10 &XI of NI-PEI is used as a modifier for epoxy

resins.

In a novel approach, Agag and ~akeichi '~' used polyimide

containing hydroxyl functional groups as a curative for DGEBA resin.

The hydroxyl functional polyimide was synthesized by reacting 3,3'-

diamino-4,4'-dihydroxybiphenyl and 2,2-bis(3,4-dicarboxypheny1)-

hexafluoropropane dianhydride in N-methyl-2-pyrrolidone (NMP)

followed by thermal imidization. The epoxy-polyimide solution in NMP

was cast into a film followed by drying at 50°C for 16 h. The cast film

was thermally cured at 100°C for 1 h, 200°C for 1 h and then at 250°C for

3 h. It was observed that the tensile strength of polyimide modified

epoxy increases with increase in imide content. Viscoelastic analyses of

polylmide-epoxy showed that the glass transition temperature shifted to

higher temperature with the increase of polyimide content. Polyimide-

epoxy films have excellent solvent resistance and are thermally more

stable.

Modification with polyamic a'cid: Kakimoto et al. 129-131

synthesized polyamic acid (PAA) from oxydianiline and pyromellitic

dianhydride in tetrahydrofuran and used it as a curative for diepoxy

(DGEBA) to obtain epoxyIPI semi- interpenetrating networks. DGEBA

was mixed with P h i in THFImethanol mixture and applied on a copper

backed Kapton film and cured under pressure at 125OC for 1 h and at

250°C for 2 h to simultaneously cure the epoxy and imidize the PAA in

the film. About 1230 g/cm adhesive peel strength (180" peel) was

obtained for epoxy-polyimide composition of 90: 10. The addition of even

small amounts of hardener, pyromellitic anhydride resulted in the large

improvement in adhesive strength. Morphological studies using SEM

revealed that in the systems cured by only PAA, the mixtures did not

phase separate and formed molecularly interlocked interpenetrating

network^.'^' On the other hand, when additional monomeric hardener

such as PMDA was used along with PAA, phase segregated domains

were observed. The inhomogeneities increased in size with increase in

PMDA content. The reaction rate of PMDA-DGEBA reaction is

relatively higher than that of PAA-DGEBA. The above difference in the

reaction rates causes the formation of crosslinked epoxy phase due to

PMDA-DGEBA reaction with vely little epoxy ring opening occuring

due to PAA-DGEBA reaction. Thus the difference in the reaction rates

cause phase separation to occur in the early stages of cure, with the

morphologies of the systems being determined at low degrees of

conversion.

Polyimide-epoxy films were synthesized by Nema et by reacting PAA with DGEBA. These films exhibited lower water intake and

retained its excellent macroscopic properties.

Modification with bismaleimides: In an attempt to improve the

high temperature performance of epoxy resins bismaleirnides were

blended with epoxy resins and curatives, and co-cured. 133,134 The

following five reactions can take place: (i) epoxy + bismaleimide adduct

formation, (ii) epoxy + amine addition and crosslinking, (iii)

bismaleimide + amine-extended bismaleimide, (iv) amine-extended

bismaleimide + epoxy resin - crosslinking and (v) self-polymerization of

bismaleimide.

Musto et al.69 investigated the blend system in

TGDDM/diaminodiphenylsulfone (DDS) resin was mixed with a

bismaleimide viz., N,N'-bismaleimido-4,4'-diphenylmethane monomer

and cured. The kinetics and mechanism of the curing process as

investigated by FTIK reveal the following: (i) two different molecular

networks are formed during the process: the f is t due to the bismaleimide

homopolymerization and the second due to the crosslinking of the

TGDDWDDS pair and these two networks possess a substantial degree

of interpenetration, (ii) the bismaleimide network grows at a higher rate

than that formed by the TGDDhUDDS pair and (iii) the bismaleimide

network is likely to be defective as the final conversion of bismaleimide

does not exceed 70%. The presence of BMI, through a molecular mobility

effect, enhances the formation of small cyclic structures in the

TGDDWDDS network, thus lowering its crosslink density.

Saraf et investigated the curing of DGEBA (Araldite LY

556)- 4,4'-diaminodiphenylmethaneI4,4'-diaminodipheny1~~1fone system

and bismaleimide blends. Thermal, thermomechanical and SEM studies

reveal the formation of interpenetrating networks.

Kim and ~ a ~ n ' ~ ' investigated the cure characteristics of a mixture

of novolac epoxy resin, phenol-novolac resin, bismaleimide and

diaminodiphenylmethane. It was observed that diaminodiphenylmethane

is more reactive with bismaleimide than with epoxy resin and phenol

novolac resin is more reactive with epoxy resin than with bismaleimide.

Generally, the T, of the epoxy-bismaleimide resin system increases with

the content of bismaleimide irrespective of the extent of conversion.

35

Woo et al.136 investigated intercrosslinked networks formed by co-

curing two thermosets, viz., TGDDM/4,4'-diaminodiphenylsulfone and

bismaleimides derived from 4,4'-diaminodiphenylmethane and

1,3-tolylenediamine. No phase separation was observed in the

intercrosslinked network system and the high temperature properties of

these systems are between those provided by the individual components.

Chandra et studied the cure kinetics of DGEBA resin and

4,4'-bismaleimidodiphenylmethane using different amine curing agents

and it was observed that the cure kinetics follow the autocatalytic kinetic

first order reactions.

Li-Nan and ~ i - ~ e n ' ~ ' studied the morphological changes in the

curing process of DGEBA resin/4,4'-diaminodiphenylsulfone ' and

4,4'-bismaleimidodiphenylmethane. TEM results confirmed the formation

of interpenetrating network of BMI and epoxy resin. The degree of

interpenetration is cor~trolled by the first stage cure temperature.

Modification with maleimide containing novolac resins: Gouri

et a1.139 cured Araldite GY 250 and Araldite EPN 1139 with maleimide-

novolac resins. The adhesive properties and thermal stability of these

systems were superior to those of self-cured maleimide-novolac resins.

1.5 End-uses of modified epoxy resins

Epoxy resins modified with rubber toughening agents, siloxane

modifiers, thermoplastics and polyimides find wide ranging applications

in aerospace, electronics and adhesive industries. Rubber toughened

epoxy resins are used as engineering adhesives. 11,30,140 Epoxy resins

modified with polydimethylsiloxanes are used as encapsulants for 41.42 integrated circuits. Hydrolyzed alkoxysilane functionalized epoxy

resins are used as coatings possessing good abrasion re~ is tance .~~

Polysulfone modified epoxy resins find applications as adhesives,

coatings and composites. 49.50.54

Epoxy-hide resins are used as matrix resins for composites 89-92,121

which find applications in the field of aerospace and microelectr~nics.~~~

They are also used as organic insulators, padding and encapsulating

compounds and as adhesives. 79,80,105,141 Polyimide-epoxy alloys are used

for making films having good tensile strength, low water absorption and

high diffusion coeffi~ient. '~~ Phosphorous containing epoxy-imides are

used as flame-retardant materials. 115-117

1.6 Objective and scope of the present investigation

The survey of literature on the modification of epoxy resins

suggests that considerable attention has been paid toward modlfylng

epoxy resins with thermally stable imide groups. These resins are

synthesized by three different methods as discussed in the previous

section. The published literature reveals that the attention is mainly

focused on the synthesis and thermal properties of epoxy-hide resins and

limited information is available on their end-use as adhesives, coatings

and matrix resins for composites.

The present investigation aims at developing epoxy-imide resins

mainly through the reaction of epoxy resins with imide-diacidsldiimide-

diacids and siloxane linkage containing diimide-diacid, and evaluating

their end-uses as adhesives, coatings and matrix resins for composites.

In the present study, three types of epoxy resins, viz., a

difunctional epoxy .resin (Araldite GY 250), a polyfunctional epoxy resin

(Araldite EPN 1138) and an aliphatic internal epoxy resin [epoxidized

hydroxyl-terminated polybutadiene (EHTPB)] derived from hydroxyl-

terminated polybutadiene (HTPB) have been used. In order to understand

the structure-property correlations, in addition to varying the nature of

37

epoxy resin, it is also essential to vary the type of imide group containing

curatives. For t h~s purpose, imide-diacids such as N-[4- and

3-(carboxy)phenyl]trimellitimides were synthesized from 4- and

3-aminobenzoic acids. N-[4- and 3-(carboxy)phenyl]trimellitimides were

selected in order to study the influence of position of carboxyl group on

the cure reaction and on the mechanical and thermal properties. The

curing of epoxy resins with imide-diacids proceeds through carboxyl-

epoxy addition reaction and the cure reaction was followed by DSC and

IR . The end-use of the epoxy-imide resins as adhesives for stainless steel

substrate has been extensively investigated. The effect of variation of

imide-diacid to epoxy ratio (which in turn influences the imide content

and aromatic content) on the adhesive lap shear strength at room

temperature and at 100, 125 and 150°C has been studied. Attempts have

been made to correlate the adhesive properties with the structure of

epoxy-imide resins. The effect of variation of imide-diacid to epoxy ratio

on the thermal properties has also been studied.

Literature survey reveals that only limited information is available

on the use of diimide-diacids as curatives for epoxy resins. When

diimide-diacids are used as curatives instead of imide-diacids, the imide

and aromatic content can be increased considerably. In addition to this,

the availability of a wide variety of diarnines provides the scope for the

synthesis of different types of diimide-diacids. It is expected that a

detailed study on the thermal and mechanical properties of epoxy-imides

obtained from various diimide-diacids will be useful in arriving at

structure-property correlations. In the present study, epoxy-imide resins

were synthesized from Araldite GY 250 and Araldite EPN 1138 and 2,2-

bis[4(4-trimellitimidophenoxy)phenyl]propane (DIDA-V). DIDA-V

which contains two ether linkages was synthesized from 2,2-bis[4-(4-

aminophenoxy)phenyl]propane. The adhesive and thermal properties of

38

epoxy-imide resins for different compositions of epoxy resin and

DIDA-V have also been studied. The adhesive and thermal properties of

the above epoxy-imide resins were compared with those of the epoxy-

imide resins based on other diimide-diacids synthesized from different

aromatic and aliphatic diamines. Feasibility of using Araldite

GY 250-IAraldite EPN 1138-DIDA-V system as a matrix resin for

composite has also been studied.

Epoxy-imide resins, by and large, are brittle in nature and hence,

attempts have been made to toughen them by blending with reactive

liquid rubbers such as EHTPB and liquid carboxyl-terminated butadiene

acrylonitrile copolymer (CTBN-L), and solid carboxyl-terminated

butadiene aclylonitrile copolymer (CTBN-S). The adhesive and thermal

properties of the rubber-modified systems have been studied and

compared with the unmodified epoxy-imides. The morphology of

unmohfied and rubber-modified epoxy-imide systems were studied by

SEM.

It is known that the presence of siloxane linkage in a polymer

backbone imparts flexibility to the system. Thus, it would be of interest to

incorporate siloxane linkage in epoxy-imide systems which are brittle in

nature. It is reported that the siloxane-imides are synthesized through the

reaction of siloxane containing amine with aromatic dianhydrides142-'" or

through the reaction of siloxane containing dianhydrides with aromatic

diamines followed by chemical irnidization. 145.146 In the present

investigation, attempts have been made to synthesize siloxane-imides

using the synthetic approach adopted for the preparation of epoxy-imides.

Thus, siloxane-imides containing ester linkages or in other words, epoxy-

imide resins containing siloxane linkages were synthesized by the

following three different approaches: (i) by curing siloxane-epoxy resin

with diimide-diacid, (ii) by curing Araldite GY 250 with siloxane

39

containing diimide-diacid and (iii) by curing siloxane-epoxy with

siloxane containing diimide-diacid. The adhesive and thermal properties

of the above systems have been studied.

Survey of literature reveals that imide-diacids and diimide-diacids

on epoxidation with epichlorohydrin readily give epoxy resins containing

preformed imide lmkagge. 102-104, I18 In the present investigation, the above

synthetic approach has been adopted to synthesize a novel epoxy resin

having preformed imide and siloxane linkages. The advantage of this

approach is that siloxane linkage, imide linkage and epoxy functional

groups are present in. the same molecule. The presence of imide linkages

toughen the system, siloxane llnkages offer flexibility to the system and

epoxy groups offer polymerization/crosslinking sites. Siloxane linkages

present in the resin, in addition to imparting flexibility to the system, may

also impart atomic oxygen (AO) resistance. A 0 formed by the

photodissociation of' molecular oxygen in the upper atmosphere is a

severe threat to spacecraft materials placed in low earth orbit. 147-150

Hence, they require protective coatings. Poly(carborane-siloxane)~, 151,152

decaborane-based polymers, 148,149,153 polysiloxanes,154 siloxane- epoxy, 1". 156 polysilanes,'57 polycarbosilanes,157 phosphorus containing

polymers 158-160 and siloxane-imidesi6' were reported as A 0 resistant

coatings. In the present work, attempts have been made to evaluate

siloxane-imide-epoxy resin as A 0 resistant coatings. This resin has been

suitably modified by reacting with siloxane-epoxy, amine-terminated

siloxane resin and siloxane containing diamine and the modified resin has

been evaluated as A 0 resistant coatings for substrates such as polyimide

film, carbon-polyimide composite and glass-polyimide composite which

are susceptible to A 0 attack. The coated and uncoated samples were

exposed to A 0 in a plasma barrel system and the mass loss of the samples

at regular time intervals was measured to understand the protective ability

of the coatings. SEM analysis of the uncoated and coated samples after

exposure to A 0 has also been carried out to understand the changes that

take place on the substrates and the coating on exposure to AO.

In the present investigation, attempts have also been made to

synthesize addition-type epoxy-imides. Addition-type imides, in general,

are brittle in nature due to excessive crosslinking. In an attempt to reduce

the extent of crosslinking, structural units containing amide-imide'62, 86,87 ester-imide and siloxane-imide14' linkages were incorporated between

the crosslinked sites. A promising approach in the above direction would

be to incorporate structural units derived from a siloxane epoxy viz., 1,3-

bis(3-glycidyloxypropyl)tetramethyldisiloxane. Addition-type epoxy-

imides have been synthesized by two different synthetic approaches. In

the fust approach, adhtion-type epoxy-irnides have been synthesized

through the reaction of the above epoxy resins with monoitaconamic

acids derived fiom aromatic diamines where the reaction proceeds

through epoxy-amine reaction. In the second approach, they have been

synthesized through the reaction of epoxy resins with maleimido benzoic

acids, where the reaction proceeds through the carboxyl-epoxy addition

reaction. These addition-type epoxy-hides have been converted to

crosslinked matrices by thermal polymerization. The adhesive and

thermal properties of these systems have been evaluated.