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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.