Chapter 1

Introduction

A general description about epoxy resins and their properties are summarised in

this chapter. The important curing agents and modifiers for epoxy resin are

discussed. Elastomers and thermoplastics are effective in enhancing the fracture

toughness of epoxy resins. Thermoplastics are the choice in demanding

conditions. Phase separated blends were found to be more effectlve in improving

the fracture toughness of epoxy resins. Phase separation occurs by either

nucleation and growth or spinodal decomposition mechanism. The ultimate

properties were dependent on the curing agent and curing conditions, amount and

type of modifier and interaction between the blend components. Various

toughening mechanisms in rubber and thermoplastic toughened epoxy resins are

also discussed. The importance and main objectives of the present study are

described towards the end of the chapter.

2 Chapter 1

P olymer blending has gained lot of attention as an easy and cost effective way

to develop materials having desired properties. The ultimate properties of the

blends can be manipulated according to the end use by the proper selection of

component polymers. A range of polymer blends like thermoplastidthermoplastic,

thermoplastiirubber, rubberlrubber, therrnosetlrubber and thermosetlthermoplastic

blends are available for a variety of applications. Multicomponent systems based

on thermoseVthermoplastic blends have attractive properties which made them

useful for structural applications.

Thermosetting materials can be defined as those materials, which change

irreversibly under the influence of heat from a fusible and soluble material to

one which is insoluble and infusible through the formation of a covalently

crosslinked and thermally stable network. During crosslinking or curing

reaction, the reactive monomers grow in molecular weight to form branching

structure and eventually a three dimensional network structure is formed.' The

most important thermosetting resins include phenolics, amino resins,

unsaturated polyesters urethane foams and epoxy resins. Among these,

epoxy resins are the most versatile.

1.1 Epoxy resins

Epoxy resins were generally accepted as workhorse raw material among the

various thermosetting resins due to their outstanding mechanical properties

and good handling characteristic^.^^^ They were made available commercially

in 1946. They are now used for a variety of applications from coatings to

military and aerospace applications. These resins are characterised by high

chemical and corrosion resistance, good mechanical and thermal properties,

outstanding adhesion to various substrates, low shrinkage upon cure, flexibility

in processing, good electrical properties and ability to be processed under a

variety of conditions6

Epoxy reslns are molecules conta~n~ng more than one a-epoxy group capable of

belng converted to a useful thermoset form They are character~sed by a three

membered nng known as epoxy, epox~de, oxlrane or ethoxylene group conslstlng of

an oxygen atom attached to two Interconnected carbon atoms as shown in Fig 1 1

Figure 1.1: Epoxy group

The term epoxy resin is applied to both the prepolymers and the cured resins.

Epoxide group is usually a 1,2- or a-epoxide that appears in the form.

0 / \ - CH-, - CH - CH2

, called the glycidyl group, which is attached to the rest of

the molecule by an oxygen, nitrogen or carboxyl linkage and is termed as glycidyl

ether, glycidyl amine or glycidyl ester respectively.

Epoxy resins are available in various physical forms ranging from low viscosity

liquids to tack free solids. The most commonly used epoxy resin is diglycidyl ether

of bisphenol-A (DGEBA). Other aromatic glycidyl ether resins, which gained

commercial importance are epoxy phenol novolac (EPN) and epoxy cresol novolac

(ECN) resins. Due to high functionality, phenolic resins have high crosslink density

and better thermal and chemical resistance than bisphenol-A resins. Multifunctional

glycidyl amine resins, which gained commercial importance are triglycidyl-p-

aminophenol (TGAP) and tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM).

TGDDM is used as binders in graphite reinforced composites and for military

applications.

1.2 Curing of epoxy resins

The ultimate properties of epoxy resin are achieved by crosslinking it into insoluble

and infusible network. This is made possible by using chemical compounds known

as curing agents or hardeners. The chemistry of cure begins by the formation and

linear growth of a chain that soon begins to branch and then to crosslink As the

curing reaction proceeds, the molecular weight increases rapidly and eventually

becomes linked together into networks of infinite molecular weight This sudden

and irreversible transformation from viscous to elastic gel. is called the gel point.

Gelation is critical in polymer processing since the polymer does not flow and is no

longer processable beyond this point. Gelation is dependent on the functionality,

reactivity and stoichiometry of the reactants and it does not inhibit the curing

reaction. Beyond gel point, reaction proceeds towards the formation of one infinite

network possessing the dimension of the reaction vessel with substantial increase

in crosslink density, glass transition temperature (T,) and ultimate physical

properties. This phenomenon that occurs during curing is called the vitrification of

the growing chain or network. The transformation from viscous liquid or elastic gel

to a glass begins to occur as the T, of these growing chains or network becomes

equal to the curing temperature. The onset of vitrification causes a shift from

chemical control to diffusion control of reaction and in practical cases vitrification

brings an abrupt halt to curing reaction. Vitrification is a reversible process and cure

may be resumed by heating to devitrify the partially cured thermoset. Various

phenomena occurring during curing are illustrated in the time-temperature-

transformatii (m) diagram given in Fig. 1.2. In the diagram the times to gelation

and vitrification are plolted as a function of the isothermal cure temperature. The S-

shaped gelation and vitrification curves divide the TTT diagram into four distinct

stages of the thermosetting curing process: liquid, gelled rubber, ungelled glass and

gelled glass. At temperatures below the T, of unreacted resin (T,,), reaction is

confined to the solid state and is therefore very slow to occur. T,, defines the storage

temperature for unreacted resins. The resin will react above Tm Between Tm and gel

T,, the liquid will react until its continuously rising T, becomes equal to the cure

temperature at which stage vitrification will commence and the reaction becomes

diffusion controlled and is eventually quenched when vitrification is complete. Between

gel T, and glass transition temperature of fully cured epoxy resin (T,.,), gelation

precedes vitrification and a crosslinked rubbery network forms and grows until its T,

coincides with the cure temperature. At this stage the reaction will be quenched. T,

represents the minimum cure temperature required to acquire complete cure. Above

this temperature, the thermoset will remain in the rubbery state after gelation unless

other reactions like oxidative crosslinking or chain scission occur The handling,

processing and development of ultimate properties are dependent on gelat~on and

vitrification.

Figure 1.2: Time-temperature-transformation diagram (L. V. Mc Adams J. A Gannon, Encyclopedia of Polymer Science and Engineering, Vol. 6, 2"* Edn., Wiley Interscience, 1986)

A schematic representation of the physical changes during curing reaction of

epoxy resin is given in Fig. 1.3

Gel

Monomers 0 a-

L~quid Glass

Viscosity Increase Gelallon Virificatian

Figure 1.3: Schematic representation of the physical changes occurring during the curing of epoxy resin

6 Chapter 1

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

can react with many nucleophilic and electrophilic reagents. Compounds with

active hydrogen atoms such as arnines, phenols, alcohols, thiols, carboxylic acids

and acid anhydrides are widely used as curatives due to their workability and

availability. The chemical reactions between epoxy resin and various curing

agents are shown in Scheme 1.1

+ -R-W2 I I

HO-C-C-NHR-

amine I I

0 / \ I I

- c - C - + -R-COOH ---cI tD-C-C-COOR-

1 I acid I I

+ -- R - SH I I - HO-C-C-SR-

ttid 1 I

Scheme 1.1: Reactions between epoxy and different curing agents

Today, a wide range of chemical compounds are available for curing epoxy resins.

This is mainly due to hyo reasons. During the course of time, novel epoxy resins

were developed to meet the requirements for specific applications. As a result,

many organic compounds other than the traditional ones' could be used as curing

agents. Secondly, depending on the end use, there is a growing need to modify

the presently available curing agents. Curing agents for epoxy resins are broadly

classified Into two, namely catalytic and coreactive curing agents.

1.2.1 Catalytic curing agents

Catalytic curing agents act as initiator for epoxy homopolymerisation, as a

supplemental curing agent with polyamines or polyamides or as accelerators

along with anhydrides. The rate of reaction is low at room temperature. Hence

these are mainly used as accelerators for other curing agents. Lewis bases like

benzyldimethylamine (BDMA), 2.4,6-tris(diniethylaminomethy1 phenol) and Lewis

acids like borontrifluoride monoethylamine are examples for catalytic curing

agents. Another rapidly growing method is photoinitiated cationic cure.' The

catalysts include aryldiazonium, diaryliodonium and onium salts of group Vla

elements.

1.2.2 Coreactive curing agents

Coreactive curing agents act as a comonomer in the polymerisation process. A

variety of chemically reactive compounds are used as coreactive curing agents

Amines, mercaptans, isocyanates, melamine, urea and phenol-formaldehyde

resins, carboxylic acids, acid anhydrides etc. were used as coreactive curing

agents.

1.3 Amine curing of epoxy resin

Amines are the most extensively used curing agents for epoxy resin. A range of

aromatic and aliphatic amines were used for curing epoxy resins. The ultimate

properties of the resin were dependent on the nature of the amine used. Aliphat~c

amine offers ambient cure temperature and low viscosity. Aromatic amine cured

resins have good elevated temperature properties, good chemical resistance, long

pot life and low molsture absorption. The reaction between epoxy resin and an

amine curing agent is given in Scheme 1.2.

8 Chapter 1

Scheme 1.2: Reaction between amine curing agent and epoxy resin

Reaction of primary amine with epoxy yields a secondary amine and secondary

hydroxyl group. The secondary amine in turn reacts with an epoxy to give a

tertiary amine and two secondary hydroxyl groups. The tendency of the

etherification reaction to take place depends on the temperature8,' and the

basicity1' of the diamine and increases with the initial epoxylamine ratio.'

A variety of techniques such as differential scanning calorimetry (DSC),"."

thermal scanning rheometry,lg dielectric spectroscopy,20 Raman s p e c t r o s ~ o p y ~ ' ~ ~ ~

and fourier transform infrared spectroscopy (FTIR)~'~' have been used to monitor

the curing reaction of epoxy resins. The kinetics of curing reaction has also been

studied in detail. Epoxy-amine curing reaction was found to follow autocatalytic

mechanism.2628 The curing kinetics was found to be influenced by the changes in

the chemical structure of curing agents2' The chemical structure of a series of

amine curing agents are given in Fig. 1.4.

introduction 9

MDEA MCDEA

BAPS

BAPP

Figure 1.4: Chemical structures of amine curing agents

Grillet et found that the reactivity of amines varied in the order

DDM>BAPP>BAPS>3,3'-DDS>4,4'-DDS, from size exclusion chromatography and

viscosity measurements, where DDM. BAPP, BAPS. 33'-DDS and 4,4'-DDS

represent 4.4'-diaminodiphenylmethane, 2,2'-bis[4-(4-aminophenoxy)phenyl]propane,

bis[4-(4-aminophenoxy)phenyl]sulfone. 3,3'-diaminodiphenylsulfone and

4.4'-diaminodiphenylsulfone respectively. Basicity of the amines determined their

reactivity. In DDS, sulfone group being an electron withdrawing group, 'decreased

the basicity of the diamine. 4,4'-DDS was least reactive because the electron

10 Chapter I

transfer from -NH2 to -SO2 1s the easiest for para substitution. BAPP and BAPS

are more reactive than DDS and less reactive than DDM due to the action of

phenoxy group. In another study, Reydet et a13' found that the reactivity of amine

decreases in the order MDA>MDEA>DDS>MCDEA for DGEBA epoxy resin. MDA,

MDEA and MCDEA exhibited same secondary to primary amine reactivity ratio of

0.65 but DDS exhibited a different value (0.45). The lower value may be due to the

lower number of stable conformat~ons afforded by -SO2 group in comparison to

-CHZ group.

MDA - 4,4'-methylenedianiline

MDEA - 4,4'-methylenebis(2,6-diethylaniline)

DDS - 4,4'diaminodiphenylsulfone

MCDEA - 4.4'-methylenebis(3-chloro-2,6-diethylaniline)

1.4 Properties of cured epoxy resin

Epoxy resins, once cured have highly crosslinked structures. Because of this

highly crosslinked structure they have good mechanical and thermal properties

and high modulus. In addition they have very good adhesive properties. The

properties of the cured resins are dependent on the curing agent. Amine cured

epoxy resins have excellent electrical properties coupled with excellent chemical

resistance and relatively high heat distortion temperature (HDT). Epoxy resins

cured with anhydrides have excellent properties with better outdoor weathering

resistance. Some of the anhydride cured resins have high HDT and excellent

retention of strength at high temperature. These systems have low viscosity and

long pot life

1.5 Additives and diluents for epoxy resin

In addition to curing agents, numerous other materials are added to epoxy resin to

modify the properties of the resin both in the cured and uncured states. These

materials help to develop desirable properties in the cured resin or improve the

processability of the uncured resin.32

1.5.1 Diluents

Diluents are usually employed as a means of reducing viscosity in order to aid

general processability as well as allowing for greater incorporation of other

formulatory ingredients such as fillers and as a means of improving characteristics

such as wetting and the incorporation of resins into various fibrous reinforcements.

It is also noted that other properties will be also modified. The extent of

modification depends on the type and quantity of diluent used. Diluents for epoxy

resin are classified into reactive and non-reactive diluents.

1.5. I . I Reactive diluents

Two major classes of reactive diluents are available. These are epoxy based and

non-epoxy based diluents. Epoxy based diluents constitute mono-epoxy

compounds and poly-functional epoxies. In both the cases the diluent becomes

part of the cured network as a result of the reaction with curing agent. Mono-epoxy

compounds incorporated into epoxy resin reduced the viscosity as well as the

functionality of the reacting system. This will reduce the crosslink density of the

cured resin and consequently the high temperature properties are affected.

Important mono-epoxides are octylene ox~de, p-butyl phenyl glycidyl ether, butyl

glycidyl ether, cresyl glycidyl ether, styrene oxide, phenyl glycidyl ether etc.

Polyfunctional epoxy diluents are used to retain the physicallmechanical

properties of the cured resins. Since the functionality of the resin was not much

affected, crosslink density as well as the ultimate properties did not show

deterioration. Typical polyfunctional epoxy diluents are butadiene dioxide,

divinylbenzene dioxide, diglycidyl ether, butanediol diglycidyl ether, limonene

oxide and vinylcyclohexene dioxide. Important non-epoxy based reactive diluents

are triphenyl phosphate and lactone compounds such as butyrolactone. These

materials have effective viscosity reducing characteristics but frequently at the

expense of elevated temperature capability.

1.5.1.2 Non-reactive diluents

These materials are used to reduce the viscosity of epoxy resin. Examples are

toluene, xylene, dibutyl phthalate, styrene etc. Addition of these materials

12 Chapter 1

decreased the viscosity of DGEBA epoxy and also increased the extent of curing

reaction because of the better mobility of the reacting species. However, at higher

concentrations, the properties of the cured resin decreased and have low

chemical resistance.

1.5.2 Fillers

Fillers are the most common formulatory ingredients employed in majority of

epoxy formulations. In addition to reducing the cost, fillers are added to modify the

properties and characteristics of epoxy. The major drawback is the increase in

viscosity which drastically affects the processability of the resin. Table 1.1

summarise the advantages and disadvantages of fillers.

The important fillers used in epoxy technology are aluminium, alumina, aluminium

silicate, aluminium trioxide, arsenic pentoxide, barium sulphate, beryllium oxide.

calcium carbonate, calcium sulphate, calcium silicate, carbon black, copper.

colloidal silica, fibrous glass, graphite, glass microballoons, kaolin clay, lithium

aluminium silicate, mica, molybdenum disulphide, quartz, sand, silica, silver.

titanium dioxide, talc and zircon~um silicate. Each of these materials is used for

specific property improvement.

Advantages

Reduced formulation cost

Reduced shrinkage

Improved toughness

Improved abrasion resistance

Reduced water absorption

lncreased heat deflection temperature

Decreased exotherm

lncreased thermal conductivity

Reduced thermal expansion coefficient

Disadvantages

Increased weight

Increased viscosity

Machining difficulties

Increased dielectric constant

Table 1.1: Advantages and disadvantages of filler incorporation

The addition of particulate fillers generally reduces the tensile and flexural

properties but substantially enhanced the modulus depending on the filler type

and loading. Exception is the additlon of aluminium or alumina, which increased 33 - the strength of structural adhesive formulations. F~llers generally did not provide

enhancement in T, or other measures of high temperature distortion. Incorporation

of fillers substantially diminishes the exothermic heat generation by reducing the

quantity of epoxy in the formulation and by increasing the thermal conductivity.

Fillers also reduce the thermal expansion coefficient of epoxy resins which is

advantageous in electronic and structural adhesive applications.

Filler incorporation reduces shrinkage by replacement of resin with an inert

compound, which does not participate in the crosslinking reaction. Careful

selection of fillers can s~gnificantly increase the conductivity for electronic and

electrical applications.34 The incorporation of fillers invariably results in increased

viscosity. Fibrous fillers exert greater viscosity enhancing effects, on an equivalent

weight basis than particulate fillers. In particulate fillers, the particle size exerts

dominating effect with fillers of small particle size tending to increase viscosity to a

greater extent than the corresponding f~llers of greater particle size. This is

attributed to the greater surface area of the former.35 But this has been made use

in structural adhesive formulation requiring anti-sag properties using various types

of silica capable of exerting thixotropic effect. It was also found that the

incorporation of particulate fillers like silica, glass microspheres and aluminium

trihydrate Increased the fracture toughness of epoxy resin along with improvement

in r n o d u l ~ s . ~ ~ ~ Optimum toughness enhancement was dependent on variables

such as particle size, particle size distribution, particle surface chemistry and

volume fraction. Crack pinning mechanism has been proposed to explain the

increase in fracture t o ~ ~ h n e s s . ~ ~ ~ ~ ~

1.5.3 Resinous modifiers

These materials are used with epoxy resin either as a means of cost reduction or

to impart property modifications. Typical examples include combinations of

epoxies with nylons, polysulphides, polyvinyl formal and butyral, polyurethanes,

14 Chapler 7

styrene-butadiene copolymers, chlorosulphonated polyethylene, fluorinated

polymers, silicones, isocyanates, furfural and acrylate resins. Alloying epoxy resin

with resinous materials has also been shown to be an effective means of

enhancing t ~ u ~ h n e s s . ~ ~ , ~ ~ Many of the resinous materials especially the halogen

containing ones are effective in improving the hydrophobic character of epoxy

r e ~ i n . ~ . ~ ~

1.5.4 Flexibilising/pasticizing additives

Plasticizers or flexibilising agents are added to alleviate the brittle nature of epoxy

resins. The major difference between them is that plasticizers do not react with

epoxy resin whereas flexibilisers react with epoxy system during cure and become

part of the cured network.

1.5.4.1 Plasticizers

The flexibilisation achieved by plasticizers is comparatively lower than the reactive

systems. These materials often phase separate from the base epoxy. Hence they

are not extensively used in epoxy resin technology.

1.5.4.2 Reactive flexibilising additives

These materials are effective in reducing the exotherrn and shrinkage in addition

to reducing the basic hardness and rigidity of epoxy resins. The flexibilised nature

of the crosslinked network can result in a more strain tolerant system, better able

to relieve internal stresses, which can develop in cured networks. They can

improve the adhesive joint properties such as lap shear strength and peel strength

in addition to improvements in impact strength and low temperature crack

resistance. The disadvantages are that the reactive flexibilisers yield cured

systems that are less strong mechanically and have reduced electrical, chemical

and solvent resistance in comparison to their non-flexibilised counterparts. The

important reactive flexibilisers are polyamides, carboxyl terminated polymers,

polysulphides and polyglycol diepoxides.

Numerous other ingredients are added to epoxy resin to improve its properties.

For example coupling agents are used to Improve the adhesive properties of

epoxy resin. Organosilanes are used as a coupling agent as well as to improve

the properties of filled epoxy systems. For appl~cations involving fire retardency,

halogen containing resin or curing agents4' or incorporation of flame retarding

fillers such as antimony trioxideJ5 or somewhat more esoteric additives such as

amino alkyl phosphates or halogenated [ i -~actones~~ are used Solvents are

important formulatory ingredients for surface coating applications.4g Most common

solvents used for this purpose are methyl ethyl ketone, 2-ethoxyethanol, xylene.

2ethoxy ethylacetate, toluene, or blends of various solvents, where greater

solvating power has been demonstrated

Very recently, nanoparticles are used to enhance the properties of epoxy resin.5053

The cure kinetics, mechanical properties and fracture toughness of nanoparticles

filled epoxy systems have been analysed in detail. Ratna et studied the

properties of DGEBA epoxylclay nanocomposites cured with diethyltoluene

diamine (DETDA). A modest increase in T, and significant increase in storage

modulus were achieved as a result of incorporation of clay. The formation of

nanocomposite was confirmed by wide-angle X-ray analysis. The higher impact

strength of the nanocomposite compared the DGEBA matrix was due to increased

plastic deformation of the matrix. Intercalated nanocomposites of modified

montmorillonite clays in a glassy epoxy were prepared by crosslinking with

commercially available aliphatic diamine curing agents. These materials are

shown to have improved Young's modulus with reduction in ultimate strength and

strain to failure. The fracture toughness of these materials was investigated and

improvements in toughness values of 100% over unmodified resin were

dem~nstrated.~~

Two other important modifiers for epoxy resin are elastomers and thermoplastics.

These are extremely important modifiers because substantial improvement in

fracture toughness was obtained by modifying epoxy resin with elastomer or

16 Chapter 1

thermoplastic. A detailed description is given in the coming sections of the

chapter.

1.6 Toughening of epoxy resin

Epoxy resins are extensively used as adhesives and as matrices for fibre

reinforced composites because of their outstanding mechanical and thermal

properties. All these properties are attributed to their high crosslink density.

Because of high crosslink density, these materials have very low resistance to

crack initiation and propagation. It is very important to increase the toughness of

these materials without causing major losses in other desirable properties. Of all

the methods that have been considered and adopted in an attempt to alleviate the

brittle characteristics of epoxy resin, elastomeric and thermoplastic modifications

were found to be most successful.

1.6.1 Elastomer modification of epoxy resin

The various types of elastomers that have been considered for rubber modification

of epoxies are reactive butadiene acrylonitrile rubbers, polysiloxanes,

fluoroelastomers, acrylate elastomers etc. The most widely investigated and used

modifier was reactive butadiene acrylonitrile rubbers.

Mc Garry and c o - w o r k e r ~ ~ ~ ~ were the first to report on the toughening of epoxy

resin using rubber. They have used low molecular weight liquid carboxyl

terminated poly(butadiene-acrylonitrile) (CTBN) copolymers to toughen liquid

DGEBA epoxy resin. The reaction of the epoxy group with the carboxyl terminal

groups of CTBN produced alternating block copolymers of DGEBA-CTBN that

eventually precipitated as rubbery domains. The morphologies generated and the

resultant properties depend on the initial amount of modifiers, particle size and

cure cycle and the presence of other additives in the formulation.

Bascom et a ~ . ~ ' used a combination of liquid and solid CTBN rubbers to produce a

dual particle size distribution of 05pm and 1 - 2pm respectively. Smaller particles

were found to deform principally by voiding and induced local shear y~elding, and

Introduction 17

larger particles produced localised yielding in the surrounding matrix, which was

facilitated by the presence of smaller part~cler; The compatibility of the rubber and

epoxy was found to be controlled by the acrylonitrile (AN) content of the rubber

modifier as well as the curing condition^.^^ The higher the AN content, better is the

compatibility of CTBN with epoxy in terms of solubility parameter and slower is the

precipitation of rubber phase in the epoxy matrix. Moreover, greater AN 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 dynamic mechanical spectrum. This indicates

either complete miscibility or absence of part~cles above a critical size. Dissolution

of rubber is reflected in the mechanical properties, especially in the impact

strength of epoxy.

Siebert and ~ i e w ~ ' explained the chemistry of rubber particle formation in DGEBA

based liquid epoxy resin, CTBN and piperidine system. They suggested an ;n situ

formation of epoxy-CTBN-epoxy adduct, which is further chain-extended and

crosslinked with additional epoxy resin. This explained the chemical bond between

dispersed rubber phase and matrix resin in the presence of piperidine. But most of

the other curing agents favour either epoxy-epoxy or epoxy-amine reaction and

the carboxyl-epoxy reaction is suppressed. They showed that non-reactive CTBN

does show a second phase on curing with piperidine. However the fracture energy

was not improved. Mathur and co-workerss2 reported that chemical interaction

between epoxy cresol novoloac resin and CTBN occurred via formation of ester

linkage between epoxy groups and the carboxyl groups of CTBN Fracture

toughness increased with the addition of CTBN Arias et al.63 investigated the

damage zone around the crack tip of 15phr CTBN modified DGEBA cured with

piperidine and 4,4',-diamine-33-dimethyldicyclohexylmethane (3DCM). Optical

and transmission electron microscopic studies revealed dilatation bands and

massive shear yielding in the region close to crack tip. In 3DCM cured system, a

region of cavitated particles without shear deformation was also observed.

18 Chapter 1

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 al.@

Maximum adhesive strength was obtained when 10phr CTBN was used. Addition

of CTBN also increased the bulk tensile strength and impact energy. In CTBN

toughened DGEBA epoxy resin, the fracture behaviour showed a transition from

cohesive failure to interfacial failure on increasing the molecular mass between

crosslinks The transition was due to the prevention of crack from propagating

cohesively by increased ductility of the matrix, the formation of weak boundary by

the enlarged mismatch of elastic constant and the agglomeration of rubber

particles at the metallepoxy interface.65

The effect of CTBN on epoxy-graphite composites was studied by Liao and

a an^" who observed an increase in the fracture toughness with 10-15% addition

of CTBN. Sohn and emersonB7 investigated the interfacial tension between CTBN

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

composition using digital image processing techniques. With the increase in AN

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

corresponding decrease in the domain phase in the epoxy matrix was observed.

Considerable improvement in toughening was reported by ~ e e ~ ' with small

amount of poly(methy1 methacrylate)-natural rubber graft copolymer (PMMA-NR)

content in epoxy than with higher concentration 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 Olpm.

Klung and seferis6' examined the influence of silica on rubber phase separation in

CTBN modified epoxy resin. Phase growth rates were depressed by silica

resulting in a lower percentage of toughening domains. Cure kinetics of epoxy-

amine system modified with CTBN and amine terminated butadiene acrylonitrile

(ATBN) was examined by Wise et a[." The rate of epoxy-amine reaction

increases with the addition of CTBN and decreases with the addition of ATBN.

Recently, it was reported that the thermal stability of epoxylCTBN blend cured with

piperidine has high initial degradation temperature and higher activation energy for

introduction 19

decomposition both in air and nitrogen atmosphere 71 This showed better thermal

and thermo-oxidative stability of the blends compared to pr~stine epoxy resin.

Zhang et studied the bimodal phase separation in DGEBA epoxy modified

with hydroxyl terminated butadiene acrylonitrile random copolymer (HTBN) cured

with tetrahydrophthalic anhydride (THPA) using time resolved small angle light

scattering (SALS). DSC and digital image analysis. The bimodal size distribution

was explained qualitatively by nucleation and growth coupled with spinodal

decomposition and competition between phase separation and polymerisation

reaction.

Verchere et a ~ . ' ~ found that the domain size was influenced by the nature of the

curing agent and curing conditions. In DGEBNepoxy terminated butadiene

acrylonitrile random copolymer blends, the domain size changed with cure

temperature and curing agent. DDM showed maximum size and 3DCM showed

minimum size for the domains.

Cardwell and ~ e e ' ~ 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 also studied. These authors observed that the fracture toughness of

unmodified epoxy does not depend on the testing conditions while that of the

rubber-modified samples increases with a decrease in testing rate or with an

increase in temperature.

Ramaswamy and coworker^'^ reported the toughening effect of epoxidised

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 10phr This is attr~buted to the higher toughness produced

by the dispersed rubber particles. At higher EHTPB content, the rubber phase

became continuous and flexibilisation effect predominated over toughening effect

of EHTPB.

20 Chapter 1

Recently, it was found that carboxyl terminated polybutadiene (CTPB) improved

the fracture toughness of epoxy cresol novolac resins." lOphr addition gave

maximum improvement in flexural strength, tensile strength and impact properties.

The change in properties was due to the change in the morphology from

homogeneous to heterogeneous due to the addition of rubber. Ramos et a ~ . ' ~

reported that CTBN and hydroxyl terminated polybutadiene (HTPB) increased the

impact strength of DGEBA epoxy resin cured with piperidine. Maximum

improvement in impact strength for HTPB was obtained by 3phr addition.

Siloxane polymers are also used to modify epoxy resins. Siloxane modifiers are

used owing 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, siloxanes tend to migrate to the air-polymer interface and

provide a hydrophobic surface for the substrate. Riffle et a ~ . ~ ' synthesised

functionally terminated polydimethylsiloxanes and utilised them to modify epoxy

resin. DGEBA resin (Epon 828) was mixed with siloxane modifier and an amine

curative and cured at 160°C for 2hrs. The behaviour of the resultant network was

found to depend on the nature of the end-functional group of the modifier. Network

prepared with secondary amine terminated system, e.g., piperazine functionality

was able to produce homogeneous cured network. Electron spectroscopy for

chemical analysis (ESCA) revealed that piperazine capped oligomers significantly

enriched the surface with siloxane structures. Yorgitis et at." synthesised siloxane

modified epoxy prepolymers by reacting DGEBA type resin with piperazine

terminated polydimethylsiloxane and its statistical copolymers with either

methyltrifluoropropylsiloxane or diphenylsiloxane. These prepolymers were cured

with cycloaliphatic diamines. Increasing the percentage of rnethyltrifluoropropyl or

diphenyl units relative to the dimethylsiloxane content of the oligomers enhanced

the compatibility with epoxy resin. This enhancement produced smaller rubber

particles and altered particle morphology. Improved fracture toughness relative to

the cycloaliphatic diamine-cured control resin was achieved in resins modified with

polysiloxane copolymers containing 40% or more of methyltrifluoropropyl units or

20% diphenylsiloxane units.

Introduction 21

Lln and ~ u a n ~ " synthesised siloxane modified epoxy resin by reacting dangling

hydroxyl group with methoxy or silanol terminated polydimethylsiloxane in

presence of a tetraisopropyl titanate and was cured with 2.4,6-tris-(dimethyl

aminomethyl)phenol. Thermogravimetric analys~s (TGA) 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 tmprovement of

the copolymer. The thermal degradation study made by the same authorsa' 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 revealed that siloxane modified epoxy copolymer with

phenyl enriched siloxane oligomer provides higher thermal stability than the

dimethylsiloxane modified copolymer. Liu et a1." found that the morphology of

triethoxysilyl terminated polycaprolactone elastomer (PCL-TESi) modified DGEBA

epoxy resin changed with the curing agent used. The fracture toughness was also

dependent on the type and amount of curing agent used.

From the foregoing discussion it was found that the addition of different types of

elastomers increased the fracture toughness of epoxy resin. The rubber

incorporated into the epoxy resin was distributed in the matrix as fine domains.

The domains initiated several toughening mechanisms. The important toughening

mechanisms in rubber modified epoxy resins are summarised below.

1.6.2 Toughening mechanisms in elastomer modified epoxy resins

1 . 6 2 1 Crazing of the rnatr~x

Rubber particles initiate crazes in the surrounding matrix at the same time they act as

craze terminators preventing uninhibited craze growth resulting in premature failure.

The multiple crazing processes consume significant amount of energy and hence

increase the fracture toughness. This mechanism was proposed on the basis of

0bSe~ations like stress whitening ahead of the crack tip, strong dependence of

fracture toughness on particle size and pressure sensitivity of yielding.

22 Chapler I

1.6.2.2 Rubber particle bridging

Good interfacial adhesion 1s a prerequisite for this mechanism to operate. Rubber

particles span the crack and exert closure tractions on the fracture surfaces. This

process effectively reduces the stress intensity at the crack tip. Alternatively,

rubber particles are stretched and torn to fracture, which consumes additional

fracture energy. This mechanism was proposed on the basis of microscopic

observation of the fracture surfaces. Th~s mechanism was considered as a

secondary toughening mechanism in rubber modified epoxy resin. However.

rubber particle bridging has been shown to be important for highly crosslinked

epoxies modified by relatively large particles.

1.6.2.3 Formation of plastic zone in the matrix

Rubber particles increase the size of plastic zone ahead of the crack tip and

hence increase the fracture toughness. This was attributed to the suppression of

microcrack formation in the presence of a highly flexibilised interface. This

mechanism suggests that extensive plastic deformation in the matrix is the major

energy dissipative process in rubber modified epoxies.

1.6.2.4 Rubber cavitation and induced shear deformation in the matrix

Rubber particle cavitation is the primary toughening mechanism in rubber modified

epoxies especially in relatively lower crosslink density epoxy resins. Rubber

particles cavitate because of the triaxial tension around the crack tip. This

cavitational process relieves the plane strain constraint ahead of the crack tip.

allowing the stress concentrations associated cavitated particles to activate

extensive shear deformation of the matrix in the form of dilatational void growth.

1.6.2.5 Crack bifurcation and/or deflection by rubber particles

This mechanism was considered to be a secondary toughening mechanism.

Rubber particles cause the main crack to fork into many secondary cracks.

distributing the local stress intensity of the main crack to multiple cracks andlor

deviating the crack off the principal plane of propagation, which increases the area

of the crack surface and induces a mixed mode of crack propagation. This

Introduclion 23

mechanism is important for highly crosslinked epoxies modified with relatively

large particles.

A schematic representation of various toughening mechanisms is shown in Fig 1.5.

Figure 1.5: Schematic representation of toughening mechanisms in rubber

toughened epoxy resin, (1) crack deflection. (2) rubber tearing,

(3) crack bridging, (4) massive shear banding, (5) rubber particle

cavitation and plastic void growth and (6) particle induced craze

formation (R. A. Pearson, L. Pruitt, in Polymer Blends, Vol 2,

Performance, Eds. D. R. Paul, C. B. Bucknall, John Wiley & Sons,

New York, 2000)

1.6.3 Thermoplastic modification of epoxy resin

Elastomer toughening was effective in increasing the fracture toughness of epoxy

resin, but it often resulted in reduction in high temperature properties and

modulus. Additionally. elastomers are ineffective in increasing the fracture

toughness of highly crosslinked epoxy resins. Hence, as an alternative to rubber

toughening, thermoplastics were used to modify epoxy resin. Thermoplastic

modification resulted in enhanced fracture toughness with retention of high

temperature properties. The important thermoplastics used to modify epoxy resin

and the resulting properties are discussed in the coming sections

24 Chapter 1

1.6.3.1 Polysulfone modified epoxy resin

Polysulfone (PSF) is an important class of engineering thermoplastic. They are

amorphous thermally stable polymers with excellent mechanical properties and

high T,. Blends of epoxy resin with polysulfones were the most studied among the

various thermoplastic modified epoxy resins. Extensive studies were conducted on

the phase separation, mechanical properties, fracture toughness and morphology

evolution during curing. Bucknall and partridgea3 were the first to use PSF to

modify epoxy resin. They observed no increase in fracture toughness with the

addition of poly(ether sulfone) (PES) to a tetrafunctional epoxy resin. One of the

earlier studies involving the toughening of DGEBA epoxy resin was reported by

Hedrick et a ~ . ' ~ They used phenolic hydroxyl and amine terminated PSF oligomers

to toughen epoxy resin. High molecular weight PSF ( M n - 8200) (15%) increased

the fracture toughness by 100% without any loss in thermal proper tie^.'^ Although

toughness increased with amine terminated PSF, it was not as effective as the

hydroxyl terminated oligomer. Fu and suns6 also observed increase in fracture

toughness and fracture energy in PSF modified epoxy resin. Electromagnetically

cured blends also showed increase in fracture toughness with less developed

morphology.87

The curing kinetics of the blends followed autocatalytic mechanism and the

kinetics was followed using different analytical t e ~ h n i ~ u e s . ~ ~ ~ ~ ~ Jenninger et al."

studied the curing kinetics of PES and PSF modified DGEBA epoxy resin cured

with DDM. Autocatalytic model was used to explain the cure kinetics of the blends.

Decrease in cure temperature or increase in thermoplastic led to less perfect

crosslinked structure and hence low T,. The influence of trifunctional epoxy on the

cure kinetics of DGEBNPES blends cured with MCDEA was investigated in detail."

Reaction order of three was found to fit with the experimental data well. PES with

terminal reactive phenoxy group was found to accelerate the reaction rate.

The rheological properties of DDM cured PES modified DGEBA epoxy changed

with the progress of cure reaction. The onset and endset of phase separation

covers the full viscosity fluctuat~on reglon. Viscosity was affected because the

system was transformed from a single phase to a multiphase system and the

composit~on varied with time. Another factor influencing rheological behaviour is

the variation of the v~scoelastic properties of the phase separated blend The

viscosity fluctuation due to the phase separation of PES was due to h~gher

viscosity of PES compared to the low molecular weight epoxy medium. Phase

separation was found to occur much earlier than gelation.92.93

The infrared spectroscopic studies of curing of phenolic hydroxyl terminated PSF

modified DGEBNDDS system revealed interesting results.94 Most of the primary

amine groups were consumed by reaction before gelation. Also, it was found that

the epoxy primary amine reaction was not affected by cure temperature due to low

system viscosity upto gelation. The reaction of secondary amine starts only when

most of the primary amines were exhausted and a certain amount of side

reactions occurred during curing. Kim and lnoueg5 used dynamic mechanical

analysis (DMA) and FTlR to follow the curing reaction during the late stages of

DGEBAIPESIDDS system. They observed changes in T, on increasing cure time

Two T,'s were obtained from the dynamic mechanical spectrum of the blends

Yamanaka and lnoueQ6 studied the phase separation mechanism in PES

toughened amine cured epoxy resin. From light scattering studies, these authors

found that phase separation occurred by spinodal decompos~tion. Swier and

~ele''.~' used modulated temperature differential scanning calorimetry to study

the phase separation mechanism in PES modified epoxy resin. They reported that

careful selection of cure schedule will lead to PES modified epoxy with tailor made

morphologies. They were able to detect phase separation earlier than the

conventional cloud point measurements. In MDA crosslinked system, complex

morphology with interphase at certain cure schedules was obtained. The heat

capacity signal for 20phr PESIDGEBAIMDA system showed two step decrease,

first one was due to phase separation of PES and the second one due to epoxy

vitrification." In linearly polymerising aniline cured system, better segregation of

the phases was observed due to the close proximity of T, with cure temperature

Investigation of reaction induced phase separation (RIPS) in 20phr PESlepoxy

26 Chapter 1

blends cured with aniline revealed that phase separation and morphology

advancement affect heat capacity signal in temperature modulated differenttal

scanning calorimetry (TMDSC) measurement^.'"^'^' GUO'O~ reported that the ultimate

morphology of phenolphthalein poly(ether sulfone) (PES-C)/epoxy blends was

dependent on the type of curing agent used. Amine cured (DDS and DDM) blends

gave homogeneous morphology because of H-bonding whereas phthalic anhydride

(PA) cured blends gave heterogeneous systems. The reaction mechanism was step

growth polymerisation in amine curing and chain growth in anhydride cured blends. In

poly(ether sulfone):poly(ether ether sulfone) (PES:PEES) copolymerlepoxy resin

cured with 3,3'-DDS and MDEA, MDEA cured blends were heterogeneous while

DDS cured blends were homogeneous. The scanning electron micrographs of

MDEA cured epoxy resin blended with PES:PEES copolymer is shown in Fig. 1.6.

(a) (b) Figure 1.6: SEM microphotographs of MDEA cured Epon828 system blended with

different amounts of thermoplastic: (a)l5O/0 40:60 PES : PEES and (b)

30% 40:60 PES : PEES ( I . Blanco, G. Cicala, 0. Molta, A. Recca, J.

Appl. Polym. Sci., 94, 361, 2004)

The difference in morphology was due to different viscosity and solubility effects of

the b ~ e n d s . ' ~ ~ . ' ~ ~ But according to Bucknall and coworker^,'^^ molar mass of epoxy

resin was the main factor in controlling the phase separation in epoxy1PES blends.

Alig et a~.'~\tudied the RIPS in PES modified epoxy resin using SALS.

Ratna et al.'07 synthesised amine terminated plyamide sulfone (ATPS) and

Introduction 27

blended it with epoxy resin The HY 951 cured resin formed homogeneous blends

before and after curtng.

At a particular composition the important parameters affecting morphology are

reactivity of epoxy resin, initial solution viscosity and the demixing

temperature. 108.109 Oyanguren et alU0 observed that the initial concentration of

PSF was the determining factor in the morphology of methyltetrahydrophthal~c

anhydride (MTHPA) cured epoxy resin. They also found that 10phr PSF formed

bicontinuous morphology. The primary phase separation in this system was due to

spinodal decomposition and secondary phase separation occurred by nucleation

and growth mechanism. Post curing changed the composition of both the

phases."1 Gianotti et a lU2 found that changing the epoxy resin changed the

morphology of 15phr PSF modified epoxy resin cured with DDS. Resin with

considerable polydispersity resulted in dispersed PSF phase. The morphology of

DGEBAIPES blends was also dependent on the curing agent to epoxy ratio.'13 In

DDM cured blends with lower stoichiometric ratio, spherical domains were

observed and at stoichiometric ratio cocontinuous morphology was observed.

Above stoichiometric ratio. interconnected globular structure was observed

Secondary phase separation was observed in some cases. The secondary phase

separation in PES/epoxy/4.4'-methylene-bis(2.6-dimethylaniline) (DIM-DDM) was

due to hydrodynamic flow due to interface motion. This will cause the geometrical

coarsening to become too fast for diffusion to follow and leading to spontaneous

secondary phase separation.'14

Commercial PSF are miscible with epoxy resin. But in some cases the

improvement in fracture toughness was less. Therefore it is desirable to have

specific interaction between the two components or even primary chemical bonds

to a certain extent. Phenolic hydroxyl terminated PSF were reported to increase

the fracture toughness and it was discussed earlier that the extent of improvement

was strongly dependent on the morphology of the blends. 115-117 The Improvement

in toughness often depends on the molecular weight of the modifier. Ratna eta^."^ reported that low molecular weight ATPS acted as a flexibiliser for epoxy resin

28 Chapter 1

Pasquale et a1.'19 used amino functionalised PES to increase the fracture

toughness of epoxy resin. The improvement in fracture toughness was due to the

decrease in crosslink density. PES with pendent amino group and vinyl benzyl

groups were used to modify epoxy r e ~ i n . ' ~ ~ . ' ~ ' The amount of amine groups in the

polymer was critical in enhancing the fracture toughness and morphology. The

presence of dicumyl peroxide (DCP) gave more fracture toughness compared to

benzyl-PSF modified epoxy resin lijima et a ~ . ' ~ ~ reported that chloro terminated

PES was more effective in enhancing the fracture toughness than epoxy

terminated PES. The increase in toughness was due to plastic deformation of

matrix prior to failure. Huang et al.lZ3 obtained homogeneous blends of epoxy and

bisphenol-A PSF cured with DDM having 20% increase in fracture toughness. The

morphology and properties of bisphenol-A PES modified biphenyl type epoxy resin

were dependent on the curing conditions employed.'" Lower curing temperature

favoured homogeneous morphology and higher cure temperature favoured

heterogeneous morphology. Fig. 1.7 shows the fracture toughness of epoxyIPES

blends cured at 140 and 180'C.

Figure 1.7: Plot of fracture toughness vs modifier content in epoxyIPES blends

cured at 140 and 180°C (K. Mimura, H. /to, H. Fujioka. Polymer, 41,

4451, 2000)

Introduction 29

In both cases fracture toughness increased without decreasing other mechanical

properties. In phase separated systems the increase in fracture toughness was

due to crack branching and in homogeneous blends the increase was due to

induced shear deformation of the matrix. In DDM cured epoxy resin modified wlth

PSF, morphology was dependent on the cure temperature. Fracture toughness

increased by 15% and heterogeneous morphology favoured increase in fracture

toughness.'25 The addition of PES and coreshell rubber particle with

polybutadiene core and poly(methy1 methacrylate) (PMMA) shell increased the

fracture toughness of DGEBA epoxy resin cured with DDS. The toughening

mechanism was rubber cavitation followed by plastic deformation of the matrix.126

DGEBAIPSF blends cured with DDS having morphology spectrum was found to

have more toughness compared to specimens with uniform distribution of PSF.'~'

Different morphologies include homogeneous region, inverted sea-island morphology,

nodular structure, sea-island and neat epoxy region with less than 10% PSF. The

increase in fracture toughness was due to plastic deformation of PSF rich phase.

The thermal stability of DGEBNPSF blends cured cationically was found to

increase up to 30phr PSF.'~' Mimura and 1 t 0 ' ~ ~ found that rate of decomposition in

PES modified biphenyl epoxy cured with phenol novolac resin was dependent on

the morphology of the blends. Cocontinuous blends decomposed fast. The

physical ageing in PESlepoxylDDS blends was also i n~es t i~a ted . '~~ . '~ '

Raman spectroscopy was a useful tool for studying the reaction between epoxy

resin and amine terminated co-PES. Reaction between the blend components

was confirmed in the absence of curing agent.13' Amine curing of multifunctional

epoxy resin followed autocatalytic mechanism. The reaction became more

diffusion controlled with decrease in cure temperature and increase in PSF

content.13? The curing reaction of TGAPIDDS system decreased with increase in

PES ~ 0 n t e n t . l ~ ~ From calorimetric studies it was found that the curing exotherm

decreased linearly with increase in PES content. The magnitude of curing

exotherm and T, of the blends are given in Table 1.2.

PES content AH

(%) (JJg) -.

("C)

0.0 666.7 237.3

5.4 624.0 220.0

11.0 618.4 217.3

15.6 566.3 214.3

20.5 483.2 206.7

26.7 475.6 212.4

30.0 448.7 215.0

34.6 444.8 213.8

39.1 412.6 209.4

Table 1.2: Heat of cure and T, for epoxyIPES blends from calorimetric studies

(A. J. Mc Kinnon, S. D. Jenk~ns, P. T Mc Grail, R. A. Pethrick,

Macromolecules, 25, 3492, 1992)

The rheological properties during curing were investigated and found that PES

delayed the curing r e a ~ t i 0 n . l ~ ~ Hydroxyl terminated PSF showed two phase

morphology on curing. Gel time decreased at 20% modifier content. The activation

energy for gelation and vitrification decreased with increase in PSF because

gelation and vitrification occurred at lower con~ers ion. '~~ Hydroxyl, amine and

non-functional PES were blended with TGAP cured with DDS showed two phase

morphology. 137.138 Clarke's method was used to predict the phase diagram of

triepoxy/polyarylsulfone blend cured with DDS. Good correlation with the results

obtained from SALS and SEM studies was observed and the model could explain

the secondary phase separation to some extent.13' The time temperature

transformation diagram was made with the help of DSC, torsional braid analysis

(TBA), light scattering and dynamic viscoelastic analyses.'40 Increase in

concentration of PES decreased the rate of phase separation in tetrafunctional

epoxyIPSF blends. Secondary phase separation also occurred in the blends.14'

F~nal morphology was governed by the kinetics of cure in dicyandiamide (DICY)

cured blends.'42 Positron annihilation studies showed a complex free volume

distribution in the blend '43 The two-phase morphology in MDEA cured PES

modified epoxy resin was investigated using micro-Raman spectr~scopy. '~~

Gumen and ones'^^ used group interaction model to examine the contribution of

individual components to thermo-mechanical properties of the blend.

Varley et a ~ . ' ~ ~ studied the properties of a trifunctional epoxy resin modified with

PSF and cured with DDS. They found that side reactions occurred during post

curing. Fracture toughness increased with the addition of PSF for prereacted and

non-prereacted systems. The fracture toughness of TGAP cured with DDS was

increased by the addition of amino PES. 20% addition gave distinct increase in

fracture toughness. Dielectric studies revealed the suppression of molecular

mobility on gelation and vitrification at molecular A mixture of TGAP and

diglycidyl ether of phenol-formaldehyde in the ratio 2 : l was modified with hydroxyl

end capped PES. Cocontinuous morphology was observed at 30 and 40%

modifier and phase inverted morphology was observed at 50% PES on curing with

3,3'-DDS. Cocontinuous morphology facilitated more uniform stress distribution

under load and avoids premature failure.14' Raghava 149.150 obtained bimodal

particle size distribution for MY 720lhydroxyl PES blends. The fracture toughness

increased with the testing temperature. Young's modulus decreased slightly

indicating plasticizing effect of PES. The cure reaction of anhydride cured

tetrafunctional epoxy modified with PSF followed first order kinetics after an

induction period. At higher cure temperature phase separation occurred at lower

conversion. Fracture toughness increased for 10 and 15phr blends.15' The

properties of the blends are also dependent on the type of PES used to modify

epoxy resin. On changing from non-functional PES to functional PES, the particle

size decreased. On adding 20 to 30phr functional PES, the morphology changed

from cocontinuous to phase inverted one and maximum toughness was observed

in this region.lS2 Incorporation of triepoxy to diepoxy was found to be more

effective than changing the molecular weight of PES. Influence of molecular

weight was more marked in toughness. Homogeneous morphology was observed

up to 8% reactively terminated PSF modified TGAPIDDS blends and above this

concentration phase separation occurred. Fracture toughness and fracture energy

~ncreased for phase separated systems.15% full interpenetrating polymer network

32 Chapter I

(IPN) with increased fracture toughness was obtained on adding TGDDM to

diglycidyl ether of bisphenol-S (DGEBS)/PES:PEES copolymer blend cured with

DDS. T, was also increased with the addition of TGDDM."~

1.6.3.2 Pdyetherirnide modmed epoxy ms~n

PolyeMerimide (PEI) being a high performance thermoplastic, was used in toughening

both 'functional and multifunctional epoxy reslns. PEI has good mechanical properties.

tlame resistance and due to the presence of biiphenol group, it is flexible too. Amine

cured blends are the most studied in the case of DGEBA resins. Both sohrtion casting

and melt blending techniques were used for mixing purpose. However, FTlR

investigations in Me absence of curing agents revealed that PEI did not affect the

epoxae conversion. PEI does not have any functional group and hence did not exert

any effect on the pnoducbon of hydroxyl groups.'56 Kinetic sWks by Barral et al. 156.157

using 13-bis(aminomethyl)cyclohexane (1.3-BAC) as curing agent revealed that the

cure reaction followed autocatam mechanism in spite of Me presence of PEI. The

decrease in extent of reaction and increase in Me rate of curing reaction is evident from

the plot of reaction rate vs time given in Fig. 1.8.

Figure 1.8: Plot of reaction rate versus time for neat epoxy resin and blends cured

with 1,5BAC cured at 100°C (L. Barral, J. Cano, J. Lopez, I L. Bueno,

P. Noguena, M. J. Abad, C Ram~rez. Polymer, 41, 2657, 20W)

The activation energies calculated from dynamic DSC measurements for the

blends are high compared to the neat resin This means that the presence of PEI

hindered the curing reaction. But the rate of reaction increased with increase in

PEI content. The overall conversion decreased with increase in PEI content. The

experimental and theoretical values calculated using the autocatalytic model was

in good agreement with inclusion of diffusion factor. In a trifunctional epoxy cured

with MCDEA, a more complex behaviour is observed.g0 The experimental and

theoretical values show large discrepancies due to the different reactivities of the

three epoxide groups and the poss~bility of side reactions. Although TGAP showed

overall reaction rate higher compared to DGEBA, the rate was not influenced by

the presence of PEI. According to Bonnet et a ~ . ' ~ ' the rate of reaction depends on

the composition of the blends. At low concentration of PEI. < lo% no sudden

change in kinetic rate was observed. For higher concentration >30%, increase in

rate after phase separation was observed due to less dilute epoxy-amine phase.

The cure kinetics of DGEBAIPEI blends cured with liquid aromatic diamine, diethyl

toluene diamine (DETDA 80) was studied using DSC up to 15 weight%. Cure

mechanism remained autocatalytic. Rate of reaction increased with increase in

PEI content. Diffusion control was used to describe cure at later stages and it

increased with increase in PEI content.'59 The maximum cure rate was found to

increase with increase in cure temperature."'0

The rheological behaviour was found to be strongly dependent on the final

morphology and composition of the blend. 161,162 Viscosity increased initially at all

compositions but different behaviour was observed after phase separation. At low

thermoplastic component, viscosity decreases due to separation of highly viscous

PEI phase but at high PEI content (33%) where PEI forms continuous phase.

viscosity increases on phase separation Complex viscosity behaviour was

observed at intermediate compositions. Tho change in rheological properties with

respect to PEI concentration is shown in Fig 1.9.

34 Chapter 1

Curing time (linear arbitrary scale) Figure 1.9: Influence of PEI concentration on the rheological behaviour of

DGEBNPEI blends cured with MCDEA. TEM micrographs illustrate

the final morphologies obtained (A. Bonnet, J. P Pascault, H.

Sautereau, Y Camberlin, Macromolecules, 32, 8524, 1999)

The uncured blends exhibited upper critical solution temperature (UCST)

behaviour. Bonnaud et al.lK3 found that the initial miscibility was improved by the

addition of TGAP and also by the addition of MCDEA. The cloud point

temperature of the TGAPIDGEBAIPEI blend was predicted by the Flory-Huggins

equation according to a procedure developed by Kamide et al.lK4 The addition of

PEI delayed the beginning of cure reaction and prolonged the cure time. PEI

addition reduced the heat of polymerisation due to dilution effect andlor earlier

vitrification of PEI. Thermodynamic analysis based on Flory-Huggins-Staverman

approach revealed that the decrease in entropy due to increase in molar mass of

epoxy-amine oligomer and decrease in the interaction parameter (x) with extent of

cure reaction gave the impetus for phase separation. Uniform globules are formed

conriected with each other at period~c distances and rate of cure reaction was

higher than that of phase separation The miscibility was more with lower molar

lntroducfion 35

mass PEI. The type of curing agent also influenced the miscibility. DDS lowered

miscibility whereas MCDEA increased the miscibility. Synchrotron small angle X-

ray scattering (SAXS) stud~es revealed rhat phase separation occurred by

spinodal decomposition rnechan~sm.'~~ The reaction between epoxy and curing

agent was monitored by dielectric spectroscopy.'66 Phase separation was

detected by a sharp Maxwell-Wagner-Sillars (MWS) effect. The reaction

advancement and onset of phase separation increases at high temperature due to

higher solubility in PEI but maximum value was found to be 045.'"An extensive

study on the various factors controlling the phase separation in PEI modified

system was done by Cui et all* Increase in cure rate has no effect on the phase

separation rate. Slow reacting DDS gave clear phase separation in the blends. If

the modifier has reactive functional group like amine as in this case, fast cure rate

and phase separation occurred earlier giving small particles and short interparticle

distance compared to the non functionalised PEI. Morphological analysis revealed

that DGEBAIPEI blends cured with MDA was homogeneous while those cured

with DDS was heterogeneou~.'~~ Phase separation in MTHPA cured DGEBAIPEI

blends occurred by spinodal decomposition mechanism. lncrease of curing

temperature accelerated curing reaction and increase of blend viscosity pinned

phase separation from proceeding. Low curing temperature was favourable for

better adhesive proper tie^.'^' The morphology of DGEBAIPEIIMTHPA blends in

presence of benzyldimethylamine (BDMA) accelerator was found to be

cocontinuous whereas it was spherical domain in the absence of accelerators.

The coarsening process of epoxy droplets and the final morphology are affected

by viscoelastic effect^.'^'^"^ The phase separation in PEllDGEBA blends cured

with imidazole revealed that sponge like structures are formed at 10 - 25phr and

homogeneous morphology at 5phr. Sponge like structures are due to strong

viscoelastic effects in the early stage of phase ~eparation."~

Change in molecular weight of the PEI oligomers changed the final morphology.

Low molecular weight oligomer (inherent viscos~ty 0.39) formed phase separated

morphology while high molecular weight oligomer (inherent viscosities 0.58, 0.61

and 0.67) formed cocontinuous In high molecular weight species

36 Chapter 7

phase separation occurred early due to the poor miscibility. Morphology

development studies revealed spinodal decomposition mechanism for low

molecular weight species. For high molecular weight species large domains of

10 - 20pm were formed first and the boundary becomes clearer without any

further increase in size. The T, was found to increase with curing for low molecular

weight species. For latter species T, of PEI rich phase increased fast while that of

epoxy rich phase slowly. The difference in phase behaviour was due to the

difference in phase separation mechanism. In the high molecular weight species,

large domains are formed initially due to poor miscibility and further phase

separation occurred in the separated phases.17' Wu et a ~ . ' ~ ~ considered

PEI/DGEBA/imidazole as a quasi binary due to the following reasons. (i) imidazole

acts as a catalyst only. (ii) weight fraction is small and (iii) reaction mechanism is

simple. Compared to other PEI modified systems, there are two exotherms, one

due to adduct formation and second due to etherification.

The phase separatiin mechanism in epoxyIPEl blends was dependent on the

initial thermoplastic content and varied with the type of curing agent used '77 In

DDS cured blends, the critical concentration was 9.8wt% and for MCDEA it was

10.7wt0/~. Phase separation studied by SAXS, light scattering (LS) and

transmission electron microscopy (TEM) studies revealed phase separated

morphology.'78 Phase separation mechanism near critical concentration was

spinodal decomposition whereas for off critical composition it was nucleation and

growth. Also the phase separation process can continue during post cure due to

devitrification of both phases and further curing reaction. Although T, increased

with addition of PEI, thermal stability de~reased."~

The fracture toughness was strongly dependent on the morphology of the blends.

PEI did not improve the mechanical properties and toughness when a

homogeneous morphology was formed in the case of DGEBAlPEl cured with

3-diethylaminopropylamine (DEAPA).'~' Bicontinuous morphology gave maximum

toughness and the morphology was strongly dependent on cure temperat~re.'~'

The increase in toughness was due to micro cracks and crack pinning effect. The

high degree of interaction between phases in bicontinuous morphology facilitates

more uniform stress distribution in the material under stress and results in

enhanced toughness.'82 The creep behaviour of 5phr PEIIDGEBAIMCDEA blend

was studied. The blends formed phase inverted structures and strong debonding

takes place even at low ~tress."~

The cure kinetics studies of PEITTGDDMIDDS blends revealed that the rate of

reaction was increased by the addition of PEI and the extent of conversion

increased from 82 to 94% on increasing cure temperature from 177 to 1 9 7 " ~ ' ' ~

The phase morphology development was similar to that of difunctional resinlPEl

blends. Fracture energy of the blends showed eight fold increase. The toughness

improvement is more than that predicted by the rule of mixtures. The toughness

increase was attributed to good interfacial strength and phase separation as

revealed by dynamic mechanical analysis. At low thermoplastic content yielding in

resin matrix was the mechanism while between 10 and 30%, ductile drawing of

thermoplastic phase was the toughening me~hanism."~ Morphological analysis of

the blends revealed that maximum toughness was observed with phase inverted

morphology due to maximum ductile yield The morphology at a particular

composition varied with cure conditions used. For example 20phr PElIepoxy blend

cured at 140°C for 2hrs and 190°C for 2hrs gave phase separated morphology

while those cured at 70°C for 8hrs, 140°C for 2hrs and 190°C for 2hrs each gave

cocontinuous morphology because at low temperature, system was gelled in the

initial stages of phase separation. Due to increase in viscosity of the system

smaller particles are obtained from increasing PEI from 30 to 5 0 ~ h r . " ~ Although

the addition of PEI increased the fracture toughness of TGAP, no correlation was

observed between morphology and fracture toughness. There is a levelling off

after 15% PEI. Post curing increased T, due to the reaction between epoxy and

PEI leading to a grafl copolymer. crosslinking reaction of thermoplastic or loss of

plasticizer.''' In another study conducted by Hourston et al l" modified a series of

mixed epoxies of di, tri and tetrafunctionalities with PEI. A mixture of difunctional

and trifunctional epoxy gave maximum properties.

38 Chapter I

Since the fracture toughness was increased by increase in interfacial adhesion,

functionally terminated PEI like aminated and hydrolysed PEI were used to

toughen epoxy resin. A tetrafunctional epoxy N.N,N'.N'-tetraglycidyl-a,a'-bis-(4-

aminopheny1)-p-diisopropyl benzene cured with diamine was modified with

hydrolysed PEI. When hydrolysed PEI was used, the fracture toughness was

higher than the unmodified system. Due to the presence of carboxyl and amide

groups, an IPN structure was formed at the interface resulting in improvement of

interfacial strength. Due to the presence of chemical bonds stress is more

efficiently transferred to the PEI phase. The hydrolysis time was also important as

the properties deteriorated at longer times. Thirty minute hydrolysis was observed

to give optimum pmperties.'89~'90 The fracture toughness of epoxy resin was

reported to increase by the addition of aminated PEI, due to the formation of

strong interface. Addition of aminated PEI with minimum amine gave maximum

fracture toughness. The extent of improvement was limited due to highly

crosslinked struct~re.'~'

7.633 Polycarbonate modified epoxy resin

Epoxy resin was found to react with polycarbonate (PC). The miscibility of

PClepoxy blends was studied using various techniques and found to be miscible

at various compositions.'s2 The reaction between epoxy and PC prior to cure was

investigated and it was observed that at 150°C transesterification occurred

between secondary hydroxyl and carbonate of PC and it increased at 200°C. The

large increase was due to the generation of hydroxyl groups from the degradation

of P C . ' ~ ~ Transesterification reaction in epoxylPC blend was catalysed by

quarternary ammonium salts. The reaction passes through a transition state. With

stoichiometric composition of PC in the blend, a network of crosslinked chains

were formed with carbonate and chain scission occurred in non-stoichiometric

system.lg4 The as prepared DGEBNPC blends were homogeneous and on

heating with 50% PC at 200°C for various times showed an increase in T, up to

50°C. The main reaction was bond exchange between -OH in epoxy and carbonyl

in PC. On heating. PC chains are fractured and grafted to -OH of epoxy At high

extent of reaction all PC chains break and graft to -OH sites. Some epoxles have

Introduction 39

multiple -OH groups and hence a crosslinked structure will result.'95 Diepoxides

and inchain carbonates react readily in the presence of quarternary ammonium

salts and reaction was absent if no catalyst was present The crosslink density can

be controlled by changing the epoxylcarbonate ratio.'96

Aminopolycarbonate/epoxy blends cured with DDM exhibited autocatalytic

mechani~m.'~' TGDDMIDDSIPC blends exh~bited nm order reaction mechanism.'"

The epoxy cure rate was increased by the addition of PC. Su and

investigated the diffusion controlled reaction mechanism in tetrafunctional epoxy

resin modified with PC. They were able to predict the diffusion control on kinetics

and rate of reaction for full range.

EpoxyIPC blends were transparent and no phase separation was observed from

scanning electron microscopy (SEM) or TEM and DMA studies. Addition of

bisphenol-A PC decreased T, of the blends. The miscibility was attributed to the

similar chemical structure and H-bonding interactions between carbonyl and

hydroxyl groups.'" In MTHPA cured blends with BDMA catalyst,

transesterification was found to occur and post curing forms cyclic carbonates.

Both these reactions occurred during the later stages of homopolymerisation and

it proceeded through a zwitter ion.20' PCIDGEBA blends catalytically cured with

tert-amines are found to undergo transesterification and cyclisation reaction. The

cyclic carbonate was formed through a zwitter ion intermediate.202 in

DGEBAIbisphenol-A PC blends cured with DDS or maleic anhydride (MA),

tetramethyl ammonium iodide (TMAI) was used to catalyse the reaction between

PC and epoxy resin. Melt mixing for longer times led to chain scission. At low PC

concentration (10phr) PC accelerated curing reaction depending on temperature

and at higher PC content (20phr) lower rate constant was observed due to high

viscosity of the system. Addition of TMAI affects curing reaction and brings about

less conversion, but overall kinetics was similar to that of neat epoxy.203

DGEEWPC blends cured with polyoxypropylene diamines (POPDA) were

investigated by Lin et a1204 POPDA reacted with carbonate of PC to form

N-aliphatic aromatic carbamate. During primary curing at 80"C, rest of amine

40 Chapter 1

reacts with epoxy group and N-aliphatic aromatic carbamate to form urea

structure. After secondary curing at 15O0C, N-aliphatic-aliphatic carbamate was

formed. At high temperature aliphatic-aliphatic carbamate was formed by two

routes. Mechanism of curing of epoxylPC blends cured with two different

processes was investigated. In the first method, epoxy and PC was melt mixed

and aliphatic amine was added later. Aliphatic amine reacted with PC carbonate to

form carbamate. The remaining amine reacted with epoxy. Deblock reaction of

carbamate accelerated by oxirane resulted in the formation of isocyanates at 80°C.

This will react with hydroxyl of network to form new carbamate. In the second

method, PC was dissolved in aliphatic amine and then epoxy was added. Aliphatic

amine reacted with PC to form urea and rest will react with epoxy. Most urea will

react with hydroxyl of cured resin to yield carbamates in network structure and a

fraction will remain in matrix as flexibiliser. The first system has higher T, and

lower fracture energy.2"

Transesterification reaction also occurred on curing blends with aromatic amine.'06

Due to this reaction alcoholic hydroxyl are replaced by phenolic hydroxyls and this

accelerated curing reaction and scission of long chain PC. The PC epoxy blends

are affected by minor components in epoxy monomer, PClepoxy ratio and

environment in which it was prepared.207 The chemical interactions between

TGDDM and PC were studied extensively using FTIR, nuclear magnetic

resonance (NMR) spectroscopy etc.Zo&210 When proper stoichiometry was used a

fully crosslinked system was obtained. The crosslinked network was found to

contain aliphatidaliphatic and aliphaticlaromatic carbonate. The chemical

interaction was influenced by composition and temperature. When optimum

amount of PC was present maximum reaction takes place and aliphaticlaliphatic

linkage was formed. Gel permeation chromatographic (GPC) studies of extracted

samples from PC/novoladnadic methyl anhydride (NMA) cured samples revealed

that chain scission of PC occurred. Presence of PC was not found to alter the

overall k~netics of epoxy cure. The reaction between epoxy and PC was studied by

Woo and su2" They investigated the effect of epoxyIPC ratio and time on the

reaction and found that maximum reaction occurred in 1:l ratio of epoxy and PC.

The morphology of intermediately crosslinked samples was complex. The AC

electrical properties of novolac PCIepoxylNMA blends was analysed and found to

be stable in the range 2 5 - 1 0 0 " ~ ~ ' ~ Li and chang213 reported that no

transesterification reaction occurred in PClepoxylBF,-MEA system. The only

reaction was between oxirane and oxonium group from BF3-MEA activated epoxy

group. Kinetic parameters showed first order reaction

The presence of aromatic amine. DDM promoted epoxy-amine reaction due to

catalytic effect of -OH terminated PC chains.'14 When solution casting was used.

PC crystallised. In melt blending, reaction between PC and epoxy prevented

crystallisation. PC epoxy phase separation and crystallisation can be controlled by

varying composition, PC-epoxy reaction and by using curing agents of varying

reactivity. Tetraethylenepentamine (TEPA) gave homogeneous morphology due to

H-bonding and high cure rate. DDS cured system induced homogeneity by

introducing PC-epoxy reaction or by eliminating crystallisation. Anhydride cure

system showed stronger ability to phase separate and crystallisation due to

weaker cr bond in^."^ Crystallisation process in PCIepoxy blends was analysed

using a photoresistor and good correlation with Avrami's equation was ~ b t a i n e d . ~ ' ~

The thermal and mechanical properties of the epoxyIPC blends were dependent

on the PC content, molecular weight of aliphatic amine curing agent and curing

stages.217 The fracture toughness of the homogeneous blends was higher than

that of neat epoxy. The load displacement curve showed stick slip fashion in the

blends. 20% addition of PC increased fracture energy by seven fold. SEM

revealed characteristics of stick slip propagation of crack on the fracture surface.

Localised yielding at the crack tip with consequent notch blunting was the major

source of energy dissipation in the blends218 It was observed that solution

blending of PC with epoxy resin led to crystallisation of PC. The tensile strength

remained the same in both cases. Increase in fracture toughness was observed in

melt blended samples due to bonding of PC chains to epoxy (Table 1.3)~' '

42 Chapter I

.- -- Neat 5phr 10phr 15phr 20phr

Solution blended samples

K, , (MP~~.~) 0.720 0.794 0.925 0.849 0.794

~ , , ( k ~ / m ~ ) 0.187 0.211 0.027 0.230 0.198 -

Melt blended samples

K , ( M P ~ ~ - ~ ) 0.720 0.912 1.006 0.770 0.614

~ , , ( k ~ l m ~ ) 0.187 0.281 0.307 0.207 0.135 -

Table 1.3: Fracture toughness of epoxy resinlPC blends prepared by solution

casting and melt blending (C M Don, C. H. Yeh, J. P. Bell, J. Appl.

Polym. Sci., 74, 2510, 1999)

Lee et a ~ . ~ " found that there is strong relationship between compatibility and

thermal stability in epoxylPC blends.

1.6.3.4 Polyester modified epoxy resin

Polyesters are commercially important polymers having good properties.

Polyesters like poly(ethy1ene terepthalate) (PET) and poly(butylene terephthalate)

(PBT) are semi-crystalline polymers. The effects of interchange reactions in

blends of the semicrystalline PET and amorphous epoxy resin were studied by the

effect of reactions on the crystallisatin and melting behaviour of the blendsz2'

The T, of the blends increased with increase in the thermal treatment time and the

Gordon-Taylor parameter (k) for the blends treated at 280°C for different times

increased with increase of treatment time. Hence it is deduced that interchange

reaction occurred between the hydroxyl groups of DGEBA with ester group in PET

which led to the formation of a grafted copolymer, which then transformed to

crosslinked copolymers upon further reaction. The immiscibility of the heat treated

blends in 1.1,2,2-tetrachloroethane at 120°C revealed that crosslinking reaction

occurred on heat treatment. PET was found to be miscible in DGEBA epoxy and

3.4-epoxycyclohexyl-methyl-3,4-epoxycyclohexyl carboxylate (ECY). PET showed

less reactivity with DGEBA. but was more reactive towards E C Y . ~ ~ '

Introduction 43

The influence of the semicrystalline PBT on the flow behaviour of epoxy resin was

Investigated by Wang et alZZ3 Steady shear measurements showed that the neat

and PBT modified resin were non-Newtonian fluids. An abrupt change in

rheologlcal properties was observed when the PBT content increased above

crltical gel concentration and the gel formed was thixotropic. Xu et alZz4

investigated the cure kinetics of hyperbranched polyester with DGEBA cured with

m-phenylenediamine. The addition of hyperbranched polyester was found to

increase the cure reaction of DGEBA. A two-parameter autocatalytic model,

SestAk-Berggren equation was found to be adequate for describing the cure

kinetics.

Different morphologies of crystalline PBT were obtained by dissolving it in epoxy

resin and crystallising PBT from solution by Solutions crystallised at

high temperature form thermoreversible gels by crystallisation of interspherulitic

matter around large spherulities. Solutions crystallised at low temperature form

isolated spherulites suspended in liquid epoxy. Solutions self nucleated before

cooling formed thermoreversible gels with little observable microstructure. The

crystallinity and crystallisation ability of PET in the blends decreased with increase

of thermal treatment time. From OM studies, it was found that the blends treated

for one minute crystallised completely and on increasing the treatment time, the

extent of crystallisation decrea~ed.~~' These results further support the

interchange reaction between DGEBA and PET. The half time crystallisation of

PBT in uncured epoxy decreased while it remained constant in DGEBA epoxy

resin cured with MCDEA.~'~

Oyanguren et al.'" investigated the influence of curing agent on the properties of

epoxy resin/PBT blends cured with MCDEA and DDS. No significant improvement

in toughness was observed in both the systems. PBT was more soluble in

DGEBA-MCDEA than DGEBA-DDS system. This is a significant factor which

influenced the phase separation during curing reaction. Since PBT was more

compatible with MCDEA full conversion occurred without phase separation in the

DGEBA-MCDEA system provided that T,>T,. For DGEBA-DDS formulation phase

separation always takes place during the course of curing reaction.228

In the DGEBA-MCDEA system, no crystall~sation was observed, but in DGEBA-

DDS system crystallisation of modifier inside dispersed domains took place on

cooling to room temperature.229 Small angle laser light scattering (SALLS) studies

revealed spinodal decomposition mechanism. Increased curing agent reactivity

led to small size due to earlier form of crosslinking network. The modification of

DGEBA epoxy resin with aromatic polyesters cured with methylhexahydrophthalic

anhydride (MHHPA) was reported by lijima et The molecular weight and

chain length of the polyesters were the important factors which influenced the

properties of the modified resin. The fracture toughness was found to decrease

with increase in chain length of the polyaikylene phthalates (PEP>PBP>PHP) and

medium molecular weight polyesters are found to be more effective than high

molecular weight polyesters as evident from 150% increase of fracture toughness

by the addition of 20wt% PEP (M,, =7200) at no expense of mechanical properties.

The fracture surfaces of the resin modified with 20wt% polyesters were either

rough or containing ridges or tended to macrophase separation. The improved

toughness was partially due to the dissipation of the fracture energy by blunting of the

crack tip based on delocalised plastic deformation and partly due to the reinforcement

of the matrix itself by the incorporated polyester. lijima et al.23',232 have also reported

the toughening of DDS cured epoxy resin with polyethylene phthalate (PEP).

polybutylene phthalate (PBP) and related copolyesters. They are soluble in epoxy

resin without solvents. The properties of the modified resin were influenced by the

polyester structure, molecular weight and concent ra t i~n .~~~ The presence of

relaxation peak near room temperature was effective in improving the toughness

of epoxy resin because of the increase of the plastic deformation zone attributed

to the increase of temperature at the crack front.234,235 A significant contribution to

the improvement in toughness was also due to the absorption of the fracture

energy due to ductile drawing and tearing of the thermoplastic phase.z36

Tomoi and toughened a cycloaliphatic epoxy resin cured with methyl

hexahydrophthalic anhydride using PEP and poly(ethy1ene phthalate-co-ethylene

terephthalate) The improvement in fracture toughness was attr~buted to the

reinforcement of the matrix by the polyesters, existence of new relaxations based

on polyester and the broadness of a-relaxation and the dispersion of fine polyester

rich particles. The increase in toughness of PET modified epoxy resin cured with

MA was due to cavitationlshear banding at 12phr PET and below it microcracking

led to increase in fracture toughness. The impact and tensile properties were

increased on adding 16phr PET.'^' Harani et a ~ . ~ ~ ~ used hydroxyl terminated

polyester to toughen DGEBA epoxy resin cured with imidazoline polyamine. The

increase in fracture toughness was due to localised plastic shear yielding of

matrix. Kim and Maz4' investigated the mechanism of mode I1 fracture of PBT

modified epoxy resin. Mode II fracture toughness of PBT modified epoxy resin

increased with PBT content as shown in Fig. 1.10.

PET content (phr)

Figure 1.10: Variation of mode II fracture toughness with PBT content (H. S. Kim.

P Ma, J. Appl. Polym. Sci, 69, 405, 1998)

PBT particles being tough acted as obstacles at crack initiation. The effectiveness

of hyperbranched polyester (HBP) as toughness were investigated by WLI et alZ4'

46 Chapter 1

The fracture toughness of HBP modified system did not show any improvement

compared to linear polyester. Boogh et a~.~~%also observed the same behaviour in

epoxy resin modified with HBPs.

1.6.3.5 Poly(ether ether ketone) modified epoxy resin

Polyether ether ketone (PEEK) is a semicrystalline engineering thermoplastic having

a unique combination of properties like toughness, stiffness, thermooxidative stability,

chemical and solvent resistance, electrical performance, flame retardancy and

retention of physical properties at high temperature.243 Functionally terminated PEEK

with pendent alkyl groups were used instead of virgin PEEK to modify epoxy resin.

The advantage is that the oligomers were less crystalline and easy to process. The

blends were prepared either by solution casting or melt blending. All the blends were

homogeneous before curing. The cure kinetics of DGEBA epoxy modified with PEEK

based on tert-butyl hydroquinone (PEEKT) was found to follow autocatalytic

mechanism. The rate of reaction decreased with the addition of PEEKT to epoxy resin

cured with DDS (Fig. 1.1

O - Neat Resin lOphr PEEKT

-A-2Ophr PEEKT -*-3Ophr PEEKT -,>-40phr PEEKT

SOphr PEEKT

Time (mi".)

Figure 1.11: Reaction rate against time plot for neat resin and epoxy1PEEKT

blends cured at 180°C (B. Francis, G. V. Poel. F. Posada, G.

Groeninckx, V L Rao, R. Rarnaswamy, S. Thomas, Polymer, 44,

3687, 2003)

According to Zhong et epoxylphenolphthalein poly(ether ether ketone) (PEK-C)

blends cured at 80°C using DDM as the curing agent were homogeneo~~s as

evident from DSC. DMA and SEM studies. In contrast to this when the

temperature was ratsed to 150"C, the blends were heterogene~us.'~~ Scannlng

electron microscopy and DMA studies confirmed the phase separated

morphology. The difference in phase behaviour was due to the increased mobility

of the molecules at high curing temperature. In order to establish the relationship

between the curing agent and phase behaviour, Guo et alZ4' investigated blends

cured with MA. PA and hexahydrophthalic anhydride (HHPA). Phase separated

morphology was evident from SEM micrographs for MA and HHPA cured

systems. But due to the aromatic nature of PA and PEK-C, the cured blends were

homogeneous.

The mechanical properties of DDM cured PEK-Clepoxy blends revealed that

fracture toughness and flexural properties were slightly reduced by the addition of

PEK-C. The reduction in properties was due to the homogeneous nature of the

blends. A series of amine terminated oligomers based on tert-butyl hydroquinone

(TBHQ) and methyl hydroquinone (MeHQ) were blended with epoxy resin by melt

mixing.248 Toughness increased by the addition of oligomer. The substantial

increase in fracture energy with TBHQ based oligomer was strongly dependent on

the final morphology of the blends. The phase inverted morphology gave

maximum toughness The modification of amine terminated PEEK was reported

by Cecere and Mc ~ r a l h . ~ ~ ~ The water absorption reduced by half by the addition of

40% oligomer. lijima et a ~ . ~ efkdvely used a series of synthesized poly(ary1 ether

ketone)^ to improve the fracture toughness of DGEBA resin with no expense of

mechanical properties. They obtained maximum toughness by the addition of 10wt0/o

modifier. Cocontinuous morphology gave the maximum toughness. Song et

used PEK-C to modify a tetrafunctional epoxy (TGDDM). The DDM cure blends

were homogeneous and T, initially increased and then decreased From FTlR

studies it was found that the there rema~ned certain amount of unreacted epoxy,

which led to decreased crosslink density In an interesting study, Brostow et alZs2

used fluorinated poly aryl ketone (12F-PEK) to reduce the friction of DGEBA resin

48 Chapter I

cured with triethylenetetramine (TETA). Addition of 10% fluoropolymer, reduced

friction by 30%. They also found that the properties were strongly dependent on the

curing temperature. They further observed that 5% addition of 12F-PEK to epoxy

cured at 24°C resulted in better healing afler scratching. The properties were

dependent on the cure temperature and hence can be manipulated by changing cure

temperature.2u The total surface tension of a polymer solid has strong effect on the

static friction, dynamic friction, penetration depth and residual ~cratch."~

1.6.3.6 Poly(acrylonitrile-co-butadiene-co-styrene) modified epoxy resin

The cure kinetics of poly(acrylonitrile-co-butadiene-co-styrene) (ABS) modified

epoxy/l,3-BAC system followed autocatalytic mechanism and the total heat of

reaction was not affected by the addition of A B S . ~ ~ ~ Diffusion factor was

incorporated to explain the curing reaction during the final stages of cure. The

curing of blends was studied using DMA. The appearance of significant

improvement in mechanical properties in the modified resin coincides with gelat~on of

continuous phase. The conversion of blends at gelation is lower than that of neat

resin. Maximum value of modulus was assigned to the vitrification of the blends. 15phr

ABS blend showed bicontinuous and 30phr blend had phase inverted morphology.256

From DMA it was found that activation energy for relaxation (E,) increased from

281.5kJlmol to 400kJImol on post T, and peak height of tan8 curve

decreased with the addition of ABS. 5 and 10phr blends showed droplet-matrix

morphology and domain size increased with increase in ABS Fracture

toughness improved while other mechanical properbes showed no deterioration.

Fracture surfaces were rough but SEM micrographs showed less adhesion between

epoxy and ABS phases.z59 5phr ABSlepoxy blends was found to be the most effective

The thermal stability of the blends was analysed by Abad et a~.'~' and it

was observed that ABS did not change the degradation mechanism.

1.6.3.7 Poly(2,6-dimethyl-l,4-phenylene ether)modified epoxy resin

The processing of poly(2.6dimethyl-1.4-phenylene ether) (PPE) was improved through

reacbve solvent approach. Here epoxy resin was used as a reactive solvent for PPE.

Phase inversion during curing at sufficiently high concentration of PPE was made use in

reactive solvent approach. Epoxy serves as a solvent during processing and reduces

viscosity and enhances processability of PPE. Upon curing phase separat~on occurs

and thermoplastic forms continuous phase. Bromised epoxylDICYIPPE blends exhibited

UCST behavio~r.~~' Complete phase separation was impossible because of v~tnfication

of PPE. Above 60 - 70% PPE, vitrification occurs on cooling without phase separation

Low molecular weight PPE/DGEBA also exhibited UCST behaviour. When imidazole

curing agent was used, it catalysed the reaction between phenol chain ends of PPE with

epoxy.262 In epoxyIPPVMDEA blends, UCST behaviour and critical concentration were

dependent on the molecular weight of PPE. Endcapping with methyl group and acid

modification reduced Increase in PPE increased palticle size. TEM and

DMA confirmed complete phase separation, but acid modification reduced domain size

due to incomplete phase separation. The phase separation prwess in

PPVepoxyIdiethyltoluenediamine was investigated using time resolved SALLS. The

light scattering curves for 3 W ? PPE modified epoxy resin cured for various times are

shown in Fig. 1.12.

I 2 1 * 4 v""

Figure 1.12: Scatter~ng curves for a 30w% PPE blend after curlng at 195 "C for

60s. 90s. 120s. 150s. 180s and 210s (Y. ishif, A. J Ryan.

Macromolecules, 33, 158, 2000)

50 Chapter 1

It was found that phase separation occurred by spinodal decomposition and the

driving force behind gelation was epoxy polymerisation. 264.265 lshii et a1.266

predicted the phase diagram in epoxylPPE blends. The reaction induced phase

separation was predicted using Flory-Hugg~ns theory and also determined the x parameter. Kranbuehl et al.267 used dielectric spectroscopy and DMA to study the

phase separation and reaction advancement in PPEIepoxylMCDEA system.

Dielectric studies detected phase separation sooner than DMA. Poncet et

studied the phase separation in PPEIDGEBAIMCDEA system. A 45% blend

showed peaks corresponding to phase separation and vitrification of PPE phase.

The rate of reaction decreased with the amount of PPE in the blend due to dilution

and high viscosity and lower level of conversion at vitrification due to the presence

of high T, PPE. Vitrification of PPE occurred first followed by vitrification of epoxy.

Morphology was strongly influenced by kinetics, diffusion and viscosity conditions

during phase separation.

Compatibilisers were used to improve the properties of PPElepoxy blends.269,270 In

PPEIDGEBAIMCDEA blends, MA grafted to poly(styrene-b-ethylene-co-butene-b-

styrene) was used as the compatibiliser. Polystyrene (PS) block was soluble in

PPE and on pretreatment with MCDEA, side grafted MCDEA was formed and it

can react with epoxy. Decrease in domain size was observed on morphological

analysis. TEM studies showed reduction in interfacial tension. The presence of

interface was evident from micrographs. The observation of micromechanical

transition for effectively compatibilised blends was a strong indication that a

copolymer rich interphase with a certain volume and specific properties did exist.

Liang et a12" used a random copolymer of styrene and MA with 7.5% MA content

as cornpatibiliser for brominated epoxylDICY1PPE blends. The copolymer was

miscible in both epoxy and PPE. Fracture toughness and elongation at break,

dielectric properties and water resistance increased.

Venderbosch eta1.272-274 studied epoxy1PPE blends in detail. They obtained phase

separated system and vitrification of PPE was found to be the determining factor

in controlling the morphology. The tensile strength of the blends cured at high

temperature was higher than those cured at low temperature. They also

investigated the effect of a blend of two epoxies [DGEBA and diglycidyl ether of

poly(propy1ene oxide) (DGEPPO)] on the properties of PPE blended with it. At

higher concentration of DGEPPO (20180, OIIOO), cured epoxy was in rubbery

state. Phase inverted morphology was observed in the blends with PPE. The

domain size of rubbery epoxy was higher than that of glassy epoxy. Young's

modulus showed no decrease with glassy epoxy but it decreased linearly in 20/80

blend. Rubber particles showed more fracture toughness compared to glassy one.

The oxygen and carbondioxide permeability decreased in PPEIDGEBNMCDEA

blend.275 The composition was so selected that PPE formed the matrix. Epoxy

droplets were impermeable to gas and transport takes place through PPE phase.

Tortuosity was more for PPE blend as evident from SEM micrographs.

1.6.3.8 Poly(methyl methacrylate) modified epoxy resin

Poly(methy1 methacrylate) (PMMA) has two advantages, it is miscible with DGEBA

epoxy at all proportions and did not react with epoxy even at 220°C on prolonged

heating. 195.276 The effect of PMMA on cure kinetics, mechanical and fracture

properties was investigated2" Blends were prepared by solution casting from

methylene chloride. MTHPA was used as the curing agent. No influence on

epoxylanhydride cure kinetics was observed by the addition of PMMA.

DGEBN4,4'-diamino-3,3'-dimethylcyclohexylmethanelPMMA blends were prepared

by solution casting from methylene chloride. The blends were homogeneous in the

uncured state, and did not show lower critical solution temperature (LCST) or UCST

behavio~r.~'~ In ternary blends, there was an increase in boVl gelation and vitrification

time because PMMA reduced the rate of reaction between epoxy and hardener. The

curing reaction of epoxy1PMMA blends cured with DDM was followed using near

IR and dielectric relaxation spectroscopy.278 Primary amine showed greater

reactivity than secondary amtnes and gelation and vitrification time increased with

the addition of PMMA.

DDS cured epoxylPMMA blends were visually cloudy and optical microscopy

studies revealed phase separated morphology.279 Morphology of DDS cured

52 Chapter 1

DGEBNPMMA blend was analysed using atomic force microscopy (AFM). Initial

morphology was found to be strongly dependent on the initial composition of the

b ~ e n d s . ' ~ ' ~ ~ Phase inverted morphology was obtained by 20phr addition of

PMMA to DGEBA epoxy resin cured with DDS."' The size of epoxy particles

decreased from 60pm to 10pm on increasing PMMA content from 20 to 40phr.

The amount of DDS also affected the domain size. Increase in crosslink density

increased the domain size. AFM studies revealed that all the blends were phase

separated. The extent of phase separation was affected by the precure time at

8 0 " ~ . ~ ~ , ~ ~ ~ The morphology of DGEBNPMMA blend was influenced by the type

curing agent.284 DDS and MDA cured blends showed RIPS before gelation and

rate of curing increased. MCDEA cured blends showed no phase separation

before gelation. The morphology and ultimate properties of PMMA/poly(ethylene

oxide)lDGEBNDDM was studied.285 PEO was used as compatibiliser. The

addition of PEO was found to influence the viscoelastic properties of the blend and

increased the fracture toughness of the blends. But the most efficient system was

the uncompatibilised blend.

IPNS were prepared from DGEBA epoxy and PMMA.~ '~ The IPN's were found to

have better ultimate tensile strength and modulus and fracture toughness. Semi

IPN's have less properties than full IPN's. The thermal stability was better than

that of neat epoxy resin in both the cases. In both cases extensive plastic

deformation was evident. In full IPN, crosslinked PMMA acted as rigid obstacles

resulting in particle debonding and matrix cracking. In semi IPN, crack branching.

delocalisation of particles and bridging of microcracks were evident from the SEM

micrographs.

Mondragon et al.287 investigated the viscoelastic properties of PMMA modified

DGEBA epoxy resin cured with DDM in detail. Three relaxations were observed in

the dynamic mechanical spectrum corresponding to a relaxation at high

temperature corresponding to three dimensional motion of chains between

crosslinking points and crosslink point themselves during glass rubber transition,

small w relaxation between 30 and 90°C due to the local motions in highly

crosslinked regions of the matrix and broad relaxation attributed to the highly

cooperative motional process of the glycidyl ether group in the cured epoxy

mixture. The peak position changed with respect to composition of the blends and

frequency of measurements.

1.6.3.9 Phenoxy modified epoxy resin

Teng and changzS8 studied DGEBAIDDSlphenoxy blends with l-cyanoethyl-2-

ethyl-4-methylimidazole (CEMI) as catalyst. The kinetics of reaction with and

without CEMI accelerator was studied using DSC, FTIR, and rheology. Opaque.

translucent and transparent samples were obtained depending on the

concentration of CEMl by kinetic control. Phenoxy blends with TGDDM cured with

DDS followed autocatalytic mechanism. The rate of reaction was higher for the

blends due to the catalytic effect of hydroxyl groups. The hydroxyl groups reacted

with epoxy towards the final stages of cure. The morphology of the blend was

dependent on the amount of phenoxy in the blend.289

~ u o ~ ~ ~ studied the miscibility of epoxylphenoxy blends using different hardeners

like MA, PA. HHPA, DDS and DDM The blends were miscible in the uncured

state and DDS and aromatic anhydr~de (PA), cured blends were heterogeneous.

DDM and aliphatic anhydride (MA. HHPA) cured systems were homogeneous.

Phenoxy is a proton donating polymer. So it can interact with ester groups formed as

a result of curing. Pendent OH groups can react with anhydride to form copolymer

which can act as compatibiliser. Phenoxy contains pendent OH groups which is

capable of intramolecular self associations or intermolecular interactions with other

molecules through H-bonding. DDS cured resins have no favourable interaction to

form homogeneous blends. Interdomain chemical links are formed between phenoxy

and TGDDM epoxy cured with DDS at sufficiently high temperature. In

DGEBAlphenoxy blends cured with stoichiometric amount of DDS, no chemical links

were developed between epoxy and phenoxy due to the lack of epoxy groupsw'

SEM and DMA stud~es showed difference in morphology with CEMl content in

DGEBNDDSIphenoxy blendszsz Sample without and low levels of accelerator

54 Chapter 1

were heterogeneous while higher amount of accelerator resulted in homogeneous

morphology. The partial miscibility was attFlbuted to similar chemical structure and

rate of reaction. Fracture toughness of homogeneous blends was higher than that

of phase separated blends. The homogeneous blends have high chain flexibility

and the fracture mechanisms were crack blunting and local plastic deformation.

DDS cured DGEBAlphenoxy was heterogeneous and DDM cured blend was

homogeneous. Absence of phase separation in DDM was due to fast curing and

high viscosity build up. At 2Ophr phenoxy. DGEBA blends cured with DDS, formed

cocontinuous morphology and above that phase inverted morphology was

observed. Impact, fracture toughness and tensile properties were higher for DDS

cured blends. Strong interaction between components facilitates effective stress

transfer. The modulus mismatch causes stress concentration which initiated

yielding around phenoxy particles in epoxy matrix enhancing the fracture

toughness. Localised plastic deformation and crack blunting were the major

toughening mechanisms.293

1.6.3.10 Polystyrene modIfied epoxy resIn

Syndiotactic polystyrene (sPS) is a relatively new engineering plastic introduced

by ~shihara.~" The cure kinetics and phase separation process of sPS/epoxy/4,4'-

methylenebis(3-chloro-2,6-diethylaniline) (MCDEA) blend was investigated. Phase

separation and crystallisation occurred at similar times with phase separation just

being ahead of crystallisation. In the first few minutes, sPS slowed down reaction

rate and at larger times, rate of blends increased due to phase ~eparation.~"

The clystallinky of epoxy/sPS cured with MCDEA was studied. At lower concentratins

(15, 20, 26%) of sPS, [3 crystal form was present. At high sPS (64%) pure sPS

exhibited a mixture of both a and P forms. The difference was attributed to the

processing route. The absence of y and 6 forms indicated complete phase

separation. The crystallinity of sPS increased with increase in epoxy content.296

DGEBA can act as a monomer for improved processing as it decreases T, by

20 - 60K and T, by 16 - 45K with the addition of 20 to 90% DGEBA.'~' The

morphological evolution under shear was studied for amorphous PStDGEBNMDEA

Introduction 55

blends. The evolution of phase composition was not affected by static or dynamic

curing. But the final size and shape were strongly affected by the curing

process.298 Thermodynam~c analysls of phase separation revealed that j(

decreased with progress of epoxy-amine reaction. The decrease in absolute value

of entropic contribution to the free energy of mixing remained the principal driving

force behind phase separation.29g Riccardi et a13* were able to predict the

composition of phases in the blends. The interfacial structure and properties of

immiscible deuterated PS (dPS)lepoxy bilayer films were investigated using

neutron reflectivity.30' The interfacial width and growth rate strongly depended on

composition of epoxy layer but only weakly on dPS layer. Effect of resin

crosslinker composition was due to different near surface structures that result

from different epoxy stoichiometry. Schut et a1302 investigated reactive processing

approach for sPSIDGEBAiMCDEA system. The initially miscible blends underwent

RIPS. Melt viscosity of sPS reduced by 50 - 500 folds and the reaction kinetics

was slowed down with increase in concentration of sPS. The gel point was found

to have a drastic influence on the size and shape of the final epoxy particles

(60140 PSlepoxylMCDEA) and phase separation occurred by nucleation and

1.6.3.11 Polyethylene oxide modified epoxy resin

Polyethylene oxide (PEO) was mixed with epoxy resin above the T, of PEO and

TEPA was used as the curing agent304 The blends without curing agent were

transparent just above the melting point of PEO. The presence of single T,

indicated that the blends were homogeneous in the uncured state. No T, was

observed at low PEO contents (10phr). T, depression was more as epoxy content

increased. PEO was immiscible with highly crosslinked epoxy resin.305 PEO

formed miscible blends with DGEBA epoxy cured with PA due to strong

~ntermolecular interactions306 DDM cured DGEBA blends also were

homogeneous due to H-bonding between hydroxyl group and ether bond of PEO.

DDS and pyromellitic dianhydride (PMDA) formed homogeneous blends while

diethylenetriamine (DETA) cured blends were heterogeneous. The miscibility in

DDS cured system was due to the close polarity of cured network with ~ € 0 . ~ ~ '

56 Chapter I

PEOIDGEBA epoxy cured with 1,3.5-trihydroxy benzene also formed miscible

system with single composition dependent T,. It was found that intermolecular H-

bonding decreased with crosslinking Similar behaviour was observed with PEO

blended with bisphenol-S epoxy cured with phloroglucinol. 308-310

The reaction mechanism remained autocatalytic for PEOlepoxy blends cured with

DDS.~" From isothermal studies it was found that the rate of reaction decreased

with PEO and also time to reach maximum rate was decreased. This is because

PEO acted as a diluent reducing the rate of reaction.

Different spherulitic morphology was observed on blending PEO with DGEBA 312.313 epoxy resin. The spherulitic morphology is shown in Fig. 1.13.

Figure 1.13: PEO spherulitic morphology (Tc=42"C) in (A) neat PEO and (6)

PEOIDGEBA (80120) ( Y P. Huang, J. F Kuo, E M. Woo. Polym.

Int., 51, 55, 2002)

The miscibility and morphology of novolac epoxy and PEO blends in the cured

and uncured state were reported. DSC of uncured epoxy revealed single T,, which

was composition dependent. OM studies showed Maltese cross birefringence

pattern and regular shape with defined borders314 MCDEA cured DGEBNPEO

blends were homogeneou~.~ '~ The crystallisation behaviour of the blends was

dependent on the amount of epoxy resin present in the blend. MCDEA cured

epoxy resin lowered molecular rnobil~ty and hence hindered the crystallisation of

PEO in the blends.

1.6.3.12 Pdy(t-caprolactone) modified epoxy resin

DGEBA, diglycidyl ether of bisphenol-F (DGEBF) or polyglycidylether of phenol

novolaclpoly(~-caprolactone) (PCL) blend was cured with DDM. TETA or nadic

methyl PCL was partially miscible with amine cured system due to

H-bonding. Amine cured resins have less elongation and less fracture energy

while anhydride cured resin have less modulus, high elongation and high fracture

energy. Noshay and ~ o b e s o n ~ ' ~ observed that above a critical molecular weight,

PCL phase separates out in anhydride cured epoxy due to reaction between end

groups of PCL and anhydride producing a type of block copolymer. Phase

separation mechanism in PCLIDGEBAIDDS blend was investigated in detail. 318 A

PCL content of 11% was found to be the critical concentration and phase

separation occurred by nucleation and growth mechanism. At 10% PCL, spinodal

decomposition occurred and then microphase separation of PCL occurred via

nucleation and growth mechanism. The crystallisation behaviour from melt was

found to be influenced by composition. T, and curing. Addition of epoxy reduced

overall crystallisation rate and depression in T,, however curing resulted in overall

increase in crystallisation rate and depression in T,. Curing changed nucleation

mechanism and enhanced the nucleation rate31g Cured and uncured blends were

completely miscible in the whole composition range. Cured blends have reduced

crystallisation rate and crystallinity of PCL Miscibility of cured blends was due to

enthalpic contribution. H-bonding was responsible for miscibility in cured blends.320

The morphology of DDS cured DGEBNPCL blend was dependent on composition

and curing conditions. Phase separation mechanism changed with curing

temperature due to shift of critical point in phase diagram3" The time required for

crystallisation was longer for cured blends The crystallisation of PCL was affected

by the presence of DGEBA in both cured and uncured ~ ta te . "~ Depending on

cure temperature either nucleation and growth or spinodal decomposition

occurred in 3DCM cured blends.323 Cur~ng with BAPP increased crystallisation

rate and nucleation and growth of PCL were dramatically changed.324 FTlR

studies revealed that H-bonding interactions in blends were weaker than that in

neat epoxy resin. From proton-spin lattice relaxation, it was found that the blends

were homogeneous on the size scale of 20-30nm and was dependent on

58 Chapter I

composition.3z5 Reaction induced phase separation occurred in PCL/novolac/

HTMA cured epoxy resin and the morphology was found to be dependent on the

composition of the blends.3z6

Many other thermoplastics like polyphenylene oxide, 327-330 polypropylene oxide. 331 -333

po~yoxymeth~lene.~ po~~ iny l~~r ro l idone, *~~~~ polyethylene, 337.338 polypropylene. 3 ~ ~ 4 1

polyaniline. 342,343 polyether nitrilew etc. were used to modify epoxy resin.

1.6.3.13 Nanostructured epoxy blends

Thermoplastic modified epoxy resin formed macrophase structured blends as

evident from the discussions in previous sections. Novel nanost~ctured blends based

on epoxy resin was reported towards the end of last decade. Hillmyer et al.3d5

obtained nanostructured blends with epoxy resin and amphiphilic poly(ethylene

oxide)-poly(ethy1 ethylene) (PEO-PEE) and poly(ethy1ene oxide)-poly(ethy1ene-alt-

propylene) (PEO-PEP) block copolymer. The peculiarity of these copolymers is

that the PEO block is miscible in epoxy resin. Due to the miscibility of PEO block

in uncured epoxy resin, the polyalkane-PEO block copolymer formed an ordered

morphology. Gelation tends to macrophase separation, but curing inhibits the mobility

of block copolymer preventing macrophase separation. Lipic et a ~ . ~ ~ ~ investigated the

development epoxy blends using PEO-PEP copolymer. The nanostructure

morphology was retained even after curing with MDA. Rihenthaler et al.%' obtained

transparent nanostructured structures by blending and reacting an epoxy system

with polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM)

triblock copolymers. The morphology was unchanged when MCDEA was used as

the curing agent while macrophase separation occurred on curing with DDS. The

TEM micrographs shown in Fig. 1.14 show that the morphology was the same

before and after curing with MCDEA. Before reaction, the three blocks self-

organize on a nanometer scale. In PS spheres surrounded by PB nodules while

the PMMA blocks are solubilized with the epoxy precursors, forming a swollen

corona. The final structure is composed of undiluted PS and PB blocks forming

"spheres on spheres" morphology, most of the PMMA chain remaining embedded

in the epoxy network.

Introduction 59

. *

ll", '" 1I)D .m -

(a) (b) Figure 1.14: Transrn~ssion electron m~crographs obtalned for the DGEBA-

MCDEA/50wt0h SBM blend (a) before and (b) after react~on (S. R~tzenthaler, F Court, L Davrd, E G Reydet, L Leibler, J. P. Pascaulf, Macromolecules, 35, 6245, 2002)

Bates and coworkers348 used two types of amphiphilic block copolymers,

poly(ethy1ene oxide)-poly(ethylene-alt-propylene) (PEO-PEP) and poly(methy1

methacrylate-ran-glycidyl methacrylate)-poly(2ethylhexyl methacrylate) (P(MMAran-

GMA)-PEHMA) to toughen a mixture of brominated and non brominated epoxy

resin cured with phenol novolac. In the dilute limit, block copolymers self-

assemble into disordered spherical micelles, wormlike micelles or vesicles. The

different morphologies are shown in Fig. 1.15.

>{>l~cfici; $:I;LL~Ic ,; . ~ \ t ~ l r l , . wi.nnliki: ~ i j j~ .c l l<

Figure I .I51 Transmission electron microscopy images of (a) spherical micelles (PEO- PEP8), (b) wormlike micelles (PEOPEP-IS), and (c). vesicles (P(MMA- ran-GMA)-PEHMA4), each in epoxy formulations containing 25 wt % DER 560 and cured with phenol novolac (J. M. Dean, N. E Verghese, H. Q. Pham, F. S. Bates, A4acromolecules. 36, 9267, 2003)

Spherical and wormlike micelles were obtained from PEO-PEP-9 and PEOPEP-15

diblock copolymers respectively, while vesicles were generated with the P(MMA-ran-

GMA)-PEHMA compounds

Maximum toughness was observed in blends containing work like micelles as

shown in Fig. 1.16.

weight 5.j DER 560

Figure 1.16: Fracture resistance for DER383lDER560lPN containing 5 wt %

diblock copolymer; the result denoted vesicles at 50% DER560

contains 2.5% block copolymer (J. M Dean, N. E Verghese, H. 4.

Pharn, F. S. Bates, Macromolecules, 36, 9267, 2003)

1.6.4 Toughening mechanisms in thermoplastic modified epoxy resins

The optimum properties in thermoplastic modified epoxy resin were strongly

dependent on the intrinsic properties of the component materials. In general

several toughening mechanisms operate simultaneously to produce the overall

toughening effect. The important mechanisms responsible for enhancement of

fracture toughness are given below.

Introduction 61

1 . 6 4 1 Thermoplastic-particle bridging

Thermoplastic particles span the crack, wh~ch necessitates their ductile stretching

and tearing. This mechanism provides closure traction to the crack surfaces and

effectively reduces the local stress intensity factor at the crack tip. It may induce

significant toughening effect especially for highly crosslinkled epoxies, owing to

the high yield stress of the thermoplastic modifier. This mechanism is effective

only when relatively large particles with strong interfaces are present.

1.6.4.2 Crack pinning or bowing by thermoplastic particles

Rigid thermoplastic particles act as impenetrable Objects and effectively pin the

advancing crack. The pinned crack front bows out, which consumes additional

energy. This mechanism is effective only when there is good interfacial adhesion.

1 . 6 4 3 Crack deflection and/or bifurcation by thermoplastic particles

Thermoplastic particles change the crack path by causing the crack to deviate

from its principal plane of propagation andlor the crack is split into several

secondary cracks. The deflection of the crack path increases the total surface

area of the crack surface and forces crack propagation into mixed mode. The

bifurcation of a crack reduces the local stress intensity factor at the tip by

distributing it over multiple cracks. Thls mechanism accompanies other

toughening mechanisms.

1.6.4.4 Shear banding of the matrix

This mechanism provides significant increase in fracture toughness if the intrinsic

ductility of the matrix is fully exploited. The thermoplastic acts as stress

concentrators owing to the significant modulus mismatch between the particle and

the matrix. This stress concentration causes the matrix to undergo extensive

shear deformation, generally in the form of massive shear banding between the

particles. The formation of shear bands absorbs considerable energy, thereby

increasing the fracture toughness.

62 Chapter 1

1 . 6 4 5 Mirocracking of the matrix

Thermoplastic paflicles cause stress concentration and initiate massive

microcracks in the surrounding matrix, dissipating extra energy and increasing the

fracture toughness. This mechanism is effective for highly crosslinked epoxies

when the particles are relatively rigid and capable of debonding.

1 6 . 4 6 Transformation of thermoplastic partrcles

Particles of semicrystalline thermoplastics may undergo stress induced phase

transformation in crystal structure. The dilatation usually associated with this

transformation reduces tensile stresses ahead of the crack tip, thus effectively

lowering the local stress intensity factor. This mechanism is valid only for epoxies

toughened with semicrystalline thermoplastic. A schematic representatlon of

various toughening mechanisms is shown in Fig. 1.17.

Figure 1.17: Schematic representation of toughening mechanisms in

thermoplastic toughened epoxy resin (1) crack pinning, (2) crack

deflection, (3) massive shear banding, (4) crack bridging and

(5) microcracking

1.7 Morphology of the blends

Homogeneous or heterogeneous morphologies are generated upon epoxy

resin/thermoplastic blends. Most of the thermoplastics are miscible with epoxy

resin in the uncured state. But thermoplastics like polyamide 12 (PA12) and

poly(viny1edene fluoride) 349.350 are immiscible with epoxy resin. Cur~ng the

Introduction 63

~mmiscible blend invariably resulted in heterogeneous blends However, curing the

m~scible binary epoxylthermoplastic blends can result in completely miscible or phase

separated system depending on the extent of interactions between the blend

components, curing agent and curing conditions. In the absence of strong interactions,

phase separation occurred in the blends as a consequence of the increase in

molecular weight of the progressively crosslinking epoxy resin The different

morphologies developed in epoxy resintthermoplastic blends are shown in Fig. 1.18.

Epoxy resin + Thermoplastic

Mixing

r--i Homogeneous Heterogeneous

0 * + ..* * + *.* Homogeneous Heterogeneous Hetsrogeneous

Figure 1.18: Different types of morphologies in epoxy resinlthermoplastic blends

From the previous discussions on thermoplastic modified epoxy resin, it was found

that the ultimate properties of the blends were strongly dependent on the final

morphology. Most of the blends undergo RlPS upon curing. The main advantage

of RlPS is that different morphologies can be obtained by changing the

composition, curing agent and curing conditions. A uniform dispersion of rubber or

thermoplastic was obtained by RIPS. Even homogeneous morphology was

obtained by proper control of curing conditions. Depending on the composition,

64 Chapter 1

dispersed, cocontinuous or phase inverted morphologies are generated as shown

in Fig. 1.19.

Homogeneous blend

1 Phase se~aration

during curing

d~suersed thermo~lastlc co-continuous dispersed epoxy

Figure 1.19: Schematic diagram showing the evolution of morphology during

curing for epoxy resin/thermoplastic blends

The phase separation in the blends can be followed using different techniques.

These include microscopic techniques, SANS, SAXS, dielectric spectroscopy etc.

1.8 Phase separation mechanism in blends

Most of the miscible polymer blends form single phase system over a certain

temperature range. Phase separation occurs either on heating or cooling. Phase

separation can occur by different mechanisms resulting in same composition

having different morphologies. This affords a possibility of controlling the

properties of miscible polymer blends by phase separation. The occurrence of

phase separation at a given temperature is determined by the shape of the free

energy of mixing (AG,) versus composition curve

Figure 1.20: (a) LCST curve and (b) free energy of mixing versus composition for

a binary polymer mixture (G. van Den Poel, Ph. D Thes~s,

Katholfeke University Leuven, Belgium, 2003)

Fig. 1.20 shows a typical LCST curve. The composition of the homogeneous mixture

is given by C, and C', and C", are the concentrations of the nuclei. The solid line

represents the binodal curve and the dashed line represents the spinodal curve.

Phase separation can occur either by spinodal decomposition or by nucleatmn and

growth mechanism depending on the blend composition and temperature.

In the spinodal region a small fluctuation in composition will lead to phase separation

due to decrease in free energy. There is no thermodynamic barrier to phase growth

and the composition of the phases change continuously with time Spinodal

decomposition leads to cocontinuous structure. The evolution of phase growth and

correspond~ng phase structure is schematically represented in Fig. 1.21

66 Chapter 1

Figure 1.21: Phase separation in miscible polymer blend by spinodal

decomposition mechanism (a) one dimensional evolution of

concentration profiles (b) resultant phase structure (G. van Den

Poel, Ph. D Thesis, Katholieke University Leuven, Belgium, 2003)

In the region behveen spinodal and binodal points the blend is in the meta stable

region, because small concentration fluctuation does not lead to phase separation.

At sufficiently high concentration fluctuation, demixing occurs by nucleation and

growth. Fig. 1.22 shows growth process and the corresponding phase structure.

Intmduction 67

Figure 1.22: Phase separation in a miscible polymer blend by nucleation and

growth mechanism (a) one dimensional evolution of concentration

profiles (b) resultant phase structure (G. van Den Poel, Ph. 0 Thesis,

Katholieke University Leuven, Belgium, 2003)

1.9 Conclusion

The characteristic properties which made epoxy resin suitable for various

applications are easy cure, low shrinkage on cure, flexibility in processing,

excellent adhesion, high mechanical and thermal properties. A range of

compounds is available for curing epoxy resin. The properties of the cured resin

were dependent on the curing agent and curing conditions used Numerous

components other than curing agents are added to epoxy resins to improve its

properties for specific applications. The br~ttleness of epoxy resins owing to highly

crosslinked structure was effectively reduced by the addition of rubber or

thermoplastic to epoxy resin. Thermoplastics are used to toughen epoxy resin,

where high temperature properties are required. The cure kinetics of the blends

were affected by the curing agent and type of modifier used. The ultimate

properties are influenced by the curing agent and curing conditions used,

composition of the blends, molecular weight and presence of reactive functional

groups in the modifier and morphology of the blends. Reaction induced phase

68 Chapter 1

separation occurred in most of the blends. Dispersed, cocontinuous or phase

inverted morphology was obtained as a result of reaction induced phase

separation. Secondary phase separation occurred in some blends. Depending on

the composition of the blends, phase separation occurred by spinodal or

nucleation and growth mechanism. The final morphology was dependent to a

large extent on the composition, curing agent and curing conditions. The

improvement in fracture toughness was due to several toughening mechanisms

like ductile tearing of domains, crazing, shear banding, local plastic deformation of

the matrix, crack bridging, crack path deflection and rubber particle cavitation.

1.10 Scope and objectives of the work

From the foregoing discussion it is understood that the properties of toughened

epoxy resin are dependent on the curing conditions and curing agent used.

properties of modifier and morphology of the blends. Thermoplastics are being

used to toughen epoxy resin without reduction in thermo-mechanical properties.

Thermoplastics like PES, PEI, PMMA, PEEK etc. are used to toughen difunctional

as well as multifunctional epoxy resins. Among the various epoxy resins, DGEBA

resin is being extensively used for multitudes of applications because of its

versatility. DGEBA epoxy resin cured with DDS has excellent mechanical and high

temperature properties.

From the review on thermoplastic toughened epoxy resins, it was observed that

relatively few systematic studies were conducted on PEEK toughened epoxy

resins. PEEK is a semicrystalline engineering thermoplastic with excellent

properties. Relatively less literature on PEEK toughened epoxy was due to the

difficulty in processing the blends. Miscibility of the modifier in epoxy resin before

curing is one of the prerequisites for obtaining excellent toughness. From the

literature it was found that modified PEEK with less crystallinity was used to

toughen difunctional and multifunctional epoxy resins.

The important objective of the present work is to develop an easily processable

DGEBA epoxy resin toughened with PEEK, which can be used as matrix resin for

carbon fibre reinforced composites. To achieve this goal, a series of PEEK

polymers with pendent alkyl groups having different molecular weights and

functionality were synthesised. Different alkyl groups such as ditert-butyl, tert-butyl

and methyl groups and sulfone group were introduced on the polymer in order to

change the semicrystalline nature of PEEK. Functional groups were introduced to

improve the interactions between epoxy resin and modified PEEK to have good

interfac~al adhesion between PEEK and epoxy resin. The polymers were

synthesised by the nucleophilic substitution reaction of 4.4'-difluorobenzophenone

with bisphenol compounds in high boiling solvents such as N-methyl-2-pyrrolidone

and sulfolane using potassium carbonate as catalyst. The molecular weight and

terminal functional groups were controlled using Carother's equation. The

structure and properties of the synthesised polymers were evaluated using various

analytical techniques.

The modified PEEK polymers were blended with epoxy resin and the miscibility of

modified PEEK in epoxy resin was analysed using DSC and the T,-composition

behaviour was modelled using several empirical equations. The blends were

cured with DDS. DDS was used as the curing agent in order to achieve high T,

and excellent mechanical properties. The cure kinetics of selected blend systems

were investigated since the curing conditions play an important role in determining

the ultimate properties of the blends. FTlR was used to study the interactions

between blend components and the completion of cure. The morphology and

viscoelastic properties of the blends were investigated with respect to composition

and type of modifier used. The effect of composition and modifier type on the

fracture toughness, tens~le and flexural properties was investigated in detail. The

fracture mechanisms responsible for the enhancement in fracture toughness have

been investigated. The morphology of the blends was investigated and correlated

with their properties The thermal stability of the blends was investigated using

TGA. Unidirectional laminates as well as carbon cloth laminates were made uslng

toughened epoxy as matrix. The mechanical properties and fracture energy of the

com~osites were also determined.

70 Chapter 1

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