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CHAPTER 1

INTRODUCTION

From pre-historic times, man has exploited for his own use the

properties of natural polymers such as horns, waxes and bitumens. Over the

years, it was gradually learnt that the properties of such materials could be

improved by techniques, such as purification and modification with other

substances. By the turn of the 19th

century, with the explosion of scientific

knowledge in fields such as chemistry and physics, coupled with the demands

from industry for materials with properties which could not be found in

nature, the scene was set for the development of a whole range of new

materials; among them are the early synthetic polymers.

Polymers have obviously not been discovered overnight. They came

out of long and persevering studies by a host of motivated scientists, whose

work has enriched human life. Today, polymers have become an

indispensable material for us and it is difficult to think of daily life without

them.

Just as an architect chooses bricks, stones, logs of wood etc. in

varying shapes, sizes and patterns to create various designs, a chemist

produces innumerable plastics, rubbers, foams, fibers, adhesives, composites,

etc. by judiciously combining various chemicals under desired conditions.

Thus, we can say that a polymer chemist is an “architect of molecules.”

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Depending on its ultimate form and use, a polymer can be made

available in many forms such as plastics, rubber, foam, adhesives,

composites, etc. Out of all the forms, composites are widely used in high

performance applications. High strength and light weight remain the winning

combination that propels composite materials into new arenas, but other

properties are equally important. Composite materials offer good vibrational

damping and a low coefficient of thermal expansion, characteristics that can

be engineered for specialized applications which translate into a finished

product that requires less raw material, fewer joints and fasteners, and shorter

assembly time. Composites have a proven resistance to temperature extremes,

corrosion and wear, especially in industrial settings, where these properties do

much to reduce product life-cycle costs.

Composites differ from traditional materials in that the composite

parts comprise two distinctly different components – fibres and a matrix

material (most often a polymer resin) that, when combined, remain discrete

but function interactively, to make a new material whose properties cannot be

predicted by simply summing the properties of its components. In fact, one of

the major advantages of the fibre / resin combination is its complementary

nature.

The structural properties of composite materials are derived

primarily from the fibre reinforcement. High performance composites derive

their structural properties from continuous, oriented high-strength fibre

reinforcement – most commonly carbon, glass or aramid – in a binding matrix

that promotes processability and enhances mechanical properties, such as

stiffness, and chemical and hygroscopic resistance.

The matrix material in the composite can be a polymer. In the last

forty five years there has been a flurry of activity in the synthesis and

development of high performance and high temperature polymers. This has

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been, in large part, due to the need for advanced materials required for a

diverse range of applications, including aerospace, automotive and

microelectronic industries. These applications often demand a unique

combination of properties, including high glass transition temperatures,

toughness, good adhesion, oxidative and thermal stability, and low a dielectric

constant.

In this regard, a large number of polymers have been developed,

which can be broadly categorized as either thermosets or thermoplastics.

Examples of thermosets are unsaturated polyesters, epoxies, BMI, etc., while

thermoplastics include PEEK (polyetherether ketone), polysulphones,

polyether sulphones etc.

Unsaturated polyesters are the most widely used matrix material in

composite applications, but these resins are less resistant to moisture and have

low performance properties, which limit their application in advanced

composites.

Epoxy thermosets have been widely used as matrix resins for

advanced composites due to their outstanding mechanical and thermal

properties. These properties include high modulus and tensile strength, high

glass transition temperature, high thermal stability and moisture resistance.

When cured, epoxy resins form a highly cross-linked three-dimensional

infinite network, whose microstructure provides the desirable engineering

properties. However, the highly cross linked micro structure also makes it

brittle. It also absorbs moisture that acts as a plasticizer during service, and

reduces the glass transition temperature leading to a decrease in dimensional

stability. This undesirable property restricts epoxy thermosets from

applications requiring high impact and fracture strengths.

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Another important matrix material used for advanced composites is

polyimide. Polyimides possess much better mechanical and thermal properties

than epoxies, but the drawbacks of this resin are low moisture resistance and

poor processability. Bismaleimides have better processability, but they are

excessively brittle.

Processabilty is another pivotal issue regarding the use of high

temperature thermosetting polymers for composite applications. Generally,

these systems require high temperatures for processing, and therefore, cannot

be effectively used in conjunction with low cost processing techniques like

resin transfer moulding. Therefore, attempts for developing novel high

performance thermosetting polymers with easy processing and performance

characteristics, have led to the development of cyanate ester resins. Cyanate

ester resins are a novel class of thermosetting materials, which exhibit

enhanced physical and thermal properties, when compared to traditional

polymer systems like epoxies. These cyanate esters display features including

high glass transition temperatures, low dielectric loss, low moisture

absorption, low corrosion potential, and easy processing, and thus show

promise in technical fields like aerospace and microelectronics.

With the availability of functional materials, and the feasibility of

embedding or bonding them into composite structures, new smart structural

concepts are emerging to be attractive for potentially high-performance

structural applications (Maugin 1988, Gandhi and Thompson 1992, and

Srinivasan and McFarland 2001). A smart structure is one that has surface

mounted or embedded sensors and actuators, so that it has the capability to

sense and take corrective action. Numerous conferences, workshops, and

journals dedicated to smart materials and structures, stand testimony to this

growth. The technological implications of this class of materials and

structures are immense: structures that monitor their own health, process

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monitoring, vibration isolation and control, medical applications, damage

detection, and noise and shape control.

As applications of active vibration/deflection controls in aerospace,

automobile industries and building applications, smart structures have

received considerable attention (Lowey 1997). Vibration and the shape

control of structures are essential to achieve the desirable performance in

modern structural systems. Advances made in the design and manufacturing

of smart structure systems, improve the efficiency of the structural

performance.

1.1 MATRIX MATERIAL

Although it is undoubtedly true that the high strength of composites

is largely due to the fibre reinforcement, the importance of the matrix material

cannot be underestimated as it provides support for the fibres, and assists

them in carrying the loads. It also provides stability to the composite material.

The resin matrix system acts as a binding agent in a structural component in

which the fibres are embedded. In a composite material, the matrix material

serves the following functions-

• Holds the fibres together

• Protects the fibres from the environment

• Distributes the loads evenly between fibres, so that all the

fibres are subjected to the same amount of strain

• Enhances the transverse properties of a laminate

• Improves the impact and fracture resistance of a component

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• Helps to avoid the propagation of crack growth through the

fibres, by providing an alternative failure path along the

interface between the fibres and the matrix.

• Carries interlaminar shear

The matrix plays a minor role in the tensile load-carrying capacity

of a composite structure. However, the selection of a matrix has a major

influence on the interlaminar shear, as well as on the in-plane shear properties

of the composite material. Interlaminar shear strength is an important design

consideration for structures under bending loads, whereas the in-plane shear

strength is important for torsion loads. The matrix provides lateral support

against the possibility of the fibre buckling under compression loading, thus

influencing, to some extent, the compressive strength of the composite

material. The interaction between the fibres and the matrix is also important

in designing damage tolerant structures. Finally, the processability and defects

in a composite material depend strongly on the physical and thermal

characteristics, such as viscosity, melting point, and curing temperature of the

matrix.

1.1.1 Properties of a Matrix

The needs or desired properties of the matrix which are important

for a composite structure are as follows:

• Reduced moisture absorption

• Low shrinkage

• Low coefficient of thermal expansion

• Good flow characteristics, so that it penetrates the fibre

bundles completely, and eliminates voids during the

compacting/curing process

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• Reasonable strength, modulus and elongation (elongation

should be greater than the fibre)

• Must be elastic to transfer the load to the fibres

• Strength at elevated temperature (depending on application)

• Low temperature capability (depending on application)

• Excellent chemical resistance (depending on application)

• Should be easily processable into the final composite shape

• Dimensional stability (maintains its shape)

Out of the many matrix materials known, epoxy resin is a widely

used matrix material in composite applications.

1.2 EPOXY RESIN

Epoxy resin is defined as a molecule containing more than one

epoxide groups. The epoxide group, also termed as an oxirane or ethoxyline

group, is shown in Scheme 1.1.

Scheme 1.1 Structure of Epoxide group

These resins are thermosetting polymers, and are used as adhesives,

high performance coatings, and potting and encapsulating materials. These

resins have excellent electrical properties, low shrinkage, good adhesion to

many metals, and resistance to moisture, and thermal and mechanical shock.

Viscosity, epoxide equivalent weight and molecular weight are the

important properties of epoxy resins.

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1.2.1 Types of Epoxy Resins

There are two main categories of epoxy resins, namely, glycidyl

epoxy, and non-glycidyl epoxy resins. Glycidyl epoxies are further classified

as glycidyl-ether, glycidyl-ester and glycidyl-amine. Non-glycidyl epoxies are

either aliphatic or cycloaliphatic epoxy resins. Glycidyl epoxies are prepared

via a condensation reaction of an appropriate dihydroxy compound, dibasic

acid or a diamine, and epichlorohydrin, while, non-glycidyl epoxies are

formed by the peroxidation of an olefinic double bond.

Glycidyl-ether epoxies such as, diglycidyl ether of bisphenol-A

(DGEBA) and novolac epoxy resins are the most commonly used epoxies.

1.2.2 Diglycidyl Ether of Bisphenol-A

Diglycidyl ether of bisphenol-A (DGEBA) shown in Scheme 1.2 is

a typical commercial epoxy resin and is synthesised by reacting bisphenol-A

with epichlorohydrin in the presence of a basic catalyst.

Scheme 1.2 Structure of the DGEBA

The properties of the epoxy resins depend on the value of n, which

is the number of repeating units commonly known as the degree of

polymerization. The number of repeating units depends on the stoichiometry

of the synthesis reaction. Typically, n ranges from 0 to 25 in many

commercial products.

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1.2.3 Novolac Epoxy Resins

Novolac epoxy resins shown in Scheme 1.3 are glycidyl ethers of

phenolic novolac resins. Phenols are reacted in excess, with formaldehyde in

the presence of an acidic catalyst to produce phenolic novolac resin. Novolac

epoxy resins are synthesised by reacting phenolic novolac resin with

epichlorohydrin in the presence of sodium hydroxide as a catalyst.

Scheme 1.3 Structure of novolac epoxy resin

Novolac epoxy resins generally contain multiple epoxide groups.

The number of epoxide groups per molecule depends upon the number of the

phenolic hydroxyl groups in the starting phenolic novolac resin, the extent to

which they reacted, and the degree of low molecular species being

polymerised during synthesis. Multiple epoxide groups allow these resins to

achieve high cross-link density resulting in excellent temperature, chemical

and solvent resistance. Novolac epoxy resins are widely used to formulate the

moulding compounds for microelectronics packaging, because of their

superior performance at elevated temperature, excellent mouldability and

mechanical properties, superior electrical properties, and heat and humidity

resistance.

1.2.4 Curing of Epoxy Resins

The curing process is a chemical reaction in which the epoxide

groups in the epoxy resin react with a curing agent (hardener) to form a highly

crosslinked, three-dimensional network. In order to convert epoxy resins into

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a hard, infusible, and rigid material, it is necessary to cure the resin with a

hardener. Epoxy resins cure quickly and easily at practically any temperature

from 5-150oC depending on the choice of the curing agent.

1.2.5 Curing Agents (Hardeners)

A wide variety of curing agents for epoxy resins is available,

depending on the process and properties required. The commonly used curing

agents for epoxies include amines, polyamides, phenolic resins, anhydrides,

isocyanates and polymercaptans. The cure kinetics and Tg of the cured

system, are dependent on the molecular structure of the hardener. The choice

of the resin and hardeners depends on the application, the process selected,

and the properties desired. The stoichiometry of the epoxy-hardener system

also affects the properties of the cured material. Employing different types

and amounts of hardener which, tend to control the cross-link density, vary

the structure.

The amine and phenolic resin based curing agents, described below,

are widely used for the curing of epoxy resins.

1.2.5.1 Amine based curing agents

Amines are the most commonly used curing agents for epoxy cure.

Primary and secondary amines are highly reactive with epoxy. Tertiary

amines are generally used as catalysts, commonly known as accelerators for

cure reactions. The use of an excessive amount of catalyst achieves faster

curing, but usually at the expense of working life, and thermal stability. The

catalytic activity of the catalysts affects the physical properties of the final

cured polymer.

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1.2.5.2 Phenolic based curing agents

Epoxy resins when cured with a phenolic hardener, give excellent

adhesion, strength, and chemical and flame resistance. Phenolic novolac-

cured epoxy systems are mainly used for encapsulation, because of their low

water absorption, and excellent heat and electrical resistance. The usefulness

of epoxy resins in many engineering applications is often limited by their

brittle nature and poor thermal conductivity

1.3 TOUGHENING OF EPOXY SYSTEMS

The epoxy resin is the most widely used matrix material for many

structural composites. It has many good properties like stiffness, low

shrinkage, good adhesion to glass/ carbon fibre etc. Unfortunately, the very

factor contributing to its high stiffness and heat resistance leads to its main

draw back, viz (lack of toughness), and that is its highly cross-linked

structure. Aircraft structures are often subjected to impact loads during flight,

which can cause severe damage to the structure by delamination. Toughening

can help in developing damage tolerant structural components for aircraft.

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

failure, i.e., the total amount of energy required to cause failure.A rough idea

about the fracture toughness of various materials is given in Table 1.1, which

shows that rubber toughened epoxy resins can match engineering

thermoplastics in fracture toughness values. Composites made of such matrix

systems are expected to be tougher, and hence more useful.

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Table 1.1 Fracture toughness of various materials (Bascom et al. 1989)

MaterialFracture toughness

range (kJ/m2)

Inorganic glass Very low

Epoxy/Polyester resins 10-1

Polysulphones/rubber toughened epoxy 100

Metals >10

1.3.1 Various approaches to toughening epoxy resins

The need for toughening epoxy resins was felt in the mid 1960 s.

Initially, solid rubbers like nitrile and polysulphide rubbers were

found to flexibilise epoxy resin. Such resin systems were found to be suitable

for adhesives as these rubber modified resins could sustain large

deformations, and hence, gave better peel joints than the unmodified epoxy

based adhesive. However, as they caused unacceptable levels of deterioration

of stiffness and thermal properties for composites, better tougheners were

needed.

Another method employed the introduction of inorganic as well as

organic filler materials. Of these, the organic systems, especially the one

based on elastomers, have come to the attention of material scientists. In

particular, the amine-, carboxyl- and, hydroxyl-terminatedpoly (butadiene-

co- acrylonitrile) systems (i.e., ATBN, CTBN and HTBN respectively) have

found favour, as reports of inclusion of the one of these rubbers in epoxy

matrix have indicated an increased toughness value at the expense of the

modulus and glass transition temperature (Kunz et al. 1982 and Alan Meek

1974).

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The next method is the inclusion of thermoplastics, such as

polysulphones, polyether sulphones and polyether imides into epoxy networks

(Nam Gyun Yun et al. 2004; Mimura et al. 2000; Hourtson et al. 1991; and

Akay et al. 1993). These studies show a good improvement in thermal and

mechanical properties. However, these thermoplastic polymers show poor

solubility in organic solvents, leading to difficulties in processing.

Yet another method is transverse stitching, z-spinning and 3D

weaving or braiding of reinforcement (Velmurugan et al. 2007 and Gwo-

Chung Tsai et al. 2005). It is observed that these techniques increase the

Mode I delamination toughness several times higher than that of woven fibre

reinforcement composite specimens. It is very expensive when compared to

the other toughening mechanisms.

Later toughening of epoxy was tried with interpenetrating polymer

networks (IPNs). Several studies have demonstrated that the fracture energy

of brittle matrices may be significantly improved with the formation of

interpenetrating polymer networks (Robert Vabrik et al. 1998; Harani et al.

1998 and Raymond and Bui 1998). Polyurethanes have excellent elasticity

and high impact strength; therefore, in proper ratio with epoxy resins,

materials with the desired mechanical and thermal properties can be produced

by a blending technique utilizing the interpenetrating polymer networks

(IPNs) of the two polymer components. However, these have poor mechanical

and thermal properties.

Another area which continues to attract research interest is the field

of thermoset-thermoset polymer blends, in particular, the incorporation of

inherently tough polymers into the brittle polymer systems, in order to impart

improvements in fracture toughness in the resulting blends. Hence, among the

different materials used for the modification of epoxy resins, cyanate esters

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are expected to be the best material to improve the thermomechanical

properties. As compared to epoxies, they are inherently tough, have

significantly better electrical properties and lower moisture uptake. Cyanate

esters also offer ease of handling and processing similar to that of epoxy resin

systems. In addition the dicyanate monomers are expensive to prepare, and

hence, blending a small amount of dicyanate with epoxy resin to get a

polymer resin with superior properties seems to be the best way. Thus,

blending of epoxy with cyanate ester resin continues to attract research

interest in order to impart improvements in fracture toughness, without

compromising other mechanical and thermal properties.

1.4 CYANATE ESTER RESIN

The term “cyanate ester resin” is used to describe prepolymers and

cured resins, the former containing the reactive ring forming cyanate (-OCN)

functional group. Attempts begun in 1857 to react unsubstituted phenoxide

with cyanogen halides produced only mixtures of imidocarbonates and

cyanurates. O-alkylated aryl cyanates were prepared by this method in 1960.

Synthesis via the thermolysis of 1,2,3,4 thio-triazoles was also successful, but

too expensive for commercial interests. Most alkylcyanates were found to

readily undergo exothermic isomerization to the more stable isocyanate form.

Exceptions to this are cyanates of bicyclic alcohols having the OH group in

the bridgehead position, and acidic alcohols containing electronegative

substituents such as halogens.

Based on the cyanogen halide chemistry, the first simple route to the

synthesis of cyanate esters by reacting phenol with cyanogen halide, was

discovered by Grigat and Pütter in 1963.

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ArOH + ClCN ArOCN

Base.HCl

Base+

-

Scheme 1.4 Synthesis of cyanate esters

Among the various reported routes for the Scheme 1.4 synthesis of

cyanate esters, the first reported methods were not quite as simple and

resulted in relatively low yields.

Aryl dicyanates were found to undergo nearly quantitative

conversion to cross-linked cyanate homopolymers by cyclotrimerization to S-

triazines. Tractable prepolymers of up to 50% conversion, produced

commercially by thermally quenching the cyclotrimerization reaction, were

found to be superior for pressure lamination than the low viscosity molten

monomers.

It was discovered that blends of cyanate esters and epoxides

coreacted to form cost-effective hybrids. The complex reaction pathway,

involving cyanate trimerization, epoxide insertion and ring cleavage with

additional epoxide to form substituted oxazolidinones, was not investigated

until much later. Nevertheless, blends of bis-phenol A dicyanate prepolymer

with 45-55% epoxy resin were the predominant cyanate ester, of the 1980s.

With the advancement of integrated circuit technology, interest was

generated in the 1970s, for resins with low dielectric constants to achieve

faster signal propagation via reduced resistance to the passage of

electromagnetic fields. Increased glass transition temperature, Tg, for

matching molten solder temperatures (~250°C) was also of interest for

improving the dimensional stability and reliability of multilayer circuit

boards. Bayer AG developed a prepolymer of bis-phenol A dicyanate and

commercialized this laminating solution as Triazine A resin in 1976. Two

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years later, Bayer exited this business, citing a premature market. A private

communication suggested that carbamate impurities resulting from the use of

an aqueous base during monomer synthesis and zinc co-ordination metal

catalysts, may have contributed to the severe blistering of some laminates

during the solder application. Bayer AG licensed their CE technology to

Mitsubishi Gas chemical in 1978 and to Celanese in 1984.

Mitsubishi Gas Chemical commercially introduced BT resins,

blends of bisphenol. A dicyanate and or its prepolymers with bismaleimides

BMI in 1978. The basic patent describes gel times for CE blends with several

BMIs, including a Michael addition product of methylene dianiline and

excess BMI monomer derived from the same aromatic amine. Although

isoureas produced from the reaction with cyanates and secondary aromatic

amine are known cyclotrimerization catalysts, no supporting evidence was

offered for the proposed heterocyclic rings reported in subsequent

publications, from the cycloaddition reactions between cyanates and

maleimide C=C unsaturation. Mitsubishi Gas Chemical Co. supplied BT

resins in addition to the formulated prepreg and clad laminate to circuit board

manufacturers.

Celanese Corp. (now Ciba-Geigy) initiated research in 1981 that

utilized chemically tailored bisphenols and polyphenols to expand CE

capabilities in the directions to lower dielectric loss properties, increased

hydrolytic and thermal stabilities, lower cure temperatures and liquid

monomer forms. These efforts combined with those of Dow Chemical Co.

and Allied Signal have expanded the family of commercially available CE

monomer, to the current seven structurally distinct monomers and numerous

other speciality monomers and prepolymers.

Beginning in 1979, considerable research was directed at

toughening CE resins by alloying them with initially soluble engineering

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thermoplastics that phase separated into continuous morphologies during

cure. These efforts have produced composites for aircraft primary structures

with melt processability similar to that of thermosetting epoxies, but which

pass the wet condition tests for supersonic aircraft rated at 177°C skin

temperatures. The same concept has been extended to ketone laminating

solutions for toughened circuit board laminates with improved drilling

characteristics and thin (50-100 m) double sided circuits with robust

handling properties for multichip module construction (Treliant Fang and

David A. Shimp 1995).

Today, CEs are established thermosetting resins for insulating high

speed, high density electronic circuitry, as matrix resins for aircraft

composites, geostationary broadcast satellites, radomes and antennae, and

have potential as versatile adhesives as well as passive wave guides or active

electrooptic components for processing light signals in fibre optic

communication.

1.4.1 Curing of Dicyanate

Cyanate ester resins are bisphenolic or polyphenolic derivatives

containing the ring forming cyanate (-OCN) functional groups. Chemically,

this family of thermosetting monomers and their pre-polymers are esters of

bisphenol (or polyphenol) and cyanic acid. The esters cyclotrimerize to form

substituted triazine rings on heating. Cross linking of cyanate esters occurs

via cyclotrimerization to form 3-dimensional networks of oxygen linked

triazine rings. The curing reaction is classified as an addition polymerization

and occurs without the emission of volatile by-products.

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NCO Ar OCN N

N

N

OAr

OArArO

--

--

--[

n

Three Dimensional Polycyanurate N

Catalyst

Heat/

Scheme 1.5 Formation of three dimensional networks by cyclotrimerization

Pure cyanate ester monomers will cure relatively slowly. However,

the addition of a catalyst exponentially increases the cure rate. Effective

catalysts recommended by Rhone-Poulenc for cyclotrimerization were a

combination of transition metal complexes and active hydrogen, non-volatile,

liquid phenolic compounds, eg. nonyl phenol, that serves as a co-catalyst.

The latter also promoted enhanced solubility of the metal catalyst in the

cyanate resin. In general, organic soluble compounds of most coordination

metals serve as potent trimerization accelerators. Zinc, copper, manganese

and cobalt compounds were preferred over the more reactive transition metals

like iron, tin, titanium, lead and antimony. This solution is consistent with the

observation that the latter group promotes the undesirable hydrolysis of an

otherwise stable cyanate ester linkage. Specifically, chelated metals of

acetylacetonates are less active hydrolysis catalysts (relative to the metal

carboxylates, octoates and naphthenates) and also have the added advantage

of increased latency (less trimerization activity at room temperature per unit

of elevated temperature activity) due to the lower activity of the chelate.

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1.4.2 Copolymerization with Epoxides

Early reports on the epoxide–cyanate co-reaction noted the

formation of oxazole (5-membered ring) structures without elaboration on the

mechanism or the complete reaction scheme. Publications in the period 1987

–90 noted that the trimerization of the cyanate group preceded reactions

involving epoxide, and demonstrated that cured hybrid properties were

independent of the CE prepolymerization, and thus proposed additional

epoxide consumption via polyetherification catalyzed by the triazine ring.

Monofunctional epoxy and CE model compounds were first

employed in 1988 to separate and identify reaction products by GPC and IR.

Publications by Bauer and later by Shimp, elucidated and supported a

complex reaction pathway partially described in Scheme 1.6. The sequence

of CE trimerization followed by epoxide insertion, isomerization of glycidyl

cyanurates to isocyanurates and isocyanurate ring cleavage / rearrangement

with additional epoxide to form substituted oxazolidinones, can be observed

by heating the trimer of a mono cyanate with monoepoxide. The complicated

insertion – rearrangement – cleavage sequence is expected to occur only at

higher temperature, due to both the lack of homoconjugation in the alkyl

cyanurates and the high activation barrier to breaking the isocyanurate ring.

(1) Trimerization

Trimerization

3 Ar' OCNN

N

N

OAr

OArArO

Aryl cyanurate

Aryl cyanate

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(2) Epoxide Insertion

N

N

N

OAr'

Ar'O OAr'

Aryl Cyanurate

+ 3 Ar" CH2 CH CH2O

O

N

N

N

OAlk

AlkO OAlk

Alkyl Cyanurate

Alk = CH2 CH CH2 O Ar"

OAr'

Epoxide

(3) Rearrangement

N

N

N

OAlk

AlkO OAlk

Alkyl Cyanurate

N

N

N

O

O O

Alk

Alk Alk

Alkyl Cyanurate

(4) Ring cleavage and reformation

3 Ar" CH2 CH CH2O

O

+N

N

N

O

O O

Alk

Alk Alk

Alkyl Cyanurate

3 Ar" CH2O

N

CH2

O

CH

O

Alk

Epoxide

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(5) Direct Ring Formation

Ar' OCN

Aryl cyanate

+ Ar" CH2 CH CH2O

O

Epoxide

Ar" CH2O

N

CH2

O

CH

O

Alk

Scheme 1.6 Proposed cyanate – epoxy reaction pathway

Reactions (1) and (5) are the major oxazolidinone reactions

occurring at lower temperatures (T < 150°C) (Martin et al. 1999).

1.5 SMART STRUCTURES

A smart structure involves distributed actuators and sensors, and one

or more microprocessors that analyze the response from the sensors, and use

the distributed parameter control theory to command the actuator to apply

localized strains. A smart structure has the capability to respond to a changing

external environment (such as loads and shape changes) as well as to a

changing internal environment (such as damage or failure). Many types of

actuators and sensors are being considered, such as piezoelectric materials,

shape memory alloys, electrostrictive materials, magnetostrictive materials,

electrorheological fluids and fiber optics. These can be integrated with the,

main load carrying structure by surface bonding or embedding without

causing any significant changes in the structural stiffness of the system.

Among these materials, the piezoelectric are the most popular. They undergo

surface elongation (strain) when an electric field is supplied across them, and

produce a voltage when surface strain is applied, and hence can be used both

as actuators and sensors. In an applied field these materials, however,

generate a very low strain but cover a wide range of actuation frequency. The

most widely used piezoceramics (such as Lead Zirconate Titanate) are in the

form of thin sheets, which can be readily embedded or attached to a

composite structure.

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1.5.1 Piezoelectric Materials

Piezoelectric materials have become very popular in the dynamic

sensing, actuation and control of active structural and mechanical systems.

They can act both as actuators and sensors. In the development of intelligent

structural systems, piezoelectric are widely used as sensors and actuators for

the dynamic monitoring and control of structural and mechanical systems. In

general, piezoelectric materials respond to force/pressure and generate a

charge/voltage; this is referred to as the direct piezoelectric effect; on the

other hand, the material exhibits stress/strain changes when a strong external

voltage/charge is applied, and this is called the converse piezoelectric effect.

In addition, the material would also respond to a temperature

fluctuation and generate a charge/voltage, and this is referred to as the

piezoelectric effect. It has been over 120 years since the first discovery of

piezoelectric materials. However, it is only relatively recently that engineers

and scientists have again recognized the potential of piezoelectric materials in

distributed actuators and sensors, high precision systems,

microelectromechanical systems, and adaptive structural systems.

Consequently significant efforts have been devoted to the research and

development of piezoelectric technologies in recent years. Besides mechanical

and electrical coupling and interaction, temperature can also influence the

performance of piezoelectric devices. For example, temperature variation can

introduce voltage/charge generation in piezoelectric sensors. In addition,

control voltage can cause a temperature rise in piezoelectric actuators.

1.6 SCOPE AND OBJECTIVES

In recent years, a new damping material has been developed on the

basis of a new damping mechanism. For some specialized composites,

mechanical vibrating energy is first transmitted to piezoelectric ceramic

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powders and converted into an alternating potential energy by the

piezoelectric effect. Such an energy transferring effect is referred to as the

piezo-damping effect (Sheng et al, 2008). Lead Zirconate Titanate (PZT) is

usually used as piezoelectric ceramic fillers to construct the piezo-electric

units in the polymer matrix. It is well known that reinforcement can improve

the mechanical property of the composite greatly. The damping property of

the composite may also be improved, if the reinforcement is chosen properly

and combined with the matrix in a special way, to make varied damping

mechanisms playing roles together in the composite. Thus the work throws

light on the mechanical and damping properties of the quarternary composite

with varying cyanate and PZT loading, and on analyzing the effect of cyanate

and PZT loading and on the properties.

In view of the challenges associated with the manufacturing of the

high performance composites discussed above, the present research proposal

is focused on the,

Development of new matrix material

Determination of the mechanical properties of the prepared

composites

Application of newly developed composites

The objectives of the present work are given below.

1. Development of the matrix material.

Arocy b10 (bis phenol dicyanate) is chosen to prepare

blends in different loading levels of 0%, 20%, 40% and

60% with commercial epoxy resin system (LY556) using

DDM(HT972) as curing agent.

24

A ceramic material, namely, lead zirconate titanate is

loaded in different loading levels of 10%, 20% and 30%

with the best cyanate loaded epoxy resin system.

2. Fabrication of composite plates

The fabrication of the Piezothermoelastic polymer matrix

E-glass fibre composites using various blends with the

combination of the following matrix materials:

a. Epoxy Resin

b. Cyanate Ester

c. Lead Zirconate Titanate

3. Testing and determination of the Mechanical properties

Determination of the following mechanical properties:

a. Tensile Strength

b. Tensile Modulus

c. Flexural Strength

d. Flexural Modulus

e. Fracture Toughness

4. Vibration analysis of the fabricated composites with and

without the piezoelectric material.(Experimental and Finite

Element Analysis)

Study of the natural frequency, damping factor, stiffness,

storage modulus and loss modulus

5. Performance and emission characteristics of the lead zirconate

titanate loaded cyanate modified epoxy coated combustion

chamber in a diesel engine

25

Study of the brake fuel consumption, brake thermal

efficiency, pressure, heat release rate, hydrocarbon

emission, oxides of nitrogen, carbon monoxide emission

and exhaust gas temperature.

The following are the contributions of this thesis:

I Scientific contribution

i) Development of a Piezothermoelastic cyanate based epoxy

composite material.

II Technical contribution

i) Application of an analytical expression to determine the

damping factor and modulus of the developed composite

material.

ii) a. Fabrication of the Piezothermoelastic epoxy-cyanate

ester matrix glass fibre composite.

b. Determination of the tensile properties, flexural

properties and fracture toughness of the fabricated

composite.

c. Determination of the vibration characteristics of the

fabricated composites.

III Contribution towards practical application

i) Use of the developed composite for high temperature

application in an engine, which can be extended to

aircraft also.

26

1.7 LITERATURE SURVEY

1.7.1 Epoxy Toughening

Kinloch et al (1983) studied the microstructure and fracture

behaviour of an unmodified and a rubber-modified epoxy. Values of the stress

intensity factor, KIc, at the onset of crack growth, the type of crack growth,

and the detailed nature of the associated fracture surfaces have been

ascertained. Both materials exhibit essentially the same types of crack growth,

but the values of KIc for the rubber-modified material were usually

significantly higher than those for the unmodified epoxy. The mechanisms for

this increased toughness have been considered, and one that accounts for all

the observed characteristics, has been proposed.

Smiley and Pipes (1987) have investigated the rate effect on Mode I

interlaminar fracture toughness in graphite/PEEK and graphite/epoxy

composites. The tests were performed over a range of crosshead speeds from

4.2 x 10-6

to 6.7 x 10-1

m/s. The results indicate that the toughness of both

material systems is rate sensitive. The Mode I interlaminar fracture toughness

of the graphite/PEEK material decreases from 1.5 to 0.35 kJ/m2 over five

decades of the opening rate. The fracture toughness of the graphite/epoxy

material decreases from 0.18 to 0.04 kJ/m2 over four decades of the loading

rate.

Chu (1989) has made a study of the non-linear effect on Mode-I

interlaminar fracture toughness. Large deflections occur on testing thinner

specimens or thicker specimens with larger delamination lengths using a

linear analysis; theIC

G value increases with increasing delamination growth,

and a significant difference is indicated between the first and the last test

point. However, a non-linear analysis shows much better results than a linear

analysis.

27

Madhu S.Madhukar and Drazal (1990) have studied the fiber-matrix

adhesion and its effect on composite mechanical properties for Mode-I and

Mode-II fracture toughness of graphite/epoxy composite. This method

demonstrated that there is a strong dependency of the composites

interlamination fracture toughness and failure modes on fiber-matrix

adhesion. The full potential of a composites inter laminar fracture toughness

can be realized only when the fiber matrix interface is strengthened to its

maximum level.

Verchere et al (1990) studied the effect of a bisphenol-A

diglycidylether based epoxy cured with a cycloaliphatic diamine (4,4 -

diamino-3,3 -dimethyldicyclohexylmethane, 3DCM), in the presence of an

epoxy terminated butadiene-acrylonitrile random copolymer (ETBN). Results

showed that the vitrification is slightly delayed with the rubber addition. The

maximum Tg of the rubber-modified matrix does not depend on the cure

temperature, but decreases with the initial rubber concentration. This implies

that a significant amount of rubber remains in the solution in the continuous

phase. This explains the delay in vitrification.

Bazhenov (1991) has made a study on strong bending in the DCB

interlaminar test of thin, E-glass woven –fabric-reinforced laminates. In this

case only the critical crack extension force needs to be measured to determine

the fracture toughness. The interlaminar fracture toughness of fabric-

reinforced laminates is significantly higher than that of non-woven

unidirectional composites. The crack propagation between the fabric layers is

unstable, whereas it is stable if the crack intersects some fabric layer.

Verchere et al (1991) studied the mechanical properties of a system

consisting of a bisphenol A diglycidylether epoxy, cured with a cycloaliphatic

diamine (4,4 -diamino-3,3 dimethyldicyclohexyl-methane, 3DCM), in the

presence of an epoxy-terminated butadiene-acrylonitrile random copolymer

(ETBN), as a function of the cure schedule and the initial rubber

28

concentration. Fracture toughness (KIc) and fracture energy (GIc) were

increased, while Young's modulus and yield strength decreased slightly with

increasing volume fraction of the dispersed phase. There is no significant

influence of the precure schedule and of the various observed particle

diameters on the mechanical properties for a constant rubber volume

fraction.The main deformation process in the rubber-modified epoxy

networks is shear yielding, while cavitation is negligible.

Hourston and Lane (1992) prepared a series of blends by adding a

polyetherimide, in varying proportions, to a trifunctional epoxy resin,

triglycidylparaaminophenol, cured with 4,4 -diaminodiphenylsulphone. All

the materials showed a two-phase morphology when characterized by a

dynamic mechanical thermal analysis and scanning electron microscopy.

Addition of the thermoplastic resulted in improved fracture properties (K1C

and G1C), as measured by three-point bending experiments, although no

obvious correlation with blend morphology was observed.

Roxana et al (1993) used castor oil (CO) to replace polydisperse

commercial rubbers (carboxy- or epoxy-terminated butadiene-acrylonitrile

random copolymers, CTBN or ETBN) in model systems, developed to

analyse the origin of the phase separation process in rubber-modified

thermosets. Mixtures of CO with an epoxy resin based on the diglycidyl ether

of bisphenol A showed a higher miscibility than a typical CTBN (18%

acrylonitrile)-DGEBA system. Percentage conversions at the cloud point

during the DGEBA-EDA polymerization were experimentally determined,

and compared with theoretical predictions using the pseudo-binary approach

in the framework of the Flory-Huggins lattice model. Reasonable agreement

was found, giving direct evidence of the fact that phase separation results

from the decrease in the entropic contribution to the free energy of mixing

during polymerization.

29

Dong Chen et al (1993) introduced butadiene-acrylonitrile random

copolymers with various molecular weights and end-groups into a system

consisting of the diglycidyl ether of bisphenol A and 3,3 -dimethyl-4,4 -

diamino dicyclohexyl methane (3DCM) cured at 50°C. They studied the

influence of a low level of soluble additive on the polymerization rate,

evolution of viscosity, gelation and vitrification times, and the final network

in the case of different additive molecular weights and end groups of the

additive. It was found that the additive influences the final network only by

dissolving in the matrix, and that the dilution effect plays a minor role, but the

catalytic effect of an additive can play a significant role on polymerization

depending on the additive end groups. However, in all the rubber-modified

systems, phase separation has no direct influence on the polymerization rate

and the change of viscosity.

Min et al (1993) used near infra-red spectroscopy techniques to

study the cure reactions of various epoxy resin formulations based on the

diglycidyl ether of bisphenol A resins cured with 4,4 -diaminodiphenyl

sulfone (DDS) hardener. Stoichiometric and non-stoichiometric

DGEBA/DDS resin formulations, using neat as well as thermoplastic

toughened systems containing two phenolic hydroxyl terminated polysulfones

with different molecular weights were involved in this study. This series of

quantitative analysis of the major chemical groups in several resin systems,

led to a clear understanding of not only the reaction mechanism in each

system but also the cure kinetics.

Cho et al (1993) used tetraglycidyl-4,4 -diaminodiphenyl methane

based resin with 30 phr diaminodiphenyl sulfone as the curing agent and was

toughened with poly(ether imide) (PEI). The effects of morphology on the

fracture toughness of modified epoxy resins were investigated. Morphology

was controlled by changing the curing conditions. The co-continuous

30

structure and morphology of the PEI spherical domain dispersed in the epoxy

matrix were obtained. Phase-inversed morphology with PEI matrix was also

obtained with 30 phr PEI content. The cured resin with phase-inversed

morphology showed the highest fracture toughness. The modified epoxy

resins with enhanced fracture toughness exhibited other improved mechanical

properties such as flexural strength, flexural modulus and strain at break.

Tzong-Hann Ho and Chun-Szhan Wang (1994) used dispersed

acrylate rubbers to improve the toughness of cresol-formaldehyde Novolac

epoxy resin cured with phenolic novolac resin for electronic encapsulation

application. The effect of the alkyl group of acrylate monomer on the phase

separation of the resultant elastomers from epoxy resin was investigated. The

dispersed acrylate rubbers effectively improve the toughness of cured epoxy

resins by reducing the coefficient of thermal expansion (CTE) and flexural

modulus, while the glass transition temperature (Tg) was hardly depressed.

Electronic devices encapsulated with the dispersed acrylate rubber-modified

epoxy molding compounds have exhibited excellent resistance to the thermal

shock cycling test and resulted in an extended device use life.

ZhiQiang Cao et al (1994) studied several rubbers (acrylonitrile–

butadiene copolymer) or thermoplastic (polyethersulfone) additives bearing

different chain ends introduced into pure aromatic dicyanates. The influence

of these initially miscible modifiers on the polymerization kinetics, as a

function of their chemical structure and concentration, were also studied. It

appears that apart from those bearing a labile hydrogen atom, the additives

play almost no role on the polycyclotrimerization rate; neither does phase

separation. However, the additives influence the structure of the final

networks insofar as they partially dissolve in the matrix, and thus modify both

the final Tg and the onset of vitrification, compared with the pure monomer.

31

Tzong-Hann Ho and Chun-Shan Wang (1994) investigated the use

of dispersed silicone rubbers to reduce the stress of cresol–formaldehyde

novolac epoxy resin cured with phenolic novolac resin for electronic

encapsulation application. The effects of structure, molecular weight, and

contents of the vinylsiloxane oligomer on reducing the stress of the

encapsulant were studied. Morphology and the dynamic mechanical behavior

of the rubber-modified epoxy resins were also studied. The dispersed silicone

rubbers effectively reduce the stress of the cured epoxy resins by reducing

flexural modulus and the coefficient of thermal expansion (CTE), whereas the

glass transition temperature (Tg) was hardly depressed. Electronic devices

encapsulated with the dispersed silicone rubber modified epoxy molding

compounds have exhibited excellent resistance to the thermal shock cycling

test and have resulted in an extended device use life.

Philippe Bussi and Hatsuo Ishida (1994) studied the dynamic

mechanical properties of blends of diglycidyl ether of bisphenol-A-based

epoxy resin and internally epoxidized polybutadiene rubber. It is shown that

the influence of the composition of the continuous phase and of the dispersed

phase can be studied not only from the variations of the glass transition

temperature but also from the changes in the apparent enthalpy of activation

associated with this transition. As the initial rubber content increases, the

composition of the dispersed phase remains practically constant while more

rubber is able to dissolve in the continuous phase.

Youjiang Wang and Dongming Zhao (1995) have studied the

characterization of the interlaminar fracture behavior of woven fabric

reinforced polymeric composites. A large displacement, small strain non-

linear beam model was used to calculate the interlaminar fracture toughness.

The fabrics used included fiberglass and Kevlar woven structures with

different weave patterns. This study shows that the improvement in the

32

interlaminar fracture behavior of laminated polymeric composites has

generally focused on the matrix material, the reinforcement and the fiber-

matrix interface.

Tsung-Han Ho et al (1996) used polyol or polysiloxane

thermoplastic polyurethanes (TPU) to reduce micro-cracking in cresol–

formaldehyde novolac epoxy resin cured with phenolic Novolac resin for

electronic encapsulation application. A stable dispersion of TPU particles in

an epoxy resin matrix was achieved via the epoxy ring opening with

isocyanate groups of urethane prepolymer to form an oxazolidone. The effects

of the structure and molecular weight of TPU in reducing the stress of the

electronic encapsultant were investigated. The mechanical and dynamic

viscoelastic properties and morphologies of TPU modified epoxy networks

were also studied.

Reza Bagheri and Raymond (1996) elucidated the role of particle

cavitation in toughening through a comparative examination of epoxies

modified by conventional rubber modifiers and hollow plastic particles. The

results of this study illustrate that rubber particles with different cavitation

resistance and pre-existing microvoids toughen the present epoxy matrix in

the same manner. Therefore, they concluded that the cavitation resistance of

the rubbery phase does not directly contribute to toughness, but instead

simply allows the matrix to deform by shear. An additional mechanism of

microcracking was observed when 40- m hollow plastic particles were

employed. Despite the similar behaviour in fracture toughness testing, rubber

particles and microvoids differ considerably in how they affect the

compressive yield strength of the blend. The results of this study suggest the

possible importance of inter-particle distance in the toughening of epoxies.

This concept will be examined in part 2 of this study.

33

King-Fu Lin and Yeow-Der Shieh (1998) employed a two-stage,

multistep soapless emulsion polymerization to prepare various sizes of

reactive core–shell particles (CSPs) with butyl acrylate (BA) as the core and

methyl methacrylate (MMA) copolymerizing with various concentrations of

glycidyl methacrylate (GMA) as the shell. Ethylene glycol dimethacrylate

(EGDMA) was used to crosslink either the core or shell. The number of

epoxy groups in a particle of the prepared CSP measured by chemical titration

was close to the calculated value, based on the assumption that the added

GMA participated in the entire polymerization unless it was higher than

29 mol %. Similar results were also found for their solid-state13C

-NMR

spectroscopy.

Tsung-Han Ho and Chun-Shan Wang (1999) synthesized a series of

phenol-based and naphthol-based aralkyl epoxy resins by the condensation of

p-xylylene glycol with phenol, o-cresol, p-cresol, or 2-naphthol, respectively,

followed by the epoxidation of the resulting aralkyl novolacs with

epichlorohydrin. The incorporation of stable dispersed polysiloxane

thermoplastic polyurethane particles in the synthesized epoxy resin's matrix

was achieved via epoxy ring-opening with the isocyanate groups of urethane

prepolymer to form an oxazolidone. The mechanical and dynamic viscoelastic

properties of cured aralkyl novolac epoxy resins were investigated. A sea-

island structure was observed in all cured rubber-modified epoxy networks

via SEM. The results indicate that a naphthalene containing aralkyl epoxy

resin has a low coefficient of thermal expansion, heat resistance, and low

moisture absorption, whereas phenol aralkyl type epoxy resins are capable of

imparting low elastic modulus, resulting in a low stress matrix for

encapsulation applications.

Shun-Fa Hwang and Bon-Cherng Shen (1999) have studied the

opening-mode interlaminar fracture toughness of interply hybrid composite

materials. This method demonstrated that crack growth in the three types of

34

specimens is dominated by the opening mode and the Mode-I interlaminar

fracture toughness can be approximated. For hybrid composite specimens, the

effect of geometrical non-linearity should be included. This study clearly

shows that the effect of geometric non-linearity increases with an increase of

the crack length.

Hua and Hu (2000) have investigated a new kind of simultaneous

interpenetrating polymer networks (SINs) composed of epoxy resin (epoxy)

and urethane acrylate resin (UAR) having various amounts of hard segment

and prepared with poly(oxypropylene) polyol (PPO) having different

molecular weights, and the relationship between the morphologies and

mechanical properties of these SINs were investigated in detail. It was found

that the different morphologies of these SINs were related to various

structures of the UAR network. The morphology of such SINs not only

depends on the compatibility between the poly(methyl methyacrylate)

segments existing in the UAR network and epoxy network, but is related to

the microphase separation of the UAR network as well.

Kessler and White (2001) have studied the self-activated healing of

delamination damage in woven E-glass/epoxy composites. With the ultimate

goal of self-healing in mind, two types of healing processes are studied.

Healing efficiencies relative to the virgin fracture toughness of up to 67% are

obtained when the catalyzed monomer is injected and about 19% for the self-

activated materials.

Hsieh et al (2001) have investigated polyurethanes (PU) based on

poly(butylene adipate) [PU(PBA)] and poly(oxypropylene) [PU(PPG)]

polyols as a graft agent to prepare interpenetrating polymer networks of

urethane-modified bismaleimide (UBMI) and the diglycidyl ether of

bisphenol A (Ep) (UBMI/Ep graft-IPNs). The UBMI was introduced and

partially grafted to the epoxy by PU graft agents, and then the simultaneous

35

bulk polymerization technique was used to prepare the graft-IPNs.All the PU

graft agents were characterized by infrared (IR). The tensile strength of both

the UBMI/Ep graft-IPNs with PU (PBA) and PU (PPG) graft agent systems

increased to a maximum value with increasing UBMI content in the system,

and then decreased with further increasing the UBMI content. For both kinds

of PU with various molecular weights in the UBMI/Ep graft-IPNs, the Izod

impact strength increased with the UBMI contents increasing. The better

compatibility of PU (PBA)-based UBMI/Ep graft-IPNs led to higher impact

strength.

Fellahi et al (2001) used kaolin as a modifier at different contents to

improve the toughness of diglycidyl ether of bisphenol A epoxy resin with

polyamino-imidazoline as a curing agent. The chemical reactions suspected of

taking place during the modification of the epoxy resin were monitored and

evaluated with the Fourier transform infrared spectroscopy. The glass-

transition temperature (Tg) was measured with differential scanning

calorimetry. The mechanical behavior of the modified epoxy resin was

evaluated in terms of the Izod impact strength (IS), the critical stress intensity

factor (KIC), and the tensile properties at different modifier contents. Scanning

electron microscopy (SEM) was used to elucidate the mechanisms of

deformation and toughening in addition to other morphological features.

Finally, the adhesive properties of the modified epoxy resin were measured in

terms of the tensile shear strength (TSS).

Dispenza et al (2001) chose a high molecular weight acrylonitrile

/butadiene /methacrylic acid (Nipol 1472) rubber to control the processability

and mechanical properties of a TGDDM (tetra glycidyl diphenyl methane)

based epoxy resin formulation for aerospace composite applications. The

physical blend of rubber and epoxy resin, achieved by the dissolution of all

the components in a common solvent, forms a heterogeneous system after

36

solvent removal and presents coarse phase separation during cure, which

impairs any practical relevance of this material. A marked improvement of

rubberepoxy miscibility is achieved by the reactive blending (‘pre-reaction’)

of the epoxy oligomer with the functional groups present in the rubber.

Shangjin He et al (2001) synthesized two kinds of reactive

toughening accelerators for epoxy resin, amine-terminated chain-extended

urea (ATU) and imidazole-terminated chain-extended urea (ITU) from

polyurethane prepolymer. Compared with the unmodified system, the curing

activity, dynamic mechanical behavior, impact property and fracture surface

morphology of the modified systems were systematically investigated.

Results show that the curing activity of the modified epoxy resin E-

51/dicyandiamide (dicy) systems is so greatly enhanced that the apparent

activation energy of the curing reaction decreases from 130.2 kJ/mol for the

unmodified system to 75–85 kJ/mol for the modified systems. The curing

reaction mechanism of the E-51/dicy system accelerated by ITU is different

from that of the system accelerated by ATU, and a little different from that of

the system accelerated by imidazole. Furthermore, the impact strength of the

cured systems modified with ITU is 2–3 times higher than that of the

unmodified system, while the glass transition temperatures are a little altered,

and the fracture surfaces of all modified systems display tough fracture

feature.

Fábio et al (2002) employed hydroxy-terminated polybutadiene

functionalized with isocyanate groups and in preparation of a block

copolymer of polybutadiene and bisphenol A diglycidyl ether (DGEBA)-

based epoxy resin. The block copolymer was characterized by the Fourier

transform infrared (FTIR) spectroscopy and size-exclusion chromatography

(SEC). Cured blends of epoxy resin and hydroxy-terminated polybutadiene

(HTPB) or a corresponding block copolymer were characterized by

37

differential scanning calorimetry (DSC), dynamic mechanical analysis

(DMTA), and scanning electron microscopy (SEM). All modified epoxy resin

networks presented an improved impact resistance with the addition of the

rubber component at a proportion of up to 10 wt % when compared to the

neat cured resin.

Raffaele Mezzenga et al (2002) studied the effect of the structural

buildup during the reticulation of thermoset systems containing reactive

modifiers to strongly influence the final properties of such blends. This was

studied by considering the rheological behavior during the cure of an

epoxy/amine thermoset system blended with reactive dendritic hyperbranched

polymers (HBPs). Depending on the chemical structure of the HBP used in

the blend, a phase separation could be observed. The onset and offset of the

phase separation process could be detected by observing the evolution of the

viscoelastic properties. The phase separation onsets obtained by rheological

measurements were compared with the values obtained by traditional cloud

point observations. A good agreement between the two techniques was

observed.

Kim et al (2002) synthesized amine-terminated poly(arylene ether

sulfone)–carboxylic-terminated butadiene-acrylonitrile–poly(arylene ether

sulfone) (PES-CTBN-PES) triblock copolymers with controlled molecular

weights of 15,000 (15K) or 20,000 (20K) g/mol from amine-terminated PES

oligomer and commercial CTBN rubber (CTBN 1300x13). The copolymers

were utilized to modify a diglycidyl ether of bisphenol A epoxy resin by

varying the loading from 5 to 40 wt %. The epoxy resins were cured with

4,4 -diaminodiphenylsulfone and subjected to tests for thermal properties,

plane strain fracture toughness (KIC), flexural properties, and solvent

resistance measurements. The fracture surfaces were analyzed with SEM to

elucidate the toughening mechanism. The properties of the copolymer-

38

toughened epoxy resins were compared with those of the samples modified by

the PES/CTBN blends, PES oligomer, or CTBN. The PES-CTBN-PES

copolymer (20K) showed a KIC of 2.33 MPa m0.5

at 40 wt % loading while

maintaining good flexural properties and chemical resistance.

Giannotti et al (2004) modified epoxy–aromatic diamine

formulations simultaneously with two immiscible thermoplastics (TPs),

poly(ether imide) (PEI) and polysulfone (PSF). The epoxy monomer is based

on the diglycidyl ether of bisphenol A and the aromatic diamines (ADs) are

either 4,4 -diaminodiphenylsulfone or 4,4 -methylenebis(3-chloro 2,6-

diethylaniline). The influence of the TPs on the epoxy–amine kinetics is

investigated. It is found that PSF can act as a catalyst. The presence of the TP

provokes an increase of the gel times.

Valéria et al (2005) prepared composites using epoxy resin (ER),

carboxyl-terminated butadiene acrylonitrile copolymer (CTBN) and hydroxyl-

terminated polybutadiene (HTPB), in different proportions. A chemical link

between the HTPB and the epoxy resin was promoted employing tolylene

diisocyanate (TDI). The reactions between elastomers and epoxy resin were

followed by FTIR. The mechanical properties of the composites were evaluated

and the microstructure was investigated through scanning electronic microscopy

(SEM). The results showed that the impact resistance of the CTBN-modified ER

was superior to that of the pure epoxy resin. For the composites with HTPB, the

impact resistance increased with elastomer concentration of up to three parts per

hundred parts of resin (phr). Higher concentrations of HTPB resulted in larger

particles and gave lower impact values.

Maity et al (2007) modified the diglycidyl ether of bisphenol-A

(DGEBA) resin with amine functional aniline acetaldehyde condensate

(AFAAC), and cured with an ambient temperature curing agent triethylene

tetramine. The resulting networks displayed significantly improved fracture

39

toughness. The AFAAC was synthesized by the condensation reaction of

aniline and acetaldehyde in the acid medium (pH 4) and characterized by

FTIR and NMR spectroscopy, elemental analysis, viscosity measurements,

and mole of primary and secondary amine analysis. The DGEBA and

AFAAC were molecularly miscible, but developed a two-phase

microstructure upon network formation. Epoxy/AFAAC compositions were

systematically varied to study the effect of AFAAC concentration on the

impact, adhesive, tensile, and flexural properties of modified networks. The

dynamic mechanical analysis and scanning electron microscopy studies

showed two phase morphology in the cured networks, where AFAAC

particles were dispersed. The AFAAC modified epoxy network was thermally

stable up to around 280°C.

1.7.2 Epoxy and Cyanate Ester Blends and Composites

Ian Hamerton et al (1998) presented a review article on recent

technological developments in the field of cyanate ester resin. In this article,

recent developments in the processing, toughening properties and applications

of the cyanate ester resin are reviewed.

Hwang et al (1999) studied the effect of the composition of

polysulphone (PSF) on the cyanate ester system. From the study it is found

that homogeneous bisphenol A dicyanate (BADCy) / PSF blends with a low

content of PSF (less than 10 wt%) are cured isothermally, and blends are

phase separated by nucleation and growth mechanism to form the PSF

particle structure. But with more than 20 wt% of PSF content the BADCy /

PSF blends are phase separated by spinodal decomposition to form the

BADCy particle structure, and when the PSF content was 15 wt% the blends

are phase separated by nucleation and growth, and spinodal decomposition

resulting in the formation of a combined structure having both PSF and

BADCy particle structure.

40

Roman et al (2000) investigated the effects of temperature and

moisture on the thermal and mechanical properties of thermoplastic and

elastomer toughened high-temperature cyanate ester composite material. The

thermoplastic modified cyanate ester showed increased thermal stability. The

elastomer modified cyanate ester showed the highest mode I fracture

toughness values, primarily because the toughener did not phase separate.

Kimo Chung et al (2001) evaluated the thermomechanical property

changes of carbon fibre/cyanate ester composites by DMA through time-

temperature equivalence. From the study a modelling methodology was

developed which quantitatively provided an understanding of the ageing

process of fibre reinforced composites in isothermal environments.

Ian Hamerton et al (2002) prepared carbon fibre impregnated tape

from a range of prepolymers comprising several different blends of akenyl

functionalized cyanate ester monomers with commercial cyanate ester and

BMI monomers and blends. Incorporation of akenyl-functionalised cyanate

ester monomers into commercial cyanate ester/ BMI blends raised the Tg

value while maintaining GIC and other mechanical properties. This

enhancement in neat resin fracture toughness was translated into the

corresponding composites.

Jerome Dupuy et al (2002) investigated the thermophysical

properties (heat capacity, thermal conductivity) and modelled neat resin and

glass fibre composites. The models are used to stimulate the thermal transfers

in an instrumented heated mould. The calculated local temperatures and

surface heat fluxes appear to be in very good agreement with measurements

for both the neat resin and the composite. The moisture absorption studies of

cyanate ester modified epoxy resin matrices under constant hydrothermal

conditions are an attempt to understand the so-called “reverse thermal effect.”

From the swelling study it is argued that in the initial stage of the absorption

41

process the water diffuses into the regions of volume equal to or greater than

the volume of water molecules, which does not result in swelling. In the later

stage, the water molecules penetrate the regions with a volume less than that

of the volume of water molecules, with molecular reorganization of the resin

network, resulting in swelling (Sunil K. Karad et al 2002).

Shinn-Gwo Hong et al (2003) studied the effect of copper oxide on

the thermal degradation of bis-maleimide triazine (BT) prepreg with IR, ATR

and TGA. The results indicate that the thermal degradation in the bulk BT is

mainly from the epoxy constituent while that in the copper oxide contacted

BT happens not only from the epoxy but also from the more stable cyanate

ester constituent.

Baochun Gwo et al (2003) investigated the chemical nature of the

changes in a cyanate ester-novolac epoxy resin blend caused by hygrothermal

ageing, and the effects of the residual reaction in the blends on the

hygrothermal ageing resistance. The results of the study indicate that the

long-term hygrothermal ageing may cause substantial changes in the chemical

nature of the blends when the cure extent is not sufficiently high.

Tim J. Wooster et al (2003) studied the effect of filler incorporation

on the thermal, mechanical and conductivity properties of cyanate ester

composites. From the study it is concluded that silica filler increased the

thermal conductivity, Young’s modulus and dielectric constant (slightly) and

decreased thermal expansion. It is also found that the addition of silica

resulted in a marginal decrease in strength.

Sabyasachi Ganguli et al (2003) prepared nanocomposites of

cyanate ester by dispersing organically modified layered silicate into the

resin. The inclusion of only 2.5 % by weight of organically modified layered

silicates showed a 30% increase in both the modulus and toughness.

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1.7.3 PZT Modified Polymer Composites

Edward et al (1987) presented analytical and experimental studies

on piezoelectric actuators as elements of intelligent structures with highly

distributed actuators, sensors and processing networks. Static and dynamic

analytical models are derived for segmented piezoelectric actuators that are

either bonded to an elastic substructure or embedded in a laminated

composite.

Lee (1991) developed a piezoelectric laminate theory that uses the

piezoelectric phenomenon to effect distributed control and sensing of

bending, torsion, shearing, shrinking and stretching of a flexible plate. The

reciprocal relationship of the piezoelectric sensor and actuators is unveiled.

Dimitriadis et al (1991) analytically investigated the behavior of two

dimensional patches of piezoelectric material bonded to the surface of elastic

distributed structures, and used as vibration actuators to the supported elastic

structure. The theory is then applied to develop an approximate dynamic

model of the vibration response of a simply supported elastic rectangular plate

excited by a piezoelectric patch of variable rectangle geometry.

Woo-Seok Hwang et al (1993) presented a finite element

formulation for the vibration control of a laminated plate with piezoelectric

sensors/actuators. The classical lamination theory with the induced strain

actuation and Hamilton’s principle are used to formulate the equations of

motion. The total charge developed on the sensor layer is calculated from the

direct piezoelectric equation of motion.

Ghosh and Batra (1995) showed the deflection of the centerline of a

simply supported plate and the tip deflection of a cantilever plate, both

43

deformed quasistatically, that can be controlled by applying suitable voltage

to the PZTs. The first order shear deformation theory is used to study the

infinitesimal elastic deformations. The adhesive between the PZT and the

plate is assumed to be of negligible thickness, and the displacement and

surface tractions across the interfaces between the PZTs and the plate were

taken to be continous.

Paul Heyliger and Saravanos (1995) developed exact solutions for

predicting the coupled electro mechanical vibration characteristic of simply

supported laminated piezoelectric plates composed of orthorhombic layers.

The three dimensional equations of motion and the charge equations are

solved using the assumptions of the linear theory of piezoelectricity.

Dimitris A Saravanaos et al (1997) presented the structural

mechanics for the analysis of laminated composite plate structures with

piezoelectric actuators and sensors. The theories implement the layer wise

representation of displacements and electric potentials, and can model both

the global and local electromechanical response of smart composite

laminates.Finite element formulations are developed for the quasi-static and

dynamic analyzing of smart composite structures containing piezoelectric

layers.

Vardarajan et al (1998) discussed the shape control of a laminated

composite plate with integrated piezoelectric actuator. The effectiveness of

the piezoelectric actuators and position sensors is investigated for shape

control under the influence of quasi statically varying unknown loads.

Hernandes et al (2000) investigated the free vibration behavior of

thin composite plates with surface bonded piezoelectric patches, including

stress stiffening effects. A finite element formulation is presented, based on

the Reissener mindlin theory and including non-linear strain displacement

relations to formulate a free vibration Eigen value problem in the presence of

44

a geometric stiffness matrix. The case of the symmetric laminate and ideal

linear behavior is assumed for the piezoelectric actuation.

Hori et al (2001) developed new types of piezoelectric damping

materials, piezoelectric ceramic (PZT) powder/carbon black (CB)

powder/epoxy (EP) resin composites, and studied their mechanical and

damping properties. Here, the mechanical energy of vibrations and noises

were transformed into electric energy (current) by PZT, and the electric

current was conducted to an external circuit through CB powders and then

dissipated as thermal energy through a resistor. When CB was added to

PZT/EP (70/30 in wt%), the mechanical loss factor (h), a measure of the

mechanical and damping intensity, showed a maximum value of 0.08 at the

CB content of 0.51 wt%, at which the CB particles electrically just contact

each other. In this work, it was found that the PZT/CB/EP composite of

90.0/0.5/9.5 shows a large h value of 0.15.

Hajime Kishi et al (2004) characterized the damping properties of

carbon fiber-reinforced interleaved epoxy composites. Several types of

thermoplastic-elastomer films, such as polyurethane elastomers, polyethylene-

based ionomers and polyamide elastomers were used as the interleaving

materials. The damping properties of the composite laminates with/without

the interleaf films were evaluated by the mechanical impedance method. Also,

the effects of the lay-up arrangements of the carbon-fiber prepregs on the

damping properties of the interleaved laminates were examined. The

viscoelastic properties of the interleaved polymer films were reflected in the

damping properties of the corresponding interleaved laminates. The loss

tangent of the interleaf films at the test temperature played an important role

in the loss factor of the interleaved laminates. Also, the stiffness of the films

at the resonant frequency of the laminates was another important parameter

that controlled the loss factor of the interleaved laminates.

45

Botelho et al (2005) determined the viscoelastic properties, such as

the storage modulus (E’) and loss modulus (E’’), for a glass fiber/epoxy

composite, aluminum 2024-T3 alloy and for a glass fiber/epoxy/aluminum

laminate (Glare). It was found that the glass fiber/epoxy (G/E) composites

decrease the E’ modulus during hygrothermal conditioning up to the

saturation point (6 weeks). However, for Glare laminates the E’’ modulus

remains unchanged (49 GPa) during the cycle of hygrothermal conditioning.

The outer aluminum sheets in the Glare laminate shield the G/E composite

laminae from moisture absorption, which in turn prevent, to a certain extent,

the material from hygrothermal degradation effects.

Botelho et al (2006) investigated the viscoelastic properties, such as

elastic (E’), and the viscous (E’’) responses were obtained for the aluminum

2024 alloy, carbon fiber/epoxy, glass fiber/epoxy and their hybrids aluminum

2024 alloy/carbon fiber/epoxy and aluminum 2024 alloy/glass fiber/epoxy

composites. The experimental results were compared with the calculated E

modulus values by using the composite micromechanics approach. For all the

specimens studied, the experimental values showed good agreement with the

theoretical values. The damping behavior, i.e., the storage modulus and the

loss factor, from the aluminum 2024 alloy and fiber epoxy composites can be

used to estimate the viscoelastic response of the hybrid FML

Tsantzalis et al (2007) investigated the fracture toughness of carbon

fiber reinforced polymer (CFRP) laminates doped with carbon nanofibers

(CNF) and/or piezoelectric (PZT) particles. An increase of 100% in fracture

energy was observed after the addition of 1% CNF in the matrix of the

laminates. The investigation of the fracture surfaces showed extensive fiber

bridging because of the presence of CNFs, which verifies the enhanced

fracture properties. On the other hand, the introduction of PZT particles led to

46

a reduction in the fracture toughness, mainly due to the brittle character of the

particle inclusions.

Jian Gu et al 2007 prepared and characterized the high damping

properties of the promising low density epoxy/fly ash composites, a series of

epoxy composites filled with fly ash of a different volume fraction. Damping

tests of cured epoxy composites are performed in the temperature range of

40 to 150 C and in the frequency range of 10 to 800 Hz, by using a tension-

compression mode. The results show that the values of tangent delta (tan )

reach their peak values at the glass transition temperatures for the composites

with 30–50 vol.% fly ash, and the tan values attenuate slowly with the

increase in frequency, which indicates that the damping properties of such

composites are better than those of other composites. Scanning electron

microscopy was used to observe the fractured surfaces of the composites, and

to clarify the dispersion and distribution of fly ash particulates in the matrix.

In addition, the thermogravimetric curves were also employed to characterize

the heat-resistant performance of the composites.

Tsantzalis et al (2007) studied vapor growth carbon nanofibers

(CNF), lead zirconate titanate piezoelectric (PZT) particles, as well as a

combination of these two added in an epoxy resin (EP), and their influence on

the mechanical quasi-static properties. Moreover, the prepared samples were

characterized by a dynamic thermal mechanical analysis, and optical and

scanning electron microscopy. An enhancement of the mechanical properties

was observed by the addition of the CNF. The uncured mixtures were also

used as matrix material for manufacturing unidirectional carbon fiber

reinforced laminates.

In this paper, the author, Toshio Tanimoto, summarizes the previous

works on the passive damping of carbon-fiber reinforced plastic (CFRP)

cantilever beams using: (1) interleaving of viscoelastic thermoplastic films,

47

(2) surface-bonded piezoelectric ceramics, and (3) dispersed PZT particle

interlayers. Introducing polyethylene-based film interlayers between

composite plies resulted in a significant increase in the vibration loss factor. It

is also shown that the vibration damping of CFRP laminates can be improved

passively by means of resistively shunted, surface-bonded piezoelectric

ceramic, PbZrO3–PbTiO3 (PZT) sheets.

Kostopoulos et al (2007) investigated the influence of carbon

nanofibers (CNF) and/or piezoelectric (PZT) particles on the fracture

behaviour of carbon fiber reinforced polymer laminates. For this purpose the

fillers were added as dopants in the epoxy matrix of the laminates. An

increase of 100% in the fracture energy was observed after the addition of 1%

CNF in the matrix of the laminates, while the introduction of the PZT

particles led a to reduction in the fracture energy, mainly due to the brittle

character of the particle inclusions. In addition, the acoustic emission

technique was used for monitoring the fracture process of the laminates.

Rodríguez et al (2008) investigated the catalytic performance of

3 wt.% copper supported on carbon nanofibers (CNFs) in liquid phase

oxidation, using a batch stirred tank microreactor in order to determine the

decolorization and total organic carbon (TOC) removal efficiency in washing

textile wastewater (WTW). A preliminary study was carried out in a

temperature range of 120–160 °C and two oxygen partial pressures of 6.3 and

8.7 bar. TOC removal and toxicity reduction were as high as 74.1% and 43%,

respectively at 140 °C and 8.7 bar, after 180 min reaction. The main

intermediates detected in raw wastewater were decanoic acid, methyl ester

and 1,2-benzenedicarboxylic acid, and they have been degraded by means of a

Cu/CNF catalyst. The application of CWAO to the treatment of a textile

effluent at 160 °C and 8.7 bar of oxygen partial pressure showed that the use

of a Cu/CNF catalyst significantly improves the TOC and color removal

48

efficiencies, and it can be considered as an option for a pretreatment step in

the treatment of these industrial effluents.

Sheng Tian et al (2008) prepared a new type of rigid piezo-damping

epoxy-matrix composites, containing multi-walled carbon nanotubes(CNT)

and piezoelectric lead zirconate titanate (PZT), and investigated their

electrical and damping properties. The dynamic mechanical thermal analysis

revealed that the loss factors of the composites were improved by the

incorporation of PZT and CNT under the concentration above a critical

electrical percolation. Based on this piezo-damping material, the PZT

contributes to the transformation of mechanical noise and vibration energies

into electric energy, while the CNT serves in the shorting of the generated

electric current to the external circuit. An optimum formulation for the piezo-

damping epoxy based materials can be designed on the basis of the results of

this study.

1.7.4 Engine Coating

Dennis N. Assanis (1988) deals with the transient analysis of piston-

liner heat transfer in low-heat-rejection diesel engines.A two-dimensional

finite element program has been developed to analyze the transient heat flow

paths in low-heat-rejection engine combustion chambers. This analysis tool

has been used to study the transient heat transfer performance of a ceramic-

coated piston-liner assembly and compare it with the performance of baseline

cast-iron components. The direction of the gas-to-liner and piston-to-liner

heat flux, changes several times during the cycle, and these changes occur at

different instants, as the distance from the cylinder fire-deck varies. In the

conventional metal engine, heat flows from the relatively hotter piston to the

liner, via the rings for most part of the cycle. In contrast, for the LHR

configuration, the net heat transfer is from the liner to the rings, and thus to

the piston.

49

Miyairi (1988) deals with the computer simulation of a low heat

rejection direct injection diesel engine using a two zone combustion model

and Anand’s heat transfer model. Spray penetration, deflection, growth, and

rate of entrainment were considered under the steady state condition. The

combustion and heat transfer characteristics were studied in this literature by

taking into account the high temperature swing in the low heat rejection

engines. The heat transfer through the combustion chamber components was

studied, by solving the unsteady conduction heat transfer equations. This

literature is used in this work as a basic support for the formulation of various

models for the combustion and heat transfer calculations.

Randolph A Churchill et al (1988) studied a low heat rejection

engine. Reducing heat losses from the engine cylinder makes minimal

changes to the efficiencies of the existing engines. It reduces the need for

cooling systems and their cost, reducing weight and reducing the complexity.

Partially stabilized zirconia has been developed, that decreases the magnitude

of the phase changes and is now considered a good candidate for engine use.

Recovering the available exhaust heat will also aid in controlling pollution. It

may be possible to build an unlubricated engine with 55% thermal efficiency.

Jorge J.G.Martins (2004) deals with the thermodynamic analysis of

an over expanded engine. The equation of an over-expanded engine is

developed with the equation for the otto cycle, diesel cycle and dual cycle at

part load condition. It is clear that the most efficient cycle under light and part

load condition is the miller cycle with a fixed trapped compression ratio,

particularly at very low loads (0.3 to 0.4), where it can achieve theoretical

efficiencies approaching 73%. In the diesel cycle, the amount of intake air is

the same as there is no restriction on the intake. So, the change in the cycle

configuration to a lower load will be the reduction of heat supplied during the

isobaric heating.