Polper M a w Composites- Materiuh, Processing, Characterization and

w e n t Advances

Scope of tlie chapter

@is chapter gives a brief intmdiution of the subject and highlights the recent advances in polymer mat* composites. I t also gives a 67ief description of the important tliennosettiqj a d thmop(&Ftic mat* resim, &lierent types a d

fonm of reinforcements and the effect of interface qua& on the performance of the composites. '71ie method3 adoptedfor the quality /perfoormance evaluation of the composite and interphase, mocijication of intefaces a d toughening of th mat* have been highlighted: A brief account of the recent advances in the composite arena, including nano-composites and syntactic foam composites, is aho given Tina$, the techniques adoptedfor the fomuliztion andchnracteniatwn of hI&h pefomance m a t e resim and the advantages of using reactive4 6fended m a t e resim for advartced composite appl iatww are aho presented: Qie chapter concCuds with the current dmeIbpments in thejielii of hI&h pe$omance m a t e resitls inc ld ing epogis, 6ismahimides and their co-reactive b&n& and indicates the prospects of RaZ 'D in this area:

Chapter I 2

1.1 Introduction With the advancement in science and technology a drive for higher and higher

performance materials is being felt in aerospace, military, defense and engineering

applications. As a result, the conventional structural materials are being replaced by

different types of composite materials, the properties of which can be tailored to meet

any specific requirement by proper selection and modification of the matrix,

reinforcement, interface and processing technique. A composite material can be

defined as a macroscopic combination of two or more distinct materials having

recognizable interface between them. The requirement for a composite material was

felt because no single homogenous structural material can be found that has got all

the desired attributes for a specific application. Composite technology is of an

interdisciplinary nature, which extends over a wide range of subject fields like

chemistry, physics, engineering, polymer science and engineering, material science

etc.

The world is witnessing a spectacular growth in the application of composites

in every conceivable use. In today's world, market demand for cost effective material

solution$ for replacing metals, alloys and ceramics for a wide spectrum of applications

is increasing every day.

Composites are classified by the type of matrix material (metal, ceramic.

polymer and carbon') or by the geometry of the reinforcement (particulate, flake or

fiber). The structure, properties and applications of various composites are

investigated worldwide by several authors2". Composites possess a combination of

properties like high specific strength and specific modulus, good design flexibility, high

fatigue endurance limit and thermal cycling tolerance, corrosion and wear resistance.

tailorable coefficient of thermal expansion etc. which make them superior to most of

the conventional structural materials.

The term advanced composite has been coined to cover those reinforced

plastics having more than 50% by volume of fibers. These have been developed

primarily for aerospace industry in which the demand for strong and stiff lightweight

structures overcomes the prohibitive costs of early composite materials.

The composite products, successfully tried out towards missile programmes

fall under three major categories namely structural, electromagnetic and ablative

areas. The primary structural domain includes airframe sections, foldable wing and

frame, canister and rocket motor casing. Electromagnetic components are basically

redomes for various frequencies. The ablative products consist of thermal protection

liners for rocket motors, flex nozzles, re-entry heat shields, blast tubes etc.

The carboncarbon (C-C) composites are the most sought after in satellite

applications. Satellite in orbit dissipates tremendous amount of waste heat from

absorbed solar radiation and internal heat sources. The C-C radiator panel conducts

thermal energy more effectively than other materials currently in use for dissipating

thermal energy on satellite. They are used in satellite structures like solar panels,

antenna reflectors, yoke, optical support structures etc. They also have markedly

higher specific thermal efficiency compared to aluminium and offer improved

performance for lower volume and mass. Carbon composites are used extensively in

ablative applications in re-entry vehicle nose cones, heat shield etc. and in launch

vehicles for solid rocket motor case (C- epoxy), rocket nozzles and liners, pressure

bolts, heat shields and exit cones. Carbon fiber-phenolic composites can withstand

the high temperature and erosive gases of solid rocket motor operation and the high

temperatures generated by aerodynamic heating in re-entry systems.

Extensive use of fiber reinforced polymer matrix composites has been

identified as a viable means of reducing weight and thus increasing the fuel efficiency

and payload in aerospace applications. In the development of reusable space

transport systems, one of the major challenges is the development of materials and

structures to meet the flight requirements, and withstand the loads and adverse

conditions like high temperature and aggressive plasma environment. The clear and

confident answer to these requirements is a composite. Composites, the wonder

materials with light weight, high strength to weight ratio and stiffness properties, have

Chapter 1 4

come a long way from their inception, in replacing conventional materials like metals,

woods etc8.

1.2 Polymer Composites

The rapid advancement in the field of macromolecular materials has helped to

expedite the technological revolution in the field of aerospace, telecommunication,

micro electronics, defense and transport. At present, more than 60% of the advanced

composite market is within aerospace industry, identified primarily with aircraft I

aerospace and in sports1 leisure applications. The high specific strength, easy

processability and corrosion resistance have given prime place for polymer

composites as structural materials in aerospace. Weight reduction in aircrafts I

spacecrafts and launch vehicles can obviously lead to benefits like improved

performance and fuel economy, increased range and payload capability. Some of the

superior properties and resulting applications of advanced polymer composites in

contrast to conventional materials are given in Fig 1.1

Parabolic antenna Machinery Biocompatibility

Cryogenic application Sporting goods Chemical apparatus

Measuring instruments Music instruments Pipelines

Fig.1 .l Advantageous properties of advanced composites (Carbon Fiber Reinforced Plastics) in contrast to conventional materialsq0

Chapter I 5

Polymer matrix composites (PMCs) constitute several vital components of satellite

and launch vehicles ranging from motor cases and nozzle components to systems like

honeycomb structures, equipment panels, cylindrical support structures, pressure

bottles, solar array substrates and antenna reflector^""^. In Launch vehicles meant

for small satellites, the structure is reported to be composed of PMCs to the extent of

80%".

Composites have been used in space application from the start of 1950s.

Building up on the first successful use of composite solid fuel rocket motor case in

1950s, composites have been continually developed for aircraft, launch vehicles,

missiles and spacecraft structures. The key spacecraft environment includes

transient, steady state, vibration, acoustic, pyro shock, pressure, handling and

transport. In addition, the structural design may be affected by other environmental

conditions such as salt and fog, humidity, ambient temperature and space

environment. The space environment includes naturally occurring phenomena such

as atomic oxygen, very low density atmosphere, ionizing radiation, plasma, charged

particle plasma, neutral, atomic and molecular particles. The composites meant for

aerospace applications must be capable of surviving in this hostile environment for

sufficient length of time. Advanced composites have already started to play a major

role in the game of weight saving, where every pound or fraction of a pound weight

saved may spell the cliff between success and failure. Polymer matrix composite

(PMC) materials in the form of braided, woven or stitched laminates are increasingly

being used in critical aircraft structural parts that must be damage tolerant for reasons

of safety, reliability, maintainability and supportability. This means that PMC structures

must be durable and reliable under service conditions that can expose aircraft

structure to many types of damage mechanisms.

Space launch vehicles of expendable type experience severe thermal

environment during the ascent phase in the atmospheric regime of the flight. The

reusable launch vehicle face a similar environment during the ascent phase but are

subjected to a more severe and hostile environment during the re-entry phase. With

Chapter 1 6

the high speed advancements in science and technology, the urge for the

development of miracle materials to survlve in these hostile environment and loading

conditions is badly felt. The effects of changing the reinforcements and matrix

materials for producing composites with the properties essential for the system

requirement have been studied extensively. An overview of the myriad material and

configuration options available for polymer based composites was reported by many

researchersi4.

1.3 Principles of Polymer Reinforcement

Composites are combinations of a reinforcement material such as particles,

fiber or whisker and a matrix or binder material. This also implies that the materials

are macroscopically identifiable, retaining their distinctive component properties.

Composites reinforced with dispersed fillers, fibers and fabrics are widely used to

manufacture structural components and load bearing elements in space technology

and in aircraft, automotive and ship building industries. Therefore, the measurements,

calculations and predictions of the strength of such materials are of great interest to

engineers and researchers. The main feature of fiber and fabric reinforced composites

is the strong anisotropy of properties like strength and elastic modulus. Greatest

progress in exploitation of properties of continuous fiber reinforced composites has

been made, owing to a relatively simple mechanism of failure.

The three important factors determining the performance of a composite

material are the matrix, reinforcement and interphase.

1.4 Matrix Materials and Classifications

Matrix materials are generally classified in to four groups, ceramics, metals.

carbon and polymers. Choice of the matrix depends upon performance requirement.

service environment, available processing facilities, cost and several other factors.

The matrix constitutes approximately 40% (by volume) of the composite and performs

two major roles. The matrix component of these composites dictates many key

features like toughness, resistance to heat, corrosive environment and service

temperature. The matrix material transfers loads to the reinforcement and protects it

from adverse environmental effects. In addition, matrix controls the thermo-

mechanical behavior of the composites. The resin matrix spreads the load applied to

the composite between each of the individual fibers and also protects the fibers from

damage caused by abrasion and impact.

1.5 Polymer ~atrices"

The matrix controls the processibility, upper use temperature, flammability

characteristics and corrosion resistance of the composite. The composite mechanical

performance depends to a large extent on the resin modulus, failure strain, and the

fiber- matrix bond-strength.

Polymers are broadly classified into thermoplastics and thermosets. A number

of polymer systems under each categoly have been utilized as matrices. The merits

and demerits of each of these should be evaluated based on its chemical nature, ease

of transformation into pre-preg, processing and fabrication and also on the resultant

composite properties.

1.5.1 ~hermoplastics'~

Thermoplastics permit easy repair of structures made out of them. Joints can

be made by heating together and cooling, without need for cutting or welding. Owing

to their excellent toughness and thermal performance even to cryo-temperatures,

thermoplastics are being increasingly used in high performance applications. Unlike

thermosets, thermoplastics do not require reactive cure cycles. They require only heat

and pressure with subsequent cooling for composite fabrication. Some of the

commercially available advanced thermoplastics are poly ether ether ketone (PEEK),

poly phenylene sulfide (PPS), poly sulfide (PS), poly ether imide (PEI) and poly amide I

imide (PAI). Their glass transition temperatures normally range from 140 - 275°C and

Chapter I 8

processing temperature up to 350"~". High performance thermoplastics frequently

contain rigid aromatic and I or heterocyclic repeating segments. These repeating

segments have low mobility and thus provide high glass transition temperature, which

in turn give good heat resistance and retention of material strength at elevated

temperatures. Polymers containing high aromatic or heterocyclic structures require

high energy for thermal degradation process and thus give a high char residue in the

thermal break down process. These characteristics give polymers high decomposition

temperatures and low flammability that meet major requirements needed for

advanced structural applications. Typical high performance uncrosslinked polymers

include polyimides, polybenzobisthiozoles, polybenzobisoxazole, aromatic

polyamides, polybenzimidazoles, liquid crystalline polyesters, polyether sulphons,

polyquinolines, polyamide imides, polyether ketones, polycarbonates etciattS.

1.5.1.1 ~ o l ~ c a r b o n a t e s ~ ~

Polycarbonate has got exceptional total package of inherent properties such

as impact strength, heat resistance, clarity, inherent flame retardance and good

processability. Standard grade of polycarbonates are made from bisphenol A and

phosgene. It can be blended with other polymers to enhance the specific properties

such as chemical resistance. Co- polymers of polycarbonate have also been recently

employed to achieve high thermal performance approaching that of high-end

thermoplastics. Its superior impact strength and resistance to breakage and damage

makes it suitable for appliances, automotive, computers, business equipments,

electrical and electronic components, medical devices, transportation equipments,

outdoor fixtures and household utensils. Its excellent electrical properties remain

relatively constant over a wide temperature range.

1.5.1.2 ~ol~ethersu lphones~ '

The polyether sulphone belongs to polysulphone group of thermoplastics. Its

chemical structure consists of aromatic sulphone units linked through ether bridge.

Chapter I 9

The former is mainly responsible for the high temperature stability and mechanical

strength, while the later introduces sufficient chain mobility to permit melt processing

at reasonable temperature. The properties of polyethersulphone (PES) that

distinguish it from most other thermoplastics are its high temperature performance,

good mechanical strength and low flammability. These properties are achieved

without the penalty of difficult fabrication since PES can be melt processed in

conventional moulding equipments. The thermal stability as measured by the half life

of the tensile strength at a given temperature is very high for PES making it a

candidate for class H electrical application. PES has got very high glass transitin

temperature (220°C) and hence its properties show small dependence on

temperature. It is inherently flame retardant and does not require a flame retardant

additive. Its dielectric constant and loss factor show minimum variation with

temperature up to its glass transition temperature (T,), Most of the moulding, extrusion

and finishing operations associated with thermoplastic fabrication are applicable to

PES. It has got very good creep resistance which is one of the important attributes of

engineering thermoplastics. It is a very good electrical insulator. Little variation in

dielectric constant or loss factor is shown in the range of 0 -180 'c. Its high J temperature performance, dimensional stability and inherent fire / flame retardency

make it suitable for electronic and electrical industry. They have excellent

therrnoforrnability and mechanical properties.

1.5.1.3

Polyesters (PE) are produced by the condensation polymerisation of

difunctional acids or anhydrides with difunctional alcohols or epoxy resins. Different

classes of polyester resins are general purpose resins, chlorendic resins, bisphenol A

fumerate resins, vinyl ester resins etc. Within these classes of resins, specific

polyester can be formulated by varying the starting materials. The physical properties

of the resin do affect their durability and thermal performance. PE resins are generally

used in the elevated temperature applications, especially in the electrical and

Chapter I 10

corrosion resistant areas. Monomer type also plays a vital role in the thermal stability

of PE resins. PE resin is for application in corrosion resistant tanks, pipes, ducts and

liners in chemical processes and in pulp and paper industry. Their improved UV

resistance makes them a candidate material for outdoor applications.

1.5.1.4 Polyether ether ketonez3

Among the thermoplastics, polyether ether ketone (PEEK) is a selection

candidate for many important high performance applications because of its special

properties. It does not absorb significant amount of moisture. PEEK has got 25-40 %

crystallinity. It is relatively costly thermoplastic with good mechanical properties and

toughness, and hence most apt to be employed in high performance composites. For

example Carbon- PEEK is a competitor for carbon reinforced epoxy and Aluminium-

Copper and Aluminium - Lithium alloys in aircraft industry. PEEK has got many

essential qualities required for aerospace applications. The outstanding feature of

PEEK is its excellent fracture resistance. PEEK and other aromatic hydrocarbon

matrices have low hydrogen to carbon ratio and hence do not produce large amounts

of combustible volatiles. Limiting Oxygen Index (LOI), which is a measure of

inflammability, is as high as 35 for PEEK and its composites. Owing to its high T, and

melting point (T,), it is difficult to set fire to PEEK.

1.5.2 Thermosetting Resins

Thermosets are currently the predominant matrix resin for composites due to

their availability, well established processing technologies, existence of large

database and low material cost. The advancement in the area of high performance

composite has led to a number of related R 8 D activities and availability of abundant

literature on the topic2'. The widely used classes of thermosetting resins are

polyesters, phenolics, polyimides and epoxies.

Chapter 1 11

1.5.2.1 Phenolic ~ e s i n s "

Among the thermosetting high temperature resistant polymers, phenolics

have a unique place in terms of their chemistry, applications, thermal and chemical

resistance, and processability. Phenolic resins, both novolacs and resols are widely

used commercially due to their excellent flame retardance and low c o ~ t ~ ~ . ~ ' . The low

smoke and toxic evolution on burning make phenolics the material of choice for

nonstructural applications like aircraft interiors. Phenolics have broad range of

applications varying from construction to electronics and Addition-cure

phenolics offer viable alternatives to condensation type phenolics, avoiding need for

high pressure cures0. However, most of them require high cure temperature, posing

processing difficulties. Phenolics excel over other matrix resins in terms of resistance

to fire and the very low smoke and toxic evolutions1. Good heat and flame resistance.

ablative characteristics and low cost are the major advantages of phenolics which find

numerous applications apart from aircraft interiors, automotive components, rocket

nozzles, appliance moulding etc.

Brittleness, poor shelf- life and need for high pressure curing are the major

short comings of phenolic resins for structural applications. Brittleness is obviated by

blending it with elastomers", risking the thermal properties. Blending the phenolic with

polyesterss5 and engineering thermoplastic like po~yethersul~hone" is reported to

yield tougher systems. Cyanate esters of novolac phenolic resin (Phenolic Tr iazins

have the combined advantages of high temperature resistance and flame retardancy

of phenolic resins, epoxy- like processing and handling convenience coupled with

excellent high temperature performance comparable to that of polyimides35.

1.5.2 .2 polyimides3$

Among the numerous high-temperature resistant polymers, the polyimides(P1)

have achieved by far the greatest commercial success, due to their ability to maintain

acceptable mechanical properties at elevated temperatures together with the thermal

stability for long term use at temperatures around 300°C"~~~. Polyimides are the most

Chapter 1 12

popular heat resistant polymers due to their thermal stability, solvent resistance and

retention of properties over a wide temperature range and hence have gained

acceptance for use in aerospace composites, electrical electronic and industrial

applications. It has wide-spread application as thermal control films, coatings,

sealants and adhesives. New monomers based on different types of aromatic

diamines, and aromatic di anhydrides have been synthesized with various specific

properties such as processability and improved stability under thermal, chemical,

electrical and radiation environment^^^^'. They offer improved high temperature

properties compared to the epoxy systems. Shao et a14' reported the synthesis and

properlies of fluorinated polyimides from new unsymmetrical diamine. These

polyimides showed outstanding solubility and good dielectric properties.

Thermoplastic polyimides are less expensive than their thermoset versions.

However, their maximum use temperature is comparatively low. Condensation

polyimides are formed by a two - step process involving formation of a polyamic acid

followed by its imidisation. Thus, the processing times are much longer and require

the application of high pressure for consolidation, due to evolution of water or alcohol

as condensation by-products. Condensation polyimidesU are thermoplastics and

remain melt fusible. Consequently, they are having good tractability. Structure 1.a

gives the structure of a condensation polyimide. A number of polyimide systems have

been developed that are well suited for use as matrix resins, adhesives and coatings

for high performance applications in aerospace and electronic industriesu. However,

major disadvantages of these thermoplastic systems include poor precursor shelf-life

and processing difficulties, hydrolytic instability and need for removing volatiles. The

growing demand for polyimides that are processable, solvent- resistant with good

thermal stability provided a strong incentive for developing processable addition-type

polyimide systems. These are low molecular weight, multifunctional monomers or pre-

polymers that carry functional reactive terminations and imide functions in their

backbone. The bismaleimides, itaconimides and nadimides are the versatile ones in

this category. The structure of typical addition polyimides with nadimide terminals are

given in Structures.1.b. A large variety of addition polyimides, with good processibility

and thermal stabil~ty has been successfully used as matrix reslns in high performance

and high temperature resistant composite applications

b)

Structures 1 (a) condensation polyimide (b) addition polyimide

In addition to the condensation and thermoplastic family of polyimides, the third type

based on imide pre-polymers also has gained considerable importance recently

because of their ease of processabilty. Among these, bismaleimides have been

studied extensively mainly because they offer greatest benefits in terms of cost

effectiveness and enhanced processability.

1.5.2.3 Bisrnaleirnides

Among the addition type polyimides, bismaleimides (BMls) still dominate the

high temperature polymer scenario due to their thermal and oxidative stability, flame

retardence, low propensity for moisture absorption, ease of synthesis and cost

effectiveness4'. Bismaleimides are best defined as low molecular weight, atleast

difunctional monomers or pre polymers or mixtures there of, that carry maleimide

terminations. The general structure of bismaleimide is given in Structure.2.

Chapter 1 14

Structure.2 General structure of Bismaleimide resin

The key to the improved BMI system is the properties of the co monomer employed

for the BMI resin.The building blocks used in commercial BMI resins are 4, 4'-

bismaleimido diphenyl methane, 2, 4- bismaleimido toluene. 1. 3-bismaleimido

benzene etc. Because of the toxicity problems associated with methylene dianiline

(MDA) (4, 4'-diaminodiphenylmethane), polyaromatic diamines and the BMls based

on them are of increasing interest. The structure of some of the bismaleimides based

on specific polyaromatic diamines are given in Structure. 3.

Structure 3 Bismaleimides based on polyaromatic diamines

Cross linked BMI polymers are infusible, rigid, brittle and are insoluble in all

solvents.. They have relatively high densities (1.35 -1.40 gtcc) and exhibit T,s

generally in the range of 250-300°C. The strain to failure is typically below 2%. The

moisture absorption levels in BMI resins tend to be much the same as in Epoxy resins

Chapter 1 15

(4-5%by weight) but saturation occurs faster than in epoxy resins. New products such

as Narmco X 5250" aim for superior hot wet performance, compression-after-impact

in composite, coupled with good processibility and longer shelf- life over currently

used epoxy and BMI composites.

Polymerisation of BMI resins: BMI resins are reactive towards various reactive

species. Such reactivities provide BMI resins with high versatility for various

application through proper formulation with other thermosetting oligomers. A few

curing reactions, such as thermal polymerization, addition reactions, Diels- Alder

reactions etc have been developed to increase the application performance of BMI

resins.

Thermal polymerization: The most important curing reaction of BMI resins has

been their homo polymerisation at elevated temperatures (thermal polymerisation).

The reaction produces no volatile byproducts and provides void-free thermosets. The

BMI resins thermally set to rigid, solvent resistant, highly crosslinked thermosets.

G ~ n d schober4' first reported their homo polymerization by simply heating the

monomer to a temperature between 150-400°C.

Michael Additions: BMI resins contain reactive unsaturated olefinic groups that are

further activated by two strong electron withdrawing carbonyl groups and are known to

undergo Michael additions with hydrogen active moieties such as phenols, thiols and

amines. The reactions have been used to prepare not only high molecular weight

linear polymers but also high performance thermosets4'. The properties of BMI resins

strongly depend on synthesis conditions. It can undergo a chain extension reaction

with amino compound through Michael addition of the structure (Structure.4).

Structure 4 Bismaleimide-MDA Michael addition product

Ene reaction: One important class of high performance thermosets is derived from

BMI resins and allyl phenyl oligomers. Upon thermal curing, the mixture co-polymerize

to produce polymers of high glass transition temperature. The applicability of BMI-allyl

phenol resins as matrices for advanced composites have been studied e x t e n s i v e ~ y ~ ~ . ~ ~

CIBA-GEIGY introduced a unique cross linking chemistry into bismaleimide system

by utilizing the so called "Ene" reaction with the maleimide double bond5'

Diets-Alder polymerization: Diels- Alder polymerization requires reaction of a diene

monomer and a monomeric dienophile, which is normally activated by electron

withdrawing substituents. During the polymerization, a stiff, heat resistant ring

structure is formed by every Diet-Alder reaction unit. Diels Alder reactions have been

developed to increase the application performance of BMI resins. The type and extent

of each reaction depend on the chemical and molecular characteristics of the

bismaleimide.

Alder-ene reaction: Maleimides are known to react with allyl phenols through an

Alder-ene reaction5"" to give rise to cyclic network structures and are used as matrix

resin with improved high temperature perforrnan~e~~". A number of polymer systems

based on the co reaction of BMls and allyl phenols have been reported5'. Matrimide

(Biismaleimido diphenyl methane +diallyl bisphenol A) is a typical formulation. A few

patent claims have been made on the blends of allyl phenol based novolac and

EMIS". The Alder-ene type reaction in such systems proceeding via an intermediate

Wagner-Jauregg reaction, leads to thermally stable, crosslinked, cyclic structuresm,

High performance BMI resins are based on rigid aromatic structures and thus

possess high melting points and a narrow processing window. Detailed studies were

carried out on thermal behavior of different types of bismaleimide monomers in neat

and blended forms using various thermo analytical techniques for composite

applicationsg

The major disadvantage of all bismalimide resins is their extreme brittleness.

Much research is still being directed towards toughening of EM1 resins. Tough

compositions need blending with elastomers and lor chain extension using diamines.

Several modified versions, particularly the PMR-type resins (with service

temperatures up to 300°C for long duration) are currently in use in aerospace industry.

The various members of the PMR family are designated by numbers, the PMR-15

being the best known. Ethynyl terminated imide and isoimide and norbornene

terminated imide (PMR-15) oligomers are commercially available for high temperature

applicati~ns~~. Modifications of the PMR type addition polyimides with nadic8',

paracyclophane8z and cyclobuteneS3 terminations also have been reported

1.5.2.4 Cyanate ~ s t e r s ' ~

Cyanate esters (CE) emerged as a compromise thermosetting matrix system

~n the late 80's, which encompasses several of the characteristics required for an ideal

matrix. They possess the heat and fire resistance of phenolic resins, processing and

handling convenience of epoxies and hlgh temperature performance of polyimides.

The cyanate compounds are stable at room temperature and undergo trimerisation to

form s-triazine rings at elevated temperature in presence of catalysts and generate

h~gh crosslink densty, which in combination with rigid aromatic rings, provide

excellent high temperature properties, solvent resistance, electrical properties and

mechanical strength. Moreover, highly desirable properties like built-in toughness, low

dielectric constant and dissipation factor, radar transparency and low moisture

absorption make them the resin of choice in high performance structural composites

for aerospace applications and in electronic industry. The low out-gassing, minimal

dimensional changes during thermal cycling, low moisture absorption, low shrinkage.

good long term stability, self adherend properties to honeycomb and foam corese5 and

high service temperature are the key advantages of cyanate esters over state- of- the-

art epoxy resins. Despite thew high cost they find appl~cation in electronics, printed

circuit boards, satellites and aerospace structural composites. They have been

successfully employed in composite structures in INTELSAT". Cyanate esters can be

reactively blended with other high performance matrices such as bismaleimides, and

epoxies, wherein, the w-reaction or interpenetrating networks (IPNs) formation

facilitates imparting a wide spectrum of properties to the blend, depending on

composition and nature of the additivee7. One of the potentially and commercially

important oligomers terminated with cyanate ester group is based on phenolic

(novolac) resinss8. Novolac-cyanate provides an aerobic char yield of 58% at 800°C

and an oxygen index of about 45. Novolac cyanate-graphite composite has got short

beam shear strength of 74 MPa and flexural strength of 1300 MPa at 1 7 7 ' ~ . Novolac

-cyanate based composite is reported to give good property retentione9 after 12

weeks of air heat ageing at 240°C.

Basically cyanate ester resins have good rheological properties to meet the

processing needs for laminate and composite preparation. One of the most serious

concerns of current high-performance thermosetting resins, such as epoxy resin,

BMls, nadimides, and phenolics is their ability to pick up high percentage of moisture.

Cyanate ester linkages are very resistant to hydrolysis at ambient service conditions

and hence cyanate ester homopolymers absorb less water at saturation than epoxy,

BMI and polyimide resins. Their high T, (260-28O0C), low shrinkage, excellent

adhesion, excellent dielectric properties, low moisture uptake (<I%), and higher

tensile strain (2-6%) make them a material of choice for many important

applications". The most effective way to broaden the application scope of cyanate

ester resins is the development of co-polymerisation systems. Copolymers of cyanate

Chapter 1 19

ester with polyphenols, polyamines, cyanamides, organic anhydrides and epoxy

resins yielded useful systems suitable for variety of applications.

1.5.2.5 Epoxy ~esins"

Since their introduction in 1940% epoxy resins have found a myriad of

applications due to their good thermal and chemical resistance, superior mechanical

properties and excellent adhesion to variety of substrates. These materials are

characterized by terminal epoxide groups. The alfa epoxy is the most common variety.

Di-and multifunctional glycidyl compounds play a unique role among the thermosetting

resins. Depending on the chemical structure of the synthesized epoxy component, the

structure of the curing agent and curlng conditions, uncured epoxy resins can be

synthesized in a broad variety of physical forms ranging from low viscosity liquids to

solids and from well defined di-, tri- and tetra functional glycidyl ethers to

advancement products of oligomer type. It is possible to obtain toughness, good

chemical and corrosion resistance, high adhesive strength, mechanical properties.

good heat resistance and high electrical properties. The current state -of - the - art

matrix resin in structural aerospace composites is epoxies. Their attractive features

include ease of processing and handling convenience, good mechanical properties,

excellent adhesion to a variety of reinforcements, toughness and low cost. In addition

to applications in high performance aerospace composites, they are used in coatings,

adhesives, floorings and electrical applications. Versatility in form, modification.

properties, hardening methods, curing conditions and application is probably the most

outstanding characteristic of epoxy resins. The thermoplastic bisphenol A type

novolac re~in'~.~' was sequentially reacted with epichlorohydrin in alkali media to

generate a bisphenol- A type novolac epoxy resin. Its molecular back bone consists of

aromatic rings and as a result it can achieve excellent mechanical and thermal

performance.

Nature and amount of curing agent and its concentration are critical in

deciding the properties of the cured network. Common curatives are aliphatic and

Chapter 1 20

aromatic amines, dicyandiamide, acid anhydrides and Lewis acids such as BF3, which

are selected based on the applications of the final cured network. Fluorine-based

epoxy systems exhibit low moisture uptake, improved toughness and heat

re~istance'~. Epoxy resins are not generally recommended for applications in severe

conditions due to poor hotlwet performance and high moisture sensitivity.

Epoxy resins dominate the structural composites in aerospace applications.

Light weight motor cases use combinations of Kevlar and epoxy system. Satellite

support structures, antena, yoke and solar panel substrates are preferably made of

epoxy-composites. The novolac epoxy resin being multy functional, can produce a

denser crosslinked network compared to the common epoxy resins7'. They are

characterised by an outstanding combination of both mechanical and chemical

properties in terms of high modulus, thermal stability, solvent resistance and therefore

find wide spread application as a sealing material for semiconductor devices. The acid

anhydrides are characterized by long pot life, better heat ageing in air at elevated

temperatures and better electrical properties. Principal Lewis acid used in epoxy

composite is boron tri fluoride. Phenolics have been used as curing agent for epoxy

resins where the epoxy is usually the major comp~nent~' .~~. In many cases, these

systems make up the base resins for the semiconductor packaging due to their

improved hydrophobicity relative to amine- cured epoxies, and their low cost. Ogata et

a~~~ studied in detail the effect of cross link densities on the physical properties of the

epoxy networks. The primary resin in epoxy technology is the diglycidyl ether of

bisphenol A (DGEBPA) and its higher homologues. The glycidyl ethers of various

novolac resins are the second most important class of epoxy resins. The glycidyl

n0~0laCs are characterized by better elevated temperature performance than

bisphenol A (BPA) based systems. The diglycidyl ether of bisphenol F (DGEBPF) has

a lower viscosity than the BPA epoxies with minimal loss in T,. The triglycidyl ether of

triphenyl methane (TGETPM), a relatively new comer to the epoxy family is said to

have high T,, thermal oxidative stability and excellent moisture resistance.

Tetraglycidyl methylene dianiline (TGMDA) is the most widely used epoxy resin for the

Chapter I 2 1

fabrication of carbon fiber composites for aerospace industry. The structures of some

of the higher functional epoxies are given in Structures.S.a, b 8 c.

Epoxies can be cured with various types of hardners to obtain a wide range

of physical and mechanical propertiese0. Most common curing agents are aliphatic

amines, aromatic amines, acid anhydrides, lewis acids and dicyan diamine (DICY).

Aliphatic amines and their derivatives are recommended for ambient temperature

curing. Diethylene tetramine (DETA) and triethylene tetramine (TETA) are common

aliphatic amines. Cyclo aliphatic amines are curing agents at moderate temperature

and have got good elevated temperature properties. Aromatic amines are high

temperature curing agents. Their major advantages are long pot life, excellent

chemical resistance, good electrical properties, and good elevated temperature

performance. eg. methylene dianiline (MDA), m-phenylene diamine (MPDA). In epoxy

amine curing, the reaction takes place in two steps. The efficiency of secondary

reaction of amine with epoxy is generally 25% of that of primary reaction and hence

takes more time for completion. Sulphone diamines are normally used in conjunction

with a Lewis acid accelerator. They provide the best elevated temperature

performance. 4, 4' -diamino diphenyl sulphone is a typical example. Combination of

DDS with TGMDA resin is widely used in aerospace industry for carbon fiber

composites. Acid anhydrides are characterized by long pot- life, better heat -ageing in

air at elevated temperature and better electrical properties. Eg. n-hexa hydrophthalic

anhydride (HHPA) and methyl tetra hydmphthalic anhydride (MTHPA) perform well at

elevated temperatures. They have excellent electrical properties and give colourless

laminates with superior weathering properties. But anhydrides of high functionality

cause handling pmblem and have poor solubility. The chemistly of curing of epoxies

with anhydrides is complex. It was found that Lewis acids and bases act as catalysts

for this reaction.

Chapter 1 22

a ) EPN

b) TGETM

c) TGMDA

Structure.5 Higher functional epoxy resins (a) Epoxy resin based on novolac (Poly functional) - EPN. (b) Tris (4-glycidyloxy phenyl) methane ( TGET M) (c) N, N'-tetra glycidyl methylene dianiline (tetra epoxy) - TGMDA

At first, anhydride reacts with the catalyst to form a carbonyl anion, then the

anionic process continues up to the completion of cure. The principal Lewis acid used

in epoxy composition is boron trifluoride in the form of its mono ethyl amine complex

(BF3-MEA). It has long pot life and high T, and a storage life of over six months. Many

critical produck based on epoxies were designed and developed for launch vehicles

viz. SLV, ASLV, PSLV, GSLV and INSAT programmes of ISRO. Multifunctional - tri-

Chapter 1 23

and tetra functional - epoxies are used in most of the aerospace composites" A

comparison of properties of conventional high performance thermoset matrix resins is

given in Table 1.1 and in Fig. 1.2.

Tablel.1 Comparison of Common High Performance Thermosetting Polymer Matrices

1 property I Epoxy I Phenolic I Toughened BMI

1 Tensile modulus 3 . 1 - 3 . 8 f 3-5 3 1 ; 3;;;; ( ~ ~ l r n ~ )

Use temperature RT-180 200-250

Density (glcc)

Cure Temperature ("C)

Mould Shrinkage

( mmtmm)

1.5.3 Miscellaneous Thermally Stable Systems

High temperature heterocyclic polymers like acetylene terminated polyphenyl

quinoxaline and polybenzimidazoles are of limited application due to difficulty in

processing. Other themlally stable polymers reported include poly oxazole,

polyphenylene oxide, polythiadiazole, propargyl ether resin etc. However, few have

found commercial acceptance because of their high cost and poor processability.

1.2-1.25

RT - 180

0.0006

TGA onset ('C)

Dielectric constant

1.24-1.32

150-190

~ ~ - ~ ~ - 0.002

260-340

3.8-4.5

300-360 360-400 400-420

1.2-1.3

220-300

0.007

1.1 - 1.35

180-250

0.004

Chapter 1 24

Mast 10 Daircrtrte

8

6

4

2

'Least EAra6lc 0

Emission Strength Tempernlure

Fig. 1.2 Comparison of performance of common matrices used in PMCS'~

The rigid aromatic backbones result in highT,, high melt viscosities and low solubility

and therefore, these are more intractable. Thus, the emphasis in recent years has

been to introduce structural variations that allow improved processibility, solubility, etc.

with no sacrifice of thermal properties.

The reinforcement of the composite plays a major role in determining its strength,

stiffness and dimensional stabilitya4. The different forms of reinforcements are

particles, nano particulates, whiskers, continuous and short fibers and woven

fabricsas. Fibers are the most common type of reinforcement.

1.6.1 Particulates

Particles are used as fillers to improve strength, toughness, processability,

dimensional stability, flame retardance and to lower the cost. Particles are available in

Chapter 1 25

different forms like sphere (glass), cube and block (calcite) and flakes (talc, mica,

wood flour) and the resulting composites are macroscopically isotropic. Whiskers are

single crystals with aspect ratio >I. Typical lengths are in the range 2-50 nm. They

have extra ordinary strength (up to 1000 ksi).

Ceramic whiskers like Sic, A1203, B,C and Si3N4 exhibiting high temperature

capability upto 2000°C. However, the difficulty to collect, sort and distribute them,

together with high costs have limited the application of whiskerss8. The knowledge of

the filler properties such as its size (Surface area, size distribution), shape

(crystallinity, anisotropy) and surface characteristics (roughness, particle aggregation

8 chemical structure) is essential for its effective utilization in the composite material.

The mechanical loss factor can be used to characterize the properties of the

interface region formed in the matrix around each filler particle, as well as the

effectiveness of various coupling agentss7-89. It is also known that the other factors

such as filler particle shape, particle diameter, modulus ratio of filler to polymer, also

have effect on polymer reinforcement.

Choice of the reinforcement for a particular application depends on a large

number of parameters including strength, stiffness, environmental stability, long term

characteristics and cost. At present carbon, glass and aramid fibers account for over

95% of industrial market. The stiffness, strength, and mechanical properties of the

composite along the reinforcement direction are dominated by the properties of the

reinforcements.

1.6.2 Low Density Fillers

Hollow micro spheres have recently evoked a lot of attention as low density

fillers for the fabrication of syntactic foam composites. The syntactic foam composites

have recently attracted lot of attention as low density composites for wide range of

applications go.~yntactic foams are composites consisting of hollow micro spheres

dispersed in resinous matrix so as to have porosity at the microscopic level. Bibin et

Chapter 1 26

als' studied the effect of shell thickness and concentration of hollow glass

microballoon on the physical. thermal and mechanical properties of modified epoxy

and cynate ester syntactic foam composites. This micro porous foam material has got

additional advantages like lesser anisotropy and mechanical property enhancement

including specific strength, shear strength, impact resistance and energy absorptionw.

Their favourable thermal, dielectric and damping characteristics makes these foams

multifunctional, and suitable for applications like bulkheads of ships, submarines,

automotive and personal protection, heat shielding of missiles heads and aerospace

structures. They find application in aerospace and automotive industries where

reduction in energy conservation is one of the important factorso3. Syntactic foams as

core material find application in the construction of sandwich structures.

Sankaran et a t 4 studied the effect of incorporation of small amounts of carbon

nanotubes (CNTs) on microballoon filled epoxy- arnine syntactic foam composites.

The incorporation of small percentage of CNT was found to improve the storage shear

modulus of the system by 50% while in the case of multiwalled carbon nano tube

(MWCNT), the reported increase was 24%. The scanning electron micrograph (SEM)

images indicated that the CNTs tend to occupy the interstitial spaces in the foam

thereby decreasing the void- content. The property improvement in epoxy resin by

nano-silica reinforcement was reported recentlys5.

Continuous fiber composites routinely have strengths 2-3 times greater than

discontinuous fiber composites. The fibers for resin matrix application should have the

highest modulus, lowest density and highest tensile, compressive and interfacial bond

strength between resin and fiber, while still failing in a non-catastrophic mode. The

significantly higher strength of fibers in the longitudinal direction is provided by the

orientation of molecules along the axis during drawing.

1.6.3 Inorganic Fibers

These have been developed with inorganic lattice structures. lnorganic fibers

are extremely strong although inextensible and are intended for ultra high temperature

uses. The inorganic fibers commercially available mainly include glass, boron, silicon

carbide, nitrides and alumina.

1.6.3.1 Glass ~ i b e r s ~ ' ~ ~ '

Glass fibers have been known and used for decades. Different types of glass

fibers like E. S, C and quartz, depending on their chemical composition, are

manufactured. The popularity of glass fibers results from their high tensile strength

and durability, dimensional stability, low dielectric constant and relatively low cost,

making them attractive in automotive, submarines and motor boat applications. Fibers

are usually supplied with a finish or sizing with chrome complexes and silanes for

bener handleability and adhesion to the matrix E glass fibers (Electrical grade) have

relatively high energles to failure, provid~ng their composites with good impact

resistance compared to those of other fibers. The impact resistance of composites

made from S-glass fibers is the highest of all fiber reinforced materialssg. The major

limitation of glass fibers is their relatively low modulus.

1.6.3.2 Boron and Silicon Carbide ~ i b e r s " ~

Boron (B) and silicon carbide (Sic) fibers find selective placement in the high

value aerospace market where cost is often secondary to performance. Both are high

modulus fibers, prepared by chemical vapour deposition process. The current

principal use of B fibers is as reinforcement for Blepoxy composites for high

performance applications, whose property retention at elevated temperatures is

dictated by the epoxy matrix. Boron fibers being too expensive, are replaced by

graphite fibers for commercial use. Sic fibers are also suitable for high temperature

appli~ations'~'es~ecial1y in ceramic matrix

1.6.3.3 Alumina Fibers

Alumina fibers with very high modulus ( 2 . 7 ~ 1 0 ~ MPa) are specialty fibers

introduced by Du Pont. These are used as reinforcements for metal and ceramic

Chapter 1 28

matrices owing to their stability in molten metals, as well as radar transparent nature.

Their strength and modulus are similar to Boron fiber but density is higher by around

10%'02.

1.6.4 Organic ~ibers'"

Organic fibers commonly used as reinforcements include aramids, ultra high

molecular weight polyethylene and carbon. Most of the natural organic fibers- cotton,

jute, sisal - and many synthetic organic fibers with the exception of aromatic

polyamides (aramides), are of little interest for structural applications, because of their

low modulus and modest mechanical properties.

1.6.4.1 Aramide ~ibers"'

Du Pont has developed high strength and high temperature liquid crystalline

fibers based on aromatic amide structures, by the name Kevlar in 1971. Kevlar is

made by solution spinning in three different types, high toughness Kevlar-29, high

modulus Kevlar 49 and ultrahigh modulus ~evlar-149"'. Kevlar - 49 has an axial

modulus (124GPa) in between that of E-glass and high strength graphite and specific

gravity of 1.44. Because of its higher stiffness, this is used more widely. Their

compressive behavior is unique. The onset of compressive non linearity starts at

lower stress I strain value and hence the compressive strength of composites

reinforced with Kevlar-49 fibers is significantly lower than those using other

reinforcements such as glass and graphite. The fiber has got excellent fatigue

resistance and they are strongly anisotropic, with transverse extensional modulus and

shear modulus about an order of magnitude lower than axial extensional modulus.

Due to their good strength and modulus, light weight, toughness and excellent rigidity,

they find key applications in ballistic protection, motorized combat vehicles, tire belts,

high performance pressure vessels and rocket motor cases (hoop stress for filament

wound Kevlar 49lepoxy is 450 ksi'06

Chaoter 1 29

Aramide fiber is an insulator of both electricity and heat, resistant to organic

solvents, fuels, and lubricants. As unprotected fibers, they are highly tenatious and do

not behave in brittle manner as do both glass and carbon. Eventhough they have very

high specific strength and specific modulus, they have inferior compressive properties.

1.6.4 .2 Poly Ethylene ~ibers'"

Polyethylene fibers like Spectra 900 and Himont 1900, with excellent specific

strength and modulus are manufactured respectively by extrusion and gel spinning'08.

These low density fibers have very high molecular weights (3x10~) and approximately

45% crystallinity. The fibers possess abrasion and solvent resistance, high impact

toughness even at cryogenic temperatures and resistance to radiations. But their high

temperature performance is inferior to Kevlar owing to low T, and creep problem'0s.

The major applications are in radomes, ballistic penetration prevention and low

dielectric applications.

1.6.5 Carbon Fibers 'lo

Carbon and graphite fiber technology have made enormous strides in recent

yean in combination with high performance synthetic resin matrices. Carbon fibers

were developed as reinforcements with higher strengths and modulii than glass fibers.

They are used for reinforc~ng composites for critical aerospace applications like re-

entry vehicle structures, rocket motor cases, satellite structures, tanks and pressure

vessels. Today, carbon fibers are among the highest strength and modulus materials

and have become the dominant reinforcement for advanced composites because of

their tailorable mechanical properties, strength retention at extreme high temperature

(in non-oxidising environment) and reasonable cost. The mechanical properties of the

carbon fiber are determined by the type of precursor and the temperature-time profile

of the manufacturing process.

Chapter 1 30

The carbon fiber-reinforced composites are most suitable for cry0

temperature applications. The interlaminar shear strength of carbon fiber composites

improves with lowering of temperature. They have good thermal and electrical

conductivity and these properties do not disappear at the lowest temperature,

because, the contribution by electrons is completely eliminated. These properties offer

promising application in pulsed super conductive electromagnets. The specific

stiffness of carbon fiber composite is 9.7 times that of glass composite"'. Carbon

fibers can be prepared from mesophase pitch '12 (P-100, P-120, P-75), polymers like

polyacrylonitrile (PAN)"' and rayon (T300, T600, T1000) or carbonaceous gases like

acetylene (vGcF)"'. PAN and pitch based fibers comprise 80% of the carbon fiber

market as they are more graphatisable and therefore can attain high thermal

conductivity and low electrical resistivity. The black, thin (5-lomicron dia) man made

fiber embedded in polymer matrix has initiated a "revolution in materials" even they

are reported to be a "Miracle material" having extremely low thermal expansion, better

fatigue resistance and high corrosion resistance. The family of carbon fibers

consisting of high tensile, high modulus and intermediate modulus cover a Youngs

modulus value ranging from 200-700 GNlm2 and strength 2 - 4 GNI~~. The use of

vapor grown carbon fibers in composites is reported to yield unprecedented high

thermal conductivity and caused significant reduction in coefficient of thermal

expansion of the polymer matrixH5.

1.6.5.1 Graphite ~ibers"'

Carbon I graphite fiber is rapidly establishing itself as a top candidate for high

performance applications. It is available in forms that offer high strength, high

modulus, dimensional stability, electrical conductivity, inherent lubricity, excellent

corrosion resistance, heat resistance and low density. Good adhesion between fiber

surface and matrix resin is critical in maximizing properties and minimizing sensitivity

to moisture"'. Many critical products made of epoxies were designed and developed

for launch vehicles viz. ASLV, PSLV, GSLV and INSAT programmes. Since ASLV.

Kevlar-Epoxy has been replacing glass epoxy in many of their applications such as

rocket motor cases, pressure bottles etc. due to 50% of their weight saving.

Graphite fiber sometimes referred to as carbon fiber, comprise one of the

most important classes of reinforcement with enormous potential for future growth.

Their primary advantages over glass fibers are higher modulus, lower density, better

fatigue properties, improved creep rupture resistance and lower coefficient of themlal

expansion '18. Creep at mom temperature is generally considered to be negligible. On

the negative side, because of their low strain to failure ratio, failure energies are

relatively low and hence the impact resistance of graphite fiber composites is low.

Graphite fibers of commercial importance are made from poly acrylonitrile (PAN)

fiberi1', rayon fiber, and petroleum pitchlZ0. The high strength and moduli of graphite

fiber regardless of precursor, result from their high degree of crystallinity and

orientation. Because of their high degree of internal structure orientation, graphite

fibers are strongly anisotropic. Transverse extensional modulus and shear modulus of

most fibers are generally an order of magnitude lower than axial modulus.

1.6.5.2 Carbon ~anotubes '~ '

Since the discovery of carbon nanotubes, having extremely small physical

size (diameter approximately lnm) and many unique mechanical and electrical

properties, they have been appreciated as ideal fibres for nano -composite structures.

Carbon nanotubes (CNT) are long graphine sheets wrapped in different angles (chiral

angles) with different circumferential lengths (chiral vector) depending up on which

they are able to offer wide range of mechanical, electrical and thermal properties.

Based on the number of such sheets, these carbon allotropes may be classified as

single walled nanotube (SWNT) or multy wall nanotube (MWNT). The versatility of

these materials made them attractive candidates for reinforcing polymer matrices. It

has been reported that nanotubes own remarkable mechanical properties with

theoretical Young's modulus and tensile strength as high as 1 TPa and 200 GPa

respectively. Lau et alqZ2 has given a review of the recent researches and appliiatiins

Chapter I 32

of carbon nano tubes. The surface co-valent encapsulation of MWCNT and formation

of various grafted polymers using the same is reported by Liu et allz3 The important

mechanisms of load transfer are micromechanical interlocking, chemical bonding and

Vander Waal bonding between the matrix and the nano tube.

The influence of nanotube / matrix interfacial interaction was evidenced by

Gong et alqz4. It was also reported that coating carbon fiber with Multiwalled carbon

nanotubes prior to their dispersion into an epoxy matrix improves the interfacial load

transfer possibly by the local stiffening of the matrix near the interface '16. Jianyu

~uang"', has demonstrated its stretchability to four times its original length at

2o0o0c.

1.6.6 Hybrid and Speciality Reinforcements 127,128

Hybrid reinforcements are commonly used for improving the properties,

getting some specific properties and / or lowering the cost of conventional composites.

The best properties of each of the reinforcements can be utilized. E.g. carbon fiber is

used in conjunction with ductile fibers like glass or aramid (enhanced failure strain),

PEEK (better drape and toughness), polyethylene fibers (toughness and damping

ability), elastomer particles (toughness), alloy particles and whiskers (wear and fatigue

resistance, increased transverse strength) and coating with niobium carbon nitride

(super conducting fiber)"'. The potential advantage of combining two kinds of fibres in

a single matrix has been described by many re~earchers''~~'. Investigations in the

field of hybrid composites reveal that most of the studies were concentrated on

systems which have been re~nforced with carbon fibers.

Hybrids composites of glass fiber-epoxy laminates and aluminium foil (glare)

are commonly used to obtain hybrid fiber-metal laminates (FMLS), which are

candidates for the next generation structural materials for advanced air craft^"^. The

static mechanical performance and fatigue behavior of the hybrid composites is

dominated by the behavior of the lower strain phase j3', often carbon fibers. The fiber

arrangement is an important parameter in determining the strength of the composite.

Various types of fiber arrangements are possible in hybrid composites'". Additional

possibilities are the use of woven fabrics with one fiber in the warp and another in the

weft and further variations using interspersed tows in woven configurations. Plies

containing different fibers or mixture of fibers may be stacked with the fibers aligned in

different directions to form different combination.

Owing to their unique properties, several fibers like carbon I fluoride, nitrides,

polyether ether ketone (PEEK), poly benzoxazole (PBO), poly benzothiazine (PBT)

are being developed for specific applications. Carbon1 fluoride fibers having good

thermal conductivity and electrical resistivity (100 to 10 - cm) are used as

semiconductors. Boron and silicon nitrides are reported to possess high thermal

shock resistance, low coefficient of thermal expansion and good retention of stiffness

and strength at elevated temperatures. PEEK fibers, developed by melt spinning of

aromatic PEEK are resistant to acid and alkali hydrolysis but susceptible to UV

radiation. Liquid crystalline polyester fibers like Ekonol and Xylar are noted for high

tensile strength and modulus'35.

1.6.7 Woven ~ a b r i c s " ~

Woven fabric composites are characterized by better impact resistance,

damage tolerance, dimentional stability over a large range of temperatures, subtle

conformability and ease of manufacturing. But their in- plane properties are

compromised by using the reinforcement in fabric form. The mechanical behavior of

woven fabric (W.F) composites depends upon the type of weave, fabric geometry. 137-39 fiber volume fraction, laminate configuration and the material system used . The

fiber reinforcing efficiency depends on the percentage of curved fiber length and the

severity of curvature. Two main forms of curvature are Twist and crimp. The fabric

construction determines the reinforcing efficiency, conformability to complex surfaces

(drape), resistance to distortion (stability) and extent of anisotropy. The fabric

geometry should be so chosen that it should give the best possible properties for the

application under consideration. The important fabric parameters are strand cross

Chapter 1 34

sectional geometry, strand fineness, number of counts and the weaving conditions like

balanced or unbalanced.

1.7 ~nterphase'~' The inter phase represents an interfacial region of finite volume between the

fiber and the matrix wherein the material properties vary (either continuously or in a

step-wise manner) between those of the fiber and the matrix material. The properties

of the inter phase are governed largely by the chemical I morphological nature and

physical I thermodynamic compatibility between the two constituents. Hence, a

thorough knowledge of the microstructure - property relationship at the interphase

region is an essential key to the successful and proper use of composite materials.

The importance of inter phase in composite technology has caused ever increasing

research interests in both experimental and micro-mechanical I numerical

characterisation of the inter phase. The optimization of the mechanical properties of

composites is influenced by the behavior of interface / inter phase"'. A great deal of

research has, therefore, been focused on the analysis of interfacial phenomena in

composite materials 14244

The interface is always involved in the failure of the composite, either as an

initiation point or as a locus of failure. Its potential to have large influence on the

composite performance arises from its large area compared to the composite area'".

The complexity of the inter phase can best be illustrated with the use of a schematic

model, which allows many different characteristics of this region to be enumerated as

shown in Fig.l.3. There is now considerable evidence available which demonstrate

the outstanding influence of interfaces on fracture toughness, in both transverse and

inter laminar fractures, strength and stiffness of fiber composites in various failure

modes and loading configurations. The sensitivity of the composite properties to fiber-

matrix adhesion will be governed by, how matrix and fiber are connected and how the

applied load is transferred and distributed in composite.

Chapter 1 3 5

Thermal. chsmlarl. and mecbankal ew~mnments

BUR liber

Fig 1.3 Schematic representation of fiber -matrix Inter phasef4'

In the case of transverse tension, the matrix and fibers are connected through the

fiber- matrix interface in series and all the three components fiber, matrix and

interface1 inter phase carry equal load. In such cases, the fiber -matrix adhesion

should not be expected to have a dominant effect on composite properties'40. A

comparison among the transverse flexural, tensile and short beam shear strength of

composites made of epoxy matrix and three types of carbon fibers is shown in

~ig.1.4a'~'. There is a significant difference between transverse flexural and

transverse tensile strengths. Not only is the transverse flexural strength more

sensitive to fiber matrix adhesion, but it is also significantly higher than the transverse

tensile strength. The mode-l and mode-ll -inter laminar fracture toughness of

composites are shown in Fig.l .4bI4O. The fiber- matrix adhesion in the carbon fiber is

varied by different surface treatments and is in the order AS-4C > A S 4 > AU-4.

Chapter 1 36

Fig-1.4 (a) Effect of fiber-matrix adhesion Fig.l.4 (b) Effect of fiber-matrix adhesion on transverse flexural, tensile and short on the mode I and mode II inter laminar beam shear strength of epoxy- carbon fracture toughness of epoxy- carbon composites composites

A number of experimental techniques have been developed to characterize

the interface1 inter-laminar properties of fiber reinforced ~ o m ~ o s i t e s ' " ~ . These

techniques in general are classified in to (1) testing of single fiber micro composites

and (2) testing of laminates'". The stress distributions in fibers and fiber matrix

interface have also been measured extensively using novel techniques based on

Laser Raman spectroscopy for various combinations of fibers and polymer 150.63 matrices . The fiber I matrix interfacial degradation is generally reflected in short

beam shear (SBS) testsqs4. Surface roughness, surface functional groups and

adhesion at the lnterface are some of the factors influencing the performance of the

inter-phase.

1.7.1 Adhesion at the Interface

In order to get a complete picture of the properties of a composite, intensive

study on the active involvement of interfaces is necessary. Adhesion between the

matrix and fiber (physical, chemical and frictional) should be good for maximum stress

Chapter 1 37

transfer. Higher the resistance to shear, higher will be the stresses that can be applied

to the specimen. Normal and shear stresses are effective at the interface, the latter

arising from residual thermal stresses and difference in Poisson's ratio.

Work of adhesion is expressed as,

where, y is the surface tension and 0 is the contact angle. Matrix also plays an

important role in adhesion. Usually adhesion is stronger in polar polymers like epoxy,

owing to their capability of hydrogen-bonding with the functional groups on

reinforcement surface. Ductility also is important as it provides resistance to crack

propagation.

1.7.2 Theories of Adhesion

The fiber matrix interface adhesion can be attributed to five main

mechanisms. a) Adsorption and wetting b) Interdiffusion c) Electrostatic attraction, d)

Chemical bonding and e) Mechanical adhesion. Schematic representation of Fiber-

matrix interface adhesion is shown in Fig. 1.5.

1.7.3 Fiber Surface and Interface hlodifications'"

The interaction of fiber with a matrix material, depend strongly on the

chemical / molecular features and atomic composition of the fiber surface layers as

well as i b topographical nature. A number of factors affect the microstructure of the

inter phase, which in turn control the mechanical and physical properties of the

composite. They are the topology and the chemical composition of the fiber surface,

silane structure in the treating solution and its organ0 -functionality, acidity, drying

conditions and homogeneity.

Chapter 1 38

Fig 1.5 Schematic representation of Fiber- matrix interface adhesion1"

In recent years a surface technique. Time of Flight Secondary Ion Mass

Spectroscopy (TOF SIMS) in combination with XPS has been extensively used by

Wang et al 16' to observe the chemical reaction in the silane interface region during

cure.

The inter phase material tends to have a lower glass transition temperature,

higher modulus and tensile strength and lower fracture toughness than the bulk

matrix. Since the details of the interface reaction is specific to each cornbination Of

fiber and matrix material, with totally different chemical and atomic compositions and

morphological nature, no general conclusions can be drawn regarding the ductility

and fracture toughness of the inter phase relative to the surrounding matrix. Suzuki et

al '" reported that the interlaminar fracture toughness of the glass fiber composite is

influenced by the type and concentration of the silane solution. A schematic model for

interdiffusion and inter-penetration in a silane treated glass fiber polymer matrix

composite is shown in Fig.l.6.

The interface adhesive behavior can be visualized in to three. i) A sharp

interface with a weak molecular force as in the case of a non-polar polymer with a

polar polymer ii) A sharp interface with a strong molecular force, such as dispersion

force between a nonpolar polymer and a high energy material and iii) Diffuse interface

with any molecular force; if there is sufficient molecular diffusion and entanglement,

interfacial slippage will not occur.

Chemically bonded 8 Diffused interface ---h - --, +----------.

Polymer o coupling agent

Fig.l.6 Schematic model for interdiffusion and IPN i75f silane treated glass fiber polymer matrix composite

The interfacial interaction depends on the physical and chemical structure

and polarity of the components. The interfacial adhesion will be enhanced by

chemically coupling the matrix to the reinforcement. Several processes such as

chemical treatments, photochemical treatments, plasma treatments, surface

grafting etc, have been developed to modify polymers and fiber surfaces. These

cause physical and chemical changes on the surface layer without affecting the

bulk properties. The plasma treatment"', acetylation, benzoylation, acrylation,

permanganate treatment, peroxide treatment e t ~ . ' ~ ' " ~ were found to effectively

enhance the compatibility of fibers with polymers. Tethered chain is also reported to l a 4 5 improve the mechanical properties of polymer blends .

The degree of adhesion between carbon fiber and matrix depends considerably on

the quality and state of carbon fiber surface, wh~ch in turn depend on the internal structure

of graphite (which is highly anisotropic on a nano scale). The techniques used for the

Chapter I 40

characMzaWn of c a m fiber surface like inverse gas chromatography, at infinite and finite

dilution1", surface energy measurement^'^.'" and detefmination of active s k concenb-abn

are discussed in the case of virgin, plasma treated and anodically oxidized PAN and pitch

based carbon fibers. ESCA specboscopy deconvo~ution~~ is suggested to be a very

inkresting way to assess the Oxidation Degree of Spherical CaMn (ODSC) index. This index

expresses the ratio of the surface concentration of more oxidized groups such as carboxylic

acids and esters to less oxidied groups such as alcohols and ketones. The most e w e

surface treatment of carbon fiber- epoxy resin system are reported to be oxidative in natureqm.

Schematic model for the chemical reaction between oxidized carbon fiber surface and epoxy

matrix is given in Scherne.1. These m t s have been mnjunctured to improve the b r -

mabix adhesion primally through increased chemical bonding'" or by enhanced mechanical

interlodting between the fiber and the m a d n .

Scheme.1 Schematic model for the chemical readion between oxidzed carbon fiber surface and epoxy mabixl"

~eviews'" on the surface treatment of carbon fibers show that a range of

active functional groups can be produced by different surface treatment methods,

which can be classified into oxidative and non-oxidative treatments. The oxidative

treatments are further divided in to dry oxidation in presence of gases, plasma etching

and wet oxidation, the last of which is carried out chemically or electrolytically.

Deposition of more active forms of carbon such as the highly effective whiskerization,

plasma polymerization and grafting of polymers are among the non oxidative

treatments of carbon fiber surfaces. The surface treatment of carbon fibers results in

substantial changes in the fiber surface area with associated variation in regocity,

removal of weak surface layer ( the removal being more serious in plasma etching

than in wet oxidation), increase in the polar surface energy and chemical modification

by the formation of carboxyl, hydroxyl and carbonyl groups on the fiber surface.

To improve interfacial interaction and hence compatibility, the in situ reactive

mixing of the two polymeric phases has been shown to be one of the most effective

ways"5. The interaction between component phases may, however, be brought by

addition of an agent that interacts with both the phases, and render them mutually 'I76dl compatible and by specific reaction between the two phases . The occurrence of

reaction between the two polymeric components at the inter phase is expected to lead

to a reduction in interfacial tension. The factors which directly influence the extent of

reaction that happens include the intensity and time of mixing, functionality level,

kinetics of reactive groups, and stability of the covalent bond to the processing

conditions"'. In the future, micro engineering of the fiber-matrix inter phase will be

used to optimize the properties and performance of composite materials

1.7.4 Fiber Surface and Interface ~haracterizatjon'~

Spectral and microscopic methods are mostly resorted to the fiber

surfaces and interface characterization. The characterization gives evidence of

chemical composition of the surface region as well as information on interactions

between fiber and matrix. Electron spectroscopy for chemical analysis (ESCA)"',

Mass spectroscopy. X-ray diffraction, electron induced vibration spectroscopy and

photo acoustic spectroscopy are shown to be successful in polymer surface and 103,184 interfacial interaction chara~terization""'~~. Apart from Infrared spectroscopy ,

frequently used thermodynamic methods such as wettability studies, inverse gas

chronuitography measurement, zeta potential measurement, contact angle

measurement and Neutron activation analysis are used for the characterization of

reinforced polymers.

Chapter 1 42

1.8 Fabrication of ~ornposites'~~ Different processing techniques are used for composite fabrication,

depending on the desired properties, nature and properties of the matrix and shape of

the product. Different manufacturing processes used for the fabrication of the

composite leads to varying amounts of imperfections and air inclusions eventhough all

manufacturing methods aim to avoid entrapment of air or formation of voids since

these represent gross defects.

1.8.1 Thermoset Processing

The impregnation mechanisms and consolidation quality have got profound

influence on the mechanical properties of composites. The main goal of consolidation

is to produce a monolithic structure by inducing intimate contact and adhesion

between individual plies and to minimize void content inside the composite.

Cosolidation is accompanied by applying pressure and heat at the system to squeeze

the air and volatiles out of the composite.

The simplest method of consolidation is manual lay up (wet and pre-preg).

High strength parts with irregular shapes can be manufactured by vacuum bagging.

Application of vacuum assists in compressing the plies, simultaneously removing the

volatiles. Autoclave molding gives better compression and elimination of volatiles as

these are pneumatic pressure vessels with built-in-heatei and blower units. Therefore,

this technique is used for molding complex and critical parts.

The impregnation techniques followed irre generally classified in to pre-

impregnation techniques and post-impregnation techniques. 'The former includes melt

impregnation and solvent aided impregnation and the later constitutes film stacking,

powder coating, co -weaving and hybridization.

Another important technique for fabrication of compositss is filament winding.

This method is suited for manufacture of pressure vessels, rocket motor cases, etc.

Pultrusion method is employed for the production of rods, sheets, tubes, ek. In this,

Chapter 1 43

continuous fibers are passed through a resin bath and then through the heated die so

that the resin cures in the die itself.

Compression molding (matched die molding) is performed under heat and

pressure. Depending on the form in which material is placed inside the mold, preform,

sheet molding (SMC) and batch molding compound (BMC) methods are available to

make complex shapes with good consolidation. Small parts with powder or short fiber

reinforcements are manufactured by transfer molding. Reaction injection molding is

also a fast growing process.

1.8.2 Thermoplastic ProcessinglB6

The processing of thermoplastics is similar to that of thermosets with minor

modifications depending on the matrix and fiber chosen. Instead of curing.

thermoplastics are heated into molten stage and cooled during fabrication. Pultrusion,

matched die molding, injection molding and extrusion are the common methods for

processing of a thermoplastic resin.

1.8.3 Process Monitor ing

Many process monitoring techniques are used to track the chemical reactions

that take place during the processing of composite materials. Variety of techn~ques

such as, differential scanning ca~orimetry'~'"~~, thermal scanning rheometry'",

dielectric spectroscopy'92 Raman spectroscopy'g3and F T I R " ~ ' ~ have been used to

monitor cure reaction of epoxies. Epoxy -amine reaction was reported to follow auto

catalytic mechan i~m '~ ' "~~~

The chemical reactivity and kinetics are controlling factors for the successful

polymerization of composite resin matrix. Since chemical reactivity is related

exponentially to the temperature, the heat-up rate and temperature difference within

the laminate, the knowledge of its chemical kinetics is very important for obtaining

acceptable composites. One of the mechanisms of understanding the chemical

Chapter 1 44

kinetics of a resin system is the experimental determination of its time-temperature-

transformation (TTT) diagram. These diagrams are made by subjecting the resin to

pre determined conditions and measuring the response usually by rheometric

methods. The results are usually plotted as viscosity vs. time for various

temperatures.

1.9 Nano ~ o r n p o s i t e s ~ ~ ~

The goal of nano composite developments in composite industry is to create

new materials that have unprecedented properties due to their small size. Nano-

composite technology has been described as the next great frontier of material

science. Polymer layered nano-composites represent a rational alternative to

conventional filled polymers. The field of polymer nano-composites is stimulating both

fundamental and applied research, because a minimal addition of 40% nano-scale

materials often exhibit physical and chemical properties that are dramatically different

from conventional micro composites. The polymer layered nano-composites can

exhibit increased modulus, decreased thermal expansion coefficient, reduced gas

permeability, increased solvent resistance and increased ionic conductivity compared

to the polymer.

Hackman et a1 studied the basic concepts of nano composites, the physical

and chemical requirements for attaining nanocomposites, the types of property

improvements that have been generated by researchers in this field and nano

composite fabrication techniques viz. conventional filling, intercalation and exfoliation.

The incorporation of nano particles for the modification of fiber- reinforced composites

also yielded high performance compositeszo2.

Wang et a1203 reported the improvement in barrier properties of the nano

composites over the pristine polymer. The nano particle addition was found to give

extensive improvement in mechanical properties for the polymer having sub ambient

T, while modest improvement only was observed in the case of polymers of high T,.

Weiping Liu et a1204 evaluated the performance of organo clay filled high performance

Chapter 1 45

epoxy nano composites and reported an improvement in compressive strength and

fracture toughness (Gi, and K,3 with filler concentration while the T, and yield strength

were found to be reduced.

Epoxy-nano clay composites are being studied extensively by various

researchers which include the studies on filler concentration '05, mixing methods-direct

mixing method (DMM) vs. high pressure mixing method (HPMM) 'O' and types of

smectite clays "'. Subramonian et alZos have obtained improved properties for the

exfoliated nano composites compared to the intercalated nanoclay- glass fiber

composites.

1.10 Fiber Reinforced Composites

The fiber reinforced composites are classified in to three, based on the type of

fiber used viz. chopped, continuous or woven. The fibers are the load carrying

members and the matrix which binds the fibers together, acts as a load transfer

medium and protects the fiber from environmental damages due to elevated

temperature, humidity and damage due to handling. The stiffness, strength and

mechanical properties of the composites along the reinforcement direction are

dominated by the properties of the reinforcement.

The continuous fiber composites are usually subdivided into straight fiber

laminates that have only in-plane fiber orientations and braided or woven textile

composites that have both in-plane and out-of-plane fiber orientations. Straight fiber

laminates are used extensively in aircraft structures. However, the potential for

delamination is a major problem because the interlaminar strength is matrix

dominated. Textile composites that do not have distinct laminae are not susceptible to

delamination, but their strength and stiffness are sacrificed due to fiber curvatures.

Predicting crack propagation in PMC structure is further complicated by the

existence of a multiplicity of design options arising from the availability of numerous

choices of fibers, fiber coatings, fiber orientation patterns, matrix materials, constituent

material combinations, and hybridizations207. The performance of 3D woven

Chapter 1 46

composites was found to far outstrip that of conventional 2D laminates or stitched

laminates in many important regards. Thus, 3D braided fiber reinforced PMCs can be

expected to exhibit similar, superior performance to 2D braided PMC materials.

Further, tow size, strength and distributions in space of geometrical flaws are found to

be key design and manufacturing parameters for controlling performance

characteristics of 3D woven and braided PMCs.

The bidirectional fiber arrangement in a woven composite restricts matrix

shrinkage. The small voids may not significantly affect the mechanical properties but

show considerable effect on diffusion behavior. Moisture absorption is a matrix

dominated propertyzo7. Unlike the neat resin and uni-weave composites, the first stage

diffusion of the woven fabric composite deviate from Fickian diffusion which predicts

the moisture uptake to increase as a linear function of square root time during the

early period208.

Different mechanisms have been suggested to explain the matrix to

reinforcement stress transmission. Klaus Friedrich et alZo9 have given a detailed

picture of different deformation mechanisms in knitted fabric composites. Continuous

fibers bear the maximum applied stress. Assuming a strong fiber-matrix interface, the

tensile Stress applied along the fiber direction will be distributed as

where subscripts c, m and f refer to composite, matrix and fiber and E. V, o and E

represent modulus , volume fraction, stress and strain respectively . In the case of discontinuous fibers, stress transmission depends on the critical

fiber length I,, which depends on the ultimate strength, &of the fiber and its diameter.

Chapter I 47

Where, r = shear strength between matrix and interface.

d = diameter

The critical fiber length, I, is very important since in cases where the fiber

length is shorter than this, the fiber will not be able to reinforce the matrix. For

example the typical I, for carbon I epoxy is 4mm. The properties of the ~0mposite are

affected by several factors like the size, structure and distribution of reinforcement,

matrix morphology and molecular restructuring during processing and residual

stresses that can lead to dewetting2".

The properties of the woven fabric composites are determined by the fabric

material, fiber orientation, stacking sequence, weaving pattern, void content, etc. The

effect of fiber orientation on the work of fracture obtained by notched charpy impact

test of commercial GFRP laminates are shown in Fig.1.7'"

1 2 5 10 20 50 100 200 500 F R a C T U R ENFRGY. k J

unlBlrect~onn1.V 0 . b ~ ( 8 = 0 d e t ~ n e s tlbre sxls) E

A Cl-088 plled, non-woven, V = 0.67 f Woven cloth imrnate, V, - 0 . 4 7

Fig 1.7 Work of fracture of GFRP (by notched charpy impact test)

Chapter 1 48

1.1 1 Characterization of Fiber Reinforced Composites

The composite materials are being extensively used in different areas where

they are subjected to different loading and climatic environments. Hence a thorough

understanding of their thermal, physical, mechanical and dynamic mechanical

properties under different climatic and loading environments is essential for assessing

its suitability for the specific application. The knowledge of their on -axis and off-axis

mechanical properties, fracture properties and behavior under fatigue loading

conditions are essential for their performance prediction. The properties of different

types of composites and the factors influencing their properties were extensively

studied by several authors 212-216

1.11.1 Thermophysical characterisation2"

The thermal properties of a material are characterized by its glass transition

temperature, thermal and thermo oxidatiie resistance. flame resistance, ablative

resistance, coefficient of thermal expansion, thermal decomposition temperature in

nitrogen and air, and char yield. The highly polar groups will increase the inter-

molecular attraction. The stiff and bulky groups in the polymer backbone or sidechains

inhibit the free rotation of the chain segments and increase T, and heat resistance.

1.11.1.1 Thermo Mechanical Analysis

Thermo mechanical analysis (TMA) is useful in determining the softening

point, thermal expansion coefficient and glass transition temperature of materials.

TMA is used in expansion, flexure and penetration modes to determine the glass

transition temperature of the polymers and composites. Since the composite is a

blend of the matrix and reinforcement having different thermal expansion coefficients,

and the mismatch in their expansion coefficients can lead to the failure of the

composite at the interface, the determination of expansion coefficient of composite

and its constituents is of special concern. The coefficient of linear thermal expansion-

a, undergoes a marked change in the T, region and it is determined by the point of

Chapter 1 49

intersection of the lines fit to the expansion data above and below the T, range.

Flexural TMA analysis is sensitive to the modulus of reinforcing fibers in the

composite and this will give different results depending on the nature of the fibe?".

Uniaxially oriented fiber composites have different a values in different directions

which can be computed knowing the volume fractions, a values and Poisons ratios of

the individual component^^'^. In the transverse direction, a can be more than that of

the matrix material because of the fact that the rigid fiber prevents the expansion of

the matrix in the longitudinal direction, so that the matrix is forced to expand in the

transverse direction more in this case2".

1.11.1.2 Thermo Gravimetric Analysis

Thermo gravimetric analysis (TGA) of the polymer matrix gives information on

its thermal stability, char formation, and decomposition kinetics. The preferred method

for determining the short-term thermal stability or thermo oxidative stability of high

temperature polymers has been dynamic TGA. The weight loss in dynamic TGA is

sensitive to experimental variables like heating rate, sample mass and atmosphere.

Long-term stabilities have been most conveniently determined by iso-

thermogravimetric analysis. Thermal stability is primarily determined by the bond

energies between atoms in the chain. High thermal stability in polymers has been

achieved by making full use of resonance stabilization. Cross linking generally results

in improvement in thermal stability.

1.11.1.3 Differential scanning calorimetry

Differential scanning calorimetry (DSC) monitors the enthalpy changes in the

polymeric matrix as a function of temperature. It gives useful informations on its cure

and transition temperatures and enables to define processing window and fabrication

conditions of the composite based on the cure behavior of the resin. Since the heat

capacity of a polymer changes at T,, the differential scanning calorimeter is used to

determine its T,. The temperature corresponding to the base line shifl in heat flow vs

Chapter 1 50

temperature curve gives the To of the system which dictates the performance of the

resin system under different climatic conditions.

1.11.1.4 Dynamic Mechanical Thermal ~ n a l ~ s i s ~ ' ~

Dynamic Mechanical Thermal Analyser (DMTA) is a powerful tool for

determining the glass transition temperature, understanding the cure behavior, and

hence providing the processing conditions for the polymer matrix composite. The

dynamic mechanical properties, especially damping, are sensitive to all kinds of

transitions, relaxation process, structural heterogeneities, and morphology of the

multy phase systems such as composites. Damping is often the most sensitive

indicator of all kinds of molecular motions which are going on in the material. The

DMA is the most common and preferred method of characterizing T, of organic matrix

composites. The DMA technique provides curves of dynamic storage and loss

modulus and loss tangent as a function of temperature and frequency. The tan 6

reflects the amount of energy dissipated during each cycle of loading and goes

through peak values during the To region. The damping properties of the composite

results from the inherent damping properties of the constituents especially that of the

matrix material. It is associated with the movement of small groups and chains of

molecules within the polymer structure. Laurence E Neilson "' has given a detailed

picture of the effect of various parameters like temperature, test frequency, stress /

strain amplitude, thermal history, molecular weight, crosslinking, crystallinity,

crystalline morphology, copolymerization, plasticization, molecular ori@ntation,

strength of intermolecular forces and polymer blending and grafting on the dynamic

mechanical properties of different polymer systems. In polymer mahix composites the

damping characteristics of the polymer is altered by the incorporation of the

reinforcement. The relationship between temperature and frequency for composites

are well explained by Arrhenius relationship. The apparent activation energy H for the

relaxation process at To region is calculated using the following equationm.

Where f is the measuring frequency, f,, the frequency when T approaches infinity T is

the temperature corresponding to the tan 6 peak. The activation energy Of the

composites is less than that of the neat resin. The composite damping property

depend on the inherent damping properties of the constituents. The damping of

composites can be expressed by the rule of mixtures equation2"

tan 6, = V, tan 6, + (14,) tan 6,- (1.5)

Where V, is the volume fraction of fiber and tan$, tan6, and tan& are the damping

values of the composite, matrix and the fiber respectively. In the case of rigid

inclusions the first term can be neglected and the equation becomesz2'

tan 6, = (I - V,) tan 6, (1.6)

Several methods are reported to calculate the T, from DMA data. The temperature.

heating rate and frequency employed will also affect the results. ASTM D - 4065 covers both forced and resonant vibration techniques for DMA analysis. The modulus

in the glassy state is primarly determined by the strength of the intermolecular forces

and the way the polymer chains are packed. The increase in modulii indicates good

adhesion between the matrix and the fiber. The higher value of storage modulus

below T, indicate the reinforcing ability of the matrix in that temperature range.

Equations are available, which enable the calculation of mechanical

properties from the dynamic mechanical properties and vise versazz5.

1.11.2 Mechanical ~haracter isat ion~~'

Chapter 1 52

For structural applications, a material can be designed to maximize its mechanical

properties that are normally defined by tensile, compression and flexural

characteristics under various service environments. All of these three characteristics

are defined by ultimate strength (yield and break strengths), modulus (stiffness) and

extensibility. High cross-link density combined with rigid molecular structure embrittles

a high performance thermoset and can cause early catastrophic failure of the material.

Sankar ~ a 1 1 ~ ~ ' has reported the influence of different types of fibers -glass, carbon,

graphite & aramid- on properties of epoxy unidirectional (UD) composites and also

the influence of matrix type on the high and intermediate modulus graphite fiber

composites. The effect of different types of reinforcements on the stress-strain

properties of the composite was investigated by several authorszz8

The on-axis properties (longitudinal tensile, compressive and flexural

properties) are dominated by fiber properties whereas the off-axis properties

(transverse tensile and flexural, in-plane and inter laminar shear) and inter laminar

fracture toughness are dominated by matrix and interfacial properties. The sensitivity

of the composite properties to fiber matrix adhesion is governed by how the matrix

and fibers are connected and how the applied load is transferred and distributed in the

composite. Laurence T Drzal has explained clearly the interphase and fiber- matrix

adhesion effect on the composite mechanical propertiesz2' The mechanical

properties of the PMCs under all loading environments are temperature dependent.

1.11.2.1 Tensile strength

The tensile properties of the composites depend among other things on the

properties of the individual components, their volume fraction, interface characteristics

and the quality, distribution and orientation of reinforcement. Laurence T ~rzal''' has

given a comparative evaluation of the on-axis and off-axis tensile, compressive and

flexural properties of carbon-epoxy composites without and with different surface

treatments. The fiber/ matrix volume fraction and the testing mode determine whether

the failure is fiber dominated or matrix dominated

The fiber-matrix adhesion does not play a dominant role in the case of

longitudinal tensile behavior. On the other hand, in the case of transverse tension, the

matrix and fibers are connected through the fiber- matrix interface in series and since

all the three components carry equal load, the fiber-matrix interface1 interphase is

expected to have a dominant effect.

Extensive studies were reported indicating the influence of stress directionzH

and constituent volume % on the tensile strength of CFRP I GFRP UD compo~ i tes?~~

1.11.2.2 Compressive Strength

Kink band formation is the most common compression mechanism in graphite

-epoxy composites. With the help of Micro Raman Spectroscopy (MRS) which is

capable of measuring fiber strain with 2 micrometer spatial resolution, Narayan et a12"

could monitor the strain in individual fibers in graphite -epoxy composites and

correlate the inter phase behavior and strain concentration behavior to its kink band

fomlation in compressive failure. Depending on the matrix properties, service

environment and processing, the fiber, matrix or interphase can be the critical factor in

initiating compressive failure

1.11.2.3 Flexural Strength

Flexural properties are generally evaluated by determining the strength and

modulus of the material in flexure mode either by three point bending or by four point

bending method depending on the stiffness of the material. Not only is the transverse

flexural strength more sensitive to changes in fiber- matrix adhesion, it is higher than

the transverse tensile strength. It is reportedzs4 to be due to the non uniformity in

stress distribution in the flexure test.

1.11.2.4 Inter Laminar Shear strengthz3'

The inter-laminar strength (ILSS) characteristics of the composite materials

are generally determined by Short Beam Shear (SBS) test. In the case of short beam

Chapter 1 54

shear test, increase in fiber matrix adhesion increases the ILSS, but levels off or

shows slight decrease with highest level of adhesion. The interlaminar shear strength

(ILSS) has been used for the assessment of bond strength between fiber and matrix

resin in a laminated composites. The differences in the thermal expansion coefficients

of the fiber and matrix and cure shrinkage in thermosetting matrices result in the

development of residual stresses at the fiber 1 matrix i n t e r f a ~ e ~ ~ ' ~ ~ ' . The fiberlmatrix

interfacial degradation may be reflected in such short beam shear (SBS) testszJ8.

Interfacial debonding is expected to introduce the intraiaminar crack initiation for

composite that contains weak fiberlmatrix interface bonds"'. The interlaminar shear

properties are correlatable to many of the composite strength characteristics. Dai Gil

et aiZw correlated the short beam shear properties and impact energy absorption

characteristics of fiber reinforced polymer composites. The ILSS is sensitive to poor

material fabrication practices and damage induced by improper coupon machining

and fiber alignment relative to loading direction.

1.11.2.5 Impact Resistance 243,242

The effective use of polymer matrix composites for high performance

applications demands a thorough understanding of its damage resistance

characteristics. Since the reinforcing fibers- glass, carbon or aramid - have elongation-

to-break values of 3.5 % or less and such fibers constitute significant fraction of the

structural composite, brittle failure will always be the primary mode of mechanical

degradation. The laminated UD composites used for high performance structural

applications are highly susceptible to impact damages because of their lower

transverse tensile strength. Woven fabric composites are characterized by superior

impact resistance and damage tolerance characteristics and higher fracture

toughness.

The damage mechanisms due to an ~mpact involve delamination, matrix

cracking, and fiber breakage. The first two damage modes are purely matrix

dependent. It is now widely accepted that the increase in failure strain or area under

the stress-strain curve of the matrix material could improve the residual strength of the

composite after impact. The composites with tougher matrix have better impact

resistance. This is also related to the interlaminar fracture toughness. G1, and Gllc

which are strain energy release rates to quantify resistance against delamination in

tensile or shear modes respectivelyz4. The impact resistance show systematic

increase with GI, and Gllc.

The techniques used for the evaluation of impact resistance can be classified

in to three categ0ries.i) Drop weight tests ii) Pendulum tests and iii) Ballistic testszu.

Several parameters may affect the impact behavior, including the type of impact test

machine, characteristics of the impactor, velocity and mass of the impactor, and the

specimen configuration. Many of the elastomers245 and thermoplasti~sz~~z47 used to

toughen the epoxy resin have reactive functional groups. Impact tests enable the

determination of important characteristics such as incipient damage, ductile to brittle

transition point, maximum load and total energy absorbed in fracture. It is determined

by the energy absorbed by the specimen at different stages of fracture, the relative

contribution at different stages depending on the size and geometry of the specimenz4

1 .I 1.2.6 Fracture ~ o u ~ h n e s s ' ~ ~

Basic to the evaluation of durability and reliability of the composite is the

analysis of fracture initiation and progression under static or fluctuating thermo-

mechanical loading and fast rate of loading in variable environments. Fracture

initiation is associated with defects such as voids, machining irregularities, stress

concentrating design features, damage from impacts with tools or other objects

resulting in discrete source damage (DSD), and non-uniform material properties. The

toughness evaluation of unidirectional (UD) carbon fiber composites showed that the

stiffness is largely determined by the type of fiber and the fiber content. Raghava et al

2M has given a detailed account of the effect of different toughening mechanisms

/matrix toughening on the impact strength and fracture toughness. He has given the

equations for the energy absorption by fiber, matrix and interphase. A detailed picture

Chapter 1 56

of the effect of curing agents, concentration of reactive functional groups, type and 262,253 extent of elastome?" I thermoplastic/ hard particle modification , strain rate and

test temperatureZY on the fracture toughness of thermosets was also given with

special reference to epoxies. ~ u n s t o n ~ ' ~ examined the effect of toughening of twenty

one matrix resins (thermoset, toughened-thermoset and thermoplastic) on the fracture

energy of the composites and concluded that the increase in resin toughness

enhanced the fracture toughness of their composites.

Important fracture properties are strength and strain-to-failure, fracture

toughness, strain energy release rate, interlaminar fracture energy, fatigue crack

growth resistance and compression-after-impact? The different energy absorbing

mechanisms in the composites are fiber failure- Uf, resin crazing or cracking-U,.

fiber-resin debonding-&, fiber pull out from the matrix across a failure surface-Up,

fiber relaxation and stress distribution to the matrix-U,, multiple fiber fracture- U,,,, and

multiple matrix failure-U,. If all the failure mechanisms occur in one failure situation,

the total energy absorbed (U,) is given by '"

If the interfacial strength is stronger than the matrix strength, the crack will prefer to go

through the weaker resin rather than breaking the stronger fiber and the pnmary

failure mode changes from the intelfacial failure to matrix failure. When the fiber-

matrix adhesion is strong, several energy absorbing phenomena, such as matrix

deformation, matrix cracking, fiber pull out, interfacial failure and so on take place.

The toughness of thermoplastic matrix composite is higher than that of the thermosets

and the property more commonly characterized is the inter-laminar fracture toughness

(G.) obtained using the double cantilever beam test (DCB). Several techniques are

available for evaluating the interlaminar fracture toughness in different modes such as

t ode-lZM. Mode- 11 and mixed modes269.

The low interlaminar fracture toughness of thermosetting matrix composites

has been one of the main limitations to their use in service, due to the ease with which

delamination can initiate and propagate. The matrix properties play a vital role in

controlling the interlaminar fracture resistance of composites. The role of matrix

properties on the toughness of and thermosettingz6' composites has

been reported by several authors. Latest publications on the toughening studies on

the DDScured epoxy (DGEBA)~'~ claims more than 50% increase in fracture energy

for the DDScured epoxy (DGEBA) system using PEEK with pendent ditert- butyl

group. In toughened matrices the type of reinforcement, resin- reinforcement

interaction, ply stacking sequence, fatigue properties, interleaving etc. play significant

role in determining the toughness of the laminate composite.

Hedrick et a12" used phenolic hydroxyl and amine terminated polysulphone

oligomers to toughen epoxy resin. High molecular weight PSF increased fracture

toughness (FT) by 100% without any loss in high temperature propertiesz". Epoxy

modification using several thermoplastics -poly s u ~ ~ h o n e ~ ~ ' , polyetherimide 'B8.267

polycarbonate26s~268. polyester?m'z7', poly ether ether ketonez7z, polymethyl

methacrylate z73 and poly ethylene oxide 274 was tried earlier by several others.

The important mechanisms responsible for enhancement of F.T of

thermoplastic modified epoxies are crack pinning or bowing, crack deflection and

bifurcation, shear banding, and micro cracking. The generally observed toughening

mechanisms in rubber toughened polymers are shown in Fig.l.8. It was reported in

literature that the flexibility of short glass fiber reinforced epoxy resin was improved

with HTPB modification2". Barcia et al reportedz7' that the reinforcement of epoxy

resin using carbon fiber grafted with HTPB,when used to reinforce epoxy resin,

improved the toughness and impact resistance of the composite.

The interphase also plays a significant role and a strong interphase is

required in order to achieve the potential toughness of the polymer matrix. The effect

of temperature and test rate on the fracture toughness has been examined.

Chapter 1 58

The rubber toughening was found to increases the mode-l delamination

energy more than that of mode l l It was found that generally the improvement in

G,, was considerably less than GI1, and hence GI, is not as good an indicator of

impact resistance as GI,,.

Fig.l.8 Toughening mechanisms in rubber toughened polymers. 1) Shear band formation near rubber partides 2) Rubber particle hadure after cavitation 3) Stretching 4) Debonding 5) Crack deflection by hard partide & shearing of rubber pafides 6)Trans particle fracture 7) Debonding of hard oarticles.9) Voided I cavitated Nbber oarticks 10) Crazing 11) Plastic zone at Me crack tip 12) d i s e d shear yielding 13) Shear bandl craze interactionm

The most interesting approach taken towards the toughening of BMI resin is

by the incorporation of a high modulus, high T, thermoplastic to form a semi inter

penetrating net work after cross linking of the BMI resin. ~riedrich"' concluded in his

studies that the reinforcement geometry and matrix ductility are important fracture

toughness considerations..Although various attempts have been made to increase the

toughness by adding a ductile third phase material either dispersed in the matrix or

selectively located at the interphaseZe0,there is still no good format for predicting,

apriori, the toughness of the composite system.

1.11.2.7 Fatigue Resistance 2LH.282

Fatigue is defined as the degradation of the integrity of the material as a result

of external conditions that vary with time. These external conditions can be fluctuating

Chapter 1 59

mechanical stress1 strain, thermally induced cyclic stress or cyclic moisture exposure.

Fatigue resistance is measured to understand its behavior under cyclic loading

environments encountered under certain service conditions. Extensive studies were

carried out on the fatigue resistance of different structural materials"'.

The fatigue life is determined by number of experimental, environmental and

molecular variables. The temperature, pressure and humidity are among the

environmental variables. The test frequency, type of loading -tensile, compressive.

flexure, shear and their combinations -come under the experimental variables while

the surface characteristics of the reinforcement, matrix type and its properties and

inter phase quality come under the molecular characteristics. Fatigue damage in

composite materials is very complex due to several damage mechanisms occurring at

many locations, throughout a laminate. Matrix crack, fiber fracture, longitudinal

cracking, crack coupling and delamination growth are the five major damage

mechanismsz". The S - N culves quantify the relationship between fatigue life and

applied stress amplitude.

Thermo mechanical fatigue (TMF) tests simulate the combined effect of

thermal cycling and mechanical fatigue on advanced materials to ensure that they can

withstand the arduous operating conditions (temperatures of the order of 200O0C,

under hypersonic heating rates and dynamic loading conditions) expected to be

encountered by the various structural components of the space vehicle during the

different phases of ascend, in-orbit manuaring, re-entry and landing phases. The

stiffness reduction during the fatigue of the woven material appeared to have regions

of damage (Fig. 1.9) as described by ~eifsnide?.

From experimental obse~ations, the authors hypothesize that the damage

progressed in the following way. The rapid modulus decay at the beginning of the

fatigue process was caused by the initiation and accumulation of debonds in the weft

and matrix cracks. This behavior is similar to that commonly observed in orthotropic

laminates. However, the woven system behavior differs in that these debonds occur at

Chapter 1 60

each region surrounding a fiber undulation. Matrix cracking and weft debonding were

observed to stop progressing.

Various authors have studied the effect of test frequency on the fatigue of

polymer composites. The fatigue properties of composites have been reviewed by

~ o o r e " ~ and Dickinso et a12" examined the behavior of PEEK composite in

comparison with epoxy matrix composites. The studies confirmed superior fatigue

properties for PEEK in comparison with epoxies. Considerable reduction in fatigue

resistance is reported for [+I- 451 lay up

Fig. 1.9 Damage progression during fatigue lifez6'

1.11.3 Chemical resistance

Depending on the requirement for specific application the composite should

have excellent chemical resistancezsg including resistance to solvent, adjacent

materials, solubility, solvent crazing, and swellability

1.11.4 Dielectric Properties

In addition to mechanical, thermal and chemical properties, the polymer

should resist electric failure under service conditions. The electric storage ability for a

material can be quantitatively expressed by the dielectric constant that may be viewed

as the average energy stored per unit volume per unit electric field strength. The

polymer breakdown under electric potentials can be from three possible mechanisms:

electrical, thermal and electro-mechanical breakdowns2*. High performance

thermosets used for fabricating electronic components should possess good thermal

conductivity, so as to dissipate heat, high electric strength, high surface break down

strength, low dielectric constant, low dielectric dissipation factor, and low water

absorption in addition to the necessary thermal and mechanical properties

1 .I 1.5 Physical properties

Composite physical properties of particular interest to designers and fabrkabrs

include density, fiber volume hctbn and vo!d volume. The fiber vdume fracbn can influence

the slrength chamchish of the composite. The inaease in fiber volume taction was found

to ilxxease the area under the load displacement curve and mused the change in failure

mode from propagationdominated delamination to inMiondominated delaminationm. Many

apphtions require knowledge of hygmthernnal propetties of the composite including thenal

conducliv~Q, thermal expansion and moisture expansion. The water absorption and moisture

expansion of composite materials influence the composite performance to a mnsiderable

extent

1.11.6 Scanning Electron Microscopy

The defects in the laminates like delamination, disbands, porosity or bond

lines, fiber mis-orientation, missing plies, variation in fiber volume fraction, and resin-

rich and resin-starved areas and foreign inclusions determine its performance. SEM

Chapter 1 62

photographs enable the detailed surface and inter-phase characterization of

composites 2s2.

1.11.7 Moisture resistance

Water acts aggressively on polymer - reinforcement bonds and leads to the

movement of the locus of failure from being cohesive in adhesive to pwely interfacial.

The amount of moisture absorbed by the organic matrix depends on the polymer type.

exposure time, component geometry, temperature, relative humidlty and exposure

conditions.

A number of effects results from the ingress of moisture to an adhesive joint

and these include plasticization of the system and disruption of the interfacial region

between the substrate and the organic phase. The moisture uptake characteristics of

the resin are extremely important. The absorption of water lowers T, and produces

changes in matrix and interphase related properties. There are two moisture

properties- moisture diffusivity and equilibrium moisture content. These are

determined by gravimetric methods (ASTM D 5229 1 D 5229M). The rate of moisture

absorption is controlled by moisture diffusivity. It usually follows an Arrhenius type

exponential relation with absolute temperature. It is a function only of temperature and

weakly related to relative humidity. But equilibrium moisture content is weakly related

to temperature and is a function of relative humidity of the environmentzs3.

The hygmthermal effects not only induce moisture expansion or contraction,

but also change properties such as strength, modulus, impact and fatigue resistance.

The absorption rate of the composite can be reduced drastically when diffusivity of the

outer plies is reduced 2e4. In all composites, both the neat resin and composite exhibit

similar two stage diffusion behavior, suggesting moisture diffusion in the composite is

matrix dominated. The initial fast process may be related to the cure induced voids

and the later slow process is due to the intrinsic diffusion in the matrix.

Chapter 1 63

1.12 Advances in Epoxies, Bismaleimides and their co-cured

Systems

A high performance thermoset 285 is designed for use in highly demanding

environments. The ideal high temperature polymeric system for use as a laminating

resin for structural composites should have the following characteristics: easy tape

and pre-preg preparation, good shelf life, acceptable tack and drape, acceptable

quality control procedures, no evolution of volatiles during processing, retention of

mechanical properties over the specific temperature-time ranges and environmental

conditions and acceptable repair. Its performance criteria include processibility,

fabrication procedure and physical properties. Various high performance oligomeric

segments, such as imide, quinoline, quinoxaline, amide, amide- imide, ester,

sulphone, ether- sulphone and ether- ketone have been synthesized from a number of

different monomers to meet the performance requirements. As a result of the

molecular weight reduction, the properties of a high performance thermoset become

significantly dependent on the type of reactive functional groups. The high content of

aromatic and heterocyclic segments in a thermosetting resin reduces the solubility

and increases the melting point and melt viscosity. Hence, poor processibility is a

common concern for high performance thermosetting resins. High melting

temperature causes increase in rate of polymerization at high temperature. A wide

processing window is desired to provide more room for fabrication design and

performance control. A good account of the various thermoset systems have been

given in section 1.5.2 of this chapter.

BMI constitute one class of high performance polymer. The research in the

area of improved BMI resin systems is still ongoing. Among the addition polyimides.

bismaleimides are the most important system currently used for advanced material

applications due to its high performance to cost ratio. Various arylenediamines have

been used for the preparation of BMls to provide high stiffness and heat and thermal

resistance after crosslinking. The properties of the BMI resin vary with synthetic

Chapter 1 64

conditions. The introduction of rigid rings in the structure increases its melting point

and narrows down its processing window which is of concern and unless solution

processing is employed. Lin et alZg6 have given a detailed picture of the different

characterestics of several BMI resins.

The thermally cured bis (4-maleimidophenyl) methane has excellent

mechanical properties and heat resistance. It has got high property retention at

elevated temperatures, good electrical properties-dielectric strength, volume

resistivity, dielectric constant and dissipation factor- and is useful as insulation

material in radiation environments. However, low elongation to failure and poor

toughness are concerns for structural applications. They suffer from high brittleness

due to their high crosslinking densities. Toughness improvement of the cured BMI

resin can provide the required fatigue resistance and prevent the fracture propagation.

Several approaches have been developed to toughen I increase the impact resistance

of the cured BMI resins. Many chemical concepts to modified BMI systems have been

published "'. But only few combine all the desired properties required for use in

advanced fiber composites.

1.12.1 Toughening of Bismaleimides

1.12.1.1 Changing the basic structure

The first approach towards the improvement of toughness and other physical

properties is to change the basic structure of the backbone between two maleimide

end groups. Generally, the aromatic linkages have higher rigidity than the aliphatic

ones. This rigidity normally contributes to high melting point, glass transition

temperature, thermo oxidative resistance, decomposition temperature and modulus of

these polymeric materials. However, the processability and fracture resistance of

aromatic BMI resins are drastically reduced by the increased molecular rigidity. The

introduction of ether linkages to a BMI molecule increases the strength and flexibility

of the cured resin. The undesired effect is the reduction in heat resistance related to

the decreased glass transition temperature. The modification of BMI resins with

Chapter 1 65

aromatic amines has been the most attractive and important approach for practical

use. MIS Technochemie has synthesized poly-phenyl-group functionalised poly-

arylene ether ketone high polymers and could demonstrate that these are excellent

tougheners for BMls.

1.12.1.2 Thermoplastic modification

The most interesting approach taken towards the toughening of BMI resin is

the incorporation of a high modulus, high T, thermoplastic to form a semi

interpenetrating net work after cross linking of the BMI resin. The introduction of

engineering thermoplastic segments between two maleimide groups is an attractive

way to achieve high heat resistance and improved toughness. Poly (arylene ether

sulphones) and Poly (arylene ether ketones) are important industrial high temperature

thermoplastics which possess excellent tensile properties and fractural toughness.

Polybenzimidazole (PBI) is a high temperature thermoplastic polymer with T, of 435 'C

has an excellent chemical resistance and does not melt or bum in air. The addition of

PBI causes very little difference in the thermal and mechanical properties. But it helps

in enhancing the toughness of the system. Polysulphone is reported to provide

excellent improvement in toughness, but reduces heat resistance298.

1.12.1.3 Chain Extensions

Two approaches have been generally adopted to change the performance of

the BMI thermoset- the extension of an oligomer structure between two maleimido

groups and the reaction of a basic BMI resin with modifiers.

Extension by bridging segment: The chain extension between two crosslinking

sites is one of the ways for achieving improved toughness. The increase in molecular

weight between two maleimide groups decreases the crosslink density. This

technique generally provides improvement in toughness and strength but reduces the

T,, modulus and processability.

Extension by Michael addition: The modification of BMI resins with aromatic

amines has been the most attractive and important approach for practical use. The

performance of the cured BMI resin depends on the structure of the BMI resin and the

extender, the ratio of the BMI to the diamine modifier and the curing conditions. Many

diamines have been used to modify the properties of various BMI resins. The diamine

terminated amide resins were used to modify bis (Cmaleimido diphenyl) methane to

lower the crosslinking density and to increase the t o ~ g h n e s s ~ ~ ~ . ~ h e same resin was

chain extended with aliphatic amines300. In general, the onset of curing occurs at a

reduced temperature when the aliphatic amine is used as the chain extender. The

heat of curing also decreases significantly by chain extension.

1.12.1.4 Co polymerisation

The simplest way for polymer modification is the use of two or more

monomers in a thermosetting compound. High performance BMI resins are based on

rigid aromatic structures and thus possess high melting point and a narrow processing

window. An approach using eutectic mixture has been applied to improve the

processibility and performance of the final thermoset. BMI resins are reacted with

other high performance thermosetting compounds such as cyanate ester and

bisbenzocyclobutene (BCB) resins. Mixtures of precursors of BMI resins with cyanate

esters known as BT resins are important commercial products for high performance

thermosets. BCB- BMI mixtures can be cured to high heat resistant thermosets

through a rearrangement and Dlel- Alder reaction at elevated temperatures. A Diels-

Alder co-monomer for BMI is bis (o--propenyl phenoxy) benzophenone available from

MIS Technocheme under the trade name Compimide TM123. The BMll Compimide

TM123 copolymer resin is attractive because of its extremely high temperature

stability. It shows better thermal oxidative stability than all other commercially

available BMI systems.

Chapter 1 67

Of the several approaches reported for BMI matrix toughening, co-reaction

with allyl phenol is now gaining wide spread acceptance3". Alkenylphenyl compounds

are used as co-reactants and toughners for BMI resins. The most effective method

developed so far for the toughness improvement of BMI resin is the incorporation of

alkenylphenyl compounds such as allylphenyl and propenylphenyl derivatives as co-

reactants and toughenersm2. The final crosslinked products provide improved

toughness and heat and thermal resistance due to the formation of cyclic structures

and high crosslinking density through Diels-Elder reaction, addition polymerisation

and ene reaction. A technology using alkenyl phenols or alkenyl phenol ethers to react

with BMls to form processable pre-polymers was developed by Zaire et awl to

improve the toughness and humidity resistance of BMI resins.

Zhongming Li et ato4 reported the synthesis of modified bismaleimido

diphenyl methane (BDM), with high heat and hot I wet resistance and high mechanical

properties by modifying the Bismaleimide resin using diallyl bisphenol A and diallyl p-

phenylene diamine. A copolymer of bismaleimide and a new allyl compound

containing diglycidyl ether of bisphenol A backbone, with improved tack and drape

properties was prepared by Aijuan Gu et a~'~'. Takao luima et a13" modified a three

component bismaleimide resin system composed of 4, 4'-bismaleimido diphenyl

methane (BDM), o, 0'-diallyl bisphenol A (DABA) and o, 0'4iethallyl bisphenol A

(DBA), by blending it with poly(ether ketone ketone)^ like poly(phthaloy1 diphenyl

ether) (PPDE), poly(phthaloy1 diphenyl ether -co - isophthaloyl diphenyl ether)

(PPIDE) and poly(phthaloy1 diphenyl ether-co - terephthaloyl diphenyl ether) (PPTDE).

so that the new system gave an improvement in K,, with no deterioration in flexural

strength and modulus. Co-reactive curing agents act as a monomer in the

polymerization process.

Yehai Yan et developed and characterized a resin system based on

novolac and bismaleimide by curing the allylated novolac resin using BMI and using

allyl phenyl ether as the diluent and the woven fabric composites based on this

system showed very good flexural properties and high retention rates at 200 and

Chapter 1 68

300°C. Kancheng Mai et alM' attempted the stability studies on bismaleimide- o, 0'-

diallyl bisphenol A (BMI-DABA) blends modified using thermoplastic bisphenol A

polysulphone (PSF). Polyether ketone (PEK-C) and polyether sulphone (PES-C) and

found that the stability of the thermoplastic can be ~mproved by the formation of semi-

interpenetrating polymer networks (SIPN). An alkenylphenol compound is reactive in

both maleimide and epoxy resins and a thermoset compound of these three resins

have been developed provid~ng high performance characteristics3? The Michael

addition adduct of BMI resins using excess diamines should produce amino end

groups useful for reacting with other species, such as epoxy resins, unsaturated

olefins and The combination of rigid rod benzimidazole structure. Michael

addition of BMI resin, and BMI- allylphenyl reactions has resulted in the preparation of

molecular composites with simultaneous improvement of tensile strength, modulus

and elongation. No phase separation was observed in these matrix systems3".

Modified epoxies are used in many high performance applications. Dual

photo and thermally initiated cationic polymerisation of epoxy monomers has been

reported by Crivello et.al3I2. These dual cure systems have potential application in

adhesives, potting resins and composites. The flame retardence of epoxy resin can be

improved by incorporating bromine in to the structure, but bromine increases the toxic

smoke emissi~n"~. Fluorine containing bismaleimides find application in multilayer

printed circuit boards. The chemical nature of two fluorine containing bismaleimides

investigated in Hitachi Research institute, ~ a ~ a n ~ ' ~ are given in Structure 6 Tyberg et

alms carried out cure reaction of phenolic resin using epoxies to achieve an improved

void-free system, as strong as epoxy net work, which retain the fire retardance of

phenolicss1s. Gonzalez et a1317 reported the synthesis and propertes of an epoxy

thermoset with reduced shrinkage and thermal degradability with increase in ratio of

the co-monomersdiglycidylether of bisphenol A and 6, 6 dimethyl-(4, 8 dioxaspiro

[2.5] octane- 5.7 dione).

Chapter 1 69

Structure 6 Fluorine contaming bismaleimides

Blending of trifunctional solid epoxy with difunctional liquid epoxy was found to reduce

its viscosity to a level suitable for the pre-pregging operation3".~he dry T, was found

to be lowered from 310 to 3 0 2 ' ~ by the addition of 25 weight % of the difunctional

epoxy when cured using DDS. Blending with epoxy monomer can also be used to

impart good processing quality to more interactable thermosetting resins such as

polyimides or to reduce the intrinsic brittleness of b i s m a l e i m i d e ~ ~ ' ~ ~ ~ ~ ~ . The lower

crosslink density of the epoxy component will result in the lowering of the thermal

performance of the polyimide.

The best known way to alter the properties of an epoxy resin, both in terms of

its cure kinetics and the properties in the cured state, is by changing the nature of the

chemical agent used for its polymerization. at ridge'^' has studied the effect of DDS

and dicy hardner on the chemoviscosity and T, of TGDDM epoxy. A mixture of these

two hardners can also be used so that dicy acts as an accelerator for the DDS cure.

An alternate approach to epoxy cure, which aids the introduction of flexible

crosslinks, is to use an isocyanate compound. This type of cure gives rise to both

chain extension and crosslinking via stiff heterocyclic structures, the oxazolidone and

isocyanurate ring respectively. Nevertheless the epoxy resin cured in this way

combine toughness and high T, values.

A rapid development in the formulated high performance epoxies and

bismaleimides based on the principle of dissolving a high temperature thermopiastic

Chapter 1 70

or rubber additive in the monomer often with the aid of a common solvent. Good

adhesion between the two phases can be obtained by sufficient grafting between the

additive and the monomer. The polymeric modifiers such as functionalized

thermoplastics32z yield sufficient toughness without much deterioration in high

temperature performance.

Novel thermosetting resin with very high T, based on bismaleimide and

allylated Novolac (BMAN) has been reported by Yuan Yao et atz3. BMAN -15 resin

with high allylation degree and low BMI proportion was found to give high

performance composite matrix with high strength and modulus retention at 350%. The

thermosetting resin based on BMI and allyl compounds such as bisphenol A diallyl

ethe?"allylated novolac 3258'2Band allylated xylok showed T,s up to 350%. Diallyl

bisphenol A novolac was cured with BMI to get high temperature resistant matrix

which retained its lap shear strength up to 2 0 0 ~ ~ ~ ~ ~ . Bindu et reported a co-cured

matrix composed of novolacs derived from maleim~do phenol and phenol-allyl phenol.

A dual cure maleimido phenol- epoxy system exhibited excellent thermo-adhesion

profile in contrast to novolac cured epoxy resin329.

Scope and Objective The increasing need of materials for high performance, high temperature

resistant aerospace structural composite applications pose great demand for the

development of speciality thermoset matrix resin systems which can outdo the mast

commonly used epoxy resin in terms of thermal capabilities, toughness and moisture

resistance, and bismaleimide in terms of toughness and mechanical properties.

Epoxy-based laminates largely find applications as aerospace structural components.

The most important property that has made epoxy composite laminate as a competent

aerospace material is its ease of processing, superior heat ageing, lower laminating

pressures, freedom from condensation volatiles, less complicated cure cycles and

superior flexural strength. Epoxy composites find numerous other applications in

marine, construction, tooling, electrical and sports goods. The use of epoxy

composites is increasing rapidly in commercial and military aircrafts, light compact

aircrafts, helicopters, launch vehicles, satellites and spacecrafts. The BMI emerged as

an alternative matrix system with enhanced thermal capability and replaced the

epoxies to some extent in areas where their thermo-mechanical profile is of

advantage. The inherent brittleness of the system, poor impact resistance and high

cure cycles were major concern for their wide spread applications. Blending of BMI

with allylated derivatives could solve these issues to a great extent resulting in the

emergence of commercial products based on BMI and diallyl bisphenol adduct. In

many cases a compromise matrix system, with tailored properties can be derived by

way of blending one or more polymer system to form polymer blends, alloys or lPNs

depending on the physical and chemical interactions among the components and the

resultant morphological features. A control of the parameters can impart desirable

properties to the resultant blends.

This thesis concerns the research efforts to derive a blend of epoxy resin and

bismaleimide. There are limited literature on the blends of epoxies and BMI. Epoxy - phenol systems have been reported as low T,, tough, hydrophobic matrices. Allyl

phenol- BMI systems have a success story as high temperature resistant tough

matrices. It was of interest to derive a combination of epoxy- phenol and allyl phenol-

BMI through a multi reaction ternarysystem constituted by epoxy- allyl phenol- BMI.

The literature survey on matrix resin and composites and presented in chapter 1

reveals that epoxy-allylphenol- BMI systems have been little addressed in open

literature.

The effect of modification of epoxy- phenol system by co- reacting it with

bismaleimide and the evaluation of the influence of BMI concentration on the thermo-

physical, mechanical and rheological properties of the ternary blend and their fabric

reinforced composites form the major objective of the thesis. Another objective of the

study is to examine the performance and properties of the ternary reactive polymer

blend of epoxy- diallyl bisphenol-BMI system with the structural variations of the epoxy

and bismaleimide components. The thesis concerns the properties of the composites

Chapter 1 72

with different reinforcements, fillers, fiber orientation etc. Typical applications of the

ternary matrices have also been explored. The gist of the research work carried out in

this perspective and presented in the thesis is as follows.

The first chapter gives a summary of the literature survey carried out on

polymer matrix composites with special emphasis on matrix resins, reinforcements,

interphase, composite fabrication and characterization. It also throws light on the

recent trends in matrices, reinforcements and composites with special emphasis On

the epoxy- phenol- BMI constituted polymer matrix composites.

The second chapter gives a brief description of the sources and properties of

the materials used for the study, techniques used for their characterisation.

optmisation of the processing condition of the matrix resin and composites, and the

methods used for their performance evaluation.

Prelude to the evaluation of the properties of the ternary system, the

properties of the precursor epoxy- diallyl bisphenol was studied. The third chapter

gives an account of the cure kinetics of-the novolac epoxy- diallyl bisphenol system.

Studies using dynamic DSC experiments revealed the influence of catalyst

concentration on the epoxy- phenol cure reaction, which enabled the selection of

optimum catalyst concentration and cure conditions.

Studies on the mechanical characterization of novolac epoxy- diallyl bisphenol

system are dealt with in chapter four. The effect of varying the molar ratio of epoxy

resin and diallyl bisphenol on their cure behavior, mechanical and thermo-mechanical

properties was investigated. The influence of the extent of allyl group curing on the

properties of the epoxy-phenol matrix system and its glass composite was also

evaluated.

Though the epoxy- diallyl bisphenol system provided a matrix with good

mechanical properties, it was inferior in respect of its high temperature performance.

To address this, the matrix was co- reacted with BMI and the resultant resin was

characterized for its cure behavior, thermal stability and composite properties. Thus.

the fifth cha~ter focuses on the studies on the modification of stoichiometric blend

Chapter 1 73

(1:l) of the epoxy - diallyl bisphenol matrix system by co-reaction with BMI to form an

epoxy-phenolic-bismaleimide (EPB) ternary blend. The effect of BMI concentration on

the rheological, mechanical and thermo-physical properties of the ternary blend was

examined. The system was evaluated at composite level using glass fabric as

reinforcement composites. The matrix composition with optimum concentration of BMI

was chosen for further studies.

The sixth chapter mainly concentrates on the evaluation of the effect of

different types of reinforcements on the performance of the epoxydiallyl bisphenol-

bismaleimide (EPB) composite. Comparative evaluation of the performance of the

fabric reinforced EPB composites with different reinforcements such as glass and

carbon fabrics of different fiber orientations and stacking sequences was carried out

by determining their tensile, compressive, flexural and interfacial I inter-laminar

properties and fracture toughness. Their moisture absorption characteristics,

coefficient of linear expansion, constituent content and surface morphology were also

compared. To iilustrate the utility of the new matrix system in diverse compositions,

other than laminates, the resin was evaluated as a matrix for the processing of

syntactic foam composites. Syntactic foam composites with varying concentrations of

hollow glass microballoon were characterized for their physical, thermal and

mechanical properties and the performance variation of the foam composite with

microballoon type and concentration was examined. The use of blended microballoon

fillers on the properties of the syntactic foam composite was also studied. The utility of

the matrix as high temperature adhesive was also explored.

The influence of the structural variations of both the epoxy resin and

bismaleimide on the thermal, physical and mechanical properties of the ternary blend

was examined and presented in the seventh chapter. EPB compositions were

prepared using bismaleimide, diallyl bisphenol-A and three different epoxy resin

systems viz. Novolac epoxy, LY-556 and Triepoxy, having different physical and

molecular characteristics. The effect of structural variations of BMI resin on the

performance of the EPB system was studied using EPB compositions containing three

Chapter 1 74

different types of bismaleimides in combination with the novolac epoxy and diallyl

bisphenol A. The variation in high temperature performance of these EPB systems

with nature of the bismaleimide was studied by determining their strength retention at

different climatic temperatures.

The overall conclusions from the above studies are summarized in chapter

eight and the prospects for future work is also indicated.

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