S t d w on Epoqy- Phenolic 4 ~ I k ' r n i c t k Composites - Effect of ~ n f o ~ c e n m t ,

Staceng Sequence, Fi6er Orientation and Fi&s

Part of the resuhs dwmsed in ttiis ctinpterispu6Cislied

2. %polcy- Pfienob -i8isma&de Composites; Eflect of Stac&g Sequence andFi6er Orientation" Znt . N a t i o d C o n ~ Z O a C - 2 0 0 5 , M ~ ~ lypti%yanr 21-23 Marrfi 2005

3. '3yntactic Foam Composites of ~pqiWij4cpfiewL @ismaIkimide Tmry Bhd- Effect of ~ompositionaf2/afiations on MechnuafcPIoperties"

4. "Syntactu Foam Composites of E p o ~ A ~ f c P & n d @hi-rmzIkimde Tematy Bhnd - Processing andcproperties"

J. . f . p p C i c d ~ S - ( in n*uJ)

Swpe ofthe study

?lie study main4 concentmtes on the d i t w n of the #at of hieretat types of m n ~ r c e m e n t ~ on the pqhormance of the epog-al2jlphenoC&a&midk composite. Comparative evaluation of the performance of the fat% reinforced Em composites m'tli d i i m n t mmnfbnements such as g b s a d carbon fa6rics m'th & f l i t $6er orientations and stac&ng sequences was c a d out by determining their tenelk, compressive f i m r a l and interfa&( / inter-lhminar propwties and fracture toughness. llieir maisture absqtion cliaracte~istics, coefJinent of linear qansion, constituent content andsurface chamcteristics were aho compamd llie 6isma&imi& modifiedepoay system was evaliitedas a mat* for the processing yC syntactic foam composites. Syntactic foam composites fabricated using varying concentrations of h o b minoba&ons are characterized for tlin'rphysica[ thennaladmeclianicaCprop~ies. llie perJaMnce vaziatwn of the foam composite with $& concentration w s t d i d by comparing their speafi s t q t h , moduli andsurface mqliolbay obtainedfmm the .SEN picturn of the failkd susfaces. e f i c t of shea tliu&nars (&us ratio) of the hioabw minob&on and use of minoballbon blknd reinforcement on the properties of the syntactic foam composite was d o studiikdm'ng t w o g r i s of micro6aabons m*ed in & f l i n t ratios.

6.1 Introduction In recent years there has been an increasing interest in fabric composites due

to their attractive capability for variety of structural applications. Their unique

mechanical property includes increased transverse modulus and strength, improved

shear resistance, fracture toughness and damage tolerance. The properties of th@

fabric composites can be tailored by using reinforcements of different types, f0ITI'I.S.

surface characteristics, stacking sequence and fiber orientation. When a resin system

is combined with a reinforcing fiber such as glass, carbon or polyaramide, exceptional

properties can be obtained. The development of innovative fiber architecture and

fabric manufacturing technology has significantly expanded the potential of fiber

reinforced composites. The plain weave fabric imparts high degree of crimp to the

constituent fibers and hence we can exped lower mechanical performance from the

laminates processed with this fabric. The square weave fabric allows very little

slippage to occur between the fibers. Hence this fabric style is difficult to form over

double curvature shapes. As the fabric style impart progressively less crimp to the

fibers by moving to twill weave and satin weave styles, the mechanical properties of

the resulting composite and degree of drape both improve.

The internal long fiber structure inhibits crack propagation and improves the

fatigue endurance in long fiber reinforced thermoplastics (LFRTPs). Lee and co

workers ' in their studies on glass- and carbon fiber- reinforced UD composites found

that the compressive strength of carbon composite was less than that for glass

composite at high volume fractions. Failure mode was also found to be different - simultaneous splitting and kink banding in glass composite while in carbon composite,

it was only kink banding. Inorganic fibers undergo catastrophic failure in compression

while all polymer fibers exhibit kinking in compression. Studles that investigated fiber

variations '-" have run the gamut from glass fiber, aramid fiber, PAN-based vs. Pitch-

based carbon fiber13 and various locally manufactured organic fibers". Many of these

studies have also investigated weave geometry'' and short fiber reinforcement.

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

performance applicatior~s.

Fiber properties such as its length, aspect ratio, orientation and orientation

distribution have got profound influence on the performance of the composite. A

system exhibiting good fiber-matrix adhes~on will have a lower critical fiber length (I,).

A discontinuous fiber composite can develop approximately 95% of the strength

predicted for continuous fiber composite if the fiber length exceeds the I, by a factor of

10". Control of fiber orientation during processing is probably the most important

factor underlying the efficient use of short fibers in composites". In pure shear load,

fibers are most effective when they are oriented at *45' with respect to the loading

direction1'.

The fiber orientation with respect to the test direction has got profound

influence on the performance of the composite. A decrease of tensile modulus and

strength was reported for a change in the fiber orientation angle from 0" to 90'. The

fiber arrangement could change the residual stress in a composite, which in turn

influence the kinetics of water diffusionqe. The geometry of the fiber in a composite is

also important since fibers have their highest mechanical properties along their length

rather than along their width.

The fibers used in advanced composite manufacture come in various forms,

including yarns, rovings, chopped strands, woven fabrics and mats. The application of

textile fabrics in engineering structures has been driven by various attractive aspects

like ease of handling, high adaptability, damage tolerance and if compared with UD

laminates, better out of plain stiffness properties. Different types of fibers are used in

woven fabric laminates on the basis of specific needs, and the geometry of the

reinforcement (Yarn spacing, yarn thickness, shape and the weave type) to attain the

required mechanical properties. Various studies have established the existence of

high residual curing stresses in woven and cross ply composites2'. Jose et al"

presented a simple relationship between the inter laminar fracture toughness at

specimen level of a cross ply [0/901,sand its constituent sub laminates [Oh and 190130

of carbon -epoxy composites.

The arrangement of fibers in composites can be uni directional, bi-

directional, multidirectional or random. In two directional architecture, fibers are

woven, knitted or braided in both zero and 90 directions, bringing the properties in

these directions closer to each other. The fatigue response of angle ply specimen

exhibits strong dependence on test frequencp. For laminates in which the fibers are

arranged at an angle to one another, the micro mechanics are complicated by the

presence of inter laminar shear stresses at the free edges, fiber crossovers and resin

rich areas. In practice, complex: laminates with balanced symmetrical configuration

(02/45,). are used to provide load bearing ability. Shear stresses are maximum at 45"

angle. Antony et alZ3 has given a detailed picture of the failure mechanism and

strength of different types of textile composites.

The woven fabric provides more balanced properties in the fabric plane than

UD laminates. High impact resistance, ease of handling, and low fabrication cosk

make them suitable for structural applications. Limited conformability, limited in-plane

shear resistance, reduced yarn-to-fabric tensile translation efficiency due to yarn

crimp are some of the disadvantages of woven fabrics. Tri-axial woven fabrics provide

higher isotropy and in plane shear rigidity than orthogonal weaves. The plane weave

provides maximum fabric stab~lity and firmness with minimum yarn slippage. It has got

good strength in both yarn directions. Satin weaves are favored as planar woven

fabric reinforcement due to their long floats and less crimp.

It has been shown that even well made fiber composite laminates contain

significant degree of misalignmentz4. Woven fiber composites have severe

misalignmen? and short fiber composites can be more or less completely

misa~i~ned~'~' . A criterion of 50% angle, which is the angle of pull at which the fiber

breaks at half its normal strength, is considered in certain cases. On single fiber

measurement, the 50% angles are found to be 20" for glass and 30-40" for carbon

and 45" for Kevlar ".

Several studies have been conducted to date to understand the visco-elastic

behavior of polymeric composites Z9'30. Its physical ageing is accompanied by increase

in stiffness, yield stress, density and viscosity and decrease in creep and stress

relaxation ratesa'. The stress-relaxation and creep properties of the composites are

evaluated to get an insight into their long term performance".

Syntactic foams are strong, light weight materials that find application in

products for marine, aerospace and automotive industries. The properties of the

resulting foam composition are dictated by the filler type, its volume fraction and the

quality of the binder matrix system. They have concluded that the wall thickness of

the microballoon has got little influence in the overall elastic modulus. An experimental

- numerical investigation in to the tensile, compressive and fracture behavior of

prefabricated syntactic foams has been briefly reported by Rizzi et ai ". In this chapter, the effects of type of reinforcing fabric, its weaving pattern.

fiber orientation and stacking sequence on the performance1 properties of the Epoxy-

phenol- bismaieimide polymer composites are examined. The matrix system was

evaluated for application in syntactic foam composites. The effects of type and

concentration of microballoon and the fractional volume I weight of the two types of

microballoons in the microballoon blend on the properties of the syntactic foam

composites has also been evaluated.

6.2 Materials and Methods

6.2.1 Materials

The EPB matrix system (1:l:l blend of EPN. DABA and BMIP) was used for

the processing of composites using woven fabrics with different fiber orientations and

stacking sequences. The characteristics of the glass- and carbon woven fabrics used

for the study are given in Table-2.1 in chapter.2. The properties of the glass

microballoon used for the processing of the syntactic foam composites are given in

Table 6.1.

Table 6.1:Pafticle size distribution and properties of microballoon fillers

6.2.2 Processing of composites

6.2.2.1 Processing of woven fabric composites

Average

true

particle

density

(kg1 m3)

250

370

2

The woven fabric composites are fabricated using the method mentioned in

chapter 5. Two sets of fabrics were cut from the same glass cloth, one with their fibers

at 0190' (Gl) and the second set with fibers at i45'angles (G2) with each other. The

matrix I reinforcement volume ratio was kept 40: 60 in all cases. The components of

the EPB matrix system, weighed in their stoichiometric equivalent ratios, are dissolved

in sufficient quantity of AR acetone for improved wetting of the fiber. The fabrics cut

into strips of size 150 cm xl5cm were impregnated with the matrix by dip coating. The

resin impregnated fabric is cut in the required size afler allowing it to expel the

absorbed acetone by exposing it to room temperature for sufficient length of time (-14

hours). The cure schedule optimized for the EPB matrix system is used for the

Target

Fractional

su~ival(pm)

90

90

Effective

top s~ze

(elm)

105

85

Microballoon

Identification

K-25

K-37

Microballwn size

distribution (vm. Volume%)

loth 25

20

soih 55

45

goth

90

80

fabrication of the composites. EPBcomposites of different stacking sequences were

prepared by stacking the resin impregnated cloths with fibers at 0/9OSangles and t45'

angles so as to get different types of EPB -glass composites having all cross plies

(EPB-GI), all angle plies (EPB-G2) and angle plies stacked in behveen the cross

plies (EPB-G3) as given in Table 6.2 and Fig. 6.1.; EPB-carbon composites were also

prepared using plane weave, satin weave and chopped plane weave carbon cloth (so

as to have a fiber length of lomm).. The processing conditions used were the same

for both glass and carbon composites.

Table 6.2 Identification of different EPB composites.

Description

-plane weave glass ([O/QO] composite

- plane weave glass [ 2451 composite

- plane weave glass [(0/90)d( *45) /(0/90)2 1, composite

plane weave carbon [O/QO]composite

-satin weave carbon composite

-Chopped carbon fabric composite

K37 Micro balloon composite. Particle density-370kg/mJ

K25 Micro balloon composite, Particle density-250kg/mJ

Blended micro balloon (K-37 and K-25) composite

I;

The plies are arranged in such a way as to get laminates of the following fiber

orientations and stacking sequences

Fig.6.1 Schematic representation of the three stacking sequences of the EPB-glass

composites

6.2.2.2 Fabrication of Chopped Fiber Composites

Sufficient quantity of chopped carbon fabric (70 mm length) was mixed well

with the stoichiometric blend (1:l: l) of EPB matrix system so that the volume ratio of

the matrix:reinforcement is 40:60. The .matrix system was diluted with sufficient

quantity of AR grade acetone to enable proper fiber wetting. The composite

fabrication is carried out using matched die molding following the time -temperature

cure schedule optimized for the EPB matrix system.

6.2.2.3 Processing of Microballoon Composites (Syntactic Foams)

EPB composites were processed using glass micro-balloon of different

grades (K-37 and K-25). The K-37 microballoon content in the composite was varied

from 40 weight % to 70 weight % in steps of 10%. EPB composites (50 % filled) were

also prepared using these two types of rnicrobaltoons, blended in different ratios. The

processing conditions used were the same as that for other EPB-fabric composites

except the special care given during mixing and pressure application to avoid the

microballoon breakage.

6.2.3 Characterisation o f composites

6.2.3.1 Thermal characterization

The glass transltlon temperature, which indicates the upper limit of the usable

temperature of the composite, is influenced by the thermal conductivity of the

reinforcement. The reinforcement which has got lower coefficient of thermal

conductivity than the resin, tends to increase the effective T, of the composite above

that of the base resin% The specific heat and linear expansion of these composites

were determined using 13SC and TMA techniques respectively, the analysis conditions

of which are given in chaptev.

6.2.3.2 Mechanical characterization

The composite panels processed using epoxy-phenol-bismaleimide matrix

system and different types of reinforcements were characterized for their tensile.

flexural and compressive properties. The strength measurements were carried out

using the ASTM standard test procedures mentioned in chapter.2. The interphase

properties were evaluated by interlaminar shear strength determination as per the

standard ASTM procedure D-2344.

6.2.3.3 Physical characterization

The physical properties like, density, water absorption, void content and resin

content were determined using the standard techniques mentioned in Table 2.1 in

chapter 2.

6.2.3.4 Stress relaxation studies

The stress relaxation studies were carried out on the composite specimens

using rectangular strips of lOOmm length and 5mm width and 3 to 4 mm thickness.

The specimen was subjected to 2% strain in tension mode at a crosshead speed of 10

mmlmin and the load was monitored as a function of time. The stress relaxation

modulus at different time intervals computed from this load-time data are used for the

plotting the stress relaxation curves.

6.3 Results and Discussion

6.3.1 Effect of Fabric type on Mechanical Properties of Woven Fabric

composites

The EPB composites with different types of reinforcement viz. plane weave

glass cloth (EPB-GI) and plane weave carbon cloth (EPB-C1) were characterized for

their compressive, flexural and interlaminar shear properties at ambient conditions.

The results are summarized in Table .6.3 along with their physical and thenno-physical

properties

Table 6.3 Influence of reinforr~ment type on the physiw- mechanical properties of EPB composites

Material identification

--

p G m m - - - - ? - - ~ l Water absorption (%)

The carbon fabric reinforced systems were found to have superior inter

laminar shear strength and flexural properties compared to the glass composite. The

compressive strength was found to be comparable for both the systems irrespective of

the type of fabric. The carbon composite was able to withstand more flexural load than

glass composites. Generally carbon is endowed with low thermal expansion

characteristics and it is reflected in the expansion behavior of EPBcarbon

composites. The superiority of the carbon composite is evident in their superior

specific strength. The shess-relaxation behavior of the carbon and glass fabric

reinforced EPB composites are given in Fig. 6.2.

m.m I I 0 50 100 150 200

Time (Min)

Fig.6.2Stress relaxation curves of glass and carbon fabric reinforced EPB ramposites

The relaxation modulus of EPBcarbon composite (EPB-C1) was found to be

higher when compared to that of glass composite (EPB-GI). The drop in stress

relaxation modulus of the composite is influenced by the time dependent response

characteristics of the matrix. The drop in relaxation modulus is found to be less for Me

carbon composite compared to glass composite showing better fiber-matrix

interactionlinterfacial adhesion in the former case.

6.3.2 Effect of fiber orientation and stacking sequence on the

properties of woven fabric composites

The fiber orientation is an important factor contributing to the strength and

isotropy of the composite. The strength and stiffness of the fiber reinforced composite

is reported to reduce from unity for UD to half for bidirectional to three eighth for

random reinforcements. Three sets of specimens were prepared using epoxy-

phenol- bismaleimide matrix system and same type fabric (plane weave glass cloth)

but with different fiber orientations and stacking sequences. The cross-ply and angle-

ply laminates were fabricated using the plane weave glass cloth with their fibers at

0190 degree (cross ply) and at _+45 degree (angle- ply) respectively. The stacking

sequence was altered by fabricating composites with [O/90Jm-G1. [i45]=-G2 and

[(O190)~1(~45)l(0190)~]s-G3 fiber arrangements. The composite properties were

evaluated by determining their mechanical and thermo- physical properties.

Mechanical properties were determined under compressive and flexural loading

conditions and the results are presented in Table 6.4.. Analysis of the results revealed

that the compressive, flexural and interlaminar shear strength of the cross-ply

laminate (EPB-GI) is higher in comparison to that of angle-ply laminate (EPB-G2).

On changing the stacking sequence from G2 to G3, the performance of the

composite improved considerably. The properties of G3 were found to be close to

that of G1. The trend in stress relaxation behavior of the G1 and ~2 composites with

different fiber orientations revealed that the stress relaxation behavior is also

influenced by fiber orientation. As in the case of mechanical properties, the relaxation

modulus of the EPB-G2 composite was found to be inferior in comparison to EPG-GI.

The relaxation behavior of EPB-Gland EPB-G3 was found to be comparable (Fig

6.3).

Table 6 4 Effect of fiber orientation and stacking sequence on the physico- mechanical properties of EPB composites

Material identification

ILSS (kg/cmL)

6.3.3 Effect of fiber architecture on the performance of EPB- carbon

Compressive strength (kglcmz) I 2480

composites

The effect of fiber architecture on the performance of the EPB composite was

evaluated using composites fabricated using three types of carbon reinforcements

(plane weave, satin weave and chopped plane weave carbon cloth). The properties of

these reinforcements .are given in Table 2.1 in chapter.2. The physical and

mechanical properties of these composite systems are given in Table 6.5.

1980 2560

50.0k

W

0 50 100 150 200

hm3 (mill)

Fig. 6. 3 Stress relaxation curves of EPB composites with different

fiber arrangements

Table 6.5 Effect of fiber architecture on the performance of EPB-carbon ~ p 0 S i t e S

EPB-C3

480

2000

6230

1.50

0.29

22.0

EPB-C1

520

2430

7500

1.58

0.21

21.40

EPB-C2

530

2360

7130

1.53

0.25

21.6

The variation in weave pattern under consideration was found to have caused only

marginal variation in the properties of the carbon fabric reinforced composites. Even

though the properties 01: the carbon fabrics composites using plane weave cloth (EPB-

C1) and satin weave cloth (EPB-C2) were more or less comparable, the latter was

found to have improved inter laminar shear strength, while its compressive strength

and flexural strength were found to be slightly inferior to those of EPB-C1.ThiS may be

due to the improved wetting of the fiber by the matrix caused by the reduced crimp in

the satin weave compared to that in the plane weave pattern. The properties I

performance of the chopped fiber composites (EPB-C3) were found to be minimum

among the three types considered. There was a slight reduction in its mechanical

properties, density, flexural strength and compressive strength. The water absorption

was comparatively more for the chopped fiber composite (EPB63).

This may be due to the fact that the surface area of the chopped fiber is more

compared to that of the woven cloth. Since the same volume1 weight of matrix resin is

used in all cases, the l:rend in properties leads to the conclusion that the efficiency of

wetting of the fiber with the resin varies wiUl the difference in fiber architecture.

6.3.4 Syntactic foam composites

The EPB matrix was used for processing syntactic foam composites, which

find variety of applications as low density foam insulations in different areas

(mentioned in chapterl). The mechanical properties of the syntactic foam composites

depend on its density which in turn depends on the resin I filler ratio. In the case of

syntactic foams, generally the strength properties are found to follow the trend in its

densityw. The foam c;omposites were fabricated using the EPB matfix system and

varying concentrations of hollow glass microballoon (K-37), and characterized for their

tensile. flexural and compressive properties. The propeltiis were found to vary with

microballoon concentration in the foam composite.

<;

6.3.4.1 The effect of micro balloon concentration on mechanical properties of

EPB- K37 composite

The mechanical properties of the foam composites are found to vary

significantly with the concentration of microballoon. The effect of microballoon

concentration on the compressive, flexural and tensile strength (C.S, F.S and T.S) of

the EPB-K37 foam composite is shown in Fig. 6.4.

Fig 6.4 The effect of micro balloon concentration on the mechanical properties of EPB -K37 syntactic foam composites.

The strengths of these systems, under all the three loading environments, were found

to decrease systematically with increase in filler concentration, while the elongation

was not found to be affected much. The modulus values also showed the same trend

as the strength values. The flexural, compressive and tensile moduli (F.M, C.M, T.M)

of the foam composites with different microballoon concentrations are shown Fig. 6.5.

Fig. 6.5 The effect of micro balloon concentration (40 -70 weight%)on the modulus of EPB-K-37 composites

Even though the compressive.strength was higher than the flexural strength at all

microballoon concentrations considered, the modulus value showed a reverse trend.

The failure was found to be of brittle nature in the case of tensile and flexural tests

while under compression, the mode of failure was different.

The stress- strain curves obtained for the compression test (Fig 6.6) reveal

that the failure is not abrupt. After attaining the maximum strength, a slow decrease in

strength is observed with increase in strain ultimately leading to final crushing failure

of the material. Unlike the trend observed in fiber reinforced composites, where kink

banding, shear failure, matrix I fiber interface failure are common failure modes in

compression tests, the compressive failure in these syntactic foam composites was

crushing failure at the upper and lower surfaces of the specimen in contact with the

platens of the compression test fixture, while no visual cracks were observed in the

other areas of the specimen even after crossing the maximum load point.

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Can. Sain

Fig. 6.6 The compressive stress-strain behavior of EPB- K-37 syntactic foam composites of varying microballoon concentration.

As the compression load is increased, the height of the specimen decreased without

substantial lateral volume expansion as reported by Ho Sung Kim et al". This

observation indicates that during compression, hollow microspheres are getting

broken leading to subsequent densification. There are hvo possibilities for its

breakage and densificatin. The initial slope of the compression stress-strain curve

corresponds to the elastic deformation of the foam and the low slopelplateau regime

is easily related to crushing I densification of microspheres. The performance

difference in crushing/densifmtion may be due partly to the resin content. When resin

content is high, it is possible that smearing of resin over microsphere occurs and thus

direct microsphere contact is avoided in transferring of the load. In such situations, the

densiffcation dominates.

The density of the foam composites was found to decrease with increase in

microballoon concentration. In all cases, the density measured was close to that

theoretically predicted from the weights and densities of its constituents and using the

Rule of Mixtures relationshipm. The specific tensile, compressive and flexural strength

values of these foam composites given in Table 6.6 also showed the same trend.

At lower microballoon concentrations the failure originated mainly at the interface,

due to better microballoon +ig by the matrix polymer available in sufficient

quantity, while at higher miixo balloon concenKMin, the weak microballoons are not

properly protected by the binder matrix thereby reducing the strength.

The SEM pictures of the failed surfaces (Figs. 6.7a & b) revealed the

distribution of broken microballoons. This indicates the likely initiation of failure in the

foam composite. Karthykeyan et at0 observed that the process involving resin fracture

and resin-microballoon debonding rather than crushing of microballoon dominated the

Table 6.6. The effect o f micro balloon concentration on the specific strength o f the EPB-K37 foam composites.

Micro balloon content (We~ght %) -+

Specific Tens~le strength (MPa ikg.m-3)x102

Specific Com. Strength (MPa 1kg.m~)xl0~ Specific FlexjStre~gth (MPa 1kg.m )xi0

Density (kg. ma) .

60%

3 2

8 3

4.8

500

70%

2 3

3 6

4.0

440

40%

3 9

8 9

6 1

600

50%

3 3

8 6

5.5

540

tensile failure of syntactic foams. The matrix serves as the load bearing phase in the

composite where as the microballoon only provides light weight and minimal

strengthening effect4D. The reduction in load bearing matrix resin outweighs the

increase in stiffness produced by the higher microballoon concentration. At lower

microballoon concentration, the failure surface was found to be dominated with the

matrix resin layer and the breakage is found to be caused by both interphase failure

and the breakage of the microballoons of higher size (Fig. 6.7a). Most of the lower

size microballoons were intact due to its intimate contact with sufficient quantity of the

matrix resin and resulting higher bond strength. At high rnicroballoon concentrations of

the order of 70%, the failure occurs mainly due to microballoon breakage (Fig. 6.7 b)

due to poor microballoon wetting and the resulting poor reinforcing ability. The

property decrease is quite abrupt beyond 60% microballoon loading (Table 6.6)

Fig 6.7 SEM pictures of EPB-syntactic foam composite with different microballoon

concentrations a) K-37 (40%) and b) K-37(70%)

6.3.4.2 Effect of Filler Concentration and Temperature on the Thermo-Physical

Properties of EPB-K37 Foam Composite

The thermo physical properties like the specific heat and linear expansion are

very important properties for a foam composite used for thermal insulation. These

thermo physical properties were determined using DSC and TMA techniques following

the standard procedures described in chapter 2. The specific heat of the composites

showed a systematic increase with microballoon concentration up to 60%

microballoon loading (Table 6.7) and at moderate temperatures (up to 80 OC). This

can be explained on tho basis of the specific heat difference between the matrix resin

and the hollow glass microballoon. Since the specific heat of the microballoon is

higher in comparison tc, that of matrix resin, the specific heat of the filled system will

be higher than that for the matrix and it will increase with filler content as per the rule

of mixtures. At temperatures above 80% it did not show any definite trend. The

specific heat of the microballoon foam composites with varying microballoon content

determined at different temperatures is given in Table 6.7. Specific heat was found to

be less sensitive to the test temperature. However, it showed a slight decreasing trend

with temperature at all microballoon concentrations.

Table 6.7 Effect of micro balloon concentration on the specific heat of the EPB-K37 syntactic foam composite

I Micro balloon 1 Specific heat (cal I g I C) at different test temperatures

The thermal expansion coefficient showed systematic reduction with increase in

microballoon wncentration (Table 6.8), while it was found to increase systematically

with increase in temperature at all microballoon concentrations. The values are

comparable to those of commercial composites, though the system contain hollow

microballoon.

Table 6.8 The effect of micro balloon concentration on the linear expansion of the EPSK37 foam composites.

I Micro balloon contenti I Linear Expansion coefficient ( "C")xlOJ 1

6.3.4.3 EPB Syntactic Foam composites using Microballoon Blend (EPB-BMB)

Foam composites were processed using a mixture of K-37 and K-25 micro

balloon fillers at a total filler content of 50 weight%. The proportion of the two types of

microballoons was varied to get different K-37: K-25 blend ratios (100:0, 60:40, 4050

and 0:lOO). These composites were characterized for their thermo physical and

mechanical properties and the results are summarized in Table 6.9. The properties

viz. tensile strength (T.S), flexural strength (F.S) and compressive strength (C.S) were

found to increase with increase in concentrat~on of K-37 microballoon filler in the

blend. The change in strength and modulus values with microballoon blend

composition is given in Fig.6.8 and 6.9 respectively. The specific strengths of

composites were also found to increase with increase in concentration of the K-

150-200

5.8

3.7

2.9

2.5

Temperature range

("C ) +

50-100

4.4 F3! 3.6

1.7

70% 1.1

2.4

1.5

37rnicroballoon in. the blend. The modulus values followed the same trend as the

strength values. For the same concentration of micro balloon, EPB- K 37 gave higher

density, strength and modulus compared to EPB-K 25, owing to the higher shell

thickness and true density of the K-37 mickballoon.

Fig. 6.8 The effect of micro balloon composition on the strength of the EPB-Blended microballoon composite ( 50 weight% filler)

Bibin et a14' also observed in their studies on cyanate ester syntactic foam

composites, that the strength of the K-37 foam composite is superior to K-25

composite. Since the composite breaking strength values are proportional to the shell

thickness as well as the crushing strength values of the micro balloon, -the

microballoon breakage is confirmed as the cause for the foam failure. The SEM

pictures (Fig 6.10) of the failed surfaces of the foam composites also confirmed the

same. It can be seen that the proportion of broken microballoon is more in the case of

K-25 composite (Fig 6.10b). Both interface failure and microballoon breaking (Mixed

failure mode) are clearly visible in the SEM photographs in the case of K-37 foam

composite (Fig 6.10a). As expected, the density values also showed an inverse

proportion to the concentration of K-37 micro balloon.

Fig. 6.9 The effect of micro balloon composition on the modulus of the EPB-Blended microballoon composite with (50 weight % filler)

Table 6.9 The effect of micro balloon blend composition on the specific strength of EPB-BMB syntactic foam composites (Total filler content -50 weight %).

K-37 : K-25 weight ratio+ 100:O 60:40 4050 0:lOO

Specific Compressive strength (MPa /kg.m-3)x102

Specific Flexural strength (M Pa /kg. rn4)x4 0'

Density (kg. mJ)

5.6

540

5.9

470

5.9

440

3.1

390

Fig. 6.10 SEM pictures of the failed surfaces of the EPB foam composites with a) K-37 and b) K-25 microballoon (50 weight % filled)

6.3.4.4 Effect of microballoon blend composition on specific heat and linear

expansion of EPB.-BM8 composites

The thermal expansion coefficient of composite depends not only on the type

of reinforcement and the type of matrix, but also on the geometry of the reinforcement

and its volume fractiond2. Table 6.10 shows the effect of microballoon blend

wmposition on the specific heat of a typical EPB - BMB syntactic foam composites

(50 weight % filled). The change in microballoon blend ratio and temperature was not

found to have much influence on its specific heat. Even though the specific heat

difference between the EPB-K37 and EPB- K25 is significant, the specific heat

variation with blend ratio was insignificant up to a filler ratio ( K25 :K37) of 60:40.

However, there is a decreasing trend with temperature as well as K-37 concentration

in the blend though it is not considerable.

Table 6.10. Effect of micro balloon blend composition on the specific heat of EPB- BMB syntactic foam composites (50 weight % filled).

6.4 Conclusion The carbon fabric reinforced epoxy- phenolic -bismaleimide composites were found to

have superior inter laminar shear strength and flexural properties compared to the

corresponding glass composites. The compressive strength of the carbon composite

was comparable or even slightly less than that of EPB-glass composites. The fiber

orientation influenced Me properties of the EPB composites. The compressive,

flexural and interlaminar shear properties of the cross ply laminate showed

considerable improvement over that of angle ply laminate. The properties were

affected by changing the stacking sequence of the EPB-glass composites. On

changing the stacking sequence from G2- [f45]28 to G3-[(0/90)2/ (f45) 1 (0/90)21s, the

performance of the composite improved considerably. The G1-[0190]2a stacking

sequence yielded the best properties among the three. The propettiis of EPB- GI

were found to be more or less comparable tb that of EPB-G3. The variation in weave

pattern1 fiber architecture (between EPB-C1 and EPB-C2) caused only marginal

variation in the properties of the carbon fabric reinforced composites, while the

properties of the chopped fiber c~mposite was inferior to these two.

The addition of hollow micro balloon to the EPB system has resulted in the

reduction in strength and modulus of the material in tension, compression and flexure

modes. The mechanical properties and density showed systematic decrease with

increase in microballoc,n concentration in the foam composite, but with the added

advantage of higher specific strength and specific modulus. At lower microballoon

concentrations, the failure occured at the interface, due to the better microballoon

wetting by the matrix polymer available in sufficient quantity, while at higher

microballoon concentration, the weak microballoons are not properly protected by the

binder matrix thereby reducing the strength as a result of poor interfacial interaction

between the matrix and the microballoon. Hence, both interface failure and

microballoon breakage occurred at lower microballoon concentrations. The specific

heat showed a slight increasing trend with increase in microballoon concentration up

to about 60% filler loading and at moderate temperatures (up to 8 0 ' ~ ) . The strength

and the corresponding specific strength of the K-37 wmposites were found to be

higher in comparison to those of K-25 wmposite. Since the breaking strength values

are proportional to the shell thickness as well as the crushing strength of the two types

of micro balloons, the microballoon breakage is confirmed as the cause for the foam

failure, particularly in the case of K-25 with lesser shell thickness. The failure in these

syntactic foam wmposites under compression was crushing failure at the loaded

surfaces, while no visual cracks were observed in the other areas of the specimen

especially at lower microballoon concentrations. The thermo physical properties of the

foam composites were found to vary with the wncentration of the microballoon. The

coefficient of linear thermal expansion indicated a systematic decrease with increase

in microballoon wncentration and an increase with enhancement in temperature. The

incorporation of blend of low density fillers resulted in variation in mechanical

performance of the wmposite, depending on the proportion of the lowerthigher

strength microballoon in the blend, while the thermo physical properties were not

affected much for the blend ratios investigated. This protocol provided an easy

method for composite property and density tuning.

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