Stdw Phenolic 4~Ik'rnictk Composites Effect of ~nfo~cenmt...
Transcript of Stdw Phenolic 4~Ik'rnictk Composites Effect of ~nfo~cenmt...
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.
6.5 References
1. S. H. Lee, Wans, M. Anthony, International J. Fracture (ISSN 0376- 9429).,Vo1.100, No.3,275-306 (1999).
2. T. Tsukizoe, N. Ohmae, Friction and Wear of Advanced Composite Materials. Fibre Science and Technology, 18, 265-286 (1983).
3. 2. Hanmin, H. Guoren, Y. Guicheng, Wear, 116, 59 - 68, (1987).
4. Z. Hanmin, H. Guoren, Y. Guicheng; Wear, 116, 69 - 75 (1987).
5. M. Cirino, K. Friedrich, R. B. Pipes, Wear, 121,127- 141 (1988).
6. Jacobs, K. Friedrich, G. Marom. K. Schulte, H. D. Wagner, Fretting. Wear 135, 207-216 (1990).
7. J. Bijwe, C. M. Logan,. IJ. S Tewari, Wear, 138,77 - 92, (1990).
8. B. Vishwanath, A. P.Verma, C. V. S. K. Rao, Wear,l67,93-99 (1993).
9. K. Friedrich, 2. Lu, A. M Hager; 19, 1-11 (1993).
10. K. Friedrich, 2. Lu, A. Mi. Hager, Wear, 190, 139-144 (1995).
11. S. N. Kukureka, C. J. Hooke, M. Rao. P. Liao, Y. K. Chen. Tribology International, 32, 107-1 16 (1999).
12. J. Bijwe, A. K. Indumathi, Ghosh, Wear, 256,27-37 (2004)
13. J. Flock, K. Friedrich, C!. Yuan, Wear, 304-311, 225-229 (1999).
14. A. A. El-Sayed, M. G. El-Sherbiny, A. S. Abo-El-Eu,. G. A. Aggag, Wear, 184, 45-53 (1 995).
15. J. Bijwe, J. Indumathi., A. K. Ghosh, Wear, 253, 803-812 (2002).
16. M. J. Folkes, Short fiber reinforced thermoplastics, Research studies, Wiley. New York (1982).
17. L . A. Goettler, Polym. Compos, 4, 249 (1983).
S .G. Advani . C. L Tucker Ill. Proce. 4 3 M ~ ~ ~ . ~ e c h n . Conf. SPE 31, 1113 (1985).
J. M. Whitney, C.E. Browning. In Advanced composite Materials- environmental effects. ASTM, STP 658. American Society for Testing and Materials; 43-60 (1978).
Huang, J. W. Gillespie, T. Bogetti; Compos. Struct.; 49, 303-312 (2000).
S. Jose, R. Rameshkumar, M. K. Jana, G. Venkateswara Rao, Composite Science and Technology. 61, 11 15-1 122 (2001).
V. Barren, M. Buggy, N. H. Mckenna, J. of Material Sciences, Vol. 36. no. 7,1755-1761 ( 2001).
Anthony kelly &.Carl Zweben Eds.. Comprehensive composite materials, Elsevier Science Ltd.. UK. 750-758 (2000).
S. W. Yugartis, Comp. Sci. Technol., 53 , Elsevier Science Limited, 145154, (1995).
P. I. Fju, J. C. Masters, W. C. Jakson, Comp. Sci. Technol., 53, 155-156 (1995).
P. J. Hine, N.Davidson. R. A. Duckett. I. M Ward, Comp. Sci. Technol., 53, 125-132, (1995).
J. J. McGrath, J. M. Willie,.Comp. Sci. Technol., 53, 133-144 (1995).
Marziyeh Khatibzadeh, M. R. Piggot, Composite Sci. Tech, 56, 1435-1442 (1996).
D. Dean. A. Miyase, P. H. Geil, J. Thermoplas Comp. Mater., 136-151 (1992).
J . L. Sullivan, Composite Sci. and Technol.. 39. 207-232 (1990).
L .C. E. S t ~ i k , F'olymer, 30, 815-830 (1989).
Laurence E Neilson. Mechanical properties of polymers and composites Marcel Dekker, New York, ch.3,67-128 (1974).
E. Riui, E. Papa, A. Corigliano, 1nt.J solids stmct, 37, 5773-5794 (2000).
John L Clarke Ed., Structural design of polymer composites, Publisher. E & FN, UK, 291 (1996).
John L Clarke Ed,, Structural design of polymer composites, Publisher. EBFN, UK. 288 (1996).
C .S. Karthykeyan, S. Sankaran, Kishore, Materials letters, 58, 995-999 (2004).
Ho Sung Kim. Pakorn Pubrai, Manufacturing and failure mechanisms of
syntactic foam under compression Composites Part A, 35, 1009-1015 (2004).
C. S. Karthykeyan, S. Sankaran. M. N. Jagadlsh Kumar. Kishore, J. Appl. Polym. Sci., 81, 405-411 (2001).
C. S. Karthykeyan, S. Sankaran, Kishore, Macromol. Mat. Eng., 58290, 60- 65 (2001).
Erwin. M Wouterson, freddy Y. C. Boey Xiao Hu, Shing-Chung Wong, Compos. Sci. and Technology, 65, 1840-1850 (2005).
Bipin John, K. Ambika Devi. C. P. R. Nalr, K. N. Ninan. Proceeding of ISAMPE lnt. Conf. on Composites INCCOM- IV, 249-260 (2005).
John L Clarke Ed.. Structural design of polymer composites, Publisher. E & FN, UK, 292 (1996).