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Chapter
4
EFFECT OF INTERFACE MODIFICATION ON THE MECHANICAL PROPERTIES OF SHORT SISAL
FIBRE – POLYSTYRENE COMPOSITES
Abstract
The effects of interface modification on the mechanical (tensile, impact
and flexural) properties of polystyrene- sisal fibre composites were
investigated. The interface modification was performed by treatment of sisal
fibres with benzoyl chloride, polystyrene maleic anhydride (PSMA), toluene
diisocyanate (TDI), methyl triethoxy silane and triethoxy octyl silane. These
interface modifications improve the compatibility of hydrophilic sisal fibre
with hydrophobic polystyrene matrix and enhance the tensile properties of
the composite. In all cases, except PSMA coating, interface modifications
decreases the impact strength. The PSMA coating, however, improves the
impact strength of the composite. Flexural properties were also changed by
interface modifications but to varying degree. The treated fibres were
analysed by spectroscopic techniques. Scanning electron microscopy was
used to investigate the fibre surface, fibre pullout and fibre- matrix interface.
The results presented in this chapter have been accepted for publication in
Polymer Composites
4.1 Introduction
Reinforcement of thermoplastics with natural fibres produces materials with good
mechanical properties and low specific mass. Moreover, the production of these
materials is more economical than pure polymers and imparts better strength and
toughness to the thermoplastics. However, the lack of good interfacial adhesion
and poor resistance to moisture absorption leads to debonding with age and made
the use of natural fibre reinforced composite less attractive1. These cellulose
fibres are hydrophilic in nature and are generally incompatible with hydrophobic
hydrocarbon polymers. Another factor controlling the physical properties of the
composite is the interface. The interfacial interactions can be modified by fibre
surface modification, which can be either physical or chemical methods. One of
the important chemical modifications used to improve fibre–matrix interaction
involves coupling methods. The coupling agents used contains functional group,
which can react with the fibre and polymer. The bonds formed may be either
covalent or hydrogen bonding that improves fibre- matrix interaction. Bisanda
and Ansell2 have studied the effect of alkali treatment on the physical and
mechanical properties of sisal/epoxy composites. Felix and Gatenholm3 reported
an improvement in wetting of cellulose fibres to polypropylene matrix by
modifying cellulose fibres with maleic anhydride –polypropylene copolymer.
Prasad et al.4 have studied the effect of alkali treatment on the mechanical
properties of coir/ polyester composites. Kokta and co-workers5,6,7 have reported
that the coupling agents like silanes and isocyanates improve the mechanical
properties and dimensional stability of cellulose fibre –PE and PS composites.
Short Sisal Fibre Reinforced Polystyrene Composites 126
Mieck et al.8 reported on the use of alkyl functional silanes in cellulose –PP
composites. According to them silanes do not form covalent bonds but improves
the wetability of the fibres and chemical affinity to the PP matrix. Mieck et al.8
also reported a 60% increase in shear strength in the flax silane system due to the
formation of hydrogen bonds.
The use of peroxide to improve the adhesion in cellulose fibre reinforced
thermoplastic composites has been reported by various researchers and leads to
easy processability and improved mechanical properties9-12. A significant
improvement in the mechanical properties and impact strength of HDPE /
asbestos composites by catalytic grafting of polyethylene on asbestos fibre was
reported by Wang and et al.13. The use of maleic anhydride –polypropylene
copolymer to improve mechanical properties of flax –PP composite was also
reported in literature14. The effectiveness of these coupling agents depends on the
grafting rate and on the average molar mass of the copolymer. Gassan and
Bledzki15 reported the effect of fibre treatment time and maleic anhydride – PP
concentration on the mechanical properties of jute-PP composites. The chemical
bonding between the anhydride and the hydroxyl groups of the fibre caused better
stress transfer between the fibre and matrix leading to a higher tensile strength.
In this chapter, a detailed investigation has been carried out on the
mechanical properties of sisal fibre reinforced polystyrene composites with
special reference to the effects of fibre modification. The surface modification
was done by treatment of sisal fibres with benzoyl chloride, polystyrene maleic
anhydride (PSMA), toluene diisocyanate (TDI), methyl triethoxy silane and
Effect of Interface Modification on the Mechanical Properties of …….. 127
triethoxy octyl silane. Scanning electron microscopy (SEM) was used to study
the improvements in adhesion between the treated fibres and PS matrix.
4.2 Results and Discussion
4.2.1 Characterization of treated fibres
(a) Benzoylated fibre
.
eme 4.1 –Mechanism of reaction between benzoyl chloride and sisal fibre
The chemical reaction between sisal fibre and benzoyl chloride can be
schematically represented as in scheme 4.1. The chemical structure of sisal fibre
was remarkably changed by benzoylation as indicated by the IR spectra of
untreated (Fig. 4.1) and treated fibre (Fig 4.2). Hydroxyl groups absorption at
about 3400cm-1diminished after benzoylation as a result of esterification of the
hydroxyl group. Absorption bands around 1950, 1600 and 710 cm-1 indicate the
presence of aromatic groups and the peak around 1725 and 1300 cm-1 indicate the
presence of ester groups
Fig. 4.3a and 4.3 b show the SEM photographs of the surface of untreated sisal
fibre and benzoylated fibre respectively. These figures indicate defibrillation of
the fibre upon benzoylation.
Fibre-O-Na+ + COCl Fiber-O -CO +NaCl
Fibre-OH + NaOH Fibre-O- Na+
Sch
Short Sisal Fibre Reinforced Polystyrene Composites 128
Fig. 4.1 IR spectra of untreated sisal fibre
Fig. 4.2 IR spectra of benzoylated sisal fibre
.
Effect of Interface Modification on the Mechanical Properties of …….. 129
Moreover, the treatment produce a rough fibre surface and a number of small
voids on the surface of the fibre that promote the mechanical inter locking
between the fibre and the matrix.
(a) (b)
Fig. 4.3- SEM photographs of the surface of sisal fibre (a) untreated fibre (b) benzoylated fibre
(b) Poly styrene maleic anhydride (PSMA) treated fibre
Sisal fibre, when treated with PSMA, maleic anhydride groups present in PSMA
forms hydrogen bonds with the hydroxyl groups of the fibre. Scheme 4.2 shows
the possible mechanism of the reaction between the fibre and PSMA.
O
O
O
+
O
O
O
H
H
H
FIBERSURFACE
O
O
O
H
H
H
FIBERSURFACE
O
O
O
Scheme 4.2 – A possible scheme for the formation of bond between PSMA and sisal fibre
Short Sisal Fibre Reinforced Polystyrene Composites 130
Unlike polypropylene maleic anhyride16, PSMA does not form any covalent
his is clear from the IR spectrum of untreated (Fig.4.1)
Fig. 4.4 IR spectra of PSMA treated sisal fibre
bonds with the fibre and t
and that of treated (Fig.4.4) fibre. This is also confirmed by 13C NMR spectrum
of the untreated (Fig.4.5) and treated fibre (Fig.4.6), which show no peaks
characteristics of PSMA grafting on fibre.
Fig. 4.5 13C NMR spectrum of untreated sisal fibre
Fig. 4.6 13C NMR spectrum of PSMA treated sisal fibre
Effect of Interface Modification on the Mechanical Properties of …….. 131
(c) Silane treatment
the ca OR2 groups to
lanols;
ethyl (CH3-) group for methyl triethoxy silane and
ilane. R2 represents ethoxy (-OC2H5) group in
both ca
i-O-) as well as hydrogen bonds are established between the –OH
iisocyanate (TDI) treatment
Scheme 4.4 shows the reaction between TDI and sisal fibre. The reaction
from the IR spectra of treated fibre
In se of silane treatment, the – of the silane may hydrolyses
some extent to form si
R1Si(OR2)3+ 3H2O → R1Si(OH)3
Where, R1 represents m
octyl (C8H24-) for triethoxy octyl s
ses.
When the fibres are immersed in the aqueous solution of the silane, chemical
bonds (R1-S
groups of the fibre surface and R1-Si(OH)3 molecules. Formation of these bonds
reduces the water up taking capacity of silane treated composites. Scheme 4.3
shows the mechanism of reaction between sisal fibre and silane.
OH O
OH
OH
+FIBER SURF-ACE
FIBERSURFA-CE O
O
Si-R1
(HO)3 Si-R1
H
HO
H
H
O
Scheme 4.3 Mechanism of reaction between sisal fibre and silanes
(d) Toluene d
between the fibre and TDI can be confirmed
Short Sisal Fibre Reinforced Polystyrene Composites 132
(Fig.4.7) which shows characteristic peaks at 1357 cm-1 corresponding to
carbonyl stretching and at 888 and 1626 cm-1 corresponding to aromatic groups.
Scheme4.4 Mechanism of reaction between sisal fibre and TDI
Fig. 4.7 IR spectra of TDI treated sisal fibre
FIBERSURF-ACE
OH
OH
OHN=C=O
N=C=O
3
FIBERSURF-ACE
OH
OH
O
+
CH3C
O
N
H
FIBERSURF-ACE
OH O
C O
N H
CH
Effect of Interface Modification on the Mechanical Properties of …….. 133
32
36
40
44
48
52
M206Sm206
T206B206Se206
U206
PS
Tens
ile s
treng
h(M
Pa)
4.2.2 Effect of fibre modification on tensile properties
Fig. 4.8 shows the effect of chemical treatment on the tensile strength of PS-sisal
composites. From this figure, it is clear that the fibre modification improves the
tensile strength of the composites and the improvement follows the order
M206>Sm206>B206 ≅T206>Se206>U206>PS. The maximum improvement in
tensile strength was observed with PSMA treated fibre.
he Yo shows
provement and follows the order Se206 > M206 > B206 ≅ T206 ≅ Sm206 >
U206 > PS. The improvement in the Young’s modulus may also be attributed to
the improvement in the adhesion between the fibre and matrix. The effect of fibre
treatment on percentage of elongation at break of the PS-sisal composite (Fig
4.10) follows the order PS>U206>B206 ≅ M206≅Sm206 >T206 ≅Se206.
Fig. 4.8 – Effect of fibre modification on tensile strength of sisal fibre – PS composite
T ung’s modulus of the treated composites (Fig.4.9) also
im
Short Sisal Fibre Reinforced Polystyrene Composites 134
400
600
800
1000
1200
1400Se206
M206
Sm206B206T206U206
Pa)
PS
Youn
g's
mod
ulus
(M
5.0
5.5
6.0
6.5
7.0
7.5
8.0
9.0
9.5
10.0
8.5
ELO
NG
ATIO
N A
T BR
EAK(
PS
B206M206Sm206
T206Se206
%)
U206
Elo
ngat
ion
at b
reak
(%)
Fig. 4.9 – Effect of fibre modification on Young’s modulus of sisal fibre – PS composite
Fig. 4.10 – Effect of fibre modification on elongation at break of sisal fibre –PS composite
Effect of Interface Modification on the Mechanical Properties of …….. 135
In the case of benzoylated, TDI treated and methyl triethoxy silane treated fibre
composites no appreciable change in strain was observed. However, PSMA
eated and t
provement in strain. When the fibre matrix adhesion is higher the composite
Fig. 4.11
PS composite
tr triethoxy octyl silane treated fibre composite show a sligh
im
will fail at a lower elongation. The reduced elongation values of treated
composites confirm the improved adhesion between the fibre and matrix.
Now, let us examine in details the mechanism involved in the improvement of
adhesion in each cases. The improvement in tensile properties of benzoylated
fibre composite is attributed to the presence of phenyl structure in treated fibre
similar to that of polystyrene, which improves the thermodynamic compatibility
between the fibre and polystyrene. Another contributing factor is the reduction in
the hydrophilicity of the fibre as a result of benzoylation, which makes the fibre
more compatible with hydrophobic polystyrene.
FIBER SURFACE
OHO
C=O
O
C=O
O
C=O
PS MATRIX
A hypothetical model of interface of benzoylated sisal fibre-
Short Sisal Fibre Reinforced Polystyrene Composites 136
Moreover, benzoylation makes the surface of the fibre very rough and provides
del of
interface of benzoylated sisal fibre-PS composite is shown in Fig. 4.11.
s a result
f this the fibre becomes more hydrophobic and becomes more compatible with
hydrophobic polystyrene. Moreover, the presence of polystyrene segments in the
PSMA attached to the fibre renders them thermodynamically more compatible
with the polystyrene matrix. A hypothetical model of interface of PSMA treated
sisal fibre-PS composite is shown in Fig. 4.12.
Fig 4.12
sisal fibre-PS composite
better mechanical interlocking with the polymer matrix. A hypothetical mo
In the case of PSMA coating, the maleic anhydride group of PSMA form
hydrogen bonds with hydroxyl groups of the fibre as discussed earlier. A
o
SISAL FIBER SURFACE
O O O
PS MATRIX
PSMA MATRIX
OOO OOO
O
HH H H
A hypothetical model of interface of PSMA treated
Effect of Interface Modification on the Mechanical Properties of …….. 137
The enhanced bonding in TDI treated composite is attributed to the formation of
strong covalent bonds between the –OH groups of the fibre and the –N=C=O
groups of TDI as discussed earlier.
Fig. 4.13- A hypothetical model of interface of TDI treated sisal fibre-
he benzene rings present in the treated fibre increases the thermodynamic
fibre more hydrophobic and improves the interaction with hydrophobic PS
treated sisal fibre -PS composite is given in Fig 4.13.
the case of silane treated fibre the –OR2 groups of the silane hydrolyse to some
extent to form silanols and the resulting –OH groups or –OR2 groups provides
OH
PS Mat
rix
TDI treated fibre
C=O
CH3
NH
OHO OH
C=O
CH3
NH
O
PS composite
T
compatibility of the fibre with PS matrix. Moreover, the treatment converts the
matrix. A hypothetical model of the fibre matrix interface in the case of TDI
In
link to their –OH groups by the formation of hydrogen bonds as discussed
earlier.The hydrophobic alkyl groups attached to the fibre as a result of silane
Short Sisal Fibre Reinforced Polystyrene Composites 138
treatment increases the compatibility with the hydrophobic PS matrix and
improves the mechanical properties of the composite. A hypothetical model of
the interface of silane treated sisal fibre - PS composite is shown in Fig 4.14.
OO
O
PS Ma
OH
trix
Si
R'
HO
OH
HO
OO
Si
R'
HO
HO
H
H
Silane treated fiber
H
Fig. 4.14 A hypothetical model of interface of silane treated sisal fibre- PS
composite
he improvement in adhesion between the treated fibre and PS matrix can be
T
understood from the SEM photographs of the fractured surface of untreated sisal
fibre-PS composite (Fig.4.15a) and that of treated fibre composites given in
figures 4.15 b,c, d, e and 4.15f. While the fractured surface of untreated fibre
composite shows holes and fibre ends indicting poor adhesion between the fibre
matrix, fracture surface of treated fibre composite shows fibre breakage rather
than pullout, indicating better interfacial strength.
Effect of Interface Modification on the Mechanical Properties of …….. 139
(a) (b)
(c)
(e) (f)
4.15 SEM photographs of the fractured surface of sisal fibre-PS composites (a)
(d)
Fig
untreated fibre (b) benzoylated fibre (c) PSMA treated fibre(d) methyl triethoxy silane
treated (e) triethoxy octyl silane treated and (f) TDI treated
Short Sisal Fibre Reinforced Polystyrene Composites 140
(a)
(d)
(e)
composite
(a) untreated fibre (b) benzoylated fibre (c) PSMA treated fibre (d) methyl
ted
(b)
(c)
(f)
Fig. 4.16 SEM photographs of the surface of fibre pulled out from
triethoxy silane treated (e) triethoxy octyl silane treated and (f) TDI trea
Effect of Interface Modification on the Mechanical Properties of …….. 141
The better adhesion in the case of treated fibre composites is also clear from the
SEM photographs of the surface of untreated (Fig.4.16a) and that of treated fibre
(Fig.4.16 b, c, d, e and 4.16f stripped out from the composite. The surfaces of the
treated fibre have a coating of polystyrene particles suggesting better interfacial
interactions.
4.2.3 Effect of fibre modification on impact properties
The lowering of adhesion between fibre and matrix and application of suitable
coating on the fibre that modifies the inter laminar shear stress leads to
improvement in toughness. However, very low adhesion efficiency may result in
the lowering of toughness.
0
5
10
15
20
25
30
M206
U206T206Sm206
Se206
PS
B206
Impa
ct e
nerg
y(KJ
/m2 )
Fig. 4.17– Variation of impact energy of sisal fibre –PS composite –effect of fibre modification
Fig. 4.17 shows the effect of fibre- matrix interface modification on the impact
energy of PS- sisal composites. From the figure it is clear that the impact strength
Short Sisal Fibre Reinforced Polystyrene Composites 142
decreases as the interfacial bond strength increases except in the case of PSMA
coating and the impact strength follows the order M206>U206>T206>Sm
206>Se206>PS>B206.It is interesting to note that while benzoylation, silane and
TDI treatment of the fibre reduces the impact strength of the composites, PSMA
coating on the fibre increases the impact energy of the composite. It was already
established that a strong interface between the fibre and the matrix reduces the
impact strength of the composites17-19 .At high levels of adhesion, the failure
mode is brittle and relatively little energy is absorbed. In the case of a weak
interface the triaxial stresses at the tip of an advancing crack cause debonding to
occur and a crack bunting mechanism takes place and improves the toughness of
the material 20.
The increase in the impact strength of PSMA coated fibre composite is in
agreement with results obtained for PP/PP-MA/ flax fibre system21. In the case of
PSMA coated sisal fibre composites, the coating may improve the dispersion of
the fibre. Moreover, in this case the adhesion between the matrix and the fibre
may be intermediate and leads to progressive delamination which require
additional energy and hence an improved impact strength. When the fibre-matrix
adhesion is strong, the mechanism of failure change from fibre debonding and
pullout to brittle failure and reduce the impact strength.
4.2.4 Effect of fibre modification on flexural properties
Fig.4.18, 4.19 and 4.20 show the effect of fibre modification on flexural strength,
flexural modulus and flexural strain of PS-sisal fibre composites. It is interesting
to note that while benzoylation reduces the flexural strength compared to
Effect of Interface Modification on the Mechanical Properties of …….. 143
untreated fibre composites, all other treatments improves the flexural strength and
follows the order M206>Se 206>Sm206≅ T206>U206>B206>PS.
40
60
80
100
M206
Se206
Sm206T206U206
B206
PS
Flex
ural
stre
ngth
(MPa
)
Fig. 4.18– Variation of flexural strength of sisal fibre–PS composites as a function of fibre modification
3000
4000
5000
6000
Se206
Sm206
U206M206
T206
B206
PS
Flex
ural
mod
ulus
(MP
a)
Fig. 4.19– Variation of flexural modulus of sisal fibre –PS composites as a function of fibre modification
Short Sisal Fibre Reinforced Polystyrene Composites 144
0.025
0.026
0.027
0.028
0.029
0.030
0.031
0.032Se206
M206
B206
Sm206U206
T206
PSFlex
ural
stra
in (%
)
Fig. 4.20– Variation of flexural strain of sisal fibre –PS composites as a function of fibre modification
However, the improvement in flexural strength in the case of Sm206 and T206 is
only marginal and in all other cases the flexural strength was found to be higher
than that of untreated fibre composites. Flexural modulus (Fig.4.19) shows a
decrease in the case of benzoylated and TDI treated fibre composites and shows
improvement in the case of silane treated fibre composites. PSMA treated fibre
composites, however, do not show appreciable changes in flexural modulus
compared to untreated composites. Flexural modulus values of all the composites
are higher than that of pure PS and follows the order Se206 > Sm206 > U206 ≅
M206 > T206> B206>PS. The variation of flexural strain values (Fig.4.20) of
composites with fibre modification is only marginal and follows the order PS <
T206<U206 <Sm206<B206<M206 ≅ Se206.
Effect of Interface Modification on the Mechanical Properties of …….. 145
4.3 References
1. J. Gassan and A.K.Bledzki, Angew.Makromole.Chem., 236,129, 1996.
2. E.T.N.Bisanda. and M.P. Ansell, Comp.Sci. Technol., 41, 165, 1991.
3. J.Felix. and P.Gatenholm., J.Appl.Polym.Sci., 42, 609,1991.
4. SV Prasad, C. Pavithran and P.K.Rohatgi, J. Mater. Sci., 18,1443,1983.
5. A.D.Beshay, B.V.Kokta and C.Daneult, Polym.Comp., 6(4), 261,1985.
6. D.Maldas, B.V. Kokta, R.G Raj, and C.Daneult, Polymer, 29,1255 ,1988.
7. B.V.Kokta, F.Debele and C.Daneult, in Polymer Science and Technology,
C.E.Carraher Jr. and L.H. Sperling, Eds. Plenum, New York Vol.33, p85
8. K.P.Mieck, A. Nechwatal, C.Knobelsdorf, Angew. Makromole. Chem.,
37, 225,1995.
9. S.Manrich, J.A.M. Agnelli, J.Appl. Polym. Sci., 37, 777, 1989.
10. S.Sapieha, P.Allard, and Y.H.Zang, J.Appl. Polym. Sci., 41, 2039, 1990.
11. S.Sapieha, J.F Pupo and H.P.Schreiber, J.Appl. Polym. Sci., 37, 233,1989.
12. R.A.Young., Wood Fibre, 10,112, 1978.
13. Q.Wang, S.Kaliaguine and A.Ai-Kadi, J.Appl.Polym.Sci., 45,1023,1992.
14. M.Avella, C.Bozzi, R.dell’Ebra , B.Focher, A.Marzetti and E.Martuscelli,
Angew. Mkromole.Cheme., 233,149,1995.
15. J.Gassan, A.K .Bledzki, Composites Part A, 28A ,1001, 1997.
16. N.G. Gaylord and M.K. Mishra, J.Polym.Phys.Polym.Lett.Ed., 21,23,
1983.
17. N.L. Hacox, Composites, 3,41,1971.
Short Sisal Fibre Reinforced Polystyrene Composites 146
18. M.G. Bader, J.E. Bailey and I. Bell, J. Phys.D.Appl.Phys., 6,572, 1973.
19. P.Yeung and L.J. Broutman, Polymer Eng. Sci., 18,62, 1978.
20. J. Cook and JE Gordon, Proc.Roy.Soc., A282, 508, 1964.
21. S.K.G Garkhail, R.W.H.Heijenrath and T.Peijs, Appl. Comp. Mater.,7,
351,2000.
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