short sisal fibre reinforced -...

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


Fig. 4.1 IR spectra of untreated sisal fibre

Fig. 4.2 IR spectra of benzoylated sisal fibre

.


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


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


(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


(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


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


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


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-


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


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


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.


(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


(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


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


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


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


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.


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.


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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