Chapter 4 EFFECT F INTERFACE MDIFICATIN N TE MECANICAL PRPERTIES F SRT SISAL FIBRE PLYSTYRENE CMPSITES 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. owever, 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 attractive 1. 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. ne 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 Ansell 2 have studied the effect of alkali treatment on the physical and mechanical properties of sisal/epoxy composites. Felix and Gatenholm 3 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-workers 5,6,7 have reported that the coupling agents like silanes and isocyanates improve the mechanical properties and dimensional stability of cellulose fibre PE and PS composites.
126 Short Sisal Fibre Reinforced Polystyrene Composites 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 properties 9-12. A significant improvement in the mechanical properties and impact strength of DPE / 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 literature 14. The effectiveness of these coupling agents depends on the grafting rate and on the average molar mass of the copolymer. Gassan and Bledzki 15 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. Fibre- + Na Fibre- - Na + Fibre- - Na + + Cl C Fiber- -C +NaCl Scheme 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). ydroxyl groups absorption at about 3400cm -1 diminished 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.
128 Short Sisal Fibre Reinforced Polystyrene Composites 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. + FIBER SURF ACE FIBER SURF ACE Scheme 4.2 A possible scheme for the formation of bond between PSMA and sisal fibre
130 Short Sisal Fibre Reinforced Polystyrene Composites Unlike polypropylene maleic anhyride 16, PSMA does not form any covalent bonds with the fibre and this is clear from the IR spectrum of untreated (Fig.4.1) and that of treated (Fig.4.4) fibre. This is also confirmed by 13 C 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. 4.4 IR spectra of PSMA treated sisal fibre Fig. 4.5 13 C NMR spectrum of untreated sisal fibre Fig. 4.6 13 C NMR spectrum of PSMA treated sisal fibre
Effect of Interface Modification on the Mechanical Properties of.. 131 (c) Silane treatment In the case of silane treatment, the R 2 groups of the silane may hydrolyses to some extent to form silanols; R 1 Si(R 2 ) 3 + 32 R 1 Si() 3 Where, R 1 represents methyl (C 3 -) group for methyl triethoxy silane and octyl (C 8 24 -) for triethoxy octyl silane. R 2 represents ethoxy (-C2 5 ) group in both cases. When the fibres are immersed in the aqueous solution of the silane, chemical bonds (R 1 -Si--) as well as hydrogen bonds are established between the groups of the fibre surface and R 1 -Si() 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. FIBER SURF- ACE + () 3 Si-R 1 FIBER SURFA- CE Si-R 1 Scheme 4.3 Mechanism of reaction between sisal fibre and silanes (d) Toluene diisocyanate (TDI) treatment Scheme 4.4 shows the reaction between TDI and sisal fibre. The reaction between the fibre and TDI can be confirmed from the IR spectra of treated fibre
132 Short Sisal Fibre Reinforced Polystyrene Composites (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. FIBER SURF- ACE + C 3 N=C= N=C= FIBER SURF- ACE C N N C 3 C FIBER SURF- ACE Scheme4.4 Mechanism of reaction between sisal fibre and TDI Fig. 4.7 IR spectra of TDI treated sisal fibre
Effect of Interface Modification on the Mechanical Properties of.. 133 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. 52 Tensile strengh(mpa) 48 44 40 36 PS M206 Sm206 T206 B206 Se206 U206 32 Fig. 4.8 Effect of fibre modification on tensile strength of sisal fibre PS composite The Young s modulus of the treated composites (Fig.4.9) also shows improvement 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.
134 Short Sisal Fibre Reinforced Polystyrene Composites 1400 M206 Se206 1200 Pa) Young's modulus(m 1000 800 U206 T206 B206 Sm206 600 400 PS Fig. 4.9 Effect of fibre modification on Young s modulus of sisal fibre PS composite 10.0 9.5 PS 9.0 ELNGATIN Elongation at AT break BREAK( (%) %) 8.5 8.0 7.5 7.0 6.5 6.0 Se206 T206 Sm206 M206 B206 U206 5.5 5.0 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. owever, PSMA tr eated and triethoxy octyl silane treated fibre composite show a slight improvement in strain. When the fibre matrix adhesion is higher the composite 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 C= C= C= PS MATRIX Fig. 4.11 A hypothetical model of interface of benzoylated sisal fibre- PS composite
136 Short Sisal Fibre Reinforced Polystyrene Composites Moreover, benzoylation makes the surface of the fibre very rough and provides better mechanical interlocking with the polymer matrix. A hypothetical model of interface of benzoylated sisal fibre-ps composite is shown in Fig. 4.11. In the case of PSMA coating, the maleic anhydride group of PSMA form hydrogen bonds with hydroxyl groups of the fibre as discussed earlier. As a result of 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. SISAL FIBER SURFACE PSMA MATRIX PS MATRIX Fig 4.12 A hypothetical model of interface of PSMA treated sisal fibre-ps composite
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 groups of the fibre and the N=C= groups of TDI as discussed earlier. TDI treated fibre C= C= N N C 3 C 3 PS Matrix Fig. 4.13- A hypothetical model of interface of TDI treated sisal fibre- PS composite The benzene rings present in the treated fibre increases the thermodynamic compatibility of the fibre with PS matrix. Moreover, the treatment converts the fibre more hydrophobic and improves the interaction with hydrophobic PS matrix. A hypothetical model of the fibre matrix interface in the case of TDI treated sisal fibre -PS composite is given in Fig 4.13. In the case of silane treated fibre the R 2 groups of the silane hydrolyse to some extent to form silanols and the resulting groups or R 2 groups provides link to their groups by the formation of hydrogen bonds as discussed earlier.the hydrophobic alkyl groups attached to the fibre as a result of silane
138 Short Sisal Fibre Reinforced Polystyrene Composites 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. Silane treated fiber Si R' Si R' PS Matrix Fig. 4.14 A hypothetical model of interface of silane treated sisal fibre- PS composite The improvement in adhesion between the treated fibre and PS matrix can be 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) (d) (e) (f) Fig 4.15 SEM photographs of the fractured surface of sisal fibre-ps composites (a) untreated fibre (b) benzoylated fibre (c) PSMA treated fibre(d) methyl triethoxy silane treated (e) triethoxy octyl silane treated and (f) TDI treated
140 Short Sisal Fibre Reinforced Polystyrene Composites (a) (b) (c) (d) (e) (f) Fig. 4.16 SEM photographs of the surface of fibre pulled out from composite (a) untreated fibre (b) benzoylated fibre (c) PSMA treated fibre (d) methyl triethoxy silane treated (e) triethoxy octyl silane treated and (f) TDI treated
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. owever, very low adhesion efficiency may result in the lowering of toughness. 30 M206 25 Impact energy(kj/m 2 ) 20 15 10 PS Sm206 Se206 T206 U206 5 B206 0 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
142 Short Sisal Fibre Reinforced Polystyrene Composites 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 composites 17-19.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 system 21. 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. 100 M206 Se206 Flexural strength(mpa) 80 B206 U206 T206 Sm206 60 PS 40 Fig. 4.18 Variation of flexural strength of sisal fibre PS composites as a function of fibre modification Sm206 Se206 6000 M206 U206 Flexural modulus(mpa) 5000 4000 B206 T206 3000 PS Fig. 4.19 Variation of flexural modulus of sisal fibre PS composites as a function of fibre modification
144 Short Sisal Fibre Reinforced Polystyrene Composites 0.032 0.031 M206 Se206 Flexural strain (%) 0.030 0.029 0.028 PS T206 Sm206 U206 B206 0.027 0.026 0.025 Fig. 4.20 Variation of flexural strain of sisal fibre PS composites as a function of fibre modification owever, 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.
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