Studies on mechanical performance of biofibre/glass reinforced polyester hybrid composites
Introduction
Natural fibre composites (bio-based composite materials) are mainly price-driven commodity composites that have useable structural properties at relatively low cost. The manufacture, use and removal of traditional composite structures usually made of glass, carbon and aramid fibres are considered critically because of the growing environmental consciousness 1, 2. Advantages of natural fibres over traditional reinforcing fibres such as glass and carbon are: low cost, low density, acceptable specific properties, ease of separation, enhanced energy recovery, CO2 sequesterization and biodegradability. There is a growing interest in the use of natural/biofibres as reinforcing components for thermoplastics and thermosets. Although thermoplastics have the added advantage of recycling possibilities; thermosets are targeted to obtain much improved mechanical properties as compared to thermoplastics in the resulting bio-composites.
The use of reinforced thermoset composites [3] by automakers has nearly doubled in the last decade, and is expected to increase 47% during the next 5 years through 2004. The majority of resins used in the composite industry are thermosets. About 65% of all composites produced currently for various applications; use glass fibre and polyester or vinyl ester resins. Unsaturated polyester (USP) resins are widely used, thanks to a relatively low price, ease of handling and a good balance of mechanical, electrical and chemical properties. Natural fibre reinforced polyester composites have received much commercial success in the infrastructure area primarily for low cost housing applications. The engine and transmission covers of Mecedes-Benz transit buses now contain polyester resin reinforced with natural fibre [4].
The development of composite materials based on the reinforcement of two or more fibres in a single matrix which leads to the development of hybrid composites with a great diversity of material properties is still in its infancy. Research revealed that the behavior of hybrid composites appears to be simply a weighted sum of the individual components in which there is a more favorable balance between the advantages and disadvantages inherent in any composite material. It is generally accepted that the properties of hybrid composite are controlled by factors such as nature of matrix; nature, length and relative composition of the reinforcements; fibre–matrix interface; and hybrid design etc.
Several advantages of natural fibre incorporated hybrid composites are reported earlier [5]. Sisal and glass fibres are good examples of hybrid composites 6, 7, 8, 9, 10 possessing very good combined properties. For sisal/glass fibre reinforced LDPE (low density polyethylene) composites, the effects of fibre orientation, composition and fibre surface treatment on the mechanical properties have been studied. Due to superior properties of glass fibres, the mechanical properties of the hybrid composites increase with increasing volume fraction of glass fibres. Yang et al. [10] studied the mechanical and interface properties of sisal/glass fibre reinforced poly (vinyl chloride), PVC hybrid composites before and after immersion in water. It has been found that there exists a ‘positive’ hybrid effect for the flexural modulus and unnotched impact strength. They also suggested that water might have a detrimental effect on the fibre–matrix interface leading to reduced properties. The mechanical properties of sisal-saw dust hybrid fibre composites with phenol-formaldehyde resin have been studied by Thomas et al. [11] as a function of sisal fibre loading. It has been seen that mechanical properties (tensile and flexural) increase with sisal fibre content. This is due to the fact that the sisal fibre possesses moderately higher strength and modulus than saw dust.
Pineapple leaf fibres (PALF) and sisal fibres possess moderately high specific strengths and stiffness and can be used as reinforcements in polymeric resin matrices to make useful structural composite materials. Lack of good interfacial adhesion between the fibre and the matrix and poor resistance to moisture absorption make the use of biofibre reinforced composite materials less attractive. A review by Bledzki and Gassan [12] covers the complexities involved with the compatibilization of cellulose based fibres and different polymeric matrix. Recently our research group reported on the effect of different chemical modifications on physico-mechanical properties of PALF-polyester [13] and, sisal-polyester [14] biocomposites.
The present paper describes the changes in the tensile, flexural and impact properties of PALF/glass and sisal/glass fibre hybrid polyester composites as a function of relative weight fractions of the two fibres. In addition to this, sisal/glass hybrid polyester composites of alkali treated, cyanoethylated and acetylated sisal fibre have been fabricated and their mechanical properties have been assessed. The effect of glass fiber hybridization on water absorption tendencies of the biofibre-reinforced polyester composites has also been studied.
Section snippets
Materials
Pineapple leaf fibres (PALF) are obtained from South India Textile Research Association (SITRA), Coimbatore, India and sisal fibres are collected from Keonjhar, Orissa, India. Glass fibres in the woven mat form are supplied by CEAT, India. Chemicals and solvents are of analar grade and are used without further purification. General- purpose polyester resin (FB-333), methyl ethyl ketone peroxide (catalyst) and cobalt naphthenate (accelerator) are supplied by Ruia Chemicals Pvt. Ltd., Calcutta,
Tensile strength
Important physical and mechanical properties of PALF, sisal, glass and polyester matrix are shown in Table 1. Fig. 1 shows the variation of tensile strength with glass fibre loading in PALF/glass hybrid polyester composite. The total fibre (PALF and glass) loading of the composite is maintained at 25 wt.%. The glass fibre addition varied from 0 to 12.9 wt.%. The figure clearly indicates that the tensile strength of PALF-polyester composite is significantly improved by the incorporation of glass
Conclusions
In this paper, mechanical properties of PALF/glass and sisal/glass fibre reinforced polyester composites have been described. The tensile, flexural and impact properties of PALF and sisal reinforced polyester composites are observed to have improved by the incorporation of small amount of glass fibres in these composites, showing positive hybrid effect. Optimum glass fibre loadings for PALF/glass hybrid polyester and sisal/glass hybrid polyester composites are 8.6 and 5.7 wt.% respectively
Acknowledgements
A.K. Mohanty is thankful to the Department of Science and Technology, Government of India for financial support in the form of sanctioning a Research Project to carry out the present investigation. S. Mishra is thankful for the Junior Research Fellowship in the said Research Project. Help rendered by CIPET, Bhubaneswar, India and Department of Geology, Lucknow University, Lucknow, India is gratefully acknowledged.
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