Jute fibre due to its adequate tensile strength and good specific modulus enjoys the right potential for usage in composites.The composite technology with a polymeric matrix reinforced with man-made fibres such as glass, Kevlar, carbon, etc has come of age especially with the advances in aerospace applications since 1950s. The developments in composite material after meeting the challenges of aerospace sector have cascaded down for catering to domestic and industrial applications. Composites, the wonder material with light-weight, high strength-to-weight ratio and stiffness properties have come a long way in replacing the conventional materials like metals, woods, etc. The material scientists all over the world focused their attention on natural composites reinforced with jute, sisal, coir, pineapple, etc primarily to cut down the cost of raw materials. The Eastern India has been bestowed with abundant cultivation of jute. The production of processed jute fibre in India has touched 1.44 million tonnes in 1998-99. Jute as a natural fibre has been traditionally used for making twines, ropes, cords, as packaging material in sacks and gunny bags, as carpet-backing and, more recently, as a geo-textile material. But, lately, a major share of its market has been eroded by the advent of synthetic materials, especially polypropylene. In order to save the crop from extinction and to ensure a reasonable return to the farmers, non-traditional outlets have to be explored for the fibre. One such avenue is in the area of fibre-reinforced composites. Such composites can be used as a substitute for timber as well as in a number of less demanding applications. Jute fibre due to its adequate tensile strength and good specific modulus enjoys the right potential for usage in composites. Jute composites can thus ensure a very effective and value-added application avenue for the natural fibre.
Natural fibres such as jute, sisal, banana and coir are grown in many parts of the world. Some of them have aspect ratios (ratio of length to diameter) > 1000 and can be easily woven. These fibres are extensively used for cordage, sacks, fishnets, matting and rope, and as filling for mattresses and cushions (e.g. rubberised coir). Recent reports indicate that plant-based natural fibres can very well be used as reinforcement in polymer composites, replacing to some extent more expensive and non-renewable synthetic fibres such as glass. Cellulosic fibres are obtained from different parts of plants, e.g. jute and ramie are obtained from the stem; sisal, banana and pineapple from the leaf; cotton from the seed; coir from the fruit, and so on. The maximum tensile, impact and flexural strengths for natural fibre reinforced plastic (NFRP) composites reported so far are 104.0 MN/m2 (jute-epoxy), 22.0 kJ/m2 (jute-polyester) and 64.0 MN/m2 (banana-polyester), respectively. The properties of some of the natural fibres are compared in Table 1.
There are many examples of the use of cellulosic fibres in their native condition like sisal, coir jute, banana, palm, flax, cotton, and paper for reinforcement of different thermoplastic and thermosetting materials like phenol-formaldehyde, unsaturated polyester, epoxy, polyethylene, cement, natural rubber, etc. Different geometries of these fibres, both singly and in combination with glass, have been employed for fabrication of uni-axial, bi-axial and randomly oriented composites. Amongst these various ligno-cellulosic fibres, jute contains a fairly high proportion of stiff natural cellulose. Rated fibres of jute have three principal chemical constituents, namely, µ-cellulose, hemicellulose and lignin. In addition, they contain minor constituents such as fats and waxes, inorganic (mineral) matter, nitrogenous matter and traces of pigments like b-carotene and xanthophyll.
Although the tensile strength and Youngs modulus of jute are lower than those of glass fibres, the specific modulus of jute fibre is superior to that of glass and on a modulus per cost basis, jute is far superior. The specific strength per unit cost of jute, too, approaches that of glass. Therefore, where high strength is not a priority, jute may be used to fully or partially replace glass fibre without entailing the introduction of new techniques of composite fabrication. The need for using jute fibres in place of the traditional glass fibre partly or fully as reinforcing agents in composites stems from its lower specific gravity (1.29) and higher specific modulus (40 Gpa) of jute compared with those of glass (2.5 & 30 Gpa respectively). Apart from much lower cost and renewable nature of jute, much lower energy requirement for the production of jute (only 2% of that for glass) makes it attractive as a reinforcing fibre in composites. The jute composites may be used in everyday applications such as lampshades, suitcases, paperweights, helmets, shower and bath units. They are also used for covers of electrical appliances, pipes, post-boxes, roof tiles, grain storage silos, panels for partition and false ceilings, bio-gas containers, and in the construction of low cost, mobile or pre-fabricated buildings which can be used in times of natural calamities such as floods, cyclones, earthquakes, etc.
Effect of moisture on jute fibres
There is, however, a major drawback associated with the application of jute fibres for reinforcement of resin matrices. Due to presence of hydroxy and other polar groups in various constituents of jute fibre, the moisture uptake is high (approx. 12.5% at 65% relative humidity and 20°C) by dry fibre and 14.6% by wet fibre. All this leads to (i) poor wettability with resin and (ii) weak interfacial bonding between jute fibre and the relatively more hydrophobic matrices. Environmental performance of such composites is generally poor due to delamination under humid conditions. With increase in relative humidity up to 70%, the tenacity and Youngs modulus of jute increases but beyond 70%, a decrease is observed. To reduce the moisture regain property of jute fibre, it is essential to pre-treat the fibre so that the moisture absorption is reduced and the wettability of the resin is improved.
Modification of jute fibre
In order to develop composites with better mechanical properties and environmental performance, it is necessary to impart hydrophobicity to the fibres by chemical reaction with suitable coupling agents or by coating with appropriate resins. Such surface modification of jute fibre would not only decrease moisture adsorption, but would also concomitantly increase wettability of fibres with resin and improve the interfacial bond strength, which are critical factors for obtaining better mechanical properties of composites.
Modification of jute and other natural cellulosic fibres can be done by following means:
- Chemical means
· Coating with polymeric solutions
- Graft copolymerisation.
Jute is chemically treated with isopropyl tri-isostearoyl titanate (abbreviated as titanate), g-aminopropyl trimethoxy silane (abbreviated as silane), sebacoyl chloride (SC), and toluene diisocynate (TDI). All these reagents are expected to block the hydroxy groups of jute, thus making the fibres more hydrophobic. Polymeric coating of jute fibre with phenol-formaldehyde or resorcinol formaldehyde resins by different approaches are highly effective in enhancing the reinforcing character of jute fibre, giving as high as 20-40% improvements in flexural strength and 40-60% improvements in flexural modulus. These modifications improve the fibre-matrix resin wettability and lead to improved bonding. Jute can be graft copolymerised with vinyl monomers such as methyl methacrylate, ethyl acrylate, styrene, vinyl acetate, acrylonitrile and acrylamide in the presence of different redox initiator systems such as vanadium-cyclohexanol, vanadium- cyclohexanone, etc. Grafting of polyacrylonitrile (10-25%) imparts 10-30% improvements in flexural strength and flexural modulus of the composites. Grafting of polymethyl methacrylate is also effective in this respect, though to a lower degree.
Polyester resin forms an intimate bond with jute fibres up to a maximum fibre:resin ratio (volume/volume) of 60:40. At this volume fraction, the Youngs modulus of the composite is approximately 35 GN/m2. For higher volume fraction of fibre, the quantity of resin is insufficient to wet fibres completely. In order to overcome the poor adhesion between resin matrix and jute fibres, a multifunctional resin like polyesteramide polyol has reportedly been used as an interfacial agent. Significant improvement in mechanical properties of jute fibre composites was observed by incoporation of polyesteramide polyol. Also, hybrid composites of glass (facing) and modified jute fibre (core) can be a good alternative.