Natural fibres have been used to reinforce materials for over 3,000 years. More recently they have been employed in combination with plastics. Many types of natural fibres have been investigated for use in plastics including Flax, hemp, jute, straw, wood fibre, rice husks, wheat, barley, oats, rye, cane (sugar and bamboo), grass reeds, ramie, oil palm empty fruit bunch, sisal, coir, water hyacinth, pennywort, kapok, paper-mulberry, banana fibre, pineapple leaf fibre and papyrus. Natural fibres have the advantage that they are renewable resources and have marketing appeal.
The Asian markets have been using natural fibres for many years e.g., jute is a common reinforcement in India. Natural fibres are increasingly used in automotive and packaging materials. Bangladesh is an agricultural country. Thousands of tons of different crops are produced but most of their wastes do not have any useful utilization.
Agricultural wastes include wheat husk, rice husk, and their straw, hemp fibre and shells of various dry fruits. These agricultural wastes can be used to prepare fibre reinforced polymer composites for commercial use. This report examines the different types of fibres available and the current status of research.
Natural fibres, as reinforcement, have recently attracted the consideration of researchers because of their compensations over other established materials. They are environmentally friendly, fully biodegradable, copiously available, renewable and economy and have low density. Plant fibres are light compared to glass, carbon and aramid fibres.
The biodegradability of plant fibres can contribute to a healthy ecosystem while their low cost and high performance accomplishes the economic interest of industry. When natural fibre reinforced plastics are subjected, at the end of their life cycle, to combustion process or landfill, the released amount of CO2 of the fibres is neutral with respect to the assimilated amount during their growth. The abrasive nature of fibre is much lower which leads to advantages in regard to technical process and recycling process of the composite materials in general.
Natural fibre-reinforced plastics, by using decomposable polymers as matrices, are the most environmental friendly materials, which can be self-possessed at the end of their life cycle. Natural fibre composites are used in place of glass mostly in non-structural applications. A number of self-propelled components previously made with glass fibre composites are now being manufactured using ecologically friendly composites.
Although natural fibres and their mixtures are environmental friendly and renewable (unlike traditional sources of energy, i.e., coal, oil and gas), these have several restricted access. These have: poor wettability, incompatibility with some polymeric matrices and high moisture absorption. Composite materials made with the use of unchanged plant fibres frequently exhibit inadequate mechanical properties.
To overcome this, in many cases, a surface treatment or companionable agents need to be used prior to composite fabrication. The properties can be improved both by physical treatments (cold plasma treatment, corona treatment) and chemical treatments (maleic anhydride organosilanes, isocyanates, sodium hydroxide permanganate and peroxide). Mechanical properties of natural fibres are much lower than those of glass fibres but their specific properties, especially stiffness, are comparable to the glass fibres.
Chemical modifications of natural fibre: One of the major problems related with the use of natural fibres in composites is their high moisture sensitivity leading to severe reduction of mechanical properties and delamination. The reduction in mechanical properties may be due to poor interfacial attachment between resin matrices and fibres. It is therefore necessary to transform the fibre surface to render it more hydrophobic and also more harmonious with resin matrices.
An effective method of chemical alteration of natural fibres is implant copolymerisation. The resulting co-polymer exhibits the characteristics of both fibrous cellulose and grafted polymer. One of the most explored chemical modifications is the acetylation-esterification of cellulose-OH, by reaction with acetic anhydride.
This reaction reduces hydrophobicity and swelling of lingo-cellulose and their mixtures. The effect of chemical treatment of natural fibres with sodium alginate and sodium hydroxide has also been reported for coir, banana and sisal fibres. This modification results in an increase in adhesive bonding and thus increases ultimate tensile strength up to 30%. The chemical modification of pineapple leaf fibres using alkali treatment, di-azo coupling with aniline and cross-linking with formaldehyde.
Physical surface treatment methods: The use of different kinds of physical surface treatment methods (i.e. corona discharge, cold plasma) leads to changes in the surface structure of the fibres, as well as to changes in the surface energy. The treatment of rayon fibres with oxygen plasma results in increasing the total and polar part of the free surface energy with increasing treatment time, because of the increased O/C ratio. The wettability of wood is improved with increasing level of corona treatment. In the case of wood, surface activation increases the amount of aldehyde groups.
Special types of composites based on natural fibres: In general, the mechanical and physical properties of natural fibre reinforced plastics only provisionally reach the characteristic values of glass-fibre reinforced systems. By using hybrid composites, made of natural fibres and carbon fibres or natural fibres and glass fibres, the properties of natural fibre reinforced composites can be enhanced further for compression strength. Natural fibre composites have been assessed with regard to their anti-ballistic physiognomies. Many researchers have investigated the response of composite materials to ballistic impact.
Recently, D’Almeida investigated ballistic impact damage of glass fibre reinforced epoxy composites, while Hasur reported on the response of carbon/epoxy composites under high velocity impact. Hine studied the energy absorption of woven nylon and aramid composites and UHMWPE (ultra-high molecular weight polyethylene).
Cantwel and Villanueva investigated the failure of fibre metal laminate (FML) reinforced aluminium foam sandwich structures at high velocity impact. Research on ballistic impact has been fixated only on the high performance fibres, metal and ceramics and now attempts have been made to study the performance of national fibre compound under ballistic impact. Wambua bridged the gap and investigated the response of flax, hemp, and jute fabric reinforced polypropylene composites to ballistic impact by fragment simulating projectiles.
Technical applications of natural fibre reinforced composites: Natural fibres are substituting artificial fibres as reinforcement in various matrices. The composites can efficiently be used as supernumerary for wood and also in various other practical fields, e.g. automotive parts.
Seventy years ago, nearly all resources for the production of commodities and many methodological products were materials derived from natural textiles. Textiles, ropes, canvas and also paper, were made of local natural fibres. As early as 1908, the first composite materials were applied for the fabrication of huge quantities of sheets, tubes and pipes for electronic purposes.
For example in 1996, aeroplane seats and fuel tanks were prepared of natural fibres with small content of polymeric binders. The last period has seen a diversity of applications of natural fibre composites due to their inspiring properties such as biodegradability and high specific properties.
Presently, a revolution in the use of natural fibres, as reinforcements in technical application, is taking place mainly in the automobile and packaging industries (e.g., egg boxes). In the automotive industry, textile waste has been used for years to reinforce plastics used in cars. The use of natural fibres within composite applications is being chased extensively throughout the world.
Consequently, natural fibre composite materials are being used for making many apparatuses in the automotive sector. These resources are based largely on polypropylene or polyester matrices, incorporating fibres such as flax, hemp, and jute. Thus in the upcoming cars may be moulded from cashew nut oil and hemp.
Even golf clubs may be constructed around jute fibres, and tennis racket may be stiffened with coconut hair. Bicycle frames may derive their strength from any one of the 2000 other appropriate plants. The high-tech revolution in use of natural fibres could end in replacement of artificial materials.
The diverse range of products now being produced, utilizing natural fibres and bio based resins derived from soybeans, is giving life to a new age group of bio-based composites for a number of applications. These include not only automotive vehicles (including trucking) but also hurricane-resistant housing and structures, especially in the United States.
The construction sector and the leisure industry are some of the other areas where these novel materials have discovered a market. In Germany, car manufactures are aiming to make every component of their vehicles either recyclable or biodegradable.
Natural fibres, when used as reinforcement, contest with such technical fibres as glass fibre. The compensations of technical fibres are good mechanical possessions; which vary only little, while their drawback is trouble in recycling. Several natural fibre composites reach the mechanical properties of glass fibre composites, and they are already functional, e.g., in automobile and furniture industries. Natural Fibres are renewable raw materials and they are recyclable.