Natural fibers for biocomposites
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Introduction Nature uses composite constructions to generate an amazing array of materials with tailored properties for specific applications. Although much can be learned from the intricate way nature combines components, which often in themselves have poor or limited mechanical properties, into materials with impressive properties, the development of biocomposites attempts to unravel nature’s complex system to obtain biofibers for use in conventional composites technologies. Biofibers can be used in short fiber applications such as thermoplastic injection moulding or can be made into blown or woven mats and cloth for use with thermosetting resins. This review will concentrate on lignocellulose fibers, fibers extracted from plant materials consisting largely of cellulose but with varying amounts of other substances such as lignin, pectin, waxes, and extractives. Lignocellulose fibers are the most commonly used natural fiber reinforcement in biocomposites. Chitin1 and cellulose produced from bacteria2 are also receiving some research attention, but this is largely as sources of nano-reinforcements. Nano-cellulose fiber composites are reviewed briefly in a subsequent section. The full potential for natural fibers in biocomposites is yet to be achieved, but they show potential because they are sustainable, have potentially high specific stiffness and strength, and produce less wear than glass fibers in the processing of thermoplastic composites.
It should be recognized that the term “biocomposite” is used for a variety of different composite materials in which either the matrix or reinforcement or both are a biomaterial or a biosourced material. Consequently, the term can cover a material containing bioresin + biofibers, bi-sourced resin + biofibers, synthetic resins + biofibers, bioresin or biosource resin with synthetic reinforcement, or more complex mixtures such as those found in nature. Although most synthesized biocomposites currently use lignocellulose fibers and a synthetic polymer matrix, there is growing interest in replacing the synthetic matrix with bioresins or biosourced resins.
Lignocellulose biofibers Modern materials engineers are attracted to the high stiffness of the primary cellulose crystal. The theoretical modulus of a cellulose crystal has been given as 250 GPa,3 which compares favorably with carbon fiber and other high-technology reinforcements. However, in plant fibers, the high modulus cellulose crystals are combined with materials of much lower modulus. Moreover, the cellulose crystals are often aligned at an angle to the axis of the plant fiber. Both factors reduce the overall modulus of the plant fiber. Table I shows indicative values for the modulus of a number of natural lignocellulose fibers, and it can be seen that the modulus varies from 5 to
Rowan W. Truss, School of Mechanical and Mining Engineering, and School of Chemical Engineering, The University of Queensland, Brisbane; [email protected] DOI: 10.1557/mrs.2011.207
© 2011 Materials Research Society
MRS BULLETIN • VOLUME 36 • SEPTEMBER 2011 • www.m
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