Fiber reinforced composites (FRCs) have been used for a long time in structural and semi-structural applications. FRCs have the advantage of lightweight and best property performance compared to traditional materials such as metals. In several instances, disposability of such products becomes a major issue. There has been increasing demand for use of recyclable and or biodegradable composites for automotives, especially due to the recent European Union directives. With the growth of automobiles in the global market, and a simultaneous pressure to address the issue of sustainability, there is continual need for the incorporation of natural fiber based materials into automotives. The focus of this research has been to produce biodegradable cotton fiber-based composites that can be safely disposed off after their intended use without polluting the atmosphere, in an environmentally safe manner.
This research deals with cotton-based nonwovens using blends of cotton, flax, kenaf and a biodegradable thermoplastic fiber. Biomax®, Polylactic Acid (PLA), Polyvinyl Acetate (PVAc), and Eastar®bio-copolyester (PTAT) are the chosen thermoplastic fibers that could function as the binders, thus eliminating the use of any non-biodegradable synthetic fiber such as Polypropylene (PP) or a chemical binder. The process involves the fabrication of nonwovens from blends of fibers in different proportions made by air laying or carding to form webs, molding these webs into composites, and subsequent characterization of the composites for their properties such as tensile strength, flexural strength, and acoustic properties. Results from these studies addressing the structure and properties of the composites, contribution from individual constituents, with respect to their suitability for automotive applications are discussed.
Basic studies on structure and properties of fibers showed the ability of these natural fibers to form a good bond between thermoplastic polymer such as Eastar, Biomax, and Cellulose Acetate. Fiber bonding studies reinforced this observation. Comparison of Sandwich type composites with Fiber mix type composites showed that the bonding between natural fibers and the binder polymer is better when composites are made from mixed fiber webs. Furthermore, intimate blending is the key to make a composite with good properties.
Biodegradable composites were developed from air laid webs of natural fibers (cotton, flax, and kenaf) and binder fibers (Biomax, PLA, and PVAc) by thermal bonding in a hot press. It proved that blending of flax and kenaf increases the tensile strength of the cotton composites. Further, Three point bending test showed that PLA based cotton composites have slightly lower flexural strength compared to conventional PP. Adding about 10% kenaf or flax increases flexural strength substantially, indicating that kenaf and flax act like stiffeners. Acoustics properties of the composites measured by Four point Impedance Tube method showed that blending kenaf or flax increases noise absorption quality of cotton-PLA composites. Notched Izod impact tests showed that the impact strength of PLA and PLAbico binders is higher than that of PP. Moreover blending kenaf or flax increases the impact strength of the composites substantially. Impact strength increases as the composite thickness is raised keeping same basis weight.
Comparison of binders, Biomax, PLA, and PVAc fibers in a natural fiber composite showed that PVAc provides more tensile strength and elongation to the cotton or flax rich composite, where as PLA performs similarly in kenaf rich composites. Biomax performance is very close to that of PVAc. In other words, PVAc and Biomax form better composites with cotton and flax than PLA. If PVAc stands out for its superior performance in composites containing more cotton or flax, PLA stands out for the similar performance at lower curing temperature that reduces the bad odor in composites and has processing conditions close to conventional PP. The main advantage of Biomax is its lower cost compared to both PLA and PVAc.
Process optimization studies showed that there is an optimum bonding temperature and optimum-curing time for composites. Tensile strength increases as the curing pressure or basis weight/ thickness increases. Increase in tensile strength achieved by blending kenaf or flax (even at 10% level) is substantial. However there is marginal drop in elongation.
Further on the basis of these studies, it is expected that viable composite parts containing cotton and other natural fibers can be produced with a thermoplastic binder fiber, that are biodegradable and possess the required properties that are comparable to the traditional polypropylene based composites. Such composites are suitable for automotive and many other semi-structural applications
