Biofiber mechanical properties

The structure and properties of biofibers, mainly of cellulose, were described in this chapter. First, the hierarchy microstructure of natural plant fiber and then a variety of crystal modifications of cellulose were mentioned. The ultimate mechanical properties (modulus of 138 GPa and strength of 17.8 GPa) and thermal properties (thermal expansion coefficient of 10 order) were emphasized as quite excellent for cellulosic fiber, enough for use as reinforcement in the composites. With the manifestation of these intrinsic properties in macroscopic material, the oH-cellulose composite was shown to possess excellent mechanical properties, thermal resistance, and optical transparency, besides being composed of fully sustainable resources and hence, biodegradable. Nowadays, the interest in cellulosic nanocomposites has increased considerably [60, 61] and they are expected to be used in many fields such as electronic devices, vehicles, and windmills to replace glass and/or carbon fibers. 

Biofiber-reinforced PP composites have received lot of attention because of their light weight, good mechanical properties, recyclability, and environmental-friendly features. The mechanical properties of ramie fiber-reinforced PP composites... 

In order to develop composites with better mechanical properties and environmental performance, it becomes necessary to increase the hydrophobicity of the biofibers and to improve the interface between matrix and biofibers. Graft copolymerization of biofibers is one of the best methods to attain these improvements. As of now, only few studies have reported the use of biofiber graft copolymers as reinforcing material in the preparation of composites [33], Mechanical properties of thermoplastic composites reinforced with acrylate-gro/led henequen cellulose fibers were studied. It has been found that best results could be obtained with poly(methyl methacrylate) (PMMA)-grafted cellulose fibers because of better fiber-matrix adhesion. The modulus of poly(vinyl chloride) (PVC) composites is increased when grafted or ungrafted cellulose are used as reinforcement but the composites with... 

S. 5 Biofiber-Reinforced Rubber Composites 295 Table 8.4 Mechanical properties of vulcanizates with various fiber lengths [7]. 

Diffusion and swelling studies of biofiber-reinforced rubber composites are very important as they provide information about its interface and the service performance in liquid environment. On absorption of liquid, the composite undergoes a diffusion process, which involves transfer of Hquid to its interior. Hence, the material swells as a whole, which leads to gradual deterioration in its physical and mechanical properties and subsequent premature failure. Several factors such as chemical structure and composition of the rubber compound, solvent, test piece size and shape, rubber-liquid ratio, temperature, time, presence of fillers affect the swelling behavior of elastomers. 

Blending different biofibers could result in biocomposites with balanced properties. Engineered biofibers, defined as the suitable blend of surface-treated bast and leaf fibers as shown in Figure 13.8, have been studied and reported. By manipulating the blend ratio of biofibers, an optimum balance in mechanical properties of the resulted biocomposites could be attained. For example, kenaf- and/or hemp-based composites exhibit excellent tensile and flexural properties, while leaf fiber (PALF) composites have high impact properties. The combination of bast and leaf fibers is expected to achieve a balance of flexural and impact properties of the targeted biocomposites [49]. 

The mechanical properties of biofibers as compared to conventional synthetic fibers are shown in Table 10.3 and it can be seen that the biofibers compare well with glass fibers, but are not as strong as either aramid or carbon fibers. However, the density of glass fibers is higher than those of biofibers. Therefore, in terms of specific properties, some biofibers are comparable to glass fibers on stiffness basis. They, however, have lower specific tensile strength than glass fibers. 

Alkalization is the process of subjecting a biofiber to the action of a fairly concentrated aqueous solution of a strong base (alkali] so as to produce great swelling with resultant changes in the fiber structure, dimension, morphology, and mechanical properties. It depends on the type and concentration of the alkaline solution, its temperature, time of treatment, tension of the material as well as the additives used. Most of the non-cellulosic components... 

Incorporation of biofibers in biopol5miers results in biocomposites with improved properties. Because of huge abundance of data available on the properties of biocomposites, it will not be possible to cover them all. For this reason, we shall take one most widely used biopol5nner from each of the three classes of biopol5miers and discuss the properties of their biocomposites. The biopol5miers selected for this purpose are starch, PLA, and PHB. Table 10.4 presents a comparison of physical and mechanical properties of these three types of biopolymers with a commodity thermoplastic PP. It is evident that the density and mechanical properties of biopolymers are comparable to those of PP, although their low thermal stability is a matter of concern. 

Biofiber-reinforced PLA composites are now among the most well-established biocomposites in the industry. Some of the examples of the products made from these biocomposites are urns, mobile phone shells and spare tire covers in automobiles. They are also the most widely researched of all biocomposites as is evident from Table 10.5, which lists mechanical properties of a selection of such composites. 

Starch is one of the most widely used biopolymer in biocomposites because of its low cost and versatility. A plasticizer like glycol is sometimes used to make it suitable for processing. It is also blended with other polymers like aliphatic polyesters to improve its physical and mechanical properties. Biocomposites based on starch matrices show improved properties, which are comparable to E-glass/epoxy composites. Tensile, flexural, impact, and creep properties of these biocomposites are significantly better than those of neat starch. Various biofiber surface treatments have been shown to improve the properties of starch-based biocomposites. 

PLA is another thermoplastic biopolymer that has the potential to replace commodity polymers such as PET, PS, and PC in everyday applications as well as a matrix for biocomposites. The mechanical properties of PLA are similar or in most cases are superior to petrochemical polymers, such as PR PLA is also blended with other polymers to improve its properties, particularly its lower thermal stability. PLA-based biocomposites are the most comprehensively researched of all the biocomposites. These biocomposites also exhibit excellent mechanical properties. Surface treatments of biofibers have positive effects on the properties of these composites. 

Recently, Flores-Hernandez et al. (2014) developed a green composite chitosan-starch as matrix and keratin biofibers as reinforcement. Keratin biofibers from feathers are non-abrasive, eco-friendly, biodegradable, renewable, and insoluble in organic solvents and also have good mechanical properties, low density, hydrophobic behavior, ability to dampen sound, warmth retention, and finally low cost (Meyers et al., 2008 Martinez-Hernandez and Velasco-Santos, 2011). Flores-Hernandez etal. (2014) developed the ecocomposites using three different kinds of keratin reinforcement short and long biofibers and rachis particles. These were added separately at different concentrations to the chitosan-starch matrix and the composites were processed by a casting/solvent evaporation method. 

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