Polyolefins are one of the largest classes of carbon-chain thermoplastics and elastomers, produced by the polymerization of alkenes (Fig. 8.10), such as polyethylene (PE), polypropylene (PP), polybutene-1 (PB-1), polyisobutylene (PIB), ethylene- propylene-rubber (EPR), etc. Among all polyolefins, polyethylenes and polypropylene have been the most extensively used in many different forms and applications. More interestingly, biopolyethylenes were produced from bioethanol-derived ethylene, and they exhibited similar properties as their petroleum-based counterparts [152].

The low cost and outstanding chemical resistance of polyolefin materials render them highly attractive in the plastics industry. For example, polyolefins or functionalized polyolefins are largely consumed in the manufacture of flexible packaging, grocery bags, and shopping bags [94,153].

As mentioned previously, biopolyolefins, e.g., biopolyethylene or composites, were synthesized with bio-based feedstocks. Castro et al. developed biocomposites of high density polyethylene (HDPE) produced from sugarcane ethanol and ligno- cellulosic curaua fibers [154]. To make them biodegradable, natural polymers, such as starch, were added into polyethylene. For example, biodegradable starch-LDPE films containing 30% starch were produced. Targeting structural applications, Kuciel et al. evaluated properties of a fully bio-based polyethylene composite, in which a biopolyethylene was obtained from sugarcane ethanol. The biopolyethylene

Basic structure of the main polyolefins

Figure 8.10 Basic structure of the main polyolefins: (A) polyethylene (PE); (B) polypropylene (PP); and (C) polystyrene (PS).

was filled with different fillers (25 wt%): wood flour, cellulose powder, kenaf fibers, and tuff particles. Physical, mechanical, and thermal properties, water uptake, and fracture morphology of the biocomposites were evaluated. Compared to the neat biopolyethylene, biocomposites reinforced by various natural fillers resulted in a lower density, increased stiffness, improved resistance to deformation, and better heat resistance [155].

The recent advances in natural fiber-reinforced polyolefin composite materials were comprehensively reviewed by Spiridon [156]. The commonly used natural fibers include bast fibers, leaf fibers, fruit fibers, seed fibers, wood fibers, and other grass fibers. Extrusion is the most common method for processing thermoplastic polyoelfins with natural fibers, where the fiber may be pretreated by chemical reactions and/or physical methods to improve its compatibility with the polyolefin matrix. In addition, other fillers such as waste cardboard, microcrystalline cellulose, and CNC can also be applied into polyolefin biocomposites [157].

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