Filler reinforcement can effectively reduce cost, and enhance mechanical and thermal properties of the recycled plastics. A variety of fillers, such as natural fillers, inorganic fillers and elastomers, have been successfully applied to reinforce recycled plastics.
Natural fillers have substantial inherent advantages, such as low density, low abrasion of processing equipment, relatively good specific mechanical properties, biodegradability and low cost. The natural fillers, including wood, bagasse, phor- mium tenax and wheat straw have been widely used. The natural filler-based recycled polyolefins now have been used in construction applications, such as decking and furniture, and automotive ones, including door panels and seat frames. However, incompatibility, processability and thermal degradation are main problems which limit applications of the natural fillers in the recycled polyolefins (Mengeloglu and Karakus 2008). Extensive research has been carried out to solve these problems and improve mechanical properties of the composites.
The addition of inorganic fillers to plastics results in remarkable improvements in morphological, mechanical, rheological and thermal properties. Unlike the natural fillers, the properties can be significantly improved even upon using small amount of inorganic fillers. Various inorganic fibres have been developed and widely used in the plastic reprocessing industry. They are calcium carbonate, mica, glass beads, glass fibre, talc, silver, zinc oxide, titanium dioxide, cement, fly ash, and clays. The properties of inorganic filler reinforced recycled polyolefins strongly depend on size, shape, and distribution of the fillers in the plastic matrix, and also to the extent of interfacial adhesion between the fillers and the matrix.
Elastomer fillers can effectively improve toughness and impact resistance, but have side effects of decreasing tensile strength and Young’s modulus (Clemons 2010). Elastomers, such as poly(ethylene-co-propylene) rubber (EPR), poly (ethylene-co-octene) rubber (EOR), and poly(ethylene-co-propylene-co-diene monomer) (EPDM) are often used to improve the toughness and elongation at break of recycled materials, but the yield strength, tensile strength and Young’s modulus would decrease (Liu et al. 2010). Therefore, sometimes inorganic fillers are used as well to compensate the side effect of the elastomers.