ReviewUse of recycled plastics in wood plastic composites – A review
Graphical abstract
Introduction
Recycled and waste thermoplastics are some of the major components of global municipal solid waste (MSW) and they present a promising raw material source for WPCs, especially because of the large volume and low cost of these materials. Table 1 shows that waste plastics account for 11.2% of the annual 84.2 thousands tons of the municipal waste stream generated in Tehran during 2006 (Ashori, 2008) and 12.4% of 250 million tons in the USA during 2010 (EPA, 2011).
As given in Table 2, high density polyethylene (HDPE), low density polyethylene (LDPE/LLDPE), polypropylene (PP), Polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC) are the primary constituents of plastics in MSW. The blend of the mixed waste plastics can be changed depending on the regional habits and seasons of a year and on the mode of waste collection, also (Chanda and Roy, 2007). Table 2 also shows that only 7.6% of 31 million tons (12.4% of MSW) of generated waste plastics were recycled.
Reutilizing the post-consumed polymeric materials reduces the environmental impact and the consumption of virgin plastics. Most single polymer plastics made from petroleum are relatively easy to recycle. Therefore, with an efficient collection, separation and recycling system, discarded plastics can be recycled into new products with only the addition of energy (Jayaraman and Bhattacharya, 2004).
Products manufactured from waste plastics for use are increasing, including floor carpets, flower vases, waste paper baskets, park benches, picnic tables (DeWeese, 1998) and plastic lumber (Dutta et al., 1994). Also, recycled plastics can be used in wood plastic composites (WPCs), which use will provide an additional market for recycled plastics. Trex, the largest supplier of wood-plastic composite lumber, purchases an average of over 227,000 kg of plastic scrap each day (Principia Partners, 2002). Winandy et al. (2004) listed some wood plastic composite products using recycled plastics, produced commercially in the USA.
Generally in WPC manufacturing, virgin thermoplastic polymers are widely used. The most prevalent polymers are PE, PP, PVC and PS. The waste and recycled plastics have been used for manufacturing WPCs already in 1990s and the use has significantly increased in the developed and developing countries in recent years. This paper presents a detailed review about waste and recycled plastics and the research published on the effect of recycled plastic on the physical and mechanical properties of WPCs.
Section snippets
Properties of waste and recycled plastics
If the recycled plastics are considered as new materials in WPCs production, it is necessary primarily to understand well the elemental and fundamental structure of these materials. By knowing the properties of recycled plastics, the processes for manufacturing WPCs can be well controlled and then the relationship between the properties of recycled plastics and their mechanical aspects can be better understood as well as those of the resulted WPC products.
At the end of the first life cycle of
Use of waste plastic in WPCs
The studies of WPCs based on recycled thermoplastics are limited. Most of the studies have been focused on the use of either a single type of waste plastic (for example, the milk bottle) or a combination of recycled and virgin plastics to produce the WPCs, though some also on using recycled plastic blends. However, the impact of recycled or waste thermoplastics in WPCs is still not fully understood, leaving open research opportunities for the optimization of product properties. There are
Conclusion
It has been shown that recycled thermoplastics have great potential for WPCs manufacturing. Because recycled plastics are probably obtained from various sources, exposed to different storage and reprocessing conditions, they may show different performance depending on their degradation level. The degradation of plastics due to repeated processing cycles and environmental exposure complicates recycling, so focused researches are required. Therefore, the effect of the degradation level of
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