Mechanical Reprocessing of Polypropylene Waste
Thermoplastic waste from disposable consumer packaging and products is increasing, elevating the environmental pollution and wasting useful resources. Polypropylene (PP) is one of major types of thermoplastics used throughout the world in a wide variety of applications, such as toys, containers, pipes, automotive parts, electrical components, etc. In Australia, 0.23 million tonnes of PP products were consumed in 2013, while the recycling rate was only 24% (i.e. 0.056 million tonnes) (A’Vard and Allan 2014). Legislations have been introduced around the world to limit disposal of the plastic wastes and to encourage more environmentally friendly options, to control plastic pollution (Achilias et al. 2008). Efficient recycling and recovery methods are therefore being researched and developed. According to Australian National Plastics Recycling Survey (A’Vard and Allan 2014), mechanical recycling is the most widely practiced of these methods in Australia, since it is relatively easy and economical. Technology and infrastructure required for collection and mechanical reprocessing of plastic waste is also widely available.
Mechanical recycling refers to reprocessing plastic waste into secondary raw materials and products by physical means. The mechanical recycling involves a series of treatments and preparation steps (Yin et al. 2015c). Generally, the first stage of recycling process includes collecting, sorting, shredding, milling, washing and drying the plastic waste into recycled plastic pellets, powder or flakes (Al-Salem et al. 2009). Extensive research has been carried out in this stage, especially in terms of collection and sorting techniques (Carvalho et al. 2012), such as flotation, optical sorting, density separation, electrical separation, etc. In the second stage the recycled plastic pellets, powder or flakes are molten and reprocessed into final products by resin moulding techniques (Demirel and Daver 2013), including extrusion moulding, injection moulding, blow moulding, vacuum moulding, inflation moulding, etc.
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S. Yin, Development of Recycled Polypropylene Plastic Fibres to Reinforce Concrete, Springer Theses, DOI 10.1007/978-981-10-3719-1_2
Mechanically recycled products, however, often have mediocre mechanical properties in practice, which strongly limit their applicability and market demand (Khan et al. 2010). Two factors mainly lead to unsatisfactory performance of recycled plastic products. The first is degradation of the plastic waste (Sanchez et al. 2014). When plastics undergo high temperature or shearing during their processing stage, thermo-mechanical degradation occurs (Mbarek et al. 2006). Moreover, during the service life of plastic products, long exposure to the air, light, moisture, temperature and weathering gives rise to their natural aging. The second factor is heterogeneity of the plastic waste (Brems et al. 2012). Plastic waste is a mixture of various types and grades of polymers with distinct degrees of polymerisation and chemical structures, which are mutually incompatible. Furthermore, contaminants, such as paper scraps and adhesive additives deteriorate the mechanical properties of the recycled polymers and limit their applications. The more complex and contaminated the waste is, the more difficult it is to recycle mechanically. Thus, a full separation of individual components is rarely implemented (Brachet et al. 2008).
In order to improve quality of end products of recycled PP, various workable reprocessing techniques in the second stage of mechanical recycling have been developed and widely applied in the recycling industry (Kabamba and Rodrigue 2008). This chapter critically reviews the current reprocessing techniques of recycled PP. The degradation and crystallisation behaviour accompanying with the reprocessing processes is presented. This would help us compare different reprocessing methods and choose the most suitable one for the production of recycled PP fibre.