Mechanical Reprocessing of Polyolefin Waste: A Review Shi Yin, 1 Rabin Tuladhar, 1 Feng Shi, 2 Robert A. Shanks, 3 Mark Combe, 4 Tony Collister 4 1 College of Science, Technology & Engineering, James Cook University, Queensland 4811, Australia 2 School of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 100000, China 3 School of Applied Sciences, RMIT University, Melbourne, Victoria 3001, Australia 4 Fibercon, Queensland 4051, Australia Efficient technology and applications for recycled polymer waste has become increasingly important to decrease environmental contamination and to conserve nonrenew- able fossil fuels. Mechanical recycling is the most widely practiced in Australia, since it is relatively easy and eco- nomic; and moreover, infrastructure for collection and reprocessing has been well established. In order to improve quality of end products of recycled plastics, vari- ous workable reprocessing techniques in the second stage of mechanical recycling have been developed and widely applied in the recycling industry. This article crit- ically reviews the current reprocessing techniques of recycled polyolefins. Reprocessing recycled polyolefins is always accompanied with degradation, crystallization, and consequent processability problems, which result from molecular chain scission, branching, and crosslink- ing. The present state of knowledge and technology of various reprocessing techniques, including melt blending, filler reinforcement and mechanochemistry, is then described and evaluated systematically. Each reprocess- ing technique presents its own individual advantages and special applications. POLYM. ENG. SCI., 55:2899–2909, 2015. V C 2015 Society of Plastics Engineers INTRODUCTION Thermoplastic waste from disposable consumer packaging and products is increasing, elevating the environmental pollution and wasting useful resources. According to the surveys done by United States Environmental Protection Agency [1], plastic waste accounted for less than 1%Á(w/w) of municipal solid waste stream in the 1960s, which considerably increased to 12.7%Á(w/w) in 2012. Plastic bottles and plastic bags become increasingly prevailing and have substituted glass bottles, metal containers and paper bags. In Australia, 1.5 million tonnes of plastics were consumed in 2013, while the recycling rate was only 20% (i.e., 0.3 million tonnes) [2]. Polyolefins, mainly low-density polyethylene (LDPE), high- density polyethylene (HDPE), and polypropylene (PP), are major types of thermoplastics used throughout the world in a wide variety of applications, such as grocery bags [3], toys, con- tainers [4], pipes [5], industrial wrappings and film [6], automo- tive parts [7], electrical components [8], etc. In Australia, a total of 0.8 million tonnes of polyolefins were consumed in 2013, which is 53% of the total plastics consumption in Australia. Approximately 47% of polyolefins products have duration of life shorter than one month, leading to a vast waste stream [2]. Legislations have been introduced around the world to limit disposal of the plastic wastes and to encourage more environ- mentally friendly options, to control plastic pollution [9]. Effi- cient recycling and recovery methods are therefore being researched and developed. The major categories include reuse (primary) [10], mechanical recycling (secondary) [11], chemical recycling (tertiary) [12], and energy recovery (quaternary) [13]. Depending on contamination and degradation conditions of the plastic waste, different methods can be chosen. Each method has inherent context, application, and specific recycling advantages. Reuse is usually a preferable end of life option, however it is commonly applied in the processing line itself but rarely applied among recyclers, as recycling materials rarely possess the required quality [14]. Chemical recycling is the conversion of polymers back into a monomer or new raw materials by chang- ing their chemical structures [13]. Chemical recycling is, how- ever, not yet common in Australia. Energy recovery refers to recovering energy from plastics through controlled combustion or conversion to a liquid fuel (which is subsequently burnt) [13]. There is now some controlled combustion occurring in Australia, however, there is no known conversion to liquid fuels. According to Australian National Plastics Recycling Survey [2], 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 prepa- ration steps [15]. 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 [16]. Extensive research has been carried out in this stage, especially in terms of collection and sorting techniques, such as flotation [17], optical sorting, density separation [18], electrical separation [19], etc. In the second stage, the recycled plastic pellets, powder or flakes are molten and reprocessed into final products by resin molding techniques [20], including extru- sion molding [21], injection molding [22], blow molding [23], vacuum molding [24], inflation molding [25], melt spinning [26], etc. Mechanically recycled products, however, often have medio- cre mechanical properties in practice, which strongly limit their applicability and market demand [27]. Two factors mainly lead to unsatisfactory performance of recycled plastic products. The Correspondence to: R. Tuladhar; e-mail: rabin.tuladhar@jcu.edu.au DOI 10.1002/pen.24182 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2015 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2015