Potential uses and value-added products derived from waste polystyrene in developing countries: A review

Resources, Conservation and Recycling 107 (2016) 157–165

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Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec

Review

Potential uses and value-added products derived from waste polystyrene in developing countries: A review Nhamo Chaukura a,∗ , Willis Gwenzi b , Tavengwa Bunhu c , Deborah T. Ruziwa d , Innocent Pumure e a

Department of Polymer Science & Engineering, Harare Institute of Technology, PO Box BE 277, Belvedere, Harare, Zimbabwe Department of Soil Science & Agricultural Engineering, University of Zimbabwe, Box MP 167, Mt. Pleasant, Harare, Zimbabwe c Department of Chemistry, Chinhoyi University of Technology, P. Bag 7724, Chinhoyi, Zimbabwe d Harare Polytechnic, PO Box CY 407, Causeway, Harare, Zimbabwe e School of Environmental, Physical and Applied Sciences, University of Central Missourri, 108 W South St, Warrensburg, MO 64093, United States b

a r t i c l e

i n f o

Article history: Received 2 April 2015 Received in revised form 28 October 2015 Accepted 29 October 2015 Keywords: Applications Recycling Waste polystyrene

a b s t r a c t Uses of polystyrene include industrial, packaging and household applications. However, waste polystyrene (WPS) poses serious environmental risks especially in developing countries where disposal facilities are lacking. Information on WPS disposal in these countries is limited. This review therefore (1) presents an overview of the production, uses and current disposal practices for WPS, (2) identifies potential uses of WPS in developing value-added products, and (3) highlights research gaps and proposes future research. In developed countries, WPS is converted into paints, adhesives and flocculants, yet no such documented cases exist in sub-Saharan Africa (SSA). WPS is often disposed of in waste dumps and subsequently burnt, thereby causing air pollution. The review identified several uses of WPS. In SSA, it has been used to control vector-borne diseases like malaria. Potential applications include developing ion-exchange resins for remediation of contaminated water. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4. 5.

6. 7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 Production of polystyrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 2.1. The polymerisation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 2.2. Polystyrene processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 Current applications of polystyrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 Disposal practices of polystyrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 Potential uses of WPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 5.1. Generation of styrene monomer through depolymerisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 5.2. Reprocessing of HIPS and EPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 5.2.1. Reprocessing into polymer-clay nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 5.2.2. High value products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Challenges and risks associated with recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 Proposed future uses of WPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Conclusion and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .163 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (N. Chaukura). http://dx.doi.org/10.1016/j.resconrec.2015.10.031 0921-3449/© 2015 Elsevier B.V. All rights reserved.

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1. Introduction In developed countries, plastic consumption has grown remarkably over the past two or three decades (Mudgal et al., 2013). In the developed countries, limited oil resources are used in producing a variety of plastics for a wide range of products. For most applications, the products have a few months life-cycle and then the vast majority are disposed of through open burning and in nonsanitary waste dumps thereby posing public and environmental health risks (Huysman et al., 2015). In most cases recovery of this plastic waste is not economically feasible. Unlike in the European Union, for example, where there are stringent governmental initiatives for waste management (Allacker et al., 2014), such controls and initiatives are generally less stringent or non-existent in developing countries including those in sub-Sahara Africa (SSA). Industry is increasingly moving towards reusing and reprocessing plastics for economic and environmental reasons, with many companies developing technologies and strategies for recycling plastics. Better still, efforts are being made to produce biodegradable materials from plant materials that may be used in place of plastics (Selke, 2006). Plastics are made from non-renewable resources, and are either non-biodegradable or the biodegradation process is slow. Consequently, plastic litter is usually the most obnoxious kind of persistent litter and reduces the design life of waste disposal facilities such as landfills (Venkatesan, 2006). Plastics consumption per capita in developing countries is a lot less than in developed countries. Nevertheless, growth in plastics consumption in the developing world is rapid. These plastics are often produced from expensive imported feedstocks, and that imposes a huge burden on the country’s foreign currency reserves. There is a much broader scope for recycling and conversion of waste to products in developing countries due to a number of reasons. Low labour costs make it relatively cheaper to run a recycling plant. In most cases an informal sector, which is ideally suited to running small-scale recycling activities exists. Disadvantaged people in urban areas exploit such opportunities to earn a small income. In

Current Disposal Practices:

Synthesis of PS: Distillation / pyrolysis of storax

Catalytic dehydrogenati on of ethylbenzene

the developing world, there are usually fewer legal requirements to regulate the standards of recycled materials. Feedstock transportation costs are often lower, with rudimentary and other cheap modes of haulage often being used. Low cost feedstocks give a competitive edge in the manufacturing sector. Innovative use of scrap machinery often leads to low entry costs for processing or manufacture. In these developing countries the possibilities for recycling plastics are growing as the amount of plastic being consumed inevitably increases (Armah, 1994). Polystyrene is a plastic widely used for packaging applications, its Society for Plastics Industry (SPI) code is 6, indicating the difficulty associated with its recycling (Bekri-Abbes et al., 2006). Due to its low density, it can easily be scattered by wind in the environment, creating a visible nuisance. The common forms of polystyrene widely used in industry are general purpose polystyrene (GPPS), expanded polystyrene (EPS), high impact polystyrene (HIPS), and syndiotactic polystyrene (SPS). EPS and HIPS are fairly moderately priced and have outstanding mechanical and insulating properties. They are used in resinmoulded articles like TV cabinets and polystyrene foam used for packing electric appliances (Inagaki et al., 1999; Inagaki and Kiuchi, 2001; Brennan et al., 2002). Unfortunately, a large portion of used EPS and HIPS is disposed of in landfills or by incineration in developed countries, through open burning and waste dumps in developing countries, and are barely ever recycled. This is because EPS and HIPS are relatively inexpensive and conventional recycling methods turn them into lower value materials like fuel oil or recycled resin. The problem of plastic waste cannot be solved by land-filling or incineration, because such suitable and safe disposal facilities are unaffordable, especially in developing countries. Besides, incineration contributes to emission of greenhouse gases such as NOx , SOx , COx (Fig. 1), which cause climate change, and releases carcinogens (Bekri-Abbes et al., 2006; Shibamoto et al., 2007; Bharadwaj et al., 2008; Ruziwa et al., 2015). Various research has adopted chemical recycling of WPS into the corresponding monomers or

Open-burning

post consumer waste

PS

X

Disposal in nonengineered waste dumps

Leakage of chemicals into the environment

Applications:

√

Cationic polymerisation

Production of genotoxic and carcinogenic PAHs; production of COx , NOx, SOx

WPS

Free radical polymerisation

Anionic polymerisation

Public & Environmental Impacts:

Potential uses (as is)

packaging of nonfood items; furniture; construction material; vector control

WPS-toproducts / energy

Styrene; polymer-clay nanocomposites; adhesives; paints; adsorbents; ion exchangers; nanofibres

Potential benefits Cheaper material; disease control Material recovery; low cost water treatment materials; cleaner environment

Fig. 1. A summary of the synthesis methods for polystyrene (PS), current waste polystyrene (WPS) disposal practices and their public and environmental impacts and potential uses and value added products from WPS and their potential benefits in developing countries.

N. Chaukura et al. / Resources, Conservation and Recycling 107 (2016) 157–165

hydrocarbons (William and Bagri, 2004). Because the process is inefficient owing to the low cost of pure hydrocarbons and virgin monomers relative to that obtained by recycling, there is need to find efficient techniques to recycle WPS. There is limited information on the disposal and potential uses of WPS in developing countries including those in Africa. Yet the continent uses a substantial amount of imported polystyrene for several industrial and household applications. For example, in Zimbabwe it is estimated that the total 1.57 million tonnes of solid waste generated per year, 0.2 million tonne is WPS (Ministry of Environment and Natural Resources, 2010). Therefore, the objectives of this review were; (1) to present an overview of the production, uses and current disposal practices for WPS, (2) to identify the potential uses of WPS to develop value-added products, and (3) to highlight key research gaps and propose future research. 2. Production of polystyrene Polystyrene is a polymer of styrene which was first developed by a German chemist Eduard Simon in 1839 by distilling or pyrolysing liquid storax (Andrady and Neal, 2009) (Fig. 1). Styrene was first produced on an industrial scale in 1931 by IG Farben industries in Germany and a little later by the Dow Chemical Company in USA. Both companies employed the catalytic dehydrogenation of ethylbenzene, which is still the main process used today (Wunsch, 2000). First, the polymerisation process will be discussed, followed by the processing of polystyrene into finished goods.

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synthesise and design polystyrene with desired properties. Taued (1993) investigated the polymerisation of styrene at temperatures from ambient to 373 K with different emulsifier regimes including persulphate, 2,2-azoisobutyronitrile (AIBN) and surface active initiators. Activation energies showed a strong dependence on the initiator-emulsifier systems used. McIntyre et al. (1972) demonstrated that mono disperse polystyrene with molecular weights up to 44 × 106 can be synthesised using tetrahydrofuran (THF) as the solvent and polystyryllithium in benzene as the initiator. Several other techniques have been used for the polymerisation of styrene. Atom transfer radical polymerisation (ATRP) was performed with bromoacetylated syndiotactic polystyrene as macroinitiator and copper bromide combined with N,N,N,N,Npentamethydiethylenetriamine as catalyst (Baker, 2011). Ultrasonically initiated mini-emulsion polymerisation of styrene was carried out in the presence of Fe2 SO4 nanoparticles. Adding FeSO4 nanoparticles increased the polymerisation rate considerably due to an enhanced acoustic intensity and the Fe2+ reacted with H2 O2 to produce hydroxyl radicals increasing the concentration of free radicals. The boost in co-surfactant concentration also raised the polymerisation rate (Qiu and Qu, 2006). Other methods for the synthesis of polystyrene include use of the Blatter radical and its derivatives in mediated stable free radical polymerisation (Demetriou et al., 2014). This creates new opportunities to design and develop radicals to optimise the polymerisation processes.

2.1. The polymerisation process 2.2. Polystyrene processing High molecular weight vinyl aromatic polymers, particularly polymers having weight average molar mass (Mw) greater than 300,000 have been typically produced by anionic polymerisation rather than by free radical polymerisation. This is due to the slow polymerisation rates used in free radical polymerisation of styrene to achieve high molecular weight polymers. However, anionic polymerisation processes require expensive initiators, such as organolithium compounds, and tend to produce discoloured products owing to the presence of residual lithium-containing salts. Consequently, high molecular weight polymers from vinyl aromatic monomers are produced using a free radical polymerisation process which does not exhibit these disadvantages, as well as to provide an improved process to produce bimodal compositions containing these high molecular weight polymers (Meister and Cummings, 2003). In the case of styrene, polymerisation with an organolithium initiator in either a hydrocarbon or ether solvent results in an atactic polymer of narrow molecular weight distribution (Worsfold and Bywater, 1963; McIntyre et al., 1972). Generally, high molecular weight vinyl aromatic polymers have also been produced by free radical polymerisation (Fig. 1) in the presence of a soluble organic acid having pKa of 0.5–2.5 (Shero et al., 1992). However, in this process the acid does not bind to the polymer and can migrate from the polymer during use, which can cause corrosion of mould surfaces (Meister and Cummings, 2003). Styrene has been polymerised by all four of the discrete polymerisation mechanisms: anionic, cationic, free-radical and Ziegler (Soga et al., 1990) (Fig. 1). Of these processes, free-radical polymerisation is the most commercially prevalent and forms atactic polystyrene, which is the type most widely used in packaging and durable goods. Free-radical polymerisation is used industrially in two different processes, namely continuous (mass or bulk) and suspension polymerisation. A number of parameters, for example, type and quantity of initiator, surfactant, experimental conditions like polymerisation temperature, and polymerisation technique can be varied to

Polystyrene processing is an established field, and as a result, developments concentrate on additives that modify the properties of the final product. Although GPPS is the major commercial form, there are three other types of polystyrene, namely HIPS, EPS and SPS as alluded to earlier. These are designed through different polymerisation procedures. GPPS and HIPS are made using continuous bulk mass polymerisation plants. In the early 1940s Dow invented an extrusion process for polystyrene to achieve a closed cell foam that resists moisture, which became the basis for Styrofoam products. EPS foam beads are prepared by a two-step process. The blowing agent is impregnated into pre-prepared resin beads in an aqueous suspension (Stastiny and Gaeth, 1954). It can also be prepared in a one step process in which the blowing agent is impregnated directly into polystyrene beads that are being formed by suspension polymerisation (D’Alelio, 1957; Park et al., 2003). EPS products have low mechanical strength. To counter this, polyolefin bead foams have been formulated (Lee, 2010). Although polyolefin beads are resilient, they do not retain the blow agent introduced during processing, hence the polyester foam was invented. The resilient polyester foam that is both expandable and moldable was synthesised from an amorphous polyester resin using a mixed blowing agent composed of a high-solubility compound and a low-permeability compound. Whereas the highsolubility compound permits a high degree of expansion, the low permeability compound confers secondary expandability (Park et al., 2003). Recent research in injection foam processing using the styrenemaleic anhydride (SMA) copolymer have resulted in a distinct and major increase in the performance of foam for food packaging applications (Roberts and Kwok, 2007). Technology from Dow Chemical Company has facilitated the production of ethylene-styrene interpolymers (ESI) through the copolymerisation of ethylene and styrene monomers. The properties of the interpolymers vary significantly with copolymer load.

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3. Current applications of polystyrene Polystyrene has a wide variety of applications ranging from packaging (Marsh and Bugusu, 2007) manufactured items, construction, medical applications, to arts and crafts, among others. We briefly present examples of a few of such uses below. Polystyrene foam containers are widely used in the food service and food packaging Perishable food items can be prevented from spoiling if packaged in polystyrene. Electronic goods are packed in boxes along with support material made of polystyrene which insulates and protects from damage during shipping (Park et al., 2003). Extruded polystyrene (XPS), also generally known as lightweight Styrofoam, and (EPS) have been used for coffee cups, trays, bowls, cartons, takeaway food containers, and for equipment or instrument protection (Thompsett et al., 1995; Issam et al., 2009). Polystyrene is used to make disposable plastic cutlery, household appliances like blenders, air conditioners, refrigerators, ovens, microwaves, and hand-held vacuum cleaners. Besides lightness of weight, polystyrene also imparts a pristine finish. Polystyrene is also incorporated into materials that make other household goods, for example, kitchen and bathroom accessories, garden utensils. It is also used to make toys, housing for electronic devices such as smoke detectors, televisions, CD or DVD cases and several other devices (Inagaki et al., 1999; Inagaki and Kiuchi, 2001). Polystyrene is used in resin-moulded articles such as TV cabinets and video cassette shells (Asayama, 1999). It is also used in soft shell cycle helmets (Mills, 2007), as housing for cameras, and can be cross-linked to produce extruded sheets and bun foams for footwear parts. Other uses of polystyrene include construction purposes to insulate ceilings, walls and floors (Pick and Knee, 1967). It also finds utility in sound-proofing walls of buildings. Resins of polystyrene are used in lighting and plumbing fixtures, panels and sidings. Laboratory items like test tubes, petri dishes, trays for conducting tissue culture test, diagnostic test equipment, medical cups, medical keyboards, plastic boxes are made from polystyrene. Extruded polystyrene or Styrofoam can be used for art and craft projects, models of architectural designs, candle holders and ornaments for Christmas trees (Paraskevopoulos, 1956).

the heat produced during incineration can be used for steam and hydroelectric power generation (Durlak et al., 1998). However, incineration has been criticised for the production of a complex mixture of polyaromatic hydrocarbons (PAHs), particularly at low temperature (800–900 ◦ C) (Durlak et al., 1998). Exposure to PAHs through dermal contact, ingestion and inhalation may lead to genotoxicity and carcinogenicity (Zhou et al., 2010). Studies have shown that PAHs in the body metabolise to produce reactive polar products such as diol epoxides (Ramesh et al., 2004; Zhou et al., 2010). These metabolites are either excreted or react with DNA to form PAH–DNA adducts, which initiates carcinogenesis (Zhou et al., 2010). In the presence of light, PAHs have also been reported to cause cleavage of the DNA single strand via oxidation of DNA bases (Yan et al., 2004). The quantities and species of PAHs formed during incineration depend on incineration temperature and polystyrene feed size. An investigation of the effects of feed size (100–300 ␮m) and incineration temperature (800, 900, 100 and 1200 ◦ C) on the production of PAHs revealed two key findings crucial for the operation of incinerators. First, the total number of PAH species and total mass yield of PAHs declined exponentially as temperature increased from 800 to 1200 ◦ C with practically no PAHs at 1200 ◦ C. Second, feed size effect showed that the PAH mass yield for smaller particles (104–175 ␮m diameter) was about seven times lower than those for large particles. Overall, the study by Durlak et al. (1998) indicates that better combustion of polystyrene occurs at higher temperatures with smaller size than with large ones (250–300 ␮m). However, PAHs partitioning between the gaseous and solid phase was independent of temperature (Durlak et al., 1998). Due to the public health risks associated with PAHs, most countries are considering minimisation strategies. These strategies involve reduction, recycling, reuse and recover. In the USA, reduction of polystyrene use in packaging has been achieved by the use of paper packaging as a substitute. However, from an environmental perspective, paper packaging has a higher carbon footprint than polystyrene (Hocking, 1991). They report that, compared to paper, polystyrene has less environmental impacts because, when disposed of in landfills, paper decomposes to release methane and CO2 in the ratio 2:1. Both methane and CO2 are greenhouses gases contributing to global climate change, with methane, CH4 being 20 times stronger than CO2 as a greenhouse gas.

4. Disposal practices of polystyrene Polystyrene is highly stable and resistant to decomposition, largely due to the presence of phenyl groups and single C C bonds. The persistence of polystyrene causes several environmental impacts including aesthetic effects due to high visibility, entanglement of wildlife resulting in reduced feeding efficiency, strangulation and ingestion (Davis, 2013). In addition, polystyrene also releases a mixture of polyaromatic hydrocarbons (PAHs), which are well-known carcinogens (Zhou et al., 2010). Due to low volatility and biodegradability, once they enter the body, PAHs bioaccumulate (Sese et al., 2009). To minimise these environmental and public health risks, WPS is disposed by landfilling, incineration or minimisation processes in developed countries, and in non-engineered landfills and through open burning in developing countries. Due to low density, polystyrene is bulk and reduces landfill capacity and design life (Eckhardt, 1998). Landfilling is also a missed opportunity to recover the valuable energy resource from WPS. In most developed countries, incineration has been the most common disposal practice for WPS. High temperature incineration produces predominantly CO2 , water, volatile hydrocarbons and heat. Compared to landfilling, incineration at 900 ◦ C produces 65% gases and 35% solids in the form of soot, thereby reducing the volume to about 1% of the original material (Durlak et al., 1998). Due to the high calorific value

5. Potential uses of WPS Recycling of polystyrene is also a well-established practice. However, in most countries there is a restriction on the use of recycled polystyrene in food applications (Hocking, 1991). This limits possible end uses for recycled polystyrene to packaging, insulation, floating billets/boards, patio furniture and drainage tiles (Aminudin et al., 2011; Hocking, 1991) (Fig. 1). Reduce and recover processes appear the most ideal options for minimising WPS. Direct recycling includes development of new products from WPS, while indirect recycling involves modification of the WPS to develop new products. Typical recycling examples include pyrolysis or catalytic cracking to produce volatile gases, fuels, products such as ion exchange resins and styrene monomers, and development of building materials (e.g., Kuhail and Shihada, 2003; Kuswanti, 2002; William and Bagri, 2004; Aminudin et al., 2011; Mbadike and Osadebe, 2012). For example catalytic pyrolysis has been used to develop hydrocarbon oils and gases from recycled polystyrene (William and Bagri, 2004). Marsh et al. (1994) observed that the use of heavy oils and a solvent enhances the pyrolysis process. Mohammadi et al. (2012) used EPS as a diesel additive and improved engine performance, while reducing emissions. WPS has also been used to develop cheap and light-weight

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building materials including concrete, bricks and asphalt binders (Mbadike and Osadebe, 2012; Kuhail and Shihada, 2003). Mbadike and Osadebe (2012) incorporated polystyrene granules to develop concrete matrix, while Kuhail and Shihada (2003) developed a lightweight concrete from polystyrene. Ward et al. (2006) developed a novel method for converting WPS to biodegradable plastic, namely polyhydroxyalkanoate (PHA). A rather curious application of WPS has been in disease and vector control (Fig. 1). This is particularly important in SSA where malaria and other vector-borne diseases are prevalent. In this region, low-cost control methods that are affordable to poor households are required. To this end, several laboratory and field studies have shown that WPS beads are effective in controlling human and animal diseases and vectors (e.g., Chavasse et al., 1995; Sivagnaname et al., 2005). In Kenya, Tanzania and India, 70–100% reduction in emergence, biting and microfilarial rates were observed following application of polystyrene beads. For example, using a floating layer of WPS beads to control mosquitos in Dar es Salaam in Tanzania, Chavasse et al. (1995) observed that the densities of mosquitoes dropped by 76.7% in treated sites, but increased by 84.9% and 25.6% in two untreated control sites. Given the prevalence of malaria in SSA, the application of WPS in this regard represented a rather under-utilized technology. This application of WPS is mainly related to its low density and ability to float on water surfaces and its high volume that enables it to cover large surfaces of water bodies. However, wide use of this method is likely to result in another form of water pollution due to the WPS particles suspended in water (Nhapi et al., 2002). Four common routes for the treatment of plastic solid waste material have been identified. These are: primary (re-extrusion), secondary (mechanical), tertiary (chemical) and quaternary (energy recovery) (Al-Salem et al., 2009; Kasper et al., 2011). Advanced thermo-chemical treatment methods mainly produce either fuels or petrochemical feedstocks and energy recycling by use of catalysts like silica/alumina and ZSM5 zeolite. Zhang et al. (1995) reported the use of barium oxide in the selective degradation of polystyrene into styrene monomer and its dimeric form. 5.1. Generation of styrene monomer through depolymerisation Chemical recycling into monomers over solid acids and bases has been reported. However, acids have challenges in that they quickly get deactivated due to accumulation of carbonaceous material on the surface. Zhang et al. (1995) found that solid acids like silica/alumina and ZSM5 zeolite are effective in degrading polyethylene to fuel oils while BaO worked for polystyrene degradation to styrene. Similar results were reported by William and Bagri (2004). Catalysed pyrolysis of polystyrene has been reported to give a high yield (>80% wt) of styrene monomer, as opposed to uncatalysed which produced a reduced amount of styrene and a range of aromatic hydrocarbons including: ethylbenzene, toluene and polycyclic aromatic hydrocarbons (William and Bagri, 2004). Sun et al. (2012) reported the use of platinum or rhodium catalyst combined with cerium and supported on alumina, in a continuous process, for the depolymerisation of polystyrene into its monomers. Ground WPS particles were fed into the reactor, brought into contact with the hot catalyst and pyrolysed into styrene monomer. The heat given off by the process sustained the high temperatures of the reactor, thus no extra energy input was required. Other potential products from the process include benzene, toluene, ethylbenzene or small polystyrene oligomers. A yield of about 80% styrene monomer was obtained. The costeffectiveness of such a process depends on a number of factors such as sorting, cleaning and transportation of waste plastic. Lilac and Lee (2001) investigated the degradation of polystyrene by using supercritical water, and found the selectivity for styrene

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monomer to be 71%. Selectivity for benzene, toluene and other hydrocarbons was significantly lower than that for styrene. US Patent 5672794 (Northemann, 1997) discloses the recovery of styrene from polystyrene through a thermal depolymerisation process in the presence of magnesium aluminium silicate as the heat transfer agent in a fluidized bed reactor. The WPS can be fed into the reactor in the solid or liquid phase. The reactor is operated at temperatures ranging 400–700 ◦ C (Northemann, 1997). Karmore and Madras (2002) assessed the depolymerisation of polystyrene in the presence of Lewis acids (aluminium chloride, ferric chloride, boron trifluoride etherate and tin chloride) at temperatures ranging 75–125 ◦ C. The best results were obtained using aluminium chloride.

5.2. Reprocessing of HIPS and EPS Noguchi et al. (1998) developed a prototype production system for recycling waste EPS using a natural solvent, d-limonene, which shrinks the EPS. The recycled polystyrene is suggested to have the same mechanical properties as the virgin polymer, hence can be used in applications similar to those of virgin material.

5.2.1. Reprocessing into polymer-clay nanocomposites Polymer-clay nanocomposites exhibit significant increases in tensile strength, modulus, and heat distortion temperatures, low water sensitivity and reduced gas permeability as compared to the pristine polymer (Powell and Beall, 2006). WPS can be reclaimed through processing into polystyrene-clay nanocomposites which, due to their improved tensile strength and modulus, can be exploited in packaging, manufacture of toys, and as casings of electronic goods. Mauroy et al. (2013) reported an increased thermal stability of the nanocomposites compared with the pure polystyrene. Brennan et al. (2002) investigated the recycling of acrylonitrilebutadiene-styrene (ABS) and HIPS from waste computer equipment. The individual polymer materials or blends were injection moulded and their mechanical properties assessed. The investigators highlighted the possibility of chain length shortening due to reprocessing, which probably caused the reduction in strain to failure and impact strength. The incorporation of montmorillonite nanoclay into such recycled WPS to make polystyrene-clay nanocomposites can greatly improve these properties. HIPS has been reported as a promising material for mechanical recycling since it retains its properties even after several processing up to nine cycles (Vilaplana et al., 2006; Maharana et al., 2007). The solvents do not result in polymer degradation and they can be recycled. The green recycling of WPS using the above natural solvents opens the possibility for a variety of uses including the reprocessing into nanocomposite membranes, electrospun carbon nanofibres as well as remoulding into other new articles.

5.2.2. High value products 5.2.2.1. Production of adhesives. Issam et al. (2009) reported the use of polystyrene in the manufacture of adhesive emulsions (Fig. 1). The WPS polymer was mixed in varying proportions with coumarone-indene resin and benzene as a solvent. The tackiness as measured by the rolling ball technique increased with increase in amount of WPS. The WPS needs some level of cleaning before use in the adhesives. United States patent, US5459193A (Anderson and Simmons, 1995), reported the preparation of a polystyrene-ethylene/butylenes-polystyrene block copolymer hot melt adhesive. The use of WPS in adhesive manufacturing is quite attractive in terms of producing high value products from waste materials. Most of the work on WPS use in adhesive manufacturing

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is from researchers based in Asia (Seluka et al., 2014), and it still looks quite an open field. 5.2.2.2. Production of paints. WPS can also be used as a binder in the manufacture of paints (Theiler, 1974) (Fig. 1). Osemeahon et al. (2013) explored the possibility of using WPS as a binder in paint manufacturing. Gasoline was found to be the ideal solvent for WPS dissolution as it gave greater elongation at break. The researchers found the maximum binder (polystyrene) solubility in gasoline to be less than 20% (w/v). Styrene homopolymers (in admixture with other binders) find special application in zinc dust primers (anti-corrosion), bronze lacquers, paints for mineral substrates and paper. Styrenated alkyd resin coatings dry faster, have better resistance to water, light and chemicals, and a low yellowing tendency of their films (Stoye and Freitag, 1998). It therefore is clear that the paint industry can consume a lot of this WPS. 5.2.2.3. Production of super-absorbents. WPS can also be modified into high-value products such as super-absorbents (Fig. 1). Ismail and Zulkifli (2013) reported the development of a WPS-acrylic acidbentonite super-absorbent nanocomposite which was prepared through emulsion polymerisation. The highest water absorbency (500 g/g in distilled water and 49 g/g in 0.9% NaCl) was obtained for the nanocomposite containing 3% bentonite. Such nanocomposite super-adsorbents can be used in diapers, and could potentially be used for the remediation of contaminated ground water. However, there is need to assess the risk of residual styrene monomers and other PAHs in such products. 5.2.2.4. Production of flocculants and ion exchangers. Inagaki et al. (1999) investigated the reclamation of WPS by sulphonation. The WPS included high-impact polystyrene containing rubber components or colouring agents. The sulphonation process converted the WPS into a polymeric flocculant material with superior properties relative to the conventional polyacrylamide (PAA). The authors also observed some synergy after using a combination of the two polymeric flocculants. Given the high cost and poor availability of water treatment chemicals, the production of flocculants from WPS is highly attractive. Sulphonated WPS has also been used as an adsorbent for Pb2+ and Cd2+ (Bekri-Abbes et al., 2006), and Pb2+ and Zn2+ (Ruziwa et al., 2015) from aqueous solution. Although these materials have been found to have inferior removal efficiencies to biosorbents, for example, they have proved useful in significantly removing selected heavy metals from water. WPS has also been used for the development of novel ion exchange resins (Abrams, 1956). These resins have wide industrial applications including water and wastewater treatment (Nasef and Güven, 2012), remediation of contaminated soils and the purifications of industrial products such as enzymes and hormones through bioseparation and chromatographic applications (Azam and Dahman, 2008). Ozer et al. (2013) developed a cross-linked sulphonated polystyrene ion exchange resin with high capacities (as high as 1.5–2 times that of the commercial resins) and rapid equilibration of the ion exchanges (i.e. less than 1 min). The ion exchange resins were effective in removing divalent ions such as Ca (II), Mg (II), Ni (II) and Cu (II) ions in water (Ozer et al., 2013). Polystyrene-based exchangers may represent low-cost adsorbents for the treatment of water and wastewater in SSA, where the majority of people have no access to clean drinking water. Another area of WPS application is the manufacture of ionexchange membranes for water purification after the necessary chemical modification of the WPS. Sachdeva et al. (2008) reported the synthesis of highly cross-linked anion exchange membranes for the electrodialysis of chloride ions. The composite anion exchange polystyrene membrane was homogeneously modified by gasphase nitration followed by amination using hydrazine hydrate,

and further reacted with dichloroethane and triethylamine to introduce quaternary ammonium charges on the membrane. The chloride ions are removed through electrodialysis. Tsyurupa et al. (1995) reported the successful use of hyper-crosslinked polystyrene sorbents (styrosorb) for the removal of phenol from water, with a possible application in the treatment of phenolcontaminated industrial effluent. Ion-exchange membranes can also find application in mineral processing after modification of the membranes with appropriate selective chemical groups. As with all the other possible applications of polystyrene highlighted in this paper, there is need for a cost-benefit-analysis in terms of the collection of feedstock, cleaning and reprocessing. 5.2.2.5. Production of polystyrene nanofibres. The high surface to volume ratio property inherent in carbon nanofibres can be utilised for catalysis as well as the preparation of nanopore filters (water purification and air conditioning units). Non-woven polystyrene nanoporous filters can be prepared from WPS (Fig. 1). The nanoporous filters can find application in air and water purification. Marek et al. (2009), prepared electrospun polystyrene nanofibres from regenerated polystyrene. Solvents such as tetrahydrofuran and dimethyl formamide have been used to dissolve the WPS. The polystyrene nanofibres were then crosslinked using sulphuric acid followed by reaction with ethylene diamine. The obtained cation and anion exchange polystyrene nanofibress were applied in fast water treatment. Such ion exchange nanofibres can be used as matrix support for catalytically active metals ions. Rojas et al. (2009) prepared electrospun nanocomposites from WPS loaded with cellulose nanowhiskers. The cellulose whiskers-reinforced polystyrene nanofibres possess unique structural features which enable new properties and functionalities. Electrospun nanofibres find wide application in sensors, filtration, protective clothing, wound dressing, drug delivery and tissue scaffolds, among other applications. Electrospun polystyrene nanofibres have been reported to thermally stabilise cyclodextrin inclusion complexes of volatile fragrances like menthol up to a temperature of 350 ◦ C (Uyar et al., 2009). A separate study was reported by Neoh and Yoshii (2008), in which an inclusion cyclodextrin complex of 1methycyclopropane (1-MCP) was electrospun in a polystyrene nanofibre matrix. 1-MCP inhibits ethylene induced physiological changes in various types of agricultural produce and prolongs their shelf life. 6. Challenges and risks associated with recycling There are several challenges and risks associated with the recycling of WPS. Although the technology to recycle polystyrene to produce a variety of products is well known, its recycling is limited by transportation problems due to its low density to volume ratio. This makes it uneconomic to collect, store and transport over long distances. Moreover, there are limited incentives to promote recycling due to lack of investment in infrastructure, compaction equipment and logistical systems. Due to its high stability arising from the presence of phenyl groups and single C C bonds, the decomposition or depolymerisation process into its monomers is energy-intensive requiring high temperatures and pressure. However, research shows that the use of solvents such as heavy oils enables the pyrolysis of polystyrene to occur at low temperatures of 400 ◦ C (Marsh et al., 1994). The process involved is similar to catalytic pyrolysis in the presence of zeolites (William and Bagri, 2004). Polystyrene generally has poor strain resistance, low strength and is highly flammable, characteristics which limit its use in the construction industry. For example, Mbadike and Osadebe (2012) showed that incorporation of polystyrene in concrete matrix

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reduced its strength. Similarly, Brennan et al. (2002) and Focke et al. (2009) reported that blending ABS and HIPS from outer casings of computer equipment considerably reduced mechanical properties particularly the strains to failure and impact strengths of the material. Although polystyrene has high calorific value (30 MJ/kg), its use as an energy source is limited by the emissions of toxic and carcinogenic PAHs (Durlak et al., 1998; Zhou et al., 2010). The production of these carcinogens occurs when combustion occurs under temperatures of below 500 ◦ C. Another challenge associated with the recycling of WPS pertains to perceptions and attitudes of industry and consumers. Using resins as an example, Kuswanti (2002) demonstrated that many recycling companies hesitate to use regrind and postconsumer resins (PCRs) because of the extensive testing required to identify plausible uses and processing parameters. In addition, they showed that labelling on recycled resins may be misleading and uninformative to determine moulding and mechanical parameters (Kuswanti, 2002). Because used polymers may be degraded or mislabelled, it is important to characterise the used polymer rather than track the original virgin polymer properties. Another major challenge to plastics recycling even in developed countries such as the USA is that, existing polymer databases do not contain information about regrind resins or PCRs. Besides assisting with choice of materials, such polymer databases are used to reduce the experimental time to determine moulding and mechanical properties. Overall, the case study by Kuswanti (2002) demonstrates that negative perceptions of industry arise from the financial risks and costs associated with additional testing of materials. In the context of consumers, the public health risks associated with the use of recycled plastics may also be a contributing factor. Experts on polymers the world over are very limited (McCoy and Wenzel, 2014). The problem is more pronounced in SSA for various reasons. This is largely due to the fact that there are few educational institutions offering curricula in the disciplines. Accordingly the plastics, their disposal and recycling represent a new area of research. Therefore, despite the different possible applications discussed here, there is still limited empirical evidence on the largescale industrial application of such technologies in SSA. However, as the public becomes more aware of the environmental impacts of WPS, it is envisaged that the need to recycle will be considered as one option to minimise these impacts.

7. Proposed future uses of WPS While the recycling of waste PS into construction material and artefacts is gaining acceptance in SSA, its chemical conversion into high value products is lagging behind. Future research is likely to pursue chemically converting waste PS into flocculants for heavy metal removal from solution, and for other applications like paints and adhesives. Although Hamad et al. (2013) demonstrated that mechanical recycling of WPS was preferred to chemical recycling, flocculants are especially useful since potable water is generally a problem in SSA, particularly in Zimbabwe. The cost of treating water is escalating, consequently people in urban areas consume contaminated water and hence the spread of water-borne diseases like cholera, typhoid, and the like, is recurrent (Nhapi et al., 2002). While heavy metal poisoning is not readily evident, heavy metals are persistent pollutants in industrial and mining effluent waters (Pumure et al., 2011). These are often difficult to remove in order to render the water suitable for discharge into public waterways or to make it potable. Consequently they find their way into public waterways and directly or up the food chain to humans. Heavy metals cause problems that include poisoning plant and animal life. The sequestration of heavy metals from water systems is therefore a priority. On the other hand waste plastics generally cause

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land pollution. Research into chemical conversion of WPS into a cation exchange resin through sulphonation is the subject of current research by some members of this group. Moreover, there is also need for further research to determine and compare the suitability of valued-added products (e.g., light-weight concrete) developed from WPS to conventional products. 8. Conclusion and outlook The current study investigated the potential uses and development of value-added products from WPS with particular emphasis on developing countries especially in SSA. The review showed that there is a lot of potential uses of WPS including the production of industrial products such as paints, adhesives, artefacts and garden furniture and low-cost construction materials. In SSA, where animal and human vector-borne diseases such as malaria are prevalent, after an environmental impact assessment, WPS could be used to complement current efforts to control such diseases and vectors. Moreover, WPS could be used for the development of novel synthetic materials such as ion-exchangers for remediation of contaminated media. Application of such adsorbents include treatment of portable water and wastewater, and remediation of contaminated water such as acid mine drainage. Overall, WPS recycling and conversion to products create potential value chains opportunities while minimising public and environmental impacts of current WPS disposal practices. However, before the benefits of using WPS could be realised in developing countries especially SSA, there is need to overcome several constraints and risks. Specifically, there is limited information on the development and evaluation of most of the value-added products highlighted in this study. This lack of information reflects the general lack of research on polymer or material science in SSA in particular. To overcome these challenges, government, local authorities and non-governmental organisations (NGOs) can fund research and information dissemination on the recovery and reuse of WPS particularly focusing on products for potable and waste water treatment, in addition to the other high-value products like paints, adhesives, and the like. This should include the establishment of well-equipped collaborative research facilities and agencies or centres that educate the general public on uses of WPS. This will ensure there is fundamental research that informs scaling up of operations and also policy formulation. There is also need for the development of a policy and legal framework on the use of polystyrene products and disposal of WPS, and to aid with the collection and transportation for recycling, will go a long way in addressing environmental issues associated with waste polystyrene. The use of incentives for proper disposal, and imposing some tax on the use of polystyrene by companies and penalties for dumping WPS, could significantly promote recycling of WPS. Moreover, there is also need to understand consumer perceptions and attitudes on products developed from waste materials such as WPS. Current on-going research on development and evaluation of light-weight concrete, ion exchanges and composites from WPS represents one step in that direction. However, the lack of facilities and research funding will remain a constraint in developing countries. References Abrams, I.M., 1956. High porosity polystyrene cation exchange resins. Ind. Eng. Chem. 48, 1469–1472. Al-Salem, S.M., Lettieri, P., Baeyens, J., 2009. Recycling and recovery routed of plastic solid waste (PSW): a review. Waste Manag. 29, 2625–2643. Allacker, K., Mathieux, F., Manfredi, S., Pelletier, N., De Camillis, C., Ardente, F., Pant, F., 2014. Allocation solutions for secondary material production and end of life recovery: poposals for product policy initiatives. Resour. Conserv. Recycl. 88, 1–12.

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