Synthetic polymers recycling

Recovery and reuse of synthetic polymers is by far the most acceptable way of dealing with the problem of waste. Items can rarely be reused as such but instead the polymers from which they are fabricated can be recycled. In principle, polymers can be recycled without any significant loss of their properties. For example, polycarbonate bottles can be recycled into automobile bumpers and then into articles for high performance housing. 

The Synthetic polymers are used for making so many items and it is impossible to imagine modem civilization without them. However, the main concern is that once they are made, they do not decay and tend to remain for all times. In other words, they cannot be disposed off unlike other waste products, which are degradable. This problem has been solved to a certain extent by reuse or recycling of some of the polymers. 

Obviously, there is no accumulation of natural polymers because they are recycled by biological degradation to CO2. This is then assimilated and reincorporated into biomass by photosynthesis. On the other hand, persistence is a well known property of most synthetic polymers, e.g. polyethylene (PE) and polystyrene (PS). 

In order to decrease human consumption of petroleum, chemists have investigated methods for producing polymers from renewable resources such as biomass. Nature Works polylactic acid (PLA) is a polymer of naturally occurring lactic acid (LA), and LA can be produced from the fermentation of corn. The goal is to eventually manufacture this polymer from waste biomass. Another advantage of PLA is that, unlike most synthetic polymers which litter the landscape and pack landfills, it is biodegradable. PLA can also be easily recycled by conversion back into LA. It can replace many petroleum-based polymers in products such as carpets, bags, cups, and textile fibers. 

Fortunately, we do not throw away everything. In a few states, consumers return soft drink and beer containers to stores to reclaim a deposit of 5 or 10 cents. And in many places, some types of plastics (in addition to glass bottles, metal cans, and paper and cardboard) are separated from the trash and recycled. What happens to these items We want to understand why only certain plastics are recycled, how these are recycled, what some of the limitations are that limit their reuse, and what other options might exist to better utilize synthetic polymers. 

N. Horvat and E. T. T. Ng, Tertiary polymer recycling stndy of polyethylene thermolysis as a first step to synthetic diesel fuel. Fuel, 78, 459-470 (1999). 

A. M. CunUffe and P. T. Williams, Characterisation of products from the recycling of glass fibre reinforced polyester waste by pyrolysis. Fuel, 82, 2223-2230, (2003). J. H. Harker and J. R. Backhurst, Fuel and Energy, Academic Press London, 1981. A. C. Albertson and S. Karlsson, Polyethylene degradation products, In Agricultural and Synthetic Polymers, ACS Symposium Series 433, J. E. Glass and G. Swift (eds), American Chemical Society, Washington DC, 60-64, 1990. 

Although thousands of different synthetic polymers have now been prepared, six compounds, called the Big Six, account for 76% of the synthetic polymers produced in the United States each year. Each polymer is assigned a recycling code (1-6) that indicates its ease of recycling the lower the number, the easier to recycle. Table 30.1 lists these six most common polymers, vas well as the type of products made from each recycled polymer. 

Synthetic polymers are arguably the most important materials made by chemists and used in modem society. Chapter 30 Synthetic Polymers expands on the foundations of polymers discussed in earlier Chapters 15 and 22. Of significance is emphasis on the environmental impact of polymer synthesis and use, discussed in sections on Green Polymer Synthesis (30.8), Polymer Recycling and Disposal (Section 30.9A), and Biodegradable Polymers (Section 30.9B). 

Between 50 and 60 billion pounds of synthetic polymers are manufactured each year in the United States—over 200 pounds per person. A large percentage of these polymers are tossed into our landfills after use. This represents a serious waste of precious raw materials (the petroleum products from which synthetic polymers are made), and exacerbates concerns that the landfills are quickly filling up. These factors give the recycling of polymers a high priority among our nation s concerns. 

Some synthetic polymers can be recycled and some cannot. So-called thermoplastic polymers, usually composed of hnear or only shghtly branched molecules, can be heated and formed and then reheated and reformed. Therefore, they can be recycled. On the other hand, thermosetting polymers, which consist of molecules with extensive three-dimensional cross-hnking, decompose when heated, so they cannot be reheated and reformed. This makes them more difficult to recycle. 

The degradative radiation-recycling of PTFE led to a successful pilot-scale plant producing 12 tons/year recycled powder at Sumitomo, Japan [9], For similar polymerdegrading industrial developments several other candidates are very promising. Among other synthetic polymer products, discarded automobile tires represent a major environmental concern, in an amount close to 10 Mtons/a. A promising method is mentioned in the literature [9] in which the vulcanized rubber product is crushed at low temperature, irradiated at a dose rate of 100 kGy, and milled repeatedly, if necessary. The reclaimed de-crosslinked material can be added to an extent 10 - 15% to various new rubber blends. 

Synthetic polymers can conveniently be divided into three groups commodity, engineering and specialty. Their relative annual production can be approximated by the ratio 100 10 1. Thus, it is logical that the main emphasis has been for recycling of the commodity resins (see Eigure... 

Polyolefins are synthetic polymers of olefinic monomers. They are the largest polymer family by volume of production and consumption. Several million metric tons of polyolefins are produced and consumed worldwide each year, and as such they are regarded as commodity polymers. Polyolefins have enjoyed great success due to many application opportunities, relatively low cost, and wide range of properties. Polyolefins are recyclable and significant improvement in properties is available via blending and composite technologies. 

Identification of Polymers, it is a fact of commercial life that there Is a frequent call for the rapid Identification of synthetic polymers, usually by industrial scientists and technologists Interested In a rival product. The growing possibility that recycling of plastic material may become economically attractive compared with disposal would also require Identification of different synthetic polymers for sorting purposes. Luminescence spectroscopy could provide a convenient method of rapid identification. 

Synthetic polymers are not easily biodegraded and can cause an environmental problem of disposal. One answer is for them to be recycled. 

The preceding section shows how it is possible to deerease the quantity of synthetic polymer while retaining the same fimetional properties. In this way the environmental footprint is diminished, but the produets are still not amenable to recycling processes because of the cross-linking chemistry used in the silicone phase. The feet feat these silicone coatings may be used for their barrier properties should motivate a transition toward this important area of packaging. 

It is clear that such cellulose products as paper and boards play a major economic role but, because of their poor barrier properties, they have brought little added value to the final products. The latter is aetually provided by introduction of hydrophobic synthetic polymers, but these severely eompromise the ability for recycling of the products. 

Tires are one of the most durable technological products manufactured today. They are a resilient, durable composite of fabric, steel, carbon black, natural rubber, and synthetic polymers. The qualities that make tires or other engineered rubber products a high-value item create a special challenge of disposal. Tires and other rubber products, such as conveyor belts and hydrauUc hoses, are not biodegradable and cannot be recycled like glass, aluminum, or plastic. Four potential applications for such products entering the solid waste stream have been identified ... 

Biological CLR refers to fibres that can be safely composted at end of fife to return nutrients to the soil. Technical CLR refers to the synthetic products that are not biodegradable. In textiles, this is frequently the synthetic polymer-based fibres such as polyester, acrylic and nylon. Blending of the two kinds of streams is referred to by McDonough and Braungart (2002) as a monstrous hybrid , meaning that the two kinds of waste streams cannot be effectively separated for ease of recycling. In the apparel context, monstrous hybrids abound in the form of cotton/polyester, or viscose/polyester, or cotton and spandex blends. 

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