Plastic recycling

Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products, sometimes completely different from their original state. For instance, this could mean melting down polyester soft drink bottles then making model army figures and ammunition.

Before recycling, plastics are sorted according to their resin identification code. PET, for instance, has a resin code of 1.

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Obstacles

When compared to glass or metallic materials, plastic poses some unique challenges from a recycling perspective. Chief among them is their low entropy of mixing, which is due to the high molecular weight of large polymer chains. Another way of stating this problem is that, since a macromolecule interacts with its environment along its entire length, its enthalpy of mixing is large compared to that of a small organic molecule with a similar structure; thermal excitations are often not enough to drive such a huge molecule into solution on their own. Due to this uncommon influence of mixing enthalpy, polymers must often be of nearly identical composition in order to mix with one another. To take representative samples from beverage containers, the many aluminium-based alloys all melt into the same liquid phase, but the various copolymer blends of PET from different manufacturers do not dissolve into one another when heated. Instead, they tend to phase-separate, like oil and water. Phase boundaries weaken an item made from such a mixture considerably, meaning that most polymer blends are only useful in a few, very limited contexts.

Another barrier to recycling is the widespread use of dyes, fillers, and other additives in plastics. The polymer is generally too viscous to economically remove fillers, and would be damaged by many of the processes that could cheaply remove the added dyes. Additives are less widely used in beverage containers and plastic bags, allowing them to be recycled more frequently.

The use of biodegradable plastics is increasing. If some of these get mixed in the other plastics for recycling, the recycled plastic is less valuable.

Many such problems can be solved by using a more elaborate monomer recycling process, in which a condensation polymer essentially undergoes the inverse of the polymerization reaction used to manufacture it. This yields the same mix of chemicals that formed the original polymer, which can be purified and used to synthesize new polymer chains of the same type. Du Pont opened a pilot plant of this type in Cape Fear, North Carolina, to recycle PET by a process of methanolysis, but it closed the plant due to economic pressures.

Another potential option is the conversion of assorted polymers into petroleum by a much less precise thermal depolymerization process. Such a process would be able to accept almost any polymer or mix of polymers, including thermoset materials such as vulcanized rubber tires and the biopolymers in feathers and other agricultural waste. Like natural petroleum, the chemicals produced can be made into fuels as well as polymers. A pilot plant of this type exists in Carthage, Missouri, using turkey waste as a feedstock. See the main article on thermal depolymerization. Gasification is a similar process, but is not technically recycling since polymers are not likely to become the result.

Recently, a process has also been developed in which many kinds of plastic can be used as a carbon source in the recycling of scrap steel. ^[1]

Yet another process that is gaining ground with startup companies (especially in Australia, United States and Japan) is Heat Compression. The heat compression process takes all unsorted, cleaned plastic in all forms, from soft plastic bags to hard industrial waste, and mixes the load in tumblers (large rotating drums resembling giant clothes dryers). The process generates heat from the friction of the plastic materials rubbing against each other inside the drum, eventually melting all, or most of the material. The materials are then pumped out of the drum through heated pipes into casting moulds. The most obvious benefit to this method is the fact that all plastic is recyclable, not just matching forms. But criticism rises from the energy costs of rotating the drums, and heating the post-melt pipes.

Applications

The most-often recycled plastic, HDPE or number 2, is downcycled into plastic lumber, tables, roadside kerbs, benches, truck cargo liners, trash receptacles, stationery (e.g rulers) and other durable plastic products and is usually in demand. The white plastic "peanuts" used as packing material are often accepted by shipping stores for reuse.

In Israel successful trials have shown that plastic films recovered from mixed municipal waste streams can be recycled into useful products.^[2]

Similarly, agricultural plastics such as mulch film, drip tape and silage bags are being diverted from the waste stream and successfully recycled into bulk resin commodities in Labelle, FL.^[3] Historically, these agricultural plastics have primarily been either landfilled or burned on-site in the fields of individual farms.^[4]

The environmental benefits of recycling plastic are that it produces less sulphur dioxide, less waste and less carbon dioxide.

Consumers

Plastic recycling rates lag far behind those of other items, such as newspaper (about 80%) and cardboard (about 70%). One reason is that consumers often don’t understand the types of plastics that can be recycled in their area. Types of plastics are assigned a number, which is usually stamped or printed on the bottom of containers and surrounded by a pyramid of arrows. (See the table in Plastic.) Numbers 1, 2, and 6 are the most-often recycled plastics in the United States. Many programs exist in the United States and the reduction of weight in numerous packaging applications has been significant over the last 25 years.

Consumers can find out which plastics are accepted in their local area and how to prepare and transfer them by contacting their local recycling hauler (usually the local city or county solid waste or public works department, or a private company). Generally, paper labels do not need to be removed from plastic bottles or containers, but lids should be thrown away because they typically are made from a type of plastic that is not recyclable. Plastic bottles and containers must be rinsed, squashed, and placed in recycle bins for collection. Plastic grocery bags are often accepted by stores in recycling containers placed near the entranceways. ^[5]

In the UK, the amount of post-consumer plastic being recycled is relatively low^[6], due in part to a lack of recycling facilities and services for plastic and many other materials. Only a few exist in the whole country and many people do not know what plastics can be recycled. Furthermore, most of the plastic that is re-used is sent to China to be recycled, raising ethical questions as well as concern that such recycling costs more CO2 emissions than simply burying in landfill. Others will counterclaim that many recycling programs exist in the UK, and landfill is not an option if there are no sites to fill. One of the largest recyclers of PET bottles is based there and also has initiated one of the first "bottle to bottle" recycling businesses in the world. Many other 'waste' products are sent to China for recycling, using the often empty containers that are used for delivering goods to the UK and that are sent back for re-use.

See also

• Recycling of PET Bottles
• Scrap
• Recycling

References

Books

"Polymer Recycling: Science, Technology and Applications", John Scheirs, 1998