Plastic recycling

From Wikipedia, the free encyclopedia

Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products. For instance, this could mean melting down polyester soft drink bottles then spinning the polymer into fibres.

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

1 Obstacles
2 Alternative processes
3 Applications
4 Consumers
5 See also
6 References

[edit] 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 very, very 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.

[edit] Alternative processes

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 to recycle PET by a process of methanolysis, but 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]

[edit] Applications

The most-often recycled plastic, HDPE or number 2, is recycled into plastic lumber, tables, 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]

[edit] Consumers

Currently in the United States, plastics recycling rates lag far behind those of other items, such as newspaper (about 80%) and cardboard (about 70%), and one reason is that consumers often don’t understand the kinds of plastics they can recycle 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.

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. ^[3]

In the UK, not very much plastic is currently recycled due to a lack of recycling facilities 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.