Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products, sometimes completely different in form from their original state. For instance, this could mean melting down soft drink bottles then casting them as plastic chairs and tables.
Before recycling, plastics are sorted according to their resin identification code, a method of categorization of polymer types, which was developed by the Society of the Plastics Industry in 1988. Polyethylene terephthalate, commonly referred to as PET, for instance, has a resin code of 1.
When compared to other materials like glass and metal materials, plastic polymers require greater processing to be recycled. Plastics have a low entropy of mixing, which is due to the high molecular weight of their large polymer chains. A macromolecule interacts with its environment along its entire length, so its enthalpy of mixing is large compared to that of an organic molecule with a similar structure. Heating alone is not enough to dissolve such a large molecule; because of this, plastics must often be of nearly identical composition in order to mix efficiently.
When different types of plastics are melted together they tend to phase-separate, like oil and water, and set in these layers. The phase boundaries cause structural weakness in the resulting material, meaning that polymer blends are only useful in limited applications.
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 reclaimed plastic is not recyclable because the variance in properties and melt temperatures.
Many such problems can be resolved 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, USA, 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, USA, using turkey waste as input material. 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.
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 most obvious benefit to this method is the fact that all plastic is recyclable, not just matching forms. However, criticism rises from the energy costs of rotating the drums, and heating the post-melt pipes.
Applications
Main article: Recycling of PET bottlesPost-consumer PET (number 1) is often sorted into different color fractions. This sorted post-consumer PET waste is crushed, pressed into bales and offered for sale to recycling companies. PET flakes are used as the raw material for a range of products that would otherwise be made of polyester.
Main article: PVC recyclingPVC- or Vinyl Recycling has historically been difficult to perfect on the industrial scale. But within the last decade several viable methods for recycling or upcycling PVC plastic have been developed.
The most-often recycled plastic, HDPE or number 2, is downcycled into plastic lumber, tables, roadside curbs, benches, truck cargo liners, trash receptacles, stationery (e.g. rulers) and other durable plastic products and is usually in demand.
The white plastic foam 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 household products such as buckets.
Similarly, agricultural plastics such as mulch film, drip tape and silage bags are being diverted from the waste stream and successfully recycled into much larger products for industrial applications such as plastic composite railroad ties. Historically, these agricultural plastics have primarily been either landfilled or burned on-site in the fields of individual farms.
CNN reports that Indian Dr. S. Madhu of the Kerala Highway Research Institute has formulated a road surface that includes recycled plastic. Aggregate, bitumen (asphalt) with plastic that has been shredded and melted at a temperature below 220 degrees C (428 °F) to avoid pollution. This road surface is claimed to be very durable and monsoon rain resistant. The plastic is sorted by hand, which is economical in India. The test road used 60 kg of plastic for an approx. 500m long, 8m wide, two-lane road.
Financial justification
In 2008, the price of PETE dropped from $370/ton in the US to $20 in November.
Consumer education
United States
Plastic recycling rates lag far behind those of other items, such as newspaper (about 80%) and corrugated fiberboard (about 70%). Low national plastic recycling rates have been due to the complexity of sorting and processing, unfavorable economics, and consumer confusion about which plastics can actually be recycled. Part of the confusion has been due to the recycling symbol that is usually on all plastic items. This symbol is called a resin identification code. It is stamped or printed on the bottom of containers and surrounded by a triangle of arrows. (See the table in Plastic.) The intent of these arrows was to make it easier to identify plastics for recycling. The recycling symbol doesn’t necessarily mean that the item will be accepted by residential recycling programs. They just indicate the plastic resin content.
United Kingdom
In the UK, the amount of post-consumer plastic being recycled is relatively low, due in part to a lack of recycling facilities.
Plastic Identification Code
Main article: Resin identification codeSeven groups of plastic polymers, each with specific properties, are used worldwide for packaging applications (see table below). Each group of plastic polymer can be identified by its Plastic Identification code (PIC) - usually a number or a letter abbreviation. For instance, Low-Density Polyethylene can be identified by the number 4 and/or the letters "LDPE". The PIC appears inside a three-chasing arrow recycling symbol. The symbol is used to indicate whether the plastic can be recycled into new products.
The PIC was introduced by the Society of the Plastics Industry, Inc. which provides a uniform system for the identification of different polymer types and helps recycling companies to separate different plastics for reprocessing. Manufacturers of plastic products are required to use PIC labels in some countries/regions and can voluntarily mark their products with the PIC where there are no requirements. Consumers can identify the plastic types based on the codes usually found at the base or at the side of the plastic products, including food/chemical packaging and containers. The PIC is usually not present on packaging films, as it is not practical to collect and recycle most of this type of waste.
The quantity of post-consumer plastics recycled has increased every year since at least 1990. In 2006 the amount of plastic bottles recycled reached a record high of 2.2 trillion pounds.
The amount of PET bottles recycled in 2006 increased more than 102 million pounds compared to 2005.
HDPE bottle recycling increased in 2005 to 928 million pounds.
All plastic bottles were recycled at a rate of 24 percent in 2005.
See also
- Reuse of water bottles
References
- ^ http://cygnus-group.com/use-less-stuff/Paper-and-Plastic-Grocery-Bag-LCA-Summary-3-21-08.pdf
- ^ http://www.allbusiness.com/chemicals/chemicals-overview/9060603-1.html
- ^ Steel CNN, Accessed 9.11.06
- ^ http://www.allbusiness.com/consumer-products/office-supplies-equipment/6916130-1.html
- ^ Plastic trial procedure Oaktech Environmental website, accessed 9.11.06
- ^ Agricultural plastics recycling process Agricultural plastics recycling website, accessed 07.11.08
- ^ Plastic
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