Plastic recycling
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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 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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[edit] Processing
When compared to other materials like glass and metal materials, plastic polymers require greater processing to be recycled.[citation needed] 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 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 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.[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 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.
[edit] Applications
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 "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.[2]
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.[3] Historically, these agricultural plastics have primarily been either landfilled or burned on-site in the fields of individual farms.[4]
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 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. One kilometer of one-lane road would need 60 kg of old plastic. Since the US annually uses 100 million metric tons of plastic, it could pave 1 2/3 billion km of single-lane road this way.
[edit] 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, called a resin identification code, which is usually stamped or printed on the bottom of containers and surrounded by a recycling symbol (a triangle of arrows). (See the table in Plastic.) Numbers 1, 2, and 6 are the most-often recycled plastics in the United States. Many municipal recycling programs exist in the United States.
Consumers can find out which plastics are accepted in their local area and how to recycle, 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]
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In the UK, the amount of post-consumer plastic being recycled is relatively low[6], due in part to a lack of recycling facilities. 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.
[edit] Plastic Identification Code
Seven groups of plastic polymers,[7] 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 food packaging and containers can voluntarily mark their products with the PIC. Consumers can identify the plastic types based on the codes usually found at the base or at the side of the plastic food 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.
| Plastic Identification Code | Type of plastic polymer | Properties | Common Packaging Applications |
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Polyethylene Terephthalate (PET, PETE) | Clarity, strength, toughness, barrier to gas and moisture. | Soft drink, water and salad dressing bottles; peanut butter and jam jars |
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High Density Polyethylene (HDPE) | Stiffness, strength, toughness, resistance to moisture, permeability to gas. | Milk, juice and water bottles; trash and retail bags. |
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Polyvinyl Chloride (V) | Versatility, clarity, ease of blending, strength, toughness. | Juice bottles; cling films; PVC piping |
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Low Density Polyethylene (LDPE) | Ease of processing, strength, toughness, flexibility, ease of sealing, barrier to moisture. | Frozen food bags; squeezable bottles, e.g. honey, mustard; cling films; flexible container lids. |
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Polypropylene (PP) | Strength, toughness, resistance to heat, chemicals, grease and oil, versatile, barrier to moisture. | Reusable microwaveable ware; kitchenware; yogurt containers; margarine tubs; microwaveable disposable take-away containers; disposable cups and plates. |
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Polystyrene (PS) | Versatility, clarity, easily formed | Egg cartons; packing peanuts; "Styrofoam"; disposable cups, plates, trays and cutlery; disposable take-away containers; |
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Other | Dependent on polymers or combination or polymers. | Beverage bottles; baby milk bottles. |
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 2,220,000,000 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.
[edit] References
- ^ Steel CNN, Accessed 9.11.06
- ^ Plastic trial procedure Oaktech Environmental website, accessed 9.11.06
- ^ Plastic Composite Railroad Tie Facts Plastic Composite Railroad Ties website, accessed 01.21.08
- ^ Recycling Used Agricultural Plastics James W. Garthe, Paula D. Kowal, PennState University, Agricultural and Biological Engineering
- ^ Plastic bag recycling Charity Guide Website, Accessed 11.11.06
- ^ Plastics wasteonline.org.uk, Accessed 10.18.07
- ^ Safe Use Of Plastic Food Packaging And Containers.
[edit] External Links
- Why are So Few Food Containers Made of Recyclable Plastic? about.com
- American Chemical Society: Plastic Packaging Resin Codes [1]
