• Densities of Various Solutions vs. Plastic Separation
  
• Are Polymers Photodegradable?
  
• Separation of Plastic Waste Collected
  
• Recycling Science Fair Projects and Experiments
  

Plastic recycling is the process of taking scrap or waste plastics and recovering the material for use in manufacturing. For instance, this could mean melting down polyester soft drink bottles, and spinning the polymer into fibers.

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

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 not often 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.

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].

Recycling of PET Bottles

Recycling of PET Bottles is the activity whereby bottles made out of PET are collected, sorted and processed in order to reuse the material out of which they are made.

In many countries, PET plastics are coded with the number 1 which is found inside the universal recycling symbol, usually located on the bottom of the container.

PET is used as a raw material for making packaging materials such as bottles and containers for packaging a wide range of food products and other consumer goods. Examples include soft drinks, alcoholic beverages, detergents, cosmetics, pharmaceutical products, edible oils, etc. PET is one of the most common consumer plastics used.

The empty PET packaging is discarded by the consumer after use and becomes PET waste. In the recycling industry, this is referred to as "post-consumer PET." Many local governments and waste collection agencies have started to collect post-consumer PET separately from other household waste. The collected post-consumer PET is taken to recycling centres known as Materials Recovery Facilities (MRF) where it is sorted and separated from other materials such as metal, objects made out of other rigid plastics such as PVC, HDPE, polypropylene, flexible plastics such as those used for bags (generally low density polyethylene), drink cartons, and anything else which is not made out of PET.

Bales of crushed PET bottles

Post-consumer PET is often sorted into different colour fractions: transparent or uncoloured PET, blue and green coloured PET, and the remainder into a mixed colours fraction. The emergence of new colors (such as amber for plastic beer bottles) further complicate the sorting process for the recycling industry.

This sorted post-consumer PET waste is crushed and pressed into bales, which are offered for sale to recycling companies. Transparent post-consumer PET attracts higher sales prices compared to the blue and green fractions. The mixed colour fraction is the least valuable. Recycling companies will further treat the post-consumer PET by shredding the material into small fragments. These fragments still contain residues of the original content, shredded paper labels and plastic caps. These are removed by different processes, resulting in pure PET fragments, or "PET flakes". PET flakes are used as the raw material for a range of products that would otherwise be made of polyester. Examples include polyester fibres, a base material for the production of clothing, pillows, carpets, etc., polyester sheet, strapping, or back into PET bottles.

Worldwide, approximately 1.5 million tons of PET are collected per year. Petcore, the European trade association that fosters the collection and recycling of PET, forecasts that in Europe alone, collection will exceed one million tons by 2010.

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