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MEEF - Recycling Technologies  - previous page

 


Introduction
  - See
Plastics Glossary / Plastic Abbreviations

Recycling of plastics that used to end up only at city landfills or incinerators is increasing around the world. As with any technological trend, the engineering profession plays an important role. Discarded plastic products and packaging make up a growing portion of Municipal Solid Waste(MSW). The Environmental Protection Agency (EPA) estimates that by the year 2000, the amount of plastics throw away will be 50 percent greater than at the beginning of the 1990s. EPA also says that plastic waste accounts for about one-fifth of all waste in the waste stream. Over the past two decades, recycling of plastics has dramatically increased. After years of predictions that plastics recycling would never be widespread because processes were inefficient, too expensive or not practical, the tide of waste headed to the landfill is slowly being turned.

The difference between a polymer and a plastic

The term “plastics” is used to describe a wide variety of resins or polymers with different characteristics and uses. Polymers are long chains of molecules, a group of many units, taking its name from the Greek “poly” (meaning “many”) and “meros” (meaning “parts” or “units”).

The term “polymer” is often used as a synonym for plastic, but many other types of molecules — biological and inorganic — are also polymeric. While all plastics are polymers, not all polymers are plastic. Polymers are rarely useful in themselves and are most often modified or compounded with additives (including colours) to form useful materials. The compounded product is generally termed a plastic. Most people have little contact with "polymers" because most articles that they come across are actually modified and coloured and therefore are actually plastics. Polymers can be classified in many ways, based on how they are developed and perform. For this discussion of recycling, an understanding of two basic types of polymers is helpful:
 

  • Thermoplastic polymers can be heated and formed, then heated and formed again and again. The shape of the polymer molecules are generally linear or slightly branched. This means that the molecules can flow under pressure when heated above their melting point.

  • Thermoset polymers undergo a chemical change when they are heated, creating a three-dimensional network. After they are heated and formed, these molecules cannot be re-heated and re-formed.

Comparing these types, thermoplastics are much easier to adapt to recycling.

 

  

PETE Polyethylene Terephthalate (PET) -   Soda & water containers, some waterproof packaging. Recycling PET is similar to the polyethylenes (PE). Bottles may be color sorted and are ground up and washed. Unlike polyethylene, PET sinks in the wash water while the plastic caps and labels are floated off. The clean flake is dried and often repelletized.

    Recycled PET has many uses and well established market for this useful resin. By far, the largest usage is in textiles. Carpet companies can often use 100% recycled resin to manufacture polyesther carpets in a variety of colors and textures. PET is also spun like cotton candy to makr fiber filling for pillows, quilts and jackets. PET can also be rolled ito clear sheets or ribbon for VCR and audio cassettes. In addition a substantial quantity goes back into the bottle market.  


HDPE High-Density Polyethylene -  Milk, detergent & oil bottles, Toys and plastic bags. HDPE is called natural since that is it's natural color, and it is the most valuable because it can be made into any color when it is recycled. Other products are often packed in brightly colored bottles whiched are mixed together at recycling plants into mixed color or rainbow bales. Most of this material is later dyed black after it is processed.

Recycling HDPE is a pretty simple process. The bales are broken aprt and ground into small flakes. These flakes are then washed and floated to removed and heavy (Sinkable) contaminants. This cleaned flake is then dried in a stream of hot air and may be boxed and sold in that form. More sophisticated plastic plants may reheat these flakes, add pigment to change the color and run the material through a pelletizer. This equipment forms little beads of plastic that can then be reused in injection molding presses to create new products. Some end uses for recycled HDPE are plastic pipes,lumber, flower pots, trash cans, or formed back into non food application bottles.

 

V Vinyl/Polyvinyl Chloride (PVC) - Food wrap, vegetable oil bottles, blister packages.

 


LDPE Low-Density Polyethylene - Many plastic bags. Shrink wrap, garment bags. It ic chemically similar to HDPE but it is less dense and more flexible. Most polyethylene film is made from LDPE which you often see as plastic bags and grocery sacks. This scrap may be clear or pigmented and it is hand sorted and baled at recycling processing plants.

Recycling LDPE is verry similar to HDPE except special grinders are used to handle the thin films. The films are often washed and repelletized or used directly to make new products. Some end uses for recycled LDPE are plastic trash bags and grocery sacks, plastic tubing, agricultural film, and plastic lumber.

 

PP Polypropylene - Refrigerated containers, some bags, most bottle tops, some carpets, some food wrap.

 


PS Polystyrene - Throwaway utensils, meat packing, protective packing.
 

 


OTHER Usually layered or mixed plastic.  No recycling potential - must be land filled.




These symbols are meant to indicate the type of plastic,  not its recyclability. Types 1 and 2 are commonly recycled. Type 4 is less commonly recycled. The other types are generally not recycled, except perhaps in small test programs. Common plastics polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) do not have recycling numbers. Chemical engineers will say that there are many more types and uses for polymers. But most debate in recycling focuses on these seven categories.

Uncoded Plastics

Plastic consumer goods not identified by code numbers are not usually collected. Plastic tarps, pipes, toys, computer keyboards, and a multitude of other products simply do not fit into the numbering system that identifies plastics used in consumer containers. There are actually thousands of different varieties of plastic resins or mixtures of resins. These are developed to suit the needs of particular products. There is limited recycling of some of these specific plastic products in truckload quantities from industrial sources. No one has entered the business of collecting a variety of these plastics in small quantities.

The Problem with Plastics Recycling

When glass, paper and cans are recycled, they become similar products which can be used and recycled over and over again. With plastics recycling, however, there is usually only a single re-use. Most bottles and jugs don't become food and beverage containers again. For example, pop bottles might become carpet or stuffing for sleeping bags. Milk jugs are often made into plastic lumber, recycling bins, and toys.

A recent development has been the bottles-to-bottles recycling of "regenerated" pop bottles. Though it is technologically possible to make a 100% recycled bottle, there are serious economic questions. Also, some critics claim that the environmental impact of the regeneration process is quite high in terms of energy use and hazardous by-products.

Currently only about 3.5% of all plastics generated is recycled compared to 34% of paper, 22% of glass and 30% of metals. At this time, plastics recycling only minimally reduces the amount of virgin resources used to make plastics. Recycling papers, glass and metal, materials that are easily recycled more than once, saves far more energy and resources than are saved with plastics recycling.

Consider this example: polyvinyl chloride (PVC) bottles are hard to tell apart from PET bottles, but one stray PVC bottle in a melt of 10,000 PET bottles can ruin the entire batch. It's understandable why purchasers of recycled plastics want to make sure that the plastic is sorted properly. Equipment to sort plastics is being developed, but currently most recyclers are still sorting plastics by hand. That's expensive and time consuming.  Plastics also are bulky and cumbersome to collect. In short, they take up a lot of space in recycling trucks.

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