Introduction - See
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:
Comparing these types,
thermoplastics are much easier to adapt to recycling.
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
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.
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
- Food wrap, vegetable oil bottles, blister packages.
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.
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
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.
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
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
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.