Page 211 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 7 Polymers: Structure, General Properties, and Applications

7.8 Biodegradable Plastics

Plastic wastes contribute about 10% of municipal solid waste by weight; on a vol-
ume basis, they contribute between two and three times their weight. Qnly about
one-third of plastic production goes into disposable products, such as bottles,
packaging, and garbage bags. With the growing use of plastics and great concern
over environmental issues regarding the disposal of plastic products and the short-
age of landfills, major efforts are underway to develop completely biodegradable
plastics. The first attempts were made in the 19805 as a possible solution to road-
side litter.
Traditionally, most plastic products have been made from synthetic polymers
that are derived from nonrenewable natural resources, are not biodegradable, and
are difficult to recycle. Biodegradability means that microbial species in the environ-
ment (e.g., microorganisms in soil and water) will degrade a portion of (or even the
entire) polymeric material under the proper environmental conditions and without
producing toxic by-products. The end products of the degradation of the biodegrad-
able portion of the material are carbon dioxide and water. Because of the variety of
constituents in biodegradable plastics, these plastics can be regarded as composite
materials. Consequently, only a portion of them may be truly biodegradable.
Three different biodegradable plastics have thus far been developed. They
have different degradability characteristics, and they degrade over different periods
of time (anywhere from a few months to a few years).
l. The starch-based system is the farthest along in terms of production capacity.
Starch may be extracted from potatoes, wheat, rice, or corn. The starch gran-
ules are processed into a powder, which is heated and becomes a sticky liquid.
The liquid is then cooled, shaped into pellets, and processed in conventional
plastic-processing equipment. Various additives and binders are blended with
the starch to impart special characteristics to the bioplastic materials. For ex-
ample, a composite of polyethylene and starch is produced commercially as
degradable garbage bags.
2. In the lactic-based system, fermenting feedstocks produce lactic acid, which is
then polymerized to form a polyester resin. Typical uses include medical and
pharmaceutical applications.
3. In the fermentation of sugar (the third system), organic acids are added to a sugar
feedstock. With the use of a specially developed process, the resulting reaction
produces a highly crystalline and very stiff polymer, which, after further process-
ing, behaves in a manner similar to polymers developed from petroleum.
Numerous attempts continue to be made to produce fully biodegradable plas-
tics by the use of various agricultural waste (agrowastes), plant carbohydrates, plant
proteins, and vegetable oils. Typical applications include the following:

° Disposable tableware made from a cereal substitute, such as rice grains or
wheat flour
° Plastics made almost entirely from starch extracted from potatoes, wheat, rice,
or corn
° Plastic articles made from coffee beans and rice hulls that are dehydrated and
molded under high pressure and temperature
° Water-soluble and compostable polymers for medical and surgical applications
° Food and beverage containers (made from potato starch, limestone, cellulose, and
water) that can dissolve in storm sewers and oceans without affecting marine life
or wildlife.