Page 238 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012F-600 July 26, 2001 11:16

Polymers, Recycling 747

and engineers have been devoted to making polymers good mechanical properties. While such materials have
last longer, but the problems associated with accumulated their application, it seems unlikely that such practices will
waste have shifted the focus. It should be realized that have a major impact on problems associated with the use
degradation results in a loss of polymeric material, which, of commodity or engineering plastics.
while decreasing the burden of waste accumulation, has A problem with centrally disposing of degradable poly-
some negative environmental limitations. Polymers rep- mers is that degradation does not readily occur in dry land-
resent a state of matter of low entropy (low disorder), and fills, and those with water present lead to leachants, which
degradation, where the polymers revert to lower molec- may contaminate ground water if they are not properly
ular weight materials, represents an increase in their en- contained. The best way of dealing with them is through
tropy. The second law of thermodynamics tells us that for a composting facility in which liberated gases like
isolated systems, entropy spontaneously increases (they methane can be captured and used as fuel, and the result-
become disordered) , and that an investment of energy is ing compost can be utilized to benefit agriculture. Unfor-
required to reverse this trend. Thus, the entropy increase tunately, such facilities are rare, so their lack has limited
associated with degradation is equivalent to an energy ex- the success of this approach. For example, disposable di-
penditure necessary to produce the reverse, with its conse- apers can constitute 1–2% of landfill volume, so efforts
quential environmental impact. A combination of entropy have been made to fabricate them from degradable com-
and energy considerations (thermodynamically expressed ponents. However, this effort has not gained favor, largely
as “free energy”) is necessary in considering alternatives. because of the lack of the required infrastructure.
On the positive side, degradation of waste polymers re- While the impact of degradable polymers in affecting
duces litter and decreases the need for means for otherwise the plastic waste problem may be limited, they can be of
dealing with the waste polymers, which are often energy great importance in medical applications such as for fab-
demanding and environmentally damaging. The impor- ricating biodegradable sutures which degrade as wounds
tance of such “cosmetic” measures depends on the nature heal and for controlled release of drugs which may be
of the society, and is probably more important in “first dissolved or dispersed in a biodegradable polymer matrix.
world” environments where neatness and absence of litter
are highly valued.
D. Incineration
Some successes of the employment of degradation
should be cited. The “rings” used to bind beverage cans Incineration involves the burning of polymers. To do this
together are often made from long-lasting polyethylene. in an environmentally desirable way, the liberated energy
When these are discarded, they can lead to strangling shouldbecapturedandused,usuallytogenerateelectricity
of birds and marine life. Such polyethylene can be ren- in a “trash-to-energy” facility. Polymers are an excellent
dered photodegradable by introducing ultraviolet light- fuel, with heating value comparable to oil, and better than
absorbing carbonyl groups through the use of carbon coal and most components of trash. The principal prod-
monoxide as well as ethylene in its synthesis. The ab- ucts of complete burning of most polymers are water and
sorption of light by such groups induces photochemical carbon dioxide, which are relatively environmentally be-
reactions leading to scission of the polymer chains and nign (other than effects of CO 2 as a “greenhouse gas”).
their reduction to a sufficiently low molecular weight so The procedure has not been favored by most environmen-
that further degradation by bacterial action becomes pos- talists and the public since the vision is conditioned by old
sible. The time for such degradation may be controlled by incinerators which are not state of the art and are polluting,
varying the concentration of such groups. This practice producing toxic fumes and ash.
proves economical and is sometimes required by law. From a thermodynamic point of view, trash-to-energy
Polyethylene sheets are sometimes used as “mulch” in procedures are superior in that the energy content of the
farming to cover fields and prevent the growth of weeds. plastic is recovered, so beyond reuse and recycling, it is
A problem can be the disposal of such sheets at the end a viable approach, provided the polluting aspects can be
of the growing season. By making these photodegradable eliminated. This is possible, and it has been shown that
with an appropriate lifetime, the sheets can degrade at the modern incinerators do not produce toxic fumes, regard-
end of the growing season, eliminating the need and cost less of the polymer content of their feed stock. The toxic
for their removal. fumes usually result from incomplete combustion, which
The above uses involve photodegradation. While most can be avoided by operating the incinerator at a proper
polymers resist bacterial degradation, it is possible to in- temperature with adequate oxygen supply. Furthermore,
troduce structures that are susceptible to attack by bacteria any toxic substances in the effluent that might result could
or enzymes. There is an appreciable industrial effort eco- be detected through proper monitoring of the effluent and
nomically to produce such polymers which still maintain eliminated by “scrubbing” the fumes.