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750 Polymers, Recycling

One approach that can be economical is through use of duced by these means are free of concerns about bacterial
devices that depend upon the density differences of com- contamination.
ponents. Sometimes components of a pulverized polymer
may be separated in air streams, while in other cases, the
use of liquid baths might be employed. In the latter case, IV. OVERVIEW
separation may sometimes be aided through use of deter-
gents or surface-active agents that selectively interact with It is evident that there is no single solution for polymer
one of the components and aid its floating or sinking. recycling. The choice of an approach is often governed
The effectiveness of any sorting procedure is depen- by economics, and at present it is unlikely that the ma-
dent upon its accuracy. The properties of a separated poly- jority of plastic waste can be economically recycled. This
mer can be appreciably affected by the presence of small fraction will increase with employment of more “design
amounts of a different one. The cost of the sorting pro- for recycling,” improvement of means for sorting, and de-
cess increases, as does the need for its accuracy, so it is velopment of infrastructure. Societal and legislative con-
necessary to seek a compromise between purity and cost. siderations can be as important as technical aspects in
Separation of polymers prior to processing is usually affecting such changes. However, there will always be a
essential since polymer melts are usually very viscous portion of the waste stream for which recycling is not the
and separation procedures for the melt are virtually im- best choice, so alternatives such as generation of energy
possible. This suggests the need for procedures involving are viable considerations.
depolymerization to reduce molecular weight and render
the mixture more susceptible to conventional procedures.
SEE ALSO THE FOLLOWING ARTICLES

B. Secondary and Tertiary Recycling
BIOPOLYMERS • ENERGY FLOWS IN ECOLOGY AND IN THE
These approaches involve chemical modification so as to ECONOMY • HAZARDOUS WASTE INCINERATION • RE-
reduce the molecular weight of the polymeric species, NEWABLE ENERGY FROM BIOMASS • WASTE-TO-ENERGY
often reducing them back to monomers. Such low- SYSTEMS
molecular-weight materials may then be separated by con-
ventional means such as distillation, after which they may
serve as the feed stock for repolymerization or other chem- BIBLIOGRAPHY
ical operations. The means for molecular weight reduc-
tion may involve pyrolysis, hydrolysis, or alcoholysis, the Andrews, G. D., and Subramanian, P. M. (eds.). (1992). “Emerging Tech-
choice being dependent upon the kind of bonding occur- nologies in Plastic Recycling,” American Chemical Society, Washing-
ring in the polymers involved. ton, DC.
ARC. (1999). “ARC’99. 6th Annual Recycling Conference Proceed-
A disadvantage of these procedures is that they are
ings,” SPE Recycling Division.
usually demanding of energy necessary for breaking the Brandrup, J., Bittner, M., Michaeli, W, and Menges, G. (eds.). (1996).
chemical bonds holding the monomer units together in the “Recycling and Recovery of Plastics,” Hanser, Munich.
polymer. They are in opposition to the desire for prevent- Ehrig, R. J. (ed.). (1992). “Plastics Recycling, Products and Processes,”
ingentropyincreaseinthatoneisconvertingalow-entropy Hanser, Munich.
Le Mantia, F. P. (ed.). (1993). “Recycling of Plastic Materials,” Chem
polymer species into higher entropy monomers.
Tec Toronto.
An advantage is that cleaner separations are possible, Mishra, M. K. (ed.). (1999). “Special issue on polymer and fiber recy-
and the reduced molecular weight material may be sepa- cling,” Polymer Plastics Technol. Eng. 38.
ratedfromothercomponentssuchasfillersandreinforcing Rader, C. P., Baldwin, S. D., Cornell, D. D., Sadler, G. D., and
fiber by processes such as filtration. Application is pos- Stockel, R. F. (eds.). (1995). “Plastics, Rubber, and Paper Recycling,
A Pragmatic Approach,” American Chemical Society, Washington,
sible to materials like carpets, disposable cameras, and
DC.
laminated bottles, where separation by primary recycling Stein, R. S. (1992). “Polymer recycling: Opportunities and limitations,”
means is difficult or impossible. Also, the materials pro- Proc. Natl. Acad. Sci. USA 89, 635–838.