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20.5 Recycling Issues in Materials Science and Engineering • 795interactions with the environment (natural chemicals, microorganisms, oxygen, heat,
sunlight, etc.), the material deteriorates and returns to virtually the same state in
which it existed prior to the initial processing. Engineering materials exhibit varying
degrees of recyclability and biodegradability.Metals
Most metal alloys (i.e., Fe, Cu), to one degree or another, experience corrosion and
are also biodegradable. However, some metals (i.e., Hg, Pb) are toxic and, when land-
filled, may present health hazards. Furthermore, alloys of most metals are recyclable;
on the other hand, it is not feasible to recycle all alloys of every metal. In addition,
the quality of alloys that are recycled tends to diminish with each cycle.
Product designs should allow for the dismantling of components composed of
different alloys. Another of the problems of recycling involves separation of various
alloys types (i.e., aluminum from ferrous alloys) after dismantling and shredding;
in this regard, some rather ingenious separation techniques have been devised (i.e.,
magnetic and gravity). Joining of dissimilar alloys presents contamination problems;
for example, if two similar alloys are to be joined, welding is preferred over bolting
or riveting. Coatings (paints, anodized layers, claddings, etc.) may also act as contam-
inants, and render the material nonrecyclable. These examples illustrate why it is so
important to consider the entire life cycle of a product at the beginning stages of its
design.
Aluminum alloys are very corrosion resistant and, therefore, nonbiodegradable.
Fortunately, however, they may be recycled; in fact, aluminum is the most important
recyclable nonferrous metal. Since aluminum is not easily corroded, it may be totally
reclaimed. A low ratio of energy is required to refine recycled aluminum relative
to its primary production. In addition, there are a large number of commercially
available alloys that have been designed to accommodate impurity contamination.
The primary sources of recycled aluminum are used beverage cans and scrapped
automobiles.Glass
The one ceramic material that is consumed by the general public in the greatest
quantities is glass, in the form of containers. Glass is a relatively inert material, and,
as such, it does not decompose; thus, it is not biodegradable. A significant proportion
of municipal land-fills consists of waste glass; so also does incinerator residue.
In addition, there is not a significant economic driving force for recycling glass.
Its basic raw materials (sand, soda ash, and limestone) are inexpensive and readily
available. Furthermore, salvaged glass (also called “cullet”) must be sorted by color
(clear, amber, and green), by type (plate versus container), and by composition
(lime, lead, and borosilicate [or Pyrex]); these sorting procedures are time-consuming
and expensive. Therefore, scrap glass has a low market value, which diminishes its
recyclability. Advantages of utilizing recycled glass include more rapid and increased
production rates and a reduction in pollutant emissions.Plastics and Rubber
One of the reasons that synthetic polymers (including rubber) are so popular as engi-
neering materials lies with their chemical and biological inertness. On the down side,
this characteristic is really a liability when it comes to waste disposal. Most polymers
are not biodegradable, and therefore don’t biodegrade in land-fills; major sources of
waste are from packaging, junk automobiles, automobile tires, and domestic durables.