2. RECYCLING ISSUES IN MATERIALS SCIENCE
Important stages in the materials cycle where materials science and engineering
plays a significant role are recycling and disposal.
The issues of recyclability and disposability are important when new materials are
being designed and synthesized.
During the materials selection process, the ultimate disposition of the materials
employed should be an important criterion.
From an environmental perspective, the ideal material should be either
totally recyclable or completely biodegradable.
Recyclable means that a material, after having completed its life cycle in one
component, could be reprocessed, could reenter the materials cycle, and could be
reused in another component—a process that could be repeated an indefinite
number of times.
Engineering materials exhibit varying degrees of recyclability and biodegradability.
14. Summary of recycling issues in METALS
Most metal alloys (e.g., Fe, Cu), to one degree or another experience corrosion
and are also biodegradable.
Some metals (e.g., Hg, Pb) are toxic and, when land-filled, may present health
hazards.
Alloys of most metals are recyclable; on the other hand it is not feasible to recycle
all alloys of every metal.
The problems of recycling involve separation of various alloys types (e.g.,
aluminum from ferrous alloys) after dismantling and shredding.
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 contaminants,
and render the material non recyclable.
Aluminum alloys are very corrosion resistant, and, therefore, non biodegradable.
Aluminum is the most important recyclable nonferrous metal.
Since aluminum is not easily corroded, it may be totally reclaimed.
The primary sources of recycled aluminum are used beverage cans and scrapped
automobiles.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31. 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.
Advantages of utilizing recycled glass include more rapid and increased production
rates and a reduction in pollutant emissions.
COMPOSITE MATERIALS
Composites are inherently difficult to recycle because they are multiphase in
nature.
Complete phase/material separation is virtually impossible, and recycling
procedures that require material separation are impractical.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43. PLASTICS AND RUBBER
Synthetic polymers (including rubber) are so popular as engineering materials lies
with their chemical and biological inertness. This characteristic is really a liability
when it comes to waste disposal.
Polymers are not biodegradable, and, as such, they constitute a significant land-fill
component; major sources of waste are from packaging, junk automobiles,
automobile tires, and domestic durables.
Biodegradable polymers have been synthesized, but they are relatively expensive
to produce.
Since some polymers are combustible and do not yield appreciable toxic or
polluting emissions, they may be disposed of by incineration(to burn).
Thermoplastic polymers, specifically polyethylene and polypropylene, are those
most amenable to reclamation and recycling, since they may be reformed upon
heating.
The recycled plastic is less costly than the original material, and quality and
appearance are generally degraded with each recycle.
Typical applications for recycled plastics include shoe soles, tool handles etc.
44. The recycling of thermoset resins is much more difficult since these materials are
not easily remolded or reshaped due to their cross linked or network structures.
Rubber materials present some disposal and recycling challenges.
When vulcanized, they are thermoset materials, which makes chemical recycling
difficult.
Scrap tires have been utilized as a fuel for some industrial applications (e.g.,
cement plants), but yield dirty emissions.
Recycled rubber tires that have been split and reshaped are used in a variety of
applications such as automotive bumper guards, mud flaps, door mats, and
conveyor rollers; and, of course, used tires may also be recapped.
