Page 239 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GQT Final Pages
Encyclopedia of Physical Science and Technology EN012F-600 July 26, 2001 11:16

748 Polymers, Recycling

Toxic materials in the ash are almost always not of poly- of such recycling depends on whether the economic value
mer origin, but arise from other materials, usually contain- of the resulting products exceeds the cost of the energy
ing heavy metals, that are mixed with the polymer feed necessary to bring about the recycling. Such energy con-
stock. A desirable approach is to have a crude sorting of siderations should include that expended in collecting and
the feed stock so as to prevent their mixing with the poly- sorting the waste feed stock, in addition to that utilized in
mer content. Their inclusion could be reduced through its processing.
parallel societal efforts such as making available ready
means for recycling items like batteries and possibly en-
A. Primary Recycling
couraging the use of such recycling facilities through of-
fering reverse deposits. Also, the ash can be converted to a Primary recycling involves the conversion of waste plastic
low-solubility slag through addition of materials like lime into a new product by a physical process, such as melt-
to the feed stock. In any case, incineration considerably ing and remolding, without chemical modification of the
reduces the volume of the waste so as to make its proper polymer chemistry. It is the cheapest way to recycle, but
disposal more feasible. is limited in application. It may be applied to mixed or
A consideration is what might happen to this possible “commingled” polymers or to those which are separated
content of toxic material had incineration not been done. into separate types.
It would then probably enter into the landfill, probably in a Most collected waste plastics consist of mixtures of dif-
more soluble form, and eventually leach out and enter the ferent composition and color, and the product formed by
environmentinanuncontrolledmanner.Isittoconcentrate their primary recycling usually will be inferior in proper-
it in the form of an insoluble slag of reduced volume which ties to the original components. Most plastics of different
can be disposed of in a more satisfactory manner. chemical structure are immiscible. That is, when melted,
A societal problem in providing state-of-the-art incin- they will be phase separated and consist of domains of
erators is their cost. In the short run, the approach is of- their components having a size dependent upon the pro-
ten more expensive than alternative disposal means, but cessing conditions. The boundaries between such polymer
in the long run, it becomes cost-effective and environ- phases of different composition are often weak and lead
mentally desirable. It is a problem, however, to persuade to mechanical failure when the article is subject to strain.
the public and politicians to adopt the long-range view, For this reason, articles fabricated from such commin-
particularly for the latter, where reelection is often influ- gled polymers often have inferior mechanical properties
enced by the short-range consequences of their term of leading to limited application and diminished value. They
office. may be used for applications such as park bench and pier
There is sometimes public resistance to the locating of construction and parking barriers, where the mechanical
incinerators and the adoption of the “not in my backyard” demands are not great. For such applications, they can
syndrome. However, this problem has been conquered, be superior to the usual alternative, wood, in that they
for example, by the Japanese, who combine the facilities resist rot and do not require the application of preserva-
with heated swimming pools and greenhouses, which win tives, which are often polluting. However, the number of
public acceptance. With proper design, these can be better such applications is not great enough to accommodate the
neighbors than composting facilities or landfills. amount of material available for recycling.
Boundaries between immiscible polymers can be
strengthened by use of “compatibilizers” that are often
III. RECYCLING copolymers having affinities for both components, which
serve to bind them together. A peripheral advantage in us-
The practice of recycling has won strong favor among en- ing such agents is that they usually result in the domains
vironmentalists. They have favored legislation requiring being smaller, resulting in improved mechanical and op-
specified levels for recycling. Unfortunately, such legis- tical properties. A disadvantage is that the use of such
lation sometimes ignores theromodynamic and economic agents adds to the cost. In some cases, the components
limitations, leading to their opposition by polymer pro- are sufficiently reactive toward each other that chemical
ducers and fabricators. It seems sensible to convert waste binding can occur when the mixture is heated with possi-
products into articles having a second or more use. En- ble addition of a catalyst.
tropy concepts can be applied in that the use of plastic Where appearance is important, the use of mixtures of
articles leads to their wear and eventual discarding, rep- polymers of different color can be a problem. The mixture
resenting an increase in disorder and entropy. The second assumes an intermediate color, so uses are often selected
law tells us that to convert these waste products back to where color is not important or where a dark color is ac-
useful products, energy must be expended. The viability ceptable.