Page 240 - 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

Polymers, Recycling 749

For some sophisticated processing applications, molec- nylon-66. New carpets are usually installed by profession-
ular properties such as molecular weight and its distribu- als, who remove and carry away the old carpets. This con-
tion, branching, and tacticity can be important. Thus, even stitutes a potential source of recycle feed stock, which is
though the polymers that are mixed are of the same chem- dealt with by relatively experienced personnel. A prob-
ical composition, there may be difficulties if they differ in lem is that the nylon components are usually bound to a
these molecular properties. backing, often jute, which must be separated to permit ef-
The recycle value of separated polymers is superior, but fective recycling. Efforts are underway to develop means
the difficulty of acquiring separated feed stock is greater. to do so. Most currently effective techniques involve sec-
One approach is to select readily identifiable objects that a ondary recycling, as discussed later.
consumer will have no difficulty in separating from others. Recycling means are often affected by public pressure,
A prime success story is that of beverage bottles, usually sometimes based on emotional rather than rational think-
made from polyethylene terephthalate (PET or polyester), ing. Styrofoam fast food utilities had been effectively re-
which has the added advantage that it is a high value- cycled, with collection at establishments being rather ef-
added polymer. Consumers can be encouraged to return ficient. Styrofoam was effectively converted into articles
such bottles to recycle centers, either through social pres- like plastic trays. However, in response to misguided pub-
sure or monetary encouragement, such as receiving a de- lic pressure, most such establishments have converted to
posit when the empty bottle is returned for recycling. The the use of substitute materials with questionable environ-
use of “reverse deposit machines” in supermarkets which mental consequences.
automatically identify the returned bottle, return the de- Regulations often limit the use of the products of pri-
posit to the consumer, and grind the plastic for compact mary recycling. There is concern by FDA about bacterial
shipment greatly facilitates the process. content of recycled articles that may be used in contact
The beverage bottle case serves to indicate an impor- with food, although fabrication temperatures are probably
tant aspect. Originally, such PET bottles were made with sufficiently high in most cases to sterilize the articles.
polyethylene bases. While these functioned well, they had There remains a large quantity of waste plastics arising
to be removed for effective recycling, which added to the from sources not leading to ready separation. The econ-
cost. Since the design and introduction of an all-PET bot- omy of using such mixed plastic depends on the means
tle was slightly more expensive, manufacturers were re- available for separation and collection. Consumer separa-
luctant to do so until forced to by legislation. The lesson is tion can be facilitated by the requirement that identifying
that recycling can proceed more effectively if one designs symbols be placed on plastic articles. However, the effec-
products with recycling in mind. This must sometimes be tiveness is limited by the conscientiousness and the ability
encouraged by means such as legislation. of the consumer to carry out the recommended procedures.
A task is to identify other products where a single Also, the sorting is time dependent, so the real economics
species of polymer can be readily identified and eco- is dependent upon the value of alternate use of the time of
nomically collected for recycling. The cost of collecting the consumer.
and separating mixed polymers from diverse sources is Once sorting is done by the consumer, it is necessary
great, so increasing the efficiency of this process is a chal- that these sorted polymers be kept separate during collec-
lenge. One such source is from used automobiles which tion and delivery to the recycling facility. This requires the
are turned in upon purchase of a new car. Reuse is best use of vehicles having bins to keep the various polymer
through their sale as used cars, but ultimately the state types separate. This adds to the cost of the operation.
of the car becomes sufficiently poor that it is scrapped. As an alternative to sorting by the consumer, one can
Increasingly, plastic parts are employed in cars and the employ “postconsumer sorting,” in which the process is
large-scale scrapper is sufficiently knowledgeable to iden- carried out in a central facility. This may be done man-
tify parts and classify them according to plastic type. ually or by using automatic sorting equipment. The ef-
The process can again be facilitated by “design for re- fectiveness of manual sorting depends on the availability
cycling” in which the number of different kinds of plas- of adequately trained personnel willing to perform this
tic which are used is reduced, and fasteners are such as rather monotonous procedure. There have been impres-
to permit the easy separation of plastic parts from other sive developments in devising automatic devices for sort-
components. A desirable measure is the establishment ing based upon spectroscopic and other signals. This is a
of databases for car parts so that the recycler may read- difficult task considering the variety of kinds, sizes, and
ily identify them and sort and return them to the proper shapes of articles encountered. The equipment is relatively
place. expensive and the economics is dependent on the ability
Another area of growth for recycling is for carpet, to develop equipment capable of adequate sorting at rea-
with the principal polymer components being nylon-6 and sonable cost.