Commonly Encountered Plastics

Commonly Encountered Plastics

The properties of plastics depend upon their chemical composition, along with physical properties, i.e., size, shape and orientation. These properties may be affected or radically changed by compounding (adding in) certain other plastic or nonplastic materials.

The polyolefins may be flexible or semi-rigid to rigid. The chemical makeup of polyolefins are such that the monomers used to form the polymers contain only carbon and hydrogen (PE, PP, PS), or carbon, hydrogen and oxygen (acrylics, polyesters, and the family of plastics called the cellulosics). Of course, any additives compounded into the polymers would carry with them their own chemical composition, but since there are so many possibilities, just the polymers and their properties are considered here.

In 1982, some 40 billion pounds of plastics were used in various applications in the United States, with 90 percent of this total being thermoplastics. The largest single thermoplastic in use today is polyethylene (PE), and its properties are such that if it were polymerized as high-density polyethylene (HOPE), it would be semi-rigid to rigid and suitable to be blow-molded into one of several types of bottles, containers or similar products. If it were polymerized as low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE), it would be flexible and suitable for making trash bags, agricultural film, various grades of sheet, wire and cable insulation, and other products too numerous to mention.

Polyethylene belongs to a family of polymers known as polyolefins, which also includes polypropylene (PP) and polybutylene (PB). Polybutylene is a rather small volume thermoplastic, while polypropylene’s volume is much greater, being used in automobile parts, bottles, containers, rope, and many other uses.

The second highest volume thermoplastic is polyvinyl chloride (PVC). While the polyolefins are generally used as a natural polymer (no color), PVC is not a polymer that is useful in its pure form. A stabilizer must be added to it so it won’t degrade (decompose) during processing. It assures that PVC will retain its properties later. If one desires the PVC part to be flexible, a plasticizer must be added, and in almost all cases an inert mineral filler is added to lower the cost of the compound (whenever substantial amounts of materials other than the polymer are added to the polymer, the resulting material is called a compound).

Because of the chlorine in the molecule, PVC is hard to ignite, and will burn only if there is a supporting flame present or if a substantial fire begins, usually with other materials. PVC, as well as any other polymer, may be compounded so that it becomes even more flame-retardant.

Although PVC is second to polyethylene in pound volume, it ranks first in uses. Needless to say, only a small number of those uses may be mentioned here, but common among them are phonograph records, bottles and containers, house siding, wire insulation, furniture and automobile parts and upholstery, floor tile, wall covering, clothing, film and sheet, and on and on and on.

Polystyrene is the plastic that ranks third in volume, and it might be most familiar to you as foamed insulation or foamed hot drink cups. This is only one form of polystyrene, as it may be made into sheet for skylights, lighting covers, decorative use, containers, packaging, and literally hundreds of other uses. Polystyrene may be rigid with no impact strength (easily cracked or shattered) or it may have rubber compounded into it to give it resistance to shattering or breaking upon impact.

Thermoplastic polyesters are a very high volume family of plastics, with the most important member being polyethylene terephthalate (PET). PET is the plastic used to make fibers for polyester cord tires and polyester clothing. It is also the plastic used to make the half-liter and 2-liter plastic soft drink bottles that have become so popular in the last few years.

Polyamides are commonly used in various areas, mainly in carpeting and clothing. The polyamides most familiar to the consuming public are the various grades of nylon. A very specialized polyamide is known as aramid, which is used to make protective clothing for fire fighters.

A very large volume plastic that has, like the nylons and some polyesters, uses in both commodity and engineering applications is ABS, acrylonitrile-butadiene-styrene. ABS plastics can be copolymerized (not the same process as with one polymer) with a variety of properties so it can be used in automobile and airplane interiors, campers and other recreational vehicles, canoes, construction panels and many other applications.

Acrylic plastics form a very large group of thermoplastics, and they have many uses, including glazing (both commercial and residential windows and skylights), lighting panels, automobile taillights, bus and subway car windows, signs, etc. Acrylics are usually rigid materials that are cast, extruded and/or formed into rigid parts.

There are many more thermoplastic resins, compounds and alloys, but the above represent the vast majority of the volume used.

Thermosetting plastics are much more limited in their usage because of their properties. Phenolics are usually found as electrical switches and receptacles, epoxies are generally used as coatings and adhesives, while urea-formaldehyde may be found as insulation (a use no longer common) or melamine resins used in molded and laminated products.

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