What is Plastic?
Plastics are polymers. What is a polymer? A polymer is something made of many units. Think of a polymer as a chain. Each link of the chain is the "-mer" or basic unit that is usually made of carbon, hydrogen, oxygen and/or silicon. To make the chain, many links or "-mers" are hooked or polymerized together. Polymerization can be demonstrated by linking countless strips of construction paper together to make paper garlands or hooking together hundreds of paper clips to form chains, or by a string of beads.
The Structure of Polymers
Many common classes of polymers are composed of hydrocarbons. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the backbone.
There are polymers that contain only carbon and hydrogen. Polypropylene, polybutylene, polystyrene and polymethylpentene are examples of these.
Even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved. Oxygen, chorine, fluorine, nitrogen, silicon, phosphorous and sulfur are other elements that are found in the molecular makeup of polymers. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen. There are also some polymers that, instead of having a carbon backbone, have a silicon or phosphorous backbone. These are considered inorganic polymers. One of the most famous silicon-based polymers is Silly Putty.
Molecular Arrangement of Polymers
Think of how spaghetti noodles look on a plate. This is similar to how polymers can be arranged if they are amorphous (no orderly arrangement of atoms). Controlling and quenching the polymerization process can result in amorphous organization. An amorphous arrangement of molecules has no long-range order or form in which the polymer chains arrange themselves. Amorphous polymers are generally transparent. This is an important characteristic for many applications such as food wrap, plastic windows, headlights and contact lenses.
Obviously, not all polymers are transparent. The polymer chains in objects that are translucent and opaque are in a crystalline arrangement. By definition a crystalline arrangement has atoms, ions, or in this case, molecules in a distinct pattern. You generally think of crystalline structures in salt and gemstones, but not in plastics. Just as quenching can produce amorphous arrangements, processing can control the degree of crystallinity. The higher the degree of crystallinity, the less light can pass through the polymer. Therefore, the degree of translucence or opaqueness of the polymer is directly affected by its crystallinity, or distinct pattern.
Characteristics of Polymers
Polymers are divided into two distinct groups: thermoplastics and thermosets. The majority of polymers are thermoplastic, meaning that once the polymer is formed it can be heated and reformed over and over again. This property allows for easy processing and facilitates recycling. The other group, the thermosets, can not be remelted. Once these polymers are formed, reheating will cause the material to scorch.
Every polymer has very distinct characteristics, but most polymers have the following general attributes.
- Polymers can be very resistant to chemicals. Consider all the cleaning fluids in your home that are packaged in plastic. Reading the warning labels that describe what happens when the chemical comes in contact with skin or eyes or is ingested will emphasize the chemical resistance of these materials.
- Polymers can be both thermal and electrical insulators. A walk through your house will reinforce this concept, as you consider all the appliances, cords, electrical outlets and wiring that are made or covered with polymeric materials. Thermal resistance is evident in the kitchen with pot and pan handles made of polymers, the coffee pot handles, the foam core of refrigerators and freezers, insulated cups, coolers and microwave cookware. The thermal underwear that many skiers wear is made of polypropylene and the fiberfill in winter jackets is acrylic - both polymers, or plastic materials.
- Generally, polymers are very light in weight with varying degrees of strength. There are a vast range of applications, from toys to the frame structure of space stations, or from delicate nylon fiber in pantyhose or Kevlar, which is used in bulletproof vests.
- Polymers can be processed in various ways to produce thin fibers or very intricate parts. Plastics can be molded into bottles or the bodies of a cars or be mixed with solvents to become an adhesive or a paint. Elastomers and some plastics stretch and are very flexible. Other polymers can be foamed like polystyrene (StyrofoamTM) and urethane, to name just two examples. Polymers are materials with a seemingly limitless range of characteristics and colors. Polymers have many inherent properties that can be further enhanced by a wide range of additives to broaden their uses and applications. In looking at all the superior attributes of polymers, it is equally important to remember some of the difficulties associated with the material. Plastics deteriorate but never decompose completely, but neither does glass, paper, or aluminum. Plastics make up 9.5 percent of our trash by weight compared to paper, which constitutes 38.9 percent. Glass and metals make up 13.9 percent by weight.
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