What is Plastic?

Plastics are relatively stiff materials composed of polymers and other ingredients such as fillers, pigments (for color), plasticizers, flow improvers, and stabilizers.

Chemically, Plastics are polymers, which are simply long chains of atoms bonded to one another. These chains are made up of many repeating molecular units derived from "monomers", known as "repeat units". Each polymer chain will have several 1000's of repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone, or composed of oxygen, nitrogen, chlorine or sulfur. There are also some plastics of commercial interest which are silicon based.

The evolution of plastics has developed first from the use of natural materials (e.g., chewing gum, shellac) to the use of chemically modified natural materials (e.g., natural rubber, nitrocellulose, collagen) and finally to completely synthetic molecules (e.g., epoxy, polyvinyl chloride, polyethylene).

In the nineteenth century a plastic material based on chemically modified natural polymers was discovered: Charles Goodyear discovered vulcanization of rubber (1839) and Alexander Parkes, English inventor (1813—1890) created the earliest form of plastic in 1855. He mixed pyroxylin, a partially nitrated form of cellulose (a major component of plant cell walls), with alcohol and camphor. This produced a hard but flexible transparent material, which he called "Parkesine."

The first plastic based on a synthetic polymer was made from phenol and formaldehyde, with the first viable and cheap synthesis methods invented by Leo Hendrik Baekeland in 1909, the product being known as Bakelite. Many items such as telephones and radios from the 50's and 60's were made from this material.

Subsequently poly (vinyl chloride), polystyrene, polyethylene (polyethene), polypropylene (polypropene), polyamides (nylons), polyesters, acrylics, silicones, polyurethanes were amongst the many varieties of plastics developed and have had great commercial success.

In 1959, Tho Koppers Company in Pittsburgh, PA had a team that developed expandable polystyrene foam - Styrofoam.

http://en.wikipedia.org/wiki/Plastic

One reason for plastics widespread use is their low cost and their ease of being molded to any shape very easily.

Let's say you have some plastic and you now want to shape it into a bottle or something. There are a number of processes, and you use whichever process leads to the kind of shape, thickness, and utility you need for your job. One way to go is "thermomolding," in which you form plastic sheets into parts by the application of heat and pressure.

On the other hand, to make a bottle you might use a process called "blow-molding," in which you form a tube from your plastic material and use hot gas to force the tube to expand into a mold, forming a hollow object with a definite size and shape. There are many other possible processes.
http://www.newton.dep.anl.gov/askasci/chem99/chem99146.htm