BACKGROUND ON POLYPROPYLENE
Polypropylene is chemically very similar to polyethylene, consisting of only carbon and hydrogen atoms. One of its most important properties is its low price, which is due to a relatively simple synthesis from the low cost petrochemical, propylene.
To be useful in a wider range of applications, polypropylene is often co polymerized with polyethylene, yielding a material with most of the temperature resistance, stiffness and strength of polypropylene, but better impact resistance contributed by the flexible polyethylene linkages. When polypropylene is listed in technical literature, "homopolymer" or "copolymer" is usually specified. Polypropylene is not as resistant to thermal or UV initiated degradation as polyethylene is. Even in stabilized grades it is not widely used in outdoor applications, except in black colors. (Carbon black acts as a good UV stabilizer)
A very popular property of polypropylene is the "living hinge" effect. In thin sections where the molecules are oriented, the material can be flexed almost indefinitely without failure. Molecular orientation is crucial to this function and should be induced in the mold through the part design and gate location.
Polypropylene is used widely in film, fiber, sheet, and molded applications. Most of the film is used in packaging. The fiber is used in carpeting and upholstery due to its wear and chemical resistance. Common molded applications include bottles, pipe, containers, and tanks, where the chemical resistance key. Due to its low cost, PP is used in toys, and disposable house wares. In summary, polypropylene is a versatile polymer, which through new technology is becoming more attractive to many markets including automotive and appliance, due to the low cost of the base polymer.