How Plastics Degenerate

Most plastics do not rust or corrode like metals.Plastics are made up of long strings like congealed spaghetti. Solvents seep in between the strings, weakening the structure and making it swell. The material usually softens, and all other properties are also affected. Very strong solvents may even break up and dissolve the strings.

Other chemicals, and ultra violet light, cause the chain linking reactions of the plastic to continue and accelerate. The plastic becomes hard and brittle, and small cracks begin to appear at the surface. Once these cracks appear, they continue to grow throughout the material.

The cracks usually begin in zones of local stress caused by heating, bending, glueing, or welding. Moulded articles may also have internal stress caused by uneven flow of plastic in the moulding process. Internal stress can be thought of as neighbouring strings within the plastic being stretched by different amounts when the article is made. It is very important for the moulds to be properly designed                                                                                            

An Introduction to Plastics     (Resource Notes for a Junior School Project)