TABLE OF CONTENTS In the past, results of standard tests such as tensile strength, Izod impact strength, and softening point have been given major emphasis in plastic technical literature. More recently, however, with the increasing use of plastics in more critical applications, there has been a growing awareness of the need to supplement such information with data collated from tests more closely simulating operational conditions.
For many applications the choice of material used depends upon a balance of stiffness, toughness, processability, and price. For any particular application, a compromise between these features will usually be necessary. For example, it is generally true that within a given family of grades of a particular polymer, the rigidity increases as the impact strength decreases. Again, processing requirements may well place an upper or a lower limit on the molecular weight (MW) of the polymer that can be used, and this will frequently influence the mechanical properties quite markedly. Single test values of a given property are a less reliable guide to operational behaviour with plastics than with metals, since for plastics the great mass of empirical experience and tradition of effective design which has been built up over centuries in the field of metals is not yet available. Furthermore, no single value can be placed on either the stiffness or the toughness of a plastic as:
Stiffness will vary with time, stress, and temperature.
Toughness is influenced by the design and size of the component, the design of the mould, processing...
