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Materials: Plastics

Introduction

Humans have taken advantage of the versatility of polymers for centuries in the form of oils, tars, resins, and gums. However, it was not until the industrial revolution that the modern polymer industry began to develop. In the late 1830s, Charles Goodyear succeeded in producing a useful form of natural rubber through a process known as "vulcanization." Some 40 years later, Celluloid (a hard plastic formed from nitrocellulose) was successfully commercialized. Despite these advances, progress in polymer science was slow until the 1930s, when materials such as vinyl, neoprene, polystyrene, and nylon were developed. The introduction of these revolutionary materials began an explosion in polymer research that is still going on today. Some degree of compromise is almost always necessary in designing plastic parts. Arriving at the best compromise usually requires satisfying the mechanical, thermal, and electrical requirements of the part, utilizing the most economical resin or compound that will perform satisfactorily and be attractive, and choosing a manufacturing process compatible with the part design and material choice. Probably no plastic will provide 100% of the requirements for the desired performance, appearance, processibility, and price. Selecting the best qualified material is not based simply on comparing numbers on published data sheets; such values can be grossly misleading. For example, choosing the most economical material for a part by comparing the cost per pound of various plastics is a mistake. Some plastics weigh twice as much per cubic inch as others and so would require twice as much to fill a given cavity and cost twice as much to ship. Polymers have a wide range of mechanical properties. Network polymers are often quite strong and stiff (high yield strength and modulus of elasticity), although they have poor ductility. Linear polymers have much lower strength but quite high ductility, and elastomers have very large values of ductility and a variable modulus of elasticity. Polymers are generally classified according to their structure, properties and use as:
  • Thermoplastic
  • Thermosetting
  • Elastomers
Original Source: http://engineershandbook.com/Materials/plastics.htm

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