Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

hardwood should be used with caution, because there are some hardwood timbers that are softer

than softwoods.

Plastics

In the latter part of the 20th century, plastics increasingly became the material of choice for

many applications. They are very lightweight, strong, inexpensive, and easily made into vari-

ous shapes. Over 100 million metric tons of plastic are produced annually worldwide. Of course,

this number increases as the demand for inexpensive, durable, disposable material grows. Most

of you are already familiar with examples of plastic products, including grocery and trash bags,

plastic soft drink containers, home cleaning containers, vinyl siding, polyvinyl chloride (PVC)

piping, valves, and fittings that are readily available in home improvement centers. Styrofoam™

plates and cups, plastic forks, knives, spoons, and sandwich bags are other examples of plastic

products that are consumed every day.

Polymersare the backbone of what we call plastics. They are chemical compounds that

have very large, molecular, chainlike structures. Plastics are often classified into two categories:

thermoplasticsandthermosets. When heated to certain temperatures, the thermoplastics can be

molded and remolded. For example, when you recycle Styrofoam dishes, they can be heated and

reshaped into cups or bowls or other shapes. By contrast, thermosets can not be remolded into

other shapes by heating. The application of heat to thermosets does not soften the material for

remolding; instead, the material will simply break down. There are many other ways of classi-

fying plastics; for instance, they may be classified on the basis of their chemical composition,

their molecular structure, the way molecules are arranged, or their densities. For example, based

17.3 Some Common Solid Engineering Materials 567

Because of their low cost

of production, polymer-

based products are

commonly used in our

everyday lives. Source:Stockbyte

Source:2001 PhotoDisc, Inc.

Source:2001 PhotoDisc, Inc.

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