Environmental Engineering Reference
In-Depth Information
3
AN INTRODUCTION TO PLASTICS
Back in 1922, Hermann Staudinger, a German chemist, first realized that
polymers were in fact made up of long covalently bonded molecules. Until
then, polymers were generally assumed to be colloids. Not only are polymer
molecules very large with average molecular weights reaching millions of
grams per mole (and hence called macromolecules), but they also have
long-chain-like molecular architecture. The recognition of this class of
unique giant linear molecules marked the birth of a new science and
technology that has yielded, among other materials, the ubiquitous plastics
widely used in numerous applications. The unique, impressive mechanical
properties of plastics (or polymers in general), such as high strength and
modulus, are the result of their distinctive chain-like molecular architecture.
The following discussion on polymers is not intended to be a comprehensive
introduction but provides only basic information on the subject. Many
important aspects of polymer structure, properties and characterization have
been left out. Aspects discussed here are primarily those likely to assist the
reader in understanding the content in subsequent chapters. The reader is,
however, directed to excellent monographs on polymer science for more
detailed and complete information (Coleman and Painter, 1998; Rudin and
Choi, 2012; Young and Lovell, 2011).
3.1 POLYMER MOLECULES
Structurally, polymers are long-chain-like molecules made up of repeating
structural segments (called “repeat units”) linked end to end. The desirable
consequence of this unique molecular geometry. With large molecules where
many atoms are in such close proximity, the van der Waals forces of
attraction are quite large; this is what is behind the exceptional mechanical
properties of plastics. Most common plastics, for instance, have the same
structural unit repeated along the macromolecular chain, and are called
homopolymers. In copolymers, two or more such repeat units can be mixed
within a single polymer chain. Common plastics polyethylene (PE) and
polypropylene (PP) are, for instance, homopolymers, while plastics such as
