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degradation reactions and the possibilities for conserving these plastics will
be presented in this chapter.
2 The Chemistry and Physics of Plastics
According to the Penguin English Dictionary, the noun ‘plastic’ is defined
as ‘any of the numerous synthetic organic polymers that, while soft, can be
moulded, cast, etc.into shapes and then set to have a rigid or slightly elastic
form’. In practice, although the polymer component of a plastic is always the
greatest by weight, it is usually modified by adding other materials to improve
inherent stability, longevity, physical and chemical characteristics.
Polymers are used as the base material to make plastics. Polymers are large
molecules often described as chains, with a characteristic ‘repeat’ chemical
unit acting as the links. The repeat unit is usually a small molecule known as
a monomer; typically there are between 1000 and 10,000 monomers in a poly-
mer chain. The number of repeat units can be used to describe the chain length
of the polymer, but the use of molecular weight is more common. Monomers
may contain carbon, hydrogen, oxygen, nitrogen, silicon, chlorine, fluorine,
phosphorous and sulfur. The chemical process of joining the monomers into a
high molecular weight polymer is called polymerisation. Since polymerisation
usually produces chains of varying lengths, the molecular weight is calculated as
an average weight per chain known as weight-average molecular weight (Mw).
Polymers are often named after the monomer rather than after the repeat-
ing unit in the structure; thus the polyethylene molecule, prepared from the
monomer ethylene, consists of a long chain of repeating methylene (CH 2 )
groups (Formula 1). Polymers prepared from more than one species of
monomer are called copolymers, e.g.butadiene-styrene polymers.
Formula 1. Polyethylene consists of a long chain of repeating methylene groups.
CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 (1)2.1 Preparation of Polymers
The first polymers were developed in 1862, known as semi-synthetics and
formed a technological bridge between natural (those produced by trees, plants
and insects) and fully synthetic polymers. Semi-synthetic plastics were made
by treating a natural material chemically to modify its properties, usually with
the aim of producing a mouldable product. In 1909, the first fully synthetic
polymer was produced by reacting two chemicals (monomers) together.
Preparation of semi-synthetic polymers.Cellulose plastics, particularly cellulose
nitrate and acetates, were the most commercially-important semi-synthetics,
and have been used to prepare photographic films, textile fibres and lacquers.