2.1 Introduction
Plastics were originally seen as substitute products for traditional materials such as metal and wood.
However, now they have become as irreplaceable as the classic materials themselves. Plastics have
managed this achievement because of their unique versatility and the ability to tailor their properties,
which other materials cannot match. Our modern everyday life would be inconceivable without plastics.
The use of plastics enables us to solve problems that are insoluble with the classic materials, whether it
be – to name only a few examples – in electronics, light engineering, medical technology, space
technology or machine and vehicle manufacture.
Plastics are made up of polymers and other materials that are added to them to give the desired
characteristics. Natural polymeric materials such as rubber, shellac and gutta percha have a long history
as raw materials for man. The first thermoplastic, celluloid, was also manufactured from a natural
product, from cellulose. Even today, there are still some cellulose based plastics, i.e., the cellulose
acetates (CA). Cellulose is already composed of the large molecules that are characteristic of plastics
(macromolecules). However, to manufacture CA plastics, they still have to be 'prepared' with acetic
acid. The first injection moulding machine was built and patented in 1872 in order to mould cellulose
materials.
Today the vast majority of plastics are manufactured artificially, i.e., the macromolecules are built up
from smaller molecules (predominantly from carbon and hydrogen). Basically, plastics can also be
manufactured from their basic constituents, carbon and hydrogen (coal and water). For economic
reasons, however, similar to petrol manufacture, plastics are nowadays almost exclusively manufactured
from products generated by the fractionated distillation of crude oil.
We can, therefore, divide plastics into:
1. Plastics made from natural substances, e.g., Celluloid, cellulose acetate, vulcanised fibre, casein
plastics (galalith)
2. Artificial plastics, e.g., polyethylene, polystyrene, polyamide
However, the origin of plastics, whether obtained from naturally occurring large molecules or
synthetically prepared from smaller molecules, makes no difference to the subsequent processing.
The first synthetically developed polymer was called Bakelite after its inventor, Leo Baekeland in 1907.
The material, phenol-formaldehyde, is a thermoset phenolic resin. However, up until 1924, and the
work of Herman Staudinger, there was no real understanding of the chemical structure of polymers.
Staudinger proposed the concept of linear molecular chains and macromolecules, which once accepted
by the scientific community (this actually took several more years), allowed the doors to open on the
synthesis and development of new polymeric materials. This new understanding of the structure of
polymers allowed the development of plastics such as polyvinyl chloride and cellulose acetate in the
1920s. The 1930s saw the introduction of polyamides, polystyrene and acrylics as well as the
introduction of single and twin screw extruders for polymer processing. New polymeric materials
continued to be introduced, their development fuelled by the Second World War. The 1940s saw the
introduction of epoxies, polyethylene and acrylonitrile-butadiene-styrene (ABS) to name but three. The
1950s saw the birth of the polypropylene industry as well as polyethylene terephthalate (PET) and
polycarbonate (PC). American companies developed a number of engineering materials in the 1970s
including polyphenylene sulphide and a number of fluoropolymers such as DuPont’s (US) ethylene
tetrafluoroethylene copolymer Tefzel and perfluoralkoxy plastics under the trade name Teflon PFA. In
1973 Dynamit Nobel (Germany) introduced polyvinylidene fluoride (Dyflon) into the market and the
1980s saw the development of liquid crystal polymers (LCP).
The development of new polymers has now slowed due to the expense and difficulty of synthesising
new materials, however new plastics are still being developed by mixing existing materials together.
These materials are called polymer alloys and blends. An alloy has a single glass transition temperature
(this will be explained later), and generally has better properties than the individual components. A
blend has more than one glass transition temperature and has properties between those of the original
materials. An example of a commercially successful blend is ABS.
2.2 Structure and Typical Properties of Polymers
The word polymer derives from the Greek word poli, which means many and the word meros, which
means parts. This is because polymers are made up of a number of smaller repeated units called
monomers. The simplest and most commonly used monomer is ethylene. Chemically it consists of two
carbon atoms (C) and four hydrogen atoms (H). It can be represented in the two ways shown in Figure
2.1. The lines in this diagram represent bonds that exist between the atoms to form a molecule.
It is the existence of the double bond between the carbon atoms in ethylene, which allows the creation
of polyethylene. This happens when the monomers are combined by a process called polymerisation to
form a chain such as the one shown in Figure 2.2. A chain of useful polymer may consist of 200-2000
monomers joined together. This particular type of polymerisation is called addition polymerisation.
It can be seen from Figure 2.2, that carbon atoms form the backbone of the polymer. Many polymeric
systems are made of long chains of carbon atoms such as this. But polymers are not confined to carbon
forms and injection moulding materials such as liquid silicone rubbers (LSR) have different chemical
structures as the backbone of the polymer chain. This will be illustrated later in Chapter 7. The repeat
unit structures of a number of common polymers are shown in Table 2.1.
