Production, forming and joining of polymers

People have used polymers for far longer than metals. From the earliest times, wood, leather, wool and cotton have been used for shelter and clothing. Many natural polymers are cheap and plentiful (not all, though think of silk) and remarkably strong. But they evolved for specific natural purposes - to support a tree, to protect an animal -and are not always in the form best suited to meet the needs of engineering. 

The real breakthrough came when chemists developed processes for making large molecules from their smallest units. Instead of the ten or so natural polymers and modifications of them, the engineer was suddenly presented with hundreds of new materials with remarkable and diverse properties. The number is still increasing. 

And we are still learning how best to fabricate and use them. As emphasised in the last chapter, the mechanical properties of polymers differ in certain fundamental ways from those of metals and ceramics, and the methods used to design with them (Chapter 27) differ accordingly. Their special properties also need special methods of fabrication. This chapter outlines how polymers are fabricated and joined. To understand this, we must first look, in slightly more detail, at their synthesis. 

Plastics are made by a chemical reaction in which monomers add (with nothing left over) or condense (with FljO left over) to give a high polymer. 

Polyethylene, a linear polymer, is made by an addition reaction. It is started with an initiator, such as FIjOj, which gives free, and very reactive —OFI radicals. One of these breaks the double-bond of an ethylene molecule, C2FI4, when it is heated under pressure, to give 

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