STRUCTURE OF PLASTICS AND METALS

In the paragraph on Organic chemistry of chapter 3 some atten- 

Thermoplastics consist of long chains of molecules, in the case of PE these are unbranched. Many of these chains together form a tangle which is more difficult to unravel the more branched the chains are. The branches are like hooks which cause the molecules to catch. Under the influence of relatively small external forces chains and parts of chains can slide across each other. PE is for instance used to make containers, chemical tubing and blow-moulded bottles. Some other thermoplastics are polypropylene (crates), polyvinyl chloride PVC (pipes) and polystyrene (foam). 

By means of chemical reactions thermosetting plastics form three-dimensional structures. In the example above the nitrogen compound urea reacts with formaldehyde (methanal), in which process three molecules combine and a molecule of water is formed. In this example two H atoms react, but all other H atoms ( ) enter into the same reaction. Since urea is a three-dimensional molecule, the network will also be three-dimensional. For instance switches and sockets are made of UF. Other thermosetting plastics are polyurethane PU (insulation) and melamine-formaldehyde MF (panels). 

In the synthesis above styrene reacts with butadiene to form long chains while at the same time retaining a double bond per two molecules. Subsequently sulphur is added and sulphur bridges are formed between the remaining double bonds which interconnect the chains. These bridges are represented by small dashes. SBR is for instance used in the manufacture of car tyres. Another elastomer is polyurethane rubber PUR which is used as a foam in head rests. 

The crystals of metals are built up as regular packings of positive ions among which an electron cloud of valence electrons moves. 

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