The crude oil fraction that polymers go into is the naphtha fraction. This fraction normally contains Alkanes. Ethene put under pressure and heated with a catalyst will polymerise to form poly (ethene). Note that there are no double bonds in the polymer. Poly (ethene) is an alkane. It is a saturated hydrocarbon. This process is called polymerisation. A polymer that is formed from monomers added together where no substance (other than the polymer) is produced is called an addition polymer. Examples of addition polymers are poly (ethene), poly (propene), poly (chloroethene) – PVC, and poly (phenylethene) – poly (styrene). The double bond in an alkene (like ethene) can be broken open and joined to other molecules. A molecule of ethene can be joined to another molecule of ethene. In the process, the second ethene molecule has its double bond broken, and this may be added to a third molecule of ethene, and so on. Many ethene molecules can be joined together to form a polymer. The polymer is called poly (ethene) because it is made from ethene.
Polymers may be classified as thermosoftening or thermosetting. Thermosoftening polymers are sometimes called thermoplastic. Thermosoftening polymers include poly (ethene), poly (propene), poly (styrene) and poly vinyl chloride. A polymer molecule is a long chain of carbon atoms held together by strong covalent bonds.
The forces in between the molecules are relatively weak but parts of the chains can sometimes line up side by side to form tiny crystals. These crystals can form links between the chains and hold the structure together. When the polymer is heated the crystals will melt, the material will become very soft and can flow slowly like a thick liquid. In this state the polymer can fill a mould and be cast into a shape. When the polymer cools down, new crystals can form between the chains and the new shape is fixed. The same polymer can be reheated and remoulded. Such polymers are called thermosoftening (meaning that they go soft when you heat them). Some polymers do not form crystals. This is because the molecules become bonded to each other so they won’t break and there are strong covalent bonds, which cannot be broken. They also soften when heated and harden when cooled down again. When cold, they are not crystalline but glassy. These polymers are called Thermosets or Thermosetting plastics.
Polymers may be classified as thermosoftening or thermosetting. Thermosetting polymers have their chains cross-linked by covalent bonds. The polymer is originally placed into a mould to form the desired shape. The polymer is then heated and chemical reactions occur to form the cross links between the chains. The resulting three-dimensional solid structure cannot then be changed. Further heating will not cause the polymer to soften or change shape (unlike thermosoftening polymers).
Examples of thermosetting polymers are:
1. Melamine – used in furniture.
2. Bakelite – used for saucepan handles and electric light fittings.
3. Epoxy resins – used in many glues.
Although polymers are very useful materials, there are problems with disposal of unwanted articles. Polymers are not biodegradable. This means that bacteria will not break them down. If they are buried in the ground (landfill) they simply remain as polymers in the ground. Products from plant material (wood, paper, cotton etc.) are biodegradable. When buried, bacteria and fungi break them down into useful nutrients for further plant growth. Nature recycles its own products.
Polymers produce toxic materials (poisons) when burnt, in addition to the expected products from combustion of a hydrocarbon, which are carbon dioxide, water, carbon monoxide and carbon (soot). Those that contain chlorine (PVC for example) also produce hydrogen chloride on burning. Those that contain nitrogen (nylon for example) produce hydrogen cyanide. Polymers are a fire hazard. Many people die from the smoke of burning polymers in house fires, long before the fire reaches them. Burning polymers is not a good way of disposing of them.
More and more polymers are being recycled. This is not as cost effective as recycling metals, but we don’t want to live amongst piles of (unrotting) plastic. Research into biodegradable polymers will increasingly provide useful replacements for the main polymers of today. Another solution is mechanical recycling. This is where you return the polymers to small chains again under high heat and pressure in the absence of oxygen.