These fractions are very complex mixtures. No chemical changes have yet taken place. The fractions need to be chemically altered to make them into more useful products with different melting and boiling points and different chemical properties. .
Cracking and reforming are the processes used to alter the fractions. Cracking breaks large molecules into smaller ones that are more useful. Reforming changes the internal structure of molecules to produce different compounds. By altering conditions such as temperature and pressure or the use of catalysts, the cracking and reforming techniques can be controlled to produce exactly the blend of compounds needed at a particular time.
Naphtha is cracked by mixing it with steam and heating it to eight hundred degrees centigrade. It is cooled rapidly to four hundred degrees centigrade, causing chemical changes. The mixture of C6 to C10 compounds is converted to a small number of C2, C3 and C4 compounds which contain carbon-carbon double bonds, C=C (Brown 111).
The simple compounds are often known as basic chemicals'. All of the basic chemicals are small molecules containing between two and seven carbon atoms. It is these molecules that are the monomers from which the polymers are then made.
The small monomer molecules are reacted together to form a polymer. In order to make the monomers react and join together, small amounts of catalysts are added to the polymerization reactor.
Eight of the most important polymers are produced from only three basic chemicals which come from naphtha (Harper).
Ethylene C2H4.
polymerization to form high density polyethylene (HDPE), low density polyethylene (LDPE) or linear low density polyethylene (LLDPE).
reaction with chlorine to form chloroethene * polymerization to form polyvinyl chloride (PVC).
reaction with benzene to form styrene * polymerization to form polystyrene (PS).
reaction with oxygen to form ethane oxide * further reaction and polymerization to form polyethylene-terephthalate (PET).