Pyrolysis is generally defined as the controlled burning or
heating of a material in the absence of oxygen. In plastics pyrolysis,
the macromolecular structures of polymers are broken down into
smaller molecules or oligomers and sometimes monomeric units.
Further degradation of these subsequent molecules depends on a
number of different conditions including (and not limited to)
temperature, residence time, and the presence of catalysts and
other process conditions. The pyrolysis reaction can be carried out
with or without the presence of catalyst. Accordingly, the reaction
will be thermal and catalytic pyrolysis. Since majority of plastic
used are polyolefins, so extensive research has been done on this
polymer which is summarised as below.