Foaming of WPC can be produced by using either a physical
blowing agent (PBA) or a chemical blowing agent (CBA). The
PBA can either be a gas (e.g., CO2 and N2) or a liquid (e.g.,
chlorofluorocarbon (CFC) and other aliphatic hydrocarbons).
The use of PBA like CFC is banned because of their damaging
effect on the ozone whereas the aliphatic hydrocarbons are
highly flammable [7, 10]. CBAs are known to be more expensive
and yielded less gas liberation, but they are more environmentally
friendly and easier to apply in the WPC production
process. Extensively used CBAs include azodicarbonamide
(AC), modified AC (mAC), oxybis-(benzenessulfonyl hydrazide),
toluene sulfonyl hydrazide and citric acid/sodium bicarbonate
blend. ZnO-modified AC or mAC was selected for the
current research due to its compatible decomposition temperature
range with the processing temperature of the PVC matrix
in the WPC [1, 2].