| Making
things out of plastics is like playing a game with molecules. The aim is to
re-organise them into new shapes without their changing colour, sticking to the
mould, or doing anything that could spoil the finished article. Additives help
with all these problems. In fact, processing plastics without additives is
virtually impossible. Consider
the range of plastics available and the wide variety of plastic objects in
every day use. From fizzy drinks bottles and carrier bags to window frames and
computers, it is clear that there are many different ways of forming plastics.
Most of these processes involve melting polymer powder or granules inside a
heated tube. This ''melt'' is forced through a shaped die, injected into a mould,
or rolled or blown into flat film. The ease with which this is done depends on
the physical and chemical properties of each plastic material, and can be
improved through the use of certain additives known as PROCESS AIDS. Process
aids become liquid during the moulding process and form a liquid around colour
particles so that they mix better. Other additives make the individual polymer
particles adhere more to each other inside the tube so that they melt quicker.
This means that the moulding temperature can be lower which saves energy and
prevents or minimises heat damage to the plastics. Certain plastics can be difficult to process
because they become viscous and sticky when they melt. LUBRICANTS help reduce
viscosity by creating a film between the mould and the polymer melt, and by
lubricating the polymer particles against each other. More intricate shapes can
be moulded and the moulding temperature can be lowered. Most plastics have to
be processed at over 180oC, a temperature which unfortunately can sometimes
spoil the colour and weaken or embrittle the plastic. However, these effects
can be prevented or minimised by additives known as ANTIOXIDANTS, which are
special compounds such as Vitamin E, which help protect the plastic under
hostile conditions. Other additives called HEAT STABILISERS help stop plastics
from decomposing during processing |