Agriculture Reference
In-Depth Information
4
Common Polymers and Additives
used for Processing
Since the emergence of
polymeric composites
, the practice of using only one or
two basic polymers as the matrices for composites has spread to a wider range of
polymers. Ongoing research is working out the possibilities of using combinations
of polymers, polymer blends and other, newer polymers as matrices. However, this
chapter will deal with the most common polymers, which are tried and tested and are
yielding the best results as polymeric composites from a practical and commercially
viable angle. To achieve optimum results of end products, it is essential for composite
designers, formulating chemists and producers to have a reasonable understanding of
the properties and behavioural patterns of common polymers used today.
The basic ingredient for polymers is the monomer ethylene. Ethylene is a gaseous
hydrocarbon derived, in general, from petroleum sources. It has the formula C
2
H
4
and can be identified as a pair of methylene groups connected to each other. Ethylene
is usually produced from the refining of crude oil but can also be generated by the
dehydration of ethanol. Ethylene is a stable molecule that polymerises only upon
contact with catalysts. Because the catalysts are highly reactive, ethylene must be of
high purity. The conversion is highly exothermic (i.e., the process gives out a lot of
heat). Coordination polymerisation is the most pervasive technology, hence metal
chlorides or metal oxides are used. The most common catalyst used is titanium
chloride. Ethylene can also be produced through radical polymerisation but this
method has only limited utility and requires high-pressure apparatus.
The following polymers have been selected as being the most suitable and capable
of producing commercially viable polymeric composites for a wide range of end
applications:
• Polyethylene(PE)
• Polypropylene(PP)
• Polyvinylchloride(PVC)
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