Basic Technology of Plastics - Introduction to Polymeric Composites with Rice Hulls - page 19

2.2.10 Modified Polymers
The properties of thermoplastics can be modified by adding additives such as a
plasticising agent. If a plasticising agent is added, the glass transition temperature
(T g ) shifts to lower temperatures and, consequently, the shear modulus starts to drop
at a much lower temperature. In essence, the plasticiser replaces a temperature rise
by 'wedging' these typically low-molecular-weight materials between the polymer's
molecular structures, generating free volume between the chains. This additional
free volume is equal to a temperature rise, which consequent lowers the T g , making
a normally brittle and hard thermoplastic softer and compliant.
Some amorphous thermoplastics can be made high impact-resistant (less brittle)
through copolymerisation. The most common example is ABS. Butadiene chains
vitrify at temperatures below -50 °C, so ABS is very tough at room temperature
in contrast to polystyrene (PS) and acrylics by themselves. Due to the T g of the
materials that form the blend, ABS shows two general transition regions, one around
-50 °C and the other at 110 °C, which are visible in logarithmic decrement and
the shear modulus.
2.2.11 Polymer Groups
The main group of polymers relevant to this section on polymeric composites with rice
hulls (PCRH) are PE, P P, PS and PVC, which are linear homo-polymers polymerised
from unsaturated monomers or from the transformation of analog polymers. This
group is the major commodity group of thermoplastics. Another group of polymers
that exhibit double bonds and belong to the family of elastomers are polyacetals ;
polyethers; polyaryleters ; and polyaryletherketones .
The existence of a double bond allows these polymers to undergo vulcanising
or crosslinking. Polycyclo-olefins are also an important group as high-resistant
thermoplastics. Polyacetals and polyethers are polymers that have a heteroatom
(oxygen) present in the backbone of the molecule. The heat resistance of these polymers
is increased significantly by the introduction of bulky aromatic building blocks into
the molecules, thereby significantly reducing intra-molecular mobility. The oldest
thermoplastics that are still in use today are cellulose polymers. The rigid cellulose
molecules make them difficult to process, requiring the use of plasticisers or other
polymers. The high polarity of cellulose molecules makes them highly hydrophilic
and sometimes soluble in water.