Common Polymers and Additives used for Processing - Introduction to Polymeric Composites with Rice Hulls - page 46

4.3.3 Plasticisers
Most vinyl products contain plasticisers that dramatically improve their rheological
performance and mechanical performance. Plasticisers are used as processing aids as
softening agents for polymers and also because they have the same effect of raising the
temperature of polymers. The most common plasticisers are derivatives of phthalic
acid. Plasticisers are selected based on their compatibility with the polymer, low
volatility and cost. These materials are, in general, oily colourless liquids that mix
well with PVC particles. Phthalates can be divided into two groups (high-MW and
low-MW) with the high-MW phthalates. Plasticisers are most commonly added to
alter the mechanical properties of a polymer (e.g., stiffness, toughness and strength).
For example, adding a plasticiser such as dioctyl phthalate to PVC can reduce its
stiffness and enable the mix to be soft and with reduced viscosity, as well as with other
additives for easy flow (e.g., coating on fabric in the manufacture of artificial leather).
4.3.4 Important Properties
PVC has high hardness and mechanical properties. The mechanical properties improve
with increased MW but decrease with increased temperature. The heat stability of
PVC is poor and, if the temperature reaches ≈104.4 °C, PVC starts to decompose.
Its melting temperature is 121.1 °C. The linear expansion coefficient of PVC is small
and has flame retardancy, the oxidation index is ≥45. Therefore, incorporation of a
heat stabiliser during processing is crucial to ensure the properties of the end product.
The two basic grades of PVC are: unplasticised polyvinyl chloride (uPVC) and soft
PVC (flexible PVC). Rigid PVC (uPVC) has a density of 1.3-1.45 g/cm 3 , whereas
soft PVC has a density of 1.1-1.35 g/cm 3 . PVC is a polymer with very good electrical
insulation but, because of its highly polar nature, may be inferior to non-polar
polymers such as PE and PP.
4.4 Polystyrene
PS is a synthetic polymer made from the liquid petrochemical monomer styrene. PS
can be rigid or foamed, as in expandable polystyrene (EPS). General-purpose PS is
clear, hard and brittle. It is an inexpensive resin per unit weight and used widely in
industry (especially in injection moulding and extrusion). PS is naturally transparent
but can be coloured readily. As a thermoplastic polymer, PS is in a solid glassy state
at room temperatures but flowed easily if heated above its if g . Being a thermoplastic,
the molten polymer becomes rigid again on cooling and can be re-used. It is very
slow to biodegrade unless special biodegradable additives are incorporated in the