Common Polymers and Additives used for Processing - Introduction to Polymeric Composites with Rice Hulls - page 54

Polymer foams can be produced by mechanically 'whipping' gases into the polymer
or by chemical agents or physical means. The basic stages of a foaming process are:
(i) cell nucleation; (ii) cell expansion; and (iii) stabilisation of the cell structures. Too
much cell expansion or foaming of the polymer will be harmful to the end product
due to lessening of the properties. Cell nucleation occurs if, at a given temperature
and pressure, the solubility of a gas is reduced, leading to saturation and expulsion
of excess gas to form bubbles. In EPS, for example, the polymer resin is in the form
of tiny spheres, each one with a very small amount of an inert gas. If heated, this gas
is activated to expand and form cells within the polymer. This is a chemical foaming
process that will produce closed cells. In the production of flexible PU, water acts as
the main blowing agent and sometimes methylene chloride is added to achieve very
light densities.
Many popular blowing agents are available. If a complete online process is used for
a polymeric composite extrusion line, a chemical blowing agent can be added to the
polymer matrix, rice hull powder and other additive mixes during compounding.
Some of the commonly used chemical blowing agents are: azodicarbonamides (a
pale-yellow to orange powder), hydrazinc derivatives (a slightly yellow crystalline
powder), semicarbicides (a white crystalline powder), and carbonates (white powders).
4.7.11 Colourants
Colourants can be divided into two categories: pigments and dyes. Pigments are
organic or inorganic materials that are practically incompatible with polymeric
materials, and must be dispersed into a polymer melt using a relatively intense mixing
method. Pigments come in several forms such as powders, granules, liquid/solid
concentrates and masterbatches tailored by compounders. For easy dispersion, a
composite producer can select a masterbatch where the pigment is already incorporated
into the same polymer and available in liquid or solid forms. Dyes are compatible with
polymer melts. Because dyes dissolve in the resin, there are no visible particles and
they will not affect the transparency of the material mix. When selecting colourants,
some of the important aspects are:
• Thecolourindex is a classification for pigments and dyes.
• Heatresistance : Check the highest processing temperature and exposure time.
• Lightfastness : Rating between 1 (low performance) and 8 (high performance).
• Weatherfastness : Most suppliers will provide information.