Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
Mixtures of sodium carbonate o# bicarbonate with acids are also used. With the
nitrogen compounds, the residues are largely volatile and disappear although some
may remain in solution. If the salt/acid mixtures are used, the pores will contain
carbon dioxide and the plastic will contain residues of the appropriate sodium salt.
Expanded PS sheet can be made by two different methods. One starts with PS
beads that have been impregnated, under pressure, with a liquefiable gas, usually
pentane. A nucleating agent (such as a mixture of citric acid and sodium bicarbonate)
is also added in order to produce a fine cell size. The other method uses ordinary PS
beads as the feedstock and gassing is carried out in the extruder. The blowing agent,
here, is usually a fluorinated hydrocarbon. A nucleating agent is again necessary.
6. COLOURANTS
Many packaging films are unpigmented but some are coloured by the addition of
either pigments or dyestuffs. Pigments are black, white, coloured, metallic or
fluorescent, organic or inorganic solids that are insoluble in, and remain essentially
unaffected by, the medium into which they are dispersed. They are small in particle
size, generally in the range 0.01—1.0 microns in diameter. Pigments produce colour
by the selective absorption of light, but, because they are solids, they also scatter
light. Light scattering is undesirable in a transparent film but is desirable if opacity is
required. There are some organic pigments that are extremely small in particle size
and so scatter very little light. They act, therefore, more like dyes. Organic pigments
are characterised by high colour strength, brightness, low density, high oil absorption,
and sensitivity to heat and light. Inorganic pigments are more opaque, less bright and
weaker in tint. However, they are more resistant to heat, light, chemical attack,
migration and weathering. They also have higher density, lower cost and less
antioxidant effect. One example is molybdate orange for vinyl film.
Dyes are intensely coloured, solubilised organic substances that are retained by the
medium they colour, by chemical bonding, absorption or mechanical retention. Dyes
produce colour by absorption of light without affecting transparency or optical purity.
Selection of a colourant for plastics is governed not only by the colouring
characteristics of the pigment or dye but also by the resin to be coloured its
properties and compatibility and the method by which it is processed. For example,
PVC and its copolymers liberate hydrochloric acid at high temperatures so that acidsensitive
colourants like cadmium red and ultra-marine blue must be used with care.
In addition, molten nylon acts as a strong acidic reducing agent and can decolourise
certain dyes and pigments. Colourants must be heat stable enough to withstand
processing temperatures, which are sometimes very high. Colourants may also react
with other additives used in the film, such as heat and light stabilisers. As an
example, sulphur-containing cadmium pigments react with the nickel-bearing
stabilisers used in some film grade PPs.
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