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Dow Corning revolutionized the plastics additive industry by introducing
ultra-high molecular weight siloxane polymers as the basis of its standard
additives. These unique additives are far more effective than conventional
lower molecular weight silicones as process aids and surface lubricants. So,
they provide greatly enhanced effects, and also overcome many of the
limitations of the lower molecular weight materials. This page details the
history of the highly beneficial use of Dow Corning silicone materials as
additives to thermoplastic resins.
The first commercial use of silicone materials as additives to thermoplastic
resins was the use of silanes to promote crosslinking of polyethylene and to
improve the compatibility of mineral fibers. The Sioplas process for
crosslinking polyethylene, which was developed and patented by Midland
Silicones, has since been further developed by others.
The first use of polysiloxane polymers was also in filler treatment,
especially with silica and titania. The low molecular weight
polydimethylsiloxanes used for this treatment are less expensive than silanes
and yield broadly similar results unless very high surface areas are required.
During the early 1970's, higher molecular weight polydimethylsiloxanes began
to be used as separate additives to thermoplastics.
Other types of polysiloxanes have also been used as additives, but because
they are more expensive than basic dimethyl materials, they are normally
restricted to smaller, higher value applications. Phenyl-containing siloxanes,
for example, have been used in styrenic polymers for medical applications and
where outstanding clarity is required.
While all of these silicone polymers are good lubricants, they have two
intrinsic disadvantages:
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Because they are liquid at room temperature, they present handling
difficulties to an industry accustomed to solid raw materials.
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When added directly into a polymer melt, they induce screw-slip at higher
concentrations
Masterbatches can partially overcome these problems, but the second issue has
effectively limited silicone concentrations to about 10%. While this is
sufficient for a few specific applications, it is not high enough for broad
spectrum use.
Fortunately, by using an ultra-high molecular weight siloxane as the basis of
its standard masterbatches, Dow Corning has overcome this concentration
problem by making 25-50% masterbatches. At the same time, ultra high molecular
weight siloxane additives are even more effective than conventional lower
molecular weight siloxanes as process aids and surface lubricants, thus
providing greatly enhanced effects.
Dow Corning Multibase has developed two special solid forms of plastic
additives:
1. Siloxane masterbatches
2. Siloxane powders
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