< SILICONE RUBBERS
POLYURETHANE RUBBERS
Polyurethane rubbers are based on carbon-oxygen-nitrogen polymer chain links, with a part ‘B’ catalyst to initiate setting and curing. Like the other RTV’s mentioned here, polyurethane rubbers are available in a variety of setting and curing times, the basic product’s viscosity may also be modified using thixotropic additives. Polyurethane rubbers are mixed and applied to master patterns using similar techniques to those used for making other types of RTV mould.
The polyurethane grades used in art foundries are typically very tough, strong and resistant to abrasion. This makes them an excellent choice if the mould is also likely to be used to make casts in materials like concrete and cement. The strength and tenacity of this rubber in adhering to a pattern does make polyurethene somewhat unsuitable for moulding delicate or fragile master patterns. Even robust patterns, especially those with porous surfaces, must be thoroughly sealed and treated with a release agent before moulding.
One of the most attractive features of polyurethane rubbers is their cost base, as they are among the cheapest available RTV rubber compounds by weight. This makes the rubber attractive for use when moulding large scale master patterns, especially if only a very limited number of reproductions are required.
Polyurethane rubbers can be be used with most studio casting materials (wax, plaster, resin etc), but not with low temperature metals. The rubber lasts well in storage, though some products tend to soften slightly after three years or so. One item of note, is that whilst safe under normal use, polyurethane rubber contains an isocyanate component which will produce highly toxic fumes if ignited.
Polyurethane rubbers produce perfectly satisfactory art foundry moulds, but polysulphide and silicone rubbers are better suited for moulding delicate and other special requirement patterns.
RIGID REPRODUCTION MOULDS >
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TIP: The usable life of a RTV rubber used for casting concrete and other abrasive media can be extended by treating the rubber surfaces with a detergent. When casting resins, the use of polyvinyl alcohol will provide the same benefit. |
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