< SYNTHETIC RTV ELASTOMERS
POLYSULPHIDE RUBBERS
Polysulphide rubbers are primarily composed of carbon/sulphur polymer chains, and are catalysed using a lead or copper based oxidising compound. Two part pack polysulphide moulding products are readily available with a variety of setting times and curing rates, hardnesses and viscosities. Like the other RTV rubbers, polysulphide rubbers can be custom modified by the moulder to suit each individual job, in particular THIXOTROPIC additives are used to vary the rubber’s viscosity and flow characteristics.
Different grades of the rubber are typically identified by a colour coding, the black tinted [lead peroxide catalysed] version is one of the most common rubbers used in art foundries. This and most other polysulphide grades are easily identified by a ‘bad eggs’ sulphurous odour - though it is worth noting that this usually disappears once the rubber has CURED. The base rubber is mixed with a matching CATALYST to the manufacturers' recommendations (proportions are usually given as a part A [rubber] to part B [catalyst] ratio by weight). The rubber’s usable working time after mixing with a catalyst varies according to the combination selected – though a useful characteristic of polysulphide rubber is that later applied layers usually adhere quite well to earlier layers, so it is possible to introduce fresh rubber into a partially set mould without too great a risk of layer delamination.
Polysulphide rubbers are suitable for use with a variety of mouldmaking techniques, from simple pour (drop) moulds through to multiple piece part moulds. As with the other RTV’s described in this section, mixing polysulphide rubbers with an added in thixotropic powder increases the rubber’s viscosity to assist 'paint on' applications to patterns not secured within a plaster restraining case (however a restraining case is necessary to retain very fluid rubbers or hot poured PVC). A quite viscous rubber compound is especially useful in those situations where manipulating a pattern into a more convenient position for moulding is not possible, for instance when moulding a sculpture in situ . The moulder can add in enough thixotropic to enable a rubber layer to be applied directly to vertical, or even overhead pattern surfaces. When a rubber is used in this way, the cured skin is usually supported later with a plaster case.
Most polysulphide rubber compounds are relatively soft, making these products quite suitable for moulding delicate or fragile patterns, though the rubber does have a tendency to stain (espcially porous stone or wood surfaces). Polysulphide rubbers can also react adversely with some lacquer finishes and styrene based products.
In addition to wax copies for metal casting, most polysulphide rubbers can be used to create reproductions in plaster, low exothermic resins, and numerous other castable media. The rubber is also resistant to solvents, oils, moisture absorption and distortion, making it suitable for long term use and storage. In general, standard polysulphide rubbers make an excellent general purpose art foundry moulding compound.
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