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	PRODUCT DATA of 16: Rubbers and elastomers

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16: Rubbers and elastomers

General Information

Only vulcanized-rubber items which are extruded or moulded in their final shape are covered in this section. For some other rubber-compound applications where there is either no cure or a cure-in-place application (like RTV rubbers).

Use in Spacecraft

There are many applications throughout a vehicle for rubber compounds, e.g. mechanical damping systems, seals and gaskets, electrical insulation, membranes, and bladders for fluids.

Main Categories

Commercial "rubbers" contain not only one or more rubber polymers, but many, for example, additives, fillers and pigments. Themost useful for space applications are based on polybutadiene, polychloroprene, polyurethanes, acrylics, nitrile, ethylene-propylenes, silicones and fluorinated polymers. They appear, for example, as moulded parts, films, coated textiles, extruded insulation, sleeves and shrinkable items. It is practically impossible to obtain details from the manufacturers of the formulations they sell. For critical applications it is sometimes better to use a special formulation tailored to the use with the help of a local compounder.

Processing and Assembly

Except in the case mentioned above, the user is not concerned with rubber processing. This operation is rather complicated and calls for specialized equipment. It consists in hot-mixing rubber polymers with, for example,pigments, fillers, reinforcing agents, antioxidants and vulcanizing agent, followed by shaping (extrusion, moulding) and curing. All the steps are quite sensitive to processing variables and shall be carefully controlled. On the other hand, the use of finished or semi-finished items is normally straightforward and only the bonding of the rubber to other materials or to itself is a possible problem.

Precautions

Under the same generic name, for example “butyl”, an immense number of different formulations can exist. The identification of a rubber product is difficult, but should nevertheless be made carefully. Rubbers, depending on their nature and composition and on the type of environmental exposure, have a tendency to “set” under stress, i.e. to suffer a non-reversible deformation, which should be taken into account. Cyclic stresses produce heat in rubber structures; this can lead to thermal degradation. Some rubber mixtures contain products that are corrosive to certain metals. Most rubbers are quite sensitive to chemical attack by gas, liquids and solvents. Tables of chemical resistance should be consulted. Some rubbers have a limited life time in air, this shall be considered if long duration storage is involved.

Hazardous and Precluded

Polysulphide rubbers are not stable enough in the space environment. Chlorinated rubbers are marginal in outgassing. In many types of use, the choice is dictated not by the space environment, but by the compatibility with some fluid or gas (e.g. membranes and bladders). Silicones shall not to be used for low gas permeability (pressurized systems). Rubbers containing, for example, plasticisers and extending oils, are unstable in vacuum and shall be excluded. Fillers may be leached out by a fluid and clog small apertures (e.g. the pores of a catalyst).

Effects of Space environment

Vacuum exposure provokes outgassing, which is particularly due to volatile additives, but also to depolymerization of the base polymer. Both these phenomena lead to a change in mechanical and physical properties of rubber items. The risk of contamination in the vicinity is also high. Outgassing and contamination shall be measured for each formulation: results cannot be generalized safely to a full series, except perhaps in the case of perfluorinated rubbers, which are safe, and for the silicone rubbers, which become generally acceptable only after a long post-cure at 250 ºC. Radiation attacks rubber either by hardening it (cross-linking) or by softening it to form a viscous material. Most common rubbers cannot be used if the ionizing radiation is more than a few Mrad. Polyurethanes and fluorinated rubbers can go up to 10 Mrad. Uses inside the spacecraft are not limited by these features, but care shall be taken inthe selection of external applications, particularly because of the added action of solar UV. The temperature range for useful rubber properties is rather narrow, from -100 ºC for the best low-temperature silicones to 300 ºC for short exposure of fluorinated rubbers. At low temperature, one observes hardening, stiffening and eventually crazing and crushing. High temperatures provoke decomposition. Some boron-based experimental rubbers exist now for temperatures up to 400 ºC. The temperature resistance is lessened in the presence of incompatible fluids.

Some Representative Products

As for plastics, raw products and some semi-finished items are produced by large companies, but there are many relatively small compounders manufacturing catalogue or “on-demand” items. The following materials, for which data sheets are provided, can be considered: Eccoshield SV-R, Vibrachoc VHDS, Viton B910.