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|PRODUCT DATA of 16: Rubbers and elastomers|
|Material||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|
|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: