ABLATIVES. Materials used for the outward dissipation of extremely high heats by mass removal. Their most common use is as an external heat shield to protect supersonic aerospace vehicles from an excessive buildup of heat caused by air friction at the surface. The ablative material must have a low thermal conductivity in order that the heat may remain concentrated in the thin surface layer. As the surface of the ablator melts or sublimes, it is wiped away by the frictional forces that simultaneously heat newly exposed surfaces. The heat is carried off with the material removed. The less material that is lost, the more efficient is the ablative material. The ablative material also should have a high thermal capacity in the solid, liquid, and gaseous states; a high heat of fusion and evaporation; and a high heat of dissociation of its vapors. The ablative agent, or ablator, is usually a carbonaceous organic compound, such as a phenolic plastic.
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As the dissociation products are lost as liquid or vapor, the char is held in place by the refractory reinforcing fibers, still giving a measure of heat resistance. The effective life of an ablative is short, calculated in seconds per millimeter of thickness for the distance traveled in the atmosphere. Single ablative materials seldom have all the desirable factors, and thus composites are used. Phenolic or epoxy resins are reinforced with asbestos fabric, carbonized cloth, or refractory fibers, such as asbestos, fused silica, and glass. The refractory fibers not only are incorporated for mechanical strength, but also have a function in the ablative process, and surface-active agents may be added to speed the rate of evaporation. Another composite, polyarylacetylene (PAA) reinforced with carbon fiber fabric, proved superior to carbon-reinforced phenolic in tests to develop an alternative ablative and insulative material for nozzle components of solid rocket motors. Favoring the PAA is its high (90%) char yield, lower weight loss and erosion, greater moisture resistance, and more stable ablation. Ablative paint, for protecting woodwork, may be organic silicones which convert to silica at temperatures above 2000°F (1093°C).
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