BEHAVIOR OF PURE AND REINFORCED CHARRING POLYMERS DURING ABLATION UNDER HYPERVELOCITY RE-ENTRY CONDITIONS

Abstract : Arc tests conducted on a series of cured epoxy resins indicated that a novolac structure in the polymer, and a system functionality in excess of 2.14 and possibly as high as 2.6, are necessary for ablative char formation in the systems tested. In addition, certain acids, when added to noncharring epoxies, induced char formation in arc tests. A mechanism of char layer formation and removal has been postulated wherein the polymer undergoes thermal degradation to form carbonaceous residue and gaseous products; the gaseous products percolate through the carbonaceous char, undergo further thermal decomposition, and deposit pyrolytic graphite on the carbonaceous cell walls and silica fibers (if present); the surface is removed by mechanical and chemical erosion, i.e., silicacarbon reaction and/or graphite oxidation. Photomicrographs of char layer cross-sections show details of pyrolytic graphite microstructure and graphite-silica interactions. X-ray diffraction patterns show that silicon carbide is the principal crystalline component in the surface zone of the char layer of the silica reinforced materials. Examination of arc specimens indicates that the char layer fluctuates in thickness, but shows an average increase in thickness during the exposure. The fluctuation in thickness appears to be due to a periodic weakening and removal of the char surface zone. (Author)