The term "secondary polymer layered impregnated tile" ("SPLIT") denotes a type of ablative composite-material thermal- insulation tiles having engineered, spatially non-uniform compositions. The term "secondary" refers to the fact that each tile contains at least two polymer layers wherein endothermic reactions absorb considerable amounts of heat, thereby helping to prevent overheating of an underlying structure. These tiles were invented to afford lighter-weight alternatives to the reusable thermal-insulation materials heretofore variously used or considered for use in protecting the space shuttles and other spacecraft from intense atmospheric- entry heating. Tiles of this type could also be useful on Earth as relatively lightweight components of fire-retardant structures.
The SPLIT concept admits to so many different combinations of constituent materials, spatial distributions of the materials, and fabrication processes, that it is not possible to even list, much less summarize or describe all of them. Instead, a representative example must serve to illustrate the main principles. The starting material for fabricating a typical SPLIT is a porous substrate, having a void volume fraction of about 90 percent, that comprises a rigid tile or fabric made from any of a large variety of carbon fibers and/or ceramics fibers. The fiber composition can be the same throughout the thickness or can be graded: for example, it can differ among front, middle, and rear layers.
The front layer, which is the one to be exposed directly to intense heating, is typically impregnated with a thermosetting resin (e.g., a phenolic or a silicone). This layer becomes the first line of defense against intense heating: a large amount of heat is absorbed in the pyrolysis of the front polymer layer and is dissipated to the environment through a combination of outflow of the pyrolysis gas, and thermal radiation from the char layer formed in the pyrolysis. The outflow of the pyrolysis gas also provides further protection against heating by blocking the inflow of hot ambient gas.
The middle layer (if any) is typically not impregnated. The back layer is the one to be placed in contact or proximity to the structure to be protected. The back layer is initially impregnated with a thermoplastic polymer (the secondary polymer) in solution and the solvent is allowed to evaporate, so that the fibers in the back layer become coated with the thermoplastic but the layer retains substantial porosity. The secondary polymer is chosen to be one that pyrolizes completely or nearly completely to gaseous products (i.e., leaving little or no solid residue), at a temperature much lower than the pyrolysis temperature of the front layer. For example, polystyrene and poly (methyl methacrylate) decompose at temperatures between 350 and 450 °C.
Eventually, some heat penetrates the front layer and diffuses to the back layer, where the lower-temperature pyrolysis reaction of the secondary polymer retards the transfer of heat to the structure to be protected. The pyrolysis gas from the secondary polymer escapes through the middle (if any) and front layers, thereby effectively preventing excessive heating of the underlying structure through a combination of transpiration cooling and blockage of inflow of hot ambient gas. With suitable choice of materials and processing, it is possible to delay the onset of heating of the back surface and limit the temperature rise of the back surface (see figure) to a value low enough to protect the underlying structure.
This work was done by Huy K. Tran, Daniel J. Rasky, and Christine E. Szalai of Ames Research Center; Ming-ta S. Hsu of HC Chem Research and Services Corp.; and Joseph A. Carroll of Tether Applications. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Materials category.
This invention has been patented by NASA (U.S. Patent No. 6,955,853 B1). Inquiries concerning rights for the commercial use of this invention should be addressed to
the Ames Technology Partnerships Division at (650) 604-2954.
Refer to ARC-14165-1.