Reducing the cost and weight of launch structures is essential to meeting NASA goals for reliable access to space. Currently, separate systems are used for structure and pressure containment, cryogenic insulation, and high-temperature insulation. One way of reducing this cost and weight is through the development of multifunctional materials that can eliminate parasitic weight. Combining two functional components — structure and insulation — reduces weight and structural complexity, which usually is akin to fragility in the system, and minimizes the need for parasitic thermal protection and insulation systems.
This invention is an inorganic-organic hybrid syntactic insulation that provides a manufacturing method for a structural cryogenic insulator capable of an effective laminate thermal conductivity of 0.14 W/mK, and that shows compression resistance to loads >5,000 psi.
Powdermet has been developing its inorganic-organic (IO) hybrid composite for a number of years. Its inauguration was the development towards an effective spray coat for high temperatures and for providing abrasion resistance for down-well piping. Another technology platform Powdermet has been developing is low thermally conductive syntactic composites (SComp). It was believed in the scope of this project that the development of a combination of these two technologies to create a structural insulator for a multilayer thermal protection system (TPS) could be achieved.
Formulation and processing methods had to be redesigned to incorporate hollow spheres into the IO hybrid. Hollow microspheres can be used in all standard processing methods for thermoset and thermoplastic composites, including extrusion and injection molding, and have found a variety of applications across all industries. Comparable to fly ash, but at half the density while still maintaining structural strength, hollow microspheres offer the multifunctional properties desirable for integrating into composite parts to reduce weight, improve dimensional stability, and reduce thermal conductivity. Many of these property enhancements are attributed to the hollow sphere inclusion’s low density. However, their low density, along with surface incompatibilities, makes the uniform dispersion into composites difficult.
The IO hybrid SComp design uses a high-temperature performance thermoplastic as the organic constituent, a polysilizane as the inorganic constituent, and hollow glass microspheres (with crush strength up to 4,000 psi). Two incorporating methods were conceptualized using the chemical and physical properties of the IO constituents to interlock the hollow spheres during mixing. The first method utilizes an observed physical effect, where the thermoplastic will solidify in solution after a certain concentration similar to a non-Newtonian fluid. This is believed to happen because of a low-energy solvated polymer inter-network crystalline state that is formed. Incorporating the hollow glass microspheres by mixing, and letting this inter-network crystalline form, interlocks the uniformly dispersed microspheres. The second method relies on the reactive chemistry of the polysilizane by hydrolysis to form interlinked cross-linkings. In the solution with mixing microspheres, the addition of water solidifies the mixture, locking in the uniformly dispersed microspheres.