Polyimide/Glass Composite High-Temperature Insulation

This composite was found to exhibit an unexpectedly high degree of fire resistance.

Lightweight composites of RP46 polyimide and glass fibers have been found to be useful as extraordinarily fire-resistant electrical-insulation materials. RP46 is a polyimide of the polymerization of monomeric reactants (PMR) type, developed by NASA Langley Research Center. RP46 has properties that make it attractive for use in electrical insulation at high temperatures. These properties include high-temperature resistance, low relative permittivity, low dissipation factor, outstanding mechanical properties, and excellent resistance to moisture and chemicals. Moreover, RP46 contains no halogen or other toxic materials and when burned it does not produce toxic fume or gaseous materials.

The U. S. Navy has been seeking lightweight, high-temperature-resistant electrical-insulation materials in a program directed toward reducing fire hazards and weights in ship electrical systems. To satisfy the requirements of this program, an electrical-insulation material must withstand a 3-hour gas- flame test at 1,600°F (about 871°C). Prior to the development reported here, RP46 was rated for use at temperatures from –150 to +700°F (about –101 to 371°C), and no polymeric product — not even RP46 — was expected to withstand the Navy 3-hour gas-flame test.

A typical process for applying RP46/glass-fiber composite to a wire, pipe, or other electrically conductive object that one seeks to insulate consists of the following steps:

1. The surface to be coated with the composite is prepared by roughening it, then cleaning it using methanol and acetone.
2. The roughened, cleaned surface is wrapped with either a prepreg [glass fabric or one or more layer(s) of glass fibers pre-impregnated with RP46] or a dry fabric or fiber sleeve or preform to a desired thickness.
3. If a dry sleeve has been wrapped, then at this point, it is infused with a resin solution having a suitable viscosity, by use of a vacuum-assisted resin-transfer molding (VARTM) technique. The VARTM step can be performed at either room temperature or an elevated temperature, depending on the specific resin solution used.
4. The workpiece as processed thus far is placed in an autoclave, wherein the resin is cured at an appropriate elevated temperature and pressure. If the resin has a low and stable melt viscosity, then the cure can be performed in a vacuum bag in an oven.

The figure depicts the Navy gas-flame test being performed on a copper pipe insulated with an RP46/glass-fiber composite. The same test was also performed on a similarly insulated aluminum pipe. The RP46/glass-fiber composite layers unexpectedly passed the tests, retaining their electrical-insulation integrity for more than 3 hours at 1,600±50°F (about 871±28°C). Furthermore, the composite showed remarkably high insulating capability. This was evident from the observation that while the RP46 was exposed to a temperature of 1,667°F (908°C), the temperature of the insulated conductor was only 229°F (109°C).

This work was done by Ruth H. Pater, Peter Vasquez, Richard L. Chattin, Donald L. Smith, Thomas J. Skalski, and Gary S. Johnson of Langley Research Center and Sang-Hyon Chu of the National Institute of Aerospace. LAR-17321-1