Woven fabrics comprising polybenzoxazole (PBO) fibers blended with other selected fibers have been found to exhibit combinations of properties that, in some applications, may be superior to those of fabrics made from either PBO alone or the other materials alone. This finding is the result of an investigation of improved fabrics for potential use in space suits and in structures for protecting spacecraft against micrometeoroids. Potential terrestrial applications for the improved fabrics could include bulletproof vests, protective suits for firefighters, and lightweight armor for military equipment and personnel, as well as tethers for parachutes or aircraft.
PBO is characterized by high values of thermo-oxidative stability and tensile strength — a combination of properties that make PBO desirable for many applications, including those described above. However, PBO exhibits some of the same deficiencies — in particular, limited flex life and limited resistance to abrasion — observed in other high-modulus fiber materials. Therefore, woven fiber blends with PBO were investigated in an effort to increase flex life and resistance to abrasion without sacrificing tensile strength or resistance to flames and heat.
In the investigation, enhanced performance was sought through changes in the structures of yarns and fabrics through blending PBO multifilament yarns with other high-performance filamentary materials. Specimens in the form of webs 2 in. (≈5.1 cm) wide were made from PBO and other materials as follows:
- A plied (double-cloth) fabric consisting entirely of PBO (one of two control specimens);
- A plain-weave fabric consisting entirely of PBO;
- A plied fabric consisting entirely of Kevlar aromatic polyamide (the other control specimen);
- A plied yarn hybrid of which 60 percent consisted of PBO fibers and 40 percent consisted of high-temperature-resistant, wear-resistant polyimide fibers known by the trade name "P84";
- A plied yarn hybrid consisting of 40-percent PBO and 60-percent P84 polyimide; and
- A hybrid fabric consisting of 70-percent PBO and 30-percent Nomex aramid felt.
The specimens were abraded according to a Federal specification for testing for resistance to abrasion. The specimens were tested for tensile strength before and after abrasion. They were also evaluated with respect to limiting oxygen index (LOI) (a measure of resistance to burning).
The table presents some of the data from the tests. The PBO/Nomex-blend specimen resisted abrasion better than did the pure-PBO specimen. Although the LOIs of the blend specimens were less than those of the pure-PBO specimens, they were still much higher than that of any other commercially available polymer fiber fabric of the same density. The LOIs of all samples tested satisfy the flame-resistance requirements for use in the space-shuttle cabin, which is an indication that PBO fiber blends have great potential for spacecraft applications.
This work was done by Evelyne Orndoff of Johnson Space Center, Steven Clarke of Albany International Research Co., and Rajib Dasgupta of Lockheed Martin. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.