Sandia National Laboratories
Amine-cured epoxy materials have traditionally been used in high-performance adhesive, encapsulation, coating, and fiber-reinforced structural composite applications for moderate temperature service environments owing to their relatively low cost, excellent mechanical properties, and favorable processing characteristics. The most thermally stable formulations consist of a multifunctional (f>3) aromatic glycidyl ether resin cured with an aromatic diamine. These materials have limited high temperature (>180°C) application however, due in large part to their susceptibility towards nucleophilic-initiated pyrolytic degradation as a result of the relatively strong nucleophilicity of tertiary amine groups generated during cure. This degradation reaction is coupled with significant water formation which accounts for 30-50% of observed weight loss during aging at 200-240°C. We found that the thermal stability of these materials can be improved by selecting aromatic amine curatives of lower basicity such as 4,4’-diaminodiphenyl sulfone, however, even the most optimized formulations lag significantly behind other classes of high temperature thermosets.
Given the inherent limitations of epoxy materials, we focused on developing alternative high temperature encapsulants/adhesives inspired by state of the art materials currently used in high performance aerospace applications. To that end, numerous cyanate ester (CE) and bismaleimide (BMI) materials were screened for thermal stability via isothermal TGA and processing/cure characteristics via infrared spectroscopy. The most promising formulations were also evaluated for mechanical performance via flexural strength, K1c fracture toughness, and compression testing. All toughened BMI and CE materials that were screened showed substantially better thermal stability and similar mechanical performance compared to a reference aromatic amine cured epoxy material. The 3 best performing materials were selected for an ongoing accelerated aging study at 260-200°C to assess mechanical property changes and degradative volatile production via a novel flow-thru FTIR gas analysis technique.
Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.