The specific environmental impact achieved by this project is the replacement of composite materials containing 4,4'‐methylenedianiline (MDA) for Department of Defense (DoD) weapons systems and platform applications. MDA is a known carcinogen and liver toxin, and a constituent of polyimide matrix composites (e.g. PMR‐15) currently used in legacy aircraft and engine structures. In addition to the negative human health and environmental impact of MDA‐ containing polyimides, recent government legislation to phase out carcinogenic materials in production environments, employer liability and workplace safety concerns, hazardous waste handling and disposal requirements, high manufacturing costs, worker health monitoring, and a sole source supply chain for PMR‐15 lead to a potentially catastrophic sustainability risk to the DoD. As a result, there is a critical, immediate need for qualified, aerospace grade polyimide matrix composites that can replace PMR‐15 and other MDA containing polyimides. This project will demonstrate, qualify, and mature, to high technology readiness level and high manufacturing readiness level, environmentally friendly, polyimide matrix composite that is a suitable, cost‐competitive replacement to PMR‐15.
The technical approach of this project focused on development of a low toxicity polyimide for evaluating thermal and mechanical performance versus PMR-15. The approach utilized P2 SI® 635LM, a patented resin formulation originally developed as a resin transfer molding polyimide, comprised of state-of-the-art polyimide chemistry and low cost, commercially available monomers, as a PMR-15 replacement candidate. Being based on preliminary thermo-oxidative stability and mechanical properties, processing characteristics, low toxicity non-MDA containing material and a non-International Traffic in Arms Regulations (ITAR) regulated material. The primary technical objective was to mature the P2 SI® 635LM prepreg, resin infused fiber reinforcement for composite manufacture. As well as determining the suitability of the material as a PMR-15 replacement. Additionally, an Integrated Product Team (IPT) comprised of users of the technology and relevant government agencies was assembled. The product team were involved in critical decisions made on characteristic testing properties and were involved in defining transition pathways for the materials technology. They also supported internal research and development efforts, and provided input throughout the effort. Materials work began with an initial phase of resin and prepreg manufacturing followed by process modeling. Once the material had a complete model for the range of process critical parameters there was an effort of composite study on accelerated thermo-oxidative aging and characterization of mechanical properties. This was followed up with mechanical characterization of lamina and laminate properties. In addition, fluid sensitivity of representative fluids were characterized for both short and extended duration exposures.
P2 SI® 635LM resin was manufactured according to internal processes and certifications. The prepreg was manufactured with aerospace grade T650-35/8HS de-sized fabric used as the reinforcement. This followed the industry standard polyimide prepreg specifications completed in house using existing aerospace grade proprietary processes. The product team participated in plan for the maturation of P2 SI® 635LM polyimide composites for aerospace applications. This included system manufacturers and key Tier I suppliers of high temperature polymer matrix composites. A complete process model was developed for the P2 SI® 635LM prepreg system. This included models for imidization kinetics, cure kinetics, viscosity as a function of extent of cure, and glass transition temperature as a function of extent of cure. Knowledge of the various kinetic parameters and the ability to predict this behavior was used in design of cure cycles for components of various geometries. The process models were used to develop an optimized, robust cure cycle for the P2 SI® 635LM prepreg system. Laminates up to 0.6 inches (15 mm) in thickness were manufactured with developed cure cycle. Non-destructive (ultrasonic) and destructive (optical microscopy, acid digestion) were used to characterize laminate quality, void content, and fiber volume fraction. The optimized cure cycle, autoclave bagging schemes and process boundaries were compiled into a process control document and provided to component manufactures with the material database. Testing of mechanical properties of lamina and laminate properties of the materials system was also completed under extreme thermal and environmental conditions similar to those seen in B-allowables database for National Center for Advanced Materials Performance qualified composite materials.
This project has multiple benefits to the DoD. The environmental impact associated with removal of MDA from the workplace reduces human exposure, hazardous waste disposal and handling costs, and manufacturing costs associated with this toxic material. Providing a new alternative to MDA‐ containing polyimides reduces supply chain risk, reduces costs by increasing competition, provides additional material sources, increases efficiency, and improves readiness. Since the material system is dual use, sustainment risks are minimized and affordability maintained through integration into the various commercial sectors that use polyimides.