High Temperature VARTM of Phenylethynyl Terminated Imides
Depending on the part type and quantity, fabrication of composite structures using vacuum-assisted resin transfer molding (VARTM) can be more affordable than conventional autoclave techniques. Recent efforts have focused on adapting VARTM for the fabrication of high temperature composites. Due to their low melt viscosity and long melt stability, certain phenylethynyl terminated imides (PETI) can be processed into composites using high temperature VARTM (HT-VARTM). However, one of the disadvantages of the current HT-VARTM resin systems has been the high porosity of the resultant composites. For aerospace applications, the desired void fraction of less than 2% has not yet been achieved. In the current study, two PETI resins, LaRC PETI-330 and LaRC PETI-8 have been used to make test specimens using HT-VARTM. The resins were infused into ten layers of IM7-6K carbon fiber 5-harness satin fabric at 260 or 280 °C and cured at temperature up to 371 °C. Initial runs yielded composites with high void content, typically greater than 7% by weight. A thermogravimetric-mass spectroscopic study was conducted to determine the source of volatiles leading to high porosity. It was determined that under the thermal cycle used for laminate fabrication, the phenylethynyl endcap was undergoing degradation leading to volatile evolution. This finding was unexpected as high quality composite laminates have been fabricated under higher pressures using these resin systems. The amount of weight loss experienced during the thermal cycle was only about 1% by weight, but this led to a significant amount of volatiles in a closed system. By modifying the thermal cycle used in laminate fabrication, the void content was significantly reduced (typically ∼ 3% or less). The results of this work are presented herein.
Processing and characterization of PETI composites fabricated by high temperature VARTM
The use of composites as primary structures on aerospace vehicles has increased dramatically over the past decade. As these advanced structures increase in size and complexity, their production costs have grown significantly. A major contributor to these manufacturing costs is the requirement of elevated processing pressures, during the thermal cure, to create fully consolidated composites. For certain composite parts, high temperature vacuum assisted resin transfer molding (HT-VARTM) can offer reduced fabrication costs compared to conventional autoclave techniques. The process has been successfully used with phenylethynyl terminated imide (PETI) resins developed by NASA LaRC. In the current study, two PETI resins, LARC(TradeMark) PETI-330 and LARC(TradeMark) PETI-8 have been used to make test specimens using HT-VARTM. Based on previous work at NASA LaRC, larger panels with a quasi-isotropic lay-up were fabricated. The resultant composite specimens exhibited void contents <3% by volume depending on the type of carbon fabric preform used. Mechanical properties of the panels were determined at both room and elevated temperatures. These included open-hole compressive (OHC) and short beam shear (SBS) properties. Limited process modeling efforts were carried out including infusion times, composite panel size limitations and fabric permeability characterization. Work has also been carried out to develop new PETI based resins specifically geared towards HT-VARTM. The results of this work are presented herein.
Improvement of thermoplasticity for s‐BPDA/PDA by copolymerization and blend with novel asymmetric BPDA‐based polyimides
Asymmetric biphenyl type polyimides (PI) derived from 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA) and p-phenylenediamine (PDA) or 4,4′-oxydianiline (ODA) show higher Tgs, and much better thermoplasticity than the corresponding isomeric PIs from symmetric 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA). In addition, a-BPDA-derived PIs are completely amorphous owing to their bent chain structures and highly distorted conformations, whereas the PIs from s-BPDA are semicrystalline. a-BPDA-derived PIs possessing these properties or the a-BPDA monomer were used as a flexible blend component or a comonomer to improve the insufficient thermoplasticity of semirigid s-BPDA/PDA homo polymer. The blends composed of s-BPDA/PDA (80%) with a-BPDA-derived PIs (20%), as well as the s-BPDA/PDA-based copolymer containing 20% a-BPDA, showed a certain extent of thermoplasticity above the Tgs without causing a decrease in Tg. In addition, these blends and copolymer provided comparatively low thermal expansion coefficient (ca. 18 ppm). The improved film properties for the blends are related to good blend miscibility. On the other hand, when s-BPDA/ODA was used as a flexible matrix polymer instead of a-BPDA-derived PIs, the 80/20 blend film annealed at 400°C exhibited no prominent softening at the Tg. This result arises from annealing-induced crystallization of the flexible s-BPDA/ODA component. Thus, these results revealed that a-BPDA-derived PIs are promising candidates as matrix polymers for semirigid s-BPDA/PDA for the present purpose. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2499–2511, 1999