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.
Experimental Investigation of the Controlled Atmospheric Pressure Resin Infusion (CAPRI) Process
Controlled atmospheric pressure resin infusion (CAPRI) is a variation of the vacuum-assisted resin transfer molding (VARTM) process. The CAPRI process increases the fiber volume fraction of the preform prior to infusion via debulking and applies a reduced pressure gradient during infusion to minimize thickness gradients during processing. This study experimentally investigates the effect of debulking and reduced pressure gradient on the incoming material parameters, process behavior and final dimensional tolerances. The effect of debulking on fabric permeability and compaction behavior has been investigated and shows a significant impact on the infusion time and final fiber volume fraction. Several E-glass plain weave preforms have been infused and flow, pressure and thickness data has been recorded and compared to traditional VARTM processing. A previously developed model uses the experimentally obtained permeability data and good agreement of the flow behavior is observed, the CAPRI process decreases thickness gradients to less than 1% while increasing fiber volume fraction by 5% in the composite part.
Imidization Reactions in PMR-15 Polyimide
The NASA-developed polyimide, PMR-15 is the frontrunner in the race to develop a matrix resin for composites intended for high temperature applications. The properties of PMR-15 depend on the polymer structure. Variations in the structure such as molecular weight, molecular weight distribution and crosslink density, will affect the ultimate mechanical and thermal properties of the material. The polymerization of PMR-15 is complex. Competing reactions in the initial imidization stage, e.g. anhydride formation, can lead to variations in the structure of the oligomers which will affect processing characteristics. This study examines the structure of the prepolymers produced by the thermal imidization of the monomer mixture under different time/temperature conditions. Techniques used include NMR, FTIR, GPC, mass spectrometry and dynamic mechanical spectroscopy. The polymer structures are complex with imide, amide, anhydride, ester, amine and salt all being detected in varying amounts depending on the thermal treatment. Differences in prepolymer molecular weight and molecular weight distribution are also observed. Variations are also seen in prepolymer viscosity and also in volatile evolution at elevated temperatures. Such differences could have a profound effect on processing characteristics.