Compositions of, and processes for fabricating, high-temperature composite materials from phenylethynyl-terminated imide (PETI) oligomers by resin-transfer molding (RTM) and resin infusion have been developed. Composites having a combination of excellent mechanical properties and long-term high-temperature stability have been readily fabricated. These materials are particularly useful for the fabrication of high-temperature structures for jet-engine components, structural components on high-speed aircraft, spacecraft, and missiles.
Phenylethynyl-terminated amide acid oligomers that are precursors of PETI oligomers are easily made through the reaction of a mixture of aromatic diamines with aromatic dianhydrides at high stoichiometric offsets and 4-phenylethynylphthalic anhydride (PEPA) as an end-capper in a polar solvent such as N-methylpyrrolidinone (NMP). These oligomers are subsequently cyclodehydrated — for example, by heating the solution in the presence of toluene to remove the water by azeotropic distillation to form low-molecularweight imide oligomers. More precisely, what is obtained is a mixture of PETI oligomeric species, spanning a range of molecular weights, that exhibits a stable melt viscosity of less than approximately 60 poise (and generally less than 10 poise) at a temperature below 300 °C. After curing of the oligomers at a temperature of 371 °C, the resulting polymer can have a glass-transition temperature (Tg) as high as 375 °C, the exact value depending on the compositions.
As an example, one PETI oligomer, denoted PETI-330, was synthesized as shown in the figure. First, 0.53 mole of 2,3,3',4'-biphenyltetracarboxylic dianhydride (BPDA) and 0.94 mole of PEPA were reacted with 0.50 mole of 1,3- bis(4-aminophenoxy)benzene and 0.50 mole of 1,3-diaminobenzene in NMP to form the phenylethynyl-terminated amide acid oligomer. Then, by azeotropic distillation using toluene, the phenylethynylterminated amide acid oligomer was converted to the PETI oligomer having a calculated repeat unit molecular weight of 750 g/mole.
The imide oligomer was isolated, washed well with water, and dried. The melt viscosity of the imide oligomer was found to be 0.6 poise initially at a temperature of 280 °C and 0.9 poise after 2 hours at that temperature. The imide oligomer was degassed, then injected, at a temperature of 280 °C, into a preform made of a fabric of T650 carbon fibers in 8-harness satin weave. The oligomer was then cured for 1 hour at a temperature of 371 °C and a pressure of 200 psi (˜1.4 MPa). The resulting quasi-isotropic composite was found to have an open-hole compressive strength of 39 kpsi (˜0.27 GPa) at a temperature of 23 °C and 29 kpsi (˜0.2 GPa) at a temperature of 288 °C. After aging for 1,000 hours at 288 °C in air, the open-hole compressive strength was found to be 32 kpsi (˜0.22 GPa) at 23 °C. The unnotched compressive strength was found to be 75 kpsi (˜0.52 GPa) at 23 °C. The composite exhibited no microcracking, either immediately after cure or after 200 subsequent thermal cycles from –55 °C to +288 °C. The Tg of the composite was found to be 330 °C.
Another such PETI oligomer, denoted PETI-375, was synthesized and used to make an RTM composite with a carbon fiber cloth of the type described above. The open-hole compressive strength of this composite was found to be 43.5 kpsi (˜0.3 GPa) at 23 °C, 34.8 kpsi (˜0.24 GPa) at 288 °C, and 32.2 kpsi (˜0.22 GPa) at 316 °C. The Tg of the composite was found to be 370 °C.
This work was done by John W. Connell, Joseph G. Smith, Jr., and Paul M. Hergenrother of Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.