IM7 LARC(TM)-ITPI POLYIMIDE COMPOSITES

LARC(TM)-ITPI, an isomeric variation of the better known LARC(TM)-TPI and based on 4,4'-isophthaloyldiphthalic anhydride and 1,3-phenylenedi amine, was evaluated as a matrix for high-performance composites. Five 30% poly(amide acid) solutions in N-methyl pyrrolidone, with stoichio metric offsets of 2.0, 3.0, 4.0, 4.75 and 5.5% in favour of the diamin e and end-capped with phthalic anhydride, were synthesized and their m olecular weights and molecular weight distributions determined. Import antly, high concentrations of low molecular weight species were found in all the offset compositions. Solvent/volatile depletion rates were carefully determined on thermally imidized films of the five compositi ons and were an important part of the composite consolidation studies. All films failed a solvent resistance test which involved immersion i n acetone, methyl ethyl ketone, toluene, dimethylacetamide and chlorof orm for 1 min followed by a fingernail crease. A minor modification of the polymer backbone improved solvent resistance measurably. Unidirec tional IM7 prepreg was made from each of the five resin solutions by s tandard drum-winding procedures. A workable composite consolidation cy cle was developed for the 3% offset solution by conducting a parametri c study involving residual solvent content, melt viscosity and composi te C-scan information. The basic strategy was to B-stage the prepreg t o a temperature where 98% of the volatiles were depleted while, at the same time, adequate molten resin fluidity (via incomplete imidization and residual solvent content) was retained, then apply pressure and i ncrease temperature to complete the consolidation. This moulding cycle was then applied successfully to the remaining compositions and compo sites fabricated. From the processing information and composite mechan ical properties, including short-beam shear strength, flexural strengt h and flexural modulus at room temperature, 93, 150 and 177 degrees C, the 4.75% stoichiometrically offset end-capped polymer was chosen as the optimal matrix. Composite engineering properties for this selected composition were also obtained, including longitudinal tension, trans verse flexural, longitudinal compression, interlaminar shear, short-bl ock compression, compression strength after impact and open-hole compr ession (OHC). Notably, 80% of the room temperature OHC strength was re tained at 177 degrees C, indicating that the LARC(TM)-ITPI is an excel lent high-temperature matrix material for selected future aerospace ap plications where solvent resistance is not a key requirement.