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-phenylenediamine, was evaluated as a matrix for high-performance composites. Five 30% poly(amide acid) solutions in N-methyl pyrrolidone, with stoichiometric offsets of 2.0, 3.0, 4.0, 4.75 and 5.5% in favour of the diamine and end-capped with phthalic anhydride, were synthesized and their molecular weights and molecular weight distributions determined. Importantly, 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 compositions and were an important part of the composite consolidation studies. All films failed a solvent resistance test which involved immersion in acetone, methyl ethyl ketone, toluene, dimethylacetamide and chloroform for 1 min followed by a fingernail crease. A minor modification of the polymer backbone improved solvent resistance measurably. Unidirectional IM7 prepreg was made from each of the five resin solutions by standard drum-winding procedures. A workable composite consolidation cycle was developed for the 3% offset solution by conducting a parametric study involving residual solvent content, melt viscosity and composite C-scan information. The basic strategy was to B-stage the prepreg to 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 increase temperature to complete the consolidation. This moulding cycle was then applied successfully to the remaining compositions and composites fabricated. From the processing information and composite mechanical properties, including short-beam shear strength, flexural strength 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, transverse flexural, longitudinal compression, interlaminar shear, short-block compression, compression strength after impact and open-hole compression (OHC). Notably, 80% of the room temperature OHC strength was retained at 177 degrees C, indicating that the LARC(TM)-ITPI is an excellent high-temperature matrix material for selected future aerospace applications where solvent resistance is not a key requirement.