An improved class of formulations for PMR polyimide resins retards the imidization that undesirably occurs during handling and storage. While imidization is desired at the final (deliberate polymerization) stage of production of a polyimide, imidization results in premature aging when it occurs during earlier stages of synthesis, shipping, prepregging, and fabrication layup. By retarding imidization at storage and handling temperatures, the improved class of formulations increases both shelf life and the upper limit of allowable temperature for handling and storage prior to final polymerization.
The improved class of formulations is best explained with reference to the origin of the PMR concept: U.S. Patent 3,745,149, issued on July 10, 1973, disclosed that polyimides can be synthesized from mixtures of monomeric reactants that have been prepared by using lower (primary) alcohols (usually, ethanol or methanol) to esterify anhydride end caps and aromatic dianhydrides. The primary alcohols are also used as solvents for the reactants. A solution that incorporates such reactants plus an aromatic diamine in the mole ratio of [N diester diacid: N+1 diamine: 2 ester acid end cap] constitutes a monomeric mix which, at high temperature, polymerizes ("cures"), becoming a polyimide; hence, the evolution of the term "polymerization of monomeric reactants," now known better by its abbreviation, "PMR." Numerous variations of the basic PMR concept have been published since the original patent was issued, but, until now, none of these variations has departed from the use of primary alcohols to prepare methyl or ethyl ester acids.
The major disadvantages of conventional commercial PMR formulation and processing are limited shelf life at room temperature, short working life, and high sensitivity to temperature excursions above room temperature. These disadvantages are associated with premature imidization during all phases of PMR usage (synthesis, manufacturing, shipping, handling, storage, and fabrication). For example, typical commercial PMR-15 polyimide prepregs are out of manufacturing specification after a shelf life of only one to three weeks at room temperature. In many cases, the shelf life is even shorter for other PMR formulations, e.g., PMR-II-50. While PMR shelf life and working time can be extended by refrigeration and ensured by careful monitoring of temperature histories, this practice adds to the costs of PMR products.
The improved class of formulations involves the use of secondary alcohols (usually, isopropanol) instead of primary alcohols for esterification of the anhydride end caps and dianhydride monomers and as solvents for the resulting isopropyl ester acids. In comparison with PMR resins made conventionally by use of primary alcohols, PMR resins made by use of isopropanol are less reactive at comparable storage or handling temperature, without changing typical imidization or curing temperatures. For example, PMR-15 resin made with isopropanol may be stored or handled at temperatures up to 50 °C higher than PMR-15 resin made with a primary alcohol (methanol) in order to produce comparable amounts of imide-aging products in both resins after equal handling and storage times.
This improvement affords a wide margin against mishandling of PMR solution, reducing or, in some cases, eliminating the need for refrigeration and monitoring of temperature histories. The results include not only increased shelf life (see figure) but also decreased costs of shipping and handling, more consistent processability, reduced variability among batches, improved hot-melt prepreg manufacturing, and reduced scrap rates. The improved formulations are also safer because in comparison with primary alcohols, isopropanol is less toxic and less flammable. Also, the longer retention time of isopropanol compared to methanol or ethanol during composite processing may be beneficial to some processing techniques, e.g., solvent-assisted resin transfer molding (SARTM) techniques.
This work was done by William B. Alston of the Army Research Laboratory; Daniel A. Scheiman of Dynacs Engineering, Inc.; and Gloria S. Sivko of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 —8
21000 Brookpark Road
Refer to LEW-16691.