RTM-Processable Resins Containing Thermosetting Plasticizers

John H. Glenn Research Center, Cleveland, Ohio

A number of matrix resins that contain thermosetting plasticizers have been developed for use in the resin-transfer molding (RTM) of composite-material (fiber/matrix) parts. These resins and composites are candidates for use in manufacturing lightweight components of aircraft engines and other structures that must withstand temperatures up to and perhaps somewhat above 300 °C.

RTM is attractive for this purpose because it is a relatively inexpensive process for producing medium quantities (typically, thousands) of complexly shaped parts. In RTM, a fiber preform is infiltrated with a molten resin that, typically, has been partially pre-cured ("staged"). For making high-quality parts, a resin to be used in RTM must have the following characteristics:

• Its viscosity must be low enough to allow it to flow through the preform without distorting the preform;
• It must wet the fibers adequately;
• Its pot life at the RTM processing temperature and viscosity must be at least 30 minutes; and
• It must outgas as little as possible during filling of the mold and subsequent curing of the resin.

The present resins are based on the AMB-21 resin system — a less-toxic system recently developed as an alternative to the PMR-15 polyimide system. [The industry standard polyimide matrix resin, PMR-15 was developed at Lewis Research Center (since named Glenn Research Center) in 1971. The PMR-15 monomer mixture includes methylenedianiline (MDA), a suspected carcinogen.] The basic AMB-21 prepolymer solution is a made by combining three monomeric solutions:

• 3,3',4,4'-benzophenonetetra-carboxylic ester (BTDE) in methanol;
• the monomethyl ester of norbornene-2,3-dicarboxylic acid (NE) in methanol; and
• bis(aminophenoxy) propane (BAPP) in tetrahydrofuran (THF), acetone, or toluene.

The basic AMB-21 prepolymer does not have the correct rheological properties for RTM; in particular, its viscosity is too high. The approach taken in the development of the present resins involved the reduction of viscosity through the addition of thermosetting plasticizers (more specifically, low-viscosity reactive diluents that are chemically compatible with AMB-21). Plasticizers that have shown promise include diethynyldiphenyl methane (DEDPM), phenylethynyldiphenyl methane (PEDPM), and mixtures thereof.

During RTM, the plasticizer molecules lower the viscosity of a resin. During the cure that follows RTM, when the plasticizing effect is no longer needed, the plasticizer molecules disappear via addition reactions, without evolution of volatiles. This is advantageous in that volatiles can give rise to undesired voids in the finished part. The cured resin has relatively high thermal stability. The glass-transition temperatures (T_gs) of polymerized matrix resins formulated and processed following this approach equal or exceed the T_g ≈ 300 °C) of AMB-21.

One potential drawback of this approach is that the high degree of cross-linking in these resins can suppress ductility and induce a brittle failure under single-cycle or fatigue loading, or can cause micro-cracking during cure. Therefore, the development effort has included tradeoff studies to identify the formulations of RTM-processable resins with the best high-temperature stability and ductility. The table depicts some of the formulations investigated thus far. Composite specimens made from some of these resins have exhibited promising thermal and mechanical properties. These studies were continuing at the time of reporting the information for this article.

This work was done by Robert Kovar, Nelson Landrau, Margaret Roylance, and Thomas Tiano of Foster-Miller, Inc., for Glenn Research Center.

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,
Cleveland, Ohio 44135.

Refer to LEW-16926.

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