Replication of discontinued additive saves resin requalification costs on missile program

Niche chemical products often become too expensive to manufacture and distribute, particularly when demand is limited and environmental rules tighten. Such was the case with a specialty epoxy diluent trade named Unox 4206, developed by The Dow Chemical Co. (Midland, Mich.) and distributed by H.B. Fuller (St. Paul, Minn.). The diluent, 4-vinylcyclohexene diepoxide (VCHD), was a component of a low-viscosity, high-temperature epoxy used for years by an global aerospace OEM to filament wind the composite thread rings that attach a missile radome to its body. High-strength aramid fiber is wet out with the resin and wound over a ribbed mandrel to create the threads. When cured, the part acts as a threaded connector between the missile body and radome. Winding is performed by a subcontractor to the OEM.

The original epoxy system had undergone extensive qualification testing decades earlier, but when the diluent was discontinued, a new resin system was required, especially given the increased need for missile retrofits. In late 2005, the OEM contacted family-owned Magnolia Plastics Inc. (Chamblee, Ga.) to investigate whether a replacement diluent could be developed with the same properties and performance as the original product. Magnolia’s vice president and director of R&D Bob Andrews says the new compound, when combined with the base epoxy, had to impart good winding characteristics, namely low viscosity, thorough wetout of the reinforcing fiber, long pot life and good stability. Performance requirements dictated that it have high compressive strength as well as resistance to the high temperatures experienced by the missile during supersonic flight. The OEM, its subcontractor and Magnolia collaborated in defining, developing and testing the resin system.

Rick Wells, Magnolia’s president, says the team was able to formulate a duplicate resin with the properties of the original compound: “Our solution was close enough so that the company could run equivalency tests and perform unit qualification to prove the performance specs,” Wells points out, adding, “While we’re working on perfecting the formula, we’re currently making it in batches of between 50 lb and 100 lb.” The company reports that quantities as large as 500 lb can be produced, if required.