Objective

Pre-preg, an intermediate product comprised of an organized fiber bed pre-impregnated with a reactive thermoset resin, often expires before curing due to accumulated out-time and/or freezer storage times that exceed specified limits. Expired pre-preg must be scrapped, even if it is pristine and unused, this results in waste of a high-value engineered material and added program costs. In practice, tracking pre-preg age is challenging, and there is no reliable diagnostic to assess the physical state of aged pre-preg. Furthermore, the physical changes responsible for aging of pre-cured pre-preg, whether due to out-time or freezer storage, are not well understood. The net result of these factors is unacceptably high rates of scrap and waste, with “buy-to-fly” ratios reportedly as high as 1.5-2 common for aerocomposite programs.

Technical Approach

This project will identify the mechanisms responsible for physical changes in naturally aged (precured) epoxy resins, and develop practical diagnostics to ascertain these changes to reduce production scrap. The project team will also develop and demonstrate computational engineering tools to assess the impact on material and process characteristics. This will show how these tools can be used to modify cure cycles to extend the shelf life of pre-preg beyond the manufacturer’s specification and achieve performance equivalent to in-spec pre-preg. This task will include demonstrating modified cure environments to overcome aging effects. The tasks will be undertaken in coordination with personnel at a partner Department of Defense (DoD) lab(s) that uses the same pre-preg materials. The tasks will include (1) identifying the mechanism responsible for freezer aging, (2) developing a diagnostic protocol for determining the physical state of aged pre-preg, and (3) developing tools to design cure cycles that compensate for freezer aging.

Benefits

The project will deliver benefits to DoD and the scientific community, these include: a reduction in composite production waste, DoD approved protocol for determining shelf-life beyond expiration, and methods of adjusting cure cycles to suit the physical state of resin to yield laminate performance comparable to virgin materials. The project will produce an integrated computational tool to evaluate the impact of the material state and to design modified cure cycles according to the physical state of the resin.