In this investigation, OoA prepreg (T700-2510) was aged with extended out-time in controlled environments of 70 ± 2 °F and relative humidity between 0% and 90%. The aged prepreg was then used to manufacture composite laminates with acceptable quality (void content below 2%) to assure that deviations in material properties were due to changes in the composite resin.
In the most extreme case, the combined humidity and out time led to a 35% duction in flatwise tensile (FWT) strength. Gel permeation chromatography measurements showed the aging rate as verified by changes in molecular weight, was twice as fast for high humidity conditions (90 RH%) than for dry conditions. The resin surface showed a two-phase microstructure which agreed with a bimodal glass transition temperature (Tg) as evidenced by dynamic mechanical analysis (DMA).
Furthermore, micrographs of the resin surface show the microstructure appears to change from 10’s of nanometer sized features to approximately micron sized features when out time and humidity are combined. This change in microstructure contributed to the reduction in FWT strength as shown by the presence of brittle regions on fracture surfaces of aged laminates.
Monitoring the two-phase glass transition with DMA, showed that aging resulted in a reduction in the low temperature component and a downward temperature shift for the higher temperature component which is attributed as the source of the brittle regions. This study ultimately shows that the microstructure within OoA resins is critical to laminate materials properties.
- Understand the microstructural changes that can occur as Out-of-Autoclave prepreg ages
- Investigate the multi-phase structure of cured polymer matrix composites
- Determine the environmental conditions necessary to reduce aging and assure material properties are maintained
Why Is This Important?
To reduce the cost and time constraints of curing and assembling composites, manufacturers are using Out of Autoclave (OoA) techniques to achieve fewer steps and larger structures. This has meant new materials are necessary to achieve consolidation, flow and volatile evacuation without pressure from an autoclave. A principle concern with this method is the significant time required to make large structures and therefore it is critical to know how the uncured materials are impacted by environmental exposure during fabrication and what effect it has on the ultimate properties of the large structures.