The objectives of this project were to (a) characterize the effects of freezer storage and out-time on thermoset polymer prepregs, (b) develop a method for measuring accrued shelf-life of prepreg using non-invasive in situ monitoring, and (c) identify and impose adjusted process conditions (cure cycles) to extend prepreg shelf-life to produce laminates with equivalent performance to material produced under recommended prepreg storage and processing conditions, thereby eliminating the detrimental effects of protracted storage.
The effects of storage time on laminate quality were monitored for prepreg stored at freezer temperature (to simulate long-term storage) and room temperature (to simulate out-time) by periodically curing laminates using the manufacturer’s recommended cure cycle (MRCC). Laminate microstructure was characterized for defect (void) content using light microscopy, and mechanical properties were measured using short beam shear tests. The onset of age-induced void formation or mechanical performance decline was identified for each prepreg storage condition, and—where possible—the nature and cause of deviations from expected performance were identified. Differential scanning calorimetry was used to measure the B-stage glass transition temperature of prepreg, and this metric was demonstrated to be an effective means for assessing the time accrued at room temperature. A relationship between B-stage glass transition temperature (Tg) and prepreg out-time was developed for the standard aerospace prepreg used in this study.
Insufficient resin flow (due to resin crosslinking during exposure to ambient temperature) was identified as the primary mechanism by which prepreg aging due to out-time causes defects. Freezer storage of prepreg, however, did not cause significant crosslinking; resin Tg and rheological behavior were unchanged after 20 months of freezer storage. A method for increasing resin flow in over-aged prepreg was developed by using resin cure kinetics and rheology models to estimate how cure cycle modifications changed resin flow properties. Cure cycle modifications were tailored to measurements of prepreg B-stage Tg to minimize deviations in manufacturer’s recommended processing conditions. Cure cycle tailoring was demonstrated on prepreg aged for 44 days, yielding laminates with increased mechanical performance (compared to a laminate with similar aging but cured using the MRCC).
The methods developed during this project will restore full value and performance to most prepreg materials currently considered waste, while ensuring that fabricated parts will achieve equivalent performance and exhibit fewer process-induced defects arising from protracted storage. In doing so, this work will reduce waste streams associated with time-expired materials, reducing total production costs and environmental impact.