Methods to Understand, Monitor, and Extend Prepreg Storage Time

Dr. Steven Nutt | University of Southern California

WP20-1490

Objective

OBJECTIVE

Reduce waste during composites processing by developing and demonstrating protocols to:

  1. Characterize the effects of storage time (ambient and freezer) on thermoset polymer prepregs.
  2. Measure accrued shelf-life of prepreg using non-invasive in situ monitoring.
  3. Identify and impose adjusted process conditions that extend prepreg shelf-life to produce laminates with equivalent performance, eliminating the detrimental effects of protracted storage.

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Technical Approach

Composite structures for defense applications are produced from prepregs, fiber beds pre-impregnated with a partially-cured (B-staged) thermoset polymer (e.g., an epoxy). Being catalyzed, this resin undergoes continuous polymerization/cross-linking, which can cause deviations from ideal processing behavior by modifying rheology and defect formation (voids). Currently, prepregs are assigned a specified shelf-life and out-time for sub-ambient and ambient exposures. These specs are viable (though inconvenient) for production, but greatly challenge material storage and quality assurance in unconventional processing environments (e.g., in forward deployment bases, or in the field). Prepregs after protracted storage or out-time are deemed unusable and discarded. However, proof-of-concept research by the proposers has shown that prepregs can yield defect-free parts well past their nominal out-lives (e.g., with up to twice the out-life spec) provided the material state is known and process conditions are suitably adjusted. This project will develop a comprehensive, efficient, and reliable method for practical use of this strategy.

Technical work will focus on four tasks: material characterization, in situ monitoring of prepreg shelf-age and out-time, cure cycle tailoring, and demonstrations. Material characterization efforts will develop a simple and low-cost methodology for assessing shelf-life and out-time effects on a DoD-qualified thermoset resin (through differential scanning calorimetry, rheometric analysis, and other means) and associated prepreg (through process trials). These results will provide specific quantitative knowledge about the processing behavior of prepregs with accrued age. In situ monitoring experiments will focus on maturing a proof-of-concept method for measuring prepreg storage-age (as well as attributes such as resin degree-of-cure and viscosity) using in situ dielectric analysis. Two technical goals will be pursued: streamlining an approach for correlating dielectric response to properties measured by thermal analysis and demonstrating material state detection using state-of-the-art dielectric sensors unaffected by contact with conductive carbon fibers. Collectively, advances in material characterization and in situ sensing will support the most important aspect of the technical effort: developing and demonstrating a method for tailoring cure cycles to extend storage-life and eliminate associated defects.

The proposed approach will consist of employing embedded dielectric sensing to precisely measure shelf-time for a given prepreg, using characterization data (as a numerical database or model) to predict corresponding changes in material behavior (e.g., altered cure kinetics, rheology, and flow), and identifying modified cure cycles yielding defect-free parts with equivalent performance. Criteria for determining cure cycle viability will be developed based on initial work showing that the resin flow number can serve as predictor for out-time-induced defects. Finally, representative case studies for prepreg processing using autoclave, oven, and heated blanket/tooling will be used to explore scale-up (in collaboration with industry and the sponsors) and demonstrate success, and life cycle impacts will be calculated for the proposed approach.

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Benefits

The methods developed during this project are expected to restore full value to 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 increase process robustness to sub-optimal conditions and reduce waste streams associated with time-expired materials, reducing total production costs and environmental impacts.

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Points of Contact

Principal Investigator

Dr. Steven Nutt

University of Southern California

Phone: 213-740-1634

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