One-part epoxy film adhesives are used to repair military and industrial load-bearing composite structures. The replacement costs and regulatory compliant disposal costs of the wastes generated by expired epoxy film adhesives exceeds millions of dollars per year for large aircraft repair facilities. Most composite repair materials are engineered to cure at 250°F or greater. When repair temperatures exceed 212°F, the boiling point of water, composite parts subjected to repair can delaminate as their adsorbed moisture turns to vapor. To avoid damage to the composite, additional steps must be taken to prepare the material for repair. Defective composites are scrapped, adding to the waste disposal burden.
The objective of this project was to demonstrate the feasibility of producing an extended shelf life and environmentally compliant non-volatile organic compound (VOC), non-hazardous air pollutant (HAP), epoxy-based composite repair material that can be engineered into films, resin pastes, and liquid shim adhesives. Once activated by thermal or mechanical means, the shelf-stable epoxy resin adhesive (SSERA) can be used to repair military and commercial composite structures found in aircraft, ships, and amphibious and tactical vehicles.
Infoscitex Corporation and Northrop Grumman, with support from the U.S. Air Force and Navy, Boeing, and Henkel-Loctite, developed the SSERA composite repair adhesive. Shelf-stable epoxy film, liquid shim adhesive, and composite prepreg materials that can be stored at ambient temperature for up to 1 year until intentionally activated can revolutionize the composite repair process in terms of performance, waste minimization, cost savings, and protection of the environment. To achieve shelf stability, microencapsulation was used to isolate curatives from the epoxy resin. These curatives remain isolated until intentionally released at which time curing of the material will occur at lower temperature (200°F). The latent cure epoxy resin was evaluated for storage stability prior to release of the curative and reactivity following the release of the microencapsulated curatives. Test specimens of composites repaired with the SSERA then were evaluated to determine the performance of the technology.
This project successfully demonstrated the feasibility of developing a shelf-stable adhesive system via microencapsulation of the catalyst accelerant. Thermal analysis of microcapsules made from carrageenan blends showed that they formed an effective barrier between commonly used, bisphenol-based epoxy resins and monuron, a typical catalyst accelerant. The encapsulated monuron was made available for curing by means of mechanical and thermal activation methods. The mechanical method was found to be most compatible with the overall requirements for this application. SSERA samples activated in this manner showed equivalent performance to unencapsulated control samples during lap shear tests and superior shelf-stability after accelerated thermal aging. A follow-on SERDP Project (WP-1763) is ongoing to further develop shelf-stable epoxy repair adhesives for composite materials.
The SSERA is based on an epoxy resin system that has been formulated to meet demands for the military's composite components. When combined with microencapsulation, the 100% reactive composite repair system significantly minimizes uncured and cured composite scrap waste; offers extended shelf life and reduces regulatory-compliant and expensive disposal of expired or uncured repair adhesive and resin wastes; further reduces waste by not producing water-induced delamination or damaged parts; and does not release any VOCs or HAPs during application and use.