Thermoset adhesives used in composite joining and repair include epoxy, polyurethane, organo-silane, and vinyl ester. These adhesives are applied as films or liquids in volatile and toxic solvents, including toluene, methyl ethyl ketone (MEK), acetone, and xylene, which are both volatile organic compounds (VOC) and hazardous air pollutants (HAP) that can impact workers' health. The primary chemical components and plasticizers that constitute the uncured adhesives are hazardous waste listed on the Environmental Protection Agency Toxic Release Inventory. The uncured adhesive materials require low temperature storage and have a shelf life of approximately 1 year. If not used, the uncured materials must be discarded and disposed of as hazardous waste.

The objective of this project was to evaluate a novel composite repair methodology using thermoplastics as adhesives instead of standard thermosets and using Kubota Research Associates' near-infrared (NIR) P-Wave welding technology. The 1-year effort addressed scientific and technical barriers, including selection of thermoplastic polymers as alternatives, understanding and optimizing adhesion mechanisms between thermoplastics and thermosets, optimizing the NIR system for selective heating of the thermoplastic, and mechanical performance of the thermoplastic alternatives.

Several thermoplastic polymers have properties similar to currently used thermoset adhesives, yet they have the added benefit of increased toughness. The main barrier is the level of adhesion that is possible between a thermoplastic and a pre-cured thermoset interface. Two mechanisms—surface roughness and chemical bonding facilitated by plasma treatment of the thermoplastic—were evaluated to improve and optimize adhesion. The NIR system can provide heat locally at the bondline without heating the substrate, and optimal parameters to maximize adhesion were identified. Adhesion levels were evaluated through lap-shear tests with thermoset composite substrates.

The capability of using thermoplastics as adhesives for thermosets materials was evaluated using a wide range of thermoplastics based on their adhesive strength. A Na⁺-based ionomer S1856 and an adhesion promoter ethylene metacrylic acid (EMAA) have shown high peel resistance but poor shear strength due to the high elongation of these polymers. Series of PA12 alloys have been tested, some of which were doped with Na⁺ ions for toughening and others were alloyed with adhesion promoter EMAA.  Another PA12-based epoxidized polymer was alloyed with styrene butadiene copolymer (SBC). The results showed that a lower peel and shear resistance was achieved with a 30% EMAA promoter, while good resistance was seen with a lower (15%) or no EMAA content. Little difference is seen between the 15% and the 0% EMAA PA12 alloys for both the shear and the peel test. The influence of Na⁺ toughening could not be established; as the samples showed similar values in both cases. On the other hand, the PA12-epoxidized SBC showed high peel and shear strength—0.648 kN/m for the peel strength and 20.6 MPa for lap shear tests. Both values are higher than the thermoset baselines (AF563 and FM300K); therefore, the PA12-epoxidized SBC shows great potential for structural adhesives.

The use of thermoplastics for composite repair eliminates the generation of VOCs and HAPs, has infinite shelf life, and has no storage restrictions or disposal issues. The NIR source is portable and low energy. The radiation heating process only heats and melts the resin at the bonding interface, minimizing heating and cooling time in the repair cycle to provide a fast repair (in minutes).  The short repair time enables easy application of external pressure (rollers) rather than the autoclave or other expensive alternatives. Using thermoplastics also allows for the unique capability to bond and debond repair patches, effectively allowing rapid, low-performance field repair to restore platform mobility followed by quality depot-level repair for full system performance.