The objective of this project was to address the shelf-life shortcomings of currently available composite repair film adhesives. At military repair depots, currently used one-part adhesives have such a short shelf life that significant amounts expire and need to be discarded, contributing to a financial and environmentally costly waste stream. This effort aims to develop a one-part epoxy film adhesive that is stable at ambient temperatures for up to one year or at freezer temperatures for two years or longer.

The technical approach focused on enhancement of current epoxy resin-based film adhesives through modification of the accelerator package. This was accomplished through formulation of controlled release encapsulated accelerators into a one-part epoxy resin. Henkel Aerospace film adhesive product Hysol EA 9696 was selected as the model epoxy resin system against which to develop accelerator modifications. A baseline was formulated using the primary components of the Hysol EA 9696 resin system matrix and fenuron accelerator. The experimental shelf-stable adhesive (SSA) was formulated in a manner identical to the baseline; the only differentiating factor was that the fenuron accelerator was encapsulated in a controlled release shell.

Two sets of tests were performed to establish the feasibility of encapsulating the accelerant in a one-part epoxy adhesive system. The first set of tests focused on evaluating the long-term shelf stability of the SSA adhesive. The second set of tests focused on assessing the mechanical performance of the SSA adhesive.

The tests were based on standard AMS-A-25463 as well as input from technical advisors. AMS-A-25463 provides all requisite guidance for the chemical and physical performance of film adhesives. The model epoxy resin system selected is classified as a Type I adhesive. Accordingly, performance metrics for the SSA film were based on Type I requirements. The long-term stability of each adhesive was assessed at a sustained elevated temperature (32°C/90°F). The SSA and baseline formulations were compared at each time interval using single lap shear testing (ASTM D1002) of cured specimens. Each sample set consisted of at least nine specimens.

A matrix of five different mechanical tests was performed on the baseline and candidate SSAs under six test conditions. All testing was performed on samples that were prepared with new adhesive that was meticulously stored at 10°F. A summary of the test results are shown below. All operational mechanical tests were performed at third-party facilities.

The operational mechanical and shelf stability test results show that the one-part SSA has potential as a viable upgrade for the currently available one-part adhesive systems. The SSA has been demonstrated to retain at least 75% of its adhesive strength when stored at 90°F for a year. When compared to the baseline formulated with an unencapsulated accelerant, the stability of the SSA formulation offers a significant advantage. If stored in a freezer, it is anticipated that the SSA will be stable for well over two years. The data confirms that the capsules are not having a deleterious effect on the performance of the adhesive. In every test, the mechanical properties of the conditioned SSA samples were comparable or better than those of the baseline formulation. The test program has shown that the SSA formulation with encapsulated accelerator provides a significant shelf stability advantage over the baseline without sacrificing mechanical performance.