Time-temperature dependence of thermomechanical recovery of cold-curing structural adhesives

The thermomechanical recovery behavior of a commercial cold-curing structural epoxy adhesive was investigated. Exposing the adhesive to temperatures above the glass transition temperature T-g significantly increased the latter although the curing degree increased only marginally. Cooling the adhesive from temperatures above T-g to temperatures below T-g led to full recovery of the mechanical properties (tensile stiffness and strength). The secondary bonds between the polymer chains fully reformed after cooling. Temporarily exceeding the T-g did not therefore result in any degradation of the material-on the contrary, the mechanical properties significantly improved due to post-curing. Simulating temperature fluctuations by eight consecutive cycles across glass transition also improved the mechanical properties and did not lead to any degradation. An existing model for predicting temperature-dependent mechanical properties was extended to also describe the recovery behavior. The simulations agreed well with the experimental results. (C) 2012 Elsevier Ltd. All rights reserved.