Impact of thermal and humidity conditions on structural epoxy adhesives during medium-term exposure

Abstract

An experimental program was undertaken to evaluate the performance characteristics, strengths and limitations of two commercially available two-component structural epoxy resin adhesives under varying thermal and humidity conditions, focusing on their performance in scenarios relevant to engineering applications. The following adhesives were selected for investigation, 3M Scotch DP490 and DP125 Gray. In practice, DP490, a high-rigidity epoxy, is primarily used for bonding small, unpainted metal parts, as well as metal to glass, ceramics, and stone. DP125 Gray, on the other hand, is a more flexible system, typically used for bonding metal to glass, glass to plastic, and painted or powder-coated metals. The selected adhesives were subjected to elevated temperatures (40 °C, 60 °C, 80 °C), hygrothermal conditioning (24h $\to$ 4h 40 °C/90%RH; 8h -20 °C; 8h 70 °C/90%RH; 4h 20 °C/60%RH) and freeze-thaw cycles (24h $\to$ 16h -20 °C; 8h 20 °C/45%RH). A medium-term exposure period was adopted to more accurately simulate real-world service conditions (7, 14, 28 days). The experimental characterization of these adhesives was conducted using scanning electron microscopy (chemical composition, fracture surface analysis), standard tensile tests (Young’s modulus, tensile strength), and Shore hardness tests. SEM analysis showed that both adhesives’ chemical compositions remained unchanged after environmental exposure, moreover DP125 Gray was found to contain harmful fluorine. DP490 lost elasticity at high temperatures but gained tensile strength at lower temperatures, while DP125 Gray showed improved elasticity, strength, and Shore hardness with higher temperatures. Unfortunately, both adhesives degraded at high humidity. The research described in this paper is part of a broader research program related to the application of bonding in the manufacturing of complex thin-walled profiles.