Formaldehyde is a volatile organic compound that may remain in, or be released from, epoxy adhesive formulations. Prolonged or high-level exposure can cause chronic respiratory injury and elevate cancer risk. Accordingly, developing epoxy curing agents from renewable bioresources without introducing formaldehyde is of significant importance for safeguarding human health and reducing environmental impacts. In this study, a biobased phenalkamine curing agent was synthesized from cardanol via a Mannich reaction incorporating 1,4-butanediamine and citronellal, and subsequently combined with bisphenol A diglycidyl ether to construct an epoxy adhesive system. The overall performance was systematically assessed by differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis, and mechanical testing. The adhesive system was curable at 26 °C, with an optimal curing temperature of 88 °C. The room-temperature cured specimens exhibited a tensile strength of 1.73 MPa, an impact strength of 32.59 kJ·m–2, and a lap-shear strength of 3.28 MPa. After stepwise thermal curing, the corresponding values were 5.06 MPa, 50.10 kJ·m–2, and 3.22 MPa, respectively. Dynamic mechanical analysis revealed glass-transition temperatures (Tg) of 38.9 and 48.8 °C for the room-temperature cured and stepwise thermally cured groups, respectively. Vertical burning tests showed rapid self-extinguishment of approximately 3 s and no ignition-causing dripping, indicating favorable flame retardancy of the adhesive system. This work provides experimental evidence supporting the development of formaldehyde-free, low-environmental-burden, room-temperature-curable epoxy adhesive systems.
Su et al. (Thu,) studied this question.