Thiourea modified low molecular polyamide as a novel room temperature curing agent for epoxy resin

A thiourea modified low molecular weight polyamide (TLMPA) as a room temperature curing agent was synthesized by a two-step method. Firstly, a low molecular weight polyamide curing agent (LMPA) with low viscosity and high amine value was synthesized by amidation of sebacic acid with tetraethylenepentamine, then the synthesized curing agent was modified with thiourea to increase its reactivity at room temperature. The optimal reaction conditions were studied by L₉(3³) orthogonal experiments. The structure of the prepared curing agent was analyzed by Fourier transform infrared spectroscopy (FT-IR). The kinetics of TLMPA curing of E-51 epoxy resin was analyzed using the Kissinger method with non-isothermal differential scanning calorimetry (DSC). The activation energy of TLMPA/E-51 calculated by the Kissinger method and FWO method was 38.79 kJ mol^-1 and 42.73 kJ mol^-1. The nano-SiO₂ filler was compounded with E-51 epoxy resin, TLMPA, allyl glycidyl ether diluent, and KH-560 coupling agent to prepare the room temperature curing epoxy resin (EP) system. L₉(3⁴) orthogonal experiments were carried out to study the effect of various factors on the mechanical properties of the cured resin systems. The best formulation of the system is that the content of nano-SiO₂, curing agent, diluent, and coupling agent is 3, 35, 15, 1 wt%, respectively. With the optimal formulation, the tensile and shear strength, tensile strength, impact strength, and bending strength of the cured EP system was 13.19 MPa, 53.8 MPa, 52.16 kJ m^-2, and 94.95 MPa, respectively.