Derivative thermogravimetric analysis curve for characterizing the curing process of urea-formaldehyde resins

The curing process of urea-formaldehyde (UF) resin affects the performance of cured UF resin. To characterize the curing process of UF resins, we proposed using the derivative thermogravimetric analysis (DTG) curve to simultaneously disclose the relationship between the mass-loss rate and curing of UF resin during the curing process. In this work, UF resins with F/U mole ratios of 1.60 and 0.95 and MUF resin were synthesized. The TG and differential scanning calorimetry (DSC) curves of all resins at different pH conditions were measured by TG-DSC with an open pan under atmospheric pressure. The DTG curves were obtained by the first derivative of TG curves. The structures of UF resin at different times (or temperatures) of the DTG curve were characterized by FT-IR and X-ray diffraction (XRD). The results show that the temperature of peak 1 of the DTG curve relates to the condensation reactivity of the UF oligomer. The temperature of peak 2 relates to the cross-linking structure of the cured UF resin. The curve between the two peaks reflects the variation in the degree of cross-linking of the UF resin. Analysis by DTG curves is a feasible approach to show the curing process of UF resin. Furthermore, we employed DTG curves to study the effect of NH4Cl on the curing process of UF resin. The DTG results show that NH4Cl plays different roles for UF resins with different F/U mole ratios.