Characterization of the reactions and curing behavior of phenol-formaldehyde resins

The reactions and curing behavior of phenol-formaldehyde (PF) resins and modified PF resins have been investigated by experimental and simulation methods. Differential scanning calorimetry (DSC), as a major tool, was used to measure the curing process of PF resins with other methods, such as Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and dynamic mechanical thermal analysis (DMTA). The Monte Carlo method was used to simulate the reactions of PF resins during the synthetic process. The results indicate that model-free kinetics can be used to characterize the overall curing process of PF resins and predict the isothermal behavior of PF resins from non-isothermal data. The curing process generally contains two stages, chemical and diffusion regimes. Powdered PF resins undergo a different curing process because of the absence of water, which plays an important role of diluent or plasticizer dependent on the reaction stages in the curing process and water content in the curing system. Wood makes the activation energy of reactions increase and reaction enthalpy decrease during the curing process of PF resin mixed with wood substrates. The interactions between PF resin and wood are mainly secondary forces, especially the polar forces and hydrogen bonds of hydroxyl groups at the interface of the wood and resin. Urea introduced into PF resin by co-condensation accelerates the curing process of PF resin, but the rigidity of the network decreases a little as compared to the pure PF resin. The reactions of PF resin can be simulated by the Monte Carlo method. In this simulation, the product parameters, such as molecular weight and its distribution, the type of methylene linkages and the amount of monomers remaining in the reaction system, were calculated and the effect of kinetics on the simulated resin was taken into account.