Properties and synthesis mechanism of lignin-phenol-formaldehyde resin

Phenol Formaldehyde (PF) resin is the earliest industrial synthetic polymer material, which has a history of more than 100 years. PF resin adhesive are widely used in the production of outdoor wood-based panels due to their advantages such as high bonding strength, weathering resistance, good water resistance, aging resistance, and so on, and it is the second largest wood adhesive after urea-formaldehyde resin. PF has some defects, such as high production cost, dark color, hard and brittle after curing, easy cracking, low initial viscosity, high toxicity, and so on, which lead to low production efficiency and high energy consumption of wood-based panels, thus limiting its wider application. Therefore, the study of alternatives for phenol has become the focus. Lignin is similar to phenol in chemical structures, and it has the condition and potential to replace part of phenol to prepare phenol formaldehyde resin. It can reduce the cost of the preparation of PF, and increase the biomass content of PF to improve its the biodegradability, and also realize the effective utilization of lignin resources. In order to reduce the cost of phenol formaldehyde resin, 30%, 40% and 50% alkali lignin was substituted for phenol to prepare Lignin-Phenol-Formaldehyde (LPF). Effects of substitution ratios on bonding performance, curing properties and thermal stability of LPF were studied, and the synthesis mechanism of LPF was also discussed in this paper. The results indicated that: 1) Compared with phenol formaldehyde resin, LPF had low transparency, high solid content, low free formaldehyde, high viscosity and bad operation. 2) With the increase of lignin addition, the bonding strength of LPF increased and then decreased, but all generally higher than that of phenol formaldehyde resin. The ratios of lignin substitution for phenol could be up to 50%. 3) LPF resin required a higher curing temperature, and the more the lignin addition, the higher the hot-pressing temperature. 4) The lignin addition could affect the thermal stability of LPF resin, and it's thermal stability was higher than that of phenol formaldehyde resin when lignin addition was only 40%. 5) At alkaline conditions, whether phenol, lignin phenol ring or lignin side chains were used as the starting reaction points for the synthesis of LPF, forming methylene conjugate structures by reaction mechanism based on hydroxymethyl phenol was the key. In conclusion, the research has a great significance to provide further scientific guidance for the improvement of LPF synthesis process and its practical application.