Preparation and performance study of lignin-citric acid adhesive

Sodium lignosulfonate(SL) and citric acid(CA) were utilized as raw materials for the production of an all-biomass wood adhesive called lignin-citric acid(SCA) adhesive. The adhesive's structural characteristics were examined using X-ray photoelectron spectroscopy(XPS) and Fourier transform infrared spectroscopy(FT-IR), while its curing properties were evaluated through differential scanning calorimetry(DSC) and dynamic thermomechanical analysis(DMA). Additionally, the optimal preparation process for the SCA adhesive was determined by varying the process conditions, such as the CA/SL mass ratio, hot pressing temperature, and hot pressing time. The experimental results revealed that the hydroxyl group in SL and the carboxyl group in CA underwent an esterification reaction, resulting in the formation of a new ester bond. Furthermore, the lignin-citric acid adhesive demonstrated good bonding properties and water resistance when the CA/SL mass ratio was 1.25/1. The plywood prepared using SCA5 adhesive exhibited satisfactory dry bonding strength, as well as 24-hour bonding strength and water resistance when pressed at a pressure of 1.0 MPa, temperature of 220 °C, and time of 3 minutes. Specifically, the SCA5 adhesive achieved dry bonding strength, 24-hour wet bonding strength in cold water, 63 °C wet bondinging strength in hot water, and 93 °C wet bonding strength in boiling water of 0.84 MPa, 1.15 MPa, 0.98 MPa, and 0.78 MPa, respectively. These bonding strengths met the requirements for Class II plywood according to GB/T 9846—2015 standards, which provides reference for the high value utilization of lignin and citric acid.