Environmentally friendly chitosan adhesives for plywood bonding

Chitosan is a biomass material with abundant reserves. It has been used in this work to prepare an environmentally friendly wood adhesive. By employing carbohydrates, namely glucose, sucrose and starch modified by specific oxidation as hardeners, petroleum-based aldehydes were eliminated due to their unsustainability, volatility, and toxicity. Three-layer laboratory plywood was prepared to measure the bonding performance of the chitosan adhesives prepared. The effect of different levels of oxidation by different sodium periodate additions on their bonding performance was also studied. The thermal properties of the chitosan adhesives were analyzed by differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). Fourier-transform infrared (FTIR) spectrometry and matrix assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry were used to explain the reaction mechanisms involved. Oxidized carbohydrates have been shown to effectively improve the bond strength and water resistance of chitosan adhesives, with oxidized starch showing a better improvement than glucose and sucrose. The best chitosan-oxidized starch (CS) adhesive is obtained by treatment with 8 mass% of oxidized starch mixed with a chitosan solution at ambient temperature, when 10% sodium periodate on starch weight is used for its specific oxidation. Thermal analysis shows that the chitosan-oxidized starch (CS) adhesive cures at a lower temperature than the other adhesives in this work, this being one of the reasons for CS having a better bonding performance. MALDI and FTIR showed oxidized glucose, taken as a model, to form aldehydes and condensation products due to water elimination between two aldehyde groups, followed then by their reaction with the chitosan.