Application of tung oil to improve adhesion strength and water resistance of cottonseed meal and protein adhesives on maple veneer

• Tung oil addition improves adhesion strength and water resistance of cottonseed meal and protein adhesives.
• Optical microscopy images reveal that water soaking diffused the bondline, leading to weaker bond strength.
• Tung oil prevents the adhesives from diffusing away from the bond joints.
• 0.1% Tung oil is recommended for economics and convenience.

Cottonseed meal-based products show promise in serving as environment-friendly wood adhesives. However, their practical utilization is currently limited due to low durability and water resistant properties. In this research, we tested the improvement of adhesion strength and water resistance of cottonseed meal-based adhesives by the addition of tung oil. Our data showed that the shear strength of maple wood strips glued at 100 °C by water-washed cottonseed meal (WCM) and cottonseed protein isolate (CSPI), containing tung oil, increased by 19.9% and 21.1%, respectively, compared to the tung oil-free adhesive controls. After 2 cycles of water soaking, the average of the soaked shear strength of glued maple specimens with WCM/tung oil adhesives was 3.49 MPa, representing a water resistance improvement of 46.6%, compared to the control value (2.38 MPa). For this study, the hot-press temperature (100–130 °C) was not critical for improving the water resistance of WCM or CSPI with tung oil. Optical microscopy images of the bondlines and interface showed that the improvement of adhesion strength and water resistance of cottonseed meal-based adhesives were due to the tung oil's prevention of the adhesives from diffusing away from the bond joints, while water soaking diffused the bondline, leading to weaker bond strength. The addition quantities of tung oil varied from 0.05 to 1.0% (w/v) and we observed little influence on improving the adhesive strength values, therefore, we recommend the addition of 0.1% tung oil for economics and convenience. The results of this research provide useful information on cottonseed meals as one of renewable sources of wood adhesives.