Understanding the Fundamental Influence of Wood Extractives on Wood adhesion

Abstract

Spotted gum (Corymbia citriodora) is a highly desirable Australian hardwood species, valued by the forestry industry for its fast growth, durability, strength, and attractive color, making it ideal for structural applications. Despite these advantages, the broader utilization of spotted Gum remains limited due to significant challenges associated with achieving cost-effective and reliable adhesive bonding.The difficulty in gluing Spotted Gum arises from its inherent chemical and anatomical characteristics. The wood contains a high extractives content that significantly reduces the effectiveness of most adhesives. Although extractives can inhibit adhesion in many hardwoods, in spotted gum, this issue is unusually problematic due to the species' particular sensitivity to thermal surface deactivation compared to other species. Additionally, its dense structure and the presence of blocked vessels limit the ability of adhesives to penetrate the wood matrix. Together, these factors lead to weak or inconsistent bond performance, often requiring complex and costly pre-treatment methods to ensure adequate adhesion. Overcoming these bonding challenges is crucial to unlock the full potential of spotted gum for structural and engineered wood products. Despite ongoing efforts, a commercially viable gluing protocol that consistently produces compliant structural bonds has yet to be established. This research contributed to addressing the challenge of understanding the relationship between extractives and adhesion performance in hardwood species. Specifically, it investigated how the removal of extractives through solvent extraction influences adhesive durability. Solid spotted gum wood was extracted using three solvent systems of different polarity and a delamination test was developed and applied to evaluate adhesive durability after weathering. The specific objectives of this research are to 1) develop an efficient extraction method with 3 solvent systems (dichloromethane – DCM, methanol/water – Binary mixture, and a single phase mixture DCM/methanol/water – Ternary mixture) for spotted gum for further characterization, 2) assess a newly proposed cycle-based delamination testing test to evaluate wood adhesive durability due to weathering, 3) study the influence of extractives removal on the adhesion performance of the treated solid wood specimen through delamination test.