Adhesion Fundamentals in Spotted Gum (Corymbia citriodora)

The goal of this project was to advance adhesion science and technology related to the Australian hardwood spotted gum (Corymbia citriodora). Plantation-grown spotted gum exhibits poor adhesion properties in comparison with similar woods, such as Gympie messmate (Eucalyptus cloeziana). To better understand adhesion differences between these two woods, this research compared and contrasted the surface chemistries of plantation-grown spotted gum and Gympie messmate with a particular focus on sensitivity to thermal deactivation.

Wetting measurements were performed using the sessile drop method. Initial and equilibrium contact angles, time-dependent wetting, and surface energy were determined. Time-dependent wetting and equilibrium contact angles were most informative. Initial contact angles and surface energy calculated with them were misleading and often generated anomalous results.

Heating water-saturated wood to mild surface temperatures (105 deg C, directly after evaporative cooling) severely deactivated spotted gum but not Gympie messmate. This suggests conventional kiln drying appears unsuitable for spotted gum while amenable for Gympie messmate. Spotted gum likely requires non-aqueous, low surface tension adhesives or aqueous adhesives formulated with surface active wetting agents.

Water-saturation (followed by room-temperature vacuum drying) substantially altered the surface chemistries of both woods, making them more hydrophilic. Consequently, the question was raised of whether a water-spray onto the wood surface prior to adhesive application could improve bonding. If so, this simple, industrially-feasible treatment could prove very beneficial to the wood composites industry. Water-saturation also revealed differences in the two wood's water permeability, which has implications for adhesive penetration and wood drying and may additionally help explain adhesion differences.

Analysis of the plantation-grown heartwood (inner, middle, and outer heartwood regions) revealed significant wetting differences on spotted gum with only minor differences on Gympie messmate.

The Australian woods were compared to two North American woods-loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii). Examining water wetting measurements, the Australian and North American woods exhibited some interesting similarities. However, methylene iodide wetting measurements revealed that the Australian woods were quite different from the North American samples studied here.