Forestry 485

1. Forestry 485 Lecture 2-4-1: Adhesion Theories, Part I

2. Overview of Adhesion as Science • Adhesive bonding of materials, particularly wood, has been practiced since ancient times • e.g., Veneered furniture of Egypt and Rome • “Adhesion science” developed since ca. 1920s • Yet, “fundamental knowledge about adhesion phenomenon is still very modest and fragmented…one has to admit that the mechanisms governing adhesion are still fairly imprecisely understood. Moreover, no single global theory or model can explain all the phenomena.” • Fourche, 1995

3. Adhesion Theories • Mechanical Adhesion • Specific Adhesion • Electronic Theory • Diffusion Theory • Adsorption Theory • Covalent Bonding Theory • Weak Boundary Layers • Fourche, 1995

4. Mechanical “Adhesion” – Better known as Mechanical Interlocking • Bain, 1926 – Proposed mechanical interlocking as an adhesion mechanism, based on studies of wood bonding. • This is a significant mechanism on all porous substrates, e.g., wood, paper, fabric, leather • “The roughness and porosity of substrates are generally suitable factors only insofar as the wettability by the adhesive is sufficient.” • Fourche, 1995

5. Mechanical Interlocking • Flow of adhesive resin into cell lumen, vessels, or other interstices • Subsequent hardening of the resin results in Interlocking

6. Mechanical Interlocking • For Wood Adherends: • Ready Supply of Void Spaces in Wood Furnish • Surface Wetting/Capillary Effect • Resin Acts to Reinforce Surface/Interface layers of Wood Cells • For wood substrates, adhesive penetration of approximately 6-10 cell diameters (few 100 micrometers, maximum) is regarded as necessary for optimal adhesive bonding

7. Specific Adhesion – A collection of theories: • Electronic Theory • Diffusion Theory • Adsorption Theory • Covalent Bonding Theory • Weak Boundary Layers • Fourche, 1995

8. + + + + + + - - - - - - Electronic Theory • Theory suggests the formation of a double layer of electrostatic charge across the substrate/adhesive interface • Depends on material properties that allow electron transfer across the interface • Requires intimate contact/ smooth surfaces • Interactions are very weak and rather insignificant • Mechanism not important for wood substrates

9. Diffusion Theory Adhesive Adherend

10. Diffusion Theory • Occurs as interdiffusion of polymeric adhesives and adherends at the interface • Consequence: The disappearance of the interface • Only possible when both adhesive and adherend are polymers • Polymers must be compatible

11. Vapor Development of Intermolecular Forces γLV Ө Liquid γSL γSV Solid Adsorption Theory “Thermodynamic Adsorption Theory” = “Wetting Model”

12. Covalent Bonding Theory Formation of 1o Chemical Bonds Between Adhesive and Substrate Might be possible with a reactive substrate like wood. However, so far such bonds have not been well established or accepted.

13. Covalent Bonding Theory • Some authors (e.g., Wellons, 1977) suggest that covalent bonds have been demonstrated in some systems. • Others (notably Pizzi, 1994) have subsequently argued that unequivocal evidence for covalent bonding of wood substrates has not been found.

14. Weak Boundary Layers (Bikerman Model) • Theory is based on observation of adhesion failure; rarely seen to occur at the interface, but typically is a cohesive failure of adherend(s). • Classes of “weak boundary layers”: • Air (case of poor wetting) • Contaminants (in adhesive or adherend) • Reaction products (air/adherend)

15. Weak Boundary Layers • For wood adherends • Extractives have significant effect on surface chemistry of wood adherend • Migration of extractives to surface may constitute “contamination” of surface • Physical barrier (block pores, impede wetting and flow of adhesive) • Hydrophobic (impede wetting) • Chemically reactive (alter adhesive chemistry)

16. Recap: Adhesion Theories • Mechanical Adhesion • Specific Adhesion • Electronic Theory • Diffusion Theory • Adsorption Theory • Covalent Bonding Theory • Weak Boundary Layers • Fourche, 1995