Forestry 485

Lecture 2-4-1: Adhesion Theories, Part I

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

Adhesion Theories

Mechanical AdhesionSpecific Adhesion

Electronic Theory Diffusion Theory Adsorption Theory Covalent Bonding Theory Weak Boundary Layers

Fourche, 1995

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

Mechanical Interlocking

Flow of adhesive resin into cell lumen, vessels, or other interstices

Subsequent hardening of the resin results in Interlocking

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

Specific Adhesion – A collection of theories:

Electronic TheoryDiffusion TheoryAdsorption TheoryCovalent Bonding TheoryWeak Boundary Layers

Fourche, 1995

+-

+++++- - - - -

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

Diffusion Theory

Adhesive

Adherend

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

Adsorption Theory

Ө

Vapor

Liquid

Solid

γLV

γSVγSL

Development of Intermolecular Forces

“Thermodynamic Adsorption Theory” = “Wetting Model”

Formation of 1o Chemical Bonds Between Adhesive and Substrate

Might be possible with a reactive substrate like wood.However, so far such bondshave not been well establishedor accepted.

Covalent Bonding Theory

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.

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)

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)

Recap: Adhesion Theories

Mechanical AdhesionSpecific Adhesion

Electronic Theory Diffusion Theory Adsorption Theory Covalent Bonding Theory Weak Boundary Layers

Fourche, 1995