forestry 485 lecture 2-4-1: adhesion theories, part i
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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
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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
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