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

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

3. Significant Mechanisms
For lignocellulosic substrates, a combination of adhesion mechanisms are significant:
Thermodynamic adsorption/wettability
Prerequisite for other mechanisms
An adhesion mechanism per se
Mechanical Interlocking
Perhaps the predominant means of adhesion
Diffusion
Degree to which adhesion develops is unclear
Covalent Bonding
Theoretically plausible; not well supported by experimental evidence

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

5. 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

6. 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

7. 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.

8. Weak Boundary Layers Describes adhesion in terms of failure
The boundary layer is an “interphase” with properties that match neither the adhesive nor the adherend
Failure propagation occurs in the interphase or wood adherends
Thus, not an adhesion theory per se, but the concept is useful for understanding of adhesion performance

9. Bond “Anatomy” Link 1 = Adhesive film
2 & 3 = Intra-adhesive boundary layer
4 & 5 = Adhesive-adherend interface
6 & 7 = Adherend subsurface
8 & 9 = Adherend proper 1 2 3 4 5 6 7 8 9
Marra, 1992

10. Bond Anatomy Continued
Symmetrical – Realistic?
Chain Represents Stress Transfer
Most of the Links Depend on
Surface Behavior 1 2 3 4 5 6 7 8 9

11. Furnish Factors: Strength Density/Porosity Moisture Content
Thickness – Heat Transfer 1 2 3 4 5 6 7 8 9

12. Furnish Sub-Surface Factors: Damage Imparted by Processing
Resin Penetration – Mechanical
Interlocking
Active Surface 1 2 3 4 5 6 7 8 9

13. Adhesion Three Basic Mechanisms: Mechanical Interlocking
Adsorption – Molecular Interactions
Chemical Bonding 1 2 3 4 5 6 7 8 9

14. Resin – Surface Controlled
Factors:
Not Participating in Adhesion
Influenced by Resin/Wood
Interaction
pH, Selective Absorption,
Mobility Effects 1 2 3 4 5 6 7 8 9

15. Resin - Bulk Factors: Glue Line Material Cohesion Stress Transfer
Depends on Proper Curing 1 2 3 4 5 6 7 8 9