Forestry 485 Lecture 2-3-1: Wood Surface Properties, Part I

Adhesion Theories  Several mechanisms of adhesion have been proposed, including Mechanical Interlocking, Diffusion Theory, Electronic Theory, Covalent Bonding Theory, and Adsorption Theory (these will be discussed in Lectures and 2-4-2).

Surface Properties: Wetting  Regardless of adhesion mechanism, optimal adhesion is dependent upon effective contact of adhesive and adherend; contact is dependent upon Surface Wetting phenomenon.

Surface Interactions  Liquid-vacuum, liquid-gas, liquid-liquid, or liquid- solid interfaces are the sites of “surface interactions”  Surface interaction parameters: Surface Energy Contact Angle Wetting

All molecules in a material are attracted to each other in order to minimize energy. Liquid Solid Surface Molecule Bulk Molecule Surface Energy

Surface Energy Continued Bulk Molecules experience uniform interactions that minimize energy. Surface molecules encounter discontinuous interactions. This results in higher “surface energy” also known as surface tension.

From: Fundamentals of Adhesion, ed. Lee, L.H., p.126, Plenum Press, New York, 1991 Surface Energy Measurement  Sessile Drop Method  Capillary Rise Method  Wilhelmy Plate Method

Measuring Surface Properties Slide courtesy Dr. Doug Gardner, University of Maine

Sessile Drop Contact Angle Analysis Slide courtesy Dr. Doug Gardner, University of Maine

Contact angle changes as a function of time… Slide courtesy Dr. Doug Gardner, University of Maine

Dynamic Contact Angle Measurement by the Wilhelmy Plate Method Slide courtesy Dr. Doug Gardner, University of Maine

Surface Energy Measurement of Solids Zisman Method: Measurement of Critical Surface Energy γ C γ C = 18 mN/m

Interfacial Behavior Ө Vapor Liquid Solid γ LV γ SV γ SL γ SV = γ SL + γ LV cos Ө If Ө = 0, Spreading Occurs If Ө < 90 o, Wetting is Favorable If Ө > 90 o, Wetting is not Favorable

Control of Wetting on Wood Substrates Two Approaches Alter Liquid/Resin Contact Angle will Decrease as γ LV Decreases Alter Substrate/ Wood Furnish Contact Angle will Decrease as γ SV is Increased Decreasing γ LV Increasing γ SV

Control of Wetting of Wood Substrates: Means of Improvement  Machine Wood Surface  Higher Surface Energy  Reduce Roughness  Avoid Deactivating Wood Surface  (Minimize time from machining to bonding, minimize heat treatment, etc.)  Add a Surface-Active Agent to the Resin