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Forestry 485

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

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Forestry 485

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

  2. Adhesion Theories • Several mechanisms of adhesion have been proposed, includingMechanical Interlocking, Diffusion Theory, Electronic Theory, Covalent Bonding Theory, and Adsorption Theory (these will be discussed in Lectures 2-4-1 and 2-4-2).

  3. 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.

  4. 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

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

  6. 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.

  7. Surface Energy Measurement • Sessile Drop Method • Capillary Rise Method • Wilhelmy Plate Method http://www.ksvinc.com/wilhelmy_plate.htm From: Fundamentals of Adhesion, ed. Lee, L.H., p.126, Plenum Press, New York, 1991

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

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

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

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

  12. Surface Energy Measurement of Solids Zisman Method: Measurement of Critical Surface Energy γC γC = 18 mN/m

  13. Vapor γLV Ө Liquid γSL γSV Solid γSV = γSL + γLVcos Ө If Ө = 0, Spreading Occurs If Ө < 90o, Wetting is Favorable If Ө > 90o, Wetting is not Favorable Interfacial Behavior

  14. Control of Wetting on Wood Substrates Two Approaches Alter Liquid/Resin Contact Angle will Decrease as γLV Decreases Alter Substrate/ Wood Furnish Contact Angle will Decrease as γSV is Increased Increasing γSV Decreasing γLV

  15. 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

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