Forestry 485

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

Adhesion TheoriesAdhesion 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 2-4-1 and 2-4-2).

Surface Properties: Wetting

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 InteractionsSurface 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 EnergySurface Energy

Surface Energy Continued

Bulk Molecules experience uniform interactions that minimize energy.

Surface molecules encounter discontinuous interactions. This results in higher “surfaceenergy” also known as surface tension.

From: Fundamentals of Adhesion, ed. Lee, L.H., p.126, Plenum Press, New York, 1991

Surface Energy Measurement

Surface Energy Measurement

Sessile Drop Method

Capillary Rise MethodWilhelmy Plate Method

http://www.ksvinc.com/wilhelmy_plate.htm

Measuring Surface Properties

Measuring Surface Properties

Slide courtesy Dr. Doug Gardner, University of Maine

Sessile Drop Contact Angle Analysis

Sessile Drop Contact Angle Analysis

Slide courtesy Dr. Doug Gardner, University of Maine

Contact angle changes as a function of time…

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

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 γC

Zisman Plot

0.5

0.7

0.9

1.1

10 20 30 40 50

Surface Energy (mN/m)

cos

thet

a

γC = 18 mN/m

Interfacial Behavior

Ө

Vapor

Liquid

Solid

γLV

γSVγSL

γSV = γSL + γLVcos Ө

If Ө = 0, Spreading OccursIf Ө < 90o, Wetting is Favorable If Ө > 90o, Wetting is not Favorable

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

Dec

reas

ing

γ LV

Incr

easi

ng γ

SV

Control of Wetting of Wood Substrates: Means of

Improvement

Control of Wetting of Wood Substrates: Means of

ImprovementMachine Wood Surface

Higher Surface EnergyReduce Roughness

Avoid Deactivating Wood Surface(Minimize time from machining to bonding, minimize

heat treatment, etc.)Add a Surface-Active Agent to the Resin