Forestry 485

Lecture 2-3-2: Wood Surface Properties, Part II

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 Energy: “Bond Breaking”

Surface Energy: “Bond Breaking”

Bonds broken to create surfaces=“excess energy in the surface” or “surface energy”

Surface does not exist of itself: It must be part of an interface (two substances or phases)

Liquid-vapor, liquid-solid interactions result from dispersion forces and hydrogen bonds

Surface Energy vs. Surface Tension

Surface Energy vs. Surface Tension

Strictly, surface energy DOES NOT equal surface tension

HOWEVER, it is generally impractical to measure precise values, especially for solids

THUS, the terms “surface energy” and “surface tension” are often taken as synonymous, except in theoretical treatments

Surface Energy MagnitudeSurface Energy Magnitude

Organic polymer surfaces; typical values:γ < 100 mJm-2 (low energy)

Hard inorganic surfaces (metals, ceramics, etc.)

γ > 100’s to 1,000’s mJm-2 (high energy)

Slide courtesy Dr. Doug Gardner, University of Maine

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

Contact Angle HysteresisContact Angle Hysteresis

Microscopic “surface inhomogeneities” (roughness) causes variations in contact angle

Contact angle measurement varies between advancing and receding liquid-solid interface

Dynamic contact angle measurement (e.g., Wilhelmy plate method) helps to account for these variations

Contact Angle HysteresisContact Angle Hysteresis

Another cause of hysteresis is “heterogeneous contamination” of surfaces with low-energy impurities

In the case of wood adherends, such “contaminants” are typically hydrophobic extractives

Young’s Equation: Proposed in 1805 (!) to explain the equilibrium of a drop of liquid on a solid surface

Young’s Equation: Proposed in 1805 (!) to explain the equilibrium of a drop of liquid on a solid surface

“When a droplet of liquid, L, with its vapor, V, is at rest on a solid surface, S, it takes a configuration which minimizes the energy of the system and highlights the liquid-solid interactions.”

The equilibrium condition is represented by:

γSV = γSL + γLVcos Ө- Fourche, 1995

Contact Angle

Ө

Vapor

Liquid

Solid

γLV

γSVγSL

γSV = γSL + γLVcos Ө

Or

cos Ө = (γSV – γSL)/ γLV

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

Young’s Equation:

Critical Surface EnergyCritical Surface Energy

C = Critical Surface Energy, is that surface energy at which complete wetting occurs

Notice (in the following slide) that if cos θ = 1, cos-1 θ = 0o

Zisman Plot

0.5

0.7

0.9

1.1

10 20 30 40 50

Surface Energy (mN/m)co

s th

eta

γC = 18 mN/m

Zisman Plots

cos Ө = 1 + b (C - L)

Ө = measured Contact Angleb = the slope of the lineC = Critical Surface EnergyL = Liquid Surface Energy

Dupre’s Equation: Work of Adhesion

(“postulated centuries ago”!!)

Dupre’s Equation: Work of Adhesion

(“postulated centuries ago”!!)Consider: “An elastic material of unit cross-

section is subjected to a tensile force. The material breaks, creating two new surfaces.”

Since “the new surfaces are each made of the same material, then the total energy expended must be twice the surface energy of the material.”

Thus, work of COHESION, Wcoh = 2γ» Pocius, 2002, chapter 4 21

Dupre’s Equation: Work of Adhesion

Dupre’s Equation: Work of Adhesion

NOW, Consider: “A situation in which two dissimilar materials are in intimate contact. A tensile force splits the materials into two dissimilar materials. If the sample is of a unit cross sectional area, then the energy expended should be the sum of the two surface energies…”

BUT, “because the two dissimilar materials were in contact there were intermolecular forces present that are now missing since the materials were separated. That is, an interfacial energy may have been present before the materials were split apart. As this energy is missing after the two surfaces are separated, we must subtract it from the energy used to create the two new surfaces.”

» Pocius, 2002, chapter 4

Work Of Adhesion

2

Definition: Work requiredto separate two bonded materialssolely in terms of surface energy.

Thus, we have the Dupre’ Equation:WA = γ1 + γ2 – γ12

1

1 2

Where γ1 =surface energy of material 1, γ2 =surface energy of material 2, and γ12 = interfacial energy between materials 1 and 2.

Resin – Wood Furnish Interactions

Mechanism•Application – Droplet Formation•Fluid Motion – Compression/Consolidation•Molecular Motion – Spreading/Wetting

Goal: Achieve even distribution of resin within adhesive joint, and promote intimate contact between resin molecules and furnish surface.

AcknowledgementAcknowledgement

Illustrations in slides 2, 11, 13-14 and 16 courtesy of Carter Johnson.

Literature cited is from module 2 optional readings, except for Pocius, chapter 4 (copy available on request)