effect wood surface

8/16/2019 Effect Wood Surface

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Effect of Wood Surface on Wood

Adhesive

Wood as an Adherend



Adhesion

Adhesive – a substance capable of holding

materials together by surface attachment

Adherend – a substrate held to another substrateby an adhesive

Adhesives bond substrates together via chemical

interactions and/or mechanical interlocking



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”

Bonds broken to create surfaces=“excess energy

in the surface” or “surface energy”

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

Liquid-vapor, liquid-solid interactions result from

dispersion forces and hydrogen bonds



Surface Energy vs. Surface Tension

Strictly, surface energy DOES NOT equal surface

tension

HOWEVER, it is generally impractical to measureprecise values, especially for solids

THUS, the terms “surface energy” and “surface

tension” are often taken as synonymous, exceptin theoretical treatments



Wood Adherend Variables

A number of wood characteristics can affect the

performance of an adhesive. These

characteristics may be called “wood adherendvariables.”

The “top five” wood adherend variables are

described in the following slides, but notnecessarily in priority order.




1. PorosityWood is a porous cellular

solid

Flow of resin into pores,followed by curing,

improves the “mechanical

interlocking” mechanism of

adhesion.

Note that porosity varies in

the three principal planes

of wood structure.




2. Surface RoughnessSurface roughness is related

to porosity and to surface

preparation (machining)Smoother surfaces are

generally better for bonding,

e.g., knife planed vs. sawn

Smooth surface allows formore intimate contact of

adjacent substrates via a thin

glue lineRoughness: Macroscopic

and Microscopic




3. Wood DensityLower density woods are

generally easier to bond

due to their greaterdegree of porosity

% wood failure (a

measure of adhesive

bond quality) decreaseswith increasing wood

densityLow, medium, and high density oak




4. Wood Moisture ContentWood is hygroscopic!

MC too low or too high

will impede adhesion A MC range of 6-14% is

optimal for many wood

adhesives




5. Surface ChemistryOxidation by exposure to air or

heating (drying or friction from

machining) results in poor

wetting

Contamination by airborne

particles, dirt, grease, etc. has a

negative effect on surfaceproperties




5. Surface Chemistry, continuedExtractives may be driven

to surface by drying;many are hydrophobic,may inhibit reactions ofadhesives, or blockmicropores

In extreme cases, woodswith high extractivescontent (>20%) may benon-bondable

If adhesive cannot wet the

surface, the substrate cannot be

adhesively bonded.



Surface Properties of Wood

Wood surface should be smooth, flat, and free of

machine marks and other surface irregularities,

including planer skips and crushed, torn andchipped grain.

The surface also should be free of burnishes,

exudates, oils, dirts and other debris.(because adhesives bond by surface attachment,

the physical and chemical conditions of the

adherend’s surface)



Overdrying and overheating deteriorates the physical

condition of the wood (forcing the extractives to diffuse to

the surface)Wood surfaces can be chemically inactivated with

respect to adhesion by airborne chemical contaminants,

hydrophobic and chemically active extractives from the

wood.e

The deterioration of physical and chemical also can

interfere with the cure and resulting cohensive strength

of the adhesive.



Extractive on Surfaces

Extractive on wood surfaces are the principal physical

and chemical contributors to surface inactivation, hence

to poor wettability by adhesives.

Resinous and oily exudates are hydrophobic ( repelwater)

The acidity of extractives can interfere the chemical cure

of adhesives. (the acid may accelerate the cure of analkaline phenolic adhesive, causing the adhesive to gel

prematurely and reducing the ability to wet, flow and

penetrate



knife and Abrasive Planed

SurfacesWood should be surfaced to remove extractives and

other physical and chemical contaminants.

Experience and testing have proven that a smooth, knifecut surface is best for bonding.

The crushed and burnished surface inhibits adhesive

wetting and penetration.

Damaged surfaces are inherently weak and result in

poor bond strenght.



Veneer SurfacesWood properties of veneer are essentially no different

from those lumber however manufacturing processes

change physical and chemical surface.

Peeling of log will resulted in loose side and tight side ofthe veneer.

Drying veneer to very low moisture content levels at very

high temperatures and drying at moderate temperatures

for prolonged periods, inactives surfaces, causing poor

wetting veneer hence poor bonding.

effect wood surface

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