Dimensional Stabilization of Wood

Problems of Wood• Hygroscopicity• Dimensional instability• Durability• Mechano-sorptive properties

How to solve these problems?Use wood with high natural quality

• Availability (mid term, long term)• Sustainability

Use of wood preservativesUse of new technologies for wood treatment!!

Modification methods

Thermal modificationThe Plato process

• Hydro thermal treatment / hydro thermolysis (160-190ºC 6-8 bar)

• Drying (10% MC)• Curing (heating of the altered wood) (160-190ºC)

Thermal modificationDrying

6-10% MCHigh Temp.180-230 ̊C

Heating2-3Hours

Hotpressing6-10 MPa

Staypak

Protected against burningusing steam as an inert gas

Staypak

1.25 to 1.40 g/cc10 to 18 times hardness

Chemical modification

Acetylation

• Impregnation with acetic anhydride• Reaction at elevated temperatures• Post treatment (acetic acid)

Increases naturally occurring acetyl content of wood(from 1-3% to 20%)

Acetylation

Wood(Veneer) treated with a thermosetting, fiber-penetrating resin and cured without compression is known as impreg.

Stabilizes dimensions, increases strength and resistance to water, moisture, and chemicals, and reduces cracking.

Impreg

PROCESS

• Green Wood• Dry Wood• 25-35%

Weight Gain

Soaking

• 1-2 Days• For Uniform

Distribution of the Resin

Nondrying condition • Drying

(Moderate Temp)

• Curing (Higher Temp)Drying and

Curing

Properties of Impregnated WoodImpreg has a number of properties differing from those of normal wood and ordinary plywood

15% to 20% greater Specific Gravity

1/4 to 1/3 Shrinkage/Swelling

1/10 Permeability to water vapor

Better Decay, Termite, Acid Resistance

Greatly increased heat resistance

NB: Comparative properties will be shown later

COMPREGSimilar to impreg except that it is compressed before the resin is cured within the wood.

The resin-forming chemicals (usually phenol formaldehyde) act as plasticizers

PROCESS

• Green Wood• Dry Wood• 25-35%

Weight Gain

Soaking

• 1-2 Days• For Uniform

Distribution of the Resin

Nondrying condition • Drying

(Moderate Temp)

• Curing (Higher Temp)Drying and

Curing

7-10 Mpa

Properties of Compreg Wood

Natural lustrous finish

Sp. Gravity usually 1.0 - 1.4

1/4 to 1/3 Shrinkage/Swelling

Permeability to water vapor: much less than impreg

Better Decay, Termite, Acid Resistance

Greatly increased heat resistance

NB: Comparative properties will be shown later

High strength resin impregnated compressed wood

Very high pressing pressure50 to 80 MPa

1MPa Pressing Pressure (PF Resin)1.16gm/cc MOE: 29GPa MOR: 307MPa

NaOH0.5% Aqueous

NaClO22% Aqueous

Comparative properties of modified woods

Property Impreg Compreg Staypak

Specific gravity 15% to 20% greater than normal wood

Usually 1.0 to 1.4 1.25 to 1.40

Equilibrium swellingand shrinking

1/4 to 1/3 that of normal wood

1/4 to 1/3 that of normal wood

Same as normal wood but slower

Springback None Very small when properly made

Moderate when properly made

Face checking Practically eliminated Practically eliminated About the same

Decay and termiteresistance

Better than normal wood Better than normal wood

Normal but occurs slowly

Compressive strength

Increased Increased Increased

Tensile strength Decreased significantly Increased less than proportional to specific gravity increase

Increased about in proportion to specific gravity increase

Hardness Increased 10 to 20 times that of normal wood

10 to 18 times that of normal wood

Toughness About 1/2 of value for normal wood

1/2 to 3/4 of value for normal wood

Same to somewhat greater than normal