Shrinkage and Swelling

shrinkage

occurs as bound water molecules escape from the long chain cellulose molecules; chain molecules can then move closer together upon water absorption; The amount of _____ that occurs is generally proportional to the amount of water removed from the cell wall.

Shrinkage

· As wood loses moisture below the FSP, water molecules are removed from the cell wall structure and the microfibrils in the S2 layer of the cell wall move more closely together causing the cell to shrink.

Swelling

· When the cell wall gains moisture, water molecules move into the cell wall, causing the S2 layer microfibrils to become further apart, thus resulting in swelling

bound water

· Shrinkage and swelling are caused by _______ getting into and out of the sorption sites within cell wall

ST > SR > SL

The observed dimensional changes in wood are unequal along the three structural directions:

6-12%

3-6%

0.1-0.3%

General ranges of shrinkage:

ST

SR

SL

2.0

Differential shrinkage

width

For plain sawn lumber, highest shrinkage or swelling will occur along the ______

thickness

For quarter sawn lumber, the highest dimensional changes will occur along the ______

quarter sawn

If the planks will be used for paneling; ____ lumber should be used

negligible

The longitudinal shrinkage of normal wood is_____ for most practical purposes due to the low microfibril angle (mfa) in the cell wall

Wood Rays

· are more dimensionally stable than surrounding tissues

· act as "reinforcing rods" to restrain movement in the radial direction.

· lower in value than other elements

more, stronger

· Dense latewood shrinks and swells ______ and is also_____ than earlywood.

large, small

tangential shrinking/swelling is _____ because it is controlled by the latewood, while radial shrinking/swelling is relatively ____ because it is intermediate between large latewood and small earlywood effects.

radial walls, tangential walls

are heavily pitted with large pits, and the ____ are sparely pitted with small ones.

pits

are holes on the cell wall; no shrinkage can occur to it.

Wood Species

Inherent sample variability

Magnitude of change in MC below FSP

Anatomical direction

Presence of reaction or juvenile wood

5 Factors Affecting Dimensional Changes in Wood

density

The higher the_____of the sample, the more it will tend to shrink or swell. This is due to the increase in the sorption sites where water molecules can attach themselves to or detach themselves from.

Extractives

tend to take place of the water molecules and limit the sorption sites through bulking. Thus, species with higher extractive content tend to be more dimensionally stable.

hemicellulose, lignin

· The increase in the amount of ____ with reduced ____ content (with the same cellulose content) tend to increase the dimensional changes in wood since it is more hygroscopic than lignin.

Inherent sample variability

· related to the location where the samples were obtained along the length or across it. Data on the amounts of shrinkage/swelling in sapwood and heartwood portions are not conclusive. This is also the case along the height of the tree.

Blocking of the Passages

Restraint

Replacement of Bound Water

Hydroxyl Substitution

Heat Treatment

Removal of Amorphous Region

Acid hydrolysis

7 Means of Reducing Moisture Induced Dimensional Changes