The Science of Expansion & Contraction – or Lack Thereof
- Thermapro
- Mar 20
- 3 min read
One of the key benefits of thermally modified wood is its dimensional stability. This means the wood resists expansion and contraction—the primary causes of movement, more commonly known as twisting, warping, cupping, splitting, shrinking, or swelling. To understand why non-modified wood expands and contracts while thermally modified wood remains stable, let’s take a quick dive into the science behind it.
WOOD STRUCTURE: The microstructure of wood is porous. Cell walls surround macro-voids with pits which are pathways through the walls. The cell walls are made of cellulose, hemicelluloses, and lignin.
Wood cell walls comprise complex polymers: hemicelluloses, cellulose, and lignin. These polymers contain hydroxyl groups (oxygen & hydrogen molecules -OH) which are polar and capable of forming hydrogen bonds. The hydroxyl groups of wood polymers are sorption sites, meaning they are bonding sites for other polar molecules.
WATER: Water molecules consist of two hydrogen atoms bonded to an oxygen atom. Water molecules are also polar and capable of forming hydrogen bonds. Because of its polarity, water molecules happily bond with other polar molecules like itself and hemicellulose molecules.
ABSORPTION: Wood is naturally hygroscopic, meaning it readily attracts and absorbs water. When wood absorbs water, the water can be either free (in the macro-voids) or bound to the polymers within the cell walls.
Water found in the macro-voids or capillary structure absorbs and desorbs without physically binding to the wood. It flows freely in and out without bonding.
It’s the cell walls that are hygroscopic and attract water molecules. Water molecules readily bond with the hydroxyl groups of the hemicellulose and cellulose. However, the cell walls are not porous.
If water or water vapor is available, the molecules continue to bond until the cell becomes saturated. When it reaches the over-hygroscopic range of saturation, the cell will swell. Swelling of the cell walls changes the wood’s dimensional properties causing expansion and the resulting contraction as it dries. Expansion and contraction are the main causes of movement in wood, often resulting in cracking and warping.
How does thermal modification reduce water absorption?
Remember that wood cell walls are made up of cellulose, hemicellulose, and lignin. During the thermal modification process, the hemicellulose and the amorphous part of the cellulose are removed. This means that those hydroxyl groups that hydrogen bond with water molecules are removed. And if any of the little buggers do sneak through the process, the lignin within the cells is hardened, filling most of the cell so there is little space for bonded water to be stored.
By changing the structure of the cell walls, thermal modification reduces their water absorption capacity. Since the cell walls do not bond to the water, expansion and contraction are reduced by up to 90% thus eliminating the main cause of warping, cupping, and cracking in wood.
Note that thermal modification does not collapse the macro-voids so free water can still move through thermally modified wood.
If you would like to understand more details on the water absorption of wood, visit our friends at researchgate.net or sciencedirect.com. Special thanks to Emil Thybring and Maria Fredriksson for their research work.
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Have any other burning questions you want answered about thermally modified wood? Let us know by contacting marketing@thermaprowood.com
References:
Thybring, Emil & Fredriksson, Maria. (2021). Wood Modification as a Tool to Understand Moisture in Wood. Forests. 12. 372. 10.3390/f12030372.