Water Absorption (Porosity) of Thermally Modified Wood Explained

One of my goals in continuing to educate on thermally modified wood is to clear up any misconceptions, so today we're going to explore the porosity of wood. Why? Because when you mix thermally modified wood with water, it may appear that the wood is absorbing water, but in reality, the water is just passing through.

To grasp the concept, we first need to understand how non-modified wood interacts with water.

Structure of Wood. Thybring, Emil & Fredriksson, Maria. (2021).

Water in non-modified wood can be found both within cell walls and outside of the cell walls in its porous capillary structure. Water found in the capillary structure absorbs and desorbs without physically binding to the wood - flowing in and out. Wood cell walls however are hygroscopic; made up of cellulose, hemicellulose, and lignin which act like a sponge. Water interacts with the wood polymers in the hydroxyl groups of the hemicellulose by hydrogen bonding. Since these cell walls are not porous, they swell when saturated with bonded water.

Hydrogen Bonding Diagram. Thybring, Emil & Fredriksson, Maria. (2021)

Water can be absorbed from moisture in the air and contact with liquid; the amount absorbed or held at any time in wood is known as moisture content. Different species of wood have varying capacities to absorb and bond with water. When the moisture content reaches the over-hygroscopic range, swelling of the cell walls occurs which changes its dimensional properties through expansion and contraction, often resulting in cracking and warping.

How does thermal modification reduce water absorption?

Remember that wood cell walls are made of cellulose, hemicellulose, and lignin. During the thermal modification process, the hemicellulose and the amorphous part of the cellulose are removed. This means the functional hydroxyl groups that hydrogen bond with water molecules are also removed. The lignin is polymerized and fills the inside of the cell. These changes that occur during thermal modification do two things: 1) remove the hydroxyl groups that bond with water molecules thereby forming a close porous structure with a low ability to bind with water, and 2) provide less space for hydrogen-bonded water with any hydroxyl groups that may remain.   

By changing the structure of the cell walls, thermal modification reduces their water absorption capacity which minimizes expansion and contraction by up to 90%. Since the cell walls do not bond to the water, the wood doesn’t swell (expand and contract), which is the main cause of warping, cupping, and cracking.

What you see as water absorption in thermally modified wood is water flowing through its capillary structure (absorption and desorption). So, while thermal wood does ‘take in’ water, kind of like a web of straws, it doesn’t bond with water causing expansion and contraction of the wood fiber.

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.

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.