The Science Behind Wood Preservative

Preservative Treatment Graphic

Wood preservative has been around for centuries. Coal-tar creosote was patented as a preservative treatment in 1836. Dr. Karl Wolman, a pioneer in wood preservation, first patented a fluoride-based preservative in 1907.

Water in Wood – A Diffusion Process

Wood is hygroscopic, meaning that it can attract and hold water molecules from the surrounding environment. The process which all non-pressurized wood preservative treatment delivers treatment chemicals is through water vapor diffusion. Water carries the preservative chemicals into the wood cells through absorbing the liquid solution.

Preservation is primarily concerned with sapwood – the outer layer directly beneath the bark. This is the physiologically active part of the wood and most susceptible to decay.

Firstly, moisture is removed from the lumber when the lumber is dried . In the next step a liquid solution carrying the treatment chemicals is applied to the dry lumber. For the preservative treatment to be effective the treatment chemicals must penetrate the surface. The force of diffusion draws the moisture into the fiber and delivers the preservative ingredients. The moisture then evaporates and the treatment stays within the wood fiber. To sum up this is the diffusion process.

Water Diffusion in Wood Fiber

Within the wood cells are two types of water: Bound water is held within the cell wall by absorption forces. Similarly, free water exists as either condensed water or water vapor within the cell cavity.

Fiber Saturation is when all free water is removed from the cell cavity but cell walls are fully soaked.

Diffusion forces determine the movement of free water from the cell cavity to the cell walls. This occurs as the exterior environment draws moisture toward the material surface for evaporation into the environment.

Why Wood Preservatives Work

Diffusion forces determine the movement of free water in fiber cells. In turn controlling the movement of a preservative into the cell cavities and walls.

To ensure that this action can take place the moisture content should be at or below 20% moisture content. In addition, for wood preservation to happen the lumber must accept the water-based preservative such as Wolman AG.

Concerns about the health and environmental impacts of metallic wood preservatives is ongoing. Consequently, interest in non-metallic treatment product such as Propiconazole-Tebuconazole-Imidacloprid better known as PTI.

(For a deeper dive into diffusion in wood products-click this link)

Preservatives Improve Performance

Belco XT Trim
Micro view of sapwood structure with preservative treatment. Protective sealants do not penetrate the cells. Wood preservatives penetrate the cells through diffusion.

PTI treated wood products are very well adapted for paint and stain applications with no bleed-through.

PTI treated wood products are no more corrosive than untreated wood. Additionally, PTI treated wood products and are approved for all types of metal contact, including aluminum. Wolman® AG (PTI) preservative, is a carbon-based treatment listed with the EPA. This product has earned a listing in the GreenSpec® directory of environmentally preferable products.

For more information on preservative treatments click this link

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