Wood Preservatives

by Dr. Nenad Vidovic, PhD., B.Sc. &
Lissi Jeppesen, M.Sc., B.Comm., Chem. Eng.

Premature decay of untreated wood, periodically or permanently exposed to increased moisture content (over 25%), reached serious proportions in Canada in recent years. As a result, requirements for reliable fungistatic treatments have been progressively introduced. The most acceptable pre-treatment and remedial treatment for rot and insect attack was found in the application of Boron based wood preservatives.

Apart from their proven efficiency, the borate formulations are accepted worldwide as environmentally safe wood preservatives, having a very low human and mammalian toxicity. Their lethal dosage on rabbits (LD50) has been found to be 2000 to 2500 mg per one kilo of a rabbit body. The LD50 dosage means that rabbits (having, for example, a body weight of 1 kg) must consume 2000 mg of the product to reach the rate of at least 50% rabbit mortality. For comparison, pentachlorophenol has an LD50 of 125 mg/kg.

Boracol wood preservatives are based on inorganic Boron (disodium octaborate tetrahydrate) which has the ability to diffuse in moist wood (12% moisture content or greater), has a very low vapor pressure and the ability to progressively penetrate the wood even several years after application. Throughout this period, whenever and wherever a determined concentration of Boron is found in wood, the organisms capable of degrading wood have no chance to survive or establish the infection. The only condition for proper performance is that the treated wood should not be exposed to direct contact with water (for example wood in ground contact or occasional heavy rain over construction wood). If water has access to wood, it will make it possible for Boron to "travel the opposite way" by leaching process. Therefore, a post-treatment application of the wood surfaces is recommended by applying a proven wood stain, water repellent or paint system, preventing the leaching of Boron, or covering the lumber to create a barrier for keeping water away.

Boracol products are the most famous Boron based wood preservatives that have been used in Canada over the past decade with great success. They are clear, colorless and odorless solutions of active ingredient (Boron), dissolved in environmentally acceptable propylene glycol. This "special solvent carrier" assists distribution of boron into the drier section of the timber, even into heartwood. The products are applied to wood surfaces by brush, dipping or spray equipment. Within the first 24 to 48 hours, the product will penetrate 3-5 mm deep into wood with greater than 12% moisture content and will establish a reserve of over concentrated Boron. From this, further penetration/diffusion will take place depending on moisture content.

Boracol preservatives will form a wet-appearing film on wood surfaces as soon as the products are applied. The film will stay on wood for a period necessary for the solvent (non-evaporative but penetrating propylene glycol) to penetrate the wood surface, bringing the Boron compound deeper in the wood. During this period, the treated wood surfaces should be protected from contact with water. It is advised to clean the wood before treatment. Vacuum cleaning, power washing or compressed air are good ways to remove dirt. If treated wood is intended for subsequent paint or stain application, allow at least seven days for wet film of Boracol to completely penetrate and disappear from the surface. If a slight film or crystal like particles are visible, the surface should be wiped off with a damp cloth. Slight sanding (80 grit) can be very helpful to remove boron crystals and improve adhesion of the coating. Subsequent coating or covering of the treated wood is particularly important when wood is exposed to contact with rain or condensed water that stays trapped in the wood for a longer period of time (balconies, for example).

Penetration versus Moisture Content
When a Boron wood preservative is applied on the wood surface, a layer of concentrated Boron based wood preservative is established. Due to diffusion forces, the product is slowly penetrating into the wood depths. The velocity of the penetration depends on several factors, the moisture content in the wood being of highest importance.

Outer wood layers generally have a lower moisture content than the central portions, taking up to a few days for the concentrated product to fully penetrate. When the surface moisture content of wood is 25% or more, the Boracol wood preservative penetrates into the wood faster (one to two days) than with a moisture content below 25% (four to five days). Higher moisture contents are not recommended due to dimensional changes of wood in construction and possible mildew development prior to the treatment. However, even if the wood moisture content is extremely low (e.g. 12%), the product will still penetrate into the wood due to the non-drying solvent incorporated into the Boracol formulation. More time is needed for penetration of Boracol products into wood with lower moisture content.

Spraying or misting of dry wood with water prior to Boracol treatment has been tried in order to raise its moisture content. To the best of our knowledge, this method did not give spectacular results, mainly because water was evaporating more rapidly than penetrating across the wood grain. Apart from this, some coniferous species, such as spruce, close its border pits as soon as the moisture content drops below 30% and do not absorb any liquid. However, spruce will absorb Boracol because Boracol penetrates due to the chemical diffusion process.

When applied to wood with a low moisture content, Boracol wood preservatives tend to settle on the wood surfaces, forming glass-like crystals. Even though the crystals are formed, there is no need for concern on "wasting the material" as the quality of the treatment is not affected. The crystals may stay on the wood surface for months or years before they disappear. In fact, the crystals at similar to Impel Rods (glass-like rods) and dissolve and reactivate as soon as increased moisture brings the wood into decay hazard./b>

Reprinted with permission.
All rights reserved.
The Sansin Corporation
3377 Egremont Drive
Strathroy, Ontario
N7G 3H6 Canada
519/245-4623 bus.
519/245-4759 fax



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