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
www.sansin.com
info@sansin.com
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