Note: Descriptions are shown in the official language in which they were submitted.
CA 02573211 2008-02-19
Method for treating lignocellulosic material
TECHNICAL FIELD
The present invention relates to methods for treating wood and, more
specifically, to
methods for treating wood with borate or other water-soluble preservatives.
BACKGROUND OF THE INVENTION
Unprotected wood structures are susceptible to degradation by fungi and
insects
such as termites, which can cause substantial damage to wood building
structures.
Property damage in US alone is estimated to be in the neighbourhood of 1
billion
dollars per year. Various processes and chemicals have been used to treat wood
to
protect it from insect degradation. For example, preservatives, such as Copper
Chrome
Arsenate (CCA), ACC, ACQ and borate, have been impregnated in wood by a
variety of
processes.
One popular method of integrating chemicals in the wood is by a
vacuum/pressure
treatment in the presence of an aqueous solution containing the preservative.
While the known methods are relatively successful at introducing preservatives
in wood
products, they necessitate the use of costly systems and may not be energy
efficient.
Accordingly improved methods are desirable. Also the resulting product is very
prone to
wash out by exposure to water. In particular, borate-treated lumber is
particularly prone
to leaching or wash-out by rain water and cannot be used in exterior
structures.
Methods have been developed that attempt to prevent the leaching out of
preservatives.
One such method is described in U.S. Pat. No. 6,426,118 to Barnisin. The
method
consists in drying the wood before infusing the preservative followed by
further drying
and the infusion of a waxy solid that constitutes a barrier to water and
prevent leaching
of the active ingredient. This method suffers from the need to treat the wood
with not
CA 02573211 2008-02-19
only the active ingredient but also the waxy solid which makes the treatment
longer and
more costly.
Furthermore the wax may adversely affect further treatment of the wood with
coatings
such as paint.
There is thus a need for an improved process for treating wood whereby the
impregnated water-soluble preservatives are inhibited from leaching out of the
wood.
SUMMARY OF THE INVENTION
According to an aspect of the present invention green or dry wood can be
treated with a
preservative by placing the wood in an environment of aqueous vapor containing
the
preservative such as a salt soluble in water. In an embodiment of the
invention heat
is applied to the wood therefore favoring the penetration of the water soluble
salt
preservative in the wood. The extent of wood treatment can be achieved by
controlling
the concentration of the salt and the dryness of the wood.
The invention provides also a method for treating wood which enhances fixation
(or
"locking" or "fixing") of the borate or other water-soluble preservatives in
the wood.
By locking or fixing the water-soluble preservative within the wood, the
preservative
is prevented from leaching out of the wood, which makes the wood particularly
useful in
outdoor applications where it is exposed to rain. Since the preservative is
inhibited from
leaching out of the wood when exposed to water, the wood is therefore
substantially
permanently protected from wood-destroying fungi and insects such as termites.
In general, the present invention provides a method whereby lignocellulosic
material
is treated with a preservative using aqueous vapor and water as a vector, the
method
involves as a second part the modification of the wood structure by heat, i.e.
the
extreme drying of wood to a moisture content of between 0 and 5%, and
preferably as
2
CA 02573211 2008-02-19
close to 0% as possible, such that the cellular structure of the wood is
permanently
altered, thereby locking or "fixing" the preservative (e.g. borate or other
water-soluble
preservative) in the cells of the wood. This treatment method can be performed
on
green wood or on dried wood but not on wood already modified by heat.
All prior art methods of treating wood that are known to the applicant involve
drying the
wood to a moisture content as low as 8-10%, but not any lower. This prior-art
technique
drives out free or extracellular water and even some intercellular water.
The method in accordance with the present invention modifies the wood
structure by
drying it much further, to as low as 0% moisture content, which first drives
out the
remains of the intercellular water and then drives out the intracellular
water. The
modification of the wood structure by heat in this manner effectively altered
the cells
structure in the wood, permanently changing their cellular structure, and thus
causing
any preservative which has diffused into the wood to become locked or fixed
within the
wood. Accordingly, wood treated in this manner is highly resistant to outward
diffusion
and leaching out of the preservative from the wood. The modification of the
wood
structure by heat for the purpose of fixing the water-soluble preservative in
the wood
can generally proceed in one of two methods: in a first method, a wood that is
already
preservative-treated is vapor-dried until the moisture content of the wood
falls to
within 0-5%, and preferably as close to 0% as possible. In a second method,
the
preservative can be dissolved and or introduced in an aqueous solution and
vaporized
(or sprayed) in a chamber containing the lignocellulosic material.
Alternatively, an
aqueous solution of the preservative can be pulverized in the aqueous vapor
within the
chamber to be mixed therewith. Untreated wood is vapor-dried in the presence
of a
preservative-laden vapor which infuses the preservative into the wood. The
wood is
3
CA 02573211 2008-02-19
vapor-dried until a moisture content of the wood is between 0 and 5%, and
preferably
as close to 0% as possible. With either method, the cellular structure of the
wood is
altered by the heat that locks the preservative in the wood in such a manner
that it will
not leach out in the presence of water. Without the modification of the wood
structure by
heat the wood will be treated but the salt will be subject to leaching. It
will be
appreciated that in the treating part the aqueous solution/vapor may comprises
other
solvents than water to help the penetration of the salt into the wood. The
vapor can be
used to dry the lignocellulosic material either after or during the treatment
with the
preservative. In the respect it may be advantageous to control the temperature
within
the chamber so as to maximize the penetration of the preservative and the
drying of the
material. For example, temperatures of between 15 degree C. and 60 degree C.
may be
used to treat the material with the preservative and temperatures between 60
degree C.
and 120 degree C. can be used for drying the material. The time of exposure to
appropriate the temperature for penetration of the preservative and drying of
the
material may depend on the wood species and the desired degree of penetration
and
dryness. The lignocellulosic material may also be treated without having been
subjected
to drying condition (i.e. the material can be treated green). For the
modification of the
wood structure by heat the material is heated to a temperature typically
comprised
between 180 degree C. and 240 degree C. After the modification of the wood
structure
by heat, the wood may reabsorb some of ambient moisture, and accordingly wood
modified by heat ("thermo-modified") may ultimately have moisture content
between 3%
and 5%. Even if the moisture content, after the modification by heat, rises
toward 5%,
the process of modification of the wood structure by heat has permanently
altered the
cellular structure of the wood and therefore the preservative is already
permanently
4
CA 02573211 2008-02-19
locked or fixed in the wood, thus preventing leaching out of the preservative.
Re-humidification of the wood treated by modification of the wood structure by
heat
does not lead to leaching, unlike the prior-art drying methods which are
susceptible to
diffusion due to re-absorption of water in the wood.
Further features and advantages of the present invention will become apparent
from
the following detailed description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a method of treating lignocellulosic material,
such as
wood, with a water-soluble preservative, such as preferably borate; the
treatment does
not required the use of vacuum/pressure treatment or a common treatment vessel
(i.e.
autoclave) such as the one described in U.S. Patent 5,024,861 (Vinden). The
present
invention provides a method whereby the lignocellulosic material is treated
using
aqueous vapor and water as a vector and then the modification of the wood
structure by
heat (thermo-modified) locks or fixes the preservative in the wood. Thermo-
modifying
the wood to extremely low moisture content, i.e. between 0 and 5%, and
preferably as
close to 0% as possible, not only drives out the free or extracellular water
but also the
intercellular and intracellular water. In so doing, the process of thermo-
modifying
permanently alters the cellular structure of the wood, effectively killing the
cells, and
thus locking or fixing the preservative in the wood. The preservative, once
locked by the
process of modification of the wood structure by heat, is highly resistant to
leaching out,
even in the presence of water. Woods preserved with borate and thermo-modified
in like
manner are highly useful in building outdoor structures exposed to rain and
humidity as
the preservative does not leach out and therefore the wood does not become
vulnerable
to attack from fungi or termites.
5
CA 02573211 2008-02-19
In one preferred embodiment of the invention the wood is first treated with a
wood
preservative to produce wood having a desired concentration of preservative.
The
wood is then vapor-dried to extremely low moisture content, e.g. close to 0%.
This
process of modification of the wood structure by heat changes the cellular
structure of
the wood and thus locks or fixes the preservative in the wood.
In another preferred embodiment, untreated wood is vapor-dried with a given
concentration of borate in the vapor. The preservative can be dissolved and or
introduced in an aqueous solution and vaporized (or sprayed) in a chamber
containing
the lignocellulosic material. Alternatively, an aqueous solution of the
preservative can be
pulverized in the aqueous vapor within the chamber to be mixed therewith. The
borate-
laden vapor treats and dries the wood, and the modification of the wood
structure by
heat (increased of the heat) causing the borate to become locked inside the
wood by
this process of thermo-treated wood.
In another embodiment, untreated wood can be dried to about 8-19% maximum
moisture content to drive out all free water (and perhaps some of the
intercellular water)
and then wood can be treated by vapor-drying it in the presence of a vaporized
water-
soluble preservative. In order to accelerate the penetration of borate into
the wood,
ethylene glycol or other such penetration accelerants can be added to the
vapor.
Ethylene glycol will enhance the rate of penetration and thus reduce the time
required
for the treatment process. Treatment of the wood with a preservative can be
accomplished using processes known in the art, such as but not limited to
vacuum/pressure treatment. For greater clarity, the preservative-treated wood
is dried to
a moisture content below that which is necessary to support diffusion of the
preservative
within the wood. In general, simple removal of "free" (extracellular) water is
not
6
CA 02573211 2008-02-19
sufficient to achieve this goal. One must also remove the intercellular water
and finally
the intracellular water in order to alter the cellular structure of the wood.
At the end of
the drying process, the overall moisture content of the preservative-treated
wood should
be less than 8-19% to avoid diffusion and, more preferably, between about 0
and
5%, to avoid leaching and thus locks or fixes the preservative into the wood,
which
will of course depend on the species of the wood being treated. The method is
particularly useful for anhydride and/or salt-based preservatives or any
preservatives
that would be susceptible to leaching out of the wood as a result of the
presence of
water.
In a preferred embodiment borate is used at concentrations sufficient to
destroy or
prevent the growth of certain wood-destroying fungi and insects such as
termites and
beetles.
In a preferred embodiment final concentrations of between 2 and 5 kg/m<sup>3</sup>
and
more preferably, between 2.7 and 4.5 kg/m<sup>3</sup> are used. Various forms of
borate
can be used, such as borate salts which may include but are not limited to
disodium
octaborate tetrahydrate.
The drying and/or the thermo-modification of the preservative-treated wood to
reduce the moisture content below the level necessary to support diffusion
and/or to
avoid leaching and thus locks or fixes the preservative into the wood, may be
achieved using various known drying and/or the thermo-modifying methods. These
methods may comprise, for example, steps in which the wood is dried using
water
vapor as a heat conductor and thermo-treated in an oxygen-depleted environment
(for example, by replacing oxygen with another gas such as nitrogen).
In a further embodiment, it will be appreciated that the wood may be treated
during the
7
CA 02573211 2012-03-01
(for example, by replacing oxygen with another gas such as nitrogen).
In a further embodiment, it will be appreciated that the wood may be treated
during the
drying process. For example, the preservative may be incorporated in the water
vapor
utilized to heat the wood. The heating process may also be coupled with known
processes for incorporating preservatives in wood such as but not limited to
vacuum/pressure treatment. The coupling may be accomplishing in such way that
the
drying and preservative incorporation take place substantially simultaneously.
It will also
be appreciated that treatment with the preservative may take place at some
intermediate step of the drying process. For example, the wood may be dried
until a
certain desired level of moisture is reached at which point the preservative
may be
incorporated in the wood prior to resuming the drying process. Our tests in
laboratory
demonstrate that in order to obtain a better penetration of the borate salts
in the wood
piece and humidity of the wood piece should be lower than 30% before
introducing
borates in the drying chamber.
In a preferred embodiment, the invention is carried out using the time-
temperature
profile described in U.S. Pat. No. 6,374,513, which describes a heat-treatment
chamber for drying wood. Treatment using borate preservative can also confer
fire-retardant properties to the treated wood.
In another embodiment of the invention there is also provided compositions
comprising
wood and one or more preservative and in which the moisture content is below
the level
supporting diffusion and/or to avoid leaching and thus locks or fixes the
preservative into
the wood such as can be obtained using the method described above. The wood
included in the composition may consist of but is not limited to structural
wood,
8
CA 02573211 2008-02-19
laminated wood, fibre wood panels, plywood and the like.
In view of the foregoing, it should be understood that the treatment of wood
with borate
or other water-soluble preservatives can be done in different ways. As borate
is the
preferred preservative, the process has been described with respect to borate,
although
it should be understood that other preservatives can be substituted.
The embodiments of the invention described above are intended to be exemplary
only
and should not be construed as delimiting the scope of the invention. The
scope of the
invention is defined solely by the appended claims.
9
