Note: Descriptions are shown in the official language in which they were submitted.
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TWO-PART BORATE, SILICATE AND ZINC COMPOSITIONS, AND METHOD S
FOR TREATING WOOD PRODUCTS
This application claims the benefit of U.S. Provisional Application No.
60/559,485,
filed on April 3, 2004, the entire contents of which are incorporated herein
by reference.
FIELD OF THE INVENTION
This invention relates to the preservation of wood and more particularly, the
invention
provides compositions and methods for treating wood and wood products to
provide leach-
resistant protection against insect and fungal attack, as well as resistance
to fire.
BACKGROUND OF THE INVENTION
Inorganic borate compounds have been used as wood preservatives for many years
for
protection against termites and other wood destroying insects, as well as
fungal decay.
Soluble borates such as borax, boric acid and disodium octaborate tetrahydrate
are well
known preservatives in aqueous-based systems for treating solid wood for use
in protected
environments. However, due to their water solubility they are readily leached
from treated
wood in exposed environments such as exterior and ground contact applications.
Copper chrome arsenate (CCA) is a leach-resistant wood preservative that has
been
used for many years to treat solid wood for use in exterior applications.
However, due to
environmental health and safety issues, and toxicity concerns relating to the
constituent
metals, particularly arsenic, CCA has come under increasing regulatory
pressure and is beirig
phased out of use in many areas. Even compositions containing copper witliout
chromium or
arsenic are coming into disfavor for environmental reasons, and thus it is
desirable to reduce
or eliminate copper content as well.
Solid zinc borate has proven very useful as a preservative for wood
composites, where
it is added as a solid material during manufacture of the composites. The
inherent low
solubility of zinc borate malces it resistant to leaching, even in high
moisture environments_
However, in view of its low solubility, it is not so easy to treat solid
lumber with zinc borate.
Dev et al. (J. Timb. Dev. Assoc., 1997) describes a two-stage process for
treating solid wood
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with zinc borate which involved impregnating the wood with solutions of borax
and zinc in
two separate steps.
Ammonia-based solutions have been proposed to solubilize metals such as zinc
and
copper in an attempt to fix borates in wood. U.S. Pat. No. 2,194,827 (Gordon)
discloses an
aqueous ammonia solution of copper, zinc and borate salts for the treatment of
wood.
U.S. Pat. No. 3,974,318 to Lilla discloses a method for fire retarding and
preserving
wood products, paper, cardboard, boxboard, cloth and other porous materials
having a
plurality of internal voids, in which a water soluble silicate composition is
applied to the
porous materials, penetrating into the voids, followed by drying the material.
Thereafter, a
water soluble metallic salt composition is applied, also penetrating into the
voids and reacting
in situ to form a water insoluble metallic silicate with a high degree of
water of hydration
disposed throughout the voids.
Sliiozawa (U.S. Pat. No. 5,478,598) discloses a wood preservative composition
that
includes a first solution having: a copper compound selected from the group
consisting of
copper borate, copper hydroxide, copper acetate, copper chloride, and copper
sulfate; a zinc
compound selected from the group consisting of zinc borate, zinc acetate, zinc
hydroxide,
zinc oxide, zinc chloride, and zinc sulfate; and/or a boron compound selected
from the group
consisting of boric acid and borax; sodium silicate, and a second solution
having rare earth
chloride or alkaline earth chloride. The inventor states that the inventive
composition can be
retained in the wood while the leaching thereof out of the wood is prevented.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a preservative system comprising:
(a) a first aqueous composition comprising a source of zinc selected from the
group
consisting of zinc oxide and soluble zinc salts; a source of borate selected
from the group
consisting of zinc borate, boric acid, boric oxide and water soluble borate
salts; a source of
ammonia; and water; wherein the composition comprises at least 50 wto/o water;
and
(b) a second aqueous composition, separate from the first composition,
coinprising an alkali
metal silicate and water.
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In another aspect, the invention provides a preservative system consisting of:
(a) a first aqueous composition consisting essentially of a source of zinc
selected from the
group consisting of zinc oxide and soluble zinc salts; a source of borate
selected from the
group consisting of zinc borate, boric acid, boric oxide and water soluble
borate salts; a
source of ammonia; and water; wherein the composition is essentially copper-
free and
comprises at least 50 wt% water; and
(b) a second aqueous composition, separate from the first composition,
consisting essentially
of an alkali metal silicate and water.
In still another aspect, the invention provides a method for preserving an
article
comprising wood fibers, the method comprising the steps of:
(a) applying to the article a first aqueous composition prepared by combining
ingredients
comprising:
i) a source of zinc selected from the group consisting of zinc oxide and
soluble zinc
salts;
ii) a source of borate selected from the group consisting of zinc borate,
boric acid,
boric oxide and water soluble borate salts;
iii) a source of ammonia; and
iv) water;
such that the first aqueous composition penetrates into the wood fibers,
wherein the
composition comprises at least 50 wt% of water;
(b) applying to the article a second aqueous composition comprising an alkali
metal silicate
and water; and
(c) drying the wood fibers;
such that there is deposited therein a bioeffective amount of a residual
component comprising
zinc, boron, and silicon.
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In yet another aspect, the invention provides an article comprising wood
fibers
comprising a residual component comprising zinc, boron, and silicon, prepared
by treating
the wood fibers according to the method set forth in the immediately preceding
paragraph.
In still anotlier aspect, the invention provides a method of treating a
substrate
comprising wood fibers to provide resistance to flame spread, the method
comprising the
steps of:
(a) applying to the article a first aqueous composition prepared by combining
ingredients
comprising:
i) a source of zinc selected from the group consisting of zinc oxide and
soluble zinc
salts;
ii) a source of borate selected from the group consisting of zinc borate,
boric acid,
boric oxide and water soluble borate salts;
iii) a source of ammonia; and
iv) water;
such that the first aqueous composition penetrates into the wood fibers,
wherein the
composition comprises at least 50 wt% of water;
(b) applying to the article a second aqueous composition comprising an alkali
metal silicate
and water; and
(c) drying the wood fibers;
such that there is deposited therein a flame retardant amount of a residual
component
comprising zinc, boron, and silicon.
DETAILED DESCRIPTION OF THE INVENTION
The invention provides a system and method for preservative treatment of items
comprising wood fibers. The method involves first treating the item with a
first aqueous
composition comprising ammonia-stabilized zinc and borate, followed by
treatment of the
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item with a second aqueous composition containing silicate. The preservative
compositions
are useful in the treatment of such items to provide borate leach-resistant
protection against
wood destroying organisms such as termites or other wood destroying insects,
and decay
fungi. They may also provide other benefits to items with which they are
treated, including
providing resistance to fire. The compositions may contain copper, or may be
essentially
copper free, by which it is meant that copper, if present at all, is present
only as an impurity
in the compositions of this invention, and is not purposely added. In any
case, compositions
that are "essentially copper free" contain less than 0.1% copper by weight.
As used herein, the term "residual component" refers to a material comprising
zinc,
boron, and silicon that remains in an article after being contacted with a
composition
according to the invention. It will be understood that the residual component
may vary in
composition according to the exact ratio and identity of the zinc, boron, and
silicate sources
used in the treatment compositions, as well as the amount and type of other
materials that
may be included in the compositions.
As used herein, the term "bioeffective amount" as applied to a residual
coinponent
means an amount of material sufficient to reduce or eliminate attack or
residence on a treated
article by one or both of an insect and a fungus that causes rot. Such
reduction or elimination
may be by any means, including but not limited to repelling, killing, and
prevention of
growth on or in the treated article.
As used herein, the term "flame retardant amount" as applied to a residual
component
means an amount of material sufficient to reduce or eliminate flame spread on
a treated
article.
As used herein, the term "essentially chloride ion free" means that none of
the
ingredients comprises chloride ion, other than as an impurity. In any case, a
composition that
is "essentially chloride free" contains less than 0.1 lo chlorine by weight.
As used herein, the term "sodium borate" means one or more of disodium
octaborate
tetrahydrate, sodium tetraborate decahydrate (borax), sodium tetraborate
pentahydrate,
sodium tetraborate (anhydrous borax), sodium metaborate, sodium pentaborate,
and mixtures
of any of these. The term "water soluble borate salt" means any sodium borate,
any
analogous potassium borate, any analogous ammonium borate, or mixtures of any
of these.
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References to amounts of ammonia or amines in a composition refer to the
amount of
that material calculated as its unbound form, although it will be understood
that equilibrium
processes may cause at least some of the compound to be in the form of a salt
or other
chemical species.
The preferred concentrations for the first aqueous composition are between
about 0.1
and 1 percent by weight boron (B), between about 0.2 and 2.5 percent by weight
zinc (Zn)
and between about 1 and 5 percent by weight ammonia (NH3). The preferred zinc
to boron
(Zn:B) mole ratio in the first aqueous composition is at least 0.4:1,
typically at least 1:1, and
more typically at least 1.5:1. The preferred ratio is at most 5:1, typically
at most 3:1, and
more typically at most 2:1. The preferred ammonia to zinc (NH3:Zn) mole ratio
in the first
aqueous composition is in the range of about 7:1 to about 33:1. The first and
second
compositions are aqueous mixtures, and are kept in separate containers and
applied
sequentially to the substrate being treated. It has been found that choice of
Zn:B and NH3:Zn
ratios within the ranges specified above, combined with the absolute
concentration ranges set
forth above, provides compositions having both good shelf life stability
against gelling and
precipitation and high resistance to leach-out of borate in items treated with
the compositions.
The first and second compositions typically each contain at least 50 wt%
water, but
compositions having a higher concentration of active ingredients and a less
than 50% water
content may be used according to the invention. Such compositions rnay, for
example, be
kept as concentrates and diluted as needed prior to application.
The second aqueous composition is prepared by dissolving a soluble silicate
compound in water. Suitable nonlimiting examples of soluble silicate compounds
include
alkali metal silicates and ammonium silicates. A liquid or water-soluble solid
form of silicate
may be used. Preferred silicates include the alkali metal silicates, e.g.
sodium or potassium
silicate, in liquid form. Preferably the alkali metal silicate is sodium
silicate, and more
preferably it is sodium silicate having an Si02:Na2O weight ratio greater than
3:1. Most
preferably the sodium silicate has a silicate to sodium (Si02:Na2O) weight
ratio of about
3.22:1. Such materials are commercially available under the trade names
NoSilicate and
N Clear, sold by the PQ Corporation of Valley forge, PA. The concentration of
the Si02 in
the second aqueous composition may be between about 1 and 10 wt% Si02,
according to the
invention. Typically, the concentration will be about 4 wt% Si02.
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Higher molar Zn/B ratios reduce leaching of borate from substrate, provided
that the
resulting compositions do not suffer significant precipitation or gelation.
Upper limits of
Zn/B may however be imposed by practical considerations regarding stability of
the
forrnulation, and/or by precipitation or gelling reactions that occur due to
the interaction of
high concentrations of zinc with other ingredients.
Zinc Sources
Suitable sources of zinc for use according to the invention may be provided in
the
forrn of various zinc compounds including zinc oxide, zinc chloride, zinc
acetate, zinc sulfate,
and other water-soluble zinc salts. Other zinc salts such as zinc naphthenate,
zinc
acetylacetonate, zinc gluconate, and zinc complexes with chelating agents such
as EDTA
may also be used according to the invention. Alternatively, zinc borate may be
used. In
some embodiments of the invention, zinc chloride is a preferred source of
zinc. In other
embodiments, for example where it is desired to reduce the chloride ion
content in the
composition for purposes such as prevention of corrosion, zinc acetate or zinc
sulfate may be
preferred. Zinc serves to reduce the tendency of borate to leach from the wood
upon
exposure to water after it has been treated, possibly by formation of a zinc
borate precipitate
which is not readily soluble in water. Zinc may also contribute to the
biocidal properties of
the preservative compositions of the present invention. Ammonia, such as in
the form of
amrnonium hydroxide (NH4OH), aids the dissolution of zinc in the first aqueous
composition.
Source of borate
Suitable source of borate for use according to the invention include boric
acid and the
water-soluble salts thereof. Alternatively, zinc borate may be used. Preferred
source of
borates include the sodium borates, such as disodium octaborate tetrahydrate
(commercially
available as TIM-BOR Industrial wood preservative manufactured by U.S. Borax
Inc.,
Valencia, CA), sodium tetraborate decahydrate (borax), sodium tetraborate
pentahydrate,
anhydrous sodium tetraborate, sodium metaborate and sodium pentaborate, as
well as otlier
alkali metal borates and ammonium borates such as potassium tetraborate,
potassium
metaborate and ammonium pentaborate. Boric acid and boron oxide may also be
used.
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Preparation of First Aqueous Composition
The first aqueous composition of the present invention is preferably prepared
by
dissolving a zinc compound in an aqueous ammonia solution, followed by the
addition of a
source of borate and agitating until essentially all solids are dissolved. The
source of borate
is preferably pre-dissolved in water prior to adding to the ammonia stabilized
zinc solution.
Vigorous mixing is recommended to promote the rapid dissolution of zinc and
borate solid
compounds in solution.
It has been found that mere dissolution of zinc borate in ammonia provides
preservative compositions with poor borate leach performance; that is, wood
treated with
such compositions loses borate content when contacted with water over an
extended period of
time, as measured by te:st method AWPA E11-97. In contrast, wood that has been
treated
with compositions prepared according to the invention show notably lower
borate leach rates,
and therefore may be expected to retain their preservative activity for a
longer time.
Source of Ammonia
Suitable sources of ammonia for use according to the invention may include, as
nonlimiting examples, aqueous ammonium hydroxide and anhydrous ammonia.
Wood Fibers
Wood fibers according to the invention may be fibers in a piece of wood, or
fibers
freed from wood by a pulping operation such as is commonly performed in the
pulp and
paper industry, i.e. wood pulp. As used herein, the term "wood" is to be
understood
according to its common use, and includes wood pieces or particles of any size
or shape,
including for example sawn lumber, plywood, oriented strand board, particle
board, ground
wood, sawdust, and wood/polymer composite materials. The temi "wood" according
to this
use therefore refers to wood that has not been subjected to a pulping
operation.
As used herein, -the term "wood pulp" refers to wood that has been subjected
to a
pulping operation, including but not limited to Kraft pulping, sulfite
pulping, and cherni-
thermomechanical pulping. Wood pulp treated according to the invention may be
in any
form, including but not limited to unconsolidated (loose) pulp fibers,
including for example
blown insulation, and paper. Paper that comprises wood pulp treated according
to the
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invention may be paper in any fomi, including but not limited to sheet paper,
corrugated
board, and paper coniprising a surface of gypsum wallboard.
Application of the Preservative Compositions
The preservative compositions may be applied to the item to be treated by any,
conunercially acceptable method, as long as sufficient composition penetrates
into the item
so as to result in the deposition of a bioeffective or fire retardant amount
of a residual
component. It should be noted that the residual component, which is the
material that is
active for deterrence of biological attack or attack by fire, may comprise
zinc, boron, and/or
silicon in the form of the compounds that were used to prepare the
compositions. They may
however represent the result of subsequent chemical reactions in the treated
substrate. One
possible nonlimiting example is formation of zinc borate in the treated
article, but other
chemical reactions may occur in addition or instead, or none may occur at all.
Similarly,
ammonia may be chemically bound in the treated item, or it may be essentially
absent due to
other chemical reactions or to volatilization out of the item. Regardless of
the exact form and
location of the zinc, boron, ammonia, and silicon after the treatment is
complete, there
remains a residual component that provides the preservative properties of the
invention.
Resistance to insects, wood-decay fungi, and/or fire is thereby achieved.
The composition containing zinc and borate is applied first to the item to be
treated.
After completing the treatment with the zinc/borate solution, the item is
treated with the
silicate composition. The relative amounts of first and second aqueous
compositions applied
to the item are such that there is sufficient Si content to immobilize
substantially the
components of the treatment system in the substrate. In one exemplary
embodiment of the
invention, the molar ratio of Si/Zn is about 0.6:1, althougli higher or lower
ratios may be used.
In general, higher levels of silicate appear to improve immobilization of the
components.
For wood products, application of the first and/or second compositions may
involve a
method such as vacuum and/or pressure treatment or dip treatment under
atmospheric
pressure. Preferably, treatment may involve both vacuum and pressure, wherein
a vacuum is
first applied to the wood product, prior to application of the first and/or
second aqueous
composition. The solution is then applied to the wood product and pressure is
then applied to
force the solution into the pores of the wood. Preferably, after the wood
product has been
treated, the treated wood may be dried to improve the leach resistant
properties of the wood.
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Drying may be performed at an elevated temperature, preferably no higher than
90 C, even
more preferably no higher than 70 C, with about 60 C being typical. It has
been found that
the use of lower drying temperatures for a given amount of drying time tends
to reduce the
borate leach rate of items treated with these compositions. Thus in some
embodiments of the
invention, drying is performed under ambient temperatures, typically between
about 20 C
and 25 C, optionally aided by the use of vacuum or blown air. Methods for
drying wood, and
the desired moisture level of dried wood, are well known in the art.
For applications in which resistance to biological attack is the desired
result, it is
believed that the amount of borate in the treated substrate should be at least
0.1 %, measured
as boric acid equivalents (BAE). A level of at least 0.5% will typically be
used. In general,
increased BAE provides increased resistance to biological activity, as well as
to fire.
Methods for applying the compositions include spraying, roll coating, dipping,
and any other
means known in the art relevant to the particular form of the wood or wood
pulp.
EXAMPLES
Example 1
A series of five ammonia-stabilized zinc and borate-containing solutions, each
having
a zinc to boron (Zn:B) mole ratio of 0.4:1, were prepared according to the
methods of the
invention, as examples of the first aqueous composition. The solution
concentrations for the
five solutions were designed -to provide target retentions of about 0.13 Jo,
0.25%, 0.50%,
0.75% and 1.5% B203 in the -wood after treatment. The amounts of each
ingredient used in
each of the five solutions, and the resulting concentrations of boron, zinc,
and ammonia, are
summarized in Table I. The method of preparation is described below, using the
target
0.75% B203 solution as an example.
Ammonium hydroxide (69 mL of a 30% NH4OH solution) was stirred into 200.00 g
of deionized water. Zinc chloride (10.36 g ZnC12) was added to the water-
ammonia solution
and the mixture was vigorously stirred until the zinc chloride was completely
dissolved. In a
separate container, 9.80 g of TIM-BOR disodium octaborate tetrahydrate
(manufactured by
U.S. Borax Inc.) was dissolved in 110.89 g deionized water. The TIM-BOR
solution was
then poured slowly with agita.tion into the zinc/ammonia solution, providing
an essentially
clear solution containing 0.51 % B, 1.24% Zn, and 2.51 % NH3.
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A series of five solutions of sodium silicate was prepared as follows, each
paired with
one of the ammonia stabilized zinc and borate compositions described above. In
a separate
container, the amount of NoSilicate (manufactured by PQ Corporation, Valley
forge, PA)
indicated in Table I was added to the indicated amount of deionized water and
stirred until
the silicate was completely dispersed. The resulting solution was the second
aqueous
composition.
Table I
Comp. Target 1oB20
No. in Wood 4 0.13 0.25 0.50 0.75 1.50
1 H20 (g) 227.51 225.82 222.44 200.00 121.50
1 30 lo NH4OH (mL) 50 50 50 69 137
1 ZnC12 (g) 1.80 3.49 6.87 10.36 20.62
1 H20 (g) 118.99 117.39 114.19 110.89 101.19
1 TIM-BOR (g) 1.70 3.30 6.50 9.80 19.50
2 H20 (g) 280.89 279.29 276.10 272.80 263.11
2 N-Silicate (g) 1.70 3.30 6.49 9.79 19.48
1 Wt. fo Boron (B) 0.09 0.17 0.34 0.51 1.02
1 Wt.% Zinc (Zn) 0.22 0.42 0.82 1.24 2.47
1 Wt.% Ammonia (NH3) 1.82 1.82 1.82 2.51 4.99
2 Wt.% Silicate (Si02) 0.17 0.34 0.66 0.99 1.98
Example 2
An ammonia-stabilized zinc and borate-containing solution each having a zinc
to
boron (Zn:B) mole ratio of 1:1, was prepared according to the methods of the
invention, as an
example of the first aqueous composition. The method of preparation was as
follows.
Ammonium hydroxide (56 mL of a 30% NH4OH solution) was stirred into 200g of
deionized water. Zinc chloride (17.64g ZnC12) was added to the water-ammonia
solution and
the mixture was vigorously stirred until the zinc chloride was completely
dissolved. In a
separate container, 6.68g of TIM-BOR disodium octaborate tetrahydrate
(manufactured by
U.S. Borax Inc.) was dissolved in 120g of deionized water. The TIM-BOR
solution was
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then poured slowly with agitation in_to the zinc/ammonia solution, providing
an essentially
clear solution.
An acidified solution of sodium silicate was prepared as follows. In a
separate
container, a solution of 40g of N Silicate in 119g of deionized water was
added with mixing
to a solution of 5.7g of concentrated sulfuric acid in 119g of deionized
water. The resulting
solution was the second aqueous cornposition.
The first aqueous composition was used to treat 3/4" x 3/4" southern pine wood
blocks
according to AWPA method El 1-97. The wet, treated wood blocks were then
soaked in the
second aqueous composition for 5-10 minutes until a gel was visible on the
surface of the
wood blocks. Then, the wood bloclks were removed from the composition and
dried
overnight (about 16 hours) at 60 C. The wood blocks were then leached
according to AWPA
method E11-97 to produce the following results:
Boron Loading in Boron Retention in % Boron
Wood, %BAE Wood, %BAE Retention
2.18 0.51 23.4
The preservative solutions of this invention are suitable for treating wood
products to
provide leach-resistant protection against biological attack from a variety of
wood-destroying
organisms, including insects and fungal decay. Various changes and
modifications of the
invention can be made, and to the eKtent that such changes and modifications
incorporate the
spirit of this invention, they are intended to be included within the scope of
the appended
claims.
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