Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
[0001] Compositions and Methods for Inhibiting Tannin Release
CROSS-REFERENCES TO RELATED APPLICATIONS
[0002] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED
RESEARCH OR DEVELOPMENT
[0003] Not applicable.
REFERENCE TO A SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM
LISTING APPENDIX SUBMITTED ON A COMPACT DISC
[0004] Not applicable.
BACKGROUND OF THE INVENTION
[0005] The invention relates generally to the field of reducing tanun staining
of
cellulose-containing materials. Tannins are a group of polyphenolic compounds
that occur in
the vascular tissues of terrestrial plants.
[0006] Synthetic lumber has been used as a substitute for wood in areas where
wood can
deteriorate quickly due to environmental conditions. Although in the past, its
commercialization was limited by costs, modern recycling techniques and low
cost extrusion
manufacturing capability have permitted greater penetration by polymer-fiber
composite
materials into the commercial and residential markets. One such product
manufactured under
the trademark TREX, by Trex Company, LLC, Winchester, Va., consists of a
polyethylene-
wood fiber blend which is extruded into board dimensions for decking
applications.
Polyethylene-wood composite boards in 5/4 inch thicknesses have sufficient
rigidity to be
used as declcing planks, but typically are not recommended for structural wood
substitutes,
such as the lattice structure often used as a support for decks.
[0007] Many types of synthetic lumber comprise cellulose fibers derived from
terrestrial
plants. For examples, synthetic lumber products comprising wood flour or paper
fibers are
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known. The cellulosic materials can contain tannins. Natural wood products
also contain
tannins. Under certain environmental conditions, tannins that are present in
the cellulosic
materials of natural or synthetic lumber products can leach to the surface of
the product and
be deposited there. Such deposition is considered unsightly and undesirable
for maaiy
applications, particularly where the appearance of the lumber product is
important. This
problem is recognized in the art, and is referred to as tannin staining.
[0008] Prior art methods of iuubiting tannin staining involve sealing a lumber
product in
such a way that water cannot reach the cellulosic materials of the product. If
water does not
contact the cellulosic materials, leaching of tannins therefrom and deposition
on the product
surface are prevented. Numerous primer, paint, and sealing products are
commercially
available. Such products are suitable for protecting lumber products from rain
and humidity,
but can crack, peel, or degrade over time. Application of such products to
lumber products
can also inhibit application of other desired products, such as finishing
paints or stains.
[0009] Growth of molds and fungi ("microbial growth") on and in lumber
products can
detract from their appearance, their strength, and their ability to bind
appropriately with
fmislung products such as paints, primers, stains, and sealers. Microbial
growth can occur
under conditions in which lumber products are ordinarily stored. Furthermore,
natural wood
products and some synthetic lumber products contain compounds that can be
metabolized by
microbes, further enhancing the rate and extent of microbial growth on those
products. For
example, calciwn carbonate is sometimes used as a filler in synthetic hunber
products, but
can be metabolized by many molds. For this reason, mold growth on calcium
carbonate-
containing synthetic lumber products can be a particular problem.
BRIEF SUMMARY OF THE INVENTION
[0010] The invention relates to a method of treating a substrate that
comprises cellulose
and a tannin. The method comprises contacting a surface of the substrate with
an aqueous
composition comprising an agent that binds covalently with the tannin. The pH
of the
composition should not be greater than 9.0, nor less than 3Ø Preferably, the
pH of the
composition is in the range 4.0 to 8.5, 5.0 to 8.0, or 6.0 to 7.5. A preferred
pH range is 6.5 to
7.2. Compositions having a pH of substantially 7 are suitable.
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[0011] In one embodiment of tlus method, the antimicrobial agent is a compound
that has
the chemical formula X - L - Tbm. In this formula, X is a chemical moiety that
exhibits
antimicrobial activity, L is an optional linking moiety, and Tbm is a tannin-
binding moiety.
For example, X can a quaternary ammonium moiety, such as one having the
chemical
formula ((R1)3-N+)-. In this formula, each Rl can be independently selected
from the group
consisting of -H and branched and straight-chain C1 to C24 alkyl radicals. L
can, for
example, be a branched or straight-chain C 1 to C24 alkyl diradical,
optionally having one or
more -O- or -N- diradicals in its backbone. Tbm can be selected from the group
consisting of
a silane moiety, and an alkoxysilyl moiety, although it can also be other
tannin-binding
moieties.
[0012] In another embodiment of the methods described herein, the
antimicrobial agent is
selected from the group consisting of an
octadecylaminodimethyltrihydroxysilylpropyl
ammonium salt and an octadecylaminodimethyltrimethoxysilylpropyl ammonium
salt.
[0013] The composition used in the methods described herein can comprise the
conjugate
base of an organic acid. Suitable organic acids include dicarboxylic acids
such as oxalic acid,
malonic acid, succinic acid, fumaric acid, glutaric acid, adipic acid, and
malefic acid: Owing
to its known utility in certain applications, oxalic acid is suitable for
decking applications, for
example.
[0014] The methods described herein are useful for treating a variety of
substrates.
Examples of suitable substrates include polymer - cellulose composites such as
those
comprising polyvinylchloride. Wood and other exterior building materials are
also suitable
substrates. The methods are useful for a variety of purposes, such as treating
planks and
other decking materials.
BRIEF SUMMARY OF THE SEVERAL VIEWS OF THE DRAWINGS
[0015] . Not applicable.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The invention relates to the discovery that tannin staining of lumber
products can
be reduced during application of water or aqueous compositions by avoiding
extremes of pH
in the water or composition during its application. Furthermore, prolonged
inhibition or
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prevention of tannin staining and prevention of microbial growth can be
achieved with
lumber products by treating such a product with a composition comprising an
aqueous
composition of an antimicrobial agent that covalently binds with a tannin in
the product at a
relatively neutral pH.
[0017] Definitions
[0018] An antimicrobial agent is a compound that inhibits or prevents growth
or
replication of a mold or fungus or that kills a mold or fungus.
[0019] Lumber products include natural wood products (e.g., whole cut wood),
manufactured wood products (e.g., plywoods, chip boards, and particle boards),
and polymer-
cellulose fiber composites (e.g., composites of polyvinylchloride or
polyethylene with wood
flour or paper fiber).
[0020] Detailed Description
1 S [0021] It has been discovered that water-based leaching of tannins from a
natural or
synthetic lumber product can be reduced, inhibited, or prevented by
maintaining the pH of
aqueous compositions that contact the lumber product relatively neutral.
Advantageously,
both microbial growth and tannin staining can be inhibited by treating a
lumber product with
a tannin-binding antimicrobial agent at a relatively neutral pH.
[0022] pH
[0023] It has been discovered that the pH of compositions with which
substrates that
comprise both cellulose and one or more tannins are treated is important for
determining the
rate, extent, or both, of subsequent tannin release from the composition.
Tannins have been
discovered to be more soluble in very alkaline aqueous systems than in less
aqueous systems.
For this reason, the pH of any aqueous solution used to treat a lumber product
should be
maintained not greater than about 9.0, and preferably not greater than 8.5,
8.0, or 7.5.
Tannins have also been discovered to be more soluble in very acidic aqueous
systems than in
more neutral systems. For this reason the pH of any aqueous solution used to
treat a lumber
product should be maintained not less than 3.0, and preferably not less than
4.0, 5.0, or 6Ø
An optimal pH range for aqueous solutions used to treat lumber products is 6.5
to 7.2. Other
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suitable ranges of pH include 6.0 to 7.5, 5.0 to 8.0, and 4.0 to 8.5, although
the more neutral
pH ranges are preferable. In one embodiment, the pH of any aqueous solution
used to treat a
lumber product is maintained at about 7Ø
[0024] The way in which the pH of any aqueous solution with which a lumber
product is
treated is not critical. A simple way of adjusting the pH of such a solution
is to add an acid or
base to the solution. The identity of the acid or base is not critical. Simple
inorganic acids
and bases (e.g., HCl and NaOH) can be appropriate. However, certain acids or
bases or types
of acids or bases can be preferred for substrates to be used in various
applications. When
human contact with the treated substrate is anticipated, the acid or base
should be selected so
as to minimize toxicity and irritation towards humans and to improve
appearance. For
example, if a synthetic lumber product is to be treated as described herein
and thereafter used
in a residential decking application, and if the treatment involves contacting
the product with
an antimicrobial composition that is ordinarily very alkaline, then the pH of
the antimicrobial
composition can be made more nearly neutral by adding oxalic acid (a compound
known to
be acceptable in residential decking applications) thereto.
[0025] Tannin-Binding Agents
[0026] Inhibition of tannin release from a substrate that comprises cellulose
and the
tannin can be achieved by increasing the molecular weight of the tannin, by
increasing the
hydrophobicity of the tannin, or both. The molecular weight of a tannin can be
increased by
attaching a moiety to the tannin (e.g., attaching a silane or siloxane
compound thereto) or by
crosslinking the tannin with another tannin or with another component of the
substrate (e.g.~
with cellulose in the substrate). Regardless of the reagent used to derivatize
or cross-link the
tannin, the reagent should be used in an aqueous formulation having a
relatively neutral pH as
described herein.
[0027] The important characteristics of the tannin-binding agents used as
described
herein depend on the end use of the substrate to be treated therewith. Many
lumber products
are used in construction applications in which fire resistance and relative
non-toxicity upon
incineration are important characteristics. In such applications, tannin-
binding agents can be
selected to maximize fire resistance and to minimize toxicity upon
incineration. In
applications in which human contact with the treated substrate is anticipated
(e.g., use of
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treated synthetic lumber products in residential decks, rails, and siding),
the tannin-binding
agent should be selected so that the treated product exhibits low toxicity and
does not exhibit
undesirable side effects (e.g., pain, burning, or discoloration) with animal
tissues.
[0028] Numerous silane and siloxane compounds are known in the art, and
substantially
any of these can be used to bind with or cross-link a tannin. Examples of such
compounds
include silanes, alkoxysilanes, chlorosilanes, and other derivatized silanes
offered for sale by
companies such as Dow Corning and others. Silane and siloxane compounds can be
used
alone or in conjunction with other compounds.
[0029] Diacids and diacid anhydrides can be used to cross-link tannins, as
described in
U.S. Patent Application publication number 20030004232. Examples of suitable
diacids
include oxalic acid, malonic acid, succinic acid, fumaric acid, glutaric acid,
adipic acid, and
malefic acid. Oxalic acid is well lmown for use in cleaning and brightening
residential lumber
products, and its relative non-toxicity and suitability for residential
applications recommend
its use for substrates with which human contact is anticipated.
[0030] A preferred tannin-binding agent is an anti-microbial agent that binds
covalently
with one or more taniuns in the substrate. Such,an agent both increases the
molecular weight
of the tannin (i.e., thereby decreasing its proclivity to leach from the
substrate) and inhibits
microbial growth on the substrate, within the substrate, or both. Such an
agent will often be a
molecule wherein one part of the molecule exhibits antimicrobial activity and
a different part
of the molecule is capable of binding covalently with the tannins) in the
substrate. The two
parts of the molecule can be separated by a linker. If one or more of the
linker, the
antimicrobial part, or the tannin-binding part of the molecule exlubits
significant
hydrophobicity, then the hydrophobic nature of the agent can further inhibit
tannin release
from the substrate.
[0031] In one embodiment, the tannin-binding agent has the following
structure.
[0032] X - L - Tbm
[0033] In this structure, X is a chemical moiety that exhibits antimicrobial
activity, L is
an optional linking moiety, and Tbm is a tannin-binding moiety.
[0034] Many chemical moieties that exhibit antimicrobial activity are known,
and X can
be substantially any of them. Preferably, X does not exhibit significant
adverse effects when
external human tissues contact it. Suitable X moieties include quaternary
ammonium
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moieties such as those having the structure ((R1)3-N+)-, wherein each R1 is
independently
selected from the group consisting of -H and branched and straight-chain C 1
to C24 alkyl
radicals. In one embodiment, at least one of the R1 moieties is a C 12 to C24
alkyl radical,
such as a C 1 g alkyl radical (i.e., a CH3(CH2) 17- radical).
[0035] The R1 moieties can, optionally, be substituted with one or more
hydroxyl or
primary, secondary, or tertiary amine groups, and the backbone of the radicals
can have one
or more -O- or -N- diradicals therein. Furthermore, the charged nitrogen atom
can be part of
a ring system, preferably an unsaturated ring system. The antimicrobial
properties of
quaternary ammonium salts are known in the art, and substantially any such
salt can be
adapted for use herein simply by connecting it with the linker (if present)
and the tannin-
binding moiety. The identity counter ion, if any, used with the quaternary
ammonium moiety
is not critical. Standard counter ions (e.g., chloride or acetate) can be
used.
[0036] When the linking moiety is present, its identity is not critical. The X
moiety can
be linked directly with the Tbm moiety. Alternatively, a short (e.g., C1 to
C6), long (e.g.,
C12 to C24), or intermediate (e.g., C6 to C12) alkyl radical can be interposed
between X and
Tbm. The linker can be flexible (e.g., a straight chain alkyl radical),
relatively inflexible
(e.g., a hydrocarbon backbone having one or more unsaturated bonds), or rigid
(e.g., an
unsaturated ring system).
[0037] The identity of the Tbm moiety is not critical, so long as the moiety
is able to~ bind
covalently with one or more functional group present in a tannin of the
substrate to be treated.
Suitable examples of Tbm moieties include silane and siloxane moieties and
carboxylic acid
moieties. Silane moieties occur in many known wood-treatment products, and the
Tbm
moiety can replicate the silane moiety of substantially any of those products.
Silane moieties
having one or more alkoxy substituents (i.e., alkoxysilyl moieties) are also
suitable.
[0038] The tannin-binding agents described herein are either commercially
available or
can be synthesized by ordinary chemical synthetic methods based on known
compounds.
[0039] Treated Substrate
[0040] The compositions and methods described herein
[0041] Tannins occur in many sources of cellulose fibers , including recycled
paper
products, such as agriculturally-derived fibers, pulp, newsprint, soft woods,
such as pine, or
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hard woods from deciduous trees. Hard woods are generally preferred for
incorporation into
synthetic lumber products because they absorb less moisture. Although hard
wood is the
primary source of fiber for synthetic lumber products, additional fiber make-
up can be
derived from a number of secondary sources including soft wood fibers, natural
fibers
including bamboo, rice, sugar cane, and recycled or reclaimed fiber from
newspapers,
cardboard boxes, computer printouts, etc. The synthetic lumber products can
also be made
using wood flour of about 10-100 mesh, preferably 20-30 mesh.
[0042] The observations made herein relating to the pH of aqueous compositions
used to
treat cellulose-containing substrates are not limited to lumber products. The
observations are
equally applicable to aqueous compositions contacted with substantially any
material that
contains both cellulose and one or more tannins. By way of examples, such
materials include
finished wood products, paper products, leather products, food products, and
the like.
[0043] Treatment of Substrates
[0044] Many known methods of treating wood or another cellulose- and tannin-
containing substrate result in tannin staining, either upon performance of the
method or
thereafter upon subsequent wetting of the treated substrate. It has been
discovered that taimin
staining that occurs during and after such treatments can be reduced if the
treatment is
normally performed at a relatively extreme pH. Release of tannin from the
substrate is
reduced by making the pH of any aqueous reagents) used in the treatment more
neutral. The
pH of the reagents) is made more neutral by adding acid (to reduce pH) or base
(to increase
pH) to the reagents) prior to application of the reagents) to the substrate.
Suitable pH
values and suitable pH-adjusting agents are described herein.
[0045] The method by which a pH-adjusted reagent is applied to the substrate
is not
important. Reagents can be applied using and known method or equipment for
applying
liquids to solid substrates. For example, brushes, rollers, spray apparatus,
foggers, dunking
or dipping tanks, and the like can be used to apply a liquid to a substrate.
[0046] Foamed Polymer-Fiber Composite Lumber
[0047] In an important embodiment, the substrate that is treated as described
herein is a
foamed polymer-fiber composite, such as one described in U.S. Patent No.
6,344,26. Such
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composites generally contain about 35-75 wt. % resinous materials, such as
thermoplastic and
thermosetting resins. Examples of suitable materials include PVC,
polyethylene,
polypropylene, nylon, polyesters, polysulfones, polyphenylene oxide and
sulfide, epoxies,
cellulosics, and the like. A preferred thermoplastic material for the is PVC.
[0048] PVC thermoplastics comprise the largest volume of thermoplastic
polymers in
commercial use. PVC used in synthetic lumber products can be combined with
impact
modifiers, thermal stabilizers, lubricants, plasticizers, organic and
inorganic pigments, fillers,
biocides, processing aids, flame retardants, or other commonly available
additive materials,
as is known. PVC can also be combined with other vinyl monomers in the
manufacture of
polyvinyl chloride copolymers. Such copolymers can be linear copolymers, graft
copolymers,
random copolymers, regular repeating copolymers, block copolymers, or the
like. Examples
of monomers that call be combined with vinyl chloride to form vinyl chloride
copolymers
include acrylonitrile; alpha-olefins such as ethylene and propylene;
chlorinated monomers
such as vinylidene dichloride; acrylate monomers such as acrylic acid,
methylacrylate,
methyl-methacrylate, acrylamide, and hydroxethyl acrylate; styrenic monomers
such as
styrene, alpha methyl styrene, and vinyl toluene; vinyl acetate; and other
commonly available
ethylenically unsaturated monomer compositions. Such monomers can be used in
an amount
of up to about 50 mole%, the balance being vinyl chloride. PVCs can be
compounded to~be
flexible or rigid, tough or strong, to have high or low density, or to have
any of a wide
spectrum of physical properties or processing characteristics, as is known in
the art. PVC
resins can also be alloyed with other polymers, such as ABS, acrylic,
polyurethane, and
nitrile rubber to improve impact resistance, tear strength, resilience, or
proccessability. They
can be produced water-white in either rigid or flexible compositions, or they
can be
pigmented to almost any color.
[0049] In the preferred embodiments of this invention, rigid PVC, optionally
containing a
small amount of plasticizer, is employed. This material is a hard and tough
and can be
compounded to have a wide range of properties, including impact resistance and
weatherability, e.g., fading color to a wood grey appearance. It also has a
tensile strength of
about 6,000-7,500 psi, a percent elongation of about 40-80%, and a tensile
modulus of about
3.5-6.0 x 106 psi. It can be acceptably used without chlorination, to about
140°F, and with
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chlorination to about 220°F. It also has a coefficient of thermal
expansion of about 3-6 x 10-
inch/inch-°F.
[0050] PVC composite building materials of this invention can be injection or
vacuum
molded, extruded and drawn, using customary manufacturing techniques for
thermoplastic
5 and thermosetting materials. In one embodiment, a mixture of PVC regrind or
virgin
compound is compounded and then heated and extruded through a die to produce
boards and
other shapes having a length of about 4-20 feet and thicknesses ranging from
0.05-6.0 inches.
The extruded thermoplastic boards can be subject to further molding,
calendaring, and
finishing to provide a wood grain or fanciful texture.
[0051] Synthetic lumber products can contain cellulose fiber, such as fiber
derived from
wood (e.g., red oak powder) or cotton. When present, the cellulosic material
preferably
comprises about 25-60 wt. % of the final lumber product. The methods described
herein for
inhibiting tannin release from the cellulosic material are particularly
applicable when
materials having a relatively high tannin content are used. Many woods (e.g.,
cedar,
redwood) are known to have a relatively high tannin content, wlule cotton
products generally
contain much less tannins. Additionally, inorganic fillers, such as calcium
carbonate, talc,
silica, etc. can be incorporated into the synthetic lumber product.
[0052] Foamed polymer-fiber composite synthetic lumber products are made by
combining a resin (e.g., PVC) and a cellulosic material (e.g., wood flour)
with a chemical
blowing agent, or by introducing a gaseous medium into a molten mixture of the
resin and
cellulosic material to produce a series of trapped bubbles prior to thermo-
forming the
mixture, for example, by molding, extrusion or co-extrusion. Such processes
for making rigid
foam articles are generally known.
[0053] Chemical blowing agents can be any of a variety of chemicals which
release a gas
upon thermal decomposition. Chemical blowing agents may also be referred to as
foaming
agents. The blowing agent, or agents, if more than one is used, can be
selected from
chemicals containing decomposable groups such as azo, N-nitroso, carboxylate,
carbonate,
hetero-cyclic nitrogen-containing and sulfonyl hydrazide groups. Generally,
they are solid
materials that liberate gas when heated by means of a chemical reaction or
upon
decomposition. Representative compounds include azodicarbonamide,
bicarbonates,
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dinitrosopentamethylene tetramethylene tetramine, p,p'-oxy-bis
(benzenesulfonyl)-hydrazide,
benzene-1,3-disulfonyl hydrazide, aso-bis-(isobutyronitrile), biuret, and
urea.
[0054] The blowing agent can be added to the polymer in several different ways
which
are known to those skilled in the art, for example, by adding the solid power,
liquid or
gaseous agents directly to the resin in the extruder while the resin is in the
molten state to
obtain uniform dispersion of the agent in the molten plastic. Preferably the
blowing agent is
added before the extrusion process and is in the form of a solid. The
temperature and
pressure to which the foamable composition of the invention are subjected to
provide a
foamed composition will vary within a wide range, depending upon the amount
and type of
the foaming agent, resin, and cellulosic fiber that is used. Preferred foaming
agents are
selected from endothermic and exothermic varieties, such as
dinitrosopentamethylene
tetramine, p-toluene sulfonyl semicarbazide, 5-phenyltetrazole, calcium
oxalate, trihydrazino-
s-triazine, 5-phenyl-3,6-dihydro-1,3,4-oxandiazin-2-one, 3,6-dihydro 5,6-
diphenyl-1,3,4
oxadiazin-2-one, azodicarbonamide, sodium bicarbonate, and mixtures thereof.
[0055] 111 addition to the above, a coloring agent can be added to the
compounded
mixture, such as dyes, colored pigments, or fly ash, or a mixture of these
ingredients
depending on the resulting color, and cost considerations. Such additives can
provide
"weatherability" or a faded grayish coloring or a permanent tint, such as
blue, green or
brown. This invention can be fuxther understood by reference to the following
examples.
[0056] After a synthetic lumber product is formed, it can be treated using a
composition
described herein to inhibit microbial growth thereon and to prevent or inhibit
tannin staining
of the product if it is exposed to moisture.
[0057] The disclosure of every patent, patent application, and publication
cited herein is
hereby incorporated herein by reference in its entirety.
[0058] While this invention has been disclosed with reference to specific
embodiments, it
is apparent that other embodiments and variations of this invention can be
devised by others
skilled in the art without departing from the true spirit and scope of the
invention. The
appended claims include all such embodiments and equivalent variations.
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