WO1998045053A1 - Stain inhibiting wood sealer - Google Patents

Abstract

A process of treating wood to reduce the tannin staining of coatings subsequently applied includes the steps of providing a solution of a zirconyl compound in a carrier liquid such as water, applying the solution to a wood surface, and, drying the solution. The pore structure of the wood is modified or sealed so that staining of coating compositions applied subsequently over the surface is reduced. The process is particularly beneficial in cases where the coating composition is a clear or a light-colored latex paint, especially white. The preferred zirconyl compound is zirconyl acetate. The treating solution may be modified by addition thereto of a UV light protective compound and/or fungicide.

Description

STAIN INHIBITING WOOD SEALER

RELATED APPLICATION:

This application is a continuation-in- part of application Serial No. 08/833,867 filed on April 10, 1997.

BACKGROUND OF THE INVENTION:

This invention relates to compositions and processes for modifying wood surfaces, particularly to reduce tannin staining of coatings subsequently applied thereto. In a more general sense, the composition and processes are also useful for coating a variety of substrates to prevent coatings subsequently applied thereto from being stained by colored materials on or in the substrate.

Tannin staining, an undesirable spontaneous phenomenon, is a well known problem in conjunction with aqueous white coatings or aqueous clear coats, especially when if applied on hardwood species, for example, oak, of high water soluble tannin contents, which are characterized by high staining potential. Other examples are redwood and red cedar in connection with which dark brown discoloration of coatings or clear coats is observed. Such discoloration develops cumulatively, causes aesthetic degradation and limits the service life of the coatings.

Notably, tannin staining of solvent based opaque coatings or clear coats is less prevalent, even on wood substrates of high staining potential. However, under federal and state legislative pressure, intended to minimize VOC emissions, significant R&D efforts of the paint and coating industry are currently invested into development of high performance water-based wood coatings. Enhancement of the tannin stain inhibitive capacity of such coatings is one of the challenges of the related development work.

It is informative to refer briefly to some elements of wood chemistry, the mechanism of the tannin staining process and to the available preventive technologies. Wood consists mainly of cellulosic and ligninous materials. Diverse wood species contain also variable amounts of extractables, some of which are water soluble, colored and consequently cause staining. It is well known, however, that not all water soluble colored extractables are tannins and not all tannins are water soluble. The content of such extractables varies widely among diverse wood species, the quantitative distribution of the same being variable even among distinct anatomical regions of the same tree. The chemistry of colored extractable species is typically complex and not always well known.

In addition to tannins, of which redwood, for example, contains approximately 4-12%, other water soluble, colored organic products of complex structure, such as quinones, flavonoids and tropolones are also present in various wood species. It is interesting to observe that thujic acid, an intensely colored member of the tropolone family, is present in western red cedar, a widely used, but highly staining wood species. Wood is also a highly porous material, characterized by a remarkable high average value of capillary specific surface area of about 2000 cm²/g. The cellular walls, and to limited extent, the cellular cavities (lumens) are microscopic storage sites for soluble materials, concentrated predominantly in heartwood, the physiologically inactive anatomical region of living trees. Wood is hygroscopic, as well, having the capacity to absorb water vapor up to approximately 30% by weight, the fiber saturation point achievable at

100% relative humidity. Wood also desorbs moisture in a dynamic equilibrium with the moisture content of the surrounding air. The absorption-desorption process is one of the major causes of physical degradation of wood coatings, involving up to 30% weight increase and dimensional instability marked by repeated swelling and shrinking of the wood. Additionally, upon long term exposure to water, wood's water content can reach 200% by weight, without additional swelling, however. Swelling peaks at the fiber saturation point.

Notably, polar organic solvents such as aliphatic alcohols, amines, glycols and derivatives of the same, common components of aqueous or solvent based paint formulations, are also absorbed by wood, as the result of their ability to form hydrogen bonding with cellulosic hydroxy groups.

Tannin staining, the spontaneous and complex process which results in loss of decorative value or, if extensive, limits the useful service life of coatings on wood is an indirect consequence of porosity and hygroscopicity of wood. During staining several processes occur concurrently in a complex and dynamic equilibrium. The staining process starts at the moment of application, as a paint film's water content penetrates at a relatively high rate and rehydrates the substrate's surface layer.

Consequently, the cellular walls, typically collapsed in dry wood materials, are restored to their expanded and porous structure, characteristic of green wood. As the result, water soluble staining species, normally stored in cellular walls or lumen cavities of wood materials, are solubilized, mobilized by absorbed water and thus made available for transport. Consequently, mobilized staining species, driven by pertinent concentration gradients diffuse spontaneously through substrate-coating interfaces into coating applications and toward coating-air interfaces, where cumulative staining occurs. Thus, tannin staining occurs at a particularly high rate during the film forming stage of curing of freshly applied aqueous coatings. The coatings, therefore, are to a large extent discolored at the end of the curing process and hence at the very beginning of their service life. For example, acrylic clear applications on oak substrates are known to develop extensive brown-purple discolorations during the film forming period.

It is emphasized, however, that tannin staining progresses continuously at variable, although comparatively lower, rates all during the service life of aqueous wood coatings, resulting in ever increasing accumulated discoloration or staining. Under normal service conditions it is the surrounding atmosphere's relative humidity which controls the rate of the discoloration process. Under condensing humidity conditions, the substrate becomes saturated with water and the staining rate is diffusionally limited by the coating's chemical composition and physical structure as well as by the substrate's tannin concentration.

As will be thus appreciated, tannin staining, is a dynamic, complex phenomenon which includes several concurrent processes such as water or water vapor absorption by coated wood substrates, solubilization of staining constituents, diffusion into the coating and gradual accumulation at the coating-air interface of soluble matter, including tannin species, thus resulting in progressive discoloration of the coating.

By definition, tannin staining inhibition implies such capacities of wood coating systems as interaction with dissolved and diffusing tannins, causing immobilization of staining species in situ in coatings, thus obstructing the accumulation thereof at the coating-air interface and minimizing the overall rate of the discoloration process.

The employment of pigment grade "stain inhibitors" or "tannin blockers¹¹ constitutes the state of the art with respect to procedures available for inhibition of wood coating's aesthetic degradation by tannin staining. Reactive stain inhibitors such as synergistic pigment composites as disclosed in my U.S. Patent No. 5,529,811, provide highly effective tannin stain inhibitive and moderate fungus growth control capacities to water or solvent-based paint formulations, of which they are a function components .

Tannin stain inhibition by functional pigmentation constitutes an effective procedure available to prevent degradation of coatings by highly staining wood substrates. The limitations of this procedure, however, result from the fact, observed during the development of the present invention, that tannin staining of aqueous coatings freshly applied on wood substrates, occurs to a large extent during the film forming process of the same.

Considering the typical volumetric composition of a common, 33% solids and 33% P.V.C. (pigment volume concentration) paint formulation, the reason for the stain inhibitor pigment's apparent ineffectiveness, observed during the critical curing period of aqueous coatings, becomes evident. In this respect, it will be noted that typically about 66% by volume of paint formulations are represented by a solvent phase available for diffusion of staining species, and less than 4%, or approximately 10% of the P.V.C. (for simplicity reasons, the pigment phase's density is considered to be approximately 1) , are occupied by active stain inhibitor pigments. Obviously, freshly applied paint's high liquid volume ratio indicates a system susceptible to solubilization and diffusion of tannin species, a characteristic which, considering the low volume ratio occupied by the inhibitor pigment phase, is primarily accountable for high staining rates observable during the film formation process. As a result of solvent evaporation, however, coatings collapse to approximately 1/3 of their initial volume and their characteristics change dramatically by the end of the curing process.

The above-described qualitative model provides a plausible explanation, as well, of the relatively higher tannin staining inhibitive capacity displayed by solvent-based coatings on wood, in comparison with corresponding aqueous systems, which includes such considerations as lower solubility of tannins in organic solvents.

Often, according to the actual industrial practice, one solvent based clearcoat sealer is applied directly on wood substrates, followed typically by pigmented multiple aqueous paint applications. Such practices, adopted frequently in an effort to enhance the overall tannin staining inhibitive performance of aqueous coatings, constitute severe limitations of such technologies, since the primary objective of eliminating volatile organic compound emissions is compromised. SUMMARY OF THE INVENTION:

The above described shortcomings of aqueous paint and coating technologies intended for wood protection, are minimized according to the present invention by providing aqueous "sealer" compositions and treatment procedures effective on wood surfaces. The aqueous "sealer" composition, according to the present invention, if applied pursuant to an appropriate treatment procedure, alters certain surface characteristics of wood substrates, specifically those related to diffusion of dissolved species across the substrate-air or substrate-coating interfaces, without causing, however, significant discoloration, or texture alteration such as grain raising, or solid deposit formation on the substrate. As a result of the "sealing" treatment, undesirable properties of wood substrates such as, tannin staining susceptibility, staining rate, grain raising tendency or sensitivity to UV radiation are spectacularly minimized. Consequently, the aqueous "sealer" compositions and treatment procedure, according to the present invention are applicable in combination with aqueous clearcoat or pigmented white coating technologies intended for wood protection.

As used herein, "wood" is intended to include reconstituted materials such as wood- containing particle board, chip board, or the like, in addition to natural wood substrates, provided that the materials are of a tannin- containing type. Many other substrates may contain staining components either within their structure or on their surface. Such substrates, in addition to wood, may include plastic materials, stucco, concrete, paper, old painted surfaces, etc. As it was discovered pursuant to the present invention, the aqueous "sealer" composition is also applicable on old coatings pre-existent on wood substrates characterized by high staining potential, such as redwood. It is particularly beneficial, when restoration of mechanically damaged or stain-damaged existing coatings, on intensely staining substrates are attempted. Performed usually by aqueous overcoat applications, such restoration operations' success are often limited due to localized staining of the fresh coatings. In such cases, localized or general application of "sealer" compositions, performed prior to the overcoat application, is beneficial. It prevents localized staining and allows overcoatings of uniform appearance (see Example 13) .

It was learned according to the present invention, that various and specifically water soluble zirconyl compounds, when applied as clear aqueous solutions onto wood surfaces and subsequently dried, promote "sealing" of such substrates, without discoloration of, without causing grain raising or formation of any visible solid deposit or film on them. The sealing effect becomes superbly evident, particularly on wood species characterized by high tannin staining capacity, such as redwood. Any aqueous white paint formulation applied and cured over such substrates, previously "sealed," will form coatings substantially less discolored by tannin staining in comparison with the same on identical "unsealed" surfaces.

Briefly, the invention provides a process of treating substrates including wood, to reduce staining of coatings subsequently applied which includes the steps of providing a solution of a zirconyl compound in a carrier liquid such as water, applying the solution to a substrate surface, and, drying the application. The pore structure, in the case of wood, is modified or sealed so that staining of coating compositions applied subsequently over the surface is reduced. The process is particularly beneficial in cases where the coating composition is a clear formulation or a light-colored latex paint, especially white. The preferred zirconyl compound is zirconyl acetate. DETAILED DESCRIPTION: The "sealing" effect on surfaces, especially wood, observed according to this invention is explicable, considering the well known polymeric structure, especially prevalent in aqueous media, and the ability of zirconyl compounds to crosslink molecular species by forming ionic or covalent bonds with -OH or -COOH functional groups. The chemical structure of wood, considering the typical molecular structure of polysacharides such as cellulose, offers numerous sites for crosslinking. It is speculated that "sealing" of wood substrates, according to the present invention, occurs by crosslinking of polysaccharide moieties by zirconyl species, thus transforming the microscopic pore structure of wood, presumably at the cellular wall level.

Considering the chemical structure of tannins, it is plausible to suppose, that the same chemical mechanism, by crosslinking, could immobilize tannin species in situ , as well. The practical realization of the present invention includes several procedures, the most important among them being the application of dissolved zirconyl compounds onto wood substrates. For that purpose, aqueous solutions of zirconyl compounds are applied by common techniques, such as spraying, brushing, rolling, dipping, etc. on selected wood substrates, followed by drying. Since the "sealing" effect is not necessarily strictly the consequence of surface phenomena, the application can be performed under diverse temperature and pressure conditions, as well.

Although aqueous solutions of varying zirconyl contents are applicable in the practice of the present invention, concentrations of 2 to 25% by weight, expressed as % Zr0₂, are preferred.

The "sealing" efficiency of aqueous applications, varies considerably as a function of the zirconyl specie's specific consumption or spreading rate. In this respect, it was observed that spreading rates of 0.5 to 50, and preferably of 3 to 10 mg Zr0₂/square inch result in optimal "sealing" performance on any wood species, redwood and oak included.

It was also found, that since the "sealing" effect is not strictly the consequence of a surface process, the procedure's effectiveness, is proportionally enhanced by the time allowed for absorption of zirconyl species by porous substrates, such as wood. A minimum of 3 to 5 minutes is necessary but a significantly longer time, up to 30-40 minutes, from the time of the application until force drying, is applicable. Thus, while the process of the present invention appears to involve the surface of a porous substrate and some depth thereunder, it will be referred to generally herein as "sealing".

It is obvious, that any procedure able to increase the rate of relevant diffusion processes, such as pre-wetting of the substrates, performing multiple successive applications on the same substrate or performing the operation under vacuum or elevated temperature and humidity conditions, could potentially shorten the diffusion time and/or enhance the procedure's effectiveness. Alternatively, the employment of surface-active agents (cationic, non-ionic or amphoteric, pH- compatible with the media) which reduce the surface tension of aqueous media, could potentially shorten the diffusion time or enhance the procedure's effectiveness.

As to the kinetics of the crosslinking process, the chemical mechanism of the "sealing" process, a modest rate is plausible. Consequently, force drying of the application (after allowing for appropriate diffusion time) is preferred in order to complete the crosslinking process. During the development of the present invention, drying of applications was typically performed at 130- 140°F for about 5 minutes. It will be apparent, however, that diverse drying conditions may be used, such as ambient temperatures for longer periods of time, for example, 12 hours.

After drying, wood substrates treated according to the present invention, as above disclosed, display negligible discoloration, a limited degree of surface hydrophobicity, and more importantly, low tendency for swelling, grain raising and deformation of the substrates. Considering that such treated wood substrates' natural color and texture are preserved, the "sealing" process, according to the present invention, could itself be considered as wood surface finishing procedures, useful in specific applications. In a typical sequence of steps common to wood finishing processes, substrates treated according to the foregoing procedure, are ready for the subsequent application of aqueous- clearcoats or aqueous pigmented white primers, performed pursuant to various coating procedures known in the art. The overall tannin stain inhibitive performance of such aqueous clear or white pigmented coatings will be superior compared to similar aqueous systems and equivalent or superior to common solvent-based systems, when applied on identical wood substrates. It will be noted, that wood substrates treated in accordance with the present invention, are generally compatible with solvent based clearcoats or pigmented white primers, as well.

With respect to specific zirconyl compounds applicable to the practice of the invention, it will be observed that essentially any water soluble compounds, "cationic", "anionic" or "neutral" are suitable. This categorization refers to the ionic character, rather than the pH, of the zirconyl moiety of diverse zirconyl compounds, a consequence of the polymeric structure and the usually undefined stoichiometry in aqueous solutions, of the dissolved species.

As for the pH of aqueous solutions of soluble zirconyl salts, acidic, basic and neutral compounds exist in all of these ionic categories.

A partial inventory of available water soluble zirconyl compounds includes: cationic compounds, such as: nitrate, ZrO(N0₃)₂; hydroxynitrate; oxychoride; hydroxychloride, Zr(OH)OCl; anionic compounds, such as: orthosulfate, H₂Zr0₂(S0₄)₂; zirconyl ammonium carbonate,

(NH₄)₂[Zr (C0₃) ₂(OH)₂] ; sodium zirconium carbonate; zirconyl potassium carbonate, K₂[Zr (C0₃)₂(OH) ₂] ; zirconyl potassium hexafluoride, K₂ZrF₆; neutral compounds, such as: acetate, Zr (OOC-CH₃)_n; propionate, Zr(OH)₂₆ (OOC-C₂ H₅ )_{1 4}; formate, Zr(OOC-H)_n, where n>4.0; zirconium acetylacetonate, Zr (C₅H₇0₂)₄; neutral zirconyl salt solution (such as ProtecZA7, commercialized by Magnesium Electron, Inc.) These applicable zirconyl compounds are given by way of example, and the invention is not intended to be limited thereby since zirconyl species are the active moiety of such compounds as used in practice of the present invention. It will be within the scope thereof to use related zirconyl compounds of any chemical composition, provided that the solubility requirement is satisfied.

Considering the intent of the present invention to provide practically zero VOC "sealer" technology for wood protection, water solubility of applicable zirconyl compounds is an important preferred property. It will be noted however, that the present invention's object can be realized by employing zirconyl compounds dissolved in organic solvents, as well.

Extreme pH values, which often characterize the aqueous solutions of some inorganic zirconyl salts, (such as ZrO(N0₃)₂, Zr(OH)0Cl, orthosulfate, all strongly acidic or the quite basic zirconyl ammonium carbonate) when applied, could cause significant discoloration of highly staining wood substrates such as redwood and consequently, limit the usefulness of such compounds in combination with clearcoats. It will be noted, however, that such discoloration due to extreme pH values does not necessarily constitute a limitation when water soluble zirconyl compounds are used in combination with pigmented white coatings. Quite on the contrary, as discovered pursuant to the present invention, some of the highly acidic or basic zirconyl salts, which promote considerable degree of discoloration specifically on intensely staining wood substrates such as redwood, display also significant degree of stain inhibitive efficiency in preventing the discoloration of subsequently applied pigmented white coatings. The tannin stain inhibitive efficiency of diverse soluble zirconyl salts is extensively demonstrated in Examples 10 and 11.

The chemical composition of aqueous solutions of zirconyl compounds applicable according to the present invention is variable between relatively large limits, defined primarily by assay (expressed in Zr0₂) and by acid component/Zr0₂ or alternatively, in the case of basic solutions, by basic component/ Zr0₂ molar ratios.

For example, the quality parameters of the mildly acidic aqueous solution of the zirconyl acetate, one of the preferred zirconyl compounds of the present invention, are variable as follows: assay = 2 to 25% Zr0₂, molar ratio of acetic acid /Zr0₂ = 1.4 to 2.2 and pH = 3-4. Preferred compositions are disclosed in the accompanying examples.

It was also discovered pursuant to the present invention, that it is beneficial to modify the composition of aqueous zirconyl compounds solution by introduction of various cationic species, in variable amounts, compatible with the pH of the medium. Although solubility is a limiting factor in this respect, the list of suitable cationic species includes those formed by Group IIA metals (i.e., Mg(II) , Ca(II), Sr(II)), as well as Cr(III), Fe (II or III), Mn(II) , Cθ(II), Ni(II), CU(II), Ag(I), Cd(II), Hg(II), Pb(II) , Ti(IV), Hf(IV), among others.

It was found highly beneficial, however, with respect to usefulness as a surface "sealer", to modify the chemical composition of aqueous solutions of zirconyl salts by introduction of Zn(II) , Al(III), lanthanides and more especially of Ce(III) or Ce(IV) species, or mixtures thereof.

By considering the well documented UV absorbing capacity of Ce compounds, the benefit of introducing such species into zirconyl salt solutions becomes evident.

Additionally to improve stain inhibitive performance as aqueous sealer, Ce modified zirconyl salt solutions provide enhanced protection against UV radiation, as well. Such protective characteristics are significant, for example, with respect to applications on wood substrates, which are known to be vulnerable to UV radiations, and particularly in conjunction with clearcoats. Notably, however, it is also contemplated to use, in lieu of lanthanides or Ce, commercially available organic UV absorber products, or combinations thereof, for the same purpose. Water soluble or dispersable organic UV absorbers are suitable, such as Tinuvin 1130 commercially available from Ciba-Geigy Corporation.

As for the benefit realized by introducing Cu(II), Zn(II) and Al(III) species into the zirconyl salts solution according to the present invention, it will be noted that the former and especially Cu(II) , are known for their effective fungicidal and mildewicidal activity. It is important to observe in this sense, that the service conditions of high humidity and warm climate, which promote tannin staining of wood coatings, support also the growth of various fungi on the same. In such conditions, in addition to the aesthetic degradation caused by dark fungal colonies, fungal attack promotes the accelerated breakdown of coatings and ultimately of wood substrates, as well. Consequently, fungal growth control capacity is an important attribute of wood coatings, which able extension of the service life and improvement of the overall protective performance of such systems.

It was learned pursuant to the present invention, that aqueous zirconyl salt solutions modified by addition of Zn(II), Cu(II), Al(III), lantanides and more specifically Ce(III) and Ce (IV) species or mixtures thereof, when applied as "sealer" on wood substrates, display complex protective functionalities, including tannin stain inhibition, fungus growth control and protection against UV radiation. As for the practical preparation of aqueous solutions of zirconyl compounds modified by various cationic species, as for example methods for the preparation of aqueous zirconyl acetate solution modified by Zn(II), Ce(III), Al(III), etc. species, will be apparent to those informed in the art. According to the present invention, however, oxides, freshly precipitated hydroxides, acetates, carbonates and borates of the cationic species, and more specifically ZnO, A1(0H)₃ or aluminum acetate, Ce(III) -carbonate, Cu- borate, are the preferred precursors of the added cationic species.

As shown in the following examples, the preparation of aqueous solutions of zirconyl acetate modified by various cationic species according to the present invention includes the preparation of mixed suspensions containing basic zirconyl carbonate and one or more of the above specified precursors, and solubilization of the solid phases by acetic acid addition, agitation and heating. It will be noted that U.S. Patents Nos. 3,183,118 and 3,291,635, among others, disclose such procedures in the preparation of diluted (assay < 2%) zirconyl acetate solutions containing Cu(II) , Hg(II) and Ni(II).

The chemical compositions of the modified aqueous solutions of zirconyl salts, according to the present invention, are variable in large ranges, using zirconyl acetate as an example, as follows: assay = 4 to 24%, (expressed in weight % of total oxides) ; molar ratio of acetic acid/cationic species = 1.4 to 2.2; molar ratio of Zr/added cationic species = 20 to 1. Preferred values of these quality parameters are disclosed in the several following specific examples.

Aqueous solutions of zirconyl salts, and more specifically diluted ones, are known to be unstable due to "gelling" at temperatures exceeding ambient temperatures. An undesirable behavior which limits the applicability of such solutions as a "sealer", "gelling" can be prevented by employment of various additives, inclusive of hydroxy carboxylic acids, as suggested by Stewart et al in U.S. Patent No. 3,741,782. Without any intent of limitation, however, tartaric acid, the stabilizing additive preferred according to the present invention, was found to be compatible with "sealer" applications of zirconyl acetate solutions. (See Example 1) .

It will be obvious, however, to one skilled in the art, that aqueous solutions of various zirconyl salts can be modified by addition of diverse anionic species, constituents of inorganic or organic acids, other than tartric, such as boric, citric, lactic, or glycolic. It is believed that such additional anionic species do not specifically contribute to the tannin stain inhibitive activity of basic zirconyl salts. It was also learned pursuant to the present invention, that aqueous solutions of zirconyl salts and specifically modified solutions of zirconyl acetate, are compatible with cationic and non-ionic surface-active agents. Considering the surface-activity, as well as the bactericide and fungicide activity of some quaternary ammonium salt compounds, the employment of such materials as additives to aqueous "sealer" compositions according to the present invention, is understandably preferred.

With no intent of limitation, a commercial product (Dowicil 75 Preservative, available from Dow Chemical Co.), a highly effective bactericide recommended for preparation of aqueous paint formulations, was chosen in order to demonstrate the compatibility of quaternary ammonium salts with aqueous solutions of zirconyl acetate. (See Example 8) .

Evidently, however, solubility in water does not necessarily constitute an exclusive quality requirement for fungicides applicable in combination with aqueous zirconyl salt solutions. Water-dispersable or emulsionable organic fungicides, compatible with and stable in the specific pH environment of any particular zirconyl salt solution, are suitable. For example, a water dispersible, organic broad spectrum fungicide for aqueous systems, sold by Troy Corporation under the trade name "Troysan Polyphase WD17" was found to be compatible with aqueous solutions of zirconyl acetate.

As above mentioned, the practical realization of the present invention is based on application of aqueous solutions of zirconyl compounds, directly on wood substrates, in order to promote "sealing" of the related surfaces and consequently, to inhibit tannin staining of subsequently applied clearcoats or white coatings. Notably, as is known in the art, zirconium compounds interact strongly and in a complex fashion, with diverse polymeric systems as well as with finely divided, dispersed inorganic solids (fillers) both of which are typically present in aqueous paint formulations. It is assumed that attempts to achieve

"sealing" of the interface by introduction of zirconyl compounds directly into paint systems prior to their application on wood substrates, would have comparatively limited success. In such cases, the crosslinking capacity of zirconyl compounds would be consumed by chemical interactions with resin and dispersed solid components of the formulations, consequently reducing their availability to promote "sealing" of the substrate coating interface. Based on the above specified and earlier disclosed kinetic considerations, it can be concluded, that in order to achieve effective "sealing", direct application of aqueous solutions of zirconyl compounds on wood substrates constitutes the most effective mode of practice of the invention.

EXAMPLES The following examples illustrate preferred embodiments of the invention, without intention, however, to limit its applicability in any sense, and are intended to provide technical details regarding selected examples of the invention as well as to demonstrate the resulting substantial contribution to the art of surface treatment of wood substrates.

GENERAL Exemplification of the present invention's reduction to practice includes a brief description of zirconyl acetate solution preparation, and of its modified versions preferred in the practice of the present invention, and more specifically, includes practical details with respect to application of aqueous solutions of various zirconyl salts on wood substrates. Surface finished redwood and oak veneer panels were selected for that purpose. Zirconyl salt solutions of known concentrations were uniformly applied by brushing on such panels of known surface area. The specific spreading rate of zirconyl solution, expressed in g Zr0₂/square inch, was determined gravimetrically or volumetrically, considering the zirconyl salt solutions' assay, applied amounts and the treated wood surfaces' dimensions. The "sealing" process of the treated exhibits' surfaces was completed by allowing 5-20 minutes for absorption at ambient conditions (considered from the moment of completion of the applications) and by subsequent force-drying, typically performed at 140°F for 5 minutes .

In order to demonstrate the practical effectiveness of the invention, further relevant operations were performed, such as: application of aqueous acrylic clearcoat on "sealed" test exhibits and unsealed oak veneer substrates, respectively (see Example 9) and the application of white pigmented aqueous paints onto previously "sealed" and on identical, but "unsealed" redwood substrates, the latter being considered control exhibits (see Example 3) .

Notably, the applied aqueous paint formulations, containing active stain inhibitive pigments, were based on two different commercially available resin (latex) components, characterized by quite different tannin staining inhibitive capacities. Curing of the paint applications was performed by keeping them overnight at ambient temperature . In order to evaluate quantitatively the tannin staining inhibitive efficiency of the "sealing" treatment, the tannin staining performance of "sealed" redwood surfaces was measured (on white paint applications) , comparatively to identical untreated panels. For that purpose, redwood panels, prepared as above disclosed, were subjected to condensing humidity conditions for several days and the magnitudes of resulting discolorations of pertinent paint applications were measured by means of computer assisted reflectance spectrophotometer, before and after exposure.

Relevant color values (expressed in CIELab color system) measured as dE values versus identical paint applications on non-staining aluminum panels, considered as color standard, allow quantitative evaluation of the "sealing" treatment's effectiveness in inhibition of the redwood substrate's tannin staining activity. Observing that dE values (measuring the extent of discoloration, as above described) are inversely proportional to tannin staining inhibitive performance of related coatings, it will be evident that such values pertinent to control panels (symbolized hereafter as dEc) , compared to those of treated panels (symbolized as dE) , allow quantitative characterization of the stain inhibitive performance of the "sealing" treatment. The "sealing" treatment's efficiency index (Is) can be calculated according to % Is = 100 (dEc -dE)/dE. It is important to note, that by measuring the extent of discoloration, as above described, but performed at completion of the drying process, similar characterization of the "sealing" treatment relative to the paint application's drying period, is possible. The evaluation of the "sealing" treatment's stain inhibitive efficiency on oak, on which acrylic clearcoat applications were applied, was performed in similar fashion as above described. Such data is presented in Example 9.

The following examples disclose various means of exploitation of the present inventions objective, related specifically to tannin staining inhibition.

Example 1. An aqueous solution of zirconyl acetate was prepared following traditional procedures, known in the art.

For that purpose, 100.0 g. of aqueous zirconyl carbonate paste, available with an assay of approximately 39-40% Zr0₂, was re-slurried in 200 ml water and subsequently reacted, at normal temperature and agitation, with 39.0 g of glacial acetic acid, in approximately 1:2 stoichiometrical ratio.

The reaction was finalized by keeping the obtained solution at about 60 'C for approximately one hour and by subsequent introduction of 600 ml. water. Approximately 930 g. Of clear solution was recovered.

The prepared clear solution, characterized by pH= 4.0, and assay (determined gravimetrically) of 4.7% Zr0₂ by weight, was used in all examples of "sealer applications" of the present invention, unless otherwise noted.

Notably, the aqueous solution of zirconyl acetate as above described, displayed a definite tendency for gelling when exposed to higher than ambient temperatures for a longer period of time, for example, 140°F for 48 hours. As expected, however, the gelling process was found to be reversible at normal temperatures. In such conditions, the complete liquification of gelled solution was observable in a short period of time.

A stabilized version of an aqueous zirconyl acetate solution, as above described, was prepared by addition of 6.0 g. tartaric acid (as aqueous solution of 30%) to 930 g. of the former. Approximately 950 g. of stabilized, but generally unmodified clear solution resulted. The solution, stored at 140°F for several days, displayed no tendency for gelling during that period of time.

Example 2. White pigmented paint formulations identified as 2.1 and 2.2, recommended for wood protection and applied in context of the present invention are presented below. It will be observed, that both formulations contained a commercially available tannin staining inhibitive pigment.

Table 2.1

Components of Trade names & Parts by Weight formulations suppliers of 2.1. 2.2. components

Water 222.0 203.0

Ti02 RCL-535(1) 153.0 150.0

Filler Piginent Gammaspers 80(2) 119.0 116.0

Stain inhibitor pigment * 34.0 33.0

Coalescent solvent Butyl carbitol(3) 9.5 Ethylene glycol 19.5 Texanol (4) 5.5

Freeze stabilizer/ coalescent Propylene glycol 49.0 Stabilizer Surfynol 104 A (5) 3.5 2.0 Thickener Acrysol SCT 270 (6) 23.5 Acrysol QR-708 (6) 5.5 Natrosol 250 MR (7) 1.5 0.5

Dispersant Colloid 226 (8) 8.0 Tamol 681 (6) 12.0

Defoa er Colloid 643 (8) 4.0 Biocide Nopcocide N-40D (9) 11.5 Skane M-8 (6) 2.0

Neutralizer AMP 95 (10) 1.5 Ammonia, 28% 1.0

Latex Resin Synthemul 40-412(11) 430.0 Maincote MV-23LO(6) 520.0

1070.0 1069.5

Suppliers of components are:

(l)SCM Chemicals, (2)Georgia Marble Co. , (3) Union Carbide Co. , (4) Eastman Chemical Co., (5)Air Products and Chemicals, (6) Rohm and Haas Co., (7)Aqualon, (8) Rhone-Poulenc Ag.Co., (9) Henkel Co., (10) Angus Chemical Co., (11) Reichold Chemicals, Inc.

* commercially available stain inhibitor pigment.

Example 3. The testing of resultant solutions was performed on four surface-finished redwood panels, of about 20 square inches, each. For that purpose, two of the experimental panels were surface "sealed" according to the present invention, by brush application of zirconyl acetate solution prepared according to Example 1. ; it was performed by applying 2.0 g. of zirconyl acetate solution per exhibit, determined gravimetrically, at an approximate spreading rate value of 4.7-5.0 g. Zr0₂/square inch, followed by about 20 minutes of absorption time and subsequent forced-drying, at 140 °F for 5 minutes.

Visual examination did not reveal "grain raising" on, or any significant discoloration of the "sealed" substrates, comparatively to the "untreated" control panels.

Paint formulations 2.1. and 2.2., according to Example 2, were applied using a 3 mil letdown bar on each of the control and "sealed" panels, after which all exhibits were allowed to dry overnight at ambient temperature. In order to assess the extent of discoloration which occurred during drying, the color value of all obtained paint applications was measured and compared with non-staining coatings on aluminum panels, which were considered as color standards. Consecutively, all exhibits were exposed to condensing humidity conditions, continuously, for 7 (seven) days (at 100^* F), after which the extent of discoloration which occurred was assessed again in identical fashion, by measuring the related color values, compared to the color standards.

The effectiveness in respect to tannin staining inhibition of the "sealing" treatment, according to the present invention, was evaluated by Is, the efficiency index, calculated for both, the paint applications' curing time and the period of seven days exposure to condensing humidity conditions. Data characterizing the "sealing" treatment's tannin staining inhibitive performance, relevant to the coatings' curing time and to the condensing humidity exposure period (for seven days), are presented in Table 3.1 and Table 3.2, respectively.

Applied paint of Example 2 dE dEc Is, 2.1. 8.0 4.0 100 2.2. 4.2 0.8 425

Table 3.2

2.1. 50.0 12.0 316

2.2. 11.0 2.0 450

Is values presented in Tables 3.1 and 3.2, ranging 100-450, or more typically 300-450, indicate that the aqueous "sealing" composition and treatment, performed pursuant to the present invention, was highly effective with respect to reduction of redwood substrates' tannin staining capacity. Based on related practical expertise, it will be observed, that tannin staining inhibition of comparable magnitude is not achievable by active pigmentation of aqueous or solvent-based protective coatings, the alternative technique known by the prior art.

Example 4. Modified aqueous solution of zirconyl acetate, containing Zn(II) and acetic acid in approximate molar ratios of n(Zr) : n(Zn) = 3:1 and n(acetic acid): [n(Zn) + n(Zr)] = 1,5:1, respectively, was prepared according to as follows:

100.0 g. of aqueous paste of zirconyl carbonate

(see Example 1) and 8.5 g. of commercially available high grade ZnO, was reslurried in 150.0 ml. water, and subsequently reacted with 40.0 g. of glacial acetic acid in similar fashion as described in Example 1.

The solubilization process was completed in about 2 hours, after which 50.0 ml. of water were added to the reaction medium. The obtained clear solution was characterized by pH = 4.1 to 4.3 and assay by weight (determined by ignition at 600°C) of approximately (Zr0₂+ZnO ) = 15.5%. Example 5.

Aqueous solution of zirconyl Acetate modified by introduction of Ce(III) species, containing Ce(III) and acetic acid in molar ratios of approximately n(Zr) :n(Ce) = 4:1 and n(acetic acid) : [n(Ce)+ n(Zr)] = 1.7:1, respectively, produced pursuant to the following procedure.

Initially, an aqueous mixed suspension was prepared by dispersing 166.0 g. of wet zirconyl carbonate (see Example 1) and 36.0 g. of Ce₂(C0₃)₃(H₂0)₃ (technical grade, commercially available from Molycorp Inc.) in 160.0 ml. water. The mixed suspension was subsequently solubilized by gradual introduction of 72.0 g. glacial acetic acid with extensive agitation at 40-45°C, the process being completed by maintaining these conditions for about 4 hours. Approximately 400 g. of clear but slightly yellow solution resulted, characterized by the following quality parameter values: assay (by ignition at 600°C) , as (Zr0₂ + Ce0₂) = 21%; pH = 3.5 to 4.0; specific gravity = 1.24; yield = approximately 400 g.

Obviously, the recommended amounts of cerium-carbonate are replaceable by equivalent amounts of cerium-acetate. It will be noted, as well, that alternatively, cerium-carbonate can be substituted for other lanthanides or mixed- lanthanide (Ln) compounds (available from the same supplier) , such as La-carbonate and Ln-Carbonate, respectively. Example 6. Aqueous solution of zirconyl acetate modified by addition of Cu(II) species, containing Cu and acetic acid in molar ratios of approximately n(Zr) : n(Cu) = 9:1 and n(acetic acid) : [n(Zr) + n(Cu) ] = 1.6:1, respectively, produced essentially according to the procedure presented in Example 5. In this case, however, 192.0 g. of wet zirconyl carbonate (see Example 1), 10.0 g. of commercially available Cu(B0₂)₂, (Cu(II) -borate) or alternatively, 3.5 g. Cu(OOC-

CH₃)₂H₂0, (Cu(II)- acetate) was dispersed in 150 ml water and dissolved in 68.0 g. glacial acetic acid. The resulted clear, moderately blue solution was characterized by the following quality parameter values: assay (by ignition) of about

20%, as (Zr0₂+CuO) ; pH = 3 to 4; specific gravity = 1.23/24°C; yield = approximately 400 g.

Notably, by using Cu(B0₂)₂ as above specified, equivalent amounts of borate (as B0₂(-) species) are also introduced into the aqueous solution of zirconyl acetate.

It will be observed however, that borates are known fungicides and can be also incorporated into zirconyl acetate solution as H₃B0₃ or as various salts of the pyro-boric acid.

Example 7. Aqueous solution of zirconyl acetate modified by addition of Al(III) species, containing Al and acetic acid in molar ratios of approximately n(Zr) :n(Al) = 4:1 and n(acetic acid) : [ n(Zr) + n(Al)] = 1.7:1, respectively, was produced as follows: 153.0 g. of wet zirconyl carbonate (see Example 1) and freshly precipitated, washed aluminum hydroxide paste containing 9.5 g. of A1(0H)₃, was dispersed in 180.0 ml. water and subsequently dissolved by gradual addition of 72.0 g. glacial acetic acid, under extensive agitation at approximately 50°C. The resulted clear solution was characterized by the following quality parameter values: assay (by ignition) approximately 18%, as (Zr0₂ + A1₂0₃) ; pH = 3 to 4 ; yield = approximately 400.0 g.

Example 8. Aqueous solution of zirconyl acetate was modified by addition of organic cationic species, such as typical for quaternary ammonium salts. For that purpose 1.77 g. of l-(3-chloroallyl) - 3,5,7- triaza-1-azoniaadamantane chloride, as aqueous solution of 5.0% (available from The Dow Chemical Co. under the trade name of Dowicil 75

Preservative, containing 67.5% of active ingredient) was gradually introduced by agitation into 400.0 of zirconyl acetate solution obtained according to Example 4. The preparation process was finalized by agitation until a clear solution of similar quality as described in Example 4, was obtained. The final product's calculated content of quaternary ammonium salt was approximately 0.3%. Example 9.

An application of the present invention was performed on surface finished oak panels in combination with an aqueous clearcoat. The intent was to demonstrate the tannin stain inhibitive effectiveness of the "sealing" treatment on oak, as well as to prove the compatibility of such surface treated substrates with aqueous clear applications.

For that purpose 1.0 ml. of Ce(III) - modified aqueous solution of zirconyl acetate, (prepared according to Example 5 of the present invention and diluted in 1:1 ratio with water) was applied by brush to one-half of the surface of an oak veneer panel (approximately 28 square inches surface area was covered) , allowed for 15 minutes to absorb and subsequently force-dried at 120°F for 5 minutes. The spreading rate was approximately 4.5 mg. of (Zr0₂+Ce0₂) /square inch. On visual examination, the "sealed" portion of the oak substrate did not display significant discoloration or any texture modification, inclusive "grain raising".

Subsequent to the treatment as above described, the entire surface (inclusive the "sealed" section) of the oak veneer panel was coated by three successive brush applications of a commercial clear acrylic latex (obtained commercially from Deft Coatings under the trade name of "Safe & Easy" Interior Wood Finish) . One hour of drying time was allowed and sanding was performed between coats.

After drying overnight under ambient conditions, the test panel was later exposed to condensing humidity conditions at 100 'F for 24 hours and subsequently the related dEc and dE values were measured, following the previously described experimental technique.

It is important to note, in this case the freshly "sealed", veneer surface which was uncoated and not exposed to condensing humidity was chosen as the color standard against which all of the dE values were measured.

Is, the value of the Efficiency Index of the "sealing" treatment was calculated as above specified, considering dEc = 13.8 and dE = 6.1, determined to be the discoloration values after humidity exposure, measured on the "sealed" and on the untreated sections of the test panel, respectively. The calculated value of Is = 126%, indicates highly effective tannin stain inhibitive performance on oak by the "sealer" treatment, applied in combination with aqueous acrylic clearcoats.

Example 10. A comprehensive list of commerciall available solid or aqueous solutions of zirconyl compounds (other than zirconyl acetate) applicable according to the present invention, is presented below:

Table 10 . 1

Name Chemical Ionic Typical assay pH formula character as Zr0₂ %

nitrate ZrO(N0₃)₂ cationic 20 < 1 hydroxy- nitrate Zr(0H)0(N0_; 30 1 -2 oxy- chloride ZrOCI - 20 < 1

Hydroxy- chloride Zr(0H)0CI 20 1 -2 ortho- sulphate H-Zr0₂(S0₄)- anionic 1 8-20 < 1 basic sulphate Zr₆O_β(S0₄)₂ 38 1 -2

K,Zr-hexa- fluoride K₂ZrF_β ammonium zirconium carbonate (NH₄)-[Zr(CO-)₂(OH)] " 20 9-9.5 potassium zirconium carbonate K-[Zr(C0₃)₂(OH)-] 20 1 1 sodium zirconium carbonate Na₂[Zr(C0₃)₂(OH)₂] 10 partially neutralized zirconium acetate* Zr(Ac)n + (NH₄-Ac)m " 20 zirconium formate Zr(HCOO)n neutral 20-22 3-4 zirconium acetyl- acetonate Zr(C₆H70₂)₄ neutral 15

* neutral zirconium salt solution, such as ProtecZA7 commercialized by Magnesium Electron, Inc.

Example 11.

In order to demonstrate the applicability of aqueous solutions of diverse zirconyl compounds, in addition to zirconyl acetate, as tannin stain inhibitor "sealers" for wood substrates, several commercially available products listed in Table 10.1 were tested in similar fashion to Example 3.

The application of diverse solutions (at an approximate spread rate of 5 mg. Zr02/ sq inch) , more specifically, those characterized by extreme pH values, allowed significant discoloration of the substrate redwood panels, which were measured against the original color of the substrates. The measured values of substrate discoloration are expressed in CIE Lab system on the (+,-) dL scale, corresponding respectively to lighter (+) , or darker (-) appearance in comparison to the original color values of the same, as subsequently presented.

Is = 100(dEc-dE)dE, the above defined "sealer" efficiency index, which quantifies the stain inhibitive efficiency of the zirconyl compounds tested, when applied at the same spread rate, was determined in identical fashion to that described in Example 3, by overcoating all "sealed" panels with an application of the white primer formulation 2.2 described in Example 2 , and by subsequent exposure of the same to condensing humidity conditions for 7 days. Values of dE were measured on the control [dEc(unexposed) and dEc] and test coating applications [dE (unexposed and dE] before and after exposure to condensing humidity conditions, respectively. On the control panel, where no zirconyl salt solution was applied, values of dEc (unexposed) =9 and dEc=15 were measured and dLc=0 was determined.

Table 11.1

Zirconyl compound dL dE(unexposed) dE ls,% tested

control 0 9 15 - formate 4.8 7 1 1 .5 30 nitrate -8.5 2.5 5.5 170 oxy-chloride -6.0 2.5 6 150 ortho-sulphate -16.5 2.5 10 50 partially neutralized acetate 1 .2 4 5 200 ammonium zirconium carbonate - 6 13.5 10

As it can be seen, the Is values indicate significant tannin stain inhibitive efficiency for several water soluble zirconyl compounds. Example 12

In order to demonstrate dual protective activity, against tannin staining and UV radiation, of Ce(III) modified aqueous solution of zirconyl acetate, control and test panels were prepared in identical fashion to Example 9 , except they were applied on surface finished cedar substrates and exposed to outdoor conditions for 3 months .

The conclusions based on visual examination of the control and test panels are presented below: Table 12 . 1

Thus, due to the "sealer" application, the test panel was significantly better protected against outdoor destructive factors such as UV radiation and condensed water in comparison to the unsealed control. Example 13 As above stated, aqueous "sealer" compositions according to the present invention are also applicable on old coatings pre-existent on diverse substrates. Such applications are particularly beneficial when restoration (by overcoating) of mechanically damaged or stain- damaged coatings, pre-existent on intensely staining substrates (such as redwood) is performed.

In order to demonstrate this, a commercially available, solvent based primer was applied on two surface finished redwood panels (in similar fashion as described in the applicable section of Example 3) . The fresh applications were subsequently damaged, on the test section of each panel, by scraping. The thickness of the applications having been significantly reduced, the applications were allowed to dry for 24 hours.

Further test and control panels were prepared in identical fashion as presented in Example 3, with and without "sealer" application, respectively, and by subsequent coating application of formulation 2.2 according to Example 2.

After 72 hours drying time, the test and control panels were evaluated instrumentally by measuring the extent of discoloration of the test section of each. The extent of discoloration of the control panel (dEc=5.2) was significantly larger in comparison to that of the test panel (dE=3.3). The latter, however, was practically equal to the value of discoloration measured on the undamaged section of both panels, dE=3.1. It can be concluded, that aqueous "sealer" compositions according to the present invention, are applicable, with full benefit of stain inhibition, on old pre-existent coatings on intensely staining wood substrates, as well as on new wood.

Claims

CLAIMS :

1. A process of treating wood to reduce the tannin staining of coatings subsequently applied thereto comprising: providing an aqueous solution of a zirconyl salt, applying said solution to a wood surface, drying said solution; and, subsequently applying a coating composition to said wood surface, whereby tannin staining of a coating on said wood surface formed by said coating composition is reduced.

2. A process according to claim 1 wherein said coating composition comprises a light-colored latex paint.

3. A process according to claim 1 wherein said coating composition comprises a clear sealer composition.

4. A process according to claim 1 wherein said salt comprises zirconyl acetate.

5. A process according to claim 1 wherein said salt is selected from the group consisting of ZrO(N0₃)₂, Zr (OH)0(N0₃) , ZrOCl₂, Zr(OH)OCl, H₂Zr0₂(S0₄)₂, Zr₅0₈(S0₄)₂, K₂ZrF₆, (NH₄)₂[Zr(C0₃)₂(OH)], K₂[Zr (C0₃) ₂ (OH) ₂] ,

Na₂[Zr(C0₃)₂(OH)₂] , Zr(CH₃COO)n, partially neutralized zirconyl acetate, Zr(HCOO)n and Zr(C₅H_?0₂)₄ or mixtures thereof.

6. A process of inhibiting of the staining of a film forming finish applied to a tannin containing wood substrate which comprises the step of applying to the wood substrate prior to or concurrently with the film forming finish, a protective coating containing an effective amount a zirconyl salt, to inhibit the migration of tannins from said substrate into said finish.

7. A wood substrate having a surface prepared for application thereto of a coating composition, the surface being treated with a aqueous solution containing zirconyl acetate and dried, whereby tannin staining of coating compositions applied subsequently over said surface due to migration of tannins from said wood substrate into said coating is reduced.

8. A substrate according to claim 7 wherein said aqueous solution also contains an antifungal composition or a UV absorbing composition.

9. A substrate according to claim 7 wherein said substrate comprises previously painted wood having a deteriorated coating thereon.

10. A substrate according to claim 7 wherein said solution also contains anionic species selected from borates, tartrates, lactates and citrates.