One part moisture curable urethane composition

Abstract: The present invention relates to one-part, moisture-curable, urethane compositions suited as a coating for wood. The composition comprises trisocyanate prepolymer, an epoxy silane, and compatible solvent.

Description: Background of the Invention This invention relates to a one-part moisture-curable urethane composition, a method of providing a protective coating by application of the composition to a substrate, and to substrates such as wood coated therewith. Coatings containing polyurethane are very popular because they provide a clear coating on wood which has flexibility, toughness, abrasion resistance, and chemical resistance. The polyurethane coating may be either a two-component or a one-component system. The two-component systems are generally inconvenient to use because they have a short pot life and are difficult to apply. The use of two-component systems as a wood finish is thus generally limited to professionals. The one-component, moisture-cureable polyurethane systems are well known as clear wood finishes. One type, the isocyanate modified drying oil or urethane oil, depends for its final curing upon the oxidative polymerization of double bonds of the drying oil fatty acid portion. Another type, the so-called blocked isocyanate coating system, involves the heat curing of an end-capped isocyanate which is blocked by phenol or ketoxime that is curable with a polyol via thermal regeneration of the isocyanate. U.S. patent No. 4,222,909, issued to D. W. Brixius et al, discloses a moisture curable coating composition comprising low molecular weight isocyanate functional acrylic polymer having a terminal thioalkyl group based on the copolymerized isocyanatoalkyl methacrylate. U.S. Patent No 4,217,436, issued to Richter et al, discloses the use of a novel bis cyclic urea blend with polymeric polyols to form one-component room temperature stable curable composition. The bis cyclic urea is easily converted to aliphatic diisocyanate simply by heating. U.S. Patent No. 4,565,857, issued to Grant, discloses a protective coating system comprising a polymerizable ethylenically unsaturated compound having an isocyanate group reacted with the hydroxyl groups of the cellulose ester This thermoplastic polymer can be coated on a wood surface by conventional methods and, following a final cure, thermoset by exposure to ultra violet light. U.S. Patent No. 4,600,790, issued to Eck et al discloses a moisture curable composition containing SiC-bonded biuret groups, which is prepared by reacting amino functional silane with mono or diisocyanates, that serves as an additive to lacquers. U.S. Patent No. 4,289,869, issued to Zengel et al, describes a moisture curable mixture containing polyisocyanate and polyepoxide with an N,N′-substituted imidazolidine or hexahydropyrimidine derivative as a moisture activated hardener. U.S. Patent No. 4,625,012, issued to Risk et al, discloses the preparation of a moisture curable polyurethane polymer containing terminal isocyanate and alkoxysilane by a two-step reaction route, to provide a sealing material for non-porous surfaces such as glass and ceramic. U.S. Patent No. 4,539,345, issued to Hansen, teaches one-part moisture-curable polyurethane adhesive, coating, sealant, or casting compositions based on a blend of isocyanate-functional prepolymer, terpene-phenolic resin, and a silane compound. The polyurethane coating compositions disclosed in the cited prior art have problems or disadvantages. For example, the cellulose-type urethane coatings lack the ability to provide heat and stain resistance, oil-type urethane systems require the use of heat and UV radiation for curing, polymerizable isocyanate-silane polymers have a relatively short pot life due to high molecular weight, and some coatings are complicated to use and/or are relatively expensive to prepare because they use sophisticated materials. Many of these disadvantages can be overcome by the present invention. Summary of the Invention The present invention provides a one-part, moisture-curable composition that comprises triisocyanate prepolymer, an amount of at least 0,1 parts by weight per 100 parts by weight of prepolymer of epoxy silane effective to crosslink the prepolymer, and sufficient compatible liquid vehicle to make the composition suitable for application. Preferred prepolymers are selected from the group consisting of a reaction product of a diisocyanates and a triol. The diisocyanate may be aromatic or aliphatic, but is preferably aliphatic, if yellowing associated with the aromatic is a concern. Blends of triisocyanate prepolymer with diisocyanate prepolymer are also preferred since the presence in the coating of diisocyanate tends to enhance the flexibility of the cured coating. Preferred prepolymer blends may be obtained by reaction of diisocyanate with trimethylol propane and 1,3-butandiol. Preferred isocyanates are toluene 2,4-diisocyanate and aliphatic isophorone diisocyanate. The epoxy silanes used in the present invention are characterized by having a silicon atom having trialkoxy functionality and an epoxy group. Such epoxy silanes are very stable, even when mixed with the isocyanate prepolymer, so long as the other components are free of active hydrogen atoms. The epoxy silane is thought to hydrolyze to hydroxy and then crosslink with triisocyanate to form a coating. Preferred epoxy silanes are selected from the group consisting of gamma glycidoxypropyltrimethoxy silane and beta-(3,4-epoxycylohexyl)ethyltrimethyloxy silane. The composition of the invention may include a catalyst to accelerate cure time, but such catalysts are not required since the cure times of compositions without the catalyst are quite adequate for most applications. Useful catalysts include stannous octoate and dibutyl tin dilaurate. Any compatible liquid vehicle may be used in the practice of the invention. Useful liquid vehicles are solvents which are free of active hydrogen atoms and moisture. The composition of the invention may be used by a method which has the following steps: (a) preparing the surface of a substrate surface for treatment, if needed, usually by conventional methods; (b) applying the composition to the surface; and (c) subjecting the coating applied in (b) to conditions which facilitate the cure of the composition. Application may be by any of a variety of methods such as brushing, spraying and wiping. Since the composition has a high solids content and low viscosity it may be easily applied in a "wipe-on" application to wood surfaces as a very quick, convenient, and precise method preferred by the do-it-yourself consumer. This invention provides many advantageous features. It provides a coating composition which has an extremely stable shelf-life, low viscosity and high solids content to facilitate easy application. The coating exhibits excellent hardness, good adhesion to wood surfaces and other non-porous or porous surfaces such as ceramic, cement, etc., good stain and chemical resistance, and heat resistance. The coating composition of the invention is particularly useful to treat wood in the consumer market to provide a clear finish. The composition of the present invention provides considerable improvement over commercially available products which are based upon cellulose, urethane oil and thermoplastic urethane. The present composition also has excellent penetration into the pores of wood and other porous materials and develops excellent bonding with wood cellulose and other surfaces. Once treated, a wood surface is provided with an excellent durable finish, highlighting the wood grain. The treated wood surface feels waxy and smooth, but not greasy. The treated surface is easy to clean, and has a high resistance to food stains, household chemicals, and damage caused by contact with hot items. The composition also has a shorter drying time to a sandable condition than other known commercial products which allows users to apply multiple coatings within a shorter time. The applied composition has excellent weatherability, showing less discoloration as well as less wood grain raising. Description of the Invention In the practice of the present invention, the moisture curable compositions can be reacted with atmospheric or added moisture to produce a crosslinked polyurethane clear coating. The triisocyanate-functional prepolymer (hereafter sometimes referred to as the "prepolymer") contains sufficient isocyanate groups to enable the composition to be crosslinked by the epoxy silane upon exposure to atmospheric or added moisture. The amount of silane compound preferably is adjusted to provide desired handling characteristics before cure and a desired level of adhesion (measured initially and after exposure to environmental conditions) after cure. This amount will vary depending upon the particular prepolymer employed, the other ingredients present in the composition, and the substrate and environmental conditions with which the composition will be used. In general, an effective amount of silane compound is an amount which will provide coherent protective coating of the composition when it is applied to a substrate with substantially no unreacted starting materials. Expressed numerically, a preferred amount of silane compound is about 0.1 to 25 parts by weight of silane compound per 100 parts of prepolymer, more preferably about one to 10 parts of silane compound per 100 parts of prepolymer. The triisocyanate prepolymers used in the present invention are commercially available from National Lead Chemicals, Highstown, N.J. The compositions and the structures of these prepolymers are described as follows: where A is a backbone consisting of at least three -CH₂O-groups derived from polyhydric alcohols, such as glycerol, trimethylol propane, pentaerythritol, dipentaerythritol, sorbitol, or inositol. B is a moiety consisting of urethane (-COHN-) linkage to an aliphatic or aromatic segment derived from diisocyanates such as cyclohexyl diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, if aliphatic, and 2,4,toluene diisocyanate, 4,4′-diphenylmethane diisocyanate, 4,6′-xylylene diisocyanate, if aromatic. The prepolymer is usually prepared as a solution with a suitable solvent. Some examples of useful prepolymer solutions include the following. It should be noted that the solids content and the particular solvent is given for illustrative purposes and both the solids content and the solvent may be changed depending upon the needs of the user. Prepolymer Solution A This is a solution of an aromatic prepolymer which is a reaction product of trimethylol propane (TMP) with 3 moles of toluene diisocyanate (TDI). The isocyanate prepolymer structure is as follows: The solution contains 60% solids of the isocyanate and 40% methoxy propyl acetate as solvent. The prepolymer solution has an available NCO content of 10% by weight. Prepolymer Solution B Prepolymer Solution B is a solution of a blend of prepolymers which are the mixed reaction products of trimethylol propane and 1,3-butandiol with toluene diisocyanate (TDI). The reaction product structures are as follows. Prepolymer Solution B contains 75% solids as a blend of the two polyisocyanates at a molar ratio of II:III of 2:1, and 25% methyl ethyl ketone as solvent. The available NCO content is 12.5% by weight. Prepolymer Solution C Prepolymer Solution C is a solution of a blend of prepolymer reaction products of trimethoyl propane (TMP) and 1,3-butandiol with isophorone diisocyanate (IPDI). The reaction products have the following structures. Prepolymer Solution C contains 60% solids of the blend in a molar ratio of IV to V of 2.1:1, and 40% 50/50 methoxy propyl acetate/xylene as solvent. The available NCO content is 7.5% by weight. Prepolymer Solution D Prepolymer Solution D is the same as Prepolymer Solution C except the ratio of IV to V is 1.7:1, it has an available NCO of 8.8% by weight, and it contains 65% solids and 35% 50/50 methoxy propyl acetate/toluene as solvent. Prepolymer Solution E Prepolymer Solution E is a solution of a trimer of isophorone diisocyanate prepolymer, having the following formula. where R = This solution had a solids content of 70% by weight, an NCO content of 10.2% by weight and 50/50 by weight methoxy propyl acetate/xylene solvent. The prepolymers typically have relatively low molecular weights, i.e., from about 400 to about 800. The branched triisocyanate in the polymer structure plays an important role in controlling the degree of crosslinking in curing process. The diisocyanate prepolymer can be considered as a modifier that adds more linear chains to provide a more flexible coating. The prepolymers used in the invention are preferably substantially free of the reactants such as unreacted toluene 2,4-diisocyanate and isophorone diisocyanate, which have a relatively high vapor pressure and may be somewhat toxic. The epoxy silane compound is free of active hydrogen-containing functional groups which could cause premature gelation of the prepolymer when combined therewith. Thus, epoxy silanes containing hydroxy, mercapto, amino, ureido functional groups are not suitable because they contain undesirable reactive groups. Preferably, the epoxy silane compound contains one or more alkoxy groups, e.g., methoxy or ethoxy, with methoxy groups being most preferred. The epoxy silane is characterized by having a polymerizable (preferably terminal) epoxy group and a polymerizable silane group. These groups are connected by a linking moiety which is free of functional groups which will hydrolyze in the presence of the prepolymer. This linking moiety may be aliphatic or aromatic or be formed of both aliphatic and aromatic moieties. The linking moiety may contain nitrogen although it is preferably free of nitrogen atoms. The linking moiety may contain oxygen atoms which are typically within the molecule as ether linkages and most preferably are present only adjacent to the epoxy group. The linking moiety may be substituted, as is well known in the art, so long as the substitution does not greatly affect the ability of the epoxy-terminated silane to cross link the prepolymer. Example substituents which may be included in the linking moiety include NO₂, alkyl [e.g., -CH₂(CH₂) n CH₂- where n is from 0 to 20], alkoxy (e.g., methoxy), halogen, etc. Preferred epoxy silanes useful in the practice of this invention are compounds of general formulae: where R is a divalent hydrocarbon radical (aliphatic, aromatic, or mixed aliphatic-aromatic containing) of less than 20 carbon atoms or a divalent radical of less than 20 carbon atoms consisting of carbon, nitrogen, sulfur and oxygen atoms, with the oxygen atoms present as ether linkages and with no two heteroatoms being adjacent within the backbone of the divalent hydrocarbon radical. More preferred epoxy silanes are represented by the following formulae. wherein R³ is a non-hydrolyzable divalent hydrocarbon radical of fewer than 20 carbon atoms or a divalent group of fewer than 20 carbon atoms and backbone of which is composed of only carbon, nitrogen, sulfur and oxygen atoms with no two adjacent heteroatoms, and R² is an aliphatic hydrocarbon group (e.g., alkyl) or acyl group of fewer than 10 carbon atoms. In the epoxy silane compositions of formulae VII -X, n is from 0 to 1, m is 1 to 3, R may be any divalent hydrocarbon radical such as methylene, ethylene, decalene, phenylene, cyclohexylene, cyclopentylene, methylcyclohexylene, 2-ethylbutylene, and allene or an ether radical such as -CH₂-CH₂-O-CH₂-CH₂-, -(CH₂CH₂O)₂-CH₂-CH₂-, -O-CH₂-CH₂-, and -CH₂O-(CH₂)₃-. R¹ can be any aliphatic hydrocarbon radical of less than 10 carbon atoms such as methyl, ethyl, isopropyl, butyl, vinyl, alkyl, or any acyl radical of less than 10 carbon atoms such as formyl, acetyl, propionyl, or any radical of the formula (CH₂CH₂O)) k Z in which k is an integer of at least 1, and Z is hydrogen. The most preferred epoxy silanes are those represented by the formulae: wherein R is an alkyl group of up to 6 carbon atoms, and m and n are independently 1 to 6. Catalysts in the present invention are generally used in the amounts of from 0.01 to 5% by weight of the reactive ingredients in the curable composition. Preferably in the range of 0.05 to 1.0% by weight. The most preferably catalysts are dibutyl tin dilaurate and tin octoates. Other useful catalysts include triethylene diamine, dimethyl piperazine, bis[2-(N,N-dialkylamino)alkyl] ethers and dimorpholino dialkyl ethers, and other urethane-forming catalysts known to those skilled in the art. Suitable solvents used in this invention are generally free of active hydrogen. Examples of useful solvents include ketones (such as acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl isoamyl ketone), aromatics (such as benzene, toluene, and xylene), acetates (such as ethyl acetate, methoxy propyl acetate, dipropylene glycol methyl ether acetate) and chlorinated solvents (such as 1,1,1 trichloroethane). The preferred solvents are methoxy propyl acetate, methyl isobutyl ketone, toluene and the blend of these. The range of solvent content can be from about 0 to about 95% by weight, depending upon different prepolymer and silane formulations used. The preferred solvent content is from 20 to about 80% by weight. Coloring agents, including organic dyes, may be incorporated into the composition of the invention to form compositions which can stain and finish a substrate. These materials are, for example, metallic azo dyes, and organic dyes (such as phthalocyanine dye and perylene dye). The weight percent of such additives is preferably from 1% to about 90% of the total solids. Other conventional additives may also be added. The coating composition is preferably packaged in conventional containers used for this purpose such as metal cans or containers made of high density polyethylene, nylon-coated polyethylene, polytetrafluoroethylene-coated polyethene, or in a sealed pouch made of a laminate of polyethylene and aluminum foil. The coating composition may include a suitable aerosol propellant and be packaged in a spray can, or in a conventional can for brush or "wipe on" application. The compositions of the invention are cured by exposure to water, e.g., water vapor or moisture. Ambient humidity usually is adequate to promote cure. Heat or high humitidy will accelerate cure, and low temperatures (e.g., 5°C or less) or low humidity (e.g., 15% R.H. or less) will retard cure. Reference to the following examples will provide further understanding of the present invention. All parts are by weight, unless otherwise stated. Examples 1-15 Examples 1-15 demonstrate the use of different prepolymers and silanes in a 1:1 molar ratio of available NCO in prepolymer to in the silanes in comparison with the use of prepolymers alone. The composition of Examples 1-15 is given in Table I. Curing time was recorded for 3 drops of a test formulation on glass surface until a tack-free condition was reached under ambient laboratory conditions. The physical property testing was accomplished by the following procedures. Hardness was measured by ASTM Pencil Hardness Test D3363-74 (1980) to determine the degree of crosslinking and mar resistance. Stain resistance involved the application of four different types of stain to the surface being tested. The types of stains were as follows: (a) Blue-dyed ethanol-water solutions with an ethanol concentraion of 80%, 70%, and 60%, respectively, each solution containing 0.2% of a dye which is commercially available as Sandan Blue E-HRL (from Sandoz Colors and Chemicals). (b) Fingernail polish commercially available from Revlon Inc, New York, New York as BLAZE OF RED 55 and which was composed of a mixture of butylacetate, ethylacetate, toluene, isopropyl alcohol, resins and pigments. (c) Marking pen commercially available as Berol Marker 8800 (red color), which contained a red coloring material dissolved in organic solvents. (d) Shoe polish commercially available from KIWI as Black Liquid Wax which was thought to contain hydrocarbon solvent, resins and wax. These household items were selected because they are common items which often cause staining problems. The stains were applied to the test coating which had been coated on a wood (e.g., maple, oak, etc.) block, after the test coating had dried. After being permitted to dry, the stains were removed by wiping with a cheese cloth soaked with acetone or fingernail polish remover. The surface condition of the test coating was then rated on a scale from 1 to 5, according to the following criteria: 1- coating was stripped during stain removal or stain penetrated deeply to coating; 2- coating was discolored or stain partially remains; 3- stain remained weakly visible in coating; 4- stain remained vaguely visible in coating; and 5- surface unchanged from condition prior to staining. The heat resistance test envolved the use of a 2 3/4 inch (6.9 mm) diameter 3 inch (7.5 mm) high stainless steel cylinder weighing 5 lbs (2.27 kg) which was heated in oven at 152°C for 15 minutes. The block was quickly removed from the oven and placed on its circular end onto a test coated surface which had previously been covered with one layer of cheese cloth, maintaining the cylinder on the surface for 30 minutes. After removal of the cylinder and cheese cloth, the coating was observed and rated from 1 to 5 according to the following criteria: 1- heavy discoloration and marking; 2- heavy discoloration or marking; 3- light discoloration and marking; 4- light discoloration or marking; and 5- no change from original surface. Overall stain and heat resistance results, shown in Table II, reveal that coatings according to the invention which contain prepolymer and silane have better performance than coatings made with the prepolymer alone. Example 16 This example demonstrates the chemical stability of the one-component, moisture-curable composition according to the invention. A coating formulation was prepared using 63 parts of Prepolymer Solution A, 21 parts of gamma-glycydoxy propyl trimethoxysilane, 63 parts of toluene, and 0.25 part of dibutyltin dilaurate. The solution was divided into two parts. One part was kept as a control and the other part was aged by heating at 60°C for 72 hours. Both samples were analyzed by nuclear magnetic resonance spectrometry and infrared spectrometry and found to have the same molar ratio of active ingredients. Examples 17-26 Comparative Examples 17-26, described in Table III, were prepared to demonstrate the useful range of the two active ingredients used in terms of weight ratio and molar ratio. The test data shown in Table IV shows that optimum performance is at a 1/1 molar ratio and 3/1 weight ratio of prepolymer to silane, and that the preferred range is from 10/1.0 to 1.0/1.0 (weight ratio) and from 3.37/1.0 to 0.34/1.0 (molar ratio). Example 27 This example compares the performance of commercially available coating products with the coating of Example 3, a coating composition of the invention. The penetration is determined by two factors - "spread" and "absorption". Spread is the ability of a one ml volume of a test coating composition to spread on a pine surface before it dries. Absorption measures the height of a test coating carried vertically up an oak column (1 inch in diameter) 10 seconds after the column is placed end wise in a container of the test coating composition. The larger the spread area and the higher the height reached on the column, the better the penetration into wood. The product of the invention was found to be superior to the commercially available products in all categories. The comparison of properties is shown in Table V.

Claims: A one-part, moisture-curable polyurethane composition curable to form clear coatings comprising a triisocyanate functional prepolymer which prepolymer is of formula : wherein A is a backbone consisting of at least three -CH₂O- groups derived from polyhydric alcohols and B is a moiety consisting of a urethane linkage to an aliphatic or aromatic segment derived from a diisocyanate, or is a trimer of isophorone diisocyanate of formula : Where R is : an amount of at least 0.1 parts by weight per 100 parts by weight of prepolymer of an epoxy silane compound effective to cross-link said prepolymer; said epoxy silane being free of active hydrogen-containing functional groups to avoid premature gelation of the pre-polymer, optionally diisocyanate functional prepolymer, and optionally sufficient compatible liquid vehicle to make the compositiom suitable for application. The composition of Claim 1 characterized by said triisocyanate functional prepolymer consisting of a reaction product of a diisocyanate with a triol. The composition of claim 2 characterized by said diisocyanate being selected from the group consisting of aromatic and aliphatic diisocyanates. The composition of any of claims 1 to 3 characterized by said epoxy silane being selected from the group consisting of gamma glycidoxypropyltrimethoxy silane and beta-(3,4-epoxy-cyclohexyl)ethyltrimethoxy silane. An article characterized by being a substrate treated with a cured moisture-curable composition according to any one of claims 1-4, said substrate having a clear coating of said cured composition thereon. The article of claim 5 further characterized by said substrate being wood. A method of treating a substrate surface to provide it with a protective surface, said method comprising: (a) preparing the surface of said substrate for treatment, if needed; (b) applying to said surface a one-part moisture-curable polyurethane composition; and (c) subjecting the coating applied in (b) to conditions which facilitate the cure of said composition characterized by said one-part moisture-curable polyurethane composition being according to any one of claims 1-4. The method of claim 7 further characterized by said substrate being wood.

Inventors: FONG JAMES J C O MINNESOTA MIN, FONG, JAMES J. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1988
Main classification: C08G18
IPC codes: C09D175, C08G18