A versatile two-component polyurethane resin and coatings formed therefrom

Abstract

A two-part polyurethane system is provided. The system includes a resin and a crosslinker component. The resin has units obtained from a polyol and a polyisocyanate. The polyol has free carboxyl groups and from 0.5% to 3% by weight of free hydroxyl groups. The polyisocyanate has ‑NCO groups reactive with the free hydroxyl groups of the polyol in amounts to provide a molar ratio of -NCO / -OH of from 0.1 to 0.3 such that the resin thus formed contains minimal free hydroxy groups and sufficient free carboxyl groups to provide an acidity from 5-50 KOH / g. The crosslinker component includes a carbodiimide compound, polyaziridine compound, or a combination thereof. The crosslinker component is configured to react with the free carboxyl groups of the resin to form a cured polyurethane coating.

Description

A VERSATILE TWO-COMPONENT POLYURETHANE RESIN AND COATINGSFORMED THEREFROMTechnical Field

[0001] The present invention relates to a two-component polyurethane system including a polyurethane resin formed from a pre-reaction and a crosslinker component comprising a carbodiimide, polyaziridine, or a combination thereof; coatings formed therefrom; and methods of making the same.Background of the Invention

[0002] The US paint and coatings industry includes more than 1,000 companies with combined annual sales in excess of $20 billion and continues to grow. Typical coatings used for indoor applications, such as on drywall in a house, comprise a waterborne latex. Such waterborne latex coatings at least include a polymer dispersed in water. Upon applying a waterborne latex-based coating to a substrate, the water evaporates, and the remaining polymer coalesces to form a continuous, cured film on the substrate. Though waterborne latex-based coatings are low-cost, easy to apply to a surface, and easy to cleanup, drying of waterborne latex-based coatings vary depending on the humidity and temperature of the application surface and surrounding environment. Additionally, dried waterborne latex -based coatings are prone to scratches and cracking.

[0003] To achieve a highly protective coating that is less prone to scratching, cracking, and other damage by various environmental conditions (e.g., humidity, temperature, sunlight, etc.), polyurethane-based coatings may be used. For example, a polyurethane-based coating may be used on wood furniture, outdoor surfaces, indoor trim, marine applications, or other surfaces that are exposed to harsh conditions. One-component polyurethane coatings cure by crosslinking when exposed to water and / or heat. In two-component polyurethane coatings, a first component is mixed with a second component before applying the coating to a surface. Two-component polyurethane coatings typically do not require water or heat for curing, and instead may depend on the specific ratio of components mixed together and / or a crosslinker agent in the two-component polyurethane system for curing. Although two-component polyurethane coatings may have higher mechanical properties compared to one-component polyurethane systems, there are environmental, health, andsafety concerns associated with the reaction upon mixing the two components to form a cured, two- component polyurethane coating.Summary of the Invention

[0004] In accordance with an embodiment of the present invention, a two-part polyurethane system is provided. The system may comprise a resin and a crosslinker component. The resin has units obtained from a polyol and a polyisocyanate. The polyol has free carboxyl groups and from 0.5 to 3% by weight of free hydroxyl groups. The polyisocyanate has -NCO groups reactive with the free hydroxyl groups of the polyol in amounts to provide a molar ratio of -NCO / -OH of from 0.1 to 0.3 such that the resin thus formed contains minimal free hydroyxl groups and sufficient free carboxyl groups to provide an acid value (referred to herein as “acidity”) of from 5-50 mg KOH / g. Acidity (or acid value) can be determined using conventional titration techniques, such as ASTM D4662 and ASTM D7253. In the present invention, the acidity of from 5-50 mg KOH / g is obtained based on the ratio of free hydroxyl groups and free carboxyl groups present in the polyurethane, with the goal being to adjust these to obtain the desired acidity. The actual hydroxyl number is not necessary to measure for the final polymer. The crosslinker component may comprise a carbodiimide compound, a polyaziridine compound, or a combination thereof and is configured to react with the free carboxyl groups of the resin to form a cured polyurethane coating.

[0005] In accordance with an embodiment of the present invention, a method of forming a coating is provided. The method includes mixing a resin comprising polyurethane with a crosslinker component comprising a carbodiimide compound, a polyaziridine compound, or a combination thereof. The mixture of resin and crosslinker component is applied to a substrate. The mixture of resin and crosslinker component is cured to form a cured polyurethane coating on the substrate. The resin comprises free carboxyl groups and has units obtained from a polyol and a polyisocyanate. The polyol contains free -OH groups, and the polyisocyanate contains free -NCO groups. A molar ratio of the -NCO groups in the isocyanate to -OH groups in the polyol is from 0.1 to 0.3.

[0006] These and other objects of this invention, alone or in combination, have been satisfied by the discovery of a two-component polyurethane system comprising a resin and crosslinker component. The polyurethane system and coating composition made therefrom will be further described in the following detailed description and appended claims.Brief Description of the Drawings

[0007] The invention may take physical form in certain parts and arrangements of parts, a preferred embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof, and wherein:

[0008] FIG. 1 illustrates a chemical drawing of some embodiments of a reaction between free carboxyl groups of a polyurethane resin with a carbodiimide-based crosslinker component to form a cured crosslinked polyurethane coating comprising carbonyl urea groups.Detailed Description o f the Invention

[0009] The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

[0010] To the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the present application, such terms are intended to be inclusive in a manner similar to the term “comprising.” The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Additionally, the terms "a," "an," "the," "at least one," and "one or more" are used interchangeably. Thus, for example, a coating composition that contains "an" additive means that the coating composition can include "one or more" additives.Approximating language, as used herein throughout the specification and claims, may be applied to modify a quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term such as “about” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Moreover, unless specifically stated otherwise, a use of the terms “first,” “second,” etc., do not denote an order or importance, but rather the terms “first,” “second,” etc., are used to distinguish one element from another.

[0011] The term “comprises” and variations thereof do not have a limiting meaning where these terms appear in the description and claims.

[0012] As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and / or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances an event or capacity can be expected, while in other circumstances the event or capacity cannot occur - this distinction is captured by the terms “may” and “may be.”

[0013] The term "acrylic" as used herein includes (meth)acrylic acid, (meth)alkyl acrylate, (meth)acrylamide, (meth)acrylonitrile and their modified forms such as (meth)hydroxyalkyl acrylate. Throughout this document, the word fragment "(meth)acryl" refers to both "methacryl" and "acryl". For example, (meth)acrylic acid refers to both methacrylic acid and acrylic acid, and methyl (meth)acrylate refers to both methyl methacrylate and methyl acrylate.

[0014] The term “aliphatic” when used in the context of a carbon-carbon double bond includes both linear (or open chain) aliphatic carbon-carbon double bonds and cycloaliphatic carbon-carbon double bonds, but excludes aromatic carbon-carbon double bonds of aromatic rings.

[0015] The term "aqueous" composition or dispersion herein means that particles are dispersed in an aqueous medium. An "aqueous medium" herein has a continuous phase of water that makes up at least 50 weight percent of the aqueous medium, wherein the remaining composition of the aqueous medium comprises particles and water-miscible compound(s) such as, for example, alcohols, glycols, glycol ethers, glycol esters, and the like.

[0016] The term “(co)polymer” as used herein includes both homopolymers (polymers containing units from a single monomer) and copolymers (polymers containing units from two or more different monomers), unless otherwise specifically stated.

[0017] The term “crosslinker” as used herein refers to a molecule capable of forming a covalent linkage between polymers or between two different regions of the same polymer.

[0018] The term "glass transition temperature" or "Tg" in the present invention can be measured by various conventional techniques including, for example, differential scanning calorimetry ("DSC") or calculation by using a Fox equation. DSC data and methods described herein are in accordance with ASTM D6604-00.

[0019] The term “on,” when used in the context of a coating applied on a substrate, includes both coatings applied directly or indirectly to the substrate. Thus, for example, a coating applied to a primer layer overlying a substrate constitutes a coating applied on the substrate.

[0020] The terms "preferred" and "preferably" refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

[0021] As used herein, the term “structural units,” also known as polymerized units, of the named monomer refers to the remnant of the monomer after polymerization, or the monomer in polymerized form.

[0022] Embodiments of the invention disclosed herein relate to a two-part polyurethane system. The two-part polyurethane system comprises a first part and a second part that when combined, are configured to react to form a cured polyurethane coating. The first part of the two-part polyurethane system comprises a resin having free carboxyl groups. The second part of the two-part polyurethane system comprises a crosslinker component. The crosslinker component may comprise one or more crosslinkers, including, but not limited to a carbodiimide compound, a polyaziridine compound, or a combination thereof. The crosslinker component is configured to react with the free carboxyl groups of the polyurethane resin. In some embodiments, the carbodiimide is more effective as a crosslinker component. Thus, in some embodiments, the crosslinker component comprises carbodiimide alone or a combination of carbodiimide and polyaziridine. In such combination embodiments, it may be more effective to have more carbodiimide than polyaziridine present in the combination. A cured polyurethane coating is formed once the first and second parts are mixed and react with one another. While the final coating is crosslinked and may not formally be considered apolyurethane polymer, the overall system is referred to as a polyurethane system throughout this disclosure because the system works by crosslinking an initially formed polyurethane resin.

[0023] The two-part polyurethane system and corresponding cured polyurethane coating are formed from common, low-cost raw materials that also do not contain any substances of very high concern (SVCH), that are not very persistent and very bioaccumulative (vPvB), and that are not carcinogenic, mutagenic, or toxic (CMR). Thus, the two-part polyurethane system has favorable health, safety, and environmental qualities. Additionally, the first component of the two-part polyurethane system has a high molecular weight, which ensures proper film formation and high mechanical properties of the resulting cured polyurethane coating. Further, the reaction between the first and second components of the two-part polyurethane system improves the flexibility and adhesion properties of the system such that the resulting cured polyurethane coating reliably protects the substrate without peeling.

[0024] The resin that makes up the first part of the two-part polyurethane system has units obtained from a polyisocyanate and from a polyol. The poly isocyanate has free -NCO groups, and the polyol has free carboxyl groups and free hydroxyl groups. The amount of free hydroxyl groups and free carboxyl groups in the polyol are controlled such that when the polyol is mixed with the polyisocyanate, almost all of the free hydroxyl groups of the polyol react with the -NCO groups of the polyisocyanate to form the polyurethane resin. Further, the resulting polyurethane resin still has a significant number of free carboxyl groups such that the polyurethane resin can later react with the second part of the two-part polyurethane system.

[0025] The polyol used in the formation of the polyurethane resin comprises about, for example, preferably 0.5% to 3.0% by weight of free hydroxyl groups, more preferably 1.0% to 2.5% by weight of free hydroxyl groups, and most preferably 1.5% to 2.0% by weight of free hydroxyl groups. Without intending to be bound by theory, it is believed that (i) if there are less free hydroxyl groups in the polyol, then not enough polyurethane polymers would be formed in the resulting resin, (ii) if there are more free hydroxyl groups than the aforementioned ranges in the polyol, then the resulting polyurethane resin may not be sufficiently reactive with the crosslinker component comprising carbodiimide, polyaziridine, or a combination thereof, and (iii) if there are more free hydroxyl groups, less crosslinking will occur, which may result in lower performance of the final coating, namely for chemical resistance, especially to alcohols.

[0026] Additionally, the polyol used to form the polyurethane resin also comprises a significant number of free carboxyl groups such that the polyol and resulting polyurethane resin each have a high acidity of, for example, preferably 5 mg KOH / g to 50 mg KOH / g, more preferably 5 mg KOH / g to 35 mg KOH / g, and even more preferably 10 mg KOH / g to 20 mg KOH / g. Without intending to be bound by theory, it is believed that (i) if the amount of free carboxyl groups in the polyol is lower, resulting in a lower acidity, then there would not be enough free carboxyl groups available to react with the second part of the two-part polyurethane system; as a result, a fully cured and crosslinked polyurethane coating would not be formed, and (ii) if the amount of free carboxyl groups in the polyol is higher, resulting in a higher acidity, then more crosslinker component from the second part of the two-part polyurethane system may be needed to fully cure the resulting polyurethane coating, or the cured resin would have remaining carboxyl reactive groups that may provide unwanted properties in the final coating; this would also increase the time to cure the polyurethane coating and increase cost of the two-part polyurethane system without improving the properties of the resulting polyurethane coating.

[0027] In some embodiments, the polyol is a polyol having film forming properties, preferably an acrylic polyol or some other suitable polyol having sufficient free hydroxyl groups reactive with the -NCO groups of the polyisocyanate. The polyol used has a high molecular weight to improve the mechanical properties of the resulting cured polyurethane coating. In some implementations, the weight average molecular weight (Mw) of the polyol is greater than typical acrylic polyols used in industrial wood applications. For example, the weight average molecular weight (Mw) of the polyol used herein may be greater than 50,000 g / mol. Other examples of the polyol that may be used to form the polyurethane resin are provided further herein. Further, in some embodiments, the polyol may be solvent based. For example, the polyol may be supplied in butyl acetate or some other suitable solvent that allows for the formulation of aromatic-free systems. Other examples of solvents that may be used to form the polyurethane resin are provided further herein.

[0028] The polyisocyanate used in the formation of the polyurethane resin comprises about, for example, preferably 15% to 35% by weight of -NCO groups, more preferably 20% to 25% by weight of -NCO groups, and even more preferably 21% to 23% by weight of -NCO groups. Without intending to be bound by theory, it is believed that (i) if the amount of -NCO groups in the polyisocyanate is higher, then too many free hydroxyl groups may be left in the polyurethane resin,which may disrupt the crosslinking step when the second part is added to the resin, and (ii) if the amount of -NCO groups in the polyisocyanate is lower, then not enough polyurethane polymers may be formed in the resulting resin.

[0029] In some embodiments, the polyisocyanate may be an isocyanate oligomer. In certain embodiments, the polyisocyanate is a member selected from the group consisting of hexamethylene- 1,6-diisocyanate (HDI trimer), pentamethylene diisocyanate (PDI), isophorone diisocyanate (IPDI), 4,4’-diisocyanato dicyclohexylmethane (HDMI), tetramethylxylene diisocyanate (TMXDI), and blocked derivatives thereof. In certain embodiments, the polyisocyanate is hexamethylene-l,6-diisocyanate (HDI trimer). In some other embodiments, some other suitable polyisocyanate that has a suitable percent by weight of -NCO groups and that is reactive with the free hydroxyl groups of the polyol may be used. Such other examples of the polyisocyanate that may be used to form the polyurethane resin are provided further herein. In some embodiments, the polyisocyanate is a solvent-free solution. Further, in some embodiments, the polyisocyanate is aliphatic to better facilitate the reaction between the polyisocyanate and the polyol. By using aliphatic structures, the resulting polyurethane coating may be more flexible which improves the adhesion properties to the substrate and reduces cracking of the cured polyurethane coating. Further, the use of aliphatic structures reduces yellowing of the cured polyurethane coating over time.

[0030] To form the polyurethane resin, particular amounts of the polyol and the polyisocyanate are combined such that the reaction between the free hydroxyl groups in the polyol and the -NCO groups in the polyisocyanate is completed. The resulting polyurethane resin has a substantial amount of free carboxyl groups and a limited amount of free hydroxyl groups. Because the free carboxyl groups are not significantly reactive with the -NCO groups, the number of free carboxyl groups in the polyurethane resin may be about equal to the number of free carboxyl groups in the polyol reactant. The polyurethane resin is formed, and thus, the reaction between the polyol and the polyisocyanate is completed when the concentration of -NCO is spectroscopically undetectable. In some embodiments, the concentration of -NCO may be measured using, for example, Flourine-19 Nuclear Magnetic Resonance (F NMR) spectroscopy, such as the method disclosed in A. Moghimi et al., Polymer Testing 33 (2014) 30-33. Less preferably, the concentration of -NCO may be measured using Fourier Transform Infrared (FT-IR) spectroscopy, middle infrared (MIR)spectroscopy, near infrared (NIR) spectroscopy, or some other suitable method. Because the -NCO groups are fully reacted, the resulting polyurethane resin may be classified as a “non-isocyanate” (NISO) product. This way, when a consumer purchases the overall two-part polyurethane system, the consumer is not exposed to the -NCO groups in either the polyurethane resin (i.e., the first part) or in the crosslinker component (i.e., the second part).

[0031] To ensure that the -NCO groups are fully reacted, then the molar ratio of the -NCO groups to the -OH groups is less than 1. For example, in some embodiments, the molar ratio of the -NCO groups in the polyisocyanate to the -OH groups in the polyol just prior to mixing the polyol and polyisocyanate to form the polyurethane resin is from, for example, preferably 0.1 to 0.3, more preferably 0.15 to 0.25, and even more preferably 0.1 to 0.2. The disclosed molar ratio of - NCO groups to -OH groups provides an optimal compromise between performance and molecular weight of the resulting cured polyurethane resin. With a higher molecular weight, the polyurethane coating forms a continuous film over the applied substrate and properly adheres to the applied substrate upon curing. Additionally, because of the higher molecular weight, the polyurethane resin has improved mechanical properties and corrosion resistance. The polyurethane resin also has a chemical resistance that is typical of other acrylic resins used in two-part polyurethane systems. In some embodiments, the molecular weight may be measured using gel permeation chromatography (GPC), mass spectrometry, or some other suitable method.

[0032] In some implementations, to form a cured polyurethane coating, the polyurethane resin may be mixed with a carbodiimide-based crosslinker component. Upon mixing, the mixture of the crosslinker component and the polyurethane resin may be applied to a substrate. As shown in FIGURE 1, the carbodiimide crosslinker component is configured to react with the free carboxyl groups present in the polyurethane resin. Over time, the crosslinker component fully reacts with the free carboxyl groups to form carbonyl urea groups, which ultimately forms a fully cured, crosslinked polyurethane coating on a substrate. By using a carbodiimide-based crosslinker component, the coating is more flexible when applied to the substrate and thus, can better adhere to the substrate upon curing. In some other implementations, the crosslinker component may comprise a polyaziridine compound or a combination of a polyaziridine compound and a carbodiimide compound.

[0033] The curing of the polyurethane coating is not sensitive to water, and thus, the polyurethane coating can still cure in humid environments. Additionally, the potlife, which is the time between mixing the first and second parts and applying the polyurethane coating to the substrate is substantially long, for example, in a range of between about 5 hours to about 24 hours, about 5 hours to about 12 hours, or about 5 hours to about 8 hours. In some embodiments, the potlife is at least 5 hours. Thus, when a user mixes the first part with the second part, the user does not have to work extremely quickly and compromise the work product when applying the polyurethane coating to the substrate. In some embodiments, the disclosed polyurethane coating system is solventborne, whereas in some other embodiments, the disclosed polyurethane coating system may be waterborne or aqueous.

[0034] In some embodiments, the two-component polyurethane system is particularly suitable for wood substrates. For example, wood substrates often have an unpredictable texture to which the high-molecular weight and flexible polyurethane coating system disclosed herein can conform. Additionally, wood substrates sometimes attract moisture, to which the two-part polyurethane system disclosed herein is not sensitive for curing. Thus, the two-part polyurethane system provides a reliable, self-sealing coating over wood substrates. It will be appreciated that this two-part polyurethane system is not limited to wood substrate applications, and thus, the two-part polyurethane system may be applied to, for example, drywall, cement, ceramic, plastic, metal, or some other suitable material. Because of the high-molecular weight of the two-part polyurethane system, fewer coats of the system may be used on the substrate to provide a protective effect to the substrate when compared to lower-molecular weight polyurethane coatings.

[0035] As discussed above, the method of forming the two-part polyurethane system includes the formation of a first part (i.e., a polyurethane resin) from a reaction between a polyol and a polyisocyanate. The resulting polyurethane resin comprises substantially no -NCO groups but comprises a significant amount of free carboxyl groups. Further, the second part (i.e., a crosslinker component comprising a carbodiimide compound, a polyaziridine compound, or a combination thereof) is provided. The first part and the second part may be produced in a manufacturing facility prior to sale to the end customer. When the first part and second part are then combined in a customer paint line, any airborne byproducts such as volatile organic compounds (VOCs) that arereleased during a flash off and curing time may be properly contained and processed within the customer painting line facility.

[0036] After the first and second parts are manufactured, a consumer may purchase the first part housed in a first container and a second part housed in a second container. When a substrate is ready for coating, the first part is mixed with the second part at a predetermined ratio. For example, the mixture may comprise about 70% to about 90% or more preferably about 80% to about 85% of the first part (i.e., the polyurethane resin). Thus, for example, the mixture may comprise about 10% to about 30% or more preferably about 15% to about 20% of the second part (i.e., the crosslinker component). In some embodiments, the second part (i.e., the crosslinker component) may be a diluted solution. The first and second parts may be mixed using a paint stick, a bucket agitator, an electric mixing attachment, or some other suitable mixing tool. The resulting mixture of the first part and the second part may be applied to the prepared substrate. Over time, the free carboxyl groups of the polyurethane resin react with the crosslinker component to seal the substrate by forming a cured, crosslinked polyurethane coating on the substrate. In some embodiments, to increase the thickness of the coating on the substrate and better protect the substrate, at least one additional polyurethane coating may be applied to the first polyurethane coating. In some such embodiments, the first polyurethane coating does not have to be fully cured and does not need to be sanded before applying a second polyurethane coating to the first polyurethane coating. Instead, the second polyurethane coating may be applied within a matter of hours, often within about 3 hours, of the application of the first polyurethane coating. Thus, the potlife of the disclosed polyurethane system is greater than the wait-time between applying a first polyurethane coating and a second polyurethane coating. This way, a consumer does not have to mix a second batch of the polyurethane system in applications where at least two coats of the polyurethane coating are used. As will be discussed further herein, the disclosed polyurethane coating may fully cure in about 3-10 days. It will be appreciated that in some embodiments, a primer coating or some other coating may be arranged between the substrate and the first polyurethane coating. Similarly, it will be appreciated that in some embodiments, at least one topcoat, such as a clearcoat, may be applied to the first polyurethane coating or the at least one additional polyurethane coatings.

[0037] In some embodiments, any suitable polyol or mixture of polyols may be used to form the polyurethane resin of the present invention given that the adequate amount of carboxyl andhydroxyl groups as described above are present in the polyol or mixture of polyols. The one or more polyols may be a monomer, an oligomer, a polymer, or a mixture thereof. In addition, the one or more polyols can be a diol, a triol, a polyol having 4 or more hydroxyl groups, or a mixture thereof. Non-limiting examples of suitable oligomer and / or polymer polyols include polyether polyols, polyester polyols, polyether-ester polyols, polyimide polyols, polyurea polyols, polyamide polyols, polycarbonate polyols, saturated or unsaturated polyolefin polyols, polyurethane polyols, and combinations thereof. Suitable polyol monomers may include, for example, glycols and / or glycerol.

[0038] In some embodiments, the polyol used to form the polyurethane resin may be solvent based. Non-limiting examples of suitable organic solvents for use in the water-based and / or solventbased coating compositions of the present invention include aliphatic hydrocarbons (e.g., mineral spirits, kerosene, VM&P NAPHTHA solvent, and the like); aromatic hydrocarbons (e.g., benzene, toluene, xylene, the SOLVENT NAPHTHA 100, 150, 200 products and the like); alcohols (e.g., ethanol, n-propanol, isopropanol, n-butanol, iso-butanol and the like); ketones (e.g., acetone, 2- butanone, cyclohexanone, methyl aryl ketones, ethyl aryl ketones, methyl isoamyl ketones, and the like); esters (e.g., ethyl acetate, butyl acetate and the like); glycols (e.g., butyl glycol); glycol ethers (e.g., ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, and the like); glycol ether esters (e.g., butyl glycol acetate, methoxypropyl acetate and the like); and mixtures thereof.

[0039] A variety of isocyanate-functional compounds may be used to form the polyurethane resin given that the adequate amount of -NCO groups as described above are present in the isocyanate- functional compound. In some embodiments, the isocyanates are incorporated into the polyurethane polymer predominantly or exclusively through urethane linkages. In other embodiments, at least some of the isocyanate compound may be incorporated into the backbone of the polyurethane polymer via one or more non-urethane step-growth linkages (e.g., urea) formed through a reaction involving an isocyanate group (-NCO) of the isocyanate compound.

[0040] The isocyanate compound may be any suitable compound, including an isocyanate compound having 1 isocyanate group; a polyisocyanate compound having 2, 3, or 4 or more isocyanate groups; or a mixture thereof. Suitable diisocyanates may include hexamethylene diisocyanate (HD I), pentamethylene diisocyanate (PDI), isophorone diisocyanate (IPDI), 4,4’-diisocyanato dicyclohexylmethane (HDMI), tetramethylxylene diisocyanate (TMXDI), isophorone diisocyanate (i.e., 5-isocyanato-l -isocyanatom ethyl- 1,3, 3-trimethyl cyclohexane); 5-isocyanato-l - (2-isocyanatoeth-l-yl)-l,3,3-trimethylcyclohexane; 5-isocyanato-l -(3-isocyanatoprop-l-yl)-l, 3,3- trimethyl cyclohexane; 5-isocyanato-(4-isocyanatobut-l-yl)-l,3,3-trimethylcyclohexane; 1- isocyanato-2-(3-isocyanatoprop-l-yl)cyclohexane; l-isocyanato-2-(3-isocyanatoeth-l- yl)cyclohexane; l-isocyanato-2-(4-isocy-anatobut-l-yl)cyclohexane; 1,2-diisocyanatocyclobutane; 1,3-diisocyanatocyclobutane; 1,2-diisocyanatocyclopentane; 1,3-diisocyanatocyclopentane; 1,2- diisocyanatocyclohexane; 1,3-diisocyanatocyclohexane; 1,4-diisocyanatocyclohexane; dicyclohexylmethane 2,4 '-diisocyanate; trimethylene diisocyanate; tetramethylene diisocyanate; pentamethylene diisocyanate; hexamethylene diisocyanate; ethylethylene diisocyanate; trimethylhexane diisocyanate; heptamethylene diisocyanate; 2-heptyl-3,4-bis(9-isocyanatononyl)-l- pentyl-cyclohexane; 1,2-, 1,4-, and l,3-bis(isocyanatomethyl)cyclohexane; 1,2-, 1,4-, and l,3-bis(2- isocyanatoeth-l-yl)cyclohexane; l,3-bis(3-isocyanatoprop-l-yl)cyclohexane; 1,2-, 1,4- or l,3-bis(4- isocyanatobuty-l-yl)cyclohexane; liquid bis(4-isocyanatocyclohexyl)-methane; and derivatives or mixtures thereof.

[0041] In some embodiments, the isocyanate compounds are preferably non-aromatic. Therefore, in certain embodiments, the polyurethane resin of the present invention (and preferably the coating composition) does not contain any structural units derived from an aromatic isocyanate compound. Pentamethylene diisocyanate (PDI) and hexamethylene diisocyanate (HMDI) are preferred non- aromatic isocyanates.

[0042] The two-part polyurethane system of the present invention may also include other optional ingredients that do not adversely affect the two-part polyurethane system or a cured coating resulting therefrom. Such optional ingredients include, for example, catalysts, dyes, pigments, toners, extenders, fillers, lubricants, anticorrosion agents, flow control agents, thixotropic agents, dispersing agents, antioxidants, adhesion promoters, light stabilizers, surfactants, and mixtures thereof. Each optional ingredient is preferably included in a sufficient amount to serve its intended purpose, but not in such an amount to adversely affect the two-part polyurethane system or a cured coating resulting therefrom.

[0043] The following example is provided to illustrate the present invention and its advantages, but should not be construed as limiting a scope of the invention.

[0044] In one embodiment, the first part of the two-part polyurethane system is formed by first preparing a hydroxy functional acrylic resin supplied in butyl acetate that will allow the formulation of aromatic-free systems. Other additives such as pigments, fillers, and the like may be added to the hydroxy functional acrylic resin. Then, a solvent-free aliphatic polyisocyanate in a liquid phase is mixed with the hydroxy functional acrylic resin to form the polyurethane resin, which is the first part of the polyurethane resin system. The solvent-free aliphatic polyisocyanate is added to the hydroxy functional acrylic resin in an amount such that a molar ratio between the amount of -NCO groups from the polyisocyanate to the amount of -OH groups from the acrylic resin is equal to about 0.25.

[0045] To facilitate the reaction between the acrylic resin and the polyisocyanate to form the first part, the mixture of polyisocyanate and acrylic resin may be stored at ambient temperature and the concentration of -NCO groups may be monitored daily. After a period of about 3-10 days, the concentration of -NCO groups is undetectable, indicating that the reaction is complete and the polyurethane resin is fully formed. Further, the molecular weight of the resulting polyurethane resin may increase by an amount in a range of between about, for example, 10% to about 40% when compared to the molecular weight of the acrylic resin reactant. It will be appreciated that if the storage temperature is greater than ambient temperature, then the time it takes for all of the -NCO groups to react with the -OH groups is decreased; similarly, if the storage temperature is less than ambient temperature, then the time it takes for all of the -NCO groups to react with the -OH groups is increased. For example, in another embodiment, the time to form the polyurethane resin may take about 10 days when stored at 10 degrees Celsius, whereas in yet another embodiment, the time to form the polyurethane resin may take about 3 days to form when stored at 35 degrees Celsius.

[0046] As a non-limiting example, Table 1 shows the change in weight average molecular weight (Mw), number average molecular weight (Mn), and -NCO concentration over time after the polyisocyanate and acrylic resin are mixed together according to the ratios disclosed herein. In particular, the data was collected for a batch of the polyurethane resin comprising various conventional additives and also for a batch of the polyurethane resin without such various conventional additives. The reaction between the polyisocyanate and acrylic resin shown in Table 1 was conducted at room temperature.TABLE 1

[0047] As exemplified by the data of Table 1, the concentration of -NCO was undetectable and thus, reaction was completed between 4 and 7 days. Further, as exemplified by the data of Table 1, the trend in increase in molecular weight over time is similar whether or not additives are present in the resin. During the curing period in each case, the number average molecular weight (Mn) was relatively unchanged (within the limits of the measurement method), while the weight average molecular weight (Mw) increased significantly during the curing.

[0048] As a further non-limiting example, Table 2 shows the change in weight average molecular weight (Mw), number average molecular weight (Mn), and -NCO concentration over time after the polyisocyanate and acrylic resin are mixed together according to the ratios disclosed herein at a reduced temperature of 10 degrees Celsius instead of room temperature. In particular, the data was collected for a batch of the polyurethane resin comprising various conventional additives and also for a batch of the polyurethane resin without such various conventional additives.TABLE 2

[0049] As shown in Table 2, at the lower temperature of reaction, the curing took longer (10 days vs. 7 days), but still occurred.

[0050] As another non-limiting example, Table 3 shows the change in weight average molecular weight (Mw), number average molecular weight (Mn), and -NCO concentration over time after the polyisocyanate and acrylic resin are mixed together according to the ratios disclosed herein at an elevated temperature of 35 degrees Celsius instead of room temperature. In particular, the data was collected for a batch of the polyurethane resin comprising various conventional additives and also for a batch of the polyurethane resin without such various conventional additives.TABLE 3

[0051] Once again, as shown in Table 3, curing takes place at the elevated temperature, and occurs in a quicker timeframe (3 days vs. 7 days). Accordingly, these data show that the product of the invention can be used over a wide range of application temperatures to form coatings.

[0052] Next, after the polyurethane resin is formed, the second part of the two-part polyurethane system is obtained, which may be a crosslinker component comprising a multifunctional polycarbodiimide, a polyaziridine, or a combination thereof. This crosslinker component may be added to the polyurethane resin, and then the mixture of the crosslinker component and polyurethane resin may be applied as a coating to the substrate. Over time, the crosslinker component and the polyurethane resin may react with one another to form a fully cured, crosslinked polyurethane coating over the substrate. The fully cured crosslinked polyurethane coating exhibited the same performance and aesthetic characteristics as a standard two-part polyurethane system.

[0053] The following are non-limiting examples of some embodiments of the present invention: Embodiment 1 . A two-part polyurethane system comprising: a resin having units obtained from: a polyol having free carboxyl groups and from 0.5 to 3% by weight of free hydroxyl groups, and a polyisocyanate having -NCO groups reactive with the free hydroxyl groups of the polyol in amounts to provide a molar ratio of -NCO / -OH of from 0.1 to 0.3, such that the resin thus formed contains minimal free hydroxyl groups and sufficient free carboxyl groups to provide an acidity of from 5-50 mg KOH / g; anda crosslinker component comprising a carbodiimide compound, , a polyaziridine compound, or a combination thereof, the crosslinker component being configured to react with the free carboxyl groups of the resin to form a cured polyurethane coating.Embodiment 2. The two-part polyurethane system of Embodiment 1, wherein the polyisocyanate is a solvent-free aliphatic diisocyanate.Embodiment 3. The two-part polyurethane system of any one of Embodiments 1 or 2, wherein the polyisocyanate has from 20 to 35% by weight of -NCO groups.Embodiment 4. The two-part polyurethane system of any one of Embodiments 1-3, wherein the polyisocyanate is an isocyanate oligomer.Embodiment 5. The two-part polyurethane system of any one of Embodiments 1-4, wherein the polyisocyanate is a member selected from the group consisting of hexamethylene diisocyanate (HD I), pentamethylene diisocyanate (PDI), isophorone diisocyanate (IPDI), 4,4’-diisocyanato dicyclohexylmethane (HDMI), tetramethylxylene diisocyanate (TMXDI), and blocked derivatives thereof.Embodiment 6. The two-part polyurethane system of any one of Embodiments 1-5, wherein the polyol is a solvent based acrylic polyol.Embodiment 7. The two-part polyurethane system of any one of Embodiments 1-6, wherein the polyol has from 1.5 to 2% by weight of free hydroxyl groups.Embodiments 8. The two-part polyurethane system of any one of Embodiments 1-7, wherein the molar ratio of -NCO / -OH is from 0.15 to 0.25.Embodiment 9. A coatings system comprising: a polyurethane resin comprising free carboxyl groups and having units obtained from: a polyol containing free -OH groups, and a polyisocyanate containing -NCO groups, wherein a molar ratio of the -NCO groups in the polyisocyanate to the -OH groups in the polyol is from 0.1 to 0.3; and a crosslinker component comprising a carbodiimide compound, a polyaziridine compound, or a combination thereof, the crosslinker component being configured to react with the free carboxyl groups of the polyurethane resin to form a cured polyurethane coating.Embodiment 10. The coatings system of Embodiment 9, wherein the polyol has from 0.5 to 3% by weight of free -OH groups.Embodiment 11. The coatings system of any one of Embodiments 9 or 10, wherein the polyol has an acidity of from 5-50 mgKOH / g.Embodiment 12. The coatings system of any one of Embodiments 9-11, wherein the polyisocyanate has from 20 to 35% by weight of -NCO groups.Embodiment 13. The coatings system of any one of Embodiments 9-12, wherein the polyisocyanate is a solvent-free aliphatic diisocyanate, and wherein the polyol is a solvent based acrylic polyol.Embodiment 14. The coatings system of any one of Embodiments 9-13, wherein the polyisocyanate is an isocyanate oligomer.Embodiment 15. The coatings system of any one of Embodiments 9-14, wherein the acrylic polyol has from 1.5 to 2% by weight of free hydroxyl groups.Embodiment 16. The coatings system of any one of Embodiments 9-15, wherein the molar ratio of the -NCO groups in the polyisocyanate to -OH groups in the acrylic polyol is from 0.15 to 0.25 Embodiment 17. A method of forming a coating comprising: mixing a resin comprising a polyurethane with a crosslinker component comprising a carbodiimide compound, , a polyaziridine compound, or a combination thereof; applying the mixture of the resin and the crosslinker component to a substrate; and curing the mixture of the resin and the crosslinker component to form a cured polyurethane coating on the substrate, wherein the resin comprises free carboxyl groups and has units obtained from: a polyol containing free -OH groups, and a polyisocyanate containing free -NCO groups, wherein a molar ratio of the -NCO groups in the isocyanate to the -OH groups in the polyol is from 0.1 to 0.3.Embodiment 18. The method of Embodiment 17, wherein the curing occurs over time as the carbodiimide compound and / or polyaziridine compound from the crosslinker component reacts with the substantial amount of free carboxyl groups from the resin.Embodiment 19. The method of any one of Embodiments 17 or 18, wherein the substrate comprises wood.Embodiment 20. The method of any one of Embodiments 17-19, further comprising: forming the resin comprising the polyurethane with the crosslinker component, wherein the resin is formed when the -NCO concentration is spectroscopically undetectable.

[0054] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any examples, or language describing an example (e.g., "such as") provided herein, is intended to illuminate the invention and does not pose a limitation on the scope of the invention. Any statement herein as to the nature or benefits of the invention or of the preferred embodiments is not intended to be limiting. This invention includes all modifications and equivalents of the subject matter recited herein as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. The description herein of any reference or patent, even if identified as "prior," is not intended to constitute a concession that such reference or patent is available as prior art against the present invention. No unclaimed language should be deemed to limit the invention in scope. Any statements or suggestions herein that certain features constitute a component of the claimed invention are not intended to be limiting unless reflected in the appended claims. Neither the marking of the patent number on any product nor the identification of the patent number in connection with any service should be deemed a representation that all embodiments described herein are incorporated into such product or service.

[0055] While the embodiments discussed herein have been related to the coatings and methods discussed above, these embodiments are intended to be examples only and are not intended to limit the applicability of these embodiments to only those discussions set forth herein.

[0056] The above description is merely illustrative of several possible embodiments of various aspects of the present invention, wherein equivalent alterations and / or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In addition, although a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.

[0057] Additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

What is Claimed is:

1. A two-part polyurethane system comprising: a resin having units obtained from: a polyol having free carboxyl groups and from 0.5 to 3% by weight of free hydroxyl groups, and a polyisocyanate having -NCO groups reactive with the free hydroxyl groups of the polyol in amounts to provide a molar ratio of -NCO / -OH of from 0.1 to 0.3, such that the resin thus formed contains minimal free hydroxyl groups and sufficient free carboxyl groups to provide an acidity of from 5-50 mg KOH / g; and a crosslinker component comprising a carbodiimide compound, a polyaziridine compound, or a combination thereof, the crosslinker component being configured to react with the free carboxyl groups of the resin to form a cured polyurethane coating.

2. The two-part polyurethane system of claim 1, wherein the polyisocyanate is a solvent-free aliphatic diisocyanate.

3. The two-part polyurethane system of claim 1, wherein the polyisocyanate has from 20 to 35% by weight of -NCO groups.

4. The two-part polyurethane system of claim 1, wherein the polyisocyanate is an isocyanate oligomer.

5. The two-part polyurethane system of claim 1, wherein the polyisocyanate is a member selected from the group consisting of hexamethylene diisocyanate (HDI), pentamethylene diisocyanate (PDI), isophorone diisocyanate (TPDI), 4,4’-diisocyanato di cyclohexylmethane (HDMI), tetramethylxylene diisocyanate (TMXDI), and blocked derivatives thereof.

6. The two-part polyurethane system of claim 1, wherein the polyol is a solvent-based acrylic polyol.

7. The two-part polyurethane system of claim 1, wherein the polyol has from 1.5 to 2% by weight of free hydroxyl groups.

8. The two-part polyurethane system of claim 1, wherein the molar ratio of -NCO / -OH is from 0.15 to 0.25.

9. A coatings system comprising: a polyurethane resin comprising free carboxyl groups and having units obtained from: a polyol containing free -OH groups, and a polyisocyanate containing -NCO groups, wherein a molar ratio of the -NCO groups in the polyisocyanate to the -OH groups in the polyol is from 0.1 to 0.3; and a crosslinker component comprising a carbodiimide compound, a polyaziridine compound, or a combination thereof, the crosslinker component being configured to react with the free carboxyl groups of the polyurethane resin to form a cured polyurethane coating.

10. The coatings system of claim 9, wherein the polyol has from 0.5 to 3% by weight of free -OH groups.

11. The coatings system of claim 9, wherein the polyol has an acidity of from 5-50 mgKOH / g.

12. The coatings system of claim 9, wherein the polyisocyanate has from 20 to 35% by weight of -NCO groups.

13. The coatings system of claim 9, wherein the poly isocyanate is a solvent-free aliphatic diisocyanate, and wherein the polyol is a solvent based acrylic polyol.

14. The coatings system of claim 9, wherein the polyisocyanate is an isocyanate oligomer.

15. The coatings system of claim 9, wherein the polyol has from 1.5 to 2% by weight of free hydroxyl groups.

16. The coatings system of claim 9, wherein the molar ratio of the -NCO groups in the polyisocyanate to -OH groups in the acrylic polyol is from 0.15 to 0.2517. A method of forming a coating comprising: mixing a resin comprising a polyurethane with a crosslinker component comprising a carbodiimide compound, a polyaziridine compound, or a combination thereof; applying the mixture of the resin and the crosslinker component to a substrate; and curing the mixture of the resin and the crosslinker component to form a cured polyurethane coating on the substrate, wherein the resin comprises free carboxyl groups and has units obtained from: a polyol containing free -OH groups, and a polyisocyanate containing free -NCO groups, wherein a molar ratio of the -NCO groups in the isocyanate to the -OH groups in the polyol is from 0.1 to 0.3.

18. The method of claim 17, wherein the curing occurs over time as the carbodiimide compound and / or polyaziridine compound from the crosslinker component reacts with the substantial amount of free carboxyl groups from the resin.

19. The method of claim 17, wherein the substrate comprises wood.

20. The method of claim 17, further comprising: forming the resin comprising the polyurethane with the crosslinker component, wherein the resin is formed when the -NCO concentration is spectroscopically undetectable.

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