Process for producing aqueous polyurethane coating and coat therefrom

Abstract: A process for producing an aqueous polyurethane coating is provided. A polyisocyanate, a polyol and a dimethylol alkane acid of the following formula, where R is an alkyl group of 2 to 10 carbon atoms, are reacted in the presence of an organic solvent having a boiling point of 50 to 120°C to thereby prepare an urethane polymer solution. The organic solvent is removed from the urethane polymer solution, and the resultant urethane polymer is then dissolved or dispersed in water. The coating is of a substantially solvent-free form without reliance on any solvent having a high boiling point which results in impaired coating environment and reduced coat quality. The coat therefrom is widely useful for construction materials, automotive parts, electrical parts and textile products in particular and also is highly effective for preventing volatile of organic compound from becoming emitted in the atmosphere.

Description: FIELD OF THE INVENTION This invention relates to a process for the production of aqueous polyurethane coating and a coat therefrom. More particularly, the invention relates to a process for producing such an aqueous polyurethane coating in which a novel urethane polymer is prepared by the reaction of specific reactants in the presence of a specific organic solvent having a low boiling point and thereafter the solvent is remove and the resultant urethane polymer is dissolved or dispersed in water. BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE RELATED ART Aqueous urethane polymers made their debut nearly 20 years ago. For their superior softness, mechanical quality and adhesiveness among various physical properties, such urethane polymers have been used widely in the field of coatings, binders, adhesives and the like. To cope with a recent movement of restricting the emission of volatile of organic compound (VOC) in the air from the viewpoint of environmental protection, for the development and application of aqueous coatings has been demanded strongly. In order to produce urethane polymers suitable for use in aqueous polyurethane coatings, many attempts have been done to hydrophilize urethane polymers on the ground that hydrophilic groups therein render the high solubility or dispersibility in water. One notable example is an anionic-type urethane polymer which has basic salt of carboxyl groups attached to the prepolymer chain and which enjoys credit since it offers relatively good resistance to water. This type of urethane polymer is obtained by reacting a polyisocyanate, a polyol and dimethylol propionic acid in the mixture of a polar organic solvent having a high boiling point and an organic solvent having a low boiling point so-as to form an urethane prepolymer having carboxyl groups and NCO groups at its terminal end, followed by neutralization of the resultant carboxyl groups with a basic material and by subsequent dispersion of the prepolymer in water (see Japanese Patent Publication Nos. 61-5485, 3-48955 and 4-488). After chain extending of the prepolymer with a polyamine compound, the solvents are then removed. In the prior art methods noted above, the polar solvent having a high boiling point such as dimethylformamide (boiling point: 153°C), N-methylpyrrolidone (boiling point: 204°C) or the like should necessarily be used in combination with the organic solvent having a low boiling point in order to dissolve the dimethylol propionic acid uniformly. This latter compound is less soluble in the organic solvent having a low boiling point such as acetone, methyl ethyl ketone or the like. The use of the polar organic solvent having a high boiling point, however, poses the problem that removal or liberation of the solvent from the urethane prepolymer is difficult at a reaction stage after hydrophilization of the urethane prepolymer. It means that the polar solvent having a high boiling point is objectionably left unremoved in the final aqueous urethane polymer. A aqueous coating agent derived from the urethane polymer so formed results in a coat of poor drying. Another serious drawback is that the polar solvent having a high boiling point left in the coat acts like a plasticizer, eventually causing physical deterioration of the coat, or impairing coating environment due to some malodor inherent to that solvent. Moreover, water resistance of the coat is not always satisfactory. To gain improved solubility of dimethylol propionic acid in the organic solvent having a low boiling point, it has been proposed as disclosed in Japanese Patent Laid-Open No. 6-313024 to use a hydrophilizing material for modifying the OH group of dimethylol propionic acid. The modified dimethylol propionic acid is more readily soluble in the solvent. The hydrophilizing materials are, for example, a lactone monomer, for example such as ε-caprolactone, and an alkylene oxide such as ethylene oxide. The resulting urethane polymer, however, has still much to be improved with respect to its resistance of water and heat. A reaction product of a urethane prepolymer and a chain-lengthening agent is also known, as disclosed in Japanese Patent Laid-Open No. 5-39340. The resulting urethane prepolymer can provide a coat with good resistance of hydrolysis as well as good appearance at low temperatures. Such prepolymer used here is composed of an organic diisocyanate, an amorphous polycarbonate diol and dimethylol propionic acid provided in its molecules with one hydrophilic group and two or more groups capable of reacting with the diisocyanate. The prepolymer, however, has still much to be improved with respect to its folwability and a control of the reaction for producing the prepolymer. SUMMARY OF THE INVENTION The present invention has been made to solve or eliminate all of the foregoing problems experienced with the aqueous polyurethane coatings of the prior art which result from use of dimethylol propionic acid and entail some high-boiling solvent put in the reaction system but having remained in the resultant urethane polymer to be unremoved by all possible means. Therefore, the present invention is to provide a process for producing a new aqueous polyurethane coating earth-friendly and a coating with good resistance of hydrolysis and appearance. More specifically, the invention provides a process for producing an aqueous polyurethane coating, which comprises the steps of: (1) reacting a polyisocyanate, a polyol and a dimethylol alkane acid of the following formula: wherein R is an alkyl group of 2 to 10 carbon atoms, in the presence of an organic solvent having a boiling point of 50 to 120°C as the only solvent to thereby prepare a solution of a urethane prepolymer having carboxyl groups and at its terminal ends NCO groups; (2) neutralizing the carboxyl groups attached to the urethane prepolymer with a basic material; (3) admixing water to the resultant urethane prepolymer solution whereby a solution or dispersion of the urethane polymer is obtained; (4) extending the polymer chain with a chain extending agent; (5) removing said organic solvent from the urethane polymer solution or dispersion obtained; or which comprises the steps of: (1) reacting a polyisocyanate, a polyol and a dimethylol alkane acid of the following formula: wherein R is an alkyl group of 2 to 10 carbon atoms, under the condition that NCO or OH groups are present in greater excess than others, in the presence of an organic solvent having a boiling point of 50 to 120°C as the only solvent; (2) conducting a chain extension using a chain extending agent in the presence of said solvent to thereby prepare a solution of a urethane prepolymer having carboxyl groups and at its terminal ends NCO groups; (3) neutralizing the carboxyl groups attached to the urethane prepolymer with a basic material; (4) admixing water to the resultant urethane polymer solution whereby a solution or dispersion of the urethane polymer is obtained; (5) removing said organic solvent from the urethane polymer solution or dispersion obtained. The present invention also provides a method of forming a polymer urethane coating from an aqueous polyurethane coating, which comprises the steps of: (1) applying an aqueous polyurethane coating to a substrate; and (2) drying said coating supported on said substrate to obtain thereby a dry coat, wherein the aqueous polyurethane coating is obtainable by the above processes. Also, the present invention is to provide a coat obtainable by the latter method which contains not more than 0.1% by weight of the organic solvent. The above and other objects, features and advantages of the invention will be apparent to those versed in the art from the following description. DETAILED DESCRIPTION OF THE INVENTION Urethane Polymers Urethane polymers useful for the purpose of the present invention are derivable from the reaction of a polyisocyanate, a polyol, a dimethylol alkane acid represented by the following formula, where R is an alkyl qroup of 2 to 10 carbon atoms, and as an optional member a chain extending agent in the presence of an organic solvent having 50 to 120°C of boiling point. Generally, the urethane polymer for use in the invention can be obtained by any known method of producing similar polymers, but with the exceptions of use of the above specified organic solvent having a low boiling point alone as a solvent and of the dimethylol alkane acid of the above structural formula. Two processes described hereunder are typified for the formation of the urethane polymer according to the invention. 1) A polyisocyanate, a dimethylol alkane acid, and a polyol are reacted in the organic solvent having a low boiling point, thereby preparing an urethane prepolymer having carboxyl groups and NCO groups at its terminal end. Thereafter, the carboxyl groups resultantly attached to the prepolymer chain are neutralized with a basic material. The prepolymer is dissolved or dispersed in water and then extended in its chain with a chaine extending agent, followed by removal of the solvent. 2) An urethane prepolymer is prepared by reacting polyisocyanate, polyol and dimethylol alkane acid under the condition of NCO or OH groups in greater excess than others, after which a chain extending is conducted with use of a chain extending agent or a polyisocyanate compound in the organic solvent having a low boiling point. After being neutralized of carboxyl group in the prepolymer incorporated with a basic material, the resulting polymer is dissolved or dispersed in water. Finally, the solvent is removed to obtain an aqueous polyurethane coating agent. The equivalent ratio of NCO groups of the polyisocyanate to OH groups of the polyol is set to be an NCO/OH of 1.1/1 to 30/1. The polyisocyanate and polyol are reacted in the organic solvent having a low boiling point in such a way that the resulting urethane prepolymer has a solid content of 20 to 90% by weight. This range of solid contents is necessary for facilitating subsequent emulsification and dispersion of the urethane polymer. The solvent here should be easily removable upon hydrophilization of the polymer so that an aqueous polyurethane coating is provided as desired. Importantly, the organic solvent should have a boiling point of 50 to 120°C, preferably 50 to 110°C, more preferably 50 to 90°C. Typical organic solvents are chosen from acetone (boiling point: 56.3°C), methyl ethyl ketone (boiling point: 79.6°C), methyl isobutyl ketone (boiling point: 117°C), tetrahydrofuran (boiling point: 66°C), 1,4-dioxane (boiling point: 101.4°C), ethyl acetate (boiling point: 76.8°C), or toluene (boiling point: 110.6°C). Particularly preferred among these solvents are acetone and methyl ethyl ketone for their good solubility of the urethane polymer and their easy remove from the reaction system at the second stage. Acetone is most preferred in practicing the invention. The reaction for obtaining the urethane polymer of the invention may be carried out generally in non-catalyzed condition. This reaction can be accelerated, if desired, by using a catalyst which is suitably selected from organometallic compounds such as dibutyltin dilaurate, dibutyltin dioctoate or stanneous octoate, and tertiary amines such as triethylenediamine, triethylamine or tributylamine. Reaction temperatures are usually at from 20 to 120°C. Reaction times depend upon the reaction temperature to be set, the solid content of an urethane polymer to be formed and other parameters at each reaction stage. They are variable and usually from 1 to about 40 hours. Also importantly, a dimethylol alkane acid of the structural formula written above should be used as one essential reactant or ingredient in producing an urethane polymer of the invention. This specific compound can be derived from aldol condensation of an alkyl aldehyde with formalin in the presence of a base catalyst, followed by oxidation of the resultant aldehyde groups through the action of a peroxide. Suitable dimethylol alkane acids include dimethylol butanoic acid, dimethylol pentanoic acid, dimethylol heptanoic acid, dimethylol octanoic acid or dimethylol nonanoic acid. Dimethylol butanoic acid, dimethylol heptanoic acid and dimethylol nonanoic acid are favored in view of cost savings and other factors. Dimethylol butanoic acid is most favorable for its emulsification at rather a high level. In the structural formula of the dimethylol alkane acids according to the invention, the substituent R denotes an alkyl group having a carbon number of 2 to 10. In the case where the alkyl group is one in carbon number, namely the dimethylol propionic acid noted above in connection with the prior art, or 11 or larger, the corresponding dimethylol alkane acid would not sufficiently be soluble in the organic solvent. For instance, the dimethylol butanoic acid contemplated under the invention has been found to show a greater solubility in acetone at 40°C, say as high as 18% by weight, than the dimethylol propionic acid being soluble at a too low level of 2% by weight under the same test conditions. In the urethane polymer of the invention, the dimethylol alkane acid should be present in an amount of 0.4 to 5% by weight, preferably 0.6 to 4% by weight, more preferably 0.8 to 3% by weight, in terms of the content of carboxyl groups and based on the solid content of the urethane polymer. The content of carboxyl groups in the urethane polymer, which is closely associated with hydrophilization of the polymer, can be adjusted to suit a particular application, but is set usually at from 0.4 to 5% by weight based on the solid content of the urethane polymer. Less than 0.4% by weight in the content of carboxyl groups would make it difficult to hydrophilize the urethane polymer. More than 5% by weight in like content would produce a physically deteriorated, commercially unacceptable coat. Adjustment of the content of carboxyl groups in the urethane polymer may be effected with the dimethylol alkane acid charged in its varied proportions to a coreactive polyol. For instance, the content of carboxyl groups (COOH) in dimethylol butanoic acid (DMBA) is calculated from the following equation. COOH content (%) = [amount of DMBA used] × [amount of molecular weight of COOH (45)] [molecular weight of DMBA (148)] ÷ [total solid content of urethane polymer] × 100 Suitable polyisocyanates for producing the urethane polymer of the invention include aromatic diisocyanates such as 4,4'-diphenylmethane diisocyanate, 2,4- or 2,6-tolylene diisocyanate, 1,5-naphthalene diisocyanate or p- or m-phenylene diisocyanate, alicyclic diisocyanates such as isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,4-cyclohexylene diisocyanate or hydrogenates of tolylene diisocyanate, aliphatic diisocyanates such as hexamethylene diisocyanate, xylene diisocyanate or m-tetramethylxylene diisocyanate. These polyisocyanates may be used singly or in combination. Eligible polyols are exemplified by polyether polyols having preferably of 400 to 5,000 in number-average molecular weight, such as polytetra-methylene ether glycol , polypropylene glycol or polyethylene glycol, bifunctional condensates derived from condensation of a diol with a dicarboxylic acid and terminated with a hydroxyl group, such as polyethylene adipate, polyethylenebutylene adipate, polybutylene adipate, polypropylene adipate, poly-hexamethylene adipate, polyneopenthyl adipate or poly-3-methyl-1,5-pentylene adipate, condensates obtained from condensation of a diol with terephthalic acid alone or with isophthalic acid and adipic acid, such as 1,6-hexanediol or 3-methyl-1,5-pentanediol, polyester polyols such as polycaprolactone or polymethylvalerolactone, polycarbonate polyols, silicone polyols, polybutadiene polyols or polyolefin polyols. In use, they may be individual or a combination of two or more polyisocyanates. When found desirable, the above listed polyols can be used in combination with either one of those compounds chosen from short-chain diols such as ethylene glycol, propylene glycol, diethylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol or 3-methyl-1,5-pentanediol, short-chain triols such as trimethylol propane or glycerin, and trifunctional polyols derived by addition of propylene oxide, ethylene oxide or ε-caprolactone to those short-chain triols. Basic materials for use in neutralizing the carboxyl groups attached to the urethane prepolymer chain are exemplified by tertiary amines such as trimethylamine, triethylamine, methyldiethylamine or tripropylamine, alkanolamines such as dimethylethanolamine, methyldiethanolamine or triethanolamine, and hydroxides and carbonates of alkaline metals such as ammonium, sodium or potassium, among which tertiary amines and alkanolamines are particularly preferred. The basic material is added usually in an equivalent weight of 0.5 to 1 per one equivalent of the carboxyl group attached to the urethane prepolymer. Chain extending agents for use in extending the chain of the NCO group-terminated urethane prepolymer of the invention include diamines such as ethylenediamine, 1,2-propanediamine, tetraethylenediamine, hexamethylene-diamine, 1,4-cyclohexylenediamine, isophorone-diamine, 4,4'-dicyclohexylmethanediamine, m-xylenediamine or phenylenediamine, polyethylene polyamines such as diethylenetriamine, triethylenetetraamine or tetraethylenepentamine, and dihydrazines such as hydrazine, piperazine, hydrazine-adipic acid compounds or hydrazine-phthalic acid compounds. In addition, the short-chain diols and short-chain triols exemplified above may be used where desired. These agents may be used alone or in combination. When the urethane polymer needs to introduce a hydroxyl group to its terminal end, the polyamine referred to here can be used together with ethanolamine or diethanolamine. If hydroxyl groups are desired to be incorporated in the urethane polymer at its backbone, then such polyamine can be used in combination with aminoethyl-aminoethanol. In forming the urethane polymer according to the invention, a specified organic solvent is employed as described above and required to be removed at the second reaction stage. Liberation of the solvent can be done, for example, by a method in which at temperatures lower than the boiling point of water, usually at from 30 to 100°C, air or nitrogen is blown over or into the resulting liquid reaction product to thereby remove the solvent, by a method in which the reactor is held under vacuum to thereby evaporate the solvent, or by a method in which a distiller of a thin-film type is utilized to collect the solvent. The present invention, in fact, contemplates providing a substantially solvent-free, aqueous-type urethane polymer for use in the production of an aqueous polyurethane coating. In fields of use where some organic solvent is approvedly made feasible in a limited amount and with accepted ventilation facilities, the solvent so named may be employed in forming an urethane polymer and can be allowed to remain in the ultimate polymer. To implement the invention, the amount of the low-boiling organic solvent having remained in the urethane polymer should be not more than 10% by weight. Aqueous Polyurethane Coating Aqueous polyurethane coatings of the present invention can be produced by the use of the urethane polymers obtained above and, if necessary, after dilution with water and also after blending with various additives described later. The ultimate coating of the invention has a solid content usually of 10 to 70% by weight. The average particle size of the urethane polymer dispersed in water is not larger than 5 µm, preferably in the range of 0.005 to 2 µm. The appearance of such dispersed polymer is rather variable with its particle size; that is, smaller particle sizes give a fluorescent solution, whereas larger particle sizes lead to a milky emulsion. Both the solution and emulsion, however, afford sufficient stability in character even after lapse of prolonged periods of time. The solid content and viscosity of the coating can be adjusted according to the purpose of use, and in this instance, the urethane polymer is controlled in its size of dispersed particles. The polymer particles dispersed tend to be in a larger size when scarce in the content of hydrophilic groups and to be in a smaller size when abundant in the same content. Further, the urethane polymer has a number-average molecular weight of 3,000 to 500,000. Here, the number-average molecular weight is determinable by dissolving 1% by weight of an urethane polymer sample in tetrahydrofuran and then subjecting the solution to gel permeation chromatography (GPC) and subsequently by converting the resulting numerical value in terms of polystyrene. Application of Aqueous Polyurethane Coatings The aqueous polyurethane coating of the present invention is suitably useful for various purposes. Though applicable as it is in a one-pack fashion, this coating is preferably used as a two-pack coating with addition of a crosslinking agent so as to attain improved durability. Such agent is chosen from aqueous blocking-type polyisocyanate crosslinking agents; water-dispersible, NCO group-nonblocking polyisocyanate crosslinking agents; melamine-based crosslinking agents; epoxy-based crosslinking agents or polyaziridine-based crosslinking agents. The amount of the crosslinking agent to be added depends on the quality and durability required for the purpose of use and on the kind of those agents used, but ranges preferably from 100 : 1 to 100 : 40, more preferably from 100 : 5 to 100 : 30 in a weight ratio based on the solid content of the urethane polymer that predominates in the coating. Smaller weight ratios than 100 : 1 would be ineffective to attain improved performance and durability of the final coat, while larger weight ratios than 100 : 40 would result in a coat of excessive hardness and objectionable fragility. The aqueous polyurethane coating of the invention may be blended with various additives which include antioxidants, ultraviolet stabilizers, pigments, colorants, defoamers, fluidity regulators, repellents, smoothing agents or fillers. Moreover, the aqueous polyurethane coating of the invention may be incorporated with different dispersions of acrylic- , polyester- , epoxy-, silicone- or fluorine-based resins on condition that those dispersions do not adversely affect the final coating. Substrates for use in supporting the aqueous polyurethane coating of the invention are selected from, though not restricted to, wood, leather, paper, plastics, metal, ceramics, glass, concrete, woven or knitted fabric or nonwoven fabric. They may be in the form of a film, plate, block, molding, thread or porous material. After being adjusted in the solid content and viscosity to meet the purpose, the aqueous polyurethane coating can be applied to a given substrate as by a sprayer, brush, roll, bar coater, applicator, doctor blade, comma coater, spin coater or dipping. The thickness of the dry coat is usually in the range of 0.05 to 500 µm which, however, is variable with the use and purpose. Temperatures at which to dry and anneal the coat depend largely upon the purpose of the coat used, the coating equipment used, the thickness of the coat set, the kind and amount of crosslinking agents used and other parameters, but range usually from 20 to 200°C. When faster drying is desired, the temperature is selected at from 60 to 200°C. Drying times may be likewise set and usually from several seconds to 24 hours. Further advantages afforded by the aqueous polyurethane coating of the invention are that the dry coat obtained above is made substantially free from the organic solvent used. Thus, the content of the solvent left in the dry coat should be held preferably at not more than 0.1% by weight, more preferably at below 0.05% by weight. An organic solvent having remained in a coat would render the coat mechanically deteriorative and hence less durable. Here, an example is cited in which N-methylpyrrolidone of a hydrophilic class is employed. In such instance, when the resulting coat is immersed in water or placed in hot and humid conditions, the N-methyl-pyrrolidone solvent having remained in the coat would take in water and thus cause blister at an interface between the coat and the mating substrate, entailing decreased resistance to water. EXAMPLES The following examples are provided to further illustrate the present invention. In these examples, all parts and percentages are by weight unless otherwise noted. Various physical properties tested in the examples were measured by the methods indicated below. Solid Content Samples were put in amounts of 1 to 1.5 g into aluminum cups and weighed with precision. Upon drying at 110°C for one hour, the residue was precisely weighed. Viscosity Viscosity at 25°C was measured on a rotary viscometer (EM type; Tokyo Instruments Co.). Average Particle Size Determination was made on a particle size distribution-measuring apparatus (LA-500 type; Horiba Limited.). Film Quality An aqueous polyurethane coating was applied by an applicator to a polypropylene releasable paper such that the resultant dry coat had a thickness of about 70 µm. Drying was done at 80°C for one hour, and the coat was allowed to stand for one day in an environment of 23°C and 60% RH (relative humidity) after which it was punched into rectangular pieces each of 10 mm x 120 mm. Tensile strength and elongation were measured by the procedure of JIS K5400 and with the use of a tensile tester (Tensilon UTM-III-100 type; Toyo Boldwin Co.). Adherence According to the cross-cut test stipulated by JIS K5400, the adhesion of a coat to the mating substrate was examined. An adhesive tape (Cellotape, tradename; Nichiban Co.) was allowed to sufficiently adhere to and over 100 cross-cuts and then held at an angle of 90° to the coated surface, followed by instant backward releasing of the coat. Judgement was done by the number of cross-cuts released. Solvent Resistance A coat supported on a plate was scrubbed 5 times with acetone-immersed gauze, and the appearance of the coat was inspected for any change. Light Resistance As stipulated in JIS L0842, a coated plate was irradiated with ultraviolet rays for 100 hours at 63 ± 3°C on a sunshine fademeter (Suga Test Instruments Co. FAL-3 type). The appearance of the coat was inspected for any change. Moist heat resistance A coated plate was allowed to stand in an environment of 70°C and 95% RH for one week. Any change was inspected in the appearance of the coat. Example 1 Into a reactor were put 98.8 parts of 4,4'-dicyclohexylmethane diisocyanate, 251.4 parts of polytetra-methylene ether glycol of 2,000 in number-average molecular weight, 16.3 parts (0.122 mol) of dimethylol butanoic acid and 91.6 parts of acetone. Reaction was carried out with stirring at 60°C for 7 hours in a nitrogen stream, whereupon an NCO group-terminated urethane prepolymer was obtained in the form of a uniform transparent solution. This polymer solution was thereafter incorporated with 10.0 parts of triethylamine, 2.0 parts of Tinuvin 144 (light stabilizer; Ciba-Geigy AG) and 69.7 parts of acetone. To the mixture was gradually added 488.0 parts of desalted water to thereby disperse the urethane prepolymer. 7.0 parts of hydrazine hydrate and 87.3 parts of desalted water were then added at 50°C, followed by removal of the acetone under vacuum. The aqueous urethane polymer (E1) thus formed was a milky emulsion with a solid content of 40%, a viscosity of 30 mPa.s at 25°C and an average particle size of 0.4 µm. Examination by GPC revealed that such polymer had a number-average molecular weight of 18,000. The ultimate film produced satisfactory results with a tensile strength on break of 28 MPa and an elongation on break of 540%. Example 2 Into a reactor were put 98.3 parts of 4,4'-dicyclohexylmethane diisocyanate, 125 parts of polytetra-methylene ether glycol of 2,000 in number-average molecular weight, 125 parts of 1,6-hexanediol type polycarbonate diol of 2,000 in number-average molecular weight, 18.5 parts of dimethylol butanoic acid and 91.7 parts of acetone. Reaction was made with stirring at 60°C for 8 hours in a nitrogen stream, thereby obtaining an NCO group-terminated urethane prepolymer as a uniform transparent solution. 11.4 parts of triethylamine, 1.1 parts of Tinuvin 144 and 70.9 parts of acetone were thereafter added to the polymer solution. The mixture was incorporated gradually with 470 parts of desalted water to thereby disperse the urethane prepolymer. 5.6 parts of hydrazine hydrate and 107.4 parts of desalted water were then added at 50°C to extend that prepolymer in its chain. Subsequent removal of the acetone under vacuum gave an aqueous urethane polymer (E2) as a milky solution. The above urethane polymer had a solid content of 40%, a viscosity of 120 mPa.s at 25°C and an average particle size of 0.4 µm. The number-average molecular weight of the urethane polymer was measured to be 20,000 by GPC. The resultant film was 30 MPa in tensile strength on break and 460% in elongation on break. Example 3 Into a reactor were put 92.5 parts of isophorone diisocyanate, 233.3 parts of polybutylene adipate of 2,000 in number-average molecular weight, 19.7 parts of dimethylol butanoic acid and 86.4 parts of acetone. Reaction was conducted with stirring at 60°C for 8 hours in a nitrogen stream, thereby obtaining an NCO group-terminated urethane prepolymer as a uniform transparent solution. This prepolymer was thereafter incorporated with 12.8 parts of triethylamine, 1.2 parts of Tinuvin 144 and 119.4 parts of acetone. To the mixture was added gradually 500 parts of desalted water so as to disperse the urethane prepolymer. 28.3 parts of isophoronediamine and 218 parts of desalted water were then added at 30°C to thereby extend the prepolymer chain. Subsequent removal of the acetone under vacuum led to an aqueous urethane polymer (E3) as a milky solution. The above urethane polymer had a solid content of 35%, a viscosity of 200 mPa.s at 25°C and an average particle size of 0.3 µm. The number-average molecular weight of such polymer was determined to be 70,000 by GPC. The resulting film had a tensile strength on break of 28 MPa and an elongation on break of 750%. Example 4 Into a reactor were put 174.0 parts of 80-TDI (a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate in a weight ratio of 80 : 20), 83.3 parts of polypropylene glycol of 1,000 in number-average molecular weight, 24.7 parts (0.167 mol) of dimethylol butanoic acid, 69.3 parts of neopentylglycol, 351.3 parts of acetone and 0.18 part of dibutyltin dilaurate. Reaction was made with stirring at 55°C for 16 hours in a nitrogen stream to thereby obtain an NCO group-terminated urethane prepolymer. This prepolymer was further incorporated with 8.7 parts of neopentylglycol and 8.7 parts of acetone. Reaction was continued for another 8 hours to thereby extend the prepolymer chain. The resulting solution was uniform and transparent. 14.1 parts of dimethylethanol-amine, 200.8 parts of acetone and 1.8 parts of Tinuvin 144 were then added, and the whole was uniformly admixed. Phase transformation was effected by the addition of 872.9 parts of desalted water, and the acetone was removed under reduced pressure, whereupon an aqueous urethane polymer (E4) was obtained as a fluorescent solution. The above urethane polymer was 30% in solid content and 1,000 mPa.s at 25°C in viscosity. Its number-average molecular weight was determined to be 12,000 by GPC. The resulting film was 45 MPa in tensile strength on break and 120% in elongation on break. Example 5 Into a reactor were put 98.3 parts of 4,4'-dicyclohexyl diisocyanate, 250 parts of an OH group-terminated polyester polyol composed of a dicarboxylic acid (terephthalic acid/adipic acid: a mol ratio of 1/1) and 3-methyl-1,5-pentanediol and having a number-average molecular weight of 2,000, 11.1 parts of dimethylol butanoic acid, 4.5 parts of 1,4-butanediol and 91.0 parts of acetone. Reaction was carried out with stirring at 60°C for 7 hours, whereby an NCO group-terminated urethane prepolymer was obtained as a uniform transparent solution. This solution was thereafter incorporated with 7.2 parts of triethylamine, 1.2 parts of Tinuvin 144 and 109.5 parts of acetone. To the mixture was added gradually 300 parts of desalted water so as to disperse the urethane prepolymer. 21.2 parts of isophoronediamine and 93.6 parts of desalted water were then added at 30°C to thereby extend the prepolymer chain. Removal of the acetone under vacuum provided an aqueous urethane polymer (E5) in milky form. The above urethane polymer showed a solid content of 50%, a viscosity of 40 mPa.s at 25°C and an average particle size of 0.5 µm. Its number-average molecular weight was determined to be 35,000 from measurement by GPC. The ultimate film had a tensile strength on break of 34 MPa and an elongation on break of 510%. Comparative Example 1 The procedure of Example 1 was followed except that 16.3 parts (the same mol) of dimethylol propionic acid was used in place of the dimethylol butanoic acid. Most of the dimethylol propionic acid remained undissolved, failing to produce any uniform prepolymer. Comparative Example 2 The procedure of Example 1 was followed except that 16.3 parts (the same mol) of dimethylol propionic acid was substituted for the dimethylol butanoic acid and that 48.5 parts of acetone and 45.8 parts of N-methylpyrrolidone were used. Reaction was conducted to obtain a uniform transparent urethane prepolymer solution (CE2). An aqueous urethane polymer was likewise prepared. A milky emulsion was provided which was 40% in solid content, 40 mPa.s at 25°C in viscosity, 0.4 µm in average particle size and 19,000 in number-average molecular weight. The resultant film was 19 MPa in tensile strength on break and 680% in elongation on break. This comparative film was smaller in strength by far than the inventive film of Example 1. Comparative Example 3 The procedure of Example 4 was followed except that 22.4 parts (the same mol) of dimethylol propionic acid was substituted for the dimethylol butanoic acid and that 281.0 parts of acetone and 70.3 parts of N-methylpyrrolidone were used. Reaction was conducted to obtain a uniform transparent urethane prepolymer solution (CE3). An aqueous urethane polymer was likewise prepared. A fluorescent solution was provided which had a solid content of 30%, a viscosity of 1,400 mPa.s at 25°C and a number-average molecular weight of 11,000. The film derived from this polymer solution was measured to be 33 MPa in tensile strength on break and 200% in elongation on break. This comparative film was lower in strength than the inventive film of Example 4. Examples 6 to 10 Performance evaluation was made of various coatings derived from the aqueous urethane polymers prepared in Examples 1 to 5. Two different crosslinking agents A and B were put to use in conducting these tests. Crosslinking agent A was an aqueous dispersion of a blocking-type polyisocyanate (Mitec, trademark; SW 500, tradename; solid content, 35% by weight; NCO-group content, 4.7% by weight; Mitsubishi Chemical Corp.), while crosslinking agent B was a water-dispersible polyisocyanate compound (solid content, 100% by weight; NCO group content, 18.9% by weight) which was derivable by addition reaction of a hexamethylene diisocyanate trimer and polyethyleneglycol monomethylether in a weight ratio of 100 : 7. To improve wettability of the coating with respect to a given substrate, use was made of a surfactant (Surfynol 104; Nisshin Chemical Industry Co., Ltd.). Desalted water was used to dilute the aqueous urethane polymer. Titanium oxide (JR 600A; Tayka Corp.) was employed as a pigment in Examples 9 and 10 and also in Comparative Example 5 below. The aqueous urethane polymer, surfactant, titanium oxide and desalted water were dispersed for 2 hours in a paint shaker in which glass beads were contained, followed by admixing with the crosslinking agents. Thus, an aqueous polyurethane coating was provided. The coating formulations, coating methods and the performance are shown in Table 1. Comparative Examples 4 and 5 As was done in Examples 6 and 9, performance evaluation was made of two coatings prepared from the aqueous urethane polymers of Comparative Examples 2 and 3 with the procedures and results shown also in Table 1. The coatings of Comparative Examples 4 and 5 developed some offensive odors attributed to the N-methylpyrrolidone used, hence rendering coating works cumbersome. No such malodors took place out of the coatings of Examples 6 to 10. The above, inventive and comparative, coatings are now compared in their respective physical properties. The coatings of Examples 6 to 10 were produced from the aqueous urethane polymers according to the invention. As shown in Table 1, these aqueous urethane polymers were obtained with use of dimethylol butanoic acid as a starting material for polymer formation and without use of any high-boiling solvent. The inventive coatings have been proved to be highly satisfactory in respect of all of the qualities tested. The coatings of Comparative Examples 4 and 5 were derived from dimethylol propionic acid, namely a conventional starting material for polymer formation, and N-methylpyrrolidone, namely solvent having a high boiling point. As evidenced by the results of Table 1, the comparative coatings were insufficient in regard to adherence, solvent resistance and resistance to elevated temperature and humidity. This is probably because such high boiling solvent remained unremoved in the coat. (Formulation of coating, Coating method and Evaluation) Example No. Comparative Example No. 6 7 8 9 10 6 7 (corresponds to Ex.6) (corresponds to Ex.9) Aqueous polyurethane resin E1 E2 E3 E4 E5 CE2 CE3 Crosslinking agent B A B B A B B Formulation of coating (parts by weight) Aqueous polyurethane resin 100 100 100 100 100 100 100 Crosslinking agent 8 15 10 5 15 8 5 Surfactant 0.06 0.07 0.09 0.07 0.07 0.06 0.07 Titanium dioxide - - - 1.8 2.5 - 1.8 Desalted water 20 15 50 15 30 20 15 Substrate ABS Glass Glass Steel Steel ABS Steel plate plate plate plate plate plate plate Coating method AP AP SP BR AP AP BR Thickness of film(µm) ca. 30 ca. 30 ca. 50 ca. 100 ca. 30 ca. 30 ca. 100 Drying conditions 60x0.5 80x0.5 80x0.5 23x24 80x0.5 60x0.5 23x24 (°Cxhr) 100x0.5 - - - - 100x0.5 - Baking conditions (°Cxhr) - 150x0.5 150x0.5 - 150x0.5 - - Odor during coating and drying almost none almost none almost none almost none almost none strong strong Aging conditions (23°C.60%RHxday) 7 7 7 7 7 7 7 Evaluation Adherence ○ ○ ○ ○ ○ × × Solvent resistance ○ ○ ○ ○ ○ × × Light resistance ○ ○ ○ Δ ○ ○ × Moist heat resistance ○ ○ ○ ○ ○ × × Judgement of Evaluation Adherence: ○ number of unreleased cross-cut/100 cross-cuts = 96-100, × number of unreleased cross-cut/100 cross-cuts = 0- 95 Solvent resistance: ○ no change in appearance, Δ small damage to coating film, × large damage to coating film Light resistance: ○ no change in appearance, Δ little yellowing, × much yellowing Moist heat resistance: ○ no change in appearance, × deterioration such as blisters Note: AP: Applicator, SP: Spraying, BR: Brushing Examples 11 and 12 In Example 11 the coating (E1) of Example 1 was used and in Example 12 the coating (E4) of Example 4. Under the same conditions as shown in Table 1, each of the coatings was applied by an applicator over a releasable paper of polypropylene (EKR78PD-HM, tradename; Lintec Corp.) such that the film thickness was approximately 70 µm on a dry basis. The coating so applied was dried at 80°C for one hour and then let to stand in an environment of 23°C and 60% RH for one day. The resultant coat after being released from the paper was immersed in tetrahydrofuran at room temperature for one day. The extract was subjected to solvent analysis by gas chromatography (GC-14A; Shimadzu Corp.) with the results shown in Table 2. As regards the results obtained with Example 11, the content of acetone was determined to be substantially nil in the dry coat, about as low as below 0.01%. Likewise, the such content was determined to be in a limited amount of 0.02% in the dry coat of Example 12. These findings are presumably due to the coats of Examples 6 and 9 being highly resistant to elevated temperature and humidity as shown in Table 1. Comparative Examples 6 and 7 In Comparative Example 6 the coating (CE2) of Comparative Example 2 was employed and in Comparative Example 7 the coating (CE3) of Comparative Example 3. In the same manner as in Examples 11 and 12, two dry coats were prepared and then subjected to solvent extraction. The extract was likewise analyzed with the results shown in Table 2. The comparative coats each contained a trace of the acetone used in preparing the urethane polymer. However, Comparative Example 6 revealed a large amount of 0.9% of the N-methylpyrrolidone solvent having remained in the coat and Comparative Example 7 a by far larger amount of 8.5%. The results here are presumed to be closely associated with the coats of Comparative Examples 4 and 5 which had been found involve reduced resistance to elevated temperature and humidity. (Formulation of coating, Coating method and Evaluation) Example No. Comparative Example No. 11 12 6 7 (corresponds to Ex.No.11) (corresponds to Ex.No.12) Aqueous polyurethane resin E1 E4 CE2 CE3 Crosslinking agent B B B B Formulation of coating (parts by weight) Aqueous polyurethane resin 100 100 100 100 Crosslinking agent 8 5 8 5 Surfactant 0.06 0.07 0.06 0.07 Titanium dioxide - 1.8 - 1.8 Desalted water 20 15 20 15 Substrate Polypropylene plate Polypropylene plate Polypropylene plate Polypropylene plate Coating method Applicator Brushing Applicator Brushing Thickness of film(µm) ca. 30 ca. 100 ca. 30 ca. 100 Drying conditions 60x0.5 23x24 60x0.5 23x24 (°Cxhr) 100x0.5 - 100x0.5 - Odor during coating and drying almost none almost none strong strong Aging conditions (23°C,60%RHxday) 7 7 7 7 Amount of residual solvent in dry film(wt%) Acetone <0.01 0.02 <0.01 0.03 N-methylpyrrolidone - - 0.9 8.5 According to the present invention, a substantially solvent-free, aqueous-type urethane polymer is easily obtainable, as stated above, without using any solvent having a high boiling point that could eventually lead to impaired coating environment and reduced coat quality. The aqueous polyurethane coating of the invention may be widely used for construction materials, automobile parts, wooden articles, electrical parts or textile products. Also advantageously, it is highly effective for reducing VOC against emission in the atmosphere.

Claims: A process for producing an aqueous polyurethane coating, which comprises the steps of: (1) reacting a polyisocyanate, a polyol and a dimethylol alkane acid of the following formula: wherein R is an alkyl group of 2 to 10 carbon atoms, in the presence of an organic solvent having a boiling point of 50 to 120°C as the only solvent to thereby prepare a solution of a urethane prepolymer having carboxyl groups and at its terminal ends NCO groups; (2) neutralizing the carboxyl groups attached to the urethane prepolymer with a basic material; (3) admixing water to the resultant urethane prepolymer solution whereby a solution or dispersion of the urethane polymer is obtained; (4) extending the polymer chain with a chain extending agent ; (5) removing said organic solvent from the urethane polymer solution or dispersion obtained. A process for producing an aqueous polyurethane coating, which comprises the steps of: (1) reacting a polyisocyanate, a polyol and a dimethylol alkane acid of the following formula: wherein R is an alkyl group of 2 to 10 carbon atoms, under the condition that NCO or OH groups are present in greater excess than others, in the presence of an organic solvent having a boiling point of 50 to 120°C as the only solvent; (2) conducting a chain extension using a chain extending agent in the presence of said solvent to thereby prepare a solution of a urethane prepolymer having carboxyl groups and at its terminal ends NCO groups; (3) neutralizing the carboxyl groups attached to the urethane prepolymer with a basic material; (4) admixing water to the resultant urethane polymer solution whereby a solution or dispersion of the urethane polymer is obtained; (5) removing said organic solvent from the urethane polymer solution or dispersion obtained. The process according to claim 1 or 2, wherein said dimethylol alkane acid is selected from the group consisting of dimethylol butanoic acid, dimethylol pentanoic acid, dimethylol heptanoic acid, dimethylol octanoic acid and dimethylolnonanoic acid. The process according to any of claims 1 to 3, wherein said dimethylol alkane acid is dimethylol butanoic acid. The process according to any of claims 1 to 4, wherein said organic solvent has a boiling point of 50 to 90°C. The process according to any of claim 5, wherein said organic solvent is acetone or methyl ethyl ketone. The process according to any of claims 1 to 6, wherein the amount of said dimethylol alkane acid contained in said urethane polymer is in the range of 0.4 to 5 % by weight in terms of the content of carboxyl groups. The process according to any of claims 1 to 7, wherein the amount of said organic solvent having remained unremoved in said urethane polymer is not more than 10 % by weight. A method of forming a polymer urethane coating from an aqueous polyurethane coating, which comprises the steps of: (1) applying an aqueous polyurethane coating to a substrate; and (2) drying said coating supported on said substrate to obtain thereby a dry coat, wherein the aqueous polyurethane coating is obtainable by a process comprising steps (1) to (5) as defined in claim 1 or 2. The method according to claim 9, wherein the amount of said organic solvent remained in said dry coat is not more than 0.1 % by weight. A coat obtainable by the process of claim 9, wherein the content of the organic solvent having a boiling point of 50 to 120°C is not more than 0.1 % by weight. The coat according to claim 11, wherein the content of the organic solvent is less than 0.05 % by weight.

Inventors: KOBAYASHI TATSUHIKO, KOBAYASHI, TATSUHIKO, Kobayashi, Tatsuhiko, c/o Mitsubishi Chem. Corp.
Applicants: NIPPON KASEI CHEMICAL COMPANY, NIPPON KASEI CHEMICAL COMPANY LIMITED
Application year: 1996
Main classification: C08G18
IPC codes: C09D5, C08G18, C09D175