PREPARATION OF A HYDROPHILIC COATING

Abstract: A process for coating a polymer surface with a hydrophi­ lic coating with low friction in wet condition. The process comprises applying to the polymer surface a solution containing between 0.05 to 40 % of a compound which comprises at least two unreacted isocyanate groups per molecule, evaporating the solvent, applying a solution containing between 0.5 to 50 % of polyvinylpyrrolidone to the thus created polymer surface and then evaporating the solvent of the last mentioned solution, and curing the coating at elevated temperature. The process is preferably carried out in the presence of a catalyst for the curing of isocyanate.

Description: The present invention relates to a process for coating a polymer surface with a hydrophilic coating which has a low coefficient of friction when wetted with a water-based liquid, as well as medical articles being coated with a hydrophilic coating according to said process. Prior art British patent No. 1600963 (Biosearch Medical Products Inc.) discloses a process for applying a hydrophilic coating which coating consists of an interpolymer of polyvinylpyrrolidone and polyurethane, said coating having a much lower coefficient of friction in wet condition than in dry condition. Such coatings are, among other things, especially useful for medical instruments intended to be entered in body cavities, because the instrument gives a good hand grip in dry condition while simultaneously becoming very slippery when it gets in contact with body liquids or wet mucuous membranes, and thus can be inserted easily without damage to the patient. This method known in the art comprises applying a polyisocyanate and a polyurethane (or polyisocyanate alone where the substrate is a polyurethane) in a solvent to the surface of the substrate to be coated. The tus treated substrate is then coated with polyvinylpyrrolidone in a solvent followed by solvent evaporation. The process disclosed in said British patent has been shown to be useful for accomplishing such coatings, but is on the other hand too complicated for being suitable for large-scale production. The process described in Example 1 in the said patent thus comprises no fewer than ten steps, whereby one of the steps takes 6 h to carry through. A further disadvantage with this known process is that cracks are often formed in the coating. Disclosure of the invention The object of the invention is to provide a process for the preparation of a hydrophilic coating that has a much lower coefficient of friction in wet condition and which process is more simple and more rapid to carry out than the process disclosed in the above-mentioned British patent No. 1600963. These objects of the invention have surprisingly been achieved in the absence of polyurethane by applying a solution containing between 0.05-40% (weight/volume, that is kg/l) of compound containing at least two unreacted isocyanate groups per molecule to a polymer surface, evaporating the solvent, applying a solution of polyvinylpyrrolidone containing between 0.5 to 50% (weight/volume) to the thus treated polymer surface and then evaporating the solvent of the last mentioned solution, and curing the coating at elevated temperature. The invention is explained in detail in the following. The process according to the invention is used for coating many different types of polymer surfaces other than polyurethane, selected from latex rubber, other rubbers, polyvinylchloride, other vinyl polymers, polyesters and polyacrylates. The process has been found to be especially useful for latex and polyvinylchloride. The process according to the invention comprises applying to the said polymer surface a compound containing at least two unreacted isocyanate groups per molecule dissolved in a solvent, by dipping, spraying or the like and then evaporating the solvent preferably by air drying. This step forms a coating with unreacted isocyanate groups on the polymer surface. Isocyanate containing compounds used are selected from 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, a pentamer of hexamethylene diisocyanate and toluene diisocyanate and trimerized hexamethylene diisocyanate biuret. The solvent for the isocyanate compound is one that does not react with isocyanate. The preferred solvent is methylene chloride but it is also possible to use ethylacetate, acetone, chloroform, methyl ethyl ketone and ethylene dichloride, for example. The isocyanate solution may advantageously contain between 0.5 to 10% (weight/volume) of isocyanate compound, and may preferably contain between 1 to 6% (weight/volume) of isocyanate compound. Generally, the solution needs to be in contact with the surface only briefly, for example 5 to 60 s. In the case of coating of for example rubber latex a longer period is desirable, for example 1 to 10 min, to obtain a strong adherence. Another method to increase the adherence is to swell the polymer surface beforehand in a suitable solvent. A still further method is to choose a solvent for the isocyanate, such that the solvent in itself has the ability to swell or dissolve the polymer surface which is to be coated. Following the evaporation of the solvent for the isocyanate from the polymer surface, the surface is coated with polyvinylpyrrolidone dissolved in a solvent, whereby a hydrophilic surface is obtained which subsequent to final curing of the isocyanate normally consists of a polyvinylpyrrolidone/polyurea interpolymer. The polyvinylpyrrolidone used should have a mean molecular weight of between 10⁴ to 10⁷, and the preferred mean molecular weight is about 10⁵. Polyvinylpyrrolidone having such a molecular weight is commercially available, for example under the trade name Kollidon® (BASF). Examples of suitable solvents for polyvinylpyrrolidone that can be used are methylene chloride (preferred), ethyl acetate, acetone, chloroform, methyl ethyl ketone and ethylene dichloride. The proportion of polyvinylpyrrolidone in the solution is preferably between 0.5 to 10% (weight/volume) and most preferred between 2 to 8% (weight/volume). The polyvinylpyrrolidone in the solvent is applied by dipping, spraying or the like for a short period of time, for example during 5 to 50 s. After the polyvinylpyrrolidone solution has been applied to the coated surface, the solvent is evaporated preferably by air drying. The residual traces of solvent are removed at the curing of the coating which is preferably performed at a temperature of 50 to 100° C, in for example an oven, and during 5 to 30 min. All the other steps in the process may be performed at ambient temperature. The purpose of the curing, which is advantageously conducted in the presence of a water-containing gas such as ambient air, is to bind the isocyanate compounds together to the formation of a stable non - reactive network that binds the hydrophilic polyvinylpyrrolidone. The isocyanate group reacts with water at the curing and yields an amine which rapidly reacts with other isocyanate groups to form a urea cross-link. According to the invention it has surprisingly been found possible to simultaneously reduce the low friction of the hydrophilic surface, to improve the adherence of the coating, and to shorten the necessary reaction times and curing times, by using some additives to the solution of isocyanate and/or the solution of polyvinylpyrrolidone. Suitable additives comprise different known catalysts for isocyanate curing. These catalysts may be dissolved in either the isocyanate solution or the polyvinylpyrrolidone solution but are preferably dissolved in the latter. Different types of amines are especially useful, for example different diamines. but also for example triethylenediamine. Preferably, an aliphatic amine is employed which is volatilizable at the drying and curing temperatures used for the coating, and which furthermore is non-toxic. Examples of suitable amines are N, N′-diethylethylendiamine, hexamethylendiamine, ethylendiamine, paradiaminobenzene, 1,3-propandiol-para-aminobenzoic acid diester and diaminobicyclo-octane. The proportion of catalyst in the polyvinylpyrrolidone solution is suitably between 0.1 to 50% by weight of the amount of polyvinylpyrrolidone, preferably between 0.1 to 10% by weight. Some of the above-mentioned amines, particularly the diamines, can also react with isocyanate and thereby contribute to the cross- linking of the isocyanate compounds that give the desired strong adherence between the hydrophilic coating and the polymer surface. Furthermore, it has surprisingly been shown to be possible to reduce the low friction for the hydrophilic surface still further, by way of dissolving a polymer other than polyurethane in the isocyanate solution which polymer has no alcoholic hydroxy groups. Examples of suitable polymers are polyesters, polyvinyl compounds such as polyvinylchloride or polyvinylacetate or copolymers of these. These otherwise substantially inert polymers are supposed to give the surprisingly reduced friction at the surface mainly because they obstruct an undesired diffusion of not yet cross- linked isocyanate compounds out into the polyvinylpyrrolidone layer. A further reason for the low friction may be that the addition of a polymer enhances the elasticity of the coating. The proportion of dissolved polymer in the isocyanate solution is suitably between 0.5 to 20% by weight of the solution, preferably between 2 to 10 by weight. The obtained hydrophilic coating evidently contains an appreciable amount of partly freely movable polyvinylpyrrolidone chains. Thus it has been shown that the coating can complex-bind a substantial amount of elemental iodine (compare Example 6), as is also the case with free polyvinylpyrrolidone. Such aniodine containing hydrophilic and antibacterial coating is advantageous for many medical uses, for example for urinary catheters which are intended for insertion in the urethra for extended periods of time and which otherwise are a common cause of infections. The iodine containing coating is suitably prepared by final dipping in a Kl/l₂-solution containing at least 1% by weight of iodine, followed by drying, possibly preceded by rinsing in a solvent. The invention is illustrated in detail in the following Examples. Example 1: A pentamer of hexamethylene diisocyanate and toluene diisocyanate of cyanurate type (named Desmodur HL, Bayer AG) was dissolved in methylene chloride to a concentration of 3% (weight/ volume). A urinary PVC catheter was dipped in this solution during 5 s. The catheter was allowed to dry at ambient temperature during 30 s, whereupon it was dipped during 5 s in a solution containing 6% (weight/volume) of polyvinylpyrrolidone (K90; mean molecular weight ∼ 360 000) and 0.6% (weight/volume) of triethylene-diamine (Dabco®), dissolved in methylene chloride. The catheter was then allowed to dry at ambient temperature during 60 s, and then cured during 20 min at 70°C above a bowl filled with water. The catheter was finally allowed to cool to ambient temperature during 20 min whereupon it was rinsed in water. The catheter had a slippery and adherent surface when wet. This experiment was repeated without any triethylenediamine being added, and in that case no slippery surface was obtained. Example 2: A trimerized hexamethylene diisocyanate of biuret type (named Desmodur L 2291, Bayer AG) was dissolved in methylene chloride to a concentration of 6% (weight/volume). A urinary PVC catheter was dipped in this solution during 30 s. The catheter was then dried at 70°C during 60 s, whereupon it was dipped during 5 s in a solution containing 33 g polyvinylpyrrolidone (K25; mean molecular weight ∼ 25 000) per 100 ml methylene chloride. This solution also contained 0.33 g triethylenediamine (Dabco®) per 100 ml solution. The catheter was then allowed to dry at ambient temperature during 60 s, and then cured during 40 min at 70°C above a bowl filled with water. The catheter was finally allowed to cool to ambient temperature and was then rinsed in water. The catheter had a slippery and adherent surface when wet. This experiment was repeated without any triethylenediamine being added, and in that case no slippery surface was obtained, even if the curing time was increased to 6 h at 70°C. Example 3: A trimerized hexamethylene diisocyanate of biuret type (named Desmodur L 2291, Bayer AG) was dissolved in methylene chloride to a concentration of 6% (weight/volume). A catheter made of softened PVC was dipped in this solution during 5 s. The catheter was then dried at ambient temperature during 30 s, whereupon it was dipped during 5 s in a solution containing 6% (weight/ volume) of polyvinylpyrrolidone (K90; mean molecular weight ∼ 360 000) and 1% (weight/ volume) of N,N′-diethylethylendiamine, dissolved in methylene chloride. The catheter was then allowed to dry at ambient temperature during 30 s and was then cured in an oven during 3 h at 70° C. The catheter was finally rinsed in water during 1 h at ambient temperature. The catheter had a slippery and adherent surface when wet. The experiment was repeated without any amine being added and in that case the hydrophilic surface was not adherent and disappeared when the catheter was rinsed in water. Example 4: A trimerized hexamethylene diisocyanate of biuret type (named Desmodur L 2291, Bayer AG) was dissolved in methylene chloride to a concentration of 6% (weight/volume). A urinary PVC catheter was dipped in this solution during 30 s and was then dried at ambient temperature during 30 s. The catheter was then dipped during 5 s in a solution containing 6% (weight/volume) of polyvinylpyrrolidone (K90; mean molecular weight ∼ 360 000) and 2.5% (weight/volume) of triethylamine, dissolved in methylene chloride. The catheter was then allowed to dry at ambient temperature during 30 s and was finally cured in an oven during 5 min at 70°C. The catheter had a slippery and adherent surface when wet. The experiment was repeated with variations in curing time of from 5 to 20 min, and it was found that a surface with optimum properties as to low friction and adherence was obtained after only 5 min curing time. Example 5: A trimerized hexamethylene diisocyanate of biuret type (named Desmodur L 2291, Bayer AG) was dissolved in methylene chloride to a concentration of 6% (weight/volume). A urinary latex catheter was dipped in this solution during 30 s and was then dried at ambient temperature during 30 s. The catheter was then dipped during 5 s in a solution containing 6% (weight/volume) of polyvinylpyrrolidone (K90; mean molecular weight ∼ 360 000) and 0.6% (weight/volume) of triethylamine, dissolved in methylene chloride. The catheter was then allowed to dry at ambient temperature during 30 s and was finally cured during 20 min at 70°C above a bowl filled with water. The catheter was finally allowed to cool to ambient temperature and was then rinsed in water and 0.2M HCl (aq.). The catheter had a slippery and adherent surface when wet. The experiment was repeated with variations in dipping time in the isocyanate bath of from 30 s to 10 min, but no advantage was obtained by increasing the dipping time beyond 30 s. Example 6: The inner of a 2 m long hose of PVC (inner diameter 8 mm; outer diameter 10 mm) was flushed with different solutions and drying agents in the following orders and periods of time: 6% (weight/volume) of a trimerized hexamethylene diisocyanate of biuret type (named Desmodur L 2291, Bayer AG) dissolved in methylene chloride and during 30 s, a stream of ambient air during 30 s; 6% (weight/volume) of polyvinylpyrrolidone (K90; mean molecular weight ∼ 360 000) and 0.06% (weight/volume) of triethylamine, dissolved in methylene chloride and during 10 s; a stream of ambient air during 30 s. The hose was then cured in an oven during 6 h at 70°C, and was then flushed with a saturated water solution of potassium iodide being saturated with elemental iodine. A section of the hose was cut open and the inside of the hose was found to be coated with a brown iodine-containing surface which was slippery when wet and which adhered to the surface even after extended soaking in water. The experiment was repeated without any triethylamine being added and it was found that no adherent coating was being formed. Example 7: In this experiment pieces (50x120 mm in size) of the following different plastics film materials were treated according to the invention and tested: a) low density polyethylene, LDPE, b) plasma treated low density polyethylene, PT-LDE, c) ethylene-2.5% vinyl acetate copolymer, d) ethylene-5% vinyl acetate copolymer, e) ethylene-18% vinyl acetate copolymer, f) ethylene-3% butyl acrylate copolymer, g) ethylene-7% butyl acrylate copolymer, h) ethylene-17% butyl acrylate copolymer, i) ethylene-methacrylic acid copolymer Surlyn (trade mark) (from Dupont). The film pieces were treated with a solution of 2% diphenylmethane-4,4′-diisocyanate in a mixture of 25% trichloroethylene and 75% methylene chloride for 15 s. Then the pieces were dried for 30 s and treated with a solution of 6% polyvinylpyrrolidone in a mixture of 25% trichloroethylene and 75% methylene chloride for 3 s. The film pieces were heated in an oven for 6 h at 70°C. After this the film pieces were wetted in water and the surface was tested for lubricity. The films of Surlyn (trade mark), butyl acrylate copolymers and vinyl acetate copolymers were found to have a permanent surface with very low friction when wet. The other films had low friction surface although the surface was not as durable.

Claims: 1. A process for placing on a polymer surface, wherein the polymer is selected from rubber, including latex rubber, vinyl polymers, polyesters and polyacrylates, a hydrophilic coating which has a low coefficient of friction when wetted with a water based liquid which comprises applying to the polymer surface a solution in an organic solvent which does not react with isocyanate wherein the solute amounts to between 0.05 to 40% (weight to volume) and substantially consists of isocyanate, wherein the isocyanate is selected from 2,4-toluene diisocyanate and 4,4-′-diphenylmethane diisocyanate, or a pentamer of hexamethylene diisocyanate and toluene diisocyanate of cyanurate type, or trimerized hexamethylene diisocyanate biuret, and evaporating the solvent, applying a second solution containing between 0.5 to 50% (weight to volume) of polyvinylpyrrolidone to the thus treated polymer surface and then evaporating the solvent of the second solution, and curing the coating at a temperature of 50 to 100°C in an oven in the presence of a water-containing gas, with the proviso that no polyurethane is present or formed in said polymer or in said coating. 2. A process according to claim 1, characterized in that the solution of polyvinylpyrrolidone or the solution of isocyanate contains a catalyst that accelerates the curing of the isocyanate. 3. A process according to claim 2, characterized in that the catalyst comprises an amine, preferably a diamine e.g. triethylene diamine. 4. A process according to claim 1, characterized in that the polymer surface to be coated comprises a polymer selected from the group consisting of latex and polyvinylchloride. 5. A process according to claim 1, characterized in that the obtained hydrophilic coating is made antibacterial by treatment with iodine. 6. Medical article intended for insertion in the body, characterized in that the article is provided with a hydrophilic coating prepared according to claims 1, 2, 3, 4 or 5. 7. Medical article according to claim 6, characterized in that it comprises a catheter.

Inventors: LAMBERT, HANS RAYMOND
Applicants: ASTRA MEDITEC AB
Application year: 1983
Main classification: C08J7
IPC codes: B05D5, B05D7, C09D175, C08J7, C08G18, A61L29