WO2016203084A1 - Method for preventing biological corrosion on surfaces - Google Patents

Abstract

The invention relates to a method for preventing biological corrosion on surfaces. In particular, the invention relates to a method for preventing corrosion as a result of the fungus, Hormoconis resinae on metal surfaces. In said method, a polyurethane coating is formed comprising an antimicrobial agent that impedes the growth of the fungus on the surface, and reduces the capacity for adhesion of spores and for the formation of a biofilm over the treated surface. The invention also relates to an antimicrobial coating that can be produced in accordance with the method according to the invention, and to the use thereof for the prevention of biological corrosion on surfaces.

Description

 PROCEDURE TO PREVENT SURFACE BIOLOGICAL CORROSION

Technical field

The present invention relates to a method for preventing corrosion of biological origin on the surface. In particular, the present invention relates to a new process capable of forming a polyurethane-based coating and biocidal properties on a surface capable of presenting biological corrosion easily and safely, as well as the new coating of antimicrobial properties obtained with the process. In particular, the invention relates to a new method for preventing biological corrosion on a metal surface. Prior art

Biological contamination of fluids is a problem in their extraction, production, distribution, operation and storage. Corrosion derived from such biological contamination has been observed, for example, in steel and aluminum, painted or not, in contact with fluids, such as fuels, contaminated with microorganisms, such as fungi.

The microorganisms can grow in certain fluids, staying active in the aqueous phase and using, for example, hydrocarbons as food. In some cases they can also use some of the additives as a source of nutrients.

Bacteria and fungi can grow in the aqueous phase that is in these fluids, as well as on the surrounding surfaces causing biological corrosion on these surfaces. In particular, biological corrosion is due to the influence of these microorganisms on the kinetics of corrosion processes of metal surfaces, caused by the adhesion of microorganisms on said surfaces.

Microbial biological contamination causes several problems, including the generation of degradation products and inlays on contact surfaces that lead to their corrosion; the transfer of scale to, for example, pipes can cause blockages of the same, and even the shutdown of an engine connected to them. In addition, if such inlays reach the combustion chamber of the engine, higher pollutant emissions occur. The type of organism and the damage they produce depends on the type of fluid and the additives found in it. Among the microorganisms that can be found in, by For example, diesel fuels can be mentioned Aspergillus flavus, Staphylococcus epidermidis, Agrobacterium tumefaciens, Ralstonia picketii, the most prominent being the filamentous fungus Hormoconis resine, which is present in approximately 70% of the cases of contamination detected in said fuel. The remaining 30% includes bacteria, other fungi and some yeasts. Fungi are well known as producers of organic acids, which are capable of contributing to microbiological corrosion.

The H. resine fungus causes the most damage for several reasons. On the one hand, its size and volume, since it produces more biomass compared to other bacteria and fungi. On the other hand, it is the major cause of biological corrosion in, for example, fuel tanks. Finally, because H. resin grows in the water and fuel inferred, starting normally in a small drop of water, coating it, and continuing the growth so that it adheres firmly to the surface in contact with the fluid. The acidic organic by-products excreted by this type of fungus dissolve or complex the copper, zinc and iron of the aluminum alloys used in, for example, aviation, automotive, pipelines or their components and unions, forming corrosion points that persist in the conditions anaerobic established under the mycelium of the fungus. Thus, in order to avoid corrosion of biological origin on a surface, for example in contact with fuel, the periodic elimination of water from them has been used, and the routine addition of water-soluble biocides to the fuel in order to avoid microbial growth and at a maximum concentration that does not significantly modify the properties of the fuel. Problems appear when the biocide has run out or the amount of water present in the fuel has been underestimated.

Attempts have also been made to form emulsions between water and fuel, but these have led to an accelerated growth of fungi by increasing the interfacial surface between water and fuel and thereby the availability of nutrients and oxygen. Another approach to address this problem has been described in Lackner et al., Prevention of Biofouling in Hydrocarbons by Antimicrobial Vessel and Pipeline Coating for Cost Savings and an Increase in Safety and Reliability, Iranian J. Oil Gas Sci. Technol., 2013, 2 (2), 1-7. This article describes the application of an epoxy coating that It contains an oxide of a transition metal, such as M0O3 and WO3, in its matrix. Said oxides exhibit a strong antibacterial effect on the surfaces of the fuel tanks, which is based on the formation of molybdic or tungstic acid by reaction of the oxides with water. In this way the surface pH is reduced to a value of approximately 4.5-5.5. The cell wall of the bacteria is destroyed by coming into contact with the acidic surface. This technology, which works by contact, presents the problem that the biocide remains on the protected surface and, as leaching does not occur, does not easily reach the interface between water and fuel, which is where the fungus H. especially grows. Resin For the same purpose, Canadian patent application CA-A-2869523 describes a composite material comprising a support material, an antimicrobial agent and at least one hydrophilic agent. The antimicrobial agent is selected from M0O3 and WO3, possibly in combination with the Mo or W metals. The support material is selected from a group that includes organic polymers, silicones, glasses, ceramics, waxes, resins, dyes, varnishes, textiles , fabrics and / or wood. The examples describe efficiency tests against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa, but not against fungi such as H. resine. One of the applications of this composite material mentioned in the Canadian patent is that it can be used as marine paints to prevent the embedding of surface organisms. In spite of the technical solutions described in the state of the art to avoid the embedding of microorganisms and the consequent biological corrosion on the surface, there is still a need to have a procedure for the protection of surfaces susceptible to presenting biological corrosion on the surface, in particular that originated by the fungus H. resine. Object of the invention

The object of the present invention is a process for preventing biological surface corrosion. In particular, the present invention provides a new process capable of forming a polyurethane-based coating with biocidal properties on a surface capable of presenting biological corrosion to easily and safely prevent biological corrosion on the surface with the coating.

Also part of the object of the invention is a surface coating obtainable in accordance with the method of the invention and the use thereof to prevent biological corrosion on a surface susceptible to biological corrosion. Also part of the object of the invention is an equipment or accessory provided with a surface capable of presenting biological corrosion and coated according to the process of the invention.

In one embodiment, the equipment or accessory can be applied in aviation, automotive, pipes and unions, in general.

In another embodiment, the surface susceptible to biological corrosion is a metal surface.

Figures

Figure 1 shows the results of the tests carried out in Example 4, with agar plates inoculated with the H. resine fungus. At the top A are the agar plates in which an aluminum plate coated with a polyurethane coating without antimicrobial agent was introduced. In the central part B are the agar plates in which an aluminum plate coated with a polyurethane coating with 3% by weight of zinc pyrithione on the weight of the dry coating was introduced, and in the lower part C they find the agar plates in which an aluminum plate coated with a polyurethane coating with 5% by weight of zinc pyrithione on the weight of the dry coating was introduced. It is observed in said figure that the presence of the antimicrobial agent generates a large zone of inhibition for the growth of the H. resin resin fungus around the aluminum plate. Detailed description of the invention

The object of the present invention is a process for preventing surface biological corrosion by forming a polyurethane coating characterized by the fact that it comprises applying a composition on the surface capable of presenting biological corrosion, the composition comprising: a ) a compound with at least two isocyanate groups, b) a polyol compound with at least two hydroxyl groups, and c) an effective amount of zinc pyrithione,

to form a polyurethane coating of antimicrobial properties The authors of the present invention have developed a method for preventing surface biological corrosion by a polyurethane coating comprising zinc pyrithione as an antimicrobial compound, which is dispersed in the matrix of the coating. Said coating has, surprisingly, two effects: on the one hand it releases in a controlled manner said antimicrobial compound, so that it inhibits microbial growth, in particular of the H. resine fungus, and on the other it presents a reduced surface tension that does not favor the adhesion of the microorganisms on the surface coated with said polyurethane.

In the present description, as well as in the claims, the singular forms "a", "a" and "the" or "the" include the plural reference unless the context clearly indicates otherwise.

Biological corrosion

Biological corrosion means that corrosion that is due to the influence of microorganisms on the kinetics of corrosion processes, mainly of metals, caused by the adhesion of microorganisms to surfaces.

The method of the invention is preferably used to prevent biological corrosion caused by a microorganism selected from the group consisting of bacteria: Aerobacter aerogenes, Achromobacter, Bacillus mycoides, B. subtilis, Bacillus sp., Brevibacterium, Clostridium, Desulfovibrio desulfuricans, Flavobacterium, Micrococcus sp., Pseudomonas aeruginosa, Ps. Fluorescens, Pseudomonas sp., Sarcina hansenii, Staphylococcus, Actinomyces, Cylindrogloea bacterifera, Nocardia sp., Sorangium sp., Sphaerotilus natans, Streptomyces; and by fungi and yeasts: Aspergillus amstelodami, A. flavus, A. flavipes, A. niger, Aspergillus sp., Alternaría sp., Cephalosporium sp., Cladosporium sp. (Hormodendrum), Conidiobolus sp., Fusaríum sp., Helminthosporium sp., Paecilomyces sp., Penicillium luteum, Penicillium sp., Phialophora sp., Pullularía sp., Spicaria sp., Stysanus sp., Stemphylium sp., Syncephamatrum, Tricho measures, Ustitago sp., Candida, Rhodotorula.

The authors of the present invention have proven that the process of the invention is also useful for preventing biological corrosion caused by Escherichia Coli (E. Coli) and Legionella.

In some documents of the state of the art, the microorganism Stysanus sp, is referred to as Stypanus sp. In the context of the invention, "preventing biological corrosion" is understood to inhibit the growth of microorganisms and the formation of biofilm, in particular of the H. resine fungus, on a surface susceptible to biological corrosion in order to avoid and / or reduce corrosion on said surface due to the presence of said microorganisms. In one embodiment, the most harmful microorganism on a surface in contact with fuel is Cladosporium resine (also known as Hormoconis resine), which is the main cause of microbial corrosion. H. Resin fungus can live in equipment or accessories in contact with fuel, particularly in tanks intended to store fuel for airplanes, and takes advantage of the hydrocarbons present in it and traces of water for its growth. The by-products excreted by said fungus generate corrosion on aluminum and its alloys that are used for the construction of fuel tanks. In a preferred embodiment, the process of the invention is particularly directed to prevent biological corrosion caused by said fungus.

The polyurethane coating By executing the process of the invention it is achieved that the surface susceptible to biological corrosion, normally the inner surface of a device or accessory, is coated with a polyurethane coating.

In one embodiment, said surface is the inner surface of an equipment or accessory in contact with fuel, such as a fuel pump, a pipe provided with a plurality of pipes, valves, connecting elements, etc., in contact with fuel fluids. .

In another embodiment, said surface is the inner surface of a device or accessory in contact with water, such as a drinking water storage tank, a pump for drinking water, a pipe provided with a plurality of pipes, valves, joining elements , etc., in contact with drinking water fluids.

In yet another embodiment, said surface is the surface of an equipment or accessory in contact with fluids capable of possessing biological contamination such as, for example, surface pretreatment fluids that are used in industry in general and in the automotive sector, in particular . The surface to be coated according to the process of the invention may be new, that is, it may correspond to a device or accessory in which a fluid has not yet been stored or circulated, or it may correspond to a device or accessory in use , that It has undergone a cleaning procedure to remove organic residues and corrosion debris, and that is treated with the process of the invention to prevent biological corrosion.

In a preferred embodiment, the polyurethane coating is applied on the internal surface of a device such as a reservoir that is coated by an epoxy resin primer. In an even more preferred embodiment, the polyurethane coating is applied on an epoxy resin primer that is not yet fully cured. The latter case is well known in the field of paints, and is called a wet application on wet (in English wet on wef). The application on an epoxy resin primer that is not completely cured allows a better anchoring of the polyurethane coating in the primer layer, since the epoxy groups thereof can react with the hydroxyl groups present in the isocyanate and polyol composition.

Also part of the object of the invention is a coating obtainable according to the process of the invention, in which the composition that forms the polyurethane coating is applied wet to wet on the epoxy resin primer layer.

Also part of the object of the invention is an internally coated equipment or accessory according to the antimicrobial coating of the invention. In a preferred embodiment, the equipment is an aircraft fuel tank.

The antimicrobial polyurethane coating is obtained by applying a composition comprising a compound with at least two isocyanate groups and a polyol compound with at least two hydroxyl groups, which additionally includes zinc pyrithione as an antimicrobial component, and, optionally, other components, which are detailed below.

A polyurethane is a polymer that is obtained by condensation of a compound with two or more isocyanate groups and a polyol compound with two or more hydroxyl groups.

The process of the invention further includes the step of drying or curing the polyurethane coating. In this step the bonds between the compound with isocyanate groups and the compound with hydroxyl groups are formed. The curing step can be carried out at room temperature for a period between 6 and 10 hours, or at higher temperatures such as, for example, at 80 ° C for approximately 30 minutes, thus ensuring complete evaporation of the solvents used in the formulation.

The compound with isocyanate groups

Compounds with two or more isocyanate groups, preferably two groups, can be aliphatic or aromatic. Among the aliphatic diisocyanates there may be mentioned, for example, 4,4'-dicyclohexylmethane diisocyanate (H12MDI); 1,4-cyclohexane diisocyanate (CDI); Isophorone diisocyanate (IPDI); 1,6-hexamethylene diisocyanate (HDI); 1,1,6,6-tetrahydroperfluorohexamethylene diisocyanate (TFDI); dimeryl diisocyanate (DDI); 5-isocyanate-

1- (2-Isocyanato-1-yl) -1, 3,3-trimethylcyclohexane; 5-isocyanate-1- (3-isocyanatoprop-1-yl) -1, 3,3-trimethylcyclohexane; 5-isocyanate- (4-isocyanatobut-1-yl) -1, 3,3-trimethylcyclohexane; 1-isocyanate-

2- (3-Isocyanate-prop-1-yl) cyclohexane; 1-isocyanate-2- (3-isocyanato-1-yl) cyclohexane; 1, 2- diisocyanate-cyclobutane; 1,3-diisocyancyclocyclobutane; 1,2-diisocyanoccyclopentane; 1, 3- diisocyan-tocyclopentane; 1,2-diisocyancyclohexane; 1,3-diisocyancyclohexane; dicyclohexyl methane-2,4'-diisocyanate; trimethylene diisocyanate; tetramethylene diisocyanate; pentamethylene diisocyanate; ethylethylene diisocyanate; trimethylhexane diisocyanate; heptamethylene diisocyanate; 2-heptyl-3,4-bis (9-isocyanatonyl) -1-pentylcyclohexane; 1, 2-, 1, 4-, and 1,3-bis (isocyanatomethyl) cyclohexane; 1, 2-, 1, 4-, and 1,3-bis (2- isocyanate-1-yl) cyclohexane; 1,3-bis (3-isocyanatoprop-1-yl) cyclohexane; 1, 2-, 1, 4- and 1, 3-bis (4- isocyanatobut-1-yl) cyclohexane. Among the aromatic diisocyanates there may be mentioned, among others, 4,4'-diphenylmethane diisocyanate (MDI) and 2,6- and 2,4-toluene diisocyanate (TDI).

In the process of the invention, a diisocyanate selected from the group consisting of 4,4'-dicyclohexylmethane diisocyanate (H12MDI), 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI), diisocyanate of 1,6-hexamethylene (HDI), 1,1,6,6-tetrahydroperfluorohexamethylene diisocyanate (TFDI), dimeryl diisocyanate (DDI), 4,4'-diphenylmethane diisocyanate (MDI) and 2,6- and diisocyanate 2,4-toluene (TDI). More preferably, 2,6- and 2,4-toluene diisocyanate (TDI) is used.

Such compounds may be commercially available, for example, under the designations Basonat ^® HB 175 MP / X (HDI-based aliphatic polyisocyanate) from BASF; Desmodur ^® L75 (aromatic polyisocyanate based on TDI) and Bayhydur ^® XP 2451 (hydrophilic aliphatic polyisocyanate based on HDI) from Bayer; Polurene ^® MT90 (HDI-based aliphatic polyisocyanate) and Polurene ^® 2391 (aliphatic aromatic isocyanurate) from Sapici; and Tolonate ^® HDB 75 Bx (aliphatic polyisocyanate biuret based on HDI) from the company Vencorex. Desmodur ^® L75 is preferably used in the process of the invention, which is an aromatic polyisocyanate based on TDI, with an isocyanate group content between 12.9% and 13.7%, and a non-volatile content between 73% and 77%. Polyol compound

The polyol compound generally has a molecular weight between 200 and 10,000 g / mol, preferably between 800 and 5,000 g / mol. The polyols that are commonly used are polyethers and polyesters comprising hydroxyl groups. Other polymers comprising hydroxyl groups can also be used in a polycarbonate, polycaprolactone, or polybutadiene structure.

In the process of the invention the polyol is preferably selected from polyether with hydroxyl groups and polyester with hydroxyl groups, more preferably it is a polyester with hydroxyl groups, and even more preferably it is a polyester with hydroxyl and saturated groups. A polyether polyol is generally obtained by reacting epoxides, such as, for example, ethylene oxide, propylene oxide, or mixtures thereof, with a monomeric polyol such as, for example, glycerin, pentaerythritol, trimethylolpropane, neopentyl glycol, ethylene glycol, or sucrose, in the presence of a catalyst such as potassium hydroxide.

A polyester polyol is generally obtained by condensation of a polyol, such as those mentioned in the preceding paragraph, with a polycarboxylic acid or its derivatives such as, for example, italic anhydride, adipic acid, ter-phthalic anhydride, or trimellitic anhydride.

Such compounds may be commercially available, for example, under the names Synthoester ^® 1 165 (saturated hydroxyl polyester) from Synthopol; Tri-Rez ^® Polyol 1030A-300 Polyester Resin (linear saturated polyol polyester) from Geo Specialty Chemicals; Baycoll ^® AD 2047 (linear polyol polyester) and Desmophen ^® 1 150 (branched polyalcohol with ester and ether groups) from Bayer; CA-2010 (polyester polyol) of the Chanda Chem company; Urethall ^® 4050-55 (linear, saturated, aliphatic polyol polyester) from Hallstar; Layer ^® 2055 (linear diol polyester) from Perstorp; Aroplaz ^® 5885 (polyester polyol) from Reichhold; and Stepanpol ^® PD-200 LV (polyester anhydride and diethylene glycol polyester) from the company Stepan. Synthoester ^® 1 165 is preferably used in the process of the invention, which is a saturated hydroxyl polyester with a hydroxyl number between 240 and 290 mg KOH / g with with respect to the non-volatile content, which represents a hydroxyl group content of approximately 8% by weight with respect to the non-volatile content.

The polyurethane coating is obtained by combining appropriate amounts of isocyanate and polyol to achieve a molar ratio between 0.9: 1.0 and 2.0: 1.0 between the isocyanate and hydroxyl reactive groups, preferably between 0.9 : 1, 0 and 1, 5: 1, 0, even more preferably between 1, 0: 1, 0 and 1, 2: 1, 0, and even more preferably 1, 0: 1, 0. Thus, for example, in this case more preferred, if Synthoester ^® 1165 is used as a source of hydroxyl groups and Desmodur ^® L75 as a source of isocyanate groups, 100 parts by weight of the first are used for every 100 parts by weight of the second. The antimicrobial component

The composition used to obtain the antimicrobial polyurethane coating comprises, in addition to the compound with isocyanate groups, and the compound with hydroxyl groups, an effective amount of zinc pyrithione as an antimicrobial component. Generally an amount of zinc pyrithione is used which is between 0.5% and 10% by weight / weight on the solids content of the coating, preferably between 1% and 7%, and even more preferably between 3% and 5%.

Effective amount is understood to be that amount of antimicrobial agent that is effective in inhibiting the vegetative growth of the H. resine fungus, and can be determined according to the method described in the examples. Zinc pyrithione is a common compound in the industry and can be easily obtained from various commercial sources.

Other components

The composition used to obtain the antimicrobial polyurethane coating may also include other components selected from the group consisting of pigments, solvents, catalyst, rheological agents, dispersing-wetting agents, anti-skin agents, and mixtures thereof. The amounts of these additional components can be easily adjusted by the person skilled in the art by routine tests and taking into account the indications of the suppliers thereof.

The pigments can be included in the composition to obtain an anticorrosive effect or improve the application results. Among them may be mentioned, for example, iron oxide, talc, calcium carbonate, clay, and titanium dioxide. The solvents are used to incorporate the composition to solubilize and / or make the components compatible with isocyanate groups with the components with hydroxyl groups, and the other components of the formulation. They also facilitate the application of the composition with an appropriate viscosity. The solvent may be a single organic solvent or a mixture of organic solvents. Examples of suitable non-limiting organic solvents for use in the compositions of the present invention include aliphatic hydrocarbons (eg, mineral alcohols, kerosene); aromatic hydrocarbons (for example, benzene, toluene, xylene, solvent naphtha 100, 150, 200); alcohols (for example, ethanol, n-propanol, isopropanol, n-butanol, isobutanol); ketones (for example, acetone, 2-butanone, 4-hydroxy-4-methylpentan-2-one, cyclohexanone, methyl ketones, there ketones, methyl isoamyl ketones); esters (for example, ethyl acetate, butyl acetate); glycols (for example, butyl glycol); glycol ethers (for example, monomethyl ether of ethylene glycol, monoethyl ether of ethylene glycol, monobutyl ether of ethylene glycol, monomethyl ether of propylene glycol); esters of glycol ethers (for example, butyl glycol acetate, methoxypropyl acetate); and their mixtures. Preferably a combination of xylene and ethyl acetate is used. In the composition used to obtain the polyurethane coating the solvent content is generally not more than 22% by weight with respect to the total weight of the composition. In this way a low solvent content is maintained in the composition, in order to comply with the regulations on the limits of the concentration of volatile compounds.

The catalyst can be included in the composition to facilitate hardening of the coating. Said catalyst can be a metallic organic compound (for example, dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin diacetate, lead naphthenate, cobalt naphthenate, lead 2-ethylhexanoate, iron 2-ethylhexanoate), a phosphoric ester (for eg, monobutyl phosphate, dibutyl phosphate), or a tertiary amine (for example, triethylene diamine).

Rheological agents can be included in the composition to improve its stability and facilitate application. Said compounds allow to modify the rheological characteristics of the composition to obtain a highly thixotropic behavior, thus reducing the problems derived from the sedimentation of the pigments and the removal of the coating when applied. They are available on the market, for example, under the name Byk ^® -D 410 of the Altana company. In the composition used to obtain the polyurethane coating the rheological agent content is generally between 0.1% and 5% by weight with with respect to the weight of zinc pyrithione, preferably between 1% and 3%, and more preferably between 2.5% and 3%.

Wetting-dispersing agents may be included in the composition to facilitate dispersion of the pigments therein. They are available on the market, for example, under the name Disperbyk ^® -2155 from the Altana company. In the composition used to obtain the polyurethane coating, the content of wetting-dispersing agent is generally between 0.1% and 5% by weight with respect to the weight of zinc pyrithione, preferably between 0.25 % and 1%, and more preferably between 0.3% and 0.5%. Anti-skin agents can be included in the composition to prevent skin formation on the surface of the container in which it is located. Among them, the methyl ethyl ketone oxime can be mentioned.

Epoxy Resin Primer

As discussed above, in a preferred embodiment, the antimicrobial polyurethane coating is applied on the inner surface of the reservoir that is coated by an epoxy resin primer. In an even more preferred embodiment, the antimicrobial polyurethane coating is applied on an epoxy resin primer that is not yet fully cured. The latter case is well known in the field of paints, and is called wet application on wet (in English wet on wef). The use of an epoxy resin primer on a metal surface is common to confer corrosion protection to it.

An epoxy resin primer is obtained from a composition comprising a polymer with epoxy groups and a hardener such as, for example, a polyamine.

For a layer between 15 and 25 μηι thick, once dry, the epoxy resin primer is generally dry to the touch at 20 minutes at 23 ° C, and is usually completely cured after 3 days at 23 ° C, between 2 and 3 hours at 60 ° C, and 1 hour at 80 ° C.

The application of the wet coating on wet is preferably carried out after 12 hours after applying the epoxy resin primer and kept at room temperature. Polymers with epoxy groups Polymers with epoxy groups suitable for obtaining an antimicrobial coating with good applicative properties can be selected from the group consisting of novolac phenolic resins with epoxy groups, novolac cresolic resins with epoxy groups, glycidylamine epoxy resins, bisphenol A epoxy resins, bisphenol F epoxy resins, aliphatic epoxy resins, and cycloaliphatic epoxy resins.

Novolac epoxy resins come from the polymerization of a phenolic compound with formaldehyde which then reacts with epichlorohydrin. Glycidylamine epoxy resins are obtained by reacting aromatic amines with epichlorohydrin. The epoxy resins of bisphenol A or bisphenol F are prepolymers obtained by reacting epichlorohydrin with bisphenol A or bisphenol F respectively. Aliphatic epoxy resins are obtained by reacting epichlorohydrin with aliphatic alcohols or polyols to give glycidic ethers, or with aliphatic carboxylic acids to give glycidic esters. Cycloaliphatic epoxy resins are obtained by reacting cycloolefins with a peracid, such as, for example, peracetic acid. Such resins can be found commercially under the names Araldite ^® EPN 1183 (polyfunctional epoxy resin) or Araldite ^® EPN1 180 from Huntsman; Epalloy ^® 8240 or Epalloy ^® 8330 X85 from Emerald Performance Materials; RoyOxy ^® RAR 9438 from Royce; Epon ^® REsin 160 or Epikote ^® Resin 154-B-80 from Momentive; DEN 438-EK85 or DEN 431 of the Dow Chemical company; Uravar ^® L19 M3-42 from the company DSM; Phenodur ^® VPW 1942 / 52WA or Phenodur ^® PR 612 / 80B from Allnex; Epiclon ^® N-740 from DIC Corp .; or Epilox ^® M 1049 from the Leuna Harze company.

Epoxy Resin Hardeners

The products commonly used as hardeners of an epoxy resin are polyamines such as, for example, diethylenetriamine, triethylene tetraamine, tetraethylenepentaamine, polyoxypropylene diamine, isophoronadiamine, m-xylylenedimine, 4,4'-methylenebis (cyclohexylamine), 1, 3 [3-dimethylamino) -propyl] urea, 2-piperazin-1-ylethylamine, benzyldimethylamine, or bishexamethylenetriamine. Such products can be found commercially under the name, for example, of Aradur ^® 30 XWM 55 (aliphatic polyamine), Aradur ^® 943 (comprising diethylenetriamine) and Aradur ^® 2973 (comprising m-xylylenediamine, 4,4'-methylenebis (cyclohexylamine) , and cycloaliphatic polymeric amines with registration number CAS 135108-88-2) of the Huntsman company; Versamine ^® EH 50 (1, 3-bis [3- dimethylamino) propyl] urea) and Versamine ^® F11 (comprising 2-piperazin-1-ylethylamine and benzyldimethylamine) from BASF; Dytek ^® BHMT Amine (bishexamethylenetriamine) from the company Invista Specialty Chemicals; Docure ^® KH-500F (modified polyamine with a total amine number between 450 and 500 mg KOH / g) from Kukdo Chemical; Epilox ^® Hardener H 12-01 (aliphatic amine adduct with an amine index between 155 and 185 mg KOH / g) from the company Leuna Harze; Epikure ^® 3213 (aliphatic amine adduct) and Epikure ^® 3192 (comprising triethylenetetraamine) from the Mometive company; ChemCure ^® 310 (cycloaliphatic amine) from Polystar, or RoyOxy ^® RAC9837 (cycloaliphatic amine with an amine index between 395 and 415 mg KOH / g) from Royce. The epoxy resin primer is applied following methods well known to the person skilled in the art or following the manufacturer's instructions.

Properties of polyurethane coating

The polyurethane and epoxy coatings can be applied on the internal surface of the tank, generally of aluminum or an aluminum alloy, by brush / brush or by spraying, according, for example, with the methods described in the UNE 48307 standard.

The general requirements that the application of coatings generally meets are the following:

- The application temperature must not be less than 10 ° C. - At 8 hours the coating must be dry to the touch.

- At 18 hours the coating must be completely dry.

- At most 24 hours the coating must be able to be repainted.

Once the coatings have been applied, the following characteristics can be determined:

- Adhesion according to ISO 4624: 2002. - Drying time according to UNE 48301: 1999.

- The weight of the coating.

- The control of porosity through the use of the Holiday detector.

- Fuel resistance according to ISO 2812-1: 2007. It has been found that the antimicrobial polyurethane coating applied on an epoxy resin primer meets the requirements of adhesion, hardness and resistance to immersion in an aviation fuel, such as JP8 fuel, required for a suitable coating to be applied to a fuel tank, in particular an aircraft fuel tank. The application of the wet polyurethane coating on wet on an epoxy resin primer has superior results compared to the application of it on the epoxy resin primer once dry.

Effects on biological corrosion The zinc pyrithione included in the matrix of the polyurethane coating is released in a controlled manner to the medium so that a permanent concentration is maintained that is slightly higher than the minimum inhibitory concentration. Such sustained release provides a long-term antimicrobial effect that minimizes maintenance operations due to microbial contamination of fuel tanks. Thus, it has been observed that the antimicrobial coating has a high capacity to prevent germination of the spores and, consequently, to inhibit the vegetative growth of H. resine. To evaluate this effect, the diffusion test can be used, which is a rapid method that consists in evaluating the antimicrobial behavior of a material by direct contact between said material and an agar plate in which the microorganism has been inoculated. Through this procedure it is possible to determine whether a coating has the ability to prevent germination of the spores and, consequently, inhibit vegetative growth. This method provides qualitative information on the degree of inhibition of such growth, but does not provide information on possible sporicidal characteristics of the material. Additionally, the incorporation of zinc pyrithione into the polyurethane coating reduces the surface tension of said coating (28 mN / m for the polyurethane coating without zinc pyrithione, and 26 mN / m for the polyurethane coating zinc pyrithione), which according to RE Baier, Surface behavior of biomaterials: the theta Surface for biocompatibilty, J. Mat. Sci. Mater. Med., 2006, 17, 1057-1062, translates into less adhesion of the biofilm in said coating, and avoids having to use silicones to reduce said surface tension, since they are generally not compatible with the fuel in the tank. According to said article, the minimum bioadherence of a material is preferably in the area between 20 and 26 mN / m. This result is surprising because in the case of including zinc pyrithione in an epoxy coating there is an increase in its surface tension (31 mN / m for the epoxy coating without zinc pyrithione and 37 mN / m for the epoxy coating with zinc pyrithione). The tests carried out in relation to the development of a biofilm show that the antimicrobial coating reduces the fixation of the spores in a logarithm compared to a control coating.

Therefore, the use of a polyurethane coating comprising zinc pyrithione is also part of the object of the invention to prevent biological corrosion on a metal surface of an equipment or accessory, preferably biological corrosion caused by the H. resine fungus.

Similar results are obtained with other microorganisms such as Pseudomonas aeruginosa (bacteria) and Candida tropicalis (yeast). A fact that shows the wide applicability of the process of the invention to different types of microorganisms.

Thus, the process of the invention confers some antimicrobial characteristics of equipment or accessory thus coated, so that, surprisingly, it inhibits the vegetative growth of microorganisms, in particular of the H. resine fungus, and has a lower spore adhesion and biofilm formation generated by these microorganisms. Some examples are included below to illustrate the present invention, although they should not be considered as limiting thereof.

Examples

Comparative example 1: Epoxy coating without antimicrobial agent

An epoxy composition was prepared without antimicrobial agent by mixing the following components: 51, 2% by weight of Araldite EPN 1183 ^®, 27.2% by weight of Aradur ^® 2973, and 21, 5% by weight of xylene.

In order to obtain a homogeneous composition, a Eurostar power control-vise 6000 (IKA ^® ) high speed agitator was used in this example, and in the following ones.

By applying said composition on a substrate an epoxy coating was obtained which did not comprise antimicrobial agent. Comparative Example 2: Epoxy Coating with Antimicrobial Agent

An epoxy composition with antimicrobial agent was prepared by mixing the following components: 49.4% by weight of Araldite ^® EPN 1 183, 26, 1% by weight of Aradur ^® 2973, 2.3% by weight of zinc pyrithione 0.2% by weight of Disperbyk ^® 180 and 22.2% by weight of xylene.

The epoxy coating obtained by applying this composition had a content of 3% by weight of zinc pyrithione.

Comparative example 3: Polyurethane coating without antimicrobial agent

A composition without antimicrobial agent was prepared by mixing the following components: 38.4% by weight of Desmodur ^® L75 (75% aromatic isocyanate in ethyl acetate, with an isocyanate group content between 12.9 and 13 , 7%), 39.0% by weight of Synthoester ^® 1165 (65% polyol in a mixture of methoxypropyl acetate and xylene in a 1: 1 ratio, with a hydroxyl group content of 8%), 1 1, 3% by weight of xylene and 1.3% by weight of ethyl acetate. When applying said composition on a substrate, a polyurethane coating was obtained which did not comprise antimicrobial agent.

Example 1: Polyurethane coating with antimicrobial agent

A composition with antimicrobial agent was prepared by mixing the following components: 37.6% by weight of Desmodur ^® L75 (75% aromatic isocyanate in ethyl acetate, with an isocyanate group content between 12.9 and 13 , 7%), 38.4% by weight of Synthoester ^® 1165 (65% polyol in a mixture of methoxypropyl acetate and xylene in a 1: 1 ratio, with a hydroxyl group content of 8%), 1, 6 % by weight of zinc pyrithione, 0.05% by weight of Disperkyk ^® 2155 (dispersing-wetting agent), 0.01% of Byk ^® 410 (rheological agent), 1 1, 1% by weight of xylene and 1 1 , 1% by weight ethyl acetate.

After applying said composition and evaporating the solvents, a polyurethane coating comprising 3% by weight of zinc pyrithione dispersed in the matrix of the coating was obtained.

Example 2: Polyurethane coating with antimicrobial agent Following a procedure analogous to that of Example 1, a composition with antimicrobial agent was prepared which, after applying said composition and evaporating the solvents, a polyurethane coating comprising 5% by weight of zinc pyrithione was obtained. Example 3: Surface tension tests

The compositions prepared in comparative examples 1, 2 and 3, and in example 1 of the invention were sprayed as a top coat onto aluminum sheets coated with a completely dry epoxy resin primer.

Said epoxy resin primer was prepared from 100 parts by weight of the Base P 65-C epoxy resin and 43 parts by weight of the Hardener H88 hardener, both commercially available through the company MAPBER (France), and 80 parts of demineralized water. Once prepared, the primer could be applied over a period of 7 hours at a temperature of 23 ° C. This primer is commonly used for coating metal aircraft structures. For each of the plates coated with the epoxy top coat without and with antimicrobial agent (comparative examples 1 and 2), polyurethane without microbial agent (comparative example 3) and polyurethane with antimicrobial agent (example 1 of the invention) the tension was determined surface according to the method of the inks, marketed in this case by the company Plasmatreat (Germany). According to said method, a test ink is quickly applied to the substrate using the brush integrated into the bottle. The test is started with a high surface tension ink, such as 72 mM / m. If the brush trace margins remain stable for 2 seconds, the surface is considered to be easily wettable and, consequently, the surface tension of the substrate is at least equal to the value of the ink tested. If the brush trace margins contract, then the next test ink with a lower surface tension should be used.

Table I shows the surface tension values obtained for the four samples using a set of test inks in the form of an ethanol solution that analyzes surfaces in the range 28 mN / m - 72 mN / m at room temperature: TABLE I

It can be seen that the incorporation of zinc pyrithione in the polyurethane (PUR) coating results in a reduction in surface tension thereof, while in an epoxy coating, an opposite effect occurs, that is, an increase in the surface tension.

The reduction of the surface tension of the coating contributes to reducing the adhesion of the biofilm produced by the microorganisms on the metal surface treated with the process of the invention.

Example 4: Antimicrobial efficacy assays

The inhibition of microbial growth was determined specifically against the strain of ATCC 20495 of H. resine by the use of two methods: diffusion assay and biofilm formation.

A) Diffusion test

The diffusion test was carried out according to the following protocol:

1) The H. resin fungus was grown on malt extract agar. A spore suspension was prepared from resuspension in the diluent (4.5 g of NaCl, 1.5 g of peptone in 1 L of distilled water) of the filamentous structures of H. resin, grown on the malt extract agar plate. This suspension was filtered through a filter plate to separate the vegetative cells from the spores. The cells were retained in the plate, while the spores passed through the filter and, consequently, the final suspension contained only spores. Said suspension was centrifuged to obtain a sediment, from which a final inoculum was prepared containing 10 ⁴ spores of fungus H. resine / m \, which is the amount recommended by ASTM E 1259 to test microorganisms in fuels with a boiling point below 390 ° C.

2) The agar plates were inoculated with 100 μΙ of the H. resine spore suspension and the plate coated with the 2 cm x 2 cm test coating was placed in the center of the plate, ensuring that the surface it contained The antimicrobial agent was in contact with the agar. The plates were incubated for 7 days at a temperature of 30 ° C. All experiments were performed in duplicate.

3) At the end of that period, the plates were observed to determine if there was vegetative growth around the plates with the coating or if an inhibition zone could be identified.

The diffusion test was subjected to aluminum sheets coated with an epoxy resin primer, as described in Example 3, which were coated with a top coat based on applying the compositions described in Examples 1 and 2 of the invention , with 3% by weight and 5% by weight, respectively, of zinc pyrithione on the weight of the dry paint, and when applying the composition of Comparative Example 3, which did not contain antimicrobial agent.

In Figure 1 it can be seen that in the agar plates in which the plates coated with the polyurethane coating containing zinc pyrithione, B and C were applied, a large zone of inhibition around said plates was produced. This demonstrates the ability of said coating to inhibit the germination of spores of the H. resine fungus and, consequently, to inhibit its vegetative growth.

On the contrary, it can also be observed that in the plates corresponding to the coating that did not contain zinc pyrithione, A, a vegetative growth of the H. resine fungus occurred so that it covered practically the entire free surface of the agar.

B) Biofilm formation

The procedure used to evaluate the ability of the H. resine microorganism to form biofilms was as follows:

1) Inoculate 1-5 x 10 ⁵ spores / ml was prepared, following the procedure described in section A), in 20 ml of diluent (4.5 g of NaCI, 1.5 g of peptone in 1 L of distilled water). 2) The aluminum plates subjected to the biofilm formation test were sterilized with ethanol and washed with water to remove any remaining ethanol.

3) Each plate was incubated in the spore suspension prepared in section 1) for 24 hours at a temperature of 30 ° C.

 4) Half of the plates were washed twice with water to remove the non-adhered spores and were placed in a container with 20 ml of diluent and were exposed to mechanical agitation to release the spores. The diluent with the spores was analyzed by classical microbiological methods to quantitatively determine the microbial content thereof.

 5) The other half of the plates were washed twice with water and stained to be visualized using optical microscopy and scanning electron microscopy.

The biofilm formation test was subjected to aluminum sheets coated with an epoxy resin primer, as described in Example 3, which were coated with a top coat based on applying the composition described in Example 1 of the invention , with 3% by weight of zinc pyrithione on the weight of the dry paint, and when applying the composition of Comparative Example 3, which did not contain antimicrobial agent.

Table II shows the results obtained with the two types of aluminum sheets coated with polyurethane with zinc pyrithione and without said antimicrobial agent. The values obtained for duplicate samples are presented, as well as the logarithmic reduction, as a result of subtracting the logarithm of said concentration for the antimicrobial agent sample from the logarithm of said concentration for the control sample:

TABLE II

It can be seen, then, that the polyurethane coating comprising 3% zinc pyrithione by weight on the weight of the dry coating has the ability to reduce the anchoring of spores in it in a logarithmic unit compared to a polyurethane coating that does not include said antimicrobial agent.

The visualization of the samples by optical microscopy and scanning electron microscope led to the verification of the presence of spores in all the plates tested.

Claims

 1. Procedure for preventing surface biological corrosion by means of a polyurethane coating, characterized in that it comprises applying a composition on the surface capable of presenting biological corrosion, the composition comprising: a) a compound with at least two isocyanate groups , b) a polyol compound with at least two hydroxyl groups, and c) an effective amount of zinc pyrithione, to form an antimicrobial polyurethane coating. 2. Method according to claim 1, characterized in that the biological corrosion is caused by a microorganism selected from the group formed by the bacteria: Aerobacter aerogenes, Achromobacter, Bacillus mycoides, B. subtilis, Bacillus sp., Brevibacterium , Clostridium, Desulfovibrio desulfuricans, E. Coli, Flavobacterium, Legionella, Micrococcus sp., Pseudomonas aeruginosa, Ps. Fluorescens, Pseudomonas sp., Sarcina hansenii, Staphylococcus, Actinomyces, Cylindrogloea bacterifera, Nocardia sp., Sorangium sp., Sphaerotilus natans, Streptomyces; and by fungi and yeasts such as: Aspergillus amstelodami, A. flavus, A. flavipes, A. niger, Aspergillus sp., Alternaría sp., Cephalosporium sp., Hormoconis resine, Conidiobolus sp., Fusarium sp., Helminthosporium sp., Paecilomyces sp., Penicillium luteum, Penicillium sp., Phialophora sp., Pullularía sp., Spicaria sp., Stysanus sp., Stemphylium sp., Syncephalastrum, Trichoderma measures, Ustitago sp., Candida, Rhodotorula. 3. Method according to claim 2, wherein the microorganism is Hormoconis resine. A - Method according to any of the preceding claims, wherein the compound with at least two isocyanate groups is selected from the group consisting of 4,4'-dicyclohexylmethane diisocyanate (H12MDI), 1,4-cyclohexane diisocyanate (CDI), Isophorone diisocyanate (IPDI), 1,6-hexamethylene diisocyanate (HDI), 1,1,6,6-tetrahydroperfluorohexamethylene diisocyanate (TFDI), dimeryl diisocyanate (DDI), 4,4'-diphenylmethane diisocyanate (MDI ) and 2,6- and 2 diisocyanate, 4-toluene (TDI). 5. Process according to claim 4, wherein the compound with at least two isocyanate groups is 2,6- and 2,4-toluene diisocyanate (TDI). 6. - Method according to any of the preceding claims, wherein the polyol compound is selected from polyether with hydroxyl groups and polyester with hydroxyl groups. 7. - Process according to claim 6, wherein the polyol compound is a polyester with hydroxyl groups. 8. Process according to claim 7, wherein the polyol compound is a polyester with hydroxyl and saturated groups. 9. Method according to any of the preceding claims, wherein the effective amount of zinc pyrithione is between 0.5% and 10% by weight / weight on the solids content of the coating. 10. Method according to any of the preceding claims, characterized in that the composition further comprises other components selected from the group consisting of pigments, solvents, catalyst, rheological agents, dispersing-wetting agents, anti-skin agents, and mixtures thereof. . 1. Method according to any of the preceding claims, wherein the antimicrobial polyurethane coating is applied on a metal surface. 12. - Method according to any of the preceding claims, wherein the antimicrobial polyurethane coating is applied on a surface previously coated by an epoxy resin primer. 13. - Method according to claim 12, wherein the polyurethane coating is applied wet on wet on an epoxy resin primer. 14. - Antimicrobial coating obtainable according to the procedure defined according to any of claims 1 to 13. 15. An antimicrobial coating according to claim 14, wherein the coating is formed by a polyurethane matrix comprising zinc pyrithione as an antimicrobial compound dispersed therein. 16. - Equipment or accessory treated according to the procedure of any of claims 1 to 13. 17. Use of an antimicrobial coating according to any of claims 14 to 15, to prevent surface biological corrosion. 18. - Use according to claim 17, wherein the biological corrosion is caused by the fungus Hormoconis resine. 19. - Use according to any of claims 17 or 18, wherein the surface to prevent biological corrosion is a metal surface of an aircraft fuel tank or a water tank.