RESIN COATING ON STEEL STRUCTURES UNDER WATER

Abstract: An epoxy resin curable composition is prepared in two parts containing (a) an epoxy resin preferably of bisphenol A type and optionally up to 50 wt% of other compatible resin(s) or coal products, and (b) a curing agent for the epoxy resin which acts under water (e.g. an aromatic amine); both parts can contain filler (e.g. CaCO₃) and a flow adjusting agent. A first coating composition is made by mixing with part (a) and/or (b) a powder of average particle size 1-300 mµ of a metal (e.g. Zn or Al) of ionisation tendency larger than that of iron, or an alloy thereof, or a powder which is coated with such metal on a core of another metal or synthetic resin, in amount of such metal of 5-57 wt% of the total composition. The metal powder can be of flat, spherical or needle particles. A second composition is made by mixing (a) and/or (b), preferably of the same components as in the first compo­ sition, with a pigment, e.g. TiO₂ or a phthalocyamine; different pigments can be added to each part. Parts (a) and (b) are mixed and the first compo­ sition is coated on a steel structure (e.g. an oil rig) underwater or in a splash zone; the metal powder ensures excellent adhesion to the steel; the second composition is coated on the first coating before or after the epoxy resin has cured, to form a coloured top layer.

Description: The present invention relates to a method for coating steel structures in water. More particularly, it is concerned with a method for coating in water, whereby corrosion and staining of steel structures placed in water or under humid conditions can be effectively prevented. In recent years, with development of the ocean, oil drilling rig or steel structures such as oil storage purge and sea plant ships, piers of long bridges constructed in the sea, and steel structures in water of sea air ports have been increasingly constructed. These steel structures are substantially impossible to move to another place for their maintenance or corrosion repairing thereof, and such operations must be carried out under bad conditions such as in the sea or at a location where water is splashed thereon. For this reason it has been desired to develop a corrosion repairing method which can be easily carried out on or in the sea. Japanese Patent Application (OPI) No. 137034/79, for example, discloses a coating method which can form a coating having excellent corrosion resistance on portions in water or splash zones of underwater structures in a simple and easy manner as on the ground (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). Coventional compositions for use in such underwater coating include a composition comprising an epoxy resin as a base resin, polyamide or polyamine as a curing agent and a filler as described in Japanese Patent Application (OPI) No. 67400/76. However, these conventional compositions have disadvantages that underwater coating properties are poor and coatings are quite easily lost by wave action during hardening because of their poor adhesion force. Even if the coatings harden, the hardened coatings have poor adhesion properties and therefore corrosion protection over a long period of time cannot be expected. GB 986064 discloses a process for forming a corrosion-resistant coating on a metal structure under water, by applying a two-pack composition comprising an epoxy (polyepoxide) resin and an excess of a specified curing agent; fillers which may be added include sand, crushed shells, rocks, aluminium powder and iron particles. Only a single layer of this composition is coated. As a result of extensive investigations to overcome the above problems, it has been found that in underwater coating of steel structures, if a paint containing a specific metal powder is coated as the first layer and a paint not containing the metal powder is then coated thereon as the second layer, a coating operation can be carried out in the sea in a simple manner as on the ground and a coating having excellent anticorrosion properties can be formed. The present invention is based thereon. In particular, it is a novel discovery that the adhesion properties of an epoxy resin to iron in water are greatly improved by using a metal powder in combination in the first layer-forming paint. Accordingly, the present invention provides a method of coating a steel structure in the presence of water by coating the structure with a layer of a composition comprising an epoxy resin and a curing agent, by using an underwater-curable composition comprising 1) a two-pack resin blend system comprising a) a blend system comprising an epoxy-based resin, and b) a blend system comprising an underwater-curable curing agent capable of hardening the epoxy-based resin blend system, and 2) a powdered additive which is a metal having an ionization tendancy greater than that of iron, or an alloy such as metal, characterised in that said additive consitutes 5 to 75% by weight of the total weight of the underwater curable composition, and that the additive may also comprise composite particles coated with such metal or alloy; and also by the further step of coating an underwater-curable composition comprising said two-pack resin blend system and a coloring pigment on the first layer as a second layer, or after the first layer is cured. Each coating composition essentially comprises an epoxy resin and curing agent therefor, with either metal powder or pigment. The term "epoxy-based resin" as used herein means both an epoxy resin and a mixture of an epoxy resin and another resin compatible therewith. Of the two "blend systems" which constitute the two-pack epoxy resin composition in the present invention, one blend system comprises an epoxy resin or a mixture of epoxy resin and another resin (polymer) compatible therewith. Each part of this system can contain additives such as a filler such as calcium carbonate, silica, talc or baryta, and/or a flowability adjusting agent such as finely powdered silica or montmorillonite, depending on the purpose of use. These additives are compounded in an amount up to 1,000 parts by weight, preferably 1 to 500 parts by weight, per 100 parts by weight of the epoxy resin or the mixed resin. In particular, a blend system having a relatively high viscosity can be formed by increasing the amount of the filler added. The epoxy resin is preferably a bisphenol type epoxy resin. In addition, a cyclic aliphatic epoxy resin, a phenol or cresol novolak type epoxy resin, a glycidyl phthalate type epoxy resin, a β-methylepichlorohydrin type epoxy resin, a polyepoxy resin, a dimer acid type epoxy resin, a polyglycol type epoxy resin and the like can be used. These epoxy resins can be used alone or as mixtures comprising two or more thereof. The epoxy resin used is not particularly limited, but generally has epoxy equivalents of from 70 to 1,000, preferably from 100 to 700. Other polymers which are compatible with the above epoxy resins include thermosetting and thermoplastic polymers. Representative examples of the thermosetting polymers are a phenoxy resin, a phenol resin, a xylene resin, an acrylic resin and an unsaturated polyester resin. Examples of the thermoplastic polymers are a polyester resin, an ethylene-vinyl acetate copolymer, a thiocol resin, an ionomer resin, a modified butadieneacrylonitrile resin, a vinyl acetate resin, coal such as coal tar and asphalt pitch, and a petroleum residue resin. One or more of those other resins can be used in combination with the above epoxy resin. Such other resins compatible with the epoxy resin can be used by replacing up to 50% by weight, preferably up to 30% by weight, of the epoxy resin. The other blend system in the two-pack epoxy resin composition used in the present invention comprises a curing agent containing an underwater-curable curing agent capable of curing the epoxy resin even in water. In addition, additives such as a filler and a flowability adjusting agent as described above are compounded thereto depending on the purpose of use. These additives are compounded in an amount up to 1,000 parts by weight, preferably from 1 to 500 parts by weight, per 100 parts by weight of the curing agent. A composition having a relatively high viscosity can be formed by increasing the amount of the filler added. Curing agents for the epoxy resin which are sparingly soluble in water are replaceable with a water molecule, and active hydrogen can be used as the underwater-curable curing agent. Examples of the curing agent are amine-based curing agent such as aromatic amines, polyalkylenepolyamines, cyclic aliphatic polyamines, polyamideamines, modified polyamines, and ketemines; polymercaptans; and the like. Those are used alone or in combination with each other. In combination with the underwater-curable curing agent, cold-curable curing agents which are conventionally used in the atmosphere can be used. Examples of such cold-curable curing agents are aliphatic polyamines, polyamideamines, amine-containing adducts and separated adducts. The cold-curable curing agent is used by replacing up to 40% by weight, preferably up to 30% by weight, of the underwater-curable curing agent. A metal powder which is contained in the composition for the formation of the first layer and having the ionization tendency larger than that of iron includes a zinc powder, an aluminum powder, a magnesium powder, a chromium powder, a zirconium powder, etc., powders of alloys of the above metals, and composite powders resulting from plating or vacuum deposition of the above metals. These metal powders can be used alone or in combination with each other. The composite powder is a powder wherein the surface of particles is composed of the above described metals. For example, the composite powder is obtained by coating such metal on the surface of metal powder (including iron) having ionization tendency smaller than that of iron or the surface of synthetic resin powder by means of, for example, deposition. Use of such metal powders in combination greatly increases the adhesion of the epoxy-based resin to iron in water. Of those powders, a zinc powder and an aluminum powder are particularly preferred. The metal powder of the present invention preferably has the average particle size of from 1 to 300 µm. If the average particle size is less than 1 µm, the effect of increasing underwater adhesion properties is insufficient. On the other hand, if the average particle size is more than 300 µm, a coating having a poor appearance is undesirably obtained. The shape of the particles may be any of shapes such as flat, spherical and needle-like and the effect of increasing underwater adhesion properties can be obtained regardless of the particle shape. The metal powder must be added in an amount of from 5 to 75 wt% based on the total weight of the two-pack resin blend system (including additives) and the metal powder. If the amount of the metal powder added is less than 5 wt%, the effect of increasing underwater adhesion properties cannot be obtained. On the other hand, if the amount of the metal powder added is more than 75 wt%, the cohesive force of the resulting composition and the adhesion force of the composition to a steel material are undesirably decreased. The mixing proportion of the blend system comprising the epoxy resin and the blend system comprising the underwater-curable curing agent is generally such that the active hydrogen equivalent in the underwater curing agent component is from 0.2 to 2.0, preferably from 0.5 to 1.5, per equivalent of the epoxy group in the epoxy resin. If the proportion is too small, the curing rate becomes slow, and if the proportion is too large, the properties of the cured product deteriorate. The metal powder can be previously incorporated in either or both of the blend system comprising the epoxy resin and the blend system comprising the underwater-curable curing agent for the epoxy resin, or can be added at the time of mixing the blend system comprising the epoxy resin and the blend system comprising the underwater-curable curing agent for the epoxy resin. The adhesion force to iron is an important factor for the first layer coating as an undercoating layer. For this reason, the metal powder-containing composition is essential, while on the other hand, coloring is limited due to the presence of the metal powder. Accordingly, the underwater-curable composition comprising the two-pack resin blend system and the coloring pigment is used as the second layer (overcoating layer). The underwater-curable composition does not substantially contain the metal powder. On the other hand, the coloring pigment can be added to the metal powder-containing composition used for the first layer coating, if desired. Further, the epoxy-based resin and the underwater-curable curing agent which constitute the metal powder-containing composition for the first layer coating can be the same as or different from those in the composition for the second layer coating. All coloring pigments which are used in the conventional paints can be used as the coloring pigment for the preparation of the second layer coating composition. Examples thereof include oxide type coloring pigments such as chromium oxide, iron oxide and titanium dioxide, pigments such as carbon black and graphite, and organic coloring pigments such as phthalocyanine blue and phthalocyanine green. It is particularly preferred to add sparingly water-soluble pigments. The coloring pigment can be added to any one or both of the blend system comprising the epoxy resin and the blend system comprising the underwater-curable curing agent. Where the coloring pigment is added to both the blend systems, pigments having different hues can be added to the respective blend system, so that a measure of extent of stirring when the two blend systems are mixed and stirred can be obtained. The two-pack epoxy resin composition is constituted by the combination of the blend system comprising the epoxy resin (hereinafter referred to as a "main component") and the blend system comprising the underwater-curable curing agent (hereinafter referred to as a "curing agent"). The curing reaction proceeds by mixing the main component and the curing agent, resulting in the formation of an epoxy resin-based cured product. In connection with the properties of the two-pack epoxy resin composition, the main component and the curing agent are prepared in the form of a high viscosity composition (paste-like) or a low viscosity composition (paint-like; viscosity: about 3,000 ps or less) so that the composition can be mixed and coated by the wet hand method. Depending on the properties, the composition can be coated using, for example, a brush, a trowel, a spatula or a roller. Although the coating thickness varies depending on the viscosity and properties, coating is usually carried out such that the total thickness of the two coatings is from 100 µm to 10 mm.As to the thickness of each layer, the first and second layer coating each generally has a thickness of from 50 µm to 8 mm. In the practice of coating, rust, sea animals and the like on the steel surface in water are first removed by techniques such as a disc sander, a water jet and a sand water jet, the underwater-curable composition containing the metal powder is coated on the iron surface as the first layer, and the underwater-curable composition containing the coloring pigment is then coated thereon as the second layer. The second layer can be coated either after the first layer is cured or before the first layer is cured, i.e., the first layer is uncured. In accordance with the present invention in which an epoxy resin, a curing agent to cure the epoxy resin, and a metal powder having an ionization tendency greater than that of iron are used, the excellent coating properties and curing properties inherent in the epoxy resin can be maintained and, at the same time, due to the mutual action with the metal powder, underwater coating properties are excellent and underwater adhesion properties are greatly increased. Therefore, the composition applied is not lost by wave action or the like and is cured as such, and the cured coating strongly adheres to an underwater steel structure because of its great adhesion force. Furthermore, since the cured coating possesses inherent excellent characteristics of the epoxy resin, the coating exhibits a quite excellent anticorrosion effect in combination with the above characteristics, and effectively protects the underwater structure. Furthermore, when the main component of the overcoating composition for the second layer is the same as or the similar type to that of the undercoating composition for the first layer, not only overcoating is extremely easy but also the adhesion properties between the layers after curing are excellent. In addition, the multilayer coating provides the advantage that coating defects such as pinholes are markedly decreased. Further, in the second layer of the overcoating according to the present invention, glass flakes together with a coloring pigment can be added to the two-pack resin blend system. The glass flakes are added to increase the anticorrosion effect. The glass flakes comprises as a main component, ones which pass a 32 mesh (500 µm) sieve and do not pass 350 mesh (45 µm) sieve (the sieves are defined according to JIS Z8801). The "main component" means at least 50% by weight, preferably at least 60% by weight and more preferably at least 70% by weight, based on the total weight of the glass flake. Further, the glass flakes generally used in the present invention must all pass the 16 mesh (1,1000 µm) sieve (as defined above). The thickness of glass flake is preferably 0.5 to 10 µm. The amount of glass flake added is 5 to 30% by weight, preferably 10 to 25% by weight, per the total weight of the composition. By using the glass flake within the above described specific range, a further excellent anticorrosion effect can be obtained. Although the method of the present invention is described as a coating method in water, it can also be applied to wet surfaces such as splash zones and high and low tide zones of structures. Moreover, the method of the present invention can be applied to structures on the ground under wet conditions. The present invention is described in greater detail by reference to the following Examples and Comparative Examples. Unless otherwise indicated, all percents, parts, ratios and the like are by weight. EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 AND 2 90 Parts of a bisphenol A type epoxy resin having an epoxy equivalent of 185 to 192, 10 parts of phenyl glycidyl ether as a reactive diluent, 40 parts of calcium carbonate, and 2 parts of superfinely divided anhydrous silica (specific surface area: 200 m²/g) as a tag-preventing agent were mixed and stirred at 50°C to prepare an epoxy resin blend system. On the other hand, 50 parts of modified aromatic polyamine having an active hydrogen equivalent of 95, 15 parts of calcium carbonate and 1 part of superfinely divided anhydrous silica (specific surface area: 200 m²/g) as a tag-preventing agent were mixed at 50°C in a mixing vessel to prepare a curing agent blend. 142 g of the epoxy resin blend system and 66 g of the curing agent blend were weighed, and a zinc powder (average particle diameter: 50 µm) was added in the proportions shown in Table 1 below and mixed to prepare undercoating compositions 1, 2, 3, 4 and 5 used for the formation of the first layer. The overcoating composition used for the formation of the second layer was prepared by adding titanium oxide as a coloring pigment to the epoxy resin blend system and phthalocyanine blue B as a coloring pigment to the curing agent blend system and then mixing the respective compositions in a mixing vessel at 50°C in the same manner as in the preparation of the undercoating composition. Each mixing proportion (parts) is shown in Table 2. An SS 41 steel plate (9 x 100 x 100 mm) which had been ground with a disc sander was dipped in sea water for 4 hours as a material to be coated. The compositions 1, 2, 3, 4 and 5 shown in Table 1 each was coated on the plate in the sea with a spatula. After 30 hours, the overcoating composition was coated with a roller as the second layer. The thickness of the first layer was about 200 µm and the thickness of the second layer was about 200 µm, and the total thickness was about 400 µm. Coating workability at the time of coating, the appearance of the coated surface after coating, and the adhesion force of the coating after curing are shown in Table 3. A coated sample which had been dipped in sea water for 1 month was taken out in the air and dried over 1 day and night. This coated sample was measured for adhesion force using an adhesion tester. EXAMPLE 4 Example 1 was repeated except that an aluminum powder (average particle diameter: 15 µm) was used in place of the zinc powder. The results obtained are shown in Table 4 below. EXAMPLE 5 Example 2 was repeated except that a zirconium powder (average particle diameter: 20 µm) was used in place of the zinc powder in the undercoating composition. The results obtained are shown in Table 4 below. EXAMPLE 6 Example 3 was repeated except that a chromium powder (average particle diameter: 60 µm) was used in place of the zinc powder in the undercoating composition. The results obtained are shown in Table 4 below. EXAMPLE 7 Example 1 was repeated except that composite particles prepared by coating the surface of glass beads (average particle diameter: 60 µm) with aluminum in an average thickness of 1 µm was used in place of the zinc powder in the undercoating composition. The results obtained are shown in Table 4 below. COMPARATIVE EXAMPLE 3 Example 1 was repeated except that 120 parts of titanium white was used in place of the zinc powder in the undercoating composition. The results obtained are shown in Table 4 below. EXAMPLE 8 Example 2 was repeated except that 30 parts of a cyclic aliphatic polyamine (isophoronediamine-modified product) having an active hydrogen equivalent of 110 was used in place of the modified aromatic polyamine in the undercoating composition. The results obtained are shown in Table 5 below. EXAMPLE 9 Example 2 was repeated except that 100 parts of a cyclic aliphatic polyamine (isophoronediamine-modified product) having an active hydrogen equivalent of 110 was used in place of the modified aromatic polyamine in the undercoating composition. The results obtained are shown in Table 5 below. EXAMPLE 10 Example 2 was repeated except that a bisphenol F-type epoxy resin (epoxy equivalent: 175) was used in place of the bisphenol A-type epoxy resin in the undercoating composition. The results obtained are shown in Table 5 below. EXAMPLE 11 Example 2 was repeated except that a glycidyl phthalate-type epoxy resin (epoxy equivalent: 140) was used in place of the bisphenol A-type epoxy resin in the undercoating composition. The results obtained are shown in Table 5 below. EXAMPLE 12 Example 1 was repeated except that the thickness of the first layer was 100 µm, the thickness of the second layer was 100 µm, and the total thickness thereof was 200 µm. The results obtained are shown in Table 5 below. EXAMPLE 13 Example 3 was repeated except that the thickness of the first layer was 5 mm, the thickness of the second layer was 3mm, and the total thickness thereof was 8 mm, and that the second layer coating was conducted using a spatula. The results obtained are shown in Table 5 below. EXAMPLE 14 Example 2 was repeated except that calcium carbonate was not used in the undercoating composition and the overcoating composition. The results obtained are shown in Table 5 below. EXAMPLE 15 Example 1 was repeated except that barium sulfonate was used in place of calcium carbonate in the undercoating composition, 200 parts of barium sulfonate was added to the epoxy resin blend system, 100 parts of barium sulfonate was added to the curing agent blend system, and the amount of the zinc powder was 40 parts. The results obtained are shown in Table 5 below. EXAMPLE 16 Example 2 was repeated except that diethylene glycol diacrylate (acrylic resin) was used in place of phenylglycidyl ether (reactive diluent) in the undercoating composition. The results obtained are shown in Table 5 below. EXAMPLE 17 Example 2 was repeated except that 10 parts of coal tar was added to the undercoating composition. The results obtained are shown in Table 5 below. EXAMPLE 18 Example 2 was repeated except that 10 parts of bisphenol A-type epoxy resin (epoxy equivalent: 500) was added to the undercoating composition. The results obtained are shown in Table 5 below. EXAMPLE 19 Example 1 was repeated except that the total amount of calcium carbonate used in the overcoating composition was replaced by the same amount (weight) of a glass flake (thickness: 3 µm, all 16 mesh pass, the proportion of 32 mesh pass and 350 mesh not pass: 75 wt%). The adhesion forces after 1 month and 12 months after curing in water were measured. The results obtained are shown in Table 6 below. EXAMPLE 20 Example 1 was repeated except that the total amount of calcium carbonate used in the curing agent blend system of the overcoating composition was replaced by the same glass flake as used in Example 19. The results obtained are shonw in Table 6 below. EXAMPLE 21 Example 2 was repeated except that the total amount of calcium carbonate used in the overcoating composition was replaced by the same glass flake as used in Example 19. The results obtained are shown in Table 6 below. EXAMPLE 22 Example 3 was repeated except that the total amount of calcium carbonate used in the overcoating composition was replaced by the same glass flake as used in Example 19. The results obtained are shown in Table 6 below.

Claims: A method of coating a steel structure in the presence of water by coating the structure with a layer of a composition comprising an epoxy resin and a curing agent, by using an underwater-curable composition comprising 1) a two-pack resin blend system comprising a) a blend system comprising an epoxy-based resin, and b) a blend system comprising an underwater-curable curing agent capable of hardening the epoxy-based resin blend system, and 2) a powdered additive which is a metal having an ionization tendancy greater than that of iron, or an alloy of such metal, characterised in that said additive consitutes 5 to 75% by weight of the total weight of the underwater curable composition, and that the additive may also comprise composite particles coated with such metal or alloy; and also by the further step of coating an underwater-curable composition comprising said two-pack resin blend system and a coloring pigment on the first layer as a second layer, or after the first layer is cured. A method as claimed in Claim 1, wherein the epoxy resin is admixed with up to 50% by weight of other polymers. A method as claimed in Claim 2, wherein said other polymers comprise a phenoxy resin,phenol resin, xylene resin, acrylic resin, unsaturated or saturated polyester resin, copolymer of ethylene and vinyl acetate, thiocol resin, ionomer resin, modified butadiene, acrylonitrile resin or vinyl acetate resin. A method as claimed in Claim 1, wherein the epoxy resin is mixed with up to 50% by weight of coal tar, asphalt pitch or a petroleum residue resin. A method as claimed in any preceding claim, wherein the epoxy resin is in admixture with a filler and/or an agent for adjusting its viscosity. A method as claimed in any preceding claim, wherein the epoxy resin is a bisphenol type epoxy resin. A method as claimed in any preceding claim, wherein the curing agent is an aromatic amine, polyalkylenepolyamine,, cyclic aliphatic polyamine,, polyamideamine,, polyamine,, polyamideamine,, modified polyamine,, ketemine or polymercaptan. A method as claimed in any preceding claim, wherein the metal powder is of zinc, aluminum, magnesium, chromium, zirconium or an alloy of any of these metals. A method as claimed in any preceding claim, wherein the metal powder has an average particle size of 1 to 300 millimicrons. A method as claimed in any preceding claim, wherein the epoxy resin and curing agent are used in a ratio such that the active hydrogen equivalent in the curing agent is 0.2 to 2.0 per epoxy group equivalent.

Inventors: ASOSHINA, EISHI C/O NITTO ELEC. IND. CO., LTD., DOI, KAZUYUKI C/O NIPPON STEEL CORPORATION, HAMABUCHI, KAZUFUMI C/O NITTO ELEC. IND. CO., LTD., HITOSHI, SHUNICHI C/O NIPPON STEEL CORPORATION, SHIMIZU, MASATO C/O NITTO ELEC. IND. CO., LTD., YOSHIDA, KOTARO C/O NIPPON STEEL CORPORATION
Applicants: NITTO DENKO CORPORATION
Application year: 1986
Main classification: B05D5
IPC codes: E02D31, B05D7, B05D1, E02D19, E01D19, E02D29, C08G59, E01D101, B05D5, C08K3