MXPA98007766A - Soldile coating, that inhibits the corrosion, resistant to deterioration, which contains iron powder, for substrates of me - Google Patents

Abstract

A heat-curable, heat-curable liquid coating composition is provided for the steel, which exhibits improved deterioration resistance, without damaging the weldability characteristics of the coating. For this purpose, the composition contains a conductive welding aid for iron powder. This weldable coating, when applied to steel and cured to a dry film, allows spot welding of coated steel, without requiring special welding equipment and techniques

Description

SOLDIER COATING, THAT INHIBITS CORROSION. RESISTANT TO DETERIORATION, WHICH CONTAINS IRON POWDER, FOR METAL SUBSTRATES Field of the Invon This invon relates to organic coatings, which inhibit corrosion, for metal substrates and, more particularly, to weldable organic coatings, which inhibit corrosion, which have improved resistance to deterioration, without sacrificing the ability to weld , and a method of using them to weld two pieces of metal together.

BACKGROUND OF THE INVON For many years, organic coatings that inhibit corrosion have been applied to rolls or metal sheets, before being formed into finished articles. The design with pre-painted metals provides the metal finisher with many benefits, such as the elimination of painting operations at home, reduction of associated environml responsibilities and improvemin the quality of the paint finish. One of the problems encountered in using the previously painted metal is that if such items are to be assembled, they must be joined together by adhesives or fasteners without welding, since the organic coatings are insulating in nature and are not weldable or soldered with difficulty and only using special techniques and equipm These techniques include the welding of points with high curr or longer welding times. However, such unorthodox methods are time consuming and costly. Also, excessive temperatures are usually generated in the welding areas, which, in turn, cause vaporization and expulsion of the metal out from between the welding electrodes. This results in lower welds as well as rapid deterioration of the electrode tips. Other techniques include decreasing the thickness of the protective film, which sacrifices corrosion protection for weldability. Recy, there has been an increasing demand for "weldable" organic coatings. The organic coatings, which are electroconductive and allow the welding of electrical resistance through the cured coating films, without resorting to special equipmand techniques, are said to be "weldable" or have capacity through welding. Various types of weldable corrosion-resistant liquid coatings have been proposed, which typically contain powdered conductive metals or alloys, to reduce the electrical resistivity of the coating film. The patof E. U. A., No. ,001,173 (Anderson et al.) Discloses some commercially popular weldable sizing materials, which contain high concations of powdered conductive ferro-alloys, such as ferro-phosphorus (a mixture containing di-iron phosphide and iron phosphide), and zinc dust. Zinc dust alone is not considered a good welding aid. Also, one of the problems encountered with weldable coatings rich in ferro-phosphorus, is its appearance. Ferro-phosphorus is a very dark gray pigm and when it is supplied in coatings in a high pigmto binder ratio, necessary to impart the desired weldability, it tends to produce very dark gray films, which are not conveniin certain applications . For example, the resistance to deterioration is almost nil and even marks with the fingernail are highly visible. In solution, the dark gray coating film tends to detract from the appearance of any top coat finish applied there. It is usually necessary for the top coating to form a thick film for proper concealmor concealmof the size, which, in turn, is very costly. Attempts have been made to clarify the weldable sizes to improve the deterioration resistance and the hiding ability by adding clear pigm of standard color, such as titanium dioxide, without much success. The standard pigm are insulators in restrictive form and the high pigmconcationd Z. , necessary to displace the dark shape, tends to damage the weldability. One solution to this problem has been to return to the use of standard, non-weldable, deterioration-resistant coatings. Even without welding aids in the formulations, the very thin films (ie no larger than about 25.4 microns in thickness) required for the weldability, are below the minimum thickness necessary to supply adequate opacity and corrosion resistance of the film. Another approach has been to use a weldable size of two coatings, as described in U.S. Patent No. 5,260,120 (Moyle et al.), In which a ferro-phosphorous size is coated on top with an extremely thin layer of a protective coating, rich in titanium dioxide, not weldable. The thin protective film provided does not significantly interfere with the weldability of the conductive sizing, and still supplies a light colored surface film, which has a greatly improved resistance to deterioration. The protective film also smoothes the abrasive state of the underlying ferro-phosphorous sizing. However, the use of such a two stage coating process consumes time and is expensive.

Another problem encountered with the ferro-phosphorus-rich sizes is its abrasive form, which arises with respect to excessive wear of the stamping die and forming, during the metal finishing operations. The abrasive texture, of sandpaper, of the film finish is due to the hardness of ferro-phosphorus. As mentioned before, the patent of Moyle et al. it provides a solution to the problem, but again it requires an inconvenient two stage coating procedure. A further problem with ferro-phosphorus-rich sizing is that, during welding, they produce toxic fumes, such as phosphine gas, along with objectionable odors, when subjected to the required welding temperatures. While the toxicity does not become environmentally damaging nor the levels of insecurity physiologically, workers have been known to complain of unpleasant odors produced during welding. It is difficult, of course, to reduce toxic effluents and eliminate unpleasant odors produced by ferro-phosphorus sizing, without sacrificing welding capacity. Yet another problem encountered with ferro-phosphorus-rich sizes is that the film finish has a very high coefficient of friction. During the finishing of the metal, the stamping and forming dies tend to escape out of the coating film. The protection of corrosion in these areas is thus lost. Likewise, paint scrapes tend to accumulate and eventually cause the finishing line to be suspended. Internal lubricants, such as polytetrafluoroethylene, have been incorporated into conductive coatings to decrease surface friction, which allows finishing operations without destroying the coating, as disclosed in US Patent No. 5,624,978 (Soltwedel et al. ). Weldable sizes also invariably shorten the life of welding electrodes. The copper-tipped electrodes in the welds of resistance points, are easily degraded by taking the coating during welding. The number of spot welds that can be made on the previously coated metal, before a corrective action is required, is drastically reduced compared to that on the uncoated metal. This results in reduced productivity of the need to change or coat the electrodes more frequently as well as the inconsistent quality of the weld. Weldable coatings that prolong the life of the electrode are continuously sought. Other types of weldable liquid coatings, containing metallic welding aids, in addition to the ferro-phosphorus or zinc powders, have all been disclosed, but all suffer from drawbacks. For example, U.S. Patent No. 5,047,451 (Barrett et al.) Discloses a weldable liquid anticorrosive sizing containing a nickel powder welding agent, a non-weldable corrosion inhibitor of aluminum or powder stainless steel, a polyethylene suspension agent to prevent the finely divided metal from separating, a thixotropic agent of silicon dioxide, treated with silane, a polytetrafluoroethylene extraction agent and a hygroscopic agent. However, the nickel powder is dark gray and thus inconvenient to improve the resistance to deterioration and the concealment of the top coating. Nickel powder is also an expensive and inexpensive material for use in weldable coatings. The prior US Patent No. 2,666,835 (Elleman) discloses a solderable, liquid, corrosion-resistant zinc chromate sizing that contains up to 30% by volume of the sizing solids of a magnetic metal powder, such as Non-oxidized forms of nickel powder, sweet iron powder, stainless steel powder, steel powder and nickel alloy powder. However, nickel powder is clearly preferred due to its inherent possession of magnetic remanence, which causes the metal particles to join together naturally and form conductive chains of the paint film. While coatings containing sweet iron powder are mentioned, Elleman suggests the need to chemically reduce the thin oxide layer, normally present in the iron powder, before incorporating it into the coating. This special procedure, by the inclusion of only sweet iron powder, substantially not oxidized, is time consuming and expensive. Elleman also resorts to other special techniques to generate the weldable coating. For example, Elleman suggests the need to expose the liquid coating to a magnetic field, before drying it on the metal, in order to uniformly align the metal particles and thus impart the necessary conductivity to the film. This adds a time consuming step to the welding process, which, in turn, leads to reduced productivity and increased costs. These sizes also require zinc chromate. While chromate pigments, which include zinc chromate, strontium chromate, calcium chromate and lead chromate, are excellent corrosion inhibitors, they are bright yellow insulating pigments and tend to produce darker coatings that have a reduced resistance to deterioration and greater concealment of the upper coating. A corrosion-inhibiting liquid coating is needed, which forms a dry electroconductive film on metal substrates, which has improved deterioration resistance, improved topcoat cover, reduced abrasivity, reduced friction, reduced toxic and odor emissions unpleasant, prolonged electrode life, without sacrificing welding ability and corrosion resistance, and that can weld together, in a cured state, two pieces of metal, such as steel, coated with it, without the need for equipment and special techniques.

SUMMARY OF THE INVENTION Therefore, it is an object of this invention to provide a solderable liquid coating for metal substrates, such as steel, that does not have the above drawbacks. It is another object of this invention to provide a weldable coating that has improved resistance to deterioration and concealment of the topcoat, without sacrificing weldability and corrosion protection. Yet another object of this invention is to provide a weldable coating having a relatively non-abrasive texture to prevent die wear. Still another object of this invention is to provide a weldable coating having a low coefficient of friction, to prevent destruction of the coating film during metal finishing.

And another object of this invention is to provide a weldable coating that emits low levels of toxic effluents and unpleasant odors during welding. A further object of this invention is to provide a weldable coating that can be welded without rapidly deteriorating the life of the welding electrodes. It is still another object of the present invention to provide a weldable coating having excellent corrosion resistance. It is a related object to provide a method of welding together metal articles having coated and cured on them a weldable coating of the previous character, without the need for special equipment or techniques. The above objects and others are achieved by providing a metal-weldable liquid coating composition, in which a conductive iron powder welding aid is incorporated into the liquid coating to impart weldability without substantially obscuring the coating, such that when the coating is applied and cured on a metal substrate, the coating film not only has an improved resistance to deterioration and good cover-up, but also allows the coated metal to be welded by electrical resistance without requiring equipment and special welding techniques. The iron powder particles found most useful are smooth and smooth irregular spheroids, produced by the atomization of water jets. No chemical reduction of the iron powder is required immediately, before incorporation into the liquid coating. In addition, the iron powders do not require magnetization and remain randomly oriented in the liquid coating. The liquid, weldable, preferred coating composition of this invention comprises a urethane coating, with epoxy, thermosetting, solvent-borne epoxy, which is characterized by a solvent mixture of: a) a functional hydroxy resin, which forms films, preferably a mixture of functional hydroxy polyester resins and bisphenol A epoxy resins; b) an interlayer for the resin, which affects a urethane cure, preferably a mixture of blocked isocyanate resins and aminoplast resins; c) a catalyst; d) an effective amount for welding of conductive iron powder particles of the aforementioned character, dispersed in the liquid, to impart the desired welding capacity to the coating film; e) an optional suspension aid, still preferred, to prevent the iron powder particles from separating; f) an optional internal lubricant, still preferred, comprising polytetrafluoroethylene, to decrease the coefficient of friction of the film; and g) smaller amounts of light-colored insulating pigments, in which the composition is further characterized in that it is free of ferro-phosphorus and other ferro-alloys and nickel-welding auxiliaries, and may also be free of non-weldable anticorrosive chromate pigments. This weldable coating not only has an improved deterioration resistance and good hiding ability, without sacrificing weldability and corrosion protection, but also exhibits reduced abrasivity, to prevent excessive wear of the die during finishing, a reduced coefficient of friction, to prevent the destruction of the film during finishing, little effluent emissions toxic and unpleasant odors during welding operations, sustains the life of copper-tipped welding electrodes and has the ability to weld together two pieces of coated metal using a welding cycle similar to that for uncoated steel. The above objects and others are also achieved by using a liquid coating, of the aforementioned character, to pair together two pieces of metal together. The liquid coating is applied to sheets or rolls of metal, which are cured with heat to form a hardened dry film. Two pieces of coated metal are then welded together, for example using a standard spot welding machine, without requiring equipment and special techniques.

The various objects, features and advantages of the invention will become more apparent from the following description and the appended claims.

Detailed Description of the Preferred Modes of the Invention Through this specification, all parts and percentages specified herein are by weight, unless stated otherwise. In this invention, a solderable coating composition, which inhibits corrosion, in liquid form, is applied to a metal substrate. The liquid coating becomes a solid dry film, which binds to the. metal substrate by heating at elevated temperatures. The heating evaporates the solvents in the liquid layer and initiates the curing of a film-forming resin to provide a weldable film of protective coating permanently adhered to the substrate.

RESIN OUE SHAPES THE FILM The solderable coating composition of this invention includes a film forming resin component. A wide variety of traditional film-forming resins can be employed in this invention, such as polyester, epoxy, urethane, acrylic and methacrylic resins.

These resins generally include a plurality of interlacing functional groups, to initiate cure in a dry film. The preferred component of resin is a functional hydroxy resin. Functional hydroxy resins provide building blocks to produce flexible urethane coatings, which are convenient in this invention. A suitable class of functional hydroxy resins, useful herein, include functional hydroxy polyester resins. These polyester resins can be prepared by any of the methods well known to those of ordinary skill in the art. For example, the condensation reactions can be carried out between one or more aliphatic or cycloaliphatic, di- or polyhydric alcohols, and one or more aliphatic, cycloaliphatic or aromatic, di- or polycarboxylic acids, or the corresponding anhydrides. Among the polyester resins which are useful here are linear polyesters derived from aromatic dicarboxylic acids and alkylene glycols. Examples of suitable aromatic dicarboxylic acids include terephthalic acid, bibenzoic acid, ethylene-bis-p-oxy-benzoic acid, tetramethylene-bis-p-oxy-benzoic acid, 2,6-naphthalic acid, orthophthalic acid and isophthalic acid. Mixtures of terephthalic acid and isophthalic acid are particularly useful. Examples of suitable ethylene glycols include ethylene glycol, trimethylene glycol, pentamethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, 1, 4-Cyclohexanedimethanol and polyethylene glycol. The linear polyesters can also be derived from mixtures of aromatic dicarboxylic acids and aliphatic dicarboxylic acids and alkylene glycols. Examples of suitable aliphatic dicarboxylic acids include maleic acid, fumaric acid, adipic acid, pimelic acid, suberic acid, sebacic acid, azelaic acid, oxy-dibutyric acid, 5-oxa-l, 10-decane-dioic acid, 4- n-propyl-suberic, dodecan-dioic acid and tridecan-dioic acid. The relative amounts of aromatic dicarbnoxylic acids and aliphatic dicarboxylic acids may vary in order to obtain polyesters having different characteristics. In general, the ratio of the aromatic acid to the aliphatic acid will be about 2: 1 to 1: 2 and more often about 1: 1 on an equivalent weight basis. The ratio of the dicarboxylic acid to the dihydric alcohol can also vary, however, with the diol being generally present in excess. The ratio of the dicarboxylic acid to the diol is generally from about 1: 1 to 1: 2 on an equivalent basis by weight. The reaction between the mixture of the dicarboxylic acid and the mixture of the dihydric alcohol is carried out in a conventional manner, typically by encouraging the mixture at an elevated temperature, in the presence of catalysts. Tin catalysts are especially useful, which include dibutyl tin oxide and dibutyl tin dilaurate. Antimony oxide can also be used as a catalyst. Functional hydroxy polyesters, prepared in this manner, will generally have molecular weights ranging from about 5,000 to 50,000, and in addition will have hydroxyl numbers between about 5 and 35. In a preferred embodiment, the polyester resin comprises between about 20 and about 60. % by weight of total solids and, more preferably, between about 35 to 45% by weight. The resin component forming the film of the solderable coating composition may also contain other resins that are capable of modifying the properties of the polyester-rich mixture, such as epoxy resins, which improve the adhesion and flexibility of the film. coating, through the incorporation of pending epoxy groups (pendants) in the urethane compound. Epoxy resins generally refer to the condensation reaction products of an epihalohydrin and a hydroxy-containing compound or a carboxylic acid. These epoxy resins may be of the ether or ester type, although ether-type epoxy resins are preferred. The ether-type epoxy resins are formed by the reaction of an epihalohydrin, such as epichlorohydrin, and a compound containing at least two free alcohol hydroxyl groups and / or phenolic hydroxyl per molecule. The condensation reaction is typically carried out under alkaline conditions or, alternatively, in the presence of an acid catalyst. The products of these reactions are generally complex mixtures of glycidyl polyethers. Ether-type epoxides can be derived from aliphatic alcohols, such as ethylene glycol, diethylene glycol and poly (oxyethylene) -glycosols, propan-1,2-diol and poly (oxypropylene) glycols, propan-1, 3- diol, poly (oxytetramethylene) -glycols, pentan-l, 5-diol, hexan-2,4,6-triol, glycerol, 1,1-trimethylolpropane, pentaerythritol, sorbitol; of cycloaliphatic alcohols, such as resorci-tol, quinitol, bis (4-hydroxycyclohexyl) methane and 2,2-bis (4-hydroxycyclohexyl) propane; and alcohols having an aromatic nucleus, such as n, n'-bis (2-hydroxyethyl) aniline Y p, '-bis (2-hydroxyethylamino) diphenylmethane. The esters can also be obtained from mononuclear phenols, such as resorcinol and hydroquinone; of polynuclear phenols, such as bis (4-hydroxyphenyl) methane (bisphenol F), 4,4'-dihydroxydiphenyl, bis (4-hydroxyphenyl) sulfone, 1,1,2,2-tetrakis (4-hydroxyphenyl) ethane, 2, 2-bis- (4-hydroxyphenyl) -propane (bisphenol A), 2,2-bis (3,5-dibromo-4-hydroxyphenyl) -propane; and of Novolak resins, formed of condensates of aldehydes, such as formaldehyde, acetaldehyde, chloral and furfuraldehyde, with phenol, chlorinated phenols, such as 4-chlorophenol, 2-methylphenol and 4-tert. -butylphenol. Particularly preferred epoxy resins, useful herein, are the diglycidyl ethers of bisphenol A, which are formed from the condensation reaction of epichlorohydrin with bisphenol A, in the presence of an alkaline catalyst. Bisphenol A type epoxy resins are commercially available from a wide variety of sources. Exemplary epoxy resins of the bisphenol A type include those sold under the tradename "Epon" by the Shell Oil Company. Other suitable epoxy resins include the diglycidyl ethers of other bisphenol compounds, such as bisphenol B, F, G and H. Another suitable class of epoxy resins, useful in the present invention, are epoxidized Novolaks resins, such as queens of epoxy-cresol and epoxy-phenol. Aliphatic or cycloaliphatic epoxy resins can also be used in the present invention. Epoxy resins, prepared in this manner, will generally have molecular weights ranging from about 300 to 100,000 and equivalent epoxide weights between about 150 and 10,000. In a preferred embodiment, the epoxy resin comprises between about 0.5 and 10% by weight of total solids and, more preferably, between about 1 and 2% by weight. The total amount of the film-forming resin in the weldable coating of this invention is between about 30 and 60% by weight of the total solids and, preferably, between about 40 and 50% by weight. It will be apparent to those skilled in the art that other suitable resins that form films can be employed in the coating composition of this invention, although the aforementioned resins are most preferred.

INTERLATING AGENT The curing agent or interlacing agent for the film forming resin component can be selected from a variety of curing agents traditionally known to be useful in curing such resins. As mentioned previously, a urethane curing system is preferred. Suitable curing agents for curing the urethane include blocked isocyanates and isocyanates, although blocked isocyanates are more preferred. Free isocyanates are generally not used in this invention, since the solderable coating composition is usually coated in circles on the metal substrate from a reservoir. Therefore, the coating must have a long jar life properly, so that it does not cure and harden prematurely in the tank. The blocked isocyanate resins are based on the reaction products of blocking agents of aliphatic, cycloaliphatic or aromatic di- and polyisocyanates, and isocyanates, which prolong the life in the coating jar. Standard methods can be used to prepare the blocked isocyanates, for example, biuretization, dimerization, trimerization, urethanization and uretidionization of the starting monomeric isocyanates. Examples of suitable aliphatic diisocyanates include the 1,4-tetramethylene diisocyanate and 1,6-hexamethylene diisocyanate. Examples of suitable cycloaliphatic diisocyanates include 1,4-cyclohexyl diisocyanate, isophorone diisocyanate and 4,4'-methylene-bis-cyclohexyl isocyanate. Examples of suitable aromatic diisocyanates include the 4,4 * -diphenylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate and 2,4-toluene diisocyanate. Examples of suitable polyisocyanates include 1,2-, 4-benzene triisocyanate, polymethylene polyphenyl isocyanate, and the like. The blocking agent is typically selected from those materials that react with the functional groups of the isocyanate, so as to form adducts stable at room temperature, but which can be dissociated at elevated tempratures. Examples of suitable blocking agents include lactams, such as caprolactam and butyrolactam, lower alcohols, such as methanol, ethanol and isobutyl alcohol, oximes, such as methyl ethyl ketoxime and cyclohexanone oxime, phenols, such as phenol, pt-butyl-phenol and cresol, and pyrazoles, such as 3, 5-dimethylpyrazole. In a preferred embodiment, the blocked isocyanate crosslinking agent comprises between about 0.4 to 10% by weight of total solids and, preferably, between about 2 to 5% by weight. In addition to the aforementioned interleavers, it is generally preferred to include other interleavers to provide the desired final properties of the urethane film, such as hardness, adhesion, flexibility and solvent resistance. A suitable class of interlayers is that of the aminoplast resins.

Aminoplast resins are based on the reaction products of formaldehyde with compounds bearing an amido group. A wide variety of aminoplast resins are useful in the practice of this invention. Examples of suitable inoplast resins include the condensation products of aldehydes, particularly formaldehyde, with melamine, urea, dicyanodiamide, benzoguanamine and glycouryl. Aminoplasts that are modified with lower alkanols, having about 1 to 4 carbon atoms, are preferred. The melamine-formaldehyde condensates of hexametoxymethyl-melamine and butylated melamine-formaldehyde are especially preferred. The aminoplast resins facilitate the hardening of the skeleton of the entangled urethane resin. Pheoplasts and carbamates can also be used. In a preferred embodiment, the aminoplast interlayer comprises between about 0.5 and 10% by weight of the total solids and, preferably, between 2 and 5% by weight. In order to achieve the remarkable properties that make these weldable coatings particularly useful, it is desirable that the amount of the interlayer be sufficient to react, substantially completely, with the functionalities present in the resin component forming the film.

The total amount of the interlayer in the weldable coating of this invention is usually between about 0.5 and 10% by weight of the total solids and, preferably, between about 2 and 5% by weight. Other suitable interleavers will be apparent to those skilled in the art.

CATALYST The coating composition of this invention may also include a curing catalyst or accelerator, to increase the rate of the crosslinking reaction, between the resin that forms the film and the interlayer. A large variety of catalysts traditionally used for curing urethane systems can be used. Examples of suitable catalysts include tertiary amines, such as triethylene diamine, organometallic salts, particularly organic tin compounds, such as dibutyl tin dilaurate, dilauryl dibutyl tin mercaptide, dibutyl tin maleate, dichloride of dimethyl tin, dibutyl tin di-2-ethylhexoate, dibutyl tin diacetate, stannic chloride, ferric chloride, potassium oleate and acid catalysts, such as phosphoric acid, alkyl or aryl acid phosphates, such as butyl acid phosphate or phenyl acid phosphate, and sulfonic acids, such as methanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, naphthalene sulphonic acid, dodecylbenzenesulphonic acid and dinonylnaphthalene sulphonic acid. Acid catalysts blocked with amines and pyridines are also useful in improving shelf stability. The catalyst is generally employed in an amount effective to initiate the crosslinking reaction to commercially acceptable regimes. In a preferred embodiment, the catalyst comprises between about 0.1 and 1% by weight of total solids and, more preferably, between about 0.3 and 0.5% by weight. For a further description of coating systems, which form urethane films, liquids, particularly useful, reference can be made to the patents of US Pat. Nos. 5,001,173, 5,260,120 and 5,624,978, the descriptions of which are hereby incorporated by reference in their entirety.

WELDING AGENTS The conductive ferrous metal powder, particularly the iron metal powder, in this invention is employed as the soldering or auxiliary welding agent. Iron powder is a very cheap material. Even more significantly, powdered iron offers very little color to the coating, which drastically improves the deterioration resistance and good hiding ability of the weldable coating film, without sacrificing weldability, film opacity and corrosion protection . Powdered iron also reduces the abrasion capacity of the finished coating, does not cause the coating to emit high levels of toxic effluents and unpleasant odors during welding, sustains the life of the welding electrodes and forming dies and converts the coating into a composition having welding characteristics similar to those of an uncoated steel. The adhesion, flexibility and convenient formability of the coating are also undamaged when using powdered iron. The preferred iron powder employed in this invention comprises finely divided iron particles having glossy, silvery, uncorrogated and smooth surfaces, and are in the form of irregular spheroids, which resemble ball bearings. Some irregular spheroids are traditionally produced by water jet atomization methods. It must be understood that the geometry of the iron powder varies significantly with its production method.

The atomization of water jet, in particular, involves the introduction of molten iron in stream, which is emptied from a ladle into a spray chamber, where the current is routed past one or more nozzles, which direct pressurized jets of water to collide against the molten metal stream emptied. The current is divided into multiple droplets, which cools and coagulates quickly, forming solid particles of iron dust that fall to the bottom of the atomizing chamber, while solidifying. The iron particles, thus formed, are then collected, preferably in water, and subsequently removed from most of the water for example by heating drying followed by magnetic separation. The particles are usually classified at this point to eliminate large unwanted particles, which can be reworked. The dried particles are then collected and passed through a tempering furnace at about 760 ° C, in a hydrogen-reducing atmosphere, and then the iron powder particles are finally collected in the form of irregular, smooth and glossy spheroids (i.e. sprayed) ) for a further description of the water jet atomization techniques, reference can be made to US Patents Nos. 3,764,295, 3,909,239 and 4,274,864, the disclosures of which are incorporated herein by reference in their entirety.

The preferred size of iron particles is less than about one mesh of 100 (150 microns) and, more preferably, less than one mesh of 325 (45 microns). Commercial powders containing about 85 to 95% of the iron particles smaller than the 325 mesh and the remainder between the 100 mesh and the 325 mesh, are more convenient. The bulk density of the iron powder is preferably between about 2.85 and 3.30 g / cc. Exemplary of such atomized iron powder is that sold under the trade name "Anchor AT -230", by Hoeganaes, of Riverton, NJ. Iron powders can also be produced by other traditional methods, which produce spheroids, such as the atomization of air, which gives irregular spheroids, or the dissociation of iron carbonyls, which gives more uniform ultrafine spheroids. Methods that produce spongy iron particles, such as direct reduction of iron ore, are generally not recommended in this invention, since it has been found that iron particles with a lustrous surface are far superior to fluffy iron particles. . Weldable coatings containing fluffy iron powder do not readily weld in areas under standard conditions. Also, a spongy, pumice-like surface tends to darken the iron powder and consequently reduce the resistance to deterioration of the coating films. In a preferred embodiment, the iron powder comprises up to 50% by weight of total solids and, more preferably, between about 30 to 40% by weight. The weldable coating composition of this invention is further characterized in that no chemical reduction of the natural oxide films on the surface of the iron powder is necessary prior to incorporation into the liquid coating. Also, the liquid coating is not subjected to a magnetic field after the incorporation of the powder iron and, therefore, the non-magnetized iron particles remain randomly oriented in the liquid. The weldable coating composition of this invention is still further characterized as being essentially free of dark gray welding aids, such as ferro-phosphorus and nickel powders.

SUSPENSION AGENT Conveniently, a suspending agent is used to ensure that the powdered iron remains stably suspended in the liquid coating and does not separate and settle and form a hard mass. A suitable suspending agent is polyamide wax. Exemplary suspending agents are those sold under the trade name "Disparlon 6900-20X" by King Industries, of Nor alk, CT, which are dispersions of swollen particles of polyamide wax in low boiling point alcoholic solvents, such as xylene. Other suitable suspending agents include silicon dioxide, for example, fumed silica, silica treated with silane, phosphoric acid, alkylated or arylated phosphoric acid, and magnesium aluminum silicate treated with quaternary amines. The suspending agents also serve as thixotropic agents to prevent gelation of the coating prior to application. The silicon dioxide additionally functions as a hygroscopic agent or water scavenger in the coating composition. In a preferred embodiment, the amount of the suspending agent present in the coating composition is between 0.3 and 2% by weight of the total solids, and preferably between 0.4 and 0.6% by weight.

INTERNAL LUBRICANT An internal lubricant can be incorporated into the coating composition to lower the coefficient of friction of the coating film. Polytetrafluoroethylene (PTFE) is the preferred internal lubricant due to its ability to drastically decrease the coefficient of friction of the film finish, thus allowing the formation and finishing of metal without destroying the coating film. Other halogen-containing thermoplastic polymers can also be used, although PTFE has superior lubricating properties. Mixtures of PTFE and polyethylene (PE) are also useful. Other suitable internal lubricants, such as glycerol esters, fatty acids, fatty acid esters, fatty acid amides, fatty acid salts, fatty alcohols and molybdenum disulfide, can also be used. Conveniently, the PTFE has particles in a size ranging from about 0.01 to 30 microns and, more preferably, from about 1 to 15 microns. Suitable PTFE is sold under the trade name "Polyfluo 190" by Micro Powders, of Scarsdale, NY. The weldable coating composition preferably contains internal lubricants, conveniently PTFE, in an amount ranging between about 0.2 and 1.5% by weight of the total solids and, more preferably, between about 0.5 and 1%. in weigh.

PIGMENT The weldable coating composition of this invention can also contain light colored insulating pigment powders, to further improve the deterioration, hiding and opacity resistance of the coating film, as well as to provide the desired final appearance, yet without sacrificing the ability to weld the coating. The selection of the pigment will depend on the particular color or colors desired in the coating. The pigments can be organic or inorganic, although inorganic pigments are generally used. Suitable inorganic pigments include metal oxides, especially titanium dioxide. Other metal oxides include zinc oxide, aluminum oxide, magnesium oxide, iron oxide, chromium oxide, lead oxide, nickel oxide, silver oxide, tin oxide and zirconium oxide. Other inorganic pigments which can be used include the inorganic sulphides, sulfoselenides, ferrocyanides, aluminates, phosphates, sulfates, borates, carbonates and especially titanates. The pigment may be present in the coating at reduced concentrations and still achieve the desired deterioration and concealment resistance. The ability to decrease the concentration of the non-weldable pigments drastically improves the weldability of the coating, especially in the formation of thick dry films desired for the desired coverage, opacity and corrosion protection. In this invention, the coating film resistant to deterioration is as thick as 25.4 microns thick coated on each side of the metal surface that remains weldable, without resorting to special equipment and techniques. In a preferred embodiment, the insulating pigment comprises no more than about 25% by weight of total solids and, more preferably, between 10 and 20% by weight. The ratio of the pigment to the binder is usually not greater than about 2 and, preferably, between about 1 and 1.5.

AGENT INHIBITING CORROSION The coating composition of this invention may also contain an agent that inhibits corrosion to increase corrosion protection of the underlying metal substrate. In this invention, an agent that inhibits corrosion is optional, since the powdered iron conductive welding aid also serves as a sacrificial anode and thus provides cathodic protection against galvanic corrosion of the metal substrate. Suitable agents that inhibit corrosion include finely divided metals, such as powdered zinc spheroids or flakes. Typically, zinc dust is prepared through the distillation of zinc dust or by atomization of molten zinc air. The zinc powder typically has a particle size ranging from about 1 to 15 microns, preferably about 2 to 6 microns. Zinc powder, in particular, improves the corrosion resistance of the coating, even without significantly obscuring the coating film. Other corrosion inhibitors may be employed which include anticorrosive chromate insulator pigments, such as strontium chromate. However, it is generally preferred that the weldable coating composition of this invention is further characterized as being essentially free of anti-corrosive pigments, which include strontium chromate, calcium chromate, zinc chromate and lead chromate, since these Pigments damage the deterioration resistance and hiding ability, as well as the weldability of the coating film. However, in certain circumstances, they may be convenient. The corrosion inhibiting agent, conveniently zinc powder, if employed, may comprise up to 10% by weight of total solids and, more preferably, between 3 and 10% by weight, although it is more preferred not to use the same .

OTHER ADDITIVES In addition to the components described above, the solderable coating composition of this invention may contain common functional additives, which are well known in the art, such as flow control agents, for example, polyacrylic resins. These flow control agents are usually present in amounts of up to 1% by weight of the total solids and, preferably, between 0.06 and 0.5% by weight. Polyacrylic resins generally include methyl (meth) acrylate resins, ethylene vinyl acetate resins, and the like. Other thixotropic agents, light stabilizers, moisturizing agents, dispersion aids, leveling agents, antioxidants, flocculation agents, foam control agents, etc., may also be employed. Inorganic fillers, such as calcium carbonate, can also be included in the coating. Adhesion promoters are usually also present in amounts of up to 0.5% by weight of the total solids and, preferably, between 0.01 and 0.1% by weight. A suitable class of adhesion promoters are the epoxy phosphate esters, which are generally prepared by the reaction of an epoxy resin with the phosphoric acid. This phosphoric acid can also be considered as an adhesion promoter.

SOLVENT The mentioned components of the coating composition are mixed together in a vehicle or carrier suitable for the solids, such as an aqueous or organic solvent, to facilitate the formulation and application of the liquid. Suitable organic solvents include aromatic and aliphatic petroleum distillates, such as Aromatic 100, Aromatic 150, Aromatic 200, dibasic esters (DBE), V M & P naphtha, hexane and the like, ketones, such as isophorone, ethyl ethyl ketone, methyl isobutyl ketone, methyl isoamyl ketone, diisobutyl ketone, acetone and the like, alcohols, such as ethyl alcohol, propyl alcohol, alcohol of diacetone, 2-ethyl-hexanol, n-butanol, and the like, dimethyl phthalate and mono- and dialkyl ethers of ethylene and diethylene glycol, such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monoethyl ether acetate, propylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether and diethylene glycol diethyl ether, xylene and the like. Dibasic esters are especially useful solvents, which are typically available as mixtures of refined dimethyl esters of adipic, glutaric and succinic acids. The solderable coating composition typically contains sufficient solvent to produce the desired viscosity for the particular liquid coating method. In a preferred embodiment, the viscosity of the uncured liquid coating preferably ranges between 20 and 50 Zahn at 25 ° C and, more preferably, between 28 and 32, approximately.

PREPARATION OF THE COATING The constituents of the weldable coating composition are mixed together in any convenient manner known to those skilled in the art. Also, no chemical reduction of the powdered iron is necessary before incorporation into the liquid coating. Likewise, the liquid coating is not subjected to a magnetic field, after the incorporation of powdered iron.

METAL SUBSTRATES The weldable coating composition is usually applied as a sizing on a variety of metal substrates. In some cases, this sizing can also serve as the final finish. Metal substrates of current interest include zinc, zinc / nickel alloy and zinc-iron alloy steels, which include various zinc containing forms of galvanized steel, steel having a chromium conversion coating (with or without zinc in it) on its galvanized or unglazed surface, steel that has a zinc-rich sizing on any such surface, and steel having a pretreatment coating, FIRST COAT®, of aqueous epoxy resin / chromium trioxide (as described in the aforementioned US Patent No. 6,001,173) which is commercially available from Morton International of Chicago, IL. baked on any of its surfaces. Other suitable metal substrates include cold-rolled and hot-rolled steel, aluminized steel and galvanized steel, such as hot-dip and electro-galvanized steel, galvalume, galvanneal, etc.

METAL COATING The liquid weldable coating can be applied to the metal substrate by any conventional technique, which includes, for example, dipping, spraying, roller coating and bar coating. The use of reverse roller coating techniques is preferred to apply this coating. After application, the weldable coating is heat cured and dried in a furnace to a cured and hardened film finish on the surface of the substrate. The wet coating is usually cured at elevated temperatures between 199 and 2602C peak metal temperature and, preferably between about 224 and 252 seconds, for an adequate period of time to fully cure the coating, usually between about 20 and 60 seconds and preferably, about 40 seconds. The weldable coating is generally applied on the substrate in sufficient quantities to provide a dry film coating having a thickness of up to 25.4 microns on each surface and usually between about 10.16 and 15.24 microns. It has been advantageously found that two pieces of metal can be coated on both sides with about 25.4 microns of a coating film and still be welded together using a welding cycle similar to that of uncoated steel.

WELDING COATED METAL PREVIOUSLY After curing the coating, coated metal substrates can be welded together by any standard welding technique, such as electric resistance (dot) welding, Mig welding, Tig welding and arc welding. Spot welding, in particular, involves placing together two pieces of pre-coated metal articles, to form an assembly and then inserting the assembly between two copper-tipped electrodes of a dot welding machine. When this welder is activated, an initial compression cycle is performed, in which the two coated steel plates are also forced together between the welding electrodes. A subsequent welding cycle is performed, where a sufficient current flows through the assembly, which includes the coating, and finally a retention and displacement cycle is completed before the welding electrodes are released and the welded assembly is removed from the machine . The formation of a palacas between the welded parts represents excellent welding. Any of the standard non-weldable top coatings can be applied to pre-coated metal surfaces, after welding, for decorative appearance or increased corrosion protection. The invention will be further clarified by considering the following non-limiting examples. EXAMPLE 1 Solvent Resistant, Deterioration Resistant Urethane The following ingredients were mixed together in the order and manner given, to provide a solderable solvent-carrying liquid sizing according to this invention.

% Polyester Solution, Dynapol L-205 - a solvent solution of 30% of the Dynapol L-205 polyester resin with an approximate molecular weight of 15,000 and a hydroxyl number of around 5-10, is believed to be derived of isophthalic acid, terephthalic acid, ethylene glycol and neopentyl glycol, and which is commercially available from Hüls of Somerset NJ, in DBE.

Ti-pure R-900, is a Ti? 2 pigment, which is commercially available from DuPont of Wilmington, DE.

AeroSil 200, is fumed silica, which is commercially available from Degussa of Ridgefield Park, NJ.

Mixed Dibasic Esters (DBEs) are a commercial mixture of dibasic esters that is commercially available from DuPont of Wilmington, DE.

Panasol AN-3N, is an aromatic solvent, which is commercially available from Ashland Chemical of Columbus, OH.

Coroc A-620-A2, an acrylic resin solution, which is an acrylic flow modifier, which is commercially available from Cook Paint & Varnish, from Kansas City, MO. of Polyester Solution, Mor-Ester 49001, is a solvent solution of 40% Mor-Ester 49001 polyester resin with an approximate molecular weight of 35,000 and a hydroxyl number of about 9, derived from terephthalic acid, isophthalic acid, azeic acid and etllen-glycol, and that is available from Morton International's Chicago, II, in MEK. 8 of Polyester Ester, Mor-Ester 4120, is a solvent solution of 55% of the polyester resin of Mor-Ester 4120 of about 13,000 molecular weight and about 20.28 hydroxyl number, derivative of soft acid , terephthalic acid, hexane-diol and neopentyl-glycol, and which is available from Morton International of Chicago IL, in xylene.

Epoxy Resin, Epon 8282, is an epoxy resin based on bisphenol A / epichlorohydrin with a molecular weight of about 350-450, and about 175-210 equivalent weight of epoxide, and which is commercially available from Shell Chemical Company , of Houston, TX.

Desmodur BL 3175, is a blocked isocyanate crosslinker resin of 1,6-hexamethylene diisocyanate blocked with methyl ethyl ketoxime, which is commercially available from Bayer, Pittsburgh, PA. 11 Resimene 747 is an aminoplast hexametoxymethyl melamine crosslinker resin, which is commercially available from Monsanto of St. Louis MO. 12 Nacure 1051 is a sulphonic acid catalyst of dinonylnaphthalene sulfonic acid (DNNSA), which is commercially available from King Industries of Norwalk, CT. 13 of Phosphoric Acid Solution, is a 10% solution of phosphoric acid catalyst in isophorone. 14 Metacrure T-12 is a dibutyl tin dilaurate catalyst, which is commercially available from Aire Products of Allentown, PA.

Anchor ATW-320 is an atomized iron powder, containing about 95% finer iron particles of a 325 mesh and the remainder between 100 and 325 mesh, which is commercially available from Hoeganaes of Riverton, NJ. 16 Disparlon 6900-20X is a suspension agent for a dispersion of swollen particles of polyamide wax in xylene, which is commercially available from King Industries of Norwalk, CT.

Two steel panels, cold rolled, were individually coated on each side with the above liquid solderable sizing and then baked at a peak metal temperature of about 232 seconds for about 45 seconds to provide a dry and cured white coating film. about 25.4 microns thick, on each side of the two panels. The weldability of the coating film deposited on the cold rolled steel panels was determined by trying to spot weld the two coated panels together. These coated panels were successfully spot welded together between 6.35 mm copper tipped electrodes, which use a welding cycle similar to that of uncoated steel. The characteristics of the corrosion resistance of the coating film deposited on the cold-rolled steel panels were determined by subjecting the coated panels to a saltwater spray test, according to the ASTM B-117 test method. Despite the absence of anticorrosive chromate pigments in the sizing composition, the corrosion resistance of the coating film was similar to that of the chrome sizing systems in 240 hours of salt spraying. The performance of the weldable sizing in 580 hours of the salt spray was significantly worse than expected, without the protection of chromates. Still, the improvement in deterioration resistance and weldability are consequences that can not be obtained with standard chrome sizing systems.

EXAMPLE 2 Sizing of Weldable Urethane, Internally Lubricated. Resistant to Deterioration The following ingredients were mixed together in the order and manner given to provide another solderable solvent-containing liquid sizing according to this invention.

DBE Mixed 1.36 Total Weight 100.00 1 The 11-3071 Fast Yellow HGR is a pigment C.l. Pigment Yellow 101 (inorganic titanate) which is commercially available from Hoechst Celanese of Charlotte, NC. 2 The Cab-O-Sil M5 is smoked silica that is commercially available from Cabot Corporation of Bellerica, MA. 3 Ei Polyfluo 190 is an internal PTFE lubricant, which is commercially available from Micro-Powders of Scarsdale, NY.

Two cold-rolled steel panels were individually coated on each side with the above liquid solder and then baked at a peak metal temperature of about 232 seconds for about 45 seconds to provide a yellow coating film of dry plaster. and cured about 25.4 microns thick, on each side of the two panels. The weldability of the coating film deposited on the cold rolled steel panels was determined by trying to spot weld the two coated panels together. These coated panels were successfully spot welded together between 6.35 mm copper tipped electrodes, which use a welding cycle similar to that of uncoated steel.

The invention has been described in the above embodiments and examples. Other embodiments of the invention will be apparent to those skilled in the art. The invention is not intended to be limited to the modalities and examples, which are considered exemplary only. Therefore, reference should be made to the appended claims to assess the true spirit and scope of the invention, in which exclusive rights are claimed.

Claims (24)

1. A liquid coating composition, weldable, resistant to deterioration, which comprises a solvent mixture of: a) a film-forming resin, having interlacing functional groups; b) an interleaver for this resin; and c) an effective amount for welding,. of a welding aid of finely divided iron metal particles, randomly dispersed in the liquid coating, this coating is essentially free of a ferro-alloy and nickel-welding aids and anticorrosive chromate pigments, and the coating is capable of forming a film of dry coating on a metal substrate, after curing, which can be welded.

2. The composition of claim 1, wherein: the iron particles are present up to 50% by weight of the total solids.

3. The composition of claim 1, wherein: the iron particles are substantially smooth irregular spheroids.

4. The composition of claim 1, wherein: the iron particles have a particle size below about a 100 mesh.

5. The composition of claim 1, wherein: a plurality of the iron particles have a size below about a 325 mesh, and the remainder is between a mesh of size 100 and about 325 size.

6. The composition of claim 1, which further comprises: d) a catalyst.

7. The composition of claim 1, which further comprises: e) a pigment.

8. The composition of claim 1, which further comprises: f) an internal lubricant, including polytetrafluoroethylene.

9. The composition of claim 1, which further comprises: g) a suspending agent, for stably suspending the iron particles in the liquid coating.

10. The composition of claim 1, wherein: the dry weldable coating film, formed after curing, is a urethane-containing compound, with a pendant epoxy, which is the reaction product of: a) a resin that film form, comprising a mixture of at least one functional hydroxy polyester resin and at least one epoxy resin of bisphenol A, and b) the crosslinker comprises a mixture of at least one blocked isocyanate and at least one aminoplast.

11. A liquid coating composition, weldable, resistant to deterioration, which comprises a mixture of solvents of: * a) a film-forming resin, which includes a mixture of one or more functional hydroxy polyester resins and one or more resins of epoxy; b) an interlayer for the resin, which performs the cure of the urethane; c) a catalyst; dj up to 50% by weight of total solids of iron powder particles randomly dispersed in the liquid coating, these iron powder particles include irregular spheroids having a particle size below 100 mesh; e) a pigment; f) a suspending agent for the iron powder, this coating is essentially free of ferroalloys and nickel welding auxiliaries and anticorrosive chromate pigments, and the coating is capable of forming a dry coating film on a metal substrate after the cure, which can be welded.

12. A liquid coating composition, weldable, resistant to deterioration, which comprises a solvent mixture of: a) a film-forming resin, having interlatable functional groups; b) an interlayer for the resin; * c) an internal lubricant, which includes polytetrafluoroethylene; and d) an effective amount for solar, of a welding auxiliary of iron metal particles, finely divided, randomly dispersed in the liquid coating, this coating is essentially free of ferroalloys and nickel welding aids and the coating is capable of forming a dry coating film on a metal substrate after curing, which can be welded.

13. The composition of claim 12, wherein: the coating is essentially free of anticorrosive chromate pigments.

14. The composition of claim 12, wherein: the iron particles are present in up to 50% by weight of the total solids.

15. The composition of claim 12, wherein: the iron particles are substantially smooth irregular spheroids.

16. The composition of claim 12, wherein: the iron particles have a size below about 100 mesh.

17. The composition of claim 12, wherein: a plurality of iron particles have a size below about one mesh of 325 and the remainder between about one mesh of 100 and 325.

18. The composition of claim 12, which further comprises: e) a catalyst.

19. The composition of claim 12, which further comprises: f) a pigment.

20. The composition of claim 12, which further comprises: a suspending agent, for stably suspending the iron particles in the liquid coating.

21. The composition of claim 12, wherein: the dry weldable coating film, formed after curing, is a urethane-containing compound with a pendant epoxy, which is the reaction product of: a) a resin that forms films, which includes a mixture of at least one functional hydroxy polyester resin, and at least one epoxy resin of bisphenol A, and b) the interlayer comprises a mixture of at least one blocked isocyanate and at least one aminoplast.

22. A liquid coating composition, weldable, internally lubricated and resistant to deterioration, which comprises a solvent mixture of: a) a film-forming resin, which includes a mixture of one or more functional hydroxy polyester resins and one or more epoxy resins; b) an interlayer for the resin, which performs the cure of the urethane; c) a catalyst; d) up to 50% by weight of total solids of iron powder particles randomly dispersed in the liquid coating, these iron powder particles include irregular spheroids having a particle size below 100 mesh; e) a pigment; f) an internal lubricant, comprising polytetrafluoroethylene; and g) a suspending agent for the iron powder, this coating is essentially free of ferro-alloys and nickel welding aids and the coating is capable of forming a dry coating film on a metal substrate after curing , which can be welded.

23. A welding method, which comprises: a) applying the solderable liquid coating composition according to claim 1 on a metal substrate; b) heat-cure the coating to a dry film; and c) welding the coated metal substrate to another similarly coated substrate or an uncoated metal substrate, using a welding cycle similar to that for the uncoated metal.

24. A welding method, which comprises: a) applying the solderable liquid coating composition according to claim 12 to a metal substrate; b) heat-cure the coating to a dry film; and c) welding the coated metal substrate to another similarly coated substrate or an uncoated metal substrate, using a welding cycle similar to that for the uncoated metal.