PROCESS FOR THE CONTINUOUS COATING OF A BAND-SHAPED SUBSTRATE WITH A THERMOSETTING PAINT COMPOSITION

Abstract: The invention relates to a process for the continuous coating of a band-shaped substrate with a thermosetting paint composition comprising, first forming a film from a thermosetting paint in viscous form, applying said film to the band-shaped substrate, and subsequently curing the paint film. The paint composition comprises a binder composition consisting of a hydroxyl-functional polyester resin and a crosslinker having blocked isocyanate groups.

Description: PROCESS FOR THE CONTINUOUS COATING OF A BAND-SHAPED SUBSTRATE WITH A THERMOSETTING PAINT COMPOSITION The present invention relates to a process for the continuous coating of a band-shaped substrate with a thermosetting paint composition comprising, first forming a film from a thermosetting paint in viscous form, applying said film to the band-shaped substrate, and subsequently curing the paint film. An example of such a process is disclosed in EP- A-369,477 which describes a process in which a band-shaped metal substrate is unwound from a coil, prepared and dried. A fluid powder paint composition is directly applied by an extruder with a film applicator at a temperature below the reaction temperature of the crosslinker, after which the paint composition is applied to the coil. The temperature is then increased to above the reaction temperature of the crosslinker and the paint is distributed and cured after which the substrate is cooled and rewound. The reaction between epoxy and acid functional groups is the most important curing reaction used in practice for crosslinking thermosetting powder paint compositions. The so-called hybrid powder coatings in which carboxyl functional polyesters are cured with bisphenol-A-epoxy resins and polyester powder coatings where trisglycidylisocyanurate is used as curing agent are the best representatives of this group of products. However, if these binder compositions are applied in the process according to EP-A-369,477, the film applicator soon becomes completely clogged with prematurely cured paint. Consequently, it is a drawback of the process according to EP-A-369,477 (the complete disclosure of which is incorporated herein by reference) that the equipment becomes fouled very soon as a result of premature curing of a preferred prior art binder composition consisting of a carboxyl functional polyester and TGIC. Furthermore, EP-A-369,477 does not describe a paint composition suitable for using with such a process. The object of the present invention is to provide an improvement of a process for the continuous coating of a band-shaped substrate with a thermosetting paint composition which overcomes the stated problem. The invention is characterized in that the paint composition comprises a binder composition consisting of a hydroxyl-functional polyester resin and a crosslinker having blocked isocyanate groups. The specific choice of this binder composition of a countless number of known binder compositions for use in powder paint formulations, results in a process which makes it possible to use powder paints in applications on tape or shaped parts by coil coating. Misev, "Powder Coa¬ tings, Chemistry and Technology", pp. 42-173 (John Wiley 1991; the disclosure of which is incorporated herein by reference), elucidates that there exist a countless amount of binder compositions. A binder composition is defined as a composition comprising the combination of resin and crosslinker. A paint composition is defined as a mixture comprising a binder composition and various additives such as for example pigment, catalyst and flow agent. A coating results after the curing of the paint formulation. The powder paint formulation is preferably fed in particulate form to a hot melt compounding unit, where it is converted, above the glass transition temperature of the binder system, into viscous form before being applied. In the context of this invention, particulate form means that the composition consists of chunks, chips, powder, lumps or any other kinds of processible loose solids. Thus, it is not necessary for the paint to be processed in the form of very fine particles, as long as the particles are in processible form. The hot melt compounding unit can, for example, be a single or twin screw extruder or a melting pot. Preferably, a hot melt compounding unit is employed in which heating as well as a certain degree of motion can occur, optionally with a build-up of pressure. Preferably, an extruder is employed. If a twin screw extruder is used, it is possible to allow thorough mixing to occur in the extruder. This makes it feasible to feed a dry blend paint formulation comprising substantially unmixed raw materials to the extruder, which yields substantial upstream savings in paint preparation. The blend is mixed, melted and compounded in the extruder to obtain a paint. A twin screw extruder also enables the practitioner to introduce other additives and pigments, into the extruder simultaneously with the aforementioned constituent elements, to obtain a good homogeneous blend. If a single screw extruder is used, the binder composition is preferably added in already compounded form, or in a form in which the constituent parts are at least partly mixed. The binder composition becomes viscous in the hot melt compounding unit and it is thereafter converted into film form. The latter is preferably carried out by pressing the viscous paint composition through a slit- shaped aperture (die). An example of a die with a suitable aperture for this purpose is a flat film die, which is well-known to those skilled in the extrusion arts. An example of a flat film die is a so-called coathanger die, which provides for a uniform outflow rate across the entire width. A die having an aperture width of 0.5-1 mm is preferred. The film extruded through such a die is relatively homogeneous. On application the film will generally have a thickness corresponding with the desired thickness of the coating, for example 20 μm - 100 μm. In the process according to the present invention the film is preferably applied pressureless, i.e. with a gap between the outflow aperture of the extruder and the surface of the band-shaped material: the so-called R-gap. This gap is preferably between 0.1 and 2 cm, and more preferably is narrower than 1 cm. If the gap is too wide, there will be excessive necking of the film between the outflow aperture and the band-shaped material. The aperture width is set such that necking does not exceed the desired width of the coating. The outflow aperture will therefore generally be wider than the width of the material to be coated. In general, the outflow velocity of the film from the outflow aperture will be set relative to the displacement velocity of the band-shaped material such that the film is stretched by a factor of 5 to 20. Preferably the film is stretched by a factor of 8 to 12. The film is stretched so that the film is applied in a thickness of approximately 25 to 75 μm. Surprisingly, the film is found to be sufficiently elastic for such stretching. In general, the film will readily adhere to most kinds of band-shaped material. To promote adhesion and avoid air-filled voids, the applied film is preferably pressed onto the band-shaped substrate. Pressing the applied film can be accomplished using various means, such as a pressure roller, for example a polishing roll, or with compressed air, for example an air knife. The pressure roller is preferably cooled and has a polished, chrome-plated or teflon-coated surface. The roll is pressed onto the band-shaped material as close as possible to the outflow aperture (and should therefore preferably not be too large) but at the same time not too close to the outflow aperture, because the roll will then be in contact with the film over too great an area, with the risk of excessive cooling of the latter. After being coated, the band-shaped material is re-wound. The winding speed of the band-shaped material and the paint composition extrusion speed must be adjusted to one another. In a practical embodiment the winding speed of the material will generally lie between about 30 and about 150 /min, and preferably between about 60 and about 100 m/min, with the aim generally being to maximize the speed. The band-shaped material can have a thickness of up to about 2.5 mm or more and this thickness is usually between about 0.2 and about 0.8 mm. The width of the material generally ranges between 0.05 and about 2.5 m. The band-shaped material is heated in a pre¬ heating oven to a temperature To between about 50°C and about 200°C, and preferably to a temperature between 100°C and about 170°C. More preferably, To is about 120°C to about 150°C. The film can, in principle, be applied in any desired thickness, but is preferably applied in a thickness greater than 20 μm, and more preferably in a thickness of about 50 to about 100 μm, and most preferably in a thickness of about 60 to about 100 μm. The paint formulation is extruded at a temperature T x at which the paint cannot yet cure. In general, this is a temperature higher than 90°C, because otherwise the viscosity of the paint is too high to permit successful extrusion. T x is preferably between about 80°C and about 150°C and more preferably between about 90°C and 130°C. After application, the paint formulation is generally cured in an oven. For economic reasons, the throughput time in the oven is generally kept as short as possible and generally ranges between about 10 and about 60 seconds. Preferably, the curing occurs in 15 to 50 seconds and more preferably in about 20 to about 30 seconds. The curing oven can be an infrared oven, a gas oven or other suitable type of oven. The curing temperature T 2 generally lies between about 130°C and about 350°C and preferably between about 160°C and about 300°C and more preferably between about 230°C and about 260°C. It should be noted that T 2 must always be greater than T x . The temperature T 2 means the temperature of the band-shaped material (the so-called "peak metal temperature") and the applied coating. The temperature of the oven may be higher than T 2 . Suitable band-shaped substrates include for example aluminum, cold-rolled steel and (hot dip) galvanized steel. The substrate may be pretreated, for example with a primer. Such a primer is generally applied as a thin layer in wet form. The band-shaped material can, if desired, be successively repeatedly coated according to the process of the present invention. The same or different paint compositions can be applied in each successive treatment. On most existing equipment, preference will be given to applying the paint in a single operation. By preference, the paint is applied in a single coating treatment, i.e. single operation, due to the limitations imposed by equipment installed in most present-day factories. Polyesters are generally obtained by reaction of aliphatic polyalcohols and polycarboxylic acids. The polycarboxylic acids generally are selected from the group consisting of aromatic and cycloaliphatic polycarboxylic acids because these acids tend to have a Tg increasing effect on the polyester. In particular two- basic acids are used. Examplary polycarboxylic acids are isophthalic acid, terephthalic acid, hexahydro terephthalic acid, 2,6-naphthalene dicarboxylic acid and 4,4-oxybisbenzoic acid and, in so far as available, their anhydrides, acid chlorides or lower alkyl esters such as e.g. the dimethylester of naphthalene dicarboxylic acid. Although not required, the carboxylic acid component usually comprises at least about 50 mol%, preferably at least about 70 mol%, isophthalic acid and/or terephthalic acid. Other suitable aromatic cycloaliphatic and/or acyclic polycarboxylic acids useful herein include, for example, 3,6-dichloro phthalic acid, tetrachloro phthalic acid, tetrahydro phthalic acid, hexahydro terephthalic acid, hexachloro endomethylene tetrahydro phthalic acid, phthalic acid, azelaic acid, sebacic acid, decane dicarboxylic acid, adipic acid, succinic acid, trimellitic acid and maleic acid. These other carboxylic acids can be used in amounts of up to at most 50 mol% of the total amount of carboxylic acids. These acids may be used as such, or, in so far as available at their anhydrides, acid chlorides or lower alkyl esters. Hydroxy carboxylic acids and/or optionally lactones can also be used, such as, for example, 12-hydroxy stearic acid, hydroxy pivalic acid and ε-caprolactone. Monocarboxylic acids, such as, for example, benzoic acid, tert.-butyl benzoic acid, hexahydro benzoic acid and saturated aliphatic monocarboxylic acids, can, if desired, be used in minor amounts. Useful polyalcohols, in particular diols, reactable with the carboxylic acids to obtain the polyester include aliphatic diols such as, for example, ethylene glycol, propane-l,2-diol, propane-1,3-diol, butane-l,2-diol, butane-l,4-diol, butane-l,3-diol, 2,2-dimethylpropanediol-l,3 (= neopentyl glycol), hexane-2,5-diol, hexane-1,6-diol, 2,2-bis-(4- hydroxy-cyclohexyl)-propane (hydrogenated bisphenol-A) , 1,4-dimethylolcyclohexane, diethylene glycol, dipropylene glycol and 2,2-bis[4-(2-hydroxy ethoxy)-phenyl] propane, the hydroxy pivalic ester of neopentyl glycol. Small amounts, such as less than about 4 wt.% C dibutyltin oxide or tetrabutyl titanate. Preparation conditions and the COOH/OH ratio can be selected so as to obtain end products that have a hydroxyl number within the targeted range of values. The polyester can be a crystalline polyester, although amorphous polyesters are preferred. Mixtures of crystalline and amorphous polyesters can be used. Amorphous polyesters have a viscosity generally within a range of between 100 and 8000 dPas (measured at 158°C, Emila). Crystalline polyesters usually have a lower viscosity in the range of about 2 to about 200 dPas. Hydroxyl functional polyesters can be prepared in a manner known per se by the use of a sufficient excess of glycol (polyalcohol) in the polyester synthesis. The hydroxyl value of the polyester is preferably between 10 and 80 mg KOH/g resin and more preferably between 15 and 60 mg KOH/g resin. The Tg of the polyester is chosen such that the Tg of the polyester-crosslinker mixture is sufficiently high (preferably > 30°C) to ensure physical stability of the powder paints or binders prepared from them at room temperature. If so desired, combinations of polyester and crosslinker having a lower Tg can be used for preparing a powder coating composition. However, to maintain powder stability such powders are stored chilled. The Tg of the polyester is preferably 40-90°C and more preferably 45- 75°C. In the case of crystalline polyesters, the Tg may also be significantly lower than the aforementioned value of 30°C. The crystallization temperature must then be higher than 30°C, and preferably higher than 45°C. Isocyanates include, among others, aliphatic, cycloaliphatic and aromatic di-, tri- and tetraisocyanates such as, for example, 1,5-naphthalenediisocyanate, 4,4'- diphenylmethanediisocyanate, 4,4 '-diphenyldimethyl- methanediisocyanate, di- and tetraalkyldiphenylmethane- diisocyanate, 4,4 '-dibenzyldiisocyanate, 1,3- phenylenediisocyanate, 1,4-phenylenediisocyanate, isomers of toluenediisocyanate, l-methyl-2,4-diisocyanate- cyclohexane, 1,6-diisocyanate-2,2,4-trimethylhexane, 1,6- diisocyanate-2,4,4-trimethylhexane and 1-isocyanate- methyl-3-isocyanate-l,5,5-trimethylcyclohexane, chlorinated and brominated diisocyanates, phosphorus- containing diisocyanates, isophoronediisocyanate (IPDI), 4, '-diisocyanatephenylperfluorethane, tetramethoxy-1,4 '- diisocyanate, butane-l,4-diisocyanate, hexane-1,5- diisocyanate, hexane-1,6-diisocyanate, dicyclohexylmethanediisocyanate, cyclohexane-1,4- diisocyanate, ethylenediisocyanate, bis-isocyanate ethyl phthalate, l-chloromethylphenyl-2,4-diisocyanate, 1- bromomethylphenyl-2,6-diisocyanate, 3,3-bis- chloromethylether-4,4 '-diphenyldiisocyanate, tetramethylxylenediisocyanate, adducts containing isocyanate groups and isocyanurates of the aforementioned diisocyanates. The volatility can be suppressed by trimerization, for example, or by reaction with isocyanate-reactive compounds. In principle, any blocking agent can be employed, as long as the deblocking temperature is higher than the temperature at which extrusion takes place. Illustrative blocking agents usefully employed to block an isocyanate herein include, for example, phenols, alcohols, thioalcohols, thiophenols, oximes, β-dicarbonyl compounds, lactams, aminimides, amines, amides, imides, nitrilocarbonates and isocyanate-dimers. Preferably, methylethylketoxime or caprolactam is employed. Methylethylketoxime deblocks at 130-150°C and caprolactam at 180°C. Blocked isocyanates are described, for instance, in Wicks "Progress in Organic Coatings", 3_:73-99, (1975), and in Misev, "Powder Coatings, Chemistry and Technology", at page 60-65 and 108-117, the complete disclosures of which are incorporated herein by reference. By preference an internally blocked isocyanate is used. Internally blocked isocyanates are commercially available, e.g. Vestonal BF 1540™ (produced by Hills). An exemplary internally blocked isocyanate compound is an urethidione derived from IPDI. The isocyanate-hydroxyl curing reaction for use in the preparation of powder paints is further described in Misev, Powder Coatings, Chemistry and Technology, pp 56-58. In addition, the usual additives such as fillers, gloss agents, pigments, antioxidants, stabilizers, flow agents and/or catalysts can be added to the resin during mixing. In the practice of coating band-shaped substrates, it is unnecessary for an extrusion operation to continue indefinitely without problems, but it should be possible to process a single batch in a single operation with a minimum of problems. In practice, this means that application should be able to continue for 5 to 48 hours and preferably for up to 72 hours without problems. Figure 1 shows a schematic representation of equipment on which the process according to the invention might be carried out. In the Figure reference numeral (1) designates a coil of band-shaped material, metal foil for example. A band (2) of the coiled material is unwound and transported, horizontally to the right in Figure 1. Although horizontal coating is generally preferred, the plant configuration can be adapted to any particular configuration. The configuration shown in Figure 1 is therefore not presently regarded as being the only means for setting up the apparatus for carrying out the present processes. The band is heated in an oven (3) to a desired temperature T x . The paint formulation is supplied by the schematically represented extruder (4) at a temperature Tl, after which the paint formulation is applied by the extruder die (5) as a film to the band at temperature T 0 as shown by reference numeral (6), drawn as a thickening of band 2. The distance between the extruder die (5) and necessary in order to prevent the resultant mixture from agglomerating before it can be introduced into the hot melt compounding unit. This means that an additional and meticulous dispersion step must be included in the process. The above-referenced process involves pressing a cold block of material against a heated moving sheet. This has the further disadvantage of creating major shearing forces between the material and the heated moving sheet. In addition, the quantity of the coating being applied is difficult to control. Another process for coating a band-shaped material with an extruded coating is described in JP-A-54- 158448, the complete disclosure of which is incorporated herein by reference. The examples of JP-A-54-158448 teach to use a binder system based on an epoxy resin or on a polyester-TGIC system or on an acrylate polymer. These examples do not indicate to use the binder composition according to the present invention. JP-A-54-157142 describes a process for coating a band-shaped material with an extruded coating. The coating in JP-A-157142 is based on a resin curing under the influence of air, such as an unsaturated polyester resin or an epoxy resin, for example. This has the drawback that the resin must be stored in the absence of air and that it cannot be used to produce thicker-layer coatings, because the interior of a thick layer does not come into sufficient contact with air to cure sufficiently. The curing rate of the system according to JP-A-54-157142 is also far lower than the curing rate of the system according to the present invention. GB-A-1443292, DE-A-3328133 and US-A-4528355 are examples of publications which describe powder paint compositions based on a mixture of a hydroxyl functional polyester and a blocked isocyanate. However, these applications do not relate to a process for the continuous coating of a band-shaped material. US-A-3503823 discloses a process for coating sheet metal strips on to a reel with a thermosetting resin wherein the resin is applied directly to the moving metal strip. It relates to extrusion coating of a metal substrate comprising heating the coated substrated at least on the interface between the coating and the substrate to a temperature which is higher than that of the coating material at the time when the coating material is extruded onto the substrate. The substrate may be heated immediately before or after the coating is extruded onto the metal. US-A-3503823 does not disclose or suggest to use the binder composition according to the present invention. The invention will be elucidated with reference to the following examples, without being restricted thereto. Example I A powder paint based on 480 parts hydroxyl- functional polyester resin Uralac P1460™ (DSM Resins), 120 parts isocyanate group-containing crosslinker (Vestonal, BF 1540; Hϋls AG) , 300 parts titanium dioxide, 9 parts flow agent and 4.5 parts benzoin was heated and mixed at a temperature of 100-120°C in a twin screw extruder. The composition was extruded as film using a flat film die (about 30 cm) of the coathanger type on a coil-coating line (about 280 mm), with a cooled and polished pressure roller pressing the film onto the coil, which was pre-heated to 170°C. The aluminium coil coated in this way was then cured in an oven at a peak metal temperature ("PMT") of 250°C. A coating having a good gloss, a good flow appearance and a good homogenity of layer thinkness was thus obtained, and the process was continued for a long period without problems. Example I I The process of Example I was followed, with the pressure roller being replaced by an air knife which used compressed air to press the film onto the band-shaped material. Comparative experiment A The process according to Example I was repeated using a paint consisting of 558 parts carboxyl-functional polyester resin Dralac P5000™ (DSM Resins), 42 parts TGIC as crosslinker, 300 parts titanium dioxide, 9 parts flow agent and 4.5 parts benzoin. After a period of extrusion, this process led to a film which was not homogeneously distributed as a result of necking on either side of the flat film die. This necking was caused by curing reactions occurring in the film die, which led (locally) to increases in viscosity, which in turn led (locally) to longer residence times, more curing, etc. This problem already set in after a relatively short and unacceptably period of extrusion (15 min - 1 hour), although the extrusion temperature (110°C) was significantly lower than the targeted curing temperature. Comparative experiment B Onto a band-shaped material as described in Example I, a powder paint layer was sprayed in various thicknesses between 25 and lOOμm in powder form using a conventional method of powder paint application. It was not possible to achieve a line speed as high as in the examples according to the present invention. Moreover, the surface of the film produced was of lower quality because of orange peel. Compared to application of the paint in powder form, as in this experiment, the process according to the present invention has the additional advantage that several process steps can be omitted, allowing the product to be obtained considerably more economically. There exist a variety of coil coating lines working according to this comparative experiment. This is described, for example, in an article by Dr Bernd Meuthen entitled "Powder coil coating lines" (ECCA Congress, Brussels, 25-26 November 1991, pp.1-4; the complete disclosure of which is incorporated herein by reference). The lines operating in this manner currently have a maximum line speed of only 15-20 m/min. Based on the experience gained with these lines, the maximum feasible speed is limited to 30-40 m/min, which is substantially lower than is feasible with the process according to the invention. Comparative experiment C Onto band-shaped material as described in Example I, a wet coating of the following composition was applied in layers of various thickness. po yester res n rom DSM Res ns, Zwo le 2 from Dyno Cyanamid C.V. 3 from King Industries 4 from George H. Langer & Co. , GmbH 5 solvent, from Exxon Chemicals 6 solvent With this kind of paint, it proved impossible to achieve coating thicknesses greater than 20 μm, because above this limit blistering occurs in the coating. This blistering is caused by simultaneous evaporation of the solvent and curing of the coating in the oven. With thicker coatings evaporation takes too long, and this can cause blistering due to solvent "boiling". Solvents have the additional drawback of being environmentally undesirable and requiring extensive provisions for recycling the evaporated solvent or for incinerating the solvent in so-called after-burners. The use of solvents also constitutes unnecessary consumption of resources and requires extra provisions in view of their fire hazard.

Claims: C L I M S 1. A process for the continuous coating of a band-shaped substrate with a thermosetting paint composition comprising, first forming a film from a thermosetting paint in viscous form, applying said film to the band-shaped substrate, and subsequently curing the paint film, characterized in that said paint composition comprises a binder composition consisting of a hydroxyl-functional polyester resin and a crosslinker having blocked isocyanate groups. 2. The process according to claim 1, characterized in that the paint composition is fed to a hot melt compounding unit where it is converted into viscous. 3. The process according to claim 2, characterized in that the hot melt compounding is conducted in a twin screw extruder. 4. The process according to claim 3, characterized in that the paint formulation is fed to the extruder as a dry blend comprising substantially unmixed raw materials, wherein this blend is then mixed, melted and compounded. 5. The process according to claim 1, characterized in that in said process the viscous paint composition is urged through a flat film die with an aperture width of 0.5-1 mm. 6. The process according to claim 5, characterized in that there is a gap of 0.1 to 2 cm between said aperture and the band-shaped substrate, and the film is applied pressureless to the band-shaped substrate. 7. The process according to claims 5 or 6, characterized in that the film is stretched by a factor of 5-20. 8. The process according to claim 1, characterized in that the film is pressed onto the band-shaped substrate. 9. The process according to claim 1, charcaterized in that in said process the band-shaped substrate is transported at a speed of 60-100 m/min relative to the point wherin said film is applied thereto. 10. The process according to claim 1, characterized in that the band-shaped substrate is preheated to a temperature between about 50°C and 200°C, the film is extruded at a temperature between 80°C and 150°C and the applied film is cured at a temperature between 130°C and 350°C.

Inventors: BESAMUSCA JOHANNES WILHELMUS, BINDA PAUL HERMAN GUILLAUME, PLOEG MARTINUS, BESAMUSCA, JOHANNES, WILHELMUS, BINDA, PAUL, HERMAN, GUILLAUME, PLOEG, MARTINUS
Applicants: BESAMUSCA JOHANNES WILHELMUS, BINDA PAUL HERMAN GUILLAUME, DSM NV, PLOEG MARTINUS, BESAMUSCA, JOHANNES, WILHELMUS, BINDA, PAUL, HERMAN, GUILLAUME, DSM N.V., PLOEG, MARTINUS
Application year: 1995
Main classification: B05D1
IPC codes: B05D1, C08G18, C09D175