WO2008071638A1

WO2008071638A1 - Metal substrate coating process

Info

Publication number: WO2008071638A1
Application number: PCT/EP2007/063533
Authority: WO
Inventors: Peter Ambrosi; Heiko Hachmann-Thiessen
Original assignee: W.R. Grace & Co.-Conn.
Priority date: 2006-12-14
Filing date: 2007-12-07
Publication date: 2008-06-19
Also published as: DE102006059115A1

Abstract

The present invention relates to a process for coating a metal substrate comprising (1) applying by curtain coating a thermoreactive polymeric composition that is solid or has a viscosity at room temperature of more than 9000 mPas onto a metal substrate; and (2) curing the thermoreactive polymeric composition coated onto the metal substrate. The process according to the invention allows for a coating process that will release little or no VOCs. Furthermore, the process of the invention can be performed using existing coating lines with only small adaptations.

Description

Metal substrate coating process

Background of the Invention

Metal is used in many areas for the production of a wide variety of articles. In many cases, such articles comprise metal in coated form. For instance, metal containers for receiving foods and beverages generally have one or more coatings to prevent contact between the filled product and metal in order to prevent or minimize corrosion to the metal by the product as well as any disadvantageous influences from the metal on the quality of the product. For producing containers of this kind, there can for instance be provided metal in the form of sheets or coils that is pre-coated with a suitable coating and is then processed to obtain the desired articles by shaping (such as for three-piece can production) or deformation (such as for deep drawing processes) . In producing cans for food and beverages, coatings are required which are flexible and have a low order of toxicity. Furthermore, filled food cans are often sterilized under application of temperatures up to 135 ⁰C. The coating must therefore be sufficiently stable at those temperatures and must be capable of adhering to the surface of the metal .

Traditionally, coatings are applied onto metal substrates by roller coating a liquid lacquer followed by thermal curing, as e.g. disclosed in EP 0 248 985 Al. The thickness of a coating layer applied by this method is usually in the range of from 4 to 6 μm. The maximum thickness that can be applied in one pass is about 12 to 14 μm. Lacquers that are applicable by roller coating are either organic solvent-borne or water-borne. In either case, such lacquers contain organic liquids which are released during curing. Such release of organic compounds during curing can give rise to defects in the coating layer. Fur- ther, the release of organic compounds, such as volatile organic compounds (VOCs), in these processes is highly undesirable for environmental reasons.

A report published by a European Union project (RO2002/IB/EN- 02) aimed at minimizing VOC emissions associated with the production of coated cans names high efficiency roller coating as currently best available technology. In order to further reduce VOC emissions, it is suggested to substitute organic solvent-borne lacquers by water-borne lacquers. However, as water-borne lacquers also contain organic liquids, VOC emission can only be reduced, but not eliminated by such technology.

More recently, coating lines have been designed to apply a straight polymer film directly onto a metal either by lamination of preformed polymer films or by extrusion of polymers directly onto the metal, as disclosed e.g. in US-Patent 5,639,514. In those processes, the distance between the substrate surface and the outlet of the extrusion coating die is usually between 2 μm and 150 μm. In some cases, if this process is designed to not apply additional adhesives, no organic liquids are released during the process. However, it is not possible to apply the coating to the substrate in a continuous manner, and to apply a coating onto the metal, whereby margins free of coating are left on the metal. The latter is highly desired for the production of three-piece cans, where the margins are used to produce the welding seam. Moreover, the coating applied to the substrate is often not uniform in thickness, and, due to the substantial differences in machinery required by this process, existing coating lines designed for roller coating cannot be used or easily adapted for this application .

In view of the foregoing disadvantages of the prior art, there is a need for a novel coating process for metal substrates which involves very little or substantially no organic solvents. Preferably, such coating process would be applicable on traditional coating lines after limited adaptations whereby most of the existing components could be kept.

Summary of the Invention

In surmounting the disadvantages of the prior art, the present invention provides a process for making a polymer coated metal substrate comprising (1) applying by curtain coating, preferably under force of pressure, a thermoreactive polymeric composition that is solid at room temperature (about 23 ⁰C) or has a viscosity at room temperature (about 23 ⁰C) of more than 9000 mPas onto a metal substrate, and (2) curing the thermoreactive polymeric composition coated onto the metal substrate. It is preferred that the thermoreactive polymeric composition is solid or has a viscosity at room temperature (about 23 ⁰C) above 10000 mPas, more preferably above 15000 mPas, even more preferably above 20000 mPas, even more preferably above 25000 mPas, even more preferably above 30000 mPas, even more preferably above 35000 mPas, even more preferably above 40000 mPas, and most preferably above 45000 mPas such as above 50000 mPas .

Viscosity in this context shall mean the Newton viscosity, as measured according to ISO 3219.

According to a further aspect, the present invention provides a process for coating a metal substrate comprising (1) applying by curtain coating a thermoreactive polymeric composition that is substantially free of solvent onto a metal substrate, and (2) curing the thermoreactive polymeric hot melt composition coated onto the metal substrate. The present invention further provides a process for producing metal articles, comprising (1) forming a polymer coated metal substrate using the coating process of the invention, (2) optionally lubricating the coated metal substrate, and (3) forming the coated metal substrate into a metal product selected from containers, cans, caps and closures, can ends, aerosol containers, and steel strips.

As used within the context of this application, the term "curtain coating" means a method wherein a dispenser (e.g. a heated bath) with an aperture in the base allows a continuous curtain of thermoreactive polymer coating material to either fall by gravity or, preferably, be forced by pressure onto a metal substrate moving at a controlled speed beneath the aperture and thereby receive a relatively uniform coating of the thermoreactive polymer on its surface. The distance between the surface of the substrate and the aperture in the base of the dispenser is at least 1500 μm, preferably at least 3000 μm, more preferably from 5000 to 50 000 μm, such as from 10 000 to 25 000 μm, and most preferably from 10 000 to 20 000 μm. Preferably the polymeric coating material is forced by pressure through the aperture onto said metal surface by means of one or more gear pumps. The gear pump may be an internal gear pump or an external gear pump, and ensures consistent distribution of the coating material even on different metal substrates. Advantageously, the gear pump shall be constructed in a manner that the entire heated bath with an aperture in the base is homogeneously supplied with the thermoreactive polymeric composition. This can be achieved, for example, by providing multiple pathways for the thermoreactive polymeric composition from the gear pump to the aperture of the dispenser. Preferably, the exits of these pathways are uniformly spaced from each other. Equipment suitable for curtain coating is well known in the industry and includes, for example the slotcoater available from Coatema Coating Machinery GmbH or the Slot Die Coating Station from TSE Troller Schweizer Engineering AG or the B70 contactless curtain coating head available from Nordson Corporation or the flat sheet nozzle (Breitschlitzdϋse) including pumping system available from Inatec GmbH.

Preferably the gear pump is operated with a pump pressure of from 0.5 to 100 kg/cm², more preferably from 10 to 75 kg/cm², most preferably 30 to 50 kg/cm². This makes it possible to maintain a low temperature of the thermoreactive polymeric composition immediately before application to the substrate. Advantageously, the temperature of the thermoreactive polymeric composition immediately before application to the substrate is from about ambient temperature (about 23°C) up to about 150⁰C, preferably from 40 to 110⁰C.

The polymeric composition contemplated to be used in the present invention is thermoreactive. As used herein, the term "thermoreactive polymeric composition" means a polymeric composition comprising at least one polymer capable of chemical crosslinking upon application of heat. Where the thermoreactive polymeric composition comprises more than one polymer, chemical crosslinking can occur between molecules of the same polymer and/or molecules of different polymers.

The term "substantially free of solvent", as used herein, means a thermoreactive polymeric composition that contains no solvent, or at least no more than a de minimus amount of solvent, e.g. less than 5 % by weight, based on the total weight of the composition, preferably less than 2 % by weight, more preferably less than 1 % by weight, even more preferably less than 0.5 % by weight, and most preferably less than 0.1 % by weight . The metal substrate coating process according to the present invention provides a coating that has suitable flexibility, scratch resistance, adherence as well as pasteurization and sterilization resistance when processed in contact with food or process water. In particular, the coating is stable at elevated temperatures of up to 135 ⁰C, and when processed at extended periods of time of up to 60 minutes at such temperatures. Metal substrates coated according to the invention are thus suitable for the production of a variety of metal products .

The metal substrate coating process according to the present invention is applicable with thermoreactive polymeric compositions that have substantially higher viscosity than compositions that can be used in roller coating. Particularly, the coating process is suitable for thermoreactive polymeric compositions that are substantially free of solvent. Thus, the process of the invention will release little or no VOCs, and defects resulting from the release of organic compounds are substantially prevented. Furthermore, the process of the invention can be performed using existing coating lines, such as roller coating lines, after exchanging the roller coating unit by a curtain coating unit while keeping all other components of the coating line unchanged.

A further advantage of the metal substrate coating process according to the present invention is that it allows for high flexibility with respect to the thickness of the coating. While coatings having a thickness of 4 to 6 μm are possible where desired, the process also allows for substantially thicker coatings of up to 50 μm, preferably up to 20 μm, as well as thinner coatings down to about 1 μm. Thus, the process of the present invention will provide coatings of about 1 μm to about 50 μm, preferably about 2 μm to about 20 μm, more preferably about 4 μm to about 12 μm. At a dry film density of 1 g/cm³ this corresponds to coatings with a dry film weight of about 1 g/m² to about 50 g/m². Other film densities are possible, such as from 0.8 up to 2.5 g/cm³. Thus, the inventive process can provide thicker coatings in a single pass that would have to be applied in several steps when using, for instance, roller coating or it can apply thinner coatings in order to save resources.

Furthermore, the coating process according to the invention allows for accurate and reproducible control of the coating layer thickness. This results in a highly defined coating layer thickness, preferably having a thickness variation of less than 3%, preferably less than 1 % based on the average thickness of the coating layer. Moreover, the coating speed can be increased as compared to e.g. roller coating, thus allowing for an increase in throughput of the coating line.

The reproducible control of the coating layer thickness in the coating process according to the invention also allows for much faster adaptation of the coating process to changes in the desired coating parameters. By contrast, whenever changes are made to the coating system in roller coating applications, the coating layer thickness can only be determined empirically after several samples have been coated and dried. Such extensive test runs are not necessary in the inventive coating process. Thus, the coating process is more flexible and waste of metal substrate as well as coating material is reduced.

Moreover, in the coating process according to the invention, the method of applying the coating does not involve any tool directly contacting the metal surface to be coated. Thus, any contamination on the metal surface will not be carried along the path of the coating and the number of defects is reduced. Further, in roller coating one rubber roller will generally be used for only one specific coating format. In comparison, the inventive process also involves less expendables.

The high uniformity of the coating layer and the substantial prevention of defects result in a very desirable appearance of the coatings provided by the inventive process. Particularly, the appearance of the inventive coatings is clearly improved over those obtainable by roller coating processes.

In comparison with processes wherein polymers are co-extruded directly on the metal, the inventive process has the further advantage that not all of the metal surface has to be coated. This also allows for coating a metal substrate in the form of stripes. This is particularly important in producing three- part cans where a stripe of the metal surface must remain un- coated. Such application is not possible by either lamination of preformed polymer films or by extrusion of polymers directly on the metal. Moreover, this process can be used for coil and sheet application.

Further advantages and features of the present invention are discussed hereinafter.

Detailed Description of the Invention

All weight percentages provided herein are based on the total weight of the polymeric composition.

In one aspect, the thermoreactive polymeric composition used in the coating process according to the invention is solid or has a viscosity of more than 9000 mPas, preferably above 10 000 mPas, more preferably above 15 000 mPas, even more preferably above 20 000 mPas, even more preferably above 25000 mPas, even more preferably above 30 000 mPas, even more preferably above 35 000 mPas, even more preferably above 40 000 mPas, and most preferably above 45 000 mPas at ambient temperature (23 ⁰C) . According to a further aspect, the ther- moreactive polymeric composition used in the coating process according to the invention is substantially free of solvent.

Preferably, the thermoreactive polymeric composition used in the coating process according to the invention melts at a temperature in the range of between 50 ⁰C and 200 ⁰C, more preferably between 70 ⁰C and 180 ⁰C.

The thermoreactive polymeric composition used in the coating process according to the invention is heat curable. Preferably, the minimum temperature at which the thermoreactive polymeric composition is heat curable is at least 10 ⁰C above its melting point, more preferably at least 30 ⁰C above its melting point.

The thermoreactive polymeric composition is preferably cured at a temperature in the range of from 120 ⁰C and 250 ⁰C, more preferably from 150 ⁰C to 230 ⁰C, and preferably for a time period of from 0.5 to 20 minutes, more preferably 1 to 15 minutes, most preferably 1 to 5 minutes.

In the coating process according to the invention, the thermoreactive polymeric composition comprises at least one heat curable polymer. Preferred heat curable polymers include polyesters, such as linear or branched hydroxy or carboxy functional polyesters which are saturated or unsaturated, preferably linear or branched carboxy functional polyesters; polyeth- ers, such as hydroxy or carboxy functional polyethers; polyu- rethanes, such as polyurethane resins, or blocked or unblocked isocyanates ; amino-formaldehyde resins, such as benzoguanamin- formaldeyde, melamin-formaldeyde, urea-formaldeyde or gluco- ryl-formaldeyde resins; phenolic-formaldehyde resins, such as resins based on phenol, kresol, butylphenol, bis-phenol-A and/or their mixtures condensated with formaldehyde which are etherified or not; epoxy resins, such as epoxy resins based on BPA plus BADGE or homologues, preferably ester functional epoxy resins; acrylic resins, such as thermosetting acrylic resins; alkyd resins; and PVC copolymers, such as copolymers of vinylchloride with monomers having reactive groups (e.g. hydroxy or carboxy groups) such as vinylalcohol or acrylate.

The thermoreactive polymeric composition can optionally comprise 1 to 50 % by weight, based on the total weight of the composition, preferably 1 to 20 % by weight, of one or more non-reactive organic components. A non-reactive organic component is one that is not thermoreactive within the meaning used herein. Preferred non-reactive organic components include plasticizers, such as polyurethanes or polyesters, thermoplastic acrylic resins, polyvinylchloride (PVC) homopolymers, polyamides, novolaks such as phenol-formaldehyde novolaks, polycarbonates, and vinylic resins. It will be understood that generally any thermoplastic polymer can be used as a plasti- cizer in the thermoreactive polymeric composition. Other preferred non-reactive organic components include solvents, such as aromatic hydrocarbons such as xylene, glycolethers such as ethylene glycol or propylene gylcol ether, glycolesters, alcohols such as n-butanol, ketones, esters such as acetic acid ester, or carbonates. Preferred are solvents having a boiling point of or above 100 ⁰C, preferably of or above 150 ⁰C. Most preferably, however, solvents are not included.

The thermoreactive polymeric composition can optionally comprise 0.1 to 2 % by weight, based on the total weight of the composition, preferably 0.1 to 1 % by weight, of one or more catalysts suitable for controlling the curing reaction. Suitable catalysts include acidic catalysts, such as phosphoric acid or sulfonic acids (e.g. p-toloyl sulfonic acid), metal salts, such as metal salts of organic acids (e.g. octoates) and organometallic compounds.

The thermoreactive polymeric composition can optionally comprise 1 to 60 % by weight, based on the total weight of the composition, preferably 1 to 40 % by weight, of pigments and/or coloring agents. If desired any commercially available pigment may be used, such as pigments based on aluminum, titanium dioxide or organic pigments.

The thermoreactive polymeric composition can optionally comprise 0.1 to 10 % by weight, based on the total weight of the composition, preferably 0.1 to 5 % by weight, of a wax. Preferred waxes include carnauba wax, polyethylene, oxidized polyethylene, polytetrafluoroethylene (PTFE) , polypropylene, Fischer-Tropsch, Petrolatum, Lanoline, Amides and mixtures thereof .

Metal substrates contemplated for use in the invention preferably comprise steel, such as tinplated steel or electrolyti- cally chromecoated steel (ECCS) , stainless steel, or aluminum. Tinplated steel, chrome-plated steel or aluminum are preferred substrates. In some cases, a specific pre-treatment of the substrate surface, such as heating, may be desired in order to improve the adhesion of the thermoreactive polymeric composition on the metal.

In a further embodiment, there are provided two or more heated baths containing different coating materials and arranged sequentially along the path of travel of the metal substrate to be coated. Parallel continuous curtains of coating material are allowed to fall by gravity or forced by pressure onto the metal substrate moving at a controlled speed. Thus, it is possible to apply two or more different layers of coating in a single step. In a further aspect, the invention provides a process for producing metal articles, comprising (1) forming a polymer coated metal substrate using the coating process of the invention,

(2) optionally lubricating the coated metal substrate, and (3) forming the coated metal substrate into a metal product. Preferred metal articles include containers; cans, such as two- part cans (e.g. single-drawn cans, DRD (drawn and re-drawn) cans, and DWI (drawn and wall-ironed) cans of which single- drawn cans and DRD cans are especially preferred) , three-part cans, casks, barrels, drums and hobbocks; caps and closures, such as crowns (pry-off and twist-off) , RO (roll-on) caps, ROPP (roll-on pilfer proof) caps, stell-caps, lug-caps, PT

(pressed-in thread) caps, and CT (continuous thread) caps; can ends, such as easy-open ends, full-aperture easy-open ends, peel-off ends; aerosol containers, such as aerosol cans, aerosol cones and aerosol domes; and steel strips.

Processes for forming the coated metal substrate into a metal product are well-known in the art. Examples of such methods include welding, deep-drawing, stamping, embossing and the like.

Optionally, in a process for producing metal articles one or both sides of the metal substrate can be printed and/or roller coated before and/or after the metal substrate coating process according to the invention. Preferably, all coating steps are realized by the metal substrate coating process according to the invention.

The metal product may be further lined with a sealant. In particular, such lining may be desirable in the case of producing caps and closures, can ends or aerosol cones and domes. Example

The invention is illustrated with reference to Figure 1 which is a schematic view of a device which can be used for performing the process according to the present invention.

According to Fig. 1 a metal substrate 9 is moved by means of a backing roller 6 in the direction of arrow 10. A thermoreac- tive polymeric composition is molten in storage container 1 and pumped through a heated conduit 2 via a gear pump 3 which feeds a heated dispenser 4 comprising an aperture (not shown) in its base, which is in the form of one or more nozzles. The apertures in dispenser 4 are arranged in such a manner that the thermoreactive polymeric composition is discharged in the form of a continuous curtain. The necessary pressure is generated by means of one or more gear pump(s) 3. After passing through the nozzle, the thermoreactive polymeric coating composition falls onto the surface of the metal substrate 9 thereby forming a coating 11 on the substrate surface. The distance 8 between the surface of the substrate 9 and the aperture in the base of the dispenser 4 is at least 1500 μm. Optionally, a release roller 5 may be used to avoid air entrapment and ensure adhesion of the coating 11 to the substrate 9.

Finally, the substrate coated with the thermoreactive polymeric composition will be passed through an oven (not shown) in order to cure the coating.

In a preferred embodiment, the metal substrate 9 is moved at a higher velocity compared to the velocity with which the curtain of the thermoreactive polymeric composition is discharged from dispenser 4, resulting in the coating 11 being thinned when coming in contact with the moving metal substrate 9.

Claims

Claims :

1. A process for making a polymer coated metal substrate comprising

(a) applying by curtain coating from a dispenser with an aperture in the base under pressure a thermoreactive polymeric composition which, at 23 ⁰C, is solid or has a viscosity of more than 9000 mPas, onto a metal substrate; and

(b) curing the thermoreactive polymeric composition coated onto the metal substrate.

2. The process according to claim 1, wherein the distance between the surface of the substrate and the aperture in the base of the dispenser is at least 1500 μm.

3. The process according to claim 2, wherein the distance between the surface of the substrate and the aperture in the base of the dispenser is from 5000 to 50 000 μm.

4. The process according to claim 3, wherein the distance between the surface of the substrate and the aperture in the base of the dispenser is from 10 000 to 25 000 μm.

5. The process according to any one of the preceding claims, wherein the polymeric coating material is forced by pressure through the aperture onto said metal surface by means of one or more gear pumps operated with a pump pressure of from 0.5 to 100 kg/cm².

6. The process of claim 5, wherein the gear pump is operated with a pump pressure of from 10 to 50 kg/cm².

7. The process of claim 5, wherein the gear pump is operated with a pump pressure of from 20 to 75 kg/cm².

8. The process according to any one of the preceding claims, wherein the thermoreactive polymeric composition comprises less that 5 wt . % solvent, based on the total weight of the composition .

9. The process according to any one of the preceding claims, wherein the thermoreactive polymeric composition melts at a temperature in the range of from 50 ⁰C to 200 ⁰C, preferably from 70 ⁰C to 180 ⁰C.

10. The process according to any one of the preceding claims, wherein the thermoreactive polymeric composition coated onto the metal substrate is cured at a temperature of at least 10 ⁰C above its melting point, preferably at least 30 ⁰C above its melting point.

11. The process according to claim 10, wherein the curing is conducted at a temperature in the range of from 120 ⁰C to 250 ⁰C, preferably from 150 ⁰C to 230 ⁰C.

12. The process according to any one of the preceding claims, wherein the curing is conducted for a time period of from 0.5 to 20 minutes, preferably from 1 to 15 minutes.

13. The process according to any one of the preceding claims, wherein the thermoreactive polymeric composition comprises at least one heat curable polymer selected from the group consisting of:

polyesters; polyethers; polyurethanes ; blocked isocyanates; amino-formaldehyde resins; phenol-formaldehyde resins; epoxy resins; acrylic resins; alkyd resins; and PVC copolymers.

14. The process according to any one of the preceding claims, wherein the thermoreactive polymeric composition further comprises at least one material selected from the group consisting of:

(a) 1 - 50 % by weight, based on the total weight of the composition, preferably 1 - 20 % by weight, of one or more non-reactive organic components;

(b) 0.1 - 2 % by weight, based on the total weight of the composition, preferably 0.1 - 1 % by weight of the composition, of one or more catalysts for controlling the curing reaction;

(c) 1 - 60 % by weight, based on the total weight of the composition, preferably 1 - 40 % by weight, of one or more pigments and/or coloring agents; and

(d) 0.1 - 10 % by weight, based on the total weight of the composition, preferably 0.1 - 5 % by weight, of a wax .

15. The process according to any one of the preceding claims, wherein the metal substrate comprises a metal selected from steel, stainless steel, or aluminum.

16. A process for producing metal products comprising

(a) forming a polymer coated metal substrate using the process of any one of claims 1 to 15; (b) optionally lubricating the coated metal substrate;

(c) forming the coated metal substrate into a metal product selected from containers, cans, caps and closures, can ends, aerosol containers, and steel strips .

17. The process according to claim 16, wherein one or both sides of the metal substrate are printed and/or roller coated prior to and/or after the metal substrate coating process (1) .

18. The process according to claim 16 or 17, wherein the metal product is further lined with a sealant.

19. A polymer coated metal substrate which has been coated using a process according to any one of claims 1 to 15.

20. A metal product which has been produced using a process according to any one of claims 16 to 18.