GB2362605A

GB2362605A - Printing on metal surfaces and compositions therefor

Info

Publication number: GB2362605A
Application number: GB0012730A
Authority: GB
Inventors: Kristian John Sime; Phillip Gareth Bentley; Alan Lionel Hudd
Original assignee: Becker Industrial Coatings Ltd
Current assignee: Becker Industrial Coatings Ltd
Priority date: 2000-05-26
Filing date: 2000-05-26
Publication date: 2001-11-28
Anticipated expiration: 2020-05-26
Also published as: WO2001092025A1; GB0012730D0; AU2001258621A1; GB2362605B

Abstract

An improved process for digitally printing onto a substrate, particularly a metal coil, and novel paint compositions for use in the process. The paint composition comprises a binder system of a polymer having a number average molecular weight of at least 7500 and a cross-linking agent, such as melamine, one or more thermally activated catalysts and at least one pigment. A high boiling point carrier solvent may also be included in the composition which is applied to a substrate by an industrial inkjet printer and thermally cured.

Description

2362605 1 Title: An improved process for printing, novel paint

compositions for use in said process and to a method of forming said compositions.

DESCRIPTION

The present invention relates to an improved process for printing, particularly for printing on metal coil coatings, to novel paint compositions that may be used in said process and to a method for forming said compositions.

Metal coil coatings, for example, of steel or aluminium, are used for a wide variety of purposes, such as the production of architectural cladding, aluminium caravan bodies and street signs. Generally, the bulk metal coil, which may weigh in excess of five tons and have a width of 1 to 1.5 metres, is fed through a pretreatment section. It is then provided with a base primer coating which is then cured by passing the uncoiled metal through a curing oven. A pigmented top coating is then applied to the primer by passing the primed metal through application rollers, and the top coating is then cured in a second oven. The normal mode of application of such paint formulations to a metal coil is by roller coating. Generally, this applies a uniform, continuous, full-width coating of a single colour. At the end of the line, the metal is re- coiled for easy storage and distribution.

Additionally, the metal coils may be subsequently provided with surface decoration by means of a separate silk screen printing process using industrial and specialized coatings. However, the silk screening printing process does have a number of drawbacks in that the pattern to be applied to the metal is restricted in size. In this respect the pattern is applied by means of a cylindrical screen in the 2 form of a roller, the circumference of which dictates the length of the pattern. As a result, the pattern has to be repeated at regular intervals along the length of the coil, limiting the design of the pattern. Larger roller screens could be provided but this is undesirale due to the high cost of large screens and the problems associated with storage of such screens. The high cost of the screens also means that each type of screen must have a minimum run length to be economic.

Furthermore, the silk screen patterns have only limited definition and therefore the patterns used in the process must not be complex and involve a lot of intricate detail. This reduces the desirability of the d6cor and the variety of patterns on offer. Each screen must also be stored for any repeat prints that are required. Additionally this silk screening method can only apply one colour at a time. Multicolour designs require additional processing.

It is an object of the present invention to provide an improved printing process, particularly but not exclusively for printing on to a metal coil, that aims to overcome the abovementioned drawbacks.

A flirther object of the present invention is to provide novel paint compositions, particularly but not exclusively for use in the improved printing process.

Yet a further object of the present invention is to provide an improved method of forming said novel paint compositions.

Accordingly, a first aspect of the present invention provides a process for digitally printing on a substrate, particularly but not exclusively for printing on metal, the process comprising the steps of applying an inkJettable paint composition to a substrate and thermally curing the paint composition to the substrate.

3 The process is particularly suitable for digitally printing coil coating quality paints on a substrate.

The composition is preferably thermally cured at a temperature of at least 180'C. The substrate is preferably a metal, more preferably a metal coil.

Preferably, the composition is printed directly onto a pre-coated substrate that has been produced by means of conventional coil coating techniques. Preferably, various coloured paints are used to provide the multicoloured decorative design. More preferably, an overcoat is provided over the paint composition. It is preferable that the cured paint composition is overcoated with a clear, thermally activated crosslinkable lacquer to protect the decorative paint layer. More preferably, the paint composition and lacquer are applied in succession and subsequently cured together. The overcoat is preferably applied by airbrush techniques. The composition of the overcoat will depend upon the composition of the base coat. The lacquer preferably comprises a hydroxy functional polyester resin and a melamine resin.

A second aspect of the present invention provides a paint composition, particularly but not exclusively for use in an inkjet printing process, the composition comprising a binder system of a polymer having a number average molecular weight of at least 7500 and a cross-linking agent, one or more thermally activated catalysts and at least one pigment.

The paint composition preferably has a minimum curing temperature of 1800C. Preferably, the polymer has a number average molecular weight of <10,000.

Preferably, the polymer is a polyester, more preferably, a hydroxy functional saturated polyester resin. However, other resins, such as acrylics, fluorocarbans or 4 polyurethane, may be used provided the number average molecular weight is manipulated to provide a composition having the required characteristics for a jettable fluid that may pass through the printer head of an industrial inkjet-type printer.

The cross-linking agent is preferably a melamine crosslinker, such as those sold under the trade name Cymel. However, the crosslinker will depend upon the polymer of the binder system. Other suitable cross linking agents include polyisocyanates (for cross-linking of, for example, polyurethane), polycarboxylic acids, epoxy compounds or the like. The thermally activated catalyst is preferably one that is activated at a temperature of less than 1800C. The catalysts may be, for ex ample blocked acid catalysts or organometallic compounds.

Preferably, the pigments are light and heat stable. More preferably, the pigments are stable at temperatures up to 300'C. However, it is to be appreciated that the required stability of the pigrnent will depend upon the temperature of the curing oven and the duration that the substrate has to remain in the oven. The pigments may be either inorganic, organic or a mixture of inorganic and organic pigments. For example, the pigments may be cyan, yellow, magneta, white and black. Suitable organic pigments include copper phthalocyanine (Heliogen Blue L701F), quinacridone red (Paliogen Pink E), quinophthalone yellow (Paliotal Yellow L0962HD) and carbon black (Raven Black 12SS). Suitable inorganic pigments include titanium dioxide.

Preferably, a nwnber of paint compositions containing different pigments are provided to form a set wherein a combination of one or more compositions may be applied to a substrate using industrial inkjet printheads to produce a coloured decoration, for example a five colour set comprising cyan, yellow, magneta, white and black. However, if a white basecoat has been applied to the metal sheet, the set may omit the white pigment. Further pigments may also be provided to provide a hexachrome system. Alternatively, the composition may be provided in the form of pre-mixed spot colours. The compositions may all be applied using industrial inkjet printheads, such as the TridentTm Ultrajet drop-on demand piezo printhead.

It is preferable to provide a high boiling point carrier solvent within the composition. The solvent provides a composition with the required flow characteristics to ensure that the various colour compositions do not merge during or after the printing process. Furthermore, the solvent does not swell the pre-coat or primer of the substrate and satisfactorily wets the pre-coat or primer when the composition is applied thereto. Preferably, the solvent has a boiling point of at least 17 50C. More preferably, the solvent is a lactate ester, such as ethyl hexyl lactate and/or dodecyl lactate. Preferably, the high boiling solvent is provided with at least one other solvent. More preferably, the co-solvent is an aromatic hydrocarbon or a mixture of aromatic hydrocarbons, such as the mixture marketed under the trade name Solvesso 150.

The composition preferably include additional components, such as deaerators to control foaming of the paint, wetting agents and/or dispersants.

The paint compositions of the present invention are particularly suitable for applying to metal sheets, including coated, pre-coated or uncoated metal sheets. The compositions are particularly suitable for application to aluminium or steel coil coatings. The compositions may especially be used for decorating pre-coated or 0 6 primed metal coils. The compositions enable complex colour images to be printed on metal sheets.

Preferably, the composition has high pigment to binder ratios, more preferably ratios of 1: 1 or greater. lt is preferable for at least 50%, more preferably at least 75%, of the composition to be made up of solvent. More preferably, around 50% of the composition is made up of the cosolvent and 25 - 35%, more preferably around 30% is made up of the lactate solvent.

The composition preferably includes similar or equal proportions of the resin and the crosslinking agent in the binder system. Preferably, the paint composition has between 3 and 15% of a polymer with a number average molecular weight greater than 7500. More preferably still, around 12 % of the composition is made up of the binder system. The composition may include around 3% of dispersant and preferably includes minor amounts of catalyst and other components (i.e generally each making up <1% of the total composition).

A third aspect of the present invention provides a method of forming a novel paint formulation, the method comprising the steps of dispersing the paint composition using high speed or shear mixing to form a premix and milling the premix to form a desired particle sized dispersion.

The premix is preferably milled in a horizontal bead mill. The dispersed composition may then be let down in the carrier solvent to the required concentration for printing. Preferably, the dispersion is filtered to 1 gm prior to use.

The present invention will now be described in further detail with reference to the following Example in which a paint formulation (A) according to one embodiment of the present invention is prepared and its properties as an inkjettable 7 fluid compared with another paint formulation (B) that had been modified using only conventional paint components to produce an inkjettable fluid. Table 1 below illustrates the components making up each formulation (A) and (B) respectively.

Table 1

Formulation A Formulation B Component % Component % Function Paliogen Pink E 4.5 Paliogen Pink L47690 1.64 Pigment Crodapol E 1636 6.25 Crodapol E1239 17.4 Binder Resin (Molecular weight > (Molecular weight < 7500) 7500) Cymel 325 2.3 Cymel 325 4.23 Melamine Crosslinker Cymel 303 3.8 Beetle BE3747 7.07 Melamine Crosslinker 1786B Acid Catalyst 0.25 1786B Acid Catalyst 0.48 Catalyst Acid Catalyst CV011 0.01 Acid Catalyst CVO 11 0.12 Catalyst Dynoadd F l 0.38 Dynoadd Fl 1.71 Wetting Agent EFKA 4047 3.0 Solsperse 22000 0.05 Dispersant Solsperse 24000 0.55 Dispersant Tego Airex 9 10 0.06 Tego Airex 910 1.00 De-aerator Defbarner PC 1244 0.11 Defoarner Ethyl Hexyl Lactate 26.5 Butyl Diglycol 18.01 Solvent Solvesso 150 50 Solvesso 150 8.30 Solvent Dodecyl Lactate 3 Estasol 39.33 Solvent 8 The use of the paint formulation (A) for jet printing of metal coils was investigated and found to have a number of the required characteristics that result in the paints of the present invention being particularly suitable for such a purpose. In this respect, an industrial inkjet printer is required to deliver the paint composition to the substrate. Simple modification of conventional paint formulations, such as dilution to a viscosity typically used for the formulation of inkjet inks was found not to provide a reliable composition.

The paint formulation (A) is of a high molecular weight to enable the paint to pass satisfactorily through the print head used in the printing process and then be thermally cured to the metal surface. Additionally, the formulation was found to have the required application characteristics to prevent merging of the different colours upon application of the paint to the metal surface. The formulation also produces a final coating that is highly durable and capable of long terin exterior exposure. The unique binder system of the formulation provides a rapidly reacting system with a long pot life at room temperature but requires a high temperature to initiate the curing process. The paint formulation also includes pigments that are heat stable and light fast, enabling it to withstand the processing conditions typically used for coil coating (around 3000C The other paint formulation (B) was also found to possess the aforementioned characteristics but the formulation was found not to be as superior for providing satisfactory printing using an inkjet-type process. The fluid (B), though jettable, exhibited face-plate wetting, which compromised its continuous print performance and additionally, the fluid did not wet the substrate efficiently but instead swelled the cured undercoat.

9 In contrast, the paint formulation (A) overcame all the problems associated with the conventionally modified paint formulation. The incorporation of lactate carrier solvents that are not conventionally used for coil coating paint formulations resulted in a formulation that did not swell the cured basecoat and that provided satisfactory wetting of the basecoated surface. The polymer binders also allow the highly loaded pigmented coatings to flex during panel forming without damage occurring to the coatings.

Hence, the paint formulations of the present invention are suitable for use in an inkjet-type printer and may be used for the decoration of products derived from coated metal (primarily aluminium or steel) coil. Examples, of the use of such formulations include the decoration of architectural cladding, aluminium caravan bodies and street signs.

Another factor in the development of the paint formulations was the incorporation of suitable process techniques into the paint manufacturing procedure. Such techniques comprise the production of a dispersion concentrate in two-stage process in which a premix of the dispersion was produced using high speed/high shear mixing. This premix was subsequently milled in a horizontal bead mill to obtain the required particle sized dispersion. This dispersion was then let down to the appropriate concentration to form the required paint. The paints were filtered to 1 gm prior to use. The use of an aromatic organic co-solvent such as Solvesso'll' in addition to the lactate carrier solvent enables more dispersant to be incorporated into the formulation than if lactate solvents are used alone, giving the composition the required stability whilst the high boiling solvents, such as lactate esters, allow excellent dwell performance in the printhead, with dwell times in excess of 30 minutes being observed for these compositions. The printhead could also be started after leaving overnight with only minimal priming.

The compositions of the present invention allow metal coils to be decorated with multi-coloured complex designs by means of a jet-type printer with the compositions being thennally cured after printing. In contrast, conventional ink forinulations; for passing through an inkjet-type printer are dried by air or are UV cured, but not cured by thennal means. The primer, paint compositions and/or lacquer may be applied successively and then cured since the ability to apply the paint composition by means of a non-contact industrial inkjet printer means that the compositions can be applied to a wet substrate. The digital printing process allows intricate details to be printed on to metal coatings and to be stored electronically thereby greatly reducing the amount space required for storage of images for enabling repeat prints. The designs have increased resolution and may be applied on a la rge scale to articles, such as caravans, to enable such articles to be decorated with designs of intricate detail.

11

Claims

1. A process for digitally printing on a substrate comprising the steps of applying an inkjettable paint composition to a substrate and thermally curing the paint composition to the substrate.

2. A process as claimed in claim 1, wherein the substrate is a metal.

3. A process as claimed in claim 1 or claim 2, wherein the composition is thermally cured at a temperature of at least 1800C.

4. A process as claimed in claim 2 or 3, wherein the composition is printed directly onto a pre-primed or pre-coated metal substrate.

5. A process as claimed in claim 4, wherein the primer and composition are cured following application of the paint composition.

6. A process as claimed in any one of the preceding claims, wherein an overcoat is applied over the paint composition.

7. A process as claimed in claim 6, wherein the overcoat is a clear lacquer.

8. A process as claimed in claim 7, wherein the lacquer is a hydroxy functional polyester resin and a melamine resin.

9. A process as claimed in claim 6, 7 or 8 wherein the paint composition and overcoat are applied in succession to the substrate and subsequently cured together.

10. A paint composition comprising a bin der system of a polymer having a number average molecular weight of at least 7,500 and a cross-linking agent, one or more thermally activated catalysts and at least one pigment.

12 A paint composition as claimed in claim 10 having a minimum curing temperature of 1800C. 12. A paint composition as claimed in claim 10 or 11, wherein the polymer has a number average molecular weight of -,<1 0, 000. 13. A paint composition as claimed in claim 10, 11 or 12, wherein the polymer is selected from the group consisting of a polyester, acrylic, fluorocarbon, and a polyurethane. 14. A paint composition as claimed in claim 13 wherein the polymer is a polyester. 15. A paint composition as claimed in claim 14 wherein the polymer is a hydroxy functional saturated polyester resin. 16. A paint composition as claimed in any one of claims 10 to 15 wherein the crosslinking agent is selected from the group consisting of a melamine crosslinker, polyisocyanates, polycarboxylic acids and epoxy compounds. 17. A paint composition as claimed in claim 16, wherein the cross-linking agent is a melamine cross-linker. 18. A paint composition as claimed in any one of claims 10 to 17 wherein the catalyst is activated at a temperature of less than 1800C. 19. A paint composition as claimed in any one of claims 10 to 18, wherein the pigment is an inorganic pigment, organic pigment or mixture of inorganic and c)rganic pigments. 20. A paint composition as claimed in any one of claims 10 to 19, wherein a carrier solvent having a boiling point of at least 1750C within the composition.

21. A paint composition as claimed in claim 20 wherein the solvent is a lactate ester.

13 22. A paint composition as claimed in claim 21 wherein the solvent is ethyl hexyl lactate, dodecyl lactate or a mixture thereof 23. A paint composition as claimed in claims 20, 21 and 22 wherein the carrier solvent is provided with at least one co-solvent. 24. A paint composition as claimed in claim 23 wherein the co-solvent is a aromatic hydrocarbon or a mixture of aromatic hydrocarbons. 25. A paint composition as claimed in claim 10 to 24 wherein the composition has a pigment to binder ratio of at least 1: 1. 26. A paint composition as claimed in any one of claims 20 to 25, wherein at least 50% of the composition is solvent. 27. A paint composition as claimed in any one of claims 10 to 26 wherein the composition is between 3 to 15% polymer having a number average molecular weight greater than 7500. 28. A paint set comprising at least two paint compositions according to any one of claims 10 to 27, each paint composition being provided with a different pigrnent. 29. A paint set as claimed in claim 28 wherein a five colour set is provided comprising cyan, yellow, magneta, white and black. 30. A paint set as claimed in claim 28 wherein a six colour set is provided to provide a hexachrome system. 31. A paint composition comprising at least two paint compositions according to any one of claims 10 to 27, the compositions containing different pigments and being premixed to form spot colours.

14 32. A method of forming the paint composition according to claims 10 to 27 comprising the steps of dispersing the paint composition by means of high speed or high shear mixing to form a permix and milling the premix to form a desired particle sized dispersion. 33. A method as claimed in claim 32 wherein the premix is milled in a horizontal bead mill. 34. A method as claimed in claim 32 or 33 wherein the dispersed composition is let down in a carrier solvent to the required concentration. 35. A method as claimed in claim 32, 33 or 34 wherein the dispersion is filtered to Imin prior to use.