GB2357088A

GB2357088A - Printing onto a ceramic

Info

Publication number: GB2357088A
Application number: GB0104164A
Authority: GB
Inventors: Stephen Hoskins; David Fortune; Richard Anderton
Original assignee: University of The West of England
Current assignee: University of The West of England
Priority date: 1997-06-04
Filing date: 1998-06-04
Publication date: 2001-06-13
Anticipated expiration: 2018-06-04
Also published as: GB2340791A; GB0104164D0; GB2340791B; GB2357088B; GB9928796D0

Abstract

There is disclosed a water miscible ceramic screen printing ink comprising a ceramic enamel dispersed in an aqueous screen printing medium, and which additionally comprises a water-proofing agent. Also disclosed is a method for preparing the ink, a transfer comprising the ink and a process for decorating a ceramic.

Description

2357088 PRINTING ONTO A HEAT-RESISTANT BASE This invention relates to a

water miscible screen printing medium and enamel composition for use in the decoration of a heat-resisting base, such as a ceramic material (pottery), glass or vitreous enamels. This invention also relates to a method of making the medium and enamel composition, to a water-slide decal which is used to transfer a printed image (the image having been printed using the medium/enamel composition of the invention) onto the said heat-resisting base and to a process for decorating a heat-resisting base using the water-slide decal of the invention.

As used herein, the expression "ceramic enamel" means a composition comprising a pigment and a fluxing agent for the pigment. The fluxing agent may be a glass frit which melts at high temperatures, but may be any substance which fuses the pigment to the heat resistant base. Typical ceramic enamels are available from ceramic manufacturers, e.g. Cookson Matthey, Degussa and Haraeus.

To ensure adhesion to a ware the ceramic enamel comprising pigment and frit is normally available in a particulate form, as obtained by grinding to form a coloured powder, the particles of which are normally smaller than about 50gm. Ceramic enamels are produced to very exacting requirements and must be able to withstand high firing temperatures and be resistant to chemical attack. There are therefore only a small number of pigment systems available for ceramic enamels; these include spinels (MgA1204) and cadmium sulphur selenides (communally reds and yellows), and the more recently developed zirconium encapsulated pigments where the pigment is protected by a zircon shell. The colour of enamels depends on its 3 5_ composition and is usually determined by the metal ion in'the pigment. The flux, typically a glass frit, is responsible for wetting and coating the enamels and protecting the ware and image from chemical attack.

Frits are usually lead borosilicate glasses with a melting point of between 700 and 12000C depending on the application temperature. Additives to the frit control its properties for different applications. For example, aluminium is added _to improve resistance, while alkali metals are added to lower the melting point. Lead-free systems have been developed recently due to concerns about lead leaching from ceramic ware and lead intake by workers in the ceramics industry.

In the manufacture of water-slide decals which are to be used to transfer images onto ceramic materials, or other heat-resisting bases, ceramic colours (usually enamels, underglaze or oxides) are mixed with a screen printing medium and screen printed onto a water leaf base paper (commonly known as simplex or twincal paper) and allowed to dry. A covercoat layer is then printed over the image. The covercoat layer is based on polymeric methacrylate resins dispersed in organic solvents (such as toluene, acetone, methylethylketone) with additives such as plasticisers (phthalates chlorinated diphenyls and polyphenols). The decal is releasable from the stock by the dissolution of an underlying, starch based water-soluble gum layer or a similar carrier. The decal is then applied to the surface of the ceramic or other article to be decorated. Residues of gum allow the resultant film to adhere to the ceramic surface and act as a lubricant to aid the positioning of the image. The article is fired at a temperature, normally above about 7500C, at which the ceramic enamels in the image bond permanently to the ware.

Typically, organic media are mixed with ceramic enamel to give an ink suitable for screenprinting. The media dry to form a solid film from the liquid phase.

The nature of the drying process varies according to the physical and chemical processes involved. The most common drying method is by evaporation of the solvent.

These media burn off without residue at an early stage during the firing, before the frit fluxes to the glaze.

These media are based on synthetic resins, including formaldehyde resins, polyvinyl acetate resins, polymethacrylate systems. The type of polymer used determines the use of solvents. Common solvents are aliphatic hydrocarbons (e.g. white spirit), aromatic hydrocarbons (such as xylene and toluene) and oxygenated compounds such as ethers, alcohols and ketones. Other additives may also be present in the medium to control its physical properties. These may be plasticisers, dryers and retarders. In addition to the hazards associated with inhalation of the volatile solvents, they have been shown to give rise to contact dermatitis on prolonged exposure.

Water miscible ceramic printing inks for use in the decoration of ceramics are known, for example in hand decorated material, but these have not been considered suitable for use in mass-production processes for decorating ceramics using water-slide decals for use in screen printing. This is due to their water solubility leading to breakdown of the image when immersed in water in order to release the image and covercoat from the base paper. In addition, water-based ceramic inks are known which are used for direct printing onto flat ware.

In order to solve the problems associated with the use of organic solvents in the mass-production of ceramic screenprinting for the decoration of ceramic articles, a water-proof, water miscible ceramic decal ink is now proposed. Thus, in accordance with a first aspect of this invention, there is provided a water miscible ceramic screenprinting ink comprising a ceramic enamel dispersed in an aqueous screen printing medium, and which additionally comprises a water proofing agent.

The water-proofing agent is included in the ink to permit it to be used in the preparation of water-slide decals, by rendering the image insoluble in water.

Typically, the ceramic ink of the invention will contain approximately 55'% by weight of the ceramic enamel. This amount is variable, for example by as much as 15%, depending on the ceramic enamel used.

The aqueous screen printing medium in which the ceramic enamel is dispersed is preferably polymeric and a typical example is a medium based on polyethylacrylate or a water miscible emulsion of polymethylmethacrylate. Various conventional screen printing additives may be present in the medium. The amount of screen printing medium present varies according to the physical properties of the ceramic enamel used but is typically greater than 20% by weight. Typically the medium present may be approximately 35% by weight of the ink, although more could be present depending on the particular circumstances.

The water-proofing agent is generally present in an amount of up to 10% by weight of the ink. A preferred group of water-proofing agents which is an 80% modified polyfunctional aziridine. The preferred amount of such a water-proofing agent is about 2% by weight of the ink.

The ceramic printing ink of the invention may comprise one or more of the following: a drier, a retarder and an extender.

A water miscible retarder may be present, typically in an amount of approximately 3-0o of the sum total weight of the mixed ink. The retarder may be added to slow the drying time of the ink and thereby 1 -5- acts to retain the f luidity of the ink for screen printing. In some instances, the retarder may be replaced by water.

The water miscible screen printing ink composition of the invention may be prepared by the following steps:

(a) mixing a ceramic enamel with an aqueous screen printing medium; and (b) mixing a water-proofing agent with the mixture obtained from step (a).

The water-proofing agent is added as the final component, even when the other components mentioned above are present in the composition, as it shortens the shelf lif e of the ink and is theref ore mixed is shortly bef ore printing commences.

Optionally, a water miscible retarder may be mixed with the components, preferably at step (a).

Prior to printing, the screen is preferably washed with a mild alkaline solution in order to prevent crosslinking and the medium drying into the screen, thus prolonging the time available for printing.

Screens for printing are made in the conventional manner for water based screenprinting in the paper and board industry. A typical example is a 150t monof ilament polyester mesh stretched to 14 Newtons coated with Folex dc200 direct screen emulsion. The ink composition of this invention in combination with screens such as these can be printed and has been tested on a range of screen printing machines including hand benches, semi automatics and cylinder presses.

The ink composition of the invention may be employed in the preparation of improved transfers for use in the decoration of a heat-resisting base. Such transfers comprise a substrate and the printed image separated from each other! by a release layer, such as a gum, which is capable of releasing the printed image from the substrate on soaking of the transfer in water and which additionally serves as an adhesive for adhering the printed image to the heat-resisting base.

Normally the substrate is a paper. In addition, the transfer comprises a water-insoluble covercoat which acts as a support for the printed image. The covercoat may be located or positioned on the side of the image away from the substrate, or may be located on the side of the image against the substrate, albeit separated from the substrate by the release layer.

It has unexpectedly been found that highly advantageous results may be achieved using a transfer in which the covercoat is located between the release layer and the printed image formed from the water-proof ink of the invention. In conventional systems, the covercoat has always been an outer layer (hence the term covercoat) so that, when the image and associated cover layer are released from the substrate and applied to an article to be decorated, the cover layer is outermost. This was believed necessary to ensure that, on firing, the decomposition products of the covercoat would not interfere with the printed image. It has now been discovered, however, that when the printed image is one derived from a water miscible enamel composition of the invention, the covercoat may be located between the release layer and the image. This has important practical and commercial implications because it means that the covercoat can be laid down on the substrate, by a printing process, for example, and the image then printed on top of the covercoat. This ensures that the water miscible ink does not interact with the release layer which is often a water-soluble gum.

In a particularly preferred embodiment, the covercoat is based on cellulose acetate butyrate (C.U) This may be Oispersed in a mix of acetone and m ethylethylketone with diethylphthalate plasticiser.

The covercoat layer may be up to 5GAm thick and typically is of the order of 10)um thick.

Thus, the improved transfers of this invention may be used in a process for decorating a heat-resisting base. In such a process, the transfer is soaked in water for a period sufficient to allow removal of the covercoat layer with the printed image from the substrate. The image is subsequently located on the heat-resisting base with the covercoat located against the base and is fixed thereto on firing, leaving no discernible residue. Firing typically takes place at a temperature in the range of 650-9000C. However, this may vary depending on the required application.

It has been found that the use of water slide decals made in accordance with the present invention give particularly advantageous results, in terms of a shortened time required for the drying operation, when used in methods of production which employ forced air methods of drying, for instance using a tunnel dryer or a fan dryer.

According to a further aspect of the present invention, there is provided a transfer comprising a substrate and a printed image which are separated from each other by a release layer, which release layer is capable of releasing the printed image from the substrate on soaking of the transfer in water and which additionally serves as an adhesive for adhering the printed image to a heat resisting base, wherein the transfer further comprises a water-insoluble covercoat which is located an the side of the image against the substrate, albeit separated from the substrate by the release layer.

According to another aspect of the present invention, there is provided a kit for use in preparing 35- a water-slide decal, comprising:

a decal paper, one surface of which is provided with a release layer; a water-proofing agent for addition to a water- miscible screen printing ink, and which is capable of rendering the dried image formed from the ink water insoluble.

The kit may additionally comprise a retarder to slow drying of the ink and/or a supply of a suitable screen printing medium.

A suitable covercoat, for example a cellulose acetate butyrate (CAB) layer may be provided over the release layer of the decal paper to a depth of, for example, 1OAm.

The kit is used in a method of preparing a water- slide decal. In this method, the water-proofing agent is incorporated in the water-miscible screen printing ink containing the ceramic enamel in an amount suitable to render a dried image from the ink water-insoluble.

The ink (or a plurality of inks) are screen printed onto the surface of the decal paper provided with the release layer. The resultant decal may then be used in the normal way whereby the printed image is released from the decal paper and applied to the surf ace of a heat-resisting base, prior to firing in the traditional way.

In a preferred embodiment of the invention, a water-insoluble covercoat is provided over the release layer to act as a support f or an image to be printed thereon.

The decal paper may be a so-called Duplex or Simplex paper. The release layer may be a conventional gum- The kit may also include suitable instructions for use.

EXAMPLE

The following trials were conducted on the water miscible medium developed for ceramic transfers. Press facilities included a cylinder press (Sias) a semi automatic (Svecia), and hand bench trials. A four colour design was chosen and was prepared in Folex DC waterproof emulsion on 150 mesh polyester screens.

The composition used was as follows:

Ceramic enamel Water-proof water miscible medium 20% Waterproofing agent 1010k In addition, a retarder was added (in an amount of up to 3% of the sum total of the composition).

All the ingredients, other than the waterproofing agent, were mixed together, and the waterproofing agent was added just before printing was due to commence. It was f ound that the shelf lif e of the ceramic enamel /medium/retarder mix is about 3-6 months, whereas that of the ink after the addition of the waterproofing agent is a maximum of 24 hours.

The ceramic enamels selected were a commercial ceramic four colour printing set: (CMYK) yellow, cyan, magenta and black, as supplied by Heraeus. Initially, the inks were prepared by mixing these ceramic enamels with the medium at a ratio of 3.5:1 (700-.:20%).

However, it was found that the inks obtained were too viscous to be usable in this instance due to the nature of the particular ceramic enamel, and the ceramic enamel:medium ratio was progressively reduced to 2.5:1, at which point the ceramic enamel were judged to have an acceptable printing viscosity. To each of these inks was added 3% of the retarder, and the mixture was then triple roll-milled. Instead of triple roll milling, Hobart mixing could be used. However, all the inks appeared to mill satisfactorily. Immediately prior to printing, 10% of the waterproofing agent was added to each ink and well mixed in by hand.

The substrate chosen was a commercially available transfer paper, known as Ceramicol, which carried a 101= coating of cellulose acetate butyrate (CAB) the applicants have applied transfers prepared on this substrate directly after printing the inks, i.e., without the use of a covercoat on top of the transfers, in accordance with the claimed invention.

The inks were printed in the usual order: yellow, cyan, magenta, black, and racked between each colour.

prior to printing, the screen was washed thoroughly with an alkaline preparation (the commercial cleaner Mr Muscle which is ammonia-based). The washing was important, as traces of acid still present in the emulsion after engraving can cause the ink to cross link and dry in the mesh. Printing speeds were relatively high at approximately 200 per hour. All is washing up between colours was undertaken with water.

After printing each colour the sheets were racked at room temperature; they were sufficiently dry to be handled and printed with the next colour within about is minutes compared with several drying days necessary to print a conventional solvent based four colour set.

The transfers were applied to 250 mm bone china coupe plates without further processing, i.e., no covercoat on top of the printed image was employed.

The plates were then subjected to a number of different firing cycles, and the results were as follows:

1. Laboratory test kiln Ambient - 8OCC @ 18CC hour-'; 20 minute soak:

Good result - some evidence of radial cracks, and a zigzag fault along the top left quadrant of the design, almost certainly due to application problems.

2. Industrial Tunnel kiln Ambient - 8300C, 20 minute soak.

3. Fast-fire kiln 8700C These and other tests have shown results which are comparable to conventional solvent based equivalents and the laboratory test kiln results.

The suitability of the present invention to large- scale production processes has been evaluated.

In these tests, transfers were prepared by printing on a Svecia Printmaster 3/4 automatic machine with hand feed automatic take off with squeegee speeds in excess of those usually used for making transfers for ceramic articles. A rate of production of 1000 transfers per hour was obtained.

Drying was carried out on a Registerprint Jet Air Tunnel Dryer with a belt speed the equivalent of that required to print at 1200 sheets per hour (with the appropriate 80mm gap between sheets).

in all cases, full sheet control was maintained is through the dryer with no flutter and no sheet curling.

In fact, all the sheets remained perfectly flat. An almost imperceptible amount of shrinkage occurred, being approximately 0.25mm. It is not possible to tell whether this was caused by the application of heat to the sheets or whether this occurred naturally as the room in which the drying took place was at about 220C.

In any event, this amount of movement is very much within acceptable limits. In all cases the final cross linking of the inks took place within 15-30 minutes.

Claims

I A transfer comprising a substrate and a printed image which are separated f rom each other by a release layer, which release layer is capable of releasing the printed image from the substrate on soaking of the transfer in water and which additionally serves as an adhesive for adhering the printed image to a heat resisting base, wherein the transfer further comprises a water-insoluble covercoat which is located on the side of the image against the substrate, albeit separated from the substrate by the release layer.
2. A kit for use in preparing a water-slide decal, comprising:

a decal paper, one surface of which is provided is with a release layer and precoated with a cab layer to an approximate depth of 10 microns; a water-proofing agent for addition to a water- miscible screen printing ink, and which is capable of rendering the dried image formed from the ink water insoluble.
3. A water miscible ceramic screen printing ink comprising a ceramic enamel dispersed in an aqueous screen printing medium, and which additionally comprises a water-proofing agent.
4. A water miscible ceramic screen printing ink according to claim 3, wherein the ceramic enamel is present in an amount of about 55-0. by weight of the ink.
5. A water miscible ceramic screen printing ink according to claim 3 or 4, wherein the aqueous screen printing medium is polymeric.
6. A water miscible ceramic screen printing ink according to claim 5, wherein the aqueous screen printing medium is based on polyethylacrylate.
7. A water miscible ceramic screen printing ink according to claim 5, wherein the aqueous screen printing medium is a water miscible emulsion of polymethylmethacrylate.
8. A water miscible ceramic screen printing ink according to any one of claims 3 to 7, wherein the screen printing medium is present in an amount of greater than 20-1. by weight of the ink.
9. A water miscible ceramic screen printing ink according to any one of claims 3 to 8, wherein the water-proofing agent is present in an amount of up to lo-. by weight of the ink.
10. A water miscible ceramic screen printing ink according to any one of claims 3 to 9, wherein the water-proofing agent is a modified aziridine.
11. A water miscible ceramic screen printing ink according to any one of claims 3 to 10, further is comprising one or more of a drier, a retarder, and an extender.
12. A water miscible ceramic screen printing ink according to claim 11, wherein the retarder is present in an amount of about 3% of the sum total weight of the mixed ink.
13. A water miscible ceramic. screen printing ink substantially as hereinbefore described with reference to the accompanying ex ample.
14. A method of preparing a water miscible screen printing ink, which method comprises the steps of:

(a) mixing a ceramic enamel with an aqueous screen printing medium; and (b) mixing a water-proofing agent with the mixture obtained from step (a).
15. A method of preparing a water miscible ceramic screen printing ink substantially as hereinbefore described with reference to the accompanying example.
16. A transfer comprising a substrate and a printed image formed from a water miscible ceramic screen printing ink according to any one of claims 3 to 13, wherein the substrate and the printed image are separated f rom each other by a release layer which is capable of releasing the printed image f rom the substrate on soaking of the transfer in water and which additionally serves as an adhesive for adhering the printed image to a heat-resisting base.
17. A transfer according to claim 16, further comprising a water-insoluble covercoat which is located or positioned on the side of the image away from the substrate.
18. A transfer according to claim 16, further comprising a water-insoluble covercoat which is located on the side of the image against the substrate, albeit separated from the substrate by the release layer.

is
19. A transfer according to any one of claims 16 to 18, wherein the covercoat comprises cellulose acetate butyrate.
20. A transfer according to any one of claims 16 to 19, wherein the covercoat layer has a thickness of about 10,um.
21. A transfer substantially as hereinbefore described with reference to the accompanying example.
22. A process for decorating a heat-resisting base, which process comprises the steps of:

(a) soaking a transfer according to any one of claims 16 to 21 in water for a period sufficient to allow removal of the covercoat layer with the printed image from the substrate; (b) locating the image on the heat-resisting base; and (c) fixing the image to the heat-resisting base by firing.
23. A process for decorating a heat-resisting base substantially as hereinbefore described, with reference to the accompanying example.