DESCRIPTION

TECHNICAL FIELD

The invention relates to transfer printing of the kind in which the desired image is printed onto a temporary or decalcomania substrate of sheet material, e.g. paper, and is subsequently transferred and secured to a target surface. More particularly the present invention relates to transfer or decalcomania printing of the kind described above for use with ceramics, porcelain, glass, vitreous enamelled substrates and the like articles, to form, for example, a decoration on the article. Such articles necessarily undergo a firing process during their manufacture, which firing process involves firing temperatures of 500° or more. During this firing all organic matter is volatilised to leave inorganic pigments

and fluxes. The pigments and fluxes used in such transfers must thus be capable of withstanding firing temperatures.

BACKGROUND ART A problem associated with the printing of ceramics transfers is that, conventionally, printing inks used for such transfers are solvent-based and require extended periods of time to dry. This involves holding the transfers for long periods under clean conditions and in an atmosphere of controlled humidity and this is expensive to achieve. Conventionally such printing involves so-called silk screen printing, and since this is a contact printing process, this means that where the image to be printed is multicoloured each colour must be allowed to dry before the next colour is applied. The time required in production of the transfers is correspondingly multiplied.

In printing generally, it is well known to cure printing inks by ultra-violet (UV) light, which is very quick in comparison to the curing of solvent-based inks and has the added advantage that solvents are not released during the curing step, which may possibly require expensive solvent recovery processes to avoid their discharge into the atmosphere. It would therefore be desirable to apply UV curable inks to ceramics transfers but in practice this has proved troublesome. EP-B-0, 106,628 of JOHNSON MATTHEY PLC describes a radiation-curable and firable printing ink and a decalcomania for applying a decoration to a ceramic, vitreous or metallic substrate using the mk.

The production of ceramics transfers usually involves printing onto a paper or other sheet substrate coated with a release material such as dextrin or wax, depending on the particular transfer application method employed, i.e. water moistening (water slide) or heat softening (heat release) . A covercoat of solvent-based lacquer is normally applied over the top of the printed image to aid the transfer process and to protect the integrity of the printed image during transfer. Our early trials showed either attack by the solvents used in the covercoat system on the finished print and/or "frizzle" i.e. disturbed print during firing. It had been thought that this frizzle was due to under-cure of the polymer or that the different firing rates of different layers of overprinted materials in the image was leading to a breakdown of their intersurface bonding. Through numerous trials we discovered that although the system will frizzle if under or overcured, the majority of the problems we were encountering were due to the poor surface bonding between the print and the paper. This caused firing imperfections because the adhesive layer of dextrin or wax had not been carried by the prints and therefore, they were not secured to the substrate properly, and tended to lift either prior to, or during the early stages of firing to produce frizzle, or alternatively the bond between print and paper coating was so poor that it allowed the covercoat to penetrate between the print and the paper, thus eliminating the adhesive properties required.

It is an object of the invention to provide a ceramics transfer in which the printing inks are UV cured.

DISCLOSURE OF INVENTION

From one aspect the invention is a method of making a ceramics transfer or decalcomania having a substrate of sheet material having a face formed with a release coat comprising the steps of applying a layer of unpigmented resin to the said face of the substrate to form a primer layer, applying UV curable ink to the substrate to form an image, and irradiating the ink with UV light to cure the ink. We have found that the use of a primer layer substantially improves the bond between the ink and the release coat and ultimately between the ink and the target surface. The primer layer of unpigmented resin may be applied to the substrate before the UV curable printing ink is applied, or alternatively the printing ink may be applied to the substrate below the unpigmented primer layer, depending on whether the transfer is to be applied to its target surface face up or face down. The primer layer may be applied only to those parts of the substrate to be printed. The primer layer may be of a UV curable resin.

Whether the resin of the base layer is UV curable or is solvent-based, it is preferably cured before a layer of ink is applied.

The method may comprise the step of applying two or more differently pigmented UV curable inks to the substrate. Where the inks are printed by a contact method,

e.g. silk screen printing, then each colour is preferably cured before the next is applied. Where a non-contact printing method is employed, say mk-jet printing, then it might be possible to UV cure all of the applied inks simultaneously.

The method may comprise the step of applying a covercoat layer of a lacquer over the cured printed image to stabilize the image during the transfer step. It is possible, however, that the primer layer may be sufficient m itself to stabilize the image so as to remove the need for the covercoat.

The composition of the primer layer may include a UV light curable multifunctional acrylate resin and a photoinitiator. The composition of the primer layer may also include a structuring agent which may be a thixotropic vegetable oil derivative. The primer layer composition may further include a flow agent in the form of a surfactant.

From another aspect the invention is a ceramics transfer comprising a primer layer of unpigmented resin. From yet another aspect the invention is a ceramics transfer made by the method described above.

From a further aspect the invention is a primer layer composition for a ceramics transfer comprising a UV light curable multifunctional resin, e.g. of acrylate. The primer layer may have a composition as follows:- (a) a base resin manufactured by Coates Group Pic under their reference 66-9105 and which is a multifunctional UV light curable acrylate resin;

(b) Structuring and flow additives as required and in the form of a thixotropic vegetable oil derivative and a surfactant respectively, and

(c) a photoinitiator manufactured by Coates Group Pic under their reference 10-909.

Addition levels of photoinitiator to the composition are typically 6% to 10% by weight of the resin.

The composition of the inks may be the same as that of the primer layer with the addition of pigments as required. UV curable resin products are usually supplied ready initiated. We have discovered that by adding the initiator after pigmentation, and just prior to printing, we are able to produce a more stable ink system.

BEST MODE FOR CARRYING OUT THE INVENTION A printing method for producing ceramics transfers comprises the following steps:-

1. The printing substrate is so-called "transfer" paper, which may be paper or other sheet material in cut sheet or reel form, and which is usually coated either with dextrin gum for water slide or with wax for heat release applications, although other release coats are possible.

2. A thin print of UV curable primer is printed beneath all print areas and is UV cured to form a downcoat.

3. Pigmented UV curable resin ink system is screen printed over the primer layer to form the desired image. Where different colours are involved,

each print colour is UV cured before the next colour is applied. 4. The finished image is covercoated using a ceramics transfer covercoat lacquer. This lacquer could be a conventional covercoat i.e. resin solution drying by solvent evaporation, or might be a UV curable resin.

The covercoat holds the printed area together during the application of the transfer onto the article. The novel primer layer may however be of sufficient integrity to obviate the need for the covercoat.

The same basic UV curable resin system may be used for the primer printing ink and covercoat. Such a resin system may be a UV curable free radical acrylate system. It will be appreciated that in some circumstances the decalcomania may be printed in a reverse fashion on the substrate, i.e. with the image printed before the base layer is applied. Particularly in this case the covercoat may be omitted. The decorated article will need to go through a firing process, i.e. heated to temperatures in excess of 500°C. Suitable articles for decoration in this manner include ceramics, porcelain, glass and enamelled substrates.

EXAMPLE A transfer was printed on waterslide paper sold under the Trade Mark "TRUFLAT" by Britains. Downcoat/Primer Formulation 100 parts Coates resin reference 66-9105 by weight

6 parts Coates photoinitiator reference 10-909 by weight

When printing on heat release paper (wax coated) we would add a flow agent/surfactant typically as follows:- 0.6 parts by weight flow additive from Coates reference R1656.

The rolling ratio is typically 1:1 to 2:1 parts by weight of colour to medium.

The flux is a clear "colour" print to increase gloss and durability of the fired print. Thixotropic print medium

100 parts by weight resin (Coates) reference 66-9105 2 parts by weight Rilanit special micro wax from Henkel chemicals.

Semi-Thixotroϋic Print Medium 100 parts by weight Coates resin reference 66-9105 1 part by weight Rilanit special micro wax.

Photoinitiator Coates reference 10-909 is added to the pigmented resin system at a level of 10% by weight calculated on the resin content only. The covercoat used is a typical solvent based air drying lacquer used for the production of ceramics and glass transfers i.e. an acrylic resin solution in hydrocarbon solvent.

The printing sequence is typically as follows:- 1. Primer coat

2. Yellow print

3. Magenta print

4. Cyan print

5. Black print

6. Flux

7. Covercoat

N.B. The primer coat and each print coat is UV light cured before the next print is applied.

The finished transfer is then water slid onto the article to be decorated, squeegeed down, dried and them fired.