Method of decorating hard materials

Details Method of decorating hard materials Method of decorating hard materials

2000-05-11

2002-03-19

Barlow, John (Department: 2853)

Incremental printing of symbolic information

Ink jet

Fluid or fluid source handling means

C347S100000, C347S101000

Reexamination Certificate

active

06357868

ABSTRACT:

DESCRIPTION
The invention relates to a method of decorating the surface of hard materials, especially materials which can be fired for the purpose of decoration, by direct or indirect printing using a colour paste containing a pigment and a thermoplastic medium (thermoplastic colour paste), the colour paste being printed onto the material to be decorated or onto a transfer material by means of inkjet technology. The method is directed especially towards the decoration of surfaces of glass, porcelain, enamel and other ceramic surfaces.
The production of decorations by printing directly or indirectly on an object to be decorated, including paper, metals, plastics, glass, porcelain, enamel and other ceramic materials, using pigmented colour pastes is known per se. In the case of the decoration of materials which can be fired for the purpose of decoration, such as glass, porcelain and other fired, biscuit-fired or unfired ceramic articles, application of the decoration to the surface to be decorated is followed by firing at from 500 to 1400° C., whereby the decoration is fixed and, optionally, a partially fired or unfired article is also glazed. Although inkjet technology, a digital printing process, has become widely accepted in conventional printers for the office sector as well as in graphic technology, it has not yet been possible to use inkjet technology successfully for the decoration of ceramic materials.
In the inkjet process, droplets of colour, or ink, under the control of a digital data store, are applied pointwise to the material to be printed. The drops can be produced continuously or discontinuously. An overview of inkjet technology is given by David H. Fishman in American Inkmaker/June 1997, pages 36-39. In conventional inkjet printing processes, the printing inks used are organic colour solutions having a very low viscosity—inkjet inks having a viscosity in the range of from 1 to 10 mPa·s are usual. Although inkjet inks may also contain pigments, especially micropigments, in small amounts, it is then difficult to maintain the required low viscosity. The use of inorganic pigments and especially inks having a high content of inorganic pigments leads to problems in the inkjet printer because the specifically heavy pigments settle out readily. It is also known to use colour pencils in wax form instead of low-viscosity colours (“solid inkjet”). Such colour pencils in wax form also contain as the colouring component organic colourings or pigments dissolved therein having a density which is close to that of the wax surrounding them, generally, therefore, organic pigments such as carbon black, settling out of the pigments and hence problems with the functioning of the inkjet printer thus largely being avoided.
From WO 97/42040 there are known transfers, the decorative layer of which is applied by means of an inkjet printer. Single-colour or multi-colour inkjet printers may be used therefor. In addition to the conventional aqueous colour systems for inkjet printers, that process also uses so-called “solid inks”, that is to say systems of colours or pigments dispersed in a wax-like medium. That document gives no indication of whether the pigments are organic or inorganic pigments. That document does not indicate that the transfers can be applied to ceramic articles and the decoration can subsequently be fixed by decorative firing. This, as well as the fact that, to the knowledge of the inventors of the present Application, commercially available solid inks contain substantially no inorganic pigments, are indications that the pigments according to WO 97/42040 are substantially organic pigments and not inorganic pigments.
Attempts at printing ceramics colours by the inkjet process and hence making the inkjet process available also for decorating ceramic articles, such as glass, enamel and porcelain, have hitherto always failed owing to the pronounced tendency of the specifically heavy and coarse colour powders to form a sediment. The specific gravity of overglaze and glass colours is from 3.5 to 6.0 kg/
1
, and the mean fineness of grain of those products is from 3 to 5&mgr;m. Those products settle out from aqueous or alcoholic suspensions having the conventional inkjet processing viscosity within a few seconds to the extent of in some cases 50%. Such suspensions would rapidly lead to blocking of the print nozzles and of the entire inkjet printing apparatus. Liquid colour pastes having a substantially higher viscosity, for example 5000 mPa·s, such as are used in screen printing for decorating glass, tend to separate and must be stirred again after standing for 15 minutes.
The object of the present invention is to provide a method by which colour pastes having a high content of inorganic pigments and/or ceramics decorative colours can be printed by inkjet technology. A further object is to decorate materials which can be fired for the purpose of decoration, such as glass, enamel, porcelain and other ceramic materials, using colour pastes containing ceramics decorative colours and using an inkjet process, the decoration being applied by direct or indirect printing to the material to be decorated. A further object consists in printing by means of inkjet colour pastes having a content of over 50 wt. % inorganic materials, especially inorganic pigments and glass flux. These and other objects which will become apparent from the description are achieved by the method according to the invention.
A method of decorating hard materials, especially materials which can be fired for the purpose of decoration, has been found, which method comprises applying to a surface of the material to be decorated a decorative layer based on a colour paste containing a pigment and a thermoplastic medium (thermoplastic colour paste) by means of direct or indirect printing, the colour paste being printed by means of inkjet technology, which method is characterised in that the colour paste, which contains at least 30 wt. % inorganic solids from the group of the pigments, glass frits and other glass-forming components, is applied to the surface of the material to be decorated or to a transfer material by means of a heatable inkjet print head, the temperature of which is kept above the melting point of the colour paste. The dependent claims relate to preferred embodiments and to those methods which are directed towards the decoration of materials which can be fired for the purpose of decoration.
Surprisingly, molten thermoplastic colour pastes, such as are used for decorating container glass, and accordingly have a very high content of inorganic solids, namely glass colours, exhibit virtually no settling out. Once the heating in the inkjet print head and/or in the supply element of the inkjet printing device has been switched off, the thermoplastic colour paste solidifies immediately, so that no sedimentation occurs even in the case of a long standing time.
The thermoplastic medium generally has a melting point in the range of from 30 to 100° C., preferably from 40 to 80° C. and especially from 45 to 60° C. It was not foreseeable that thermoplastic colour pastes having a content of over 30 wt. % inorganic solids could be printed by means of inkjet technology without problems. Thermoplastic colours such as are to be used for the decoration of ceramic articles preferably contain over 50 wt. % inorganic solids.
Underglaze colours advantageously contain colour powders, that is to say mixtures of one or more inorganic pigments and a glass flux, in an amount of approximately from 65 to 75 wt. %, enamel, stoneware and porcelain colours in an amount in the range of from 70 to 80 wt. % and glass colours in the range of from 80 to 90 wt. %.
The thermoplastic colour pastes (that term is kept even though a pasty to liquid state of the colour pastes is not achieved until they have been melted) to be used according to the invention can contain one or more inorganic pigments of any desired structure and any desired chemical composition. For ceramics purposes, pigments which are substantially stable under the firing conditi

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