WO1992010547A1

WO1992010547A1 - Method and composition for jet printing

Info

Publication number: WO1992010547A1
Application number: PCT/GB1991/002201
Authority: WO
Inventors: Peter John Lyon
Original assignee: Willett International Limited
Priority date: 1990-12-11
Filing date: 1991-12-11
Publication date: 1992-06-25
Also published as: AU9041591A; GB9026896D0

Abstract

The present invention provides a method for forming an image on a substrate, which method comprises applying an ink containing an image forming ingredient in an aqueous carrier or solvent medium as a series of discrete droplets to a substrate by means of an ink jet printer, characterised in that the ink contains an image protecting agent which has been temporarily rendered water-soluble by treatment with a volatile water-soluble base. The invention also provides an ink composition for use in the method of the invention.

Description

METHOD AND COMPOSITION FOR JET PRINTING

The present invention relates to a method and composition, notably to a method for printing water resistant inks and to a composition for use therein.

BACKGROUND TO THE INVENTION:

Ink jet printers have been widely used to print a variety of forms of ink onto a wide range of substrates. However, when an ink is to be applied directly to a food stuff, for example to the shell of an egg, problems arise in the acceptability of the components of the ink for possible ingestion by humans. Thus, many forms of ink use an organic solvent as the carrier, but many such solvents, for example the commonly used MEK solvent, are hazardous both by virtue of operational risks, and also because they are carcinogenic, toxic or otherwise physiologically unacceptable. Some solvents, for example water or ethanol are physiologically acceptable, but are limited as to the materials they will dissolve and thus severely restrict the nature of the inks which can be applied to foodstuffs. A further problem arises in that the printed product may have to be exposed to conditions which are directly contradictory to those required to achieve acceptable print quality. For example, it is often desirable that a date or other code be retained on a food product even when that has been cooked in boiling water, for example a date coding on an egg. The ink may be a water based formulation which is prima facie more suitable from a physiological aspect than an organic solvent formulation, but a water soluble formulation would readily be removed from the foodstuff when exposed to hot water.

In order to protect the printed ink image from being removed by water, it has been proposed to include a water resistant binder in the ink composition which forms a coating to the printed image. However, if the coating is to be effective, it should not be water soluble and this poses problems in attempting to incorporate the binding agent into the desirable water based ink formulation. Typical coating agents acceptable for use in foodstuffs include cellulose derivatives, for example sodium carboxymethylcellulose and methylcellulose, natural or synthetic resins or gums such as guar gums, agar-agar, rosin derivatives and shellac. Most of these are soluble in water and would therefore be unsuitable for forming water resistant coatings.

Shellac, rosin and certain rosin derivatives are insoluble and contain free carboxylic acid groups which can be neutralised with sodium hydroxide to render these agents water soluble. However, even though such neutralised materials can be incorporated into a water based formulation and applied to a foodstuff, they do not provide a water resistant binder for the composition.

We have now devised a method by which water resistant printed droplets can be applied to a substrate; and an ink composition for use therein.

SUMMARY OF THE INVENTION;

Accordingly, the present invention provides a method for forming an image on a substrate, which method comprises applying an image forming ingredient in an aqueous carrier or solvent medium as a series of discrete droplets to a substrate by means of an ink jet printer, characterised in that the ink contains an image protecting agent which has been temporarily rendered water soluble by treatment with a volatile water soluble base. We have found that the volatile base readily evaporates from the printed droplet so that the solubilised protective agent reverts to a water resistant form in situ in the printed droplet. The invention thus enables the protective agent to be incorporated in the water based composition for printing, which was hitherto not possible with the conventional protective agents; and enables the printed image to become water resistant, which was not possible with water soluble protective agents.

The invention also provides an ink composition for use in the method of the invention which comprises: a. an image forming ingredient, typically a food grade water soluble dye; b. an aqueous solvent or carrier medium for the components of the composition, preferably a mixture of a water soluble or miscible lower alkanol and water; and c. a normally water resistant binder for the composition which has been rendered temporarily water miscible or soluble by treatment with a water soluble volatile base.

As indicated above, the ink is preferably for application to a foodstuff or to a wrapping closely associated with a foodstuff. Therefore, all the components of the ink composition as applied are physiologically acceptable. Alternatively, the dried composition, after loss of its volatile components (including the base and volatile components of the carrier medium, such as the ethanol and any denaturing methyl alcohol therein), results in a residual film which is physiologically acceptable.

The composition is to be applied by means of an ink jet printer to a substrate. The ink jet printer can be of the drop on demand type in which discrete droplets of ink are ejected from an array of nozzles past which the substrate passes, the nozzles being activated at the desired frequency and in the desired order to form the desired image on the substrate. Thus, the printer can be one in which ink under pressure flows to the nozzles via valving means which are actuated under the control of a computer or the like to allow ink to flow to the required nozzle to eject a droplet from that nozzle. Alternatively, ink can be fed to an ink chamber provided with a piezoelectric crystal which modifies the shape of the chamber when a voltage is applied to the crystal so as to eject a droplet of ink from a nozzle outlet to the chamber. For convenience such types of printer will be designated generally hereinafter as drop on demand printers.

Alternatively, the droplets can be applied by what is known as a continuous ink jet printer in which ink is fed under pressure from a reservoir through a nozzle to form a jet of ink. This jet is broken up into discrete substantially uniformly sized and spaced apart droplets by applying vibration or pressure pulses to the ink, for example by vibrating the nozzle or the nozzle assembly by means of a piezoelectric crystal. The ink is charged, by applying a voltage between the ink jet before it breaks up into droplets and a charge electrode, so that each drop carries a known charge. The charged droplets then pass through a deflection electric field where they are deflected from their straight line of flight by the deflection field applied. The extent of deflection will determine the point at which the droplets strike a substrate passing the printer and the charge and/or deflection fields are varied to direct the droplets to the desired location on the substrate. Droplets which are not to be printed are not deflected but are caught in a catcher or gutter and are returned to the ink reservoir for re-use. Such ink jet printers are generally denoted as continuous ink jet printers hereinafter. Many forms of the above types of printer are known and commercially available and may be used without modification in the method of the invention.

Where the ink is to be applied through a continuous ink jet printer, it will usually be necessary to incorporate one or more polar or ionic components into the ink so that it will have sufficient conductivity to accept the charge to be induced in it. Typically, it will be desired to use an ink composition with a conductivity of from 500 to 3500 microSiemens at 20°C. This can be achieved by the addition of a salt to the composition, for example by the use of a dye which already contains a salt. The ammoniation of the binder in the composition may provide at least part of the conductivity required in the composition, but this will usually be insufficient to achieve the total desired conductivity. For convenience, the invention will be described hereinafter in terms of an ink for use in a drop on demand ink jet printer where the ink need not necessarily contain conductive or polar materials, although such materials may coincidentally be present.

The ink composition for present use contains an image forming component. This can be of any suitable form having regard to the purpose to which the image on the substrate is to be put. Thus, the component can be one which fluoresces under ultra violet light or which can be detected by a magnetic reader. However, it is preferred that the component be one which forms a visible image on the substrate, for example a water soluble or water miscible dye or a pigment. For convenience, the invention will be described hereinafter in terms of a water soluble food grade dyestuff. Such dyestuffs can be selected from a wide range of known dyestuffs commercially available for use in ink jet ink compositions. As indicated above, the dye may contain salts due to its method of manufacture and the dyestuff may be used in its commercially available purity without the need for further purification for present use.

The ink composition contains a binder component which is to serve as the protective coating for the printed droplet on the substrate. The binder can be selected from a wide range of natural or synthetic film forming materials which are normally water insoluble but which can be rendered temporarily water soluble or miscible by treatment with a volatile base. Such materials include hydrocarbon resins carrying one or more carboxylic acid groups, for example carboxylated acrylic acid esters, styrene maleate or fumarate resins, carboxylated styrene acrylic polymers or copolymers, vinyl ether/maleic acid polymers or copolymers. Particularly preferred binders for present use include carboxymethyl cellulose, methylcellulose, ethylcellulose, hydroxypropylcellulose, guar gum, xanthan gum, gum tragacanth, gum arable, rosin and shellac.

For convenience, the invention will be described hereinafter in terms of the use of shellac as the binder.

In its un-neutralised state, shellac is substantially water insoluble, that is it has a water solubility of less than 0.1 gs per litre at 25°C, and forms a water barrier film when applied in a solvent such as MEK to a substrate. However, when neutralised with a volatile base, it becomes more readily soluble in water or a water/alkanol mixture and can be incorporated into the ink composition of the invention in amounts sufficient to act as the protective coating for the applied droplets. Neutralisation is carried out to a pH above 7 , notably to a pH value of from 8 - 10, as measured by a glass combination electrode. The neutralised shellac typically has a solubility in water greater than 200 gs per litre at 25°C. The base used to neutralise the acid groups in the shellac must be one which is sufficiently volatile to escape from the droplet printed on the substrate so as to cause the shellac to revert to its water insoluble form in situ in the printed droplet. Typically, the base will be one which has a vapour pressure in excess of 70 mm Hg at ambient temperature when measured for a 10% aqueous solution of the base. The base should also be one which is physiologically acceptable and does not leave unacceptable residues in the ink composition, nor should it adversely affect the other components in the ink composition. A particularly preferred base for present use is aqueous ammonia, for example 10% aqueous ammonia solution, or a momoalkylamine such a monomethylamine.

The ink composition is put up in an aqueous solvent or carrier medium, notably in de-ionised or distilled water. However, it may be desirable to incorporate one or more other co-solvents, such as a lower alkanol, to aid dissolution of the shellac. The co-solvent may also affect the surface tension and/or viscosity of the ink composition. We have found that the use of water/alkanol mixtures containing from 0.1 to 1 parts of alkanol per 1 part by weight of water are particularly preferred. It is particularly preferred that the lower alkanol contain from 1 to 3 carbon atoms in the alkyl moiety thereof, for example methanol, ethanol or isopropanol.

The ink composition may contain other ingredients which enhance the solubility of the binder or its protective film forming properties. Thus, the binder may be used in admixture with one or more drying agents.

Typically the ink composition will comprise from 0.1 to 5%, preferably from 1 to 3%, by weight of the image forming component; from 0.5 to 10% by weight of the binder. although an upper limit may be dictated by the desired viscosity of from 2 to 10, preferably 2 to 4, cps at 25°C for the ink composition; and ethanol, for example in the form of industrial methylated spirit, and water in weight ratios of from 3:2 to 1:10, notably about 1:1, to provide the solvent or carrier medium.

The ink composition of the invention can readily be made by mixing the ingredients in the desired proportions in the required solvent or carrier medium. However, it is preferred to incorporate the shellac into part of the alkanol to be used to form the carrier for the total composition. This alkanol composition can then be admixed with the aqueous/alkanol solution of the other components, optionally with heating and agitation.

The ink composition can be applied as a conventional water based formulation using an ink jet printer. However, it may be necessary to store the composition in vapour tight containers to minimise premature loss of the ammonia or other volatile base from the composition before use; and to vent the printing area to remove ammonia vapours as they are released from the printed image. If desired, evaporation of the base and the aqueous carrier medium can be assisted by applying heat to the printed substrate and/or by passing heated air over the printed substrate.

The invention can be applied to the printing of any type of image onto a wide range of substrates where a water resistant image is required. However, the invention is of especial application in printing images on foodstuffs, such as eggs, cheese, pastry products such as meat or fruit pies, or other solids foodstuffs; or in printing images onto packaging immediately associated with foodstuffs, for example the paper wrapping for fruit or dairy products or the wax coating on cheeses and the like. The invention will now be illustrated by the following examples in which all parts and percentages are given by weight unless stated otherwise.

Example 1:

An ink composition was made by mixing 1 part of shellac with 1 part of IMS to give a solution of the shellac. 2 parts of food grade red dye were dissolved in 46 parts of deionised water and 42 parts of IMS at 20° C. Aqueous ammonia (10% NH₃, 2 parts) was added to the aqueous alcohol mixture. The shellac solution was added to the resultant mixture to give an homogeneous solution containing shellac (4 parts), IMS (46 parts), deionised water (46 parts), dyestuff (2 parts), and aqueous ammonia (10% solution, 2 parts) . The resultant solution had a pH of approximately 9 and had a conductivity of 2 - 3000 microSiemens at 20°C.

The solution was jetted through a commercially available continuous jet ink jet printer to print a date or other code onto the surface of an egg. The printed droplets dried to a hard finish which resisted boiling water for 5 minutes.

By way of comparison, when the shellac was neutralised with sodium hydroxide, it was possible to make an ink similar to that described above. However, when the printed droplets were washed with cold water, they smeared and began to dissolve, showing that the shellac was still in a soluble form and did not protect the printed droplets.

By way of further comparison, attempts were made to incorporate un-neutralised shellac into the ink composition. Even when the composition was heated and the shellac vigorously stirred into the water base, it was possible to achieve a dissolved content of the shellac of less than 0.1 parts per 100 parts of the ink composition. Such a composition was unstable and the shellac readily separated out if the mixture was allowed to stand or was cooled, making it impossible to pass the ink through an ink jet printer.

Example 2:

An ink composition was made using the process described in Example 1, except that the dye stuff used was Patent Blue V (E131) and the total weight proportions of the ingredients used in the preparation of the ink composition were shellac (4 parts), dye stuff (2 parts), de-ionised water (46 parts), IMS (46 parts) and 10% aqueous ammonia (2 parts). The ink was printed using a continuous jet ink jet printer to form a water resistant image on an egg.

Claims

CLAIMS :

1. A method for forming an image on a substrate, which method comprises applying an ink containing an image forming ingredient in an aqueous carrier or solvent medium as a series of discrete droplets to a substrate by means of an ink jet printer, characterised in that the ink contains an image protecting agent which has been temporarily rendered water soluble by treatment with a volatile water soluble base.

2. An ink composition for use in the method of claim 1, characterised in that the composition comprises: a. an image forming ingredient; b. an aqueous solvent or carrier medium for the components of the composition; and c. a normally water resistant binder for the composition which has been rendered temporarily water miscible or soluble by treatment with a water soluble volatile base.

3. A method or composition as claimed in either of claims 1 or 2, characterised in that the volatile base is ammonia or a monoalkylamine.

4. A method or composition as claimed in any one of the preceding claims, characterised in that the carrier or solvent medium comprises a mixture of water with a water soluble or miscible lower alkanol.

5. A method or composition as claimed in any one of the preceding claims, characterised in that the image protecting agent is a selected from a hydrocarbon resin carrying one or more carboxylic acid groups, rosin or shellac.

6. A method or composition as claimed in any one of the preceding claims, characterised in that the ink composition comprises ammoniated shellac; ethyl or isopropyl alcohol; water; and a water soluble or miscible dyestuff.

7. A composition or method as claimed in either of claims 1 or 2, characterised in that the ink composition comprises from 1 to 3% by weight of the image forming component; from 0.5 to 10% by weight of the image protecting agent; and a carrier medium comprising ethanol and water in weight ratios of from 3:2 to 1:10.

8. A method or composition as claimed in any one of the preceding claims, characterised in that the ink composition has a conductivity of from 500 to 3500 microSiemens at 20°C.

9. A method as claimed in any one of the preceding claims, characterised in that the ink composition is applied using a continuous jet ink jet printer.

10. A method as claimed in any one of the preceding claims, characterised in that removal of the vloatile base from the printed droplets is assisted by heating or by passing an airflow over the printed substrate.