GB2345654A - Receiver medium for ink jet printing - Google Patents

Abstract

A receiver medium for use with oil-based ink jet printing ink comprises a substrate having an ink receiving surface bearing a coating of a mixture of polyisoprene emulsion (eg a natural rubber latex) and a compatible polymer emulsion of higher Tg (eg an acrylic or methacrylic polymer or copolymer). The coating is capable of absorbing oil and can thus enable oil-based inks to be printed successfully onto non-absorbent substrates, including optically transparent polyethylene terephthalate films for use in OHPs. The substrate may alternatively absorb some of the oil from applied ink.

Description

Title: Receiver medium for ink jet pnntmg Field of Invention This invention concerns a receiver medium for use in ink jet printing, particularly for use with oil-based ink jet printing inks, and also relates to a method of making such a medium and a method of printing using such a medium. The term"oil-based ink"is used to mean a substantially non-aqueous ink composition employing solvents or diluents other than water and containing one or more oils.

Background to the Invention Ink jet printing is a widely used printing technique. In general, ink jet printing inks are water-based compositions, and such inks are widely used in a range of different ink jet printers, for commercial, office and domestic use, including desk-top printers.

Oil-based ink jet printing inks comprising a low viscosity dispersion of pigment in non-volatile non-aqueous diluent comprising a major amount of aliphatic hydrocarbon (oil) and a minor amount of oleyl alcohol are also known ; see WO 96/24642. Such oil-based inks have the advantages of enabling printing to be performed very rapidly and also producing a water-resistant end product. Such inks are not, however, widely used commercially, and currently the only commercially available ink jet printers designed to use oil-based inks are wide-format printers designed to print on large rolls of paper, typically about 1 metre in width. Because oil-based inks are non-volatile, the diluent must be absorbed or otherwise permanently accommodated by the receiver medium to produce an acceptable print. Such oil-based inks produce good results when printing on paper and similar absorbent materials, with the oil being rapidly absorbed by the porosity of the paper or other material while leaving the pigment near the surface. However, such oil-based inks are not capable of providing adequate prints on non-absorbent media eg transparent media (which cannot have macroscopic pores as these scatter light and render the material opaque), and so cannot be used, for example, in production of transparency sheets for use in overhead projectors (OHPs).

They are also unsuitable for use with currently available glossy media. US Re 34933 concerns receiver sheets for use in offset lithography and similar printing techniques using solvent-based inks containing oil. The receiver sheets may comprise a transparent substrate, eg of transparent polyester such as polyethylene terephthalate, carrying a transparent ink-receptive polymeric layer comprising one or more polymers or copolymers, eg a copolymer of n-butyl methacrylate and isobutyl methacrylate. Such receiver sheets may be used to produce transparent printed images, eg by offset lithography. The receiver sheets are not, however, suited to use in ink jet printing using oil-based printing inks, as different considerations apply. For use with mechanical printing techniques, such as offset lithography, a receiver medium requires properties of mechanical stability and abrasion resistance that are not necessary when printing using ink jet techniques. Further, the solvent-based inks used for this purpose are rather different from oil-based ink jet printer inks, typically being high viscosity compositions with high solids content, and having a much smaller content of oil as compared with oil-based ink jet printer inks. These prior art receiver sheets are not capable of absorbing the high oil content of oil-based ink jet printer inks, nor of adequately coping with the low viscosity properties of such inks.

The present invention thus aims to provide a novel receiver medium suitable for use with ink jet printers using oil-based printing ink.

Summary of the Invention According to one aspect of the invention there is provided a receiver medium for use with oil-based ink jet printing ink, comprising a substrate having an ink-receiving surface bearing a coating of a mixture of polyisoprene emulsion and a compatible polymer emulsion of higher glass transition temperature (Tg).

Polyisoprene (rubber) emulsions have low Tgs, usually around-70 C, and coatings of rubber emulsion are able rapidly to absorb oil from oil-based ink jet printing ink applied thereto and give good pigment adhesion with the dispersed pigment being permanently fixed on the surface. However, rubber emulsion coatings are very soft and subject to mechanical damage. By including a harder polymer, in the form of a polymer emulsion of higher Tg, the mechanical robustness of the coating is increased, providing a coating that is able rapidly to absorb large quantities of oil on application of oil-based ink, having good pigment adhesion with the dispersed pigment being permanently fixed on the surface, and being reasonably robust and resistant to damage. Use of such a mixture results in precipitation of a soft polymer and a harder polymer as an interpenetrating network, resulting in a porous polymer matrix. The harder polymer provides the resulting coating with mechanical robustness, with the soft polymer providing oil-absorption properties.

Because the coating is capable of absorbing oil, there is no need for the substrate itself to be able to absorb oil. By applying such a coating to non-absorbent substrates it is thus possible for oil-based inks to be printed successfully onto non-absorbent substrates such as glossy white film materials in a way that has not hitherto been possible. In preferred embodiments, the coatings are optically transparent and so can be used on transparent substrates eg for the production of transparency sheets for use in OHPs. The substrate may alternatively be of other materials including metal, plastics, wood etc, and materials having metallised or other non-absorbent finishes.

There is, however, no need for the substrate to be non-absorbent, and the coating can be equally well applied to an absorbent substrate, including absorbent paper, card etc.

In this event the substrate itself may also act to absorb some of the oil from ink applied in use, in which case the coating may be made thinner than would be required on a non-absorbent substrate.

It will thus be apparent that the substrate can be selected from a very wide range of materials.

The substrate is typically in the form of a film or sheet, but the physical form is not important as the coating can be applied to substrates of a wide variety of physical forms.

Typical substrate materials include polymeric materials having suitable properties including dimensional stability, optical transparency, translucency or opacity, tensile strength, adhesion characteristics, thermal stability, hardness etc for the intended purpose. Transparent polymeric substrate materials suitable for use in the production of transparencies include sheets or films of polyester eg poly (ethyleneterephthalate) (rET) such as Melinex (Melinex is a Trade Mark) or poly (ethylenenaphthalate) (PEN). Polycarbonate sheets may also be used for this purpose. Such transparent sheets typically have a thickness of about 50 to about 150m. Other possible polymeric materials include polysulphones, polyvinyl chloride, polystyrene, polyimides, polyolefins, polymethyl methacrylate, cellulose esters such as cellulose acetate etc. A wide range of paper and card materials may also be used as the substrate.

The substrate may be pre-treated, eg in known manner, prior to application of the coating. For example, the substrate may be pre-treated with an adhesion-promoting priming layer, eg of parachlorometacresol (PCMC).

The polyisoprene emulsion typically comprises a natural rubber latex (ie a stable aqueous dispersion). Such latices are readily available commercially with a range of different droplet sizes, ammonia contents etc. Alternatively a latex of synthetic polyisoprene may be used.

The other polymer emulsion (of higher Tg than that of the polyisoprene emulsion) preferably has a Tg > -15 C, more preferably 20 C, yet more preferably 220 C. The other polymer emulsion must be compatible with the polyisoprene emulsion, ie capable of being mixed therewith to produce a stable mixture, eg without resulting in a mass of rubber precipitating therefrom on mixing. A wide range of polymer emulsions may be used for this purpose including, eg, acrylic polymers and copolymers, methacrylic polymers and copolymers. Such polymers are readily available commercially. A mixture of such other polymer emulsions may be used to constitute said other polymer emulsion.

The polyisoprene emulsion and other polymer emulsion should be mixed in suitable proportions to produce a coating having desired properties. Typically the other polymer emulsion is present in an amount of up to 150% of the weight of the polyisoprene emulsion (20-120% being preferred), eg with equal amounts be weight of the two emulsions being used.

The emulsions may be mixed in any convenient manner producing acceptable results.

On a laboratory scale a magnetic stirrer may be used, but it is preferred to use a high shear mixer as this gives more complete dispersion of the components.

The coating materials may be applied by any suitable coating technique, including those known in the field, eg by use of a Meier bar, by rolling coating, rod coating, slide coating, curtain coating, doctor coating etc.

After application, the materials are typically dried. Suitable drying conditions (time and temperature) can be readily determined by experiment for any combination of chemicals and coating thickness. A drying temperature in the range 110 to 140 C, preferably in the range 120 to 130 C, is generally suitable, with curing times at up to 120 seconds generally being appropriate.

The coating may be applied to the entire surface of the substrate or to only selected areas of the substrate surface. In the case of a sheet or film of substrate, the coating will typically be applied to at least one surface and possibly both surfaces (e. g. to enable double-sided printing).

The coating thickness will typically be in the range 30 to 100Lm for non-absorbent substrates, eg about 80pm, with thinner coatings, eg about 5pm, being suitable for absorbent coatings, with coating thickness being selected depending on substrate properties and desired characteristics of the receiver medium.

The coating desirably includes particulate filler material, to modify mechanical properties of the coating and in particular to enhance stiffness and rigidity, making the coating less soft. Suitable materials for this purpose include inorganic, organic or polymeric particulates such as silica including amorphous silica, crystalline silica, fumed silica, aluminium trihydrate, calcium carbonate, glass, clays, aluminium silicates, polyolefin particulates, organic pigments and mixtures thereof. It is preferred to use porous inorganic particulate material for this purpose, eg silica; in this event the porous filler material may also functional to absorb some of the oil from ink applied to the receiver medium. Particulate filler material has a tendency to increase light scattering, reducing coating transparency, so this factor must be taken into consideration in relation to transparent substrates and coatings, while being of no relevance to opaque receiver media. The particulate filler material may additionally act to increase surface roughness of the coating, thus reducing the tendency of the coating to block ie stick by wetting action to adjacent surfaces: this tendency arises from the low Tg of the coating. Filler material particles suitably have a primary size in the range 5nm-50pm. Fillers with a dimension much smaller than the wavelength of light can be used at higher loadings than larger fillers (because of their lower scattering) and therefore make a greater contribution to the mechanical properties of the coating, but are less efficient at creating surface roughness than are fillers with a major dimension of comparable size to the coating thickness. It is often desirable to incorporate fillers of two different sizes in order to optimise the overall properties of the coating.

The receiver medium may include an optional top coat (or supercoat) over the emulsion coating. A top coat desirably has the following characteristics: 1) The top coat should be capable of absorbing oil from applied oil-based ink reasonably rapidly.

2) The top coat should be of higher Tg than the coating so as to reduce the tendency of the receiver medium to block.

3) The top coat should exhibit good adhesion to pigment of applied ink.

The top coat conveniently comprises one or more polymers, and one example of a top coat formulation is a mixture of polybutadiene, styrene butadiene rubber and polystyrene.

The top coat is typically much thinner than the emulsion coating, eg having a thickness in the range 0.2 to 5um. me top coat desirably includes particulate filler material, eg as discussed above, to improve anti-blocking properties and possibly also to improve pigment adhesion and other receiver medium properties. Where the receiver medium includes a top coat, possibly comprising particulate filler material, it may nevertheless be desirable to include particulate filler material in the emulsion coating to perform a stiffening function. Again the light-scattering effect of particulate filler materials must be borne in mind when dealing with transparent receiver media. By use of a relatively thin top coat containing filler, the anti-blocking effect can be maximised without introducing too much light scattering, as smaller particles can be used than would be required in the thicker crosslinked polyalkene coating.

A top coat may be applied by any suitable coating technique, for example those discussed above in connection with the emulsion coating.

A crosslinked structure may optionally be included in the emulsion coating, to make the coating more robust, although this may also have the disadvantageous effect of reducing the rate of oil absorption. For example, a self-crosslinking emulsion may be used, as is known in the art, with bonds being formed between individual particles of the emulsion during or after coalescence, by bonding to surfactant on the surface of the polymer emulsion particles. Altematively, the rubber itself may be crosslinked (vulcanised).

Other additives may optionally be included in the coating to improve properties of the coating. For example, lubricants and release agents, such as waxes and silicones, may be included to reduce friction and/or adhesion at the coating surface.

In a further aspect, the invention provides a method of making a receiver medium for use with oil-based ink jet printing ink, comprising applying to an ink-receiving surface of a substrate a coating of a mixture of polyisoprene emulsion and a compatible emulsion of higher Tg.

The receiver medium is used by oil-based ink jet printing ink being applied thereto by an inkjet printing technique, eg in known manner, using known ink-jet printing apparatus. The ink may be, eg, generally as described in WO 96/24642 discussed auove. On impingement on the receiver medium, the oil of the ink is rapidly absorbed by the ink-receiving surface (ie the emulsion coating and possibly also by the top coat if present) and may also in part be absorbed by the substrate if absorbent, as discussed above.

In another aspect, the invention thus provides a method of printing, comprising applying oil-based ink to the ink-receiving surface of receiver medium in accordance with the invention by an ink jet printing technique.

The coatings described in the present application may optionally be used in conjunction with coatings described in the specifications of our co-pending UK applications filed on even date herewith under references C268.00/I, C269.00/I and C275.00/I. For example, a polyisoprene emulsion coating as described in the present specification may be applied over a crosslinked polyalkene coating as described in case reference C268.00/I.

The invention will be further described, by way of illustration, in the following examples.

Examples Samples of experimental receiver media were made and tested for their ability to absorb oil-based ink (comprising organic pigments dispersed in aliphatic hydrocarbon oil with oleyl alcohol, generally as described in WO 96/24642 by placing a few drops of ink on the edge of the coating, and drawing a Meier bar (24 llm unless otherwise stated) over the surface. The ink was then viewed obliquely in order to determine the drying time. The ability of the prints to withstand abrasion was also determined by drawing a rubber-gloved finger lightly over the printed area. Transparency was normally judged simply by looking at the samples, and this was confirmed from time to time by placing representative samples on an OHP.

Example 1 Experiments were performed using natural rubber emulsions (approximately 60% solids) known by the designations NC411 and NC405 from Lewis and Peat. The emulsions were coated onto Melinex O transparent PET film using lS0pm and 100) 1m Meier bars, and dried in an oven at 110 C for 120 seconds. This resulted in coatings about 80pm and 50pm thick, respectively, after solvent evaporation. The coatings were tested with a 24m layer of magenta ink, by the technique described above. Ink was rapidly absorbed, with the drying time being about 30 seconds.

The coatings were very soft, with poor mar resistance, and the oil-plasticised images were very easily damaged by abrasion, although the pigment adhesion seemed acceptable.

Example 2 The emulsions used in Example 1 were mixed with an equal volume of Esi-cryl 752 from ESI/Kromachem (Esi-cryl 752 is a Trade Mark) which is an acrylic polymer emulsion having a high butyl acrylate content and a Tg of-22 C, and the mixed emulsion coated and tested as described in Example 1. The ink absorption was very fast, and the films were much more robust, although still too soft for convenient handling, and with high light scattering.

Example 3 A series of coatings were made and tested as described in Example 1 in order to determine the effect of the addition of various modifying latices on the mechanical and drying properties of the coating.

Modifier Drying time, Pigment Comments sec adhesion.

Glascol LS24 30 30 no tack, mech stab 5 mins Glascol LS20 30 30 slight tack, mech stab 2 mins i Glascol LS26 30 30 no tack, mech stab mins Vinnapas EP400 not tested tacky and very hazy Vinnapas EPI not tested tacky and very hazy i CEFl 9 precipitated not tested tacky and very hazy Vinnapas EF41 precipitated not tested tacky and very hazy Vinnapas EZ36 not tested tacky and very hazy Texicryl 13-802 30 30 slight tack, mech stab 2-5 mins All modifiers were at 1: 1 additions to NC411, as weight of latices, Meier bar coated at 100 J. m and dried for 120s at 110 C.

Vinnapas formulations are comparative examples.

Component Manufacturer Tg, C Glascol LS24 Acrylic copolymer Allied Colloids 43 Glascol LS20 Acrylic copolymer Allied Colloids-5 Glascol LS26 Acrylic copolymer Allied Colloids 22 Glascol LS28 Acrylic copolymer Allied Colloids 100 Vinnapas EP400 EVA Wacker Chemicals Vinnapas EPl EVA Wacker Chemicals Vinnapas CEF19 EVANC Wacker Chemicals Vinnapas EF41 EVA/VC Wacker Chemicals Vinnapas EZ36 EVA Wacker Chemicals Texicryl 13-802 Acrylic copolymer Scott Bader-12 Joncryl 74 Acrylic copolymer Specialty-16 Chemicals Joncryl 77 Acrylic copolymer Specialty 21 Chemicals Joncryl 142 Acrylic copolymer Specialty 10 Chemicals EVA = ethylene vinyl acetate copolymer. EVAIVC = ethylene vinyl acetate/vinyl chloride copolymer.

Comments about mechanical stability relate to time taken for image to recover (after ink absorption) to a level where only very minor disruption of the coating occurs with a gloved finger and light pressure.

All the modified coatings showed some unwanted haze, which develops with time as the coatings regain moisture from the atmosphere.

Examples 4 Further experiments were carried out generally as described in Example 1 with different amounts of different acrylic copolymer latices added to NC411 to determine the effect on mechanical properties and haze. Materials used were as follows:

% Acrylic Glascol LS24 Glascol LS26 Glascol LS28 5 Tacky Tacky Tacky 10 Tacky Tacky Tacky 20 Less Tacky Less Tacky V Slight Tack 40 No tack No tack No tack All acrylics were added as % w/w of latices to NC411, Meier bar coated at 100 Rm and dried for 120 seconds at 110 C.

Example 5 Yet further experiments were carried out generally as described in Example 1 to investigate the haze and tack of further polyisoprene and acrylic blends, as follows:

Ref NC411 Texicryl Glascol Glascol Drying Tack Comments 13-802 LS28 LS24 time/s 1 50 30 20-30 No tack Quite Hazy 2 50 40 10-30 V Slight Hazy 3 50 30-20 30 V Slight Hazy 4 40 30-30 30 No Tack Lowest Haze The mixtures were Meier bar coated at 100 llm and dried for 120 seconds at 110 C.

All as weights of emulsions not actual solids.

It would appear that at least 20% of acrylic is necessary significantly to reduce tack, but that too high a proportion of acrylic would be expected to have reduced ink absorption capacity.

Example 6 Further coatings were made, generally as described in Example 1, and haze measurements of the coatings were made using a standard technique at 20 C and 60% relative humidity (RH) on an EEL Spherical Hazemeter. Results were as follows:

Sample Haze, % Total light transmission, % NC41 1/Texicryl 13-802, 32 88 1: 1 NC411/Glascol LS20, 1 : 1 31 89 NC411/Joncryl 74, 1 : l 40 87. 8 NC411/Joncryl 142, 1 : 1 27 89. 5 NC411/Joncryl 77, 1 : 1 51 85. 6 NC411/GlascolLS26, 1 : 1 55 85. 1 NC411/Glascol LS24,1: 1 69.5 85.1 NC4111GlascolLS28, 1 : 1 72 76. 3 Ref. 1 (Example 5) 57.5 84.6 Ref. 2 (Example 5) 42.5 87.2 Ref. 3 (Example 5) 36.7 88.5 Ref. 4 (Example 5) 31.5 88.7 NC411 : GlascolLS26 19 : 1 45.5 90.5 NC411 : Glascol LS26 9 : 1 42 89. 6 NC411 : Glascol LS26 4 : 1 56 86. 8 NC411 : Glascol LS26 3 : 2 58.9 85.5 NC411 : Glascol LS24 19: 1 47 91. 8 NC411 : Glascol LS24 9: 1 51 90. 7 NC411 : Glascol LS24 4: 1 66.5 88.2 NC411 : Glascol LS24 3: 2 69 87. 4 NC411 : Glascol LS28 4: 1 71.7 83.5 NC411 : Glascol LS28 3: 2 74.5 77.5 Sample Ref 4 and the NC411/13-802 mixture gave particularly favourable properties of low haze and high transmission.

Addition of 2% w/w Microperl solid glass spheres, mean diameter 20pm from Sovitec S. A., Belgium (Microperl is a Trade Mark) aided their handleability. Images were printed arrd gave a drying time of about 60 seconds. The projected image was slightly dull but acceptable. There was no evidence of bleed between any of the colours and only one area of puddling (coalescence of drops on the surface before absorption by the coating).

Example 7 In order to improve the haze and reduce the occurrence of gel particles in the coatings, some of the emulsions were mixed in a high-shear mixer. A Dispersermat mixer (from VMA Getzmann GmbH) with a 3 cm toothed disk was used to stir at 1000 rpm 24g of Texicryl 13-802 in a 100 ml beaker. 16g of Glascol LS24 was added slowly while stirring. A sample was taken at this point and coated on Melinex O using a 1001lm Meier bar, followed by drying for 120 seconds at 110 C. The sample was free from gels but slightly hazy. 40g of NC411 was then added again slowly and with stirring to the acrylic mixture. Once this was completed a further coating was made in the same way. Again the coating was free from gels but slightly less hazy than the acrylic mixture. The speed of stirring was limited in order to reduce bubble formation in the mixture.

Example 8 In order to reduce friction in the final coating, fillers were added: 0.87 g of Microperl was added, this first being wetted with 1 g of water and 0.43 g of Aqua Polysilk 19 (Micronised Polyethylene/PTFE wax, 4 llm, from Micropowders Inc (Aqua Polysilk 19 is a Trade Mark)) added from 3 g methanol and 1 g water; both were added to the Texicryl before any other additions were made. Once this solution was completed it suffered from an even greater incidence of air entrainment than the first attempt. It could not be degassed in a vacuum within a reasonable time. When the preparation was repeated using a stirrer with a low vortex impeller, there was some air entrainment but to a much lesser extent.

After 30 minutes under vacuum (800 mbar) the solu

Claims (1)

1. Claims 1. A receiver medium for use with oil-based ink jet printing ink, comprising a substrate having an ink-receiving surface bearing a coating of a mixture of polyisoprene emulsion and a compatible polymer emulsion of higher Tg.

   2. A receiver medium according to claim 1, wherein the substrate comprises a film or sheet of transparent material.

   3. A receiver medium according to claim 2, wherein the substrate comprises polyethylene terephthalate.

   4. A receiver medium according to claim 1, wherein the substrate is capable of absorbing oil.

   5. A receiver medium according to any one of the preceding claims, wherein the polyisoprene emulsion comprises a natural rubber latex.

   6. A receiver medium according to any one of the preceding claims, wherein the polymer emulsion has a Tg > -15 C, preferably > 0 C, more preferably > 20 C.

   7. A receiver medium according to any one of the preceding claims, wherein the polymer emulsion comprises an acrylic polymer or copolymer and/or a methacrylic polymer or copolymer.

   8. A receiver medium according to any one of the preceding claims, wherein the coating includes particulate filler material.

   9. A receiver medium according to any one of the preceding claims, further comprising a top coat over the emulsion coating. . A receiver medium according to claim 9, wherein the top coat includes particulate filler material.

   11. A receiver medium according to any one of the preceding claims, wherein a crosslinked structure is included in the emulsion coating.

   12. A method of making a receiver medium for use with oil-based ink jet printing ink, comprising applying to an ink-receiving surface of a substrate a coating of a mixture of polyisoprene emulsion and a compatible emulsion of higher Tg.

   13. A method of printing, comprising applying oil-based ink to the ink-receiving surface of a receiver medium in accordance with any one of claims 1 to 11 by an ink jet printing technique.