WO2000041889A1

WO2000041889A1 - Receiver medium for ink jet printing

Info

Publication number: WO2000041889A1
Application number: PCT/GB1999/004346
Authority: WO
Inventors: Richard Anthony Hann; Alan Butters
Original assignee: Imperial Chemical Industries Plc
Priority date: 1999-01-12
Filing date: 1999-12-22
Publication date: 2000-07-20

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 optionally substituted polybutadiene emulsion, particularly polybutadiene emulsion or polyisoprene emulsion (e.g. a natural rubber latex), and colloidal inorganic material, e.g. an alumina or silica sol. The coating is capable of absorbing oil and use of such a coating can thus enable oil-based inks to be printed successfully onto non-absorbent substrates, including optically transparent media such as polyethylene terephthalate films, e.g. for the production of transparency sheets for use in OHPs, in a way that has not hitherto been possible. The substrate may alternatively be absorbent and may itself act to absorb some of the oil from applied ink. The invention also covers a method of making the medium and a method of printing using the medium.

Description

Title: Receiver medium for ink jet printing

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 optionally substituted polybutadiene emulsion and colloidal inorganic material.

The emulsion is in the form of a rubber or rubber-like emulsion, which may be natural or synthetic, which is referred to herein as a rubber emulsion for brevity. The emulsions have low Tgs, usually around -70°C or below, and coatings of such 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 material, in the form of a colloidal inorganic material such as a silica or alumina sol, 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, and is reasonably robust and resistant to damage. Use of such a mixture results in precipitation of a soft polymer and a harder material as an interpenetrating network, producing a porous polymer matrix. The harder material 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) (PET) 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 150μm. 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 emulsion comprises polybutadiene emulsion or substituted polybutadiene emulsion, preferably comprising polymers of butadiene or alkyl-substituted butadienes, especially methylbutadiene (ie isoprene).

A polyisoprene (polymethylbutadiene) 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.

Suitable polybutadiene emulsions are commercially available, eg in the form of Rovene 8331 and 8329 (Rovene is a Trade Mark) polybutadiene latices from Mallard Creek Polymβffi, Inc.

A mixture of rubber emulsion materials may be used.

The colloidal inorganic material preferably comprises silica eg in the form of a silica sol. Suitable colloidal silicas are commercially available eg in the form of Syton W30 (Syton W30 is a Trade Mark) from Monsanto which has an average particle size of 125nm. Silica sols having smaller particles are also available. For example, good results have been obtained using a 40% (by weight) silica sol having a particle size of 14nm, eg in the form of Ludox HS40 and Ludox AS40 from Dupont (Ludox HS 40 and Ludox AS40 are Trade Marks). Ludox HS40 is stabilised with sodium ions whereas Ludox AS40 is stabilised with ammonia. Use of Ludox AS40 is found to provide coatings that develop reduced surface bloom on exposure to air as compared to coatings using Ludox HS40. Such bloom scatters light when the receiver medium is used in an OHP and so is undesirable. Mixtures of silica sols may be used.

The colloidal inorganic material may alternatively comprise alumina, eg in the form of an alumina sol, or aluminosilicate materials.

The rubber emulsion and the colloidal inorganic material should be mixed in suitable proportions to produce a coating having the desired properties. Typically the proportion by weight of colloidal inorganic material to rubber emulsion is in the range 1:5 to 1:1. If too little colloidal inorganic material is present the coating is undesirably soft, and if too much colloidal inorganic material is present there is undesirably high light scattering. Good results have been obtained at a ratio of about 1 :3 to 1 :4 for Ludox silica sols with polyisoprene emulsions and at a ratio in the range 1 : 1 to 1 :2 for colloidal silica with Rovene polybutadiene emulsions.

The materials 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 eβating 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 1 10 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 20 to 30μm for non-absorbent substrates, eg about 25μm, with thinner coatings, eg about 5μm, being suitable for absorbent substrates, 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 the 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 - 50μm. 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 one or more optional top coats (or supercoats) 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.

It has been found advantageous to provide the receiver medium with a top coat of a high boiling point organic liquid of low polarity, eg paraffin oil or one or more unsaturated fatty alcohols with greater than 14 carbon atoms eg stearyl alcohol. Stearyl alcohol is currently preferred. It is found that such a top coat can inhibit over extended periods of time the formation of bloom on the surface of the coating which may otherwise develop after exposure to air. Such bloom acts to scatter light when the receiver medium is used in an OHP, and is therefore undesirable.

A top_cgat of stearyl alcohol or similar material is found also to have the effect of softening the emulsion coating, so it is desirable to increase the proportion of colloidal inorganic material in the emulsion coating to make it initially harder. Such harder emulsion coatings have poorer transparency initially but become transparent on being overcoated with stearyl alcohol or similar material.

The top coat is typically much thinner than the emulsion coating, eg having a thickness in the range 0.2 to 5μm. A top coat of stearyl alcohol or similar material does not generally remain as a separate entity but becomes absorbed into the emulsion coating. In embodiments having more than one top coat including a top coat of stearyl alcohol or similar material, the order of application of the top coat of stearyl alcohol or similar material is not generally important, with known considerations applying to the order of application of other top coats.

The 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_.jcoalescence, by bonding to surfactant on the surface of the polymer emulsion particles. Alternatively, the rubber itself may be crosslinked (vulcanised).

Other additives may optionally be included in the emulsion 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 optionally substituted polybutadiene emulsion and a colloidal inorganic material.

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 above. 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 any top coats 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 inkjet 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 Nos. 9900488.9, 9900489.7 and 9900490.5. For example, a polyisoprene emulsion coating as described in the present specification may be applied over a crosslinked polyalkene coating as described in the specification of our application No. 9900488.9.

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 μm 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

The following materials were mixed together using a magnetic stirrer:

Neutracon, NC411 and Tospearl are Trade Marks. NC411 is a natural rubber emulsion having approximately 60% solids.

The mixture was coated onto Melinex O transparent PET film using a 50μm Meier bar, and dried in an oven at 110°C for 120 seconds. This resulted in a coating about 25μm thick after solvent evaporation.

The coating was tested with a 24μm layer of magenta ink, by the technique described above. Ink was rapidly absorbed, with the drying time being about 50 seconds.

The coating had high ink absorbency and good transparency and toughness, and gave good projected brightness of ink.

Experiments applying the same mixture in coatings of different thicknesses showed that using a lOOμm Meier bar (giving a coating about 50μm thick after solvent evaporation) gave a coating having undesirably increased light scatter, while using a 36μm Meier bar (giving a coating about 18μm thick after solvent evaporation) gave a coating that was undesirably soft after ink application.

Experiments using the same mixture but without Tospearl showed that the silica sol itself does not significantly affect the surface roughness of the coating. It is therefore desirable to include Tospearl (or equivalent particulate filler) to provide surface roughness and perform an anti-blocking function.

Experiments using different proportions by weight of Ludox to NC411 showed that if the proportion of Ludox is too low (less than 1 :5) the film is undesirably soft, while if the proportion of Ludox is too high (greater than 1 : 1.5) there is undesirably high light scattering.

Example 2

Receiver media were made generally as described in Example 1, using mixtures of the following materials.

Ludox HS 40 4.4g

NC 411 13.5g

and

Ludox AS40 4.4g

NC 411 13.5g

Both samples were coated onto Melinex O transparent PET film using a 50μm Meier bar and dried at 110°C for 120 seconds. The resulting coatings both gave an ink absoφtion time of about 50 seconds for a 24μm layer of magenta ink.

The initial clarity of the sample made with Ludox AS40 was higher than that made with Ludox HS40, but the biggest difference was observable after storage in air for a week, when the Ludox AS40 sample was significantly less hazy than the Ludox HS40 sample.

Example 3

Receiver media were made generally as described in Example 1 using the following mixture of materials. Ludox AS40 7-7g

NC411 7.8g

Neutracon 0.8g

Tospearl 12 μm 0.5g

The mixture was coated with a 50μm Meier bar onto Melinex O transparent film and dried at 115°C for 60 seconds.

Part of the surface was overcoated with stearyl alcohol using a 4μm Meier bar, and the resultant film treated at 90°C for 60 seconds.

The initial coating was moderately light-scattering and hard to the touch. The coated area was clear, and rubbery to the touch. Both areas showed an ink absoφtion time of about 50 seconds for a 24μm layer of magenta ink.

After storage in the air for 1 month, the overcoated area showed no sign of bloom formation, showing that stearyl alcohol will inhibit the formation of a bloom on the surface of these coatings.

Example 4

Further experiments were performed using polybutadiene latex with colloidal silica. Two sorts o " polybutadiene latex were used, with two grades of colloidal silica. Overall, the results were very similar, giving firms of good transparency and rapid absoφtion, which were not excessively rubbery in feel but still had rapid absoφtion.

Receiver media were made generally as described in Example 1 using the following mixture ofmaterials.

Rovene 8331 5g

Ludox AS40 5g Decon 90 0.5g

Decon 90 is a cationic surfactant from Decon (Decon 90 is a Trade Mark).

These were mixed together, coated onto Melinex O transparent PET film using a lOOμm Meier bar and dried at 110°C for 120 seconds to give a slightly hazy coating with good handling properties and not easily scratched.

A further formulation:

Rovene 8331 5g

Ludox AS40 2.5g

was prepared in the same way as the above. The resulting coating was clear, with moderately good handling properties, but could be scratched with a fingernail.

The coatings both gave an ink absoφtion time of about 40 seconds for a 24μm layer of magenta ink. No bloom occurs on exposure to air for a month.

It is therefore preferred for the coatings to be less than or equal to 2:1 Rovene/Ludox, in order to preserve good transparency, but greater than or equal to 1:1 to preserve good toughness.

Simiførresults were obtained by replacing Rovene 8331 with Rovene 8329. Rovene 8329 and 8331 are both polybutadiene latices made by Mallard Creek Polymers, Inc differing in the stabilising surfactant. Both have a Tg of-80°C.

Similar results were obtained by replacing Ludox AS40 with Syton W30. Syton W30 is a colloidal silica made by Monsanto. It has an average particle size of 125nm.

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 optionally substituted polybutadiene emulsion and colloidal inorganic material.

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 emulsion comprises a polyisoprene emulsion.

6. A receiver medium according to claim 5, wherein the polyisoprene emulsion comprises a natural rubber latex.

7. A receiver medium according to any one of claims 1 to 4, wherein the emulsion comprises a polybutadiene emulsion.

8. A receiver medium according to any one of the preceding claims, wherein the colloidal inorganic material comprises silica.

9. A receiver medium according to claim 8, wherein the silica comprises a 40% silica sol having a particle size of 14nm.

10. A receiver medium according to any one of the preceding claims, wherein the proportion by weight of colloidal inorganic material to optionally substituted polybutadiene emulsion is in the range 1:5 to 1:1.

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

12. A receiver medium according to any one of the preceding claims, further comprising a top coat over the emulsion coating.

13. A receiver medium according to claim 12, wherein the top coat includes particulate filler material.

14. A receiver medium according to claim 12 or 13, including a top coat of stearyl alcohol.

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

16. 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 optionally substitutedpolybutadiene emulsion and colloidal inorganic material.

17. 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 15 by an ink jet printing technique.