EP2235259A1

EP2235259A1 - Improved coated ink jet paper

Info

Publication number: EP2235259A1
Application number: EP09705906A
Authority: EP
Inventors: Marguerite Quinn; Peter Sinclair
Original assignee: ArjoWiggins Papiers Couches SA
Current assignee: ArjoWiggins Papiers Couches SA
Priority date: 2008-01-31
Filing date: 2009-01-30
Publication date: 2010-10-06
Also published as: WO2009095697A1; GB0801815D0; AU2009208801A1

Abstract

An improved coated sheet product especially useful in conjunction with inkjet printers comprises a substrate carrying a coating on at least one surface, characterised in that the coating includes (i) a pigment comprising calcium carbonate; (ii) a binder for component (i) comprising a major proportion of a polymer carrying -O-, -CO-, -OCO- and/or -COO- groups in its side-chains; and (iii) at least at the surface of said coating, a water-soluble salt of a Group II, Group III or transition metal.

Description

Improved Coated Ink Jet Paper

The present invention relates to an improved coated paper especially useful in conjunction with inkjet printers.

InkJet printers are widely used both for the domestic market and, increasingly, for the commercial market. Currently, one major drawback of using inkjet printers is the provision of suitable papers, particularly those capable of carrying photo-quality prints having a gloss or semi-gloss finish. Current photo-quality papers, although technically satisfactory, are coated papers which achieve their good results by the use of extremely expensive coating materials or processes, making them unsuitable for bulk use. Typically, the coatings present on such papers are based on swellable polymer coatings, micropigments based on silicates or aluminates, or coatings which receive a gloss finish as a result of a cast coating process. Silica is widely used as a pigment. All of these approaches provide good technical results, but are very expensive.

The results obtained when using papers designed for use in offset printers in inkjet printers are unsatisfactory. Such papers generally contain a coating comprising a pigment such as calcium carbonate together with a binder such as a styrene/butadiene latex.

GB 2,301,845 describes a recording sheet, especially a tracing sheet or an uncoated cellulose-based paper, for use in inkjet printers, wherein at least a usable surface includes a combination of a water soluble cationic substance, for example a soluble polyvalent metal salt (for example magnesium sulfate) or a cationic polymer, and a water soluble binder. Suitable binders are polyvinyl pyrrolidone, polyvinyl alcohol, starches, and gelatine.

WO 00/44568 describes a recording sheet for use on a digital press which includes either an insoluble mineral filler including a Lewis acid, for example insoluble aluminium trihydrate, in the base paper, or a soluble or insoluble metal from Groups II and III or the Transition metals, for example magnesium sulfate, at the surface. EP 1775141 describes a similar recording sheet, in which a composition comprising a divalent metal salt and a starch is applied to the surface of a substrate comprising ligno cellulosic fibres.

JP 2005-066925 discloses a specific pigment which has very small nanoparticles, and which is prepared by complexing zinc into a calcium carbonate pigment (i.e. there is no free salt present in the final coating containing the pigment). The resulting pigment is used in the presence of a polymer system containing a minor proportion of hydrophilic polymers, together with an allylic polymer and cationic (polyamine) polymer.

WO 2003/104336 describes a coating composition comprising a blend of at least two dye fixatives, at least one of which is a cationic polymer, and at least one of which is a polyvalent metal salt. JP 2007-160746 describes an inkjet recording sheet consisting of an ink receptive layer comprising a water-insoluble resin and a water-soluble resin together with an inorganic pigment and a water-soluble magnesium salt. WO 2006/049545 describes a pigment composition in the form of an aqueous dispersion or slurry comprising porous aggregates and extender particles. WO 2006/116878 discloses a coating composition comprising a styrene acrylate copolymer; and organic cationic polyelectrolyte; at least one inorganic pigment; a binder; and wetting agent; and a lithium, sodium or potassium salt. US 2006/0172093 discloses an inkjet recording material having a pigment layer and an ink recording layer, both of defined composition. None of these documents discloses the use of a specific combination of ingredients having the particular characteristics of the present invention.

There remains a need for a photo-quality paper or paper-like product which can be used with good effect in inkjet printers and which can be manufactured at reasonable cost.

We have now found that the use of a very specific combination of ingredients in a coating provides an economical paper capable of carrying photo-quality prints delivered by an inkjet printer.

Accordingly, the present invention provides a sheet product comprising a substrate carrying a coating on at least one surface, characterised in that the coating includes (i) a pigment comprising calcium carbonate; (ii) a binder for component (i) comprising a major proportion of a polymer carrying -O-, -CO-, -OCO- and/or -COO- groups in its side-chains; and (iii) at least at the surface of said coating, a water-soluble salt of a Group II, Group III or transition metal.

Although other pigments in addition to calcium carbonate may be present in component (i) if desired, preferably no aluminate or silicate is present. In one embodiment of the invention, the pigment comprises a major proportion (i.e. at least 50% by weight), preferably at least 80% by weight, of calcium carbonate. For example, calcium carbonate may be the only pigment present. In another embodiment of the invention, one or more additional pigments may be present. In this embodiment, a calcined clay may be present, for example in an amount of from 10 to 40, preferably from 20 to 30% by weight, based on the combined weight of calcium carbonate and clay.

The calcium carbonate may be in the form of precipitated calcium carbonate or ground calcium carbonate, the latter being preferred. In general, component (i) will be derived from a dispersion, for example a dispersion of calcium carbonate containing appropriate dispersion stabilizers. Preferably the pigment has a mean particle size of 15 microns or less, for example 10 microns or less. In general, the smaller the particle size of the pigment, the higher the gloss obtainable on the sheet product. For a very high gloss finish, particles having a mean particle size of 5 microns or less, especially 2 microns or less, are preferred.

Preferably the coating on the sheet product according to the invention is free from pigments in microparticulate form, especially silicates and aluminates. Microparticulate pigments (often known as nanoparticulate pigments) are generally synthetic pigments prepared by precipitation from a solution. The primary mean particle size is generally less than about 50nanometers, especially less than 30nanometers, and typically 10-20 nanometers. The particles may, however, form agglomerates where the agglomerates have particle sizes of up to about 200-3 OOnanometers (i.e. 0.2-0.3microns). The pigment used in the present invention preferably has a mean particle size of greater than 0.2microns, preferably greater than 0.3microns.

The binder used in the present invention comprises a major proportion (i.e. at least 50% by weight), and preferably at least 80% by weight, of the specified binders, which are hydrophilic binders. Suitable binders for use in component (ii) include polyvinyl acetate, vinyl acetate copolymers, polyvinyl alcohol, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, sodium alginate or starch, or mixtures thereof. Preferably the binder is used in the form of a latex, i.e. in the form of a stabilised emulsion 5 comprising either liquid droplets or solid particles dispersed in a continuous phase. Vinyl polymers are preferred, with the use of polyvinyl acetate, especially self crosslinking vinyl acetate homopolymer, being preferred. Two or more different binders of the type specified may be present, for example the binder may comprise a major proportion of polyvinyl acetate together with a minor proportion of another of the specified binders, for example 0 polyvinyl alcohol (which has the effect of enhancing the efficiency of optical brighteners) or a cellulose derivative such as hydroxyethyl cellulose or sodium carboxymethyl cellulose (which is effective as a rheology modifier and a water retention aid). Alternatively, polyvinyl acetate may be the only binder present in component (ii).

5 The specified polymers are generally anionic in nature, and therefore the coatings used in the present invention preferably contain only a low proportion, or no, cationic polymer. Preferably the weight ratio in the coating of cationic polymer to the polymer carrying -O-, -CO-, -OCO- and/or -COO- groups in its side-chains is less than 1:1, especially less than 1 :2, most preferably less than 1 :5. 0

The ratio of component (i) to component (ii) may vary within conventional limits, for example with component (i) comprising at least 55% by weight, for example from 65 to 95%, especially from 75 to 95%, by weight based on the combined weight of components (i) and (ii). 5

Component (iii) may be any water-soluble salt of a metal from Group II or Group III or a transition metal. Salts of Group II or Group III metals or first row transition metals, for example calcium, magnesium, aluminium, manganese or zinc, especially calcium, magnesium or manganese, are preferred. The salt may for example be a halide, sulfate or O nitrate, for example calcium, magnesium or manganese sulfate or nitrate. The use of calcium chloride, manganese sulfate or, especially, magnesium sulfate, is preferred. Preferably the quantity of water-soluble salt present is in the range of from 2 to 35, preferably 5 to 25, more preferably 10 to 20, parts by weight per hundred parts dry weight of calcium carbonate.

Thus, a preferred sheet product according to the present invention comprises (i) a pigment comprising at least 80% by weight of calcium carbonate; (ii) a binder for component (i) comprising at least 80% by weight of vinyl acetate; and (iii) at least at the surface of said coating, a water-soluble salt of a Group II, Group III or transition metal salt, especially magnesium sulfate, preferably in an amount of from 5 to 25 parts by weight per hundred parts dry weight of calcium carbonate.

The coating of the sheet product according to the invention may consist of a single layer, or it may contain two or more separate layers. The binder of component (ii), e.g. polyvinyl acetate, acts as a binder for the pigment of component (i), and must therefore be present in the same layer as the pigment. Component (iii) may be present in the same layer, or a different layer, provided that it is present at the surface of the overall coating. In the simplest embodiment of the invention, all three of components (i), (ii) and (iii) are present in a single layer. In a further embodiment, two (or more) layers may be present, with component (iii) being present in at least the layer furthest away from the surface of the substrate. The use of a single layer including components (i) and (ii) (and optionally component (iii)) is possible, or two (or more) such layers may be present. A higher gloss appearance is obtained if a first layer containing at least components (i) and (ii) adjacent the surface of the substrate is covered with a second layer containing components (i) and (ii), the mean particle size of the pigment in said second layer being smaller than the mean particle size of the pigment in said first layer. The gloss of the sheet can also be readily manipulated by processing after application of the coating, for example by calendering.

A solution of component (iii) may if desired be applied on top of one or more layers comprising components (i) and (ii) to produce the finished coating. This solution may be a simple aqueous solution of component (iii), or it may contain other additives, for example one or more viscosifiers. Many substances conventionally used as binders may also be used as viscosifiers. Thus the solution of component (iii) may if desired contain additional binder (acting as a viscosifier) in addition to the binder already present in association with the pigment. If additional binder is present, this may the same binder as that of component (ii), or a different binder.

The coating or any layer comprised in the coating may comprise any other desired additive which does not interfere with the performance of the finished paper. The coating may for example contain optical brighteners, colourants, dispersants, surfactants, defoaming agents, preservatives, water retention aids and pH control agents. Some of these ingredients may have multiple functions, for example, certain water retention aids may also function as binders. Starch may be present, but preferably said coating is free from starch.

The substrate of the sheet product of the invention is preferably paper, although plastic sheet materials which simulate the properties of paper (so-called "synthetic paper") can alternatively be used. The substrate may be of any desired weight or quality suitable for use in inkjet printers. The weight or quality is generally determined by the intended application. For example, business forms and transactional documents may have a low substrate weight such as 45 to 60g/m², giving after coating a finished weight of around 60 to lOOg/m². Papers aimed at promotional print may have finished weights around, say, 135g/m², and those aimed at publishing grades will be higher, say, around 160g/m². Board weights may be used for applications such as covers, menus, greetings cards, and packaging, and in this case the substrate could be in the range of 250 to 300g/m² or higher.

Different types of inkjet printers are used for different substrate weight ranges. The finished sheet according to the invention may if desired be incorporated into a composite, for example to form a label, or laminated, for example to produce a heavyweight board, for example for packaging.

The coating or any layer comprised in the coating may be applied to the substrate by any conventional method, for example by size press treatment, dip coating, roll coating, extrusion coating, airknife coating, curtain coating or, especially, blade coating. If present, a separate aqueous solution of component (iii) may additionally be applied as a wash or a spray.

The invention also provides a method of printing an image, which comprises applying water-based ink to a sheet product according to the present invention, using an inkjet printer and a water-based ink. Any water-based inkjet ink may be used in association with the sheet product of the present invention, for example a dye-based ink or a pigment-based ink, with the use of pigment-based inks generally being preferred.

The invention further provides the use of a combination of

(i) a pigment comprising calcium carbonate; (ii) a binder for component (i) comprising a major proportion of a polymer carrying -O-, -CO-, -OCO- and/or -COO- groups in its side- chains; and (iii) a water-soluble salt of a Group II, Group III or transition metal salt; in a coating for a sheet product, in order to render said sheet product suitable for use in an inkjet printer using a water-based ink.

The following Examples illustrate the invention.

Example 1

A paper sheet coated with a pre-coat layer of the composition shown in Table I was prepared as follows.

Table I

Polyvinyl alcohol (Mowiol 4-98 (Trade Mark) manufactured by Kuraray Co., Ltd) was prepared in water at 25% solids at 90⁰C. The mixture was allowed to cool and a portion of 0.07 parts by weight of caustic soda was added to 0.8 parts by weight of the polyvinyl alcohol. A portion of 90.2 parts by weight of ground calcium carbonate (Carbital 60 (Trade Mark) supplied by Imerys as slurry of 77% solids and having a 60% content of particles less than 2 microns and a mean particle size (diameter) of 1.83 microns) was added to the liquid mixture and allowed to stir for 10 minutes.

A portion of 9 parts by weight of a water based dispersion (a latex) of self cross-linking vinyl acetate homopolymer (Emultex 378 (Trade Mark) supplied by Synthomer Ltd) was added. The mix was stirred for 10 minutes and 0.07 parts by weight of a polyacrylic thickener (Viscolam 30 (Trade Mark) supplied by Lamberti SpA) was added followed by 0.24 parts by weight of optical brightener (Optiblanc KLN (Trade Mark)) and 0.03 parts of antifoam (Nopcomaster (Trade Mark). The mix was allowed to stir for 30 minutes. The aqueous mix was coated in a dry solids amount of 20g/m² on one side of the paper sheet using conventional blade coating.

The resulting sheet was provided with a further, glossy, ink receptive layer, of the composition shown in Table II. Table II

The polyvinyl alcohol was prepared in water at 25% solids at 9O⁰C. The mixture was allowed to cool and a portion of 0.4 parts by weight of the polyvinyl alcohol was added to a portion of 12.2 parts by weights of a 10% magnesium sulfate solution.

A portion of 81 parts by weight of ground calcium carbonate (Setacarb 80 (Trade Mark) supplied by OMYA as a slurry of 76% solids and having an 80% content of particles less than 2 microns and a mean particle size of 0.73 microns) was added to the liquid mixture and it was allowed to stir for 10 minutes.

A portion of 0.2 parts of calcium stearate was added as a lubricant to give the final coating anti-slip properties. To this mixture, a portion of 0.08 parts by weight of caustic soda was added followed by 5.3 parts by weight of Emultex 378. The mix was stirred for 10 minutes and 0.02 parts by weight of Viscolam 30 was added followed by 0.3 parts by weight of Optiblanc KLN. The mix was allowed to stir for 30 minutes, and then coated by blade in a dry solids amount of 5g/m on top of the previously coated pre-coat layer. This was followed by a supercalendering process to give a gloss measurement of 46% at 75 degree angle using a Gardner 75° micro-gloss meter.

Example 2 The following coating mix was prepared in the laboratory showing an ink receiving layer consisting of 85 parts by weight of calcium carbonate (Setacarb 80 (Trade Mark) supplied by OMYA as a slurry of 76% solids and having an 80% content of particles less than 2 microns) and 15 parts by weight of a water based dispersion (a latex) of self cross-linking vinyl acetate homopolymer (Emultex 378 (Trade Mark) supplied by Synthomer Ltd),followed by a non-pigmented layer containing a salt as described in Table III.

Table III

The Mowiol 4-98 was prepared in water at 25% solids at 9O⁰C. The mixture was allowed to cool and a portion of 20 parts by weight of the polyvinyl alcohol was added to a portion of 71 parts by weights of a 12% magnesium sulfate solution. 12 parts of an aqueous, anionic solution of an acrylic copolymer was added (Sterocoll Bl (Trade Mark) as supplied by BASF) to adjust the rheology of the mix. The final mixture was coated at 1.5 g/m²by rod on top of the initial coating one side of the substrate sheet.

Example 3 An ink-receiving layer was applied to one side of a paper sheet as described in Example 1. This was carried out at laboratory scale using scaled-down quantities of materials. The resulting sheet was provided with a further layer of the composition shown in Table IV. Table IV

The polyvinyl alcohol was prepared in water at 25% solids at 9O⁰C. The mixture was allowed to cool and a portion of 0.4 parts by weight of the polyvinyl alcohol was added to a portion of 6 parts by weights of a 5% manganese sulfate solution.

A portion of 88 parts by weight of Setacarb 80 was added to the liquid mixture and it was allowed to stir for 10 minutes. A portion of 0.2 parts of calcium stearate was added as a lubricant to give the final coating anti-slip properties. To this mixture, a portion of 0.09 parts by weight of caustic soda was added followed by 6 parts by weight of Emultex 378. The mix was stirred for 10 minutes and 0.02 parts by weight of a Viscolam 30 was added followed by 0.4 parts by weight of optical brightener. The mix was allowed to stir for 30 minutes, and then coated in a dry solids amount of 5g/m² on top of the previously coated pre-coat layer on one side of the substrate sheet.

Example 4

Samples of paper previously machine coated with a layer of a composition corresponding to Table I above, prepared as described in Example 1, were provided with a further layer of the composition shown in Table V. This second coating was applied in the laboratory.

Table V

Hydroxyethylcellulose (QP-100 MH Cellosize (Trade Mark), a nonionic water-soluble polymer derived from cellulose supplied by Dow Chemical Company Ltd) was prepared in cold water at 12% solids. A portion of 4 parts by weight of the polyvinyl alcohol was added to a portion of 12.5 parts by weight of a 10% magnesium sulfate solution.

A portion of 83 parts by weight of Setacarb 80 (Trade Mark) was added to the liquid mixture and it was allowed to stir for 10 minutes before being coated in a dry solids amount of 5g/m² on top of the previously pre-coated layer.

Example 5

Samples of paper previously machine coated with a layer of a composition corresponding to Table I above, prepared as described in Example 1 , were provided with a further layer of the composition shown in Table VI. This second coating was applied in the laboratory.

Table VI

A portion of 75 parts by weight of Setacarb 80 was added to a portion of 11 parts by weight of a 10% magnesium sulfate solution.

Partially hydrolysed polyvinyl alcohol, Mowiol 40-88 (Trade Mark, manufactured by Kuraray Co., Ltd) was prepared in water at 12% solids at 9O⁰C. The mixture was allowed to cool and a portion of 13.5 parts by weight of the polyvinyl alcohol was added to the mixture and allowed to stir for 30 minutes. The mix was coated in a dry solids amount of 5g/m² on top of the previously pre-coated layer on one side of the substrate sheet. Example 6a (comparative)

A paper sheet was coated on one side only at laboratory scale with a simplified version of a typical offset litho recipe. This consisted of 85 parts by weight of calcium carbonate (Setacarb 80 (Trade Mark) supplied by OMYA as a slurry of 76% solids and having an 80% content of particles less than 2 microns) and 15 parts by weight of styrene butadiene copolymer Latex, SB 083 (supplied by Latexia). The coat weight was 20gsm.

Example 6b

A similar coating was prepared as Example 6a except that styrene butadiene copolymer Latex, SB 083 was replaced by a water based dispersion (a latex) of self cross-linking vinyl acetate homopolymer (Emultex 378 (Trade Mark) supplied by Synthomer Ltd) and salt (magnesium salt) was added in ratio of 15 parts of salt to 100 parts of pigment.

Example 7 A paper sheet was coated on one side only at laboratory scale with a coating mix prepared according to the formulation shown in Example 1 , Table II. The coatweight was 20gsm.

Example 8

A paper sheet was coated on one side only at laboratory scale with a coating mix prepared according to the formulation shown in Example 1, Table II, except that the Emultex 378 was replaced by Mowiol 40-88. The coatweight was 20 gsm.

Example 9: Print quality evaluation

The products produced in Examples 1 to 8, together with a conventional product intended for use in an offset litho printer as a comparison (Example 6a) were printed on an inkjet printer (Kodak Easyshare Printer 5300) using water-based inks.

Coated papers intended for printing by offset litho are generally unsuitable for water based inkjet printing. Demanding images such as pink or green text on a trichromatic black background provide a demanding test due to the need to mix a number of inks to produce both the background and foreground colours. This results in a relatively high coverage of the different inks. Therefore this type of image was used in the testing. The conventional offset paper demonstrated poor absorbance of the ink and irregular wetting of the coating surface. Poor definition of the text, ragged edges to the characters, and bleed of the colours into the surrounding black and vice versa, was observed. This caused the print to be distorted. Also, the distribution of the colour was patchy and uneven. In the case of the trichromatic black, this appeared as severe mottle or graininess, as though a black watercolour paint had been applied to polythene and allowed to reticulate as it dried.

The results were quantified on a visual scale by assigning a zero to the comparison product. A score of 100 would be given to a perfect print comprising text with vivid, intense colours showing no distortion or bleed, with well defined edges and a background with excellent uniformity of ink coverage and no sign of mottle or graininess. The results are shown in Table VIII. It can be seen that the products made according to the present invention all gave acceptable results, much improved over the comparison product.

Table VII

Example IQa A paper sheet was coated on one side with a coating mix prepared according to the formulation shown in Table VIII. Coating weights of 5, 10, 15 and 20g/m² were obtained. This example contains clay in addition to the main pigment, calcium carbonate. The clay used was a coating grade, KI 1020-2 supplied in slurry form by Kaolin International B.V., containing solids having a mean particle size of 5.04 microns.

Table VIII

Example IQb

A coating formulation and coating were prepared as in Example 10a. The Kaolin KI 1020 was replaced by Opacarb A40. This is a precipitated calcium carbonate supplied in slurry form by Speciality Minerals, based in Belgium, containing solids having a mean particle size of 0.62 microns.

Example IQc A coating formulation and coating were prepared as in Example 10a. The Kaolin KI 1020 was replaced by Kaocal. This is a calcined clay supplied in slurry form by Thiele Kaolin Company of Sandersville, GA, USA., containing solids having a mean particle size of 3.48 microns.

Example IQd (comparative)

This formulation was selected to represent a typical coated paper used in offset litho. It contained 90 parts of Setacarb 80 and 10 parts of Kaolin KI 1020. A styrene butadiene copolymer latex, SB083 (supplied by Latexia) was used as binder at a level of 10 parts. Polyvinyl alcohol, 0.4 parts, was added since this small level is typically used in these coatings to enhance the effectiveness of optical brightening agents. No salt was added. Example 11: Print quality evaluation

The four coated papers prepared as described in Example 10 were printed using an Epson Stylus D 120 inkjet printer using water based inks.

The papers containing polyvinyl alcohol as binder and magnesium sulfate gave prints which were dry to the touch as the printed papers emerged from the printer. The coated paper produced according to Example 1Od had very poor ink drying properties and the prints could readily be smeared even after a delay of 10 minutes. This would not be acceptable on fast commercial ink jet presses where papers are often two-side printed and wound up in rolls.

The print qualities were good for examples 10a, 10b and 10c. Example 1Od showed poor infill of printed lines due to poor wetting of the coating surface by the ink.

Claims

1. A sheet product comprising a substrate carrying a coating on at least one surface, characterised in that the coating includes (i) a pigment comprising calcium carbonate; (ii) a

5 binder for component (i) comprising a major proportion of a polymer carrying -O-, -CO-, -OCO- and/or -COO- groups in its side-chains; and (iii) at least at the surface of said coating, a water-soluble salt of a Group II, Group III or transition metal.

2. A sheet product as claimed in claim 1, in which the pigment comprises a major 0 proportion of calcium carbonate.

3. A sheet product as claimed in claim 2, in which calcium carbonate is the only pigment present. 5

4. A sheet product as claimed in any one of the preceding claims, in which the calcium carbonate is in the form of ground calcium carbonate.

5. A sheet product as claimed in any one of the preceding claims, in which the pigment has a mean particle size of 15 microns or less. 0

6. A sheet product as claimed in any one of the preceding claims, in which the pigment has a mean particle size of greater than 0.2 microns.

7. A sheet product as claimed in any one of the preceding claims, in which the binder 5 comprises at least 80% by weight of a polymer carrying -O-, -CO-, -OCO- and/or -COO- groups in its side-chains.

8. A sheet product as claimed in any one of the preceding claims, in which the binder comprises polyvinyl acetate, a vinyl acetate copolymer, polyvinyl alcohol, carboxymethyl O cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, sodium alginate, and/or starch.

9. A sheet product as claimed in claim 8, in which the binder comprises polyvinyl acetate.

10. A sheet product as claimed in any one of claims 1 to 9, in which component (iii) comprises a salt of calcium, magnesium, aluminium, manganese or zinc.

11. A sheet product as claimed in claim 10, in which the salt is calcium chloride, manganese sulfate or magnesium sulfate.

12. A sheet product as claimed in claim 11, in which the salt is magnesium sulfate.

13. A sheet product as claimed in any one of the preceding claims, in which the quantity of water-soluble salt present is in the range of from 2 to 35 parts by weight per hundred parts dry weight of calcium carbonate.

14. A sheet product as claimed in claim 13, in which the quantity of water-soluble salt present is in the range of from 5 to 25 parts by weight per hundred parts dry weight of calcium carbonate.

15. A sheet product as claimed in any one of the preceding claims, in which said coating contains two or more separate layers, component (iii) being present in at least the layer furthest away from the surface of the paper substrate.

16. A method of printing an image, which comprises applying water-based ink to a sheet product as claimed in any one of claims 1 to 15, using an inkjet printer and a water-based ink.

17. A method as claimed in claim 16, in which a pigment-based ink is used.

18. The use of a combination of (i) a pigment comprising calcium carbonate; (ii) a binder for component (i) comprising a major proportion of a polymer carrying -O-, -CO-, -OCO- and/or -COO- groups in its side-chains; and (iii) a water-soluble metal salt of a Group II, Group III or transition metal; in a coating for a sheet product, in order to render said sheet product suitable for use in an inkjet printer using a water-based ink.