WO2006011799A1

WO2006011799A1 - Recording medium

Info

Publication number: WO2006011799A1
Application number: PCT/NL2005/000555
Authority: WO
Inventors: Jacko Hessing; Willem Johannes Van Baak; Akira Kase
Original assignee: Fuji Photo Film B.V.
Priority date: 2004-07-30
Filing date: 2005-07-29
Publication date: 2006-02-02

Abstract

The present invention relates to a recording medium, in particular an ink-jet recording medium of photographic quality that has good curling behaviour. According to the present invention a recording medium is provided, comprising a support and a receiving layer adhered to said support, wherein the receiving layer comprises an overlayer and an underlayer, wherein said overlayer comprises at least one gelatin and at least one polyvinyl alcohol (PVA) based polymer with a weight ratio of gelatin / PVA based polymer from 1 / 1 to 1 / 3 and said underlayer comprises at least one gelatin and at least one PVA based polymer with a weight ratio of gelatin to polymer from 5 / 1 to 2 / 1. The present invention is further directed to methods for obtaining and using such a medium.

Description

Title: Recording medium

The invention Field of invention

The present invention relates generally to a recording medium, in particular an ink -jet recording medium of photographic quality that has excellent ink absorption speed, a good image printing quality and good drying characteristics, in particular an improved curling behaviour, as well as to methods for preparing and using such media.

Background of the invention In a typical ink-jet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye, and a relatively large amount of solvent in order to prevent clogging of the nozzle. The solvent, or carrier liquid, typically is made up of water and organic material such as monohydric alcohols. An image recorded as liquid droplets requires a receptor on which the recording liquid dries quickly without running or spreading. High quality image reproduction using ink -jet printing techniques requires receptor supports, typically sheets of paper or opaque or transparent film, that readily absorb ink droplets while preventing droplet diffusion or migration. Good absorption of ink encourages image drying while minimizing dye migration by which good sharpness of the recorded image is obtained.

A very important property of inkjet media is the image quality with aspects such as vivid images, good whiteness, high gloss, good behaviour on bleed, beading and matte appearance at high density parts, and a good lightfastness, viz. the printed images must not fade over longer periods of time.

Other important aspects are the physical properties such as a pleasant non-sticky feel, good adhesion properties, low curl and brittleness and no cockling. A general problem with many inkjet recording sheets is their curling behaviour. A change in relative humidity of the environment often results in a contraction or a relaxation of the coated layers in a different way than the support. Especially at high or low humidities curling of the sheet can cause jamming in the printer. Curling can also occur after printing which is undesired by the end user.

Another phenomenon related to humidity is cockling. Cockling can be described as a wave-like appearance, or as wrinkled or puckered appearance. Occurring after printing the cause can be found in differences in relaxation of the polymers in the ink receiving layer between printed and non- printed parts of the sheet.

In the prior art several attempts can be found to improve the curling and cockling behaviour of inkjet recording sheets.

A generally encountered technique is the coating of a polymer containing layer on the backside of the substrate, as is suggested in

WO-A-99/65700, EP-A-791475 and in US-A-2002/182 376. By balancing the coated amount the curling of the ink receiving layer can be counteracted by the backside layer. In general applying a backside layer does not directly improve the cockling behaviour because cockling is usually caused by local differences in curling between different areas of the inkjet sheet.

EP-A-856414 suggests to coat the ink receiving layer at both sides of the support to prevent curling.

EP-A-O 524635 claims to inhibit the occurrence of curl under a wide range of environmental conditions by providing a backcoat layer containing a platy inorganic pigment having a high aspect ratio; also cockling is claimed to be improved.

US-A-6 406 775 attributes the reduction of curl to the addition of a non-ionic surfactant to the toplayer, whereas US-A-5 352 503 claims a beneficial effect from incorporating a polyether such as polyethyleneglycol into the coating mixture. A completely different approach is described in US-A-5 635 969 in which the curling is prevented by a special printer construction with a separate printhead for applying a precursor liquid onto the medium surface to condition that surface prior to applying the ink. WO-A-98/52765 describes a single ink-receiving layer comprising fish gelatin and alkylcelluloses or PVP derivatives to obtain a glossy ink- receiving medium without cockling or curling problems. A sealing layer is present to prevent ink penetration from the toplayer to the paper substrate.

An adequate solution to the problem of curling and cockling that can be applied to a variety of inkjet recording media has not yet been found.

It is the aim of this invention to provide formulations for recording media in which the occurrence of curling and cockling is prevented without having to accept a deterioration of other quality aspects.

Summary of the invention

The object of the present invention is to provide a recording medium having good overall properties, said recording medium more in particular being suited to produce images of photographic quality, wherein said medium has improved physical properties. It is a further objective of this invention to provide a recording medium where the medium has an improved curling as well as a good resistance against cockling.

At the same time it is desirable that the media of the present invention maintain other favourable properties with respect to good drying properties, good gloss, good image printing quality, long-lasting stability against dye fading by ozone or light, having at the same time good behaviour on bleed, beading and matte appearance at high density parts.

It was found that these objectives can be met by providing a recording medium comprising a support and an ink-receiving layer adhered to said support, wherein the ink-receiving layer comprises an overlayer and an underlayer (viz. a layer that is closer to the support than said overlayer), wherein said overlayer comprises at least one gelatin and at least one polyvinyl alcohol (PVA) based polymer with a weight ratio of gelatin to polymer from 1/1 to 1/3 and said underlayer comprises at least one gelatin and at least one polyvinyl alcohol (PVA) based polymer with a weight ratio of gelatin to polymer from 5/1 to 2/1.

Detailed description

The invention is directed to a recording medium comprising a support and a receiving layer adhered to said support, wherein the receiving layer comprises an overlayer and an underlayer, wherein said overlayer and said underlayer both comprise at least one gelatin and at least one PVA based polymer with a weight ratio of gelatin to polymer from 0.2 to 5. This invention is also related to the manufacturing of such a recording medium and the use of this medium.

Although the invention is described herein with particular reference to inkjet printing, it will be apparent to the skilled person that the high quality recording media of the present invention are not limited to inkjet recording media (viz. media suitable to be printed on using inkjet printers), but that it is within the scope of the present invention to provide recording media that are suitable for creating high quality images by using other techniques as well, such as Giclee printing, colour copying, screen printing, gravure, dye- sublimation, flexography, and the like.

Both the overlayer and the underlayer of this invention may be a multilayer of sublayers. The total number of sublayers is not particularly limited and depends largely on the available technique for application of layers and the required ink receiving properties of the ink receiving layer. The total number of sublayers may be from 2 to 25, more preferably from 2 to 17. The phenomenon of curling is related to the contraction of the polymers in the receiving layer during the drying process and is further influenced, by the relative humidity of the environment during storage of the recording medium. As described in e.g. US-A-5 352 503 the addition of certain softening compounds such as polyethylene glycol can have a beneficial effect by lowering the extent of contraction upon drying. Other compounds that can be applied as softener are glycerol, die thy lene glycol, triethanolamine, ethylene glycoldiacetate and the like. Our research has shown that such compounds do have a positive effect on curling but on the other hand negatively influence the long term bleeding behaviour, as these compounds enhance dye diffusion, especially at high humidity conditions. Surprisingly during our research it was found that it is not necessary to add softening agents to improve the curling. Restricting the ratio of gelatin and water soluble polymer was found to be a good method to achieve a good curling behaviour. Ratios between 5 tol and 1 to 5 are giving good results; the optimum ratio depends on the type of gelatin and the type of water soluble polymer used. When the gelatin/polymer ratio is less than 0.2, the impact of the gelatin becomes low. The gelatin plays a role in the network throughout the layer structure of the final product. On the other hand, when the gelatin/polymer ratio is more than 5 the curling becomes increasingly worse as the influence of the polymer is small. Without wishing to be bound by theory the relaxation effect that is the basis of the improved curling is thought to be caused by shielding the gelatin molecules from each other. The water soluble polymer molecules reduce the interaction between the gelatin strands. Thus the best results are obtained with those water soluble polymers that have no or little interaction with gelatin. This mechanism may work with mixtures of other polymers than gelatin as well.

Since not all water soluble polymers that are suitable to be used are very well miscible with gelatin, for some water soluble polymers the suitable ratios are further restricted. The optimum ratio can be determined experimentally for each specific water soluble polymer and gelatin type used, as shall be further illustrated hereinbelow. Other factors that may be given consideration include lightfastness, beading and bleeding.

A phenomenon related to curling is cockling. Cockling is the result of different curling behaviour between the non-printed areas and the printed areas where ink has been applied to the recording sheet. Usually ink contains several humectants such as glycerol to inhibit the evaporation of solvent. These humectants do not evaporate upon drying but are absorbed by the sheet and locally have a relaxing effect on the gelatin molecules comparable to the effect described above. As a result the areas that are relaxed by the humectant show less curling behaviour than the non-printed areas. Because the curling of the unprinted sheet is compensated for in the manufacturing process by applying onto the sheet a backside coating containing polymers, the compensation of curl at the printed areas becomes too strong, leading locally to a negative curl, while the non-printed areas generally show no curl or a only a slight positive curl. The result is a sheet having areas with positive curl and areas with negative curl, particularly under conditions of low humidity. According to the present invention the curling tendency of the receiving layer of the unprinted sheet is reduced, making it possible to reduce the thickness of the backside coating. Not only is this economically favourable, but also the magnitude of the cockling will be improved by this. The backside coating comprises gelatin or a water soluble polymer in an amount ranging preferably from 1 to 20 g/m², more preferably from 4 to 15 g/rα², even more preferably from 5 to 13.5 g/m², e.g. about 10 g/m². The optimum amount of the backside coating depends on the type of gelatin, the type of water soluble polymer and on the composition of the layers at the ink receiving side of the medium. The preferred polymer for the backside coating is gelatin.

A distinction can be made between the optimal ratio of gelatin and water soluble polymer for the overlayer - which largely determines factors like beading and gloss — and the optimal ratio for the underlayer - which influences properties such as drying speed. In this respect, it is noted that the types of gelatin and the types of water soluble polymer need not to be the same for the overlayer and the uiiderlayer. For the over layer the ratio of gelatin and water soluble polymer is preferably between 4:1 and 1:4, more preferably between 1:1 and 1:3. For the underlayer the ratio of gelatin and water soluble polymer is preferably between 5:1 and 1:3, more preferably between 5:1 and 2:1. In case the overlayer and/or the underlayer are a multilayer of sublayers the ratio of gelatin and water soluble polymer may be different for each sublayer. But the ratio for each sublayer falls within the ranges as specified above. Also the type of water soluble polymer and the type of gelatin may differ between sublayers.

The gelatin used in the receiving layer can be any gelatin, whether lime-processed or acid processed, hydrolysed or modified, made from animal collagen, preferably gelatin made from pig skin, pig bone, cow skin, cow bone or fish gelatin. Examples of modified gelatins are amino group deactivated gelatin such as acetylated gelatin, phthalated gelatin, succinated gelatin, quaternary ammonium modified gelatin, et cetera.

The gelatin is preferably applied to the substrate for the overlayer in an amount ranging from 0.1 to 10 g/m²,more preferably from 0.2 to 8.0 g/m² and for the underlayer from 0.2 to 12.0 g/m², more preferably from 0.4 to 9.0 g/m².

Water soluble polymers that are suitable for using in the receiving layer of the invention include polyvinyl alcohol- (PVA-)based polymers, such as fully hydrolysed or partially hydrolysed PVA, carboxylated PVA, acetoacetylated PVA, quaternary ammonium modified PVA, copolymers and terpolymers of PVA with other polymers, watersoluble cellulose derivatives such as alkyl cellulose {e.g. methyl cellulose), hydroxy alkyl cellulose {e.g. hydroxyethyl cellulose or hydroxypropyl cellulose), carboxyalkyl cellulose {e.g. carboxymethyl cellulose), dextrin, casein, gum arabic, dextran, poly aery lie acid and its copolymers or terpolymers, polymethylacrylic acid and its copolymers or terpolymers, and any other polymer, which contain monomers of carboxylic acids such as acrylic acid, methacrylic acid, maleic acid and crotonic acid, polyvinylpyrolidone (PVP), polyacrylamide, polymers of 2-pyrrolidone and its derivatives such as N-(2-hydroxyethyl)-2-pyrrolidone and N-cyclohexyl-2- pyrrolidone, urea and its derivatives such as imidazolidinyl urea, diazolidinyl urea, 2-hydroxyethylethylene urea, and ethylene urea.

Most preferred are PVA-based polymers. In general a large variety of PVA-based polymers can be used, but the preferred PVA-based polymers are those which have been modified to give a good miscibility with aqueous gelatin solutions. These modifications are such, that in the PVA-based polymer back bone groups are introduced which provide a hydrogen bonding site, an ionic bonding site, carboxylic groups, sulphonyl groups, amide groups and the like, thus providing a modified PVA-based polymer. A modified PVA-based polymer giving very good results is a poly(vinyl alcohol) -co-poly (n- vinyl formamide) copolymer (PVA-NVF). Very suitable PVA-NVF copolymers for use with the present invention are the copolymers described in WO-A-03/054029, which have the general formula I:

0 OH N-R₁ N-R₃ NH₂

C=O C=O C=O

1 I I

CH3 R2 R4 ,J, wherein n is between 0 and about 20 mole percent; m is between about 50 and about 97 mole percent; x is between 0 and about 20 mole percent; y is between 0 and about 20 mole percent; z is between 0 and about 2 mole percent and xή-y is between about 3 and about 20 mole percent; R₁ and R3 are independently H, 3-propionic acid or Ci-Cβ alkyl ester thereof, or is 2-methyl-3-propionic acid or C₁-Ce alkyl ester thereof; and K.2 and R4 are independently H or Ci-Cβ alkyl.

The water soluble polymer is preferably applied for the overlayer in an amount ranging from 0.5 to 15 g/m², more preferably from 1.0 to 10.0 g/m² and for the underlayer from 0.5 to 15 g/m², more preferably from 1.0 to 8.0 g/m².

For optimising the curl and cockling behaviour of the recording medium the following steps can be taken:

Determining the optimum ratio of gelatin and water soluble polymer for the underlayer and for the overlayer, selected to fulfil all quality requirements of the medium.

- Tuning the quantity - and if desired the composition - of the backside coating.

To give an example of this optimising process, for the underlayer, for instance, low priced ingredients are preferred that fulfil the basic requirement of liquid absorption. Suitable gelatins are alkali-treated bone gelatins that are produced in large volumes and are readily commercially available. Suitable water soluble polymers are polyvinylalcohol (PVA) based polymers, such as fully or partially hydrolysed PVA, cellulose derivatives, such as hydroxymethylcellulose, hydroxyethylcellulose and carboxymethylcellulose and PVP. Good miscibility is obtained with a ratio higher than 2:1, so for these compounds the ratio gelatin / water soluble polymer is preferably selected between 5:1 and 2:1, e.g. 4:1. From the point of view of cost, a partially hydrolysed PVA is preferred. For the overlayer on the other hand, the quality requirements are different, making it necessary to select other types of gelatin and water soluble polymer. Parameters such as gloss, beading, bleeding, dye stability against ozone and light fading, are to a large extent related to the overlayer properties. To fulfil these quality requirements, a higher content of water soluble polymer is desired. Therefore a water soluble polymer having a good miscibility with gelatins is preferred. An example of such polymer is a PVA-NVF copolymer that has a good miscibility with all kinds of gelatin. The preferred ratio of gelatin / water soluble polymer is between 1:1 and 1:3, e.g. 1:2.

After determining the optimal composition and ratio of gelatin and water soluble polymer the next step is the tuning of the back side coating. A preferred polymer for the back side coating is a low priced gelatin. As is shown in the examples described below, reducing the ratio of gelatin and water soluble polymer in the underlayer from 6:1 to 4:1 makes it desirable to decrease the dry coated amount at the back side from 13.5 to 10 g/m², to maintain a good curling behaviour. By this change the cockling behaviour improved significantly.

The overlayer may further comprise water insoluble particles, inter alia to regulate the slip behaviour and optionally surfactants and other additives to optimise the surface properties. A further improvement can be obtained by including in the overlayer one or more fluorosurfactants, preferably in the amount between 2.5 mg/m² and 250 mg/m². It was found that this kind of surfactants improves amongst others the gloss and beading. Beading is defined as the phenomenon that large ink dots become visible on the printed image. The mechanism of "beading" is not clear yet. One hypothesis is that several small ink drops coalesce with each other on the surface of the ink jet media and form large ink droplets.

The term "fluorosurfactant" as used herein, refers to surfactants {viz. molecules having a hydrophilic and a hydrophobic part) that contain fluorocarbon or a combination between fluorocarbon and hydrocarbon as the hydrophobic part. Suitable fluorosurfactants may be anionic, non-ionic or cationic. Examples of suitable fluorosurfactants are: fluoro C2-C20 alkylcarboxylic acids and salts thereof, disodium N-perfluorooctanesulfonyl glutamate, sodium 3-(fluoro-C6-Cπ alkyloxy)-l-C3-C4 alkyl sulfonates, sodium 3-(omega -fluoro-Ce-Cs alkanoyl-N-ethylamino)-l-propane sulfonates, N-[3- (perfluorooctanesulfonamide)-propyl]-N,N-dimethyl-N-carboxymethylene ammonium betaine, perfluoro alkyl carboxylic acids (e.g. C7-Ci3-aIkyl carboxylic acids) and salts thereof, perfluorooctane sulfonic acid diethanolamide, Li, K and Na perfluoro C4-C12 alkyl sulfonates, Li, K and Na N-perfluoro C₄-C₁S alkane sulfonyl— N- alkyl glycine, fluorosurfactants commercially available under the name Zonyl^® (produced by E.I. Du Pont) that have the chemical structure of RfCH₂CH₂SCH₂CH₂CO₂Li or RfCH₂CH₂O(CH₂CH₂O)XH wherein Rf = F(CF₂CF₂)_3-8 and * = 0 to 25, N-propyl- N-(2-hydroxyethyl)perfluorooctane sulfonamide, 2-sulfo-l,4- bis(fluoroalkyl)butanedioate, 1,4-bis (fluoroalkyl)-2-[2-N,N,N- trialkylammonium) alkyl amino] butanedioate, perfluoro Ce-C₁O alkylsulfonamide propyl sulfonyl glycinates, bis-(N-perfl.uorooctylsulfonyl-N- ethanolaminoethyl)phosphonate, mono-perfluoro Ce- Ci6 alkyl-ethyl phosphonates, and perfluoroalkylbetaine. Also useful are the fluorocarbon surfactants described e.g. in US-A-4 781 985 and in US-A-5 084 340. Preferably the fluorosurfactant is chosen from Li, K and Na N- perfluoro C4-C13 alkane sulfonyl— N- alkyl glycine, 2-sulfo-l,4- bis(fluoroalkyl)butanedioate, 1,4-bis (fluoroalkyl)-2-[2-(N,N,N- trialkylammonium alkyl amino] butanedioate, perfhioroalkyl subsitituted carboxylic acids commercially available under the name Lodyne^® (produced by Ciba Specialty Chemicals Corp.) and fluorosurfactants commercially available under the name Zonyl^® (produced by E.I. Du Pont) that have the chemical structure Of RfCH₂CH₂SCH₂CH₂CO₂Li or RfCH₂CH₂O(CH₂CH₂O)_* H wherein Rf = F(CF₂CF₂)S-S and x = 0 to 25.

The overlayer may optionally include matting agents, thickener agents, biocides, crosslinking agents and further various conventional additives such as colorants, colored pigments, pigment dispersants, mold lubricants, permeating agents, fixing agents for ink dyes, anti-oxidants, dispersing agents/non-fluorosurfactants, anti-foaming agents, leveling agents, fluidity improving agents, antiseptic agents, brightening agents, viscosity stabilizing and/or enhancing agents, pH adjusting agents, anti-mildew agents, anti-fungal agents, agents for moisture-proofing, agents for increasing the stiffness of wet paper, agents for increasing the stiffness of dry paper and anti¬ static agents. The above-mentioned various additives can be added ordinarily in a range of 0 to 10 weight % based on the solid content of the ink receiving layer composition.

In a specific embodiment a toplayer e.g. the layer farthest away from the support, is coated on top of the overlayer. This protective toplayer preferably comprises a modified gelatin and optionally a fhiorosurfactant. Suitable fhiorosurfactants are those for application in the overlayer, as described hereinabove. Because the toplayer may be thin, preferably having a dry thickness of less than 3 μm, more preferably below 2 μm, the contribution of this layer to the curling is small. By using a modified gelatin a high gloss is obtained. Good results are obtained with a modified gelatin, of which at least 30% of the NH2 groups is modified by a condensation reaction with a compound having at least one carboxylic group as described among others in DE-A-19721238. The compound having at least one carboxylic group can have an other functional group like a second carboxylic group and a long aliphatic tail, which in principle is not modified. Long tail in this context means from at least 5 to as much as 25 C atoms, e.g. 6 - 20 C atoms. This aliphatic chain can be modified still to adjust the properties like water solubility and ink receptivity. Specially preferred gelatins of this type are succinic acid modified gelatins in which the succinic acid moiety contains an aliphatic chain from 5 to 25 carbon-atoms, e.g. 6 to 20 C atoms, where the chain can still be modified to a certain extend to adjust the water soluble properties or ink receptive properties. Preferably the succinic acid moiety contains an aliphatic chain from 7 to 18 carbon-atoms. Most preferred is the use of dodecenylsuccinic acid modified gelatin, in which at least 30% of the NH2 groups of the gelatin have been modified with said dodecenylsuccinic acid.

Another method for obtaining modified gelatin is described in EP-A- 0 576 911, where said gelatin is formed from gelatin containing pendant amine groups and pendant carboxylic groups wherein at least one amine group of said gelatin is modified to form an amide of the formula -NHCOR. The process typically involves reaction of an amine group with an activated carboxyl, i.e. a reaction product of a carboxyl activating agent and carboxylic acid, i.e., RCOOH wherein R represents substituted or unsubstituted alkyl of 1-10 carbons, substituted or unsubstituted aryl of 6-14 carbons, or substituted or unsubstituted arylalkyl of 7-20 carbons.

Other suitable methods are described by V.N. Izmailova, et at. (Colloid Journal, vol. 64, No. 5, 2002, page 640-642), and by O. Toledano, et al. (Journal of Colloid and Interface Science 200, page 235-240) wherein hydrophobic groups are attached to gelatin molecules by reacting gelatin with respectively N-hydroxysuccinimide ester of caprylic acid and N- hydroxysuccinimide ester of various fatty acids (CVCiβ).

Other modified gelatins giving good results are gelatins modified to have quaternary ammonium groups. An example of such a gelatin is the "Croquat™" gelatin produced by Croda Colloids Ltd.

Also the underlayer can be a multilayer of sublayers. It was found that in case the underlayer is a multilayer it is beneficial to apply different concentrations of gelatin and water soluble polymer in the sublayers of the underlayer. A lower concentration of gelatin and water soluble polymer in the sublayer closest to the support enables a lower viscosity of the mixture which improves the coatability and allows higher coating speeds.

In a specific embodiment between the support and the underlayer an adhesion promoting layer is applied to enhance the adhesion of the coated layers onto the support. This adhesion promoting layer may be coated in a separate step or simultaneously with the receiving layers. The adhesion promoting layer is a thin layer, preferably having a dry thickness of less than 3 μm, more preferably below 1 μm. The contribution of this layer to the curling of the medium is small. If desired, the gelatin and/or the water soluble polymer can be cross- linked in the image -recording elements of the present invention in order to impart mechanical strength to the layer. This can be done by any cross-linking agent known in the art. For gelatin, there is a large number of known cross-Unking agents, also known as hardening agents. Examples of the hardener include aldehyde compounds such as formaldehyde and glutar aldehyde, ketone compounds such as diacetyl and chloropentanedion, bis (2-chloroethylurea), 2-hydroxy-4, 6- dichloro-l,3,5-triazine, reactive halogen-containing compounds disclosed in US-A-3 288 775, carbamoyl pyridinium compounds in which the pyridine ring carries a sulphate or an alkyl sulphate group disclosed in US-A-4 063 952 and US-A-5 529 892, divinylsulfones, and the like. These hardeners can be used singly or in combination. The amount of hardener used, preferably ranges from 0.1 to 10 g, and more preferably from 0.1 to 7 g based on 100 g of gelatin contained in the ink-receiving layer. For PVA, for example, it is preferable to choose a cross-linking agent selected from borax, glyoxal, dicarboxylic acids and the like.

The ink receiving layer (i.e. the overlayer, the underlayer of both) may further comprise an UV absorbing agent. Any UV agent known in the art can be added. Suitable UV agents are selected from the group consisting of purine compounds, pyrimidine compounds, benzimidazole compounds, imidazolidine compounds, urazole compounds, pyrazole compounds, triazole compounds, benzotriazole compounds, tetrazole compounds, pyrazine compounds, cinnamate compounds, aminobutadien compounds and mixtures thereof.

Examples of UV agents are those described in Research Disclosure RD24239, RD290119, RD30326, EP-A 0 673 783, GB-A 2088 777, EP-A 0955180, EP-A-O 738 718, US-A-4926190 and in Ullmann's Encyclopedia of Industrial Chemistry, 5^th completely revised edition 1992, volume 20, page 468-471. Suitable UV agents are also compounds containing a triazine skeleton. These compounds are described, for example, in JP-A-46-3335, JP-A- 55-152776, JP-A-5-197074, JP-A-5-232630, JP-A-5-307232, JP-A-6-211813, JP- A-8-53427, JP-A-8-234364, JP-A-8-239368, JP-A-9-31067, JP-A-10-147577, JP- 10-182621, JP-T-8-501291 ("JP-T" means published searched patent publication). EP-A- 0711804 and DE-A- 19739797 are preferable.

Preferred UV agents are benzotriazole compounds, such as 2-(2- hydroxy-5'methylphenyl)benzotriazole,2-(2'-hydroxy-3',5'-di-t- butylphenyl)benzotriazole,2-(2'-hydroxy-3',5'-di-t-butylphenyl)-5- chlorobenzotriazole,2-(2'-hydroxy-5'-t-butylphenyl)benzotriazole, 2-(2'-hydroxy- 5'-t-butylphenyl)-5-chlorobenzotriazole,2-(2'-hydroxy-3'-sec-butyl-5'-t- butylphenyl)benzotriazole,2-(2'-hydroxy-3'-sec-butyl-5'-t-butylphenyl)-5- chlorobenzotriazole,2-(2'-hydroxy-4'-n-hexyloxyphenyl)benzotriazole,2-(2'- hy droxy- 5'-isoocy tlphenyl)benzotriazole , 2- (2' -hydroxy- 3' , 5' - di-t- amylphenyl)benzotriazole,2-(2'-hydroxy-5'-isooctylphenyl)-5'-methyl- benzotriazole, 2-(2'-hydroxy-3',5'-di-t-amylphenyl) benzotriazole, 2-(2'-hydroxy- 3'-t-butyl-5'mehtylphenyl)benzotriazole,2-(2'-hydroxy-3'-sec-dodecyl- 5'mehtylphenyl) benzotriazole, as well as the benzotriazole compounds described in EP-A-O 738 718, the benzotriazole compounds described in US-A- 4926190, and mixtures thereof. The UV agent can be added in a suitable solvent or as a component of an oil in water emulsion. Also UV agents linked to gelatin can be used. The UV agent may be added in the amount from 0.03 g/m² to 10 g/m². Preferable between 0.03 g/m² and 5 g/m².

The ink receiving layer may further comprise an optical brightener. Suitable optical brighteners are disclosed in e.g. RD11125, RD9310, RD8727, RD8407, RD36544 or Ullmann's Encyclopedia of industrial chemistry (Vol. A18 pl53-167), and comprise thiophenes, stilbenes, triazines, imidazolones, pyrazolines, triazoles, bis(benzoxazoles), coumarins and acetylenes. The optical brightener can be added in a suitable solvent or as a component of an oil in water emulsion. Also optical brighteners linked to gelatin can be used. The optical brightener is preferably present in the ink receiving layer in an amount lower than 1.0 gram/m²,more preferably between 0.005 and 0.5 g/m²; even more preferably 0.005 or more, but less than 0.2 g/m²; e.g. less than 0.1 g/m². The under layer may further contain the following ingredients in order to improve the ink receiving layer properties with respect to ink receptivity and strength:

- One or more fillers; both organic and inorganic particles can be used as fillers. Useful filler examples are represented by silica (colloidal silica), alumina or alumina hydrate (aluminazol, colloidal alumina, a cat ion aluminum oxide or its hydrate and pseudo-boehmite), a surface-processed cat ion colloidal silica, aluminum silicate, magnesium silicate, magnesium carbonate, titanium dioxide, zinc oxide, calcium carbonate, kaolin, talc, clay, zinc carbonate, satin white, diatomaceous earth, synthetic amorphous silica, aluminum hydroxide, lithopone, zeolite, magnesium hydroxide and synthetic mica. Useful examples of organic fillers are represented by polystyrene, poly me thacry late, polymethyl-methacrylate, elastomers, ethylene-vinyl acetate copolymers, polyesters, polyester-copolymers, polyacrylates, polyvinylethers, polyamides, polyolefins, polysilicones, guanamine resins, polytetrafluoroethylene, elastomeric styrene-butadiene rubber (SBR), urea resins, urea-formalin resins. Such organic and inorganic fillers may be used alone or in combination.

- One or more mordants. Mordants may be incorporated in the underlayer of the present invention. Such mordants are represented by cationic compounds, monomeric or polymeric, capable of complexing with the dyes used in the ink compositions. Useful examples of such mordants include quaternary ammonium block copolymers. Other suitable mordants comprise diamino alkanes, ammonium quaternary salts and quaternary acrylic copolymer latexes. Other suitable mordants are fluoro compounds, such as tetra ammonium fluoride hydrate, 2,2,2-trifluoroethylamine hydrochloride, 1- (alpha, alpha, alpha -trifluoro-m-tolyl) piperazine hydrochloride, 4-bromo- alpha, alpha, alpha -trifluoro-o-toluidiiie hydrochloride, difluorophenylhydrazine hydrochloride, 4-fluorobenzylamine hydrochloride, 4- fluoro- alpha, alpha -dimethylphenethylamine hydrochloride, 2- fluoroethylaminehydrochloride, 2-fl.uoro-l-methyl pyridinium -toluene sulfonate, 4-fluorophenethylamine hydrochloride, fluorophenylhydrazine hydrochloride, l-(2-fluorophenyl) piperazine monohydrochloride, 1-fiuoro pyridinium trifluorom ethane sulfonate.

- One ore more conventional additives, such as: • pigments: white pigments such as titanium oxide, zinc oxide, talc, calcium carbonate and the like; blue pigments or dyes such as cobalt blue, ultramarine or phthalocyanine blue; magenta pigments or dyes such as cobalt violet, fast violet or manganese violet; • biocides;

• pH controllers;

• preservatives;

• viscosity modifiers;

• dispersing agents; • anti-oxidants;

• antistatic agents; and/or

• anionic, cationic, non-ionic, and/or amphoteric surfactants, typically used in amounts ranging from 0.1 to 1000 mg/m², preferably from 0.5 to 100 mg/m². These additives may be selected from known compounds and materials in accordance with the objects to be achieved.

The above-mentioned additives (fillers/pigments, mordants, conventional additives) may be added in a range of 0 to 30% by weight, based on the solid content of the water soluble polymers and / or gelatin in the underlayer. The particle sizes of the non water-soluble particulate additives should not be too high, since otherwise a negative influence on the resulting surface will be obtained. The used particle size should therefore preferably be less than 10 μm, more preferably 7 μm or less. The particle size is preferably above 0.1 μm, more preferably about 1 μm or more for handling purposes. The process for producing a recording medium according the invention can be described by the following steps: preparation of at least one mixture comprising a gelatin and a PVA based polymer in a ratio between 1:1 and 1:3 for the overlayer; - preparation of at least one mixture comprising a gelatin and a PVA based polymer in a ratio between 5:1 and 2:1 for the underlayer; in case a top layer is applied, preparing a mixture for the top layer; providing a support for receiving said mixtures; optionally applying a backside coating on one side of the support in an amount optimized for a good curling and cockling behaviour; and coating the resulting formulations for the overlayer, the underlayer (and for the toplayer) consecutively or simultaneously to the side opposite to the backside of the support by any method known in the art. The coating methods are for example, a curtain coating, an extrusion coating, an air-knife coating, a slide coating, a roll coating method, reverse roll coating, dip coating processes and a rod bar coating.

An important characteristic of the inkjet recording medium is the gloss. It has been found that the gloss of the medium can be improved by selecting the appropriate surface roughness of the used support. It was found, that providing a support having a surface roughness characterised by the value Ra being less than 1.0 μm, preferably below 0.8 μm a very glossy medium can be obtained. A low value of the Ra indicates a smooth surface. The Ra is measured according to DIN 4776 using a UBM equipment, software package version 1.62, with the following settings: (1) Point density 500 P/mrα (2) Area 5.6 x 4.0 mm² (3) Cut-off wavelength 0.80 mm (4) Speed 0.5 mm/sec.

The base paper to be used as the support for the present invention is selected from materials conventionally used in high quality printing paper. Generally it is based on natural wood pulp and if desired, a filler such as talc, calcium carbonate, Tiθ2, BaSO₄, and the like can be added. Generally the paper also contains internal sizing agents, such as alkyl ketene dimer, higher fatty acids, paraffin wax, alkenylsuccinic acid, epichlorhydrin fatty acid amid and the like. Further the paper may contain wet and dry strength agents such as a polyamine, a poly-amide, poly aery lamide, poly-epichlorhydrin or starch and the like. Further additives in the paper can be fixing agents, such as aluminium sulphate, starch, cationic polymers and the like. The Ra value for a normal grade base paper is well above 1.0 μm typically above 1.3 μm. In order to obtain a base paper with a Ra value below 1.0 μm such a normal grade base paper can be coated with a pigment. Any pigment can be used. Examples of pigments are calcium-carbonate, Tiθ2, BaSO₄, clay, such as kaolin, styrene- acrylic copolymer, Mg-Al-silicate, and the like or combinations thereof. The amount being between 0.5 and 35.0 g/m² more preferably between 0.5 and 20.0 g/m². This pigmented coating can be applied as a pigment slurry in water together with a suitable binders like styrene-butadiene latex, methyl me thacry late -butadiene latex, polyvinyl alcohol, modified starch, polyacrylate latex or combinations thereof, by any technique known in the art, like dip coating, roll coating, blade coating or bar coating. The pigment coated base paper may optionally be calendered. The surface roughness can be influenced by the kind of pigment used and by a combination of pigment and calendering. The base pigment coated paper substrate has preferably a surface roughness between 0.4 and 0.8 μm. If the surface roughness is further reduced by super calendaring to values below 0.4 μm the thickness and stiffness values will generally become below an acceptable level. The ink receiving multilayer of the present invention can be directly applied to the pigment coated base paper. In another embodiment, the pigment coated base paper having a pigmented top side and a back-side is provided on both sides with a polymer resin through high temperature co-extrusion giving a laminated pigment coated base paper. Typically temperatures in this (co- )extrusion are above 280 ⁰C but below 350 ⁰C. The preferred polymers used are poly olefins, particularly polyethylene. In a preferred embodiment the polymer resin of the top side comprises compounds such as an opacifying white pigment e.g. Tiθ2 (anatase or rutile), ZnO or ZnS, dyes, coloured pigments, including blueing agents, like e.g. ultramarine or cobalt blue, adhesion promoters, optical brighteners, antioxidant and the like to improve the whiteness of the laminated pigment coated base paper. By using other than white pigments a variety of colors of the laminated pigment coated base paper can be obtained. The total weight of the laminated pigment coated base paper is preferably between 80 and 350 g/m². The laminated pigment coated base paper shows a very good smoothness, which after applying the ink receiving layer of the present invention results in a recording medium with excellent gloss.

Other supports used in this invention may suitably be selected from a synthetic paper or a plastic film in which the top and back coatings are balanced in order to minimise the curl behaviour.

Examples of the material of the plastic film are polyolefins such as polyethylene and polypropylene, vinyl copolymers such as polyvinyl acetate, polyvinyl chloride and polystyrene, polyamide such as 6,6-nylon and 6-nylon, polyesters such as polyethylene terephthalate, polyethylene-2 and 6- naphthalate and polycarbonate, and cellulose acetates such as cellulose triacetate and cellulose diacetate. The support may have a gelatin subbing layer to improve coatability of the support. The support may be subjected to a corona treatment in order to improve the adhesion between the support and the ink receiving layer. Also other techniques, like plasma treatment can be used to improve the adhesion. The swellable ink-receiving layer preferably has a dry thickness from 1 to 50 micrometers, more preferably from 5 to 25 and most preferably between 8 and 20 micrometers. If the thickness of said ink receiving layer is less than 1 micrometer, adequate absorption of the solvent will not be obtained. If, on the other hand, the thickness of said ink receiving layer exceeds 50 micrometers, no further increase in solvent absorptivity will be gained.

The recording medium of the invention can be used for forming a permanent, precise inkjet image by bringing ink into contact with the medium in the pattern of a desired image.

The present invention will be illustrated in more detail by the following non-limiting examples. Unless stated otherwise, all ratios given are based on weight.

EXAMPLES

A. Preparation of solution 'A^; for the back side.

A 10 wt.% solution of a lime bone gelatin with an IEP of 5.0 and average MW of 250 kD (determined by the method described in the Journal of Colloid and Interface Science 243, 476-482, 2001) was adjusted to pH 8.5 with NaOH. This solution was agitated gently at a temperature of 40 ⁰C for about 30 minutes.

B. Preparation of underlaver solution 'B' of the ink receiving layer. A 20 wt.% solution of a lime bone gelatin with an IEP of 5.0 and average MW of 250 kD was adjusted to pH 8.5 with NaOH. An aqueous solution of 10 wt % polyvinyl alcohol (PVA Mowiol™ 4-88 from Kuraray Specialties Europe), was prepared at pH 8.5. A homogeneous mixture (viz. no phase separation occurred), of gelatin and PVA having a weight ratio of 6:1 was made by adding 143 weight parts of said PVA solution and 429 weight parts of water into 428 weight parts of said gelatin solution at a temperature of 40 °C. This mixture was agitated gently for about 30 minutes.

C. Preparation of underlaver solution 'C of the ink receiving layer. A 20 wt.% solution of a lime bone gelatin with an IEP of 5.0 and average MW of 250 kD was adjusted to pH 8.5 with NaOH. An aqueous solution of 10 wt % polyvinyl alcohol (PVA Mowiol™ 4-88 from Kuraray Specialties Europe), was also prepared at pH 8.5. A homogeneous mixture (viz. no phase separation occurred), of gelatin and PVA having a weight ratio of 4:1 was made by adding 200 weight parts of said PVA solution and 400 weight parts of water into 400 weight parts of said gelatin solution at a temperature of 40 °C. This mixture was agitated gently for about 30 minutes.

D. Preparation of the overlaver solution 'D' of the ink receiving layer.

A solution containing 27 weight parts of modified gelatin (dodecenyl- succinic modified acid treated gelatin from Stoess GmbH, Germany; modification grade 40%) having an IEP of 5.4, 54 weight parts of PVA-NVF copolymer (CGPS-910, melting range 210-230 °C, CIBA Specialty Chemicals) and 919 weight parts of water was prepared at 40 ⁰C. The pH of the solution was adjusted to 9.5 by adding NaOH. The ratio of gelatin to water soluble polymer of this solution was thus 1/2.

E. Preparation of the toplaver solution Ε' of the ink receiving layer. A solution containing 32 weight parts of modified gelatin (dodecenyl- succinic modified acid treated gelatin from Stoess GmbH, Germany with a modification degree of 40% and an IEP of 5.4), 1 weight part of Zonyl^® surfactant (a fluoro-carbon type of surfactant) and 967 weight parts of water was prepared at 40 °C. The pH of the solution was adjusted to 8.5 by adding NaOH. The backside coating was made by feeding solution A into a slide coating machine, commonly known in the photographic industry, and coating it on the backside of a photographic grade paper having polyethylene laminated at both sides. The flow of the emulsion was chosen so that the coated gelatin dry weight was 13.5 gram per square metre as a standard back side coating. After coating, the coated solution was chilled at a temperature of ca. 15 ⁰C to set the gelatin and then dried with dry air at a maximum temperature of 40 °C.

The underlayer, overlayer and toplayer solutions mentioned above were fed into a slide coating machine, commonly known in the photographic industry, and coated on a photographic grade paper having polyethylene laminated at both sides and having a backside coating as described above. After coating, the solution was chilled at a temperature of ca. 15 ⁰C to set the gelatin and then dried with dry air at a maximum temperature of 40 ⁰C. An overview of the layer structures of these examples is given below.

Layer structure

Various recording media were produced by applying on a substrate three layers: an underlayer, an overlayer and a top layer as shown in the scheme below. In the examples each layer is applied in the indicated amount, given as the wet coated amount in cc/m².

Toplayer

Overlayer

Underlayer

Photographic grade paper with polyethylene laminated on both sides

(Laminated Substrate)

Backside coating

Example 1 — comparative example In the order recited, the following layers were coated on the substrate:

Underlayer: 80 cc/m² of underlayer solution 'B' Overlayer: 80 cc/m² of overlayer solution 'D' Toplayer: 17 cc/m² of toplayer solution 'E' Backside layer: 135 cc/m² of solution 'A'

Example 2

Identical to example 1 except that for the underlayer solution C is used instead of solution B and the backside layer was applied in an amount of 10 g/m² dry weight.

Evaluation

The curling of these inkjet recording sheets is evaluated as follows. A piece of 10 x 15 cm is cut out of the inkjet recording medium. This piece is than stored for at least 1 hr at a condition of 10% relative humidity and 20 ⁰C ambient temperature. At this condition the height of each corner of the paper is measured as it curls up from a flat underground. This height is recorded in mm, as result the average value of the four corners is taken.

Cockling is evaluated on an A4 sheet on which one half is printed completely black and the other half is not printed. The cockling is not measured but evaluated visually. The following classification has been defined:

O: no or slight cockling

X: severe cockling

Results

The results of the different coated examples as mentioned above are shown in Table 1.

Table 1

Example 2 shows the benefit of reducing the ratio of gelatin/water soluble polymer: without adjusting the back side coating the curling becomes more negative which makes it necessary to reduce the amount of gelatin coated on the backside. Reducing the backside coated amount from 13.5 to 10 g/m² brings the curling back to about the same level. At the same time the cockling is improved significantly to a level which is almost not visible.

Claims

1. Recording medium comprising a support and a receiving layer adhered to said support, which receiving layer is a multilayer comprising an underlayer and an overlayer, which underlayer and overlayer both comprise one or more (sub-)layer(s), wherein said overlayer comprises at least one gelatin and at least one polyvinyl alcohol (PVA) based polymer with a weight ratio of gelatin/PVA based polymer of from 1/1 to 1/3 and said underlayer comprises at least one gelatin and at least one polyvinyl alcohol (PVA) based polymer with a weight ratio of gelatin/PVA based polymer of from 5/1 to 2/1.

2. Medium according to claim 1, wherein the type of PVA based polymer in said overlayer is different from the type of PVA based polymer used in said underlayer.

3. Medium according to any one of the previous claims, wherein said PVA-based polymer is selected from the group consisting of fully hydrolysed or partially hydrolysed PVA, carboxylated PVA, acetoacetylated PVA, quaternary ammonium modified PVA, copolymers and terpolymers of PVA with other polymers , and combinations thereof.

4. Medium according to any one of the previous claims, wherein said PVA based polymer in the overlayer is used in an amount of 0.5 to 15 g/m², more preferably from 1.0 to 10.0 g/m². 5. Medium according to any one of the previous claims, wherein said

PVA based polymer in the underlayer is used in an amount of 0.

5 to 15 g/m², more preferably from 1.0 to 8.0 g/m².

6. Medium according to any of the previous claims, wherein on the opposite side of said support a backside coating is applied comprising a gelatin or a water soluble polymer in an amount of 1 to 20 g/m².

7. Medium according to any one of the previous claims, wherein said gelatin is selected from the group consisting of alkali-treated gelatin, acid- treated gelatin, hydrolysed gelatin, fish gelatin, acetylated gelatin, phthalated gelatin, alkyl quaternary ammonium modified gelatin, succinated gelatin, alkylsuccinated gelatin, alkenylsuccinated gelatin, gelatin chemically modified with N-hydroxysuccinimide ester of fatty acid, and combinations thereof.

8. Medium according to any of the previous claims, wherein the gelatin in the overlayer is used in an amount of 0.1 to 10 g/m², more preferably from 0.2 to 8.0 g/m².

9. Medium according to any of the previous claims, wherein the gelatin in the underlayer is used in an amount of 0.2 to 12 g/m², more preferably from 0.4 to 9.0 g/m².

10. Medium according to any of the previous claims, wherein a toplayer is coated on top of the overlayer where said toplayer comprises a modified gelatin of which at least 30% of the NH2 groups is modified.

11. Medium according to any of the previous claims, wherein in the underlayer the concentration of gelatin and water soluble polymer in the sublayer nearest to the support is lower than in the sublayer further away from the support.

12. Medium according to any one of the previous claims, wherein the support is selected from a paper, a base paper, a pigment coated base paper, a laminated pigment coated base paper, a laminated paper, a synthetic paper or a film support.

13. Medium according to any one of the previous claims, wherein the support has a surface roughness Ra smaller than 1.0 μm, preferably smaller than 0.8 μm.

14. Process for producing a recording medium, comprising the steps of: a) preparation of at least one mixture comprising a gelatin and a PVA based polymer in a ratio between 1:1 and 1:3 for the overlayer; b) preparation of at least one mixture comprising a gelatin and a PVA based polymer in a ratio between 5:1 and 2:1 for the underlayer; c) providing a support for receiving said mixtures; d) optionally applying a backside coating on one side of the support in an amount optimized for a good curling and cockling behaviour; and e) coating said mixtures consecutively or simultaneously on the side opposite to the backside of said support, followed by drying the coated support.

15. A method of forming a permanent, precise inkjet image comprising the steps of: a) providing a recording medium as defined in any of the claims 1-13 or made by the process according to claim 14; and b) bringing ink into contact with the medium in the pattern of a desired image.