Description

The present invention relates to a method of producing a printed matter, the method comprising the steps of applying an aqueous primer composition onto the surface of an absorbent substrate, drying and subsequently forming an image on the primed substrate by inkjet printing using a water-based inkjet printing ink. The aqueous primer composition comprises at least one dispersed polymer which is not film forming at the pH and temperature of its application and is selected from specific acrylic or styrene homo- or co-polymers. The invention also relates to printed matter produced by said method.

Ink-jet printing is a known way for forming images on pervious packaging materials such as paper, paper board or corrugated board. Modern ink-jet printing systems produce colored images on paper substrates that can be used in paper packaging applications. The ink-jet printing systems employ various digital technologies, inks, and ink-jet printers to produce high quality printed images on the paper substrates. In a typical ink-jet printing process, liquid ink is squirted through very fine nozzles of a printer onto the substrate. This results in a printed image being formed on the surface of the substrate. Many inks used in ink-jet printing devices are aqueous-based inks containing water as their primary component. The aqueous ink-jet inks are often basic with a pH above 7. The aqueous-based inks contain molecular dyes or pigmented colorants. Small amounts of water-miscible solvents, such as glycols and glycol ethers, may be present. The substrate may be coated with one or more layers of specially designed compositions that make the substrate capable of receiving and holding the aqueous-based inks effectively so as to generate a printed image.

The paper substrates employed in ink-jet printing methods generally need to meet requirements such as the density of printed dots to be high and the hue to be bright and clear; ink needs to be quickly absorbed and when printed dots are superimposed, ink neither is to flow nor spread; diffusion of the printed dots in the horizontal direction is not larger than required and the border is smooth and results in sharpness.

WO 2017/205608 describes a method for single pass ink-jet printing on corrugated board comprising applying a primer composition to a substrate, drying the primer layer and printing on the primer layer. The primer composition may be a water-based composition comprising an acrylic polymer emulsion. EP 2529941 describes a coating material for forming an ink-jet ink receiving layer comprising a specific crosslinked vinyl polymer emulsion. EP 2687550 describes a vinyl chloride polymer emulsion which can be used for forming a coating as a receiving layer of a recording sheet.

Despite the progress to date, there is an existing need and a desire for substrates having good ink-jet print performance properties, in particular having good fixation properties (such as dot gain and color strength) of water based ink-jet inks printed on absorbing substrates and for substrates that can be produced in economic ways. It is an object of the invention to provide substrates for ink-jet printing methods meeting said needs.

The invention provides a method of producing a printed matter, the method comprising the steps of

(i) first applying an aqueous primer composition onto the surface of an absorbent substrate, and drying the aqueous primer composition; wherein the aqueous primer composition comprises at least one dispersed polymer, which preferably is non-crosslinked, wherein the dispersed polymer is not film forming at the pH and temperature of its application to the absorbent substrate; wherein the dispersed polymer has a glass transition temperature of from 30 to 200 °C, preferably from 70 to 130 °C, measured by differential scanning calorimetry according to ASTM D 3418-08 as the midpoint temperature of the second heating curve at a heating rate of 20 °C/min; and wherein the dispersed polymer is selected from the group consisting of acrylic homopolymers, acrylic copolymers made of at least 60 wt.% of acrylic monomers, styrene homopolymer and styrene acrylic copolymers made of at least 60 wt.% of a mixture of styrene and acrylic monomers;

(ii) subsequently forming an image on the primed substrate by ink-jet printing using a water- based ink-jet printing ink.

The ink-jet printing ink preferably is a non UV-curable ink. The ink-jet printing ink preferably has a pH above 7.

The aqueous primer composition preferably has a pH of from 1 to 9, preferably from 1.5 to 8. If film formation of the polymer is pH dependent, then the pH of the primer composition is set to a value at which the polymer is not film forming.

In the text below, the designation "(meth)acryl..." and similar designations is used as an abbreviating notation for "acryl... or methacryl...". The expression "Cx alkyl(meth)acrylate" encompasses alkyl acrylates and alkyl methacrylates having x C atoms in the alkyl group.

The term “aqueous” or “aqueous composition” means a solvent system consisting exclusively or predominantly (50% or more, preferably at least 70%, at least 90% or more than 95% by weight of all solvents) of water.

The terms “dispersed polymer” means a polymer which is dispersible in water and which is dispersed in an aqueous composition.

Glass transition temperatures are determined by Differential Scanning Calorimetry (ASTM D 3418-08, ’’midpoint temperature” of second heating curve, heating rate 20 °C/min).

Film-forming polymers are a group of polymers that leave a pliable, cohesive and adherent covering over a surface to which they are applied. The polymers are not film-forming at the pH and temperature of their application. For example, a polymer comprising acid groups which is not film-forming with its acid groups in the unneutralized form but which is film-forming with neutralized acid groups is applied at a pH at which the acid groups are unneutralized, e.g. at pH below 7.

The minimum film forming temperature (MFFT) of the polymers of the aqueous primer composition is preferably higher than room temperature (23 °C), preferably higher than 40 °C, even more preferably at least 60 °C. MFFT can be determined according to ASTM D2354 using a Rhopoint MFFT bar 90 instrument and as described in the examples.

In one aspect of the invention the polymer has free acid groups which are available on the surface after application of the aqueous primer composition and after the drying. This can for example be achieved by applying the polymer at a pH at which the acid groups are in their unneutralized form, e.g. at a pH below 7. Free acid groups which are available on the surface after application of the aqueous primer composition and after drying can also be achieved by applying the polymer at a pH at which the acid groups are in their neutralized form (e.g. at a pH of 7 or above 7), if a volatile base is used for neutralization of at least a part of or all the acid groups. A volatile base is a base with a high vapor pressure at ordinary room temperature so that it can evaporate on drying. Preferred volatile bases are ammonia and monoethanolamine.

The dispersed polymers preferably have a weight average molecular weight of from 10000 to 500000, preferably from 12000 to 300000. Molecular weight determination is performed by gel permeation chromatography (GPC) against polystyrene standards.

The average size of the polymer particles dispersed in the aqueous dispersion is preferably less than 400 nm, more particularly less than 300 nm. With particular preference the average particle size is between 60 and 250 nm. By average particle size here is meant the d ₅ o of the particle size distribution - that is 50 wt.% of the entire mass of all the particles have a diameter smaller than the d ₅ o- The particle size distribution can be determined in a known way by Dynamic Light Scattering.

Polymers

The one or more dispersed polymers are selected from the group consisting of acrylic homopolymers, acrylic copolymers made of at least 60 wt.% of acrylic monomers, styrene homopolymer and styrene acrylic copolymers made of at least 60 wt.% of a mixture of styrene and acrylic monomers. The polymers can be prepared by aqueous emulsion polymerization by radical polymerizing ethylenically unsaturated and radically polymerizable compounds (monomers). The monomers are selected so that the glass transition temperature of the polymer is from 30 to 200 °C, preferably from 50 to 160 °C, and even more preferably from 70 to 130 °C. An acrylic homopolymer is for example methyl methacrylate homopolymer.

The radically polymerizable monomers preferably comprise a) at least one vinyl monomer selected from the group consisting of acrylic acid esters and methacrylic acid esters, b) optionally one or more vinyl monomers with at least one acid group, preferably (meth)acrylic acid, c) optionally at least one aromatic vinyl monomer, preferably styrene, d) optionally one or more further monomers, different from monomers a), b) and c).

Monomer amounts of polymers are % by weight, based on the total amount of monomers of the respective polymer, unless otherwise noted.

If the dispersed polymer is made from at least one monomer with acid groups, the polymer preferably has an acid value of from 15 to 400 mg KOH/g, more preferably from 30 to 350, and even more preferably from 50 to 300 mg KOH/g.

For polymers with acid groups, the radically polymerizable monomers may for example comprise a) at least 30% by weight, preferably from 50 to 90% by weight of at least one vinyl monomer selected from the group consisting of acrylic acid alkyl esters and methacrylic acid alkyl esters with 1 to 8 carbon atoms in the alkyl group, b) from 1 to 70%, preferably from 10 to 30% by weight of vinyl monomers of at least one monomer selected from acrylic acid and methacrylic acid, c) from 0 to 20% by weight, preferably from 10 to 15% by weight of styrene, d) from 0 to 30% by weight of one or more further monomers, different from monomers a), b) and c), the monomer amounts are based on the total amount of monomers.

Preferred acrylic copolymers are prepared by polymerization of components comprising: a) at least one vinyl monomer selected from the group consisting of acrylic acid esters and methacrylic acid esters, b) one or more vinyl monomers with at least one acid group, preferably (meth)acrylic acid, c) optionally one or more further monomers, different from monomers a) and b).

More preferred acrylic copolymers are prepared by polymerization of components comprising: a) at least 30% by weight, preferably from 50 to 90% by weight of at least one vinyl monomer selected from the group consisting of acrylic acid alkyl esters and methacrylic acid alkyl esters with 1 to 8 carbon atoms in the alkyl group, b) from 1 to 70% by weight, preferably from 10 to 40% by weight of vinyl monomers selected from the group consisting of acrylic acid and methacrylic acid, c) from 0 to 30% by weight of one or more further monomers, different from monomers a) and b); the monomer amounts are based on the total amount of monomers.

An acrylic copolymer is for example prepared by polymerization of: a) 70 to 90% by weight of at least one vinyl monomer selected from the group consisting of methyl acrylate, methyl methacrylate and ethyl acrylate, b) from 10 to 30% by weight of vinyl monomers of at least one monomer selected from acrylic acid and methacrylic acid, the monomer amounts are based on the total amount of monomers.

Preferred styrene acrylic copolymers are prepared by polymerization of components comprising a) styrene, b) one or more vinyl monomers with at least one acid group selected from the group consisting of acrylic acid and methacrylic acid, c) optionally at least one vinyl monomer selected from the group consisting of acrylic acid esters and methacrylic acid esters, d) optionally one or more further monomers, different from monomers a), b) and c).

More preferred styrene acrylic copolymers are prepared by polymerization of components comprising a) from 10 to 90% by weight, preferably from 20 to 85% by weight, or from 20 to 65% by weight of styrene, b) from 1 to 70%, preferably from 2 to 40% by weight, or from 10 to 40% by weight of at least one monomer selected from acrylic acid and methacrylic acid, c) from 0 to 50% by weight, preferably from 1 to 50% by weight of at least one vinyl monomer selected from the group consisting of acrylic acid alkyl esters and methacrylic acid alkyl esters with 1 to 8 carbon atoms in the alkyl group, d) from 0 to 30% by weight of one or more further monomers, different from monomers a), b) and c), the monomer amounts are based on the total amount of monomers.

Acrylic vinyl monomers are used in the acrylic polymers in an amount of preferably at least 30% by weight more preferably from 50 to 90% by weight or from 70 to 90% by weight, based on the total amount of vinyl monomers.

Acrylic vinyl monomers are used in the styrene acrylic polymers in an amount of preferably from 0 to 50% by weight, more preferably from 1 to 50% by weight, or from 2 to 40% by weight, based on the total amount of vinyl monomers.

The nature and amount of the monomers are preferably such that the glass transition temperature of the polymer prepared by emulsion polymerization is in the above-mentioned range. The monomers (a) are preferably selected from alkyl(meth)acrylates, in particular C1 to C8 alkyl (meth)acrylates or C1 to C8 alkyl (meth)acrylates, for example methyl methacrylate, methyl acrylate, n-butyl acrylate, ethyl acrylate, and 2-ethylhexyl acrylate or mixtures thereof. Especially preferred are methyl acrylate, methyl methacrylate and ethyl acrylate and also mixtures of these monomers. More particularly acrylic polymers are preferably composed to an extent of at least 60 wt.% of methyl acrylate, methyl methacrylate, ethyl acrylate or their mixtures.

The vinyl monomers with acid groups are used in amounts of preferably from 1 to 70% by weight or from 2 to 40% by weight, more preferably from 10 to 40% by weight, or from 10 to 30% by weight, based on the total amount of monomers. Examples of acid monomers are ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids, and vinylphosphonic acid. Ethylenically unsaturated carboxylic acids used are preferably alpha, beta- monoethylenically unsaturated monocarboxylic and dicarboxylic acids having 3 to 6 C atoms in the molecule. Examples thereof are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, and vinyllactic acid. Examples of suitable ethylenically unsaturated sulfonic acids include vinylsulfonic acid, styrenesulfonic acid, acrylic monomers containing sulfonic acid groups such as for example 2-acrylamido-2-methylpropane sulfonic acid, sulfopropyl acrylate, and sulfopropyl methacrylate. Preference is given to acrylic acid and methacrylic acid and a mixture thereof; acrylic acid is particularly preferred. The amount of acid monomers is such that the acid value of the polymer prepared by emulsion polymerization preferably is from 15 to 400 mg KOH/g, preferably from 30 to 350, and even more preferably from 50 to 300 mg KOH/g.

Vinyl aromatic monomers are used in an amount of preferably from 10 to 90% by weight, more preferably from 20 to 85% by weight, or from 20 to 65% by weight. Vinylaromatic compounds are for example styrene, vinyltoluene, alpha- and para-methylstyrene, alpha-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene. A preferred vinyl aromatic monomer is styrene.

The monomers of the dispersed polymer may comprise a mixture of at least one soft monomer in an amount of for example from 1 to 50% by weight or from 2 to 40% by weight, based on the total amount of vinyl monomers and at least one hard monomer in an amount of for example from 20 to 90% by weight, based on the total amount of vinyl monomers, wherein soft monomers are monomers having a glass transition temperature of below 0 °C when polymerized as homopolymers, preferably alkyl acrylates with 2 to 6 carbon atoms in the alkyl group, and hard monomers are monomers having a glass transition temperature of above 0 °C when polymerized as homopolymers (excluding acid monomers), preferably methylmeth acrylate or styrene. Suitable soft monomers are for example ethyl acrylate, n-propyl acrylate, n- butyl acrylate, n-hexyl acrylate and 2-ethylhexyl acrylate. Suitable hard monomers are for example styrene, methyl acrylate, tert-butyl acrylate and the alkyl methacrylates with 1 to 4 C- atoms in the alkyl group such as methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate. The monomers of the dispersed polymer optionally comprise one or more further monomers, different from monomers a), b) and c) in an amount of from 0 to 30% by weight, preferably up to 10 wt.% or up to 5 wt.%, preferably 0.1 to 10 wt.%, more preferably 0.2 to 5 wt.%. Further monomers (d) can for example be nonionic, hydrophilic vinyl monomers in amounts of for example up to 5 wt.%, preferably 0.1 to 5 wt.%, more preferably 0.2 to 4 wt.%. Nonionic, hydrophilic vinyl monomers are nonionic and have a water solubility of greater than 100 g/l at 20 °C, for example selected from the group consisting of acrylamide, methacrylamide, diacetone acrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate and mixtures thereof.

Examples of further monomers (d) are vinyl esters of carboxylic acids having 1 to 20 C atoms, for example, vinyl laurate, vinyl stearate, vinyl propionate, Versatic acid vinyl esters, and vinyl acetate; nitrile monomers such as acrylonitrile and methacrylonitrile; vinyl halides such as ethylenically unsaturated compounds substituted by chlorine, fluorine, or bromine, preferably vinyl chloride and vinylidene chloride; vinyl ethers such as those of alcohols comprising 1 to 4 C-atoms, for example vinyl methyl ether or vinyl isobutyl ether, hydrocarbons having 4 to 8 C- atoms and two olefinic double bonds such as butadiene, isoprene, and chloroprene; or ethylene glycol dimethacrylate.

In one aspect of the invention no other monomers besides (meth)acrylic acid ester monomers (a) and vinyl monomers with at least one acid group (b) are used for preparing the acrylic copolymer. One particularly preferred acrylic copolymer is composed of a) 70 to 90% by weight of at least one vinyl monomer selected from the group consisting of methyl acrylate, methyl methacrylate and ethyl acrylate, b) from 10 to 30% by weight of vinyl monomers of at least one monomer selected from acrylic acid and methacrylic acid, the monomer amounts are based on the total amount of monomers.

In one aspect of the invention no other monomers besides vinyl aromatic monomers (a), vinyl monomers with at least one acid group (b) and (meth)acrylic acid ester monomers (c) are used for preparing the styrene acrylic copolymer. One particularly preferred styrene acrylic copolymer is composed of a) from 20 to 65 % by weight of styrene, b) from 10 to 30% by weight of vinyl monomers of at least one monomer selected from acrylic acid and methacrylic acid, c) from 25 to 50 % by weight of at least one vinyl monomer selected from the group consisting of methyl acrylate, methyl methacrylate and ethyl acrylate, the monomer amounts are based on the total amount of monomers.

The dispersed polymers may be prepared by emulsion polymerization, the product then being an emulsion polymer. In the emulsion polymerization it is usual to use ionic and/or nonionic emulsifiers and/or protective colloids, and/or stabilizers, as interface-active compounds, in order to support the dispersing of the monomers in the aqueous medium. A comprehensive description of suitable protective colloids is found in Houben-Weyl, Methoden der organischen Chemie, volume XIV/1 , Makromolekulare Stoffe [Macromolecular compounds], Georg-Thieme- Verlag, Stuttgart, 1961 , pp. 411 to 420. Emulsifiers contemplated are anionic, cationic, and nonionic emulsifiers. As accompanying interface-active substances it is preferred to use exclusively emulsifiers, whose molecular weights, in contrast to the protective colloids, are usually below 2000 g/mol. Where mixtures of interface-active substances are used, the individual components must, of course, be compatible with one another, something which in case of doubt can be verified using a few preliminary experiments. Preference is given to using anionic and nonionic emulsifiers as interface-active substances. Suitable emulsifiers are, for example, ethoxylated Cs to C36 or C12 to Cis fatty alcohols having a degree of ethoxylation of 3 to 50 or of 4 to 30, ethoxylated mono-, di-, and tri- C ₄ to C12 or C ₄ - to Cg alkylphenols having a degree of ethoxylation of 3 to 50, alkali metal salts of dialkyl esters of sulfosuccinic acid, alkali metal salts and ammonium salts of Cs to C12 alkyl sulfates, alkali metal salts and ammonium salts of C12 to Cis alkyl sulfonic acids, and alkali metal salts and ammonium salts of Cg to Cis alkylaryl sulfonic acids. Cationic emulsifiers are, for example, compounds having at least one amino group or ammonium group and at least one C8-C22 alkyl group.

In one aspect of the invention the acrylate polymer is polymerized in the presence of at least one anionic surfactant in an amount of preferably from 0.05 to 5 parts per 100 parts by weight of monomers.

Further suitable emulsifiers are compounds of the general formula in which R ⁵ and R ⁶ are hydrogen or C4 to C14 alkyl and are not simultaneously hydrogen, and X and Y may be alkali metal ions and/or ammonium ions. Preferably, R ⁵ and R ⁶ are linear or branched alkyl groups having 6 to 18 C atoms, or hydrogen, and more particularly having 6, 12, and 16 C atoms, with R ⁵ and R ⁶ not being both simultaneously hydrogen. X and Y are preferably sodium, potassium, or ammonium ions, with sodium being particularly preferred. Particularly advantageous are compounds in which X and Y are sodium, R ⁵ is a branched alkyl group having 12 C atoms, and R ⁶ or R ⁵ is hydrogen. Use is frequently made of technical mixtures which include a fraction of 50 to 90 wt.% of the monoalkylated product, an example being Dowfax ^® 2A1 . Suitable emulsifiers are also found in Houben-Weyl, Methoden der organischen Chemie, Volume 14/1 , Makromolekulare Stoffe [Macromolecular compounds], Georg Thieme Verlag, Stuttgart, 1961 , pages 192 to 208. Emulsifier trade names are, for example, Dowfax ^® 2 A1 , Emulan ^® NP 50, Dextrol ^® OC 50, Emulgator 825, Emulgator 825 S, Emulan ^® OG, Texapon ^® NSO, Nekanil ^® 904 S, Lumiten ^® l-RA, Lumiten ^® E 3065, Disponil ^® FES 77, Lutensol ^® AT 18, Steinapol ^® VSL, Emulphor ^® NPS 25. Also suitable are copolymerizable emulsifiers which comprise a radically polymerizable, ethylenically unsaturated double bond, examples being reactive anionic emulsifiers such as Adeka® Resoap SR-10.

The emulsion polymerization takes place in general at 30 to 130 °C, preferably at 50 to 95°C or at 50 to less than 90°C. The polymerization medium may consist only of water, or of mixtures of water and liquids miscible therewith such as methanol. Preference is given to using just water. The emulsion polymerization may be carried out as a batch operation or in the form of a feed process, including staged or gradient regimes. Preference is given to the feed process, in which a portion of the polymerization batch is introduced as the initial charge and is heated to the polymerization temperature, polymerization is commenced, and the remainder of the polymerization batch is supplied to the polymerization zone, usually via a plurality of spatially separate feeds, of which one or more comprise the monomers in pure form or in emulsified form, the additions taking place continuously, in stages, or under a concentration of gradient, with the polymerization being maintained. For more effective setting of the particle size, for example, it is also possible in the polymerization to include a polymer seed in the initial charge.

For the emulsion polymerization it is possible to use the typical and known auxiliaries, such as water-soluble initiators and chain transfer agents, for example. Water-soluble initiators for the emulsion polymerization are, for example, ammonium salts and alkali metal salts of peroxydisulfuric acid, e.g., sodium peroxodisulfate, ammonium peroxodisulfate, hydrogen peroxide, or organic peroxides, e.g., tert-butyl hydroperoxide. Also suitable are what are called reduction-oxidation (redox) initiator systems. The redox initiator systems are composed of at least one usually inorganic reducing agent and one organic or inorganic oxidizing agent. The oxidizing component comprises, for example, the initiators already specified above for the emulsion polymerization. The reducing components comprise, for example, alkali metal salts of sulfurous acid, such as sodium sulfite, sodium hydrogensulfite, alkali metal salts of disulfurous acid such as sodium disulfite, bisulfite addition compounds of aliphatic aldehydes and ketones, such as acetone bisulfite, or reducing agents such as hydroxymethanesulfinic acid and the salts thereof, or ascorbic acid. The redox initiator systems can be used together with soluble metal compounds whose metallic component is able to occur in a plurality of valence states.

Examples of typical redox initiator systems include ascorbic acid/iron(ll) sulfate/sodium peroxy- disulfate, tert-butyl hydroperoxide/sodium disulfite, tert-butyl hydroperoxide/Na-hydroxymethane sulfinic acid, or tert-butyl hydroperoxide/ascorbic acid. The individual components, the reducing component for example, may also be mixtures, an example being a mixture of the sodium salt of hydroxymethane sulfinic acid and sodium disulfite. The stated compounds are used usually in the form of aqueous solutions, with the lower concentration being determined by the amount of water that is acceptable in the dispersion, and the upper concentration by the solubility of the respective compound in water. In general the concentration is 0.1 to 30 wt.%, preferably 0.5 to 20 wt.%, more preferably 1.0 to 10 wt.%, based on the solution. The amount of the initiators is generally 0.1 to 10 wt.%, preferably 0.5 to 5 wt.%, based on the monomers to be polymerized. It is also possible for two or more different initiators to be used for the emulsion polymerization.

For the purpose of removing the residual monomers, it is typical for initiator to be added after the end of the actual emulsion polymerization as well.

In the polymerization it is possible to use chain transfer agents to regulate molecular weight, in amounts, for example, of 0 to 0.8 part by weight, based on 100 parts by weight of the mono mers to be polymerized, thereby reducing the molar mass. Suitability is possessed, for example, by compounds having a thiol group such as tert-butyl mercaptane, thioglycolic esters, such as 2-ethylhexyl thioglycolate (EHTG), mercaptoethanol, mercaptopropyl trimethoxysilane, n-dodecyl mercaptan, or tert-dodecyl mercaptan (t-DMK). Preference is given to isooctyl mercaptopropionate. Chain transfer agents without a thiol group, such as C6 to C20 hydrocarbons, for example, are those which form a pentadienyl radical when hydrogen is abstracted, an example being terpinolene. In one embodiment the emulsion polymer is prepared using 0.05 to 0.7 wt.% or less than 0.4 wt.%, based on the monomer amount, of at least one chain transfer agent to regulate molecular weight.

The aqueous primer composition for use in accordance with the invention are dispersions of polymers in an aqueous medium. The aqueous medium may, for example, be exclusively water, or may alternatively be mixtures of predominantly water with a water-miscible solvent such as ethanol or isopropanol. It is preferred not to use organic solvents. The solids contents of the dispersions are preferably from 15 to 65 wt.%, more preferably from 20 to 55 wt.%. The solids content may be realized for example through corresponding adjustment to the monomer amounts and/or to the amount of water used in the emulsion polymerization. The pH of the aqueous primer composition is set to a pH of preferably from 1 to 9, more preferably from 1.5 to 8. The polymers are typically dispersed at pH below 7 but might be colloidal or soluble at higher pH. The pH is set to a value at which the polymer is not film forming.

If the aqueous primer composition comprises a polymer with acid groups and has a pH of 7 or above 7, then the acid groups of the polymer are preferably at least partly neutralized with a volatile base, so that free acid groups are available after drying. A volatile base is a base with a high vapor pressure at ordinary room temperature so that it can evaporate on drying. Preferred volatile bases are ammonia and monoethanolamine.

The aqueous primer composition preferably has a viscosity of from 10 to 2500 mPa-s. Viscosity is measured at 25 °C on a Brookfield viscometer model LVT and using the appropriate spindle, giving readings between 20 to 80% of the full scale reading. Thickeners can be used to adjust the desired viscosity to higher values, if viscosity is not sufficient for the desired application method.

The aqueous primer composition may optionally comprise one or more additives customary therefor. The choice of suitable conventional additives for the primer composition depends on the particular end use and can be determined in the individual case by the person skilled in the art. Suitable additives comprise, for example, antioxidants, UV absorbers/light stabilizers, metal deactivators, antistats, reinforcers, fillers, antifogging agents, biocides, plasticizers, lubricants, emulsifiers, colorants, pigments, rheology agents, impact tougheners, adhesion regulators, optical brighteners, flame retardants, antidripping agents, nucleating agents, wetting agents, thickeners, protective colloids, defoamers, and mixtures thereof. The aqueous primer composition preferably does not comprise inorganic particles.

The aqueous primer composition preferably comprises

(a) from 18 to 50% by weight (based on the total composition) of the dispersed polymer;

(b) from 0 to 10 % by weight, preferably from 1 to 5% by weight (based on the total composition) of at least one additive selected from thickeners, defoamers, dispersing agents, wetting agents, surface modifiers, rheology modifiers, emulsifiers, protective colloids, and levelling agents; and (c) from 40 to 80% by weight water.

Suitable thickeners are for example polyethylene thickeners, polyurethane thickeners or polyacrylic thickeners. Suitable defoamers are for example polysiloxane defoamers or mineral oil defoamers.

Suitabel additves are for example mono- or dialkyl sulfosuccinate surfactants, alcohol alkoxylate surfactants, siloxanes, polyacrylic acid, polyurethanes, acrylic polymer additives (e.g. acrylic polymer levelling agents), Efka® formulation additives, Hydropalat® wetting agents, Rheovis® rheology modifers and thickeners, Dispex® dispersing agents, BYK® additives, TEGO® additives etc.

Preferred additives are Loxanol® Ml 6730 wb, Loxanol® PL 5814 wb, Loxanol® PL 5824 wb, Loxanol® PL 5830 wb, FoamStar® SI 2216 wb, FoamStar® SI 2240 wb, Disperbyk® 185, Disperbyk® 190, Disperbyk® 193, Disperbyk® 2091 , Rheovis® AS 1127 wb, Rheovis® AS 1130 wb, Rheovis® AS 1337 wb, Rheovis® PE 1330 wb, Rheovis® PU 1191 wb, Rheovis® PU 1331 wb, Orotan® 731 , Tego® Dispers 750 W, Solsperse® 44000, Dispelair® CF 16,

Dispelair® P 431 , BYK® 022, BYK® 024, Hydropalat® WE 3120 wb, Hydropalat® WE 3158 wb, Hydropalat® WE 3197 wb, Hydropalat® WE 3221 wb, Hydropalat® WE 3475 wb, Hydropalat® WE 3650 wb, Drewplus® T-3211 , Dapro® DF 7007, Agitan® 103, Agitan® 217, Foamaster® MO NXZ wb, Foamaster® MO 2111 wb, Foamaster® MO 2122 wb, Foamaster® NO 2331 wb, Foamaster® WO 2310 wb, FoamStar® PB 2724 wb, FoamStar® SI 2210 wb, FoamStar® SI 2213 wb, Byk® 346, Byk® 348, Tego® Glide 410, Tego® Glide 432.

Ink-jet printing ink

The aqueous ink-jet printing ink has a pH preferably above 7, in particular, if the polymer of the primer comprises acid groups.

The aqueous ink-jet printing ink preferably has a viscosity of from 1 to 50 mPa-s. Viscosity is measured at 25 °C on a Brookfield viscometer model LVT and using the appropriate spindle, giving readings between 20 to 80% of the full scale reading.

The aqueous ink-jet printing ink preferably comprises pigments, pigment dispersing resin, water, organic solvents, binder and additives such as surfactants, antifoam agents, and waxes. The ink-jet printing ink preferably comprises

1 to 30% by weight, more preferablyl to 12% by weight, more preferably from 2% to 8%, of at least one pigment,

10 to 90% by weight, preferably 15 to 60% by weight, more preferably 30 to 80% by weight of water,

3 to 60% by weight, preferably 3 to 50% by weight of organic solvent,

8 to 60% by weight of at least one binder, and 0-20% humectants ,

0.1 to 15% by weight of further additives such as surfactants, antifoam agents, and waxes. Amounts are based on the total weight of the ink.

The printing inks may contain the usual ink additives to adjust flow, surface tension, and gloss of a printed ink. Such additives typically are water-miscible or water-soluble materials such as polymeric dispersants, surfactants, waxes, humectants, chelating agents, defoamers, buffers, biocides, fungicides, viscosity modifiers, bactericides, anti-curling agents, anti-bleed agents and surface tension modifiers or a combination thereof. These additives may function as leveling agents, wetting agents, fillers, dispersants, defrothers or deaerators, or additional adjuvants may be added to provide a specific function. The amount of humectant used in the ink typically ranges from 1% to 20%, preferably from 5% to 15%, by weight, based on the total weight of the ink. The printing inks may contain a polymeric dispersant to disperse the pigment during mixing and grinding operations in the solvent.

All components of the ink may be blended together and ground to reduce the pigment particles to the desired size distribution, typically 10 microns or less, or alternatively the pigment and the polymeric dispersant can be premixed and ground in the solvent to form a "base" which is subsequently blended with the remaining components of the ink composition. The ink components may be mixed in a high speed mixer until a slurry consistency is reached and then passed through a media mill until the pigment is reduced to 10 microns or smaller. Other useful colorants, solvents and adjuvants can be identified by consulting The Printing Ink Manual.

The ink-jet ink composition can include a surfactant that aids in control of droplet formation, surface wetting and coalescence of droplets in the printed image. Anionic and nonionic surfactants are preferred. Typically the amount of surfactant is effective to provide a surface tension of the ink-jet ink composition of from 22 dynes/cm to 36 dynes/cm. Preferably, the ink-jet ink comprises at least one pigment dispersing resin. Examples of commercially available products of the pigment dispersing resin include styrene-acrylic acid resins (e.g. JONCRYL series of BASF)

As the pigment contained in the water-based ink-jet ink, conventionally known pigments can be used. As black pigments contained in the water-based pigment ink, carbon blacks produced by a furnace method or a channel method can be exemplified. For example, carbon blacks preferably have characteristics such as a primary particle diameter of 11 to 40 nm, a specific surface area according to the BET method of 50 to 400 m ² /g, a volatile content of 0.5% to 10% by mass, and a pH value of 2 to 10.

Examples of yellow pigments contained in the water-based pigment ink include C.l. Pigment Yellow 1 , 2, 3, 12, 13, 14, 16, 17, 20, 24, 74, 83, 86, 93, 94, 95, 109, 110, 117, 120, 125, 128, 137, 138, 139, 147, 148, 150, 151 , 154, 155, 166, 168, 180, 185, and 213.

Examples of magenta pigments contained in the water-based pigment ink include C.l. Pigment Red 5, 7, 9, 12, 31 , 48, 49, 52, 53, 57, 97, 112, 122, 147, 149, 150, 168, 177, 178, 179, 202, 206, 207, 209, 238, 242, 254, 255, and 269; and C.l. Pigment Violet 19, 23, 29, 30, 37, 40, and 50.

Examples of cyan pigments contained in the water-based pigment ink include C.l. Pigment Blue 1 , 2, 3, 15:3, 15:4, 16, and 22; and C.l. Vat Blue 4 and 6.

As the pigments contained in the water-based pigment ink, pigments of colors other than those described above, self-dispersing pigments, and the like can also be used. These pigments may be used singly or in combination of two or more kinds, in an ink of each color. The content of the pigment contained in the water-based pigment ink is, as a weight ratio, preferably in the range of 0.1% to 20% by weight, and more preferably 0.1% to 12% by weight, in the ink.

A suitable medium that is used in the ink is preferably a mixed solvent of water and a water- soluble solvent. Preferably, the water is not the common water containing various ions, but ion- exchanged water (deionized water). As the water-soluble solvent that is used in a mixture with water, glycol ethers and diols are preferred. Among them, (poly) alkylene glycol monoalkyl ethers, and alkanediols having 3 to 6 carbon atoms are effective. These solvents penetrate into paper very rapidly. These solvents penetrate into a base material having low solvent absorptive ness, such as coated paper or art paper, rapidly, too. Accordingly, drying is achieved fast at the time of printing, and accurate printing can be realized. Furthermore, since the solvents have high boiling points, they sufficiently work as moisturizers. Specific examples of glycol ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, diethylene glycol monopentyl ether, diethylene glycol monohexyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monobutyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether. Specific examples of diols include 1 ,2-propanediol, 1 ,3-propanediol, 1 ,2- butanediol, 1 ,3-butanediol, 1 ,2-pentanediol, 1 ,5-pentanediol, 1 ,2-hexanediol, 1 ,6-hexanediol, and 2-methyl-2,4-pentanediol. Among these, highly effective compounds are diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, triethylene glycol monomethyl ether, 1 ,2- propanediol, 1 ,2-butanediol, 1 ,3-butanediol, 1 ,2-hexanediol, and 2-methyl-2,4-pentanediol. These solvents may be used singly, or a mixture of plural solvents can also be used. Furthermore, depending on the type of the base material to be used in printing, water-soluble nitrogen-containing heterocyclic compounds such as 2-pyrrolidone, N-methylpyrrolidone, N- ethyl pyrrolidone, N-methyl oxazolidinone, and N-ethyl oxazolidinone can also be added for the purpose of enhancing solubility.

The content of the water-soluble organic solvent such as described above in the ink is preferably in the range of from 3% to 60% by mass, and more preferably in the range of from 3% by mass to 50% by mass, relative to the total mass of the ink. Furthermore, the content of water is preferably in the range of from 10% to 90% by mass, and more preferably in the range of from 30% to 80% by mass, relative to the total mass of the ink.

The water-based pigment ink may contain an aqueous polymer emulsion. When the ink contains an aqueous polymer emulsion, the resistance of the printed coating film can be enhanced without much increasing the ink viscosity. Thereby, water resistance, solvent resistance, scratch resistance and the like are enhanced. An enhancement of resistance to a certain extent can also be expected by adding a water-soluble resin, but the viscosity of the ink tends to rise. In the case of an ink-jet ink, there is a suitable range of viscosity for the ejection of the ink through nozzles. If the viscosity is too high, the ink may not be ejected, so that it is important to suppress an increase of the viscosity. The content of the aqueous polymer emulsion such as described above in the ink, as the solids concentration of the emulsion, is preferably in the range of 2% to 30% by weight, and more preferably in the range of 3% to 20% by weight, relative to the total weight of the ink.

The aqueous primer composition is coated onto an absorbent substrate. Suitable absorbent substrates are for example paper, card board, in particular corrugated card board. Suitable paper is for example high-quality paper, medium-quality paper, coated paper, art paper, gloss paper, paper for newspaper, various communication papers or various special papers.

The aqueous primer composition can be applied to the substrate as in-line coating immediately before forming the image on the coated substrate by ink-jet printing. Or the aqueous primer composition can be applied by one manufacturer and the primed substrate is used later by the same or another manufacturer to form the image on the coated substrate by ink-jet printing.

Coating the substrate with the primer can be carried out by procedures known to a person skilled in the art, for example by flexographic coating, gravure coating, roll coating, blade coating, air knife coating, gate roll coating, bar coating, a size press method, die coating, lip coating, comma coating, spin coating, curtain coating, spray coating, digital printing or ink-jet printing. Preferred coating methods are flexographic coating, gravure coating, curtain coating, spray coating, digital printing and ink-jet printing. The aqueous primer composition is applied with a coating weight (dry weight) of preferably from 0.2 to 40 g/m ² , more preferably from 0.5 to 20 g/m ² or from 2 to 6 g/m ² .

Drying methods are not particularly limited to, but hot air drying, an infrared method, or a reduced pressure method may be exemplified. The drying conditions may depend on the film forming properties of the coating agent, the amount of coating, or the selected additives, however, hot air heating at about 60 to 180 °C may be employed, or infrared heating at about 20 to 60 °C may be employed. As a result of the application as described above, an ink-jet ink receiving layer is formed on the substrate material, and thus, a recording medium for forming an image can be obtained. When printing is carried out with an ink-jet ink on the coated substrate, a printed matter having an image formed thereon can be obtained. The primer composition may be coated on only one surface of the substrate material, or may be provided on both surfaces.

Furthermore, if necessary, the substrate material can also be subjected to a super calender treatment, a cast treatment or the like for the purpose of planarization, gloss enhancement or surface strength enhancement of the ink-receiving layer.

Furthermore, the aqueous primer composition can be filled in an ink cartridge of an ink-jet printer, and printing can be carried out by an ink-jet system simultaneously with or prior to an ink-jet printing ink.

An image is formed on the coated substrate by ink-jet printing using a water-based ink-jet printing ink. Ink-jet printing can be done in industrial scale with ink-jet printing speeds of preferably more than 1 m/min, for example from more than 1 and up to 400 m/min, or from 10 to 150 m/min.

The present invention further relates to the printed matter produced by the method described herein.

Benefits of the method of the invention are good fixation of, good dot gain, higher color strength of water-based ink-jet printing inks printed on absorbing substrates..

The invention is now illustrated by the following non-limiting examples.

Examples

Abbreviations AA acrylic acid

MAA methacrylic acid

EA ethyl acrylate

BA n-butyl acrylate

MMA methyl methacrylate

2-EHA 2-ethylhexyl acrylate

STY styrene

GMA glycidyl methacrylate

DAAM diacetone acrylamide

AMS a-methyl styrene

Reasoap Adeka Reasoap® SR1025; anionic ether sulfate surfactant MFFT minimum film formation temperature

Method of measuring glass transition temperature (Tg):

Differential Scanning Calorimetry (ASTM D 3418-08, ’’midpoint temperature” of second heating curve, heating rate 20 °C/min)

Method of measuring viscosity:

Viscosity is measured at 25 °C on a Brookfield viscometer model LVT and using the appropriate spindle, giving readings between 20 to 80% of the full scale reading.

Method of measuring molecular weight:

Molecular weight determination is performed by gel permeation chromatography (GPC) against polystyrene standards. The polymer is dissolved in THF containing a small amount of TFA and 50 mI_ injected on a Waters Alliance 2690 Separations Module equipped with a Waters 2414 refractive index detector and THF (containing 0.10% TFA) as eluent.

Method of measuring film formation:

Film formation is determined according to ASTM D2354 using a Rhopoint MFFT bar 90 instrument. A thin layer (75 pm) of liquid aqueous polymer composition is applied with a block applicator on an aluminum foil that is heated on the gradual heating plate of the instrument. The minimum film formation temperature (MFFT) is defined as the temperature above which the polymer forms a continuous homogeneous film. Below this temperature for example a cracked film will form.

Inks:

All tests are done with HP - 953XL cartridge series Additionally Epson 664 cartridge series are used for cross checks

Print tests

Brown craft paper or white paper was coated with a test sample of primer with a coating weight of 1 g/m ² (dry) at room temperature and dried for 1 minute at 60 °C. The primer coated paper was then printed with an ink-jet printer with various ink-jet ink colors.

Method of measurino color strenoth:

A brown craft paper substrate is printed with cyan, yellow, magenta and black ink-jet ink colors. Color strength is measured with an X-rite 500 Series spectrodensitometer instrument. First, the color density of the paper background is measured according to the “density blank” mode of the instrument. This background color density is automatically subtracted from the following color density measurements. The instrument is set to the to be measured color (cyan, yellow, magenta or black). The color density of 3 different areas per color of the printed substrate is measured and an average is given.

Method of measuring dot gain:

A white absorbing (paper) substrate is printed with an area of magenta ink-jet ink color with an adjacent line of cyan. What is desired is a smooth band of blue between magenta and white. If the dot gain is not good, the ink still flows upon landing and the band gets wider and more blurred. The print is evaluated visually and ranked on a scale of from 1 (bad) to 5 (very good). Method of measuring ink lav:

A paper substrate (craft paper) is printed with an area of magenta or black ink-jet ink color.

What is desired is a homogeneous lay of the magenta or black ink. If the ink lay is not good, the image will become more grainy. The print is evaluated visually for homogeneity and ranked on a scale of from 1 (bad) to 5 (very good).

Preparation of aqueous primer compositions

(examples P1 to P22 according to the invention; examples CP1 to CP4 comparative)

The primer polymers are prepared by typical emulsion polymerization. In a typical example, part of the surfactants and water are added to a reactor and heated to 85 °C. Next, an aqueous solution of polymerization initiator is added to the reactor. Subsequently, an emulsion of the monomers in water, stabilized by the remainder of the surfactants is added over 120 minutes at 85 °C. After the addition, the reactor content is stirred for another 60 minutes at 85 °C. The product is then cooled, filtered and treated with preservative.

Standard neutralizing agents such as ammonia or sodium bicarbonate are used to neutralize the primer polymers and obtain the desired pH.

Monomer compositions and physical parameters of the examples are summarized in table 1. Table 1 : Aqueous primer compositions

¹⁾ weight average molecular weight

²⁾ amount in parts by weight

³⁾ Neutralization with NaOH:ammonia 1 .84 : 0.78 weight ratio

⁴⁾ Neutralization with sodium bicarbonate ⁵⁾ Neutralization with ammonia The results of the application tests are summarized in table 2.

Table 2: Results of application tests

¹⁾ sum of dot gain black + dot gain magenta + ink lay

The data in table 2 prove that use of the primers according to the invention leads to high color strength and good dot gain and ink lay results.