RECORDING MEDIA FOR INK

Abstract: Described are recording media for ink having excel­ lent ink absorbing properties and excellent waterproof properties. The recording medium comprises a substrate containing a composition which comprises polyvinyl alco­ hol, chitosan and an aldehyde compound.

Description: The present invention relates to recording media for ink and more particularly, to recording media for aqueous ink for use in ink jet printing or in pen-type printing, which media are highly waterproof and have excellent absorption properties. In recent years, the ink jet printing system has been increasingly applied to facsimile machines and different printers because this system minimizes noise, makes color printing easy, and facilitates high speed recording. In pen-type printing, the use of aqueous ink has become popular because the tip of a pen hardly becomes dry, the tip of a pen moves smoothly etc. Ordinary paper has heretofore been employed as carrier or recording medium for these recording systems. However, as the recording machines are improved to perform high speed recording, multicolor printing, etc., the recording media for ink need to have better properties as well. The first property they must possess is a high ink absorption rate, the second is the property of not causing the ink to blot, as in the case of blotting the diameter of an ink dot would be undesirably broadened on the recording medium for ink jet printting. Thirdly, the recording media for ink must have the property of being highly waterproof. In order to satisfy these requirements, various attempts have been made, using for example paper or water soluble resin as materials for absorbing aqueous ink. However, when the absorption rate of ink is increased, serious blotting occurs. For example, in the case of recording media for ink jet printing, the diameter of an ink dot becomes undesirably broad or waterproof properties become extremely poor. EP-A-0 187 673 discloses a composition which comprises polyvinylalcohol, a chitosan and an aldehyde compound, having water resistant properties, and a heat-sensitive recording sheet. It is an object of the present invention to provide recording media for ink which satisfy the requirements described above and, in particular, have excellent ink absorption properties and are highly waterproof. It is another object of the present invention to provide recording media for ink having excellent transparency which can be used for projecting recorded images with optical equipments such as slide projectors or overhead projectors (OHP) onto a screen, or for the transmission of light as in color display devices etc. As a result of extensive investigations to solve the problems described above and to achieve the objects described above, the present inventors have developed recording media for ink which comprise a substrate containing a composition which comprises water soluble polyvinyl alcohol (hereafter polyvinyl alcohol is simply referred to as PVA), chitosan and a water soluble compound having at least one aldehyde group in an aqueous solution. In particular, the recording media for ink according to the invention comprise a substrate, on the surface of which there is provided an ink absorbing layer containing a composition which comprises water soluble PVA, chitosan and a water soluble compound having at least one aldehyde group in an aqueous solution. These recording media exhibit excellent ink absorbing properties and waterproof properties. The present invention will be described hereinafter in detail. As PVA which can be used in the present invention, any PVA is suitable if it is water soluble. In addition to ordinary PVA, the following PVAs can also be used: silane-modified PVA, anion-modified PVA such as carboxyl group-modified PVA, sulfonic acid group-modified PVA, phosphoric acid group-modified PVA, cation-modified PVA, or modified PVAs obtained by copolymerization with e.g. ethylene, vinyl ethers having a long chain alkyl group, vinyl esters, (meth)acrylamides or α-olefins. The polymerization degree of these PVAs is not particularly limited but is generally chosen from a range of 100 to 3000. The degree of saponification is not particularly limited as long as the PVA is water soluble, but is generally chosen from a range of 70 to 100 mol%. A chitosan which can be used in the present invention, is chitosan containing 40 mol% or more, preferably 80 mol% or more amino groups. A part or all amino groups of the chitosan may be converted with an acid into ammonium groups. The molecular weight of the chitosan is not particularly limited. However, when a chitosan is used, whose molecular weight is such that the viscosity (by BL type viscometer) of a 1 wt% aqueous solution at 20°C exceeds 7 x 10⁻² Pas, the viscosity of the coating solution comprising the PVA, the chitosan and an aldehyde compound is too high. If the concentration of the coating solution is too high, the recording medium is not sufficiently waterproof. Therefore, a chitosan having a viscosity of a 1 wt% aqueous solution at 20°C of 1 to 70 cp, more preferably 1 to 30 cp, is especially suitable. The chitosan used in the present invention may be dissolved in water or in an aqueous solution containing an acid. The acid generally used is acetic acid, formic acid, glycolic acid, lactic acid, citric acid, benzoic acid, sulfamic acid, hydrochloric acid, phosphoric acid, fumaric acid, maleic acid, etc. The water soluble compound used in the present invention may be any compound which is water soluble and provides a substrate having at least one aldehyde group in an aqueous solution. Specific examples of aldehyde compounds include monoaldehydes such as formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, polyvalent aldehydes, such as glyoxal, glutaraldehyde, dialdehyde starch, condensation products of formaldehyde and ammonia such as hexamethylenetetramine, a methylolamide such as dimethylol urea, N-methylolacrylamide, urea-formaldehyde resins and melamine-formaldehyde resins. Mixtures of such aldehydes may also be used. Among the aldehydes described above, glyoxal and hexamethylenetetramine are preferred because of easy handling (odorless property, viscosity stability, etc.) of the coating solution comprising PVA, chitosan and the aldehyde compound and the effect of imparting waterproof properties. The composition of the recording medium advantageously contains 1 to 50 parts by weight, preferably 5 to 30 parts by weight, of chitosan and 0.1 to 10 parts by weight, of the aldehyde compound, preferably 0.5 to 5 parts by weight, of the aldehyde compound, based on 100 parts by weight of PVA. If the chitosan content is less than 1 part by weight, the chitosan is ineffective, while with a chitosan content of more than 50 parts by weight, the viscosity of the coating solution becomes too high which is undesirable. On the other hand, if the aldehyde content is less than 0.1 part by weight, the aldehyde compound has no effect while with a content of more than 10 parts by weight, the viscosity stability of the aforesaid coating solution becomes poor. The composition comprising PVA, chitosan and an aldehyde compound can be employed as it is but may also be used in combination with other water soluble or water dispersible resins or polymers. Examples of such resins or polymers are albumin, gelatin, casein, starch, gum arabic; cellulose derivatives such as methyl cellulose, hydroxyethyl cellulose, etc.; nonionic water soluble resins such as polyamide resins, melamine resins, poly(meth)acrylamide, polyvinylpyrrolidone, etc.; anionic water soluble resins such as CMC, sodium poly(meth)acrylate, water soluble polyesters, etc.; cationic water soluble resins such as polyethyleneimine, polyvinylamine, polyallylamine, polyallylamine-sulfone copolymers or ammonium salts thereof, cationated starch, cationated poly(meth)acrylamide, cationated polyamide resins, etc.; water dispersible resins such as SBR latex, NBR latex, vinylacetate emulsions, ethylene/vinyl acetate copolymer emulsions, (meth)acrylic acid ester emulsions, vinyl chloride emulsions, etc. If these water soluble or water dispersible resins are used in combination with the composition of the present invention, they are employed in an amount of less than 100 parts by weight, preferably less than 50 parts by weight, based on 100 parts by weight of PVA. The composition comprising PVA, chitosan and an aldehyde compound can also be used in combination with fillers such as silica, clay, talc, diatomaceous earth, zeolite, calcium carbonate, alumina, zinc oxide, satin white, etc. In this case, the amount to be used varies depending on whether the recording media are to possess excellent transparency or other properties; in general, the weight ratio of PVA filler is in the range of 1/100 to 100/1, preferably 5/100 to 100/5. The composition comprising PVA, chitosan and an aldehyde compound can be used as an ink absorbing layer because the composition itself has excellent ink absorbing properties. However, the composition may also be used as a waterproofing layer. In this case, a coating comprising the composition is formed on top of the ink absorbing layer which comprises other ink absorbing materials. As substrate suitable in the present invention, any known transparent or opaque substrate can be used. Examples of transparent substrates are films or sheets of polyesters, polystyrene, polyvinyl chloride, polymethyl methacrylate, cellulose acetate, polycarbonate, polyimide, cellophane, celluloid, or paper of high transparency. Examples of opaque substrates are ordinary paper, pigment-coated paper, cloth, wood, sheet metal or foil, synthetic paper or a film or sheet of an opaque synthetic resin. In the case of highly transparent recording media for ink which is an object of the present invention, a transparent substrate is used. Suitable methods for incorporating the composition comprising the PVA, chitosan and an aldehyde compound in the substrate are methods wherein the substrate is immersed in an aqueous solution of the composition described above, or in an aqueous solution or aqueous dispersion of a mixture of the composition with other water soluble or water dispersible resins or fillers or wherein a coated layer(s) is formed on the upper surface, or lower surface or both surfaces of the substrate by conventional coating means such as a size press, an air knife coater, a roll coater, a bar coater or a blade coater. Furthermore, the substrate may be coated with an adhesive resin onto which the aforesaid composition may be coated. Examples of adhesive resins are a composition of a polyurethane and an isocyanate, a rubber adhesive and an emulsion adhesive. If the substrate is paper, the aqueous solution or aqueous dispersion described above can also be incorporated during paper making. The amount of the composition comprising PVA, chitosan and an aldehyde compound contained in the recording media for ink of the present invention is not particularly limited but is generally 0.1 to 200 g/m², preferably 1 to 100 g/m². The recording media for ink of the present invention are mainly useful as recording media for ink jet printing (non impact printing) but are not limited thereto. They are also useful as recording media for pen-type printing or recording pens where a liquid ink, especially an aqueous ink is directly contacted with the recording media. The aqueous inks used for printing on the aforementioned media of the present invention are aqueous compositions which mainly comprise a water soluble dye, a wetting agent and a solvent which contains not less than 20 weight % water, more preferably not less than 50 weight % water and less than 80 weight % of a water soluble organic solvent, more preferably less than 50 weight % of a water soluble organic solvent. Examples of suitable organic solvents are alcohols with 1 to 4 carbon atoms such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol and isobutyl alcohol; amide compounds such as dimethylformamide and dimethylacetamide; ketones such as acetone and diacetone alcohol; ethers such as tetrahydrofuran and dioxane; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; alkylene-glycols or polyols with 2 to 6 carbon atoms such as ethylene glycol, propylene glycol, 1,2,6-hexanetriol, hexylene glycol, diethylene glycol and glycerin, and ethers of polyols such as ethylene glycol methyl ether, diethylene glycol methyl ether and triethylene glycol monomethyl ether; and pyrrolidones such as 2-methylpyrrolidone. The reasons why the recording media for ink of the present invention have excellent ink absorbing properties and are highly waterproof, are not known. The present invention will be described in more detail with reference to the examples below but is not deemed to be limited thereto. In the examples, "%" and "part" are all based on weight, unless otherwise. indicated. EXAMPLES (1) Method of ink jet printing With a recording device equipped with an "on-demand" type ink jet printing head having a discharge orifice with a diameter of 60 µm, a color ink jet printing was made using the 4 color inks described below. The recording properties were evaluated. (1) Yellow ink (composition) C.I. Acid Yellow 2.3 2 parts Diethylene glycol 30 parts Water 70 parts (2) Magenta ink (composition) C.I. Acid red 32 2 parts Diethylene glycol 30 parts Water 70 parts (3) Cyan ink (composition) C.I. Direct blue 86 2 parts Diethylene glycol 30 parts Water 70 parts (4) Black ink (composition) C.I. Direct black 19 2 parts Diethylene glycol 30 parts Water 70 parts (2) Rate of absorbing ink After recording with ink, the printed areas on the recording sheet were rubbed with a finger for a specified time period until no change was observed. It was found that the shorter the time period was, the larger was the ink absorbing rate. (3) Blotting degree The diameter of a dot of the printed area was measured by a stereoscopic microscope, in order to determine the enlargement of the original diameter of the ink droplet. It was found that the lower the enlargement was, the lower was the blotting degree. (4) Degree of transparency Transmittance (%) of visible light at non-printed areas on the recording medium was measured with a spectrophotometer using visible light having a wavelength of 500 nm. The transmittance thus measured is the degree of transparency. It was found that the larger the transmittance was, the higher was the degree of transparency. (5) Waterproof property After the recording with ink the printed areas on the sheet were wetted with water. The criterion in evaluating the waterproof property was whether or not the printed areas were dissolved or got blotted when rubbed with the finger. 5: No change 4: Slight blotting but no dissolution 3: Blotting and swelling but no dissolution 2: Blotting and slight dissolution 1: Blotting and dissolution Example 1 100 parts of PVA (Kuraray Poval PVA-405) having a polymerization degree of 550 and a saponification degree of 80 mol% and 15 parts of chitosan having a viscosity of 13 cp as a 1% aqueous solution at 20°C (viscosity measured with BL type viscometer) and having an amino group content of 89 mol% were dissolved in an acetic acid-acidic aqueous solution. A 15% aqueous solution was obtained. A coating solution was prepared by adding a 40% glyoxal solution to this aqueous solution in an amount of 2% calculated as the solids content based on PVA. The coating solution was coated onto a polyester sheet having a thickness of 50 µm and a transparency degree of 95% at a dry coating weight of 20 g/m² and was dried to give a recording medium for ink. The rate of ink absorption, the blotting degree, transperency degree and waterproof properties obtained in ink jet printing are shown in Table I. Examples 2 to 8 The procedure was performed in a manner similar to that of Example 1 except that the PVA or chitosan content was changed as shown in Table I. The results are also shown in Table I. Comparative Examples 1 to 4 The procedure was performed in a manner similar to that of Example 1 except that the following resin was used in place of the composition used in Example 1. The results are also shown in Table I. Resin used in Comparative Example 1: 15% aqueous solution containing 100 parts of Poval PVA-405 and 2 parts of glyoxal Resin used in Comparative Example 2: 10% aqueous solution containing chitosan alone as used in Example 1 Resin used in Comparative Example 3: 10% aqueous solution containing polyvinylpyrrolidone alone Resin used in Comparative Example 4: 15% aqueous solution of a cation-modified PVA containing 2 mol% of cation groups, and having a polymerization degree of 1750 and a saponification degree of 88 mol%. (Note 1) PVA-420: PVA having a polymerization degree of 1750 and a saponification degree of 80 mol% Carboxyl-modified PVA: PVA containing 1 mol% of carboxyl group units and having a polymerization degree of 1750 and a saponification degree of 88 mol% Sulfonic acid-modified PVA: PVA containing 1 mol% of sulfonic acid group units and having a polymerization degree of 1800 and a saponification degree of 88 mol% Cation-modified PVA: PVA containing 2 mol% of cationic group units and having a polymerization of 1750 and a saponification degree of 88 mol% Examples 9 to 12 100 parts of a modified PVA containing 2 mol% of cation group units and having a polymerization degree of 1750 and a saponification degree of 80 mol% and 10 parts of chitosan having the viscosity as shown in Table 2 as a 1% aqueous solution at 20°C (viscosity measured with BL type viscometer) and having an amino group content of 85 mol% were dissolved in a lactic acid-acidic aqueous solution. A 15% aqueous solution was obtained. A coating solution was prepared by adding a 40% glyoxal solution to this aqueous solution in an amount of 1% calculated as the solids content based on PVA. The coating solution was coated onto a transparent polyester sheet having a thickness of 75 µm at a dry coating weight of 5 g/m² and was dried to give a recording medium for ink. The rate of ink absorption and the waterproof properties in ink jet printing are shown in Table II. Examples 13 to 15 The procedure was performed in a manner similar to that of Example 9 except that the amount of glyoxal added was changed as shown in Table II. The results are also shown in Table II. Examples 16 to 20 The procedure was performed in a manner similar to that of Example 9 except that the aldehyde compounds as shown in Table II were used in amounts shown in Table II in place of glyoxal (1%/PVA). The results are also shown in Table II. Example 21 The composition used in Example 8 was coated onto the polyvinylpyrrolidone layer of the recording medium for ink obtained in Comparative Example 3 in a dry coating weight of 5 g/m² and dried to give a recording medium for ink. The results are shown in Table III. Example 22 The composition used in Example 1 was coated onto the cation-modified PVA layer of the recording medium for ink obtained in Comparative Example 4 in a dry coating weight of 3 g/m² and dried to give a recording medium for ink. The results are shown in Table III. Example 23 The procedure was performed in a manner similar to that of Example 8 except that art paper was used as a substrate. The ink absorbing rate was 25 seconds, the blotting degree was 2.1 times and the waterproof property was 5. Example 24 Wood free paper was used as a substrate. A coating solution having a composition described below, which was obtained using the composition employed in Example 1, was coated onto the substrate in a dry coating weight of 20 g/m² by means of a bar coater and dried to give a recording medium for ink. The ink absorbing rate, blotting degree and waterproof property of this sheet are shown in Table IV. Non-colloidal silica powder 100 parts Composition used in Example 1 25 parts Water 500 parts Comparative Example 5 The procedure was performed in a manner similar to that of Example 24 except that the PVA employed in Comparative Example 1 was used in place of the composition used in Example 24. The results are also shown in Table IV. Example 25 Wood free paper was used as a substrate. The coating solution obtained in Example 2 was coated onto the substrate in a dry coating weight of 5 g/m² and dried to give a recording medium for ink. The properties of this sheet are shown in Table V. Comparative Example 6 The procedure was performed in a manner similar to that of Example 25 except that polyvinylpyrrolidone was used in place of the composition used in Example 25. The results are also shown in Table V. Example 26 The recording medium obtained in Example 9 and pens with 4 colored inks as employed in the ink jet printing were used. Straight crossing lines and painted circles overlapping with each other were drawn onto the medium by a pen-type printer. Immediately after the pen recording, the recorded portions were rubbed with a finger but no change on the printed areas was noted. Nor was any change of the recording observed in the crossed portions of crossing straight lines and the overlapping painted circles. Blotting of the respective inks or damage of the coated layer was not observed either. The waterproof property after the recording was 5. Comparative Examples 7 and 8 The procedure was performed in a manner similar to that of Example 26 except for using the following recording media. Recording medium used in Comparative Example 7: Recording medium obtained in Comparative Example 1. Serious blotting of ink was noted in crossed portions of crossing straight lines and overlapping circle areas. The waterproof property was 1. Recording medium used in Comparative Example 8: Recording medium obtained in Comparative Example 3. Breakage due to scratching with the pen occurred in crossed portions of crossing straight lines and overlapping circles areas. At the same time, serious blotting of ink was noted. The waterproof property was 1.

Claims: A recording medium for ink, comprising a substrate containing a composition which comprises water-soluble polyvinyl alcohol, chitosan having a viscosity of 1 x 10⁻³ to 7 x 10⁻² Pas measured at 20°C in a 1 wt% aqueous solution, and a water-soluble compound having at least one aldehyde group in an aqueous solution. The recording medium as claimed in claim 1 wherein an ink absorbing layer containing a composition which comprises polyvinyl alcohol, chitosan and an aldehyde compound is provided on said substrate. The recording medium as claimed in claim 1 or 2 comprising polyvinyl alcohol, 1 to 50 parts by weight of chitosan and 0.1 to 10 parts by weight of the water-soluble compound based on 100 parts by weight of the polyvinyl alcohol. The recording medium as claimed in any one of claims 1 to 3 wherein the chitosan has a viscosity of 1 x 10⁻³ to 3 x 10⁻² Pas measured at 20°C in a 1 wt% aqueous solution. The recording medium as claimed in any one of claims 1 to 4 wherein the water-soluble compound is at least one compound selected from the group consisting of water-soluble monoaldehydes, polyvalentaldehydes, condensation products of an amine and formaldehyde, methylolamides, urea-formaldehyde resins and melamine-formaldehyde resins. The recording medium as claimed in any one of claims 1 to 5 wherein the water-soluble compound is glyoxal or hexamethylenetetramine. The recording medium as claimed in any one of claims 1 to 6 wherein the substrate is a transparent film or sheet. The recording medium as claimed in any one of claims 1 to 6 wherein the substrate is paper. The recording medium as claimed in any one of claims 1 to 8 wherein the recording medium is a recording medium for ink jet printing. The recording medium as claimed in any one of claims 1 to 8 wherein the recording medium is a recording medium for pentype printing.

Inventors: MARUYAMA, HITOSHI, ONO, ISAO, SHIRAGA, SADAHIKO, YAMAUCHI, JUNNOSUKE
Applicants: KURARAY CO., LTD.
Application year: 1988
Main classification: B41M1
IPC codes: D21H19, B41M5