JPH10151848A - Back print film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a back print film with outstanding printability and water resistance. SOLUTION: This back print film has an ink absorbing layer, to which a binder containing nitric esters carboxyalkyl cellulose having a degree of substitution of nitric ester group of 0.2-2.95 and a degree of substitution of carboxyalkyl ether group of 0.05-2.8 per one unit of glucose anhydride is applied, formed on one of the faces of a transparent film base. Alternatively, the back print film has the ink absorbing layer to which a binder containing 3-80wt.% of nitric esters and 97-20wt.% of cation-modified polyvinyl alcohol is applied, formed on one of the faces of the transparent film and further, a porous layer formed on the ink absorbing layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a back print film. More specifically, an ink-absorbing layer mainly composed of a specific nitrate ester or the nitrates and a cation-modified polyvinyl alcohol is formed on one surface of a transparent film substrate, and a porous layer is further provided thereon. The present invention relates to a back print film having excellent printability and water resistance.

[0002]

2. Description of the Related Art Conventionally, various recording systems such as a wire dot recording system, a thermosensitive coloring recording system, a thermosensitive thermal transfer recording system, a thermal sublimation thermal transfer recording system, an electrophotographic recording system, and an ink jet recording system have been used. There is. Among them, the ink jet system has the features that plain paper can be used as a recording sheet, printing cost is low, the apparatus is compact, there is no noise, high-speed recording is possible, and colorization is possible. For this reason, attention has been paid to the use as a printer for outputting image information such as characters and figures created by a computer or the like at high speed and accurately. In addition, because it is easy to colorize and the pattern is clear, computer-generated image information is recorded on a transparent recording sheet by an inkjet printer, and is used as a manuscript for an overhead projector (OHP) at conferences, presentations at various conferences, etc. It has been used for. In addition, there is an increasing demand for recording on a recording sheet of a white polyester film as a base material and using it as a design media such as a large poster, a display, an advertisement such as a flyer, in addition to various presentation manuscripts. These are generally called by a name such as a gloss (glossy) sheet and a gloss (glossy) film.

[0003] The back print film is usually
It has a structure in which an ink absorbing layer (holding layer) is provided on a transparent substrate, and an ink transmitting layer (transporting layer) is provided thereon. The features of this structure are that the ink is held between the base material and the ink permeable layer, and that the ink permeable layer has a function of protecting the printing surface. Thereby, abrasion, deterioration, and the like from the printing surface side can be prevented.

[0004] Unlike an OHP sheet or a gloss sheet, the purpose of the film is to observe it from the side opposite to the surface printed by an ink jet printer or the like. As a method of using the film, there are uses such as a bulletin board such as a timetable, a flyer, an advertisement, and a poster or a display requiring designability. Further, it can be used even at night by illumination (backlight) from the printing surface side, and its use has been increasing in recent years.

[0005] Important characteristics required for the back print film include printability and water resistance when used outdoors. To maintain these properties,
Various materials have been selected and used for the ink permeable layer and the ink absorbing layer. It is necessary that the ink permeable layer has liquid permeability and has a function of rapidly absorbing and transmitting ink that has landed on its surface. At this time, the affinity must be high for the solvent in the ink, but must be low for the coloring agent such as the dye or pigment in the ink. Further, it is necessary to have a function as a protective layer of the ink absorbing layer.

On the other hand, the ink absorbing layer needs to have a function of absorbing and retaining the ink or the coloring agent transferred from the ink transmitting layer. At this time, in order to stably absorb and supplement the ink temporarily absorbed by the ink permeable layer, the absorbing power for the ink must be stronger than that of the ink permeable layer. Therefore, in particular, it must have a high affinity for the color former.

The influence of water from the side to be observed (substrate side) does not cause any particular problem. Therefore, it is necessary to impart water resistance. About this water resistance, in the conventional back print film,
The fact is that it is not always enough. For example, Japanese Patent Application Laid-Open No. 63-34176 discloses an example in which a cation-modified PVA is used for an ink absorbing layer. However, although the printability is relatively good, the water resistance is blurred by the ink. Was inadequate.

Japanese Patent Application Laid-Open No. 62-242576 discloses an example in which polyvinylpyrrolidone is used for the ink absorbing layer. However, according to the study of the present inventors, the water resistance was insufficient. . Further, Japanese Unexamined Patent Application Publication No.
No. 126 discloses an example in which alumina hydrate having a pseudo-boehmite structure and silica gel powder are used for an ink absorbing layer. Since the ink absorbing layer utilizes a void such as an alumina material, it is impossible to prevent the intrusion of moisture from the outside (that is, because the water resistance is insufficient). In addition, there is a problem that the amount of absorbed ink is insufficient and ink overflow occurs after printing.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a back print film in which an ink absorbing layer is provided on one side of a transparent film substrate and a porous layer is further provided thereon, and has excellent printability and water resistance. The purpose of the present invention is to provide a back print film.

[0010]

Means for Solving the Problems The present inventor has found that in order to solve the above problems, nitrates of carboxyalkylcellulose or a binder comprising the nitrates and cation-modified polyvinyl alcohol are useful. Thus, the present invention has been completed. That is, the present invention is as follows. (1) On one surface of the transparent film substrate, the degree of substitution of nitrate ester groups per anhydrous glucose unit is 0.2 to 2.9.
5, the carboxyalkyl ether group substitution degree is 0.05 to
A back print film, comprising: an ink absorbing layer coated with a binder containing a carboxyalkyl cellulose nitrate ester of 2.8, and a porous layer further provided thereon. (2) The binder forming the ink absorbing layer has a nitrate ester substitution degree of 0.2 per anhydrous glucose unit.
To 2.95, a carboxyalkyl ether group substitution degree of 0.05 to 2.8, 3 to 80% by weight of nitrates of carboxyalkyl cellulose and 97 to 20% by weight of cation-modified polyvinyl alcohol. 2. The back print film as described in 1 above, which is characterized by:

Hereinafter, the present invention will be described in detail. The back print film referred to in the present invention, on one side on a transparent substrate,
It has an ink absorbing layer (holding layer) and an ink permeable layer (transporting layer) on it, and is used for observing recorded images from the opposite side (substrate side) of the printing surface side. Can be As the transparent film substrate used in the present invention, a plastic film such as a polyester film, a polyvinyl chloride film and a polyolefin film can be used. As the polyester film, known ones may be used, and polyethylene terephthalate and its copolymers, blends, cross-linked products and the like can also be used. As the polyvinyl chloride film, known ones may be used, and polyvinyl chloride and its copolymers, blends, crosslinked products and the like can also be used. As the polyolefin film, known films may be used, and polyethylene, polypropylene, and the like, copolymers thereof, blends, cross-linked products, and the like can also be used. In addition, these films are non-stretched, or uniaxially stretched, or biaxially stretched,
Either one may be used.

Further, the surface of the film is subjected to corona discharge treatment in air or another atmosphere for the purpose of imparting wettability of a binder, adhesion to a binder coating layer, or application workability. Alternatively, a surface-treated film subjected to a primer treatment or a chemical treatment can be used. The thickness of the film is preferably from 1 to 500 μm, more preferably from 10 to 300 μm, and still more preferably from 20 to 250 μm.

The carboxyalkyl cellulose nitrate used in the present invention is defined as having a nitrate ester substitution degree contained in carboxyalkyl cellulose of 0.2 to 0.2.
2.95, the degree of carboxyalkyl ether group substitution is 0.
05 to 2.8, specifically, the aqueous cellulose derivatives disclosed in JP-A-5-39301 and JP-A-5-39302.

If the nitrate ester group substitution degree is less than 0.2,
Water resistance is insufficient, and if it exceeds 2.95, hydrophilicity becomes insufficient. If the degree of substitution of the carboxyalkyl ether group is less than 0.05, the solubility in water becomes insufficient, and the ink absorbency becomes insufficient.
The water resistance of the coated surface becomes insufficient. Further, the preferred range of the degree of substitution of the nitrate groups of the nitrates of the carboxyalkyl cellulose is 0.2 or more and 2.2 or less,
The preferred range of the degree of carboxyalkyl ether group substitution is 0.05 or more and 1.5 or less.

The carboxyalkylcellulose nitrates used in the present invention may have some or all of the carboxyl groups contained therein neutralized. The neutralization increases the solubility in water and a water-soluble organic solvent containing water as a main component, and facilitates handling. To neutralize nitrates of carboxyalkyl cellulose, alkali metal ions, alkaline earth metal ions,
One or more cations such as an ammonium ion and an organic amine can be used. The degree of neutralization in this case can be arbitrarily determined according to the composition of the target solution such as water and organic solvent, but generally, 50% or more of the contained carboxyl groups are medium. Preferably, they are summed.

The water-soluble organic solvents which can be used for dissolving the carboxyalkyl cellulose nitrates used in the present invention include alcohols, polyhydric alcohols, derivatives of polyhydric alcohols, ketones, esters, carbonates. Or a combination of two or more of these. As alcohols, methanol, ethanol, n-propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, n-nonyl alcohol, n-
-Decanol, n-undecanol, or isomers thereof, cyclopentanol, cyclohexanol and the like, and preferably alcohols having 1 to 6 alkyl carbon atoms.

The polyhydric alcohols include ethylene glycol, propylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 2-cyclohexanediol, 1,7
-Heptanediol, 1,8-octanediol, 1,
Examples include 9-nonanediol, 1,10-decanediol, glycerin, pentaerythritol and the like.

Derivatives of polyhydric alcohols include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol isopropyl ether, ethylene glycol monobutyl ether, ethylene glycol isobutyl ether, propylene glycol monomethyl ether, propylene Glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, cellosolve acetate, and the like, or a solvent pocket book edited by The Society of Synthetic Organic Chemistry (p. 479-p. 574; September 20, 1990)
And polyhydric alcohols and their derivatives.

Examples of ketones include acetone, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, diisopropyl ketone, cyclopentanone and cyclohexanone. As the esters, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, amyl acetate, lactate ester, butyrate ester, dibutyl phthalate, dioctyl phthalate, and cyclic esters such as ε-caprolactone and ε-caprolactam Esters and the like.

As ethers, diethyl ether,
Isopropyl ether, n-butyl ether, tetrahydrofuran, tetrahydropyran, 1,4-dioxane and the like can be mentioned. Examples of carbonates include dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate and the like.

In the present invention, cation-modified polyvinyl alcohol can be used in combination with the above-mentioned carboxyalkylcellulose nitrates as a component of the binder for forming the ink absorbing layer. The cation-modified polyvinyl alcohol used in the present invention refers to a polyvinyl alcohol having a cation modification amount of 0.01 mol% or more. Examples of the cationic modifying group include an amide group, an imide group, a primary amino group, a secondary amino group, a tertiary amino group,
Examples thereof include at least one kind of cation group selected from a primary ammonium base, a secondary ammonium base, a tertiary ammonium base, and a quaternary ammonium base, and those containing these groups are preferable. More preferred are primary amino groups, secondary amino groups, primary ammonium bases and secondary ammonium bases, and particularly preferred are primary amino groups or primary ammonium bases.

The cation modification amount may be 0.01 mol% or more, preferably 0.1 to 30 mol%, and more preferably 0.1 to 15 mol%. When the modification amount of the cationic group is less than 0.01 mol%, the water resistance of the ink absorbing layer is not always sufficient. As the average degree of saponification,
40 to 100 mol% is preferred. If the saponification degree is less than 40 mol%, the ink absorbency tends to decrease. The average degree of polymerization is preferably in the range of 100 to 5000, more preferably 200 to 3000. Average degree of polymerization is 10
If it is less than 0, the water resistance of the ink absorbing layer is reduced, and
If it exceeds, the solution viscosity may be too high and the workability may deteriorate.

The mixing ratio of the carboxyalkyl cellulose nitrates to the cation-modified polyvinyl alcohol is 97% to 20% of the carboxyalkyl cellulose nitrates to 3 to 80% of the carboxyalkyl cellulose nitrates to 3% to 80% of the cation-modified polyvinyl alcohol. Is preferred. This mixing ratio can be appropriately selected according to the balance of the degree of substitution between the nitric acid ester group and the carboxyalkyl group of the nitric acid ester of the carboxyalkyl cellulose.

In the present invention, the use of a specific nitric acid ester of carboxyalkylcellulose improves the printability and water resistance of the ink absorbing layer in a well-balanced manner. By using a binder containing (A) as a main component, the water resistance of the ink-absorbing layer is remarkably improved. This is presumed to be expressed by polyion complex formation between the nitrates and the cation-modified polyvinyl alcohol.

In the present invention, the thickness of the ink absorbing layer is 1
-100 μm, preferably 1-50 μm, more preferably 5-30 μm. If the thickness is too small, the ink absorbency will be insufficient, and the printability and ink dryness will be insufficient. On the other hand, if the thickness is too large, the water resistance deteriorates, or the coating becomes difficult. An aqueous polymer is mixed with the carboxyalkylcellulose nitrates and the cation-modified polyvinyl alcohol used in the present invention for the purpose of improving the ink drying property, the film properties of the ink absorbing layer, gloss, and image clarity. May be.

Examples of the aqueous polymer include gelatin derivatives such as lime-treated gelatin, acid-treated gelatin and enzyme-treated gelatin, and specifically reacted with dibasic acid anhydrides such as phthalic acid, maleic acid and fumaric acid. Various gelatins such as gelatin, oxidized starch, cationized starch, starches such as etherified starch, methylcellulose,
Cellulose derivatives such as hydroxyethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, carboxymethylcellulose, polyalkylene oxides and their derivatives, polyacrylic acid, polymethacrylic acid or their esters, salts and copolymers thereof, polyhydroxy Ethyl methacrylate and its copolymer, polyvinylpyrrolidone,
Vinylpyrrolidone / vinyl acetate copolymer, polyvinylpyridium halide, polyvinyl alcohol of various saponification degrees, carboxy-modified, amphoteric polyvinyl alcohol, polyethylene glycol, polypropylene glycol, polyvinyl ether, alkyl vinyl ether / maleic anhydride copolymer, styrene Maleic acid copolymers and salts thereof, synthetic polymers such as polyethyleneimine,
Conjugated diene copolymer latex such as styrene / butadiene copolymer, methyl methacrylate / butadiene copolymer, polyvinyl acetate, vinyl acetate / maleic acid ester copolymer, vinyl acetate / acrylic acid ester copolymer, ethylene・ Acrylic polymers such as vinyl acetate copolymer latex such as vinyl acetate copolymer, acrylate polymer, methacrylate polymer, ethylene acrylate copolymer, and styrene acrylate copolymer Latex of coalesced or copolymer,
Aqueous adhesives such as a vinylidene chloride copolymer latex or a functional group-modified polymer latex with a functional group-containing monomer such as a carboxyl group of these various polymers, a melamine resin, a thermosetting synthetic resin such as a urea resin, And polymethyl methacrylate, polyurethane resin, unsaturated polyester resin, vinyl chloride / vinyl acetate copolymer,
Synthetic resins such as polyvinyl butyral and alkyd resins can be mentioned. These can be contained alone or in combination.

The ink absorbing layer comprising a nitrate ester or a nitrate ester and a cation-modified polyvinyl alcohol used in the present invention has not only printability and water resistance but also characteristics of drying property and heat blocking property. Known additives can be used in the ink absorbing layer of the present invention as long as the properties of the present invention are not impaired. For example, matting agents such as calcium carbonate, basic magnesium carbonate, and crystalline cellulose, defoamers, coating improvers, thickeners, antistatic agents, antioxidants, ultraviolet absorbers, water resistance agents, and light resistance improvers , Storage stability improvers, printability improvers such as colloidal silica and alumina sol.

The porous layer in the present invention is for allowing the ink to quickly pass through the ink absorbing layer, and preferably has a structure having cracks and a large number of voids inside the layer. In the present invention, the porous layer preferably has a light diffusing property because it is used for observing a recorded image from the side opposite to the printing (recording) surface. Examples of a material for forming such a porous layer include polystyrene, polymethacrylate, elastomer, ethylene-vinyl acetate copolymer, styrene-acryl copolymer, polyester, polyacryl, polyvinyl ether, polyamide, polyolefin, and polyolefin. Silicone, polytetrafluoroethylene, styrene-
Butadiene rubber, urea resin, resin powder such as urea formalin resin, emulsions and suspensions thereof, and further inorganic pigments, for example, synthetic silica, clay, talc,
At least one of diatomaceous earth, calcium carbonate, titanium oxide, zinc oxide, calcium sulfate, barium sulfate, zinc sulfide, titanium white, aluminum silicate, lithopone, aluminum hydroxide, calcium silicate and the like is used according to the purpose.

The size of the inorganic pigment or the organic fine particles to be used is preferably 0.1 to 100 μm in primary particle diameter. When such spherical pigment particles are used, the whiteness of the back print film becomes high, The contrast of the image is significantly improved. The thickness of the porous layer depends on the amount of ink droplets, but is preferably 1 to 200 μm, and more preferably 3 to 80 μm.

In the formation of the ink absorbing layer and the porous layer in the present invention, an air knife coater, a blade coater,
Bar coater, gravure coater, curtain coater,
Known coating means such as a roll coater, an applicator, a die coater, an edge coater, and a comma coater can be applied. The viscosity of the coating solution and the conditions for drying the coating film are not particularly limited, but the drying conditions for the coating film are desirably performed within a range that does not adversely affect the properties of the film.

In the present invention, by using a nitric acid ester of carboxyalkylcellulose or a binder containing cation-modified polyvinyl alcohol, printability, drying property and water resistance are good, and the heat-resistant blocking property of the coated surface is improved. This gives a back print film excellent in quality.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail by way of examples. In addition, the measuring method of various characteristics in the present invention is as follows. [Evaluation method of printability] Inkjet printer (MJ
810C; manufactured by Seiko Epson Corporation). After recording, the sharpness and the degree of bleeding were visually judged and evaluated. ：: good, Δ: somewhat good, ×: bad [Method of evaluating water resistance] A solid-printed recording sheet was immersed in water for 2 minutes, air-dried, and visually evaluated for ink bleeding. ○: No change, ×: Bleed

[0033]

[Production Example 1] Carboxymethylcellulose (Selogen P)
L-15; manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd .; carboxymethyl ether group substitution degree = 0.5) 100 g and sulfuric acid / nitric acid / water = 63/25/12% by weight 5000 g of an acid mixture.
Is placed in a 5 liter reaction vessel, and a nitrification reaction is carried out while stirring at 5 to 10 ° C. for 60 minutes. The reaction product is deacidified with a centrifuge and immediately washed with a large amount of water. Further, the reaction product was made into a 10% slurry in an autoclave, treated at 140 ° C. for 30 minutes, and then separated by a Buchner funnel to collect the reaction product.

The reaction product was dried at 80 ° C. for 2 hours,
29 g of carboxymethylcellulose nitrate were obtained. The degree of substitution of the nitric ester group per anhydroglucose unit of this nitrate ester is the degree of substitution of the carboxymethyl ether group of carboxymethyl cellulose (0.5).
And measurement of the nitrogen content with a CHN coder.
It turned out to be 9.

Next, 100 g of carboxymethylcellulose nitrate having a degree of substitution of 1.9 and a degree of substitution of carboxymethyl ether of 0.5 per unit of anhydrous glucose obtained per 100 g of anhydrous glucose units, and ion-exchanged water 5
98.6 g and 250 g of butyl cellosolve were put into a closed mixer and stirred. Further, 51.4 g of 10% aqueous ammonia was added as a neutralizing agent. The resulting mixture was completely dissolved while heating to obtain a solution having a solid content of 10% (this is referred to as liquid A).

[0036]

[Production Example 2] Carboxymethylcellulose (Cellogen 5
A; manufactured by Daiichi Kogyo Seiyaku Co., Ltd .; carboxymethyl ether group substitution degree = 0.7), the acid mixture was adjusted to sulfuric acid / nitric acid / water = 59/20/21% by weight, and 1
90 g of dried nitric acid ester of carboxymethyl cellulose was obtained in the same manner as in Production Example 1 except that the treatment was performed at 30 ° C. for 30 minutes. The nitric ester group substitution degree per anhydroglucose unit of this nitrate ester was 1.2 as determined by measuring the carboxymethyl ether group substitution degree (0.7) of carboxymethyl cellulose and the nitrogen content by a CHN coder. understood.

Next, 100 g of carboxymethylcellulose nitrate having a degree of substitution of nitrate ester of 1.2 and a degree of substitution of carboxymethyl ether of 0.7 per unit of anhydrous glucose unit obtained above were mixed with ion-exchanged water. 8
42.8 g and 200 g of isopropyl alcohol were put into a closed mixer and stirred. Furthermore, 10
57.2 g of aqueous ammonia was added. The obtained mixture was completely dissolved while heating to obtain a solution having a solid content of 10% (this is referred to as liquid B).

[0038]

Example 1 Cation-modified polyvinyl alcohol (CM
318: Kuraray Co., Ltd.) 100 g, ion-exchanged water 80
0 g and 100 g of isopropyl alcohol were placed in a mixer and stirred until completely dissolved to obtain a solution having a solid content of 10% (this is referred to as solution C).

The liquid A and the liquid C were converted to a weight ratio of 20:80 (ie, 20% by weight of nitrates of carboxymethyl cellulose and cation-modified polyvinyl alcohol 80).
% By weight) to prepare a binder for a back print film. This binder was applied on a transparent polyester film with a bar coater so that the film thickness after drying was 10 to 30 μm, and dried. Furthermore, on top of this
A mixed slurry (solid content: 30% by weight, solid content ratio: 5: 1) of titanium oxide (primary particle size: 0.2 μm) and polyvinyl alcohol aqueous solution is coated with a bar coater so that the film thickness after drying becomes 5 to 20 μm. The back print film was obtained by processing and drying. The evaluation results were as follows.

Printability: 、, water resistance; ○

[0041]

Example 2 The binder of liquid A alone was applied on a transparent polyester film by a bar coater so that the film thickness after drying was 10 to 30 μm, and dried. Further, a mixed slurry (solid content: 10% by weight, solid content ratio: 5: 1) of a silica gel powder (primary particle diameter: 0.1 μm) and an aqueous polyvinyl alcohol solution is dried thereon to a thickness of 5 to 20 μm. As described above, a back print film was obtained by coating with a bar coater and drying. The evaluation results were as follows.

Printability: 、, water resistance: ○

[0043]

Example 3 Liquid B and liquid C of Example 1 were converted to a weight ratio of 60.
A binder for a back print film was prepared by mixing with a mixture of No. 40 (that is, 60% by weight of nitrates of carboxymethyl cellulose and 40% by weight of cation-modified polyvinyl alcohol). This binder is coated on a transparent polyvinyl chloride film with a thickness of 10 to 30 after drying.
It was coated with a bar coater to a μ and dried. Further, a mixed slurry (solid content: 10% by weight, solid content ratio: 5: 1) of a spherical polymethyl methacrylate resin (primary particle size: 0.2 μm) and an aqueous solution of polyvinyl alcohol was dried thereon to a film thickness of 5 to 5 μm. A back print film was obtained by coating with a bar coater and drying to 20 μm. The evaluation results were as follows.

Printability: 、, water resistance; ○

[0045]

By using the back print film of the present invention, a printed matter excellent in printability and water resistance can be obtained. For this reason, the back print film of the present invention is used as a back print film for inkjet, and has posters, displays, flyers, advertisements, calendars, stickers, cards (business cards, etc.) having various sizes.
It can be suitably used for applications such as.

Claims (2)

[Claims] 1. A method according to claim 1, wherein one side of the transparent film substrate has a degree of nitrate ester substitution per anhydroglucose unit of 0.2.
To 2.95, and a binder containing a carboxyalkyl cellulose nitrate ester having a carboxyalkyl ether group substitution degree of 0.05 to 2.8 is formed, and a porous layer is further formed thereon. A back print film, characterized in that a back print film is provided. 2. A binder for forming an ink absorbing layer,
3 to 80% by weight of nitric acid esters of carboxyalkylcellulose having a degree of substitution of nitrate ester groups of 0.2 to 2.95 and a degree of substitution of carboxyalkyl ether groups of 0.05 to 2.8 per anhydroglucose unit. 2. The back print film according to claim 1, comprising 97 to 20% by weight of a cation-modified polyvinyl alcohol as a main component.