JP2006038948A - Wet electrophotographic recording sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wet electrophotographic recording sheet having extremely excellent toner fixing property and transferring property when printed by using a wet electrophotographic system using a wet toner in a wet electrophotographic printing system. <P>SOLUTION: The wet electrophotographic recording sheet has one or more coating layers applied on at least one surface of a sheet substrate, wherein the coating layer contains a kind of pigment which includes particles having less than 2 μm particle size by ≥95 mass%, particles having less than 1 μm particle size by ≥75 mass% and particles having less than 0.2 μm particle size by ≤11 mass%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a recording sheet for wet electrophotography, and more specifically, wet electrophotography having extremely excellent toner fixability and transferability when printed using a wet electrophotographic system using a wet toner. This relates to a recording sheet for use.

  Applications for printing using the electrophotographic method are not limited to terminal PC printers, fax machines, or copiers, but can also be put into practical use in the so-called on-demand printing field, which enables various types of small lot printing and variable information printing. Progress and technical progress is remarkable. In recent years, with the improvement of printing speed and image quality, the number of printed copies has started to reach the area that has been covered by printing such as offset and gravure.

  Electrophotographic printing has the advantage of being able to handle variable information, but offset and gravure printing are inferior in terms of quality and price because they have achieved very high quality printing at a low price due to many years of accumulated technology. It was. For this reason, technological development for higher image quality, higher speed, lower power consumption, and lower costs is being promoted from both sides of printing machines and printing sheets. Printed materials are required to have higher definition and better color reproducibility, but this is in conflict with speeding up and cost reduction, as well as the performance of toner and printing machines, as well as electrophotography. As for recording sheets for printing, more stringent quality demands such as toner fixing property, transfer property, color reproducibility and running property are gathered.

  Among such electrophotographic printing methods, the dry electrophotographic method is a method typified by an office copying machine and the like, and a solid powder toner made of a pigment and a synthetic resin is used as a toner for forming an image. In the image forming method, printing is performed by a method in which toner is adsorbed to an electrostatic image generated by corona charging, and this toner is heat-pressed to a transfer object. However, in this method, if the toner is made finer, it is likely to be scattered to the surrounding environment, and if this is inhaled, a health problem is caused, so that the working environment is deteriorated and the printed matter is stained. For this reason, there is a limit to miniaturizing the solid powder toner, and it is difficult to obtain high resolution. Furthermore, since the thickness of the transfer object is not uniform, the charge density on the transfer object surface varies due to corona discharge, and an undesired image called fog occurs in the non-image area, or at a somewhat high temperature. There are many problems such as having to melt and solidify.

  On the other hand, another promising printing method that can handle variable information is an inkjet method. The inkjet method is a method in which fine ink droplets are ejected onto the surface of a transfer object to form an image. Recent technological advances have been tremendous, with extremely high image quality, and the weak weather resistance has been greatly improved with the advent of pigment-type inks. However, compared with offset printing or the like, there is a problem that the printing speed is slow and a relatively expensive dedicated sheet must be used to obtain high image quality.

  Therefore, when considering variable information in the on-demand printing area and satisfying conditions such as high image quality, high speed, and cost, the wet electrophotographic method is the most promising method among various printing methods. This is because the wet electrophotographic method disperses the toner in the liquid medium, so that the scattering of the powder is not a problem, and the toner can be reduced to 1/10 or more compared to the dry electrophotographic method. This is because the pigment can be used as a coloring material and there is no problem of weather resistance, there is no three-dimensional unevenness of the image, and the printing finish is close to offset printing.

  Wet toner does not use charge or the like when transferring an image from a blanket roll to a sheet. The image is transferred so that the adhesiveness of the wet toner to the printing sheet is higher than that of the blanket roll. That is, the transfer capability of the wet toner from the blanket roll to the printing sheet is important. If the toner adhesion ability is increased more than necessary, the fixing to the sheet is improved, but at the same time, it becomes difficult to peel off from the blanket roll, so that the toner adhesion ability cannot be increased more than necessary.

  For this reason, in the case of wet electrophotographic printing, there are drawbacks that sufficient toner strength cannot be obtained even if printing is performed, and that toner particles are not sufficiently transferred to the sheet. Therefore, as a means for obtaining sufficient toner fixing strength, there is a generally known method called “sapphire treatment”. This is a technique in which a drug mainly composed of polyethyleneimine is applied to a sheet. In order to provide wet electrophotographic printing suitability without using sapphire treatment, there are also methods in which an imine compound is incorporated or applied at the paper production stage. However, these are also basically the same method as the sapphire treatment.

  In addition to this, the amount of ash in the base paper is defined and zirconium is applied to the surface (for example, refer to Patent Document 1), or the surface smoothness and the sizing agent are devised (for example, Patent Document). See 2.) There is an example. These are effective for plain paper (non-coated) type printing paper. In the case of a coated sheet, there is an example of solving the problem by defining the type of pigment or binder and the blending amount of the binder (see, for example, Patent Document 3). However, in actuality, the sheet obtained by these means has a low wet electrophotographic printing suitability as compared with the sheet treated with an imine compound.

  For these reasons, in actual wet electrophotographic printing, it has been a reality to use a sapphire-treated sheet. However, a sheet treated with an imine compound has extremely poor weather resistance. Although these sheets depend on the amount of the compound adhering to the sheets, the sheets are discolored after about one month, and the color feeling and appearance of the printed matter are remarkably lowered.

  In order to solve these problems, there is an example in which the production of an electrophotographic recording sheet having wet toner suitability is achieved by controlling the area on the surface of the coating layer that can interact with wet toner (for example, a special feature). Application 2002-343237). This method is an extremely excellent method, and has higher wet electrophotographic aptitude in a normal printing than a material subjected to sapphire treatment. Therefore, it can usually be printed smoothly, but still has insufficient wet electrophotographic printability.

This is because a very high level of wet toner fixability and transferability may be required depending on the printing pattern. Therefore, the technology up to now has provided such a high level of wet electrophotographic aptitude to the sheet. The manufacture of things was difficult.
JP-A-10-171146 Japanese Patent Laid-Open No. 10-171148 JP-A-9-281739

  An object of the present invention is to provide a recording sheet for wet electrophotography having extremely excellent toner fixing property and transferability when printed using a wet electrophotographic method using a wet toner in the wet electrophotographic printing method. It is.

  As a result of intensive studies to solve the above problems, the present inventors have invented the following wet electrophotographic recording sheet.

  That is, in a wet electrophotographic recording sheet in which one or more coating layers are provided on at least one side of a sheet-like substrate, particles having a particle diameter of less than 2.0 μm are 95% by mass or more in the coating layer. It contains a pigment of a type in which particles having a diameter of less than 1.0 μm are 75% by mass or more and particles having a particle diameter of less than 0.2 μm are 11% by mass or less.

  In the present invention, the pigment having the particle size distribution is preferably heavy calcium carbonate.

  The wet electrophotographic recording sheet of the present invention can provide a wet electrophotographic recording sheet having excellent wet toner fixability and transferability.

  Hereinafter, the wet electrophotographic recording sheet of the present invention will be described in detail.

  As a result of intensive studies on the relationship between the wet electrophotographic recording sheet and the wet toner in the wet electrophotographic system, the present inventors have invented the following wet electrophotographic recording sheet.

  As a result of extensive research on the toner fixability of the wet electrophotographic recording sheet in the wet electrophotographic system, the present inventor has found that other coating sheets can be used in order to exhibit the wet toner fixability and transferability at a high level. It has been found that a new coating sheet characteristic that is completely different from the above is required. That is, it has been found that these can be achieved by strictly controlling the voids of the coating layer.

  The interaction between the wet toner and the sheet occurs only on the very surface of the sheet. An important factor for fixing the wet toner is the surface area of the pigment on the surface of the coating layer that can come into contact with the wet toner. The wet toner in contact with the pigment is adsorbed on the pigment surface by physical and chemical interaction with the pigment. Therefore, the interaction area can be increased by using a pigment having a large specific surface area, that is, a pigment having a small particle diameter and a large surface irregularity.

  However, if finer pigment particles are present than necessary, the voids normally present in the coating layer are blocked. Although the interaction between the wet toner and the sheet occurs only on the very surface of the coating layer, the influence of the depth direction is also related to some extent. If the gap becomes smaller than necessary, the contribution ratio in the depth direction decreases. Further, even if the area where the wet toner and the pigment interact increases, if the foot of the toner entering the coating layer becomes too thin, the strength is not expressed. According to the experiments by the inventors, the most effective coating layer void size is several hundred nm.

  Therefore, the pore size in the coating layer can be controlled by finely controlling the particle size of the pigment used in the coating layer. From the relationship of specific surface area, it is necessary that 95% by mass or more of particles having a particle size of less than 2.0 μm and 75% by mass or more of particles having a particle diameter of less than 1.0 μm. In order to keep the void diameter not too small and to be an optimum place, it is important that the particles having a particle diameter of less than 0.2 μm be 11% by mass or less. By including a pigment having such a particle size distribution in the coating layer pigment, it is possible to provide a void suitable for fixing a wet toner in the coating layer.

  The pigment having a particle size distribution as described above is preferably used in an amount of 50% by mass or more based on the total pigment, and particularly 70% by mass or more is preferable because wet toner suitability at a higher level can be provided. However, the surface quality may be slightly lowered when 95% by mass or more is added.

  Among the pigments having such a particle size distribution, heavy calcium carbonate is preferably used because it has good coating suitability and high white paper properties of a sheet appearing in whiteness, surface quality, white paper gloss, and the like. Two or more kinds of pigments having the particle size distribution as described above may be mixed and used.

  The particle diameter of the pigment referred to in the present invention refers to a product measured by a light scattering type microtrack particle size distribution measuring method.

  Examples of pigments having a particle size distribution according to the present invention include “R-21” of titanium oxide manufactured by Sakai Chemical Co., Ltd., “OP-2A” of heavy calcium carbonate manufactured by Phimatech, and heavy calcium carbonate “Kaba curve” manufactured by Shiroishi Calcium. Or the like. Of course, as long as the particle size distribution satisfies the conditions, the present invention is not limited thereto.

As for the coating amount of the wet toner receiving layer, suitability of wet toner can be expressed by coating 0.5 g / m ² or more on one side. The surface of the sheet-like substrate can be covered evenly at 3.0 g / m ² or more, which is preferable. However, if it exceeds 40 g / m ² , the problem of the strength of the coating layer occurs.

  The wet electrophotographic recording sheet in the present invention is not limited to use as a wet electrophotographic recording sheet, but may be used in other printing methods such as a dry electrophotographic recording sheet, an offset printing sheet, and a thermal transfer sheet. it can.

  As the base material of the wet electrophotographic recording sheet used in the present invention, plant fibers such as wood pulp, cotton, hemp, bamboo, sugarcane, corn, kenaf, animal fibers such as wool, silk, rayon, cupra, lyocell, etc. Cellulose recycled fibers, semi-synthetic fibers such as acetate, polyamide fibers, polyester fibers, polyacrylonitrile fibers, polyvinyl alcohol fibers, polypropylene fibers, polyvinylidene chloride fibers, polyurethane fibers, and other chemical fibers, glass A fiber, metal fiber, carbon fiber or other inorganic fiber in the form of a sheet or a resin film layer provided thereon is used.

  In the case of using a fiber sheet-like base material, as a manufacturing method for making each fiber into a sheet, a general paper making process, a wet method, a dry method, a chemical bond, a thermal bond, a spun bond, a spun lace, a water jet, a melt blow, One or more are appropriately selected from each process such as needle punching, stitch blowing, flash spinning, and tow opening.

  These fibers include light calcium carbonate, heavy calcium carbonate, talc, clay, kaolin, titanium dioxide, aluminum hydroxide and other fillers, binders, sizing agents, fixing agents, retention agents, paper strength enhancers, etc. These various compounding agents are suitably blended according to each step and each material. Furthermore, a resin coat layer may be provided on these fiber sheets.

  Base materials include high quality paper, medium quality paper, color quality, book paper, cast paper, fine coated paper, lightweight coated paper, coated paper, art paper, medium coated paper, gravure paper, Indian paper, coated ivory, Uncoated ivory, art post, coated post, uncoated card, special board, coated ball, tracing paper, type paper, PPC paper, NIP paper, continuous slip paper, foam paper, copy paper, carbonless paper, thermal paper, Inkjet paper, thermal transfer paper, synthetic paper, and other paper and paperboard, non-woven fabric, or various resins, plastics, and metals formed into a film are also included.

  The substrate before coating may be subjected to various surface treatments and calendar treatments in order to obtain the required density, smoothness, and air permeability.

  In the present invention, the pigment that can be used when the coating layer is provided on the sheet-like substrate is not particularly limited, and for example, purification of various kaolins, talc, heavy calcium carbonate (ground calcium carbonate), etc. Natural mineral pigments, light calcium carbonate (synthetic calcium carbonate), composite synthetic pigments of calcium carbonate and other hydrophilic organic compounds, satin white, lithopone, titanium oxide, silica, alumina, aluminum hydroxide, zinc oxide, magnesium carbonate , Calcined kaolin, plastic pigments and the like.

  Binders used in the coating layer include starch purified from natural plants, hydroxyethylated starch, oxidized starch, etherified starch, phosphate esterified starch, enzyme-modified starch and those obtained by flash drying. Cold water soluble starch, natural polysaccharides such as dextrin, mannan, chitosan, arabinogalactan, glycogen, inulin, pectin, hyaluronic acid, carboxymethylcellulose, hydroxyethylcellulose, oligomers thereof, and modified products thereof.

  Further, natural proteins such as casein, gelatin, soybean protein, collagen, and modified products thereof, and synthetic polymers and oligomers such as polylactic acid and peptides can be mentioned.

  In addition, copolymer latex such as styrene-butadiene, acrylic, polyvinyl acetate, ethylene vinyl acetate, polyvinyl alcohol, modified polyvinyl alcohol, sodium polyacrylate, polyethylene oxide, polyacrylamide, urea, or melamine / formalin resin, Examples thereof include water-soluble synthetic compounds such as polyethyleneimine and polyamidepolyamine / epichlorohydrin. One or more of these can be used. In addition, the use of a known natural or synthetic organic compound is not particularly limited.

  In addition, as a thickener used in the coating solution, water-soluble polymers such as carboxymethylcellulose, sodium alginate, methylcellulose, hydroxyethylcellulose, casein, and sodium polyacrylate, polyacrylate, styrene maleic anhydride copolymer And synthetic polymers such as silicates and inorganic polymers such as silicates.

  In addition, various auxiliary agents that are usually used such as a dispersant, an antifoaming agent, a water-proofing agent, and a coloring agent, and those obtained by cationizing these auxiliary agents are suitably used as necessary.

  In the present invention, the method for coating the coating layer is not particularly limited, and various film transfer coaters such as size press, gate roll, shim sizer, air knife coater, rod coater, blade coater, direct fountain coater, Each system such as a spray coater or cast coater is appropriately used.

  Furthermore, in order to obtain the density, smoothness, air permeability, and appearance required for coated and dried coated sheets in a series of operations, various finishing processes such as calendering are performed as necessary. The

  Examples of the present invention will be described below, but the present invention is not limited to these examples. Further, “parts” and “%” shown in the examples indicate parts by mass and mass% unless otherwise specified.

(Example 1) to (Example 7) and (Comparative Example 1) to (Comparative Example 5)
According to the following content, the recording sheet for wet electrophotography of Examples 1-7 and Comparative Examples 1-5 was produced.

<Sheet substrate>
LBKP (Drainage 400 mlcsf) 70 parts NBKP (Drainage 480 mlcsf) 30 parts Light calcium carbonate (indicated by ash content in base paper) 8 parts Commercial cationized starch 1.0 parts Commercial cationic polyacrylamide yield improver 0.03 parts Pulp and internal chemicals were adjusted according to the above formulation, and a base paper was made with a basis weight of 87.9 g / m ² .

To this base paper, 0.80 g / m ² of both sides of oxidized starch was adhered by a size press to obtain a base paper for coating.

<Coating solution>
About the pigment of a coating liquid, the pigment of the content of Table 1 was mix | blended as Table 2. FIG. In the table, heavy calcium means heavy calcium carbonate, light calcium means light calcium carbonate, titanium means titanium dioxide, and kaolin means kaolin clay. The particle size of each pigment was calculated from the microtrack particle size distribution. In the table, 1 is “OP-2A” by Phematech, 2 is “R-21” by Sakai Chemical, 3 is “FMT-100” by Phematech, and 4 is “Hydra Curve 60” by Shiraishi Calcium. “5” is “WH-90” of Hyogo Clay, “Kaolax HS” of Shiroishi Calcium, and “Brilliant 15” of Shiroishi Calcium.

The formulation other than 100 parts of pigment is as follows.
Commercially available styrene butadiene latex binder (average particle size 100 nm, gel content 40%, Tg 5 ° C.) 9 parts Commercial phosphate esterified starch 6 parts Commercial calcium stearate 0.6 parts Commercial carboxymethylcellulose thickener (CMC) 0.1 Adjusted to pH 9.6 with sodium hydroxide

The above coating solution was applied at a concentration of 60% and an operation speed of 1000 m / min using a fountain / blade coater at a coating amount of 10 g / m ² on one side and 20 g / m ^{2 on} both sides. After coating, supercalender treatment was performed on each of the front and back 4 nips at a linear pressure of 200 kN / cm and a temperature of 80 ° C. to obtain a wet electrophotographic recording sheet.

  The wet electrophotographic recording sheets obtained in Examples 1 to 7 and Comparative Examples 1 to 5 were measured by the following measuring methods, and the evaluation results are listed in Table 3.

1. Printing Evaluation was performed using “HP-indigo press 1000 (formerly E-print 1000)” manufactured by HP as a wet electrophotographic transfer machine. As a print image to be used for evaluation, an image having a solid print portion for each of four colors of black, cyan, magenta, and yellow was output. As a printing area of the solid printing portion, at least a square with a side of 3 cm or more was used. Fixing evaluations such as the following 2 to 5 were performed on the printed solid portions of each color of black, cyan, magenta, and yellow. In addition, the evaluation result of a table | surface showed the thing which was the lowest among these evaluation results as wet suitability.

2. Tape stripping Both sheet-fed and take-up types have Nichiban's cellophane adhesive tape “Cello Tape (R) No. 405” with a width of 18 mm on the printed sample that has passed 24 hours after printing so that there is no unevenness in the printing of each color. The tape was peeled off slowly at a speed of about 5 mm / sec. The degree of fixing of the toner after peeling to the sheet was visually determined, and a four-step evaluation was performed according to the following criteria.
A: The toner does not peel at all for each color.
○: Most of the toner remains on the sheet for each color.
(Triangle | delta): The toner peeled off from the sheet | seat with one part color, and there exists a part which fell out white in the printing part.
X: For each color, the toner is peeled off from the sheet, and a white portion of the printed portion is noticeable.

3. For rubbing evaluation, the horizontal method of JIS P8147 “Testing method for friction coefficient of paper and paperboard” was applied. For each of the examples and comparative examples, a blank wet electrophotographic recording sheet was attached to the horizontal plate, and the recording sheet having a printing portion printed by the printing machine was attached to the weight. Then, when the recording sheet having the printing portion was attached to the weight, the printed surface of the printing portion was rubbed against the blank recording sheet attached to the horizontal plate.

  As described above, after the test piece is attached to the horizontal plate and the weight, the weight is slid on the horizontal plate under the conditions described in JIS P8147. In the “friction test”, the weight is slid on the horizontal plate only once with one test piece combination. In this evaluation, the weight is slid on the horizontal plate 50 times with one test piece combination. Let The test piece used was 24 hours after printing.

Thereafter, the printing portion attached to the weight was observed, and the degree of toner dropout due to rubbing between the sheets was observed. The remaining condition of the toner in the printing portion was visually determined, and a four-step evaluation was performed according to the following criteria.
(Double-circle): The density reduction of a printing part is not recognized at all for each color.
○: It can be seen that the print density decreases slightly for each color.
Δ: It can be seen that the print density decreases for each color.
X: It can be seen that the print density decreases for each color, and there is a part that is partially whitened.

4). Eraser Peeling After printing, 24 hours after printing, the printed portion of each color was reciprocated five times with a uniform force using the corners of the unused eraser “dragonfly pencil, PE-01A”. The degree of fixing of the toner to the sheet after the reciprocation was judged visually, and a four-step evaluation was performed according to the following criteria.
A: The toner does not peel from the sheet at all for each color.
○: For each color, the toner hardly peels from the sheet.
Δ: Some toner is peeled from the sheet, but less than 20%.
X: For each color, the toner is easily peeled off from the sheet, and the white portions are conspicuous.

5. Transferability After printing 500 copies on each printing machine, cleaning was performed with a print cleaner, and transferability was judged by how the print cleaner was stained.
A: Almost no dirt
○: Very small dirt adheres.
Δ: Small dirt adheres.
X: It becomes dirty clearly.

6). Applicability of coating When each coating solution prepared with the above-mentioned formulation was applied with a blade coater, the occurrence of streaks was observed and evaluated according to the following criteria.
◎: 1 bottle / min or less.
Δ: 1 to 3 / min.
X: 4 pieces / minute or more.

7). Blank paper feeling The prepared wet electrophotographic sheet was obliquely observed under a fluorescent lamp. In addition, it was placed on a desk under the same fluorescent lamp and observed from directly above. Then, visual evaluation was performed in three stages according to the following criteria. Evaluation was made into the average value of 3 persons.
A: No unevenness is observed due to oblique light.
○: Slight unevenness due to oblique light, but not observed when viewed from directly above.
(Triangle | delta): There is unevenness by oblique light, and even when seen from right above, the unevenness is faint but not noticeable.
X: Unevenness is also noticeable when viewed from directly above.

<Evaluation results>
If a pigment having a particle size distribution defined in the present invention is not contained as in Comparative Examples 1 to 5, high wet toner suitability is not exhibited. On the other hand, when a pigment having a particle size distribution defined by the present invention is included as in all the examples, high wet toner suitability is exhibited. The effect of the invention can be obtained even if there are a plurality of pigments having the particle size distribution defined in the present invention as in Example 3. As can be seen from a comparison of Examples 2 and 4, a pigment having a particle size distribution defined in the present invention has a higher effect when it is 70 mass% or more. However, as can be seen from a comparison of Examples 5 and 6, the surface quality is slightly inferior at 95% by mass or more. As can be seen from a comparison of Examples 6 and 7, heavy calcium carbonate is a preferred pigment with a good balance between coating suitability and blank paper properties.

  The present invention can be applied to a wet electrophotographic recording sheet by a wet electrophotographic method that can handle variable information in an on-demand printing area.

Claims (2)

  In a recording sheet for wet electrophotography in which at least one coating layer is provided on at least one side of a sheet-like substrate, 95% by mass or more of particles having a particle size of less than 2.0 μm and a particle size of less than 2.0 μm in the coating layer. A wet electrophotographic recording sheet comprising a pigment of a type in which particles less than 1.0 μm are 75% by mass or more and particles having a particle diameter of less than 0.2 μm are 11% by mass or less.   The wet electrophotographic recording sheet according to claim 1, wherein the pigment having a particle size distribution according to claim 1 is heavy calcium carbonate.