EP0728594A1 - Recording paper - Google Patents

Abstract

Provided is a recording paper, especially suitable for full color ink jet recording, having on one side of a base paper a recording layer comprising at least a pigment and a water-base binder, wherein the specific surface area of the pigment, Sw, determined at the void rate ε of 0.8 by the constant-pressure air permeability method and the thickness of the recording layer, L, satisfy the following relation: $${\text{5×10}}^{\text{-4}}{\text{≦ Sw}}^{\text{2}}{\text{×L ≦ 2×10}}^{\text{-2}}{\text{(m}}^{\text{5}}{\text{/g}}^{\text{2}}\text{)}$$

Description

• The present invention relates to an ink jet recording material for recording with water-base ink and, more particularly to, an ink jet recording material which has suitability for full color printing and excellent ink absorptiveness.
• In ink jet recording, fine ink drops ejected by a wide variety of mechanisms are made to adhere to a recording material to form a dot image thereon. In contrast with a dot impact recording system, the ink jet recording is noiseless, and enables easy formation of full color images and high-speed printing.
• The ink used for ink jet recording, on the other hand, is usually water-base ink using a direct dye or an acid dye. Therefore, it has poor drying properties.
• Thus, the paper used in the ink jet recording system is required to have the following properties of:
• (1) enabling high-speed drying of the ink adhering thereto,
• (2) ensuring high optical density in the images printed thereon,
• (3) causing neither running nor blurring of ink, and
• (4) not swelling in undulation by the absorption of ink.
• Since color printers of ink jet recording type have come on to the market in recent years, the aforementioned properties have been more severely required of ink jet recording paper. This is because, for instance, in such a color printer a wide variety of colors are formed by combining four kinds of monochromatic ink, namely cyan, magenta, yellow and black ink, in various ways, so the ink volume of each colored dot often becomes two or three times as much as that of a monochromatic dot. Accordingly, if the ink-absorbing capacity of recording paper is insufficient, the ink drops on the recording paper cause print-through or running phenomena.
• Further, in a boundary region between mixed colors, e.g., green and red colors, the ink volume is much greater because of overlap of many ink drops, and so rapid absorption of ink becomes difficult in such a region; as a result, a blurring phenomenon tends to occur.
• Such being the case, various proposals have so far been made for the purpose of improving upon the ink absorption of recording paper. For instance, there have been proposed the coated paper-type recording paper having its Stökigt sizing degree in the specified range, in Japanese Tokkai Sho 52-53012; the recording paper having at least two coats of a synthetic silica-containing coating material to increase its coverage rate, in Japanese Tokkai Sho 57-107879; the recording paper provided with the coated layer having a special void structure, in Japanese Tokkai Sho 58-110287; the recording paper provided with the coated layer cracked into many sections having a specified size, in Japanese Tokkai Sho 58-119888; and the recording papers using pigments having specific surface areas defined by BET method in the ranges specified respectively, in Japanese Tokkai Sho 60-174684 and 60-204390.
• However, those conventional papers are still insufficient in filling the latest requirements for full-color ink jet recording. More specifically, a present-day tendency in ink jet recording is in that an ink jet printer is designed so as to provide highly precise full-color prints, so there is a greater need for high-resolution recording paper than before. In order to prevent the ink from blurring at the ink-rich boundaries where mixed colors are adjoined, it is therefore necessary for the recording paper to be sufficiently high in ink-absorbing speed as well as in capacity for absorbing ink. Further, it is important in high-precision printing that the ink drops ejected from an ink nozzle should not be spread to excess on the recording paper and the dot diameter be controlled to an appropriate dimension. Conventional papers, on the other hand, show a tendency to spread dots thereon when they have sufficient ink-absorbing capacity, so they are unsatisfactory from the viewpoint of high precision.
• In order to solve the aforementioned problems, the Inventors have made intensive studies, and found that the ink jet recording suitability of a recording paper, especially an ink absorbing capacity thereof, is affected definitely by the quantity of ink passing through vacant spaces in the recording layer and this quantity correlates with the flow rate of a gas passing through a pigment-filled layer, and further that the recording paper having a particular relation between the specific surface area of a pigment present in the recording layer and the thickness of the recording layer can have not only high ink-absorbing capacity but also be free from a print-through phenomenon and have a properly controlled dot diameter, thereby achieving the present invention.
• The foregoing object of the present invention is attained with a recording paper having on one side of a base paper a recording layer comprising at least a pigment and a water-base binder, wherein the specific surface area of the pigment, Sw, determined at the voids ε of 0.8 by the constant-pressure air permeability method and the thickness of the recording layer, L, satisfy the following relation: $${\text{5×10}}^{\text{-4}}{\text{≤ Sw}}^{\text{2}}{\text{× L ≤ 2×10}}^{\text{-2}}{\text{(m}}^{\text{5}}{\text{/g}}^{\text{2}}\text{)}$$

  
• The recording paper prepared in accordance with the present invention is well suited for ink jet recording with water-base ink. Especially in full-color printing, the present recording paper can provide full-color images of high quality because it has excellent ink absorbing properties to achieve a small dot diameter and high optical density.
• The recording layer having ink jet suitability is rich in vacant spaces, so it can be regarded as an aggregate of capillaries. A fluid diffusing through such a layer can be expressed in terms of the Kozeny-Carman equation, and the flow rate u can be defined by the following equation: $${\text{u = ε}}^{\text{3}}{\text{×Δp × g/ {K × η × ρ}}^{\text{2}}{\text{× (1- ε)}}^{\text{2}}{\text{×Sw}}^{\text{2}}\text{× L }}$$

  

  wherein u is a flow rate, ε is void rate, Δp is a drop in pressure, g is the acceleration of gravity, K is the Kozeny constant, η is the viscosity of a fluid, ρ is a density, Sw is a specific surface area, and L is a layer thickness.
• In the foregoing equation, the factors variable in a recording layer are ε, ρ, Sw and L. Of these factors, ε and ρ are numerical values depending on a pigment present in the layer, but their individual variable ranges are around 20 % at the most. In contrast to these factors, Sw and L each have a much wider variable range, or the variable range of the order of about 1,000 %. Accordingly, the ink absorbing capability of a recording layer can be approximately expressed by the terms of Sw² and L.
• So far as the ink absorptivity of a recording layer is in a specific range, excellent recording quality can be obtained. Thereby, certain limits are placed upon the flow rate u. Consequently, the product of Sw ² and L is required to be in a specified range.
• When Sw² × L is below the appropriate range, u becomes great to result in an occurrence of print through and a lowering of print density; while when that product is above the appropriate range, not only u becomes small but also Sw or L is great, so ink drops spread without undergoing proper control of their diameter to result in a lowering of print density.
• Although the term Sw is essentially the specific surface area of the pigment present in a recording layer, which is to be determined using ink, it has now been proved that Sw can satisfactorily be expressed by the specific surface area determined by the so-called constant-pressure air permeability method.
• The specific surface area according to the constant-pressure air permeability method can be measured with an apparatus for measuring the specific surface area of a powder, e.g., Model SS-100, made by Shimazu Seisakusho Ltd.
• In the measurement according to the foregoing method, it occurs depending on the properties of a sample to be examined that the specific surface area of the sample is difficult to be determined at the prescribed voids. In such a case, the specific surface area at the prescribed voids can be determined by interpolation or extrapolation.
• The recording layer thickness L can be arbitrarily set up so far as the value Sw²×L is in the range 5×10^-4-2×10^-2 m⁵/g², preferably 1×10^-3-1×10^-2 m⁵/g². In the practical expression, the thickness L is preferably in the range of the order of 5-25 µm, wherein the recording layer can cover the surface of a base paper and sufficient print density can be secured. The layer thickness can be determined with electron microphotographs of cross sections of the recording paper.
• The pigment used in the present recording layer has no particular restrictions so far as its specific surface area Sw is in the range 8-30 m²/g, preferably 15-25 m²/g, when determined by the constant-pressure air permeability method. However, it is especially advantageous to use synthetic silica such as amorphous silica or the like. When the pigment hasa specific surface area (Sw) smaller than 8 m²/g, the ink absorbing capacity is lowered, and so the running of ink sometimes occurs; while when the specific surface area of the pigment is greater than 30 m²/g, the ink absorbing capacity becomes too high, and thereby a lowering of print density is apt to be caused.
• The term "synthetic silica" as used in the present invention is intended to include those defined in Kagaku Binran Oyo Kagaku Hen (which means "Handbook of Chemistry, Applied Chemistry Edition"), at page 256 (compiled by the Japanese Chemical Society, published by Maruzen in October 15, 1986), namely silica gel, white carbon, anhydrous silica and so on. In the present invention, it is especially desirable to use white carbon.
• The specific surface area determined by the constant-pressure air permeability method in the present invention has no particular correlation with the specific surface area determined by the BET method. This is demonstrated by the data set forth in Table 1. Table 1

  Sample	Constant-Pressure Air Permeability Method	BET Method
  	Specific Surface Area (m2/g)	Specific Surface Area (m2/g)
  Synthetic Silica A	6	40
  Synthetic Silica B	2	700
  Synthetic Silica C	14	275
  Synthetic Silica D	19	125
  Synthetic Silica E	24	260
  Synthetic Silica F	20	265
  Synthetic Silica G	10	410
  Synthetic Silica H	35	200

• The binder used in a recording layer of the present recording paper has no particular restrictions, except that it is required to be a water-base binder. Suitable examples of such a binder include starches such as oxidized starch, esterified starch, etc., cellulose derivatives such as carboxymethyl cellulose, etc., polyvinyl alcohol and the derivatives thereof, polyvinyl pyrrolidone, casein, gelatin, soybean protein, a styrene-acrylic resin and the derivatives thereof, a styrene-butadiene latex, an acrylic emulsion and a vinyl acetate emulsion.
• In preparing the coating composition for the present recording layer, it is desirable to admix a high solid-content emulsion from the standpoint of increasing the workability upon coating operation. Further, for the purpose of controlling the dot diameter, it is more desirable to use a styrene-butadiene latex as one constituent of the binder, and that in combination with at least one other constituent binder. The constituent binder(s) combined therein may be those selected from the binders cited above.
• The proportion of total constituent binders mixed with a pigment is preferably from 10 to 100 parts by weight per 100 parts by weight of the pigment. The optimum value thereof varies within the limits defined above depending on the specific surface area of the pigment used. However, the binder proportion is not necessarily limited to the foregoing range. In other words, the proportions beyond the foregoing limits are also allowable, provided that sufficient binding is ensured to the pigment and a porous structure appropriate for ink absorption is not broken.
• The coating composition used in the present invention is prepared as a water-base composition comprising at least the pigment and the binder as described above. To the water-base composition can be optionally added a pigment dispersing agent, a water holding agent, a thickener, an antifoaming agent, antiseptics, a coloring agent, a water resistance providing agent, a wetting agent, a fluorescent dye, an ultraviolet absorbent, an cationic polyelectrolyte and so on.
• As for the coating method applicable to the foregoing composition, the method using a known coating machine can be adopted. As suitable examples of such a coating machine, mention may be made of a blade coater, an air knife coater, a roll coater, a brush coater, a kiss coater, a squeeze coater, a curtain coater, a bar coater, a gravure coater, a comma coater and so on.
• The composition coated can be dried by not only an usual ventilation method but also the drying method in which the coating is pressed against a heated mirror-finished surface while it is still wet, such as cast methods including the so-called direct cast method, re-wetting cast method and coagulation cast method.
• The present invention will now be illustrated in more detail by reference to the following examples, but it should be understood that these examples are not to be construed as limiting the scope of the invention in any way. Additionally, the tests carried out in the following examples and comparative examples, and the measurement methods and evaluation criteria adopted therein are described below:
• (1) Printed Image Quality
  Recording paper samples having the images printed with a bubble jet color printer (Model BJC-400J, trade name, a product of Canon Inc.) are examined for extent of ink absorption, dot density, dot diameter and print through, and these properties are evaluated by visual observation. The evaluation criteria adopted herein are the following:
  • a) Extent of Ink Absorption
    After a 3-second lapse therefrom, the solid area is rubbed with a finger, thereby examining for the extent of ink absorption. The evaluation result is expressed in the following marks;
    • Complete absorption of ink ....○
    • Incomplete absorption of ink...×
  • b) Dot Density
    Dots are printed with cyan ink, and reflection densities of 5 dots arbitrarily selected among them are measured with Konica Microdensitometer, PDM-5 (trade name, a product of Konica Co., Ltd.). The dot density is shown as the average of these 5 measurements.
  • c). Dot Diameter
    Dots are printed with cyan ink, and diameters of 10 dots arbitrarily selected among them are measured with Image Analyzer (trade name, a product of ADS K.K.). The dot diameter is shown as the average of these 10 measurements.
  • d). Print Through
    Two kinds of colored ink are superposed by printing operation to form a solid area. The solid area is examined on the print through phenomenon, and the examination result is expressed in the following marks;
    • Absence of the print through phenomenon . . . ○
    • Presence of the print through phenomenon. . . ×
• (2) Thickness of Coating
  The cross sections of the recording paper samples are observed with an electron microscope, and thereby the average coating thickness is determined.
• (3) Operation Suitability
  Coatability, dryability and so on are synthetically judged, with those in Example 5 being taken as standards.
EXAMPLE 1
• Ninety parts by weight of LBKP (C.S.F: 300 ml), 10 parts by weight of precipitated calcium carbonate 0.02 part by weight of an internal sizing agent (of alkylketene dimer type) and 0.5 part by weight of cationized starch were compounded, and therefrom a raw paper having a basis weight of 81.4 g/m² was made by means of a Fourdrinier machine. Then, 100 parts by weight of Synthetic SilicaD (shown in Table 1) having the specific surface area of 19 m²/g, determined by the constant-pressure air permeability method, was dispersed into 300 parts by weight of water.
• The pigment dispersion thus obtained was admixed with a solution prepared by dissolving 30 parts by weight of styrene-butadiene latex (Naugatex SN-307, trade name, a product of Sumitomo Daw Co. Ltd.) and 40 parts by weight of polyvinyl alcohol (PVA 105, trade name, a product of Kuraray Co., Ltd.) in 530 parts by weight of water to prepare a coating composition. The coating composition thus obtained was coated on the aforementioned raw paper by means of a roll coater, and dried. The thus prepared ink jet recording paper had a coating thickness of 10.µm. The evaluation results thereof are set forth in Table 2.
EXAMPLE 2
• An ink jet recording paper was prepared in the same manner as in Example 1, except that Synthetic Silica G (shown in Table 1) was used as pigment instead of Synthetic Silica D and the coating thickness was changed to 7 µm. The evaluation results thereof are also set forth in Table 2.
EXAMPLE 3
• An ink jet recording paper was prepared in the same manner as in Example 1, except that Synthetic Silica E (shown in Table 1) was used as pigment, the styrene-butadiene latex content was reduced to 25 parts by weight and the coating thickness was changed to 22 µm. The evaluation results thereof are also set forth in Table 2.
EXAMPLE 4
• An ink jet recording paper was prepared in the same manner as in Example 1, except that the styrene-butadiene latex content was reduced to 15 parts by weight. The evaluation results thereof are also set forth in Table 2.
EXAMPLE 5
• An ink jet recording paper was prepared in the same manner as in Example 1, except that Synthetic Silica F (shown in Table 1) was used as pigment, the polyvinyl alcohol content was increased to 45 parts by weight and the styrene-butadiene latex was not used, and the coating thickness was changed to 18 µm. The evaluation results thereof are also set forth in Table 2
COMPARATIVE EXAMPLE 1
• An ink jet recording paper was prepared in the same manner as in Example 1, except that Synthetic Silica A (shown in Table 1) was used as pigment and the coating thickness was changed to 7 µm. The evaluation results thereof are also set forth in Table 2.
COMPARATIVE EXAMPLE 2
• An ink jet recording paper was prepared in the same manner as in Example 1, except that Synthetic Silica G (shown in Table 1) was used as pigment and the coating thickness was changed to 3 µm. The evaluation results thereof are also set forth in Table 2.
COMPARATIVE EXAMPLE 3
• An ink jet recording paper was prepared in the same manner as in Example 1, except that Synthetic Silica H (shown in Table 1) was used as pigment, the styrene-butadiene latex content was reduced to 25 parts by weight and the coating thickness was changed to 25 µm. The evaluation results thereof are also set forth in Table 2.
COMPARATIVE EXAMPLE 4
• An ink jet recording paper was prepared in the same manner as in comparative example 1, except that 25 parts by weight of a vinyl acetate-ethylene copolymer emulsion (Sumicaflex 470, trade name, a product of Sumitomo Chemical Co., Ltd.) was used instead of 30 parts by weight of the styrene-butadiene latex and the coating thickness was changed to 5 µm. The evaluation results thereof are also set forth in Table 2.

  
• As can be seen from Table 2, the ink jet recording papers according to the present invention, those prepared in Examples 1 to 5, showed complete absorption of ink and provided prints which were high in dot density, small in dot diameter and free from print through. On the other hand, the ink jet recording papers beyond the scope of the present invention, those prepared in Comparative Examples 1, 2 and 4, were inferior in most of the properties exmined, and the ink jet recording paper prepared in Comparative Example 3 was inferior in dot density although it had satisfactory ink absorbing capacity.

Claims (9)

1. A recording paper having on one side of a base paper a recording layer comprising at least a pigment and a water-base binder, wherein the specific surface area of the pigment, Sw, determined at the void rate ε of 0.8 by the constant-pressure air permeability method and the thickness of the recording layer, L, satisfy the following relation: $${\text{5×10}}^{\text{-4}}{\text{≦ Sw}}^{\text{2}}{\text{×L ≦ 2×10}}^{\text{-2}}{\text{(m}}^{\text{5}}{\text{/g}}^{\text{2}}\text{)}$$

   
2. A recording paper as claimed in Claim 1, wherein the specific surface area of the pigment is in the following range: $${\text{8 ≦ Sw ≦30 (m}}^{\text{2}}\text{/g)}$$

   
3. A recording paper as claimed in Claim 1 or 2, wherein the water-base binder comprises at least two constituents, one of which is a styrene-butadiene latex.
4. A recording paper as claimed in any one of Claims 1-3, wherein the pigment is synthetic silica.
5. A recording paper as claimed in any one of Claims 1-4, wherein the value Sw²XL is from 1X10^-3 to 1X10^-2 m⁵/g².
6. A recording paper as claimed in any one of Claims 1-5, wherein the thickness of L is from 5 to 25 µm.
7. A recording paper as claimed in any one of Claims 1-6, wherein the proportion of the binder is from 10 to 100 parts by weight to 100 parts by weight of the pigment.
8. A recording paper as claimed in Claim 2, or any one of Claims 3-7 as appended thereto wherein the specific surface area Sw is from 15 to 25 m²/g.
9. A recording paper as claimed in Claim 4, or any one of Claims 5-8 as appended thereto wherein the synthetic silica is white carbon.