JPS6321185A - Recording paper for thermal transfer printer - Google Patents

Abstract

PURPOSE:To prevent excessive permeation or diffusion of a molten ink as in the case of a woodfree paper, by providing a coated layer comprising a specified lowly heat-conductive pigment on the surface of a raw paper, thereby providing micropores in an ink-receiving surface. CONSTITUTION:A coated layer comprising a pigment is provided at an ink- receiving surface of a recording paper in such a construction that a pigment selected from (A) an organic pigment having a thermal conductivity of a pigment constituting substance of not more than 0.3kcal/m.hr. deg.C, (B) an organic pigment having a thermal conductivity of secondary aggregate particles of not more than 0.3kcal/m.hr. deg.C and (C) an amorphous inorganic pigment having a thermal conductivity of secondary aggregate particles of not more than 0.3 kcal/m.hr. deg.C constitutes 30-100wt% of the pigment component in the coated layer, and the remainder of the pigment component is a crystalline inorganic pigment. With this construction, heat loss in a molten ink due to the coated layer is restrained, permeation of the ink into the coated layer is facilitated, and transfer of the ink through release from an ink ribbon is satisfactorily achieved with suppression of blurring on the coated layer.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a recording paper for a thermal transfer printer that performs recording by transfer of heat-melting ink. More specifically, the present invention relates to a recording paper that is a pigment-coated paper that can improve image quality and is suitable for color printers using a thermal melt transfer method.

[Conventional technology]

Simply put, the transfer section of a printer using the heat-melt transfer method is made up of an ink ribbon coated with heat-melt ink, a recording paper that is in close contact with the ink-coated surface, and a thermal head that is in contact with the non-ink-coated surface. There is. The principle of heat-melting transfer is that the thermal head is heated in response to an electric signal applied to the thermal head, and the amount of heat is transmitted through the ink ribbon to melt the heat-fusible ink on the back side. The molten ink is transferred to the surface of the recording paper, cooled and solidified, and then the recorded paper and the ink ribbon are separated. Applying the thermal head, heating time is 1 to 2 milliseconds, heating temperature is 200 to 450℃
It is. Hot-melt ink is prepared by mixing pigments, dyes, etc. with natural wax such as carnauba wax or synthetic wax such as ester wax.

Regarding ink transfer in thermal melting and transfer methods, (a) the contact area between the paper and the melted ink, (b) the affinity between the melted ink and the paper surface, and (c) the viscosity of the melted ink that makes it easy for the ink to transfer. It is said that three conditions are related to transfer efficiency. Conventionally, high-quality paper with Bekk smoothness (hereinafter simply referred to as smoothness) of a certain level or higher has been used as recording paper for thermal-fusion transfer printers, and the smoothness is 100 seconds or more for monochrome and 300 seconds for color. More than that is needed.

[Problem that the invention seeks to solve]

Recently, there has been an increasing demand for color gradation recording, but it has been difficult to form rich and vivid images by simply adjusting the smoothness of the high-quality paper. This is because the size of the unit dots printed on the surface of the high-quality paper varies due to the unevenness of the paper surface and the bleeding of the ink, and the shape becomes disordered, causing the color density to vary.

On the other hand, when using general-purpose coated paper suitable for general multicolor printing, it is difficult to accurately transfer minute dots due to the insufficient adhesion of the molten ink, and when the ink ribbon and recording paper are separated, the transfer A phenomenon occurs in which the periphery of the dot is curled up or chipped, and it is not possible to cope with multicolor images and higher speeds.

As a result of repeated research into the causes of the above-mentioned defects in general-purpose coated paper, the present inventor found that in addition to the above three conditions, it is necessary to keep the thermal conductivity of the coating layer below a certain level. . In other words, in the case of high-quality paper, the thermal conductivity of the ink-receiving surface is thought to be as low as around 0.2 Kcal/hr/°C due to the pores created by the wood fibers and their overlap, so the heat-melted transfer ink is transferred to the paper surface. After permeation, it is cooled, but in the case of general-purpose coated paper, the pigments usually used are crystalline inorganic compounds such as kaolin and calcium carbonate, and have a thermal conductivity of 2 to 6 KcaUm-hr・℃.
This is thought to be due to the fact that when the heated and melted ink contacts the coating layer, it radiates heat, increasing the ink viscosity and making it difficult to penetrate into the pores of the coating layer.

[Failure to solve the problem]

The technical idea of the present invention is that in order to reduce the thermal conductivity of the coating layer, that is, to increase the heat insulation, it is necessary to select and use pigments whose thermal conductivity is below a certain level, or to cause secondary aggregation of pigments. Pigments that are formed into particles and exhibit thermal conductivity below a certain level due to the air in their micropores, and porous materials that are
The purpose of this method is to select and use pigments that have fine voids, such as pigments that exhibit thermal conductivity below a certain level because they are sub-agglomerated particles.

Examples of these pigments include amorphous inorganic pigments such as polystyrene pigments, urea resin pigments, silica gel, and silica alumina gel, as will be described later. It is known to mix them into paper stocks to improve fc and reduce the weight of paper, or to coat them on paper for similar purposes. Others - In general printing, it is known that coating with urea resin pigments is useful for improving the drying properties of printing inks (% 1987-103509). However, as in the present invention, for thermal transfer printers, especially for thermal transfer color printers, the selection of coating pigments is important.
The idea that the transfer efficiency of heat-fusible ink can be increased by lowering thermal conductivity is not known, and furthermore, the quantitative relationship between the effects of using low thermal conductive pigments is not known at all. The present invention suppresses heat loss of molten ink by the pigment coating layer, facilitates ink permeation into the coating layer, and reduces bleeding on the coating layer while perfecting peeling transfer from the ink ribbon. It is now possible to print high quality color images similar to coated paper.

[Purpose of the invention]

An object of the present invention is to provide a recording paper for thermal transfer printers, which is a beautiful coated paper that has high whiteness, smoothness, and gloss, and is capable of recording color gradations and achieving high speed. 〇 [Structure of the Invention] According to the present invention, in a recording paper for a thermal transfer printer that performs recording by transfer of heat-melting ink, the pigment coating layer provided on the ink-receiving surface of the recording paper is A: pigment. Organic pigment whose constituent substances have a thermal conductivity of 0.3 Kc 8177 m -hr -°C or less, B: The thermal conductivity of secondary agglomerated particles is 0.3 Kc 817 m
-hr - organic pigment below -℃, C: thermal conductivity of secondary agglomerated particles is 0.3 Kcat/m
The pigment coating layer contains 30 to 100% by weight of the pigment component in the pigment coating layer, and the remaining pigment component is a crystalline inorganic pigment. A recording paper for a thermal transfer printer is provided.

Table 1 shows specific examples of the low thermal conductivity pigment specified by the present invention and necessary physical properties.

Table 1 Among the white pigments shown in Table 1, powdered polystyrene and crosslinked polystyrene are those whose constituent materials have a thermal conductivity below a specific value. The thermal conductivity of the urea resin material is 0.
36KcatAn-hr・℃ (hereinafter abbreviated as Kcat)
, amorphous silica gel, and amorphous silica alumina gel have thermal conductivities of 1.0 Kcal and 1.5) (e a
j, but due to the formation of secondary agglomerated particles, all of them show below a specific value. These pigments can be used alone or in crystalline white pigments such as kaolinite clay, halloysite clay, calcium carbonate, and aluminum hydroxide, which are used in general-purpose coated paper, art paper, etc., to increase the gloss of the coating layer. Can be used in combination with pigments. The average thermal conductivity of crystalline pigments is usually 2 to 6 Kcaj, but since they are plate-shaped, needle-shaped, or spiky, their thermal conductivity has a directional property, especially regarding the crystal C axis. is even higher than the value of On the contrary, the pigment specified by the present invention has a spherical shape (
(styrene pigments, urea resin pigments) is also one of the important factors. In the present invention, as will be described later, it is not necessarily necessary to replace all of the low thermal conductivity pigment with the pigment for general-purpose coated paper and art paper, but at least 25% by weight of the total pigment in the coating layer is replaced. That's enough. In order for the present invention to be a recording paper with high glossiness comparable to that of general-purpose coated paper, and to ensure that printed images also have high glossiness and high-precision printing, it is necessary to incorporate a low thermal conductivity pigment into the coating layer. It is desirable that the remaining amount be 30 to 70% by weight of the total pigment in the pigment, and the remainder be crystalline kaolinite clay, halloysite clay, etc., which are effective for producing gloss in general-purpose coated paper.

(Determination of thermal conductivity of pigments) In general, there are two methods for measuring the thermal conductivity of a single substance: measurement under steady state of heat flow (steady method) (JIS-R2616), and measurement under unsteady state such as flash method and probe method. The measured values are recognized as physical constants. We calculated the thermal conductivity of pigments that are in the form of powder or thin layers with a thickness of microns.

(When the pigment consists essentially of secondary agglomerated particles) (2) Amorphous silica gel and amorphous alumina silica gel are both powders of porous substances with a whiteness of 94% or more (Hunter whiteness, hereinafter the same).

Those obtained by the synthesis method are single particles with a particle size of 10 to 3 Qnrn at the time of production, but as they grow, the particle size becomes 0.5 to 7.
micrometer secondary agglomerated particles), and the dry powder has a particle size of 20 μm.
Forms tertiary agglomerated particles of ~200 μm. It is known that they are dispersed in the coating liquid as secondary agglomerated particles. The same applies to naturally produced products. The thermal conductivity of secondary agglomerated particles is the first
The appearance in the table was calculated using specific gravity and the following formula.

V, , Vg are the volume fractions of solid and air, respectively (Vs + V, = 1), and dA, dT are the apparent specific gravity and true specific gravity, respectively, then dA = dTV8 The thermal conductivity of secondary agglomerated particles λ8g is Using the multifoam model (:[,,'l'opper:'Analysis
of porous゛IT Thermal In3ula
jlng MJLierla1g' Ind, and
Eng.

chem, 471377 (1955)) λ3:
Thermal conductivity of solid, λg: Thermal conductivity of air 0.0208 Kcal/m
-hr-℃ Therefore, the apparent specific gravity of amorphous silica gel is 0.
．． 18, true specific gravity 2.0, thermal conductivity (silica glass) 1
．． When OKcat/m-hr・℃ is introduced, the thermal conductivity of the secondary agglomerated particle amorphous silica gel is 0.09 Kca4/
in・hr・℃.

Silica alumina grout has an apparent specific gravity of 0.4 and a true specific gravity of 2.7.
, the thermal conductivity is 1.5 Kcal/m-hr-℃ is introduced, the thermal conductivity of the secondary agglomerated particle silica alumina gel is 0.1
8Kca4/rn@hr-'C.

(2) Urea resin pigments Urea resin pigments are produced, for example, by the method disclosed in JP-A-54-135893. The product form is, for example, a cake-like product with a solid content of 25%, in which spherical primary particles with a particle diameter of 0.2 μm aggregate to form secondary aggregated particles with a diameter of about 5 μm. This pigment has a true specific gravity of 1.45, an apparent specific gravity of 1.04,
Since the inherent thermal conductivity of the substance is 0.36 Kcal, according to the above formula, the thermal conductivity of the secondary agglomerated particles is 0.25 Kcal.
It is calculated as al.

(When the pigment consists of primary particles) Powdered polystyrene pigments consist of extremely small spherical primary particles with a particle size of 0.25 to 0.5 μm. The crosslinked polystyrene pigment consists of spherical primary particles with a particle diameter of 4 to 6 μm. In both cases, the particle size distribution is extremely narrow.

These thermal conductivities are measured by the above-mentioned steady method, unsteady method, etc., and are values recognized as material constants (for example, Polymer Materials Handbook, P L 2913-1299, Polymer Science Society Cotton, 1972 2). Corona Co., Ltd.) is adopted in the present invention, and 0.07 to 0.14 Kcal/m-h
It was set to r・℃.

Modified starch, polyvinyl alcohol, modified casein, which are commonly used as pigment adhesives without any particular restrictions.
One type of emulsion, such as a hydrophilic polymer such as carboxymethyl cellulose, styrene butadiene latex, methyl methacrylate butadiene latex, acrylic acid latex such as acrylic acid ester and methacrylic acid ester, vinyl latex such as polyvinyl acetate, etc. Select and use two or more types. These adhesives are usually added in an amount of 20 to 50 parts by dry weight per 100 parts by weight of the pigment. Coating is carried out on the base paper using the air knife method, blade method, barcode method, etc., and the coating amount is 7 to 15 g/dry weight.
lr? After binding and drying, finish to a smoothness of 300 seconds or more using a calendar, cross calendar, super calender, etc. Gloss level is 20% or more (75° angle photometer, same below)
Preferably, the finish is 40X or higher. The adhesives used for preparing the coating solution must all have low thermal conductivity.
In addition, not only low thermal conductive pigments consisting of primary particles but also low thermal conductive pigments forming secondary aggregated particles are dispersed in the coating layer without breaking down in the agglomerated state, so the experimental examples described below , the effects shown in the examples can be obtained.

〔Effect of the invention〕

Since the recording paper for thermal transfer printers of the present invention has a coating layer containing a specific low thermal conductivity pigment applied to the surface of the base paper, ■ fine pores are formed on the ink-receiving surface, making it look like high-quality paper. The thermal conductivity of the molten ink is almost at the same level as the low thermal conductivity of the base paper (high-quality paper) surface, and the molten ink does not cool down unnecessarily quickly, allowing it to sufficiently permeate into the coating layer and cool and peel off from the ink ribbon. As a result, clear images are formed. The shape of the pixels (dots) becomes constant, making it possible to create a color image with gradation.

[Experiment example]

The following experiment was conducted to confirm the effect of adding the low thermal conductivity pigment of the present invention. Aluminum hydroxide (Showa Aluminum (Dog, Heijirai) H42) [Thermal conductivity 3Kcal/
fn・hr・℃, specific gravity 2.54] and powdered polystyrene (
Asahi Kasei Corporation, Plastic Pigment 8801, solid content 48%) [thermal conductivity 0.12-0.14KCaIA-
hr・℃, true specific gravity 1.04-1.09] were mixed in various proportions by dry weight and dispersed in water, and then modified starch (Nihon Shokuhin Kako Co., Ltd., MS3800) was added to 100 parts of the mixed pigment.
) 15 parts of aqueous solution as solid content and styrene-butadiene latex (Sumitomo Naugatax Co., Ltd., N-275)
2E,)'e was added as a solid content of 50 parts, and coated on high-quality paper (Mishima Paper Oak Bond 65 g/n?) using the Meyer barcode method with a dry weight of 8 to 9 g/-. After drying, it was subjected to super calendering to obtain a recording paper having a smoothness of 900 to 100 ρ and a fall gloss of 40 to 50%. After printing with a thermal transfer color printer (Mitsubishi Electric MS4234), the print densities of black, magenta, and cyan inks were measured using a reflection densitometer (Macbeth Co., Ltd., RD514). Similarly, the uncoated high-quality paper was also measured! == Shown in the figure. The solid line is the reflection density increase curve of the recording paper according to the present invention, and the dotted line is the reflection density level of the high quality paper. 1 indicates psic ink, 2 indicates magenta ink, and 3 indicates cyan ink.

According to the Toki diagram, when the powdered polystyrene mixing ratio is 0%,
In other words, the reflection density with only aluminum hydroxide pigment is slightly inferior to that of high-quality paper for each color, but when powdered polystyrene pigment is mixed with aluminum hydroxide pigment, the reflection density improves as the mixing ratio increases. Beyond the quality paper level, 3
The increasing curve is remarkable up to 0% by weight, and thereafter the improvement becomes gradual. The printed dot shape showed a precise rectangle. These experimental results show that when a low thermal conductivity pigment is mixed with a high thermal conductivity crystalline pigment, the mixing ratio is 25% by weight or more, and preferably ν′ is 30% by weight or more, although it depends on the type of ink color. Rapid cooling of the hot melt ink can be alleviated by changing the mixing ratio, and the effects of the present invention can be achieved.

A similar tendency was observed for pigments such as crosslinked polystyrene, urea resin, silica gel, and silica alumina gel. Incidentally, for pigments such as polystyrene pigments that are easy to obtain gloss, the mixing ratio can be 25 to 100% by weight, but for pigments that are difficult to obtain gloss, the mixing ratio is preferably 25 to 70% by weight.

[Example 1] Kaolinite clay (Japan ITCC Co., Ltd., Premier) [
Thermal conductivity 2-31 (cal/m-hr-t:: :11
00 parts by weight, urea resin pigment (Mitsui Koatsu Co., Ltd., Knee Pearl M) [thermal conductivity 0.25 Kcal/m-hr-C]
454 (solid content: 22%), 186 parts by weight of water and 0.2 parts by weight of sodium polyphosphate were added and dispersed using a disintegrator.

Next, modified starch (Nihon Shokuhin Kako Co., Ltd., M838
00) 200 parts by weight of 10% solution, styrene phtadiene rubber (Sumitomo Naugatex Co., Ltd. S N, 307) 4
A coating liquid was prepared by adding an 8% dispersion liquid over 120 heavy-duty areas.

The solid content of the coating liquid is 25%, and the color viscosity is 63 cp (25%).
℃). As a control, the same kaolinite clay 20 was used as a control.
A coating solution with the same treatment was also prepared in which only 0 part by weight was added and no urea resin pigment was added. These coating liquids were applied to general-purpose coated paper (Desumi Kokusaku Pulp Co., Ltd., SK coat lO5g/
-) was coated with dry weight FLOG/R on one side using the Mayer bar method. Smoothness 230 by super calendering
A recording paper of the present invention with a whiteness of 81% and a gloss of 23% was obtained. The control coated paper has a smoothness of 2300 seconds and a whiteness of 80%.
, and the gloss level was 56X.

The recording paper of the present invention was printed with a test pattern using a thermal transfer color printer (MS4234, manufactured by Mitsubishi Electric Corporation) on both the recording paper and the control paper. As a result, the reflection was 35% with black ink, 40% with cyan, and 60% with magenta. An increase in concentration was observed. The printed dot shape was a precise rectangle and clear.

[Example 2] Precipitated calcium carbonate (PFIZER-QUIGL)
EY Co., Ltd., full bag gloss [F], thermal conductivity 4 Kca
A mixture of 20 parts by weight of l/m-hr-'C) and 80 parts by weight (dry weight) of the powdered polystyrene used in the experimental example was mixed with 2 parts of water
20 parts by weight and 0.4 parts by weight of sodium chloride.

Subsequently, 150 parts by weight of the modified starch 1O% solution and 100 parts by weight of styrene-butadiene latex (Sumitomo Naugatax Co., Ltd., N2752E1 solid content 48%) were added to give a solid content of 25.4% and a viscosity of 13°5 cp ( 20℃
) was prepared. The coating solution was applied to base paper (Sanyo Kokusaku Pulp Co., Ltd., high-quality paper 65 g/m") using the Mayer barcode method to obtain single-sided coated paper with a dry weight of 8 g/-. A recording paper of the present invention with a coated surface smoothness of 5000 seconds, a gloss level of 59%, and a whiteness level of 78% was prepared by super-calendering.A control paper in which the pigment was mixed only with precipitated calcium carbonate in the above-mentioned process was prepared. The coated paper has a coating weight of 8g/-1, a smoothness of 5000 seconds, a gloss level of 23%, and a whiteness level of 8.
It was 0%.

As a result of printing test patterns on both coated paper and control paper using a thermal transfer color printer (Mitsubishi Electric Corporation, M84234.), the coated paper with black ink was 20% lower than the control paper, and the coated paper with cyan was 50% lower than the control paper. %, and a 70% increase in reflection density was observed for magenta. The printed dot shape was a precise rectangle, and it was observed that the ink transfer was excellent.

[Example 3] 70 parts by weight of the precipitated calcium carbonate was added with amorphous silica gel (Mizusawa Chemical Co., Ltd., Mizuka Seal P78) [thermal conductivity 0. IKCal/m-hr・℃, apparent specific gravity 0.18
) were mixed and dispersed in 300 parts by weight of water. Polyvinyl alcohol (electrochemistry (1), A30) 10
% solution and 150 parts by weight of styrene-butadiene latex (
Sumitomo Naugatax Co., Ltd., SN-307, solid content 4
8%) was added to prepare a coating liquid with a solid content of 25.4% and a viscosity of 15.5 (!p (25°C)). After coating on the high-quality paper used in the experimental example and drying. The recording paper of the present invention subjected to super calendering had a coating weight of 7 g/n?, smoothness of 400 seconds, gloss of 22%, and whiteness of 84%.

Commercially available high-quality paper for thermal transfer printers (65g/rrl)
A thermal transfer color printer (Seiko Electronics Co., Ltd., Multicolor Hard Copy CM5312) was used as the control paper.
), the reflection density of the present invention increased by 20% for black ink, 20% for cyan, and 40% for magenta compared to the above-mentioned high-quality paper. The printed dot shape showed a precise rectangle.

[Example 4] Cross-linked polystyrene microbeads (Sumitomo Chemical Co., Ltd.)
, Fine Pearl @ 3000F) C thermal conductivity 0.1 K
cal/m-hr-°C) was dispersed in 300 parts by weight of water and 0.5 parts by weight of sodium polyphosphate. Then, 150 parts by weight of the 10% modified starch solution and styrene-butadiene latex (Sumitomo Naugatax, SN
307, solid content 48X) was added to obtain a coating liquid having a solid content of 25.4% and a viscosity of 8.0 cp (25°C). This coating liquid was applied to base paper (Sanyo Kokusaku Pulp Co., Ltd., High Quality Paper 6).
5g/i) with Meyer-Harney = l-) method L, super calender, coating drying type i-7,0g/f
f! 1. Smoothness 300 seconds, whiteness 78%, gloss 25%
The recording paper of the present invention was obtained.

Commercially available coated paper (Sanyo Kokusaku Pulp ((E), S
K coat 106 g/y? , smoothness 2000 seconds, whiteness 80%, gloss 64%) and thermal transfer color printer (Shinko Electric Co., Ltd., color hard copy CHCl35)
) to print a test pattern.

Reflection density is 20% black ink compared to control coated paper
, the reflection density increased by 60% for cyan ink and by 60% for magenta ink. The printed dot shape was a precise rectangle, whereas the control coated paper had transfer defects.

[Example 5] Silica alumina gel (Mizusawa Chemical Industry Co., Ltd., Sil) y9
MH)C thermal conductivity 0.18 Kcal/m-hr-℃,
The apparent specific gravity of the secondary particles was 100 parts by weight (specific gravity 0.4), and sodium polyphosphate and modified starch were added to 300 parts by weight of water under the same conditions as in Example 4.
) coating solution was obtained. It was coated on base paper (Yamaba Kokusaku Valve Co., Ltd., high-quality paper 40 g/rrl) using the air knife method, and then subjected to super calendering to give a coating dry weight of 8 g/rr. A recording paper of the present invention having a smoothness of 450 seconds, a whiteness of 75%, and a gloss of 12% was prepared.

The results of printing with the aforementioned Mitsubishi Electric thermal transfer color printer were as shown in Example 1. Compared to the coating of kaolinite clay alone, the reflection density was 50% for black ink, 77% for cyan ink, and 77% for magenta ink. This was an increase of 89%. The printed dot shape was a precise rectangle and clear.

[Brief explanation of the drawing]

The figure is a graph showing the relationship between the mixing ratio of low thermal conductivity pigment and the reflection density of thermal transfer recording. The solid line shows the reflection density increase curve of the present invention, and the dotted line shows the reflection density level of the high-quality paper. 1; Black ink, 2; Magenta ink 3; Cyan ink

Claims (4)

[Claims] (1) In recording paper for thermal transfer printers that performs recording by transfer of heat-melting ink, the pigment coating layer provided on the ink-receiving surface of the recording paper has: A: The thermal conductivity of the pigment constituent material is 0.3 Kcal/m・h
Organic pigment below r°C, B: Thermal conductivity of secondary agglomerated particles is 0.3 Kcal/m·h
Organic pigment below r°C, C: Thermal conductivity of secondary agglomerated particles is 0.3 Kcal/m·h
The pigment coating layer contains 30 to 100% by weight of the pigment component in the pigment coating layer, and the remaining pigment component is a crystalline inorganic pigment. Recording paper for thermal transfer printers. (2) The thermal conductivity of the pigment component is 0.3 Kcal/m・
2. The recording paper for a thermal transfer printer according to claim 1, wherein the organic pigment having a temperature of hr.degree. C. or less is a polystyrene pigment. (3) The thermal conductivity of secondary agglomerated particles is 0.3 Kcal/m・
2. The recording paper for a thermal transfer printer according to claim 1, wherein the organic pigment having a temperature of hr.degree. C. or less is a urea resin pigment. (4) The thermal conductivity of secondary agglomerated particles is 0.3 Kcal/m・
2. The recording paper for thermal transfer printers according to claim 1, wherein the amorphous inorganic pigment having a temperature of hr.degree. C. or less is silica gel or silica alumina gel.