BACKGROUND OF THE INVENTION
[0001] The present invention relates to an anti-fogging and surface-treated calcium carbonate
pigment for the coating layer of heat-sensitive recording paper, a process for preparing
the same, a coating composition comprising the calcium carbonate for heat-sensitive
recording paper, and heat-sensitive recording paper coated with the composition.
[0002] Calcium carbonates include light calcium carbonate and heavy calcium carbonate which
are up to 10 m²/g in BET specific surface area, and fine calcium carbonate in excess
of 10 m²/g in BET specific surface area. Calcium carbonate is widely used as a pigment
for papers, coating compositions, inks, etc. and also as a filler for rubbers, plastics,
papers, sealing materials, etc.
[0003] In recent years, calcium carbonate is increasingly used also as a pigment for the
coating layer of information recording papers such as heat-sensitive recording paper,
ink-jet recording paper, etc.
[0004] Calcium carbonate is incorporated into coating compositions for heat-sensitive recording
paper generally in order to improve the whiteness, non-transparency, surface-smoothness,
writablity and like properties of heat-sensitive recording paper as in the case of
usual papers. Yet the main object of incorporation of the calcium carbonate is to
give the following effects. When heat-sensitive recording paper is heated by the thermal
head of a heat-sensitive facsimile system or heat-sensitive printer to develop a color
for recording, a higher fatty acid amide or like chromogenic sensitivity adjusting
agent, crystal violet lactone or like dye, etc. adhere to the thermal head as residual
matter (hereinafter referred to as "residue") to obscure the recorded characters or
to reduce the runnablity, i.e., the ability of recording paper to smoothly pass, so
that calcium carbonate is incorporated in the coating composition for such paper to
inhibit or prevent the adhesion of the residue (hereinafter referred to as "effect
to remove residue") and also to achieve heat transfer with enhanced efficiency from
the thermal head to the surface of heat-sensitive recording paper.
[0005] Heat-sensitive recording paper is a recording material comprising a chromogenic material,
a color developer, a pigment and other components, and is required to have:
(1) an ability to give an image of distinct color;
(2) a background having high whiteness, i.e. free of fogging;
(3) an ability to produce an image exhibiting a high weatherability even during long-term
storage.
[0006] Use of highly oil-absorbent calcium carbonate pigment has been considered desirable
for improvement of the effect to remove residue. Such known calcium carbonates include,
for example, bodies of intertwined needle- to pillar-like calcium carbonate particles
as disclosed in Japanese Examined Patent Publication No.8048/1988 and highly oil-
and water-absorbent calcium carbonate as described in Japanese Unexamined Patent Publication
No.230424/1989. These calcium carbonates are excellent in the effect to remove residue,
but are not always fully satisfactory in anti-fogging property, ability to form images
with high resistance to weather, especially to light, heat transfer efficiency, etc.
[0007] Currently attempts are being made in the art to develop a heat-sensitive recording
paper which is able to achieve speedy recording, and there is a tendency to use more
highly oil-absorbent calcium carbonates capable of producing an improved effect to
remove residue and giving a higher heat transfer efficiency. Nevertheless calcium
carbonate has yet to be developed which is fully satisfactory in anti-fogging property
of heat-sensitive recording paper, ability to form images of high weatherability,
etc.
[0008] It is an object of the invention to provide a calcium carbonate pigment which is
sufficiently highly oil-absorbent to provide an improved effect to remove residue,
and also outstanding in anti-fogging property, ability to form images of high weatherability,
etc.
[0009] We thought that the above object might be accomplished by surface-treating a highly
oil-absorbent calcium carbonate pigment with a certain substance. For investigation,
we conducted extensive research to provide a surface-treated calcium carbonate which,
when used as a pigment for the coating layer of heat-sensitive recording paper, exhibits
not only an excellent effect to remove residue and a high chromogenic sensitivity,
but also an excellent property of forming images of high weatherability with substantially
no fogging. As a result of the research, we found the following. When a specific calcium
carbonate surface-treating agent is added to an aqueous suspension of calcium carbonate
particles having a BET specific surface area of about 17 to about 55 m²/g, and the
mixture is stirred to surface-treat the carbonate, the thus treated carbonate is given
a surface acidity of solid of about 33 to about 38 µmol/g with substantially no reduction
of oil absorption of the starting calcium carbonate, and is rendered able to achieve
remarkable effects unattainable by known calcium carbonates. The present invention
has been accomplished based on this novel finding.
[0010] According to the present invention, there is provided a calcium carbonate pigment
for heat-sensitive recording paper, the pigment having a BET specific surface area
of about 17 to about 55 m²/g, preferably about 19 to about 55 m²/g, an oil absorption
of about 90 to about 220 ml/100 g as determined by the Ogura method and a surface
acidity of solid substantially represented by the equation (Z):

[0011] wherein x is a BET specific surface area (m²/g) and y is a surface acidity of solid
(µmol/g).
[0012] The present invention also provides a process for preparing the foregoing calcium
carbonate pigment for heat-sensitive recording paper, the process comprising the steps
of (i) adding a calcium carbonate surface-treating agent to an aqueous suspension
of calcium carbonate having a BET specific surface area of about 17 to about 55 m²/g,
preferably about 19 to about 55 m²/g, the surface-treating agent being at least one
member selected from the group consisting of hydroxide of alkaline earth metal, hydroxide
of alkali metal, carbonate of alkali metal, bicarbonate of alkali metal, sodium aluminate,
aluminum acetate and C₄₋₁₄ aliphatic amine acetate; and (ii) stirring the mixture.
[0013] The invention further provides a coating composition for heat-sensitive recording
paper characterized in that the composition contains the above calcium carbonate pigment
for heat-sensitive recording paper.
[0014] Throughout the specification and claims, the BET specific surface area was determined
by the low-temperature nitrogen adsorption method (see "Particle Size Determination
Techniques," edited by the Research Association of Powder Technology, Nikkan Kogyo
Press, 1st edition, 1975, pp. 299-305). The surface acidity of solid was determined
by an amine titrimetric determination method (see Kozo Tabe and Tsuneichi Takeshita,
"Acid-base Catalyst," Sangyo Tosho, 1st edition, 1966, pp.164-167). The method comprises
titrating a calcium carbonate pigment as a solid acid in benzene with p-dimethylamino-azobenzene
(dimethyl yellow) as an indicator using normal butyl amine. The oil absorption was
determined by the Ogura method (see Junzo Matsumoto and Masateru Ogura, "Pigments,
Coloring Materials and Inks," Kyoritsu Shuppan Co., Ltd., 4th print, 1950, pp. 66-67,
with use of boiled linseed oil according to JIS K 5421).
[0015] Table 1 below shows the physical properties of the calcium carbonate pigment of the
invention for heat-sensitive recording paper in comparison with those of known calcium
carbonates.

[0016] The known calcium carbonates (i) to (iv) listed in Table 1 are as follows.
(i) Highly oil- and water-absorbent calcium carbonate as disclosed in Japanese Unexamined
Patent Publication No.230424/1989
(ii) Fine precipitated calcium carbonate
(iii) Light calcium carbonate
(iv) Bodies of intertwined needle-like to pillar-like calcium carbonate particles
as disclosed in Japanese Examined Patent Publication No.8048/1988
[0017] The properties such as bulk in Table 1 were determined by the following methods.
- Bulk:
- Pigment-testing method according to JIS K 5101
- Sedimentation volume:
- 5 g of calcium carbonate was placed into a 100 ml measuring cylinder, and water was
added to make 100 ml of a suspension. After shaking for 20 seconds, the suspension
was left to stand for 60 minutes to determine the volume of sediment.
- Hiding power:
- Pigment-testing method according to JIS K 5101
- True specific gravity:
- Pigment-testing method according to JIS K 5101
- Crystal system:
- Determined by X ray diffraction method
- Mean particle size:
- Median particle size as measured by light transmission particle size determination
method.
[0018] Table 1 shows that the surface-treated calcium carbonate pigment of the invention
has a surface acidity of solid of as low as 33 to 38 µmol/g compared with the known
calcium carbonate (i) having a BET specific surface area of about 25 to 55 m²/g, and
retains high oil absorption which is another important property of calcium carbonate
pigment for heat-sensitive recording paper.
[0019] Our research revealed that calcium carbonate having a BET specific surface area of
about 17 to about 55 m²/g has the problem of being generally not always satisfactory
in the anti-fogging property presumably because of its high surface acidity of solid
due to its great BET specific surface area irrespective of the other physical properties
such as particle shape, sedimentation volume, etc. In other words, generally calcium
carbonates having a BET specific surface area of about 17 to about 55 m²/g would be
likely to cause fogging when used as a pigment for heat-sensitive recording paper.
Therefore the surface-treating method of the invention shows a particular significance
when employed to treat calcium carbonates having a BET specific surface area of about
17 to about 55 m²/g, preferably about 19 to about 55 m²/g. The method of the invention
reduces the surface acidity of solid of calcium carbonate to the specific range of
about 33 to about 38 µmol/g in the specific relation represented by the above-defined
equation (Z) with substantially no decrease in the BET specific surface area or in
the oil absorption.
[0020] Now we turn to the properties of the calcium carbonate pigments of the invention.
The calcium carbonate pigments less than 17 m²/g in BET specific surface area have
an inherently low surface acidity of solid like the starting calcium carbonate and
are virtually free of fogging problem, but tend to have a low effect to remove residue
because of generally low oil absorption. On the other hand, the treatment of calcium
carbonate pigments more than 55 m²/g in BET specific surface area requires an increased
amount of a surface-treating agent which is needed in order to diminish the high surface
acidity of solid of the starting calcium carbonate to 38 µmol/g as specified above
according to the equation (Z), so that the obtained calcium carbonate tends to have
a decreased oil absorption. Therefore such calcium carbonates are undesirable. To
give a surface acidity of solid of less than 33 µmol/g to calcium carbonate, an increased
amount of surface-treating agent is required, resulting in reduction of oil absorption.
Thus such calcium carbonates are undesirable. In this case, presumably due to the
relatively high surface basicity of solid, the obtained heat-sensitive recording paper
is apt to form images of low weatherability. Above 38 µmol/g of a surface acidity
of solid, a fogging problem may arise.
[0021] The calcium carbonate pigment of the invention is usually prepared by the following
process. To an aqueous suspension of calcium carbonate having a BET specific surface
area of about 17 to about 55 m²/g is added at least one calcium carbonate surface-treating
agent selected from the group consisting of hydroxide of alkaline earth metal, hydroxide
of alkali metal, carbonate of alkali metal, bicarbonate of alkali metal, sodium aluminate,
aluminum acetate and acetic acid salt of C₄₋₁₄ aliphatic amine. Then the mixture is
stirred to treat the surface of the pigment particles.
[0022] Useful starting calcium carbonates having a BET specific surface area of about 17
to about 55 m²/g include a variety of known calcium carbonates. Among them, it is
desirable to use those as disclosed in Japanese Unexamined Patent Publication No.230424/1989
and Japanese Examined Patent Publications No.31530/1982 and No.30815/1982.
[0023] More desirable among these starting calcium carbonate pigments are the calcium carbonate
pigment as disclosed in Japanese Unexamined Patent Publication No.230424/1989 which
is 25 to 55 m²/g in BET specific surface area, between 5 and 110 in the ratio of BET
specific surface area (m²/g)/mean particle size (µm), at least 120 ml/100 g in oil
absorption as determined by the Ogura method and at least 1.8 g/g in water absorption,
and the calcium carbonate pigment which is 17 to 55 m²/g, preferably 19 to 55 m²/g,
in BET specific surface area, among the bodies of intertwined needle-like calcium
carbonate particles as disclosed in Japanese Examined Patent Publication No.31530/1982,
the particles being formed of bodies of needle-like primary particles of calcium carbonate
three-dimensionally intertwined irregularly and having mean dimensions of 0.5 to 10
µm in length (L) and 0.05 to 0.2 µm in width (W) as observed under an electron microscope,
an aspect ratio (L/W) of from 10 to 50, a void volume of 1.8 to 3.3 ml/g as measured
by a porosimeter (mercury intrusion porosimetry) and an oil absorption of 50 to 100
ml/100 g as measured according to JIS K 5101.
[0024] The oil absorption of calcium carbonate pigment contributes to the effect to remove
residue. From the viewpoint of the effect to remove residue, it is suitable in the
invention to use starting calcium carbonate pigments of preferably about 90 to about
220 ml/100 g, more preferably about 93 to about 220 ml/ 100 g, in oil absorption.
It is preferable that the starting calcium carbonate pigments are those having a bulk
of about 6.5 to about 15 ml/g, preferably about 7 to about 15 ml/g.
[0025] Of calcium carbonate surface-treating agents useful in the invention, typical hydroxides
of alkaline earth metals are hydroxides of magnesium, calcium or like metals, preferable
hydroxides, carbonates or bicarbonates of alkali metals are sodium or potassium hydroxides,
carbonates or bicarbonates, and preferred C₄₋₁₄ aliphatic amine acetic acid salts
are butyl amine, octyl amine and lauryl amine acetic acid salts. The amount of the
calcium carbonate surface-treating agent which is added to the aqueous suspension
of calcium carbonate of 17 to 55 m²/g in BET specific surface area is in the range
of about 0.1 to about 5 parts by weight, preferably about 0.2 to about 4 parts by
weight, per 100 parts by weight of the calcium carbonate. When the amount is less
than 0.1 part by weight, the surface acidity of solid can not be suppressed as contemplated
and the heat-sensitive paper incorporating the treated carbonate is not imparted an
improved anti-fogging property. When the amount is more than 5 parts by weight, the
obtained heat-sensitive paper has an improved anti-fogging property but tends to have
an impaired effect to remove residue due to reduced oil absorption. Therefore the
amount of surface-treating agent outside the above range is undesirable.
[0026] The concentration of solids in the aqueous suspension of starting calcium carbonate
is not specifically limited, but is usually in the range of about 5 to about 30% by
weight, preferably about 7 to about 25% by weight. The mixture is stirred for surface
treatment preferably at a temperature of about 15 to about 35°C. The aqueous suspension
of starting calcium carbonate is stirred by stirring means which can stir uniformly
the whole suspension, such as a propeller-type stirrer, high-speed impeller dispersing
means, an oar-type stirrer, turbine-type stirrer, a stirrer of the type capable of
blowing air or like gas, etc. The stirring time is not specifically limited, but is
preferably in the range of about 10 to about 30 minutes.
[0027] The calcium carbonate pigment of the invention having the BET specific surface area
and the surface acidity of solid related to each other as represented by the equation
(Z) is prepared by surface-treating the starting calcium carbonate of the above-specified
BET specific surface area under the above conditions. As compared with the starting
calcium carbonate, the obtained calcium carbonate is slightly reduced or increased,
but not substantially reduced to an unacceptable degree, in oil absorption, and has
suppressed surface acidity of solid in the range of about 33 to about 38 µmol/g.
[0028] The calcium carbonate pigment obtained by the above surface treatment may be used
in the form of a paste which is obtained by dewatering the suspension resulting from
the surface treatment by a filter press or like dewatering means, or in the form of
a powder prepared in the conventional manner by drying the paste, pulverizing the
solid and classifying the particles.
[0029] The thus obtained calcium carbonate pigment of the invention, when used for forming
a coating layer in heat-sensitive recording paper, was found to exhibit, due to the
above specific properties of the surface, the excellent properties which have not
been found in known calcium carbonates. Therefore, the present invention also relates
to coating compositions for heat-sensitive recording paper which comprise about 5
to about 90% by weight of the above-obtained calcium carbonate pigment based on the
total solids. Given below are examples of coating compositions of the invention.
(1) Composition for forming a heat-sensitive recording layer of heat-sensitive recording
paper
[0030] This composition comprises about 5 to about 60 wt. % of the calcium carbonate pigment
of the invention as admixed with a known colorless to pale-colored basic dye, a color
developer such as phenolic compound for causing the dye to produce a color when heated,
a chromogenic sensitivity adjusting agent, a binder, etc. The heat-sensitive recording
papers obtained with use of the composition are comparable to those incorporating
the conventional calcium carbonate in the effect to remove residue and are superior
to the latter in color density, anti-fogging property, resistance to light, etc.
[0031] The components of the coating composition for heat-sensitive recording paper other
than the above calcium carbonate pigment, i.e. the basic dye, color developer, chromogenic
sensitivity adjusting agent, binder, and the like can be a wide variety of those conventionally
used for heat-sensitive paper. Typical examples of these components are as follows.
(a) Colorless to pale-colored basic dyes
[0032] Triarylmethane dyes such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide,
fluoran dyes such as 3-diethylamino-6-methyl-7-anilinofluoran, spiropyran dyes such
as 3-methyl-spiro-dinaphthopyran, diphenylmethane dyes such as N-halophenyl-leucoauramine,
thiazine dyes such as benzoylleucomethyleneblue, etc.
(b) Color developers
[0033] Phenolic compounds such as 4-tert-butylphenol, 4-hydroxydiphenoxide, 4,4′-isopropylidenediphenol
(bisphenol A), 2,2′-methylenebis(4-chlorophenol) and novolak-type phenolic resins,
aromatic carboxylic acids or derivatives thereof such as benzoic acid, p-tert-butyl-benzoic
acid, p-hydroxybenzoic acid, methyl p-hydroxybenzoate, isopropyl p-hydroxybenzoate,
benzyl p-hydroxybenzoate, lauryl gallate, stearyl gallate, salicylanilide, 5-chlorosalicylanilide,
5-tert-butylsalicylic acid, hydroxynaphthoic acid and zinc or like metal salts of
these acids or derivatives thereof.
(c) Chromogenic sensitivity adjusting agents
[0034] Higher fatty acid amides such as palmitic acid amide, stearic acid amide, oleic acid
amide, hydroxystearic acid amide, methylolated fatty acid amide, ethylenebis fatty
acid amide and methylenebis fatty acid amide.
(d) Binders
[0035] Water-soluble high-molecular-weight compounds such as polyvinyl alcohol, methyl cellulose,
carboxymethyl cellulose, hydroxyethyl cellulose, hydroxpropyl cellulose, starch, casein,
gelatin and gum arabic.
[0036] When required, various auxiliary agents for use in known coating compositions for
heat-sensitive paper can be further incorporated into the composition of the invention.
Examples of such additives are release agents, defoaming agents, ultravio1et absorbers,
fluorescent dyes, coloring dyes, preservatives, etc. When required, another pigment
can be added in an amount of up to about 20% by weight, based on the total amount
of the composition. Examples of useful other pigments are aluminum hydroxide, silica,
calcined kaolin, kaolin, talc, urea resin, etc.
[0037] The components of the coating composition according to the invention and the proportions
(solid contents) thereof can be changed according to the contemplated purpose. Usually,
the composition comprises the following proportions (solid contents) of components
based on the total solids.
- Basic dye
- about 3 to about 10 wt.% (preferably about 5 to about 10 wt.%)
- Color developer
- about 15 to about 50 wt.% (preferably about 20 to about 40 wt.%)
- Chromogenic sensitivity
- about 6 to about 30 wt.%
- adjusting agent
- (preferably about 10 to about 30 wt.%)
- Binder
- about 16 to about 22 wt.% (preferably about 16 to about 20 wt.%)
- Calcium carbonate
- about 5 to about 60 wt.%
- pigment of the
- (preferably about 10 to
- invention
- about 55 wt.%)
[0038] The coating composition of the invention can be prepared in a known manner, for example,
by the following method. First, the basic dye, color developer and chromogenic sensitivity
adjusting agent are each separately milled in an aqueous solution of binder. When
a ball mill, for example, is used for this purpose, each component is added to the
binder solution usually to a solids concentration of about 15 to about 30 wt. %, and
the mill is operated for 2 days to obtain fine particles of about 1 to about 5 µm
in size. The calcium carbonate pigment of the invention, when required together with
other pigments, is dispersed in an aqueous solution of binder in the usual manner
using a dispersant to prepare a pigment dispersion containing about 20 to about 30
wt. % of solids. Subsequently, the pigment dispersion and the aqueous suspensions
of finely divided components are mixed together in the conventional manner, giving
a coating composition for forming a heat-sensitive recording layer of heat-sensitive
paper according to the invention. The composition is in the form of a dispersion containing
about 15 to about 30 wt. % of solids.
[0039] The composition is applied to a substrate and dried in the usual manner, and the
coated substrate is calendered when required to give heat-sensitive paper. The coating
composition of the invention is applied usually in an amount of about 3 to about 15
g/m², preferably about 5 to about 10 g/m², calculated as dry weight, although the
amount is variable, for example, with the properties of the heat-sensitive paper to
be obtained.
[0040] Thus, the present invention provides a heat-sensitive recording paper which is characterized
in that it comprises a substrate and a heat-sensitive recording layer formed thereon
by applying the above coating composition and drying the resulting coating. Examples
of useful substrates are various papers, synthetic resin sheets or films and the like
which are conventionally used in the art.
Coating composition for forming an intermediate layer between heat-sensitive recording
layer of heat-sensitive recording paper and substrate thereof
[0041] Our research has revealed that when the composition containing about 5 to about 40
parts by weight of a binder per 100 parts by weight of the calcium carbonate pigment
of the invention is applied to a substrate to form an intermediate layer and a heat-sensitive
recording layer is formed over the intermediate layer, the resulting paper is higher
in color density, anti-fogging property, resistance to light, and the like than when
the conventional calcium carbonate is used.
[0042] The binder to be used for the coating composition for forming the intermediate layer
can be any of the binders already mentioned in item (1) for the heat-sensitive recording
layer. Also useful are latices of styrene-butadiene copolymer, polyvinyl acetate,
polyurethane, polyacrylic acid, polyacrylate, vinyl chloride-vinyl acetate copolymer,
polybutyl methacrylate, ethylene-vinyl acetate copolymer, styrene-butadiene-acrylic
copolymer and the like. When required, the coating composition may have further incorporated
therein other pigments (such as calcined kaolin, silica, etc.), dispersants, surfactants,
defoaming agents, coloring dyes, preservatives, etc.
[0043] While the proportions of components (calculated as solids) of the coating composition
for forming the intermediate layer are variable over a wide range, a binder is used
preferably in an amount of, calculated as solids, about 5 to about 40 parts by weight,
preferably about 10 to 35 parts by weight, per 100 parts by weight of the calcium
carbonate pigment of the invention. The amount of another pigment to be added when
required is up to about 40 parts by weight, preferably about 10 to about 40 parts
by weight, per 100 parts by weight of the combined amount of the calcium carbonate
pigment of the invention and the binder calculated as solids.
[0044] The coating composition for the intermediate layer can be prepared by uniformly dispersing
the calcium carbonate pigment of the invention and another pigment when so required
in an aqueous solution of binder in the usual manner using a dispersant or the like
to obtain a pigment dispersion containing about 25 to about 35 wt. % of solids. The
composition can be applied to a substrate by various methods, for example, by a coater
such as steel blade, air knife, roll, flexographic press, Mayer bar or the like. The
amount of the composition to be applied, although widely variable, is generally about
1 to about 15 g/m², preferably about 3 to about 10 g/m², calculated as dry weight.
The coating, when dried, is preferably calendered.
[0045] A heat-sensitive recording layer is formed by applying the coating composition for
forming a heat-sensitive recording layer as stated above in item (1) in the conventional
manner to the intermediate layer thus formed, and drying the coating layer, whereby
a heat-sensitive recording paper is obtained which is excellent in effect to remove
residue, anti-fogging property, color density, resistance to light and other properties.
[0046] Accordingly, the present invention further provides a heat-sensitive recording paper
comprising a substrate, the above intermediate layer formed thereon and the heat-sensitive
recording layer formed over the intermediate layer from the coating composition for
a heat-sensitive recording layer which composition contains the calcium carbonate
pigment of the invention as described above in item (1). Examples of useful substrates
are paper, synthetic resin film and the like which are conventionally used in the
art. The heat-sensitive recording layer can be any of those heretofore known other
than those described above in item (1), such as one containing a colorless to pale-colored
basic dye, color developer, chromogenic sensitivity adjusting agent, binder and the
like as exemplified in item (1), and various other layers.
[0047] The reason still remains to be clarified why the above calcium carbonate pigment
of the invention used for the coating layer of heat-sensitive recording paper produces
the above excellent effects. However, the reason could presumably be explained as
follows. Calcium carbonate has some characteristics of solid acid as its surface properties,
and the surface acidity of solid thereof is considered to be associated with the anti-fogging
property of heat-sensitive recording paper. While the surface of particulate calcium
carbonate is relatively low in surface acidity of solid, calcium carbonates of greater
BET specific surface area tend to have a higher surface acidity of solid. It is assumed
accordingly that in a heat-sensitive recording layer, the calcium carbonate pigment
of greater BET specific surface area has a larger surface area in contact with a basic
dye acting as a chromogenic material, resulting in reduction of anti-fogging property.
Calcium carbonate pigments for heat-sensitive paper are also required to have a high
oil absorption which is another important property of such pigment. Presumably, by
surface-treating the calcium carbonate of about 17 to about 55 m²/g in BET specific
surface area according to the specific surface-treating method of the invention which
induces substantially no reduction in the BET specific surface area or the oil absorption,
the calcium carbonate pigment of the invention is endowed with a surface acidity of
solid in the specific range, and thereby a heat-sensitive recording paper improved
in anti-fogging property, color density and effect to remove residue is obtained.
The calcium carbonate of the invention used also provides a heat-sensitive recording
paper with higher resistance to light. However, the effect is unpredictable from the
relation with the surface acidity of solid, and the reason for this improvement remains
unclear.
[0048] Furthermore, the calcium carbonate of the invention used for heat-sensitive paper
can enhance the whiteness and non-transparency of the paper.
[0049] The calcium carbonate pigments of the invention also have the following advantages.
1) When used as a filler for paper, the calcium carbonate of the invention can increase
the retention of filler and enhance the whiteness and non-transparency of paper.
2) When incorporated into a coating composition for coat paper, the pigment of the
invention gives a coating layer having a porous surface which can effectively absorb
the applied printing ink, thereby accelerating the drying.
Examples
[0050] The present invention will be described below in more detail with reference to the
following examples. In the following examples, "parts" and "percentages" are all by
weight unless otherwise specified.
Example 1
[0052] A 100 kg quantity of an aqueous suspension of calcium carbonate having a BET specific
surface area of 20 m²/g, adjusted to a concentration of 20 wt.% and a temperature
of 20°C, was placed into a reactor equipped with an impeller disperser. Thereafter,
1.4 kg of an aqueous solution of sodium aluminate adjusted to a concentration of 10
wt.% was added to the aqueous suspension with stirring at 500 r.p.m., and the resulting
mixture was further agitated for 15 minutes. After completion of the agitation, the
aqueous suspension of calcium carbonate was dewatered by a filter press. The solid
product obtained was dried, pulverized and classified, giving 20 kg of a calcium carbonate
pigment for heat-sensitive recording paper of the present invention.
Examples 2 to 7
[0053] Calcium carbonate pigments for heat-sensitive recording paper of the invention were
prepared in the same manner as in Example 1 with the exception of using the conditions
listed in Table 2. Table 2 also shows the conditions employed in Example 1.

[0054] In Table 2, the calcium carbonates used as starting materials in Examples 1 and 2
were prepared by a process disclosed in Japanese Examined Patent Publication No. 31530/1982,
and those used in Examples 3 to 7 were produced by a process set forth in Japanese
Unexamined Patent Publication No. 230424/1989.
Comparative Examples 1 to 5
[0055] Comparative calcium carbonate pigments were prepared in the same manner as in Example
1 with the exception of employing the conditions as described below in Table 3. The
calcium carbonates used as starting materials in Comparative Examples 1 and 2 were
produced by a process disclosed in Japanese Examined Patent Publication No. 31530/1982,
and those used in Comparative Examples 3 to 5 were obtained by a process described
in Japanese Unexamined Patent Publication No. 230424/1989.
[0056] In Comparative Examples 2, 3 and 5, no surface-treating agent was used.

[0057] Table 4 below shows the physical properties of the calcium carbonate pigments obtained
in Examples 1 to 7 and Comparative Examples 1 to 5.

[0058] From the results with respect to Examples 2 and 3 and Comparative Examples 2 and
3 as shown in Table 4, it is revealed that the calcium carbonate pigments of the invention
are somewhat increased or decreased in BET specific surface area and oil absorption
as compared with calcium carbonate used as the starting material. However, substantially
no problem is caused even in the case where the calcium carbonate pigment of the invention
is decreased in BET specific surface area or oil absorption. This indicates that the
surface acidity of solid of the calcium carbonate pigment of the invention is suppressed
to a specific range.
Example I
[0059] Coating compositions for forming the heat-sensitive recording layer of heat-sensitive
recording paper were prepared by the following procedure using the calcium carbonate
pigments of the invention (those obtained in Examples 1, 3 and 7). The compositions
were used for preparing heat-sensitive recording papers.
[0060] First, a colorless dye, phenolic compound and fatty acid amide were each separately
milled into fine particles according to the following formulations A, B and C. For
this purpose, ball mills were operated for 2 days.

[0061] According to the following formulation D, the calcium carbonate pigment of the invention
was made into a pigment dispersion containing 25% of solids using an impeller-type
agitator.

[0062] The dispersions A to D prepared according to the formulations A to D were mixed together
in the ratio by weight of A:B:C:D = 1:5:3:5 to prepare coating compositions of the
invention for forming a heat-sensitive recording layer.
[0063] Using a coating rod, the coating composition thus prepared was applied to one surface
of wood-free paper, weighing 50 g/m², in the usual manner in an amount of 6 g/m² on
dry basis. After drying the coating at room temperature, the coated paper was calendered
to obtain heat-sensitive paper.
[0064] Table 5 showing the characteristics of the paper thus obtained indicates that the
paper retains substantially the same degree of whiteness before and after the heating
of the paper and is excellent in anti-fogging property. Further, the paper is outstanding
in resistance to light and has high effect to remove residue.
Comparative Example I
[0065] Comparative coating compositions for forming a heat-sensitive recording layer were
prepared in the same manner as in Example I except that the comparative calcium carbonate
pigments obtained in Comparative Examples 1 to 5 were used. The compositions were
used for preparing heat-sensitive recording paper. Table 5 shows the characteristics
of the paper obtained. Table 5 also shows the results obtained using known calcium
carbonate (i) (which is disclosed in Japanese Unexamined Patent Publication No. 230424/1989).
The physical properties of the known calcium carbonate (i) used are as follows. (The
same applies in the following description.)
- BET specific surface area
- 38 m²/g
- Surface acidity of solid
- 44 µmol/g
- Oil absorption
- 140 ml/100 g
- Bulk
- 10 ml/g
- Sedimentation volume
- 75 ml/60 min
- Hiding power
- 35 cm²/g
- True specific gravity
- 2.60
Example II
[0066] Calcium carbonate pigments of the invention (those obtained in Examples 1, 3 and
7) were used for forming the intermediate layer of heat-sensitive paper. More specifically,
the calcium carbonate pigment was uniformly dispersed according to the following formulation
to obtain a coating composition for forming the intermediate layer.

[0067] Using a coating rod, the coating composition was applied to one surface of wood-free
paper, weighing 50 g/m², in the usual manner in an amount of 7 g/m² on dry basis.
After drying the coating, the coated paper was falendered to obtain pigment-coated
paper.
[0068] Dispersions A, B, C and D prepared according to the formulations A to D in Example
I were mixed together in the ratio by weight of A:B:C:D = 1:5:3:1 to obtain a coating
composition, which was then applied to the pigment-coated paper in an amount of 6
g/m² on dry basis using a coating rod. After drying the coating at room temperature,
the resulting paper was calendered to obtain heat-sensitive recording paper having
an intermediate layer. Table 5 showing the characteristics of the heat-sensitive recording
paper thus prepared indicates that the paper retained substantially the same whiteness
before and after the heating of the paper, and was outstanding in anti-fogging property
and resistance to light. Further, the paper had sufficiently high effect to remove
residue.
Comparative Example II
[0069] Comparative coating compositions for forming an intermediate layer were prepared
in the same manner as in Example II except that the comparative calcium carbonates
obtained in Comparative Examples 1 to 5 were used. Comparative heat-sensitive paper
having an intermediate layer was obtained in the same manner as in Example II using
each of the comparative coating compositions. The characteristics of the heat-sensitive
recording paper thus obtained were shown in Table 5, which also shows the result achieved
using known calcium carbonate (i) similarly.

1) Test for anti-fogging property
[0070] A piece of heat-sensitive paper (white paper) was set in a test apparatus maintained
at a temperature of 60°C and was allowed to stand for 24 hours. The degree of discoloration
(fogging) of the paper was determined by checking the whiteness of the paper (degree
of 0°-45° reflection was measured with a blue filter using a deformation photometer
manufactured by Murakami Shikisai-Giken Co.) before and after the heating of the paper.
2) Test for resistance to light
[0071] A piece of heat-sensitive paper on which images were formed with use of a commercially
available facsimile machine (type G-III) was exposed to sunlight for 8 hours. The
color densities of the images were measured before and after the exposure to sunlight
using reflective densitometer (model DM-400, for black-and-white and color use, product
of Dainippon Screen Co., Ltd.). The image retentivity was determined by the following
equation.

3) Effect to remove residue
[0072] Degree of stain (due to adhesion of residue) of the thermal head of commercially
available facsimile machines was examined. The symbol "A" means that substantially
no residue adheres to the head and the paper can be used without problem. The symbol
"B" means that a large amount of residue adheres to the head and the paper is not
practically usable.
[0073] As clear from the results shown in Table 5, the calcium carbonate pigment of the
present invention is excellent not only in anti-fogging property but in resistance
to light in comparison with known calcium carbonate pigments.
1. A calcium carbonate pigment for heat-sensitive recording paper, the pigment having
a BET specific surface area of about 17 to about 55 m²/g, an oil absorption of about
90 to about 220 ml/100 g as determined by the Ogura method and a surface acidity of
solid substantially represented by the equation (Z):

wherein x is a BET specific surface area (m²/g) and y is a surface acidity of solid
(µmol/g).
2. A calcium carbonate pigment according to claim 1 which has a BET specific surface
area of about 19 to about 55 m²/g.
3. A process for preparing the calcium carbonate pigment of claim 1 for heat-sensitive
recording paper, the process comprising the steps of (i) adding a calcium carbonate
surface-treating agent to an aqueous suspension of a calcium carbonate having a BET
specific surface area of about 17 to about 55 m²/g, the surface-treating agent being
at least one member selected from the group consisting of hydroxide of alkaline earth
metal, hydroxide of alkali metal, carbonate of alkali metal, bicarbonate of alkali
metal, sodium aluminate, aluminum acetate and acetic acid salt of C₄₋₁₄ aliphatic
amine, and (ii) stirring the mixture.
4. A process according to claim 3 wherein the calcium carbonate has a BET specific surface
area of about 19 to about 55 m²/g.
5. A process according to claim 3 wherein the surface-treating agent is used in an amount
of about 0.1 to about 5 parts by weight per 100 parts by weight of the calcium carbonate.
6. A process according to claim 3 wherein the surface-treating agent is used in an amount
of about 0.2 to about 4 parts by weight per 100 parts by weight of the calcium carbonate.
7. A coating composition for forming a heat-sensitive recording layer of heat-sensitive
recording paper characterized in that the composition comprises about 5 to about 60%
by weight of the calcium carbonate pigment of claim 1 based on the total solids.
8. A coating composition according to claim 7 which comprises, calculated as solids,
about 3 to about 10 wt.% of a basic dye, about 15 to about 50 wt.% of a color developer,
about 6 to about 30 wt.% of a chromogenic sensitivity adjusting agent, about 16 to
about 22 wt.% of a binder, and about 5 to about 60 wt.% of the calcium carbonate pigment
of claim 1, based on the total solids.
9. A coating composition according to claim 7 which comprises, calculated as solids,
about 5 to about 10 wt.% of a basic dye, about 20 to about 40 wt.% of a color developer,
about 10 to about 30 wt.% of a chromogenic sensitivity adjusting agent, about 16 to
about 20 wt.% of a binder, and about 10 to about 55 wt.% of the calcium carbonate
pigment of claim 1, based on the total solids.
10. A coating composition for forming an intermediate layer between a heat-sensitive recording
layer of heat-sensitive recording paper and the substrate thereof, characterized in
that the composition contains a binder in an amount, calculated as solids, of about
5 to about 40 parts by weight per 100 parts by weight of the calcium carbonate pigment
of claim 1.
11. A heat-sensitive recording paper which comprises a substrate and a heat-sensitive
recording layer formed thereon and containing the calcium carbonate pigment of claim
1, a chromogenic sensitivity adjusting agent, a colorless to pale-colored basic dye,
a color developer for causing the dye to produce a color when heated and a binder.
12. A heat-sensitive recording paper according to claim 11 characterized in that it is
obtained by applying the coating composition of claim 8 to the substrate and drying
the coating layer.
13. A heat-sensitive recording paper according to claim 11 characterized in that it is
obtained by applying the coating composition of claim 9 to the substrate and drying
the coating layer.
14. A heat-sensitive recording paper which comprises (i) a substrate, (ii) an intermediate
layer formed thereon and containing the calcium carbonate pigment of claim 1 and (iii)
a heat-sensitive recording layer formed on the intermediate layer and containing the
calcium carbonate pigment of claim 1, a chromogenic sensitivity adjusting agent, a
colorless to pale-colored basic dye, a color developer for causing the dye to produce
a color when heated and a binder.
15. A heat-sensitive recording paper according to claim 14 which comprises (a) a substrate,
(b) an intermediate layer formed by applying to the substrate the coating composition
of claim 10 for forming an intermediate layer and drying the coating layer and (c)
a heat-sensitive recording layer formed by applying the coating composition of claim
8 for forming a heat-sensitive recording layer to the intermediate layer and drying
the coating layer.
16. A heat-sensitive recording paper according to claim 14 which comprises (a) a substrate,
(b) an intermediate layer formed by applying to the substrate the coating composition
of claim 10 for forming an intermediate layer and drying the coating layer and (c)
a heat-sensitive recording layer formed by applying the coating composition of claim
9 for forming a heat-sensitive recording layer to the intermediate layer and drying
the coating layer.