GB2145074A

GB2145074A - Calcium carbonate

Info

Publication number: GB2145074A
Application number: GB8414936A
Authority: GB
Inventors: Tsuneyoshi Shiraishi; Ryogo Tsukisaka; Toshio Kaneguchi
Original assignee: Shiraishi Central Laboratories Co Ltd
Current assignee: Shiraishi Central Laboratories Co Ltd
Priority date: 1983-06-16
Filing date: 1984-06-12
Publication date: 1985-03-20
Also published as: JPS59232916A; JPS638048B2; GB8414936D0; GB2145074B

Abstract

This invention provides a body comprising needle- to pillar-like primary particles of aragonite calcium carbonate irregularly three-dimensionally twined about one another and having an average length (L) of 0.3 to 6.0 mu m and an average width (W) of 0.04 to 0.5 mu m as observed electron microscopically and an aspect ratio (L/W) of 2 to 50, the body containing at least 50% by weight of aragonite crystals as determined by X-ray diffraction and having bulkiness of 5 to 10 ml/g and a specific gravity of 2.75 to 2.93 according to JIS K 5101, and a process for preparing the same.

Description

SPECIFICATION Calcium Carbonate This invention relates to novel bodies of intertwined needle- to pillar-like calcium carbonate particles, a process for preparing the same and coating compositions containing such bodies for heat-sensitive record material.

The components of coatings on heat-sensitive record materials include a colourless dye, a phenolic compound or like color developing material for causing the dye to form a color when heated, an agent for adjusting color forming sensitivity such as higher fatty acid amide and a binder such as a water-soluble high-molecular-weight compound. In addition to these components, calcium carbonate is widely used an an inorganic pigment for the following purposes.

Inorganic pigments are generally incorporated into coating compositions for heat-sensitive record materials for use in heat-sensitive facsimile, heat-sensitive printer and like systems in order to inhibit or prevent adhesion of the color forming sensitivity adjusting agent and like residual components (hereinafter simply referred to as "residue") to the thermal head which is used to heat the paper for color formation and printing. In addition to the effect of preventing adhesion of such residue, the inorganic pigment also has the effect of preventing unintended color formation of the heat-sensitive record material during handling or storage (hereinafter referred to as "whiteness retentivity") and the effect of giving improved weather resistance to the record material after color formation (hereinafter referred to as "record preservability").As is the case with usual papers, the inorganic pigment further improves the whiteness, opacity, writability, surface smoothness and like properties of the record material itself.

Presently calcium carbonate is said to be the best inorganic pigment for serving these purposes for use in heat-sensitive record materials.

With an increase in the amount of calcium carbonate used for the heat-sensitive materials, the above characteristics improve but the density of the color formed by heating decreases, so that the smaller the amount of calcium carbonate, the more desirable from the viewpoint of color density. Accordingly it has been desired to provide calcium carbonate which, even when used in a small amount, is high in oil absorption so as to achieve remarkable pigment effects such as the effect of preventing adhesion of the residue to the thermal head.

Heretofore known as particles of such calcium carbonate having relatively high oil absorption are those having projections and disclosed in Examined Japanese Patent Publication No.

30815/1972, and intertwined bodies of calcium carbonate particles disclosed in Examined Japanese Patent Publication No. 31530/1972. (See also U.K. Patent GB 2023561B.) However, our research revealed that such calcium carbonate particles, even when used as pigments for heat-sensitive materials, do not always achieve good results. More specifically, the foregoing known calcium carbonate particles having relatively high oil absorption is to some extent effective for preventing adhesion of residue but results in a greatly reduced color density and is not always satisfactory in whiteness retentivity, record preservability, whiteness, opacity, surface smoothness of the record material, etc.

Accordingly we have carried out research to overcome the foregoing drawback and found that when an aqueous suspension of calcium hydrozide and calcium carbonate which serves as crystal nuclei and which contains specific needle- to pillar-like aragonite crystals is subjected to carbonation reaction by introducing a carbon dioxide-containing gas into the suspension under specific conditions, the crystal nuclei can be effectively grown into needle-to pillar-like aragonite crystals which, as primary particles, are irregularly three-dimensionally twined about one another in the form of bodies of calcium carbonate having specific bulkiness and a great specific gravity.

We have further found that the bodies obtained, when incorporated into a coating composition for heat-sensitive record material, enable the record material to form a color with high color density, further producing outstanding results in the effect to prevent adhesion of residue and in other pigment effects. The present invention has been accomplished based on these novel findings.

The present invention provides a body comprising needle- to pillar-like primary particles of aragonite calcium carbonate irregularly three-dimensionally twined about one another and having an average length (L) of 0.3 to 6.0 lim and an average width (W) of 0.04 to 0.5 ym as observed electron microscopically and an aspect ratio (L/W) of 2 to 50, the body containing at least 50% by weight of aragonite crystals as determined by X-ray diffraction and having bulkiness of 5 to 10 ml/g and a specific gravity of 2.75 to 2.93 according to JIS K 5101.

The present invention further provides a process for preparing the above-mentioned bodies of intertwined needle- to pillar-like calcium carbonate particles characterized in that an aqueous suspension comprising (a) calcium carbonate serving as the crystal nuclei and containing at least 55% by weight of needle- to pillar-like aragonite crystals of calcium carbonate as determined by X-ray diffraction and (b) 10 to 500 parts by weight of calcium hydroxide per 100 parts by weight of the calcium carbonate serving as crystal nuclei, and having a total solids concentration of 3 to 30% by weight and a temperature of 10 to 50"C is subjected to a carbonation reaction by introducing a gas containing at least 10% by volume of carbon dioxide into the aqueous suspension at a rate of 2 to 1 5 liters/min per kilogram of the calcium hydroxide, the aragonite crystals having an average length (I) of 0.2 to 4 pm and an average width (w) of 0.035 to 0.35 ym as observed electron microscopically and an aspect ratio (I/w) of 1.5 to 40.

The present invention further provides a coating composition for heat-sensitive record materials which contains 10 to 60% by weight of such bodies of intertwined calcium carbonate particles.

As will be apparent from the electron photomicrograph of Fig. 1 showing novel intertwined bodies of calcium carbonate of the invention, the body resembles a sea urchin or the husk of a chestnut and is made up of primary particles of aragonite calcium carbonate which have grown and tightly twined about one another three-dimensionally. Table 1 below shows the main physical properties of the body of calcium carbonate of the invention in comparison with those of known calcium carbonate particles.

Table 1 Known calcium carbonate Calcium carbonate Properties of the invention A B Content of aragonite 50 < < 5 < 5 crystals (%) Specific gravity 2.75-2.93 2.60 2.60 Note: A: Calcium carbonate particles having projections and disclosed in U.K. Patent GB 2023561B (Examined Japanese Patent Publication No. 30815/1972) B: Body of intertwined needle-like calcium carbonate particles disclosed in U.K. Patent GB 2023561B (Examined Japanese Patent Publication No. 31530/72) The properties listed in Table 1 were determined by the following methods.

Content of aragonite crystals: Determined with use of calibration diagram from the peak height ratio between aragonite (d = 3.396A,' 111 plane) and calcite (d = 3.035A, 104 plane) obtained by X-ray diffraction.

Specific gravity: According to JIS K 5101.

The body of intertwined needle- to pillar-like particles of calcium carbonate of the present invention is prepared by the following process. First, an aqueous suspension is prepared which comprises (a) calcium carbonate serving as crystal nuclei and containing at least 55% by weight of needle- to pillar-like aragonite crystals of calcium carbonate and (b) 10 to 500 parts by weight of calcium hydroxide per 100 parts by weight of the calcium carbonate serving as crystal nuclei, and which has a total solids concentration of 3 to 30% by weight and a temperature of 10 to 50"C, the aragonite crystals having an average length (1) of 0.2 to 4 pm and an average width (w) of 0.035 to 0.35 pm and an aspect ratio (1 /W) of 1.5 to 40.

It is critical that the calcium carbonate serving as crystal nuclei contain at least 55% by weight of needle- to pillar-like aragonite crystals of calcium carbonate having the above-specified dimensions. If the calcium carbonate contains less than 55% by weight of aragonite crystals and more than 45% by weight of calcium carbonate of calcite or other crystal system or amorphous calcium carbonate, the product of the carbonation reaction has a marked tendency to contain large amounts of cubic primary particles of calcite and secondary agglomerates thereof and becomes less than 2.75 in specific gravity and lower in bulkiness. When the content of aragonite crystals is not smaller than 55% by weight, the product substantially comprises bodies each resembling a sea urchin and is almost free from cubic primary particles or secondary agglomerates thereof.This appears to indicate that calcium carbonate of calcite crystal system, amorphous calcium carbonate or the like is incorporated into the bodies during the carbonation reaction, especially during the growth and intertwining of needle- to pillar-like aragonite crystals.

This phenomenon, although still remaining to be fully established, is partly substantiated by the fact that cubic primary particles are sometimes found to be present in the central region of the body of intertwined aragonite crystals under a scanning electron microscope.

The calcium carbonate serving as crystal nuclei and containing at least 55% by weight of needle- to pillar-like aragonite crystals of calcium carbonate having the specified dimensions is prepared, for example, from an aqueous suspension of calcium hydroxide having a concentration of 10 to 35% by weight and a temperature of 1 5 to 60"C by adding to the suspension a barium compound or strontium compound serving as an agent for forming crystal nuclei, in an amount of about 0.003 to about 0.4 mole per mole of the calcium hydroxide, and thereafter introducing into the suspension a gas containing at least about 15% by volume of carbon dioxide at a rate of about 5 to about 80 liters/min per kilogram of the calcium hydroxide.

Examples of useful barium compounds and strontium compounds are halides, oxygen compounds, oxyacid salts, organic acid salts and like compounds of barium and strontium, such as chlorides, fluorides, hydroxides, oxides, sulfates, nitrates, phosphates, acetates and oxalates of barium and strontium.

The amount of calcium hydroxide per 100 parts by weight of the calcium carbonate serving as crystal nuclei is variable depending on the size and content of aragonite crystals contained in the calcium carbonate, the desired size of the bodies to be eventually obtained, the desired dimensions of the primary particles forming the bodies, etc. However, if the amount of calcium hydroxide is less than 10 parts by weight, the needle- to pillar-like aragonite particles are unable to fully twine about one another, giving a product which is excessively bulky, whereas if the amount if above 500 parts by weight, large primary particles are obtained which form agglomerates of reduced bulkiness instead of giving desired bodies of intertwined particles.In order to obtain bodies having bulkiness of 5 to 10 ml/g as determined according to JIS K 5101,.it is desirable to use 20 to 300 parts by weight of calcium hydroxide per 100 parts by weight of the calcium carbonate serving as crystal nuclei.

Our research has revealed that when at least one additive selected from the group consisting of aminopolycarboxylic acids, water-soluble salts thereof, hydroxycarboxylic acids and watersoluble salts thereof is added to the aqueous suspension of calcium carbonate serving as crystal nuclei and calcium hydroxide, the aragonite primary particles of calcium carbonate freshly grown from the needle- to pillar-like aragonite calcium carbonate contained in the calcium carbonate serving as crystal nuclei by carbonation reaction twine about one another more tightly, forming bodies which will not readily collapse into primary particles when the product is pulverized after drying or when it is used. Examples of useful aminopolycarboxylic acids are iminodiacetic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, etc.Examples of useful water-soluble salts of such acids are alkali metal salts, ammonium salts, etc. Examples of useful hydroxycarboxylic acids are citric acid, tartaric acid, malic acid, etc., and examples of water-soluble salts thereof are alkali metal salts, ammonium salts, etc. The additive is used in an amount of up to about 20 parts by weight, preferably about 0.01 to about 20 parts by weight, more preferably about 0.05 to about 10 parts by weight, per 100 parts by weight of calcium hydroxide.

According to the present invention, the solids concentration and temperature of the aqueous suspension are suitably determined in view of the velocity of carbonation, economy, etc. Usually the concentration is 3 to 30% by weight, preferably 5 to 15% by weight. The temperature is generally 10 to 50"C, preferably 1 5 to 40"C. When the concentration is less than 3% by weight or when the temperature is lower than 10"C, it is very likely that fine particles of calcium carbonate will be formed which become mixed with primary particles to form agglomerates having bulkiness of less than 5 ml/g.On the other hand, if the concentration is above 30% by weight or if the temperature is higher than 50"C, primary particles intertwine completely, and many primary particles remain as such. Thus concentrations and temperatures outside the foregoing ranges are undesirable since the bodies then obtained have varying sizes.

The aqueous suspension can be prepared by a known method, for example, by preparing a suspension of the calcium carbonate serving as crystal nuclei first, adding to the suspension an aqueous suspension of calcium hydroxide with or without the additive contained therein and mechanically stirring the mixture.

Next, the aqueous suspension obtained is subjected to a carbonation reaction by introducing a carbon dioxide-containing gas thereinto by a known method, for example, by blowing. The carbon dioxide concentration of the gas is determined in view of the solids concentration of the aqueous suspension, economy, etc. and is usually preferably at least 10% by volume.

Commerically it is advantageous to use a carbon dioxide gas having a concentration of about 1 5 to about 40% by volume and obtained by purifying the waste gas resulting from the calcination of limestone. The carbon dioxide-containing gas is introduced into the suspension at a rate of about 2 to about 1 5 liters/min per kilogram of the calcium hydroxide. When the rate of supply of the gas is lower than 2 liters/min per kilogram of the calcium hydroxide, there arises an increased tendency for the reaction to proceed locally, producing agglomerates and particles of varying sizes. If the supply rate exceeds 1 5 liters/min, the bodies of intertwined particles obtained are not satisfactory and are uneven in size, hence objectionable. Even if the reaction system is slightly fluctuated by the introduction of the carbon dioxide-containing gas, primary particles can be grown and intertwined satisfactorily to give bodies resembling sea urchins, whereas if the reaction system is vigorously stirred or agitated, primary particles, although grown, will not intertwine, failing to give desired bodies resembling sea urchins.

The carbonation reaction is usually carried out with the reaction system maintained at a temperature of about 10 to about 80"C. The time taken for the reaction is usually about 0.5 to about 20 hours although greatly variable with the concentration of calcium hydroxide, and the concentration and rate of supply of the carbon dioxide-containing gas.

The aqueous suspension resulting from the carbonation reaction is filtered and dewatered by usual methods to obtain a paste which has a solids concentration of at least about 30% by weight and which may be used as it is. When required, the paste is further dried and pulverized into a powdery product.

The present invention also relates to a coating composition for heat-sensitive record material which contains bodies of intertwined needle- to pillar-like aragonite particles of calcium carbonate thus obtained. The coating composition of the invention is prepared by admixing such calcium carbonate bodies with a colorless dye, a phenolic compound for causing the dye to form a color when heated, a color forming sensitivity adjusting agent, a binder, etc. The present composition is superior to those incorporating conventional calcium carbonate particles in the density of color formed on heat-sensitive record material, in the effect to prevent adhesion of residue to the thermal head, and also in preservability characteristics. The reasons why such superior effects can be achieved, although not known fully, will be as follows.Because the bodies of calcium carbonate of the invention have the unique characteristics of being composed of primary particles in a specified range of sizes, containing at least 50% by weight of aragonite crystals and having bulkiness of 5 to 10 ml/g and a specific gravity of 2.75 to 2.93, it is presumed that these bodies are different from conventional particles of calcium carbonate in the state in which they are incorporated in the heat-sensitive record layer. Stated more specifically, the present bodies of calcium carbonate, which have a controlled bulkiness, form a uniform surface of compact texture on the coating and therefore permit the dye to form a uniform color and to thereby produce an increased color density.This will be apparent also from the fact that the heat-sensitive record material obtained with use of the present coating composition has a considerably higher surface smoothness than those obtained with use of the composition incorporating conventional calcium carbonate. The increased color density assured by the present invention is attributable also to the fact that the specific gravity of the present body of aragonite calcium carbonate, which is 2.75 to 2.93, is higher than that of the conventional calcite calcium carbonate, which is 2.60 to 2.65, such that the present pigment occupies a smaller volume in the color forming layer than the conventional one when used in a given amount.Additionally it is thought that the present bodies having a special shape and uniformly distributed through the heat-sensitive record layer effectively absorb or enclose therein fatty acid amide or like residue which is released when the layer is heated for color formation, thus acting effectively to prevent the residue from adhering to the thermal head. Furthermore, the heatsensitive color forming layer incorporating present bodies of controlled bulkiness is homogeneous, has a low density and is smooth-surfaced, consequently giving improved whiteness and opacity and effective preservability to the record material.

The components of the present coating composition for heat-sensitive record material other than the calcium carbonate bodies of the invention, i.e. the colorless dye, phenolic compound, color forming sensitivity adjusting agent, binder, etc, can be those conventionally known for use in heat-sensitive record material. Typical of such components are as follows.

(a) Colorless dyes Colorless dyes which form a color by contact with phenolic compounds, such as 3, 3-bis(pdimethylamino-phenyl)-6-dimethylaminophthalide and like triarylmethane-based dyes, 3-diethylamino-6-methyl-7-anilinofluoran and like fluoran-based dyes, 3-methyl-spiro-dinaphthopyran and like spiropyran-based dyes, N-halophenyl-leucoauramine and like diphenylmethane-based dyes, benzoyl-leucomethylene blue and like thiazone-based dyes, etc.

(b) Phenolic compounds 4-Tertiary-buty phenol, 4-hydroxydiphenoxide, 4, 4'-isopropylidenediphenol (bisphenol A), 2, 2'-methylenebis (4-chlorophenol), novolak-type phenol resins, etc.

(c) Color forming sensitivity adjusting agents Palmitic acid amide, stearic acid amide, oleic acid amide, hydroxystearic acid amide, methylolated fatty acid amide, ethylenebis fatty acide amide, methylenebis fatty acid amide and like higher fatty acid amides. These are used singly, or at least two of them are used in admixture.

(d) Binders Polyvinyl alcohol, methylcellulose, carboxy-methylcellulose, hydroxycellulose, starch, casein, gelatin, gum arabic and like water-soluble high-molecular-weight compounds.

When required, various auxiliary agents heretofore known for use in coating compositions for heat-sensitive record materials can be incorporated into the present composition. Examples of such agents are release agents, defoaming agents, ultraviolet absorbers, fluorescence dyes, coloring dyes. antiseptics, etc. Other pigments are also usable in an amount of up to 20% by weight based on the whole amount of the composition.

The components of the present coating composition and the proportions thereof are usually as follows although changeable in accordance with the object.

Colorless dye 3-6 wt.% Phenolic compound 15-30 wt.% Color forming sensitivity 6-12 wt.% adjusting agent Binder 16-22 wt.% Calcium carbonate bodies 10-60 wt.% of the invention The coating composition of the invention is prepared by a known method, for example, in the following manner. First, the colorless dye, phenolic compound and color forming sensitivity adjusting agent are each separately pulverized in an aqueous solution of the binder. When a ball mill, for example, is used as the pulverizer, the component is adjusted to a solids concentration of about 20% and treated for 2 days to obtain fine particles of about 3 ,zm in size. Calcium carbonate bodies of the invention, and the other pigments which are used when required are made into a slurry having a solids concentration of about 40% in the usual manner with use of a dispersant.Subsequently, the slurry and the pulverized components are mixed together in the usual manner to obtain a coating composition for heat-sensitive record material according to the invention.

The composition is applied to base sheet made of paper, plastic film, etc., and dried by usual methods, and the coated sheet is calendered, whereby heat-sensitive record material is prepared.

The amount of the present composition, although variable with the properties of the heatsensitive record material to be obtained, etc., is usually about 3 to about 1 5 g/m2, preferably about 5 to about 10 g/m2, in terms of dry weight.

The following reference example shows the preparation of the calcium carbonate serving as crystal nuclei.

Reference Example 1 Into a reactor was placed 1000 kg of aqueous suspension of calcium hydroxide having a concentration of 20% by weight and adjusted to a temperature of 20"C. Strontium chloride was added to the suspension in an amount of 0.03 mole per mole of the calcium hydroxide. A gas containing 30% by volume of carbon dioxide was introduced into the suspension at a flow rate of 30 liters/min per kilogram of the calcium hydroxide for carbonation reaction, giving 270 kg of calcium carbonate serving as crystal nuclei and containing 95% by weight of needle- to pillarlike aragonite crystals of calcium carbonate.

When observed under an electron microscope, the aragonite crystals contained in the resulting calcium carbonate were found to be 0.5 to 1.0 m in length, 0.05 to 0.1 ym in width and 5 to 20 in aspect ratio.

The present invention will be described in greater detail with reference to the following examples and comparison examples, in which the parts and percentages are all by weight unless otherwise specified.

Example 1 Into a container equipped with a stirrer was placed 1400 kg of aqueous suspension having a temperature of 20"C and containing 5% of the calcium carbonate serving as crystal nuclei and obtained in Reference Example 1. To the suspension was added 293 kg of aqueous suspension of calcium hydroxide (31 parts of Ca(OH)2 per 100 parts of the calcium carbonate serving as crystal nuclei) adjusted to a temperature of 20"C, and the mixture was stirred to obtain an aqueous suspension.

Subsequently the aqueous suspension was placed into a reactor, and a gas containing 30% by volume of carbon dioxide was blown into the suspension, maintained at a temperature of 20 to 25"C, at a flow rate of 250 liters/min to carry out a carbonation reaction until the reaction mixture became neutral.

The resulting suspension of calcium carbonate was dewatered by a press, dried and pulverized, giving 100 kg of bodies of intertwined needle-to pillar-like aragonite particles of calcium carbonate according to the invention. The electron photomicrograph (10,000X) of Fig.

1 shows that the body resembles a sea urchin and is composed of needle- to pillar-like primary particles twined about one another irregularly three-dimensionally, and that the primary particles have an average length (L) of 1 to 2 m, an average width (W) of 0.05 to 0.2 ym and an aspect ratio (L/W) of 5 to 40. Table 2 shows the bulkiness and specific gravity of these bodies as measured according to JIS K 5101 and the content of aragonite crystals thereof as determined by X-ray diffraction.

Next, the bodies of calcium carbonate thus obtained were made into a coating composition for heat-sensitive record paper. First, a colorless dye, a phenolic compound and a fatty acid amide were each separately pulverized according to the following formulations A, B and C, using a ball mill which was operated for 2 days.

Formulation A 3-Diethylamino-6-methyl-7-anilinofluoran 100 parts (trademark: "ONE DYE BLACK", product of Yamamoto Kagaku Gosei Co., Ltd., Japan) 5% Aqueous solution of polyvinyl alcohol 400 parts Formulation B Bisphenol A 100 parts 5% Aqueous solution of polyvinyl alcohol 400 parts Formulation C Fatty acid amide (trademark: "ARMID HT-P", 100 parts product of LION AKZO Co., Ltd., m.p. 98"C, mixture of 22% palmitic acid amide, 75% stearic acid amide and 3% oleic acid amide) 5% Aqueous solution of polyvinyl alcohol 400 parts According to the following formulation D, the bodies of calcium carbonate of the invention were made into a slurry containing 40% of solids, using an impeller-type stirrer.

Formulation D Bodies of calcium carbonate 100 parts 5% Aqueous solution of polycarboxylic 20 parts acid dispersant Water 230 parts A 20% aqueous solution of polyvinyl alcohol (hereinafter referred to as "mixture E") prepared in the usual manner by heating was used as a binder.

The mixtures A to D prepared according to Formulations A to D and the mixture E were mixed together in the proportions of A:B:C:D:E: = 1:5:2:5:2 (by weight) to obtain a coating composition for heat-sensitive paper according to the invention.

The coating composition was applied by a usual method to one side of wood-free paper weighing 50 g/m2 in an amount of 7 g/m2 based on dry weight, using a coating rod. After drying the coating at room temperature, the paper was calendered under specified conditions to prepare heat-sensitive paper.

Table 3 showing the properties of the heat-sensitive paper indicates that the heat-sensitive color forming layer is highly smooth-surfaced, forms a black color of high color density when heated and is low in the density of color transferred (i.e. high effect to prevent adhesion of residue) and high in preservability.

Example 2 Bodies of intertwined needle- to pillar-like aragonite particles of calcium carbonate of the invention were prepared in the same manner as in Example 1 except that 0.55 kg of sodium nitrilotriacetate was added to the aqueous suspension of calcium hydroxide.

When observed under an electron microscope, the body was found to be similar to a sea urchin and composed of needle-to pillar-like primary particles twined about one another irregularly three- dimensionally and having a length of 1 to 2,um, a width of 0.05 to 0.1 pm and an aspect ratio of 10 to 40. Table 2 shows the physical properties of the bodies.

A coating composition and heat-sensitive paper according to the invention were prepared in the same manner as in Example 1 with the exception of using these bodies. Table 3 shows the properties of the paper obtained.

Examples 3 to 5 Bodies of calcium carbonate were prepared in the same manner as in Example 1 or 2 with the exception of varying the size and content of needle- to pillar-like aragonite crystals contained in the calcium carbonate serving as crystal nuclei and using different amounts of calcium hydroxide and carboxylic acid additive as listed in Table 2. When observed under an electron microscope, the bodies were found to be similar to sea urchins and composed of needle- to pillar-like primary particles twined about one another irregularly three-dimensionally and having a length of 0.3 to 5.0 zm, a width of 0.04 to 0.4 m and as aspect ratio of 2 to 50. Table 2 shows the properties of these bodies.

Coating compositions and heat-sensitive papers according to the invention were prepared in the same manner as in Example 1 with the exception of using these bodies. Table 3 shows the proporties of the papers obtained.

Comparison Example 1 The same aqueous suspension of calcium carbonate crystal nuclei as used in Example 1 was directly dewatered, dried and pulverized to obtain aragonite calcium carbonate. Electron microscopically, the crystal nuclei remained primary particles without forming bodies of intertwined particles. Table 2 shows the properties of the calcium carbonate particles.

A coating composition and heat-sensitive paper were prepared in the same manner as in Example 1 with the exception of using these particles. Table 3 shows the properties of the paper obtained.

Comparison Example 2 Calcium carbonate was prepared in the same manner as in Example 1 with the exception of using 880 kg of aqueous suspension of calcium hydroxide (600 parts of Ca(OH)2 per 100 parts by weight of the calcium carbonate serving as crystal nuclei). The resulting calcium carbonate was in the form of agglomerates of primary particles when observed electron microscopically, and was low in bulkiness and specific gravity. Table 2 shows the properties thereof.

A coating composition and heat-sensitive paper were prepared in the same manner as in Example 1 except that the above calcium carbonate was used. Table 3 shows the properties of the paper obtained.

Comparison Example 3 Calcium carbonate was prepared by carrying out a carbonation reaction in the same manner as in Example 1 except that 0.55 kg of sodium nitrilotriacetate was added to the aqueous suspension of calcium hydroxide without using the aqueous suspension of calcium carbonate serving as crystal nuclei. The resulting calcium carbonate was also found to be in the form of aggolmerates of primary particles when observed under an electron microscope and was low in bulkiness and specific gravity. Table 2 shows the properties thereof.

A coating composition and heat-sensitive paper were prepared in the same manner as in Example 1 with the exception of using the above calcium carbonate. Table 3 shows the properties of the paper.

Table 2 Calcium carbonate crystal nuclei Amount of Amount of Content of Example Content of calcium carboxylic Specific aragonite No. aragonite hydroxide acid Bulkiness gravity crystals Size 1) crystale 3) additive 5) 6) 7) 2) 4) (um) (%) (parts) (parts) (ml/g) (%) Example L = 0.5-1.0 1 W = 0.05-0.1 95 31 0 10 2.90 90 L/W = 5-20 L = 0.5-1.0 2 W = 0.05-0.1 95 31 1.2 8 2.90 92 L/W = 5-20 L = 1.0-2.0 3 W = 0.05-0.1 95 74 0 6 2.88 85 L/W = 10-40 L = 1.0-2.0 4 W = 0.05-0.1 95 295 2.5 5 2.86 80 L/W = 10-40 L = 0.5-1.0 5 W = 0.05-0.1 75 31 0 7 2.83 70 L/W = 5-20 Table 2 (continued) Calcium carbonate crystal nuclei Amount of Amount of Content of Example Content of calcium carboxylic Specific aragonite No. aragonite hydroxide acid Bulkiness gravity crystals Size 1) dcrystale 3) additive 5) 6) 7) 2) 4) (um) (%) (parts) (parts) (ml/g) (%) Comp.

Example L = 0.5-1.0 1 W = 0.05-0.1 95 0 0 15 2.91 95 L/W = 5-20 L = 0.5-1.0 2 W = 0.05-0.1 95 600 0 5 2.68 25 L/W = 5-20 3 - - - 1.2 3 2.60 0 1) Size The average size of needle-to pillar-like aragonite crystals of calcium carbonate as observed electron microscopically. L stands for the length, W for the width and L/W for the aspect ratio.

2) Content of aragonite crystals Determined with use of calibration diagram from the peak height ratio between aragonite (d = 3.396 , 111 plane) and calcite (d = 3.035 , 104 plane) obtained by X-ray diffraction.

3) Amount of calcium hydroxide The amount per 100 parts of the calcium carbonate serving as crystal nuclei.

4) Amount of carboxylic acid additive The amount per 100 parts of the calcium hydroxide.

5) Bulkiness According to JIS K 5101.

6) Specific gravity According to JIS K 5101.

7) Content of aragonite crystals The same as the method 2) above.

Table 3 Color forming and adhesion Surface preventing test 2) Preservability 3) Example smoothnese Formed color Transferred Color-formed No. of blank paper density color density Blank paper paper 1) Example 1 440 1.25 0.16 A A 2 420 1.20 0.14 A A 3 400 1.19 0.15 A A 4 430 1.20 0.14 A A 5 390 1.18 0.17 A A Comp.Ex.

1 370 1.10 0.25 A B 2 270 1.09 0.23 B C 3 190 1.05 0.22 B C Commercial paper 400 1.15 0.20 B A 1) Surface smoothness of blank paper According to JIS P 8119.

2) Color forming and adhesion preventing test An assembly of heat-sensitive paper and art paper placed over the color forming layer was passed between hot calender rolls (130"C, pressure 50 kg/cm, feed speed 20 m/min) twice and thereafter checked for formed color density and transferred color density.

The formed color density means the density of the color formed on the surface of the color forming layer. The greater the value, the higher is the density. The transferred color density means the density of the color transferred onto the art paper. The smaller the value, the higher is the effect to prevent adhesion of residue to the thermal head. The color densities were measured by a coldr and black-and-white reflection densitometer (Model DM-400, product of Dainippon Screen Co., Ltd., Japan).

3) preservability A piece of heat-sensitive paper was subjected to the same conditions as in the color forming test 2) above for color formation and a blank piece of the same paper were allowed to stand for 24 hours in a container maintained at 50"C and 90% RH and checked for color changes. The results were evaluated according to the following criteria.

A:-Very low degree of color change.

B:-Low degree of color change.

C:-Usual degree of color change.

Comparison Examples 4 and 5 Coating compositions and heat-sensitive papers were prepared in the same manner as in Example 1 with the exception of using the known calcium carbonate A (Comparison Example 4) or calcium carbonate B (Comparison Example 5) listed in Table 1 above. Table 4 below shows the properties of the papers measured or evaluated as above.

Table 4 Color forming and adhesion Comp. Surface preventing test 2) Preservability 3) Example smoothness No. of blank paper Formed color Transferred Color-formed 1) density color density Blank paper paper 4 250 0.97 0.22 B C 5 330 0.94 0.23 B B The results listed in Tables 3 and 4 reveal that the heat-sensitive papers obtained with use of coating compositions which contain bodies of intertwined needle- to pillar-like aragonite particles of calcium carbonate according to the invention are superior in smoothness, color density, effect to prevent adhesion of residue to the thermal head and preservability to those incorporating calcium carbonate particles of Comparison Examples or known calcium carbonate A or B.

Claims

1. Calcium carbonate in the form of bodies comprising needle- to pillar-like primary particles of aragonite irregularly three-dimensionally twined about one another and having an average length (L) of 0.3 to 6.0 ym and an average width (W) of 0.04 to 0.5,um as observed electron microscopically and an aspect ratio (L/W) of 2 to 50, the bodies containing at least 50% by weight of aragonite crystals as determined by X-ray diffraction and having bulkiness of 5 to 10 ml/g and a specific gravity of 2.75 to 2.93 according to JIS K 5101.

2. A process for the preparation of calcium carbonate according to claim 1, in which process an aqueous suspension comprising (a) calcium carbonate serving as crystal nuclei and containing at least 55% by weight of needle- to pillar-like aragonite crystals as determined by Xray diffraction, the aragonite crystals having an average length (1) of 0.2 to 4,um and an average width (w) of 0.035 to 0.35 lum as observed electron microscopically and an aspect ratio (I/w) of 1.5 to 40, and (b) 10 to 500 parts by weight of calcium hydroxide per 100 parts by weight of the calcium carbonate, and having a total solids concentration of 3 to 30% by weight and a temperature of 10 to 50"C, is subjected to a carbonation reaction by introducing a gas containing at least 10% by volume of carbon dioxide into the aqueous suspension at a rate of 2 to 1 5 litres/minute per kilogram of the calcium hydroxide.

3. A process according to claim 2 wherein the aqueous suspension comprising 20 to 300 parts by weight of calcium hydroxide per 100 parts by weight of the calcium carbonate serving as crystal nuclei.

4. A process according to claim 2 or claim 3 wherein the aqueous suspension has a total solids concentration of 5 to 15% by weight.

5. A process according to any of claims 2 to 4 wherein the aqueous suspension has a temperature of 1 5 to 40"C.

6. A process according to any of claims 2 to 5 wherein the carbonation reaction is carried out with the reaction system maintained at a temperature of 10 to 80"C.

7. A process according to any of claims 2 to 6 wherein the aqueous suspension contains at least one additive selected from aminopolycarboxylic acids, water-soluble salts thereof, hydroxycarboxylic acids and water-soluble salts thereof in an amount of up to 20 parts by weight per 100 by weight of the calcium hydroxide.

8. A process according to claim 7 wherein the additive is iminodiacetic acid, nitrilotriacetic acid or ethylene-diaminetetraacetic acid.

9. A process according to claim 7 wherein the additive is an alkali metal salt or ammonium salt of iminodiacetic acid, nitrilotriacetic acid or ethylenediaminetetraacetic acid.

10. A process according to claim 7 wherein the additive is citric acid, tartaric acid or malic acid.

11. A process according to claim 7 wherein the additive is an alkali metal salt or ammonium salt of citric acid, tartaric acid or malic acid.

1 2. A process according to any of claims 7 to 11 wherein the additive is used in an amount of 0.01 to 20 parts by weight per 100 parts by weight of the calcium hydroxide.

1 3. A process according to any of claims 7 to 1 2 wherein the additive is used in an amount of 0.05 to 10 parts by weight per 100 parts by weight of the calcium hydroxide.

14. A process according to any of claims 2 to 1 3 wherein the calcium carbonate serving as crystal nuclei is prepared from an aqueous suspension of calcium hydroxide having a concentration of 10 to 35% by weight and a temperature of

1 5 to 60"C by adding to the suspension a barium compound of strontium compound serving as an agent for forming crystal nuclei, in an amount of 0.0003 to 0.4 mole per mole of the calcium hydroxide, and thereafter introducing into the suspension a gas containing at least 15% by volume of carbon dioxide at a rate of 5 to 80 litres/minute per kilogram of the calcium hydroxide.

1 5. A process according to claim 14 wherein the barium compound or strontium compound is a chloride, fluoride, hydroxide, oxide, sulphate, nitrate, phosphate, acetate or oxalate of barium or strontium.

1 6. A coating composition for heat-sensitive record material, the coating composition comprising from 10 to 60% by weight of calcium carbonate according to claim 1.

1 7. A coating composition according to claim 16, the coating composition comprising from 3 to 6% by weight of a colourless dye, from 15 to 30% by weight of a phenolic compound, from 6 to 12% by weight of a colour forming sensitivity adjusting agent and from 1 6 to 22% by weight of a binder.

18. A heat-sensitive record material prepared by applying a coating composition according to claim 1 6 or claim 17 to base sheet in an amount of 3 to 15 g/m2 in terms of dry weight, drying the coating and calendering the resulting sheet.