There has been well-known heat-sensitive recording materials which utilizing the colorforming reaction between a colorless or pale colored electron donating dye and an organic or inorganic electron accepting compound, in which color images are produced by heating to contact with each other of the electron donating dye and the electron accepting compound. Recently, a considerable progress has been made in the field of heat-sensitive recording systems, and heat-sensitive facsimiles, heat-sensitive printers and the like become possible to make the recording speed very higher. For example, in heat-sensitive facsimiles a recording speed of not more than 10 seconds for a sheet of A4 size can be achieved and in heat-sensitive printers a recording speed of 120 letters per second or more can be achieved. With the improvement of hardware fields as described above, it is required for the available heat-sensitive recording material to be superior in adaptability for a high-speed recording.
As means for improving the recording sensitivity of the heat-sensitive recording material, there has been a well-known method in which a heat-fusible material having a melting point lower than that of each of the electron donating dye and the electron accepting compound is added as shown in Japanese Laid-Open Patent Publication No. 34,842 of 1974, Japanese Laid-Open Patent Publication No. 39,139 of 1978 and the like. However, it is difficult to obtain one kind of the heat-fusible material which can dissolve both of the electron donating dye and the electron accepting compound. Further, when two or more kinds of heat-fusible materials are used together, there occurs such a problem as the initiation temperature of developing color is lowered by the depression of melting point owing to the eutectic phenomenon and resultantly the whiteness is lowered.
Furthermore, Japanese Laid-Open Patent Publication No. 15,394 of 1981 discloses a method for improving the record sensitivety of a heat-sensitive recording material by finely pulverizing the mixture of an electron donating dye, an electron accepting compound and a heat-fusible material. Japanese Laid-Open Patent Publication No. 69,089 of 1983 or U.S. Pat. No. 4,717,593 discloses a method for producing an improved heat-sensitive recording material in which a horizontal sand mill is used to finely pulverize an aqueous dispersion of an electron donating dye or an electron accepting compound. Additionally, Japanese Laid-Open Patent Publication No. 15, 394 of 1981 teaches that the pulverization degree of dye composition strongly effects upon the recording sensitivity of a heat-sensitive recording material.
However, it has been found that any particularly remarkable pulverizing effect can not be obtained even if a fatty acid amide which is a heat-fusible material used in the above methods is pulverized together with the dye with a horizontal sand mill. Further, when the particle size of media used in the horizontal sand mill is relatively large such as 1.0 mm to 3.0 mm, the pulverizing efficiency becomes bad. Therefore, those methods are not practical.
The dye, electron accepting compound and heat-fusible material used for producing a heat-sensitive recording material are generally used after pulverizing until the particle size is not more than a few microns. However, as described above, a remarkable improvement of the recording sensitivity is required with the remarkable speed-up of the recording instrument and the like and recently the requirement of the pulverization becomes strong.
The object of the invention is, therefore, to provide a method for producing a heat-sensitive recording material and particularly to a method for efficiently producing a heat-sensitive recording material superior in adaptability for a high-speed recording.
We have been studied methods for pulverizing the dye which affects the recording sensitivity, particularly methods for simultaneously pulverizing the dye and the heat-fusible material to obtain more improved recording sensitivity. Resultantly, it has been found that a great difference appears in the pulverizing efficiency by the kind of the heat-fusible materials used together with the dye. Particularly, the difference is remarkable in the case of the horizontal sand mill. Further, by widely studying the heat-fusible materials pulverized together with the dye and the pulverizing methods, it has been found that desired mixture dispersions having a volume average particle size of not more than 1.0 .mu.m, preferably 0.5.about.1.0 .mu.m, is obtained efficiently with use of a specific heat-fusible material in the condition of that the heat-fusible material is mixed with a dye dispersion and the dispersion is pulverized with a horizontal sand mill. In this way, the invention has been achieved.