Heat-sensitive recording material

Description

[0001] The present invention relates to a heat-sensitive recording material, more particularly, to a highly sensitive heat-sensitive recording material that has rapid thermal response and that causes minimum deposition of tailings on the surface of a thermal head.

[0002] Heat-sensitive recording materials are generally comprised of a support such as paper, plastic film or syn­thetic paper that is coated with one or more heat-sensitive color-forming layers that contain a binder and a thermally color-forming substance as main ingredients.

[0003] Heat-sensitive recording materials need only be heated to produce an image and they can be used with rela­tively simple and compact recording apparatus. Because of these and other advantages, heat-sensitive recording materials are extensively used in information recording applications.

[0004] The machine design of facsimiles and printers that use heat-sensitive recording materials has been remarkable improvements and high-speed recording of information with these equipment which has heretofore been very difficult to achieve is now possible. With the introduction of such fast operating machines and hardware, heat-sensitive recording materials that are to be used with these machines are required to have improved recording sensitivity and many proposals have been made in this regard.

[0005] It has heretofore been held that in order to improve the recording sensitivity of heat-sensitive recording materials, the provision of an undercoat layer that not only smoothes the surface of a support but also imparts heat-­insulating property is effective. To this end, it has been proposed that the particles of a hydrophobic polymer that has a smaller heat conductivity than inorganic pigments be incorporated in the undercoat layer (see Japanese Patent Public Disclosure No. 204595/1984 and 2397/1985) or that hollow fine particles be incorporated in the undercoat layer in order to impart the heat-insulating property of the air (see Japanese Patent Public Disclosure Nos. 171685/1984 and 225987/1984). These methods are effective in improving the recording sensitivity but on the other hand, they have been still unsatisfactory from the viewpoint of reduced deposi­tion of tailings on the surface of a thermal head, which is no less important than the recording sensitivity when practical applications are considered.

[0006] To deal with this problem, it has been proposed that the deposition of tailings on the surface of a thermal head be prevented by forming an undercoat layer in which silicon oxide capable of high oil absorption is combined with the particles of a hydrophobic polymer (see Japanese Patent Public Disclosure No. 89883/1986). However, the addition of such silicon oxide increases the absorption of the color-forming fusible ingredient in the heat-sensitive color-forming layer by the undercoat layer, thus producing a dark image.

[0007] It is therefore an object of the present invention to provide a heat-sensitive recording material that has high recording sensitivity, that effectively prevents the deposi­tion of tailings onto the surface of a thermal head, and that yet prevents the production of a dark image even if color formation is effected with high energy.

[0008] This object of the present invention can be attained by a heat-sensitive recording material that comprises a support coated with a heat-sensitive recording layer con­taining an electron donating colorless dye and an electron accepting compound, which recording material is charac­terized in that an undercoat layer containing both poly­styrene and a pigment that is capable of oil absorption in an amount of less than 100 mℓ/100 g is provided between the support and the heat-sensitive color-forming layer.

[0009] As a result of intensive studies conducted in order to attain the objects of the present invention, the present inventors found that by using polystyrene as a filler in the undercoat layer formed beneath the heat-sensitive color-­forming layer, a higher recording sensitivity could be achieved than when an inorganic pigment was used as a filler. However, if polystyrene alone is used as a filler, the resulting heat-sensitive recording material has only an insufficient capability of absorbing tailings and hence is not completely satisfactory. Thus, in accordance with the present invention, an inorganic pigment is incorporated as an additional filler to provide an improved capability for absorbing tailings, whereby a desirable heat-sensitive recording material is obtained that has not only high recording sensitivity but also a greater ability to absorb tailings.

[0010] Polystyrene has a sufficiently high heat conductivity to provide a high recording sensitivity. The polystyrene to be used is generally a fine particulate polystyrene that has an average particle size of up to 5 µm, preferably in the range of 0.5 - 2 µm. Polystyrene particles larger than 5 µm are incapable of forming a highly smooth undercoat layer. On the other hand, it should be taken into consideration that polystyrene particles smaller than 0.5 µm have a tendency to deteriorate, rather than improve, the ability to absorb tailings.

[0011] The pigment to be incorporated in the undercoat layer together with polystyrene is such that it absorbs an oil in an amount of less than 100 mℓ/100 g (JIS K 5101). Examples of the pigment that satisfies this requirement include calcined clay, clay, talc, calcium carbonate and magnesium carbonate. Pigments that absorb 100 mℓ or more of an oil per 100 g contribute to an improvement in the ability to absorb tailings but, on the other hand, the density of prints obtained by printing with high energy is not satis­factorily high.

[0012] Polystyrene and the pigment capable of oil absorption in an amount of less than 100 mℓ/100 g are used in combina­tion, typically at ratios of 1:4 to 4:1 (by weight). If the pigment to polystyrene ratio exceeds 4, it is impossible to maintain a desirably high recording sensitivity. On the other hand, if the polystyrene to pigment ratio exceeds 4, it is impossible to maintain a desirably high ability to absorb tailings.

[0013] An adhesive is used to form an undercoat layer that contains the polystyrene and the pigment capable of oil absorption in an amount of less than 100 mℓ/100 g. The adhesive is used in an amount of 1 - 20 parts (by weight) per 100 parts of the sum of the polystyrene and the pigment. Generally known water-soluble polymeric aqueous emulsions may be used as pigments and specific examples are listed below: polyvinyl alcohol, methyl cellulose, hydroxyethyl cellulose, starch, starch derivatives, styrene/maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, polyamide, SBR latex, polystyrene-acrylate ester emulsion, and polyvinyl acetate emulsion.

[0014] The polystyrene, the pigment capable of oil absorp­tion in an amount of less than 100 mℓ/100 g, and the adhe­sive are dispersed in a desired order in an aqueous medium and they are mixed to prepare a coating solution, which is then applied onto a support to form an undercoat layer having a dry weight of 1 - 15 g/m². Coating operations can be performed with conventional applicators.

[0015] The support to be used in the present invention may be selected from among appropriate materials such as paper, plastic films, synthetic paper and metal foils.

[0016] The so formed undercoat layer is then overlaid with a heat-sensitive color-forming layer, which may be formed in the usual manner. The colorless or pale-colored electron donating dye may be selected from among the following compounds:
Crystal violet lactone 3-(N-ethyl-N-isopentylamino)-6-­methyl-7-anilinofluoran;
3-diethylamino-6-methyl-7-anilinofluoran;
3-diethylamino-6-methyl-7- (o,p-dimethylanilino)fluoran;
3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran;
3-pyrrolidino-6-methyl-7-anilinofluoran;
3-dibutylamino-6-methyl-7-anilinofluoran;
3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran;
3-diethylamino-7- (o-chloroanilino)fluoran;
3-diethylamino-7- (m-trifluoromethylanilino)fluoran;
3-diethylamino-6-methyl-7-chlorofluoran;
3-diethylamino-6-methylfluoran; and
3-cyclohexylamino-6-chlorofluoran.

[0017] The following compounds may be enumerated as typical examples of electron accepting compounds (developing agents):
Bisphenol A;
Benzyl-p-hydroxybenzoate;
n-butyl di(4-hydroxyphenyl)acetate
Bisphenol S;
4-hydroxy-4′ -isopropyloxydiphenylsulfone;
1,1-di(4-hydroxyphenyl)cyclohexane; and
1, 7-di(hydroxyphenylthio)-3,5-dioxaheptane.

[0018] Materials that are conventionally incorporated in heat-sensitive color-forming layers may be contained, as appropriate, in the heat-sensitive color-forming layer of the present invention. For example, known heat-fusible organic compounds that melt at 50 - 150°C may be used as sensitizers and typical examples thereof are listed below:
Phenyl p-hydroxynaphthoate;
p-benzylbiphenyl;
Benzyl naphthyl ether;
Benzyl terephthalate;
Benzyl p-benzyloxybenzoate;
Diphenyl carbonate; and
Dinitrile carbonate.

[0019] The organic and inorganic pigments that can be used in the present invention are not limited in any particular way such as with respect to the oil absorbing capability, and the following may be used as required: inorganic fine particulate compounds such as calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, zinc hydroxide, barium sulfate, clays, talc, as well as surface-­treated calcium carbonate and silica; and organic fine particulate compounds such as urea-formaldehyde resin, styrene/methacrylic acid copolymer and polystyrene resin.

[0020] Various waxes may also be incorporated as required in the heat-sensitive color-forming layer, and known waxes may be used such as paraffin, amide-based waxes, bisimide-based waxes and metal salts of higher aliphatic acids.

[0021] Useful adhesives include the following: water-­soluble polymers such as polyvinyl alcohols with various molecular weights, starches, derivatives thereof, cellulose derivatives such as methoxycellulose, carboxymethyl cellu­lose, methyl cellulose and ethyl cellulose, poly(sodium acrylate), polyvinylpyrrolidone, acrylamide/acrylate ester copolymer, acrylamide/acrylate ester/methacrylic acid terpolymer, alkali salts of styrene/maleic anhydride copolymer, polyacrylamide, sodium alginate, gelatin and casein; and latices of polyvinyl acetate, polyurethane, styrene/butadiene copolymer, polyacrylic acid, polyacrylic acid ester, vinyl chloride/vinyl acetate copolymer, polybutyl methacrylate, ethylene/vinyl acetate copolymer, styrene/butadiene/acrylic copolymer, etc.

[0022] The present invention provides a heat-sensitive recording material that has a sufficiently rapid heat response to insure high sensitivity, that prevents the production of a dark image even if color formation is effected with high energy, and that yet causes little deposition of tailings on the surface of a thermal head.

[0023] The following examples are given for the purpose of further illustrating the present invention. Unless other­wise noted, all "parts" and "%" in the examples are on a weight basis.

Example 1
Water	100 parts
Calcined clay ("Ansilex" of Engelhard Minerals & Chemicals Corporation; oil absorption, 80 mℓ/100 g)	20 parts
Dispersion of polystyrene particles ("L 8801" of Asahi Chemical Industry Co., Ltd.; solids content, 48%; average particle size, 0.5 µm)
40 parts

[0024] These ingredients were dispersed with a homogenizer for 5 minutes and thereafter mixed with 50 parts of 10% polyvinyl alcohol ("GH 17" of the Nippon Synthetic Chemical Industry Co., Ltd.) to prepare a coating solution for undercoat layer. This solution was applied onto a sheet of wood-free paper (50 g/m²) to give a deposit of 7 g/m² and dried.

[0025] A coating solution for heat-sensitive color-forming layer was prepared by the following procedure.

(1) Dispersion A
3-(N-isopentyl-N-ethylamino)-6-7-anilinofluoran	20 parts
Polyvinyl alcohol (10% aq. sol.)	10 parts
Water	70 parts

[0026] A composition consisting of these ingredients was pulverized with a sand grinder to an average particle size of 2 µm.

(2) Dispersion B
Benzyl p-hydroxybenzoate	20 parts
Polyvinyl alcohol (10% aq. sol.)	10 parts
Water	70 parts

[0027] A composition consisting of these ingredients was pulverized with a sand grinder to an average particle size of 2 µm.

(3) Formation of Heat-Sensitive Color-Forming Layer

[0028] Dispersion A (75 parts), dispersion B (125 parts), calcium carbonate (30 parts), 10% aqueous polyvinyl alcohol (200 parts), 30% paraffin dispersion (17 parts) and a dis­persion of 30% zinc stearate (17 parts) were mixed with stirring to prepare a coating solution. This solution was applied onto the previously formed undercoat layer to give a dry coating weight of 4.5 g/m², whereby a heat-sensitive recording material was produced.

Example 2
Water	100 parts
Precipitated calcium carbonate ("Calulite SA" of Shiraishi Central Laboratory Co., Ltd.; oil absorption, 60 mℓ/100 g)	20 parts
Dispersion of polystyrene particles ("L 8801" of Asahi Chemical Industry Co., Ltd.; solids content, 48%)	40 parts

[0029] These ingredients were dispersed with a homogenizer for 5 minutes and thereafter mixed with 50 parts of 10% polyvinyl alcohol ("GH 17" of The Nippon Synthetic Chemical Industry Co., Ltd.) to prepare a coating solution for under­coat layer. This solution was applied onto a sheet of wood-­free paper (50 g/m²) to give a deposit of 7 g/m² and dried.

[0030] The resulting undercoat layer was overlaid with a heat-sensitive color-forming layer as in Example 1, thereby producing another sample of heat-sensitive recording material.

Comparative Example 1
Water	100 parts
Synthetic silica ("Sorex CM" of Tokuyama Soda Co., Ltd.; oil absorption, 150 mℓ/100 g)	20 parts
Dispersion of polystyrene particles ("L 8801" of Asahi Chemical Industry Co., Ltd.; solids content, 48%)	40 parts

[0031] These ingredients were dispersed with a homogenizer for 5 minutes and subsequently processed as in Example 1 to prepare a coating solution for undercoat layer. This solu­tion was applied onto a sheet of wood-free paper (50 g/m²) to form an undercoat layer, which was overlaid with a heat-­sensitive color-forming layer as in Example 1.

Comparative Example 2
Water	100 parts
Ansilex	40 parts

[0032] These ingredients were dispersed with a homogenizer for 5 minutes and subsequently processed as in Example 1 to prepare a coating solution for undercoat layer. This solu­tion was applied onto a sheet of wood-free paper (50 g/m²) to form an undercoat layer, which was overlaid with a heat-­sensitive color-forming layer as in Example 1.

Comparative Example 3
Water	100 parts
Dispersion of polystyrene particles ("L 8801" of Asahi Chemical Industry Co., Ltd.; solids content, 48%)	82 parts

[0033] These ingredients were dispersed with a homogenizer for 5 minutes and subsequently processed as in Example 1 to prepare a coating solution for undercoat layer. This solu­tion was applied onto a sheet of wood-free paper (50 g/m²) to form an undercoat layer, which was overlaid with a heat-­sensitive color-forming layer as in Example 1.

[0034] The samples of heat-sensitive recording material prepared in Examples 1 and 2 and in Comparative Examples 1 - 3 were measured for sensitivity and the deposition of tailings, and the results are shown in Table 1 below.

Table 1

	Color density		Deposition of tailings
	0.44 mJ	0.54 mJ
Example 1	1.25	1.25	O
2	1.23	1.23	O
Comparative Example 1	1.25	1.15	O
2	1.15	1.15	O
3	1.25	1.25	X

[0035] The data in Table 1 shows that the samples of the present invention not only exhibited a high color density at the two energies applied but they also had high resistance to the deposition of tailings. The ability of these samples to maintain a constant color density over a broad range of energies (≧ 0.1 mJ) is very desirable for the purposes of designing machines such as printers and producing good image quality on heat-sensitive recording materials.

[0036] The sample prepared in Comparative Example 1 used a pigment capable of greater oil absorption than 100 mℓ/100 g and the image density obtained at 0.54 mJ was lower than when an energy of 0.44 mJ was applied. The sample prepared in Comparative Example 2 did not use polystyrene and it was incapable of producing a high color density at either of the energies applied. The sample prepared in Comparative Example 3 used polystyrene alone. Although it exhibited high color densities over a broad range of energies, this sample was unacceptable in terms of resistance to the deposition of tailings.

[0037] The measurements of sensitivity and the deposition of tailings were conducted by the following procedures.

(1) Sensitivity

[0038] Sensitivity measurements were conducted with a test apparatus adapted from a commercial thermal facsimile. Pulse width modulation was effected to achieve a line speed of 10 msec/line and a scan density of 8 × 8 dots/mm, and 64 lines were printed with an energy of 0.44 mJ or 0.54 mJ being applied per dot. The resulting color densities were measured with a Macbeth densitometer RD- 514 and used as indices of recording sensitivity.

(2) Deposition of Tailings

[0039] The deposition of tailings on the thermal head was inspected visually and the results were evaluated by the following criteria: 0, completely acceptable with little deposition of tailings; Δ, reasonably acceptable despite some deposition of tailings; X, unacceptable on account of extensive deposition of tailings.

Claims

1. In a heat-sensitive recording material comprising a support coated with a heat-sensitive color-forming layer containing a colorless or pale-colored electron donating dye and an electron accepting compound, the improvement wherein an undercoat layer containing both polystyrene and a pigment capable of oil absorption in an amount of less than 100 mℓ/100 g is provided between the support and the heat-sensitive color-forming layer.

2. Heat-sensitive recording material according to Claim 1 wherein the ratio of polystyrene and a pigment capable of oil absorption in an amount of less than 100 mℓ/100 g is within 1:4 to 4:1.

3. Heat-sensitive recording material according to Claim 1 wherein the ratio of the sum of the polystyrene and the pigment and an adhesive is within 100:1 to 100:20.

4. Heat-sensitive recording material according to Claim 1 wherein an average particle size of polystyrene is up to 5 µm.

5. Heat-sensitive recording material according to Claim 1 wherein the undercoat layer is provided so as to have a dry weight of 1 - 15g/m².