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(11) | EP 0 284 378 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Light-fixable heat-sensitive recording material |
| (57) A light-fixable heat-sensitive recording material has a layer of a diazonium salt
component and a coupler capable of reacting with the diazonium salt component to form
color wherein the diazonium salt component comprises at least two diazonium salts,
one having a maximum light absorption wavelength of 410 ± 10 nm and the other having
a maximum light absorption wavelength of 385 ± 10 nm, preferably in a respective ratio
by weight of 9:1 to 1:9. The salt or coupler is preferably microencapsulated in a wall which becomes permeable when heated. The layer preferably includes a basic substance which acelerates coloration, and/or a sensitiser; there may be also an overcoat layer and/or a backing layer. The material is locally thermally heated by a thermal head and overall exposed to fix it, to give an image of high density on a white ground. |
[0001] This invention relates to a heat-sensitive recording material and, more particularly, to a diazo type heat-sensitive recording material capable of being fixed by light. Still more particularly, it pertains to a light-fixable heat-sensitive recording material providing a high color density and, upon fixing, forming a white background by light exposure only in a lower exposure amount.
[0002] As recording materials for heat-sensitive recording processes, leuco color-forming heat-sensitive recording materials have usually been used. However, such heat-sensitive recording materials have the defect that they form color even in unexpected areas to stain the recorded image when handled roughly after recording or when heated or when a solvent adheres thereto. As heat-sensitive recording materials having no such defect, diazo color-forming heat-sensitive recording materials have been intensively investigated in recent years. One example of the thus investigated techniques is described in Japanese Patent Application (OPI) No. 123086/82 (the term "OPI" as used herein means an unexamined published Japanese patent application), and Journal of the Society of Image Electronics, 11, 290 (1982), which comprises heat-recording a recording material containing a diazo compound, a coupling component and a basic component (including a substance which becomes basic when heated), then decomposing non-reacted diazo compound by light irradiation to stop further color formation. This process truly can stop color formation in areas where no recording is necessary (hereinafter referred to as "fixing"). However, even recording materials based on this process can undergo gradual pre-coupling during storage to cause unfavorable color formation (fog). In order to prevent this pre-coupling, a technique has been devised wherein one of the color-forming components is incorporated in the form of discontinuous particles (a solid dispersion) to prevent the components from coming into contact with each other. Such a technique, however, still has the defects that the storage life of the recording material (hereinafter referred to as "shelf stability") is insufficient and that the recording material undergoes reduction in thermal color-forming properties. As another technique, it is known to incorporate the diazo compound and the coupling component in different layers in order to minimize the contact therebetween (described in, for example, the foregoing Japanese Patent Application (OPI) No. 123086/82). This technique can improve shelf stability. However, it causes serious reduction in thermal color-forming properties, and hence recording materials based on this technique fail to respond to short pulse width high-speed recording, thus this technique is not practical to use. Further, as a technique for meeting the requirement for shelf stability and thermal color-forming properties, it is also known to encapsulate one of the coupling component and the basic substance using a non-polar waxy substance (as described in Japanese Patent Applications (OPI) Nos. 44141/82 and 142636/82) or a hydrophobic high molecular-weight substance (as described in Japanese Patent Application (OPI) No. 192944/82) to thereby isolate them from other components. In these encapsulating techniques, however, wax or a high molecular-weight substance is dissolved in its solvent, and a color-forming component is dissolved or dispersed in the solution to form capsules. Therefore, the capsules are different in nature from ordinary capsules wherein a shell covers a core substance. Therefore, when a color-forming component is dissolved, the color-forming component does not form a core of a capsule but is uniformly mixed with an encapsulating substance, thus pre-coupling gradually proceeds at the interface of the capsules during storage to spoil shelf stability. On the other hand, when a color-forming component is dispersed, thermal color-forming properties are deteriorated since the color-forming reaction does not take place untill the capsule wall is made molten by heat. In addition, there is involved a problem in the production process of removing the solvent which has been used for dissolving the wax or the high molecular-weight substance after completion of encapsulation; thus the technique is still not fully satisfactory.
[0003] In order to solve these problems, an excellent heat-sensitive recording material has been obtained by a technique of microencapsulating at least one of the components relating to the color-forming reaction as a core substance by forming a wall around the core substance through polymerization (Japanese Patent Application (OPI) No. 190886/84).
[0004] This recording material can truly give high coloration density, can show good stability, and can be fixed by light. In practical use, however, when a diazonium salt is used in an amount to give a sufficient coloration density, a long time is required for fixing by light. Thus, in apparatus containing a heat-recording zone and a light-fixing zone, the recording material requires that the speed of conveying be slowed down or that several fixing lamps be used in a wide fixing zone which has caused serious problems with respect to increasing recording speed and reducing size of the apparatus, as are currently required.
[0005] The term "fixing" as used herein means both that a diazonium salt, one of the color-forming components, is decomposed by light and becomes unable to form color any more even when heated and that a diazonium salt which often appears yellow loses this yellow color when exposed to light to form a white non-printed background (or to become colorless). From a practical viewpoint, the latter is often of importance because the yellow color residue gives an unfavorable yellowish appearance to users, and paper coming out of a recording machine is therefore required to be white in non-printed areas. On the other hand, even when a non-reacted diazonium salt remains in a small amount, it practically causes no problems since it will soon be naturally decomposed, during being handled, by light from lighting fixtures or by natural light.
[0006] It is, therefore, an object of the present invention to provide a heat-sensitive recording material having excellent shelf stability and high thermal color-forming properties.
[0007] Another object of the present invention is to provide a heat-sensitive recording material which can be fixed by photo decomposition of a non-reacted diazo compound after heat recording.
[0008] A further object of the present invention is to provide a light-fixing type heat-sensitive recording material which provides high coloration sensitivity and coloration density and gives a white background when fixed by light exposure in a small exposure amount.
[0009] As a result of intensive investigations, the inventors have achieved the above-described objects by providing a light-fixable heat-sensitive recording material comprising a diazonium salt component and a coupler capable of reacting with the diazonium salt component to form color, wherein the diazonium salt component comprises at least two diazonium salts, one having a maximum light absorption wavelength of 410 ± 10 nm and the other having a maximum light absorption wavelength of 385 ± 10 nm.
[0010] The diazonium salts to be used in the present invention are in the form of diazo compounds represented by the general formula ArN₂⊕X⊖ (wherein Ar represents an aromatic moiety, N₂⊕ represents a diazonium group, and X⊖ represents an acid anion). These compounds have varying maximum light absorption wavelengths (hereinafter referred to as "λmax") depending upon the position and the kind of substituents in the Ar moiety.
[0011] The present invention is characterized in that there is used said combination of at least two of a diazonium salt having a λmax of 410 ± 10 nm and a diazonium salt having a λmax of 385±10 nm among the above-describd diazo compounds. Their structure is not particularly limited as long as the above-described requirements are satisfied.
[0012] Specific examples of the diazonium salt having a λmax of 410 ± 10 nm include 2,5-dibutoxy-4-morpholinobenzenediazonium hexafluorophosphate, 2,5-dibutoxy-4-morpholinobenzenediazonium tetrafluoroborate, 2,5-dibutoxy-4-morpholinobenzenediazonium chloride-zinc chloride, 2,5-dibutoxy-4-{N-(2-ethylhexanoyl)piperadino}benzenediazonium hexafluorophosphate, 2-5-diethoxy-4-[N-{2-(2,4-di-tert-amylphenoxy)butyryl}-piperadino]benzenediazonium 1,5-naphthalenedisulfonate, 3-(2-ethylthioethoxy)-4-pyrodinobenzenediazonium tetraphenylborate, 3-(2-octyloxyethoxy)-4-morpholinobenzenediazonium perfluorooctanesulfonate.
[0013] Specific examples of the diazonium salt having a λmax of 385 ± 10 nm include 4-[N-{2-(2,4-di-tert-amylphenoxy)butyryl}piperadino]benzenediazonium hexa fluorophosphate, 4-dioctylaminobenzenediazonium tetraphenylborate, 4-{N-(2-ethylhexanoyl)piperadino}benzenediazonium 1,5-naphthalene-disulfonate, 4-dihexylamino-2-hexyloxybenzenediazonium hexafluorophosphate, 4-N-ethyl-N-hexadecylamino-2-ethoxy-benzodiazonium hexafluorophosphate, 3-chloro-4-dioctylamino-2-octyloxybenzenediazonium hexafluorophosphate.
[0014] These diazonium salts are also characterized in that they can undergo coupling reactions with the following couplers to form colors with various hues of from short wavelength to long wavelength depending upon the kind of coupler. Black coloration, which is popularly favored, can easily be attained by a suitable combination with the coupler.
[0015] The coupling components (couplers) to be used in the present invention are those which couple with a diazo compound (diazonium salt) in a basic environment to form a dye. Specific examples thereof include resorcin, phloroglucin, sodium 2,3-dihydroxynaphthalene-6-sulfonate, 1-hydroxy-2-naphthoic acid morpholinopropylamide, 1,5-dihydroxynaphthalene, 2,3-dihydroxynaphthalene, 2,3-dihydroxy-6-sulfanylnaphthalene, 2-hydroxy-3-naphthoic acid morpholinopropylamide, 2-hydroxy-3-naphthoic acid anilide, 2-hydroxy-3-naphthoic acid-2ʹ-methylanilide, 2-hydroxy-3-naphthoic acid ethanolamide, 2-hydroxy-3- naphthoic acid octylamide, 2-hydroxy-3-naphthoic acid-N-dodecyl-oxy-propylamide, 2-hydroxy-3-naphthoic acid tetradecylamide, acetanilide, acetoacetanilide, benzoylacetanilide, 1-phenyl-3-methyl-5-pyrazolone, 1-(2ʹ,4ʹ,6,ʹ-trichlorophenyl)-3-benzamido-5-pyrazolone, 1-(2ʹ,4ʹ,6ʹ-trichlorophenyl)-3-anilino-5-pyrazolone, 1-phenyl-3-phenylacetamido-5-pyrazolone. These coupling components may be used alone or in combination of two or more to obtain an image with any hue.
[0016] In the present invention, the amount of the coupling component is preferably 0.1 to 10 parts by weight per part by weight of the diazonium salt component.
[0017] The amount of the diazonium salt component used in the present invention is preferably 0.05 to 5.0 g/m².
[0018] In the heat-sensitive recording material of the present invention a basic substance for accelerating coloration may preferably be included. As the basic substance, sparingly water-soluble or water-insoluble basic substances and those substances which generate alkali when heated may be used.
[0019] The basic substances include nitrogen-containing compounds such as inorganic and organic ammonium salts, organic amines, thiazoles, pyrimidines, piperazines, guanidines, imidazoles, imidazolines, triazoles, morpholines, piperidines, amidines, formamidines, pyridines, etc.
[0020] Specific examples thereof are ammonium acetate, tricyclohexylamine, tribenzylamine, octadecylbenzylamine, stearylamine, 2-benzylimidazole, 4-phenylimidazole, 2-phenyl-4-methyl-imidazole, 2-undecylimidazoline, 2,4,5-trifuryl-2-imidazoline, 1,2-diphenyl-4,4-dimethyl-2-imidazoline, 2-phenyl-2-imidazoline, 1,2,3-triphenylquanidine, 1,2-ditolylguanidine, 1,2-dicyclohexylguanidine, 1,2,3-tricyclohexylguanidine, guanidine trichloroacetic acid salt, N,Nʹ-dibenzylpiperadine, 4,4ʹ- dithiomorpholine, morpholinium trichloroacetate, 2-aminobenzothiazole, 2-benzoylhydrazino-benzothiazole. These basic substances may be used in combination of two or more.
[0021] The heat-sensitive recording material of the present invention may be obtained by finely dispersing two or more of the above-described diazonium salts, couplers, and preferably basic substances, and, if necessary sensitizing agents and other additives in a binder. In view of shelf stability, etc., at least one of the above-described substances is preferably microencapsulated in a manner described in the foregoing Japanese Patent Application (OPI) No. 65043/83.
[0022] In this case, the microcapsules are not of the type used in conventional recording materials which type are ruptured by heat or pressure to permit the reactive substance contained in the core of the microcapsules to come into contact with a reactive substance outside of the microcapsules and cause a coloration reaction, but are the type which, when reactive substances existing in the core of the microcapsules and outside of them are heated, permit them to permeate through the microcapsule wall and undergo reaction therebetween.
[0023] As an embodiment of microencapsulation, a process of dissolving or dispersing a reactive substance to be retained in the core substance of the microcapsules in a water-insoluble or sparingly water-soluble organic solvent and, after emulsifying the solution or dispersion, forming a microcapsule wall around the droplets is particularly preferable. This process is disclosed in U.S. Patents 3,726,804, 3,796,669, 3,281,383, 3,773,675 and 3,793,268.
[0024] Microencapsulation of at least one of the color-forming components, that is, a diazonium salt, a coupler, a basic substance, and a sensitizing agent is markedly effective for improving shelf stability. In particular, microencapsulation of a diazonium salt or a coupler is effective. In the case of microencapsulating a diazonium salt, two or more diazonium salts may be microencapsulated in different capsules or in the same capsule. In the case of using these microcapsules, the use of a sensitizing agent is preferably in veiw of sensitivity.
[0025] As such sensitizing agents, those which are described in Japanese Patent Applications (OPI) No. 201986/85, 40190/86 and 84283/86, may favorably be used.
[0026] The sensitizing agent is preferably used in the present invention in an amount of 0.1 to 20 parts by weight per part by weight of the diazonium salt component.
[0027] In the present invention, the ratio of diazonium salt having a λmax of 410 ± 10 nm to that having a λmax of 385 ± 10 nm to be used is suitably 9/1 to 1/9 by weight.
[0028] The ratio may properly be decided in view of the kind of coupler to be used, hue of formed color, coloration density and fixing speed.
[0029] For the purpose of preventing sticking to a thermal head and improving writing properties, the heat-sensitive recording material of the present invention may contain fine powder of pigments such as silica, barium sulfate, titanium oxide, aluminum hydroxide, zinc oxide, or calcium carbonate, or of styrene beads or urea-melamine resin.
[0031] In preparing the heat-sensitive recording material of the present invention, coating may be conducted by using a proper binder.
[0032] As the binder, various emulsions of polyvinyl alcohol, methylcellulose, carboxymethylcellulose, hydroxypropylcellulose, gum arabic, gelatin, polyvinylpyrrolidone, casein, styrene-butadiene latex, acrylonitrile-butadiene latex, polyvinyl acetate, a polyacrylic acid ester or an ethylene-vinyl acetate copolymer may be used.
[0033] In the present invention, citric acid, tartaric acid, oxalic acid, boric acid, phosphoric acid, or pyrophosphoric acid may be added as acid stabilizers in addition to the above-described ingredients.
[0034] The heat-sensitive recording material of the present invention can be obtained by preparing a coating solution containing a diazo compound, a coupling component, and other additives, coating the solution on a support such as paper or synthetic resin film according to a bar-coating process, a blade-coating process, an air knife-coating process, a gravure-coating process, a roll-coating process, a spray-coating process or dip-coating process, and then drying the coating.
[0035] The heat-sensitive recording material of the present invention may have an overcoat layer on the heat- sensitive layer for the purpose of preventing sticking or imparting water resistance, and may have a backing layer on the back side of the support for the purpose of preventing curling and generation of static charge, and for improving running properties, and resistance to water and oil.
[0036] The heat-sensitive recording material of the present invention may be used as paper for printers such as a facsimile machine, a CRT, a bar code printer, etc. which require high-speed recording. After heat-recording, the heat-sensitive recording material can be fixed by exposure to thereby decompose unreacted diazo compound.
[0037] The recording material is characterized in that it provides high coloration density and a white background even when high-speed printing is conducted.
[0038] The recording material of the present invention may also be used as so-called heat-developable copying material which is to be thermally developed after photo recording.
[0039] The present invention is now illustrated in greater detail by reference to the following examples which, do not limit the invention. All "parts" are by weight.
EXAMPLE
Preparation of diazonium capsule solution A:
[0040] Diazonium salt of the following formula:3.5 parts
Tricresyl phosphate 6.0 parts
Methylene dichloride 14.0 parts
Trimethylolpropane trimethacrylate 17.0 parts
Takeneito D110N (75 wt% ethyl acetate solution)(made by Takeda Chemical Industries, Ltd.) 23.0 parts
[0041] The above-described ingredients were mixed, and the resulting mixture was added to a mixture of:
Polyvinyl alcohol aqueous solution (8 wt%) 45 parts
Distilled water 110 parts
followed by emulsification and dispersion at 20°C to prepare an emulsion containing droplets of 2 µm in average size. The thus obtained emulsion was stirred at 40°C for 3 hours to obtain capsule solution A.
Preparation of diazonium capsule solution B:
[0043] Capsule solution B was obtained in the same manner as capsule solution A except for changing the amount of diazonium salt to 5.3 parts. The resulting sample had a λmax of 408 nm.
Preparation of diazonium capsule solution C:
[0044] Capsule solution C was obtained in the same manner as capsule solution B except for changing the diazonium salt to a diazonium salt having the following structure:
and excluding trimethylolpropane trimethacrylate. A sample obtained by coating this solution and drying the coat had a λmax of 387 nm.
Preparation of diazonium capsule solution D:
[0045] Capsule solution D was obtained in the same manner as capsule solution A except for changing the diazonium salt in the diazonium capsule solution B to the following:
Preparation of dispersion:
[0047] The resulting mixture was dispersed for 24 hours in a sand mill to obtain a black color-forming dispersion of 1.5 µm in average particle size.
[0048] The above-described capsule solutions and the dispersion were mixed with each other in amounts shown in Table 1 below, then coated on paper using a No. 12 coating rod, followed by drying to obtain recording paper having the composition shown in Table 1. Each of the thus obtained recording materials was subjected to thermal recording using a GIII mode thermal printer (Hi-Fax 700, made by Hitachi, Ltd.), then exposed for 5 seconds using a Ricopy Super-Dry 100 (Ricoh high pressure mercury lamp (jet light: made by Oak Co., Ltd.)) to fix.
[0049] The visual density of the thus formed image-recorded areas was measured using a Macbeth densitometer. The yellow density of the background areas was also measured in the same manner. In addition, the hue of each of the resulting images was observed with the naked eye.
[0050] The results thus obtained are shown in Table 1 below together with the results of comparative examples.
[0051] Comparative examples 1 and 2 show that to raise the image density by using a diazonium salt having a λmax of long wavelength causes an extreme increase in background yellowing. Comparative examples 1 and 3 show that a diazonium salt having a λmax of short wavelength fails to give black coloration, although high image density is obtained with less background yellowing.
1. A light-fixable heat-sensitive recording material comprising a diazonium salt component
and a coupler capable of reacting with the diazonium salt component to form color,
wherein the diazonium salt component comprises at least two diazonium salts, one having
a maximum light absorption wavelength of 410 ± 10 nm and the other having a maximum
light absorption wavelength of 385 ± 10 nm.
2. A light-fixable heat-sensitive recording material as in Claim 1, wherein said diazonium
salt component is microencapsulated in a microcapsule wherein the diazonium salt component
permeates through the microcapsule wall when heated.
3. A light-fixable heat-sensitive recording material as in Claim 1 or 2, further including
a basic substance.
4. A light-fixable heat-sensitive recording material as in Claim 1, 2 or 3, wherein
the diazonium salts are in the form of diazo compounds represented by the general
formula ArN₂⊕X⊖ wherein Ar represents an aromatic moiety, N₂⊕ represents a diazonium group, and X⊖ represents an acid anion.
5. A light-fixable heat-sensitive recording material as in any of Claims 1 to 4, wherein
the ratio of the diazonium salt having a maximum light absorption wavelengh of 410
± 10 nm to that having a maximum light absorption wavelength of 385 ± 10 nm is 9/1
to 1/9 by weight.
6. A method of forming a visible image which comprises imagewise locally heating the
surface of a material as claimed in any preceding claim and overall exposing it to
light to fix it.