GB2085179A - Thermal recording sheet - Google Patents

Abstract

A thermal recording sheet has a paper or plastics support coated with a layer which color-forms on heating containing essentially (a) an electron- donating colorless dye as particles of volume average size not exceeding 2.0 mu m and (b) particles of an electron acceptor. The particles are formed by ball milling or sand milling the colorless dye (chromogen), e.g. by using balls of size 1 to 5 mm, in an aqueous solution of a protective colloid, e.g. polyvinyl alcohol. The dispersions are mixed together and with other coating components e.g. pigment and binder. The chromogen can be a triaryl methane, diphenyl methane, xanthene, thiazine or spiro compound. The electron acceptor can be a phenolic or aromatic carboxylic acid derivative, especially a bisphenol. The fine particle size of the chromogen gives rapid color forming with adequate color density with short heat pulses of 2 ms.

Description

SPECIFICATION Thermal recording sheet This invention relates to a thermal recording sheet.

Thermal recording orthermography produces a recorded image by using the physical or chemical change of a substance due to thermal energy, and a great number of processes have been studied on thermal recording. "Wax type" thermal recording sheets that use the physical change of a substance due to heat have been known for many years, and they are used, e.g., in electrocardiograms. Several thermal recording sheets that depend on various color-forming mechanisms using the chemical change of a substance due to heat have been proposed, and "two-color forming component" thermal recording sheets are typical of this type of sheet.

To make a two-color forming component thermal recording sheet, a dispersion of fine particles of two thermally reactive compounds is mixed with binder particles to form a coating solution, which is applied to a base in such a manner that the two thermally reactive compounds are separated from each other by the binder particles. This sheet forms an image by means of a color-forming reaction that is initiated upon contact between the two compounds when one or both compounds are melted by heat. The two compounds are generally referred to as an electron donor compound and an electron acceptor compound. There are very many known combinations of these compounds, and they generally consist of those which form an image of metallic compounds and those which form a dye image.Examples of the combinations that form a metallic compound image use phenols and other organic reducing agents, chelating agents, sulfur compounds and amino compounds as the electron donor compound, and organometallic salts as the electron acceptor compound. Examples of the combinations that form a dye image use electron donating colorless dyes as the electron donor compound and phenols and other acidic substances as the electron acceptor compound, as described, e.g., in Japanese Patent Publications Nos. 4/160/68 and 3,680/69.

These two-color forming component thermal recording sheets have advantages for use as a thermal recording sheet, e.g., (1) they depend only on the primary color forming reaction, and require no separate developing step, (2) the texture of the sheet is similar to that of ordinary paper, and (3) they are easy to handle. The sheets using colorless dyes as the electron donor compound have greater commercial value, in that they achieve not only the three advantages described above but also the following two additional advantages: (4) they provide a higher color density and (5) thermal recording sheets providing various colors can be easily obtained. In view of these advantages, the latter type of sheets are used most frequently as a thermal recording material.

Thermal recording sheets having the features described above have been studied as image-receiving paper for use in facsimile communications. The use of thermal recording sheets as the recording paper for facsimiles has two advantages: (1 ) since no developing step is necessary, the construction of the equipment is simplified, and (2) the fact that the recording paper itself is the only consumable part is advantageous with respect to equipment maintenance. However, the very fact that they rely on thermal recording presents a serious problem, namely, slow recording speed. This is considered to be due primarily to the slow thermal response of the recording head and sheet.Recently, some thermal recording heads having quick thermal response are being developed, but none of the current thermal recording sheets have satisfactorily quick response, and the development of quick response sheets would be very desirable.

Therefore, one object of this invention is to provide a thermal recording sheet having quickthermal response that is capable of high-speed recording.

Another object of this invention is to provide a recording paper that achieves an adequate color density with heat pulses of a duration of 2 ms (milliseconds), which is quite shorter than the duration of 5 ms that has been required for thermal recording by the conventional method.

According to this invention a thermal recording sheet has a support coated with a thermal color-forming layer containing an electron-donating colorless dye and an electron acceptor compound, said electrondonating colorless dye being in the form of fine particles having a volume average size of 2.0 lem or less.

To provide a thermal recording sheet adapted for high-speed recording, the following methods have been used: (1) the color forming temperature is reduced by addition of a low-melting point thermofusible substance or a low-melting point electron acceptor compound (hereinafter, referred to as a "developing agent"); (2) the heat transfer efficiency has been increased by improving the smoothness of the recording surface; and (3) more color-forming component is used.

However, sheets made by these methods undergo increased fog formation in the background during storage, its surface property is inferior to that of ordinary paper, and the production cost is high. To solve these problems, extensive studies of the color-forming mechanism of thermal recording sheets have been made. As a result, it has now been found that in almost all dye-type thermal recording sheets, color is formed by the sequence of (1) melting of a developing agent and/or a low-melting thermofusible substance, (2) mutual dissolution of the melted developing agent and low-melting compound, (3) dissolution of an electron donating colorless dye in a molten eutectic mixture of the developing agent and low-melting compound (said dye is hereunder referred to as a "chromogen"), and (4) the color-developing reaction between the developing agent and the chromogen.It has also been found that the speed of dissolution in the third step controls the rate of the color-forming reaction. On the basis of these results, we prepared thermal recording sheets using fine particles of the chromagen of various sizes, and found the following: The particle size has little effect on the recording properties when the volume average size is between about 3 and 10 ,um, but when the volume average size is 2.0 lim or less, a dramatic increase in color density occurs, and the difference from the case of using a chromogen of larger particle size is particularly great in high-speed recording mode. Even better results are obtained if the chromogen has a volume average particle size of 1 Rm or less.

The volume average size i of the particles used in the present invention is represented by the following equation: Thetotal 4 The total volumes of particles 1/3 3z The total number of particles In the past, no suitable means for making fine particles of chromogen has been known, and the minimum size of chromogen particles in dispersion that could previously be attained has been about 3 or 4 um. As described above, if the particle size in dispersion is 3 um or more, the effect of the particle size on the color-forming properties is relatively small, so heretofore, the advantage of this invention that is achieved by reducing the particle size in dispersion to 2 um or less has not been known.The average particle size of the chromogen to 2 um or less has been successfully reduced partly by making special provisions in the dispersing machine and partly by mixing several water-soluble polymers as a protective colloid of the dispersion to decrease the viscosity of the dispersion.

The recording sheet of this invention has quick response to thermal pulses of very short duration, and achieves adequate color density with a small amount of heat energy.

A process for producing the thermal recording sheet of this invention is now described. The particles of chromogen and developing agent are dispersed separately in a solution of a water-soluble polymer in a ball mill or other suitable dispersing means, e.g., the fine chromogen particles are produced by dispersing them for a sufficient period of time (about 16 to 24 hours) using balls of different sizes mixed in a given proportion.

The use of a sand mill (Dyno mill produced by Wiely A. Bachofen AG Maschinenfabrik in Switzerland) can also be effective.

For example, when the dispersion is carried out using a ball mill in order to prepare the chromogen in the form of fine particles having a volume average size of 2.0 um or less, it is desirable to use balls of fine size (e.g., 1 to 5mm, preferably 1 to 3mm, in diameter) in combination with the ordinary dispersion balls (e.g., 2 to 4 cm in diameter) in a suitable proportion. The suitable proportion of fine balls is 10 to 100 % by volume, preferably 20 to 50 % by volume, per volume of the ordinary balls. Preferred fine ball materials include hard glass, alumina, zircon and Ottowa sand.When the dispersion is carried out by putting the substance to be dispersed into a vessel such as a sand mill containing such mixture of balls, and stirring the resulting mixture, the desired dispersion particle size of 2 um or less can be more easily obtained. However, other dispersion procedures may be used.

The water-soluble polymer (protective colloid) present in solution in the dispersion step can be polyvinyl alcohol, hydroxyethyl cellulose, polyacrylamide or starch. When sodium caseinate is added simultaneously with the dispersion, increase of the viscosity during dispersion and the increase of the temperature caused thereby are preferably prevented, and as a result, a more suitable dispersion solution can be obtained; the amount of sodium caseinate used is 0.1 to 5 % by weight, preferably 0.5 to 2 % by weight, per weight of the dispersion solution.

The resulting polymeric dispersions of chromogen and developing agent are mixed, and preferably blended with an inorganic pigment, wax, higher aliphatic acid amide, metal soap, and optionally UV absorber, antioxidant, and a latex binder to form a coating mixture. These additives may be incorporated when the dispersions are prepared. The coating mixture is applied to a base (support) in such an amount that the coating weight of the chromogen is generally from 0.1 to 0.6 g/m2 (gram/square meter) and dried. The base is most typically made of paper, but plastic may also be used. The lower limit of the coating weight is determined by the desired color density and the upper limit by the production cost. A particularly preferred range is from 0.2 to 0.4 g of the chromogen per m2.

There is no particular limitation on the chromogen used in this invention if it is of the type commonly employed in pressure-sensitive recording sheets and thermal recording sheets. Specific examples include; (1) triaryl methane compounds such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (crystal violet lactone), 3-(p-dimethylaminophenyl)-3-(1,2-dimethylindole-3-yl)phthalide, 3-(p dimethylaminophenyl )-3-(2-phenylindole-3-yl)phthalide, 3,3-bis-(9-ethylcarbazole-3-yl)-5 dimethylaminophthalide, and 3,3-bis-(2-phenylindole-3-yl)-5-dimethylamino phthalide; (2) diphenyl methane compounds such as 4,4'-bis-dimethyl-aminobenzhydrinbenzyl ether, N-halophenyl leucoauramine, and N-2,4,5-trichlorophenyl leucoauramine; (3) xanthene compounds such as rhodamine B-anilinolactam, 3-diethylamino-7-dibenzylaminofluoran, 3-diethylamino-7-butylaminofluoran, 3-diethylamino-7-(2chloroanilino) fluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3 ethyl-tolylamino-6-methyl-7-anilinofluoran, 3-cyclohexyl methylamino-6-methyl-7-anilinofluoran, 3 diethylamino-6-chloro-7-(ss-ethoxyethyl)aminofluoran, and 3-diethylamino-6-chloro-7-(ychloropropyl)aminofluoran; (4) thiazine compounds such as benzoyl leucomethylene blue, and pnitrobenzoyl leucomethylene blue; and (5) spiro-compounds such as 3-methylspiro-dinaphthopyran, 3-ethylspiro-dinaphthopyran, 3-benzylspiro-dinaphthopyran, 3-methyl naphtho(3-methoxybenzo)spiropyran, and mixtures.A specific chromogen is selected depending on the particular use and properties desired.

The developing agent used in this invention is preferably a phenolic derivative or an aromatic carboxylic acid derivative, and a bisphenol is particularly preferred. The bisphenol compounds useful as a developing agent in this invention are represented by the formula

wherein R1 and R2 each represent a hydrogen atom or an alkyl group containing 1 to 12 carbon atoms or R1 and R2 combine to form a carbocyclic ring; or derivatives thereof.Specific examples include: (1) phenols such as p-octylphenol, p-tert-butylphenol, p-phenylphenol, 1,1 -bis-(p-hydroxyphenyl)-propane, 2,2-bis(p- hydroxyphenyl)propane, 1,1 -bis(p-hydroxyphenyl)pentane, 1 ,1-bis(p-hydroxyphenyl)hexane, 2,2-bis(p- hydroxyphenyl)hexane, 1,1 -bis(p-hydroxyphenyl )-2-ethyl hexane, and 2,2-bis(4-hydroxy-3,5dichlorophenyl)propane; and (2) aromatic carboxylic acid derivatives such as p-hydroxybenzoic acid, ethyl p-hydroxybenzoate, butyl p-hydroxybenzoate, 3,5-di-tert-butylsalicylic acid, 3,5-di-a-methyl benzyl-salicylic acid, and polyvalent metal salts of these carboxylic acids.For melting and causing the color-forming reaction at a desired temperature, these developing agents are preferably added in the form of a eutectic mixture with a low-melting point thermofusible substance, or fine particles to the surface of which a low-melting compound is fused.

Examples of the low melting point thermofusible compound include paraffin wax, carnauba wax, microcrystalline wax, and polyethylene wax, as well as higher aliphatic acid amides such as stearoyl amide and ethylene bisstearamide, and higher aliphatic acid esters.

Examples of the metal soap include polyvalent metal salts of higher aliphatic acids such as zinc stearate, aluminum stearate, calcium stearate, and zinc oleate.

Examples ofthe inorganic pigment are kaolin, burnt kaolin, talc, pyrophyllite, diatomaceous earth, calcium carbonate, aluminum hydroxide, magnesium hydroxide, magnesium carbonate, titanium oxide and barium carbonate.

The wax, metal soap, and inorganic pigment are dispersed in a binder and mixed with the dispersions of chromogen and developing agent. Alternatively, they are dispersed together with the particles of chromogen and developing agent. The binder is generally water-soluble and examples are polyvinyl alcohol, hydroxyethyl cellulose, hydroxpropyl cellulose, ethylene-maleic anhydride copolymer, styrene maleic an hydride copolymer, isobutylene-maleic anhydride copolymer, anhydride copolymer, polyacrylic acid, starch derivative, casein and gelatin. For making these binders waterproof, they may be mixed with a water-proofing agent (such as a gelling agent or crosslinking agent) or hydrophobic polymer emulsions such as styrene-butadiene rubber latex and acrylic resin emulsion.

This invention is now described in greater detail by reference to the following examples. In the examples all parts are by weight.

Percentages refer to the content of the material in an aqueous solution or (for kaolin) dispersion.

In the accompanying drawings: Figure 1 is a graph showing the relation between the volume average size of the particles of the electron donating colorless dye used in the sample of thermal recording sheet prepared in Example 1 and the resulting color density.

Figure 2 is a graph showing the same relation of the sample prepared in Example 2.

Example 1 Two hundred grams of a chromogen, 2-diethylamino-6-methyl-7-anilinofluoran, was dispersed in 2.5 wt % polyvinyl alcohol (degree of polymerization: 500; saponification value: (99 %) with a ball mill. The ball mill consisted of a porcelain pot of a capacity of 7 liters containing 1.5 kg of alumina balls 20 mm in size and 500 g of zircon balls 3 mm in size. The dispersion was sampled after 6, 12, 24,48 and 72 hours of ball milling. The volume average size of the chromogen particles dispersed in the respective samples was 5.4,3.6,2.0, 1.4 and 1.01lm.

Eighty grams of a developing agent, 2,2'-bis(p-hydroxyphenyl)propane, and 120 g of stearoyl amide were dispersed in 5 % polyvinyl alcohol with a ball mill to provide a dispersion comprising particles having an average size of 3.5 um.

One part of the dispersion of chromogen was mixed with 5 parts of the dispersion of developing agent, 2 parts of 40 % kaolin, 1 part of 20 % zinc stearate and 5 parts of 10 % polyvinyl alcohol, to prepare a coating solution. The solution was applied to uncoated paper having a basis weight of 50 g/m2 until the coating weight of the chromogen was 0.4 g/m2, dried and calendered at a pressure of 20 kgw/cm and passed through a facsimile machine (PANAFAX-7000) of Matsushita Electrical Industrial Co., Ltd.) where pulses of varying widths were applied to develop color throughout the recording sheet. The color density at pulse times of 1.5 ms and 2.7 ms versus the size of the chromogen particles dispersed is shown by curves (1) and (2) in Figure 1, respectively.The relation between the particle size and the color density obtained by holding the thermal recording sheet in contact with a heating stamp at 1500C for 5 seconds is also shown in Figure 1 by line (3).

The particle size was measured with a particle size distribution meter, Model TA-II of Coulter Counter Corp.

The color density was measured with a visual filter on a MacBeth reflection densitometer, Model RD-514 ("MacBeth" is a registered Trade Mark).

Example 2 Chromogen dispersions were prepared as in Example 1, except that the chromogen was 3-diethylamino-6 chloro-7-((3-ethoxyethylamino)fluoran. The average size of the chromogen particles in the respective dispersions was 4.6, 3.1, 1.9, 1.2 and 0.8 clam. The dispersions were subsequently treated as in Example 1 to prepare thermal recording sheets. The relation between the color density and the average size of the chromogen particles is shown in Figure 2.

From Figures 1 and 2, one can see that by reducing the size of chromogen particles to the value defined in this invention, the color density at high-speed recording can be increased significantly.

Claims (12)

1. A thermal recording sheet comprising a support coated with a thermal color-forming layer containing particles of an electron-donating colorless dye and of an electron acceptor compound, said particles of colorless dye having each a volume average size of not more than 2.0 microns.

2. A thermal recording sheet as claimed in Claim 1, wherein said volume average size of the dye particles is not more than 1.0 microns.

3. A thermal recording sheet as claimed in Claim 1 or 2, wherein the colorless dye is coated in an amount of from 0.1 to 0.6 grams per square metre.

4. A thermal recording sheet as claimed in Claim 3, wherein the colorless dye is coated in an amount of from 0.2 to 0.4 g/m2.

5. Athermal recording sheet as claimed in any preceding claim, wherein the electron-donating colorless dye is a triaryl methane, diphenyl methane, xanthene, thiazine or spiro compound.

6. Athermal recording sheet as claimed in any preceding claim, wherein the electron acceptor compound is a phenolic derivative or an aromatic carboxylic acid derivative.

7. A thermal recording sheet as claimed in Claim 6, wherein the electron acceptor compound is a bisphenol compound represented by the general formula

wherein R1 and R2 each represents a hydrogen atom or an alkyl group containing 1 to 12 carbon atoms or R1 and R2 combine to form a carbocyclic ring; or a derivative thereof.

8. A thermal recording sheet as claimed in any preceding claim, wherein the particles of colorless dye and/or electron acceptor were prepared from a dispersion containing also a polymeric protective colloid which reduced the viscosity of the dispersion.

9. Athermal recording sheet as claimed in Claim 8, wherein the protective colloid was polyvinyl alcohol, hydroxyethyl cellulose, polyacrylamide, starch or sodium caseinate.

10. A thermal recording sheet as claimed in Claim 9, wherein the dispersion contained 0.1 to 5 weight % of sodium caseinate.

11. A thermal recording sheet as claimed in any preceding claim, wherein the particles of colorless dye and/or electron acceptor were obtained by dispesion in a ball mill or sand mill.

12. A thermal recording sheet as claimed in Claim 1, substantially as hereinbefore described with reference to Example 1 or 2.