GB2028526A - Heat-sensitive recording sheet - Google Patents

Abstract

A heat-sensitive recording sheet comprises a support, e.g. of paper, bearing a thermosensitive layer comprising (A) a chromogenic substance which is normally colourless or light-coloured; (B) an acidic organic substance, such as a phenolic compound, which is capable of making the chromogenic substance develop a colour on heating; (C) at least one wax selected from condensates of higher fatty acid amides and formaldehyde and condensates of higher fatty acids and ethylene diamine (the weight ratio of component C to component A suitably being from 0.1:1 to 5.0: 1); and (D) at least one wax selected from vegetable waxes and mineral waxes (the weight ratio of component D to component A suitably being from 0.2:1 to 6.0:1). The thermosensitive layer may also contain (E.) an inorganic filler (suitably in a weight ratio of component E to component A of from 0.5:1 to 5.0:1) and/or (F) a binder comprising a starch or modified starch together with a dialdehyde starch (suitably in an amount such that component F forms from 10 to 50% by weight of the thermosensitive layer). The surface of the thermosensitive layer preferably has a Beck's smoothness of from 1,000 to 1,800 seconds.

Description

SPECIFICATION Heat sensitive recording sheets The present invention relates to heat-sensitive recording sheets of the type comprising a support bearing a heat-sensitive layer which contains a colour-developable or chromogenic substance which is normally colourless or light-coloured and an acidic organic substance capable of making said chromogenic develop a colour when heated.

Various recording systems are used and are being developed in the information recording art. The heat-sensitive recording system has the merits that (i) the developing process can be dispensed with as heat-sensitive recording media can develop a colour upon heating, (ii) the relevant apparatus is easy to assemble and maintain, (iii) heat-sensitive recording sheets for use therein resemble ordinary paper in feel and is generally of moderate cost. Thus, the heat-sensitive recording system has been put to practical use in the printers of computers outputs and portable electronic computers, the recorder part of metrological instruments for medical care, and in facsimile recording, automatic booking machines and heat-sensitive copying apparatus.

Heat-sensitive recording sheets useful for these purposes are disclosed, for example in Japanese Patent Publication No. 4160/1968 and Japanese Patent Publication No. 14039/1970, but these prior art heat-sensitive recording sheets have defects. Firstly, since they are inferior in heat-responsiveness, they cannot bring about a sufficient recording density adapted to increase the rate of recording.

Secondly, the heat-sensitive composite used therein is fused at the time of recording and sticks to the thermal pen or thermal head and, as a result, problems arise ia feeding the recording paper, the substance sticking to the head or pen is transferred to the recording paper to cause the so-called "tailing" phenomenon, and moreover, accumulation of the adherent material lowers the recording density leading to indistinctness of the recorded image.

These properties are coliectively called "head-matching" and constitute the most important factor to be taken into account when examining a heat-sensitive recording sheet. Various means have been proposed for improving these properties, such as those disclosed in Japanese Patent Open No.

1 9231/1 973 and Japanese Patent Publication No. 27599/1976. That is, Japanese Patent Open No.

1 9231/1 973 discloses that the use of a wax having a melting point in the range of from 400 to 1000C as carrier can enhance the heat-responsiveness. Japanese Patent Publication No. 27599/1976 discloses that the joint use of a higher fatty acid and a petroleum wax can improve the heatresponsiveness and alleviate the sticking of fused composite to the thermal head.

However, the means disclosed in these documents still leave something to be desired in order to obtain a satisfactory heat-sensitive recording sheet. In other words, from the point of view of ensuring recording fidelity and ameliorating the indistinctness of the recorded image arising from the sticking of fused composite onto the thermal pen when high-speed recording by a thermal pen recorder, such as an electrocardiograph in which a tip-type pen made of a thermal head type resistor is now taking the place of a metal rod pen, satisfactory results cannot be expected from these proposals.

It has now been found, in accordance with the present invention, that the incorporation in the heat sensitive layerofa specific wax leads to an improved heat sensitive material.

According to the present invention there is provided a heat-sensitive recording sheet, comprising a support hearing a heat-sensitive layer, which heat-sensitive layer comprises (A) a chromogenic substance which is normally colourless or light-coloured; (B) an acidic organic substance which is capable of making said chromogenic substance develop a colour on heating; (C) at least one condensate of a higher fatty acid amide and formaldehyde anchor condensate of a higher fatty acid and ethylene diamine; and (D) at least one vegetable wax and/or mineral wax.

-Thus, a heat sensitive recording sheet according to the present invention comprises a support (for example consisting of paper, synthetic paper, synthetic resin film or metal-iaminated paper) and a heat-sensitive layer having a specific composition formed on the support.

The incorporation in the heat-sensitive layer of at least one wax which is a condensate of a higher fatty acid amide and formaldehyde or a condensate of a higher fatty acid and ethylene diamine (component (C)), improves the heat-responsiveness of the layer and leads to the production of a recorded image of high density.However, if component (C) is simply mixed with the chromogenic colour developable substance (component (A)) and the organic substance capable of making the chromogenic substance develop a colour on heating (component (B)), the problem of sticking to the thermal pen and thermal head cannot be sufficiently amelionated, However, the incorporation in the heat-sensitive layer of at least one wax selected from vegetable waxes and mineral waxes (component (D)) markedly reduces the sticking of fused composite onto the thermal head and the abrasion of the thermal pen and thermal head.The use of a petroleum wax such as polyethylene, polypropylene; paraffin wax or the like is undesirable because this tends to induce such sticking to the thermal head and accordingly the adhesion of residual substances to the head would increase Examples of materials for use as component (C) include: a condensate of stearic acid amide and formaldehyde (methylene-bis-stearomide CtH35CONHCH2NHCOCa7H3s), a condensate of stearic acid amide and formaldehyde (methylolstearoamide C17H35CONHCH2OH), a condensate of palmitic acid amide and formaldehyde (methylene-bis-palmitoamide N 1CONllC 0111, a condensate of lauric acid and ethylene diamine (ethylene-bis-laurylamide C11H23CONHCH2CH2NHCOC11H23).

Component (C) is not limited to the foregoing providing that its melting point should be from 800 to 1500C.

Examples of ingredient (D) include carnauba wax, sugar cane wax and cotton wax (which may be directly obtained from vegetable sources), montan wax extracted from brown coal resulting from the partial decomposition of vegetable matter, and modified waxes derived from such waxes.

The chromoenic substance (component A) for use in the present invention which is normally colourless or light coloured is generally a leuco-base such as triphenyl methane, fluorane, phenothiazine, Auramine a spiropyrane derivative and specific examples thereof include: 3,3-bis(p-dimethyminophenyl)-6-dimethyl aminophthalide, 3,3-bis(p-dimethylaminophenyl)phthalide, 3,3-bis(p-dimethylam inophenyl)-6-diethyl am inophtha lide, 3,3-bis(p-dimethylaminophenyl)-6-chlorophthalide, 3-(N-p-tolyl-N-ethyl-amino)-6-methyl-7-(N-phenyl-amino) fluorane, 3-diethyl amino-7-chlorofluorane, benzoyl Leucomethylene Blue, 6'-chloro-8'-methoxybenzoindolino-pyrylospiran, 6'-bromo-8'-methoxy-benzoindolino-pyrylospiran, by oxidizing these starches with sodium hypochlorite.Dialdehyde starches can be easily obtained by oxidizing these starches with periodate.

In order to prepare a heat-sensitive recording sheet according to the present invention, components A, B, C, D, and optionally E, are dispersed in a suitable liquid, generally water, individually or upon mixing together excepting ingredient A, by means of a disperser such as bali-mill, attriter or sand-mill, after adding a protective colloidal substance, a surface active agent, and, if necessary, further adding a binder (i.e. component F) thereto, whereby a heat-sensitive layer forming liquid is prepared.

This heat-sensitive layer forming liquid may also contain other ingredients such as defoaming agent; wetting agents, and furtheer organic pigments, bridging agents, etc.

The weight ratio of component C to component A is suitably from 0.1:1 to 5.0:1, preferably from 0.2:1 to 3.0:1; that of component D to component A is suitably from 0.2:1 to 6.0:1 preferably from 0.2:1 to 4.0:1; and that of component E to component A is suitably from 0.5:1 to 5.0:1. Component F is suitably used in an amount of from 10 to 50% by weight of the heat-sensitive layer.

When using ingredient F in the preparation of the heat-sensitive layer forming liquid, it is preferable to use it by converting it into a paste by dispersing the starch or.starch derivative in water and heating the resulting dispersion up to a temperature 2-[,6-bis(diethyl amino)-9-80-chloroanilino)xanthyi]benzoic lactam, and 2-[N-(3'-trifluoromethyl phenyl)amino]-6-diethyl aminofluorane.

Examples of acidic organic substances (component B) for use in the present invention include phenolic materials such as algha-naphthoi, beta-naphthol, 4-t-butyl phenol, 4-t-octyl phenol, 4-phenyl phenol, 2,2-bis(p-hydroxyphenyl)propane, 2,2-bi's(p-hydrnxyphenyl) butane, 4A-cyclohexilidene diphenol, 2,2-bis(2,5-dibromo-4,4-hydroxyphenyl)propane, 4,4'-isopropylidene-bis(2-t-butyl phenol) and 2,2-methylene-bis(4-chlorophenol). The heat-sensitive layer may also include an organic filler (component E) such as clay, calcium carbonate, magnesium carbonate or kaolin, preferably kaolin or calcium carbonate having a particle size of from 0.05 to 5 microns.Such compositions are suitable for practical use but it has been found that the further incorporation of a binder comprising starch or a starch derivative, together with dialdehyde starch, (component F), in the heat-sensitive layer can prevent the sticking of fused composite onto the thermal head more effectively and enhance the colourdevelopability and thus there can be obtained a very satisfactory heat-sensitive recording sheet Examples of starches and starch derivatives for use in the present invention include maize starch, wheat starch, sweet potato starch and oxidized starches obtained of 800 to 950C.In the case of dialdehyde starches which are difficult to dissolve under the same conditions, it is advisable to use such a starch by converting it into a paste by adding a buffer solution made of sodium acetate, borax or the like and heating the resulting mixture up to a temperature of 800 to 950C.

Subsequently, the thus obtained heat-sensitive layer forming liquid is coated onto a support, suitably at a rate of 4 to 10 g/m2 in terms of solid matter, and is dried.

It is advantageous to subject the heat-sensitive layer of the resulting heat-sensitive recording sheet to surface treatment so as to attain a Beck's smoothness of 1,000 to 1,800 seconds.

Thus, when the heat-sensitive layer its subjected to surface treatment with a gloss calendar or super calendar or the like so as to attain a Beck's smoothness of 1,000 to 1,800 seconds, the heatresponsiveness and the recording fidelity are markedly improved particularly when the sheet is employed in a thermal pen recorder or high speed serial printer. Even if the Beck's smoothness is from 100 to 500 seconds or thereabouts, the heat-sensitive layer displays a superior heat-responsiveness but by virtue of surface treatment to attain a Beck's smoothness of more than 1,000 seconds, the recording fidelity is further enhanced. However, if the Beck's smoothness exceeds 1,800 seconds, the ground of the heat-sensitive recording sheet will be exposed under pressure and become excessively transparent, thereby impairing the commercial value thereof.

The following examples are given by way of illustration only.

EXAMPLE 1 Two liquids, A1 and E, were prepared by pulverizing the mixtures having the formulations given below in a magnetic ball-mill for a day.

LIQUID A, 2-[-(3'-tnfiuoromethyl phenyl)amino]-6-diethyl aminofluorane 1 50 g 5% aqueous solution of polyvinyl alcohol (POVAL205, manufactured by KURARE K.K.) 150 g LIQUID E1 talc 150 g 5% aqueous solution of methyl cellulose (METHOLOSE SM-1 5, manufactured by SHINETSU KAGAKU K.K.) 150 g water 200 g Further, three liquids, B1, C1, and D, were prepared by powdering the mixtures having the formulations given below in an attriter for 2 hours.

LIQUID B1 2,2'-bis(p-hydroxyphenyl)propane 120 g 5% aqueous solution of polyvinyl alcohol 120 g Water 160 LIQUID C1 consensate of stearic acid amide and formaldehyde (in the presence of basic catalyst), Ct,H35CONHCH2OH) 80 g 5% aqueous solution of methyl cellullose 80 g nonionic surface active agent (an alkylphenol,thylene oxide adduct:: NOIGEN EA-80, manufactured by DAIICHI KOGYO SEIYAKU K.K.) 2 g water 238 g LIQUID D, carnauba wax 80 g 5% aqueous solution of methyl cellulose 80 g nonionic surface active agent 2g water 238 g Subsequently, a heat-sensitive layer-forming liquid was prepared by mixing liquids A1-E1 with a 20% aqueous solution of polyvinyl alcohol (POVAL205) in the following ratio.

liquid A, 20 g liquid B, 40 g liquid C, 60 g liquid D, 80 g liquid E, 40g 20% aqueous solution of polyvinyl alcohol 1159 Then, this heat-sensitive layer forming liquid was coated on a commercial slick paper (weighing 53 g/m2) at a rate of 7 to 8 g/m2 in terms of dry solid matter by means of a wire bar coater and was dried. Thereafter, the thus coated slick paper was further subjected to surface treatment with a calendar so as to attain the Beck's smoothness of 200 to 400 seconds, whereby a heat-sensitive recording sheet was prepared.

The heat-sensitive recording sheet was tested and the results are shown in Table 1. As is evident from Table 1, this heat-sensitive recording sheet had high heat-responsiveness, was free from such phenomena as sticking of fused composite, adhesion of residual substance, tailing of recorded image and ghaust, and had good head-matching properties.

The following test methods were employed.

(1) Heat-responsiveness was tested using a heat gradient Tester equipped with a heated iron piece (manufactured by Toyo SEIKI K.K.). The iron piece, at constant temperature, was kept in contact with the heat-sensitive recording sheet under a pressure of 2.0 Kg/cm2 for 1 second to make it develop a colour and the density of the colour thus developed was measured with a Macbeth's reflection expressed as Dmax., and the heat-responsivenessiwas expressed by the temperature at which a density of 0.8 Dmax was attained. This means that the higher the said temperature the lower is the heat-responsiveness, (2) The sticking of fused composite was examined by conducting solid printing with the overall width of the thermal head.The adhesion of residual substances, the tailing of recorded image, the ghaust and the distinctness of recorded image were evaluated by employing 20 pieces of Fascimile Test Charts for comparison.

(3) The recording density was evaluated by measuring the density of the solid area of the recorded image according to the foregoing charts by use of a Macbeth's densitometer.

EXAMPLE 2 A mixture having the following composition was pulverized in a laboratory attriter for 2 hours, to give liquid C-D.

Liquid C-D modified montan wax 40g (S-Wax manufactured by HOECHST JAPAN CO.) condensate of stearic acid and ethylene diamine (C1,H35CONHCH2CN2NHCOC,7H35) 40 g 5% aqueous solution of methyl cellulose 80 g nonionic surface active agent 29 water 238 g Subsequently, a heat-sensitive layer forming liquid was prepared by combining this liquid C-D with liquids A1, B1 and E1 of Example 1 and a 20% aqueous solution of polyvinyl alcohol in the following ratio.

liquid A1 20 g liquid B1 40g liquid C-D 140 g liquid E1 40 g 20% aqueous solution of polyvinyl alcohol 115g This liquid was employed to make a heat-sensitive recording sheet following the procedures of Example 1. This heat-sensitive recording sheet was tested as described in Example 1 and the results are shown in Table 1 , attesting to its superior quality as a heat-sensitive recording sheet.

COMPARATIVE EXAMPLE 1 A mixture having the following composition was pulverized in a laboratory attriter for 2 hours, to give liquid C2.

Stearic acid amide 80 g 5% aqueous solution of methyl cellulose 80 g nonionic surface active agent 2g water 238 g Subsequently, a heat-sensitive layer forming liquid was prepared from liquid C2 together with liquids A1, B1, and E1 of Example 1 in the ratio given below and then by applying the same procedure as in Example 1, a heat-sensitive recording sheet was prepared.

liquid A1 20 g liquid B 40g liquid C2 60 g liquid E1 40 g polyethylene wax emulsion (solid content: 30%) (POLYON A, manufactred by CHUKYO YUSHl K.K.) 53 g This heat-sensitive recording sheet was subjected to the same tests as in Example 1 and the results are shown in Table 1.

COMPARATIVE EXAMPLES 2-8 Seven varieties of comparative heat-sensitive recording sheets were prepared by applying the same procedure as in Example 1 except that the wax (ingredient C and/or ingredient,D) was altered as shown in Table 1. These heat-sensitive recording sheets were subjected to the same tests as in Example 1 and the results are shown in Table 1.

TABLE 1

Ingredient C Heat Examples and/or D responsive Ingredient D max. ness Example 1 condensate of stearic acid amide and formaldehyde and carnauba wax 1.15 91"C condensate of stearic acid and Example 2 ethylene diamine and modified 1.16 980C montan wax Comparative Example 1 stea-uc acud amide 1.16 91C Comparative Example 2 carnauba wax 1.16 1200C Comparative condensate of stearic acid amide Example 3 and formaldehyde 1.16 91 C @C (C17H35CONHCH2 OH) Comparative Example 4 stearic acid amide 1.17 1250C Comparative Example 5 calcium stearate 1.17 125"C Comparative Example 6 polyethylene wax 1.14 11000 Comparative Example 7 modified montan wax 1.15 12000 Comparative Example8 parrafin wax and C17H35CONHCH2OH 1.15 91 C TABLE 1 - (CONT.)

Result of test on quality Examples Head-matching property Recording Adhesion of Trailing Ghaust; density Sticking residuary of distinct substance recorded ness of image recorded image Example 1.11 a a a a Example 2 1.05 a a Comparative Example 1 1.11 b c b d Comparative Example 2 0.90 b b a c Comparative Example 3 1.11 d c d c Comparative Example 4 1.12 e e e Comparative Example 5 0.85 e e e e Comparative Example 6 0.95 1 c d b c Comparative Example 7 0.93 b b a c Comparative Example8 1.10 b C I b d NOTE Symbols a, b, c, d and e represent values as graded from "satisfactory" down to "unsatisfactory".

EXAMPLE 3 Two mixtures having the following compositions were pulverized in a magnetic ball-mill for 1 day, to prepare liquids A2 and E2.

Liquid A2 3,3-bis(p-dimethyl aminophenyl)-6-dimethyl aminophenyl 150 g aqueous solution of polyvinyl alcohol (POVAL 205) 150 g water 200g Liquid E2 kaolin 150 g 5% aqueous solution of methyl cellulose (METHOLOSE SM--15) 150 g water 200 g Subsequently, a heat-serisitivelayer-forming liquid was prepared by compounding these liquids A2 and E2 with liquids B1, C, and D1 of Example 1, a 20% aqueous solution of oxidized starch and a 10% aqueous solution of dialdehyde starch in the following ratio.

liquid A2 20 g liquid B2 40 g liquid C1 60 g liquid D1 80 g liquid E2 40 g 20% aqueous solution of oxidised starch 60 g 10% aqueous solution of dialdehyde starch 120g Then, this heat-sensitive layer forming liquid was coated on a commercial slick paper (weighing 53 g/m2) at a rate of 5 to 6 g/m2 in terms of dry solid matter by means of a wire bar coat and was dried.

Thereafter, the thus coated slick paper was further subjected to surface treatment with a calendar so as to attain a Beck's smoothness of 1,000 to 1,200 seconds, whereby a heat sensitive recording sheet was prepared.

This heat-sensitive recording sheet was tested and the results are shown in Table 2. As is evidence from Table 2, this heat-sensitive recording'proved to have good heat-responsiveness, highspeed recording properties and fidelity in recording, and was free from causing the sticking of fused composite to the thermal head and the abrasion of the thermal head.

Table 2 also shows the results obtained for these were collectively shown the case wherein heatsensitive recording sheets prepared in the same manner had surface treatment with a calendar to give Beck's smoothness of 500 to 550 seconds (Example 4) or to give a Beck's smoothness of 300 to 400 seconds (Example 5).

The following test methods were employed.

(1) The heat-responsiveness was evaluated in the same way as in Example 1.

(2) The high-speed recording properties and the recording fidelity were evaluated by comparing the result of the colour-developability test conducted on each recording sheet by using a linear recorder equipped with an IC pen (Thermal Pen Recorder manufactured by WATANABE SOKKI SEISAKUSHO, LTD.) applying a 50 Hz sine wave as input and setting the feed rate of heat-sensitive recording sheet at 500 mm/sec.

(3) The abrasion hardness was evaluated through a running test conducted on each recording sheet by employing the linear recorder mentioned in (2) above, applying a 60 Hz sine wave as input and setting the feed rate of heat-sensitive recording sheet at 100 mm/sec., and was expressed by the distance of the travel, in kilometers, made by the IC pen until its resistance increased by about 10% to adversely affect the drawing efficiency thereof.

(4) The sticking of fused composite onto the thermal head and the distinctness of recorded image was evaluated by comparing the result of the test conducted on each recording sheet by employing a trial printer equipped with a segment-type thick head (manufactured by NIPPON TO-KI K.K.) and a baby printer PC-1 00 (manufactured by TEXAS INSTRUMENT Inc.) for use in a portable electronic computer.

(5) The water-resistance properties are evaluated by comparing the result of measurement of the density diminution rate of each recording sheet after dipping in water for 5 minutes and then rubbing the surface of the heat-sensitive layer with absorbent cotton two times.

TABLE 2

Examples Ingredient C Ingredient Ingredient Beck and E F Smoothness Ingredient D Example 3 condensate of stearic oxidized 1,100 acid amide and kaolin starch, formaldehyde dialdehyde 2,000 and carnauba wax starch sec.

Example 4 " " " 500 Example 5 " " " 300 TABLE 2 - (CONT.)

Result of test on quality Examples Heat High Recording Sticking to Abrasion Water responsive- speed fidelity thermal hardness resisting ness recording head properties properties 3 90"C a a a 500 a 4 98 or b d a 400 a 5 90"C b d c 180 d Note: Symbols have same meanings as in Table 1.

Claims (12)

1. A heat-sensitive sheet, comprising a support bearing a heat-sensitive layer which heat-sensitive layer comprises (A) a chromogenic substance. which is normally colourless or light-coloured; (B) an acidic organic substance which is capable of making said chromogenic substance develop a colour on heating, (C) at least one wax selected from condensates of higher fatty acid amides and formaldehyde and condensates of higher fatty acids and ethylene diamine; and (D) at least one wax selected from vegetable waxes and mineral waxes.

2. A heat-sensitive recording sheet according to claim 1 in which the weight ratio of component C to component A is from 0.1 :1 to 5.0:1.

3. A heat-sensitive recording sheet according to claim 1 or claim 2, in which the weight ratio of component D to component A is from 0.2:1 to 6.0:1.

4. A heat-sensitive recording sheet according to any one of claims 1 to 3, also containing (E) an organic filler.

5. A heat-sensitive recording sheet according to claim 4 in which the inorganic filler is calcium carbonate and/or clay.

6. A heat-sensitive recording sheet according to claim 4 or claim 5 in which the weight ratio of component E to component A is from 0.5:1 to 5.0:1.

7. A heat-sensitive recording sheet according to any one of the previous claims also containing (F) a binder comprising a starch or modified starch together with a dialdehyde starch.

8. A heat sensitive recording sheet according to claim 7, in which the heat-sensitive layer contains from 10 to 50% by weight of component F.

9. A heat-sensitive recording sheet according to any one of the preceding claims in which said higher fatty acid of component C is palmitic acid or stearic acid.

1 0. A heat-sensitive recording sheet according to any one of the preceding claims in which ingredient D is carnauba wax, montan wax or modified carnauba or montan wax.

11. A heat-sensitive recording sheet according to any one of the preceding claims in which the Beck's smoothness of the heat-sensitive layer surface is from 1 ,000 to 1,800 seconds.

12. A heat-sensitive recording sheet according to claim 1 substantially as hereinbefore described with reference to the examples.