Abstract:
The present invention provides a heat-sensitive recording sheet which is composed of a support and a heat-sensitive layer formed on said support, said heat-sensitive layer consisting essentially of a color-developable substance and an organic acid substance capable of making said color-developable substance develop a color when heated and containing at least one member selected from the group consisting of condensate of higher fatty acid amide and formaldehyde and condensate of higher fatty acid and ethylene diamine and at least one member selected from the group consisting of vegetable waxes and mineral waxes.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a heat-sensitive recording sheet, and particularly it relates to a heat-sensitive recording sheet which is composed of a support and a heat-sensitive layer formed thereon, said heat-sensitive layer consisting essentially of a color-developable substance (developer) which is normally colorless or light-colored and an organic acid substance capable of making said color-developable substance develop a color when heated and also containing a specific wax. 
     (b) Description of the Prior Art 
     With the recent social need for increase in the amount of information, economy of resources, labor-saving, prevention of public nuisance, etc., utilization and improvement of varieties of recording systems are in progress in the field of the information recording art. Especially, the heat-sensitive recording system has merits such that (i) it can dispense with the developing process as it can develop a color upon heating, (ii) the relevant apparatus is easy to assemble and maintain, (iii) the heat-sensitive recording sheet for use therein resembles ordinary papers in feel and is rather moderate in cost, etc., and therefore it has been put to practical use as the printer part for computer outputs, portable electronic computer, etc., the recorder part of metrological instruments for medical care, facsimile, automatic booking machine, heat-sensitive copying apparatus, etc. 
     As the heat-sensitive recording sheet useful for these purposes, there are known such ones as disclosed in Japanese Patent Publication No. 4160/1968, Japanese Patent Publication No. 14039/1970, etc., but these heat-sensitive recording sheets in the prior art still have defects such as follows: 
     First, since they are inferior in heat-responsiveness, they cannot bring about a sufficient recording density adapted to the speed-up of the recording. Second, the heat-sensitive composite contained therein is fused at the time of recording and sticks to the thermal pen or thermal head, and as a result, inconvenience is caused in feeding the recording paper, the sticking substance is transferred to the recording paper to cause the so-called &#34;tailing&#34; phenomenon, and moreover, accumulation of the sticking substance lower the recording density, entailing indistinctness of the recorded image. 
     These properties are collectively called &#34;head-matching&#34; and constitute the most important factor to be taken into account at the time of examining a heat-sensitive recording sheet. There have admittedly been proposed various means for improving these properties, such as disclosed in Japanese Patent Open No. 19231/1973, Japanese Patent Publication No. 27599/1976, etc. That is, Japanese Patent Open No. 19231/1973 disclosed that the use of a wax having a melting point in the range of from 40° to 100° C. as carrier can enhance the heat-responsiveness. And, Japanese Patent Publication No. 27599/1976 disclosed that joint use of a higher fatty acid and a petroleum wax can improve the heat-responsiveness and alleviate the sticking of fused composite onto the thermal head. 
     However, as a matter of fact, the means disclosed in these literatures still leave something to be desired in order to obtain a fully satisfactory heat-sensitive recording sheet. In other words, from the view point of ensuring the recording fidelity and ameliorating the indistinctness of the recorded image arising from the sticking of fused composite onto the thermal pen at the time of high-speed recording by a thermal pen recorder, such as an electrocardiograph, etc., in which a tip-type pen made of a thermal head type resistor in now taking the place of a metal rod pen, a satisfactory result can, in fact, not be expected from only the arts disclosed in these literatures. 
     SUMMARY OF THE INVENTION 
     In view of the existing circumstances as above, the present inventors have made a series of studies and examinations in order to obtain a heat-sensitive recording sheet having a wide range of use in the thermal printer, thermal pen recorder, etc. As a result, they have come to the finding that application of a heat-sensitive layer consisting essentially of a color-developable substance and an organic acid substance capable of making said color-developable substance develop a color when heated (to wit, a developer), with the addition of a specific wax, can eliminate the aforementioned defects of the prior art. The present invention has been accomplished on the basis of this finding. 
     A primary object of the present invention is to provide a heat-sensitive recording sheet which is superior in heat-responsiveness and high in recording density. Another object of the present invention is to provide a heat-sensitive recording sheet which renders a sufficiently distinct image even at the time of high-speed recording. A further object of the present invention is to provide a heat-sensitive recording sheet which is free from such a trouble that the heat-sensitive materials contained therein are fused at the time of recording and stick onto the thermal head. A still further object of the present invention is to provide a heat-sensitive recording sheet which is free from change in quality despite prolonged storage. 
     In other words, the present invention provides a heat-sensitive recording sheet, which is composed of a support and a heat-sensitive layer formed thereon, said heat-sensitive layer consisting essentially of a color-developable substance which is normally colorless or light-colored and an organic acid substance which is capable of making said color-developable substance develop a color when heated, and is characterized in that said heat-sensitive layer contains at least one member selected from the group consisting of condensate of higher fatty acid amide and formaldehyde and condensate of higher fatty acid and ethylene diamine and at least one member selected from the group consisting of vegetable waxes and mineral waxes. 
     That is, a heat-sensitive recording sheet according to the present invention comprises a support (consisting of paper, synthetic paper, synthetic resin film, metal-laminated paper or the like) and a heat-sensitive layer having a specific composition as formed on said support. 
     Inasmuch as this heat-sensitive layer contains at least one kind of wax selected from the group consisting of condensate of higher fatty acid amide and formaldehyde and condensate of higher fatty acid and ethylene diamine (this wax is hereinafter referred to as ingredient C), it is superior in heat-responsiveness and renders a recorded image of high density. However, in the case where this ingredient C is simply mixed in the color developable substance (hereinafter referred to as ingredient A) and the organic acid substance capable of making the color-developable substance develop a color (hereinafter referred to as ingredient B), the property of sticking to the thermal pen and thermal head cannot be sufficiently ameliorated. 
     While, in the case where at least one kind of wax selected from the group consisting of vegetable waxes and mineral waxes (hereinafter referred to as ingredient D) is added thereto, the sticking of fused composite onto the thermal head and the abrasion of the thermal pen and thermal head can be drastically ameliorated. On this occasion, the use of a petroleum wax such as polyethylene, polypropylene, paraffin wax or the like is undesirable because it would rather induce said sticking to the thermal head and accordingly the adhesion of residuary substances thereto would increase. 
     As the ingredient C (i.e., a kind of wax selected from the group consisting of condensate of higher fatty acid amide and formaldehyde and condensate of higher fatty acid and ethylene diamine), there can be cited 
     condensate of stearic acid amide and formaldehyde (methylene-bis-stearoamide C 17  H 35  CONHCH 2  NHCOC 17  H 35 ), 
     condensate of stearic acid amide and formaldehyde (methylol stearoamide C 17  H 35  CONHCH 2  OH), 
     condensate of palmitic acid amide and formaldehyde (methylol palmitoamide C 15  H 31  CONHCH 2  OH), 
     condensate of stearic acid and ethylene diamine (ethylene-bis-stearoamide C 17  H 35  CONHCH 2  CH 2  NHCOC 17  H 35 ), 
     condensate of lauric acid and ethylene diamine (ethylene-bis-laurylamide C 11  H 23  CONHCH 2  CH 2  NHCOC 11  H 23 ), etc., 
     but the applicable wax is not limited to the foregoing providing the melting point should be in the range of from 80° to 150° C. 
     As the ingredient D (i.e., a kind of wax selected from the group consisting of vegetable waxes and mineral waxes), there can be cited carnauba wax, sugar cane wax and cotton wax collected directly from vegetables, montan wax extracted from brown coal resulting from the partial decomposition of vegetable matter and their modifications. 
     The color-developable substance (i.e., ingredient A) for use in the present invention which is normally colorless or light-colored is a leuco-base such as triphenyl methane, fluorane, phenothiazine, Auramine, spiropyrane, etc., and to cite concrete examples, there are 
     3,3-bis(p-dimethyl aminophenyl)-6-dimethyl aminophthalide, 
     3,3-bis(p-dimethyl aminophenyl)phthalide, 
     3,3-bis(p-dimethyl aminophenyl)-6-diethyl aminophthalide, 
     3,3-bis(p-dimethyl aminophenyl)-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&#39;-chloro-8&#39;-methoxybenzoindolino-pyrylospiran, 
     6&#39;-bromo-8&#39;-methoxy-benzoindolino-pyrylospiran, 
     2-[3,6-bis(diethyl amino)-9-(O-chloroanilino)xanthyl]benzoic lactam, 
     2-{N-(3&#39;-trifluoromethyl phenyl)amino}-6-diethyl aminofluorane, etc. 
     but the applicable substance is not limited to the foregoing. 
     As the organic acid substance (i.e., ingredient B) for use in the present invention, there can be cited α-naphthol, β-naphthol, 4-t-butyl phenol, 4-t-octyl phenol, 4-phenyl phenol, 2,2-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)butane, 4,4-cyclohexilidene diphenol, 2,2-bis(2,5-dibromo-4-hydroxyphenyl)propane, 4,4&#39;-isopropylidene-bis(2-t-butyl phenol), 2,2-methylene-bis(4-chlorophenol), etc., but the applicable substance is not limited to the foregoing. 
     A heat-sensitive recording sheet prepared by forming a heat-sensitive layer comprising the aforementioned ingredient A, ingredient B, ingredient C, ingredient D and an inorganic filler (hereinafter referred to as ingredient E) such as kaolin, calcium carbonate, etc. is admittedly well servable for practical use, but the present inventors have conducted further examinations and come to the finding that, mixing of a binder composed of at least one member selected from the group consisting of starches and their derivatives together with dialdehyde starch in the heat-sensitive layer can prevent the sticking of fused composite onto the thermal head more effectively and enhance the color developability. 
     In this connection, Japanese Patent Publication No. 31500/1956 discloses a heat-sensitive layer comprising (a) a color-developable substance, (b) biphenyls, (c) a water-soluble bonding agent containing carboxyl radical, (d) a water-resisting agent for making said bonding agent waterproof, (e) powdery starch, (f) a petroleum wax and (g) a higher fatty acid amide type wax together with a metallic salt of higher fatty acid, but a heat-sensitive layer of such a composition cannot produce a satisfactory heat-sensitive recording sheet. 
     On the contrary, when a binder (i.e., ingredient F) such as described in the foregoing is applied, there can be obtained a very satisfactory heat-sensitive recording sheet. As the starch or derivative thereof for use in the present invention, there can be cited maize starch, wheat starch, sweet potato starch and oxidized starches obtained by oxidizing these starches with sodium hypochlorite. Further, the aforesaid dialdehyde starch can be easily obtained by oxidizing these starches with periodate. 
     In order to prepare a heat-sensitive recording sheet according to the present invention, the ingredient A, ingredient B, ingredient C, ingredient D and an inorganic filler as ingredient E (as the filler for use herein, clay, calcium carbonate, magnesium carbonate, kaolin, etc. are applicable, but it is preferable to use kaolin or calcium carbonate having particle size of 0.05 to 5.0 μm) are dispersed, individually or upon mixing together excepting ingredient A, by means of a disperser such as ball-mill, attriter, sand-mill, etc. after adding a protective colloidal substance, a surface active agent, etc. and, if necessary, further adding a binder (i.e., ingredient F) thereto, whereby a heat-sensitive layer forming liquid is prepared. Besides, this heat-sensitive layer forming liquid can be mixed with some defoaming agent, wetting agent, and further organic pigment, bridging agent, etc. 
     Referring to the appropriate mixing ratio of these ingredients C, D and E on this occasion, as for ingredient C, it is in the range of from 0.1 to 5.0 times, preferably from 0.2 to 3.0 times, as much as ingredient A by weight, as for ingredient D, it is in the range of from 0.2 to 6.0 times, preferably from 0.2 to 4.0 times, as much as ingredient A by weight, as for ingredient E, it is in the range of from 0.5 to 5.0 times as much as ingredient A by weight, and as for ingredient F, it is in the range of from 10 to 50% by weight of the heat-sensitive layer. 
     At the time of using ingredient F in preparing this heat-sensitive layer forming liquid, it is preferable to apply it upon converting into paste through the process of dispersing a starch (inclusive of derivatives of starch) in water and heating the resulting dispersion up to a temperature of 0° to 95° C. And, in the case of dialdehyde starch which is difficult to dissolve under the same conditions as that in the case of converting ordinary starches into paste, it is advisable to apply it upon converting into paste through the process of adding a buffer solution made of sodium acetate, borax or the like and heating the resulting mixture up to a temperature of 80° to 95° C. 
     Subsequently, the thus obtained heat-sensitive layer forming liquid is coated on a support to the extent of 4 to 10 g/m 2  in terms of solid matter and is dried. It is more advantageous to subject the heat-sensitive layer of the resulting heat-sensitive recording sheet to the surface treatment so as to attain the Beck&#39;s smoothness of 1,000 to 1,800 seconds. 
     That is, when the heat-sensitive layer is subjected to the surface treatment with a gloss calendar, a super calendar or the like so as to attain the Beck&#39;s smoothness of 1,000 to 1,800 seconds, the heat-responsiveness and the recording fidelity improve remarkably in the thermal pen recorder, high speed serial printer, etc. in particular. Even when the Beck&#39;s smoothness is in the range of from 100 to 500 seconds or thereabouts, the heat-sensitive layer displays a superior heat-responsiveness, and yet by virtue of the surface treatment to attain the Beck&#39;s smoothness of more than 1,000 seconds, the recording fidelity is further enhanced. However, if the Beck&#39;s smoothness exceeds 1,800 seconds, the ground of the heat-sensitive recording sheet will be exposed under pressure and becomes excessively transparent, thereby impairing the commercial value thereof. 
     A heat-sensitive recording sheet of the present invention does not feature a mere accumulation of the effects of individual materials constituting the heat-sensitive layer thereof, but it is characterized in that a synergistic effect of all ingredients can be manifested, thereby accomplishing the intended objects. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     Two varieties of mixtures having the following composition respectively were pulverized within a magnetic ball-mill for a day, whereby liquids [A 1  ] and [E 1  ] were prepared. 
     Composition of liquid [A 1  ] 
     
         ______________________________________    2-{N-(3&#39;-trifluoromethyl phenyl)amino}-6-                            150 g    diethyl aminofluorane    5% aqueous solution of polyvinyl alcohol                            150 g    (POVAL 205, the manufacture of KURARE K.K.)    water                        200 g______________________________________ 
    
     Composition of liquid [E 1  ] 
     
         ______________________________________ talc                     150 g 5% aqueous solution of methyl cellulose                          150 g (METHOLOSE SM-15, the manufacture of SHINETSU KAGAKU K.K.) water                    200 g______________________________________ 
    
     Further, 3 varieties of mixtures having the following composition respectively were pulverized within a laboratory attriter for 2 hours, whereby liquids [B 1  ], [C 1  ] and [D 1  ] were prepared. 
     Composition of liquid [B 1  ] 
     
         ______________________________________ 2,2&#39;-bis(p-hydroxyphenyl)propane                          120 g 5% aqueous solution of polyvinyl alcohol                          120 g water______________________________________ 
    
     Composition of liquid [C 1  ] 
     
         ______________________________________    condensate of stearic acid amide and formaldehyde                            80 g    (in the presence of basic catalyst,    C.sub.17 H.sub.35 CONHCH.sub.2 OH)    5% aqueous solution of methyl cellulose                            80 g    nonionic surface active agent                            2 g    (adduct of alkylphenyl to ethylene oxide:    NOIGEN EA-80, the manufacture of DAIICHI    KOGYO SEIYAKO K.K.)    water                        238 g______________________________________ 
    
     Composition of liquid [D 1  ] 
     
         ______________________________________ carnauba wax              80 g 5% aqueous solution of methyl cellulose                           80 g nonionic surface active agent                           2 g water                     238 g______________________________________ 
    
     Subsequently, a heat-sensitive layer forming liquid was prepared by mixing these liquids [A 1  ] through [E 1  ] with 20% aqueous solution of polyvinyl alcohol (namely, POVAL 205, the manufacture of KURARE K.K.) at the following ratio. 
     
         ______________________________________liquid [A.sub.1 ]          20 gliquid [B.sub.1 ]          40 gliquid [C.sub.1 ]          60 gliquid [D.sub.1 ]          80 gliquid [E.sub.1 ]          40 g20% aqueous solution of polyvinyl alcohol                      115 g______________________________________ 
    
     Then, this heat-sensitive layer forming liquid was coated on a commercial slick paper (weighing 53 g/m 2 ) to the extent of 7 to 8 g/m 2  in terms of dry solid matter by means of a wire bar, and was dried. Thereafter, the thus coated stick paper was further subjected to the surface treatment with a calendar so as to attain the Beck&#39;s smoothness of 200 to 400 seconds, whereby a heat-sensitive recording sheet was prepared. 
     When this heat-sensitive recording sheet was tested, the result was as shown in the following Table-1. As is evident from Table-1, this heat-sensitive recording sheet was high in heat-responsiveness, free from such phenomena as sticking of fused composite, adhesion of residuary substances, tailing of recorded image, ghaust, etc., and superior in head-matching property. 
     In this context, the method of test applied was as follows. 
     (1) The test of the heat-responsiveness was conducted by the use of heat gradient Tester equipped with a heated iron piece (the manufacture of Toyo SEIKI K.K. ) and through the procedure that said iron piece as regulated to have a constant temperature was kept in contact with the heat-sensitive recording sheet under a pressure of 2.0 Kg/cm 2  for 1 hour to make it develop a color, the density of the color thus developed was measured with Macbeth&#39;s reflection densitometer, the saturated density thus obtained was expressed by Dmax., and the heat-responsiveness was expressed by the temperature at the time of attaining a density of 0.80 based on this Dmax. This means that the higher is said temperature, the more superior is the heat-responsiveness. 
     (2) The sticking of fused composite was examined by conducting solid printing with the overall width of the thermal head, and as for the adhesion of residuary 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 the use of Macbeth&#39;s densitometer. 
     EXAMPLE 2 
     A mixture having the following composition was pulverized within a laboratory attriter for 2 hours, whereby liquid [C-D] was prepared. 
     Composition of liquid [C-D] 
     
         ______________________________________    montan wax modification      40 g    (S-Wax, the manufacture of HOECHST    JAPAN COMPANY)    condensate of stearic acid and ethylene diamine                            40 g    (C.sub.17 H.sub.35 CONHCH.sub.2 CH.sub.2 NHCOC.sub.17 H.sub.35)    5% aqueous solution of methyl cellulose                            80 g    nonionic surface active agent                            2 g    water                        238 g______________________________________ 
    
     Subsequently, a heat-sensitive layer forming liquid was prepared by combining this liquid [C-D], liquids [A 1  ], [B 1  ] and [E 1  ] of Example 1 and 20% aqueous solution of polyvinyl alcohol at the following ratio, and by applying the same procedure as in Example 1, a heat-sensitive recording sheet was prepared. 
     
         ______________________________________ liquid [A.sub.1 ]         20 g liquid [B.sub.1 ]         40 g liquid [C-D]              140 g liquid [E.sub.1 ]         40 g 20% aqueous solution of polyvinyl alcohol                           115 g______________________________________ 
    
     When this heat-sensitive recording sheet was subjected to the same test as in Example 1, the result was as shown in Table-1, attesting to its superior quality as heat-sensitive recording sheet. 
     COMPARATIVE EXAMPLE 1 
     A mixture having the following composition was pulverized within a laboratory attriter for 2 hours, whereby liquid [C 2  ] was prepared. 
     Composition of liquid [C 2  ] 
     
         ______________________________________ stearic acid amide        80 g 5% aqueous solution of methyl cellulose                           80 g nonionic surface active agent                           2 g water                     238 g______________________________________ 
    
     Subsequently, a heat-sensitive layer forming liquid was prepared by using this liquid [C 2  ] together with liquids [A 1  ], [B 1  ] and [E 1  ] of Example 1 at the following ratio, and then by applying the same procedure as in Example 1, a heat-sensitive recording sheet was prepared. 
     
         ______________________________________    liquid [A.sub.1 ]            20 g    liquid [B.sub.1 ]            40 g    liquid [C.sub.2 ]            60 g    liquid [E.sub.1 ]            40 g    polyethylene wax emulsion (solid content: 30%)                            53 g    (namely, POLYLON A, the manufacture of    CHUKYO YUSHI K.K.)    2% aqueous solution of polyvinyl alcohol                            115 g______________________________________ 
    
     When this heat-sensitive recording sheet was subjected to the same test as in Example 1, the result was as shown in Table-1. 
     COMPARATIVE EXAMPLES 2 THROUGH 8 
     Seven varieties of comparative heat-sensitive recording sheets were prepared by applying the same procedure as in Example 1 save for changing the wax (ingredient C and/or ingredient D). When these heat-sensitive recording sheets were subjected to the same test as in Example 1, the result was as shown in Table-1, respectively. 
     
                                           TABLE 1__________________________________________________________________________                     Result of test on quality                            Head-matching Property                                   Adhe-   Ghaust;                                           -  Heat- Re-  sion                                           of Tail- dis-                        re- cord-  resid-                                       ing of                                           tinct-  Ingredient C          spon-                            ing    uary                                       rec-                                           ness of  and/or             D  sive-                            den-                               Stick-                                   sub-                                       orded                                           recorded  Ingredient D       max.                        ness                            sity                               ing stance                                       image                                           image__________________________________________________________________________  condensate of stearic acidExample 1  amide and formaldehyde and                     1.15                        91° C.                            1.11                               ⊚                                   ⊚                                       ⊚                                           ⊚  carnauba was  condensate of stearic acidExample 2  and ethylene diamine and                     1.16                        98 ° C.                            1.05                               ⊚                                   ⊚                                       ⊚                                           ⊚  montan wax modificationComparativeExample 1  stearic acid amide and polyethylene wax                     1.16                        91° C.                            1.11                               ○                                   ○Δ                                       ○                                           ΔComparativeExample 2  carnauba wax       1.16                        120° C.                            0.90                               ○                                   ○                                       ⊚                                           ○Δ  condensate of srearic acidComparative  amide and formaldehyde                     1.16                        91°  C.                            1.11                               Δ                                   ○Δ                                       Δ                                           ○ΔExample 3  (C.sub.17 H.sub.35 CONHCH.sub.2 OH)ComparativeExample 4  stearic acid amide 1.17                        90° C.                            1.12                               x   Δ                                       Δ                                           ΔComparativeExample 5  calcium stearate   1.17                        125° C.                            0.85                               x   x   x   xComparativeExample 6  polyethylene wax   1.14                        110° C.                            0.95                               ○ Δ                                   Δ                                       ○                                           ○ΔComparativeExample 7  montan wax midification                     1.15                        120° C.                            0.93                               ○                                   ○                                       ⊚                                           ○ΔComparative  parrafin wax andExample 8  C.sub.17 H.sub.35 CONHCH.sub.2 OH                     1.15                        91° C.                            1.10                               ○                                   ○Δ                                       ○                                           Δ__________________________________________________________________________ 
    
     (Remark) 
     Marks  o ,    ,  Δ , Δ and x represent the values as graded from &#34;satisfactory&#34; down to &#34;unsatisfactory&#34;. 
     EXAMPLE 3 
     Two varieties of mixtures having the following composition respectively were pulverized within a magnetic ball-mill for 1 day, whereby liquids [A 2  ] and [E 2  ] were prepared. 
     Composition of liquid [A 2  ] 
     
         ______________________________________ 3,3-bis(p-dimethyl aminophenyl)-6-dimethyl                           150 g aminophenyl 5% aqueous solution of polyvinyl alcohol                           150 g (namely, POVAL 205, the manufacture of KURARE K.K.) waer                      200 g______________________________________ 
    
     Composition of liquid [E 2  ] 
     
         ______________________________________    kaolin                       150 g    5% aqueous solution of methyl cellulose                            150 g    (namely, METHOLOSE SM-15, the manufacture    of SHINETSU KAGAKU K.K.)    water                        200 g______________________________________ 
    
     Subsequently, a heat-sensitive layer forming liquid was prepared by compounding these liquids [A 2  ] and [E 2  ], liquids [B 1  ], [C 1  ] and [D 1  ] of Example 1, 20% aqueous solution of oxidized starch and 10% aqueous solution of dialdehyde starch at the following ratio. 
     
         ______________________________________ liquid [A.sub.2 ]         20 g liquid [B.sub.1 ]         40 g liquid [C.sub.1 ]         60 g liquid [D.sub.1 ]         80 g liquid [E.sub.2 ]         40 g 20% aqueous solution of oxidized starch                           60 g 10% aqueous solution of dialdehyde starch                           120 g______________________________________ 
    
     Then, this heat-sensitive layer forming liquid was coated on a commercial slick paper (weighing 53 g/m 2 ) to the extent of 5 to 6 g/m 2  in terms of dry solid matter by means of a wire bar and was dried. Thereafter, the thus coated slick paper was further subjected to the surface treatment with a calendar so as to attain the Beck&#39;s smoothness of 1,100 to 1,200 seconds, whereby a heat sensitive recording sheet was prepared. 
     When this heat-sensitive recording sheet was tested, the result was as shown in the following Table-2. As is evident from Table-2, this heat-sensitive recording sheet proved to be superior in heat-responsiveness, high-speed recording property and fidelity in recording, and free from causing the sticking of fused composite to the thermal head and the abrasion of the thermal head. 
     In Table-2, there were collectively shown the case wherein a heat-sensitive recording sheet was subjected to the surface treatment with a calendar to the extent of the Beck&#39;s smoothness of 500 to 550 seconds as Example 4 and the case wherein the same recording sheet was subjected to the surface treatment with a calendar to the extent of the Beck&#39;s smoothness of 300 to 400 seconds as Example 5. 
     In this context, the method of test applied was as follows: 
     (1) The heat-responsiveness was evaluated in the same way as in Example 1. 
     (2) The high-speed recording property and the recording fidelity were evaluated by comparing the result of the color-developability test conducted on each recording sheet by using a linear recorder equipped with an IC pen (namely, Thermal Pen Recorder, the manufacture of WATANABE SOKKI SEISAKUSHO, Ltd.) applying 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 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 terms of kilometer made by the IC pen until its resistance increased by about 10% to deteriorate the drawing efficiency thereof. 
     (4) The sticking of fused composite onto the thermal head and the distinctness of recorded image were evaluated by comparing the result of the test conducted on each recording sheet by employing a trial printer equipped with a segment-type thick film head (manufactured by NIPPON TOKI K.K.) and a baby printer PC-100 (the manufacture of TEXAS INSTRUMENT Inc.) for use in portable electronic computer. 
     (5) The water-resisting property was 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__________________________________________________________________________                     Result of test on quality                         High                         speed                             Re-                                Stick                     Heat-                         re- cord-                                ing    Water-                     re- cord-                             ing                                to Abra-                                       resist-Ingredient C Ingre-            Ingre-   spon-                         ing fi-                                ther-                                   sion                                       ingand          dient            dient                Smooth-                     sive-                         prop-                             del-                                mal                                   hard-                                       prop-Ingredient D E   F   ness ness                         erty                             ity                                head                                   ness                                       erty__________________________________________________________________________            oxi- condensate of            dized stearic acid            starch,                1,100- amide and        kaolin            di- 1,200Example 3 formaldehyde            alde-                sec. 90° C.                         ⊚                             ⊚                                ⊚                                   500 ⊚ and carnauba            hyde wax        starch            oxi- condensate of            dized                500- stearic acid            starch                550  98° C.                         ○                             Δ                                ⊚                                   400 ⊚Example 4 amide and        kaolin            di- sec- formaldehyde            alde- and carnauba            hyde wax        starch condensate of            oxi-                300-Example 5 stearic acid            dized                400  90° C.                         ○                             Δ                                ○Δ                                   180 Δ amide and        kaolin            starch,                sec. formaldehyde            di- and carnauba            alde- wax        hyde            starch__________________________________________________________________________