Novel thermosensitive recording sheet

A novel thermosensitive recording sheet having a coated film containing a color-forming lactone compound, an acidic substance and a sensitizer, said sensitizer being an o-hydroxy aromatic acid ester represented by formula ##STR1## wherein R.sub.1 denotes an alkylene group, a cycloalkylene group, an oxyalkylene group, a thioalkylene group, a phenylene group, a xylylene group or a napthylene group, R.sub.2 denotes ##STR2## R.sub.3 denotes ##STR3## in which X denotes a hydroxy group or a hydrogen atom, provided a benzene ring and a naphthalene ring in R.sub.1, R.sub.2 and R.sub.3 may be substituted by a lower alkyl group, a nitro group or a halogen atom.

This invention relates to a thermosensitive recording sheet, and more 
specifically, to a thermosensitive recording sheet having a coated film 
containing a color-forming lactone compound, an acidic substance and a 
specific o-hydroxy aromatic acid ester as a sensitizer. 
Thermosensitive recording sheets are designed to display images such as 
characters and geometric figures by thermal energy, and have recently 
found applications in various printer recorders, facsimiles, POS labels, 
automatic ticket examination, etc. There are various methods of 
thermosensitive recording. From the viewpoint of the clearness, resolution 
and color of images, the most prevalent method is to use a color-forming 
lactone compound such as Crystal Violet Lactone (CVL) which is a dye 
precursor and an acidic substance capable of causing the lactone compound 
to form a color. In this method, a phenolic compound such as bisphenol A 
which is solid at room temperature but upon heating, is melted and acts as 
an acid component has previously been used as the acidic substance. 
Usually, to obtain a brilliant color, the sheets must be maintained at a 
temperature of about 140.degree. to 150.degree. C. for a period of time 
above a certain limit. Hence, various approaches have been made in order 
to obtain brilliant colors more rapidly and more easily. For example, 
there are a method in which stearamide is added as a sensitizer (Japanese 
Laid-Open Patent Publication No. 139740/1979), and a method in which 
benzyl p-hydroxybenzoate is used as the acidic substance (Japanese 
Laid-Open Patent Publication No. 74762/1979). The methods described in 
these patent documents are still not entirely satisfactory to meet the 
latest demands for higher speed of recording and energy saving although 
they can increase color-forming sensitivity. 
It is an object of this invention to remedy the foregoing drawbacks in the 
prior art. 
The present inventors have made studies to achieve the object and as a 
result, discovered that use of a specific hydroxy aromatic acid ester as a 
sensitizer can give a thermosensitive recording sheet having a markedly 
high color-forming sensitivity than in the case of using conventional 
sensitizers. 
According to this invention, there is provided a thermosensitive recording 
sheet having a coated film containing a color-forming lactone compound, an 
acidic substance and a sensitizer, said sensitizer being an o-hydroxy 
aromatic acid ester represented by formula (I) 
##STR4## 
wherein R.sub.1 denotes an alkylene group, a cycloalkylene group, an 
oxyalkylene group, a thioalkylene group, a phenylene group, a xylylene 
group or a naphthylene group, R.sub.2 denotes 
##STR5## 
R.sub.3 denotes 
##STR6## 
in which X denotes a hydroxy group or a hydrogen atom, provided a benzene 
ring and a naphthalene ring in R.sub.1, R.sub.2 and R.sub.3 may be 
substituted by a lower alkyl group, nitro group or a halogen atom. 
Examples of an alcohol component as one component constituting the 
o-hydroxy aromatic acid ester of formula (I) include aliphatic diols such 
as ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, 
1,4-butylene glycol, 1,5-pentamethylene glycol, 1,6-hexamethylene glycol, 
hexyleneglycol(2-methyl-2,4-pentanediol), 
neopentylglycol(2,2-dimethyl-1,3-propanediol), 1,3-butylene glycol, 
1,4-cyclohexanediol and pinacol(tetramethylene glycol); aliphatic oxydiols 
such as diethylene glycol, dipropylene glycol, triethylene glycol and 
tripropylene glycol; aliphatic thiodiols such as 
thiodiethyleneglycol-(2,2'-thiodiethanol) and 
ethylenebis-(2-hydroxyethylsulfide); and aromatic diols such as 
resorcinol(m-dihydroxybenzene), catechol(o-dihydroxybenzene), 
hydroquinone(p-dihydroxybenzene) and p-xylylene glycol. Examples of acid 
component as the other component constituting the o-hydroxy aromatic ester 
of formula (I) include o-hydroxy aromatic carboxylic acids such as 
salicylic acid, o-cresotinic acid, 5-chlorosalicylic acid, 
1-hydroxy-2-naphthoic acid, 3-hydroxy-2-naphthoic acid and 
1-hydroxy-4-chloro-2-naphthoic acid; and carboxylic acids such as benzoic 
acid, toluic acid, anisic acid, chlorobenzoic acid, nitrobenzoic acid and 
naphthoic acid. 
Among these o-hydroxy aromatic acid esters, those having a boiling point of 
50.degree. to 140.degree. C. are preferable because they are excellent in 
stability at room temperature and color-forming sensitivity in heating. 
Specific examples include the following o-hydroxy aromatic acid esters (1) 
to (23). 
(1) Ethyleneglycol di(salicylate) 
(2) 1,3-Propyleneglycol di(salicylate) 
(3) 1,2-propyleneglycol di(salicylate) 
(4) 1,4-Butyleneglycol salicylate-p-toluylate 
(5) 1,4-Butyleneglycol di(salicylate) 
(6) Ethyleneglycol salicylate benzoate 
(7) Ethyleneglycol salicylate-1-hydroxy-2-naphthoate 
(8) 1,5-Pentamethyleneglycol di(salicylate) 
(9) 1,5-Pentamethyleneglycol salicylate-1-hydroxy-2-naphthoate 
(10) 1,6-Hexamethyleneglycol di(salicylate) 
(11) Ethyleneglycol di(5-chlorosalicylate) 
(12) Diethyleneglycol di(salicylate) 
(13) Diethyleneglycol salicylate-1-hydroxy-2-naphthoate 
(14) Diethyleneglycol di(1-hydroxy 2-naphthoate) 
(15) Triethyleneglycol di(1-hydroxy-2-naphthoate) 
(16) Thiodiethyleneglycol di(salicylate) 
(17) Neopentylglycol di(1-hydroxy-2-naphthoate) 
(18) Resorcinol di(salicylate) 
(19) Resorcinol salicylate-p-toluylate 
(20) p-Xylyleneglycol di(salicylate) 
(21) Ethyleneglycol di(o-cresotiate) 
(22) Ethyleneglycol di(m-cresotiate) 
(23) 1,4-Cyclohexanediol di(salicylate) 
Of these, the o-hydroxy aromatic acid esters (1), (2), (3), (6), (10), 
(16), (21) and (22) are most preferable. 
Examples of the color-forming lactone compound used in this invention 
include fluoranphthalides such as 3,3-bis(p-dimethylaminophenyl)phthalide, 
3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (also known as 
Crystal Violet Lactone; CVL for short), 
3,3-bis(p-dimethylaminophenyl)-6-aminophthalide, 
3,3-bis(p-dimethylaminophenyl)-6-nitrophthalide, 
3,3-bis(p-dimethylaminophenyl)phthalide, 
3,3-bis-3-dimethylamino-7-methylfluoran, 3-diethylamino-7-chlorofluoran, 
3-diethylamino-6-chloro-7-methylfluoran, 3-diethylamino-7-anilinofluoran, 
3-diethylamino-6-methyl-7-anilinofluoran, 
3-piperidino-6-methyl-7-anilinofluoran, 
3-(N-ethyl-p-toluidino)-7-(N-methylanilino)fluoran, 
3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran 
3-N-ethyl-N-isoamylamino-6-methyl-7-anilinofluoran, 
3-N-methyl-N-cyclohexylamino-6-methyl-7-anilinofluoran and 
3-N,N-diethylamino-7-o-chloroanilinofluoran; lactams such as Rhodamin B 
lactam; and spiropyrans such as 3-methylspirodinaphtopyran, 
3-ethylspirodinaphthopyran and 3-benzylspironaphthopyran. These compounds 
should be colorless or pale-colored and react with acidic substances to 
form colors. 
The acidic substance used in this invention may be any acidic substance 
which is solid at room temperature and when heated to about 60.degree. to 
180.degree. C., is melted and opens the lactone ring of the color-forming 
lactone compound. It functions well in the presence of sensitizers. 
Examples of the acidic substance include 4-phenylphenol, 
4-hydroxyacetophenone, 2,2'-dihydroxydiphenyl, 
2,2'-methylenebis(4-chlorophenol), 
2,2'-methylenebis(4-methyl-6-t-butylphenol), 4,4'-isopropylidenediphenol 
(also known as bisphenol A), 4,4'-isopropylidenebis(2-chlorophenol), 
4,4'-isopropylidenebis(2-methylphenol), 4,4'-ethylenebis(2-methylphenol), 
4,4'-thiobis(6-t-butyl-3-methylphenol), 
1,1-bis(4-hydroxyphenyl)-cyclohexane, 2,2'-bis(4-hydroxyphenyl)-n-heptane, 
4,4'-cyclohexylidenebis(2-isopropylphenol), 4,4'-sulfonyldiphenol, 
salicylanilide, novolak-type phenolic resin and benzyl p-hydroxybenzoate. 
The acidic substance is used in an amount of usually 10 to 1,000 parts by 
weight (all parts hereinafter are by weight), preferably 100 to 500 parts, 
per 100 parts of the color-forming lactone compound. 
The sensitizer is used in an amount of usually 1 to 1,000 parts, preferably 
30 to 100 parts, per 100 parts of the acidic substance. 
The color-forming lactone compound, the acidic substance and the sensitizer 
are used in the form of fine particles, preferably fine particles having a 
particle diameter of less than several microns. 
Various known methods can be used to produce the thermosensitive recording 
sheet. Usually, there may be used (1) a method which comprises preparing a 
coating dispersion of the color-forming lactone compound, the acidic 
substance and the sensitizer in water, and coating the coating dispersion 
on a sheet substrate, and (2) a method which comprises dispersing the 
color-forming lactone compound and the acidic substance separately in 
water, including the sensitizer into at least one of the aqueous coating 
dispersions, and coating the coating dispersions in superimposed relation 
on a sheet substrate. An aqueous binder should be added to the coating 
dispersions. Examples of the binder are polyvinyl alcohol, methyl 
cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, starches, and 
styrene/maleic acid copolymer. Besides, ultraviolet absorbers (for 
example, benzophenone compounds and triazole compounds) fillers such as 
calcium carbonate, lubricants such as polyethylene wax and paraffin wax, 
agents for imparting water resistance, and other various chemicals may be 
added to the coating dispersions in order to improve their performance. 
Various dispersing agents for dispersing the various chemicals in the 
above coating dispersions may also be added. 
The coating dispersion is coated on a sheet substrate so that its dry 
weight becomes generally 2 to 12 g per m.sup.2 of the sheet substrate, and 
then dried at room temperature to about 50.degree. C. to give the 
thermosensitive recording sheet of the invention. 
Paper is generally used as the sheet substrate, but plastic sheets and 
nonwoven sheets may also be used. 
The thermosensitive recording sheet of this invention has very high 
color-forming sensitivity and its colored and non-colored portions have 
excellent stability.

The following Examples, Comparative Examples and Test Example illustrate 
the present invention more specifically. It should be understood that the 
invention is not limited at all by these examples. All parts and 
percentages in these examples are by weight. 
EXAMPLE 1 
______________________________________ 
Dispersion A (containing a dye) 
______________________________________ 
3-(N--isoamyl-N--ethylamino)-6-methyl- 
1.0 part 
7-anilinofluoran 
o-Hydroxy aromatic acid ester (1) 
2.0 parts 
[ethyleneglycol (disalicylate)] 
10% Aqueous solution of polyvinyl 
3.0 parts 
alcohol 
Water 5.0 parts 
Total 11.0 parts 
______________________________________ 
Dispersion B (containing an acidic substance) 
______________________________________ 
bisphenol A 3.0 parts 
Calcium carbonate 3.0 parts 
Zinc stearate 0.5 part 
10% Aqueous solution of polyvinyl alcohol 
7.0 parts 
Water 10.0 parts 
Total 23.5 parts 
______________________________________ 
Dispersions A and B were separately prepared by mixing the indicated 
ingredients and pulverizing and dispersing them by a paint conditioner. 
Subsequently, 11.0 parts of dispersion A and 23.5 parts of dispersion B 
were mixed to form a thermosensitive coating dispersion. It was coated on 
high-quality paper at a rate of 64.5 g/m.sup.2 so that its amount upon 
drying became 8 g/m.sup.2, and then dried to obtain a thermosensitive 
recording sheet in accordance with this invention. 
This sheet had excellent color-forming sensitivity and excellent stability 
at the colored and non-colored portions. 
EXAMPLES 2-23 
Thermosensitive recording sheets in accordance with this invention were 
prepared in the same way as in Example 1 except that the o-hydroxy 
aromatic acid esters (2) to (23) were used instead of the o-hydroxy 
aromatic acid ester (1). 
These sheets had excellent color-forming sensitivity and excellent 
stability at the colored and noncolored portions. 
EXAMPLE 24 
A thermosensitive recording sheet in accordance with this invention was 
prepared in the same way as in Example 1 except that the amount of the 
o-hydroxy aromatic acid ester (1) added was changed to 1.0 part and the 
amount of dispersion A was changed to 10 parts. 
The sheet had excellent color-forming sensitivity and excellent stability 
at the colored and non-colored portions. 
EXAMPLE 25 
A thermosensitive recording sheet in accordance with this invention was 
prepared in the same way as in Example 1 except that the amount of the 
o-hydroxy aromatic acid ester (1) added was changed to 4.0 parts and the 
amount of dispersion A used was changed to 13 parts. 
The sheet had excellent color-forming sensitivity and excellent stability 
at the colored and non-colored portions. 
COMATIVE EXAMPLE 1 
A thermosensitive recording sheet for comparison was prepared in the same 
way as in Example 1 except that the o-hydroxy aromatic acid ester (1) was 
not added and the amount of dispersion A used was changed to 9.0 parts. 
The sheet had inferior color-forming sensitivity. 
COMATIVE EXAMPLE 2 
A thermosensitive recording sheet for comparison was prepared in the same 
way as in Example 1 except that stearamide was used instead of the 
o-hydroxy aromatic acid ester (1). 
The sheet was inferior in color-forming sensitivity and stability at the 
colored and non-colored portions. 
COMATIVE EXAMPLE 3 
A thermosensitive recording sheet for comparison was prepared in the same 
way as in Example 1 except that the o-hydroxy aromatic ester (1) was not 
added, the amount of dispersion A used was changed to 9 parts and benzyl 
p-hydroxybenzoate was used instead of bisphenol A. 
The sheet was inferior in color-forming sensitivity and stability at the 
colored portion. 
TEST EXAMPLE 1 
The thermosensitive recording sheets obtained in Examples 1 to 25 and 
Comparative Examples 1 to 3 were tested for dynamic image density, and the 
stability of the colored and non-colored portions by methods described 
below. The results are shown in Tables 1 and 2. 
Measurment of the dynamic image density 
An image was printed on the thermosensitive recording sheet by means of a 
thermal head printing device (Model MSI, made by Matsushita Electronic 
Components Co., Ltd.) with a pulse width of 0.5 millisecond, and the 
density of the image was measured by a Macbeth densitometer (RD-918, made 
by Macbeth Co., U.S.A.). 
Evaluation of the stability of the colored and non-colored portions 
The sheet was pressed against a hot plate at 140.degree. C. under a 
pressure of 2.0 kg/cm.sup.2 for 1 second. The colored portion and the 
remaining non-colored portion were left to stand at 40.degree. C. and 90% 
RH for 24 hours. The degree of whitening or background fog of the colored 
and non-colored portions was visually observed and evaluated on the 
following scale. 
(1) Scale of evaluation of the colored portion 
.circleincircle. : No whitening 
.circle. : Hardly any whitening 
.DELTA.: Whitening occurred 
X: Marked whitening 
(2) Scale of evaluation of the non-colored portion 
.circleincircle. : No background fog 
.circle. : Hardly any background fog 
.DELTA.: Background fog occurred 
X: Marked background fog 
TABLE 1 
______________________________________ 
Dynamic Stability Stability of 
image of colored 
non-colored 
density portion portion 
______________________________________ 
Example 1 1.20 .circleincircle. 
.circleincircle. 
Example 2 1.23 .circleincircle. 
.circleincircle. 
Example 3 1.15 .circleincircle. 
.circleincircle. 
Example 4 1.05 .circleincircle. 
.circleincircle. 
Example 5 1.05 .circleincircle. 
.circleincircle. 
Example 6 1.12 .circleincircle. 
.circleincircle. 
Example 7 1.02 .circleincircle. 
.circleincircle. 
Example 8 1.06 .circleincircle. 
.circle. 
Example 9 1.02 .circleincircle. 
.circleincircle. 
Example 10 
1.25 .circleincircle. 
.circleincircle. 
Example 11 
1.08 .circleincircle. 
.circleincircle. 
Example 12 
1.12 .circleincircle. 
.circle. 
Example 13 
1.02 .circleincircle. 
.circleincircle. 
Example 14 
1.00 .circleincircle. 
.circleincircle. 
Example 15 
1.02 .circleincircle. 
.circleincircle. 
______________________________________ 
TABLE 2 
______________________________________ 
Dynamic Stability Stability of 
image of colored 
non-colored 
density portion portion 
______________________________________ 
Example 16 1.17 .circleincircle. 
.circleincircle. 
Example 17 1.05 .circleincircle. 
.circleincircle. 
Example 18 0.98 .circleincircle. 
.circle. 
Example 19 1.09 .circleincircle. 
.circleincircle. 
Example 20 1.08 .circleincircle. 
.circleincircle. 
Example 21 1.20 .circleincircle. 
.circleincircle. 
Example 22 1.15 .circleincircle. 
.circle. 
Example 23 1.01 .circleincircle. 
.circleincircle. 
Example 24 1.02 .circleincircle. 
.circleincircle. 
Example 25 1.26 .circleincircle. 
.circleincircle. 
Comparative 
0.42 .circle. .circleincircle. 
Example 1 
Comparative 
0.80 .DELTA. .DELTA. 
Example 2 
Comparative 
0.78 X .circle. 
Example 3 
______________________________________