The present invention relates to a binder for heat-sensitive recording
materials and, more specifically, to improvements in the water resistance of a
coating layer of said recording materials and the storage stability of an undeveloped
Heat-sensitive recording materials are now enjoying wide use for
output recording of facsimiles, electronic computers, automated ticket dispensers,
medical examinations and the like as well as hard copying of CRT, partly because
they can develop a clear image by heating and partly because they are advantageous
in that the recorders used are relatively of a compact size and maintenance-free,
the sheet substrate applied are usually paper which may be more inexpensive than
other recording materials (e.g., dielectric recording sheets). Such heat-sensitive
recording materials are obtained by applying onto a sheet substrate such as paper,
a film or synthetic paper an aqueous solution or dispersion composed mainly of
a colorless or light-colored leuco dye, a developer for allowing said dye to develop
a color by heating and a binder, followed by drying. As the binders used to this
end, it has been proposed to make use of water-soluble high-molecular compounds
such as, for instance, polyvinyl alcohol or modified products, starch, or modified
products and its derivatives, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose,
gelatin, casein, polyvinyl pyrrolidone, polyacrylamides, polyacrylates, styrene/maleic
anhydride copolymers and diisobutylene/maleic anhydride copolymers, optionally
with a waterproofing agent (formalin, glyoxal, chrome alum, glutar aldehyde, epoxy
resins and melamine/formalin resins). It has also been proposed to use a hydrophobic
resin, and high-molecular latices or emulsions of styrene/butadiene copolymers,
polyvinyl acetate and polyacrylates ester. Whether they are water-soluble or added
thereto with the waterproofing agent, however, such binders are poor in water
resistance, and are troublesome in handling of said heat-sensitive recording sheets.
For instance, when the heat-sensitive recording material is wetted by droplets
of water or for some reasons, the coated layer may become sticky, or may peel from
the sheet substrate. In addition, after recording, there may be a drop of the developed
image density. In some cases, it may be impossible to make out the developed image.
Use of the hydrophobic resin is disadvantageous in lowering of heat sensitivity
and residue build-up. Some of the waterproofing agent with the water-soluble resin
may cause the coagulation of the coating color which brings the coating trouble.
Alternatively, there is a fear that ordinary low-temperature drying may lead to
insufficient waterproofing (since drying at a temperature of higher than 100°C
may possibly incur color development, drying is usually carried out at a temperature
not exceeding 100°C). In view of the aforesaid considerations, attempts have been
made to afford waterproofness to recording materials by various methods. However,
none of these methods gives any satisfactory waterproofness. Even when the resulting
materials have satisfactory waterproofness, they are found to have other shortcomings.
For instance, the compound which is used as the waterproofing agent for the aforesaid
water-soluble binders, and has in its molecule at least two 1,2-epoxy ring structures
(Japanese Publication Patent No. 51-29947) or at least two ethylenimine groups
(Japanese Laid-Open Patent No. 49-32646) gives rise to a lowering of heat sensitivity,
which results in a drop of the developed image density and a drop of the stability
of the coating liquid due to its coagulation or increases of viscosity, and brings
about deteriorations in the storage stability of the heat-sensitive recording materials.
Japanese Laid-Open Patent Nos. 52-73047 and 54-1040 disclose an isobutylene/maleic
anhydride copolymer, and Japanese Laid-Open Patent Nos. 54-80136 and 58-89397 teaches
use of hydroxyethylcellulose and use of methylcellulose or carboxymethylcellulose
with an isobutylene/maleic anhydride copolymer or its water-soluble salt, respectively.
Although these compounds are found to give sufficient waterproofness to coated
layer, they do not give any satisfactory brightness (of higher than 75 %) to products
which are formed into heat-sensitive recording sheets. Further, the brightness
drops (to 70 % or less) due to atmospheric moisture with the lapse of time, thus
leading to deteriorations in the aesthetic appearance of the products to be used
as recording sheets. Still further, when bar code pattern is recorded, there is
a serious problem in which scaning becomes impossible.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to solve the foregoing
problems of the prior art binders and waterproofing agents. That is to say, this
invention is concerned with a specific binder which brings sufficient water resistance
of thermal coating layer accompanied with high brightness and its less reduction
with storage time under atmospheric moisture.
In consequence of extensive and intensive studies made with a view
to solving some problems as mentioned above, it has been found that a heat-sensitive
recording material having sufficient water resistance and allowing an undeveloped
portion to show a high brightness that does not substantially change with time
is provided by a heat-sensitive recording material comprising a sheet substrate
having applied thereto a heat-sensitive, color-developing layer containing as the
main components a colorless or light-colored leuco dye, a developer for permitting
said dye to develop a color by heating and a binder, in which the binder used
is an ammonium salt of a diisobutylene/maleic anhydride copolymer expressed in
terms of the following general formula:
wherein R stands for a diisobutylene group,
said ammonium salt being modified by substituting part of it
by an ammonium salt comprising a primary, secondary or tertiary ethanol amine
(for instance, H+3 NCH&sub2;CH&sub2;OH, (CH&sub3;)&sub2;H&spplus;NCH&sub2;CH&sub2;OH,
H&sub2;&spplus;N(CH&sub2;CH&sub2;OH)&sub2;, (CH&sub3;)H&spplus; N(CH&sub2;CH&sub2;OH)&sub2;,
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For instance, the leuco dyes used in the present invention usually
include the following colorless or slightly colored, lactone, lactam or spiropyran
Crystal violet lactone
Malachite green lactone
As the developers used in the present invention, use may be made
of any substances capable of being liquefied or gasified at a temperature exceeding
normal temperature, preferably higher than 70°C and reacting with the color-developing
dye for color development, which include, for instance, acidic organic substances:
4-4'-isopropylidenediphenol (bisphenol A)
4-hydroxyacetophenol; and the following acidic inorganic substances:
As is the case with the leuco dyes, however, the present invention is not exclusively
limited to the above exemplified substances.
The heat-sensitive color-develping layer according to the present
invention may contain the following various substances, if required, in addition
to the aforesaid leuco dye, acidic substances and binders. For instance, use may
be made of white pigments such as talc, clay, silica or titanium oxide for the
purpose of further improving writing ability, brightness and sticking; various
thermoplastic substances such as various waxes, metal salts of higher fatty acids
or higher fatty acid amides for the purpose of further improving image quality
and sticking and inhibiting color-development under pressure; and dispersants,
wetting agents and anti-foaming agents for the purpose of improving dispersity
and coatability of coating color.
The heat-sensitive recording materials according to the present invention
may be prepared by any conventionally used methods. For example, of the aforesaid
components, the leuco dye and the color-developing agent are at least separately
pulverized and dispersed by means of a dispersing device such as a ball mill, an
attritor or a sand mill, until a dispersion particle size of 1 to 3 microns is
obtained. Thereafter, the resulting particulate product is formulated according
to the given recipe, thereby preparing a liquid to define a heat-sensitive, color-developing
layer. Then, that liquid is applied and dried on a sheet substrate formed of, e.g.,
paper, a film or synthetic paper, and calendering gives the heat-sensitive recording
material according to the present invention.
In the following, consideration will now be given to the reasons
why the effects as mentioned above are attained by the present invention.
I WATER RESISTANCE
The reason why improvements in water resistance are achieved is that
the aforesaid compounds remain in the form of the water-insoluble diisobutylene/maleic
anhydride copolymer through volatilization of ammonia by drying in the course of
formation of said heat-sensitive, color-developing layer.
II IMPROVEMENTS IN BRIGHTNESS
Such improvements are considered to be attained by the color-developing
reaction of the leuco dye with carboxylic acid from the fact that the aforesaid
compounds are dissociated in an aqueous solution into carboxylic acid and ammonium
hydroxide and, even after the application and drying of a coating liquid for forming
the heat-sensitive, color-developing layer containing the binder, the ammonium
salt remains so that the aforesaid reaction proceeds gradually with atmospheric
moisture to set free the carboxylic acid. Accordingly, when a part of the ammonium
salt is substituted by a primary, secondary or tertiary ethanol amine, the aforesaid
dissociation reaction is considered to be inhibited, so that the high brightness
of the undeveloped portion is kept, and does not substantially change with time.
As the binder in the present invention, it is possible to use the
ammonium type containing the ethanol amine of diisobutylene/maleic anhydride copolymers
alone or in combination with other binders, if required, which may include a water-soluble
resin such as polyvinyl alcohol or starch, and latices and emulsions of styrene/butadiene
copolymers. However, it is required that the binder used be composed mainly of
said water-soluble salt of diisobutylene/maleic anhydride copolymers for the purpose
of the present invention.
In the following, the present invention will now be explained in
further detail with reference to the following examples. Examples 1 ~ 5
The following respective components were dispersed for 24 hours with
the use of a ball mill to prepare dispersion liquids A to C.
Bis-(P-hydroxyphenyl)sulfone was used in place of 4-4'(isopropylidenediphenol
in Liquid B; and the remaining components were the same as in B.
Subsequently, the liquids A and B or C were mixed with 10.0 weight
parts of the 20 solid % binder that was a liquid D, E or F comprising ammonium
salt of a diisobutylene/maleic anhydride copolymer, 25 % of which was substituted
by monoethanolamine, diethanolamine or triethanolamine respectively as shown in
Table 1, and the resulting mixture was applied on one side of wood free paper (having
a basis weight of 50 g/m²) to form a heat-sensitive, color-developing film in a
coated amount of 5-7 g/m², thereby obtaining the heat-sensitive recording sheet
according to the present invention.
Comparative Examples 1 and 2
Example 1 was repeated, except that as the binder use was made of
10.0 weight parts of an ammonium salt of a diisobutylene/maleic anhydride copolymer
or 10.0 weight parts of 20 % polyvinyl alcohol, thereby to obtain control heat-sensitive
Then, the thus obtained seven heat sensitive recording sheets were
printed by means of a label printer (HP-9303, manufactured by Tokyo Denki Kabushiki
Kaisha), whereby the testing in connection with color-developing properties, residue
build-up, sticking, water resistance and brightness of the sheet substrate were
carried out. The results are set forth in Table 2. It is noted that the testing
methods applied are as follows.
CONCENTRATION OF COLOR DEVELOPMENT
The density of the color-developed portion obtained with a label
printer was measured with a Macbeth densitometer RD-514.
WATER RESISTANCE 1
The color-developed portion obtained with a label printer was immersed
in water at normal temperature and, after 24 hours, the density thereof was measured
with a Macbeth densitometer RD-514.
WATER RESISTANCE 2
Water was added dropwise onto the heat-sensitive, color-developing
layer, which was then rubbed with fingers to examine tackiness and destraction
of the coated layer.
The whiteness of the undeveloped portion was measured with a Hunter
The sample was allowed to stand for 24 hours at 40°C and 93 % RH
to measure the brightness of the undeveloped portion.
As shown in Table 2, the heat-sensitive recording materials of the
present invention excel in the color-developing properties, and the undeveloped
portions thereof had a high degree of brightness. The printed images hardly discolored
in water, and the degree of whiteness hardly deteriorated during storage. However,
Comparative Example 1 was so poor in moisture resistance that the brightness dropped
in water with time. Referring to Comparative Example 2, the coated layer exhibited
no water resistance at all, and the printed image discolored in water to such
an extent that it was illegible.
Wärmesensitives Aufzeichnungsmaterial, das eine Substratschicht umfaßt, die
eine darauf angebrachte wärmesensitive Farbentwicklungsschicht aufweist, die als
Hauptkomponenten einen farblosen oder leicht gefärbten Leukofarbstoff, einen Entwickler
zum Ermöglichen, daß dieser Farbstoff mittels Erwärmen eine Farbe entwickelt wird
und ein Bindemittel enthält, in welcher das verwendete Bindemittel ein Ammoniumsalz
eines Diisobutylen/Maleinanhydridcopolymers ist, dargestellt in Form der allgemeinen
worin R für eine Diisobutylen-Gruppe steht,
wobei dieses Ammoniumsalz gekennzeichnet ist durch seine teilweise Substitution
durch ein Ammoniumsalz, das ein primäres, sekundäres oder tertiäres Ethanolamin
A heat-sensitive recording material comprising a sheet substrate having applied
thereto a heat-sensitive, color-developing layer containing as the main components
a colorless or light-colored leuco dye, a developer for permitting said dye to
develop a color by heating and a binder, in which the binder used is an ammonium
salt of a diisobutylene/maleic anhydride copolymer expressed in terms of the following
wherein R stands for a diisobutylene group,
said ammonium salt being modified by substituting part of it
by an ammonium salt comprising a primary, secondary or tertiary ethanol amine.
Matériau d'enregistrement thermosensible comprenant un substrat en feuilles
sur lequel est appliquée une couche thermosensible de développement des couleurs,
contenant comme principaux composants un leuco-colorant incolore ou légèrement
coloré, un révélateur permettant audit colorant de développer une couleur par chauffage,
et un liant, dans laquelle le liant utilisé est un sel d'ammonium d'un copolymère
de diisobutylène/anhydride maléique répondant à la formule générale suivante :
dans laquelle R représente un groupe diisobutylène, ledit sel d'ammonium étant
modifié en ce qu'il est partiellement substitué par un sel d'ammonium comprenant
une éthanolamine primaire, secondaire ou tertiaire.