FIELD OF THE INVENTION
[0001] The present invention relates to a heat-sensitive recording material which is good
in environmental stability and, more particularly, it relates to a heat-sensitive
recording paper for self-adhesive labels which has no protective layer.
BACKGROUND OF THE INVENTION
[0002] Hitherto, heat-sensitive recording papers have been used as a heat-sensitive recording
type, self-adhesive label.
[0003] In the case of such a heat-sensitive recording type, self-adhesive label, when pasticizers
present in plastic films, such as dioctyl adipate (DOA) and dioctyl phthalate (DOP),
penetrate into a heat-sensitive, color-forming layer, there was found a problem of
decolorization of the printed portion (color-formed portion). Accordingly, it cannot
be used as a self-adhesive label for plasticizer-containing plastic wrapping film.
Further, when water or oils penetrate into the heat-sensitive, color-forming layer,
the printed image smears or the image density reduces with a lapse of time. Moreover,
in the case that the printing is carried out by bringing the heat-sensitive, color-forming
layer into direct contact with a thermal head, if the amount of a binder present in
the heat-sensitive, color-forming layer is too high, the residue build-up and sticking
readily occur. Accordingly, in order to prevent such occurrence, the amount of the
binder in the heat-sensitive, color-forming layer is necessarily reduced, which, however,
leads to a drawback that the surface strength is weak. Thus, in the case that the
printing is carried out on the surface of the heat-sensitive, color-forming layer,
the heat-sensitive, color-forming layer is entrained in the rubber roll side, whereby
the printing is no longer possible.
[0004] In order to remove the above drawbacks, an attempt to not only prevent the transver
and penetration of plasticizers, water and oils into the heat-sensitive, color-forming
layer but also impart it with a surface strength has hitherto been made by providing
a protective layer on the heat-sensitive, color-forming layer for the purpose of adapting
such a heat-sensitive recording material to self-adhesive labels.
[0005] As a main component of the protective layer, there are used water-soluble high molecular
weight compounds such as, for example, polyvinyl alcohol or modified products thereof,
starch or modified products thereof, etc.
[0006] However, in the case of the conventional heat-sensitive recording type, self-adhesive
labels having a protective layer mainly composed of a water-soluble high molecular
weight compound, though the decolorization of the printed portion by plasticizers
and the surface strength are improved, there may occur the transfer and penetration
of water and oils from the edge portions and back side of the label, resulting in
decolorization of the printed portion by water and oils. Further, by providing the
protective layer on the heat-sensitive, color-forming layer, when the color formation
is carried out by means of a thermal head, there occur problems that the heat responsibility
reduces and the color formation sensitivity lowers. Still further, because of the
increase of a production step for providing the protective layer, there is a disadvantage
of cost increase.
SUMMARY OF THE INVENTION
[0007] An object of the present invention is to provide a heat-sensitive recording material
which can overcome the above-described drawbacks of the conventional techniques and
which can not only prevent the decolorization of the color-formed image by plasticizers,
water and oils but also be produced at a low manufacturing cost.
[0008] In order to achieve the above object, the present inventors have made extensive investigations
and found that this object can be attained by a heat-sensitive recording material
comprising a heat-sensitive, color-forming layer containing as color-forming components
a colorless or pale-colored leuco dye and an acidic compound capable of allowing the
leuco dye to color develop by heating, wherein a color former comprising a specified
fluoran derivative, a developer comprising a specified sulfone compound, and a binder
comprising a specified high molecular weight compound are combined with each other,
which finding accomplished the present invention.
[0009] That is, the present invention is to provide a heat-sensitive recording material
comprising a support having on one surface thereof a heat-sensitive, color-forming
layer comprising a color former comprising a fluoran derivative represented by the
following general formula (I):
wherein R
i and R
2 each represents a lower alkyl group, and X represents a halogen atom, a lower alkyl
group, a cyano group, or a nitro group, a developer comprising bis(p-hydroxyphenyl)sulfone
or a derivative thereof represented by the following general formula (II):
wherein R
3 and R
4 each represents a hydrogen atom, an alkyl group, an aryl group, an alkoxy group,
a phenoxy group, a carbonyl group, a nitro group, or a cyano group, and a binder comprising
an ammonium salt of a diisobutylene/maleic anhydride copolymer and a styrene/butadiene
copolymer, wherein in the amount of said styrene/butadiene copolymer in said binder
is from 0,5 to 1.0 parts by weight based on parts by weight of said ammonium salt
of diisobutylene/maleic anhydride copolymer the styrene content of which is 58 to
62 mol%.
[0010] In the heat-sensitive recording material according to the present invention, the
printing or recording of images can be carried out on one side of the support in a
heating manner, and when a self-adhesive layer is provided on the other side of the
support, the heat-sensitive recording material can be used as various price labels,
measuring labels, name plates, seals, etc.
DETAILED DESCRIPTION OF THE INVENTION
[0011] Examples of the fluoran derivative represented by the above-described general formula
(I) include 3- dibutylamino-7-o-chloroanilinofluoran, 3-dibutylamino-7-o-fluoroanilinofluoran,
3-diethylamino-7-o-chloroanilinofluoran, and 3-diethylamino-7-o-fluoroanilinofluoran,
with 3-dibutylamino-7-o-fluoroanilino- fluoran being particularly preferred.
[0012] The color former represented by the general formula (I) which is used in the present
invention can be used in combination with other color formers.
[0013] Specific examples of the compound represented by the general formula (II) include
bis(p-hydroxyphenyl)sulfone, bis(3-aryl-4-hydroxyphenyl)sulfone, bis(2,5-dimethyl-4-hydroxy)sulfone,
bis(2,5-diethyl-4-hydroxy)sulfone, bis(2,5-dipropyl-4-hydroxy)sulfone, bis(2,5-dibutyl-4-hydroxy)sulfone,
bis(2,5-dibenzyl-4-hydroxy)sulfone, bis(2,5-dimethoxy-4-hydroxy)sulfone, bis(2,5-diethoxy-4-hydroxy)sulfone,
bis(2,5-dipropoxy-4-hydroxy)sulfone, bis(2,5-dibutoxy-4-hydroxy)sulfone, bis(2,5-diphenoxy-4-hydroxyphenyl)sulfone,
bis(2,5-dibenzyloxy-4-hydroxyphenyl)sulfone, bis(2,5-dimethoxycarbonyl-4-hydroxyphenyl)sulfone,
bis(2,5-diethoxycarbonyl-4-hydroxyphenyl)sulfone, bis(2,5-diacetyl-4-hydroxyphenyl)sulfone,
bis(2,5-dibenzoyl-4-hydroxyphenyl)sulfone, bis(4-hydroxy-3-nitro)su)fone, bis(3-cyano-4-hydroxy)sulfone,
5-ethyl-4-hydroxy-2-methylphenyl 4'-hydroxy-2'-methoxy-5'-methylphenyl sulfone, 5'-ethoxy-4-hydroxy-2-methylphenyl
5'-ethyl-2'-ethoxy-4'-hydroxyphenyl sulfone, and 5-benzyloxy-4-hydroxy-2-methylphenyl
4'-hydroxy-2'-methoxy-5'-methylphenyl sulfone.
[0014] The developer represented by the general formula (II) which is used in the present
invention is known per se from JP-A 59 169 887 and can be used in combination with
other developers.
[0015] According to the above-described combination of the color former with the developer,
the plasticizer resistance, water resistance and oil resistance are improved to a
considerable extent, but when brought into contact with a plastic film containing
a plasticizer, water or an oil for a long period of time, there was observed a tendency
of decolorization of the color-formed portion. Further, the surface strength of the
heat-sensitive, color-forming layer is still weak, and even though in order to improve
the surface strength, a styrene/butadiene copolymer used in coated paper was applied
as a binder, the residue build-up and sticking occurred.
[0016] In order to improve these points, the present inventors have made further extensive
investigations and found that such an object can be obtained by using, as a binder
for the heat-sensitive, color-forming layer, an ammonium salt of a diisobutylene/maleic
anhydride copolymer (preferably one mol of a diisobutylene group being reacted with
one mol of a maleic anhydride group) represented by the following general formula
(III):
wherein Rs represents an diisobutylene group together with a styrene/butadiene copolymer
(the styrene content being from 58 to 62 mol%) represented by the following general
formula (IV):
[0017] As the ammonium salt of diisobutylene/maleic anhydride copolymer used as the binder
in the present invention, those having a molecular weight of from 10,000 to 200,000
are suitable, and one having one mol of an isobutylene group bonded to one mol of
a maleic anhydride group is known per se from JP-A 58 193 187 and JP-A 59 169 887
and is particularly effective for improving the preservability. Further, the styrene/butadiene
copolymer has an effect for improving the surface strength of the heat-sensitive,
color-forming layer, and one having a styrene content of from 58 to 62 mol% is effective
for the improvement in surface strength of the heat-sensitive, color-forming layer.
If the styrene content is too high, the heat-sensitive, color-forming layer becomes
rigid to cause a reduction in surface strength, whereas if the butadiene content is
too high, the residue build-up and sticking readily occur upon printing by means of
a thermal head.
[0018] With respect to the binder, a combination use of the ammonium salt of diisobutylene/maleic
anhydride copolymer with the styrene/butadiene copolymer is essential in the present
invention. If only the ammonium salt of diisobutylene/maleic anhydride copolymer is
used, the surface strength of the heat-sensitive, color-forming layer is low, whereas
if only the styrene/butadiene copolymer is used, the plasticizer resistance is low
and the residue build-up and sticking readily occur.
[0019] From 0.5 to 1.0 part by weight of the styrene/butadiene copolymer is mixed with one
part by weight of the ammonium salt of diisobutylene/maleic anhydride copolymer. Further,
the binder of the present invention can be used in combination with other binders
such as polyvinyl alcohol, hydroxyethyl cellulose, carboxymethyl cellulose, starch,
etc.
[0020] The heat-sensitive, color-forming layer of the present invention can be formed by
known methods. As the support, a sheet-like material, such as paper, synthetic paper
or plastic films, can be used. Onto one side of the support is coated a coating solution
containing the above-described fluoran derivative, bis(p-hydroxyphenyl)sulfone and
binder, to thereby form a heat-sensitive, color-forming layer.
[0021] If desired, the heat-sensitive, color-forming layer can contain known auxiliary additives
such as fine white pigments such as calcium carbonate, talc, clay, silica, titanium
oxide, and urea/formalin resin, and heat-fusible substances such as various waxes,
higher fatty acid metal salts, and higher fatty acid amides, to thereby improve the
sharpness of the color-formed image.
[0022] On the other hand, on the other side of the support is provided a pressure-sensitive
adhesive layer by known methods such that it can be adhered to plastic films or wrapping
paper, and a release substrate is laminated to the back side thereof. There can be
thus obtained a heat-sensitive recording type, self-adhesive label.
[0023] The thus obtained heat-sensitive recording material according to the present invention
excels in recording properties and environmental stability of the heat-sensitive,
color-forming layer, without residue build-up and sticking during the printing. Furthermore,
the heat-sensitive recording type self-adhesive label obtained by applying a pressure-sensitive
adhesive onto the back side of the support using the heat-sensitive recording material
of the present invention is more excellent than ever.
[0024] The present invention will now be explained in more detail with reference to the
examples, but it is to be understood that the invention is not limited thereto.
EXAMPLE 1
[0025] The following components were mixed together for 24 hours in a ball mill to prepare
dispersions A and B.
[0026] The dispersions A and B were mixed together with as a binder a mixture of 6 parts
by weight of a 20% diisobutylene/maleic anhydride copolymer ammonium salt aqueous
solution (molecular weight: about 50,000) and 1.6 parts by weight of a 50% styrene/butadiene
copolymer latex (molar percentage of styrene: 58 to 62%), and the mixture was coated
on one side of wood free paper (basis weight: 50 g/m
2) and then dried to form a heat-sensitive, color-forming layer. There was thus obtained
a heat-sensitive recording material according to the present invention.
EXAMPLE 2
[0027] The following components were mixed together for 24 hours in a ball mill to prepare
dispersions A and B.
[0028] The dispersions A and B were mixed together with as a binder a mixture of 6 parts
by weight of a 20% diisobutylene/maleic anhydride copolymer ammonium salt aqueous
solution (molecular weight: about 50,000) and 1.6 parts by weight of a 50% styrene/butadiene
copolymer latex (molar percentage of styrene: 58 to 62%), and the mixture was coated
on one side of wood free paper (basis weight: 50 g/m
2) and then dried to form a heat-sensitive, color-forming layer. There was thus obtained
a heat-sensitive recording material according to the present invention.
COMPARATIVE EXAMPLE 1
[0029] Heat-sensitive recording materials 1-A and 1-B for comparison were obtained in the
same manner as in Example 1 except that the 3-dibutylamino-7-o-fluoro
qnilinofluoran used in the dispersion A was replaced by 3-diethylamino-7-p-chloroanilinofluoran
and 3-(N-methyl-N-cyclohexylamino)-6-methyl-7-anilino- fluoran, respectively.
COMPARATIVE EXAMPLE 2
[0030] A heat-sensitive recording material for comparison was obtained in the same manner
as in Example 1 except that the bis(p-hydroxyphenyl)sulfone used in the dispersion
B was replaced by 4,4'-isopropylidene diphenol.
COMPARATIVE EXAMPLE 3
[0031] A heat-sensitive recording material for comparison was obtained in the same manner
as in Example 1 except that 10 parts by weight of a 20% diisobutylene/maleic anhydride
copolymer ammonium salt aqueous solution was used as the binder.
COMPARATIVE EXAMPLE 4
[0032] A heat-sensitive recording material for comparison was obtained in the same manner
as in Example 1 except that 4 parts by weight of a 50% styrene/butadiene copolymer
latex was used as the binder.
COMPARATIVE EXAMPLE 5
[0033] A heat-sensitive recording material for comparison was obtained in the same manner
as in Example 1 except that a mixture of 2.5 parts by weight of a 20% diisobutylene/maleic
anhydride copolymer ammonium salt aqueous solution and 3 parts by weight of a 50%
styrene/butadiene copolymer latex was used as the binder.
[0034] On the back side of each of the heat-sensitive recording materials was laminated
a silicone resin- coated release paper having thereon an acrylic-based pressure-sensitive
adhesive layer at a coverage of 20 g/m
2, through the pressure-sensitive adhesive layer, and the laminate was cut off into
a suitable size to obtain a heat-sensitive self-adhesive label.
[0035] The thus obtained seven heat-sensitive self-adhesive labels were printed by means
of a label printer (HP-9303, made by Tokyo Denki K.K.), whereby a color formation
test, a residue build-up and sticking test, a environmental stability test, and a
surface strength test were carried out.
[0036] The results are shown in the following table.
Each of the tests was carried out in the following manner.
Color Density
[0037] The color formation was carried out by printing by means of a label printer, and
the color-formed portion was measured by a Macbeth densitometer RD-514. (Hereinafter,
the densities were all measured by the Macbeth densitometer RD-514.)
Background Density
[0038] The density of the background portion was measured.
Plasticizer Resistance
[0039] A portion which had been subjected to color formation by printing by means of the
label printer was covered by a soft polyvinyl chloride film and after standing at
room temperature for 24 hours, the density thereof was measured.
Water Resistance
[0040] A portion which had been subjected to color formation by printing by means of the
label printer was dipped in water at room temperature for 24 hours, and the density
thereof was then measured.
Oil Resistance
[0041] A portion which had been subjected to color formation by printing by means of the
label printer was applied with a sesame oil, and after standing at room temperature
for 24 hours, the density thereof was measured.
Surface Strength
[0042] The heat-sensitive recording material was printed with an ultraviolet light-curable
ink (made by Toka Shikiso K.K.) by means of an RI tester (Akashi Seisakusho Co., Ltd.),
and the transfer amount of the heat-sensitive, color-forming layer to the rubber roll
side was observed.
[0043] It is clear from the above table that the heat-sensitive recording material according
to the present invention Example 1 and 2 is of such a high quality that it is excellent
in color formation, has a high whiteness in the background portion, is so good in
environmental stability that the color does not disappear by the plasticizer, water,
and oil, and is printable and free from residue build-up and sticking. On the other
hand, in Comparative Examples 1-A and 2, the printed portion was decolorized by the
plasticizer, oil and water and became unreadable; in Comparative Example 1-B, the
background density was high as 0.25 and the image of the label was markedly reduced;
in Comparative Example 3, although the density of the printed portion after the preservation
test by the plasticizer, oil and water was 1.10 or more and a sharp image was kept,
the surface strength was quite unsatisfactory; and in Comparative Examples 4 and 5,
the printed portion was decolorized by the plasticizer and caused the residue build-up
and sticking, and the recording materials cannot be put into practical use.