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Description  |
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This invention relates to a heat-sensitive recording sheet and particularly
to a single color or multicolor heat-sensitive recording sheet of high
sensitivity being good in image retention over a long period of time and
excellent in chemical resistance and causing no trailing or sticking when
recording is made thereon with a heat-sensitive facsimile or a
heat-sensitive printer.
Dye color-developing type heat-sensitive recording sheets being in wide
practical use are now produced by (a) dispersing a colorless dye
precursor, a color developer which causes said dye precursor to develop a
color when heated, and a wax as color-developing aid, in fine particles,
(b) adding thereto an organic or inorganic oil-absorbing pigment, a
binder, a surfactant, a fluorescent dye and the like to prepare a
heat-sensitive coating color, and (c) coating the color on a support and
drying the coated support. These heat-sensitive recording sheets are
required to have characteristics such as:
High sensitivity recording is made by a heat-sensitive facsimile or a
heat-sensitive printer.
Formed images are stable and retained over a long period of storage under
natural conditions.
No white powder is formed on images.
No color disappearance or discoloration of images occurs when these images
come in contact with chemicals.
The background has a white color.
There occurs no trailing or stacking during recording.
Heat-sensitive recording sheets currently in wide use do not sufficiently
satisfy the above requirements. For example, as the sensitivity of these
heat-sensitive recording sheets increases, blooming or powdering, trailing
and the like appear more frequently. Further, color disappearance or
discoloration of images occuring due to their contact with chemicals can
hardly be prevented in the case of a recording sheet wherein a
heat-sensitive recording layer is coated on a support in a single layer,
and this phenomenon is currently being dealt with by overcoating a high
molecular substance on said-high sensitive recording layer (Japanese
Laid-Open Patent Application No. 12834/1979), however, this requires
multiple coating and consequently a higher production cost.
The heat-sensitive recording sheet according to this invention solves all
these drawbacks of conventional heat-sensitive recording sheets and
further enables color development in multicolor by using at least two
kinds of microcapsules each containing a different dye precursor which
produces a different color and by controlling the temperature of the
heat-sensitive head of a printer.
Heat-sensitive recording sheets being able to give a multicolor with only a
single sheet are described, for example, in Japanese Laid-Open Patent
Application No. 69738/1974, Japanese Laid-Open Patent Application No.
65239/1974, Japanese Patent Publication No. 27708/1974, Japanese Laid-Open
Utility-Model Application No. 81065/1980 and so forth. In production of
these sheets, either multilayer coating or multistep local coating is
used.
The process of producing the heat-sensitive recording sheet of this
invention differs from conventional processes of producing the multicolor
heat-sensitive recording sheets, in that a multicolor heat-sensitive
recording sheet can be produced only with one time coating (single
coating) and microcapsules are used.
The process of producing the heat-sensitive recording sheet of this
invention will be explained in detail below.
Microcapsules of this invention must contain therein at least (a) a
colorless dye precursor as color former, (b) a color developer and (c) a
waxy, color-developing aid being solid at the normal temperature and
melting when heated. As the colorless dye precursor as color former, there
are mentioned, for example, Crystal Violet Lactone,
3-indolino-3-p-dimethylaminophenyl-6-dimethylaminophthalide,
3-diethylamino-7-chlorofluoran, 3-diethylamino-7-cyclohexylaminofluoran,
3-diethylamino-5-methyl-7-t-butylfluoran,
3-diethylamino-6-methyl-7-anilinofluoran,
3-diethylamino-6-methyl-7-p-butylanilinofluoran,
2-(N-phenyl-N-ethyl)aminofluoran, 3-diethylamino-7-dibenzylaminofluoran,
3-cyclohexyl-amino-6-chlorofluoran,
3-diethylamino-6-methyl-7-xylidinofluoran,
2-anilino-3-methyl-6-(N-ethyl-p-toluidino)fluoran,
3-pyrrolidino-6-methyl-7-anilinofluoran,
3-pyrrolidino-7-cyclohexylaminofluoran,
3-pyperidino-6-methyl-7-toluidinofluoran,
3-pyrrolidino-6-methyl-7-(p-toluidino)fluoran,
3-pyperidino-6-methyl-7-anilinofluoran,
3-N-methylcyclohexylamino-6-methyl-7-anilinofluoran,
3-diethylamino-7-(m-trifluoromethylanilino)fluoran and the like. Of
course, other dye precursors may be used.
As the color developer usable in this invention, there are, for example,
4-phenylphenol, 4-hydroxyacetophenon, 2,2'-dihydroxydiphenyl,
2,2'-methylenebis(4-chlorophenol),
2,2'-methylenebis(4-methyl-6-t-butylphenol),
4,4'-isopropylidenebis(2-methylphenol), 4,4'-ethylenebis(2-methylphenol),
1,1'-bis(4-hydroxyphenyl)-cyclohexane, 2,2-bis(4'hydroxyphenyl)propane,
4,4'-cyclohexylidenebis(2-isopropylphenol), novolak type phenolic resin,
3,5-di-t-butylsalicylic acid, 3,5-di-.alpha.-methylbenzylsalicylic acid,
3-methyl-5-t-butylsalicylic acid, phthalic acid monoanilide,
p-ethoxybenzoic acid, p-benzyloxybenzoic acid, benzyl p-hydroxybenzoate
and the like. Of course, other color developers may be used.
As the waxy, color-developing aid usable in this invention, there are, for
example, animal waxes such as beeswax, spermaceti, Chinese insect wax,
lanolin and the like; vegetable waxes such as candelilla wax, carnauba
wax, Japan wax, rice wax, sugar cane wax and the like; mineral waxes such
as montan wax, ozokelite, ceresine, lignite wax and the like; petroleum
waxes such as paraffin wax, microcrystalline wax and the like; modified
waxes such as montan wax derivative, paraffin wax derivative,
microcrystalline wax derivative and the like; hydrogenated waxes such as
castor wax, opal wax and the like; low molecular weight polyethylene and
its derivative; synthetic waxes such as Acra wax, distearylketone and the
like; saturated fatty acid amide waxes such as capronamide, caprylamide,
pelargonamide, capramide, lauramide, tridecylamide, myristamide,
stearamide, behenamide, ethylenebisstearamide and the like; unsaturated
fatty acid amide waxes such as caproleamide, myristoleamide, oleamide,
elaidamide, linolamide, erucamide, ricinoleamide, linoleamide and the
like; and higher resin acids. They can be used alone or in combination. Of
course, other compounds may be used.
Microcapsules of this invention can be produced by any of known methods
such as the complex coacervation method, the in situ method and the
interfacial polymerization method.
The point needing attention in microcapsule production is how to produce
microcapsules without causing color development. In general, when a
colorless dye precursor as color former and a color developer are
contained in the same microcapsule, they are liable to develop a color
during microcapsule production. As a method to solve this problem, there
is a technique in the field of pressure-sensitive recording sheets wherein
a volatile desensitizer is contained in microcapsules and this
desensitizer vaporizes into the air when the microcapsules are broken by
pressure and thereby a colorless dye precursor as color former and a color
developer are caused to develop a color (Japanese Laid-Open Patent
Application No. 82919/1973). However in the field of heat-sensitive
recording sheet, such a color-developing mechanism by capsule breakage is
not utilized, and it is required that color development occurs in
microcapsules only when these microcapsules are heated without capsule
breakage.
The present inventors made studies on an additive to be contained in
microcapsules which does not cause color development during
microencapsulation and also even when microcapsules are pressurized but
which does cause color development only when microcapsules are heated. As
a result, aforementioned waxy substances which are solid at the normal
temperature and melt when heated, have been found to be effective as such
an additive.
Various investigations were made on specific processes of encapsulation
and, as a result, the following process has been found to be effective
because it causes no color development during encapsulation.
Firstly, a colorless dye precursor as color former and a color-developing
aid are melted together followed by being emulsified as finely as possible
(Step 1). Then, a color developer and a color-developing aid are melted
together followed by being emulsified as finely as possible (Step 2). Two
emulsions obtained in Step 1 and Step 2 are mixed and therein is added an
emulsifier necessary for encapsulation (Step 3). Therein is further poured
a melted color-developing aid and an emulsion having particle diameters of
several microns is obtained (Step 4). The emulsion wherein there are mixed
a colorless dye precursor, a color developer and a color-developing aid is
obtained. Next, a capsule wall-forming agent and other additives are added
and the whole mixture is stirred at a predetermined temperature to obtain
a coating color containing microcapsules.
It is supposed that in the microcapsule-containing coating color thus
obtained there exist, as impurities, microcapsules containing a co-melt of
a colorless dye precursor and a color-developing aid, microcapsules
containing a co-melt of a color developer and a color-developing aid and
microcapsules containing only a color-developing aid, to some extent.
These microcapsules as impurity cause no oozing of contents out of
microcapsules and accordingly no color development when heated, and
therefore do not impair the characteristics of the heat-sensitive
recording sheet of this invention. Microcapsules containing a colorless
dye precursor as color former, a color developer and a color-developing
aid develop a color inside themselves when heated.
In the process of encapsulation, each of the emulsions obtained in
afore-mentioned step (1) and step (2) need not be a co-melt with a
color-developing aid. Instead, they may be particles in the form of a
powdery dye precursor or color developer coated with a color-developing
aid.
There is another process of encapsulation. A colorless dye precursor as
color former and a color-developing aid are melted together and then
emulsified as finely as possible. By using a known microencapsulation
process, there are prepared pseudo-microcapsules having a very thin shell
thickness compared with ordinary shell thicknesses (Step I). Shells of
these pseudo-microcapsules are very thin and barely forms a film, and may
be somewhat porous. Next, a color developer and a color-developing aid are
melted together and emulsified as finely as possible, and then in the same
manner as in step (I), pseudo-microcapsules are prepared (Step II). Two
kinds of pseudo-microcapsules thus obtained in steps (I) and (II) are
mixed, and to the resulting dispersion of these two kinds of
pseudo-microcapsules is added an emulsifier necessary for encapsulation
and emulsification. To the resulting emulsion, is added a capsule
wall-forming agent and the whole mixture is stirred at a predetermined
temperature, whereby microcapsules are formed (Step III). Thus, there is
obtained a coating color containing microcapsules containing two kinds of
pseudo-microcapsules prepared in step (I) and step (II) and each having a
very thin shell film. In microcapsules present in the coating color
obtained, a colorless dye precursor as color former and a color developer
are contained in respective pseudo-microcapsules each having a very thin
shell film, and accordingly do not develop a color at the normal
temperature because they are separated from each other by the very thin
films. Only when heated, the dye precursor and the color developer cause
color development inside microcapsules because the very thin films
covering pseudo-microcapsules are broken and the dye precursor and the
color developer come in contact in microcapsules through a
color-developing aid.
When microcapsules containing two kinds of pseudo-microcapsules prepared in
step (I) and step (II) are formed in step (III), it is desirable viewed
from the effect of color development that these pseudo-microcapsules be
contained in microcapsules in a coagulated form like a cluster of grapes.
Also it is advantageous from the standpoint of heat sensitivity that the
wall films of pseudo-microcapsules be made of a thermoplastic resin.
There is still another process of encapsulation. A finely dispersed, solid,
colorless dye precursor as color former is contained, by the use of a
known microencapsulation process, in pseudo-microcapsules having a very
thin film thickness compared with ordinary film thicknesses (Step i).
Next, in the same manner as in step (i), a finely dispersed solid color
developer is contained in pseudo-microcapsules (Step ii). Two kinds of
pseudo-microcapsules prepared in step (i) and step (ii) are mixed and then
dispersed in a molten color-developing aid (Step iii). To this dispersion
is added an emulsifier and emulsification is carried out. Then, a
film-forming agent is added thereto and the whole mixture is stirred at a
predetermined temperature whereby microcapsules are formed (Step iv). In
the microcapsules formed, pseudo-microcapsules containing a colorless dye
precursor as well as pseudo-microcapsules containing a solid color
developer are surrounded by a color-developing aid, and no color is
developed at the normal temperature but, when heated, the wall films of
the pseudo-microcapsules are broken and a color is developed by the action
of the color-developing aid present as a medium outside the
pseudo-microcapsules.
The proportion of (a) a colorless dye precursor as color former, (b) a
color developer and (c) a color-developing aid is suitably 1 to 10 parts
by weight of (b) and 1 to 10 parts by weight of (c) relative to 1 part by
weight of (a). The proportion is determined by the melting points and the
color-developing abilities of (a), (b) and (c).
Next, a process for coating a support with the above-obtained coating color
containing microcapsules will be explained.
In microcapsules of this invention, a color is developed within
microcapsules when heated. But, a colored substance does not ooze out of
the microcapsule film and hence an oil-absorbing organic or inorganic
pigment as used in heat-sensitive recording sheets currently in wide use
is not necessarily required. Just mixing of a microcapsule-containing
coating color and a binder giving a low level of sticking to a thermal
head followed by coating the mixture on a support and drying is
sufficient. As the usable binder, most binders having adhesivity can be
used. They are, for example, polyvinyl alcohols or their modified
products, starch or its modified product, polyacrylamides, casein,
styrene-maleic anhydride resin, isobutylenemaleic anhydride resin,
carboxymethyl cellulose, hydroxyethyl cellulose, carboxymodified
polyethylenes, styrene butadiene latex, carboxymodified styrene butadiene
latex and so forth.
It is also possible that the microcapsule-containing coating color of this
invention be spray-dried, then to the resulting powder-like microcapsules
be added an appropriate additive and the resulting printing ink be printed
on a support.
As the support, a paper is generally used. The support may also be a film,
a synthetic paper, a metal foil or their composite sheet.
When the microcapsule-containing coating color is coated on a support, a
dispersant and/or a surfactant may also be used so as to give improved
stability.
Hereinafter, this invention will be explained specifically by referring to
Examples.
EXAMPLE 1
Production of microcapsules which develop a color at low temperatures
3 Parts by weight of 3-diethylamino-6-methyl-7-anilinofluoran and 6 parts
by weight of oleamide (m.p. 70.degree. to 75.degree. C.) were melted
together by heating. The co-melt, while hot, was poured into 50 parts by
weight of an aqueous solution dissolving 0.5% of methyl cellulose of the
normal temperature. The mixture was emulsified so as to give particle
diameters of 1 to 2 .mu., by the use of a TK-homomixer (manufactured by
Tokushu Kika Kogyo K. K.) to obtain an emulsion A. Separately, 3 parts by
weight of benzyl-p-hydroxybenzoate and 6 parts by weight of oleamide were
melted together by heating. The resulting co-melt still being hot was
poured into 50 parts by weight of an aqueous solution dissolving 0.5% of
methyl cellulose of the normal temperature. The mixture was emulsified so
as to give particle diameters of 1 to 2 .mu. by the use of the
TK-homomixer to obtain an emulsion B.
20 parts by weight of the emulsion A and 50 parts by weight of the emulsion
B were placed in a beaker and they were thoroughly stirred to obtain an
emulsion C. The emulsion C was added to 100 parts by weight of an aqueous
solution of pH 4.0 dissolving 5% of styrene-maleic anhydride copolymer as
emulsifier and a small quantity of sodium hydroxide, and emulsification
was further conducted. Thereto was added 15 parts by weight of molten
oleamide and emulsification was conducted with vigorous stirring. The
emulsion consisted of particles of the color former and oleamide,
particles of the color developer and oleamide, and oleamide. When emulsion
particles had diameters of 4 to 5 .mu., emulsification was terminated.
Separately, 8 parts by weight of melamine, 20 parts by weight of 37%
aqueous formaldehyde solution, and 52 parts by weight of water were mixed
and, after adjusting its pH to 9.0 with sodium hydroxide, were heated to
60.degree. C. In 15 min, the mixture became transparent and a
melamine-formaldehyde precondensate was obtained. This precondensate was
added to the above emulsion being stirred and they were reacted for 2 hr
with the mixture temperature being kept at 70.degree. C. The obtained
microcapsules did not develop a color and was milky white.
Subsequently, there was prepared a coating color (solid content 25% by
weight) containing 10 parts by weight (as solid) of the microcapsules and
2 parts by weight of a polyvinyl alcohol as binder. The coating color was
coated on a base paper of 50 g/m.sup.2 by the use of a Meyer bar so that
the coated quantity became 7.5 g/m.sup.2 (as solid).
On the coated sheet obtained, recording was made by the use of a
heat-sensitive facsimile, whereby a clear black image was obtained.
When ethanol was coated on the image developed, there was no image
spreading or discoloration. When dioctyl phthalate (plasticizer) was
coated, there was no image disappearance. Also, when non-heated portions
were pressurized, there was no color development.
COMPARATIVE EXAMPLE 1
2 Parts by weight of 3-diethylamino-6-methyl-7-anilinofluoran and 0.2 part
by weight (0.02 part by weight as solid) of 10% polyvinyl alcohol as
binder were mixed and dispersed to obtain a dispersion containing a color
former.
5 Parts by weight of benzyl p-hydroxybenzoate and 0.5 part by weight (0.05
part by weight as solid) of 10% polyvinyl alcohol as binder were mixed and
dispersed to obtain a dispersion containing a color developer.
2 Parts by weight of oleamide and 0.2 part by weight (0.02 part by weight
as solid) of 10% polyvinyl alcohol were mixed and dispersed to obtain a
dispersion containing a color-developing aid.
These three dispersions were mixed with 20 parts by weight (2 parts by
weight as solid) of 10% polyvinyl alcohol to obtain a heat-sensitive
coating color.
The heat-sensitive coating color was coated on a base paper of 50 g/m.sup.2
by the use of a Meyer bar so that the coated quantity became 5.5 g/m.sup.2
as solid.
On the coated sheet obtained, recording was made by the use of a
heat-sensitive facsimile, whereby a black image was obtained.
When ethanol was coated on the image developed, the whole image portion
became black in a short period of time and the image became obscure and
unrecognizable. When dioctyl phthalate was coated, the image became white
and disappeared in a short period of time. Further, when the non-heated
portions were pressurized, a color developed.
EXAMPLE 2
(1) Microcapsules were formed and a microcapsule-containing coating color
was obtained in the same manner as in Example 1, except that
3-diethylamino-6-methyl-7-anilinofluoran in Example 1 was replaced by
Crystal Violet Lactone.
(2) Microcapsules were formed and another kind of microcapsule-containing
coating color was obtained in the same manner as in Example 1, except
that 3-diethylamino-6-methyl-7-anilinofluoran and oleamide in Example 1
were replaced by 3-diethylamino-6-methyl-7-chlorofluoran and lauramide
(m.p. 80.degree. to 90.degree. C.), respectively.
(3) A coating color (solid content 25% by weight) consisting of 5 parts by
weight (as solid) of the microcapsule-containing coating color obtained in
(1), 5 parts by weight (as solid) of the microcapsule-containing coating
color obtained in (2) and 2 parts by weight of a polyvinyl alcohol as
binder, was prepared. It was coated on a base paper of 50 g/m.sup.2 by the
use of a Meyer bar so that the coated quantity became 7.5 g/m.sup.2 as
solid.
On the coated sheet obtained, high temperature recording was made by the
use of a heat-sensitive facsimile, whereby an image of violet color which
is an intermediate color between blue and red was obtained. Low
temperature recording was also made likewise, whereby an image of blue
color developed.
Both images did not cause discoloration and fading when ethanol or dioctyl
phthalate was coated on these images. When the non-heated portions were
pressurized, there occured no color development.
EXAMPLE 3
3 Parts by weight of Crystal Violet Lactone and 6 parts by weight of
stearamide (m.p. 97.degree. to 102.degree. C.) were melted by heating. The
co-melt still being hot was poured into 50 parts by weight of an aqueous
solution dissolving 0.5% of methyl cellulose of the normal temperature.
The mixture was emulsified by the use of the TK-homomixer so as to give
particle diameters of 1 to 2 .mu. to obtain an emulsion D. Separately, 3
parts by weight of benzyl p-hydroxybenzoate and 6 parts by weight of
stearamide were melted by heating. The co-melt still being hot was poured
into 50 parts by weight of an aqueous solution dissolving 0.5% of m | | |
