Thermal dye sublimation transfer recording element

Thermal dye sublimation transfer recording element for receiving sublimable basic dye-precursors, comprising a support having thereon a dye-developing layer containing a dye-developing copolymer having sulfonic acid side-groups that can react with the basic dye-precursor to produce a dye image, characterized in that said dye-developing vinyl copolymer comprises plasticizing comonomers, the weight percentage of plasticizing comonomers in the dye-developing vinyl copolymer being such that the glass transition temperature of the dye-developing vinyl copolymer is between 30.degree. C. and 90.degree. C.

DESCRIPTION 
This invention relates to thermal dye sublimation transfer recording, and 
more particularly, to dye-developing layers of recording elements for use 
in thermal dye sublimation transfer recording. 
In thermal dye sublimation transfer recording a donor element coated with a 
sublimable coloring material is brought into contact with a recording 
element and information-wise heated, for example, with a thermal head 
provided with a plurality of juxtaposed heat generating resistors. 
Coloring material from the selectively heated regions of the donor element 
is transferred to the recording element and forms a pattern thereon. The 
shape and density of this color pattern is in accordance with the pattern 
and intensity of heat applied to the donor element. 
As the coloring material there is used a substance which is prepared by 
kneading a binder and sublimable dye. Any dye can be used provided it is 
transferable to the dye-receiving layer of the recording element by the 
action of heat. Examples of dyes for use in thermal dye sublimation 
transfer are described in, e.g., EP 209990, EP 209991, EP 216483, EP 
218397, EP 227095, EP 227096, EP 229374, EP 257577 and EP 257580. 
Also sublimable dye-precursors can be used which, when heated, sublimate to 
react with a dye-developer present in the recording sheet to produce a dye 
image. These dye-precursors can be colorless or colored., if colored their 
color may change by reaction with dye-developer. 
A dye-developer contains functional groups which serve as color-developing 
sites. Sublimated dye-precursors penetrate into the dye-developing layer 
and chemically combine and/or absorb on the color-developing sites. 
The dye-developer can be a low molecular weight compound that is mixed with 
a binder to form the dye-developing layer. Alternatively, the 
color-developing sites can be incorporated into the polymeric binder 
itself, for example, by copolymerization with comonomers containing the 
color-developing sites. The polymeric binder containing the 
color-developing sites then forms the color-developing layer. 
When basic dye-precursors are used acid groups serve as color-developing 
sites. For example such acid groups as described in Japanese published 
patent application no. 84/101395 can be used. Preferably these acid groups 
are incorporated into the polymeric binder, for example, by 
copolymerization of monomers containing acid groups such as styrene 
sulfonic acid or 2-acrylamido,2-methylpropane sulfonic acid with vinyl 
monomers such as ethylene, propyleen, vinyl chloride, vinylidene chloride, 
vinyl fluoride, vinylidene fluoride, styrene, vinyl alcohol, acrylic acid, 
methyl acrylate, methacrylic acid, methyl methacrylate, vinyl acetate and 
acrylonitrile. Unfortunately depending upon the type of binder 
incorporating these acid groups the density of the developed color image 
is not always very high. 
It is an object of the present invention to provide a dye-developing layer 
for basic dye-precursors that yield developed color images with improved 
density. 
Other objects will become apparent from the description hereinafter. 
According to the present invention there is provided a recording element 
for receiving sublimable basic dye-precursors, which comprises a support 
having thereon a dye-developing layer containing a dye-developing vinyl 
copolymer having sulfonic acid side-groups that can react with the basic 
dye-precursor to produce a dye image, characterized in that said 
dye-developing vinyl copolymer comprises plasticizing comonomers, the 
weight percentage of plasticizing comonomers in the dye-developing vinyl 
copolymer being such that the glass transition temperature of the 
dye-developing vinyl copolymer is between 30.degree. C. and 90.degree. C. 
When the glass transition temperature of the dye-developing vinyl copolymer 
is lower than 30.degree. C., the dye-developing layer may pose problems of 
adhering to the donor element when heating the assemblage of donor and 
recording element. 
When the glass transition temperature of the dye-developing vinyl copolymer 
is higher than 90.degree. C., the dye-developing layer cannot sufficiently 
receive the dye-precursor transferred from the donor element, whereby a 
clear developed color image cannot be obtained. 
By incorporating plasticizing comonomers into the dye-developing vinyl 
copolymer having sulfonic acid side-groups the glass transition 
temperature of the copolymer decreases., thus the penetration of the 
dye-precursor into the dye-developing layer is improved, leading to higher 
density of the developed color image. 
The main constituent units of the dye-developing vinyl copolymer can be, 
for example, units of ethylene, propyleen, vinyl chloride, vinylidene 
chloride, vinyl fluoride, vinylidene fluoride, styrene, vinyl alcohol, 
acrylic acid, methyl acrylate, methacrylic acid, methyl methacrylate, 
butyl methacrylate, vinyl acetate or acrylonitrile. 
Preferably the comonomer containing the sulfonic acid side-group is styrene 
sulfonic acid or 2-acrylamido,2-methylpropane sulfonic acid or derivatives 
thereof. 
The amount of sulfonic acid side-groups in the dye-developing vinyl 
copolymer is not very critical as long as a minimum amount of 
approximately 4 wt % comonomers containing the sulfonic acid side-groups 
is reached. 
Plasticizing comonomers that can be used according to the present invention 
are, for example, ethyl acrylate, butyl acrylate, octyl acrylate, 
2-ethylhexyl acrylate, dodecyl acrylate, butyl methacrylate, heptyl 
methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl 
methacrylate, tetradecyl methacrylate, vinyl acetate, vinyl propionate, 
vinyl butyrate, vinylmethyl ketone, vinylethyl ketone, vinylbutyl ketone, 
vinyl butyral, vinylidene chloride, vinylidene fluoride, silane, 
butadiene, isoprene. 
The approximate weight percentage of plasticizing comonomers necessary to 
obtain the desired glass transition temperature of the dye-developing 
vinyl copolymer (T.sub.g) can be calculated in accordance with the formula 
##EQU1## 
wherein w(n) refers to the weight fractions of the comonomers, whereas 
T.sub.g (n) refers to the glass transition temperatures of the 
corresponding homopolymers. 
Examples of copolymers according to the present invention are listed in 
table 1. 
TABLE 1 
__________________________________________________________________________ 
##STR1## (1.1) 
__________________________________________________________________________ 
with n=1.8 a 2.0, x=65.8 wt %, y=24.5 wt %, z=4.4 wt %, w=4.8 wt % 
##STR2## 
with y=80.4 wt %, z=15.1 wt %, w=4.5 wt % 
##STR3## 
with x=27.1 wt %, y=63.3 wt %, z=5 wt %, w=4.6 wt % 
The dye-developing vinyl copolymer according to the present invention can 
be used as a water-dispersible latex or as a solution in an organic 
solvent. 
In order to improve the light resistance of recorded images and stabilities 
against other influences, UV absorbers and/or antioxidants may be 
incorporated into the dye-developing layer. 
The support onto which the dye-developing layer of the present invention is 
coated may be a transparant film such as a poly(ethylene terephthalate), a 
poly(ether sulfone), a polyimide, a cellulose ester or a poly(vinyl 
alcohol-co-acetal). The support may also be reflective such as 
baryta-coated paper, polyethylene-coated paper or white polyester 
(polyester with white pigment incorporated therein). 
The recording element described above is used in combination with a donor 
element comprising a support and a coloring material layer containing a 
basic dye-precursor. 
Examples of basic dye-precursors are listed in table 2. 
TABLE 2 
__________________________________________________________________________ 
##STR4## (2.1) 
##STR5## (2.2) 
##STR6## (2.3) 
##STR7## (2.4) 
##STR8## (2.5) 
##STR9## (2.6) 
##STR10## (2.7) 
##STR11## (2.8) 
##STR12## (2.9) 
##STR13## (2.10) 
##STR14## (2.11) 
##STR15## (2.12) 
##STR16## (2.13) 
##STR17## (2.14) 
##STR18## (2.15) 
##STR19## (2.16) 
__________________________________________________________________________ 
Suitable basic dye-precursors are selected from among the above 
dye-precursors taking into consideration the heat transfer temperature and 
efficiency, hue, color rendering and weatherability. 
The dye-precursor is dispersed in a suitable synthetic resin binder and 
then applied onto the support. The following polymeric binders can be 
used: cellulose derivatives, e.g., cellulose acetate hydrogen phthalate, 
cellulose acetate, cellulose acetate propionate, cellulose acetate 
butyrate, cellulose triacetate; vinyl resins and derivatives, such as 
poly(vinyl alcohol), poly(vinyl acetate), poly(vinyl butyral), poly(vinyl 
pyrrolidone), polymers and copolymers derived from acrylates and acrylate 
derivatives, such as poly(acrylic acid), poly(methyl methacrylate) and 
styrene-acrylate copolymers, polyester resins, polycarbonates, 
copoly(styrene-acrylonitrile), polysulfones, poly(phenylene oxide), 
organosilicones, such as polysiloxanes, epoxy resins and natural resins, 
such as gum arabic, or mixtures thereof. 
Any material can be used as the support for the donor element provided it 
is dimensionally stable and capable of withstanding the temperatures 
involved, up to 400.degree. C. over a period of up to 20 msec, yet thin 
enough to transmit heat applied on one side through to the dye-precursor 
on the other side to effect transfer to the recording element within such 
short periods, typically from 1 to 10 msec. Such materials include 
polyesters such as poly(ethylene terephthalate), polyamides, 
polyacrylates, polycarbonates, cellulose esters, fluorine polymers, 
polyethers, polyacetals, polyolefins, polyimides, glassine paper and 
condenser paper. The support may also be coated with a subbing layer, if 
desired. 
The coloring material layer of the donor element may be coated on the 
support or printed thereon by a printing technique such as a gravure 
process. 
A barrier layer preventing wrong-way transfer of dye-precursor into the 
support may also be employed in the donor element between its support and 
the coloring material layer. 
The reverse side of the donor element may be coated with a slipping layer 
to prevent the printing head from sticking to the donor element. Such a 
slipping layer would comprise a lubricating material such as a surface 
active agent, a liquid lubricant, a solid lubricant or mixtures thereof, 
with or without a polymeric binder. The surface active agents may be any 
of the surface active agents which are known in the art such as 
carboxylates, sulfonates, phosphates, aliphatic amine salts, aliphatic 
quaternary ammonium salts, polyoxyethylene alkyl ethers, polyethylene 
glycol fatty acid esters, fluoroalkyl C.sub.2 -C.sub.20 aliphatic acids. 
Examples of liquid lubricants include silicone oils, synthetic oils, 
saturated hydrocarbons and glycols. Examples of solid lubricants include 
various higher alcohols such as stearyl alcohol, fatty acids and fatty 
acid esters. 
The donor element employed in certain embodiments of the invention may be 
used in sheet form or in a continuous roll or ribbon. If a continuous roll 
or ribbon is employed, it may have only one dye-precursor thereon or may 
have alternating areas of different dye-precursors leading to different 
colors, such as cyan, magenta, yellow. 
The coloring material layer of the donor element or the dye-developing 
layer of the recording element may also contain a release agent that aids 
in separating the donor element from the recording element after transfer. 
The release agents can also be applied in a separate layer on at least 
part of the coloring material layer or of the dye-developing layer. For 
the releasing agent solid waxes, fluorine- or phosphate-containing 
surfactants and silicone oils can be used. 
The coloring material layer of the donor element is placed in face-to-face 
relation with the color-developing layer and heat printing is carried out 
from the back of the donor element. The transfer of the dye-precursor is 
accomplished by heating for about several milliseconds at a temperature of 
400.degree. C. 
In addition to thermal heads, laser light, infrared flash or heated pens 
can be used as the heat source for supplying heat energy. 
A multicolor image can be obtained by using a donor element containing 
three primary color dye-precursors and sequentially performing the process 
steps described above for each color.

The present invention will now be described in detail with reference to the 
following examples that by no means limit the scope of the present 
invention. 
EXAMPLE 1 
As support of the donor element a 5 .mu.m polyethylene terephthalate film 
was used. The back-side of this support was coated with a slipping layer 
comprising polyvinyl butyral acetal and a silicon lubricant 
(polydimethylsiloxane-polyether). 
The coloring layer comprised a basic dye-precursor selected from among the 
basic dye-precursors listed in table 2, dispersed in cellulose acetate 
propionate binder (dye-precursor/binder 1:1 weight ratio) with 
tetrahydrofuran or butanone as solvent (total concentration of solids: 1 
wt %). 
As recording element I a polyethylene coated paper provided with a subbing 
layer and coated with a dye-developing layer containing: 5 g of 
dye-developing latex copolymer (1.1) of table 1 (concentration of solids: 
31.2 wt %), 0.6 g of polyurethane latex 40 wt %, 0.6 g of polybutyl 
acrylate latex 20 wt %, wetting agents and B g of water was used. 
The above described donor elements were each heat-transfer printed in 
combination with the above described recording element. The densities of 
the obtained dye images (D.sub.max) measured through different color 
filters are listed below in table 3. 
TABLE 3 
______________________________________ 
no. basic D.sub.max (filter) 
dye-precursor 
red green blue 
______________________________________ 
2.1 0.14 1.68 1.05 
2.3 1.22 0.31 0.15 
2.4 0.06 0.09 0.82 
2.6 0.08 0.84 1.03 
2.7 1.29 1.34 0.29 
______________________________________ 
EXAMPLE 2 
A donor element, prepared as in example 1, containing dye-precursor (2.7), 
was used. 
The following recording elements with dye-developing layers as described 
below were prepared. 
Recording element II: 4 g of latex copolymer (1.1) of table 1, 0.6 g of 
polybutyl acrylate latex, 1.8 g of polysiloxane-polyether 5 wt % as 
wetting agent and 10 g of water. 
Recording element III: 10 ml of latex copolymer (1.2) of table 1 (18% in 
water), 0.6 ml polysiloxane-polyether 5 wt % and 10 ml water. 
Recording element IV: 10 ml latex copolymer (1.3) of table 1 (19% in 
water), 0.6 ml polysiloxane-polyether 5% and 10 ml water. 
Recording element V: 10 ml of copolymer (23% in water) consisting of 68.5 
wt % of acrylonitrile and 31.5 wt % of 2-acrylamido,2-methylpropane 
sulfonic acid, 1 ml of polysiloxane-polyether 5 wt % and 1 ml of polybutyl 
acrylate latex 20 wt %. 
Recording element VI: 10 ml of copolymer (25% in water) consisting of 74.5 
wt % of acrylonitrile and 25.5 wt % of styrene sulfonic acid, 1 ml of 
polysiloxane-polyether 5 wt % and 0.25 ml of polybutyl acrylate latex 20 
wt %. 
Recording element VII: 10 ml of copolymer (10 wt % in DMSO) consisting of 
67 wt % of acrylonitrile and 32 wt % of 3-methacryl propane sulfonic acid, 
0.5 ml of polysiloxane-polyether 5 wt %. 
Recording element VIII: 7 ml of copolymer (30 wt % in water) consisting of 
5.6 wt % of styrene and 94.4 wt % of styrene sulfonic acid, 0.6 ml of 
polysiloxane-polyether 5 wt % and 10 ml water. 
The above described recording elements were each heat-transfer printed in 
combination with the above described donor element. the densities of the 
obtained dye images (D.sub.max) were measured through a red filter. 
The T.sub.g values of the dye-developing copolymers were measured with a Du 
Pont Model 910 Differential Scanning Calorimeter (DSC). 
TABLE 4 
______________________________________ 
recording dye-developing 
D.sub.max (filter) 
element copolymer T.sub.g 
red 
______________________________________ 
II 85.8.degree. C. 
0.79 
III 64.3.degree. C. 
1.30 
IV 44.4.degree. C. 
0.88 
V 120.4.degree. C. 
0.46 
VI 120.4.degree. C. 
0.66 
VII 134.9.degree. C. 
0.53 
VIII .+-.100.degree. C. 
weak 
______________________________________ 
The results listed in table 4 clearly show that when the T.sub.g value of 
the dye-developing copolymer is between 30.degree. C. and 90.degree. C. 
(as in recording elements II, III and IV) the dyeability of the 
dye-developing copolymers is very good (D above 0.7). With a T.sub.g above 
100.degree. C. the dyeability is poor (recording elements V, VI, VII and 
VIII).