[0001] This invention relates to thermal transfer receiver sheets which are used in combination
with a thermal transfer dyesheet containing thermal transfer dyes and employing heating
means (such as thermal heads) to transfer dye from the dyesheet to a dye-receiving
layer on the receiver sheet, according to image signals applied to the heating means.
The invention relates especially to an improved dye-receiving composition.
[0002] Thermal transfer printing systems have been developed in recent years for producing
images by causing thermal transfer dyes to transfer to a receiver sheet in response
to thermal stimuli. Using a dyesheet comprising a thin substrate supporting a dyecoat
containing one or more such dyes uniformly spread over an entire printing area of
the dyesheet, printing is effected by heating selected discrete areas of the dyesheet
while the dyecoat is pressed against a dye-receptive surface of a receiver sheet,
thereby causing dye to transfer to corresponding areas of the receiver. The shape
of the image thus formed on the receiver is determined by the number and location
of the discrete areas which are subjected to heating.
[0003] High resolution photograph-like prints can be produced by thermal transfer printing
using appropriate printing equipment, such as programmable thermal heads or laser
printer, controlled by electronic image signals derived from a video, computer, electronic
still camera, or similar signal generating apparatus. Thus for example a thermal print
head has a row of individually operable tiny heaters spaced to print typically six
or more pixels per millimetre. Selection and operation of these heaters is effected
according to the electronic image signals fed to the printer.
[0004] Full colour prints can be produced by printing with different coloured dyecoats sequentially
in like manner, and the different coloured dyecoats are usually provided as discrete
uniform print-size areas in a repeated sequence along the same dyesheet.
[0005] Receiver sheets comprise a substrate sheet supporting a dye-receiving layer containing
a material having an affinity for the thermal transfer dye molecules, and into which
they can readily diffuse when an adjacent area of dyesheet is heated during printing.
Such materials are mainly constituted by various aromatic or aliphatic polyesters,
as described for example in JA-A-57-107,885, JA-A-61-258,790, JA-A-1-269,589 and US
4,474,859.
[0006] When previously known aromatic and aliphatic polyesters were used for the dye-receiving
layer of the receiver sheet, one or more of a number of problems were usually experienced.
Thus with some polyesters, the maximum optical densities that could be obtained under
normal printing conditions, were insufficient, due to inferior dye-receiving properties
of the polyester used. Other problems that were often experienced included poor stability
of the image, giving deterioration of the image during storage, a fault which can
be accelerated for measuring by exposing the printed sheet to high temperature and
high humidity conditions. Light fastness of the printed image, and resistance to the
development of finger prints, are also desirable properties that are influenced by
the material selected for the dye-receiving surface. We have now devised a new receiver
sheet having an improved balance of such properties.
[0007] According to the present invention, a thermal transfer printing receiver sheet comprises
a substrate supporting a dye-receiving layer on its surface, characterised in that
the main constituent of the dye-receiving layer is a copolyester in which the acid
component comprises at least one alicyclic dicarboxylic acid, and the alcohol component
comprises at least one alicyclic diol.
[0008] Materials suitable for use as the substrate of the receiver sheet may variously include
for example, thermoplastic films, synthetic papers and cellulose fibre papers. Examples
include films or sheets of polyester, polyvinyl chloride, polypropylene, polyethylene.
polycarbonate, polyimide, polyamide, polyamideimide, especially biaxially orientated
polyethyleneterephthalate film. Examples of synthetic papers include those of fabricated
and moulded polyolefines, polystyrene, and polyesters etc. as the polymer component.
Cellulose fibre papers, woodfree paper, coated paper, art paper, synthetic rubber
latex impregnated paper, cast-coated paper, etc, can also be used. The above may be
used on their own, although a laminated substrate using a combination of two or more
of the above, may be preferred for some applications.
[0009] The substrate supports the dye-receiving layer, which is provided as a coating on
its surface for the purpose of receiving the dyes transferred from selected areas
of the dyesheet, thereby to form an image. The main constituent of the dye-receiving
layer of the present invention is a copolyester formed by using an acid component
comprising at least one alicyclic dicarboxylic acid.
[0010] Examples of alicyclic dicarboxylic acids which can be used for this purpose, include
cyclopropane dicarboxylic acid, cyclobutane dicarboxylic acid, cyclohexane-1,2-dicarboxylic
acid, cyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid, methylhexahydrophthalic
acid, and norbornene dicarboxylic acid. In furtherance of this invention, the alicyclic
acid components of the copolyester can be introduced as the acid or as the corresponding
anhydride, as appropriate. Examples of such anhydrides, for example, include hexahydrophthalic
acid anhydride, and methyl-hexahydrophthalic acid anhydride.
[0011] The acid component of the copolyester may also include other polybasic carboxylic
acids, such as those previously used on their own without the present alicyclic acids.
Such other dicarboxylic acids may include, for example, aromatic dicarboxylic acids
such as terephthalic acid, isophthalic acid, orthophthalic acid, 2,6-naphthalene dicarboxylic
acid, etc, aliphatic dicarboxylic acids, such as succinic acid, adipic acid, azelaic
acid, sebacic acid, dodecane dicarboxylic acid, etc. Tri- and tetra-carboxylic acids,
such as trimellitic acid, trimesic acid, pyromellitic acid, etc, may also be added
to the alicyclic dicarboxylic acid.
[0012] When the acid component of the copolyester comprises a mixture of alicyclic dicarboxylic
acids and other polybasic carboxylic acids, we prefer the molar ratio of the alicyclic
dicarboxylic acids to the other carboxylic acids in the mixture to be within the range
5/95 - 95/5, especially within the range 10/90 - 50/50, the range 10/90 - 30/70 being
particularly preferred with some acid combinations.
[0013] The alcohol component of the copolyester of the present invention comprises at least
one alicyclic diol. Examples of diols that can be used for this purpose include 1,4-cyclohexanedimethanol,
cyclohexane-1,2-diol, cyclohexane-1,3-diol, cyclohexane-1,4-diol, cyclopentane-1,2-diol,
cyclopentane-1,3-diol, tricyclodecanedimethylol, and ethylene oxide adduct of hydrogenated
Bisphenol A.
[0014] The alcohol component of the copolyester may also include other polyols, such as
those previously used on their own in the absence of the present alicyclic diols,
for example. Such additional other polyols may include ethylene glycol, propylene
glycol, butanediol, neopentyl glycol, diethylene glycol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol,
2-n-butyl-2-ethyl-1,3-propanediol, and ethylene oxide and/or propylene oxide adduct
of Bisphenol A, and polyols such as trimethylol propane, glycerin, pentaerithritol,
polyglycerin, etc.
[0015] When the alcohol component of the copolyester comprises a mixture of alicyclic diols
and other polyhydric alcohols, we prefer the molar ratio of the alicyclic diols to
the other alcohols in the mixture, to be within the range 5/95 - 95/5, especially
within the range 10/90 - 50/50, the range 10/90 - 30/70 being particularly preferred
with some polyol combinations.
[0016] Dye-receiving layers of the present invention can be made by coating a substrate
with a composition containing the copolyester, using any of the normal coating techniques,
such as roll coating or gravure printing, for example. The viscosity of the coating
composition can be adjusted by altering the amount of solvents present, and these
are subsequently removed from the applied layer by drying. Receivers made according
to the invention, can show properties superior to those obtained with previously known
polyesters, but such improvements are only shown when both alicyclic dicarboxylic
acid and alicyclic diol moieties are present; ie not when only one or the other is
present.
[0017] In order to improve release from the thermal transfer dye-sheet after printing, the
thermal printing receiver may be provided with a release agent in the dye-receiving
layer or on its surface. Examples of release agents include solid waxes such as polyethylene
wax, amide wax, polytetrafluoroethylene powder, fluorine- or phosphate-type surfactants,
and especially silicone oils.
[0018] Both oil type and solid type silicone oils can be used, but curable silicone materials
are preferred. Where such release system is to be used on the surface of the receiver,
the uncured silicone materials are first applied as an uncured coating composition
on the dye-receiving layer, dried as appropriate, and thereafter cured in situ. Where
the release system is to be contained in the dye-receiving layer, the curable materials
are mixed with the copolyester in a coating composition, which is then applied to
the substrate and dried, curing of the release system then being effected in situ.
Such cross-linked matrices these produce within the dye-receiving layer help to stabilise
it. They can be reaction curing types, photocuring type or catalytic curing type,
for example, but the reaction curing type is preferred.
[0019] Examples of reaction curing release systems include the reaction products of amino-modified
silicone oils, such as KF-393, KF-857, KF-858, X-22-3680, X-22-3810 (these being products
of Shin-Etsu Chemical), and M468 (ICI), reacted with an epoxy-modified silicone such
as KF-100T, KF-101, KF-103 (these also being products of Shin-Etsu Chemical), or with
an organic oligoepoxide free from silicone, such as Diepoxide 126, Diepoxide 183 (both
being products of Degussa), and Araldite GY 1558 GB (Ciba Geigy). Preferred quantities
of such release systems in the dye-receiving layer are 0.05-20% of the copolyester.
[0020] The cross-linking of the copolyester can also be enhanced, by including an effective
amount of a cross-linking agent in the coating composition, applying this as a layer
to the substrate, and carrying out the cross-linking reaction in situ, ie in essentially
the same manner as that described above for the release system. Examples of crosslinking
agents include compounds having isocyanate groups, compounds having active methylol
or alkoxy methyl groups, acid anhydride, carboxy groups, or epoxy groups. It is generally
preferred to limit the crosslinking agent to 0.5-20 wt % of the copolyester.
[0021] In addition to the copolyester of the present invention and any polymeric release
agent that might be employed, the dye-receiving layer may also contain a minor amount
of other polymeric materials intimately mixed with the copolyester. Examples of such
other polymeric materials include polyvinyl chloride, polyvinyl acetate, vinyl chloride/vinyl
acetate coplymer, polystyrene, polycarbonate, acrylic and methacrylic polymers, cellulosic
polymers, polyacetal, polyethylene, and polypropylene; and even thermosetting resins
such as epoxy, melamine, urethane and urea type resins can be present in small quantities.
By minor amount, we mean less than 50 % by weight, and prefer there to be less than
20 % of the other polymer. For most purposes we particularly prefer that no such other
polymer be included in the dye-receiving layer, any polymeric material in the dye-receiving
layer being limited to the copolyester and any polymeric release agent.
[0022] The dye-receiving layer may also contain stabilisers for combating fading and/or
a UV absorber. Suitable quantities are 0.5-20 wt % of the copolyester.
[0023] To obtain a smoother gradation of colour, especially when using a hard substrate,
can be obtained by providing an intermediate compliant layer between the substrate
and the dye-receiving layer. Such layers may be in the form of a compliant cushion
or a porous layer, and these may be formed from polyurethane, polyester, polyamide,
acrylic or methacrylic polymers, or synthetic rubber. The thickness of such layers
may suitably be in the range 1-50 µm.
[0024] In order to control the formation of static electricity, during the fabrication process
or while travelling through the printer with the dyesheet, an antistatic agent can
be coated onto or incorporated with one or more coating of the receiver. We prefer
that antistatic treatments be applied to both sides of the substrate.
EXAMPLES
[0025] Evaluation of thermal printing receiver sheets according to the invention was made
by transferring images onto the dye-receiving layer using a thermal head. The receiver
sheet was placed against a dyesheet with its dye-receiving layer in contact with the
dyecoat of the dyesheet (details of the latter being specified in the example below).
Heat was applied to the back of the dyesheet by a thermal head having an output energy
of 0.32 W/dot, a head heating time of 6 ms and a dot density of 6 dots/mm. The printed
sheets thus formed were evaluated in respect of their optical density, light fastness,
storage stability, and finger print resistance, in the following manner.
Optical density
[0026] The image density was measured on a SAKURA optical density meter PDA 85.
Light fastness
[0027] The hue of the image was measured before and after accelerated aging of the print
by ATRAS HPUV, to determine the change in hue which occurred during the aging cycle.
Storage stability
[0028] The printed receiver sheet was stored in an atmosphere of 45°C and 85% relative humidity,
for 2 weeks. The hue was again measured before and after treatment, to determine the
change of hue occurring under those conditions.
Finger print resistance
[0029] The optical density of the printed image was determined. A finger print was then
made on the image surface, and the print left in an atmosphere of 45°C and 85% relative
humidity for 2 weeks. After that time the optical density was again measured of the
part of the print on which the finger print had been made. The change in optical density
was recorded as a percentage of the initial measurement.
Production of copolyester (A)
[0030] 245.6 parts of dimethyl terephthalic acid, 69.4 parts of ethylene glycol, 2.9 parts
of trimethylol propane, 211.1 parts of 2-n-butyl-2-ethyl-1,3-propanediol, 118.5 parts
of 1,4-cyclohexane dimethanol and 0.1 part of zinc acetate were placed into a flask
installed with a thermometer, nitrogen gas inlet tube, reflux dehydrator and stirrer,
and it was heated at 170°C - 220°C for 5 hours. During the heating, methanol formed
was evaporated outside the system. Then it was cooled at 170°C, 105.1 parts of isophthalic
acid and 97.5 parts of hexahydrophthalic anhydride were added and the mixture heated
at 170°C - 230°C for 6 hours. During the heating, water formed was evaporated outside
the system. The reflux dehydrator was then replaced with a vacuum pressure reducer,
and after an addition of 0.1 part of antimony trioxide, it was heated at 260°C and
5 mmHg pressure for 4 hours, to perform the reduced pressure condensation reaction
from which the copolyester (A) was obtained.
[0031] The reactants used for the production of the copolyester (A) are summarised below.
![](https://data.epo.org/publication-server/image?imagePath=1992/12/DOC/EPNWA2/EP91307857NWA2/imgb0002)
Example 1
Preparation of thermal transfer receiver sheet
[0032] Receiver sheet (a′) was prepared by wire bar coating one surface of a polyester film
with an ink composition (a), and drying this to produce a dye-receiving layer of approximately
5 µm thickness. The polyester film was Melinex 990 (Melinex is a trade mark of Imperial
Chemical Industries PLC, hereinafter referred to as ICI). Ink composition (a) contained
copolyester (A) as the main constituent of the dye-receiving layer, a release system
also being present as a minor constituent. The full composition of ink composition
(a), is set out below.
![](https://data.epo.org/publication-server/image?imagePath=1992/12/DOC/EPNWA2/EP91307857NWA2/imgb0003)
Preparation of the thermal transfer dye sheet
[0033] A slipping layer of silicone oil was formed on one side of a 6 µm polyester substrate
film (Lumilar Toray product). Then an ink composition (I) for the thermal transfer
printing was prepared as described below, and was coated onto the other surface of
the substrate from that coated with the slipping layer. The ink composition (I) was
then dried to form a 1.0 µm thick dyecoat, to complete the thermal transfer dye sheet
(I′). Details of the ink composition were as follows:
![](https://data.epo.org/publication-server/image?imagePath=1992/12/DOC/EPNWA2/EP91307857NWA2/imgb0004)
[0034] Then the thermal transfer dye sheet (I′) and the thermal transfer printing receiver
sheet (a′) were held together, and an image was formed in the dye-receiving layer
by heating with a thermal head. The optical density, light fastness, storage stability
and finger print resistance of the printed sheet thus produced, were evaluated. The
results are shown in Table I.
Example 2
[0035] An ink composition (b) for forming a dye receiving layer was prepared in the similar
manner to that in Example 1, but using copolyester (B) as the main constituent. A
thermal transfer printing receiver sheet (b′) was prepared by coating this ink onto
Melinex 990 and drying, as described above. Using the thermal transfer dyesheet (I′)
to provide the transfer dyes, a printed sheet was obtained by forming an image in
the dye-receiving layer of receiver sheet (b′), by heating with a thermal head. The
optical density, light fastness, storage stability and finger print resistance were
then evaluated. The results are shown in Table I.
Example 3
[0036] An ink composition (c) was prepared in the similar manner to the two preceding Examples,
but using copolyester (C) as the main constituent, and thermal transfer printing receiver
sheet (c′) was prepared and printed in like manner. The optical density, light fastness,
storaae stability and finger print resistance of the resultant print were then evaluated,
and the results are shown in Table I
COMPARATIVE EXAMPLES
[0037] Copolyester resins (D) and (E) were prepared from the compositions shown below, in
similar manner to those of Examples 1 to 3 above.
Copolyester (D)
[0038]
- dimethyl phthalic acid
- 187.0 parts
- isophthalic acid
- 240.0 parts
- ethylene glycol
- 77.0 parts
- neopentyl glycol
- 195.4 parts
Copolyester (E)
[0039]
- dimethyl phthalic acid
- 187 0 parts
- isophthalic acid
- 240.0 parts
- trimellitic anhydride
- 13.7 parts
- ethylene glycol
- 77.0 parts
- neopentyl glycol
- 195.4 parts
Comparative Example C1
[0040] An ink composition (d) for forming the dye-receiving layer was prepared in a similar
manner to that of Example I but using copolyester (D) as the main constituent, and
a corresponding thermal transfer printing receiver sheet (d′) was prepared, again
using Melinex 990 as substrate. Thermal transfer dyesheet (I′) and thermal transfer
printing receiver sheet (d′) were passed together through a thermal printer, and a
printed sheet was obtained by forming an image in the receiver layer by heating with
the thermal head. Optical density, light fastness, storage stability and finger print
resistance of the printed sheet, were evaluated. The results are shown in Table 1.
Comparative Example C2
[0041] An ink composition (e) for forming the dye receiving layer was prepared in similar
manner to those of the preceding Examples, but using copolyester (E) as the main constituent.
Receiver sheet (e′) was prepared by coating Melinex 990 with a layer of e. This was
printed as before using thermal transfer dyesheet (I′) and the optical density, the
light fastness, the storage stability and the finger printing resistance were evaluated.
The results are shown in Table I.
![](https://data.epo.org/publication-server/image?imagePath=1992/12/DOC/EPNWA2/EP91307857NWA2/imgb0005)
[0042] To summarise these observations, when general polyester resins as used previously,
were used as the main constituent of the dye receiving layer, it was difficult to
satisfy all the required properties of optical density, light fastness, storage stability
and finger print resistance. However, by using copolyesters obtained by reacting at
least one alicyclic dicarboxylic acid as the acid component for the copolymerization
and at least one alicyclic diol as the alcohol component, these properties can be
largely improved. Such receivers enable a printed sheet with an image of high quality,
having a superior balance of properties with respect to optical density, light fastness,
storage stability and finger print resistance, to be obtained.