 |
Description  |
|
|
This specification describes an invention relating to thermal transfer
printing (TTP), especially to a TTP sheet carrying a dye or dye mixture
In thermal transfer printing a heat-transferable dye is applied to a
sheet-like substrate in the form of an ink, usually containing a polymeric
or resinous binder to bind the dye to the substrate, to form a transfer
sheet. This is then placed in contact with the material to be printed, the
receiver sheet, and selectively heated in accordance with a pattern
information signal whereby dye from the selectively heated regions of the
transfer sheet is transferred to the receiver sheet and forms a pattern
thereon in accordance with the pattern of heat applied to the transfer
sheet.
Important criteria in the selection of a dye for TTP are its thermal
properties, its brightness of shade, its fastness properties, such as
light fastness, and its facility for application to the substrate in the
preparation of the transfer sheet. For suitable performance the dye should
transfer evenly, in proportion to the heat applied to the TTP sheet so
that the depth of shade on the receiver sheet is proportional to the heat
applied and a true grey scale of coloration can be achieved on the
receiver sheet. Brightness of shade is important in order to achieve as
wide a range of shades with the three primary dye shades of yellow,
magenta and cyan. For this reason anthraquinone dyes are preferred
candidates for use in TTP processes.
As the dye should be sufficiently mobile to migrate from the transfer sheet
to the receiver sheet at the temperatures employed, from
200.degree.-400.degree. C., it is generally free from water-solubilising
and ionic groups, and is thus not readily soluble in aqueous or
water-miscible media, such as water and alkanols. Many suitable dyes are
also not readily soluble in the hydrocarbon solvents which are commonly
used in, and thus acceptable to, the printing industry. Although the dye
can be applied to a dispersion in a suitable solvent, it has been found
that brighter, glossier and smoother final prints can be achieved on the
receiver sheet if the dye is applied to the substrate from a solution. In
order to achieve the potential for a deep shade on the receiver it is
desirable that the dye should be readily soluble in the ink medium,
particularly if it has a relatively low extinction coefficient, as in the
case with anthraquinone dyes. It is also important that a dye which has
been applied to a transfer sheet from a solution should be resistant to
crystallisation so that it remains as an amorphous layer on the transfer
sheet for a considerable time.
According to a first aspect of the present invention there is provided a
thermal transfer printing sheet comprising a substrate having a coating
comprising an anthraquinone dye of the formula:
##STR2##
wherein R is C.sub.1-6 -alkyl, C.sub.4-8 -cycloalkyl or C.sub.2-6
-alkenyl;
R.sup.1 is C.sub.1-6 -alkyl or C.sub.2-6 -alkenyl;
and
R.sup.2 is H or C.sub.1-6 -alkyl or C.sub.2-6 -alkenyl.
It is preferred that group represented by R is branched alkyl and more
especially C.sub.3-5 -alkyl; an especially preferred species being
iso-propyl. Examples of other groups represented by R are sec-butyl,
iso-butyl, t-butyl, allyl, n-propyl, 2-methylbutyl and cyclohexyl.
It is preferred that R.sup.2 is H and that R.sup.1 is in a para position
with respect to the amino bridging group. It is especially preferred that
R.sup.1 is methyl. Examples of other groups represented by R.sup.1 and
R.sup.2 are ethyl, n-propyl, iso-propyl, t-butyl, n-butyl and n-hexyl.
Rings A nd B may be substituted in the remaining positions by non-ionic
groups, preferably those which are free from acidic hydrogen atoms unless
the latter are positioned so that they form intra-molecular hydrogen
bonds. Examples of suitable substituents are halogen, especially bromine
and chloride, alkyl, especially C.sub.1-6 -alkyl, and hydroxy, especially
in positions adjacent to the 9,10-carbonyl groups of the anthraquinone
nucleus.
Specific examples of preferred dyes of Formula I for use in the present
invention are set out as follows:
______________________________________
Dye R R.sup.1 R.sup.2
______________________________________
1 CH(CH.sub.3).sub.2
p-CH.sub.3 H
2 CH.sub.3 p-CH.sub.3 H
3 CH.sub.3 m-CH.sub.3 H
##STR3## m-CH.sub.3 H
5
##STR4## p-CH.sub.3 H
6 CH.sub.3 p-(n-C.sub.4 H.sub.9)
H
7 CH(CH.sub.3).sub.2
p-(n-C.sub.4 H.sub.9)
H
8 CH(CH.sub.3).sub.2
p-CH.sub.3 m-CH.sub.3
9 n-C.sub.6 H.sub.13
p-CH.sub.3 H
10 CH(CH.sub.3)CH.sub.2 CH.sub.3
p-CH.sub.3 H
11 CH.sub.2 CHCH.sub.2
p-CH.sub.3 H
12 CH.sub.3 CH.sub.2 CHCH.sub.2
H
______________________________________
The dye of Formula I has good thermal properties giving rise to even prints
on the receiver sheet, whose depth of shade is accurately proportional to
the quantity of applied heat so that a true grey scale of coloration can
be attained.
The dye of Formula I has strong coloristic properties and good solubility
in a wide range of solvents, especially those solvents which are widely
used and accepted in the printing industry, such as alkanols, e.g. ethanol
& butanol, aromatic hydrocarbons, such as toluene and ketones such as MEK,
MIBK and cyclohexanone. This facilitates the application of the dye to the
substrate from a solution and thus aids in the achievement of bright,
glossy prints on the receiver sheet. The combination of strong coloristic
properties and good solubility in the preferred solvents allows the
achievement of deep and even shades.
The substrate may be any convenient sheet material capable of withstanding
the temperatures involved in TTP, up to 400.degree. C. over a period of up
to 20 milliseconds (msec) yet thin enough to transmit heat applied on one
side through to the dye on the other side to effect transfer to a receiver
sheet within such short periods, typically from 1-10 msec. Examples of
suitable materials are paper, especially high quality paper of even
thickness, such as capacitor paper, polyester, polyacrylate, polyamide,
cellulosic and polyalkylene films, metallised forms thereof, including
co-polymer and laminated films, especially laminates incorporating a
polyester layer. An especially preferred substrate comprises a laminate of
a polyester layer sandwiched between two heat resistant layers of a
polymer, such as a UV-cured acrylic resin. The acylic resin serves to
protect the polyester from the heat source during printing and to inhibit
diffusion of dye into the transfer sheet. The thickness of the substrate
may vary within wide limits depending upon its thermal characteristics but
is preferably less than 50 .mu.m and more preferably below 10 .mu.m.
The coating preferably comprises a binder and one or more dyes of Formula
I. The ratio of binder to dye is preferably from at least 1:1 up to at
least 10:1 and more preferably from 1.5:1 to 4:1 in order to provide good
adhesion between the dye and the substrate and inhibit migration of the
dye during storage.
The binder may be any resinous or polymeric material suitable for binding
the dye to the substrate which has acceptable solubility in the ink
medium, i.e. the medium in which the dye and binder are applied to the
transfer sheet. Examples of binders include cellulose derivatives, such as
ethylhydroxyethylcellulose (EHEC), hydroxypropylcellulose (HPC),
ethylcellulose, methylcellulose, cellulose acetate and cellulose acetate
butyrate; carbohydrate derivatives, such as starch; alginic acid
derivatives; alkyd resins; vinyl resins and derivatives, such as polyvinyl
alcohol, polyvinyl acetate, polyvinyl butyral and polyvinyl pyrrolidone;
polymers and co-polymers derived from acylates and acrylate derivatives,
such as polyacrylic acid, polymethyl methacrylate and styrene-acrylate
copolymers, polyester resins, polyamide resins, such as melamines;
polyurea and polyurethane resins; organosilicones, such as polysiloxanes,
epoxy resins and natural resins, such as gum tragacanth and gum arabic.
It is, however, preferred to use a binder which is readily soluble in one
of the aforementioned commercially-acceptable organic solvents. Preferred
binders of this type are EHEC, particularly the low and extra low
viscosity grades, and ethylcellulose.
The coating may also contain other additives, such as curing agents,
preservatives, etc., these and other ingredients being described more
fully in EP No. 133011A, EP No. 133012A and EP No. 111004A.
According to a further feature of the present invention there is provided a
transfer printing process which comprises contacting a transfer sheet
coated with a dye of Formula I with a receiver sheet, so that the dye is
in contact with the receiver sheet and selectively heating areas of the
transfer sheet whereby dye in the heated areas of the transfer sheet may
be selectively transferred to the receiver sheet.
The receiver sheet is conveniently a white polyester base, suitable for
photographic film, preferably having a superficial coating of a
co-polyester into which the dye readily diffuses in order to promote
transfer of dye from the transfer sheet to the receiver sheet.
The invention is further illustrated by the following examples in which all
parts are by weight unless otherwise indicated.
Ink 1
A solution of 3 g of 1-iso-propylamino-4-(4-methylphenylamino)-AQ in 20 g
of cyclohexanone, 30 g of toluene and 17 g of MEK was prepared and stirred
for 5 minutes after which 30 g of a 20% solution of EHEC (extra-low
viscosity grade) in toluene was added. The ink was stirred for a further
30 minutes with gentle heat to ensure complete dissolution of the solid
ingredients.
Ink 2 to Ink 8
A further 7 inks were prepared by dissolving a sample of each of the dyes
defined in Table 1 (all of Formula I) in chloroform to make a solution
containing 0.45% of dye followed by sufficient EHEC to give a binder level
of 0.9% (dye:binder 1:2).
TABLE 1
______________________________________
Ink Dye R R.sup.1 R.sup.2
______________________________________
1 1 CH(CH.sub.3).sub.2
p-CH.sub.3
H
2 2 CH.sub.3 p-CH.sub.3
H
3 3 CH.sub.3 m-CH.sub.3
H
4 4
##STR5## m-CH.sub.3
H
5 5
##STR6## p-CH.sub.3
H
6 6 CH.sub.3 p-(n-C.sub.4 H.sub.9)
H
7 7 CH(CH.sub.3).sub.2
p-(n-C.sub.4 H.sub.9)
H
8 8 CH(CH.sub.3).sub.2
p-CH.sub.3
m-CH.sub.3
______________________________________
Example 1
A transfer sheet, hereinafter called TS1, was prepared by applying Ink 1 to
a 6 micron sheet of polyethyelene terephthalate using a wire-wound metal
Mayr-bar to produce a 2 micron layer of ink on the surface of the sheet.
The ink was dried with hot air.
Examples 2 to 8
A further 7 transfer sheets in accordance with the present invention,
transfer sheets TS2 to TS8, were prepared according to the procedure of
Example 1 using each of Ink 2 to Ink 7, respectively, in place of Ink 1.
Example 9
A sample of TS 1 was sandwiched with a receiver sheet, comprising a
composite structure based in a white polyester base having a copolyester
receptor surface with the receptor surface of the latter in contact with
the printed surface of the former. The sandwich was placed on the drum of
a transfer printing machine and passed over a matrix of closely-spaced
pixels which were selectively heated in accordance with a pattern
information signal to a temperature of <300.degree. C. for a period of
2-10 msec, whereby the dye at the position on the transfer sheet in
contact with a pixel while it is hot is is transferred from the transfer
sheet to the receiver sheet. After passage over the array of pixels the
transfer sheet was separated from the receiver sheet. The printed receiver
sheet is hereinafter referred to as RS 1.
Examples 10 to 16
The procedure of Example 9 was repeated using each of transfer sheets TS2
to TS8 in place of TS1 and the printed receiver sheets are hereinafter
referred to as RS2 to RS8.
Assessment of Ink, and Transfer & Receiver Sheets
The stability of the inks and the quality of the print on the transfer
sheet was assessed by visual inspection and the quality of the printed
impression on the receiver sheet was accessed in respect of reflection
density of colour by means of a densitometer (Sakura Digital
densitometer). The results of the assessments are set out in Table 2:
TABLE 2
______________________________________
Stability
Presence of
Colour Density
Ink TS RS of Ink Crystals on TS
of RS
______________________________________
1 1 1 good none 1.31
2 2 2 " " 0.83
3 2 3 " "
4 4 4 " " 0.69
5 5 5 " " 0.72
6 6 6 " " 0.73
7 7 7 " " 0.62
8 8 8 " "
______________________________________
* * * * *
|
|
 |
|
 |
Description |
|
