Dye diffusion thermal transfer printing

A thermal transfer sheet comprising a substrate having a coating comprising a dye of Formula (1): ##STR1## wherein: Y is --H, alkyl, alkoxy, --NHCOalkyl, NHCOaryl, --NHSO.sub.2 alkyl or --NHSO.sub.2 aryl; PA1 Ring A is unsubstituted apart from the --N.dbd.N--, --Y and --N< groups or may be substituted by from 1 to 3 substituent groups; PA1 R.sup.1 and R.sup.2 each independently is alkyl; PA1 R.sup.3 is --H, alkyl, aryl, --SO.sub.2 alkyl, --Salkyl, --Saryl, halogen, pyridyl or R.sup.6 O(CH.sub.2).sub.n -- in which R.sup.6 is alkyl, acyl, aryl and n is from 1 to 10; PA1 R.sup.4 is --CN, --NO.sub.2, --SCN or --COOalkyl; and PA1 R.sup.5 is a group of Formula (2): ##STR2## in which: Z is --H, alkyl, alkoxy, --NHCOalkyl, --NHCOaryl, --NHSO.sub.2 alkyl or --NHSO.sub.2 aryl; PA1 Ring B is unsubstituted apart from the --N.dbd.N--, --Z and --N< groups or may be substituted by from 1 to 3 substituent groups; PA1 R.sup.7 and R.sup.8 each independently is --H or alkyl; PA1 R.sup.9 independently is any of the groups defined for R.sup.4 ; PA1 R.sup.10 independently is any of the groups defined for R.sup.3 ; PA1 L is bridging group; or PA1 R.sup.5 is alkyl, provided that when R.sup.5 is alkyl R.sup.3 is pyridyl or R.sup.6 O(CH).sub.n --.

This application is a 371 of PCT/GB95/00674, filed Mar. 27, 1995. 
This specification describes an invention relating to dye diffusion thermal 
transfer printing DDTTP or D2T2 printing, especially to a transfer sheet 
carrying a dye or a dye mixture which has an improved print stability 
particularly with respect to light fastness, to a transfer printing 
process in which the dye or the dye mixture is transferred from the 
transfer sheet to a receiver sheet by the application of heat and to 
certain novel dyes. 
It is known to print woven or knitted textile material by a thermal 
transfer printing (TTP) process. In such a process a sublimable dye is 
applied to a paper substrate (usually as an ink also containing a resinous 
or polymeric binder to bind the dye to the substrate until it is required 
for printing) in the form of a pattern, to produce a transfer sheet 
comprising a paper substrate printed with a pattern which it is desired to 
transfer to the textile. Substantially all the dye is then transferred 
from the transfer sheet to the textile material, to form an identical 
pattern on the textile material, by placing the patterned side of the 
transfer sheet in contact with the textile material and heating the 
sandwich, under light pressure from a heated plate, to a temperature from 
180.degree.-220.degree. C. for a period of 30-120 seconds. 
As the surface of the textile substrate is fibrous and uneven it will not 
be in contact with the printed pattern on the transfer sheet over the 
whole of the pattern area. It is therefore necessary for the dye to be 
sublimable and vaporise during passage from the transfer sheet to the 
textile substrate in order for dye to be transferred from the transfer 
sheet to the textile substrate over the whole of the pattern area. 
As heat is applied evenly over the whole area of the sandwich over a 
sufficiently long period for equilibrium to be established, conditions are 
substantially isothermal, the process is non-selective and the dye 
penetrates deeply into the fibres of the textile material. 
In DDTTP, a transfer sheet is formed by applying a heat-transferable dye 
(usually in the form of a solution or dispersion in a liquid also 
containing a polymeric or resinous binder to bind the dye to the 
substrate) to a thin (usually &lt;20 micron) substrate having a smooth plain 
surface in the form of a continuous even film over the entire printing 
area of the transfer sheet. Dye is then selectively transferred from the 
transfer sheet by placing it in contact with a material having a smooth 
surface with an affinity for the dye, hereinafter called the receiver 
sheet, and selectively heating discrete areas of the reverse side of the 
transfer sheet for periods from about 1 to 20 milliseconds (msec) and 
temperatures up to 300.degree. C., in accordance with a pattern 
information signal, whereby dye from the selectively heated regions of the 
transfer sheet diffuses from the transfer sheet to the receiver sheet and 
forms a pattern thereon in accordance with the pattern in which heat is 
applied to the transfer sheet. The shape of the pattern is determined by 
the number and location of the discrete areas which are subjected to 
heating and the depth of shade in any discrete area is determined by the 
period of time for which it is heated and the temperature reached. 
Heating is generally, though not necessarily, effected by a line of heating 
elements, over which the receiver and transfer sheets are passed together. 
Each element is approximately square in overall shape, although the 
element may optionally be split down the centre, and may be resistively 
heated by an electrical current passed through it from adjacent circuitry. 
Each element normally corresponds to an element of image information and 
can be separately heated to 300.degree. C. to 400.degree. C., in less than 
20 msec and preferably less than 10 msec, usually by an electric pulse in 
response to a pattern information signal. During the heating period the 
temperature of an element will rise to about 300.degree.-400.degree. C. 
over about 5-8 msec. With increase in temperature and time more dye will 
diffuse from the transfer sheet to the receiver sheet and thus the amount 
of dye transferred onto, and the depth of shade at, any discrete area on 
the receiver sheet will depend on the period for which an element is 
heated while it is in contact with the reverse side of the transfer sheet. 
As heat is applied through individually energised elements for very short 
periods of time the process is selective in terms of location and quantity 
of dye transferred and the transferred dye remains close to the surface of 
the receiver sheet. 
As an alternative heating may be effected using a light source in a 
light-induced thermal transfer (LITT or L2T2 printing) printer where the 
light source can be focused, in response to an electronic pattern 
information signal, on each area of the transfer sheet to be heated. The 
heat for effecting transfer of the dye from the transfer sheet is 
generated in the dyesheet which has an absorber for the inducing light. 
The absorber is selected according to the light source used and converts 
the light to thermal energy, at a point at which the light is incident, 
sufficient to transfer the dye at that point to the corresponding position 
on the receiver sheet. The inducing light usually has a narrow waveband 
and may be in the visible, infra-red or ultra violet regions although 
infra-red emitting lasers are particularly suitable. 
It is clear that there are significant distinctions between TTP onto 
synthetic textile materials and DDTTP onto smooth polymeric surfaces and 
thus dyes which are suitable for the former process are not necessarily 
suitable for the latter. 
In DDTTP it is important that the surfaces of the transfer sheet and 
receiver sheet are even so that good contact can be achieved between the 
printed surface of the transfer sheet and the receiving surface of the 
receiver sheet over the entire printing area because it is believed that 
the dye is transferred substantially by diffusion in the molten state in 
condensed phases. Thus, any defect or speck of dust which prevents good 
contact over any part of the printing area will inhibit transfer and lead 
to an unprinted portion on the receiver sheet on the area where good 
contact is prevented, which can be considerably larger than the area of 
the speck or defect. The surfaces of the substrate of the transfer and 
receiver sheets are usually a smooth polymeric film, especially of a 
polyester, which has some affinity for the dye. 
Important criteria in the selection of a dye for DDTTP are its thermal 
properties, fastness properties, such as light fastness, and facility for 
transfer by diffusion into the substrate in the DDTTP process. For 
suitable performance the dye or dye mixture should transfer evenly and 
rapidly, in proportion to the heat applied to the transfer sheet so that 
the amount transferred to the receiver sheet is proportional to the heat 
applied. After transfer the dye should preferably not migrate or 
crystallise and should have excellent fastness to light, heat, rubbing, 
especially rubbing with a oily or greasy object, e.g. a human finger, such 
as would be encountered in normal handling of the printed receiver sheet. 
As the dye should be sufficiently mobile to migrate from the transfer 
sheet to the receiver sheet at the temperatures employed, 
100.degree.-400.degree. C., in the short time-scale, generally &lt;20 msec, 
it is preferably free from ionic and/or water solubilising groups, and is 
thus not readily soluble in aqueous or water-miscible media, such as water 
and ethanol. Many potentially suitable dyes are also not readily soluble 
in the solvents which are commonly used in, and thus acceptable to, the 
printing industry; for example, alcohols such as i-propanol, ketones such 
as methyl ethyl ketone (MEK), methyl i-butyl ketone (MIBK) and 
cyclohexanone, ethers such as tetrahydrofuran and aromatic hydrocarbons 
such as toluene. The dye can be applied as a dispersion in a suitable 
medium or as a solution in a suitable solvent to the substrate from a 
solution. In order to achieve the potential for a high optical density 
(OD) on the receiver sheet it is desirable that the dye should be readily 
soluble or readily dispersable in the ink medium. 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. Crystallisation not 
only produces defects which prevent good contact between the transfer 
receiver sheet but gives rise to uneven prints. 
The following combination of properties is highly desirable for a dye which 
is to be used in DDTTP: 
Ideal spectral characteristics (narrow absorption curve) and high 
extinction coefficient. 
Correct thermochemical properties (high thermal stability and efficient 
transferability with heat). 
High optical densities on printing. 
Good solubility in solvents acceptable to printing industry: this is 
desirable to produce solution coated dyesheets alternatively good 
dispersibility in acceptable media is desirable to produce dispersion 
coated dyesheets. 
Stable dyesheets (resistant to dye migration or crystallisation). 
Stable printed images on the receiver sheet (resistant to heat, migration, 
crystallisation, grease, rubbing and light). 
DDTTP is used for printing images on suitable substrates. 
The achievement of good light fastness in DDTTP is extremely difficult 
because of the unfavourable environment of the dye, close to the surface 
of the polyester receiver sheet. Many known dyes for polyester fibre have 
high light fastness (&gt;6 on the International Scale of 1-8) on polyester 
fibre when applied by TTP because dye penetration into the fibres is good, 
but the same dyes exhibit very poor light fastness on a polyester receiver 
sheet when applied by DDTTP because of poor penetration into the 
substrate. It is known to improve the light fastness of dyes for use in 
conventional dyeing techniques by introducing electron-withdrawing groups 
into the dye molecule but this is generally accompanied by a hyposchromic 
shift which may be undesirable. The present invention overcomes this 
problem by providing a convenient means of improving light fastness of 
dyes in DDTTP without the disadvantage of substantially changing the 
absorption maximum of the dye. 
According to the present invention there is provided a thermal transfer 
sheet comprising a substrate having a coating comprising a dye of Formula 
(1): 
##STR3## 
wherein: Y is --H, alkyl, alkoxy, --NHCOalkyl, --NHCOaryl, --NHSO.sub.2 
alkyl or NHSO.sub.2 aryl; 
Ring A is unsubstituted apart from the --N.dbd.N--, --Y and --N&lt; groups or 
may be substituted by from 1 to 3 substituent groups; 
R.sup.1 and R.sup.2 each independently is alkyl; 
R.sup.3 is --H, alkyl, aryl, --SO.sub.2 alkyl, --Salkyl, --Saryl, halogen, 
pyridyl or R.sup.6 O(CH.sub.2).sub.n -- in which R.sup.6 is alkyl, acyl, 
aryl and n is from 1 to 10; 
R.sup.4 is --CN, --NO.sub.2, --SCN or --COOalkyl; and 
R.sup.5 is a group of Formula (2): 
##STR4## 
in which: Z is --H, alkyl, alkoxy, --NHCOalkyl, --NHCOaryl, --NHSO.sub.2 
alkyl or --NHSO.sub.2 aryl; 
Ring B is unsubstituted apart from the --N.dbd.N--, --Z and --N&lt; groups or 
may be substituted by from 1 to 3 substituent groups; 
R.sup.7 and R.sup.8 each independently is --H or optionally substituted 
alkyl; 
R.sup.9 independently is any of the groups defined for R.sup.4 ; 
R.sup.10 independently is any of the groups defined for R.sup.3 ; 
L is a bridging group; or 
R.sup.5 is alkyl, provided that when R.sup.5 is alkyl R.sup.3 is pyridyl or 
R.sup.6 O(CH).sub.n --. 
Where any one of the groups represented by R.sup.1 to R.sup.10, Z or Y is 
or contains an alkyl group the alkyl group is preferably C.sub.1-10 
-alkyl, more preferably C.sub.1-6 -alkyl and especially C.sub.1-4 -alkyl, 
such alkyl groups may be straight or branched chain alkyl groups. 
Where R.sup.3, R.sup.6, R.sup.10, Y or Z is or contains an aryl group the 
aryl group is preferably phenyl or naphthyl, more preferably phenyl. 
Where R.sup.3 or R.sup.10 is halogen it is preferably --F, --Cl or --Br. 
Where R.sup.3 or R.sup.10 is pyridyl it may be pyrid-2-yl, pyrid-3-yl or 
pyrid-4-yl. 
Where R.sup.6 is acyl it is preferably C.sub.1-10 -alkylCO-- or phenylCO-- 
and more preferably C.sub.1-4 -alkylCO-- or phenylCO--. n is preferably 
1-2. 
Where Y or Z is alkoxy it is preferably C.sub.1-6 -alkoxy more preferably 
C.sub.1-4 -alkoxy. 
The bridging group represented by L is preferably alkylene, phenylene or an 
ester group of Formula (3): 
##STR5## 
in which: m is from 1 to 6 
p is from 0 to 4 
q is from 1 to 6 
m is preferably from 2 to 4. q is preferably from 2 to 4. 
Where L is an alkylene group it is preferably C.sub.1-12 -alkylene, more 
preferably C.sub.1-10 -alkylene and especially C.sub.2-10 -alkylene, such 
alkylene groups may be branched or straight chain alkylene groups. Where 
an alkylene group is branched the branching is preferably .alpha.- to the 
nitrogen atom to which L is attached. 
The groups represented by R.sup.1 to R.sup.10, Y, Z, L Ring A or Ring B may 
be optionally substituted and the optional substituents are preferably 
selected from --CN, --SCN, --NO.sub.2, --F, --Cl, --Br, --SC.sub.1-4 
-alkyl, --Sphenyl, C.sub.1-4 -alkoxy and --COOC.sub.1-4 -alkyl. 
The presence of at least one .alpha.-branched alkyl group in compounds of 
Formula (1) improves the light fastness properties of the compound. 
In a preferred sub group of dyes of Formula (1) R.sup.1 is C.sub.1-4 
-alkyl, especially methyl or ethyl, R.sup.2 is C.sub.1-6 -alkyl, R.sup.3 
is C.sub.1-4 -alkyl, pyridyl or R.sup.6 O(CH.sub.2).sub.n -- in which 
R.sup.6 is C.sub.1-4 -alkyl, C.sub.1-4 -alkylCO--, phenylCO-- or phenyl 
and n is from 1 to 2, R.sup.4 is --CN, R.sup.5 is a group of Formula (2) 
in which L is C.sub.1-10 -alkylene, R.sup.7 and R.sup.8 each independently 
is --H or C.sub.1-6 -alkyl, R.sup.9 is --CN, R.sup.10 is C.sub.1-4 -alkyl, 
pyridyl or R.sup.6 O(CH.sub.2).sub.n in which R.sup.6 and n are as defined 
above, Y and Z each independently is --H, C.sub.1-4 -alkyl, 
--NHCOC.sub.1-4 -alkyl or --NHSO.sub.2 C.sub.1-4 -alkyl, especially --H, 
--CH.sub.3, --NCOCH.sub.3 or --NHSO.sub.2 CH.sub.3 and Rings A and B carry 
no further substituents. 
In a further preferred sub group of dyes of Formula (1) R.sup.1 is 
C.sub.1-4 -alkyl, especially methyl or ethyl, R.sup.2 is C.sub.1-6 -alkyl, 
R.sup.3 is pyridyl or R.sup.6 O(CH.sub.2).sub.n in which R.sup.6 is 
C.sub.1-4 -alkyl, C.sub.1-4 -alkylCO--, phenylCO-- or phenyl and n is from 
1 to 2, R.sup.4 is --CN, R.sup.5 is C.sub.1-6 -alkyl, Y is --H, C.sub.1-4 
-alkyl, --NHCOC.sub.1-4 -alkyl or --NHSO.sub.2 C.sub.1-4 -alkyl preferably 
--H, --CH.sub.3, --NHCOCH.sub.3 or --NHSO.sub.2 CH.sub.3 and especially 
--CH.sub.3 and Ring A carries no further substituents. 
An especially preferred sub group of dyes of Formula (1) is that in which 
R.sup.1 is methyl or ethyl, R.sup.2 is C.sub.1-4 -alkyl, R.sup.3 is 
pyridyl or C.sub.1-4 -alkylOCH.sub.2 --, R.sup.4 is --CN, R.sup.5 is 
C.sub.1-4 -alkyl, and Y is C.sub.1-4 -alkyl. 
The dyes of Formula (1) are novel and form a further feature of the present 
invention. 
The dyes of the present invention may be prepared by conventional methods 
for preparing azo dyes for example by methods disclosed in U.S. Pat. No. 
4,960,873. 
In addition to the use described above in D2T2 printing the present dyes of 
Formula (1) are useful as colorants for a variety of applications 
particularly in inks for use in ink jet printing, as toners for use in 
reprography and as dyes for dyeing and printing textile materials such as 
polyester and blends thereof. 
The Coating 
The coating suitably comprises a binder together with a dye or mixture of 
dyes of Formula (1). The ratio of binder to dye is preferably at least 
0.7:1 and more preferably from 1:1 to 4:1 and especially preferably 1:1 to 
2:1 in order to provide good adhesion between the dye and the substrate 
and inhibit migration of the dye during storage. 
The coating may also contain other additives, such as curing agents, 
preservatives, etc., these and other ingredients being described more 
fully in EP 133011A, EP 133012A and EP 111004A. 
The Binder 
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. It is preferred however, that the dye is soluble in the 
binder so that it can exist as a solid solution in the binder on the 
transfer sheet. In this form it is generally more resistant to migration 
and crystallisation during storage. 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, polyvinyl acetoacetal and polyvinyl pyrrolidone; polycarbonates 
such as AL-71 from Mitsubishi Gas Chemicals and MAKROLON 2040 from Bayer 
(MAKROLON is a trade mark); polymers and co-polymers derived from 
acrylates and acrylate derivatives, such as polyacrylic acid, polymethyl 
methacrylate and styrene-acrylate copolymers, styrene derivatives such as 
polystyrene, 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. 
Mixtures of two or more of the above resins may also be used, mixtures 
preferably comprise a vinyl resin or derivative and a cellulose 
derivative, more preferably the mixture comprises polyvinyl butyral and 
ethylcellulose. It is also preferred to use a binder or mixture of binders 
which is soluble in one of the above-mentioned commercially acceptable 
organic solvents. 
The dye or mixture of dyes of Formula (1) 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 or mixture of dyes of Formula (1) also has strong absorbance 
properties and is soluble in a wide range of solvents, especially those 
solvents which are widely used and accepted in the printing industry, for 
example, alkanols, such as i-propanol and butanol; aromatic hydrocarbons, 
such as toluene, ethers, such as tetrahydrofuran and ketones such as MEK, 
MIBK and cyclohexanone. Alternatively the mixture of dyes maybe dispersed 
by high shear mixing in suitable media such as water, in the presence of 
dispersing agents. This produces inks (solvent plus mixture of dyes and 
binder) which are stable and allow production of solution or dispersion 
coated dyesheets. The latter are stable, being resistant to dye 
crystallisation or migration during prolonged storage. 
The combination of strong absorbance properties and good solubility in the 
preferred solvents allows the achievement of good OD of the dye or mixture 
of dyes of Formula (1) on the receiver sheet. The transfer sheets of the 
present invention have good stability and produce receiver sheets with 
good OD and which are fast to both light and heat. 
The Substrate 
The substrate may be any sheet material preferably having at least one 
smooth even surface and capable of withstanding the temperatures involved 
in DDTTP, i.e. up to 400.degree. C. for periods up to 20 msec, yet thin 
enough to transmit heat applied on one side through to the dyes on the 
other side to effect transfer of the dye onto a receiver sheet within such 
short periods. Examples of suitable materials are polymers, especially 
polyester, polyracrylate, polyamide, cellulosic and polyalkylene films, 
metallised forms thereof, including co-polymer and laminated films, 
especially laminates incorporating a smooth even polyester receptor layer 
on which the dye is deposited. Thin (&lt;20 micron) high quality paper of 
even thickness and having a smooth coated surface, such as capacitor 
paper, is also suitable. A laminated substrate preferably comprises a 
backcoat, on the opposite side of the laminate from the receptor layer, 
which, in the printing process, holds the molten mass together, such as a 
thermosetting resin, e.g. a silicone, acrylate or polyurethane resin, to 
separate the heat source from the polyester and prevent melting of the 
latter during the DDTTP operation. The thickness of the substrate depends 
to some extent upon its thermal conductivity but it is preferably less 
than 20 .mu.m and more preferably less than 10 .mu.m. 
The DDTTP Process 
According to a further feature of the present invention there is provided a 
dye diffusion thermal transfer printing process which comprises contacting 
a transfer sheet comprising a coating comprising a dye or mixture of dyes 
of Formula (1) with a receiver sheet, so that the coating is in contact 
with the receiver sheet and selectively applying heat to discrete areas on 
the reverse side of the transfer sheet whereby the dye on the opposite 
side of the sheet to the heated areas is transferred to the receiver 
sheet. 
Heating in the selected areas may be effected by contact with heating 
elements, which can be heated to 200.degree.-450.degree. C., preferably 
200.degree.-400.degree. C., over periods of 2 to 10 msec, whereby the dye 
mixture may be heated to 150.degree.-300.degree. C., depending on the time 
of exposure, and thereby caused to transfer, substantially by diffusion, 
from the transfer to the receiver sheet. Good contact between coating and 
receiver sheet at the point of application is essential to effect 
transfer. The density of the printed image is related to the time period 
for which the transfer sheet is heated. 
The Receiver Sheet 
The receiver sheet conveniently comprises a polyester sheet material, 
especially a white polyester film, preferably of polyethylene 
terephthalate (PET). Although some dyes of Formula (1) are known for the 
coloration of textile materials made from PET, the coloration of textile 
materials, by dyeing or printing is carried out under such conditions of 
time and temperature that the dye can penetrate into the PET and become 
fixed therein. In thermal transfer printing, the time period is so short 
that penetration of the PET is much less effective and the substrate is 
preferably provided with a receptive layer, on the side to which the dye 
is applied, into which the dye mixture more readily diffuses to form a 
stable image. Such a receptive layer, which may be applied by co-extrusion 
or solution coating techniques, may comprise a thin layer of a modified 
polyester or a different polymeric material which is more permeable to the 
dye than the PET substrate. While the nature of the receptive layer will 
affect to some extent the depth of shade and quality of the print obtained 
it has been found that the dyes of Formula (1) give particularly strong 
and good quality prints (e.g. fastness and storage properties) on any 
specific transfer or receiver sheet, with the benefit of improved light 
fastness compared with other dyes of similar structure which have been 
proposed for thermal transfer printing processes. The design of receiver 
and transfer sheets is discussed further in EP 133,011 and EP 133012. 
The invention is further illustrated by the following examples and 
comparative examples in which all parts and percentages are by weight. 
Ink Preparation 
The inks were prepared by dissolving 0.15 g of the dye in a solution 
containing 5 g of a 6% w/w solution of ethylhydroxyethyl cellulose (EHEC) 
in tetrahydrofuran and 4.85 g tetrahydrofuran (THF). 
Transfer Sheet TS1 
This was prepared by applying Ink 1 to a 6 .mu.m polyester film (substrate) 
using a wire-wound metal Meyer-bar (K-bar No 3) to produce a wet film of 
ink on the surface of the sheet. The ink was then dried with hot air to 
give a dry film on the surface of the substrate. 
Printed Receiver Sheet RS1 
A sample of TS1 was contacted with a receiver sheet, comprising a composite 
structure based in a white polyester base having a receptive coating layer 
on the side in contact with the printed surface of TS1. The receiver and 
transfer sheets were placed together on the drum of a transfer printing 
machine and passed over a matrix of closely-spaced elements which were 
selectively heated using a constant power of 0.37 W/pixel for periods from 
2 to 10 msec, whereby a quantity of the dye, in proportion to the heating 
period, at the position on the transfer sheet in contact with an element 
while it was hot was transferred from the transfer sheet to the receiver 
sheet. After passage over the array of elements the transfer sheet was 
separated from the receiver sheet. 
Evaluation of Inks Transfer Sheets and Printed Receiver Sheets 
The stability of the ink was assessed by visual inspection. An ink was 
considered to be stable if there was no precipitation over a period of two 
weeks at ambient. 
The light fastness of receiver sheets was assessed by calculating the 
colour difference of the receiver sheets before and after exposure to 
xenon light as follows: 
Half of the receiver sheet was covered before exposure, in an Atlas Ci35 
Weatherometer, to xenon arc light at 0.8 W/m.sup.2 at a black panel 
temperature of 45.degree. C. and relative humidity of approximately 50% 
for 24 hours. The colour difference (.DELTA.E) between the exposed and the 
unexposed areas on the receiver sheets which correspond to a print time of 
10 msec was measured using a Minolta Chromameter utilising the following 
equation: 
##EQU1## 
where L.sup.*1, a.sup.*1 and b.sup.*1 are the values before exposure and 
L.sup.*2, a.sup.*2 and b.sup.*2 are the values after exposure to xenon 
light. The smaller the value of .DELTA.E the more light fast is the dye on 
the receiver sheet.

The invention is further illustrated by the following Examples 1 and 2 and 
comparative Examples A and B. 
The dyes used to prepare inks and transfer sheets and for printing on 
receiver sheets are of Formula (1) in which the substituents are as shown 
in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Dye 
R.sup.1 
R.sup.2 
R.sup.3 
R.sup.4 
R.sup.5 
Y 
__________________________________________________________________________ 
1 --CH.sub.3 
--C.sub.2 H.sub.5 
pyrid-3-yl 
--CN 
--C.sub.2 H.sub.5 
--CH.sub.3 
A --H --(CH.sub.2).sub.2 CH.sub.3 
pyrid-3-yl 
--CN 
--C.sub.2 H.sub.5 
--CH.sub.3 
2 --CH.sub.3 
--C.sub.2 H.sub.5 
--CH.sub.2 OCH.sub.3 
--CN 
--(CH.sub.2).sub.3 CH.sub.3 
--NHCOCH.sub.3 
B --H --(CH.sub.2).sub.2 CH.sub.3 
--CH.sub.2 OCH.sub.3 
--CN 
--(CH.sub.2).sub.3 CH.sub.3 
--NHCOCH.sub.3 
__________________________________________________________________________ 
The colour differences (.DELTA.E) were measured for each of Dyes 1 and 2 
and A and B as described above and the results are shown in Table 2. 
TABLE 2 
______________________________________ 
Dye .DELTA.E 
______________________________________ 
1 8.35 
A 26.15 
2 5.16 
B 7.14 
______________________________________ 
The dyes in Examples 1 and 2 which have branched chain N-alkyl 
substituents, have lower .DELTA.E values than the analogous straight chain 
N-alkyl substituted dyes in Examples A and B and thus the dyes of the 
present invention are more light-fast.