Heat transfer recording material

A heat-transfer recording material comprising a thin film support having a thermally meltable or thermally sublimable ink layer applied on one side thereof, said thin film support being made of a stretched, cross-linked polyethylene film. The ink layer contains preferably 2 to 15% by weight an oily antistatic agent. The heat transfer recording material is excellent in antisticking property and suitability for film.

The present invention relates to a heat transfer recording material, and 
more particularly, to a heat transfer recording material having an 
antisticking property and an excellent suitability for film using as a 
thin film support a stretched crosslinked polyethylene film. 
Recently, in thermal printers, thermal facsimile and the like, heat 
transfer recording materials comprising a thin film support coated with a 
thermally meltable or thermally sublimable ink (referred to as heat 
transfer ink by the abbreviations, hereinunder) have been used to provide 
sharp and fast images on receiving sheets. The principle of the heat 
transfer recording may be explained as follows: That is, a heat transfer 
recording material having a heat transfer ink layer is overlaid on a 
receiving sheet with the surface of the ink layer facing to the sheet, and 
heated on the opposite side of the recording material to the ink layer 
with a thermal head operated by an electrical signal to heat selectively a 
part of the ink which is to be transferred to the receiving sheet. The 
recording is completed by separating the recording material from the 
receiving sheet. 
Thin film supports to be used in the heat transfer recording material are 
required to have such a heat resistant property as to be capable of 
withstanding the temperature to which the thermal head may be heated 
(250.degree. to 350.degree. C.). Preferred materials for the support have 
been condenser insulating sheets and cellophane sheets which are 
unmeltable, and heat resistant films such as polyimide and Teflon films 
which are capable of withstanding the high temperature of the heated 
thermal head. Other films such as polystyrene, polyethylene, polyvinyl 
chloride, polyvinylidene chloride, polyethylene terephthalate, and 
polycarbonate films have a lower melting point than the temperature of the 
heated thermal head o that during printing, they may fuse and stick to the 
thermal head causing a phenomenon so-called "sticking" which prevents the 
thermal head from running. 
In the case the supports which may cause where sticking phenomenon are 
used, for preventing it, Japanese Patent KOKAI (Laid-open) No. sho 55-7467 
proposes that a heat resistant protective film made of a resin selected 
from the group consisting of Silicone, epoxy, melamine, phenolic, 
fluorocarbon and polyimide resins, or nitrocellulose is provided on the 
surface of the support on the side to be in contact with the thermal head. 
Japanese Patent KOKAI (Laid-open) No. sho 56-155794 discloses those 
comprising a plastic film having a stick-preventing layer applied on one 
side thereof which consists of a highly slippery inorganic pigment and a 
heat setting or high softening point resin material. 
Japanese Patent KOKOKU (Post Exam.) No. sho 57-74195 discloses those 
comprising a plastic film having a stick-preventing layer applied on one 
side thereof which is selected from three dimensionally cross-linked 
layers of silicon oxides or multi-functional (meth)acrylates. 
The present applicant also proposes a method of preventing the sticking by 
coating or impregnating a thin film support with wax, and/or substances 
which are in a liquid or paste state at room temperature, as disclosed in 
Japanese Patent KOKAI(Laid-open) No. sho 59-148697. The procedures such as 
coating and impregnating, however, add one stage to the production line 
resulting in an increase in cost. 
As disclosed in Japanese Patent KOKAI (Laid-open) Nos. sho 60-210494 and 
sho 60-245595, the present applicant also proposes the use of a 
polyethylene film having a density of 0.935 or more (the former 
application), or having a density not less than 0.935 and a weight average 
molecular weight not less than 200,000 (the latter application) as a thin 
film support. However, the polyethylene films used in the applications are 
produced by an inflation method or T-die method, and suffer from inferior 
suitability for film in that as the films are thinner, they become more 
prone to wrinkling during winding up, and that when the films are coated 
with a heat transfer ink, they are apt to be elongated, and again during 
winding up they are prone to wrinkling. 
It is an object of the present invention to provide a heat transfer 
recording material having an antisticking property and a high suitability 
for film. 
According to the present invention, there is provided a heat transfer 
recording material comprising a thin film support having a thermally 
meltable or sublimable ink layer applied on on side thereof where said 
thin film support is made of a stretched cross-linked polyethylene film. 
Preferably, the stretched cross-linked polyethylene films to be used in the 
present invention are produced by melt-extruding a polyethylene resin into 
a film by the T-die method, irradiating the film with an electron beam, 
and then heating and stretching the irradiated film. 
It is also preferable that the stretched cross-linked polyethylene films 
consist essentially of a polyethylene resin having a density not less than 
0.935, and more preferably not less than 0.950. 
The ink layer preferably contains an oily antistatic agent in an amount of 
2 to 15% by weight based on the weight of the ink layer. 
The present invention will be in detail explained hereinbelow. 
The stretched cross-linked polyethylene films used in the present invention 
are those having a reducing cross-linking degree in a depth direction from 
the surfaces and having been uniaxially or biaxially stretched. A process 
for producing the films comprises preferably supplying a polyethylene 
resin to a conventional extruder, melt-extruding the resin into a film, 
irradiating the film with an electron beam to cross-link it, and then 
stretching the irradiated film under heating. In the melt-extrusion, a 
flat sheet can be produced as by extruding the resin through a 
conventional T-die. The cross-linking of the resulting sheet may be 
achieved by subjecting the sheet to a cross-linking condition from the 
both sides thereof in such a manner as a cross-linking degree is 
progressively reduced in a depth direction from the surfaces through 
thickness. The cross-linking degree may be expressed in terms of gel 
fraction. The cross-linked sheet has preferably a lower cross-linking 
degree (0 to less than 5%) in the inner portion, and a higher degree (5% 
or more) in the outer portion. A method of cross-linking may comprise 
irradiating the sheet at both sides thereof with an electron beam in a 
dosage of 5 to 50 Mrad., and preferably 5 to 30 Mrad. The stretching may 
be performed by any one of conventional rolling, tentering, tubulating, 
and roll-milling methods to stretch uniaxially or biaxially the irradiated 
sheet so as to produce a stretched film. 
The cross-linked polyethylene films as described above are easily 
stretchable due to the cross-linking effected with electron beam so that 
they can be retained in the non-wrinkled uniform state. 
In printing with heat transfer recording materials, the temperature of a 
thermal head in use may reach 250.degree. to 350.degree. C. so that 
conventional thermoplastic resin films are generally subjected to melting 
and quenching while running. More recently, thermal printers and thermal 
facsimiles work at such a high speed that the films are not cooled up to 
room temperature upon quenching and run still in the heated state, though 
the temperature may vary depending upon the type of the apparatus. 
The sticking phenomenon is influenced by the temperatures of the support 
and the thermal head in the heated or the cooled state, a period of time 
during which they are in the heated or cooled state, and even by the 
melting point and the density of the support. It is also delicately 
influenced by whether a line head or a serial head is used. 
The stretched cross-linked polyethylene films of the present invention are 
not subjected to thermal deformation at the time of contacting with the 
thermal head due to their high density, though they may locally melt 
instantaneously. The thermal head is operated to run while melting locally 
the polyethylene film. It may be speculated that the polyethylene is less 
adhesive to the thermal head, and rather has a releasing property acting 
as a lubricant causing no sticking phenomenon. 
As a result of the above fundamental consideration, the present inventors 
have found that among various materials, polyethylene films have a good 
antisticking property, (see, Japanese Patent KOKAI (Laid-open) No. sho 
60-21094), and achieved the stretched cross-linked polyethylene films 
which do not wrinkle during winding up, that is, have a high suitability 
for film. 
In order to obtain a heat transfer recording material having such a high 
antisticking property as required in a certain thermal printer, the ink 
layer preferably contains an oily antistatic agent in an amount of 2 to 
15% by weight based on the weight of the ink layer. The inclusion of the 
oily antistatic agent improves the antisticking property, which may be 
considered to be caused for the following reasons: 
A support is coated with a thermally meltable ink containing 2 to 15% by 
weight of an oily antistatic agent by a conventional applying method. The 
resulting heat transfer recording material is usually preserved in a 
rolled state. The oily antistatic agent migrates to the surface of the ink 
layer with time, and is deposited to the side bearing no ink layer of an 
adjacent portion of the support. Therefore, the same situation as that 
where the side bearing no ink layer has been precoated with the oily 
antistatic agent is realized. The heat transfer recording material having 
the oily antistatic agent exhibits an enhanced running property owing to 
the lubricant effect of the oily antistatic agent when an energy is 
supplied by contacting with the thermal head. 
Moreover, the inclusion of the oily antistatic agent in the ink layer 
improves also the antistatic property of the heat transfer recording 
material. 
Oily antistatic agents to be used in the present invention include nonionic 
surfactants, anionic surfactants, cationic surfactants, and ampholytic 
surfactants. For example, polyoxyethylene oleyl ether, sorbitan palmitate 
ester, sorbitan fatty acid esters, glycerin fatty acid esters, polyoxy 
fatty acid amides, sorbitan laurate, alkyl phosphates, aromatic phosphate 
esters, alkyltrimethylammonium chlorides, oxyethylene dodecyl amines, 
lauryl betaines, stearyl betaines, dimethylalkyl betaines and the like may 
be mentioned. 
The stretched cross-linked polyethylene films for use as the thin film 
support of the present invention should have a thickness of not higher 
than 30 .mu.m, preferably 20 .mu.m or less, more preferably 15 .mu.m or 
less. 
The heat transfer recording materials of the present invention comprise a 
polyethylene film having a thermally transferable ink layer applied on one 
side thereof, said heat transferable ink layer comprising a thermally 
meltable or thermally sublimable ink. 
Thermally meltable inks are mainly composed of a colorant, wax and a resin. 
Colorants to be used include, for example, Benzidine Yellow G for yellow, 
Rhodamine Y lake for magenta, Phthalocyanine Blue for cyan, and carbon 
black for black, and the like. Waxes to be used include, for example, 
paraffin wax, carnauba wax, microcrystalline wax, lower molecular weight 
polyethylene wax, polyethylene oxide wax, and synthesized wax. Resins to 
be used include, for example, ethylene/vinyl acetate copolymers, 
ethylene/ethyl acrylate copolymers, fatty acid based hydrocarbon resins, 
and aromatic hydrocarbon resins. Other additives such as pigment 
dispersants, oil, and the like may be added to the ink, if necessary. 
Thermally sublimable inks are mainly composed of a dyestuff, a binder, and 
a solvent for dissolving or dispersing the binder. The dyestuff should 
have a sublimation temperature in the range of, preferably 70.degree. to 
400.degree. C., most preferably 150.degree. to 250.degree. C. For example, 
disperse dyes such as Disperse Blue 20 (available under the tradename, 
Duranol Blue 2G), Disperse Yellow 42 (available under the tradename, 
Resulinn Yellow GR), and Disperse Red 1 (available under the tradename, 
Celition Scarlet B); quinalizarin dyes, dispersive mono-azo dyes, 
dispersive anthraquinone dyes, dispersive nitrodiphenylamine dyes, 
anthracene dyes, and the like may be used. Binders to be used include 
cellulose based resins such as methylcellulose, ethylcellulose, 
hydroxyethylcellulose; acrylic resins; vinyl resins such as polyvinyl 
alcohol resins, polyvinyl acetate resins; rosin based resins; polyamide 
resins; phenolic resins; alkid resins; polyurethane resins; and the like. 
Solvents used for dissolving or dispersing the binders include alcohols 
such as methanol, ethanol, propanol, and butanol; cellosolves such as 
methyl cellosolve, and ethyl cellosolve; aromatic solvents such as 
benzene, toluene and xylene; esters such as ethyl acetate, and butyl 
acetate; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; 
hydrocarbons such as ligroine, cyclohexanone, and kerosine; dimethyl 
formamide; and the like. 
The aforementioned thermally meltable or thermally sublimable inks may be 
applied by a hot-melting or solvent coating method using any one of 
coaters equipped with a rod, gravure screen, reversible roller, or direct 
roller. 
The stretched cross-linked polyethylene films to be used as the thin film 
support in the heat transfer recording materials of the present invention 
are excellent in antisticking property and suitability for film. This may 
be presumably caused by the less adhesive, rather releasing property of 
the polyethylene material which exhibits a sort of lubricant effect when 
heated with the thermal head. This leads to prevention of the films from 
sticking to the thermal head. Moreover, it is considered that the 
suitability for film relating to the wrinkling upon winding up has been 
enhanced by irradiating the polyethylene films with an electron beam after 
extruded, but before stretched, to increase the film strength to such an 
extent as causing no wrinkling due to the cross-linking of the 
polyethylene in the process for producing the stretched cross-linked 
polyethylene films. Moreover, the heat transfer recording materials 
comprising the aforementioned stretched cross-linked polyethylene film 
having an ink layer containing 2 to 15% by weight oily antistatic agent 
applied on one side thereof allow the oily antistatic agent to migrate to 
the surface of the ink layer which with time, and to deposit on the side 
bearing no ink layer of an adjacent portion of the support, resulting in 
the same situation as the side bearing no ink layer which has been 
precoated with the oily antistatic agent. This is considered leading to 
the heat transfer recording materials which have an enhanced running 
property and hence an extraordinary high antisticking property owing to 
the lubricant effect exhibited by the oily antistatic agent when an energy 
is supplied by contacting with the thermal head. 
For the above reasons, the heat transfer recording materials of the present 
invention have such effects as not achieved by the prior art. 
The present invention will be in detail illustrated with reference to 
Examples hereafter.

EXAMPLES 1 TO 7 AND COMATIVE EXAMPLE 2 
High density polyethylene (density, 0.956 g/cm.sup.3 ; MI, 0.5 g/10 
minutes) was used and formed by the T-die method into sheet. Thereafter, 
the sheet was treated by irradiating both sides thereof with an electron 
beam in a dosage of 20 Mrad, and stretched biaxially to produce a 
stretched cross-linked polyethylene film having a thickness of 15 .mu.m 
which was employed as a support. This film had a high transparency and a 
breaking strength in tension (MD 80%; TD 60%), and was good in that it did 
not wrinkle during winding up. The thus obtained film was coated with a 
coating composition containing a thermally meltable ink consisting of the 
following components and an oily antistatic agent in amounts as shown in 
Table to form an ink layer in a proportion of 3 g/m.sup.2 in terms of 
solid content. 
______________________________________ 
A. Solid formulation for the thermally meltable ink 
carbon black 15% by weight 
black dye 5% by weight 
paraffin wax 40% by weight 
carnauba wax 30% by weight 
ethylene-vinyl acetate resin 
10% by weight 
B. Oily antistatic agent 
dimethyl lauryl betaine 
______________________________________ 
The heat transfer recording material as produced above and a receiving 
sheet (TTR-T, a heat transfer receiving sheet, manufactured by Mitsubishi 
Paper Mills Ltd.) were placed in an overlapping relationship in the 
thermal printing device (manufactured by Matsushita Electronic Parts), and 
subjected to printing. The running property was evaluated by observing 
whether the heat transfer recording material could correctly run without 
causing any sticking while the printing was conducted. Moreover, the 
antifouling property was evaluated by observing whether any fouling was 
caused by rubbing the printed images with hands. 
COMATIVE EXAMPLE 1 
The procedure of Example 5 was repeated, except that the stretched 
cross-linked polyethylene film was replaced by a polyester film having a 
thickness of 16 .mu.m. The results are shown in Table. 
TABLE 
______________________________________ 
Composition of ink 
layer 
Ink, Oily antistatic 
Evaluation 
parts agent, parts 
Running Antifouling 
(wt.) (wt.) property property 
______________________________________ 
Example 1 
100 0 .DELTA. .largecircle. 
Example 2 
99 1 .DELTA. .largecircle. 
Example 3 
98 2 .largecircle. 
.largecircle. 
Example 4 
95 5 .largecircle. 
.largecircle. 
Example 5 
90 10 .largecircle. 
.largecircle. 
Example 6 
88 12 .largecircle. 
.largecircle. 
Example 7 
85 15 .largecircle. 
.largecircle. 
Comparative 
90 10 X -- 
Example 1 
Comparative 
80 20 .largecircle. 
X 
Example 2 
______________________________________ 
Evaluation of the running property and the antifouling property: 
.largecircle. . . . Excellent 
.DELTA. . . . Good 
X . . . Bad 
In Examples 3 to 7, an excellent running property and antifouling property 
were obtained. On the other hand, in Examples 1 and 2, the running 
property was somewhat inferior to that in Examples 3 to 7 because of the 
content of the oily antistatic agent less than 2%, though an excellent 
antifouling property was achieved. In Comparative Example 2, the thermally 
meltable ink layer was so soft that when rubbing the layer with hands, 
significant fouling was caused indicating a poor antifouling property, 
though an excellent running property was achieved. 
In Comparative Example 1, the heat transfer recording material could not 
run at all because of the polyester film sticking to the thermal head when 
printing. This indicates that the inclusion of an oily antistatic agent 
alone can scarcely improve the running property. This would be presumably 
brought about because the polyester film having a higher softening point 
melts with the heat of the thermal head to stick thereto due to the 
adhesiveness of the polester film. Incidentally, antifouling property 
could not be evaluated because the heat transfer recording material could 
not run as mentioned above and hence no printed image could be obtained. 
The heat transfer recording materials comprising a thin film support having 
an ink layer containing preferably an oily antistatic agent in accordance 
with the present invention does not require any heat resistant layer on 
the side to be in contact with the thermal head as do the conventional 
ones using polyester films so that simply by applying the ink layer to the 
stretched cross-linked polyethylene films, the heat transfer recording 
materials can be produced. Therefore, the present invention achieves a 
great practical effect in that the production of the heat transfer 
recording materials can be accomplished in less cost and investment.