Ink transfer material for printer

An ink transfer material for printers which comprises a biaxially oriented polyester film and a transfer ink layer deposited on one side of the polyester film, the biaxially oriented polyester film having a thickness in the range of 1 to 15.mu., an F-5 value in the longitudinal direction in the range of 11 to 16 kg/mm.sup.2, a refractive index in each of the longitudinal and lateral directions in the range of 1.650 to 1.675, and a birefringence of not more than 0.02, and the rough surface having a center line average height thereof in the range of 0.02 to 1.mu. and a maximum height in the range of 0.2 to 10.mu.. This ink transfer material has no disadvantage such as longitudinal tear, plastic deformation, or thermal shrinkage. Thus, it is highly useful in thermal transfer printers and impact printers.

BACKGROUND 
This invention relates to an ink transfer material for printers, and more 
particularly to an ink transfer material which is excellent in dimensional 
stability and durability, virtually free from plastic deformation, and 
useful for typewriters and other similar impact printers and thermal 
transfer printers. 
Polyester film is utilized as the substrate of an ink transfer material for 
printers because this film possesses outstanding properties such as high 
crystallinity, a high melting point, excellent thermostability and 
chemical resistance, high tensile and impact strengths, and high tensile 
modulus. 
The ink transfer material, when used in impact printers such as 
typewriters, is required to endure tension and printing pressure and 
warrant repeated use. In thermal transfer printers, the extremely thin 
substrates are required to increase thermal conductivity. Therefore, the 
substrates for the ink transfer material are required to possess high 
tensile and impact strengths and small deformation including thermal 
shrinkage. 
In the ink transfer material using an ordinary biaxially oriented polyester 
film as the substrate thereof, however, there often occurs problems of 
longitudinal elongation and plastic deformation in dotted parts during the 
transfer of ink, therefore it is unsatisfactory for a printer ribbon which 
is quite susceptible of high tension and high printing pressure. 
The ink transfer material using the typical biaxially oriented polyester 
film available on the market is embossed under the impacts of printing 
types and, because of the prominent and persistent projections left in the 
film, is not smoothly rewound in the spool or the cassette of a limited 
capacity. 
The plastic deformation or embossing is caused by the property of the film 
whereby it is distorted under impact pressure and does not return to be 
flat, after the impact pressure is released. 
The ink transfer material for the thermal transfer printers is desired to 
be a good thermal conductivity and, therefore, is expected to use a thin 
substrate as far as possible. If the ordinary biaxially oriented polyester 
thin film available on the market is used as the substrate, it still fails 
to make a satisfactory ink transfer material for thermal transfer printers 
because of insufficient tensile strength. 
When the ordinary tensilized polyester film whose F-5 value in the 
longitudinal direction exceeds 16 kg/mm.sup.2 is used as the substrate of 
an ink transfer material for impact printers, the film is liable to 
sustain tear in the longitudinal direction during ink transfer by impact 
printer, and as the substrate of an ink transfer material for thermal 
printers, its thermal shrinkage is too large to make it useful. 
SUMMARY 
An object of this invention is to provide an ink transfer material which is 
excellent in dimensional stability and durability, free from the 
aforementioned drawbacks of the conventional ink transfer material, and 
useful for printers. 
Another object of this invention is to provide an ink transfer material for 
printers, which is adequately strong, break-resistant and resistant to 
plastic deformation. 
A still another object of this invention is to provide an ink transfer 
material for thermal transfer printers, which avoids the problem of 
thermal shrinkage and possesses enough strength to endure heat even in a 
reduced thickness. 
A yet still another object of this invention is to provide an ink transfer 
material for printers, which has a high resolution and can produce clean 
and clear prints. 
Namely, this invention relates to an ink transfer material for printers, 
comprising a biaxially oriented polyester film and a transfer ink layer 
deposited on one side of the polyester film, the biaxially oriented 
polyester film having a thickness in the range of 1 to 15.mu., an F-5 
value in the longitudinal direction in the range of 11 to 16 kg/mm.sup.2, 
refractive indices in each of the longitudinal and lateral directions in 
the range of 1.650 to 1.675, and a birefringence of not more than 0.02, 
possessing a rough surface on at least one side thereof, and the rough 
surface having a center line average height thereof in the range of 0.02 
to 1.mu. and a maximum height in the range of 0.2 to 10 .mu.. 
THE PREFERRED EMBODIMENTS 
By the term "polyester" as used in this invention is meant a thermoplastic 
linear polyester as well known to the art. Desirably, this polyester is a 
polymer selected from the group consisting of polyethylene terephthalate, 
polyester copolymers having ethylene terephthalate units as main repeating 
component units thereof, and polymer blends having such polyesters as main 
components thereof. 
As well known by skilled in the arts, a thermoplastic linear polyester is 
obtained by the polycondensation of (A) a dicarboxylic acid or an 
ester-forming derivative thereof with (B) a glycol. It is desired that at 
least 80 mol % of the component (A) is a terephthalic acid or an 
ester-forming derivative thereof and at least 80 mol % of the component 
(B) is ethylene glycol. 
Especially, polyethylene terephthalate homopolymer is used most widely. 
In the case of a polymer blend, it is preferable to contain not less than 
80% by weight of the aforementioned polyester. The polyester to be used in 
this invention may contain various additives such as thermal stabilizer, 
coloring agent, antioxidant, and lubricant. 
The polyester film to be used in this invention is a biaxially oriented 
film of the aforementioned polyester. This film is required to have an F-5 
value in the longitudinal direction thereof in the range of 11 to 16 
kg/mm.sup.2, preferably 11.5 to 15 kg/mm.sup.2. If the F-5 value is less 
than 11 kg/mm.sup.2, the film is readily stretched and exhibits poor 
elastic recovery and, as used in an ink transfer material for impact 
printers, it is undesirable because of plastic deformation. 
Namely, when the biaxially oriented polyester film has an F-5 value of less 
than 11 kg/mm.sup.2 in the longitudinal direction, it cannot be rewound in 
the space available on the rewind reel or in the space of the cassette 
having a limited capacity resulting from excessive embossing of the film 
where it is struck by the typewriter keys. 
When the film of the foregoing description is used for thermal transfer 
printers, it is used in a reduced thickness to ensure better 
heat-conductivity. The decrease of the thickness brings the decrease of 
the strength of the film, therefore, the obtained ink transfer material 
tends to be ruptured. 
If the F-5 value exceeds 16 kg/mm.sup.2, the film becomes too rigid, so it 
tends to tear under the impact of printing types, or undergoes serious 
thermal shrinkage under the thermal transfer printing. 
The refractive indices of the film, both in the longitudinal and lateral 
directions, is required to fall into the range of 1.650 to 1.675, 
preferably 1.655 to 1.670. If the refractive indices are less than 1.650, 
the film possesses insufficient strength and, therefore, deformed under 
the impact of printing types. If the refractive indices exceed 1.675, the 
film tends to tear under the impact of printing types or yields readily to 
thermal shrinkage under the thermal transfer printing. 
The birefringence of the film is required to be not more than 0.02, 
preferably 0.015. If the birefringence exceeds 0.02, the balance of 
mechanical properties in the longitudinal and lateral directions of the 
film is lost and the drawbacks mentioned above comes out. 
The thickness of the polyester film to be used in this invention is 
required to fall into the range of 1 to 15.mu., preferably 2 to 10.mu.. If 
the thickness exceeds the upper limit of the range defined above, the film 
no longer suits high-speed recording because the resolution of the printed 
matter or thermal conduction becomes insufficient. If the thickness is out 
of the lower limit of the range, tensile and impact strength of the film 
is insufficient and operation for application of the ink transfer layer 
onto the film becomes difficult. 
The polyester film of the present invention has a rough surface at least 
one side thereof. The roughness of said rough surface is such that the 
center line average height (Ra) thereof is required to fall into the range 
of 0.02 to 1.mu., preferably 0.04 to 0.8.mu., and the maximum height 
(Rmax) to fall in the range of 0.2 to 10.mu., preferably 0.4 to 8.mu.. If 
the magnitudes of Ra and Rmax are out of the lower limits of the 
respective ranges, slipperiness of the film becomes poor, the film tends 
to wrinkle, and stick to the thermal head in the thermal printer. If they 
exceed the upper limits, it impairs resolution, impedes uniform transfer 
of ink, and accelerates wear of the thermal head. The aforementioned 
surface roughness can be attained by a proper method known to the art, for 
example, addition of inorganic or organic particles to the polymer 
composition for forming the film, acceleration of crystallization of the 
melt extruded film, or surface treatment of the film such as sand 
blasting, chemical etching and mat coating. Particularly the addition of 
inorganic particles of an average particle diameter within the range of 
0.02 to 20.mu., preferably 0.05 to 10.mu. in an amount of 0.05 to 5% by 
weight to the polymer composition is preferable. 
Now, the manufacturing method of the ink transfer material of this 
invention will be described below. 
The biaxially oriented film to be used in the present invention is produced 
generally by stretching an extruded sheet first in the longitudinal 
direction and then in the lateral direction and optionally restretching 
the film in the longitudinal direction. In this first longitudinal 
stretching, there is employed the so-called multi-stage longitudinal 
process which effects the required stretching in two or more separate 
zones. 
To be more specific, the biaxially oriented polyester film is obtained by 
first melting polyester, extruding the molten polyester in the form of a 
sheet through a slit die, cooling and solidifying the extruded unstretched 
sheet on a cooling drum, stretching the sheet longitudinally in a 
multi-stage, i.e. heating the sheet to a temperature in the range of 
80.degree. to 130.degree. C. and stretching the sheet in two or more zones 
at a total stretching ratio in the range of four to seven times the 
original length by virtue of suitably varied peripheral speeds of rolls 
then laterally stretching the sheet at a temperature in the range of 
90.degree. to 130.degree. C. at a ratio of 3.0 to 4.5, and subjecting the 
stretched sheet to a heat treatment at a temperature in the range of 
180.degree. to 240.degree. C., preferably 200.degree. to 230.degree. C. 
Optionally, the biaxially oriented polyester may be obtained by inserting 
after the step of the lateral stretching in the procedure described above 
a re-stretching in the longitudinal direction at a temperature in the 
range of 90.degree. to 130.degree. C., preferably 95.degree. to 
110.degree. C., at a stretching ratio of not more than 1.10 times, 
preferably not more than 1.05 times to the length before the treatment, 
and subjecting heat treatment mentioned above. 
Incidentally, the polyester film which is produced by the sequential 
longitudinal-lateral biaxial stretching method described as in U.S. Pat. 
No. 2,823,421 or British Pat. No. 838,708 generally possesses higher 
orientation to the lateral direction under the influence of the lateral 
stretching which follows the longitudinal stretching. As the result, this 
film acquires a refractive index of this film becomes less than 1.650 in 
the longitudinal direction and an F-5 value becomes less than 11 
kg/mm.sup.2. If, on the other hand, the ratio of stretching is greater in 
the longitudinal direction than in the lateral direction, then the 
uniformity of stretching becomes poor and it causes thickness variation. 
The so-called tensilized polyester film having enhanced orientation in the 
longitudinal direction which is produced by the 
longitudinal-lateral-longitudinal three-stage stretching method described 
in British Patent No. 811,066 and the lateral-longitudinal stretching 
method described in Japanese Patent Publication No. 37-1588 proves to be 
undesirable because it has an F-5 value in the longitudinal direction in 
excess of 16 kg/mm.sup.2, a refractive index in the lateral direction 
below 1.650, and a birefringence exceeding 0.02. 
Then, a transfer ink layer is formed on the biaxially oriented polyester 
film of the present invention obtained as described above. 
The biaxially oriented polyester film may be subjected, when necessary, to 
a surface treatment as by means of corona discharge in air or in an inert 
gas, to a frame treatment or a reverse spattering treatment. It may be 
given an undercoating layer. 
The polyester film of this invention is desired, though not essentially, to 
possess specific surface resistivity of not more than 10.sup.15 ohm/sq., 
preferably 10.sup.13 ohm/sq., so as to preclude the problems of 
electrostatic deposition of dust on the film surface, unsmooth movement of 
the film, and infliction of damage to the electric circuit of the printer. 
To obtain the polyester film having specific surface resistivity of not 
more than 10.sup.15 ohm/sq., there may be suitably adopted a method such 
as an application of an antistatic agent on the film surface, a method 
forming a thin layer of a metal or a metal compound on the film surface, a 
method adding an antistatic agent to the composition of raw monomers at 
the stage of polymerization prepared for the formation of film, or a 
method mixing the polyester with an antistatic agent prior to the 
formation of the film. For example, a method which comprises adding an 
anionic surfactant (such as, for example, sodium alkylbenzene sulfonate or 
sodium alkyl sulfonate) and a polyalkylene glycol to the raw material for 
the polyester before the stage of polycondensation, subjecting the 
resultant mixture to polycondensation, and blending the resultant 
polyester with a film-grade polyester is recommended. 
The transfer ink to be used in the ink transfer material of this invention 
is not specifically defined. Any of the transfer inks known as available 
for use in impact printers or thermal transfer printers can be used. To be 
specific, the transfer ink is composed of a binder and a coloring agent as 
main ingredients and, optionally, other additives such as softening agent, 
plasticizer, melting point regulator, lubricant, and dispersant. In short, 
it is produced by suitably combining materials known to the art. 
Examples of the main ingredients include well-known waxes such as paraffin 
wax, carunauba wax, and ester wax or various high molecular compounds of 
low melting points as binders and carbon black, various organic and 
inorganic pigments, and dyes as coloring components. Optionally, the ink 
to be selected may be of a sublimating type. 
The deposition of the transfer ink layer on one of the surfaces of the film 
of this invention can be accomplished by any of the known methods. 
Examples of the method include a method of applying the ink in the form of 
a hot melt or solvent coating process such as gravure roll, reverse roll, 
or the slit die. 
In the case of the ink transfer material for the thermal transfer printers, 
the film may be provided on the opposite side of the transfer ink layer 
with a fusionproofing layer for the purpose of preventing the material 
from sticking to the thermal head, if necessary. 
Examples of the fusionproofing agent having good thermostability include 
silicone resin, melamine resin, fluorine resin, epoxy resin, and phenol 
resin. The fusionproofing agent comprising a mixture of (A) having high 
lubricity and releasability such as wax, higher fatty acid amide, or 
higher alcohol with (B) a thermoplastic resin such as acrylic resin, 
polyester resin, cellulose type resin, or vinyl chloride-vinyl acetate 
copolymer are also usable. 
Since the ink transfer material of the present invention comprises a 
specific polyester film and a transfer ink layer deposited on the 
polyester film, it avoids sustaining tear in the longitudinal direction 
under the impact of printing types and yields only minimally to plastic 
deformation after exposure to the impact of printing types, and excels in 
durability. 
Thus, the ink transfer material of this invention can improve the 
disadvantage of the difficulty to rewind on a spool or the cassette of a 
limited capacity. 
The ink transfer material of this invention, when used for thermal transfer 
printers, brings about the advantage that the polyester film has high 
strength enough to permit an ample reduction in the thickness as compared 
with the ink transfer material using an ordinary biaxially oriented 
polyester film, the material enjoys improved heat conductivity, and has 
less thermal shrinkage than the ink transfer material using a tensilized 
polyester film. 
Moreover, since the polyester film possesses specific surface roughness, 
the ink transfer material avoids sticking to the thermal head, moves 
smoothly in the printer interior, permits smooth rewinding within the 
spool, and produces printed images of high clarity. Owing to the 
outstanding properties shown above, the ink transfer material of this 
invention permits miniaturization as required for incorporation in small 
cases such as cassettes. Therefore, it is highly useful as an ink transfer 
material of the types as the small cassettes. (Measuring methods for 
determination of properties and standards for evaluation) 
The measuring methods used for the determination of properties defined by 
this invention are as follows: 
(1) F-5 value (tensile stress at the elongation of 5%): 
On a tensile tester of Instron type according with ASTM D-882, a specimen 
having 10 mm of width and 100 mm of length is set. Under the conditions of 
200 mm/min. of stretching speed, 20.degree. C. of temperature, and 65% RH 
of humidity, the sample is stretched by 5%. The strength of the stretched 
sample is measured. 
(2) Refractive index: 
In an Abbe refractive index meter fitted with an analyzer. The refractive 
index in the longitudinal and lateral directions of the sample is measured 
with a sodium D ray at room temperature and under normal atmospheric 
pressure (20.+-.2.degree. C. and 65% RH). 
[The principle of determination is described in Journal of Applied Polymer 
Science, Vol. 8, page 2717 (1964)]. 
(3) Birefringence: 
Under a polarizing microscope fitted with a Berek compensator, a sodium D 
ray is projected perpendicularly upon the surface of a specimen and 
retardation is measured under the conditions of room temperature and 
normal atmospheric pressure (20.+-.2.degree. C., 65% RH). The 
birefringence is calculated by dividing the value of retardation by the 
thickness of the sample. 
(4) Surface roughness: 
Center line average height (Ra) and the maximum height of rough surface 
(Rmax) are determined by the method defined in DIN 4768. 
(Example) 
Now, the preferred embodiment of the present invention will be described 
below with reference to working examples. Wherever "parts" mentioned, they 
are meant as "parts by weight".

EXAMPLES 1-3 AND COMATIVE EXAMPLES 1-2 
Polyethylene terephthalate having an inherent viscosity of 0.61 as measured 
in a O-chlorophenol solution at 35.degree. C. and containing 0.2% by 
weight of calcium carbonate particles having 3.0.mu. in average particle 
diameter was melt extruded through a T-die attached to the exit of an 
extruder. The extruded sheet was quenched on a water-cooled casting drum. 
It was solidified and an amorphous sheet 70 to 120.mu. in thickness was 
obtained. Samples of this sheet were stretched by the three methods A, B, 
and C indicated below and subjected to a heat treatment, to produce 
biaxially oriented films A, B and C. 
Method A: Stretching by a multi-stage stretching device adapted to perform 
a three-stage longitudinal stretching, comprising the first stage at a 
temperature of 80.degree. C. and a stretching ratio of 2.1 times, the 
second stage at a temperature of 100.degree. C. and a stretching ratio of 
1.1 times, and the third stage at a temperature of 125.degree. C. and a 
stretching ratio of 2.6 times, giving a total stretching ratio of 6.0 
times. In a tenter oven, the film was stretched laterally at 120.degree. 
C. at a stretching ratio of 3.5 times, then subjected to a heat set at 
220.degree. C., cooled, and wound. 
Method B: In the same device as in Method A, a two-stage longitudinal 
stretching was carried out, comprising the first stage involving only 
application of heat and no stretching, the second stage at a temperature 
of 110.degree. C. and a stretching ratio of 1.9 times, and the third stage 
at a temperature of 115.degree. C. and a stretching ratio of 2.4 times, 
giving a total stretching ratio of 4.6 times. Thereafter, under the same 
condition as those of Method A, the film was laterally stretched, 
subjected to a heat set, cooled, and rewound. 
Method C: The procedure of Method B was followed to perform a two-stage 
longitudinal stretching. Then, in a tenter oven, the film was subjected to 
lateral stretching at a temperature of 110.degree. C. and a stretching 
ratio of 3.5 times, and re-stretching longitudinally at a temperature of 
100.degree. C. at a stretching ratio of 1.02 times, subjected to a heat 
set at 220.degree. C., cooled, and wound. 
The properties of the three films were measured as mentioned above. The 
results are shown in Table 1. 
For comparison, an ordinary sequential biaxially oriented polyester film D 
was obtained by longitudinal stretching at a temperature of 95.degree. C. 
and a stretching ratio of 3.6 times, lateral stretching at a temperature 
of 110.degree. C. and a stretching ratio of 3.2 times, and a heat set at 
225.degree. C. Separately, a longitudinally tensilized polyester film E 
was obtained by longitudinal stretching at a temperature of 90.degree. C. 
and a stretching ratio of 2.75 times, lateral stretching at a temperature 
of 100.degree. C. and a stretching ratio of 3.4 times, and again 
longitudinal stretching at a temperature of 130.degree. C. and a 
stretching ratio of 2.0 times, a heat set at 215.degree. C. 
The properties of these films so produced were measured as mentioned above. 
The results are shown in Table 1. 
Then, on the samples of the aforementioned films A, B, and C and the 
comparative films D and E, a layer of a copolymer of methyl methacrylate 
and butyl acrylate was deposited in a thickness of 2.mu. and a layer of a 
composition of the following components was superposed in a thickness of 
10.mu. (as solids) and dried to form an impact transfer ink layer. 
______________________________________ 
Parts 
______________________________________ 
Vinyl chloride-vinyl acetate copolymer (87%/13%) 
10 
Lanolin 6 
Vegetable oil 4 
Carbon black 5 
Toluene 25 
Methyl ethyl ketone 50 
______________________________________ 
Meanwhile, on the polyester films A, B, C, D, and E, a composition of the 
following components as thermal transfer ink layer was applied by the hot 
melt coating method using a heated roll in a thickness of 5 .mu.. 
______________________________________ 
Parts 
______________________________________ 
Canauba wax 30 
Ester wax 35 
Carbon black 12 
Polytetrahydrofuran 
10 
Silicone oil 3 
______________________________________ 
The films A, B, and C having the transfer ink applied thereon represent 
Examples 1, 2, and 3 respectively and the films D and E having the 
transfer ink applied therein represent Comparative Examples 1 and 2 
respectively. 
The transfer materials so produced were tested in a dot impact type printer 
and a thermal transfer type printer. 
The transfer materials using the films A, B, and C of this invention as 
substrates, namely, Examples 1-3, produced prints of very fine quality. 
In contrast, the transfer material using the substrate D, namely 
Comparative Example 1, had heavy plastic deformation in the test with the 
dot impact type printer. The same transfer material, in the test with the 
thermal transfer printer, produced sag due to insufficient strength. The 
transfer material using the substrate E, namely Comparative Experiment 2, 
teared under the impact of printing types. The thermal transfer material 
using the substrate E, in the test with the thermal transfer printer, 
deformed so seriously because of thermal shrinkage and could not be moved 
through the printer. 
TABLE 1 
__________________________________________________________________________ 
Division 
Comparative 
Example Example 
Film 
Item A B C D E 
__________________________________________________________________________ 
Thickness (.mu.) 
5.7 6.1 5.8 6.0 6.9 
F-5 value, MD 
13.0 11.8 12.3 10.2 18.3 
(kg/mm.sup.2) TD 
10.7 10.5 10.2 10.5 12.0 
Refractive index 
MD 1.6628 
1.6580 
1.6620 
1.6471 
1.6725 
TD 1.6602 
1.6598 
1.6551 
1.6730 
1.6364 
Birefringence 
0.0030 
0.0018 
0.0082 
0.0263 
0.0364 
Surface 
Center line 
0.14 0.15 0.10 0.16 0.08 
rough- 
average height 
ness (.mu.) 
Maximum 1.13 1.28 0.90 1.15 0.80 
height (.mu.) 
__________________________________________________________________________ 
Note 
MD: Longitudinal direction 
TD: Lateral direction 
COMATIVE EXAMPLE 3 
One side of a biaxially oriented film 8.mu. in thickness obtained by 
following the procedure of Method A of Example 1 was roughened by the sand 
mat treatment. On the other side of the film, a transfer ink layer of the 
same composition for thermal transfer printing as in Example 1 was 
superposed. The roughness of the matted surface in the center line average 
height was 1.1.mu. and the maximum height was 12.5.mu.. When the ink 
transfer material so produced was used in a thermal transfer printer, the 
ink could not transfer uniformly and it produced prints lacking clarity.