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(11) | EP 0 322 771 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Image-receiving sheet for heat sensitive transfer |
| (57) Disclosed herein is an image-receiving sheet for use in heat sensitive transfer,
comprising a monoaxially or biaxially stretched polyester film containing minute closed-cells
and having an apparent specific gravity of 0.4 to 1.3, an opacifying power of not
less than 0.2 and an air leakage index of 50 to 10,000 sec. |
BACKGROUND OF THE INVENTION:
[0001] The present invention relates to an image-receiving sheet for heat sensitive transfer recording. More specifically, it relates to an excellent image-receiving sheet for use in heat sensitive transfer recording, capable of obtaining clear images with no printing unevenness, excellent dimensional stability, with less heat shrinkage and with no deformations such as curling, by using a polyester film containing minute closed-cells at the surface and the inside thereof.
[0002] There has been a remarkable development in recent information processing technics and, accompanying therewith, functions of the hard copy technics have been made more versatile and improved. In the hard copy technic, a heat sensitive transfer recording system has been employed as one of the recording methods. Generally, the heat sensitive transfer recording comprises overlaying, with each other, a transfer sheet having a transfer layer containing a subliming or volatile dye or a dye fusible under moderate heating and an image receiving sheet; heating the transfer sheet thereby sublimating, volatiling or melting the dye contained in the transfer layer; dyeing or transferring the dye onto the image receiving sheet and forming dye images on the image receiving sheet. At present, the function of the recording method in this type has also been made more versatile and improved, for example, increase in the printing speed, improvement in the resolution power, improvement in the printing quality, etc.
[0003] Now, the image-receiving sheet used in such heat sensitive transfer recording can include, for example, conventional printing paper such as cellulose paper, calendered gravure paper, coated paper such as art paper or coat paper prepared by coating the surface unevenness or pores of paper with a pigment comprising fine particles thereby obtaining paper surface of excellent smoothness and gloss. Furthermore, synthetic paper excellent in the strength, dimensional stability, dust-freeness, etc. is also used. With the recent development for increasing the performance, there has been a strong demand for improving the quality also in these image receiving sheets. Specifically, there has been requested those image-receiving sheets improved with overall characteristics such as mechanical strength, satisfactory dimensional stability and, moreover, excellent heat resistance and whitening power, and free from printing unevenness upon heat sensitive transfer. Therefore, cellulose paper, synthetic paper or plastic film conventionally employed can not satisfy all of these requirements.
[0004] Specifically, conventional printing paper such as cellulose paper has drawbacks in that the thickness can not be reduced since the strength is lowered to cause easy tearing, the dust-free property is poor and, in addition, the water resistance is low. Further, since the unevenness on the paper surfaces is marked, printing unevenness is liable to be caused and it is difficult to obtain clear images.
[0005] On the other hand, although synthetic paper is more preferred to cellulose paper in view of the strength, dimensional stability and dust-free property, it can not always satisfy the demand for the improved quality. That is, further improved flatness of the surface has been demanded in view of the printability and, in addition, it can not be used in those cases exposed to high temperature since the heat resistance is poor.
[0006] Now, polyester films have been utilized generally in various industrial fields owing to their excellent heat resistance, mechanical property, chemical resistance, weather resistance, etc. Among all, biaxially oriented poly(ethylene terephthalate) films, being particularly excellent in the dimensional stability, strength, flatness, etc., are used as image-receiving sheets for heat sensitive transfer recording, original pictures for overhead projectors, etc. However, there have been several problems in the case of using, as a substitute for conventional paper, biaxially oriented poly(ethylene terephthalate) films as the image receiving sheet for use in heat sensitive transfer recording. That is, since the biaxially oriented poly(ethylene terephthalate) films are tough and rigid material and lack in flexibility, they involve a drawback that the close contact between the transfer sheet and the image-receiving sheet at the thermal head is not sufficient, which causes printing uneveness, leading to a defect that no clear images can be obtained. In addition, whiteness and opacifying power are necessary for preventing images from seeing-through the film when used as a substitute for the conventional paper. A white pigment is added for providing such performance. However, since an extremely great amount of pigment has to be added in order to provide a sufficient opacifying power in this method, the film is made more rigid to impair the flexibility, failing to obtain clear images. In addition, this is unfavorable also in view of handling, since it causes cutting injury to fingers, etc. Furthermore, since the specific gravity of the poly(ethylene terephthalate) film is as large as about 1.4, if it is used as a substitute for conventional paper, the weight of documents becomes so heavy and a problem is brought out in view of transportation or storage space.
[0007] The present inventors have made an earnest study for overcoming such problems and, as a result, have found that the foregoing problems can be overcome and an image-receiving sheet for use in heat sensitive transfer recording, which is excellent over conventional paper or synthetic paper can be obtained by using a polyester film containing minute closed-cells and satisfying specific properties. The present invention has accomplished based on this finding.
SUMMARY OF THE INVENTION:
[0008] In an aspect of the present invention, there is provided an image-receiving sheet for heat sensitive transfer recording which comprises a monoaxially or biaxially oriented minute-cellular polyester film having an apparent specific gravity of 0.4 to 1.3, an opacifying power of not less than 0.2 and an air leakage index of 50 to 10,000 sec.
DETAILED DESCRIPTION OF THE INVENTION:
[0009] The present invention relates to an image-receiving sheet for heat sensitive transfer recording, which comprises a monoaxially or biaxially oriented minute-cellular polyester film having an apparent specific gravity of 0.4 to 1.3, an opacifying power of not less than 0.2 and an air leakage index of 50 to 10,000 sec.
[0010] The polyester referred to in the present invention includes those polyesters prepared by polycondensating an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid or the ester thereof, and a glycol such as ethylene glycol, diethylene glycol, 1,4-butanediol, neopentyl glycol or 1,4-cyclohexane dimethanol.
[0011] For preparing a polyester from such an acid component and a glycol component, any of customarily employed methods can be used. For instance, there may be used a method which comprises forming a bisglycol ester of the aromatic dicarboxylic acid or the polymer thereof of a low polymerization degree by an ester-exchange reaction between a lower alkyl ester of an aromatic dicarboxylic acid and a glycol, or by a direct esterification of an aromatic dicarboxylic acid with a glycol, then polycondensating them under a reduced pressure and at a temperature of not lower than 240°C. In this case, usual catalyst, stabilizer, various additives, etc. can be used optionally.
[0012] As typical examples for such polyesters, there can be mentioned poly(ethylene terephthalate), poly(ethylene naphthalate) or poly(butylene terephthalate), etc. The polyester may be a homopolyester or copolyester. Further, it may be a mixture of two or more of polyesters. In any of the cases, a preferred polyester is one in which not less than 70 mol%, preferably, not less than 80 mol% and, more preferably, not less than 90 mol% of the constitutional repeating units is ethylene terephthalates units, ethylene naphthalate units, butylene terephthalate units or a mixture thereof.
[0013] Further, in the present invention, if the polymerization degree of such a polyester is excessively low, it is not preferred since the mechanical strength is lowered. The intrinsic viscosity of the polyester is, therefore, not less than 0.4, preferably, from 0.5 to 1.2 and, more preferably, from 0.55 to 0.85.
[0014] In the present invention, a film is prepared by using such a polyester. The film used in the present invention contains minute closed-cells at the surface and the inside thereof and it is necessary that the film has an apparent specific gravity from 0.4 to 1.3 and, preferably, from 0.6 to 1.3. If the apparent specific gravity is in excess of 1.3, it is not preferred since the amount of cells contained in the film is reduced, which deteriorates the flexibility and causes uneven printing. On the other hand, if the apparent specific gravity is less than 0.4, it is neither preferred since the mechanical strength of the film becomes insufficient to cause tearing upon heat sensitive transfer.
[0015] The opacifying power of the polyester film is not less than 0.2 and, preferably, not less than 0.3. If the opacifying power is less than 0.2, it is not preferred since remarkable see-through of transferred images at the back of the film occurs to worsen the contrast and the images are difficult to be seen, as well as they are difficult to be read.
[0016] It is preferable for the polyester film that the air leakage index is from 50 to 10,000 sec., more preferably, 100 to 5,000 sec. If it is less than 50 sec, it is not preferred since the close contact between the transfer sheet and the image receiving sheet in the heat sensitive transfer head comes to insufficient, the transfer efficiency is reduced and the printing unevenness is increased. On the other hand, if it exceeds 10,000 sec, it is neither preferred since the slipperiness of the film is extremely worsened to deteriorate the film feeding property, and causing running troubles such as paper jamming upon heat sensitive transfer recording.
[0017] It is also an important factor that the polyester film is oriented at least monoaxially by stretching. The stretch ratio is preferably not less than 4 times and, preferably, from 9 to 20 times in the areal ratio, because a non-stretched polyester film is remarkably poor in the mechanical strength failing to obtain required sufficient strength and dimensional stability when used as an image receiving sheet for use in heat sensitive transfer according to the present invention. The individual strech ratio for the longitudinal direction or the transverse direction can be properly selected under the limitation for the areal stretch ratio defined above.
[0018] As has been described above, it is necessary for the polyester film as a substrate of the image receiving sheet for use in heat sensitive transfer recording according to the present invention that it posesses characteristics as described above. So long as the film contains minute cells and can satisfy such properties and requirements as described above, there is no particular restriction for the method of manufacturing the film. For the method of manufacturing a film containing such fine cells, there can be used any of the methods, i.e., a method of adding gas or gasifiable substance as described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 50-38765 or Japanese Patent Publication (KOKOKU) No. 57-46456, a method of adding a substance capable of evolving gases upon chemical decomposition as described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 52-43871 and Japanese Patent Publication (KOKOKU) No. 58-50625, etc., or a method which comprises mixing a material for film with a substance soluble in a solvent, forming the resultant mixture in the form of a film, impregnating the film in the solvent to extract the substance from the film as described, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 51-34963 or Japanese Patent Publication (KOKOKU) No. 52-27666, etc. However, since these manufacturing methods require either special molding apparatus or complicate manufacturing steps, they can not be considered to be employed conveniently.
[0019] In view of the above, as a method of easily producing a polyester film containing minute cells used in the present invention, it is preferred to employ a method, for example, as proposed in Japanese Patent Application Laid-Open (KOKAI) No. 63-168441 by the present inventors. That is, this method comprises blending a specific polypropylene with a polyester, extrusion molding them into a form of sheet and then stretching the sheet at least monoaxially to produce a film. Specifically, this method comprises blending from 3 to 50 wt% of a crystalline polypropylene homopolymer with a melt flow index (hereinafter simply referred to as M.F.I.) of 0.2 to 120 with a polyester, melt-extruding them into a substantially amorphous sheet and at least monoaxially stretching the sheet by an areal stretch ratio of 4 times or greater, thereby forming a polyester film containing a great amount of minute cells in the surface and the inside of the film.
[0020] By the employment of this method, it is easy to obtain a film stretched (oriented) at least monoaxially, which has an apparent specific gravity from 0.4 to 1.3, the opacifying power of not less than 0.2 and an air leakage index of 50 to 10,000 sec. In addition, since this method requires no particular modification for the conventional film-forming apparatus and the film can be produced under the usual production conditions for the stretched polyesters, it can provide a large merit also in view of the production cost.
[0022] The crystalline polypropylene homopolymer means such a polymer as comprising propylene units by at least 95 mol%, preferably, 98 mol% of the constitutional repeating units. If the polypropylene to be used is amorphous, polypropylene would breed out at the surface of the sheet when preparing the amorphous sheet, and contaminate the surface of the cooling roll or stretching roll, etc. Meanwhile, if a polypropylene having, for example, 10 mol% or more of ethylene unit, the amount of minute cells contained in the film becomes insufficient.
[0023] The M.F.I. of the crystalline polypropylene homopolymer is 0.2 to 120, preferably 0.5 to 50. If the M.F.I. is less than 0.2, the size of the generated cells becomes excessively large, causing frequent bursts at the time of stretching. On the other hand, if the M.F.I. exceeds 120, the sheet slips off from clips during the stretching by using a tenter. The above-described cases are not preferable because the productivity is deteriorated.
[0024] The amount of the crystalline polypropylene homopolymer to be mixed with polyester is 3 to 50 wt%, preferably 3 to 30 wt% based on the amount of the polyester. If the amount is less than 3 wt%, the amount of generated minute cells is too small, therefore, it becomes difficult to obtain a polyester film having an apparent specific gravity of not higher than 1.3. On the other hand, if it exceeds 50 wt%, it is not preferable because burst occurs at the time of stretching.
[0025] In the present invention, it is necessary that the amorphous sheet is stretched at least in monoaxial direction. The reason for this lies in that, in addition to the above-described object of giving a mechanical strength, the minute and closed cells cannot be obtained by merely mixing the polyester and the crystalline polypropylene homopolymer, but it can be obtained by employing the stretching process.
[0026] The method of stretching does not need any special condition. It can employ the conditions similar to those employed in a usual method for producing polyester film.
[0027] That is, a mixture of polyester and the crystalline polypropylene homopolymer is melted at 250 to 320°C in an extrusion machine and is extruded into a form of a sheet through a die. Next, it is cooled down below about 70°C to be made a substantially amorphous sheet. Then, this sheet is stretched in the machine and/or transverse direction by 4 times or more, preferably 9 to 20 times in the areal stretch ratio. Then, by thermal treatment at 120 to 250°C, a polyester film having a thickness of 10 to 500 µm, preferably 10 to 250 µm, an apparent specific gravity of 0.4 to 1.3, an opacifying power of 0.2 or more, an air leakage index of 50 to 10,000 and minute cells of a diameter of 1 to 300 µm, preferably 2 to 100 µm. The biaxial stretching may be conducted in either way of simultaneously biaxial stretching or successively biaxial stretching. The stretch ratio in the machine direction is 2 to 7 times, preferably 3 to 5 times, and the stretch ratio in transverse direction is 1.5 to 7 times, preferably 2 to 5 times.
[0028] Thus, the film used in the present invention can be produced as a white polyester with a low apparent specific gravity and a high opacifying power, to which various kinds of additives may be blended with no troubles so long as they do not impair the basic properties. As such additives, there can be mentioned, for example, anti-oxidants, UV-absorbers, lubricants, antistatic agents, dyes, pigments, fluorescent whiteners, matting agents, surface active agents, etc. and they are blended each in an appropriate amount by an adequate method at an optional time as required.
[0029] In addition, the film of the present invention may be subjected to various surface treatments in order to improve the dyeability, bondability, etc. with dyes, or preventing blocking or electrostatic charging in a heat sensitive transfer apparatus. As the surface treatment, there can be mentioned, for example, coating treatment, flaming treatment, solvent treatment, plasma treatment, corona discharging treatment, UV-ray treatment, ion plating treatment and sand blasting treatment, which may be applied in an appropriate time.
[0030] Among such surface treatments, the coating treatment is employed particularly preferably in the present invention. The material for the coating and the coating thickness can freely be selected depending on other requirements and purposes so long as they do not impair the film properties as the substrate required in the present invention. For instance, a thermoplastic resin, a cross-linkable resin or a mixture of such a resin and various additives, etc. can be used as required. As the additives, there can be mentioned dyes, pigments, lubricants, antioxidants, UV-absorbers, antistatic agents, inorganic fine particles, surface active agents, etc., which may be blended each in an appropriate amount as required.
[0031] There is no particular restriction for the method of forming the coating layer and it may be coated on a film already formed or coated during a film-production. For instance, in the case of forming a coating layer to a biaxially stretched polyester film containing minute cells, a method which comprises coating a coating agent to a film monoaxially stretched in the longitudinal direction, stretching in the transverse direction before or after the coating agent is dried, and then directly applying heat treatment is employed particularly preferably because it can provide a great merit also in view of the production cost since the film formation, coating and drying can be conducted simultaneously. The coating layer may be formed either on one side or both sides. In the case of forming the coating layer on both sides, the coating agent may be identical or different with each other.
[0032] The image-receiving sheet for use in heat sensitive transfer according to the present invention, which is produced from a polyester film containing minute closed-cells at the surface and the inside thereof, is of higher strength, excellent in dimensional stability and heat resistance, free from shrinkage or curling due to the heat upon image receiving and also satisfactory in dust-free property, as compared with conventional image receiving sheets made of cellulose paper, synthetic paper, etc.
[0033] Furthermore, when compared with an image receiving sheet made of usual polyester not containing minute cells, since the film of the present invention has moderate unevenness and flexibility, clear images with extremely less printing unevenness and satisfactory contrast upon heat sensitive transfer can be obtained.
[0034] In addition, since the flexibility, i.e., pliability is obtained due to the reduction in the apparent specific gravity of the film, running troubles during transfer operation can be reduced remarkably and the handling property of the image-receiving sheet is improved.
[0035] The present invention is to be explained specifically by way of examples but the invention is not restricted only to the following examples. Measurement and evaluation for various properties in the present invention were conducted by the methods shown below.
(1) Apparent specific gravity
[0036] A 10 x 10 cm square was cut as a sample from a desired part of a given film. The volume of this sample was calculated using the average thickness obtained by measuring thicknesses thereof at nine arbitrarily chosen points of the sample with a micrometer and averaging the values of measurement. The sample was weighed and the weight thereof per unit cm³ was reported as an apparent specific gravity of the given film. This determination was conducted on five samples and the average of five values was employed as the result of the determination.
(2) Opacifying power
[0037] By the use of a densitometer, Macbeth TD-904 model, the density of transmitted light through a given film was measured under Filter G to find the opacifying power. The numerical value thus found increases with increasing opacifying power. The measurement was conducted for three points and the average value therefor was determined as a measured value.
(3) Air leakage index
[0038] Air leakage index was measured by using a Bekk flatness meter according to JIS P 8119-1976. As the value was greater, the flatness was greater at the surface. Measurement was conducted at 5 points and the average value therefor was determined as a measured value.
(4) Heat shrinkage
[0039] Heat treatment was applied under tension-free state in 180°C atmosphere for 5 min and the length of the specimen before and after the heat treatment was measured and the heat shrinkage was calculated by the following equation:
(5) Flexibility ϑ (deg)
[0040] A test piece of 12.7 mm in width and 150 mm in length was cut out from a film. This test piece was horizontally fixed with a part of 127 mm length from one end overhung. A weight of 0.9 g was attached to the front end of the thus-overhung test piece to be allowed to hang down. The horizontal distance (a) mm between a point which is vertically lower by 30 mm from the fixing point at which the test piece was fixed and the hanging film is measured. From an equation tan ϑ = a/30, the hanging angle ϑ (deg.) was obtained to be made the flexibility. The measurement was repeated three times, and the average value was employed as the resulted value. It is indicated that a smaller value shows the more excellent flexibility.
(6) Quality of printing
[0041] The film was cut into A-4 size and applied with heat sensitive transfer recording by using CX-5000 colour printer manufactured by Sharp Corp. The resultant hard copy was observed with naked eyes for the degr ee of the printed density, printing unevenness and the contrast by the following five ratings respectively.
Ratings for the evaluation
(7) Practical applicability
[0043] The film was cut into 20 sheets each of A-4 size and they were stacked and set to a image-receiving paper feeding cassette in the heat sensitive transfer recording apparatus used for the printing quality test. Heat sensitive transfer recording was conducted for successive 20 sheets to observe paper jamming at the image receiving paper feed section and the state of occurrence for operation troubles such as running failure in the apparatus. Evaluation was made as "○" for the case with no occurrence of troubles at all and as "X" with once or more times of occurrence of troubles for the test of 20 sheets.
Example 1:
[0044] Starting material prepared by blending a poly(ethylene terephthalate) chips of an intrinsic viscosity of 0.648 containing 5 wt% of white pigment comprising 0.3 µm of titanium oxide with 10 wt% of a crystalline polypropylene homopolymer chips having M.F.I of 5 was melted at 290°C in an extruder, and then extruded into a shape of sheet on a cooling drum at 40°C to obtain an amorphous sheet of 0.7 mm thickness. Then, the sheet was stretched longitudinally by three times at 85°C and transversely by 3.2 times at 95°C, and then subjected to heat treatment at 240°C for 5 sec to obtain a white biaxially stretched polyester film of 100 µm thickness. The film had an apparent specific gravity of 0.98, an opacifying power of 0.5, an air leakage index of 4,000 sec, a heat shrinkage of 1.1% in the longitudinal direction and 1.6% in the transverse direction, and flexibility ϑ of 40 deg.
[0045] When the film was cut into A-4 size as an image-receiving sheet and the printing quality was evaluated, satisfactory images were obtained with the printing density of 4, the printing unevenness of 5 and the contrast of 5, and satisfactory practical applicability was shown with neither paper jamming nor running failure during image receiving operation.
Example 2:
[0046] In the same procedures as in Example 1 except for using poly(ethylene terephthalate) with an intrinsic viscosity of 0.650 and not containing a pigment instead of the poly(ethylene telephthalate ) containing the white pigment used in Example 1 and increasing the blending amount of polypropylene to 20 wt%, a biaxially stretched white polyester film with 100 µm thickness was obtained. In this case, the amount of the polymer extruded from the extruder was controlled to make the thickness of the amorphous sheet to 0.5 mm in order to attain 100 µm thickness for the finally obtained biaxially stretched film. Also in the following comparative examples, the amount of the polymer extruded was varied to control the thickness of the amorphous sheet for controlling the thickness of the finally obtained stretched film. The thus obtained biaxially stretched film had an apparent specific gravity of 0.71, an opacifying power of 0.6 an air leakage index of 1,100 seconds.
[0047] The film also had a heat shrinkage of 1.2 % in the longitudinal direction, 1.9 % in the transverse direction and a flexibility ϑ of 35 deg. When the printing quality of the film was evaluated in the same manner as in Example 1, satisfactory images were obtained with the printing density of 5, the printing unevenness of 4 and the contrast of 5 and the practical applicability was satisfactory with neither paper jamming nor running failure.
Example 3:
[0048] A biaxially stretched white polyester film of 100 µm thickness having a coating layer of 1 µm thickness was obtained by using the same starting material as in Example 2 and applying procedures under the same conditions, however, after the longitudinal stretching step, an aqueous dispersion comprising a blend of water-dispersible polyester resin/water-dispersible polyester polyurethane resin/surface active agent = 20/79/1 (weight ratio) was coated on one side of the film by using a roll coater and then the thus treated film was directly subjected to stretching in the transverse direction. The resultant film had an apparent specific gravity of 0.72, an opacifying power of 0.6, an air leakage index on the side of the coated layer of 5,000 sec and on the opposite side of 1,100 sec. The values for the heat shrinkage and the flexibility ϑ were the same as those in Example 2. The film was set to a heat sensitive transfer apparatus such that images were received on the side of the coating layer of the film and excellent copied images were obtained, with the printing density of 5, the printing unevenness of 5 and the contact of 5. The film was excellent in the practical applicability with neither paper jamming nor running failure in the apparatus.
Comparative Example 1:
[0049] By the same procedures as those in the examples under the same film-forming conditions except for using poly(ethylene terephthalate) with an intrinsic viscosity of 0.650, and a biaxially stretched polyester film of 100 µm thickness was finally obtained.
[0050] The resultant film was a transparent and smooth film having an apparent specific gravity of 1.4, an opacifying power of 0.1 and an air leakage index of 12,000 sec.
[0051] The film also had a heat shrinkage of 1.2% in the longitudinal direction and 1.4% in the transverse direction and flexibility ϑ of 44 deg.
[0052] When the film was evaluated for the printing quality as an image-receiving sheet, it was found poor, with printing density of 3, printing unevenness of 3 and contrast of 1. Further, it was inferior in practical applicability with frequent occurrence of paper jamming or running failure in heat sensitive transfer apparatus.
Comparative Example 2:
[0053] By the same procedures as in Comparative Example 1 except for dry blending 10 wt% of calsium carbonate particles having an average particle size of 2.5 µm with the starting material used in Comparative Example 1 and then supplying them to the hopper of an extruder, a biaxially stretched polyester film of 100 µm thickness was obtained. The resultant film was extremely roughened at the surface, and having an apparent specific gravity of 1.2, an opacifying power of 0.8 and an air leakage index of 30 second. In addition, the film also had a heat shrinkage of 1.2% in the longitudinal direction and 1.4% in the transverse direction and flexibility of 43 deg. In the case of using the film as an image-receiving sheet, although it showed satisfactory practical applicability with neither occurrence of paper jamming nor running failure at all, the printing quality was extremely poor, with printing density of 2, printing unevenness of 1 and contrast of 2.
Comparative Example 3::
[0054] A biaxially stretched polyester film with 100 µm thickness was obtained in the same procedures as in Comparative Example 1 except for using poly(ethylene terephthalate) having intrinsic viscosity of 0.653 containing 15 wt% of titanium oxide of 0.3 µm particle size instead of the starting material used in Comparative Example 1. Although the resultant film had an excellent opacifying power and flat surface, with an opacifying power of 1.2 and an air leakage index of 7,000 sec, the value for the apparent specific gravity was as high as 1.5. In addition, the heat shrinkage was 1.0% in the longitudinal direction and 1.1% in the transverse direction, and the flexibility was 46 deg. When the film was evaluated for the printing quality as an image-receiving sheet, it showed no satisfactory quality, with printing density of 3 and printing unevenness of 2, although the contrast is satisfactory at 5. Further, it was poor also in the practical applicability with the occurrence of troubles such as running failure in heat sensitive transfer apparatus.
Comparative Example 4:
[0055] Film preparation was tried in the same procedures as in Example 1 except for increasing the blending amount of polypropylene in Example 1 to 50 wt%, but it was found that the productivity was extremely poor since breakage occurred frequently during stretching. By the way, a small sheet-like specimens were sampled and measured for the apparent specific gravity for a portion having 100 µ m thickness. It was found to be 0.4. However, since the mechanical strength was so poor that evaluation for the film physical property and printing quality was impossible.
[0056] Table 1 shows the blending composition for the starting material, and physical properties, printing quality and practical applicability of the films obtained in Examples 1 - 3 and Comparative Examples 1 - 4. All of the results for the evaluation of the printing quality and the practical applicability for the films of Examples 1 - 3 obtained by the present invention were satisfactory. The films had a sufficient opacifying power, therefore, printing images were not seen through from the rear face thereof. Since the heat shrinkage was not greater than 2% in any of the cases of Examples 1 to 3 and the dimensional stability was also excellent, neither waving nor curling was recognized in the film after the printing. In addition, since the flexibility was also satisfactory, the handling property was also improved in addition to the satisfactory printing quality and the practical applicability. As has been described above, the film according to the present invention is excellent as an image-receiving sheet for use in heat sensitive transfer.
[0057] On the other hand, since the films obtained in Comparative Examples 1 - 4 do not satisfy the required conditions of the present invention and can not provide sufficient effects, no evaluation results sufficient for the image-receiving sheet for use in heat sensitive transfer was obtained.
Table 1
| Example or Comparative Example | Polymer composition | Film property | Printing quality | Applicability | ||||||
| Polypropylene blend amount (wt%) | Additive (addition amount) | Apparent specific gravity (-) | Opacifying power (-) | Air leakage index (sec.) | Printing density | Printing unevenness | Contrast | Paper Jamming | Running failure | |
| Example 1 | 10 | 0.3 µm Titanium oxide (5 wt%) | 0.98 | 0.5 | 4,000 | 4 | 5 | 5 | o | o |
| Example 2 | 20 | no | 0.71 | 0.6 | 1,100 | 5 | 4 | 5 | o | o |
| Example 3 | 20 | no | 0.72 | 0.6 | 5,000 | 5 | 5 | 5 | o | o |
| Comp. Example 1 | no | no | 1.4 | 0.1 | 12,000 | 3 | 3 | 1 | x | x |
| Comp. Example 2 | no | 2.5 µm Titanium oxide (10wt%) | 1.2 | 0.8 | 30 | 2 | 1 | 3 | o | o |
| Comp. Example 3 | no | 0.3 µm Titanium oxide (15wt%) | 1.5 | 1.2 | 7,000 | 3 | 2 | 5 | o | x |
| Comp. Example 4 | 50 | 0.3 µm Titanium oxide (5 wt%) | 0.4 | - | - | - | - | - | - | - |
1. An image-receiving sheet for use in heat sensitive transfer, comprising a monoaxially
or biaxially stretched polyester film containing minute closed-cells and having an
apparent specific gravity of 0.4 to 1.3, an opacifying power of not less than 0.2
and an air leakage index of 50 to 10,000 sec.
2. The image receiving sheet for use in heat sensitive transfer according to Claim
1, wherein the polyester film containing minute closed-cells comprises a polyester
which has an intrinsic viscosity of not less than 0.4 and not less than 70 mol% of
the constitutional repeating units thereof are ethylene terephthalate units, ethylene-2,6-naphthalate
units, butylene terephthalate units or a mixture thereof; and 3 to 50 wt%, based on
said polyester, of a crystalline polypropylene which has a melt flow index of from
0.2 to 120 and not less than 95 mol% of the constitutional repeating units thereof
are propylene units.
3. A method of heat sensitive transfer recording, which comprises overlaying, with
each other, a transfer sheet having a transfer layer containing a dye and an image-receiving
sheet; sublimating, gasifying or melting said dye contained in said transfer layer
by heating said transfer sheet; and dyeing or transferring said dye to said image-receiving
sheet, thereby forming dye-images on said image-receiving sheet, wherein the improvement
comprises using, as the image-receiving sheet, a monoaxially or biaxially stretched
polyester film containing minute closed-cells and having an apparent specific gravity
of 0.4 to 1.3, an opacifying power of not less than 0.2 and an air leakage index
of 50 to 10,000 sec.
4. The method according to claim 3, wherein said polyester film containing minute
closed-cells comprises a polyester which has an intrinsic viscosity of not less than
0.4 and not less than 70 mol% of the constitutional repeating units thereof are ethylene
terephthalate units, ethylene-2,6-naphthalate units, butylene terephthalate units
or a mixture thereof; and 3 to 50 wt%, based on said polyester, of a crystalline polypropylene
which has a melt flow index of from 0.2 to 120 and not less than 95 mol% of the constitutional
repeating units thereof are propylene units.
5. A use of a monoaxially or biaxially stretched polyester film containing minute
closed-cells and having an apparent specific gravity of 0.4 to 1.3, an opacifying
power of not less than 0.2 and an air leakage index of 50 to 10,000 sec, as an image-receiving
sheet for use in heat sensitive transfer.
6. The use according to claim 5, wherein said polyester film containing minute closed-cells
comprises a polyester which has an intrinsic viscosity of not less than 0.4 and not
less than 70 mol% of the constitutional repeating units thereof are ethylene terephthalate
units, ethylene-2,6-naphthalate units, butylene terephthalate units or a mixture
thereof; and 3 to 50 wt%, based on said polyester, of a crystalline polypropylene
which has a melt flow index of from 0.2 to 120 and not less than 95 mol% of the constitutional
repeating units thereof are propylene units.