JPH0415729B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF INDUSTRIAL APPLICATION The present invention relates to a polyester film that has a small shrinkage rate in both the vertical and horizontal directions and has excellent flatness without wave wrinkles or the like. More specifically, the present invention relates to a polyester film for circuit boards of membrane switches that has low shrinkage and excellent flatness, and a method for producing the same. BACKGROUND ART AND PROBLEMS TO BE SOLVED Polyester biaxially stretched films have excellent heat resistance, mechanical properties, chemical resistance, etc., and are therefore used in various applications such as magnetic recording media. Among these uses, in electrical insulation applications, floppy disk applications, perpendicular magnetic recording media applications, liquid crystal panel substrate applications, membrane switch circuit board applications, etc., during the manufacturing process or use of recording media, liquid crystal panels, and membrane switches. In order to suppress deformation caused by heat and humidity, there is a need for films with low shrinkage in both the vertical and horizontal directions. In response to these demands, during the manufacturing process of polyester film, we have performed width relaxation in the longitudinal and/or transverse directions, stretched at a higher longitudinal stretching temperature, and variously changed the temperature and time of heat setting. Improvements have been made through in-line processes. Since these methods alone have little effect on reducing shrinkage, especially in thick films, attempts have been made to reduce shrinkage by further off-line heat treatment under low tension after producing a biaxially stretched film. However, when heat-treated off-line under low tension, the film usually shrinks, resulting in the formation of washboard-like wave wrinkles or curls in the vertical direction, and the flatness of the film is extremely deteriorated. was the biggest problem. Therefore, it has been desired to reduce shrinkage through in-line processing and to improve flatness when shrinkage is reduced offline. Means for Solving the Problems As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention found that by setting the physical properties of the film after biaxial stretching heat treatment to certain specific values, the shrinkage reduction effect is large even in in-line processing. In addition, we obtained new knowledge that it is possible to obtain a film with extremely good flatness without wave wrinkles etc. through off-line shrinkage reduction processing, and discovered that the above problems can be solved, and thus arrived at the present invention. In other words, the present invention focuses on the refractive index in the thickness direction of the film.
n=1.493 or more and the average refractive index is 1.600 or more,
1.610 or less, and the shrinkage rate for 3 minutes at 120°C is within 0.4% in both length and width;
A polyester film with excellent low shrinkage and an unstretched film with a birefringence after stretching in the first axial direction.
0.070 or less, and then heat-treating the film, which is stretched in a direction perpendicular to the uniaxial direction, heat-set, and wound, at 120°C to 170°C under a tension of 1 kg/mm ² or less. The present invention relates to a method for producing the above-mentioned polyester film, which is characterized by the following. In the present invention, polyester refers to a polyester containing aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, and naphthalene-2,6-dicarboxylic acid or esters thereof, and diols such as ethylene glycol, diethylene glycol, tetramethylene glycol, and neopentyl glycol. It is a crystalline aromatic polyester that can be obtained by condensation. The polyester can be obtained by direct polycondensation of aromatic dicarboxylic acid and glycol, or can be obtained by transesterification of aromatic dicarboxylic acid dialkyl ester and glycol, followed by polycondensation. Alternatively, it can also be obtained by a method such as polycondensation of diglycol ester of aromatic dicarboxylic acid. Typical examples of such polymers include polyethylene terephthalate, polyethylene-2,6
- naphthalate, polytetramethylene terephthalate, polytetramethylene-2,6-naphthalate, etc.; Not only the bonded polyester but also 80 mol% or more of the repeating units are ethylene terephthalate or ethylene-
It may be a copolyester consisting of 2,6-naphthalate units, in which 20 mol% or less of the repeating units are other components, or a mixed polyester obtained by adding and mixing other polymers to these polyesters. In particular, copolymerization of polyalkylene glycols such as polyethylene glycol and polytetramethylene glycol as a diol component is also a preferred means, if necessary. When adding or mixing other polymers to polyester, it is necessary to do so within a range that does not essentially change the properties of the polyester, and polyolefins, polyamides, polycarbonates, and other polyesters, etc. are added at a proportion of less than 15% by weight. You can. Further, the polyester may contain inert fine particles that act as a lubricant or the like, if necessary. The amount of inert fine particles added is usually 0.005-2wt%
It is preferable to include it. Further, the average particle size of the particles is in the range of 0.005 to 5.0 μm. Inert fine particles suitable for this purpose include high melting point organic compounds that are insoluble during melting and film formation of polyester resins, crosslinked polymers, and metal compound catalysts used during polyester synthesis, such as alkali metal compounds and alkaline earth metal compounds. So-called internal precipitated particles formed inside the polymer during polyester production due to
MgCO ₃ , CaCO ₃ , BaSO ₄ , Al ₂ O ₃ , SiO ₂ ,
Examples include clay minerals such as TiO ₂ , SiC, LiF, talc, and kaolin, celite, mica, and inert externally added particles such as terephthalates such as Ca, Ba, Zn, and Mn. In addition, inert organic compounds such as metal soap, starch, and carboxymethyl cellulose can also be cited as examples of inert fine particle compounds. Of course, in addition to these particles, dyes, pigments, antistatic agents, conductive substances, magnetic substances,
It may contain additives such as compounds such as antioxidants and antifoaming agents. In the present invention, the refractive index in the thickness direction of the film
120°C to 170°C for films with n〓 of 1.493 or more
When the film is heat-treated at a temperature of 100 mL under low tension, extremely low shrinkage can be achieved, and at the same time, a film with excellent flatness and no corrugations or wrinkles can be obtained. The average refractive index is =1/3 (n〓+n〓+n〓) where the refractive index in the thickness direction is n〓, the refractive index in the main orientation direction is n〓, and the refractive index in the direction perpendicular to the main orientation direction is n〓. Given. Here, the average refractive index is preferably 1.600 or more and 1.610 or less. If the average refractive index is lower than 1.600, it is not preferable because even if the shrinkage reduction treatment is performed, the effect will be low. On the other hand, the average refractive index n is 1.610
Exceeding this is not preferable because the mechanical strength of the film decreases. Surprisingly, films with n〓 of 1.493 or more
Compared to a film with n〓 of less than 1.493, it has a greater effect of reducing shrinkage in in-line processing, and also has a greater effect of reducing shrinkage when heat-treated off-line under low tension after film formation. It was found that the flatness was improved with less. In the present invention, the shrinkage rate that should be achieved by the shrinkage reduction step is 0.4% or less in both length and width at 120° C. and heat treatment time of 3 minutes. Preferably 0.2% or less,
More preferably, it is 0.1% or less. As a low-shrinkage formulation, various shrinkage processes can be incorporated during film production as described above.
In the present invention, the most suitable method is to carry out biaxial stretching heat setting so that n〓 is 1.493 or more, and then heat treatment under low tension. In the heat treatment, the tension of the film is preferably 1 g/mm ² to 1 Kg/mm. Further, the heat treatment temperature is preferably 120°C to 170°C for several seconds to several tens of seconds. The thickness of the film obtained by this method is not particularly limited, but is preferably 2μ to 300μ. There are no particular limitations on the usage, but
There are applications that require low shrinkage, such as electrical isolation applications, video/audio applications, floppy disks, perpendicular magnetic recording applications, liquid crystal panel substrates, and solar cell substrates. It is particularly useful as a circuit board for membrane switches. When using the membrane switch as a circuit board, a silane coupling agent, water-soluble resin, or emulsion resin may be applied to both sides of the film during or after the film manufacturing process to prevent oligomer precipitation. Useful. Next, the method for forming a polyester film of the present invention will be specifically explained. A polymer resin containing an appropriate amount of inert fine particles is dried by a conventional method, extruded through an extruder, and solidified by cooling on a rotating cooling drum to form an unstretched polyester sheet. At this time, a known cooling method such as an electrostatic application cooling method can be used. The unstretched film thus obtained is first stretched in the first axial direction, usually in the longitudinal direction, so that its birefringence Δn is 0.070 or less, and then in the direction perpendicular to the uniaxial direction at a temperature of 90 to 150°C. Stretch 2.5 to 4.5 times at 200℃ to 250℃ to create a biaxially stretched film.
Heat set for 1 second to 10 minutes. However, if necessary, it is also possible to perform re-stretching before heat setting. In the present invention, it is essential that Δn in the first axial stretching direction, usually after longitudinal stretching, be 0.070 or less.
If Δn exceeds 0.070, it is difficult to make the film's n〓 of 1.493 or more, which is not preferable. In order to improve thickness unevenness, it is also preferable to carry out the first axis stretching in multiple stages. In order to reduce the shrinkage rate, super draw or near super draw stretching may be applied to the longitudinal stretching process, or relaxation may be applied in the longitudinal and/or transverse directions after longitudinal stretching and/or transverse stretching and/or after heat setting. It is also suitable to introduce a process. The biaxially stretched heat-set film thus obtained is heat treated under low tension to further reduce the shrinkage rate. In this way, it was possible to obtain a film with extremely low shrinkage in both length and width. EXAMPLES The present invention will be explained in more detail with reference to Examples below, but it goes without saying that the present invention is not limited to these Examples. The method for measuring each physical property value of the film is shown below. (1) Birefringence Measure the retardation using a Carl Zeiss polarizing microscope, and calculate the birefringence (Δn) using the following formula.
I asked for Δn=R/d where R: Retardation d: Film thickness (2) Shrinkage rate The film to be measured was cut into lengths of 50 cm (I ₀ ) and widths of 15 mm in the longitudinal and width directions, and placed in a predetermined oven. After being heat-treated at a certain temperature for a predetermined time, the length (I) of the film was measured and its shrinkage rate was determined from the following formula. Shrinkage rate = 1 ₀ -1/1 ₀ × 100 (%) (3) Refractive index 25℃ measured by Atsbe refractometer (manufactured by Atago Co., Ltd.)
The value for the Na-D line was determined. (4) Flatness This was determined by observing the appearance of the film. ×: Severe occurrence of wave wrinkles △: Wave wrinkles are noticeable when looked closely ○: No wave wrinkles Examples 1, 2, 3, and 4 (Production method of polyester chips) 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, and acetic acid 0.07 part of calcium monohydrate was placed in a reactor, heated to raise the temperature, and methanol was distilled off to carry out the transesterification reaction.
It took an hour and a half to reach 230°C, and the transesterification reaction was substantially completed. Then 0.04 part of phosphoric acid and 0.035 part of antimony trioxide
part was added, and polymerization was carried out according to a conventional method. That is, the reaction temperature was gradually increased to a final value of 280°C, while the pressure was gradually decreased to a final value of 0.5 mmHg. After 4 hours, the reaction was completed and the mixture was made into chips according to a conventional method to obtain polyester (A). On the other hand, in the production of polyester (A), amorphous silica-containing polyester (B) was obtained in the same manner as in the production of polyester (A), except that 0.10 parts of amorphous silica with an average particle size of 1.2μ was added after the transesterification. Ta. On the other hand, polyester (C) containing amorphous silica was obtained by changing the particle size of amorphous silica to 30 mμ and adding amount to 0.05 part in polyester (B). Each polyester (A), (B), and (C) was adjusted to [η] 0.63. (Film forming method) After blending the above polyester (A), polyester (B), and polyester (C) in a ratio of 55:5:40, drying by a conventional method, melt-extruding at 285°C, cooling and solidifying to form an amorphous sheet. Obtained. The amorphous sheet was first stretched 3.4 times in the longitudinal direction at 105°C using multiple IR heaters, and Δn was 0.040.
After that, they were further stretched at 105° C. by 1.20 times and 1.30 times to give Δn=0.059 and 0.068, respectively. The longitudinally stretched film thus obtained was heated to 145℃ using a tenter.
Stretch it 3.7 times in the transverse direction, heat set it at 235℃, relax it by 0.2% in the length and width, and roll it up to 50μ.
Biaxially oriented films (Samples No. 1 and 2) were obtained. Next, this sample No. The biaxially stretched film of No. 1 was heat-treated at 150° C. under a tension of 70 g/mm ² for 5 seconds by conveying rolls in a hot air oven to obtain sample films (Samples Nos. 3 and 4). Comparative Examples 1 and 2 Example 1 except that the longitudinal stretching temperature was 85°C, the longitudinal stretching ratio was 3.6 times, the transverse stretching temperature was 120°C, and the transverse stretching ratio was 3.9 times.
Film formation was carried out in the same manner as above, and the film after heat setting and relaxation was designated as Sample No. 5, and the heat-treated film of Sample No. 5 was designated as Sample No. 6. The physical properties of these films are shown in Table 1.

[Table] *: Presence or absence of off-line shrinkage reduction treatment As can be seen from Table 1, according to the method of the present invention, a film with extremely low shrinkage rate and excellent flatness can be obtained. Effects of the Invention The refractive index n〓 in the thickness direction of the film of the present invention is
1.493 or more, average refractive index 1.600 or more, 1.610 or less,
A polyester film with a shrinkage rate of 0.4% or less in both the vertical and horizontal directions at 120°C for 3 minutes has excellent flatness and low shrinkage, and is useful as a circuit board for membrane switches.

Claims (1)

[Scope of Claims] 1. The refractive index n〓 in the thickness direction of the film is 1.493 or more, the average refractive index is 1.600 or more and 1.610 or less, and the shrinkage rate at 120°C for 3 minutes is within 0.4% in both length and width. A polyester film with excellent flatness and low shrinkage. 2 Stretch the unstretched film in the uniaxial direction so that the birefringence index after stretching in the first axial direction is 0.070 or less,
The refractive index in the thickness direction of the film is then heat-treated at 120°C to 170°C under a tension of 1 Kg/mm ^{2 or} less. A method for producing a polyester film with excellent flatness and low shrinkage, which has an n〓 of 1.493 or more, an average refractive index of 1.600 or more and 1.610 or less, and a shrinkage rate of 0.4% in both length and width at 120°C for 3 minutes. .