JPH0371976B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing a biaxially oriented polyester film. More specifically, the present invention relates to a high-speed method for forming a biaxially oriented polyethylene terephthalate film having excellent flatness, smoothness, and adhesiveness. [Prior Art] Polyester biaxially stretched film has excellent mechanical properties, thermal properties, electrical properties, and chemical resistance, and is therefore widely used for various purposes, but is particularly useful as a base film for magnetic tapes. It is a great film. In particular, with the rapid spread of video, there is a demand for high quality and inexpensive video magnetic tapes. In order to meet these demands, the biggest challenge has become how to reduce costs while maintaining the high quality of the base film. Until now, high quality base films for magnetic tapes have been achieved using a variety of methods, but among them, a film that increases the longitudinal stretching temperature and lowers the longitudinal stretching ratio compared to the commonly used one-step stretching method has been used. is known to be extremely excellent in flattening and smoothing the film, adhesion to the magnetic layer, etc. (Unexamined Japanese Patent Publication 1987)
−66936, 57−189822, 58−23323, 58−53419,
58-215722, 58-78729 and 58-160123, etc.). [Problems to be solved by the invention] Although high quality is being realized by the above-mentioned method, the film obtained by the above-mentioned method has a low orientation in the longitudinal direction, and the surface after biaxial stretching and fixing is Since the film has the lowest degree of orientation, it has the disadvantage that the stretching ratio in the longitudinal direction must be lower than that of a normal stretching recipe. However, the productivity when manufacturing polyester film is
Since it strongly depends on the winding speed, that is, the overall stretching ratio in the longitudinal direction, it is
The above method of reducing the film forming speed by 30% sacrifices productivity. Therefore, in order to eliminate the above-mentioned drawbacks, the present inventors have proposed in Japanese Patent Application No. 58-168969 and other documents a method of increasing the stretching ratio while maintaining product quality. However, since this stretching method performs longitudinal stretching in two stages,
The longitudinal stretching ratio before the transverse stretching is approximately 4.0 times, which is approximately the same as the conventionally known normal stretching ratio. Currently, if Δn after longitudinal stretching is to be 0.080 or more, even if the method of the above application is followed,
Stretching ratios of 5 times or more have also been achieved by other stretching methods (Japanese Patent Application Laid-open Nos. 50-75, 50-136365, and 54-8672, etc.). However, as in the previous proposal, if the longitudinal stretching ratio before lateral stretching was at most 4.0 times, it would be impossible to reduce the cost even if it was possible to maintain the current level. Therefore, in order to reduce costs while maintaining high quality,
There is a strong demand to further increase the longitudinal stretching ratio before transverse stretching while keeping the birefringence after longitudinal stretching as low as possible. [Means for solving the problem] In order to respond to such requests, the present inventors have
As a result of intensive study, we decided to increase the number of stretching steps in the longitudinal direction to three or more stages, incorporate so-called super draw stretching in which no orientation occurs during the initial longitudinal stretching, and keep the birefringence index low after longitudinal stretching. As a result, we have discovered a method that allows the magnetic tape to be stretched at a high magnification without worsening uneven thickness while maintaining high quality of the magnetic tape, such as flatness and smoothness and adhesion to the magnetic layer. That is, the gist of the present invention is to first prepare a substantially amorphous film containing polyethylene terephthalate as a main component so that the birefringence (Δn ₁ ) is 0.001 to 1 in the longitudinal direction.
0.020 in at least one stage (first stage stretching), and then the birefringence (Δn ₂ ) in the longitudinal direction is
Stretched in at least one step so that the birefringence (Δn 3 ) becomes 0.015 to 0.055 (second stage stretching), and then further stretched at least 1 in the longitudinal direction so that the birefringence (Δn ₃ ) becomes 0.040 to 0.080.
This is a method for producing a biaxially stretched polyester film, which is characterized by stretching in stages (third stage stretching), longitudinally stretching the film so that the longitudinal stretching ratio before the transverse stretching is 4.0 times or more, and then transversely stretching the film. The polyester used in the present invention is a polyester containing 80% by weight or more of ethylene terephthalate units, and the remaining 20% by weight or less may be a copolyester or other polymer. The polyester contains stabilizers such as phosphoric acid, phosphorous acid and their esters, titanium dioxide,
Additives such as particulate silica and kaolin, lubricants, and the like may be included. In the present invention, an unstretched polyester film is first stretched in the longitudinal direction in at least one step so that Δn ₁ is 0.001 to 0.020. The purpose of the longitudinal stretching at this stage is to suppress the crystallization and orientation of the film as much as possible, and to stretch the film at a high magnification. Therefore,
If the stretching ratio is low, it will not contribute to increasing the stretching speed.
The preferred stretching ratio in the first stage is 1.9 times or more.
On the other hand, it is virtually impossible to draw at a high magnification in the first stage and suppress crystallization to reduce Δn ₁ to 0.001 or less. On the other hand, if the stretching ratio is increased and Δn ₁ is set to 0.020 or more, the stretching ratio in the second and subsequent stages will not be increased, which is not preferable since the overall longitudinal stretching ratio cannot be increased. In order to stretch at a high draw ratio and make Δn ₁ in the range of 0.001 to 0.020, so-called super draw stretching must be applied, and it is preferable to stretch at a temperature of 100°C or higher and 150°C or lower. At 100°C or lower, orientation progresses too much, which is undesirable. On the other hand, at 150°C or higher, crystallization progresses too much, which is undesirable. It is preferable to cool the film thus obtained (hereinafter referred to as "B-0 film") to a temperature below the glass transition temperature after stretching, since the thickness can be made uniform. The lower the orientation of the B-0 film, the better the stretchability of the film in the next step, so it is also possible to perform heat treatment to relax the orientation, if necessary. At that time, the heat treatment process to relax the orientation is carried out on the film after the first stage stretching so that the surface is
The film is stretched by 1.03 times or more while being brought into contact with rolls controlled at ~150°C for 3 seconds or less. When including such orientation relaxation treatment,
Film after orientation relaxation (hereinafter referred to as "B-1 film")
That's what it means. ) must be in the range _0.001 to 0.020. In the above-mentioned relaxation step, if the roll surface temperature is less than 100°C, the orientation relaxation effect will not be exhibited, and if it is 150°C or more, the thickness uniformity will deteriorate. In addition,
If orientation relaxation is carried out for a long time, crystallization progresses too much,
There is a possibility that lateral stretchability may be deteriorated. The first above
The preferred range of Δn ₁ for the film after stage stretching is 0.001.
to less than 0.012. The thus obtained B-0 or B-1 film with Δn ₁ in the range of 0.001 to 0.020 is then preferably subjected to a birefringence (Δn ₂ ) is 0.015 to 0.055. If Δn ₂ of the film after second-stage stretching is lower than 0.015, it is not preferable to reduce Δn ₃ to 0.080 or less in the third-stage stretching because the thickness unevenness in the longitudinal direction will be extremely deteriorated. On the other hand, if Δn ₂ is set to 0.055 or more in the second stage stretching, the thickness unevenness after the second stage stretching deteriorates to such an extent that it cannot be corrected in the final stage process, which is not preferable.
However, when Δn ₂ after the second stage stretching is in the range of 0.015 to 0.055 according to the method of the present invention, the thickness unevenness after the second stage stretching is not very good, but surprisingly, In the next final stretching step, thickness unevenness can be rapidly corrected. In addition, if the longitudinal stretching is completed and the transverse stretching is performed while keeping Δn ₂ in the range of 0.015 to 0.055 without performing the third stage stretching, the thickness unevenness in the film will worsen and the mechanical properties in the longitudinal direction will deteriorate. It is not desirable because it is insufficient. On the other hand, a method is known in which a film having Δn ₂ in the range of 0.015 to 0.055 after the second stage stretching is transversely stretched in order to compensate for thickness unevenness and lack of mechanical strength, and then longitudinally stretched again. (Unexamined Japanese Patent Publication No. 58-118220, etc.) However,
This method requires a re-stretching machine for the balance film to perform re-stretching, which naturally has the disadvantage of increasing equipment costs. Note that Δn ₂ after the second stage stretching in the method of the present invention is preferably in the range of 0.020 to 0.045, more preferably in the range of 0.025 to 0.045. Thus, in order to obtain a film with excellent flatness and smoothness and thickness unevenness, according to the method of the present invention, Δn ₂ before the final stage longitudinal stretching, that is, after the second stage stretching, is set to 0.015 to 0.015.
It is essential to stop at 0.055, perform at least a third stage of longitudinal stretching, and then proceed to the transverse stretching step. If the temperature of the second stage stretching is 80° C. or lower, the film will become thick and thin due to netting stretching, which is not preferable.
On the other hand, at 120° C. or higher, the degree of crystallinity after longitudinal stretching becomes too high, making transverse stretching difficult. Furthermore, since the lower the temperature, the better the stretchability is, the temperature is more preferably 105°C or lower. The number of stages of second-stage stretching is ₁ to 3 to produce a second-stage stretched film with Δn ₂ of 0.015 to 0.055 from a B-0 or B-1 film with Δn 1 of 0.020 or less. In other words, it is also possible to extend the film to a large number of times with Δn ₂ =0.015 to 0.055. However, it is preferable to stretch in one stage. Further, if the stretching ratio in this process is 1.3 or less, the contribution to improving the stretching ratio is small and it is unsuitable for this purpose. On the other hand, in order to stretch at a high stretching ratio of 3.5 times or more, it is necessary to stretch at a high temperature, but as a result, the degree of crystallinity after longitudinal stretching becomes too high, making transverse stretching difficult, which is not preferable. Naturally, it is preferable to apply an orientation relaxation process in this process. The film thus obtained is hereinafter referred to as "B-2". Then, in the third stage of stretching, the film is stretched in the longitudinal direction at a temperature of 85° C. to 120° C. or less, preferably at a stretching ratio of 1.1 to 2.0 times, so that the birefringence index (Δn ₃ ) does not exceed 0.080. If the stretching temperature in the final stretching step in the longitudinal direction is lower than 85°C, the thickness unevenness of the biaxially stretched film will not be improved even if Δn ₃ is 0.080 or less;
If it is higher, the lateral stretchability will deteriorate. Therefore, in order to improve thickness unevenness, the temperature is preferably 90°C or higher. More preferably, the temperature is 95°C or higher. Furthermore, in the third stage stretching, Δn ₃ is 0.050 to 0.080, preferably
It is advantageous to stretch to a degree not exceeding 0.055 to 0.075, more preferably 0.060 to 0.070, for flattening and smoothing and preventing breakage during transverse stretching. The third stage stretching is usually carried out as the final stage of the longitudinal stretching process, and is preferably carried out in one stage in a short period of time. The roll surface used in the longitudinal stretching process described above is made of polyester that tends to stick, and is stretched at high temperatures.
Ceramic or elastomer (for example, copolymer mainly composed of propylene hexafluoride and vinylidene fluoride, silicone resin, ethylene propylene copolymer, chlorosulfonated polyethylene, etc.) or ethylene tetrafluoride/perfluoropropyl vinyl ether copolymer It is preferably coated with a polymer, a fluororesin such as tetrafluoroethylene, or tetrafluoroethylene filled with polyimide or the like. Note that materials other than those mentioned above may be used as long as they have a large heat transfer coefficient and are less prone to adhesion. Note that the heating in the longitudinal stretching step may be performed in combination with a method other than the above-described heat transfer from the rolls (for example, using a radiation heater, hot air, in a heat medium), etc. The longitudinally stretched film obtained by the method described above is
Although it can be used as is, it is usually stretched in the transverse direction at a stretching temperature of 90 to 140°C and a stretching ratio of 2.5 to 5.0 times, and then heat-set at 150 to 250°C to achieve excellent dimensional stability. Get the film. Furthermore,
A longitudinal tensor film, a transverse tensor film, and a longitudinal and transverse tensor film can also be produced by re-stretching. For example, in the case of longitudinal re-stretching, transverse stretching is performed at 80-140°C at a magnification of 2.5-4.5 times, and in the longitudinal direction, a wide roll is used at 90-170°C at a magnification of 1.1-3.0 times.
After double stretching, heat treatment at 160 to 250°C is recommended. According to the method of the present invention, the longitudinal stretching step before the transverse stretching is performed in three or more stages, and the longitudinal stretching ratio before the transverse stretching is 4.0 times or more, preferably 4.5 times or more. [Effects of the Invention] According to the method of the present invention, horizontal stretching can be achieved without amplifying thickness unevenness and while maintaining flatness, smoothness and adhesive properties suitable for magnetic tapes, especially video base films. Since the previous longitudinal stretching ratio can be made extremely high, manufacturing costs can be reduced. [Example] The present invention will be explained below with reference to Examples, and the methods for measuring various properties of the film are as follows. (1) Thickness unevenness Using a continuous film thickness measuring device manufactured by Anritsu Electric Co., Ltd.,
The transverse central portion of the biaxially stretched film was measured along the longitudinal direction, and the measurement was calculated using the following formula. Thickness unevenness = Maximum film thickness - Minimum film thickness / Average film thickness x 100 (%) (2) Coefficient of dynamic friction (μd) Fixed hard chrome-plated metal roll (diameter 6
mm) with a wrapping angle of 135° (θ), and a load of 53 g (T ₂ ) was applied to one end at 1 m/min.
The resistance force at the other end (T ₁
(g)) was measured, and the coefficient of friction during running was determined using the following formula. μd = 1/θln (T ₁ / T ₂ ) = 0.424ln (T ₁ / 53) (3) Center line average surface roughness (Ra) Surface roughness measuring instrument manufactured by Kosaka Laboratory Co., Ltd. (SE-3FK)
It was calculated as follows. The tip radius of the stylus is
2μm, load is 30mg. A part of standard length L (2.5 mm) is extracted from the film cross-sectional curve in the direction of its center line, and the roughness curve y=f(x )
When expressed as , the value given by the following formula is expressed in μm. However, the cutoff value is 80 μm. Ra
The average value was calculated for a total of 10 points, 5 points in the vertical direction and 5 points in the horizontal direction. 1/L∫ ^L ₀ | f(x) | dx (4) Intrinsic viscosity ([η]) Add 200 mg of the sample to 20 ml of a mixed solution of phenol/tetrachloroethane = 50/50, and heat and dissolve at approximately 110°C for 1 hour. Measured at 30°C. (5) 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 (6) Film temperature The temperature of the film at the stretched portion was measured using an infrared radiation thermometer manufactured by Burns. Example 1 (Production method of polyester) 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, 0.10 parts of calcium acetate monohydrate, and 0.17 parts of lithium acetate dihydrate were charged into a reactor, heated to raise the temperature, and distilled off methanol. The transesterification reaction takes about 4 hours after the start of the reaction.
The temperature was reached to 230°C, and the transesterification was substantially completed. This reaction product is then treated with triethyl phosphate.
After adding 0.35 part of antimony trioxide and further adding 0.05 part of antimony trioxide as a polycondensation catalyst, polymerization was carried out according to a conventional method to obtain a polyester. In the polyester, many uniform and fine precipitated particles containing calcium, lithium, and phosphorus elements with a particle size of approximately 0.5 to 1 μm were observed. The polyester A had [η]=0.65. Separately, polyester B ([η] = 0.65) containing almost no such internal precipitated particles is produced and mixed with the above polyester at a ratio of A/B = 1/1 (weight ratio) to form a raw material for film formation. And so. (Film Forming Method) An unstretched polyethylene terephthalate film was formed into a biaxially stretched film using a longitudinal stretching device and a tenter (transverse stretching and heat setting device).
The details of the film forming method are described below. First, the raw material polyester was dried and then melt extruded to obtain an unstretched film ([η]=0.62) with a thickness of 160 to 200 μm. Then,
The film was passed through a longitudinal stretching device, and the first few rolls preheated the film to 85°C, and then the film was stretched in the first stage by a factor of 2.3 at a roll temperature of 130°C due to the peripheral speed difference between the low speed roll and the high speed roll, and Δn ₁ =0.006 film was obtained. Subsequently, the film was stretched in a second stage by a factor of 2.0 at a roll temperature of 85° C. using a peripheral speed difference between a low-speed roll and a high-speed roll in the second stage to obtain a film with Δn ₂ =0.040. Further, the same film was stretched in a third stage using a third stage stretching roll at a roll temperature of 95°C and a stretching ratio of 1.25 times to obtain a film with Δn ₃ =0.063. In addition, between the stretching rolls in the first and third stage stretching, an infrared heater is also used for heating, and an infrared radiation thermometer is used to measure the first and third stretching parts from the opposite side of the infrared heater. When we measured the temperature of the film at the stage stretching section, we found that each
The temperatures were 120°C and 105°C. The longitudinally stretched film thus obtained was stretched in the transverse direction by a factor of 3.8 at 140°C using a tenter, and heat-set at 215°C to obtain a biaxially stretched film with a thickness of 15μ. Example 2 A biaxially stretched film with a thickness of 15 μm was obtained in the same manner as in Example 1, except that the second stage stretching temperature was changed from 85° C. to 102° C. and the stretching ratio was changed to 2.5 times. Comparative Example 1 Preheating was performed in the same manner as in Example 1, and the first stage stretching was performed at a stretching roll temperature of 130°C and a stretching ratio of 3.4 times to obtain a film with Δn = 0.040. Roll temperature 95℃ between stretching rolls
A longitudinally stretched film with Δn=0.063 was obtained by stretching 1.25 times. After the transverse stretching, the same treatment as in Example 1 was carried out to obtain a biaxially stretched film with a thickness of 15 μm. 1st
Infrared heater heating was used in both stage and second stage stretching. Comparative Example 2 Preheating was performed in the same manner as in Example 1, and infrared heater heating was also used on the first drawing roll to maintain the roll temperature.
The film was stretched 3.7 times in one step at 85° C. to obtain a film with Δn=0.105. This is laterally stretched using a tenter at
215℃ after horizontal stretching at 110℃ and lateral stretching ratio of 3.9 times.
A biaxially stretched film with a thickness of 15 μm was obtained by heat setting. Table 1 shows the properties of each of the above films. As is clear from Table 1, by lowering Δn after longitudinal stretching, good flatness and smoothness can be achieved, and by performing three-stage stretching, the stretching ratio can be improved despite the low Δn after longitudinal stretching. A film suitable for magnetic tape can be obtained which has extremely high thickness and small thickness unevenness. 【table】

Claims (1)

[Claims] 1. A substantially amorphous film containing polyethylene terephthalate as a main component is first stretched in at least one step in the longitudinal direction so that the birefringence (Δn ₁ ) becomes 0.001 to 0.020 ( 1st stage stretching), then stretched in at least one step in the machine direction so that the birefringence (Δn ₂ ) is 0.015 to 0.055 (2nd stage stretching), and then further stretched in the machine direction so that the birefringence (Δn _{3 )} becomes 0.015 to 0.055. ) is 0.040 to 0.080 in at least one stage (third stage stretching), and after longitudinal stretching so that the longitudinal stretching ratio before transverse stretching is 4.0 times or more, transverse stretching is performed. A method for producing a characterized biaxially oriented polyester film. 2 The first stage stretching is performed at a temperature of 100°C or more and 150°C or less, the second stage stretching is performed at a temperature of 80°C or more and 120°C or less, and the third stage stretching is performed at a temperature of 85°C or more and 120°C or less. A method for producing a biaxially stretched polyester film according to item 1. 3. The method for producing a biaxially stretched polyester film according to claim 1 or 2, wherein the first stage stretching ratio is at least 1.9 times.