JPH0773877B2 - Method for producing biaxially oriented polyester film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to a method for producing a biaxially oriented polyester film. More specifically, the physical properties are uniform along the width direction of the film, and dimensional changes and in-plane variations thereof are also present. The present invention relates to a method for producing a biaxially oriented polyester film having extremely low toroidal properties.

<Prior Art> Biaxially oriented polyester films are used for various purposes. Among them, in the applications of flexible liquid crystal panels, photographs, drawings, and magnetic disks, characteristics in both vertical and horizontal directions, especially temperature expansion coefficient, humidity expansion coefficient, and heat shrinkage. It is hoped that the rates are balanced. However, it was extremely difficult to make the physical properties of the product film uniform in the width direction in the usual sequential biaxial stretching method, that is, the method in which the longitudinal stretching is followed by the transverse stretching with a stenter. This is due to the following phenomenon.

Lateral stretching of the film in the stenter is accompanied by longitudinal shrinkage stress. However, since both end portions of the film are gripped and strongly restrained by clips or the like, the influence of the shrinkage stress is small. On the other hand, since the central portion of the film has a relatively weak restraining force, it is greatly affected by the shrinkage stress. Therefore, the central portion of the film travels with a positional delay as compared with the both end portions, and the molecular orientation is also relaxed. For example, if a straight line is drawn in the width direction on the film surface before the lateral stretching, the straight line becomes a concave curve in the film advancing direction during the lateral stretching and the subsequent tension heat treatment. This phenomenon is usually called Boeing. This bowing is inevitable in successive biaxial stretching. Due to this bowing, the film has a distribution of physical properties in the width direction, and a difference in physical properties (particularly non-uniformity of thermal expansion coefficient and humidity expansion coefficient) occurs between the central portion and both end portions. That is, the molecular alignment state differs between the center part and both end parts of the film. When such a film is reheated, a difference in heat shrinkage causes a problem of irregular shape shrinkage, and the film is meandered in a processing step such as coating.

Conventionally, various methods for improving the bowing phenomenon have been proposed. For example, in JP-A-1-165423, after transverse stretching, the film is once cooled to a transverse stretching temperature or lower, and subsequently heated in a temperature region divided into two or more while being stretched by 2 to 20% in the width direction, Then, a method of heat setting has been proposed. However, although this method can reduce the bowing ratio and the anisotropy of the coefficient of thermal expansion, the lateral heat absorption from a low temperature to a high temperature becomes significantly large. As a result, it cannot be used as a base film of a magnetic disk, for example.

Further, in JP-A-1-204723, stretching in the transverse direction is started at a temperature of 90 ° C. or higher and is raised at a rate of 10 ° C./sec or lower to a required maximum temperature, and after transverse stretching at a temperature of the maximum temperature or lower. Relax laterally 0.5-6%, then 80-120
It is proposed as a method of relaxing 0.1 to 1.0% in the vertical direction at ℃. However, this method also yields a film having a large heat absorption in the lateral direction. Then, if the transverse relaxation is performed so that the heat absorption in the transverse direction is reduced to a level at which there is no practical problem, the effect of the uniformity in the width direction is reduced because the transverse stretching step and the transverse relaxing step are continuous. Further, the method of performing the relaxation in the transverse direction in the transverse stretching machine not only complicates the equipment, but also increases the equipment cost.

<Object of the Invention> An object of the present invention is to provide a method for producing a biaxially oriented polyester film having uniform physical properties along the width direction of the film, and having extremely small dimensional change and in-plane anisotropy thereof. .

<Structure / Effect of Invention> According to the present invention, the object of the present invention is to provide a biaxially oriented polyester film by sequentially subjecting a running longitudinally stretched polyester film to transverse stretching, heat setting and heat relaxation. Therefore, (a) the transverse stretching is started at a temperature 20 ° C or more higher than the glass transition point (Tg) of the polyester, and is raised to a temperature 120 to 30 ° C lower than the melting point (Tm) of the polyester. The heat setting is started from the temperature at the end of the transverse stretching, and the temperature is raised to a temperature of (Tm-20) ° C or less while stretching the film in the width direction by 5 to 20%, and then (c) the heat setting film is heated to Tg. After cooling to the following temperature, (d) thermal relaxation is performed without restraining the film width direction and 4 to 10
It is achieved by a method for producing a biaxially oriented polyester film, which is carried out at a temperature of (Tg + 30) to (Tg + 80) ° C. for 0.3 to 20 seconds under a low running tension of Kg / cm ² .

The polyester in the present invention is an aromatic polyester represented by polyethylene terephthalate and polyethylene-2,6-naphthalate, which may be a homopolymer, a copolymer or a blend polymer. For example, it may be a copolyester obtained by copolymerizing polyethylene terephthalate with 20 mol% or less of a third component. It may also be a blend polymer obtained by blending polyethylene terephthalate or copolyester with 20% by weight or less of a third component.

Polyethylene terephthalate is produced by reacting terephthalic acid or its ester-forming derivative with ethylene glycol or its ester-forming derivative, preferably in the presence of a catalyst. In the above reaction, a copolyester can be produced by adding and reacting a compound having an ester-forming functional group as the third component. Also, a blended polymer can be produced by adding and blending the third component before or after the completion of the above reaction. The polyester in the present invention includes stabilizers (eg phosphoric acid, phosphorous acid, esters thereof, etc.), lubricants (eg titanium oxide, silica, calcium carbonate etc.),
Other modifiers such as antistatic agents and flame retardants can be included.

The longitudinally stretched polyester film in the present invention can be obtained, for example, by melt-extruding polyester into a sheet, and rapidly cooling the unstretched film to be stretched in the longitudinal direction by heating with roll heating or infrared heating. This stretching is preferably performed by utilizing the peripheral speed difference between two or more rolls. The stretching temperature is preferably higher than the glass transition point (Tg) of polyester, and more preferably 20 to 30 ° C. higher than Tg. The draw ratio depends on the physical properties of the final film, but it is 3 times or more,
Further, it is preferably 3.5 times or more. This magnification is preferably 5 times or less.

In the present invention, the longitudinally stretched polyester film is sequentially subjected to the treatments of transverse stretching, heat setting and heat relaxation to obtain a biaxially oriented film. These treatments are carried out while the film is running.

The transverse stretching process is based on the glass transition point (Tg) of polyester.
Start at a temperature above 20 ° C. Then, the temperature is raised to (120 to 30) ° C lower than the melting point (Tm) of the polyester. The stretching start temperature is preferably (Tg + 31) ° C. or lower, for example, in the temperature range of 89 to 100 ° C. in the case of polyethylene terephthalate, and in the temperature range of 133 to 144 ° C. in the case of polyethylene-2,6-naphthalate. It is preferable. The maximum stretching temperature is preferably (100 to 40) ° C. lower than Tm.

The temperature increase in the transverse stretching process may be continuous or stepwise (sequential). Usually, the temperature is raised sequentially. For example, the transverse stretching zone of the stenter is divided into a plurality of zones along the film running direction, and a heating medium having a predetermined temperature is caused to flow in each zone to raise the temperature. If the transverse stretching start temperature is too low, the film may break, which is not preferable. On the other hand, if the maximum drawing temperature is lower than (Tm-120) ° C, the heat absorption of the film becomes large, and the ratio of the physical property uniformity in the width direction becomes small, which is not preferable. On the other hand, if the maximum drawing temperature is higher than (Tm-30) ° C, the film becomes soft and the film is torn due to disturbance or the like, which is not preferable.

The ratio of transverse stretching depends on the physical properties of the final film, but it is 3
It is preferable that the amount is at least double, more preferably at least 3.5. This magnification is preferably 5 times or less.

The heat-setting treatment is carried out after the transverse stretching, but the treatment is started at the temperature at the end of the transverse stretching. And 5 in the width direction of the film
Extending by -20% and heating to a temperature below (Tm-20) ° C. This extension is what is commonly referred to as toeout and is preferably 10-15%. The difference between the temperature at the end of heat setting and the temperature at the start of heat setting is 40 ° C or less,
Furthermore, it is preferable that the temperature is 30 ° C. or lower. The temperature difference may be 1 ° C. in some cases, but is preferably 5 ° C. or higher, more preferably 10 ° C. or higher. When the elongation in heat setting is less than 5%, the isotropic region in the film width direction becomes small, which is not preferable. On the other hand, if the elongation is larger than 20%, not only the heat absorption in the lateral direction becomes remarkably large, but also the film is easily broken, which is not preferable.

The heat-fixed film is once cooled to a temperature not higher than the glass transition point (Tg) of polyester, the film end is slit with a predetermined width, separated and then subjected to heat relaxation treatment.

Thermal relaxation treatment does not restrain the width direction of the film, and 4-10Kg
/ cm ² low traveling under tension, is performed (Tg + 30) ~ (Tg + 80) 0.3~20 seconds ℃ temperature. The thickness of the film used for this heat relaxation treatment is preferably 20 to 200 μm, more preferably 30 to 150 μm. The width of the film is preferably 1 m or more. The processing temperature is, for example, about 100 to 1 in the case of polyethylene terephthalate.
It is 50 ° C. The thermal relaxation treatment is preferably performed using a heating levitation treatment device. A heated inert gas, particularly heated air, is preferably used as a medium for heating and floating the film. According to this heating floating process, the thermal relaxation process can be efficiently performed while maintaining stable film traveling.

Biaxially oriented polyester film after heat-relaxation treatment at 60 ℃, 8
Thermal shrinkage of 0.02% or less when held at 0% RH for 72 hours,
Further, it is preferably 0.01% or less, and particularly preferably 0.008% or less. Further, it is preferable that the heat shrinkage rate when kept at 105 ° C. for 30 minutes is 1% or less, further 0.5% or less, and particularly 0.4% or less. Also, the refractive index isotropic in the film width direction (the refractive index of each direction along the film width direction is obtained, and the area where the difference between the maximum value and the minimum value is 10 × 10 ^-13 or less is relative to the film width. The ratio:%) is 70% or more, and even 75
% Or more, the coefficient of thermal expansion isotropic (the coefficient of thermal expansion of each direction along the film width direction is obtained, and the area where the difference between the maximum value and the minimum value is 8 × 10 ⁻⁶ or less is the entire film. The ratio (%) obtained with respect to the width is preferably 70% or more, more preferably 75% or more.

As described above, since the method of the present invention sequentially performs biaxial stretching, heat setting and heat relaxation treatment under specific conditions, it can alleviate the bowing phenomenon and significantly improve the uniformity of physical properties in the film width direction. It is possible to produce a polyester film having a significantly low in-plane anisotropy with a lower heat shrinkage. Therefore, the product yield can be improved. Further, the product film has an excellent balance of physical properties and is useful as a base film for magnetic disks, a base film for photography and other general industrial base films.

<Examples> Hereinafter, the present invention will be further described with reference to Examples. In addition, the physical property in an example is measured by the following method.

1) Refractive index of film According to ASTM-D 542-50, methylene iodide (refractive index is 1.7425) is used as a contact liquid with an Abbe refractometer, and a sodium lamp (wavelength 589 nm) is used as a light source.

Sampling of the sample film is the product film (rolling film)
In the width direction of the sample, the center distribution is performed at 100 m / m pitch, and the maximum and minimum values are obtained by measuring the refractive index of each sample in each direction. And the difference between this maximum and minimum value is 10 × 10 ^-13
The following region in the film width direction is obtained as a ratio to the total width of the product film, and this ratio (%) is shown as the refractive index isotropic.

2) Glass transition point (Tg) and melting point (Tm) of polyester Measure with a DSC (Differential Scanning Calorimeter) II type manufactured by Perkin Elmer. The DSC measurement conditions are as follows. That is, set 10 mg of sample film in the DSC device,
After being melted at the temperature of, it is quenched in liquid nitrogen. The temperature of this quenched sample is raised at 10 ° C./min, and the glass transition point (Tg) and melting point (Tm) are measured.

3) Dimensional change rate of the film A sample film with a width of 10 mm and a length of 150 mm is cut out along the measuring direction, marked points are provided near both ends in the longitudinal direction of the film, and the distance between these marked points is measured before processing. , Leave it in the oven adjusted to the specified temperature and humidity at the free end for the specified time. After taking this out and adjusting it at room temperature, measure the gauge length again,
Obtain the dimensional change rate. For the film in-plane anisotropy,
A sample film was cut out from the original film at intervals of 10 ° over 180 °, the dimensional change rate was measured using these films, and the maximum and minimum values are shown.

4) Coefficient of thermal expansion of the film Cut the sample film into strips 150 mm long and 10 mm wide,
Set this in a thermo-hygrostat and apply a constant load (10 g). Keep the humidity constant (10% RH) and keep the temperature from 20 ℃ to 30 ℃.
The reversible dimensional change Δl when the temperature is changed to ° C is electrically converted by the operating transformer and read, and a in the following equation is defined as the temperature expansion coefficient. l ₀ is a test length of 150 mm.

a = △ l / (l ₀ × 10) mm / mm ・ ℃ The sample film is sampled in the width direction of the product film (winding film) at the center distribution of 200 m / m pitch, and the temperature expansion of each sample in each direction. Measure the coefficient to find the maximum and minimum values. Then, the area in the film width direction in which the difference between the maximum value and the minimum value is 8 × 10 ⁻⁶ or less is obtained as a ratio to the total width of the product film, and this ratio (%) is shown as the coefficient of thermal expansion isotropic.

5) Film thickness Measure with a β-ray thickness gauge.

6) Running tension Measure with a roll equipped with a tension meter (MB11A manufactured by Nireco).

Example 1 Polyethylene terephthalate (Tg69 ° C., Tm263 ° C.) was melt extruded and rapidly solidified by a cooling drum to obtain an unstretched film,
This unstretched film is rolled at 95 ° C with rolls with different peripheral speeds.
It was longitudinally stretched to 3.6 times. Subsequently, the obtained longitudinally stretched film was introduced into a tenter, and transverse stretching was started from 100 ° C., and the temperature at the time of completion of stretching was sequentially raised to 190 ° C., but transversely stretched to 3.4 times, subsequently. The heat setting was started from 190 ° C., and the heat setting was performed while the temperature was 15% stretched (toe-out) in the width direction and the temperature at the end of the heat setting was 215 ° C. The obtained biaxially oriented film is cooled to 70 ° C or lower, and then the film end is slit and separated to 4 m.
A biaxially oriented film having a width of 75 μm was obtained.

Before winding this biaxially oriented film on the winder, it is subjected to a heating levitating process without restraining the width direction, and while being levitated with heated air at 130 ° C, it is biaxially oriented for 4 seconds at a running tension of 5 Kg / cm ^2. It was relaxed in the longitudinal direction and the width direction. After performing this relaxation treatment, it was wound on a winder.

The properties of the obtained film are shown in Table 1.

Example 2 The procedure of Example 1 was repeated, except that the temperature at the completion of transverse stretching and the temperature at the start of heat setting were 175 ° C.

The properties of the obtained film are shown in Table 1.

Example 3 Example 1 except that the extension in the width direction by heat setting is 10%.
Same as.

The properties of the obtained film are shown in Table 1.

Comparative Example 1 The procedure of Example 1 was repeated, except that the relaxation treatment before winding was omitted.

The properties of the obtained film are shown in Table 1.

Comparative Example 2 Example 1 except that the expansion in the width direction by heat setting was 4%.
Same as.

The properties of the obtained film are shown in Table 1.

Comparative Example 3 Example 1 except that the temperature at the completion of transverse stretching was 120 ° C.
Same as.

The properties of the obtained film are shown in Table 1. Comparative Example 4 The procedure of Example 1 was repeated, except that the temperature during the relaxation heat treatment was 80 ° C.

The properties of the obtained film are shown in Table 1.

From the first results, the biaxially oriented polyester films obtained in Examples 1 to 3 have small in-plane dimensional change rates at 60 ° C. and 105 ° C., and have isotropic refractive index and isotropic coefficient of thermal expansion. It is clear that the uniformity of the physical properties is expanded up to near the edge in the state where the film orientation is large and the orientation anisotropy in the film width direction is considerably small. On the other hand, the biaxially oriented polyester film obtained in Comparative Example 1 has a remarkably large heat absorption in the width direction and a large in-plane dimensional change rate, and wrinkles are generated during the heat treatment during product processing, and can withstand this process. There is no such thing. For example, in the step of coating the magnetic layer on the surface of the base film, application of a magnetic coating material, drying, calendering, and curing treatment are usually performed, but these treatments include heat treatment at about 60 to 120 ° C. When the film of Comparative Example 1 is used as the base film, the film is thermally deformed and wrinkles and other defects occur.

Further, it can be seen that the biaxially oriented polyester films obtained in Comparative Examples 2 and 3 have a small refractive index isotropic coefficient in the width direction and a small coefficient of thermal expansion coefficient isotropic coefficient, and thus the product yield is low. Further, it can be seen that the biaxially oriented polyester film obtained in Comparative Example 4 has a large heat absorption and has a defect similar to that of the film of Comparative Example 1.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masami Etanaka 3-37-19 Oyama, Sagamihara-shi, Kanagawa Teijin Ltd. Sagamihara Research Center (56) Reference JP-A-2-22038 (JP, A) JP-A-1-204722 (JP, A)

Claims (1)

[Claims] 1. A method for producing a biaxially oriented polyester film by sequentially subjecting a running longitudinally stretched polyester film to transverse stretching, heat setting and heat relaxation, wherein (a) transverse stretching is carried out by a glass transition of polyester. Start at a temperature 20 ° C or more above the point (Tg) and 12 above the melting point (Tm) of the polyester
The temperature is raised to 0-30 ° C lower temperature, and (b) the heat setting is started from the temperature at the end of the transverse stretching, while being stretched by 5 to 20% in the width direction of the film, and at a temperature of (Tm-20) ° C or less. Up to Tg.
After cooling to the following temperature, (d) thermal relaxation was performed without restraining the film width direction and under a low running tension of 4 to 10 kg / cm ² .
A method for producing a biaxially oriented polyester film, which comprises performing the treatment at a temperature of (Tg + 30) to (Tg + 80) ° C. for 0.3 to 20 seconds.