JP3367129B2 - Method for producing polyester film - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing at least a longitudinally oriented polyester film. More specifically, the present invention relates to a method for producing a polyester film oriented at least in the longitudinal direction in order to improve the thickness unevenness of the film, particularly the thickness unevenness in the longitudinal direction generated in the longitudinal stretching step.

[0002]

2. Description of the Related Art Polyester films are used in various fields because of their excellent physical and thermal properties. However, there is a factor that uneven thickness occurs in various steps in this manufacturing. For example, melt extrusion,
Discharge amount variation when molded and solidified into a sheet on a cooling roll, vibration of the polymer, rotation unevenness of the roll, etc., and rotation unevenness and temperature unevenness of the roll during longitudinal stretching, etc.
The temperature unevenness and wind speed unevenness in the tenter during lateral stretching.

The deterioration of the thickness unevenness is not preferable because it causes troubles when the film is secondarily processed, the winding shape is deteriorated, and the quality of the processed product is deteriorated.

Therefore, studies have been conducted to improve the thickness unevenness in each manufacturing process.
Japanese Patent Laid-Open No. 93420 proposes to suppress uneven rotation of a casting drum for cooling and solidifying a molten polymer. Further, Japanese Patent Application Laid-Open No. 59-91121 proposes a method of modifying a polymer so that it is susceptible to an electrostatic force when the molten polymer is brought into close contact with the casting drum by the electrostatic force. Further, in the longitudinal stretching step, Japanese Patent Laid-Open No.
0-189422 proposes a method of bringing a film into close contact with a drawing roll by electrostatic force. Further, JP-A-54-56674 and JP-A-2-130125.
Japanese Patent Laid-Open Publications and the like have proposed a method of performing longitudinal stretching in multiple stages.

As described above, in order to improve the thickness unevenness,
It is important to take measures for each process, and by superimposing them, comprehensive improvement in thickness unevenness can be achieved.

[0006]

As described above, there is a strong demand for improving thickness unevenness, and various studies have been made for that purpose, but the effect is not yet sufficient, and the thickness is not sufficient for market demand. There is a problem that the effect of improving unevenness is not obtained.

In order to solve the above problems, the present invention provides a method for producing a polyester film for obtaining a film in which the thickness unevenness is remarkably improved, especially for the purpose of improving the thickness unevenness in the longitudinal stretching step. The purpose is.

[0008]

A method for producing a polyester film according to the present invention, which meets this object, comprises at least a method for producing a polyester film oriented in a machine direction,
The crystallinity of the film immediately before longitudinal stretching is 1% or more 5
% Or less, which is a method for producing a polyester film.

The present invention will be described in detail below.

The polyester referred to in the present invention is a polymer obtained by polycondensation of a diol and a dicarboxylic acid. Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, adipic acid,
Sebacic acid and the like are represented, and the diol is represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexanedimethanol and the like. Specific examples thereof include polymethylene terephthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate, and polyethylene-2,6-naphthalene dicarboxylate. Of course, these polyesters may be homopolymers or copolymers, and as the copolymerization component, for example, diethylene glycol, neopentyl glycol, polyalkylene glycol and other diol components, adipic acid, sebacic acid, phthalic acid, Examples thereof include dicarboxylic acid components such as isophthalic acid and 2,6-naphthalenedicarboxylic acid. In the case of the present invention, polyethylene terephthalate and polyethylene-2,6-naphthalate are particularly preferable from the viewpoint of mechanical strength, heat resistance, chemical resistance, durability and the like.

Various known additives such as an antioxidant, an antistatic agent, a crystal nucleating agent, and inorganic particles may be added to the polyester.

The film oriented in at least the longitudinal direction in the present invention means a film stretched in the longitudinal direction to give a molecular orientation. Naturally, transverse stretching may be performed thereafter to give biaxial orientation. Further, it may be stretched longitudinally or laterally to give a strong orientation.

The longitudinal stretching in the present invention means stretching for imparting molecular orientation to the film, and is usually performed by the difference in peripheral speed of rolls. This stretching may be performed in one stage, or may be performed in multiple stages using a plurality of roll pairs. The stretching ratio is usually about 2 to 7 times.

In the present invention, the crystallinity of the film immediately before longitudinal stretching refers to the crystallinity of the film immediately before entering the roll for longitudinal stretching, and the crystallinity is described later. Was measured and calculated.

In the present invention, this crystallinity is 1%.
Ru 5% or less is necessary or more. Normally, stress-strain curve at a drawing temperature of polyethylene terephthalate HiAkirami stretching, as shown in FIG. 1, there are flat sections of stress to strain is not increased. However, when stretching is performed in this section, strain is not fixed at one point with respect to stress, so that the stretching ratio varies depending on the location, causing uneven thickness. However, as a result of intensive studies by the authors, the stress-strain curve of the film that has been slightly crystallized comes to rise as shown in FIG. 2, and the stress and the strain have a one-to-one correspondence.
That is, when a film thus slightly crystallized is stretched, it is possible to obtain a film in which unevenness in thickness is not deteriorated. Here, when the crystallinity is less than 1% , the effect of rising the stress-strain curve is small and the effect of improving the thickness unevenness is not sufficient. Also, the crystallinity exceeds 5%
If obtaining is disadvantageous because it causes the deterioration of stretchability.

In the present invention, in order to obtain the crystallinity as described above, it is preferable to heat-treat and crystallize the amorphous unstretched film before longitudinal stretching. At this time, the temperature of the heat treatment is preferably (glass transition point + 10 ° C.) or higher and (melting point−50 ° C.) or lower of the polyester. More preferably (glass transition point + 20 ° C) or higher,
(Melting point −100 ° C.) or less. When the heat treatment temperature is (glass transition point + 10 ° C.) or less, there is no crystallization effect by the heat treatment, or a great amount of time is required to obtain the desired crystallinity. If the melting point is -50 ° C or higher, it is difficult to control the crystallinity, and the film is too soft, which hinders the handling of the film during heat treatment.

In the present invention, the above heat treatment is preferably performed using a heating roll. As the heating roll, one whose surface has been subjected to non-adhesive processing such as polytetrafluoroethylene processing or silicone rubber processing is preferable.

When the above heating roll is used, the film may stick to the roll depending on the processing temperature conditions. In such a case, it is also preferable to perform the heat treatment in an oven. The oven may be one that is heated in a windless state, but a type that blows hot air onto the film is preferably used in terms of heating efficiency. Further, as a means for gripping the film in the oven, it is not particularly limited, but a roll that allows water to pass through the roll to prevent adhesion to the surface, a tenter type that grips both ends of the film, etc. It is preferably used.

Further, a heating and levitation treatment apparatus is preferably used, in which hot air is blown from the lower surface of the film to heat-treat while the film is levitated. In the case of the heating levitation treatment device, there is no problem of sticking of the film, and since the film is gripped by the surface by the air pressure, even a softened film at the time of heating can be heat-treated without problems in running property.

The present invention is not limited to the method of heat-treating the amorphous unstretched film in order to obtain the desired crystallinity. For example, when a polyester is melt-extruded from a T-die and cooled and solidified into a sheet on a cooling roll, the temperature of the cooling roll is controlled to cause crystallization in the cooling process to obtain a desired crystallinity. Is also preferably performed. With this approach,
It is preferable in that the heat treatment apparatus as described above is unnecessary.

Further, in the present invention, it is preferable to obtain not only the desired crystallinity by thermal crystallization but also the desired crystallinity by strain crystallization. That is, before the longitudinal stretching, a preliminary longitudinal stretching is performed so that 1% or more and 5% or less.
This is what gives the lower crystallinity. After carrying out this preliminary stretching, the original longitudinal stretching is carried out in a single stage or in multiple stages at a predetermined temperature and magnification.

The pre-stretching conditions in the present invention are as follows:
At the temperature of the film, the polymer (glass transition point +10
1.degree. C. or higher and (glass transition point + 30.degree. C.) or lower, 1.
A magnification of 2 times or more and 2.5 times or less is preferable. More preferably, it is 1.5 times or more and 2.0 times or less. If the temperature is lower than (glass transition point + 10 ° C.), stretching is not performed uniformly, and a state where necking occurs partially is not preferable. Further, if it is higher than (glass transition point + 30 ° C.), the desired crystallinity cannot be obtained. Also,
If the magnification is less than 1.2 times, the desired crystallinity cannot be obtained, and if it exceeds 2.5 times, the orientation is too advanced and the original longitudinal stretching is hindered. At the stretching stage, stretching is performed in the flat section of the stress-strain curve, and the unevenness in thickness becomes worse.

In this way, the method of obtaining the desired crystallinity by strain crystallization by pre-drawing not only has the effect of improving the thickness unevenness, but also leads to an increase in the film-forming speed due to an increase in the draw ratio, which leads to an increase in productivity. It is also preferable in terms of improvement.

Further, the film produced by the production method of the present invention has a surprisingly remarkable effect of reducing the heat shrinkage rate. It is presumed that this effect is exhibited because a slight crystallized structure is formed before stretching, but a clear factor has not been clarified.

Next, the manufacturing method of the present invention will be explained.
It is not limited to such an example.

As polyester, polyethylene terephthalate pellets are thoroughly dried under vacuum. The pellets are supplied to an extruder heated to a temperature of 270 to 300 ° C., and extruded into a sheet form from a T die.

This molten sheet is adhered and solidified by electrostatic force on a drum cooled to a drum surface temperature of 10 to 100 ° C. to obtain an unstretched film in an amorphous state or having a crystallinity of 1 to 5% . The unstretched film is guided to a heated roll group and longitudinally stretched, but before that, thermal crystallization is performed to a crystallinity of 1 to 5% by a heating roll or an oven. Alternatively, preliminary stretching is performed to impart strain crystallization to the above crystallinity. The crystallized film thus obtained is
It is heated by a heating roll of ~ 120 ° C, longitudinally stretched in the longitudinal direction by 2 to 7 times in one stage or in multiple stages, and cooled by a roll group of 20 to 50 ° C.

When the film is oriented biaxially, subsequently, both ends of the film are guided to the tenter while being gripped by clips,
2 to 5 in the horizontal direction in a hot air atmosphere heated to ~ 150 ° C
Stretch twice horizontally.

The biaxially stretched film has a flatness of 150 to 150 in the tenter in order to impart flatness and dimensional stability.
After heat fixing at 240 ° C., the material is gradually cooled and then cooled to room temperature and wound.

[0030]

[Physical property evaluation method]

1. A density gradient tube of crystallinity n heptane-carbon tetrachloride system was prepared and
Measure the density of the film at ° C. From this density d, the crystallinity (%) = (d-da) / (dc-da) × 10
It was set to 0. Here, da is an amorphous density, dc is a perfect crystal density, and in the case of polyethylene terephthalate, da = 1.335 and dc = 1.455 g / cm ³ were set from literature values.

2. Glass transition point and melting point Using a differential scanning calorimeter DSC3100 manufactured by Mac Science Co., Ltd., the sample was held at 300 ° C. for 5 minutes and rapidly cooled with liquid nitrogen, and then the glass transition point and melting point were measured at a temperature rising rate of 20 ° C./minute. did.

3. Stress-strain curve T. M. A film stretcher manufactured by Long Co. preheats for 30 seconds in a predetermined temperature atmosphere, and then stretches at a speed of 2
At 000% / min, stretched in the machine direction under the constraint of transverse direction,
Stress was measured with a strain gauge attached to the clip.

4. Unevenness of film thickness Anritsu film thickness tester KG601A and electronic micrometer K306C are used to measure 3 in the longitudinal direction.
The thickness is continuously measured through a film sampled to have a width of 0 mm and a length of 10 m. From the maximum thickness value Tmax (μm) and the minimum value Tmin (μm) at a length of 10 m, R (μm) = Tmax−Tmin, and from the average thickness Tave (μm) at a length of 10 m, thickness unevenness (%) = R / Tave I asked.

5. Heat shrinkage rate of the film The film was sampled in a width of 10 mm and a length of 250 mm, and marks were provided at intervals of about 200 mm. The distance between the reference points was accurately measured and designated as To (mm).

This sample is left unloaded in a hot air oven at 150 ° C. for 30 minutes and then cooled at room temperature. After sufficiently cooling, the distance between the gauge points was measured again and was set to T (mm), the heat shrinkage rate (%) = (To−T) / To × 100.

[0036]

EXAMPLES The present invention will be described based on examples.

Example 1 Pellets of polyethylene terephthalate (intrinsic viscosity 0.65, glass transition point 69 ° C., melting point 256 ° C.) were placed at 180 ° C.
After vacuum drying for 3 hours, the mixture was supplied to an extruder heated to 270 ° C to 300 ° C, and formed into a sheet from a T die. Furthermore, an unstretched film obtained by closely adhering and solidifying this film on a cooling drum having a surface temperature of 25 ° C. by electrostatic force was obtained.

This unstretched film was introduced into a group of rolls having a silicone rubber surface heated to 140 ° C., subjected to a heat treatment for 45 seconds, and then longitudinally stretched at 100 ° C. by 3.4 times and 50 ° C.
It cooled with the roll group of -25 degreeC.

Subsequently, the longitudinally stretched film was introduced into a tenter while holding both ends of the film with clips, and laterally stretched 3.6 times in an atmosphere heated to 100 ° C. Then in the tenter 23
It was heat-set at 0 ° C., uniformly cooled slowly, and then cooled to room temperature to obtain a biaxially oriented film having a winding thickness of 12 μm.

The physical properties of the obtained film are shown in Table 1. The film crystallinity immediately before longitudinal stretching was 1%, and a biaxially oriented film with improved uneven thickness could be obtained.

Example 2 An unstretched film obtained in the same manner as in Example 1 was processed at 150 ° C.
After being introduced into an oven in a hot air atmosphere and subjected to heat treatment for 40 seconds, the same longitudinal, transverse stretching and heat setting as in Example 1 were carried out to obtain a biaxially oriented film having a thickness of 12 μm.

The physical properties of the obtained film are shown in Table 1. The film crystallinity immediately before longitudinal stretching is 4.5% ,
A biaxially oriented film having improved thickness unevenness and a small heat shrinkage ratio could be obtained.

Comparative Example 1 An unstretched film obtained in the same manner as in Example 1 was subjected to the same longitudinal, transverse stretching and heat setting as it was without heat treatment to obtain a biaxially oriented film having a thickness of 12 μm.

The physical properties of the obtained film are shown in Table 1. The film crystallinity immediately before longitudinal stretching was 0% because heat treatment was not performed, and the film had poor thickness unevenness and large heat shrinkage.

Comparative Example 2 When the temperature of the oven was set to 220 ° C. in Example 2, the film was too soft and the running property in the oven was poor, and continuous film formation was not possible. Further, the film crystallinity of the part that could be partially heat-treated was 35%, and film breakage occurred in the longitudinal stretching, and film formation was impossible.

Example 3 An unstretched film obtained in the same manner as in Example 1 was prestretched 1.8 times with a roll heated to 85 ° C.,
The film crystallinity was 2.5%. Then Example 1
12 μm thick with the same longitudinal and transverse stretching and heat setting as
A biaxially oriented film of was obtained.

The physical properties of the obtained film are shown in Table 2. Also in the crystallization by pre-stretching, a biaxially oriented film with improved thickness unevenness could be obtained as in the crystallization by heat treatment.

Comparative Example 3 In Example 3, pre-stretching was carried out 2.0 times with a roll heated to 120 ° C., and the film crystallinity was 0.3.
I only got%. The same stretching and heat setting as in Example 3 were performed to obtain a biaxially oriented film having a thickness of 12 μm.

The physical properties of the obtained film are shown in Table 2. Since the degree of crystallinity in the pre-stretching is only 0.3%, the film has poor thickness unevenness.

[0050]

[Table 1]

[Table 2]

[0051]

According to the method for producing a polyester film of the present invention, it is possible to obtain a polyester film in which unevenness in thickness is remarkably improved, and various troubles caused by unevenness in thickness can be eliminated. Further, since a slight crystallized structure is formed before stretching, it is possible to obtain physical properties such as a smaller heat shrinkage ratio. further,
In the case of the method of performing the pre-stretching, it becomes possible to increase the stretching ratio, which can contribute to the speeding up of the film forming speed, that is, the improvement of the productivity.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a stress-strain curve at 90 ° C. to 110 ° C. of an unstretched film of polyethylene terephthalate (thickness 200 μm).

FIG. 2 is a schematic diagram showing an example of a stress-strain curve of a film obtained by crystallizing an unstretched film of polyethylene terephthalate into various crystallinities by heat treatment.

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Claims (8)

(57) [Claims] 1. A method for producing a polyester film, which comprises at least a longitudinally oriented polyester film having a crystallinity of 1% or more and 5% or less immediately before being longitudinally stretched. 2. The amorphous unstretched film is heat-treated and crystallized at a temperature of (glass transition point + 10 ° C.) or higher and (melting point 50 ° C.) or lower before being longitudinally stretched. A method for producing the described polyester film. 3. The method for producing a polyester film according to claim 2, wherein the heat treatment is performed on a heating roll. 4. The method for producing a polyester film according to claim 2, wherein the heat treatment is performed in an oven. 5. The method for producing a polyester film according to claim 2, wherein the heat treatment is performed by a heating floatation apparatus. 6. A polyester is melt extruded through a T die,
The method for producing a polyester film according to claim 1, wherein an unstretched film having a crystallinity of 1% or more and 5% or less is obtained by controlling the temperature of a cooling roll when the film is solidified. 7. The crystallinity is 1% or more and 5% or less by preliminarily longitudinally stretching an amorphous unstretched film before longitudinally stretching.
The method for producing a polyester film according to claim 1, wherein: 8. Preliminary stretching is performed at a temperature of (glass transition point + 10 ° C.) or higher and (glass transition point + 30 ° C.) or lower of the polyester under a condition of 1.2 times or more and 2.5 times or less. The method for producing a polyester film according to claim 7, wherein.