JPH0346211A - Polyester film for capacitor - Google Patents

Abstract

PURPOSE:To contrive improvement in slipperiness of a film by a method wherein a plurality of microscopic roughness units are provided on the surface of the film. CONSTITUTION:The title film is a poly (1,4- cyclohexanedimetyhylenetelephthalate) (PCT) film composed of terephthalic acid containing an acidic constituent of 80mol% or more and a glycol component containing 90mol% or more of 1,4-cyclohexanedimethanol, and roughness units consisting of microscopic projections and dents having the major axis of 3mum or more with the projections as nuclei, of 50 pieces/mm<2> or more are provided on the surface of the film. The slipperiness of the film can be improved by the roughness units of the above-mentioned biaxially extended polyester film. As a result, the poor slipperiness pecurier to the PTC film can be improved, and the workability such as element winding and the like can also be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a biaxially stretched polyester film for capacitors. More specifically, the present invention relates to a two-wheel stretched polyester film for capacitors that has both excellent workability and electrical properties.

[Prior art and inventions try to solve it! ! ! ! Title] Two-wheel stretched film made of polyethylene terephthalate (hereinafter abbreviated as PET) has excellent mechanical strength, can be made extremely thin to a thickness of 1 to 2 μm, and is relatively inexpensive, making it suitable for use in capacitors. It is widely used as a dielectric material.

However, capacitors using PET film have a disadvantage that they cannot be used because their dielectric loss tangent (tan δ) increases and they generate heat at high temperatures of 80° C. or higher.

On the other hand, the dielectric loss tangent of poly(1,4-cyclohexane dimethylene terephthalate) (hereinafter abbreviated as PCT) does not increase up to about 100°C, and compared to PET film, the operating temperature range on the high-temperature side is about 20°C. C can be increased.

However, PCT film has poorer slipperiness than PET film for boat fishing, and the coefficient of friction between the films is P.
CT film is much larger.

Although the exact reason why PCT film is inferior to PET film in slipperiness is not clear, one inference is that the elastic modulus of PCT film is only about 60 to 70% of that of PET film. That is,
When two films are stacked on top of each other, the minute protrusions on the surface are in contact with the other film, but films with a weak modulus of elasticity easily deform elastically when a load (or tension) is applied to the film. The idea is that the phenomenon in which the protrusions are buried in the other film with which they are in contact becomes significant, resulting in an increase in the coefficient of friction.

A large coefficient of friction, or poor slipperiness, means that
This is a serious drawback as a film for capacitors. Film capacitors are manufactured by providing electrodes by applying metal vapor deposition to a thin film of several μm or by wrapping metal foil, then winding it around an element, and in many cases, processing the rolled element through a process of crushing it to make it flat. However, in these processes, it is essential that the film has good slipperiness.

In order to improve the slipperiness of a film, fine particles are usually incorporated into the film to form fine protrusions on the surface, but as mentioned above, PCT film is
It is less slippery than a film, and if a large amount of fine particles is added to compensate for this drawback, it will cause many internal voids and lower the dielectric breakdown voltage of the film.

In order to put PCT film into practical use as a capacitor film, it was necessary to overcome these drawbacks.

[Means for Solving the Problems] As a result of studies to solve the above problems, the inventors have found that:
The present inventors have discovered that a film having a large number of fine uneven units on the surface of a polyester film having a specific &Il precept is useful as a capacitor film, and has completed the present invention.

That is, the gist of the present invention is that 80 mol% or more of the acid component is terephthalic acid, and 90 mol% or more of the glycol component is 1.4 mol%.
- A biaxial, stretched polyester film composed of cyclohexanedimethanol, on the surface of which there are 50 or more uneven units/am'' consisting of fine protrusions and depressions with a major axis of 3 μm or more with the protrusions as cores. A polyester film for capacitors is characterized by the presence of:

The present invention will be explained in detail below.

The polyester film of the present invention is a film in which 80 mol % or more of the acid component is composed of terephthalic acid, preferably 90 mol % or more, more preferably 95 mol % or more is composed of terephthalic acid. When the terephthalic acid component is less than 80 mol %, not only the rise temperature of the dielectric loss tangent is lowered, but also the crystallinity of the film is lowered, and the thermal shrinkage rate and strength are also adversely affected. In addition, if the above range is satisfied, isophthalic acid, 2゜6
Aromatic dicarboxylic acids such as -naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, and 4,4'-biphenyl dicarboxylic acid, or aromatic hydroxy acids such as P-hydroxybenzoic acid can also be selected as copolymerization components.

In addition, in the polyester film of the present invention, 90 mol% or more of the glycol component is composed of 1,4-cyclohexanedimethanol, preferably 95 mol% or more, and more preferably 97 mol% or more of 1゜4-cyclohexanedimethanol. It is a film made up of. If the 1,4-cyclohexanedimethanol component is less than 90 mol%, the rise temperature of the dielectric loss tangent, thermal shrinkage rate, and strength will be adversely affected, as in the case of the acid component. Further, examples of the third commandment that can be used in combination within the above range include ethylene glycol, propylene glycol, 1,4-butanediol, and the like.

The cis-trans fraction of 1,4-cyclohexanedimethanol in the present invention is not particularly limited, but
The range of cis-isomer/trans-isomer=476 to O/10 is preferable.

The polymerization method of poly(1,4-cyclohexane dimethylene terephthalate) in the present invention can be a known polyester melt polymerization method, but in order to minimize thermal deterioration, melt polymerization and solid phase polymerization are used in combination. It is also possible to adopt a method of adjusting the degree of polymerization to a desired degree. Further, the degree of polymerization of the film of the present invention is determined by its intrinsic viscosity [η] when measured at 30°C in a solution of phenol:tetrachloroethane:1 (weight ratio), assuming a Higgins constant of 0.33. It is preferably 0.45 or more, more preferably 0.60 or more. If [η] is less than 0.45, not only will the stretchability be poor when processing it into a biaxially oriented film, but the rising temperature of the dielectric loss tangent will also be low, making it unsuitable as a film for capacitors. Become something.

The polyester film of the present invention contains fine particles that are inactive with respect to polyester, thereby forming fine protrusions on the surface of the film.

The fine particles are appropriately selected from inorganic or organic particles, specifically, kaolin, talc, silicon dioxide, calcium carbonate, tium fluoride, titanium dioxide, zeolite, calcium terephthalate, crosslinkable polymers, and the like.

The amount of fine particles contained in the film is usually 0.01 to 2.
．． The amount is 0% by weight, preferably from 0.02 to 1.0% by weight, and the average particle size is preferably selected from the range from 0.05 to 5.0 μm. Further, as a method for incorporating it into the film, a known method conventionally used for PET films can be used.

On the surface of the film of the present invention, there are protrusions constituting uneven units with the fine particles as cores. These uneven units can be observed by depositing a metal such as alkene on the surface of a polyester film and then observing the surface using a differential interference microscope. The size can also be determined by taking a photo and comparing it with the scale.

In the present invention, the shapes of the protrusions constituting the uneven units on the film surface include a shape in which a protrusion exists in the center with a fine particle as a nucleus and a round depression surrounding the protrusion, as well as a shape in which a protrusion is formed by biaxially stretching. The surrounding depressions may appear to expand in the direction. The shape of these may change depending on slight differences in film forming conditions, but the size of the concavo-convex unit needs to be such that the long axis of the depression extending around it is 3 μm or more, preferably 5 μm or more. If only uneven units having a long axis of depressions of less than 3 μm are present, the slipperiness will not be improved, which is not preferable. In addition, excessive unevenness units (for example, the major axis is 5
0 μm or more), the core portion forming the bottom shape is often coarsened due to poor dispersion of fine particles, etc., and this is undesirable because it tends to cause insulation defects.

Further, the density of the unevenness units of the present invention needs to be 50 or more per flIIlt, preferably 100
or more. If the density is less than 50 pieces/mm2,
The upper limit of the density, at which no improvement in slipperiness can be expected, should be set at 5,000 pieces/ffl1''.If more than this is present, the effect of improving slipperiness will not improve, and on the contrary, the surface will become too rough.

Next, the method for forming a polyester film of the present invention will be explained in detail.

A polyester containing fine particles having the above-mentioned particle size and addition amount is melt-extruded at a resin temperature of 295° C. to 330° C., and an unstretched polyester sheet is formed on a rotating cooling drum using an electrostatic cooling method.

The thus obtained unstretched sheet is stretched in the first axial direction so that its birefringence Δn becomes 0.075 or less. At this time, stretching in the first axial direction may be performed in one step,
This can also be done in multiple stages. When stretching is carried out in one step, the actual temperature of the film during stretching is 100 to 13
The temperature may be selected between 0° C., the stretching ratio may be selected within the range of 2.5 to 4.0 times, and the birefringence Δn may be adjusted to be 0.075 or less.

In addition, when the first axial stretching is performed in multiple stages, it is preferable that the birefringence Δn before the final stretching stage is 0.015 to 0.060, and 0.075 or less in the final stretching stage.

In any of the above cases, in order to form the surface unevenness units of the present invention, it is essential to suppress the orientation of the film after the first axial stretching. Suppressing film orientation at this time generally worsens film thickness unevenness, but in order to minimize this, it is preferable to carry out the first axial stretching in two or more stages.

Next, the uniaxially oriented film is stretched in a direction perpendicular to the uniaxial orientation in a second axial stretching. The second axial stretching may normally be performed in one step. At this time, the stretching temperature is 85°C to 120°C.
It is preferable to carry out the stretching in combination with a stretching ratio of 2.5 to 4.5 times within the range of .degree. The thus biaxially stretched film is then heat set. Heat setting is usually 150″C to 250°C
1 Preferably from 1 second at a temperature range of 180 to 240'C
Do this for 5 minutes.

For stretching in the first and second axial directions, you can choose between roll stretching, which utilizes the difference in circumferential speed of the rolls, and tenter stretching, which involves gripping the edges of the film with clips in a tenter. I can do it.

If roll stretching is selected for the first axis stretching, the film may stick to the roll if the heated roll is hot, but in such cases, change the roll material to a non-stick material. It is also possible to stretch the film. Non-adhesive rolls include hard chrome rolls with matte surfaces, ceramic rolls, and elastomer rolls.
Fluororesin rolls, etc. can be used.

In the two-wheel stretched polyester film of the present invention, by suppressing the orientation in the first axis direction, in the protrusions on the film surface, depressions are formed that can grow with the protrusions generated around the protrusions as nuclei, and the unevenness The unit improves the slipperiness of the film. Furthermore, when roll stretching is selected for stretching in the first axial direction, whitening of the edges of the film is suppressed by keeping this orientation low. As a result, notch breakage due to edge cracking is significantly reduced when stretching in the second axis direction in a tenter and when slitting the edges of the formed film. This is a very big advantage in forming a thin film of several micrometers as a capacitor film. In addition, PCT film has lower elastic modulus, strength, and elongation at break than PET film, so this tendency is stronger than that of PET film of the same thickness. By suppressing this, film forming continuity and productivity can be significantly improved.

〔Example〕

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to the following Examples unless the gist thereof is exceeded.

In addition, the evaluation method of the film of this invention is shown below.

(1) Birefringence Δn Using an Atago Optical Co., Ltd. Atsube refractometer, measure the maximum refractive index n& in the film plane and the refractive index n1 in the plane orthogonal to the n-a direction, and calculate the birefringence from the following formula. Δn was calculated. Note that the refractive index was measured using sodium D line at 23°C.

Δn=nα−nJ (2) Number of uneven units having depressions around the protrusions After depositing aluminum on the surface of the film, the film surface was photographed at 750x using a differential interference microscope manufactured by Chikara Lutzice.
Compared to the scale, the major axis with the protrusion as the core is at least 3
The number of uneven units consisting of μm depressions was counted over a total area of LM''.

(3) Coefficient of friction of film (coefficient of dynamic friction) Layer two films cut out to Ii 15 am and length 150 am on a smooth glass plate, place a rubber plate on top of that, and then place a load on top of it. Contact pressure of two sheets of film is 2g/cm
", and the frictional force was measured by sliding the films against each other at 20 s/mtn. The friction coefficient at the point where the films were slid 5 degrees was defined as the kinetic friction coefficient.

Note that the measurement was carried out in an atmosphere of 23°C±FC1 humidity 50%±5%.

(4) Whitening of ears The whitening of ears when stretching in the first axial direction was performed by roll stretching was ranked as follows.

O: The ears do not turn white at all.

Δ: The ears are slightly whitened, but the ears are not torn.

×: The ears are considerably whitened and easily cracked.

(5) Breakage during film formation When the film is biaxially stretched and heat-set after stretching in the first axial direction by roll stretching, edge cracking occurs when the film is rolled up. Film-forming continuity was evaluated by converting the number of breaks during stretching or slitting into per hour.

(6) Peak temperature of dielectric loss tangent (tan δ) Aluminum is deposited on both sides of the sample film, and the main electrode, counter electrode,
A guard electrode was formed and used as a measurement sample. Set the measurement sample in a constant temperature bath (To-9) and electrode (SE-70) manufactured by Ando Electric Co., Ltd., and use an LCR meter (4284A manufactured by YHP Co., Ltd.).
) was measured at a frequency of 1 kHz.

To check the peak temperature of tanδ, set the temperature of the constant temperature bath to 2°C.
The temperature was raised at a rate of /2mln, and the temperature with the highest tan δ was determined.

(7) Measurement of withstand voltage using a 100kV direct/AC withstand voltage tester at 23°C50
The voltage was increased at a rate of 1000 V/sec in an atmosphere of %RH, and the voltage was read when the film was destroyed and a short circuit occurred.

Example 1 (Preparation of polymer) 95 mol% of terephthalic acid as a dicarboxylic acid component, 5 mol% of isophthalic acid, and 1.4-mol% as a glycol component.
Using 100% cyclohexanedimethanol, 0.06 mol% of titanium oxide was added as a catalyst based on the acid component, and esterification was carried out by heating in a polymerization tank with stirring. Here, thyroid with an average particle size of 1.30 μm was added as a lubricant so that the amount was 0.40 wt% in the polymer, and then polycondensation was performed to form poly(l,4-cyclohexane dimethylene with [η)=0.65]. terephthalate) copolymer was obtained. After this polymer was made into chips, solid phase polymerization was carried out under a nitrogen stream to finally obtain a polyester having [η)=1.05.

(Film production) After drying the above polyester, it was melt-extruded at 305°C, and adhered to a casting drum kept at 40°C using an electrostatic cooling method to form an unstretched sheet with a thickness of 50 to 55 μm. I got it. This unstretched sheet was brought into contact with metal rolls adjusted to 100"C and preheated, and then stretched 2.50 times in the longitudinal direction between rolls with a difference in circumferential speed while irradiating with an infrared heater. At this time, the stretching section The actual temperature of the film was measured with a radiation thermometer and found to be 100°C.After this stretched film was once cooled to below Tg (glass transition temperature), it was placed on a metal roll adjusted to 115°C again. The films were brought into contact and preheated, and stretched 1.20 times in the longitudinal direction between rolls having different circumferential speeds while being irradiated with an infrared heater.The actual temperature of the film at this time was 115°C.

Moreover, the birefringence Δn of the film after this two-step stretching is 0.
057, and the edges of the film were not whitened at all. Next, this stretched film is introduced into a tenter and 11
It was stretched 3°60 times in the width direction at 0°C. Continuing further, 2
Heat setting was performed at 35°C for 4 seconds to obtain a biaxially oriented film with a thickness of 5 μm. A sample was prepared by slitting the edges of this film and winding it up into a roll. At this time, there was no breakage at all during slitting or transverse stretching, and the continuity of film formation was good. Furthermore, on the surface of this film, 5 to 20
The number of concavo-convex units having depressions with a long diameter of μm is 25 per 1 2
There were 0.

Table 1 shows the physical properties of the obtained film.

Example 2 Example 1 except that calcium carbonate with an average particle size of 1.45 μm was added to give a concentration of 0.33 wt% instead of the thyroid used in the production of the polymer.
A polymer was created in the same manner as . Using this polymer,
A biaxially oriented film with a thickness of 5 μm was obtained in the same manner as in Example 1. At this time, the edges of the film after longitudinal stretching did not whiten at all, and did not break during horizontal stretching or when slitting the edges.

Furthermore, on the surface of this film, there were 190 uneven units per 2 units having depressions with a major axis of 5 to 20 μm. Table 1 shows the physical properties of the obtained film.

Example 3 Using the polymer used in Example 1, Example 1
Melt extrusion and sheeting are performed in the same manner as above, and the thickness is 50 to 55 μm.
An unstretched sheet of m was obtained. This unstretched sheet is held at 105°
After being brought into contact with a metal roll adjusted to C and preheated, 3.
OO@stretched in the longitudinal direction. At this time, the actual temperature of the film in the stretching section was 105°C. Further, the birefringence Δn of the film after longitudinal stretching was 0.060, and the edges of the film were not whitened at all. Next, this stretched film was introduced into a tenter and stretched 3.60 times in the width direction at 110°C. Subsequently, heat setting was performed at 235° C. for 4 seconds to obtain a biaxially oriented film with a thickness of 5 μm. At this time, there was no breakage during transverse stretching or ear slitting. Moreover, on the surface of this film, there are IIIm! 1 hit
There were 40 of them.

Table 1 shows the physical properties of the obtained film.

Comparative Example 1 Using the polymer used in Example 1, Example 1
Melt extrusion and sheeting were performed in the same manner as above, and the thickness was 55 to 60 μm.
An unstretched sheet of m was obtained. This unstretched sheet was heated to 97°C.
After preheating it by contacting it with a metal roll adjusted to
Stretched twice in the machine direction. At this time, the actual temperature of the film in the stretching section was 97°C. In addition, the birefringence Δn of the film after longitudinal stretching was 0.086, and the edges of the film were white and easily broken.6 Next, this stretched film was introduced into a tenter and heated to 3.60 times the width at 110°C. stretched in the direction. Subsequently, heat setting was performed at 235° C. for 4 seconds to obtain a biaxially oriented film with a thickness of 5 μm. At this time, there was one break during horizontal stretching during one hour, which was thought to be caused by whitening of the ears.
This occurred twice at the slit in the ear, making it impossible to obtain a roll sample smoothly. Moreover, no uneven unit having a depression with a major axis of 3 μm or more was found on the surface of this film. Table 1 shows the physical properties of the obtained film.

Comparative Example 2 0.18 wt of thyroid with an average particle size of 1.30 μm
%, polyethylene terephthalate with [η) = 0.66 is dried, melt extruded and made into a sheet with a thickness of 55 to 60%.
An unstretched sheet of μm was obtained. This unstretched sheet was longitudinally stretched in two stages as in Example 1. That is, in the first stage, the roll temperature was set at 82°C, and the temperature was 2.5°C.
Stretched 0 times, and set the roll temperature at 92°C in the second stage to 1.
It was stretched 30 times. At this time, stretching was carried out while irradiating both the first stage and the second stage with infrared heaters, and the actual temperature of the film in the stretching section was 82°C in the first stage and 92°C in the second stage.

The birefringence Δn of the film after longitudinal stretching was 0.050, and the edges of the film were not whitened. Next, this stretched film was introduced into a tenter and stretched 3.80 times in the width direction at 11-5°C. Further, heat setting was performed at 235° C. for 4 seconds to obtain a biaxially oriented film with a thickness of 5 μm. At this time, there was no breakage during the transverse stretching or the slitting of the ears. Further, on the surface of this film, there were 120 uneven units per 2 units having depressions with a major axis of 5 to 20 μm.

The physical properties of the obtained film are shown in Table 1.
Therefore, the peak temperature of tan δ is low.

〔Effect of the invention〕

The polyester film for capacitors of the present invention is made of PC
The poor slipperiness characteristic of T film is improved, and workability such as winding elements is improved. In addition, PCT film has excellent electrical properties, particularly the rise temperature of the dielectric loss tangent, which is about 20°C higher than that of PET film, making it suitable as a film for capacitors. Furthermore, since the polyester film of the present invention has good continuity during film formation, productivity can be greatly improved, and its industrial value is high.

Claims (1)

[Claims] (1) A biaxially stretched polyester film in which 80 mol% or more of the acid component is terephthalic acid and 90 mol% or more of the glycol component is 1,4-cyclohexanedimethanol. There are 5 uneven units consisting of a protrusion and a depression with a major diameter of 3 μm or more with the protrusion as the core.
A polyester film for a capacitor, characterized in that there are 0 or more pieces/mm^2.