JPH0511369B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a polyester film for capacitor dielectrics, and more specifically, it has excellent processability during capacitor manufacturing, has a high dielectric breakdown voltage when used in capacitors, has few insulation defects, and is suitable for capacitor dielectrics. This invention relates to a biaxially oriented polyester film that is extremely excellent as a body film. [Prior Art] A biaxially stretched film made of polyethylene terephthalate is used as a dielectric material for capacitors because it has excellent mechanical properties, heat resistance, electrical properties, and the like. 5 to 5 when using the film as a dielectric of a capacitor.
There are two cases in which an element is made by stacking and winding a 10μ thick metallic electrode foil (aluminum is mainly used) and a 4 to 30μ thick film (called a foil-wound capacitor element), and when aluminum or zinc is directly applied to the film. There is a method in which the capacitor is vacuum-deposited and wound around an element (called a vapor-deposited film capacitor element). Vapor-deposited film capacitor elements are particularly prized for miniaturizing capacitors, but one of the important requirements in the capacitor manufacturing process is that the element should have good windability and crushability. It is. In other words, the film after vapor deposition is wound around a core (winding process), then pulled out from the core, pressed to flatten the element (called press process), and soldered to attach lead wires. like,
A process of spraying metal particles onto both end faces of the flattened element (called a metallicon process) must be performed. Furthermore, since some foil-wound capacitor elements undergo a pressing process or metallicon process, they are required to have good windability and crushability.
Here, "good winding properties" means that the film does not meander or shift at the edges when the film is wound onto a capacitor element using a winding machine, and "good crushability" means that the film is rolled uniformly. It is possible to flatten the element without applying too much load, and there is no gap in the end face of the element after pressing, through which metal particles can enter during the metallicon process. If the winding properties are poor, the crushability is poor, and there are voids on the end face, metallicon particles will enter, causing deterioration of the insulation resistance and dielectric loss tangent, and the product will be defective. In order to improve the windability and chafing properties, it is essential to improve the slipperiness of the film used as a dielectric, and to meet this requirement, fine inorganic particles are added to the polymer. Protrusions have been imparted to the film surface by adding or forming insoluble catalyst residues in the polymer. As mentioned above, by adding an inert inorganic compound to the polymer, forming an insoluble catalyst residue, and forming many protrusions on the film surface, the processability (for example, the windability and crushability of the film) can be improved. Although the performance is good to some extent, on the other hand, the dielectric breakdown voltage decreases, the dielectric breakdown abnormality rate increases, and the performance as a capacitor element decreases. On the other hand, if an attempt is made to make the film surface smooth in order to increase the dielectric breakdown voltage and suppress the dielectric breakdown abnormality rate, the processability will decrease. [Object of the Invention] An object of the present invention is to solve the above-mentioned problems and provide a biaxially stretched polyester film for capacitor dielectrics that has excellent processability, high dielectric breakdown voltage, and few insulation defects. . [Structure/effect of the invention] According to the invention, the object of the present invention is to
It is a 12 μm biaxially stretched polyester film, and its surface has the following characteristics (1) to (3): (1) Surface roughness Ra value [X: μm] is in the following range: 0.019≦X≦0.046 (2 ) Number of surface protrusions with a height of 0.5 to 0.7 μm [Y: pieces/
mm ² ] and the number of surface protrusions with a height of 1.1 μm or more [Z:
/ ^mm2 ] is in the following range: Y≧100 Z≦11 (3) At least a part of the above protrusion is an uneven unit consisting of a protrusion and a depression formed around the protrusion with the protrusion as the core. This is achieved by a polyester film for capacitor dielectric material, which is characterized in that it is formed and present. The polyester in the present invention refers to a polyester whose main acid component is an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, etc., and a polyester such as ethylene glycol, diethylene glycol, tetramethylene glycol, neopentylene glycol, etc. It is a polyester whose main diol component is glycol. The polyester can be produced, for example, by directly polycondensing an aromatic dicarboxylic acid and a glycol, or by transesterifying an aromatic dicarboxylic acid dialkyl ester with a glycol and then polycondensing it, or by polycondensing an aromatic dicarboxylic acid dialkyl ester and a glycol. It can also be produced by a method such as polycondensation of glycol esters. Typical examples of the polyester include polyethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate, and the like. The polyester may be a homopolymer and may contain 15 mole percent of the dicarboxylic acid component.
It may be a copolymerized polyester in which the following is a non-aromatic dicarboxylic acid component and/or 15 mol% or less of the diol component is a diol component other than aliphatic glycol. Furthermore, the polyester is blended with other polymers in a proportion of 15% by weight or less (preferably 10% by weight or less) based on the total amount.
It also includes so-called polymer blends. Examples of other polymers that can be blended include polyamides, polyolefins, and other types of polyesters (including polycarbonate). Further, the polyester may contain additives such as stabilizers, colorants, and antioxidants, if necessary. The protrusions on the surface of the film of the present invention are formed from particles of an inorganic compound added to the polymer; particles based on insoluble catalyst residues produced during polymerization of the polymer; or particles of both. A part or all of these protrusions become protrusions constituting a concavo-convex unit. In addition, the depressions that make up the unevenness unit are
In the process of stretching the film, instead of the concave shape created by mechanical stamps such as conventional embossing,
It is formed by the deformation of the film itself. When an unstretched film containing particles is stretched in the uniaxial direction, the polymer is plastically deformed without deforming the particles, so usually voids are formed at the boundary between the polymer and particles during large deformation (stretching), although it depends on the conditions. occurs. The film containing these voids is then stretched in a direction approximately perpendicular to the uniaxial stretching direction (second axial direction).
When the film is made into an axially oriented film, the voids that were created during the uniaxial stretching are further deformed in the second axial direction, as shown in Figure 1-
As shown in FIG. 1, a void 22 is formed around the protrusion 21 in a quasi-circular shape. In this case, as shown in the cross-sectional view of Figure 1-2, the particles existing in the shallow part near the film surface and the surrounding poids produce protrusions with the particles as the nucleus, but no depressions are formed around the particles. . In the present invention, the depressions in the unevenness unit are obtained by changing the above-mentioned voids to depressions on the film surface.
When an unstretched film is uniaxially stretched, the film is preheated to a high temperature before stretching, or (and) the stretching ratio is set low, so that the film that has undergone the 1st axial stretching has particles (inorganic Voids are substantially not formed around external particles due to additives or internal particles containing catalyst residues.
Next, when the stretched film in this state is stretched in the second axial direction, a depressed portion (depression) of the film with the particles as a nucleus is formed along the second axial direction. The major axis of the elliptical depression is along the second axis direction. Even if slight voids are formed around the particles during the first axial stretching, depressions are formed with these particles as cores. Figure 2 shows the unit of unevenness on the film surface after biaxial stretching.
-1 (top view), and if the stretching conditions are such that stress is concentrated around the particles during the second axis stretching, the depressed portion will be deeply depressed and extend in the second axis direction according to the degree of stress concentration. The major axis tends to increase along the length. FIG. 2-2 (cross-sectional view) shows a cross section of the film near the surface, in which protrusions 21 containing particles and depressions 24 formed around the protrusions 21 are formed in the polyester film 23. In the present invention, the depression formed around the protrusion includes a pseudo-elliptical shape eccentric in the second axis direction. The film of the present invention has a surface with a height of 0.5 to
The number (Y) of 0.7 μm surface projections is 100 (pieces/mm ² ) or more, preferably 140 (pieces/mm ² ) or more, and the height is 1.1 μm.
The number (Z) of the above surface protrusions is 11 (pieces/mm ² ) or less, preferably 7 (pieces/mm ² ) or less. At least some of these protrusions exist forming uneven units. The number of these uneven units is 5 pieces/ ^mm2 or more, and even 15 pieces/mm2 or more.
It is preferable that it is at least mm ² . Furthermore, the above-mentioned film needs to have surface characteristics such that the surface roughness Ra value (X) is in the range of 0.019 to 0.046 (μm), preferably in the range of 0.023 to 0.039 (μm). Ra value:
If X is too large or too small, it is difficult to achieve the object of the present invention. Similarly, if there are too few surface protrusions Y or too many surface protrusions Z, it is difficult to achieve the object of the present invention. Next, an example of the method for forming a polyester film of the present invention will be specifically explained, but the present invention is not limited to this example. The average particle size of kaolin, calcium carbonate, etc.
When stretching an unstretched polyester film containing 0.10 to 1.0% by weight of fine particles of 0.10 to 3.0 μm (preferably 0.3 to 1.5 μm) in the first axial direction,
Preheat to 100-125°C. When this unstretched film is preheated to about 100 to 125°C, a hard chromium-plated roll or a ceramic roll with a matte surface is preferred. The unstretched film can reach a predetermined preheat loss without sticking to the roll surface and under conditions where substantial crystallization does not occur. Of course, the unstretched film can be preheated in a non-contact manner. The unstretched film is stretched at a temperature of 100 to 125°C at a stretching ratio of 3.4 times or less (preferably 2.6 to 3.2 times). The drawing speed is preferably relatively slow, and preferably does not exceed 180 m/mm. Usually 50-140
A drawing speed of about m/mm is selected, and if the drawing speed is low, the drawing temperature can be moved to a slightly lower temperature side. Next, the second stretching is performed by cooling the uniaxially oriented film below the glass transition point or preheating it to a temperature of 100 to 150°C without cooling, and then stretching the uniaxially oriented film at approximately the same temperature. Stretch 3.0 to 4.0 times (preferably 3.2 to 3.8 times) in the direction. When the temperature of the second axis stretching is high, the boundary between the concave portions of the concavo-convex unit becomes clear, but at low temperature, the boundary is often not clear. The stretching ratio in the second axis direction does not significantly affect the frequency of occurrence of uneven units. If this biaxially stretched film has insufficient mechanical strength in the first axial direction, it can be further stretched in this direction.
It is also possible to re-stretch the film by about 1.1 to 1.6 times at a temperature of about 120 to 170°C. The biaxially oriented polyester film, which has been subjected to two-stage stretching and, if necessary, three-stage stretching, is heated at 180 to 240°C (preferably
Heat setting can be performed at a temperature of 190 to 210°C for about 0.2 to 10 seconds. In addition, the third stage in the three-stage stretching
Stage (first axial direction) re-stretching can also be carried out on a film that has undergone heat setting. The direction in which the first axial stretching is applied may be either the machine direction or the width direction of the film. Further, the second axial stretching direction is preferably approximately perpendicular to the first axial direction. Of course, high-stage (multi-stage) stretching in which stretching is further performed in the first axial direction and/or the second axial direction can be performed. In this case as well, the uneven units on the film surface remain as they are even if their shapes are slightly deformed. By appropriately combining the above-mentioned stretching conditions and stretching methods, the unevenness unit can have an appropriate frequency,
Furthermore, it is possible to produce a polyester film that satisfies the surface roughness Ra and the numbers Y and Z of surface protrusions. When there is an uneven unit on the film surface consisting of protrusions originating from fine particles and depressions formed around the protrusions as if they were nuclear protrusions, the same phenomenon as the plastic deformation that forms these depressions occurs due to the formation of the microparticles in the film. Therefore, voids do not exist or their proportion is small around the fine particles in the film, and at the same time, the refractive index in the film thickness direction becomes high. The polyester film of the present invention has all of these characteristics. For example, while the refractive index in the thickness direction of a conventional polyethylene terephthalate film for capacitor dielectrics is 1.492 or less, the refractive index in the thickness direction of the polyethylene terephthalate film of the present invention is high in the range of 1.493 to 1.499. This is extremely advantageous for increasing dielectric breakdown voltage and reducing insulation defects. The method for measuring physical properties in the present invention is shown below. 1 Surface roughness (Ra) Center line parallel roughness: Ra (unit: μm) JIS-
Value defined in clause 5 of B0601. Measurement conditions Cutoff value: 0.25mm Measurement length: 2.5mm 2 Number of surface protrusions (Y, Z) After applying aluminum vapor deposition to the film surface, interference fringes are produced at a measurement wavelength of 0.54 μm using multiple interference method, and the interference fringes are photographically projected. Then, count the number of secondary interference fringes and let the number be Y [pieces/mm ² ],
Count the number of interference fringes of order 4 or higher, and calculate the number by Z
(pieces/mm ² ). 3 Measurement of unevenness Take a photo of the film surface with a thin layer of aluminum vapor deposited on it using a differential interference microscope (for example, Nikon differential interference microscope R type), and measure its size on a scale. 4. Dielectric breakdown voltage and dielectric breakdown abnormality rate Dielectric breakdown voltage was measured by the method shown in JIS-C-2318. The average value of n = 100 was adopted. and the percentage (%) of those showing a value less than 1/3 of this average value.
Let be the dielectric breakdown abnormality rate. 5 Thermal shrinkage rate Film samples with a size of 350 mm x 350 mm were placed with gauges at a distance of 300 mm in the center in both the vertical and horizontal directions, and 10 samples were placed in a hot air constant temperature oven made by Tester Sangyo set at 150°C. was suspended without tension, held for 2 hours, taken out, the distance between the gauge points was measured again, and the heat loss rate was calculated using the following formula, and expressed as the average value of n=10. Thermal contraction rate (%) = L ₀ - L / L ₀ × 100 [However, L ₀ : Original length (distance between gauges 300 mm) L: Length after test (unit: mm)] 6 Evaluation of vapor deposition processability Aluminum vapor deposition is performed on the original film with a film width of 500 mm and a winding length of 20,000 m to improve aluminum purity.
99.99%, evaporation source temperature 1400℃, distance between evaporation surface and film surface 350mm, incident angle 40°, degree of vacuum 5×
Aluminum evaporation was performed under the conditions of 10 ^-5 Torr, evaporation speed 300m/mm, evaporation tension 20Kg, and evaporation thickness 100mμ. Regarding the evaporation processability of the original film, those that did not have any horizontal wrinkles during the evaporation processing were evaluated as ◎ and slightly horizontal. If wrinkles occur but do not lead to deposition spots or slit defects after deposition, horizontal wrinkles or, in some cases, vertical wrinkles occur, and deposition spots or slit defects after deposition occur frequently.
Items that could not be used were marked as ×. 7 Evaluation of element end face irregularities and element end face shape after flattening A film of 4 to 12μ was deposited with aluminum,
A slit of 20 mm width is wound around a core with an outer diameter of 3 mm to a length of 4 m at a winding tension of 40 g and a winding speed of 30 cm/sec. What is attached○
There are some irregularities, but the degree of such irregularities is small and there is no problem in practical use, and those that are rated △, and those that cannot be used, are rated ×. When the element is flattened by pressing, the shape of the element end face after flattening is evaluated as ○, if the film layers are flattened in a straight line and uniformly, and there are no gaps.
A film in which a slight gap was observed in some parts but did not pose any practical problem was rated Δ, and a film unusable due to non-uniform crushing and gaps between film layers was rated ×. 8 Comprehensive evaluation Comprehensive evaluation of handling workability such as crushability and windability, vapor deposition workability, electrical properties such as dielectric breakdown voltage and dielectric breakdown abnormality rate, and ◎ is good in all, and ◎ is slightly poor. ○ indicates that there is no problem in practical use, △ indicates that there is a problem in practical use, and × indicates that it cannot withstand use. [Example] Hereinafter, the present invention will be explained in detail with reference to specific examples, but these examples are not intended to limit the present invention in any way. Examples 1 to 7 and Comparative Examples 1 to 5 Calcium carbonate with average particle diameters of 0.8, 1.0, and 1.3 μm was contained in the amounts shown in Table 1, and the intrinsic viscosity was [measured at 25°C using orthochlorophenol as a solvent].
Using polyethylene terephthalate having a molecular weight of 0.65, the polyethylene terephthalate was dried at 160°C, melt-extruded at 280°C, and rapidly solidified on a casting drum kept at 50°C to obtain an unstretched film of 75 μm. Subsequently, the unstretched film was preheated by four heating rollers 31, 32, 33 and 34 as shown in FIG. One stage of stretching was performed at the stretching ratio. Further, this film was stretched 3.7 times in the transverse direction at a temperature of 105°C, and then heat-treated at 220°C. Note that the stretching speed at this time was 55 m/min. Here, while the preheating temperature of the heating rollers 31 to 34 during longitudinal stretching and the conditions of the infrared heater 38 were not changed, the film temperature immediately before the roller 35 was changed to form and stretch the film. The thickness of the obtained biaxially stretched film was 6 μm,
Thermal shrinkage rate at 150℃ is 1.6% in the vertical direction and 1.6% in the horizontal direction.
It was 1.8%. The film thus obtained was deposited with aluminum, slit to a width of 20 mm, wound around an element, and pressed to flatten the element. As a result, the vapor deposition processability, irregularities on the device end faces, and the shape of the device end faces after flattening were evaluated, and the results are shown in Table 1. Incidentally, the vapor deposition processability was good in both Examples and Comparative Examples.

【table】

Table 1 shows that Examples 1 to 7 have excellent properties as capacitor dielectric films. Furthermore, when the first stage stretching temperature was 85°C, no uneven units were generated on the film surface;
It can be seen that at 100°C, very few uneven units tend to occur, and as the temperature increases to 105°C and 115°C, more uneven units occur. Examples 8 to 13 and Comparative Examples 6 to 10 The same procedures as Examples 1 to 7 were carried out except that kaolin having an average particle size of 0.9 μm was contained in the amount shown in Table 2, and polyethylene terephthalate having an intrinsic viscosity of 0.65 was used.
The film temperature during longitudinal stretching was as shown in Table 2. The properties of the obtained film are shown in Table 2.

【table】 [Brief explanation of the drawing]

Fig. 1 shows the state of voids formed around particles, Fig. 1-1 is a plan view, and Fig. 1-2 is a cross-sectional view. Figure 2 shows a projection containing particles and a depression formed around it, Figure 2-1 is a plan view, and Figure 2-
2 is a sectional view. FIG. 3 is a schematic diagram of a stretching machine used in an example of the present invention. Figure 4 is a micrograph showing the surface of a polyester film according to the prior art, and Figure 5 is a micrograph showing the surface of a polyester film of the present invention (both are magnifications
900 times).

Claims (1)

[Claims] 1. A biaxially stretched polyester film, the surface of which has the following properties (1) to (3): (1) Surface roughness Ra value [X: μm] is in the following range: 0.019≦ X≦0.046 (2) Number of surface protrusions with a height of 0.5 to 0.7 μm [Y: pieces/
mm ² ] and the number of surface protrusions with a height of 1.1 μm or more [Z:
/ ^mm2 ] is in the following range: Y≧100 Z≦11 (3) At least a part of the above protrusion is an uneven unit consisting of a protrusion and a depression formed around the protrusion with the protrusion as the core. A polyester film for capacitor dielectric material, characterized in that it forms and exists.