Capacitor containing a biaxially oriented polypropylene-cyclic olefin polymer film as a dielectric, and use of said film

Abstract

Disclosed are capacitors containing as dielectric a biaxially oriented film comprising a mixture of polypropylene and cycloolefin polymer, the proportion of cycloolefin polymer in the mixture being between 3 and 18% by weight.

These capacitors are characterized by high temperature resistance and high dielectric strength at room temperature.

Claims

1. A capacitor comprising as dielectric a biaxially stretched film comprising a blend of polypropylene and cycloolefin polymer, with the proviso that the proportion of cycloolefin polymer in the blend is between 3 and 18% by weight.

2. The capacitor according to claim 1, wherein the proportion of cycloolefin polymer in the blend is between 3 and 14% by weight.

3. The capacitor according to claim 2, wherein the proportion of cycloolefin polymer in the blend is between 6 and 12 wt. %.

4. The capacitor according to claim 3, wherein the proportion of cycloolefin polymer in the blend is between 7 and 9 wt. %.

5. The capacitor according to claim 1 wherein the biaxially stretched film shows no phase structure in scanning electron microscopic examination after 24-hour treatment with cyclohexane at room temperature.

6. The capacitor according to claim 1, wherein the biaxially stretched film exhibits a surface structure without fibrils when examined under a light microscope.

7. The capacitor according to claim 1, wherein the cycloolefin polymer is a cycloolefin copolymer.

8. The capacitor according to claim 1, wherein the cycloolefin polymer has a glass transition temperature between 130 and 170° C.

9. The capacitor according to claim 1, wherein the cycloolefin polymer is a cycloolefin copolymer consisting of structural units derived from ethylene and norbornene.

10. The capacitor according to claim 1, wherein polypropylene is a propylene homopolymer or a propylene copolymer with a crystallite melting temperature between 100 and 170° C.

11. The capacitor according to claim 1, wherein the polypropylene is a capacitor-grade polypropylene.

12. The capacitor according to claim 1, wherein the biaxially oriented film is metallized.

13. The capacitor according to claim 1, wherein the biaxially oriented film contains no additives.

14. The capacitor according to claim 1, wherein a total content of trace metals of iron, cobalt, nickel, titanium, molybdenum, vanadium, chromium, copper, magnesium and aluminum in the biaxially stretched film is less than 10 ppm.

15. The capacitor according to claim 1, wherein the biaxially oriented film has a thickness of between 0.5 and 20 μm, measured according to DIN 53370.

16. A method for manufacturing capacitors utilizing a biaxially stretched film comprising a mixture of polypropylene and from 3 to 18% by weight of cycloolefin polymer as a dielectric, the percentage being based on the total amount of the mixture.

17. The method according to claim 16, wherein a biaxially oriented film is used which, after treatment with cyclohexane at room temperature for 24 hours, shows no phase structure in scanning electron microscopic examination and which, in light microscopic examination, shows a surface structure without fibrils.

18. The capacitor according to claim 8, wherein the glass transition temperature is between 140 and 160° C.

19. The capacitor according to claim 18, wherein the glass transition temperature is greater than 145° C. to 160° C.

20. The capacitor according to claim 10 wherein the polypropylene comprises a semi crystalline polypropylene with a melting temperature between 100 and 170° C.

Description

EXAMPLES V1, V2 AND 1 TO 5

[0090] Biaxially stretched films of polypropylene as well as polypropylene-cycloolefin copolymer blends were produced in a thickness of 6 μm and metallized and from these hermetically sealed round wound capacitors were produced.

[0091] Electrical breakdown strengths were determined on the films at 23° C. according to DIN EN 60243-2, using DC voltage and a circular electrode with a diameter of 50 mm. Table 1 below gives details of the films and capacitors used and the measurement results.

TABLE-US-00001 TABLE 1 surface Electric polymer T.sub.g roughness.sup.5) breakdown composition COC Ra of film strength.sup.3) capacity.sup.4) Example PP(%).sup.1) COC(%).sup.2) [° C.] [μm] [V/μm] [μF] V1 100 0 — 0.08 580 6.37 1 95 5 147 0.08 582 6.29 2 93 7 147 0.09 577 6.26 3 93 7 142 0.08 582 6.29 4 88 12 142 0.08 570 6.28 5 85 15 147 0.08 579 6.27 V2 80 20 142 0.08 545 19.8 .sup.1)Highly crystalline polypropylene homopolymer. capacitor film grade from Borealis .sup.2)Cyclooclefin copolymer from norbornene and ethylene .sup.3)Measured values are mean values from 10 individual measurements .sup.4)Measured values are mean values from 10 individual measurements; measurement at 1 KHz .sup.5)Surface roughness determined according to DIN 4769

[0092] These examples show that PP/COC films of examples 1 to 5 used in accordance with the invention have a better dielectric strength than PP/COC films previously proposed for use in capacitors.

[0093] Furthermore, the homogeneity of the polymer matrix and the surface is improved. Films used according to the invention thus come closer in important properties to known OPP capacitor films than PP/COC films proposed so far for use in capacitors.

[0094] FIG. 1 shows a scanning electron micrograph of a cross-section through a film according to examples V1, 1, 2, 3, 4 and 5. FIG. 2 shows a scanning electron micrograph of a cross-section through a film according to example V2.

[0095] To prepare FIGS. 1 and 2, the films were cut with a microtome and COC phases were removed by contact with cyclohexane at room temperature for 24 hours. Areas where polymer has been removed are darkened by this procedure. The scanning electron microscope used was the Hitachi S-4700 model.

[0096] From the scanning electron micrographs of the films according to Example V2, it can be seen that the COC is present in the polypropylene matrix as a separate phase. Such COC phases in PP matrices are already known and described in Research Disclosure No. 655030 of November 2018. In contrast, the films of Examples 1 to 5 used according to the invention and the OPP film of Example V1 do not show any visible phase structure (resolution about 0.1 μm).

[0097] The upper half of FIG. 3 shows an optical microscope image of the surface of a film according to Example V1, 1, 2, 3, 4 or 5. For these images, the films were not prepared. The films were viewed under oblique incident light illumination or dark field and appropriate magnification. The lower half of FIG. 3 shows an optical microscope image of the surface of a film according to Example V2. Films of examples 1 to 5 used according to the invention show the same surface structures typical for OPP capacitor films. In contrast, films made from PP/COC blends of Example V2 with a higher COC content show a different fibrillar surface structure.

[0098] Films used according to the invention thus show a surface structure similar to that of OPP capacitor films. In the case of the films from Comparative Example V2, a surface structure is visible that originates from the PP/COC blended structure. Surface structures of PP/COC blended structures are already known and described in Research Disclosure No. 655030, November 2018.

[0099] Temperature Dependence of Dielectric Strength

[0100] A test was carried out to determine the dielectric strength according to DIN EN 60243-2 at different temperatures. A single-layer film was used and electrodes with a diameter of 50 mm were employed. The results can be found in Table 2 below.

TABLE-US-00002 TABLE 2 temperature [° C.] film type 25 100 150 160 film of 579 V/μm 448 V/μm 288 V/μm 262 V/μm Example V1 film of 577 V/μm 476 V/μm 336 V/μm 323 V/μm Example 2

[0101] The dielectric strength is usually determined at room temperature.

[0102] Measurements at elevated temperature show that the film of Example 2 used in accordance with the invention had a higher dielectric strength than the standard OPP film.

[0103] Long-Term Measurements at Elevated Temperature

[0104] Measurements of long-term dielectric strength at elevated temperature under tension were carried out on capacitors made from formulations according to table 1. At the beginning of a series of measurements as well as after intervals of about 250 h, capacitance and dissipation factor were determined at 50 Hz at room temperature, and storage under elevated temperature was continued thereafter.

[0105] In a first test, capacitors from Example V1, 3 and 4 were tested.

[0106] Tables 3 and 4 below show the temperature profiles and the measurement results.

[0107] The test was terminated after failure of all capacitors from example V1.

TABLE-US-00003 TABLE 3 E-field PP-film PP-COC-film PP-COC-film strength temperature (Example V1) (Example 3) (Example 4) time [h] [V/μm] [° C.] capacity [μF]/(% change) 0 6.26 6.41 6.33  0-310 190 90 6.22 (−0.6%) 6.41 (+0%) 6.33 (+0%) 310-530 190 120 6.18 (−1.3%) 6.46 (+0.8%) 6.36 (+0.5%) 530-770 190 125 6.18 (−1.3%) 6.46 (+0.8%) 6.36 (+0.5%)  770-1030 190 130 6.14 (−1.9%) 6.47 (+0.9%) 6.35 (+0.3%) 1030-1370 190 130 6.07 (−3.0%) 6.44 (+0.5%) 6.35 (+0.3%) 1370-1640 190 133 failed 6.40 (−0.1%) 6.33 (+0.0%) 1640-1900 190 145 failed 6.14 (−4.2%) 6.26 (−1.1%) failed units at test end 3 of 3 1 of 3 0 of 3

TABLE-US-00004 TABLE 4 E-field PP-film PP-COC-film PP-COC-film strength temperature (Example V1) (Example 3) (Example 4) time [h] [V/μm] [° C.] loss factor [*10.sup.−4]/(% change) 0 3.55 3.15 3.04  0-310 190 90 3.04 (−14%) 2.82 (−10%) 2.81 (−5%) 310-530 190 120 3.74 (5%) 4.17 (18%) 3.40 (12%) 530-770 190 125 3.53 (−3%) 4.01 (13%) 3.23 (6%)  770-1030 190 130 5.81 (63%) 4.29 (36%) 4.21 (38%) 1030-1370 190 130 13.20 (271%) 4.13 (31%) 4.18 (38%) 1370-1640 190 133 failed 5.61 (57%) 4.93 (62%) 1640-1900 190 145 failed 52.5 (1566%) 6.91 (127%) failed units at test end 3 of 3 1 of 3 0 of 3

[0108] From the results, it is clear that the capacitors according to the invention exhibit significantly improved stability of properties under the influence of temperature compared with the known OPP capacitors.

[0109] In another similar long-term test, capacitors according to Examples V1, 1, 2 and 5 were tested. The capacitors of examples 1, 2 and 5 were produced using PP/COC blends with low COC contents, and 5° C. higher glass transition temperature of the COC.

[0110] Tables 5 and 6 below show the temperature curves and the measurement results.

TABLE-US-00005 TABLE 5 E-field PP-film PP-COC-film PP-COC-film PP-COC-film strength temperature (Example V1) (Example 1) (Example 2) (Example 5) time [h] [V/μm] [° C.] capacity [μF]/(% change) 0 6.23 6.33 6.35 6.35  0-250 190 120 6.08 (−2.4%) 6.33 (0%) 6.40 (+0.6%) 6.41 (+0.9%) 250-500 190 125 5.92 (−5.0%) 6.31 (−0.3%) 6.39 (+0.8%) 6.41 (+1.0%) 500-800 190 130 5.09 (−18.3%) 6.30 (−0.5%) 6.36 (+0.2%) 6.41 (+1.0%)  800-1100 190 135 failed 6.25 (−1.3) 6.32 (−0.5%) 6.41 (+1.0%)

TABLE-US-00006 TABLE 6 E-field PP-film PP-COC-film PP-COC-film PP-COC-film strength temperature (Example V1) (Example 1) (Example 2) (Example 5) time [h] [V/μm] [° C.] loss factor [*10.sup.−4]/(% change) 0 3.2 2.9 3.2 3.1  0-250 190 120 5.3 (67%) 3.7 (28%) 5.7 (82.5%) 3.5 (8%) 250-500 190 125 17.9 (465%) 4.7 (63%) 6.8 (116%) 3.6 (15%) 500-800 190 130 102 (3118%) 5.3 (82%) 7.1 (124%) 4.6 (49%)  800-1100 190 135 failed 7.2 (150%) 7.2 (129%) 4.6 (47%)

[0111] From the results, it can be seen that capacitors according to Example 2 exhibit increased temperature stability even with low COC contents.

[0112] Dielectric Strength of Capacitors in Oil

[0113] Dielectric strength tests were carried out on oil-impregnated capacitors. These were wound from the appropriate film and aluminum foil and impregnated with rapeseed oil. The electrode surface area was 2 m.sup.2. The measurements were made with DC voltage and at room temperature. The measurement results are shown in Table 7 below.

TABLE-US-00007 TABLE 7 film type dielectric strength [V/μm] OPP-film; Example V1 394 PP/COC-film; Example 2 394

[0114] Both the oil-impregnated capacitors made from the known OPP films and capacitors made from the films used in the invention showed the same dielectric strength.