Abstract:
An electrical film capacitor in which a pair of metal film electrodes and a porous layer are wound into an assembly, and in which the porous layer serves to wick a liquid dielectric into the wound assembly. The liquid dielectric is an adipic acid ester such as di-isononyl adipate. Mono-isopropyl biphenyl can be added as well.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to electrical film capacitors. 
     Electrical film capacitors are known. One type is exemplified by the Aerofoil®, Aerokraft®, and non-microwave Aeropak® capacitors sold by Aerovox, Inc. These capacitors feature strips wound or folded together. The strips include a pair of electrodes separated by a solid dielectric spacer. A portion of the capacitor is porous so that it can wick a liquid dielectric, thereby drawing the liquid dielectric into the bulk of the capacitor and causing it to fill the spaces between the strips throughout the capacitor. Examples of suitable materials for the porous portion include Kraft paper. The dielectric spacer may act as the porous portion. The porous portion can also form part of one or both of the electrodes (e.g., in the case of metallized paper electrodes). Examples of liquid dielectrics which have been used in these capacitors include aromatic phthalate esters such as di-octyl phthalate. 
     Smooth metallized film capacitors are a second type of electrical film capacitor. They typically include smooth metallized thermoplastic strips wound or folded together. Such capacitors have as their electrode the metallized portion of the strip and as the dielectric spacer the thermoplastic portion of the strip. These capacitors, however, lack a porous portion that can wick the liquid dielectric and draw it into the bulk of the capacitor. As a result, the liquid dielectric (even under vacuum impregnation) at best penetrates only the end regions of the capacitor, and even then only to a small degree (if at all). Consequently, the spaces between the strips in the interior of the capacitor (and indeed throughout most of the capacitor) are devoid of liquid dielectric. 
     Various aromatic and non-aromatic liquid dielectrics have been used to fill metallized film capacitors over the years. Ross et al., U.S. Pat. No. 3,855,508, for example, describes certain adipic acid esters (specifically, di-isooctyl adipate and di-isodecyl adipate) for this purpose. Tracy et al., U.S. Pat. No. 2,947,927 also describes certain adipic acid esters for use as liquid dielectrics in metallized film capacitors (specifically, di-isobutyl adipate, di-hexyl adipate, di-isooctyl adipate, and di-isohexyl adipate). Examples of aromatic liquid dielectrics include phthalate esters (e.g., di-octyl phthalate) and biphenyls (e.g., monoisopropyl biphenyl and halogenated biphenyls). 
     SUMMARY OF THE INVENTION 
     In general, the invention features an electrical film capacitor in which a pair of metal film electrodes and a porous layer are wound into an assembly, and in which the porous layer serves to wick an adipic acid ester liquid dielectric into the wound assembly. 
     In preferred embodiments, the adipic acid ester is di-isononyl adipate and the liquid dielectric further includes mono-isopropyl biphenyl. Preferably, the di-isononyl adipate and mono-isopropyl biphenyl are present in a 9:1 ratio. 
     In some preferred embodiments, the porous layer is a paper strip and serves as a dielectric layer, and the electrodes are metal foil electrodes. In other preferred embodiments, the porous layer is a part of at least one of the electrodes; e.g., one of the electrodes may be a metallized paper strip, with the paper serving to wick the adipic acid ester into the wound assembly. 
     The invention provides a capacitor having a liquid dielectric that not only has good dielectric properties, but is safe and has a viscosity that enables it to be readily processed as well. 
     Other features and advantages will be apparent from the following description of the preferred embodiments thereof, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are perspective views of electrical film capacitor constructions having porous portions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The capacitor is an electrical film capacitor in which a pair of thin film electrodes separated by a thin film dielectric material is wound into a wound assembly. One example of such a construction is shown in FIG. 1. Referring to FIG. 1, wound assembly 10 (in the form of a roll) includes an aluminum foil electrode 12 and a double metallized paper electrode 14 separated by a solid dielectric spacer (e.g., a polypropylene film) 16. The paper portion of electrode 14 is porous and thus wicks liquid dielectric, drawing it into the interior of the wound assembly; as a result, the spaces between strips throughout the assembly (as opposed to merely the end regions) are filled with liquid dielectric. A pair of electrical conductors 18, 20 form electrical connections. Two metallic end spray regions 22 (only one of which is shown), positioned at the top and bottom, respectively, complete the construction of the wound assembly. The assembly is surrounded by housing 23, which is sealed by a cover (not shown) to prevent leakage of the liquid dielectric. 
     Another capacitor construction is shown in FIG. 2. Wound assembly 21 includes a pair of aluminum foil electrodes 25, 24 separated by strips of porous Kraft paper 26. The paper both wicks liquid dielectric and acts as a dielectric spacer. The electrodes and spacer are wound together to form a roll (but could, as well, have been folded, to form the wound assembly). A pair of tabs 28, 30 form electrical connections. Assembly 21 is retained in housing 23. 
     The liquid dielectric is incorporated into the capacitors as follows. 
     The capacitors are first baked in an air oven for 12 hours at 120° C. (±5° C.). The capacitors are then placed under vacuum for 30 minutes, followed by a 30 minute break in vacuum; this procedure is performed for a total of 4-8 times at 120° C. (±5° C.). Next, the capacitors are held under vacuum for 40-70 hours at 120° C., the vacuum being less than 100 microns for at least the last 8 hours, followed by cooling for 2 hours under vacuum to 62° C. (±8° C.). The capacitors are now ready for incorporation of the liquid dielectric. 
     The procedure is performed at 62° C. (±8° C.). The capacitors are placed in a tank which is slowly filled under vacuum with liquid dielectric until the capacitors are fully submerged in the liquid. The vacuum is then broken and the capacitors allowed to stand for 0.5 to 1 hour, after which the vacuum is re-applied for 1-2 hours (the vacuum being maintained at less than 100 microns). Next, the vacuum is broken and the capacitors allowed to soak in the liquid for another 10-30 hours. The capacitors are then removed from the tank, allowed to cool to room temperature, and sealed. 
     The preferred liquid dielectric is a 9:1 mixture of di-isononyl adipate (DINA) and mono-isopropyl biphenyl (MIPB). DINA is commercially available from Exxon Chemical Co. as JAYFLEX® DINA. MIPB is commercially available from Koch Chemical Co. as SURE SOL®-250, Stabilizers (e.g., epoxy and/or ionol) may be added as well. 
     To determine the efficacy of the DINA/MIPB mixture, nine different liquid dielectric compositions (one of which was a 9:1 mixture of DINA and MIPB) were prepared and tested in five different capacitor constructions; not all compositions were tested in all capacitor constructions, The liquid dielectric compositions are summarized below in Table 1. 
     
                       TABLE 1______________________________________    MajorComposition    Component   Minor Component                             Additives______________________________________A        99.55% DOP.sup.1                   0%        0.35% Epoxy                             0.10% IonolB        89.60% DINA.sup.2                 9.95% MIPB.sup.3                             0.35% Epoxy                             0.10% IonolC        79.64% DOP  19.91% MIPB  0.35% Epoxy                             0.10% IonolD        89.10% DOP   9.90% MIPB   1.0% EpoxyE        99.55% DINA    0%        0.35% Epoxy                             0.10% IonolF        89.60% TINT.sup.4                 9.95% MIPB  0.35% Epoxy                             0.10% IonolG        99.55% TINT    0%        0.35% Epoxy                             0.10% IonolH        89.60% DINP.sup.5                 9.95% MIPB  0.35% Epoxy                             0.10% IonolI        99.55% DINP    0%        0.35% Epoxy                             0.10% Ionol______________________________________ .sup.1 Dioctyl phthalate .sup.2 Diisononyl adipate .sup.3 Monoisopropyl biphenyl .sup.4 Triisononyl trimellitate .sup.5 Diisononyl phthalate 
    
     The capacitor/liquid dielectric combinations were subjected to four tests: accelerated lifetime tests at two different voltage/temperature combinations, measurement of the capacitance, and dielectric breakdown tests. Not all combinations were subjected to all four tests. The results are shown in Table 2. The first accelerated lifetime test is designated &#34;a&#34; in the left-hand vertical column of the table, the second accelerated lifetime test is designated &#34;b,&#34; the dielectric breakdown (which is reported with its standard deviation) is designated &#34;c,&#34; and the capacitance is designated &#34;d.&#34; 
     The accelerated lifetime tests involved subjecting the capacitor to a particular voltage at a selected temperature for a particular amount of time, and determining when the device failed. The results are given in Table 2 in terms of number failed/number tested after a particular amount of time had elapsed. For example, &#34;0/3 @1870 h&#34; means that out of the 3 capacitors that were tested, none had failed after 1870 hours. The tests were conducted by the Aerovox AC test lab per the following protocol. 
     In the case of capacitor Nos. 1-3, the accelerated life test was conducted according to EIA 495. The voltage (at 60 Hz) was chosen to be 1.25 times the capacitor&#39;s AC voltage rating. The temperature was chosen to be the temperature rating of the capacitor plus 10° C. 
     In the case of capacitor Nos. 4-5, the accelerated life test was conducted according to EIA 454. The voltage (at 60 Hz) was chosen to be 1.5 times the capacitor&#39;s AC voltage rating. The temperature was chosen to be the temperature rating of the capacitor plus 10° C. 
     The capacitance was measured at 1 KHz in the case of capacitor Nos. 1-3 and at 100 Hz in the case of capacitor Nos. 4-5 using a Gen Rad 1657 Digibridge at 25° C. 
     The dielectric breakdown was measured by raising the DC voltage at a rate of 200-300 volts per second until the capacitor shorted or the power supply (100 Kohm output impedance) was unable to charge the capacitor to any higher voltage. It is reported as the average volts/μm, along with the standard deviation. 
     The structures of the five different capacitors used for the tests and the associated test parameters employed for the accelerated lifetime tests were as follows. 
     In capacitor No. 1, the winding included an aluminum foil electrode and a double metallized paper electrode separated by a 6 μm polypropylene dielectric. The following test parameters were employed for the accelerated lifetime tests: 
     &#34;a&#34;: AC life tested at 656 V/100° C. 
     &#34;b&#34;: AC life tested at 656 V/110° C. 
     In capacitor No. 2, the winding included an aluminum foil electrode and a double metallized paper electrode separated by a 7 μm polypropylene dielectric. The following test parameters were employed for the accelerated lifetime tests: 
     &#34;a&#34;: AC life tested at 719 V/80° C. 
     &#34;b&#34;: AC life tested at 850 V/80° C. 
     In capacitor No. 3, the winding included an aluminum foil electrode and a double metallized Kraft paper electrode separated by a 8 μm polypropylene dielectric. The following test parameters were employed for the accelerated lifetime tests: 
     &#34;a&#34;: AC life tested at 850 V/100° C. 
     &#34;b&#34;: not measured. 
     In capacitor No. 4, the winding included a pair of 0.70 mil aluminum foil electrodes separated by Kraft paper. The following test parameters were employed for the accelerated lifetime tests: 
     &#34;a&#34;: AC life tested at 550 V/80° C. 
     &#34;b&#34;: not measured. 
     In capacitor No. 5, the winding included a pair of 0.75 mil aluminum foil electrodes separated by Kraft paper. The following test parameters were employed for the accelerated lifetime tests: 
     &#34;a&#34;: AC life tested at 550 V/80° C. 
     &#34;b&#34;: AC life tested at 600 V/80° C. 
     
                       TABLE 2______________________________________Liquid Dielectric Composition______________________________________    A           B           C______________________________________Capacitor #1a        0/3 @ 1870 h                0/3 @ 1870 h                            --b        0/3 @ 802 h 1/3 @ 802 h --    0/3 @ 2104 h                1/3 @ 2104 hc        353 V/μm ± 14                407 V/μm ± 19                            --d        9.76 μF  9.59 μF  --Capacitor #2a        0/2 @ 2009 h                0/2 @ 2009 h                            --b        0/4 @ 2071 h                0/2 @ 2071 h                            --c        336 V/μm ± 27                329 V/μm ± 20                            --d        3.80 μF  3.75 μF  --Capacitor #3a        1/5 @ 2011 h                0/5 @ 2011 h                            --b        --          --          --c        345 V/      363 V/μm ± 29                            --    μM ± 23d        8.33 μF  8.15 μF  --Capacitor #4a        4/8 @ 1032 h                2/6 @  1400 h                            --    7/8 @ 2327 h                6/6 @ 1935 hb        --          --          --c        169 V/μm ± 15                191 V/μm --d        4.63 μF  4.29 μF  --Capacitor #5a        --          --          0/8 @ 2327 hb        --          0/4 @ 2021 h                            --c        --          --          181 V/μm ± 9d        --          4.64 μF  5.03 μF______________________________________    D           E           F______________________________________Capacitor #1a        --          --          3/3 @ 455 hb        3/3 @ 226 h --          3/3 @ 99 hc        381 V/μm ± 13                --          393 V/μm ± 4d        10.04 μF --          9.42 μFCapacitor #2a        --          1/1 @ 1853 h                            0/2 @ 2009 hb        --          1/1 @ 109 h 0/3 @ 2071 hc        --          364 V/μm ± 10                            350 V/μm ± 10d        --          3.72 μF  3.76 μFCapacitor #3a        4/4 @ 637 h --          5/5 @ 1108 hb        --          --          --c        333 V/μm ± 19                --          373 V/μm ± 16d        8.52 μF  --          8.05 μFCapacitor #4a        --          --          3/6 @ 887 h                            6/6 @ 1266 hb        --          --          --c        --          --          --d        --          --          4.22 μFCapacitor #5a        --          3/4 @ 1350 h                            --b        --          --          4/4 @ 685 hc        --          210 V/μm --d        --          4.44 μF  4.41 μF______________________________________    G           H           I______________________________________Capacitor #1a        --          --          --b        --          --          --c        --          --          --d        --          --          --Capacitor #2a        0/2 @ 2009 h                --          --b        1/2 @ 1040 h                --          --c        307 V/μm ± 51                --          --d        3.72 μF  --          --Capacitor #3a        --          --          --b        --          --          --c        --          --          --d        --          --          --Capacitor #4a        --          --          0/8 @ 1625 h                0/8 @ 2327 h                            1/8 @ 2327 hb        --          --          --c        --          152 V/μm ± 26                            201 V/μm ± 26d        --          4.62 μF  4.66 μFCapacitor #5a        --          0/8 @ 2327 h                            1/8 @ 2327 hb        --          --          --c        --          180 V/μm 160 V/μm ± 10d        --          4.73 μF  4.80 μF______________________________________ 
    
     As shown in Table 2, the 9:1 DINA/MIPB composition (composition B) exhibited satisfactory properties compared to the DOP-based compositions (compositions A, C, and D). DINA, however, does not suffer from the toxicity problems posed by DOP. DINA, for example, has been approved by the United States Food and Drug Administration for use in food wraps. 
     Other embodiments are within the following claims.