POLYPROPYLENE COMPOSITIONS FOR CAPACITOR FILM

Abstract

The present invention relates to a newpolypropylene composition with improved processability and heat resistance for use in a capacitor film, as well as a cast film and a biaxially oriented film for the use as capacitor film comprising such polypropylene composition.

Claims

1: Polypropylene composition comprising: a) 95.0-99.9 wt % of propylene homopolymer (A) having: (i) a melt flow rate MFR.sub.2 (230° C.) measured according to ISO 1133 in the range of 1.5 to 10 g/10 min, (ii) an ash content of <60 ppm, b) 0.1-5.0 wt % of long chain branched polypropylene (B), and c) up to 1000 ppm of beta-nucleating agent.

2: Polypropylene composition according to claim 1, wherein, the crystallization temperature (T.sub.c) of the composition is 120.0-130.0° C.

3: Polypropylene composition according to claim 1, having two melting peaks, where the melting temperature Tm.sub.α is in the range of 161.0 to 170.0° C., and the melting temperature Tm.sub.β is in the range of 148.0 to 160.0° C.

4: Polypropylene composition according to claim 1, wherein propylene homopolymer (A) is an isotactic polypropylene with an isotacticity of from 96.0% to 99.5%.

5: Polypropylene composition according to claim 1, wherein the xylene soluble content of the propylene homopolymer (A) measured according to ISO 6427 is from 0.3 wt % to 2.0 wt %.

6: Polypropylene composition according to claim 1, wherein the long chain branched polypropylene (B) has: a) a F30 melt strength of at least 15 cN, determined at 200° C. according to ISO 16790:2005; and b) a melt extensibility v30 of at least 200 m/s, determined at 200° C. according to ISO 16790:2005.

7: Polypropylene composition according to claim 1, wherein the beta-nucleating agent comprises any one or a mixture of 5,12-dihydro-quino[2,3-b]acridine-7,14-dione (CAS 1047-16-1), quino[2,3-b]acridine-6,7,13,14(5H,12H)-tetrone (CAS 1503-48-6), 5,6,12,13-tetrahydroquino[2,3-b]acridine-7,14-dione (CAS 5862-38-4), N,N′-dicyclohexyl-2,6-naphtalene dicarboxamide (CAS 153250-52-3) and salts of dicarboxylic acids with at least 7 carbon atoms with metals of group IIa of the periodic table.

8: Polypropylene composition according to claim 1, wherein the polypropylene composition has an ash content of <60 ppm.

9: Cast film comprising a polypropylene composition according to claim 1, wherein the crystallinity index (X.sub.c) measured by Wide-angle x-ray scattering (WAXS) is at least 55.0%.

10: Cast film according to claim 9, wherein the composition has a content of β-form crystals of 10-50% measured by Wide-angle x-ray scattering (WAXS)

11: Biaxially oriented polypropylene film, containing a polypropylene composition according to claim 1.

12: Biaxially oriented polypropylene film according to claim 11, wherein the film has dielectric breakdown field strength of at least 600 kV/mm, measured on films having a thickness between 5-6 μm, according to the method and statistical treatment as described in IEEE Transactions on Dielectrics and Electrical Insulation (2013), Vol. 20(3), pp. 937-946.

13. (canceled)

Description

EXAMPLES

[0108] The following definitions of terms and determination methods apply for the above general description of the invention as well as to the below examples unless otherwise defined.

1. Measuring Methods

Quantification of Microstructure by NMR Spectroscopy

[0109] Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the isotacticity and comonomer content of the polymers.

[0110] Quantitative .sup.13C{.sup.1H} NMR spectra were recorded in the solution-state using a Bruker Advance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for .sup.1H and .sup.13C respectively. All spectra were recorded using a .sup.13C optimised 10 mm extended temperature probehead at 125° C. using nitrogen gas for all pneumatics.

[0111] For polypropylene homopolymers approximately 200 mg of material was dissolved in 1,2-tetrachloroethane-d.sub.2 (TCE-d.sub.2). To ensure a homogenous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotatary oven for at least 1 hour. Upon insertion into the magnet the tube was spun at 10 Hz. This setup was chosen primarily for the high resolution needed for tacticity distribution quantification (Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443; Busico, V.; Cipullo, R., Monaco, G., Vacatello, M., Segre, A. L., Macromoleucles 30 (1997) 6251). Standard single-pulse excitation was employed utilising the NOE and bi-level WALTZ16 decoupling scheme (Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225; Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 11289). A total of 8192 (8 k) transients were acquired per spectra

[0112] For ethylene-propylene copolymers approximately 200 mg of material was dissolved in 3 ml of 1,2-tetrachloroethane-d.sub.2 (TCE-d.sub.2) along with chromium-(III)-acetylacetonate (Cr(acac).sub.3) resulting in a 65 mM solution of relaxation agent in solvent (Singh, G., Kothari, A., Gupta, V., Polymer Testing 28 5 (2009), 475). To ensure a homogenous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotatary oven for at least 1 hour. Upon insertion into the magnet the tube was spun at 10 Hz. This setup was chosen primarily for the high resolution and quantitatively needed for accurate ethylene content quantification. Standard single-pulse excitation was employed without NOE, using an optimised tip angle, 1 s recycle delay and a bi-level WALTZ16 decoupling scheme (Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225; Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 11289). A total of 6144 (6 k) transients were acquired per spectra.

[0113] Quantitative .sup.13C{.sup.1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals using proprietary computer programs.

[0114] For ethylene-propylene copolymers all chemical shifts were indirectly referenced to the central methylene group of the ethylene block (EEE) at 30.00 ppm using the chemical shift of the solvent. This approach allowed comparable referencing even when this structural unit was not present.

[0115] For polypropylene homopolymers all chemical shifts are internally referenced to the methyl isotactic pentad (mmmm) at 21.85 ppm.

[0116] Characteristic signals corresponding to regio defects (Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253; Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157; Cheng, H. N., Macromolecules 17 (1984), 1950) or comonomer were observed.

[0117] The tacticity distribution was quantified through integration of the methyl region between 23.6-19.7 ppm correcting for any sites not related to the stereo sequences of interest (Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443; Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A. L., Macromoleucles 30 (1997) 6251).

[0118] Specifically the influence of regio defects and comonomer on the quantification of the tacticity distribution was corrected for by subtraction of representative regio defect and comonomer integrals from the specific integral regions of the stereo sequences.

[0119] The isotacticity was determined at the pentad level and reported as the percentage of isotactic pentad (mmmm) sequences with respect to all pentad sequences:

[mmmm]%=100*(mmmm/sum of all pentads)

[0120] The mole fraction of ethylene in the polymer was quantified using the method of Wang et. al. (Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157) through integration of multiple signals across the whole spectral region of a .sup.13C{.sup.1H} spectra acquired using defined conditions. This method was chosen for its accuracy, robust nature and ability to account for the presence of regio-defects when needed. Integral regions were slightly adjusted to increase applicability to a wider range of comonomer contents.

[0121] The mole percent comonomer incorporation in the polymer was calculated from the mole fraction according to:

E[mol %]=100*fE

[0122] The weight percent comonomer incorporation in the polymer was calculated from the mole fraction according to:

E[wt %]=100*(fE*28.05)/((fE*28.05)+((1−fE)*42.08))

[0123] The comonomer sequence distribution at the triad level was determined using the method of Kakugo et al. (Kakugo, M., Naito, Y., Mizunuma, K., Miyatake, T. Macromolecules 15 (1982) 1150) through integration of multiple signals across the whole spectral region of a .sup.13C{.sup.1H} spectra acquired using defined conditions. This method was chosen for its robust nature. Integral regions were slightly adjusted to increase applicability to a wider range of comonomer contents.

[0124] The mole percent of a given comonomer triad sequence in the polymer was calculated from the mole fraction determined by the method of Kakugo et at. (Kakugo, M., Naito, Y., Mizunuma, K., Miyatake, T. Macromolecules 15 (1982) 1150) according to:

XXX[mol %]=100*fXXX

MFR.sub.2 (230° C.) is measured according to ISO 1133 (230° C., 2.16 kg load)

Xylene Cold Soluble Fraction (XCS Wt %)

[0125] The xylene cold soluble fraction (XCS) is determined at 23° C. according to ISO 6427.

Melt Strength and Melt Extensibility

[0126] The test described herein follows ISO 16790:2005.

[0127] The strain hardening behaviour is determined by the method as described in the article “Rheotens-Mastercurves and Drawability of Polymer Melts”, M. H. Wagner, Polymer Engineering and Sience, Vol. 36, pages 925 to 935. The content of the document is included by reference. The strain hardening behaviour of polymers is analysed by Rheotens apparatus (product of Gottfert, Siemensstr. 2, 74711 Buchen, Germany) in which a melt strand is elongated by drawing down with a defined acceleration.

[0128] The Rheotens experiment simulates industrial spinning and extrusion processes. In principle a melt is pressed or extruded through a round die and the resulting strand is hauled off. The stress on the extrudate is recorded, as a function of melt properties and measuring parameters (especially the ratio between output and haul-off speed, practically a measure for the extension rate). For the results presented below, the materials were extruded with a lab extruder HAAKE Polylab system and a gear pump with cylindrical die (L/D=6.0/2.0 mm). For measuring F30 melt strength and v30 melt extensibility, the pressure at the extruder exit (=gear pump entry) is set to 30 bars by by-passing a part of the extruded polymer. For measuring F200 melt strength and v200 melt extensibility, the pressure at the extruder exit (=gear pump entry) is set to 200 bars by by-passing a part of the extruded polymer.

[0129] The gear pump was pre-adjusted to a strand extrusion rate of 5 mm/s, and the melt temperature was set to 200° C. The spinline length between die and Rheotens wheels was 80 mm. At the beginning of the experiment, the take-up speed of the Rheotens wheels was adjusted to the velocity of the extruded polymer strand (tensile force zero): Then the experiment was started by slowly increasing the take-up speed of the Rheotens wheels until the polymer filament breaks. The acceleration of the wheels was small enough so that the tensile force was measured under quasi-steady conditions. The acceleration of the melt strand (2) drawn down is 120 mm/sec2. The Rheotens was operated in combination with the PC program EXTENS. This is a real-time data-acquisition program, which displays and stores the measured data of tensile force and drawdown speed. The end points of the Rheotens curve (force versus pulley rotary speed), where the polymer strand ruptures, are taken as the F30 melt strength and v30 melt extensibilty values, or the F200 melt strength and v200 melt extensibilty values, respectively.

[0130] Melting temperature (T.sub.m) and heat of fusion (H.sub.f), crystallization temperature (T.sub.c) and heat of crystallization (H.sub.c): measured with Mettler TA820 differential scanning calorimetry (DSC) on 5 to 10 mg samples. DSC is run according to ISO 3146/part 3/method C2 in a heat/cool/heat cycle with a scan rate of 10° C./min in the temperature range of +23 to +210° C. Crystallization temperature and heat of crystallization (H.sub.c) are determined from the cooling step, while melting temperature and heat of fusion (H.sub.f) are determined from the second heating step

Electrical Breakdown Strength

[0131] The electrical breakdown strength was measured and evaluated as described in IEEE Transactions on Dielectrics and Electrical Insulation (2013), Vol. 20(3), pp. 937-946. The method and the statistical evaluation leading to the final result are briefly outlined in the following, details can be found in the cited literature. Generally in breakdown testing, several specimens of the same material (same film) are tested and a number of breakdown results (voltage, kV) become available. Commonly, to derive an average result representative of the material, further statistical treatment of the gathered results is necessary. Specific to failure testing, as in electrical failure testing, is that the measured individual results are distributed according to a Weibull distribution [see for example Dissado L. A., Fothergill J. C., Wolfe S. V., Hill R. M., IEEE Transactions on Electrical Insulation, EI-19 (3), 227-233 (1984)]. It is common procedure to evaluate data generated in dielectric breakdown testing using Weibull statistics, as for example described in IEC 60727, part 1 & 2. As the final result, i.e. as the average breakdown strength (breakdown field) of a material, the breakdown field (kV/mm) at which 63.2% of specimens will fail is usually reported. The method described in IEEE Transactions on Dielectrics and Electrical Insulation (2013), Vol. 20(3), pp. 937-946, follows these outlined principles, namely measuring several breakdowns per material and obtaining an average breakdown result by treating the gathered data as Weibull distributed, and using the 63.2 percentile. The specific features of the method according to IEEE Transactions on Dielectrics and Electrical Insulation (20013), Vol. 20(3), pp. 937-946 is that on a reasonably large area (81 cm.sup.2) of the BOPP film, a high number of breakdowns (40) is recorded and that after a breakdown had occurred, the system continues to increase the voltage. This is an advantage over the method described IEC 60243 part 1 (1998) wherein the system short circuits after the first breakdown. The measurement is repeated 6 times, so that 240 breakdown data become available, measured on a total area of 486 cm.sup.2. These 240 breakdowns have been treated using an additively mixed 2-parameter Weibull distribution (W. Hauschild and W. Mosch, Statistical Techniques for High-Voltage Engineering, Philadephia, IET, UK, 1992).

[0132] Ash content: Ash content is measured according to ISO 3451-1 (1997)

Wide-Angle X-Ray Scattering (WAXS)

[0133] The determination of crystallinity and of polymorphic composition was performed in reflection geometry using a Bruker D8 Discover with GADDS x-ray diffractometer operating with the following settings: x-ray generator: 30 kV and 20 mA; θ.sub.1=6° & θ.sub.2=13°; sample-detector distance: 20 cm; beam size (collimator): 500 μm; and duration/scan: 300 seconds. 3 measurements have been performed on each sample. Intensity vs. 2θ curves between 2θ=10° and 2θ=32.5° were obtained by integrating the 2-dimensional spectra. The quantification of intensity vs. 2θ curves were then performed as follows:

[0134] Intensity vs. 2θ curve was acquired with the same measurement settings on an amorphous iPP sample, which was prepared by solvent extraction. An amorphous halo was obtained by smoothing the intensity vs. 2θ curve. The amorphous halo has been subtracted from each intensity vs. 2θ curve obtained on actual samples and this results in the crystalline curve.

[0135] The crystallinity index X.sub.c is defined with the area under the crystalline curve and the original curve using the method proposed by Challa et al. (Makromol. Chem. vol. 56 (1962), pages 169-178) as:

[00001]$X_c=\frac{\mathrm{Area}.Math..Math.\mathrm{under}.Math..Math.\mathrm{crystalline}.Math..Math.\mathrm{curve}}{\mathrm{Area}.Math..Math.\mathrm{under}.Math..Math.\mathrm{original}.Math..Math.\mathrm{spectrum}}\times 100$

[0136] In a two-phase crystalline system (containing α- and β-modifications), the amount of β-modification within the crystalline phase B was calculated using the method proposed by Turner-Jones et al. (Makromol. Chem. Vol. 75 (1964), pages 134-158) as:

[00002]$B=\frac{I^{\beta }\left(300\right)}{I^{\alpha }\left(110\right)+I^{\alpha }\left(040\right)+I^{\alpha }\left(130\right)+I^{\beta }\left(300\right)}$

where, I.sup.β(300) is the intensity of β(300) peak, I.sup.α(110) is the intensity of α(110) peak, I.sup.α(040) is the intensity of α(040) peak and I.sup.α(130) is the intensity of α(130) peak obtained after subtracting the amorphous halo. The wt.-% of β-form was calculated by multiplying B by 100.

2. Examples

[0137] The propylene homopolymer (A) of the present invention is the commercial product HC300BF of Borealis AG (Austria), having an MFR.sub.2 of 3.3 g/10 min and an ash content of 18 ppm.

[0138] The long chain branched polypropylene (B) of the present invention was prepared from a linear propylene homopolymer powder, by a reactive extrusion in the presence of butadiene and peroxide as described in the following. Both the butadiene and the peroxide (75% solution of tert-butylperoxy isopropyl carbonate “Trigonox BPIC-C75” of Akzo Nobel) were pre-mixed with the linear PP powder (resulting in a peroxide concentration of 0.625 wt % and butadiene concentration of 1.6 wt %, based on the weight of the linear PP powder) before the melt-mixing step in a horizontal mixer with paddle stirrer at a temperature of 65° C., maintaining an average residence time of 15 to 20 minutes. The pre-mixture was transferred under inert atmosphere to a co-rotating twin screw extruder of the type Theyson TSK60 having a barrel diameter of 60 mm and an L/D-ratio of 48 equipped with a high intensity mixing screw having 3 kneading zones and a two-step degassing setup.

[0139] The final long chain branched polypropylene (B) had an MFR.sub.2 of 2.1 g/10 min and an ash content of 170 ppm. It had a F30 melt strength of 36 cN and a v30 melt extensibility of 260 mm/s.

[0140] Commercially available beta nucleating agent Cinquasia Gold YT-923-D was used in the present invention.

[0141] The respective amounts (see table 1) of components A, B and beta nucleating agent were mixed in a twin screw extruder.

[0142] The BOPP films used for the breakdown strength measurements have been obtained by first subjecting the materials of table 1 to a compression molding (CM) process and subsequent biaxial stretching. The stabilized resins were pressed into 800 μm thick sheets, 24×24 cm at a temperature of 200° C. for 3 minutes and cooled down to 40° C. at a (slow) cooling rate of 15° C./min. For the subsequent biaxial orientation, square specimens were cut out from each sheet (8.5 cm by 8.5 cm samples). These specimens were biaxially stretched on a BOPP machine (“Karo IV laboratory stretcher” Brückner Maschinenbau GmbH, Germany). The stretching-process was done at a strain rate of 400%/s and temperature of 155° C. (for IE1, CE1, CE2, CE3). The stress-strain curves obtained from the stretching process is shown in FIG. 1, which indicates the improved draw ratio and reduced stress of the IE1 material upon CE1 and CE2 materials and reflects the improved processability of the inventive example in the present invention.

TABLE-US-00001 TABLE 1 CE1 CE2 CE3 IE1 amount of A wt % 100.0% 99.0% 100.0% 99.0% amount of B wt % — 1.0% — 1.0% MFR.sub.2 of composition g/10 min 3.3 2.9 2.7 2.8 Ash of composition ppm 18 25 15 25 beta-nucleator ppm — — 8 5 T.sub.c ° C. 117.7 126.3 124.5 125.8 T.sub.mα ° C. 164.0 166.0 170.0 165.9 T.sub.mβ ° C. 149.5 — 152.7 151.9 X.sub.c % 66.7 67.8 72.8 68.2 Beta % 25 14 94 23 BOPP film thickness μm 4.5 5.5 — 5.7 DC Breakdown field kV/mm 706 718 — 730 at 63.2% probability A: propylene homopolymer (A) B: long chain branched polypropylene (B) T.sub.c: crystallization temperature T.sub.mα: melt temperature of α-phase T.sub.mβ: melt temperature of β-phase X.sub.c: crystallinity index Beta: amount of β-modification within the crystalline phase