Polypropylene film structure having increased life time

Abstract

The present invention refers to a structure comprising a biaxially oriented polypropylene (BOPP) film having at least one layer comprising a homopolymer of propylene which layer is in contact with an oil phase, the homopolymer of propylene has a) a content of isotactic pentads in the range from 95% to 98%, and b) a content of ash of not more than 30 ppm, based on the total weight of the homopolymer of propylene, characterized in that the oil phase has an absorbance value of ≤0.1, relative to the pure oil, as determined spectrophotometrically at a wavelength of 860 nm by the reduction of transmitted light intensity. The present invention further refers to the use of a biaxially oriented polypropylene (BOPP) film for making capacitors comprising said structure, wherein the oil phase has an absorbance value of ≤0.1, relative to the pure oil, as determined spectrophotometrically at a wavelength of 860 nm by the reduction of transmitted light intensity as well as the use of the homopolymer of propylene for increasing the life time of a capacitor.

Claims

1. A structure comprising a biaxially oriented polypropylene (BOPP) film having at least one layer comprising a homopolymer of propylene which layer is in contact with an oil phase, the homopolymer of propylene having: a) a content of isotactic pentads in the range from 95% to 98%, and b) a content of ash of not more than 30 ppm, based on the total weight of the homopolymer of propylene, wherein the oil phase has an absorbance value of ≤0.1, relative to the pure oil, as determined spectrophotometrically at a wavelength of 860 nm by the reduction of transmitted light intensity, wherein the BOPP film comprises additives selected from the group consisting of stabilisers, acid scavengers, nucleating agents and mixtures thereof, wherein the stabilisers are selected from the group consisting of 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene in an amount of from 400 to 2000 ppm and a combination of 1,3,5-trimethyl-2,4, 6-tri s(3,5-di-tert-butyl-4-hydroxybenzyl)benzene in an amount of from 400 to 2000 ppm and butylhydroxytoluene in an amount of from 100 to 1000 ppm, based on the total weight of the homopolymer of propylene.

2. The structure according to claim 1, wherein the BOPP biaxially oriented polypropylene (BOPP) film comprises from 400 to 5000 ppm, based on the total weight of the homopolymer of propylene, of the additives.

3. The structure according to claim 1, wherein the stabilisers consist of 1,3,5-trimethyl-2,4, 6-tri s(3,5-di-tert-butyl-4-hydroxybenzyl)benzene in an amount of from 400 to 1000 ppm and a combination of 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene in an amount of from 400 to 1000 ppm and butylhydroxytoluene in an amount of from 100 to 800 ppm, based on the total weight of the homopolymer of propylene.

4. The structure according to claim 1, wherein the oil phase comprises at least 20% by weight of aromatic hydrocarbons.

5. The structure according to claim 4, wherein the oil phase comprises cyclic aromatic hydrocarbons.

6. The structure according to claim 5, wherein the oil phase comprises at least 50% by weight of cyclic aromatic hydrocarbons.

7. The structure according to claim 1, wherein the absorbance consists essentially of polypropylene components.

8. The structure according to claim 1, wherein the oil phase has an absorbance value of from 0.01 to 0.1, relative to the pure oil, as determined spectrophotometrically at a wavelength of 860 nm by the reduction of transmitted light intensity.

9. The structure according to claim 1, wherein the absorbance value, determined spectrophotometrically at a wavelength of 860 nm, of the oil phase of the BOPP film is at least 50% below the absorbance value of the oil phase of a BOPP film which is free of a homopolymer of propylene having a content of isotactic pentads in the range from 95% to 98%, and a content of ash of not more than 30 ppm.

10. The structure according to claim 1, wherein the structure is a capacitor.

11. The structure according to claim 1, wherein the oil phase comprises toluene derivatives.

12. A method of forming a capacitor, the method comprising forming the structure as defined in claim 1, wherein the oil phase has an absorbance value of ≤0.1, relative to the pure oil, as determined spectrophotometrically at a wavelength of 860 nm by the reduction of transmitted light intensity.

13. The method according to claim 12, wherein forming the capacitor comprises the steps of flat winding the BOPP film and an aluminium foil to obtain a wound structure and impregnating the wound structure with an oil phase.

14. Use of a homopolymer of propylene having: a) a content of isotactic pentads in the range from 95% to 98%, and b) a content of ash of not more than 30 ppm, based on the total weight of the homopolymer of propylene, in a structure as defined in claim 1 for increasing the life time of a capacitor, characterized in that the increased life time is achieved if the oil phase has an absorbance value of ≤0.1, relative to the pure oil, as determined spectrophotometrically at a wavelength of 860 nm by the reduction of transmitted light intensity.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the visual absorbance results of extraction experiments.

(2) In the following the present invention is further illustrated by means of examples.

EXAMPLES

(3) Description of Methods

(4) Quantification of Microstructure by NMR Spectroscopy

(5) Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the isotacticity and regio-regularity of the propylene homopolymers.

(6) 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.

(7) For propylene homopolymers approximately 200 mg of material was dissolved in 1,2-tetrachloroethane-d2 (TCE-d2). To ensure a homogenous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotary 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., Macromolecules 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, 15 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 (8k) transients were acquired per spectra.

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

(9) For propylene homopolymers all chemical shifts are internally referenced to the methyl isotactic pentad (mmmm) at 21.85 ppm.

(10) 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.

(11) 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., Macromolecules 30 (1997) 6251).

(12) By pentad isotacticity is meant the fraction of isotactic pentads (mmmm).

(13) 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)

(14) The presence of 2,1 erythro regio defects was indicated by the presence of the two methyl sites at 17.7 and 17.2 ppm and confirmed by other characteristic sites.

(15) Characteristic signals corresponding to other types of regio defects were not observed (Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253).

(16) The amount of 2,1 erythro regio defects was quantified using the average integral of the two characteristic methyl sites at 17.7 and 17.2 ppm:
P.sub.21e=(I.sub.e6+I.sub.e8)/2

(17) The amount of 1,2 primary inserted propene was quantified based on the methyl region with correction undertaken for sites included in this region not related to primary insertion and for primary insertion sites excluded from this region:
P.sub.12=I.sub.CH3+P.sub.12e

(18) The total amount of propene was quantified as the sum of primary inserted propene and all other present regio defects:
P.sub.total=P.sub.12+P.sub.21e

(19) The mole percent of 2,1 erythro regio defects was quantified with respect to all propene:
[21e]mol %=100*(P.sub.21e/P.sub.total)

(20) Rheology:

(21) Dynamic rheological measurements were carried out with Rheometrics RDA-II QC on compression moulded samples under nitrogen atmosphere at 200° C. using 25 mm diameter plate and plate geometry. The oscillatory shear experiments were done within the linear viscoelastic range of strain at frequencies from 0.015 to 300 rad/s (ISO 6721-10).

(22) The values of storage modulus (G′), loss modulus (G″), complex modulus (G*) and complex viscosity (η*) were obtained as a function of frequency (ω).

(23) The Zero shear viscosity (η.sub.0) was calculated using complex fluidity defined as the reciprocal of complex viscosity. Its real and imaginary part are thus defined by
f′(ω)=η′(ω)/[η′(ω).sup.2+η″(ω).sup.2] and
f″(ω)=η″(ω)/[η′(ω).sup.2+η″(ω).sup.2]

(24) From the following equations
η′=G″/ω and η″=G′/ω
f′(ω)=G″(ω).Math.ω/[G′(ω).sup.2+G″(ω).sup.2]
f″(ω)=G′(ω).Math.ω/[G′(ω).sup.2+G″(ω).sup.2]

(25) The complex viscosity ratio eta*(0.05 rad/sec)/eta*(300 rad/sec), also the shear thinning index is the ratio of the complex viscosity (η*) at 0.05 rad/sec to the complex viscosity (η*) at 300 rad/sec.

(26) The Polydispersity Index, PI,

(27) PI=10.sup.5/G.sub.c, is calculated from the cross-over point of G′(□) and G″(ω), for which G′(ω.sub.c)=G″(ω.sub.c)=G.sub.c holds.

(28) Number Average Molecular Weight (M.sub.w), Weight Average Molecular Weight (M.sub.w)

(29) Molecular weight averages Mw and Mn were determined by Gel Permeation Chromatography (GPC) according to ISO 16014-4:2003 and ASTM D 6474-99. A PolymerChar GPC instrument, equipped with infrared (IR) detector was used with 3× Olexis and 1× Olexis Guard columns from Polymer Laboratories and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) as solvent at 160° C. and at a constant flow rate of 1 mL/min. 200 μL of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 0.5 kg/mol to 11 500 kg/mol. Mark Houwink constants for PS, PE and PP used are as described per ASTM D 6474-99. All samples were prepared by dissolving 5.0-9.0 mg of polymer in 8 mL (at 160° C.) of stabilized TCB (same as mobile phase) for 2.5 hours for PP or 3 hours for PE at max. 160° C. under continuous gentle shaking in the autosampler of the GPC instrument.

(30) Melt Flow Rate

(31) Melt flow rate (MFR, MFR.sub.2) was determined according to ISO 1133 at 230° C. under the load of 2.16 kg.

(32) Melting Temperature and Crystallinity of the Film

(33) Melting temperature was measured on about 5 to 7 mg film samples with a TA Instrument Q200 differential scanning calorimetry (DSC). DSC is run according to ISO 11357/part 3/method C2 in a heat/cool/heat cycle with a scan rate of 10° C./min in the 30 temperature range of −30 to +225° C. The crystallinity is obtained by dividing the measured melting enthalpy (in J/g) by the melting enthalpy of 100% crystalline polypropylene, 209 J/g.

(34) Ash Content

(35) The ash content of the polymer was determined by combusting the polymer in a weighed platinum crucible. About 100 grams of polymer is weighed into the crucible. The crucible is then heated in a Bunsen burner flame so that the polymer slowly burns. After the polymer is completely burned the crucible is cooled, dried and weighed. The ash content is then the weight of the residue divided by the weight of the polymer sample. At least two measurements are made and if the difference between the measurements is more than 7 ppm then a third measurement is made.

(36) Absorbance

(37) 1 g of each BOPP Film was extracted with 6 ml oil at 80° C. for 24 hours using small glass flasks. Subsequently, the residual film was removed and the flasks with the oil were rapidly cooled down to room temperature (21° C.±2° C.) by placing the glass flasks in a metal block kept at room temperature.

(38) Then, within 4 hours after stopping the extraction experiment, the absorbance value of the oil phase was determined spectrophotometrically by measuring the reduction in light transmittance intensity at a wavelength of 860 nm in accordance with ISO 7027-1 by using the spectrophotometer Shimadzu UV1601 instrument.

(39) The absorbance is expressed by the formula ABS=LOG(I.sub.0/I), wherein I.sub.0 relates to the intensity of the incident light at 860 nm and I relates to the intensity of the transmitted light. The absorbance values are given relative to a blank sample of the oil which was measured first, and whose transmission was set to 100%, i.e. the effective ABS of the oil was zero.

EXAMPLES

Inventive Example 1

(40) The polymerisation process according to Inventive Example 1 of EP-A-2543684 was used for the polymerisation of propylene. Hydrogen and propylene were fed into the reactor so that in each of the polymerisation reactors a propylene homopolymer having MFR.sub.2 of about 3.4 g/10 min was produced. Into the polymer was added Irganox 1330 in an amount of 1000 ppm and BHT in an amount of 500 ppm. The propylene homopolymer had a polydispersity index of 5.5 Pa.sup.−1, a shear thinning index of 14, ash content of 8 ppm and a pentad isotacticity of 96.2%.

(41) The propylene homopolymer as described above was processed to a film by a double bubble process, the resulting film had a thickness of 11 μm. The film had a melting temperature of 167° C. and crystallinity of 66.4%.

Inventive Example 2

(42) The propylene homopolymer as described for inventive example 1 was processed to a film by a tenter process, the resulting film had a thickness of 11 μm. The film had a melting temperature of 167° C. and crystallinity of 71.4%.

Comparative Example 1

(43) The procedure of Example 1 was otherwise followed except that the polymerisation process was conducted according to Comparative Example 1 of EP-A-2543684 and that instead of Irganox 1330, Irganox 1010 in an amount of 4500 ppm and BHT in and BHT in an amount of 1000 ppm was used. The propylene homopolymer had a polydispersity index of 4.5 Pa.sup.−1, a shear thinning index of 10, ash content of 9 ppm and a pentad isotacticity of 92.2%.

(44) The propylene homopolymer as described above was processed to a film by a tenter process, the resulting film had a thickness of 11 μm. The film had a melting temperature of 166° C. and crystallinity of 70.0%.

Comparative Example 2

(45) The procedure of Example 1 was otherwise followed except that the polymerisation process was conducted according to Comparative Example 1 of EP-A-2543684 and that Irganox 1330 in an amount of 1000 ppm was used and BHT was not used. The propylene homopolymer had a polydispersity index of 4.5 Pa.sup.−1, a shear thinning index of 10, ash content of 9 ppm and a pentad isotacticity of 92.2%.

(46) The propylene homopolymer as described above was processed to a film by a tenter process, the resulting film had a thickness of 11 μm. The film had a melting temperature of 166° C. and crystallinity of 68.5%.

Comparative Example 3

(47) The propylene homopolymer as described for comparative example 2 was processed to a film by a double bubble process, the resulting film had a thickness of 11 μm. The film had a melting temperature of 166° C. and crystallinity of 67.1%.

(48) Test 1

(49) Three different extraction experiments were performed with the five comparative and inventive BOPP films using Jarylec C101 (“oil” in the remainder).

(50) Extraction 1 (HPLC): 1 g of each BOPP film was extracted with 200 ml oil at 80° C. for 24 h and then the oil was allowed to cool to room temperature. The oil was filtrated, diluted by a factor 10:1 and analysed by HPLC to determine the additive content in the oil.

(51) Extraction 2: 1 g of each BOPP Film was extracted with 6 ml oil at 80° C. for 18 hours and then the oil was allowed to cool to room temperature. The absorbance of the oil was qualitatively assessed by visual impression as documented by the photographs in FIG. 1.

(52) Extraction 3: 1 g of each BOPP Film was extracted with 6 ml oil at 80° C. for 24 hours. Subsequently, the samples very rapidly cooled down to room temperature (21° C.±2° C.) by placing the sample flasks in a metal block held at room temperature. Then, the absorbance of the oil phase was determined spectrophotometrically at a wavelength of 860 nm.

(53) The results of the extraction experiments are shown in Table 1 and FIG. 1.

(54) TABLE-US-00001 TABLE 1 Results of extraction experiments Oil phase Extraction 1 Stabilizers Extraction 2 Extraction 3 BOPP (Irganox1010/1330) BHT Absorbance Absorbance Film ppm ppm visible value CE1 Traces − + 0.13 CE2 Traces − +++ 0.82 CE3 Traces − ++ 0.39 IE1 Traces Traces − 0.05 IE2 Traces − − 0.04 Pure No No − 0.00 Oil

(55) From the HPLC analyses, it can be gathered that only traces of the additives (Irganox1010/1330 and BHT if present) were found in the oil.

(56) From the visual experiments set out in FIG. 1, it can be gathered that the oil phases of CE2 and CE3 became opaque, while the oil phase of CE1 showed only slight absorbance. The absorbance was strongest for CE2. IE1 and 1E2 remained clear. The spectrophotometric determination of the absorbance revealed that comparative examples CE1, CE2 and CE3 had an absorbance of above 0.1, relative to the pure oil. In contrast, the inventive examples had an absorbance of clearly below 0.1, relative to the pure oil.