BOPP capacitor film with peculiar roughness statistics

Abstract

The present invention provides a biaxially oriented polypropylene (BOPP) film for capacitor, which has a peculiar surface roughness derived from fiber-like supramolecular structures (peaks) on a surface thereof, which show local variations in thickness that additionally cause height deviations (outlier peaks), thereby showing a unique surface roughness statistics.

Description

[0001] EP applicant: BOREALIS AG TBK ref.: WO112056 DESCRIPTION BOPP capacitor film with peculiar roughness statistics Background of the Invention All capacitor film producers are concerned with the surface morphology of biaxially oriented polypropylene (BOPP) films, often quantified by a tactile roughness measurement to obtain a surface height profile (one-dimensional: height Zi versus distance x) from which statistical parameters like the average Z deviation (positive and negative from a center line which is also the x axis (Z = 0)) are calculated. Next to tactile methods, optical methods can provide 2D topography maps (Zi as function of x-y position on an area), and the same statistical parameters (e.g. an average deviation of heights from a center plane) are calculated. However, this average center line deviation Ra (1D case) or Sa (2D case) says nothing about details of the roughness profile, that is, a 1D roughness profile that is a zig-zag line or a sine wave function (to name just very theoretical / artificial examples) can have the same Ra, but in fact show very different surface roughness and associated characteristics. What can be of additional importance is the extreme values of roughness heights, and this can be e.g. expressed by Rmax (highest vertical distance in the profile, from valley bottom to peak tip). The reason to be interested in roughness of BOPP films is because surface roughness has impact on processability such as winding / unwinding properties and interlayer pressure and on the dielectric performance of the capacitor. That is, surface roughness provides the thin (such as about 3 µm) BOPP film with anti- cling properties (i.e. the roughness imparts a slip agent property) that allows film layers on a roll to move relative to each other, which is a necessary property for machining the film into the final capacitor, and provides a spacer effect impacting on interlayer adhesion and pressure. There is also a link of the surface roughness and the inter film layer gap width (that follows from the roughness providing spacer effect) to the dielectric performance in the capacitor. Especially metallized thin-film capacitors undergo self-clearing (the local insulation of a breakdown spot) as one of their main properties that extends their life-lime and avoids breakdown of the device. The self-clearing depends on many factors (Reed, C.W. The fundamentals of aging in HV polymer-film power capacitors, IEEE Transactions on Dielectrics and Electrical Insulation, 1994, 1, 904 - 922), among them interlayer pressure, because of the plasma (gas) formed during a breakdown. A specific surface roughness (spacers between film layers) will moderate the interlayer pressure and the breakdown behavior affecting the capacitor endurance. The prior art typically describes the surface of BOPP films in terms of amplitude parameters, that is, average(d) height of the surface protrusions or an excess volume created by the protrusions. Examples of patents claiming roughness include the following. US 2023 / 0080437 A1 is concerned with roughness of a polypropylene film, but merely focuses on average height (SpkA, SpkB) and on the extra volume (or excess area) created by the roughness. US 11,440,292 B2 is concerned with roughness of a polypropylene film, but only describes the average height of the roughness profile (Sa, Sz, Sra, SRz). US 2018 / 0068791 A1 is concerned with roughness of a polypropylene film, but is only concerned with the average height of the roughness profile (SRa). JP 2014-077057 A is concerned with roughness of a polypropylene film, but the surface roughness is merely described by average height (Sq) and by total volume of convexities of convexities with different heights. WO 2022 / 168658 A1 is concerned with roughness of a polypropylene film, but it describes only roughness heights, namely an average height (Sa) and an extreme height (Sxp). Object of the invention There is a constant need to improve the properties of biaxially oriented polypropylene (BOPP) capacitor films. It is therefore the object of the present invention to provide a biaxially oriented polypropylene (BOPP) film for capacitor having increased productivity in terms of improved anti-cling and / or (un)winding properties and machinability and / or to improve dielectric performance in the capacitor, especially in terms of improved breakdown behavior by self-clearing, which also improves capacitor endurance. It is a further object of the present invention to provide a metal laminated film for capacitor and a capacitor comprising an insulation film comprising a layer of the BOPP film. It is a still further object of the present invention to use a specific polypropylene to manufacture a BOPP film for a capacitor, a metal laminated film comprising the BOPP film and a metal film provided on at least one surface of the BOPP film, or a capacitor comprising the BOPP film for a capacitor as an insulation film, as well as a process for producing a BOPP film. Unexpectedly, this technical object has been solved by providing a biaxially oriented polypropylene (BOPP) capacitor film having a specific novel distribution of roughness peaks (and valleys), i.e. a new roughness profile, by contrast to focusing only at an averaged and / or absolute height of the roughness according to the prior art. That is, the present invention relates to a biaxially oriented polypropylene (BOPP) film for capacitor, wherein the BOPP film has a peculiar surface roughness derived from fiber-like supramolecular structures (peaks) on a surface thereof, which show local variations in thickness that additionally cause height deviations (outlier peaks), such that, when determining the surface roughness according to the description, a histogram plotting the counts (abs) (y-axis) of the detected normalized height deviations Zi (peaks and valleys) versus the normalized height deviation Zi (x-axis) shows an abnormal (positive kurtosis) and asymmetrical (positive skewed) distribution of peaks and valleys with a kurtosis Sku in the range of 8.0 to 20.0 and a skewness Ssk in the range of 1.0 to 2.6, and wherein the BOPP film is made of a polypropylene composition containing an isotactic crystalline polypropylene homopolymer and having a pentad isotacticity <mmmm> (quantitative nuclear-magnetic resonance (NMR)) in the range of 96.8 to 97.5%, a molecular weight distribution Mw / Mn(MWD, GPC) in the range of 7.0 to 9.0, a polydispersity index (PI) in the range of 4.80 to 6.00 Pa-1, and a zero shear viscosity, in the range of 9000 Pa·s to 13000 Pa·s. The present invention further relates to a metal laminated film for capacitor comprising the BOPP film and a metal film provided on at least one surface of the BOPP film and a capacitor comprising an insulation film comprising a layer of the BOPP film. The present invention further relates to the use of a specific polypropylene composition to manufacture a BOPP film for a capacitor, a metal laminated film comprising the BOPP film and a metal film provided on at least one surface of the BOPP film, or a capacitor comprising the BOPP film for a capacitor as an insulation film. The present invention further relates to a process for producing a BOPP film. Preferred embodiments of the invention are depicted in the dependent claims and a detailed description of the invention is provided in the following description. Brief description of the drawings Fig. 1a exemplary shows the statistics of a distribution in terms of kurtosis. Fig. 1b exemplary shows the statistics of a distribution in terms of skewness. Fig. 2 is a representation of surface roughness profile (height deviation Zi (mean corrected) versus distance x) plotted as histograms and as normal probability plots for the BOPP films of the inventive Example IE and the comparative Examples CE1, CE2 and CE3. Detailed description The term "comprising" (as well as terms "containing", "including" or "having") as used in the present invention does not exclude other components. For the purposes of the present invention, the term "consisting of" is considered to be a preferred embodiment of the term "comprising of" (as well as terms "containing", "including" or "having) unless specifically defined otherwise. Likewise, if hereinafter a group is defined to comprise at least a certain number of embodiments, this is also to be understood to disclose a group, which preferably consists only of these embodiments, unless specifically defined otherwise. Further, unless explicitly described otherwise, the description of the present invention is to be understood so that one or more of any of the described preferred embodiments of the invention can be combined with the invention described in its most general features. Further, where an indefinite or definite article is used when referring to a singular noun, e.g. "a", "an" or "the", this includes a plural of that noun unless something else is specifically stated. Biaxially oriented polypropylene (BOPP) film for capacitor The present invention relates to a biaxially oriented polypropylene (BOPP) film for capacitor, wherein the BOPP film has a peculiar surface roughness derived from fiber-like supramolecular structures (peaks) on a surface thereof, which show local variations in thickness that additionally cause height deviations (outlier peaks), such that, when determining the surface roughness according to the description, a histogram plotting the counts (abs) (y-axis) of the detected normalized height deviations Zi (peaks and valleys) versus the normalized height deviation Zi (x-axis) shows an abnormal (positive kurtosis) and asymmetrical (positive skewed) distribution of peaks and valleys with a kurtosis Sku in the range of 8.0 to 20.0 and a skewness Ssk in the range of 1.0 to 2.6, and wherein the BOPP film is made of a polypropylene composition containing an isotactic crystalline polypropylene homopolymer and having a pentad isotacticity <mmmm> (quantitative nuclear-magnetic resonance (NMR)) in the range of 96.8 to 97.5%, a molecular weight distribution Mw / Mn(MWD, GPC) in the range of 7.0 to 9.0, a polydispersity index (PI) in the range of 4.80 to 6.00 Pa-1, and a zero shear viscosity in the range of 9000 Pa·s to 13000 Pa·s. The deviation of the shape of a distribution from the shape of a normal distribution is called kurtosis (see Fig. 1a). Kurtosis is therefore a measure of the extent to which there are outliers. For a normal distribution (also called mesokurtic), the value of kurtosis is 3. A value for kurtosis greater than 3 (also called leptokurtic) indicates that the data contains more and / or more extreme outliers than a normal distribution, whereas a value for kurtosis lower than 3 (also called platykurtic) means that the data has fewer and / or less extreme outliers than a normal distribution. Further, skewness is a measure of the asymmetry of a distribution (see Fig 1b). The normal distribution is symmetric and has a skewness value of 0. A distribution with a positive skewness tapers off long to the right (long right flank) and thus shows predominately large outliers. A distribution with a negative skewness tapers off to the left (long left flank) and thus shows predominately small outliers. In the present invention, the BOPP film exhibits a surface roughness profile (plotted as histogram as determined in line with the example section) showing a large positive kurtosis Skuin the range of 8.0 to 20.0. A kurtosis value of more than 3 indicates that the distribution is characterized by more pronounced margins, i.e. more and / or more extreme outliers, than the normal distribution. Therefore, it is apparent that the BOPP film of the present invention exhibits a surface roughness statistics having a large positive kurtosis indicating a distribution where a significant amount of the peaks and valleys are located in the tails of the distribution instead of around the mean, thus showing an abnormal distribution of the peaks and valleys. In the present invention, the kurtosis Sku is preferably in the range of 10.0 to 18.0, more preferably 12.0 to 16.0, and most preferably 13.0 to 15.0, thereby exhibiting a unique abnormal roughness profile contributing to the performance of the present invention in an even more pronounced manner. Notably, it is to be understood that each combination of lower and upper limit(s) as given in the above ranges is regarded as being disclosed, meaning that the kurtosis Skumay range from e.g. 8.0 to 16.0, 10.0 to 15.0 etc. Importantly, this general principle applies throughout the entire application to all other mentioned ranges (e.g. skewness, standard deviation σ, contents etc.) defined by lower and upper limits unless specified otherwise or technically impossible. In the present invention, the BOPP film additionally exhibits a surface roughness profile showing a skewness Ssk in the range of 1.0 to 2.6. A rule of thumb states that a symmetric distribution shows a skewness between -0.5 to 0.5, that a moderated skewed distribution shows a skewness between -1 and -0.5 or between 0.5 and 1, and that a highly skewed distribution shows a skewness of less than -1 or greater than 1. Therefore, it is apparent that the BOPP film of the present invention exhibits a surface roughness statistics having a high positive skewness indicating that the distribution contains more large outliers (peaks) located in the right tail of the distribution than small outliers (valleys) located in the left tail of the distribution, thus showing unsymmetrical distribution dominated by large outliers (peaks). In the present invention, the skewness Sskis preferably in the range of 1.2 to 2.4, more preferably 1.4 to 2.2, more preferably 1.6 to 2.0, and most preferably 1.7 to 1.9, thereby exhibiting a unique unsymmetrical roughness profile contributing to the performance of the present invention in an even more pronounced manner. Notably, in addition to the above general principle, it is to be understood that each combination of ranges of defined characteristics is regarded as being disclosed, meaning that e.g. a kurtosis Sku in the range of for instance 10.0 to 18.0 or 13.0 to 15.0 may be combined with e.g. a skewness Ssk in the range of for instance 1.0 to 2.6 or 1.4 to 2.2 etc. Importantly, this additional general principle applies throughout the entire application to all other mentioned ranges defining different characteristics (e.g. skewness, standard deviation σ, contents etc.) unless specified otherwise or technically impossible. Based on the above, it is apparent that the BOPP film of the present invention is characterized in having an abnormal and unsymmetrical distribution of peaks and valleys dominated by more and more extreme positive outliers (see Fig. 2, IE), thus providing a unique surface roughness profile contributing to the performance of the present invention. Such extreme positive outlier peaks derive from fiber-like supramolecular structures being present on a surface of the BOPP film, which show local variations in thickness that additionally cause height deviations. Without wanted to be bound to any specific theory, this unique surface roughness profile is assumed as the reason for allowing production of a BOPP film for capacitor and a capacitor with increased productivity in terms of improved anti-cling and (un)winding properties and to attain improved dielectric performance in the capacitor, especially in terms of improved breakdown behavior by self-clearing, which also improves capacitor endurance. The BOPP film of the present invention is preferably further characterized in that the surface roughness statistics exhibits a standard deviation σ in the range of 0.10 to 0.24 μm, preferably 0.12 to 0.22 μm, more preferably 0.16 to 0.20 μm, and most preferably 0.17 to 0.19 μm. The standard deviation σ is a measure of the spread of the values of a characteristic (here normalized height deviation Zi) around its mean (arithmetic mean). Put simply, the standard deviation σ is the average distance of all measured counts of said characteristic from the mean. By means of additionally satisfying the above standard deviation σ, the present unique abnormal and unsymmetrical roughness profile contributes to the performance of the present invention in an even more pronounced manner. The BOPP film of the present invention is preferably further characterized in that the surface roughness statistics exhibits an average center line deviation Ra in a range of 0.02 to 0.15 μm, preferably 0.04 to 0.11 μm, more preferably 0.06 to 0.10 μm. The Ra value provides the average of all peaks and valleys indicating deviations from the mean line throughout the entire surface of a sample length, so that a higher value indicates a rough surface, while a lower value indicates a smooth surface. By means of additionally satisfying the above average center line deviation Ra, the present unique abnormal and unsymmetrical roughness profile contributes to the performance of the present invention in an even more pronounced manner. The BOPP film of the present invention is preferably further characterized in that the surface roughness statistics exhibits a specific ratio of peaks and valleys within specific distribution ranges of the histogram being centered on zero deviation. Specifically, it is preferred that a ratio of the peaks P0 / +0.30up to a magnitude of +0.30 μm (i.e. 0 to +0.30 μm deviation) to the valleys V0 / -0.30down to a magnitude of -0.30 μm (i.e. -0.30 to 0 μm deviation) (P0 / +0.30 / V0 / -0.30) is in a range of 0.90 to 1.15, preferably 0.95 to 1.07, more preferably 0.98 to 1.04. This indicates that the counted peaks and valleys centered on zero deviation are approximately normally distributed within the range of -0.30 to +0.30 μm. This is consistent with the data derived from the normal probability plot as shown in Fig. 2 IE (Probability versus deviation), where it is demonstrated that within the range of -0.30 to +0.30 μm, the normal probability plot approximates a straight line. In a normal probability plot, the sorted data looks close to a straight line if the data is approximately normally distributed, while deviations from a straight line suggest departures from normality. In addition, it is preferred that a ratio of the peaks and valleys PV0 / ±0.30up / down to a magnitude of ±0.30 μm (i.e. -0.30 to +0.30 μm) to the peaks and valleys PV>+0.30 / <-0.30exceeding a magnitude of ±0.30 μm (i.e. -∞ to -0.30 μm and +0.30 to ∞ μm) (PV0 / ±0.30 / PV>+0.30 / <-0.30) is in a range of 12.40 to 15.10, preferably 12.50 to 14.60, more preferably 12.60 to 14.00. This indicates that the counted peaks and valleys centered on zero deviation being approximately normally distributed within the range of -0.30 to +0.30 μm are roughly ten times more frequent than the sum of the negative outliers (valleys) exceeding a deviation of -0.30 μm and the positive outliers (peaks) exceeding a deviation of +0.30 μm. In addition, it is preferred that a ratio of the peaks P>+0.35exceeding a magnitude of +0.35 μm (i.e. +0.35 to ∞ μm) to the valleys V<-0.35exceeding a magnitude of -0.35 μm (i.e. -∞ to -0.35 μm) (P>+0.35 / V<-0.35) is in a range of 1.35 to 1.75, preferably 1.40 to 1.65, more preferably 1.45 to 1.60 and / or, preferably and, wherein a ratio of the peaks P>+0.50exceeding a magnitude of +0.50 μm (i.e. +0.50 to ∞ μm) to the valleys V<-0.50exceeding a magnitude of -0.50 μm (i.e. -∞ to -0.50 μm) a range of 2.00 to 5.50, preferably 2.50 to 4.50, more preferably 3.00 to 4.20 and / or, preferably and, wherein a ratio of the peaks P>+0.75exceeding a magnitude of +0.75 μm (i.e. +0.75 to ∞ μm) to the valleys V<-0.75exceeding a magnitude of -0.75 μm (i.e. -∞ to -0.75 μm) (P>+0.75 / V<-0.75) is in a range of 5.50 to 9.00, preferably 6.00 to 8.50, more preferably 6.50 to 8.00. This indicates that the more the counted peaks and valleys deviate from the mean zero deviation, the higher the amount of the positive outliers (peaks) as compared to the negative outliers (valleys). In other words, the above ratios quantify how positive outliers (peaks) exceed the negative outliers (valleys) in their number (counts) and their magnitude. Note in this context that the above and below ∞ indicates a theoretical construct for description only, since the magnitude of peaks and valleys is practically and technically limited. In the present invention, no valleys exceeding a deviation of about -1.0 μm are usually and preferably observed, whereas peaks up to deviation of about +2.0 μm are usually and preferably observed, so that a hypothetical ratio of the peaks P>+1.00exceeding a magnitude of +1.0 μm (i.e. +1.00 to ∞ μm) to the valleys V<-1.00exceeding a magnitude of -1.00 μm (i.e. -∞ to -1.00 μm) (P>+1.00 / V<-1.00) would mathematically be indefinite. In that the BOPP film of the present invention exhibits the above specific ratios of peaks and valleys, the present unique abnormal and unsymmetrical roughness profile contributes to the performance of the present invention in an even more pronounced manner. The BOPP film for capacitor of the present invention preferably has a thickness in the range of 1.5 to 6.5 μm, more preferably 2.0 to 5.0 μm, and most preferably 2.5 to 5.50 μm. Polypropylene The biaxially oriented polypropylene (BOPP) film for capacitor of the present invention as described herein is made by means of a polypropylene composition allowing the BOPP film to satisfy the above described essential unique surface roughness profile. Specifically, the BOPP film is made of a polypropylene composition containing an isotactic crystalline polypropylene homopolymer and having a pentad isotacticity <mmmm> (quantitative nuclear-magnetic resonance (NMR)) in the range of 96.8 to 97.5%, a molecular weight distribution Mw / Mn(MWD, GPC) in the range of 7.0 to 9.0, a polydispersity index (PI) in the range of 4.80 to 6.00 Pa-1, and a zero shear viscosity in the range of 9000 Pa·s to 13000 Pa·s. Preferably, a molecular weight distribution Mw / Mn (MWD; GPC) in the range of 7.5 to 8.8, preferably 7.8 to 8.5, and / or, preferably and a polydispersity index (PI) in the range of 4.90 to 5.80 Pa-1, preferably 5.00 to 5.70 Pa-1, more preferably 5.15 to 5.50 Pa-1, and / or, preferably and a zero shear viscosity in the range of 10000 Pa·s to 12500 Pa·s, preferably 10500 Pa·s to 12200 Pa·s. As stated above, the cause of the above described essential unique surface roughness profile of the BOPP film is fiber-like supramolecular structures at the surface and local variations in thickness that additionally cause height deviations (the extreme positive outlier peaks). Without wishing to be bound to theory, it is assumed to be the polymer’s crystallization behavior (in terms of high pentad isotacticity) and its moderate weight distribution (in terms of low polydispersity index (PI) by rheological behavior and / or MWD) that affect the formation of the fiber-like supramolecular structures at the surface and its local variations. This is apparent from the Inventive Example (IE), which satisfies the unique surface roughness profile (see Fig. 2, IE) by using a polypropylene composition containing an isotactic crystalline polypropylene homopolymer (PC-IE) and advantageously combining a high pentad isotacticity <mmmm> in the above range with a moderate molecular weight distribution Mw / Mn (MWD) and / or polydispersity index (PI) and / or zero share viscosity in the above ranges. On the other hand, the Comparative Examples CE1, CE2 and CE3, which use a polypropylene composition containing an isotactic crystalline polypropylene homopolymer (PC-CE1, PC-CE2 and PC-CE3) and lacking such an advantageously combination of high pentad isotacticity <mmmm> with a moderate molecular weight distribution Mw / Mn(MWD) and / or polydispersity index (PI) and / or zero share viscosity, do not satisfy the unique surface roughness profile (see Fig. 2, CE1, CE2 and CE3). Thus, it is assumed that the unique surface roughness profile attained by the present invention is a specific finger-print of the polymer design. Thus, there seems to be a link that attaining the surface roughness profile is facilitated by unique properties of the polymer, which finding is not described or suggested in the prior art, which did not link roughness deliberately with unique properties of the polymer. Therefore, the polypropylene composition containing an isotactic crystalline polypropylene homopolymer preferably has a pentad isotacticity <mmmm> in the range of 96.8 to 97.5%, a molecular weight distribution Mw / Mn (MWD; GPC) in the range of 7.5 to 8.8, preferably 7.8 to 8.5, a polydispersity index (PI) in the range of 4.90 to 5.80 Pa-1, preferably 5.00 to 5.70 Pa-1, more preferably 5.15 to 5.50 Pa-1, and a zero shear viscosity in the range of 10000 Pa·s to 12500 Pa·s, preferably 10500 Pa·s to 12200 Pa·s. As already stated above, the general principle applies that each of the above properties (here pentad isotacticity<mmmm>, MWD, PI, zero shear viscosity) may preferably be individually adjusted into a preferred or more preferred range, while it is even more preferred that two or more, most preferably all, of the above properties are adjusted into preferred and / or even more preferred ranges in combination. Again, this general principle generally applies throughout the entire application to other mentioned properties (as given below or above) in connection with (preferred) quantifications. Given the general principle, it is apparent that the polypropylene composition containing an isotactic crystalline polypropylene homopolymer preferably has - a pentad isotacticity <mmmm> (quantitative nuclear-magnetic resonance (NMR)) in the range of 96.8 to 97.5%, and - a molecular weight distribution Mw / Mn (MWD; GPC) in the range of 7.5 to 8.8, preferably 7.8 to 8.5, and - a polydispersity index (PI) in the range of 4.90 to 5.80 Pa-1, preferably 5.00 to 5.70 Pa-1, more preferably 5.15 to 5.50 Pa-1, and - a zero shear viscosity in the range of 10000 Pa·s to 12500 Pa·s, preferably 10500 Pa·s to 12200 Pa·s; and more preferably has - a pentad isotacticity <mmmm> (quantitative nuclear-magnetic resonance (NMR)) in the range of 96.8 to 97.5%, and, - a molecular weight distribution Mw / Mn(MWD; GPC) in the range of 7.8 to 8.5, and - a polydispersity index (PI) in the range of 5.00 to 5.70 Pa-1, preferably 5.15 to 5.50 Pa-1, and - a zero shear viscosity in the range of 10000 Pa·s to 12500 Pa·s, preferably 10500 Pa·s to 12200 Pa·s. According to a preferred embodiment, the polypropylene composition containing an isotactic crystalline polypropylene homopolymer has a number average molecular weight Mn (GPC) in the range of 36.0 to 53.0 kg / mol, preferably 38.0 to 51.0 kg / mol, more preferably 40.0 to 49.0 kg / mol, most preferably 42.0 to 46.0 kg / mol. Within these ranges, the effects of the present invention are more pronounced. According to a preferred embodiment, the polypropylene composition containing an isotactic crystalline polypropylene homopolymer has a weight average molecular weight Mw(GPC) in the range of 300.0 to 420.0 kg / mol, preferably 320.0 to 400.0 kg / mol, more preferably 330.0 to 390.0 kg / mol, most preferably 340.0 to 380.0 kg / mol. Within these ranges, the effects of the present invention are more pronounced. According to a preferred embodiment, the polypropylene composition containing an isotactic crystalline polypropylene homopolymer has a z-average molecular weight Mz (GPC) in the range of 1100.0 to 1500.0 kg / mol, preferably 1200.0 to 1480.0 kg / mol, more preferably 1250.0 to 1450.0 kg / mol, most preferably 1300.0 to 1440.0 kg / mol. Within these ranges, the effects of the present invention are more pronounced. Given the general principle, it is apparently preferred that the polypropylene composition containing an isotactic crystalline polypropylene homopolymer, preferably in addition to the above and / or below characteristics, has the Mnand Mw, and more preferably the Mn, Mwand Mzin the above ranges by setting individually or combined the preferred or more preferred ranges. According to a preferred embodiment, preferably in addition to the above and / or below characteristics, the polypropylene composition containing an isotactic crystalline polypropylene homopolymer has a xylene cold solubles (XCS) content (25°C, ISO 16152) of 0.25 to 1.60 wt.-%, preferably 0.50 to 1.50 wt.-%, more preferably 0.75 to 1.40 wt.-%, most preferably 1.00 to 1.25 wt.-%. Within these ranges, the effects of the present invention are more pronounced. According to a preferred embodiment, preferably in addition to the above and / or below characteristics, the polypropylene composition containing an isotactic crystalline polypropylene homopolymer has a melt flow rate (MFR2) (230°C, 2.16 kg, ISO 1133) of 1.00 to 7.50 g / 10min, preferably 2.00 to 6.00 g / 10min, more preferably 2.50 to 4.40 g / 10min, most preferably 2.75 to 3.75 g / 10min, most preferably 2.95 to 3.55 g / 10min. Within these ranges, the effects of the present invention are more pronounced. According to a preferred embodiment, preferably in addition to the above and / or below characteristics, the polypropylene composition containing an isotactic crystalline polypropylene homopolymer has a melting temperature (Tm) (DSC) of 150.0 to 179.0°C, preferably 155.0 to 172.0°C, more preferably 158.0 to 170.0°C, most preferably 160.0 to 168.0°C. Within these ranges, the effects of the present invention are more pronounced. According to a preferred embodiment, preferably in addition to the above and / or below characteristics, the polypropylene composition containing an isotactic crystalline polypropylene homopolymer has a crystallization temperature (Tc) (DSC) of 100.0 to 124.0°C, preferably 105.0 to 121.0°C, more preferably 108.5 to 117.0°C, most preferably 110.0 to 115.0°C. Within these ranges, the effects of the present invention are more pronounced. Given the general principle, it is apparent that the polypropylene composition containing an isotactic crystalline polypropylene homopolymer preferably has - a pentad isotacticity <mmmm> (quantitative nuclear-magnetic resonance (NMR)) in the range of 96.8 to 97.5%, - a molecular weight distribution Mw / Mn(MWD, GPC) in the range of 7.0 to 9.0, preferably 7.5 to 8.8, more preferably 7.8 to 8.5, - a polydispersity index (PI) in the range of 4.80 to 6.00 Pa-1, preferably 4.90 to 5.80 Pa-1, more preferably 5.00 to 5.70 Pa-1, most preferably 5.15 to 5.50 Pa-1, - a zero shear viscosity in the range of 9000 Pa·s to 13000 Pa·s, preferably 10000 Pa·s to 12500 Pa·s, more preferably 10500 Pa·s to 12200 Pa·s, - a xylene cold solubles (XCS) content (25°C, ISO 16152) of 0.25 to 1.60 wt.-%, preferably 0.50 to 1.50 wt.-%, more preferably 0.75 to 1.40 wt.- %, most preferably 1.00 to 1.25 wt.-%, - a melt flow rate (MFR2) (230°C, 2.16 kg, ISO 1133) of 1.00 to 7.50 g / 10min, preferably 2.00 to 6.00 g / 10min, more preferably 2.50 to 4.40 g / 10min, more preferably 2.75 to 3.75 g / 10min, most preferably 2.95 to 3.55 g / 10min, - a melting temperature (Tm) (DSC) of 150.0 to 179.0°C, preferably 155.0 to 172.0°C, more preferably 158.0 to 170.0°C, most preferably 160.0 to 168.0°C, - a crystallization temperature (Tc) (DSC) of 100.0 to 124.0°C, preferably 105.0 to 121.0°C, more preferably 108.5 to 117.0°C, most preferably 110.0 to 115.0°C, - a number average molecular weight Mn(GPC) in the range of 36.0 to 53.0 kg / mol, preferably 38.0 to 51.0 kg / mol, more preferably 40.0 to 49.0 kg / mol, most preferably 42.0 to 46.0 kg / mol, - a weight average molecular weight Mw(GPC) in the range of 300.0 to 420.0 kg / mol, preferably 320.0 to 400.0 kg / mol, more preferably 330.0 to 390.0 kg / mol, most preferably 340.0 to 380.0 kg / mol, - a z-average molecular weight Mz (GPC) in the range of 1100.0 to 1500.0 kg / mol, preferably 1200.0 to 1480.0 kg / mol, more preferably 1250.0 to 1450.0 kg / mol, most preferably 1300.0 to 1440.0 kg / mol; and more preferably has - a pentad isotacticity <mmmm> (quantitative nuclear-magnetic resonance (NMR)) in the range of 96.8 to 97.5%, - a molecular weight distribution Mw / Mn(MWD, GPC) in the range of 7.5 to 8.8, preferably 7.8 to 8.5, - a polydispersity index (PI) in the range of 4 4.90 to 5.80 Pa-1, preferably 5.00 to 5.70 Pa-1, more preferably 5.15 to 5.50 Pa-1, - a zero shear viscosity in the range of 10000 Pa·s to 13000 Pa·s, preferably 10500 Pa·s to 12200 Pa·s, - a xylene cold solubles (XCS) content (25°C, ISO 16152) of 0.50 to 1.50 wt.- %, preferably 0.75 to 1.40 wt.-%, more preferably 1.00 to 1.25 wt.-%, - a melt flow rate (MFR2) (230°C, 2.16 kg, ISO 1133) of 2.00 to 6.00 g / 10min, preferably 2.50 to 4.40 g / 10min, more preferably 2.75 to 3.75 g / 10min, most preferably 2.95 to 3.55 g / 10min, - a melting temperature (Tm) (DSC) of 155.0 to 172.0°C, preferably 158.0 to 170.0°C, more preferably 160.0 to 168.0°C, - a crystallization temperature (Tc) (DSC) of 105.0 to 121.0°C, preferably 108.5 to 117.0°C, more preferably 110.0 to 115.0°C, - a number average molecular weight Mn (GPC) in the range of 38.0 to 51.0 kg / mol, preferably 40.0 to 49.0 kg / mol, more preferably 42.0 to 46.0 kg / mol, - a weight average molecular weight Mw(GPC) in the range of 320.0 to 400.0 kg / mol, preferably 330.0 to 390.0 kg / mol, more preferably 340.0 to 380.0 kg / mol, - a z-average molecular weight Mz(GPC) in the range of 1200.0 to 1480.0 kg / mol, preferably 1250.0 to 1450.0 kg / mol, more preferably 1300.0 to 1440.0 kg / mol. Further, the polypropylene composition preferably comprises 92.0000 to 99.9990 wt.-%, preferably 95.0000 to 99.9950 wt.-%, more preferably 98.0000 to 99.9900 wt.-%, more preferably 99.0000 to 99.9000 wt.-%, most preferably 99.2000 to 99.7000 wt.-%, of the isotactic crystalline polypropylene homopolymer (component (A), based on the total weight of the polypropylene composition. The polypropylene composition preferably contains stabilizer and / or modifying additive(s) (component (B)) and may contain other ingredients (s) (component C). The requirement applies in this case that the components (A), (B) and (C), if present, add up to 100 wt.-%. This means for example in case that only components (A) and (B) are present, these components add up to 100 wt.-%, but in case that components (A), (B) and (C) are present, components (A), (B) and (C) add up to 100 wt.-%. The defined ranges of the indications of quantity for the individual components, e.g. components (A) to (C), are to be understood such that an arbitrary quantity for each of the individual components can be selected within the specified ranges provided that the proviso is satisfied that the sum of all present components, e.g. components (A) to (C), add up to 100 wt.-%. Further, as apparent from the word "propylene homopolymer", the present invention does not essentially aim at a polypropylene composition of different polymers, so that the composition preferably does not comprise other polymers than the isotactic crystalline polypropylene homopolymer. Furthermore, the homopolymer may contain trace amounts of contaminate comonomers, e.g. alpha-olefin comonomers and the term homopolymer, as used herein, refers to a propylene polymer containing at least 99.0 wt.-%, preferably at least 99.8 wt.- %, more preferably of at least 99.9 wt.-%, most preferably 100 wt.-% of propylene units (e.g. determined with13C NMR spectroscopy). In any case, the isotactic crystalline polypropylene homopolymer is preferably a polypropylene homopolymer in which only propylene units are detectable (e.g. determined with13C NMR spectroscopy) and the composition preferably does not contain other polymer components than the isotactic crystalline polypropylene homopolymer. The preparation of the polypropylene composition is in the skilled person’s knowledge and the polypropylene (PP) composition can for instance be obtained by mixing the isotactic crystalline polypropylene homopolymer (component (A)) with the component (B) and / or component (C), or the component (B) and / or component (C) may in the individual case be added already during the polymerization of the isotactic crystalline polypropylene homopolymer. For mixing, a conventional compounding or blending apparatus, such as e.g. a Banbury®mixer, a 2-roll rubber mill, Buss-co-kneader, a single screw extruder with special mixing segments or a twin screw extruder may be used. The PP composition recovered from e.g. the extruder can be in the form of pellets or in a powdery state. The stabilizer and / or modifying additive(s) (component (B)) preferably include at least antioxidant(s) and acid scavenger(s), and may include additional stabilizer and / or modifying additive(s), such as e.g. UV-stabilizer(s), and metal deactivator(s), and mixtures thereof, and wherein two or more functions can be combined in the same molecule. Given the application of the BOPP film in a conductor, the composition preferably does not essentially contain slip agent(s), antiblocking agent(s) and / or antistatic agent(s) as additive(s). The use of stabilizer and / or modifying additive(s) (component (B)) is well known and these additives can be added in necessary or required amount to stabilize and / or improve the properties and characteristics of the polypropylene composition, provided that the above described essential unique surface roughness profile can be satisfied. The additives are usually added in pure form or can in the individual case be added in the form of a masterbatch in which case the content of the masterbatch is calculated as the content of the additive(s). The amount of stabilizer and / or modifying additive(s) (component (B)) is usually 5.000 wt.-% or less, preferably 2.500 wt.-% or less, more preferably 1.500 wt.-% or less, such as preferably 0.0100 to 1.5000 wt.-% (100 to 15000 ppm), more preferably 0.0500 to 1.2500 wt.-% (500 to 12500 ppm), more preferably 0.1500 to 1.0000 wt.-% (1500 to 10000 ppm), most preferably 0.2500 to 0.8500 wt.-% (2500 to 8500 ppm), based on the polypropylene composition. As stated above, the stabilizer and / or modifying additive(s) (component (B)) preferably include antioxidant(s). The antioxidant(s) is preferably present in an amount of 2.500 wt.-% or less, more preferably 1.500 wt.-% or less, such as preferably 0.0100 to 1.2500 wt.-% (100 to 12500 ppm), more preferably 0.0500 to 1.1000 wt.-% (500 to 11000 ppm), more preferably 0.1500 to 1.0000 wt.-% (1500 to 10000 ppm), most preferably 0.2500 to 0.8000 wt.-% (2500 to 8000 ppm) based on the polypropylene composition. The antioxidant(s) is not specifically limited and include those known in the art such as sterically hindered phenols, phosphites and thioesters and their synergistic combinations. Moreover, in multifunctional stabilizers, several functions can be combined in the same molecule, and a widely used combination are sterically hindered phenols with sulfur substituents or sterically hindered phenols with copper chelating function (metal deactivators), such as Irganox®1035, Irganox®565, Irganox®1520 L, Irganox®MD 1024. Further examples of antioxidants are sterically hindered phenols (such as CAS No. 6683-19-8, e.g. Irganox 1010 (FF)™ by BASF), phosphorous based antioxidants (such as CAS No. 31570-04-4, also sold as Flostanox PAR 24 (FF)™ by Clariant, or Irgafos 168 (FF)™ by BASF), sulphur based antioxidants (such as CAS No. 693-36-7, sold as Irganox PS-802 (FF)™ by BASF), nitrogen-based antioxidants (such as 4,4’-bis(1 ,T-dimethylbenzyl)diphenylamine), or antioxidant blends. Preferably, the antioxidant(s) includes, more preferably is, a phenolic-type antioxidant(s), such as 2,6-di-tert-butyl-4-methyl phenol and / or, preferably and, pentaerythrityl-tetrakis(3-(3’,5’-di-tert-butyl-4-hydroxyphenyl)-propionate. As stated above, the stabilizer and / or modifying additive(s) (component (B)) preferably include acid scavenger(s). The acid scavenger(s) is preferably present in an amount of 0.5000 wt.-% or less, more preferably 0.2500 wt.-% or less, such as preferably 0.0010 to 0.0750 wt.-% (10 to 750 ppm), more preferably 0.0015 to 0.0500 wt.-% (15 to 500 ppm), more preferably 0.0025 to 0.0250 wt.-% (25 to 250 ppm), most preferably 0.0050 to 0.0125 wt.-% (50 to 125 ppm) based on the polypropylene composition. The acid scavenger(s) preferably includes, more preferably is, stearate-based acid scavenger(s), most preferably calcium stearate. The optional other ingredients (compound (C)) may include at least one selected from the group consisting of lubricants, utilization agents, polymer additives, fillers, coloring agents, and processing aids. Such optional other ingredients are not necessary to attain the technical effects of the present invention, but may fine-tune certain characteristics in case of individual need and may therefore be added under the premise not to impair with attaining the above described and claimed essential unique surface roughness profile. Nucleating agents are preferably not contained in the polypropylene composition of the present invention as they affect the resultant surface roughness profile. If not necessary, the optional other ingredients are not present and if present, their amount is usually 4.5000 wt.-% or less, preferably 2.5000 wt.-% or less, 0.7500 wt.-% or less, more preferably 0.1500 wt.-% or less, most preferably less than 0.0500 wt.-% based on the polypropylene composition. The isotactic crystalline polypropylene homopolymer may be produced by any procedure known in the art. However, there exists a crucial difference in the chain-microstructure between polypropylenes produced by a single site catalyst such as a metallocene catalyst and a Ziegler-Natta catalyst. The chain regularity of metallocene-based polypropylene is reduced by stereo- and regio-defects, whereas the chain regularity of Ziegler-Natta based polypropylenes is only reduced by stereo defects. It is preferred that the isotactic crystalline polypropylene homopolymer is obtained in the presence of a Ziegler-Natta catalyst yielding the isotactic crystalline polypropylene homopolymer having the above described characteristics especially in terms of high pentad isotacticity <mmmm> with a moderate molecular weight distribution Mw / Mn (MWD) and / or polydispersity index (PI) as described above. More preferably, the isotactic crystalline polypropylene homopolymer is a Ziegler-Natta catalyst-polymerized polymer, which is bi- or multimodal. The term “bi- or multi modal” means herein bimodality attained by performing polymerization under different polymerization conditions in different polymerization reactors connected in series resulting in different polymer components with different characteristic’s such as e.g. MFR2, and / or density, Mw, etc. The polymerization of propylene by using Ziegler-Natta catalyst may be performed in one or more, e.g. 1, 2, 3 and more, polymerization reactors, using conventional polymerization techniques, e.g. gas phase, solution phase, slurry or bulk polymerization. In case more than one polymerization reactor is used, the polymerization reactors are preferably connected in series. Preferably, the polymerization is a slurry polymerization process. Preferably, the polymerization is performed in at least two polymerization reactors connected in series, which are preferably slurry reactors. The term "slurry polymerization process" is understood as known in the art. Accordingly a slurry polymerization process means according to this invention a polymerization process in which solid polymer, i.e. the polypropylene, is formed in a liquid polymerization medium (diluent), the liquid medium preferably comprising at least 50 wt.-% of diluent. Further, the term "polymerization reactor" herein shall indicate that the main polymerization takes place in said polymerization reactor(s). This definition does not exclude the option that the overall process comprises for instance a pre- polymerization step in a pre-polymerization vessel. In case of multiple polymerization reactors, the main polymerization is preferably accomplished in the first two polymerization reactors, i.e. at least 50 wt.-%, more preferably at least 60 wt.-%, still more preferably at least 70 wt.-%, of the polypropylene is produced in the first two polymerization reactors. Preferably, the isotactic crystalline polypropylene homopolymer is polymerized in a slurry polymerization process, wherein at least (a) a Ziegler-Natta catalyst, preferably a titanium compound, more preferably TiCl3, (b) propylene, and (c) a diluent (D) comprising a donor agent, preferably an external donor selected from the group consisting of methyl methacrylate, butyl methacrylate, 2-ethyl 1- hexyl methacrylate, and tridecyl methacrylate, more preferably methyl methacrylate, are fed into a polymerization reactor, in case of several polymerization reactors in at least the first polymerization reactor, to conduct polymerization. The Ziegler-Natta catalyst system preferably comprises a co-catalyst such as an organic aluminum compound and may contain internal donors. Such polymerization processes are described in e.g. WO 2021 / 239594 A1 and especially WO 2013 / 004781 A1 and details about the Ziegler-Natta catalyst system including co-catalysts, internal and external donors, and how to prepare Ziegler-Natta catalyzed bi- and multimodal polypropylene polymers can be found in these references, the respective contents being incorporated herein by reference. Process for producing BOPP film The present invention further provides a process for producing a BOPP film comprising the steps of: (A) extruding the polypropylene composition as defined above to a film, (B) orienting the film in the machine direction (MD) and in the transverse direction (TD) to obtain the BOPP film, and (C) recovering the BOPP film having a peculiar surface roughness derived from fiber-like supramolecular structures (peaks) at least on one surface thereof, which show local variations in thickness that additionally cause height deviations (outlier peaks), such that, when determining the surface roughness according to the description, a histogram plotting the counts (abs) (y-axis) of the detected normalized height deviations Zi (peaks and valleys) versus the normalized height deviation Zi (x-axis) shows an abnormal (positive kurtosis) and asymmetrical (positive skewed) distribution of peaks and valleys with a kurtosis Sku in the range of 8.0 to 20.0 and a skewness Ssk in the range of 1.0 to 2.6. That is, the BOPP film attained by this process leads to the BOPP film having the peculiar surface roughness of the present invention as defined above, wherein the process uses the polypropylene composition as defined above, so that a further description thereof is dispensable at this point. Preferably, the BOPP film is made by applying a melt temperature in the range of 250.0 to 270.0°C, preferably 255.0 to 265.0°C, more preferably 258.0 to 262.0°C, and / or, preferably and, a chill roll temperature in the range of 80.0 to 100.0°C, preferably 85.5 to 95.0°C, more preferably 88.0 to 92.0°C, and / or, preferably and, a machine direction orientation (MDO) temperature in the range of 135.0 to 155.0°C, preferably 140.0 to 150.0°C, more preferably 143.0 to 147.0°C, and / or, preferably and a transverse direction orientation (TDO) temperature in the range of 172.0 to 192.0°C, preferably 177.0 to 187.0°C, more preferably 180.0 to 184.0°C, more preferably by applying a melt temperature in the range of 255.0 to 265.0°C, preferably 258.0 to 262.0°C, a chill roll temperature in the range of 85.5 to 95.0°C, preferably 88.0 to 92.0°C, a machine direction orientation (MDO) temperature in the range of 140.0 to 150.0°C, preferably 143.0 to 147.0°C, and a transverse direction orientation (TDO) temperature in the range of 177.0 to 187.0°C, preferably 180.0 to 184.0°C. Such conditions are particularly suitable to attain the peculiar surface roughness of the BOPP film of the present invention typically on the air side of the film, while the chill roll side may remain smooth. Preferably, the BOPP film is oriented by a stretching ratio of at least 4 times, preferably at least 4.5 times, and at most 6 times, preferably at most 5.5 times, in the MD and at least 8 times, preferably at least 8.5 times, and at most 10, preferably at most 9.5, in the TD. Such conditions are particularly suitable to attain the peculiar surface roughness of the BOPP film of the present invention. Preferably, the orientation of the film in the MD and in the TD to obtain the BOPP film is conducted in a sequential process. The BOPP film for capacitor can be prepared by conventional drawing processes known in the art. Accordingly, the process for the manufacture of the BOPP film for capacitor according to this invention comprises the use of the polypropylene composition as defined herein and its forming into a film preferably by the tenter method known in the art. The tenter method is in particular a method in which the polypropylene composition is melt extruded from a slit die such as a T-die and cooled on a cooling drum (chill roll) obtaining an undrawn film. Said film is pre-heated for example with a heated metal roll and then drawn in the length direction (machine direction MD) between a plurality of rolls over which a difference in peripheral speeds is established and then both edges are gripped with grippers and the sheet is drawn in the transverse direction (TD) in an oven by means of a tenter resulting in a biaxially drawn film. The temperature of said stretched sheet during the longitudinal drawing is preferably controlled as described above. Such a drawing process is described for instance in WO 2020 / 127862 A1 and details about the drawing conditions and the drawing apparatus can be found in this reference, the respective content being incorporated herein by reference. Subsequently, the BOPP film for capacitor can be treated by corona discharge in air, nitrogen, carbon dioxide gas or any of the mixtures on the surface to be metalized, to improve the adhesive strength to the metal to be deposited, and wound by a winder. Use of polypropylene composition, metal laminated film and capacitor The present invention further provides a metal laminated film for capacitor comprising the BOPP film of the present invention as described above, and a metal film provided on at least one surface of the BOPP film. There are two typical types of metal laminated film for capacitor and the two BOPP capacitor types are called film-foil capacitor and metallized film capacitor. The metal laminated film for capacitor can be prepared by conventional processes known in the art and, more specifically, the metallized film carries a metal film that was deposited by e.g. chemical vapor deposition (CVD) and the film-foil set up uses an extra metal foil. The present invention further provides a capacitor comprising an insulation film comprising a layer of the BOPP film of the present invention as described above. The present invention further provides the use of the polypropylene composition as described above to manufacture a BOPP film for a capacitor, a metal laminated film comprising the BOPP film and a metal film provided on at least one surface of the BOPP film, or a capacitor comprising said BOPP film for a capacitor as an insulation film. That is, the BOPP film of the present invention can be employed in capacitor films. In such cases, the capacitor film comprises at least 80.0 wt.-%, more preferably at least 90.0 wt.-%, yet more preferably at least 99.0 wt.-% of the BOPP film. In an especially preferred embodiment, the capacitor film consists of the BOPP film according to this invention. Examples 1. Measuring methods 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. Melt flow rate (MFR) Melt flow rate MFR2 was measured according to ISO 1133 (230°C, 2.16 kg load). Xylene cold soluble content (XCS, wt.-%) The amount of the polymer soluble in xylene was determined at 25°C according to ISO 16152; 5th edition; 2005-07-01. DSC analysis (Tc and Tm) DSC analysis was performed with a TA Instrument Q200 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC is run according to ISO 3011357 / part 3 / method C2 in a heat / cool / heat cycle with a scan rate of 10 °C / min in the temperature range of -30 to +225°C. Crystallization temperature (Tc) and crystallization enthalpy (Hc) were determined from the cooling step, while melting temperature (Tm) and melting enthalpy (Hm) were determined from the second heating step respectively from the first heating step in case of the webs. GPC (Mw, Mn, Mz, MWD) Number average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity (Mw / Mn, MWD) were determined by Gel Permeation Chromatography (GPC) according to the following method: The number average molecular weight (Mn), the weight average molecular weight Mw, the z-average molecular weight Mz, and the polydispersity (Mw / Mn, wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) is measured by a method based on ISO 16014- 1:2003 and ISO 16014-4:2003. A Waters Alliance GPCV 2000 instrument, equipped with refractive index detector and online viscometer was used with 3×TSK-gel columns (GMHXL-HT) from TosoHaas and 1,2,4-trichlorobenzene (TCB, stabilized with 200 mg / L 2,6-di tert butyl-4-methyl-phenol) as solvent at 145°C and at a constant flow rate of 1 mL / min. 216.5 μL of sample solution were injected per analysis. The column set was calibrated using relative calibration with 19 narrow MWD polystyrene (PS) standards in the range of 0.5 kg / mol to 11 500 kg / mol and a set of well-characterized broad polypropylene standards. All samples were prepared by dissolving 5–10 mg of polymer in 10 mL (at 160°C) of stabilized TCB (same as mobile phase) and keeping for 3 hours with continuous shaking prior sampling in into the GPC instrument. Quantitative nuclear-magnetic resonance (NMR) spectroscopy used to quantify the isotacticity of polypropylene homopolymers Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the isotacticity the polypropylene homopolymers. Quantitative13C{1H} NMR spectra were recorded in solution-state using a Bruker Advance III 400 NMR spectrometer operating at 400.15 MHz and 100.62 MHz for1H and13C respectively. All spectra were recorded using a13C optimized 10 mm extended temperature probehead at 125°C using nitrogen gas for all pneumatics. For polypropylene 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 rotatory 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 utilizing 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 (8k) transients were acquired per spectra. Quantitative13C{1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals using proprietary computer programs. For polypropylene homopolymers all chemical shifts are internally referenced to the methyl isotactic pentad (mmmm) at 21.85 ppm. 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). 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) Rheology (polydispersity index PI and zero shear viscosity) Small-amplitude oscillatory shear (SAOS) rheology experiments were done with the Anton Paar MCR501-CTD600 rheometer in nitrogen atmosphere, using a parallel plate setup (gap 1.3 mm, 25 mm diameter), following ISO 6721-1. Specimens of circular shape with a diameter of 25 mm and thickness of 2 to 3 mm were prepared first by compression molding (200°C, load of 100 kg·cm-2) of the polymer samples. Specimens were placed between the plates, heated to the measurement temperature (220 °C) and after removing excess material (trimming) the specimen was kept at the measurement temperature for another five minutes before the experiment begun. Frequency sweeps were done in the linear viscoelastic regime (strain 2-7 %) applying frequencies ω within 103to 10-2rad·s-1. The zero shear viscosity was obtained as the fitting parameter η0 of a Carreau- Yasuda fit to the magnitude of the complex shear viscosity |η*| plotted as function of frequency (rad·s-1) where η is the shear visosity (Pa·s), η∞ is the infinite-shear viscosity plateau (set to zero, Pa·s). η0 is the zero-shear viscosity plateau (Pa·s) , λ is an average relaxation time (obtained in seconds when shear frequency is expressed in Hz), n is the power law index describing the slope of the shear thinning power law section (slope = n-1) and parameter a describes the width of the transition from zero-viscosity plateau to power law behaviour. The polydispersity index PI (Pa-1) is defined as the inverse of the crossover modulus, Gc. The crossover modulus is the modulus level where elastic modulus of the melt (also called “storage modulus” G’) and viscous (loss) modulus G” have the same value. PI = 1 / Gc * 10^5. Metal content (Aluminum) Aluminum (Al) was determined with ICP-OES, Inductively coupled plasma with optical emission spectrometry (Optima 2000DV) from an acid digest of the residue and using acid standards. Al content are reported as ppm. Preparation of the acid digest: The residue was obtained from 25 g of the polymer, wetted with 10 ml of a sulphuric acid - acetic acid mixture (10 ml H2SO4in 400 mL acetic acid). The wetted sample was burned on a quartz dish by direct ignition. The obtained residue was then further heated for 30 minutes in a muffle furnace held at 625°C. The residue, after cooling, was dissolved in 20 mL aqueous HCl (500 mL HCl 37 % / 500 demineralized water) and filtered through a Whatmann 41 filter. The liquid phase is diluted to 100 mL with demineralized water. This digest was analysed by ICP-OES. Surface Roughness Stylus profilometer measurement. 1D line profiles (Zi vs xi data sets) were measured with a stylus profilometer (MarSurf GD 25 measuring station, cantilever RHT 6-50:1, diamond tip with radius of 3 µm, Mahr GmbH, Göttingen). The vertical resolution of this equipment as claimed by Mahr is 0.5 nm. The measuring station is controlled via the software MahrWin. A typically roughness measurement requires to select a certain condition set (in turn given by a Norm). The following settings were used. Scanning speed of 0.5 s·mm-1, force of 0.5 N, measuring length LM = 5.6 mm, sampling interval 0.5 µm (11200 measurement points on 5.6 mm). Each measurement was repeated five times at six locations, namely at the two outer sides and in the center, and this was done on both film sides. The BOPP film has a side that developed by stretching from the primary sheet side that was formerly in contact with the chill roll, while the other opposite side of the film is the air side. All statistical data and roughness parameters disclosed herein are from the 1D roughness profiles recorded on the air side. For each measurement, film specimens were placed on cylindrical slab of glass as solid support and ensuring intimate contact between the film and the glass substrate, to avoid air inclusions. Raw data processing. The software MahrWin performs post processing of the raw profile data. These postprocessing steps are part of the selected condition set. Post processing done by the software included removal of the first and final 0.8 mm of the recorded profile, resulting in an effective profile evaluation length LE = 4.0 mm. The profile was centred on zero, by subtracting the arithmetic mean m (of the original Zipopulation) from each Zito make the new mean zero. The waviness of the profile was removed to straighten the mean line. MahrWin refers to “Grenzwellenlänge” (cut off wavelength λc) above which the amplitude is removed. The cut-off wavelength is a seventh of the total measuring length, the latter thus being LM= 7·λcand LE= 5·λcand so, λc= 0.8 mm in this work. The reason for these relationships is that waviness affects some of the roughness parameters, and to calculate some of them requires to divide LEinto five subsections. For example, the maximum roughness depth Rmax, is the largest vertical deviation in the profile among five sub-section of LE. For LE = 4 mm, the length a subsection thus is 0.8 mm. Waviness with amplitude larger than this cause misleadingly large Rmax differences between the subsections. Profile digitization. For the evaluation of raw roughness profiles, it was relied on the reporting function of MarWin, using a summary sheet (pdf file) with graphs of the already pre-processed roughness profiles. For the evaluations done in this work, the graphs were digitize from the pdf file. The sampling wavelength was 2.6 – 2.8 µm. The vertical resolution of the digitization process varied, because the y axis scale of the graphs varied between 0.5 and 2 µm. For the smoothest films, the smallest vertical height difference ΔZ = |Zi – Zi+1| which was found in the digitized profiles was 1.3 nm. However, the smallest ΔZ that unambiguously was due to topography as judged by continuously increasing or decreasing Zivalues (clearly by a peak or valley in the profile) was 5 nm – which was taken as the actual vertical resolution in the profile data. Data evaluation. The Zivs xidata sets recorded by a stylus profilometer are discrete data, the height deviation Ziat a discrete lateral position xi, instead of a continuous roughness function z(x). Moreover, the stylus does not measure the roughness height, but the height of the surface at point xi. The originally recorded Zi data is therefore the roughness height zi plus a background height Bi (i.e. Zi = zi + Bi). Roughness parameters obtained from discrete data are an approximation, below indicated by (almost equal to). A common average roughness parameter for 1D profile data is “Ra”, the mean centre line deviation (of Zi). and the mean m of the profile is the arithmetic mean of Zi values. The standard deviation, STD or σ of Zidata is the square root of the variance σ². The root mean square, RMS, also called the quadradic mean is To avoid confusion, note the difference between RMS and the root mean squared error, RMSE, which is the same as the STD: The mean m of the original Zipopulation is subtracted from each Zi(Zi-m = Z*i). The new mean of this processed Z*ipopulation is zero. However, note that the magnitude |Z*i|, when averaged, is not zero but Ra^^ ≈ ^^∑^^^^ |^∗^| (7)Moreover, the variance STD (and RMSE) of the Zi- and Z*ipopulation are the same, because the calculation of these quantities either subtracts the mean from the original Zi result or subtracts zero from mean subtracted Z*i data. However, the Zi- and Z*i population differ in RMS. Unless noted deliberately, Zi herein refers to the mean corrected Z*i. Histograms (probability density function, pdf). Zi data from 15 line scans (5 measurements on the left, right and central position on the air side) were treated as one population and plotted as histogram, that is, number of Zi values within a certain interval (bin). The interval (bin) size was 0.02 µm. A histogram is an approximation of a probability density function (pdf). The shape of a histogram can be quantified by Kurtosis Sku and skewness Ssk. While Sku qualifies how sharp or blunt the central area (given by the most frequently observed values) of a pdf is. In turn, Sskmeasures the degree of symmetry around the most frequently occurring values. A Normal (Gaussian) distribution has Sku= 3.0 and Ssk= 0.0. Skewness and Kurtosis are defined as ^( − (^*^= ' ^)%& + · ^^^*and where n is the sample size, Xi is the ithX value, x̅ is the average and STD is the sample standard deviation. Normal probability plot. Normal probability plots were created for a visual and quantitative evaluation of the fit of the Zi population with the Normal distribution. A probability plot uses axis scaling based on the linearized cumulative probability density function (cdf) of a statistical distribution. To perform fitting of the experimental Zi population, individual Zi results of the population are first ranked from lowest to highest and then a cumulative probability F is assigned by an approximation, "F" _"i,N" "i-0.3" / "N+0.4" eq1 where N is the number of Zi results, and i is the rank (e.g. 1st , 2nd ,…Nth). This procedure forces the data into a probability scale. So obtained F versus Zi data are plotted according to the linearized cdf of the Normal distribution. A good fit of the experimental distribution to the Normal distribution is indicated when experimental data fall on a straight line. The procedure as described can be done in appropriate statistical software and e.g. via the plotting choices in “Origin Pro 2022”.

[0002] Examples <Isotactic crystalline polypropylene homopolymers P-CE1, P-IE, P-CE2, and P-CE4 were produced as disclosed in WO 2013 / 004781 A1, using diluent-slurry conditions (reactor cascade of five continuously stirred reactors, CSTR) and a commercial Ziegler Natta catalyst (“Lynx®900”). P-CE1 and P-IE were made using 2500 ppm (relative to the catalyst) of methyl methacrylate (MMA) as donor to achieve high pentad isotacticity. P-CE1 and P-IE also differ in reactor split, P-CE1 being composed of 30 wt.-% of polymer (melt flow rate, MFR below 1.0 g / min, 230°C, 2.16 kg) made in the first reactor and of 70 wt.-% of polymer (MFR > 50 g / 10 min, 230°C, 2.16 kg) made in the remaining four reactors, while for P-IE, this wt.-% / wt.-% ratio of first over remaining four reactors was 42 / 58. P-CE2 and P-CE3 were made with 1500 ppm and 100 ppm of MMA, respectively, and with a comparable split as P-IE. The isotactic crystalline polypropylene homopolymers P-CE1, P-IE, P-CE2, and P-CE4 were compounded with additives to give the polymer compositions PC- CE1, PC-IE, PC-CE2, and PC-CE4, respectively, according to Table 1. Specifically, the additives were a mixture of 5500 ppm of sterically hindered phenol-based antioxidants and 70 ppm of a metal stearate. Table 1 shows the characteristics of the resultant polymer compositions. Table 1 zero shear Name mmmm XCS TcTmMFR viscosity Pa·s % % °C °C g·10 min-1PC-CE1 13773 97.0 1.36 111.8 164.3 3.36 PC-IE 11787 97.0 1.16 112.3 164.6 3.25 PC-CE2 11157 95.1 2.22 112.7 162.7 3.31 PC-CE3 8903 91.9 3.37 110.9 161.1 3.18 Table 1 (continued) Name PI Mn Mw Mz Mw / Mn Al Pa-1kg·mol-1kg·mol-1kg·mol-1- ppm PC-CE1 6.18 40 374 1600 9.4 1.69 PC-IE 5.32 44 363 1400 8.3 1.90 PC-CE2 5.28 42 335 1170 8.0 1.28 PC-CE3 5.11 45 353 1250 7.8 1.95 <The polymer compositions PC-CE1, PC-IE, PC-CE2, and PC-CE3 were biaxially stretched to give the films CE1, IE, CE2, and CE3, respectively. The biaxial stretching was done using continuous two-step sequential drawing on a pilot scale line (Brückner GmbH, Siegsdorf, Germany) including a MDO (machine direction orientation) unit installed between the cast film extrusion and the tenter frame. The polymers were first extruded through a rectangular die (melt temperature 260°C) and then cast onto a chill roll (CR) set to a temperature of 90°C to give primary cast sheets with a thickness of 180 μm. The so formed primary (cast) sheets were then stretched by a factor 5.0 in machine direction (MD) via a set of rolls comprised of heating, drawing and annealing rolls. More specifically, the cast sheets were continuously fed into the MDO unit which consisted of 12 rolls, of which the first six were heated from 98 to 124°C to pre-heat the film, the subsequent two were held at 145°C for drawing and the last four are held between 122-136°C for annealing. The actual MD draw step was done at 145°C. The uniaxially drawn film was then fed to a tenter frame (installed in an oven) and was then stretched by a factor 9.0 in traverse direction (TD). In the tenter operation, the MD clip-to-clip distance was constant and the MDO film was only drawn in TD in the diverging draw zone of the tenter. The oven uses several different temperature zones for pre-heating, drawing, film relaxation and annealing. The draw temperature was 182°C, which is the temperature in the first section of the draw zone. Table 2 shows surface roughness characteristics of the resultant BOPP films according to the Inventive Example (IE) as well as the Comparative Examples CE1, CE2, and CE3 as derived from polymer compositions PC-CE1, PC-IE, PC- CE2, and PC-CE3, respectively. Table 2 Name Sku Ssk σ Ra P0 / +0.30 / V0 / -0.30PV0 / ±0.30 / PV>+0.30 / <-0.30μm µm - - CE1 4.49 0.66 0.09 0.060 1.07 144.38 IE 14.1 1.8 0.18 0.096 0.99 13.27 CE2 2.13 0.37 0.13 0.096 1.08 30.86 CE3 0.52 -0.14 0.17 0.114 1.05 12.36 Table 2 (continued) Name P>+0.35 / V<-0.35P>+0.50 / V<-0.50P>+0.75 / V<-0.75Thickness - - - µm CE1 Not applicable Not applicable Not applicable 3.92 IE 1.52 3.68 7.11 3.98 CE2 1.85 Not applicable Not applicable 4.07 CE3 1.04 0.43 Not applicable 3.84 As apparent from Table 2 and Fig. 2, the Inventive Example (IE) using the polymer composition PC-IE advantageously combining a high pentad isotacticity <mmmm> with a moderate molecular weight distribution Mw / Mn(MWD) and polydispersity index (PI) satisfies the unique surface roughness profile (see Table 2 and Fig. 2, IE), allowing production of a BOPP film for capacitor and a capacitor with increased productivity in terms of improved anti-cling and (un)winding properties and to attain improved dielectric performance in the capacitor, especially in terms of improved breakdown behavior by self-clearing, which also improves capacitor endurance.

Claims

EP applicant: BOREALIS AG TBK ref.: WO112056 CLAIMS 1. A biaxially oriented polypropylene (BOPP) film for capacitor, wherein the BOPP film has a peculiar surface roughness derived from fiber- like supramolecular structures (peaks) on a surface thereof, which show local variations in thickness that additionally cause height deviations (outlier peaks), such that, when determining the surface roughness according to the description, a histogram plotting the counts (abs) (y-axis) of the detected normalized height deviations Zi (peaks and valleys) versus the normalized height deviation Zi (x- axis) shows an abnormal (positive kurtosis) and asymmetrical (positive skewed) distribution of peaks and valleys with a kurtosis Skuin the range of 8.0 to 20.0 and a skewness Ssk in the range of 1.0 to 2.6, and wherein the BOPP film is made of a polypropylene composition containing an isotactic crystalline polypropylene homopolymer and having a pentad isotacticity <mmmm> (quantitative nuclear-magnetic resonance (NMR)) in the range of 96.8 to 97.5%, a molecular weight distribution Mw / Mn (MWD, GPC) in the range of 7.0 to 9.0, a polydispersity index (PI), determined according to the description, in the range of 4.80 to 6.00 Pa-1, and a zero shear viscosity, determined according to the description, in the range of 9000 Pa·s to 13000 Pa·s.

2. The BOPP film for capacitor according to claim 1, wherein the BOPP film has at least one of, two or more or even all of the kurtosis Skuis in the range of 10.0 to 18.0, preferably 12.0 to 16.0, and more preferably 13.0 to 15.0, and the skewness Sskis in the range of 1.2 to 2.4, preferably 1.4 to 2.2, more preferably 1.6 to 2.0, and most preferably 1.7 to 1.9, the standard deviation σ in the histogram is in the range of 0.10 to 0.24 μm, preferably 0.12 to 0.22 μm, more preferably 0.16 to 0.20 μm, and most preferably 0.17 to 0.19 μm,2 / 5 an average center line deviation Ra, determined according to the description, is in a range of 0.02 to 0.15 μm, preferably 0.04 to 0.11 μm, more preferably 0.06 to 0.10 μm.

3. The BOPP film for capacitor according to claim 1 or 2, wherein the BOPP film has at least one of, two or more or even all of a ratio of the peaks P0 / +0.30up to a magnitude of +0.30 μm to the valleys V0 / -0.30down to a magnitude of -0.30 μm (P0 / +0.30 / V0 / -0.30) is in a range of 0.90 to 1.15, preferably 0.95 to 1.07, more preferably 0.98 to 1.04, wherein a ratio of the peaks and valleys PV0 / ±0.30up / down to a magnitude of ±0.30 μm to the peaks and valleys PV>+0.30 / <-0.30exceeding a magnitude of ±0.30 μm (PV0 / ±0.30 / PV>+0.30 / <-0.30) is in a range of 12.40 to 15.10, preferably 12.50 to 14.60, more preferably 12.60 to 14.00, wherein a ratio of the peaks P>+0.35exceeding a magnitude of +0.35 μm to the valleys V<-0.35exceeding a magnitude of -0.35 μm (P>+0.35 / V<-0.35) is in a range of 1.35 to 1.75, preferably 1.40 to 1.65, more preferably 1.45 to 1.60, wherein a ratio of the peaks P>+0.50exceeding a magnitude of +0.50 μm to the valleys V<-0.50exceeding a magnitude of -0.50 μm (P>+0.50 / V<-0.50) is in a range of 2.00 to 5.50, preferably 2.50 to 4.50, more preferably 3.00 to 4.20, wherein a ratio of the peaks P>+0.75exceeding a magnitude of +0.75 μm to the valleys V<-0.75exceeding a magnitude of -0.75 μm (P>+0.75 / V<-0.75) is in a range of 5.50 to 9.00, preferably 6.00 to 8.50, more preferably 6.50 to 8.

00.

4. The BOPP film for capacitor according to any one of claims 1 to 3, wherein the BOPP film is made of a polypropylene composition containing an isotactic crystalline polypropylene homopolymer and having at least one of, two or more or even all of a molecular weight distribution Mw / Mn (MWD, GPC) in the range of 7.5 to 8.8, preferably 7.8 to 8.5, a polydispersity index (PI), determined according to the description, in the range of 4.90 to 5.80 Pa-1, preferably 5.00 to 5.70 Pa-1, more preferably 5.15 to 5.50 Pa-1, a zero shear viscosity, determined according to the description, in the range of 10000 Pa·s to 12500 Pa·s, preferably 10500 Pa·s to 12200 Pa·s.3 / 5 5. The BOPP film for capacitor according to claim 4, wherein the isotactic crystalline polypropylene homopolymer is a Ziegler- Natta catalyst-polymerized polymer, which is preferably bi- or multimodal.

6. The BOPP film for capacitor according to claims 4 or 5, wherein the polypropylene composition containing the isotactic crystalline polypropylene homopolymer having at least one of, two or more or even all of a xylene cold solubles (XCS) content (25°C, ISO 16152) of 0.25 to 1.60 wt.- %, preferably 0.50 to 1.50 wt.-%, more preferably 0.75 to 1.40 wt.-%, most preferably 1.00 to 1.25 wt.-%, a melt flow rate (MFR2) (230°C, 2.16 kg, ISO 1133) of 1.00 to 7.50 g / 10min, preferably 2.00 to 6.00 g / 10min, more preferably 2.50 to 4.40 g / 10min, more preferably 2.75 to 3.75 g / 10min, most preferably 2.95 to 3.55 g / 10min, a melting temperature (Tm) (DSC) of 150.0 to 179.0°C, preferably 155.0 to 172.0°C, more preferably 158.0 to 170.0°C, most preferably 160.0 to 168.0°C, a crystallization temperature (Tc) (DSC) of 100.0 to 124.0°C, preferably 105.0 to 121.0°C, more preferably 108.5 to 117.0°C, most preferably 110.0 to 115.0°C.

7. The BOPP film for capacitor according to any one of claims 4 to 6, wherein the polypropylene composition containing the isotactic crystalline polypropylene homopolymer having at least one of, two or more or even all of a number average molecular weight Mn (GPC) in the range of 36.0 to 53.0 kg / mol, preferably 38.0 to 51.0 kg / mol, more preferably 40.0 to 49.0 kg / mol, most preferably 42.0 to 46.0 kg / mol, a weight average molecular weight Mw(GPC) in the range of 300.0 to 420.0 kg / mol, preferably 320.0 to 400.0 kg / mol, more preferably 330.0 to 390.0 kg / mol, most preferably 340.0 to 380.0 kg / mol, a z-average molecular weight Mz(GPC) in the range of 1100.0 to 1500.0 kg / mol, preferably 1200.0 to 1480.0 kg / mol, more preferably 1250.0 to 1450.0 kg / mol, most preferably 1300.0 to 1440.0 kg / mol.

8. The BOPP film for capacitor according to any one of claims 4 to 7,4 / 5 wherein the polypropylene composition comprises 92.0000 to 99.9990 wt.- %, preferably 95.0000 to 99.9950 wt.-%, more preferably 98.0000 to 99.9900 wt.-%, more preferably 99.0000 to 99.9000 wt.-%, most preferably 99.2000 to 99.7000 wt.-% of the isotactic crystalline polypropylene homopolymer based on the total weight of the polypropylene composition.

9. The BOPP film for capacitor according to any one of claims 1 to 8, which has a thickness in the range of 1.5 to 6.5 μm, preferably 2.0 to 5.0 μm, more preferably 2.5 to 5.5 μm.

10. A metal laminated film for capacitor comprising the BOPP film according to any one of claims 1 to 9, and a metal film provided on at least one surface of the BOPP film.

11. A capacitor comprising an insulation film comprising a layer of the BOPP film according to any of claims 1 to 9.

12. Use of the polypropylene composition as defined by any of claims 4 to 8 to manufacture a BOPP film for a capacitor, a metal laminated film comprising said BOPP film and a metal film provided on at least one surface of the BOPP film, or a capacitor comprising said BOPP film for a capacitor as an insulation film.

13. A process for producing a BOPP film comprising the steps of: (A) extruding the polypropylene composition as defined by any of claims 4 to 8 to a flat film, (B) orienting the flat film in the machine direction (MD) and in the transverse direction (TD) to obtain the BOPP film, and (C) recovering the BOPP film having a peculiar surface roughness derived from fiber-like supramolecular structures (peaks) at least on one surface thereof, which show local variations in thickness that additionally cause height deviations (outlier peaks), such that, when determining the surface roughness according to the description, a histogram plotting the counts (abs) (y-axis) of the detected normalized height deviations Zi (peaks and valleys) versus the normalized height deviation Zi (x-axis) shows an abnormal (positive kurtosis) and asymmetrical5 / 5 (positive skewed) distribution of peaks and valleys with a kurtosis Sku in the range of 8.0 to 20.0 and a skewness Ssk in the range of 1.0 to 2.

6.

14. The process according to claims 13, wherein the BOPP film is made by applying a melt temperature in the range of 250.0 to 270.0°C, preferably 255.0 to 265.0°C, more preferably 258.0 to 262.0°C, and / or, preferably and, a chill roll temperature in the range of 80.0 to 100.0°C, preferably 85.5 to 95.0°C, more preferably 88.0 to 92.0°C, and / or, preferably and, a machine direction orientation (MDO) temperature in the range of 135.0 to 155.0°C, preferably 140.0 to 150.0°C, more preferably 143.0 to 147.0°C, and / or, preferably and a transverse direction orientation (TDO) temperature in the range of 172.0 to 192.0°C, preferably 177.0 to 187.0°C, more preferably 180.0 to 184.0°C.

15. The process according to claim 13 or 14, wherein the BOPP film is oriented by a stretching ratio of at least 4 times, preferably at least 4.5 times, and at most 6 times, preferably at most 5.5 times, in the MD and at least 8 times, preferably at least 8.5 times, and at most 10, preferably at most 9.5, in the TD, and / or the orientation of the flat film in the MD and in the TD to obtain the BOPP film is conducted in a sequential process.

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