Polypropylene for use in BOPP applications

Abstract

The invention relates to a biaxially oriented polypropylene (BOPP) film comprising a propylene homopolymer or propylene-ethylene copolymer having an ethylene content of at most 1.0 wt % based on the propylene-ethylene copolymer having an Mw/Mn in the range from 5.0 to 12, wherein Mw stands for the weight average molecular weight and Mn stands for the number average weight and wherein Mw and Mn are measured according to ASTM D6474-12, an XS in the range from 1.0 to 6.0 wt %, wherein XS stands for the amount of xylene solubles which are measured according to ASTM D 5492-10, a melt flow rate in the range of 1 to 10 dg/min as measured according to IS01133 (2.16 kg/230° C.) and a crystal size distribution as indicated by a height/width ratio of the highest peak of the first cooling curve of at least 0.70 W/g° C. as determined by ASTM D3418-08 using a heating and cooling rate of 10° C./min.

Claims

1. A biaxially oriented polypropylene (BOPP) film comprising a propylene homopolymer or propylene-ethylene copolymer having an ethylene content of at most 1.0 wt % based on the propylene-ethylene copolymer, and having an Mw/Mn in the range from 5.0 to 12, wherein Mw stands for the weight average molecular weight and Mn stands for the number average weight and wherein Mw and Mn are measured according to ASTM D6474-12, an XS in the range from 1.0 to 6.0 wt %, wherein XS stands for the amount of xylene solubles which are measured according to ASTM D 5492-10, a melt flow rate in the range of 1 to 10 dg/min as measured according to ISO1133 (2.16 kg/230° C.), and a crystal size distribution as indicated by a height/width ratio of the highest peak of the first cooling curve of at least 0.70 W/g° C. as determined by ASTM D3418-08 using a heating and cooling rate of 10° C./min, wherein the width is measured at half height of the peak.

2. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer or propylene-ethylene copolymer has an Mw of at least 400 kmol.

3. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer or propylene-ethylene copolymer has an Mw/Mn in the range from 6.0 to 9.0.

4. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer or propylene-ethylene copolymer has an XS of at most 4.5 wt % based on the propylene homopolymer or propylene-ethylene copolymer.

5. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer or propylene-ethylene copolymer has an isotacticity of at most 97 wt % based on the propylene homopolymer or propylene-ethylene copolymer, wherein the isotacticity is determined using .sup.13C NMR.

6. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer or propylene-ethylene copolymer has a crystal size distribution as indicated by a height/width ratio of the highest peak of the first cooling curve of at least 0.75 W/g° C., as determined by ASTM D3418-08 using a heating and cooling rate of 10° C./min.

7. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer or propylene-ethylene copolymer has a melt flow rate in the range of 2 to 6 dg/min as measured using to ISO1133 (2.16 kg/230° C.).

8. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer or propylene-ethylene copolymer are essentially phthalate-free.

9. The biaxially oriented polypropylene (BOPP) film of claim 1, further comprising up to 5000 ppm of an additive.

10. The biaxially oriented polypropylene (BOPP) film of claim 1, comprising at least 95 wt % of the propylene homopolymer and/or propylene-ethylene copolymer.

11. An article comprising the biaxially oriented polypropylene (BOPP) film of claim 1.

12. A process for the production of the propylene homopolymer or the propylene-ethylene copolymer of claim 1 comprising the step of polymerizing propylene and optional ethylene comonomers in the presence of a catalyst to obtain the propylene homopolymer or the propylene-ethylene copolymer, wherein said catalyst is obtained by a process comprising the steps of A) providing a Ziegler-Natta procatalyst comprising contacting a magnesium-containing support with i) a halogen-containing titanium compound, ii) ethylbenzoate as an activator, iii) and as internal donor an aminobenzoate compound according to formula B: ##STR00002## wherein each R.sup.90 group is independently a substituted or unsubstituted aromatic group; R.sup.91, R.sup.92, R.sup.93, R.sup.94, R.sup.95, and R.sup.96 are each independently selected from a hydrogen or a linear, branched or cyclic hydrocarbyl group, selected from alkyl, alkenyl, aryl, aralkyl, or alkylaryl groups, and one or more combinations thereof; R.sup.97 is a hydrogen or a linear, branched or cyclic hydrocarbyl group, selected from alkyl, alkenyl, aryl, aralkyl, alkoxycarbonyl or alkylaryl groups, and one or more combinations thereof; N is a nitrogen atom; O is an oxygen atom; and C is a carbon atom; and B) contacting said Ziegler-Natta procatalyst obtained in step A) with a co-catalyst and at least one external electron donor to obtain said catalyst.

13. The process according to claim 12, wherein the external donor in step B) is a phthalate free donor.

14. A process for the preparation of a biaxially oriented film according to claim 1, comprising the steps of (a) providing a sheet comprising the propylene homopolymer or the propylene-ethylene copolymer, and b) stretching the sheet comprising the propylene homopolymer and/or the propylene-ethylene copolymer of step a) in machine direction (MD) and transverse direction (TD).

15. The process according to claim 12, wherein step A) to provide the Ziegler-Natta procatalyst comprises the following steps: i) contacting a compound R.sup.4.sub.zMgX.sup.4.sub.2-z with an alkoxy- or aryloxy-containing silane compound to give a first intermediate reaction product, being a solid Mg(OR.sup.1).sub.xX.sup.1.sub.2-x, wherein: R.sup.4 and R.sup.1 are each a linear, branched or cyclic hydrocarbyl group independently selected from alkyl, alkenyl, aryl, aralkyl, alkoxycarbonyl or alkylaryl groups, and one or more combinations thereof; wherein said hydrocarbyl group may be substituted or unsubstituted and may contain one or more heteroatoms; X.sup.4 and X.sup.1 are each independently selected from the group of consisting of fluoride (F—), chloride (Cl—), bromide (Br—) or iodide (I—); z is in a range of larger than 0 and smaller than 2, being 0<z<2, x is in a range of larger than 0 and smaller than 2, being 0<x<2; ii) optionally contacting the solid Mg(OR.sup.1).sub.xX.sup.1.sub.2-x obtained in step ii) with at least one activating compound selected from the group formed of activating electron donors and metal alkoxide compounds of formula M.sup.1(OR.sup.2).sub.v-w(OR.sup.3).sub.w or M.sup.2(OR.sup.2).sub.v-w(R.sup.3).sub.w, to obtain a second intermediate product; wherein M.sup.1 is a metal selected from the group consisting of Ti, Zr, Hf, Al or Si; M.sup.2 is a metal being Si; v is the valency of M.sup.1 or M.sup.2 and is either 3 or 4; w<v; R.sup.2 and R.sup.3 are each a linear, branched or cyclic hydrocarbyl group independently selected from alkyl, alkenyl, aryl, aralkyl, alkoxycarbonyl or alkylaryl groups, and one or more combinations thereof, and wherein said hydrocarbyl group may be substituted or unsubstituted; iii) contacting the first or second intermediate reaction product, obtained respectively in step i) or ii), with the halogen-containing Ti-compound; the activator; and the internal electron donor, to obtain said Ziegler-Natta procatalyst.

16. The process according to claim 12, wherein the Ziegler-Natta procatalyst is 4-[benzoyl(methyl)amino]pentan-2-yl benzoate, and the external electron donor is di(isopropyl) dimethoxysilane.

Description

EXAMPLES

Example 1

(1) Step A) Butyl Grignard Formation

(2) A 1.7 L stirred flask, fitted with a reflux condenser and a funnel, was filled with magnesium powder (40.0 g, 1.65 mol). The flask was brought under nitrogen. The magnesium was dried at 80° C. for 2 hours under nitrogen purge, after which dibutyl ether (200 ml), iodine (0.05 g) and n-chlorobutane (10 ml) were successively added and stirred at 120 rpm. The temperature was maintained at 80° C. and a mixture of n-chlorobutane (146 ml) and dibutyl ether (1180 ml) was slowly added over 3 hours. The reaction mixture was stirred for another 3 hours at 80° C. Then the stirring and heating were stopped and the small amount of solid material was allowed to settle for 24 hours. By decanting the colourless solution above the precipitate, a solution of butylmagnesiumchloride with a concentration of 0.90 mol Mg/L was obtained.

(3) Step B) Preparation of the First Intermediate Reaction Product

(4) The solution of reaction product of step A (500 ml, 0.45 mol Mg) and 260 ml of a solution of tetraethoxysilane (TES) in dibutyl ether (DBE), (47 ml of TES and 213 ml of DBE), were cooled to 5° C., and then were fed simultaneously to a mixing device (minimixer) of 0.45 ml volume equipped with a stirrer and jacket. The minimixer was cooled to 5° C. by means of cold water circulating in the minimixer's jacket. The stirring speed in the minimixer was 1000 rpm. From the mixing device, the mixed components were directly dosed into a 1.3 liter reactor fitted with blade stirrer and containing 350 ml of dibutyl ether. The dosing temperature of the reactor was 35° C. and the dosing time was 360 min. The stirring speed in the reactor was 250 rpm at the beginning of dosing and was gradually increased up to 450 rpm at the end of dosing stage. On completion of the dosing, the reaction mixture was heated up to 60° C. in 30 minutes and held at this temperature for 1 hour. Then the stirring was stopped and the solid substance was allowed to settle. The supernatant was removed by decanting. The solid substance was washed three times using with 700 ml of heptane at a reactor temperature of 50° C. for three times. A pale yellow solid substance, reaction product B (the solid first intermediate reaction product; the support), was obtained upon drying with a nitrogen purge. The average particle size of support was 20 microns.

(5) Step C) Preparation of the Second Intermediate Reaction Product

(6) In inert nitrogen atmosphere at 20° C. in a 1000 ml glass flask equipped with a mechanical agitator was filled with 50 g of reaction product B, dispersed in 500 ml of heptane and stirred at 250 rpm. Subsequently, a solution of 2.7 ml ethanol (EtOH/Mg=0.1) in 20 ml heptane was dosed under stirring during 1 hour. After keeping the reaction mixture at 20° C. for 30 minutes, a solution of 9.5 ml titanium tetraethoxide (TET/Mg=0.1) in 20 ml of heptane was added for 1 hour. The slurry was slowly allowed to warm up to 30° C. over 30 minutes and held at that temperature for another 2 hours. Finally, the supernatant liquid was decanted from the solid reaction product (the second intermediate reaction product C; first activated support) which was washed once with 500 ml of heptane at 30° C. and dried using a nitrogen purge.

(7) Step D) Preparation of the Third Intermediate Reaction Product

(8) In inert nitrogen atmosphere at 25° C. in a 1000 ml glass flask equipped with a mechanical agitator was filled with 50 g of second intermediate reaction product C dispersed in 500 ml of heptane and stirred at 250 rpm. Subsequently, a solution of 6.3 ml ethanol (EtOH/Mg=0.3), 20.8 ml of toluene and 37.5 ml of heptane was dosed at 25° C. under stirring during 1 hour. The slurry was slowly allowed to warm up to 30° C. over 30 minutes and held at that temperature for another 3 hours. Finally, the supernatant liquid was decanted from the solid reaction product (the third intermediate reaction product D; second activated support) which was washed once with 500 ml of heptane at 25° C. and dried using a nitrogen purge.

(9) Preparation of the Catalyst H

(10) Steps A-D) are carried out as in Example 1. Step E) is carried out as follows.

(11) A 300 ml reactor-filter flask was brought under nitrogen and 125 mL of titanium tetrachloride was added, then 5.5 g of second activated support in 15 ml of heptane was added to the reactor. The contents of the reactor were stirred for 60 minutes at room 25° C. Then, 1.78 ml of ethylbenzoate, EB (EB/Mg=0.30 molar ratio) in 4 ml of chlorobenzene was added to the reactor in 30 minutes. Temperature of reaction mixture was increased to 115° C. and then the reaction mixture was stirred at 115° C. for 90 minutes (I stage of catalyst preparation). The contents of the flask were filtered, after which the solid product was washed with chlorobenzene (125 ml) at 100 to 105° C. for 20 minutes. Then, the contents of the flask were filtered. A mixture of titanium tetrachloride (62.5 ml) and chlorobenzene (62.5 ml) was added to the reactor. The reaction mixture was stirred at 115° C. for 60 minutes (II stage of catalyst preparation). Then, the contents of the flask were filtered. A mixture of titanium tetrachloride (62.5 ml) and chlorobenzene (62.5 ml) was added to the reactor. Then, 0.51 g of 4-[benzoyl(methyl)amino]pentan-yl benzoate (AB/Mg=0.04) in 4 ml of chlorobenzene was added to the reactor in 10 minutes. The reaction mixture was stirred at 115° C. for 30 minutes (III stage of catalyst preparation). Then, the contents of the flask were filtered. A mixture of titanium tetrachloride (62.5 ml) and chlorobenzene (62.5 ml) was added to the reactor. The reaction mixture was stirred at 115° C. for 30 minutes (IV stage of catalyst preparation). Then, the contents of the flask were filtered. The solid product obtained was washed five times with 125 ml of heptane starting at 60° C. with 5 minutes stirring per wash prior to filtration. The temperature was gradually reduced from 60 to 25° C. during the washings. Finally, the solid product obtained was dried using a nitrogen purge at a temperature of 25° C. for 2 hours. The composition of the solid catalyst H produced is given in Table 1.

(12) TABLE-US-00001 TABLE 1 Composition of solid catalyst H d50 Mg Ti ID Activator (EB) EtO Catalyst Example [μm] [%] [%] [%] [%] [%] H 8 22.16 19.65 2.40 8.41 6.68 1.48
Catalyst CE

(13) Catalyst CE is prepared according to the method disclosed in U.S. Pat. No. 4,866,022, hereby incorporated by reference. This patent discloses a catalyst component comprising a product obtained by: (a) forming a solution of a magnesium-containing species from a magnesium carbonate or a magnesium carboxylate; (b) precipitating solid particles from such magnesium-containing solution by treatment with a transition metal halide and an organosilane having a formula: R.sub.nSiR′.sub.4-n, wherein n=0 to 4 and wherein R is hydrogen or an alkyl, a haloalkyl or aryl radical containing one to about ten carbon atoms or a halosilyl radical or haloalkylsilyl radical containing one to about eight carbon atoms, and R′ is OR or a halogen; (c) reprecipitating such solid particles from a mixture containing a cyclic ether; and (d) treating the reprecipitated particles with a transition metal compound and an electron donor. This process for preparing a catalyst is incorporated into the present application by reference.

(14) Polypropylene was produced in a scaled down (pilot plant) version of an Innovene™ PP process (gas phase technology for the production of polypropylene (PP).

(15) The (pilot) plant process consisted of two horizontally stirred gas-phase reactors in series in with an intermediate powder transfer system and downstream powder processing units (=degassing & catalyst deactivation) for powder collection.

(16) Reactor 1 & Reactor 2 were both operated at 150° F. (65.6° C.), 320 psig (22.1 bar). H2/C3 ratios in both reactors were controlled independently such that the melt flow rate produced at Reactor 1 is the same at the melt flow rate (MFR) of the powder collected at Reactor 2. (In practice, this means that both reactors were operated at nearly the same H2/C3-ratio (Approximate H2/C3=0.012 mol/mol).

(17) Amount of catalyst, dosed to Reactor 1 via the catalyst nozzle as slurry in hexane, was such to allow for maximum production rate in the pilot plant. Cocatalyst (TEAI) and External Donor (DIPDMS) were dosed via a separate nozzle to the reactor (as a premixed mixture) and in ratio to the catalyst flow.

(18) The process conditions as given in Table 2 were used:

(19) TABLE-US-00002 TABLE 2 Process conditions. catalyst donor Al/Ti Si/Ti H.sub.2/C.sub.3 Example 1 H DiPDMS 118 8 0.015 Example 2 H DiPDMS 118 2 0.013 Comparative example 1 CE DiBDMS 59 4.4 0.0026 (CE1) Comparative example 2 CE DiBDMS 59 1.1 0.0016 (CE2) DiPDMS: di-(isopropyl)-dimethoxysilane DiBDMS: di(isobutyl)-dimethoxysilane

(20) The powder was collected and granulate was prepared by melt-mixing the powder with the appropriate additives in a single screw extruder. The additives (antioxidants, acid scavengers) were used in an amount of 1300 ppm based on the powder and mixed prior to dosing to the extruder. The temperature profile in the extruder was 20-20-30-50-100-170-220-220-240° C., at a throughput of 13 kg/h at 200 rpm.

(21) Methods

(22) MWD, Mn, Mw

(23) Mw, Mn and Mz were all measured according to ASTM D6474-12 (Standard Test Method for Determining Molecular Weight Distribution and Molecular Weight Averages of Polyolefins by High Temperature Gel Permeation Chromatography). Mw stands for the weight average molecular weight and Mn stands for the number average weight. Mz stands for the z-average molecular weight.

(24) Cold Xylene Solubles (XS)

(25) XS, wt % is xylene solubles, measured according to ASTM D 5492-10. 1 gram of polymer and 100 ml of xylene are introduced in a glass flask equipped with a magnetic stirrer. The temperature is raised up to the boiling point of the solvent. The so obtained clear solution is then kept under reflux and stirring for further 15 min. Heating is stopped and the isolating plate between heating and flask is removed. Cooling takes places with stirring for 5 min. The closed flask is then kept for 30 min in a thermostatic water bath at 25° C. for 30 min. The so formed solid is filtered on filtering paper. 25 ml of the filtered liquid is poured in a previously weighed aluminium container, which is heated in a stove of 140° C. for at least 2 hours, under nitrogen flow and vacuum, to remove the solvent by evaporation. The container is then kept in an oven at 140° C. under vacuum until constant weight is obtained. The weight percentage of polymer soluble in xylene at room temperature is then calculated.

(26) Isotacticity

(27) “APP wt. %” or “weight percentage of atactic polypropylene” as used in the present description means: the fraction of polypropylene obtained in a slurry polymerization that is retained in the solvent. APP can be determined by taking 100 ml of the filtrate (“y” in millilitres) obtained during separation from polypropylene powder after slurry polymerization (“x” in grammes). The solvent is dried over a steam bath and then under vacuum at 60° C. That yields APP (“z” in grammes). The total amount of APP (“q” in grammes) is (y/100)*z. The weight percentage of APP is (q/q+x))*100%.

(28) The isotacticity is 100 wt %−APP (in wt %).

(29) The isotacticity was measured using .sup.13C NMR.

(30) Crystal Size Distribution

(31) The crystallization temperature, the crystallinity and the melting temperature are measured according to ASTM D3418-08 at a heating rate of 10° C./min in DSC. The sample is heated up to 200° C. (first heating) and then cooled at a cooling rate 10° C./min of (to measure the crystallization temperature and crystallinity) and then heated a second time at a heating rate of 10° C./min (second heating) to measure the melting temperature (Tm) and heat of fusion (W). In order to determine the crystal size distribution, a 5 mg polymer sample was measured.

(32) Crystal size distribution is described by the height/width ratio of the highest peak in the cooling (crystallization) thermograms according to the DSC method.

(33) For determining the height/width ratio, the analysis as described by A. K. GUPTA,*S. K. RANA, and B. 1. DEOPURA, Journal of Applied Polymer Science, Vol. 44, 719-726 (1992)), was applied.

(34) FIG. 1 (FIG. 1) is the cooling thermogram of comparative example 1 (CE1) and shows the determination of the height/width ratio (crystal size distribution). In FIG. 1, the height and the width of the highest peak have been indicated (in this case, only one peak was present in the thermogram). As can be seen the width is determined at half height. The heat of fusion (W) was normalized to W/g sample.

(35) The height, the width at half height (width) and the ratio of the height/width (crystal size distribution) of the highest peak in the DSC cooling curve of examples 1 and 2 and CE1, CE2 and CE3 is indicated in Table 3 below.

(36) A higher height/width ratio exhibits a narrower crystal size distribution and therefore a more homogeneous crystallinity.

(37) Melt Flow Rate (MFR)

(38) For purpose of the invention the melt flow rate is the melt flow rate as measured according to ISO1133 (2.16 kg/230° C.).

(39) Haze

(40) The determination of the Haze and values was carried out in accordance with the standard ASTM D1003 at 23° C. at 50% relative humidity. The test specimens were BOPP films, which are prepared as described below.

(41) Stiffness (Tensile Modulus)

(42) For purpose of the present invention, stiffness of the BOPP film prepared as described below, was determined by measuring the tensile modulus (1% secant) according to ASTM D882 at 23° C., 50% relative humidity, at a speed of 25 mm/min, sample length of 250 mm in machine direction (MD), also indicated herein as (II), and transversal direction (TD), also indicated herein as (L).

(43) Preparation of BOPP Films

(44) Extrusion of PP Sheets.

(45) A non-stretching sheet, with a thickness and width of 500 μm and 270 mm respectively, was made by an extrusion line ZE25Ax42D with a discharge amount of 16 kg/hr. The extrusion was carried out at 240° C. and the chill-roll temperature was set to 35° C. The take-off speed was 2.6 m/min. No draw ratio was subjected on the MDO unit after extrusion. Approximately 10-15 m sheet was winded of each sample that was produced for further stretching trials.

(46) Stretching Using a Biaxial Stretching Machine.

(47) BOPP films were produced on the biaxial stretching machine KARO IV. The biaxial stretching was performed in the sequential or in the simultaneous stretching mode. The stretching temperature was 160° C.

(48) The conditions during the stretching process are summarized in table 4 below.

(49) TABLE-US-00003 TABLE 3 Stretching conditions on the KARO IV. Sheet Stretch. Pre- Thick- Sheet oven heat Speed Stretch. ness dimensions temp time MD/TD ratio Speed [μm] [μm] [° C.] [S] [%/s] [MD × TD] profile 500 90 × 90 160 30 400 5 × 10 SEQ

(50) The final BOPP film was prepared by stretching the extruded 500 μm sheet 5×10 in sequential mode. This stretching was performed at the temperature T=160° C.

(51) Results:

(52) The results of the experiments are shown in Table 4 below:

(53) TABLE-US-00004 TABLE 4 example Example 1 CE1 CE2 Example 2 Molecular characterization Mw (kmol) 480 400 430 430 Mn 65 75 72 55 Mz/Mw 4.3 2.9 3.3 3.9 MWD = Mw/Mn 7.3 5.3 5.9 7.8 XS (wt %) 2.0 1.9 2.9 3.4 Isotacticity (wt %) 95.4 95.4 92.7 94.2 Tm (2.sup.nd heating), (° C.) 164.7 163 163 163 Height (W/g) 3.26 2.82 3.1 3.72 Width (° C.) 4.5 5 4.8 4.2 Height/width ratio (W/g° C.) 0.72 0.56 0.65 0.89 MFI (g/10 min) 2.6 3.6 2.7 2.5 properties Tensile modulus (II) (BOPP 2.7 2.8 2.4 2.6 film) in GPa Tensile modulus (L) (BOPP 5.7 4.3 4.7 4.7 film) in GPa Haze (wt %) <1.0 <1.0 1.2 <1.0

CONCLUSION

(54) As can be seen from Table 4, the polypropylenes of the invention show a higher stiffness (high tensile modulus (L) on the BOPP film prepared from the polypropylenes of the invention (at comparable XS values) as compared to the comparative examples. For example, when comparing example 1 to comparative example CE1, the tensile modulus (L) of the BOPP film is 5.7 versus 4.3 GPa. As can be seen from the above Table 4, the tensile modulus decreases with increasing XS. Therefore, it is surprising that the tensile modulus (L) of the BOPP film of example 2 having an XS of 3.4 wt % is the same as the tensile modulus (L) of the BOPP film of the comparative example CE2 having a significantly lower XS of 2.9 wt %.

(55) In addition, Table 4 also shows that the polypropylenes of the invention show a low haze (below 1.0 wt %) when used in a BOPP film.

(56) The examples 1 and 2 of the invention show a higher height/width ratio than the comparative examples CE1 and CE2. A higher height/width ratio indicates a narrower crystal size distribution, which indicates a more homogeneous crystallinity. In Table 4, it is shown that this higher height/width ratio leads to a better BOPP film appearance in terms of lower haze and also to an increased stiffness at comparable XS.

(57) Therefore, a propylene homopolymer or propylene-ethylene copolymer having an ethylene content of at most 1.0 wt % based on the propylene-ethylene copolymer having an Mw/Mn in the range from 5.0 to 12, wherein Mw stands for the weight average molecular weight and Mn stands for the number average weight and wherein Mw and Mn are measured according to ASTM D6474-12 an XS in the range from 1.0 to 6.0 wt %, wherein XS stands for the amount of xylene solubles which are measured according to ASTM D 5492-10. a melt flow rate in the range of 1 to 10 dg/min as measured according to ISO1133 (2.16 kg/230° C.) a crystal size distribution as indicated by a height/width ratio of the highest peak of the first cooling curve of at least 0.70 W/g° C. as determined by ASTM D3418-08 using a heating and cooling rate of 10° C./min, can suitably be used for the preparation of BOPP films having a low haze and a good stiffness.

(58) In addition, such propylene homopolymer and/or propylene-ethylene copolymer can suitably be used to prepare stiffer BOPP films having a low haze without requiring nucleating agents or clarifying agents. This makes the BOPP films of the invention extremely suitable for use in packaging applications where a stiffness and low haze (high transparency) are desired, such as all applications that require flexible wrapping, such as for example the wrapping of flowers and cigarettes. In particular, the BOPP grades and BOPP films of the invention could also be suitable for applications where a very low additive concentration is desired, such as food and medical applications.

(59) In addition, since the propylene homopolymer of the invention shows such a high stiffness in the BOPP film, less antioxidant/nucleating agent can be used for achieving the same stiffness as compared to the prior art, which makes the propylene homopolymer of the invention a cost effective solution in achieving high stiffness.