BOPP film having low shrinkage

Abstract

The present invention relates to a new biaxially oriented polypropylene (BOPP) film, a process for the preparation of such film as well as the use of a polypropylene for the preparation of such film and an article comprising such film.

Claims

1. A biaxially oriented polypropylene (BOPP) film comprising a propylene homopolymer (H-PP) having a ratio of weight average molecular weight (M.sub.w) to number average molecular weight (M.sub.n) [M.sub.w/M.sub.n] of from 10.0 to 15.0 and a melting temperature (T.sub.m) measured by differential scanning calorimetry (DSC) of from 164 to 169° C., wherein the biaxially oriented polypropylene (BOPP) film has a thermal shrinkage measured after 5 min at 120° C. in transverse direction (TD) of ≦0.3%.

2. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein propylene homopolymer (H-PP) has a crystallization temperature (T.sub.c) measured by differential scanning calorimetry (DSC) of at least 115° C.

3. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer (H-PP) has a melting temperature (T.sub.m) measured by differential scanning calorimetry (DSC) of from 164 to 168° C.

4. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer (H-PP) has a melt flow rate MFR.sub.2 (230° C.) measured according to ISO 1133 of ≦7.0 g/10 min.

5. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer (H-PP) has a xylene cold soluble fraction (XCS) determined at 25° C. according ISO 16152 of ≦4.0 wt.-%.

6. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer (H-PP) has a) an mmmm pentad concentration of ≧94.0%, as determined by NMR-spectroscopy, and/or b) 2,1 erythro regio-defects of below 1.0%, as determined by .sup.13C-NMR spectroscopy.

7. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the propylene homopolymer (H-PP) has a polydispersity index of ≧5.0.

8. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the biaxially oriented polypropylene (BOPP) film comprises a propylene homopolymer (H-PP) having a ratio of weight average molecular weight (M.sub.w) to number average molecular weight (M.sub.n) [M.sub.w/M.sub.n] of from 10.0 to 14.0.

9. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the biaxially oriented polypropylene (BOPP) film has a thermal shrinkage measured after 5 min at 120° C. (a) in transverse direction (TD) of ≦0.2% and/or (b) in machine direction (MD) of from 3.0 to 6.0%.

10. The biaxially oriented polypropylene (BOPP) film according to claim 1, wherein the biaxially oriented polypropylene (BOPP) film has a tensile modulus in machine direction (MD) of at least 2 300 N/mm.sup.2 and/or a tensile modulus in transverse direction (TD) of at least 4 500 N/mm.sup.2.

11. The biaxially oriented polypropylene (BOPP) film according to claim 2, wherein the propylene homopolymer (H-PP) has a melting temperature (T.sub.m) measured by differential scanning calorimetry (DSC) of from 164 to 168° C.

12. The biaxially oriented polypropylene (BOPP) film according to claim 2, wherein the propylene homopolymer (H-PP) has a melt flow rate MFR.sub.2 (230° C.) measured according to ISO 1133 of ≦7.0 g/10 min.

13. The biaxially oriented polypropylene (BOPP) film according to claim 2, wherein the propylene homopolymer (H-PP) has a xylene cold soluble fraction (XCS) determined at 25° C. according ISO 16152 of ≦4.0 wt.-%.

14. A process for the preparation of a biaxially oriented polypropylene (BOPP) film according to claim 1, the process comprising at least the steps of a) providing a propylene homopolymer (H-PP)) having a ratio of weight average molecular weight (M.sub.w) to number average molecular weight (M.sub.n) [M.sub.w/M.sub.n] of from 10.0 to 15.0 and a melting temperature (T.sub.m) measured by differential scanning calorimetry (DSC) of from 164 to 169° C., and b) stretching the polypropylene of step a) in machine direction (MD) and transverse direction (TD), wherein the stretching in machine direction (MD) and transverse direction (TD) is carried out in consecutive steps.

15. The process according to claim 14, wherein the propylene homopolymer (H-PP) has a) a drawing temperature (T.sub.draw′) when drawn in transverse direction in the range of the inequation (II),
Tm−25≦Tdraw′≦Tm+10  (II), wherein T.sub.draw′ is the drawing temperature (T.sub.draw′) in ° C. of the first segment of the drawing zone (DZT) in the oven where the propylene homopolymer (H-PP) is drawn in transverse direction, wherein further this first segment of the drawing zone (DZT) is defined as the zone where the draw ratio of the drawn propylene homopolymer (H-PP) is at least 1.1; and Tm is the melting temperature (T.sub.m) of the propylene homopolymer (H-PP) in ° C.; and optionally b) a drawing temperature (T.sub.draw″) when drawn in transverse direction in the range of the inequation (III),
Tm≦Tdraw″≦Tm+18  (III), wherein T.sub.draw″ is the drawing temperature (T.sub.draw″) in ° C. of the heating zone (HZT) in the oven where the propylene homopolymer (H-PP) is drawn in transverse direction, wherein further the heating zone (HZT) is the zone upstream to the drawing zone (DZT); and Tm is the melting temperature (T.sub.m) of the propylene homopolymer (H-PP) in ° C.

16. The process according to claim 14, wherein the propylene homopolymer (H-PP) has drawing temperature (T.sub.draw) when stretched in machine direction in the range of the inequation (I)
Tm−50≦Tdraw≦Tm−15  (I), wherein T.sub.draw is the drawing temperature (T.sub.draw) in ° C., wherein the drawing temperature (T.sub.draw) is defined as the temperature (in ° C.) of the first roll (R1) of two successive rolls (R1, R2) of all rolls in the oven consecutively arranged in machine direction where the propylene homopolymer (H-PP) is drawn in machine direction, wherein further said two successive rolls (R1, R2) when locking in machine direction have as the first pair of successive rolls for the first time a roll speed difference of at least 20 m/min; and Tm is the melting temperature (T.sub.m) of the propylene homopolymer (H-PP) in ° C.

17. The process according to claim 14, wherein step b) is carried out in that the propylene homopolymer (H-PP) of step a) is stretched a) in machine direction (MD) with a draw ratio of ≧3.0, and/or b) in transverse direction (TD) with a draw ratio of ≧6.0.

18. An article comprising a biaxially oriented polypropylene (BOPP) film as defined in claim 1.

Description

EXAMPLES

A. Measuring Methods

(1) The following definitions of terms and determination methods apply for the above general description of the invention including the claims as well as to the below examples unless otherwise defined.

(2) Calculation of melt flow rate MFR.sub.2 (230° C.) of the second polypropylene fraction (PP2):

(3) $\begin{array}{cc}\mathrm{MFR}\left(\mathrm{PP}2\right)={10}^{\left[\frac{\log \left(\mathrm{MFR}\left(\mathrm{PP}1\text{/}2\right)\right)-w\left(\mathrm{PP}1\right)\times \log \left(\mathrm{MFR}\left(\mathrm{PP}1\right)\right)}{w\left(\mathrm{PP}2\right)}\right]}& \left(I\right)\end{array}$

(4) wherein w(PP1) is the weight fraction [in wt.-%] of the first polypropylene fraction (PP1), w(PP2) is the weight fraction [in wt.-%] of the second polypropylene fraction (PP2), MFR(PP1) is the melt flow rate MFR.sub.2 (230° C.) [in g/10 min] of the first polypropylene fraction (PP1), MFR(PP1/2) is the melt flow rate MFR.sub.2 (230° C.) [in g/10 min] of the polypropylene obtained after the second polymerization reactor (R2), i.e. of the mixture of the first polypropylene fraction (PP1) and second polypropylene fraction (PP2), MFR(PP2) is the calculated melt flow rate MFR.sub.2 (230° C.) [in g/10 min] of the second polypropylene fraction (PP2).

(5) Quantification of Microstructure by NMR Spectroscopy

(6) Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the stereo-regularity (tacticity), regio-regularity and comonomer content of the polymers. Quantitative .sup.13C{.sup.1H} NMR spectra were recorded in the solution-state using a Bruker Advance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for .sup.1H and .sup.13C respectively. All spectra were recorded using a .sup.13C optimised 10 mm extended temperature probehead at 125° C. using nitrogen gas for all pneumatics.

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

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

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

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

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

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

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

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

(15) 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)

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

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

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

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

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

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

(22) For copolymers characteristic signals corresponding to the incorporation of ethylene were observed (Cheng, H. N., Macromolecules 17 (1984), 1950).

(23) With regio defects also observed (Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253; Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157; Cheng, H. N., Macromolecules 17 (1984), 1950) correction for the influence of such defects on the comonomer content was required.

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

(25) The mole percent comonomer incorporation in the polymer was calculated from the mole fraction according to:
E[mol %]=100*fE

(26) The weight percent comonomer incorporation in the polymer was calculated from the mole fraction according to:
E[wt %]=100*(fE*28.05)/((fE*28.05)+((1−fE)*42.08))

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

(28) The mole percent of a given comonomer triad sequence in the polymer was calculated from the mole fraction determined by the method of Kakugo et at. (Kakugo, M., Naito, Y., Mizunuma, K., Miyatake, T. Macromolecules 15 (1982) 1150) according to:
XXX[mol %]=100*fXXX

(29) The mole fraction comonomer incorporation in the polymer, as determined from the comonomer sequence distribution at the triad level, were calculated from the triad distribution using known necessary relationships (Randall, J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29, 201):
fXEX=fEEE+fPEE+fPEP
fXPX=fPPP+fEPP+fEPE

(30) where PEE and EPP represents the sum of the reversible sequences PEE/EEP and EPP/PPE respectively.

(31) The randomness of the comonomer distribution was quantified as the relative amount of isolated ethylene sequences as compared to all incorporated ethylene. The randomness was calculated from the triad sequence distribution using the relationship:
R(E)[%]=100*(fPEP/fXEX)

(32) Melt Flow Rate (MFR.sub.2)

(33) The melt flow rates were measured with a load of 2.16 kg (MFR.sub.2) at 230° C. The melt flow rate is that quantity of polymer in grams which the test apparatus standardized to ISO 1133 extrudes within 10 minutes at a temperature of 230° C. under a load of 2.16 kg.

(34) Rheology:

(35) Dynamic rheological measurements were carried out with Rheometrics RDA-II QC on compression moulded samples under nitrogen atmosphere at 200° C. using 25 mm—diameter plate and plate geometry. The oscillatory shear experiments were done within the linear viscoelastic range of strain at frequencies from 0.01 to 500 rad/s. (ISO 6721-10)

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

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

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

(39) The Polydispersity Index, PI,

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

(41) Number Average Molecular Weight (M.sub.n), Weight Average Molecular Weight (M.sub.w)

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

(43) The xylene solubles (XCS, wt.-%): Content of xylene cold solubles (XCS) is determined at 25° C. according ISO 16152; first edition; 2005 Jul. 1.

(44) Melting temperature T.sub.m, crystallization temperature T.sub.c, is measured with Mettler TA820 differential scanning calorimetry (DSC) on 5-10 mg samples. Both crystallization and melting curves were obtained during 10° C./min cooling and heating scans between 30° C. and 225° C. Melting and crystallization temperatures were taken as the peaks of endotherms and exotherms.

(45) Also the melt- and crystallization enthalpy (Hm and Hc) were measured by the DSC method according to ISO 11357-3.

(46) Tensile modulus in machine and transverse direction were determined according to ISO 527-3 at 23° C. on the biaxially oriented films. Testing was performed at a cross head speed of 1 mm/min.

(47) Shrinkage of the BOPP film was measured in analogy to ISO 11501—“Determination of dimensional change on heating”. Film specimens sized 5×10 cm were cut from the BOPP film and placed in an oven in air, for 5 min at 120° C. The relative decrease in length of the so treated film compared to the original film is reported as percent shrinkage.

B. Examples

(48) The catalyst used in the polymerization process for the polypropylene of the inventive example IE1 was the commercial Ziegler-Natta catalyst ZN168M catalyst (succinate as internal donor, 2.5 wt.-% Ti) from Lyondell-Basell prepolymerised with vinylcyclohexane (before used in the polymerisation process) used along with triethyl-aluminium (TEAL) as co-catalyst and dicyclo pentyl dimethoxy silane (D-donor) as external donor.

(49) The aluminium to donor ratio, the aluminium to titanium ratio and the polymerization conditions are indicated in table 1.

(50) TABLE-US-00001 TABLE 1 Preparation of example IE1 Example IE1 Ti in cat [wt.-%] 2.7 Donor D TEAL/Ti [mol/mol] 30.5 TEAL/Donor [mol/mol] 9.9 Prepoly Reactor Temp [° C.] 30 H2/C3 [mol/kmol] 0.620 Loop Reactor Temp [° C.] 78 H2/C3 [mol/kmol] 2.602 split [wt.-%] 46 MFR.sub.2 [g/10 min] 2.2 1 GPR Reactor Temp [° C.] 85 H2/C3 [mol/kmol] 11.32 split [wt.-%] 54 MFR.sub.2 [g/10 min] 2.6 MFR.sub.2 produced in GPR1 [g/10 min] 3.0 2 GPR Reactor Temp [° C.] 90 H2/C3 [mol/kmol] 20 split [wt.-%] 19.5 MFR.sub.2 [g/10 min] 2.85 MFR.sub.2 produced in GPR2 [g/10 min] 4.6

(51) The properties of the unstretched inventive example IE1 as well as of a comparative examples CE1 and CE2 are summarized in Table 2.

(52) TABLE-US-00002 TABLE 2 Properties of unstretched samples Examples Unit CE1 CE2 IE1 MFR.sub.2 [g/10 min] 2.2 2.3 3.5 XS [wt %] 3.3 1.0 3.0 mmmm [%] 92.3 96.6 95.2 2,1 e [%] 0 0 0 T.sub.m [° C.] 159.5 163.0 165.0 T.sub.c [° C.] 111.3 114.2 124.3 PI [Pas.sup.−1] 4.3 4.9 5.5 M.sub.w/M.sub.n [—] 6.1 7.1 11.0

(53) As comparative example CE1 a commercial propylene homopolymer (available as HB311BF from Borealis AG, Austria) has been used.

(54) As comparative example CE2 a commercial propylene homopolymer (available as HC300BF from Borealis AG, Austria) has been used.

(55) BOPP films comprising either the inventive example IE1 or the comparative examples CE1 or CE2 were prepared by using a BOPP pilot line of Bruckner Machinenbau. The used BOPP pilot line closely resembles the tenter frame technology of full scale commercial lines, comprising of (i) a casting unit to create a sheet of un-oriented film, (ii) a machine orientation section to stretch the cast film in machine direction (MD) and (iii) a heated oven for the transverse orientation (TD) operation, yielding the BOPP film.

(56) In Table 3, the applied settings for the preparation of the present BOPP film are outlined.

(57) TABLE-US-00003 TABLE 3 Settings for the BOPP film preparation Length Temperature speed of relevant Draw Strain rate [° C.] [m/min] drawing section ratio (ε′) Melt 260 — 15 cm.sup.a 4 2 s.sup.−1 Cast  90 13 n.a. 0 — MDO 137.sup.b 13 .fwdarw. 60 5 mm.sup.c 4.6 ~6 s.sup.−1   TDO1 170 to 176 60 4.1 m.sup.d 1.0 0.0 TDO2 164.sup.e 60 0.8 m.sup.f 1.1-2.7 1 s.sup.−1 TDO3 160-164 60 3.3 m.sup.g 1.1-9.0 1 s.sup.−1 .sup.adistance between chill roll and die exit, .sup.btemperature of the first roll (R1) of the two successive rolls (R1, R2) = [T.sub.draw], .sup.cdistance between last heating roll and first drawing roll .sup.dthe length of the heating zone .sup.etemperature of the first segment of the drawing zone (DZT) = T.sub.draw′ .sup.flength of the first segment of the drawing zone (DZT) .sup.gthe total length of the drawing zone (DZT) MDO drawing in machine direction [T.sub.draw] TDO1 drawin in transverse direction: heating zone (HZT) [T.sub.draw″] TDO2 drawin in transverse direction: first segment of the drawing zone (DZT) [T.sub.draw′] TDO3 drawin in transverse direction: total drawing zone (DZT)

(58) Each resin was extruded through a T-die (die-gap 1 mm) and was cast onto the chill roll which was set to 90° C. The melt, before contacting the chill roll, was drawn in air by a factor 4, at a Hencky strain rate of approximately 2 s.sup.−1, as achieved by the difference in melt output rate and take up speed (13 m/min) A final cast film thickness of ˜250 μm was obtained. This cast film was continuously fed to the MDO unit.

(59) The MDO unit of the Brückner pilot BOPP line was composed of 12 rolls, of which the first 7 rolls are used to heat the cast film to the MD stretching temperature (137° C.). Rolls 8-12 were run at 60 m/min, providing the MDO drawing by a factor of 4.6 (λ.sub.MDO). The final rolls of the MDO unit anneal the MDO film at 126° C. The very small gap width between roll 7 and 8 (5-10 mm) causes a very high strain rate of ˜6 s.sup.−1. Table 4 lists the temperature of each MDO roll.

(60) TABLE-US-00004 TABLE 4 Temperatures of MDO rolls Roll 1 2 3 4 5 6 7 8 9 10 11 12 Temperature[° C.] 88 94 102 108 114 120 137 135 110 110 126 126

(61) The drawing of the MDO film in TD direction and its transport in MD direction along the length of the TDO oven was accomplished by two counter rotating belts, which run on both sides of the TDO oven, both equipped with several, equidistant clamps. The clamps of each belt, before they enter the TDO oven, automatically open and then close to grab the MDO film which is continuously fed into the TDO oven consisting of a heating-, drawing-, relaxation- and annealing-zone. Each zone is further segmented into shorter sections which can be set to a selected temperature. The temperatures in the TDO oven were typically adjusted to temperatures between 140 and 175° C.

(62) The TDO drawing was accomplished by the increase of the transversal belt-to-belt distance in the drawing zone. The belt-to-belt distance increases linearly, providing a non-constant (decreasing) TD drawing rate of the MDO film. The initial strain rate, calculated from length of the drawing section (3.3 m), line speed (60 m/min) and TD drawing ratio (×9) is ˜1 s.sup.−1.

(63) This is a typical strain rate for full scale lines. In the relaxation zone of the TDO oven, the draw ratio was slightly reduced, via a small decrease in the belt-to-belt TD-distance. The TDO film was collected on a cardboard mandrel and stored for further analyses.

(64) The properties of the biaxially oriented polypropylene (BOPP) films prepared from the polypropylenes of the inventive and comparative examples are summarized in Table 5.

(65) TABLE-US-00005 TABLE 5 Biaxially oriented polypropylene (BOPP) films Thermal shrinkage at Tensile 120° C./5 min Modulus T.sub.draw′ MD TD MD TD [° C.] [%] [%] [N/mm.sup.2] [N/mm.sup.2] CE1 164 4.82 0.86 2082 4255 CE2 166 3.28 0.38 2557 5321 IE1 164 4.42 0.12 2456 5007

(66) As can be gathered from the measured details outlined in Table 5, the inventive BOPP film has favourable film properties compared to prior art biaxially oriented polypropylene (BOPP) films. In particular, it can be gathered that the inventive BOPP film shows an exceptional low shrinkage in transverse direction (TD) compared to films made with polypropylenes of the prior art.