Calcium carbonate as cavitation agent for biaxially oriented polypropylene films

Abstract

The present invention relates to a mono or multi-layer biaxially oriented polypropylene film having a density of ≤0.72 g/cm.sup.3, a process for producing the mono or multi-layer biaxially oriented polypropylene film, the use of at least one natural calcium carbonate as cavitation agent in the mono or multi-layer biaxially oriented polypropylene film, an article comprising the mono or multi-layer biaxially oriented polypropylene film as well as the uses.

Claims

1. A mono or multi-layer biaxially oriented polypropylene film, wherein at least one layer of the film comprises at least one polypropylene in an amount ranging from 79.0 to 95.0 wt.-% and at least one natural calcium carbonate in an amount ranging from 5.0 to 21.0 wt.-%, based on the total weight of the layer, wherein the at least one natural calcium carbonate has a weight median particle size d.sub.50 from 3.2 μm to 8.0 μm and the at least one natural calcium carbonate is marble and/or limestone and/or chalk, and wherein the at least one polypropylene and at least one natural calcium carbonate comprising layer of the film, has a density of <0.62 g/cm.sup.3.

2. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one polypropylene and at least one natural calcium carbonate comprising layer of the film comprises a) the at least one polypropylene in an amount ranging from 82.0 to 93.0 wt.-%, based on the total weight of the layer, and/or b) the at least one natural calcium carbonate in an amount ranging from 7.0 to 18.0 wt.-%, based on the total weight of the layer.

3. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one polypropylene is selected from the group of propylene homopolymers, propylene random copolymers, and terpolymers.

4. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one polypropylene has a) a melt flow rate (MFR) determined according to ISO 1133 (230° C., 2.16 kg) in the range from 0.01 to 20 g/10 min, and/or b) a density determined according to ISO 1183 in the range from 0.880 g/cm.sup.3 to 0.920 g/cm.sup.3.

5. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one natural calcium carbonate is a ground natural calcium carbonate.

6. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one natural calcium carbonate is limestone and/or chalk.

7. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one natural calcium carbonate has a) a weight median particle size d.sub.50 from 3.5 μm to 8.0 μm, and/or b) a top cut particle size d.sub.98 of ≤50.0 μm, and/or c) a specific surface area (BET) of from 0.5 to 150 m.sup.2/g, as measured using nitrogen and the BET method according to ISO 9277.

8. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one natural calcium carbonate is a surface-treated natural calcium carbonate comprising a treatment layer on the surface of the at least one natural calcium carbonate comprising i. a phosphoric acid ester blend of one or more phosphoric acid mono-ester and salty reaction products thereof and/or one or more phosphoric acid di-ester and salty reaction products thereof, and/or ii. at least one saturated aliphatic linear or branched carboxylic acid and salty reaction products thereof, and/or iii. at least one aliphatic aldehyde and/or salty reaction products thereof, and/or iv. at least one mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C2 to C30 in the substituent and/or salty reaction products thereof, and/or v. at least one polydialkylsiloxane, and/or vi. mixtures of the materials according to i. to v.

9. The mono or multi-layer biaxially oriented polypropylene film of claim 8, wherein the treatment layer on the surface of the at least one natural calcium carbonate comprises at least one saturated aliphatic linear or branched carboxylic acid and salty reaction products thereof, and/or at least one mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C2 to C30 in the substituent and/or salty reaction products thereof.

10. The mono or multi-layer biaxially oriented polypropylene film of claim 8, wherein the surface-treated natural calcium carbonate comprises the treatment layer in an amount of from 0.05 to 2.3 wt.-%, based on the total dry weight of the at least one natural calcium carbonate.

11. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one polypropylene and natural calcium carbonate comprising layer of the film further comprises at least one additive selected from the group consisting of a cavitating agent, pigments, solid microspheres, hollow microspheres, metals, an antioxidant, an acid scavenger, a processing aid, an antistatic additive, an extrusion aid, a nucleating agent, a light stabilizer, an optical brightener, a blue dye, an antiblocking agent and mixtures thereof, which is/are dispersed in the at least one polypropylene.

12. The mono or multi-layer biaxially oriented polypropylene film of claim 11, wherein the at least one polypropylene and at least one natural calcium carbonate comprising layer of the film comprises the at least one additive in an amount ranging from 0.1 to 30.0 wt.-%, based on the total weight of the layer.

13. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one polypropylene and at least one natural calcium carbonate comprising layer of the film, has a) a density of from 0.4 to 0.62 g/cm.sup.3, and/or b) an opacity of ≥40%.

14. A process for producing a mono or multi-layer biaxially oriented polypropylene film comprising the steps of: a) providing a composition comprising at least one polypropylene and at least one natural calcium carbonate, and b) forming a film from the composition of step a), and c) stretching the film obtained in step b) in machine direction (MD) and transverse direction (TD) in any order, wherein the stretching in machine direction (MD) and transverse direction (TD) is carried out sequential or simultaneously, wherein the at least one natural calcium carbonate has a weight median particle size d.sub.50 from 3.2 μm to 8.0 μm, and wherein the film produced from step c) has a density of ≤0.62 g/cm.sup.3.

15. The process of claim 14, wherein the composition provided in step a) is a masterbatch obtained by mixing and/or kneading the at least one polypropylene and at least one natural calcium carbonate to form a mixture and continuously pelletizing the obtained mixture.

16. The process of claim 14, wherein the composition provided in step a) is a masterbatch comprising the at least one natural calcium carbonate in an amount between >30 and 85 wt.-%, based on the total weight of the masterbatch.

17. The process of claim 14, wherein the composition provided in step a) is a compound obtained by mixing and/or kneading the at least one polypropylene and at least one natural calcium carbonate to form a mixture and continuously pelletizing the obtained mixture.

18. The process of claim 14, wherein process steps a) and b) are carried out simultaneously.

19. The process of claim 14, wherein the composition comprising at least one polypropylene and at least one natural calcium carbonate of step a) is obtained by adding the at least one natural calcium carbonate to the polymerization process of the at least one polypropylene.

20. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one polypropylene is a propylene homopolymer.

21. An article comprising a mono or multi-layer biaxially oriented polypropylene film according to claim 1, wherein the article is selected from the group consisting of flower overwrapping, cigarette overwrapping, CD overwrapping, shrinkable films, release films, twist films, mat films, non-electrical capacitor films, food packaging, flexible packaging, candy bar wrappers, hygiene articles, labels, textiles, stationery goods, photo albums, envelopes, windows, catalogues, manuals, packaging bags, maps, audio/video cassettes, industrial tapes, pressure sensitive tapes, box sealing tapes, masking tapes, laminated metallized brochure catalogues, print laminations, carton boxes, cosmetic boxes, restaurant menus, and electrical articles.

22. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one natural calcium carbonate has a weight median particle size d.sub.50 from 4.0 μm to 6.8 μm.

23. The mono or multi-layer biaxially oriented polypropylene film of claim 1, wherein the at least one polypropylene and at least one natural calcium carbonate comprising layer of the film, has an opacity of ≥40%.

Description

EXAMPLES

(1) 1 Measurement Methods and Materials

(2) In the following, measurement methods and materials implemented in the examples are described.

(3) MFR

(4) The melt flow rate MFR is measured according to ISO 1133 (230° C., 2.16 kg load).

(5) MVR

(6) The melt volume rate MVR is measured according to ISO 1133 (250° C./2.16 kg)

(7) Crystallization Temperature T.sub.c

(8) The crystallization temperature is measured by differential scanning calorimetry (DSC) on a Mettler-Toledo “Polymer DSC instrument (Mettler-Toledo (Schweiz) GmbH, Switzerland). The crystallization curve was obtained during 10° C./min cooling and heating scans between 30° C. and 225° C. Crystallization temperatures were taken as the peak of endotherms and exotherms.

(9) Particle Size

(10) The particle distribution of the untreated ground calcium carbonate-comprising filler material was measured using a Sedigraph 5120 from the company Micromeritics, USA. The method and the instruments are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement was carried out in an aqueous solution comprising 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and supersonics.

(11) Specific Surface Area (BET)

(12) Throughout the present document, the specific surface area (in m.sup.2/g) of the filler material is determined using the BET method (using nitrogen as adsorbing gas), which is well known to the skilled man (ISO 9277:2010).

(13) Ash Content

(14) The ash content in [wt.-%] of the masterbatches and films was determined by incineration of a sample in an incineration crucible which is put into an incineration furnace at 570° C. for 2 h. The ash content was measured as the total amount of remaining inorganic residues.

(15) Film Thickness

(16) The film thickness was determined using a digital measuring slide Mitutoyo IP 66 (Mitutoyo Europe GmbH, Neuss, Germany). Measured values were reported in μm.

(17) Density of a Film or Layer

(18) The density was determined from a test piece whereby a precise area of film (100 mm×100 mm) is cut and weighed on an analytical balance. An average film thickness was determined by taking nine thickness measurements allocated over the whole film surface. The density was calculated and reported in [g/cm.sup.3]. Also an average yield in m.sup.2/kg and the unit weight in g/m.sup.2 can be calculated from these values.

(19) Brightness Rv

(20) Colorimetric values were measured using a Datacolor Elrepho spectrometer (Datacolor AG, Switzerland), Ry was measured according to DIN 53163 and CIELAB colour differences L*, a* and b* were determined according to DIN 6174.

(21) Gloss 60° (20°, 850)

(22) The gloss was measured at 600, additionally also at 200 or 850 may be measured. All measurements were done according to ISO 2813 using a trigloss Glossmeter (Byk-Gardner GmbH, Germany).

(23) Opacity

(24) The opacity measurements were done according to DIN 53146 by measuring the whiteness of a film sample on a black and a white substrate using a Byk-Gardner Spectro-Guide (Byk-Gardner GmbH, Germany). The opacity is the contrast ratio of the two measurements. The units are percent % and a perfectly opaque material will have an opacity value of 100%.

(25) Transmittance

(26) Light Transmittance (transparency) is the ratio of total transmitted light to the amount of incident light. Light transmittance was measured using haze-guard plus (Byk Gardener, Germany) test equipment according to ASTM D1003.

(27) Tensile Strength

(28) The produced film samples were tested on their tensile behaviour on a Zwick/Roell Allround Z020 equipment (Zwick GmbH & Co. KG, Germany) according to ISO 527-3. Tensile testing was done on samples taken in machine direction (MD) and in transverse direction (TD). At least five samples are tested for each formulation and average values are calculated. Tensile modulus [MPa], tensile strength [MPa], and elongation at break [%] are reported. The sample size of the film samples is 15 mm×170 mm and the testing length was 5 cm.

(29) Maximum Elongation at Break

(30) Elongation at break determination was performed according to ISO 527-3. The film specimen width was of 15 mm and the testing length was 5 cm.

(31) Tensile E-Modulus (Modulus of Elasticity)

(32) Tensile E-modulus determination was performed according to ISO 527-3. The film specimen width was of 15 mm and the testing length was 5 cm. The E-modulus corresponded to the inclination of the tensile test curve between the points at 0.02% and 2% elongation.

(33) Water Vapour Transmission Rate (WVTR)

(34) The WVTR value of the polypropylene films was measured with a Lyssy L80-5000 (PBI-Dansensor A/S, Denmark) measuring device according to ASTM E398.

(35) 2 Materials

(36) CC1 (inventive): Natural ground calcium carbonate, commercially available from Omya International AG, Switzerland (d.sub.50: 5 μm; d.sub.98: 20 m), surface-treated with 0.5 wt.-% stearic acid (commercially available from Sigma-Aldrich, Croda) based on the total weight of the natural ground calcium carbonate. BET: 2.1 m.sup.2/g.

(37) CC2 (inventive): Natural ground calcium carbonate, commercially available from Omya International AG, Switzerland (d.sub.50: 5 μm; d.sub.98: 30 μm), without surface-treatment. BET: 2.1 m.sup.2/g.

(38) CC3 (comparative): Natural ground calcium carbonate, commercially available from Omya International AG, Switzerland (d.sub.50: 3 μm; d.sub.98: 12.5 μm), without surface-treatment. BET: 2 m.sup.2/g.

(39) CC4 (comparative): Natural ground calcium carbonate, commercially available from Omya International AG, Switzerland (d.sub.50: 1.4 μm; d.sub.98: 5 μm; content of particles <1 μm=28%), surface-treated with 0.7 wt.-% stearic acid (commercially available from Sigma-Aldrich, Croda) based on the total weight of the natural ground calcium carbonate. BET: 5.1 m.sup.2/g.

(40) P1 (comparative): Polyethylene terephthalate (PET), Valox 334, commercially available from Sabic, the Netherlands, MVR (250° C./2.16 kg) of 105 cm.sup.3/10 min (ISO 1133; according to the technical data sheet).

(41) P2 (comparative): Polyethylene terephthalate (PET), Valox 3104, commercially available from Sabic, the Netherlands, MVR (250° C./2.16 kg) of 40 cm.sup.3/10 min (ISO 1133; according to the technical data sheet).

(42) P3 (comparative): Polyethylene terephthalate (PET), Valox 195, commercially available from Sabic, the Netherlands, melt viscosity of 82 000 mPas (determined according to Sabic method) (according to the technical data sheet).

(43) PO1 (polymer matrix): Polypropylene homopolymer, Dow PP H-358-02, MFR of 2.1 g/10 min (230° C., 2.16 kg), density of 0.90 g/cm.sup.3 (according to technical data sheet), commercially available from Dow Europe, Switzerland.

EXAMPLES

Example 1—Preparation of Masterbatches

(44) Polypropylene masterbatches containing the calcium carbonate fillers CC1 to CC4 and the polymers P1 to P3 and PO1 were continuously prepared on a lab scale Buss kneader (Buss PR46 from Buss AG, Switzerland). The compositions and filler contents of the prepared masterbatches are compiled in Table 1 below. The precise filler content was determined by the ash content.

(45) TABLE-US-00001 TABLE 1 Composition and filler content of prepared masterbatches. Filler or Filler content Ash content Masterbatch PBT [wt.-%].sup.a) [wt.-%] MB1 (inventive) CC1 70 68.4 MB2 (inventive) CC2 70 69.7 MB3 (comparative) CC3 70 69.8 MB4 (comparative) CC4 70 68.7 MB5 (comparative) P1 70 — MB6 (comparative) P2 70 — MB7 (comparative) P3 70 — .sup.a)refers to the overall amount of filler or PBT and PO1, this means 30 wt.-% PO1.

Example 2—Preparation of Polypropylene Cast Films

(46) Cast films were prepared on a Collin Laboratory Film Line (Dr. Collin GmbH, Germany) with a twin screw extruder with a diameter of 30 mm wide T-die and a take-up system, which had temperature controlled chill-rolls. The chilled roll was kept 20 mm from the T-die to produce a polypropylene sheet having a thickness of around 1 500 μm. The extruder and die temperatures were consistent throughout the experiment. The die temperature was set at 250° C.; the line speed was 0.8 m/min. The masterbatch or polymer was mixed with the neat polymer PO1 to receive cast films with the concentrations given in Table 2.

(47) TABLE-US-00002 TABLE 2 Compositions and properties of prepared cast films Masterbatch Filler content Ash content Film Sample or polymer [wt.-%] [wt.-%]  1 (comp.) no 0 0 2 (inv.)  MB1 15 15.3 3 (inv.)  MB1 20 19.5 4 (inv.)  MB2 15 14.0 5 (inv.)  MB2 20 19.0  6 (comp.) MB3 15 14.2  7 (comp.) MB3 20 19.0  8 (comp.) MB4 15 14.7  9 (comp.) MB4 20 18.2 10 (comp.) MB5 6 — 11 (comp.) MB5 8 — 12 (comp.) MB6 6 — 13 (comp.) MB6 8 — 14 (comp.) MB7 6 — 15 (comp.) MB7 8 —

(48) All films shown in Table 2 are cast films which were produced in good quality with visual good appearance.

Example 3—Preparation of Biaxially Stretched Polypropylene Films

(49) A biaxial lab stretcher (Model Maxi Grip 750S Bi-axial Laboratory Stretching Frame, from Dr. Collin GmbH, Germany) was used to stretch the cast films. The cast film with dimensions of 135 mm×135 mm and a film thickness of around 1 500 μm (exact values given in Table 2) was gripped by 9×9 clips and heated by the infrared system up to 135° C. measured on the cast film surface. The preheat time before drawing was fixed at 90 seconds at 145° C., then the film was stretched by simultaneously biaxially stretching to the final stretch ratio with an acceleration of 6 000 mm/s.sup.2×6 000 mm/s.sup.2 resulting in speed of 250 mm/s×250 mm/s. After being stretched to the final dimensions, the film was immediately air cooled to room temperature by a fan and then was removed from the stretcher. Films were drawn to the target draw ratio of 4.6×4.6 (360%×360%). The stretching ratio and temperatures were kept constant for all samples.

(50) The physical, optical and barrier properties of the obtained oriented films are outlined in Table 3.

(51) The results shown in Table 3 confirm that the inventive oriented polypropylene films have a good quality, a reduced density and a high opacity. The inventive films have also a high degree of whiteness, good barrier properties and good mechanical properties.

(52) By comparing the results shown in Table 3, it can be seen that surprisingly a lower film density is obtained when a coarse calcium carbonate according to claim 1 is used (see Examples 2 to 5, d.sub.50=5.0 μm), whereas the use of a finer calcium carbonate results in higher film densities (see Comparative Examples 6 and 7, d.sub.50=3.0 μm). The inventive Examples 2 to 5 show film densities between 0.58 and 0.70 g/cm.sup.3 and are not only superior to the Comparative Examples using a finer calcium carbonate but also to the Comparative Examples using an organic cavitation agent.

(53) Furthermore, it can be gathered from Table 3 that the film thickness and the WVTR are under consideration of the usual deviations for all films approximately the same and are within the usual ranges for BOPP-films. Remarkably is also the significant improvement of the opacity when using a surface treated calcium carbonate (see Examples 2 and 3).

(54) TABLE-US-00003 TABLE 3 Physical, optical and barrier properties of prepared biaxially stretched polypropylene films Film Gloss 60° Gloss 60° Brightness CIE Lab thickness Density Opacity First side Second side Ry L* WVTR Sample [μm] [g/cm.sup.3] [%] [GU].sup.# [GU].sup.# [%] [—] [g/(m.sup.2 .Math. day] 1 (comp.) 36 0.89 13 81 100 2 90 15 2 (inv.) 45 0.61 71 40 42 53 91 20 3 (inv.) 39 0.58 72 26 49 57 92 28 4 (inv.) 30 0.70 43 30 49 32 91 32 5 (inv.) 32 0.61 46 22 50 35 92 39 6 (comp.) 44 0.78 45 24 29 34 92 25 7 (comp.) 31 0.73 54 23 33 42 92 27 8 (comp.) 30 0.91 30 30 32 22 92 19 9 (comp.) 33 0.86 62 25 28 48 92 18 10 (comp.) 38 0.96 22 43 43 15 92 16 11 (comp.) 24 0.89 16 24 31 17 92 25 12 (comp.) 36 0.96 17 50 54 11 92 39 13 (comp.) 36 0.96 16 21 23 13 92 39 14 (comp.) 18 0.86 15 16 23 10 92 35 15 (comp.) 17 0.83 15 10 17 11 93 41 .sup.#Gloss units