White propylene polymer film.

p0001The invention relates to a biaxially stretch-oriented, white film having a supporting layer based on propylene polymers, which contains a mixture of particles of calcium carbonate and titanium dioxide, and on one or both surfaces having at least one cover layer.

p0002A film of this kind is known for example from EP-A 0 244 614th The support layer of this film comprises a polymer mixture of polypropylene and polystyrene, and contains 5 to 15 wt .-% powdered fillers. Suitable fillers mixtures of calcium carbonate, silica and titania also be used. The particle size is in the range 2 to 5 microns. It is also prior art to incorporate titanium dioxide pigments with a coating of silica and alumina in thermoplastic resins (DE-A-27 40 561). EP-B-0044515 is the lesson with the oxides of aluminum, zinc, titanium, zirconium and / or magnesium and an organic substance such as long chain fatty acids, alcohols or polydimethylsiloxane coated TiO₂ pigments in plastic over the course eg polyethylene paper to use. Finally, it is known from DE-A-26 46 965, to use TiO₂- or CaCO₃ particles having a coating of a fatty acid ester as a filler in thermoplastic resins, for example polypropylene. It has been further described a polypropylene film containing SiO₂ with Al₂O₃ and TiO₂ coated particles, which are optionally coated with a polymeric organic silicon compound or a polyol (DE-A-38 17 909).

p0003It is an object of the present invention to improve the whiteness and the opacity of films of the aforementioned type. The film should continue to have good aging properties. This is to understand the property that the polymer under the influence of electromagnetic radiation such as light and especially UV rays only slight signs of degradation or embrittlement show the result in a deterioration of mechanical properties and yellowing of the white film. In particular, the tendency to yellow is disruptive when the white film to effective channel wrapping of objects, such as is used as a packaging film, label or laminating on cardboard, cardboard or paper. Further, the film should provide a good barrier to light and especially UV-radiation so that it is particularly suitable for the packaging of radiation-sensitive goods such as fat-containing foods. Another object of the invention is to provide an economical method for producing this film.

p0004This object is achieved by the film with the features mentioned in claim 1 and by the method of claim 15. The dependent claims provide expedient further developments of the film or of procedure. The propylene polymers of the carrier or base layer of the film is usually an isotactic homopolymer or copolymer of propylene with ethylene or alpha-olefins having 4 to 8 carbon atoms or a mixture of propylene homopolymers and propylene copolymers and / or other polyolefins, in particular having 2 to 6 carbon atoms, wherein the mixture comprises at least 50, especially at least 75 wt .-% of propylene homopolymer contains. The isotactic homopolymer expediently has an n-heptane-soluble fraction of at most 15 wt .-%. In the copolymers, the amount of comonomer of ethylene and the alpha-olefins is from 4 to 8 carbon atoms, generally up to 10 wt .-%, based on the copolymer. Preferred comonomers are ethylene and butene (1). Suitable polyolefins in the polymer mixture are HDPE, LDPE and LLDPE, where the proportion of these polyolefins is in each case 15 wt .-%, based on the polymer mixture does not exceed. These polymers of the base layer advantageously have a melt flow index in the range of 0.5 g / 10 min to 8 g / 10 min at 230 ° C and 2.16 kg load (DIN 53 735), in particular from 1.5 g / 10 min to 4 g / 10 min.

p0005The backing layer comprises a mixture of particles of titanium dioxide and calcium carbonate. The titanium dioxide anatase or rutile, preferably predominantly of rutile, which has higher opacity than anatase. In a preferred embodiment, the titanium dioxide particles comprise at least 95 wt .-% of rutile. They can be prepared by a conventional method, for example, by the chloride or the sulfate process. Their proportion is 1.5 to 5 wt .-%, based on the support layer, the average particle size is relatively small and is preferably 0.15 to 0.30 micrometer.

p0006The titanium dioxide particles have a coating of inorganic oxides, such as is usually used as a coating for TiO₂-white pigment in papers or paints to improve lightfastness. TiO₂ is known photoactive. Upon exposure to UV rays, free radicals form on the surface of the particles. These free radicals can migrate to the film forming constituents of the paint, resulting in degradation reactions and yellowing. Particularly suitable oxides include the oxides of aluminum, silicon, zinc or magnesium or mixtures of two or more of these compounds. TiO₂-particles having a coating of a number of these compounds are described for example in EP-A-0044515 and EP-A-0078633. Furthermore, the coating contains organic compounds having polar and nonpolar groups. The organic compounds must have sufficient thermal stability during production of the film by extrusion of the polymer melt. Polar groups are, for example, -OH, -OR, -COOX (X = R, H or Na, R = alkyl having 1 to 34 carbon atoms). Preferred organic compounds are alkanols and fatty acids having 8 to 30 carbon atoms in the alkyl group, in particular fatty acids and primary n-alkanols with 12 to 24 carbon atoms, and also polydiorganosiloxanes and / or polyorganohydrosiloxanes, such as polydimethylsiloxane and polymethylhydrosiloxane.

p0007The coating on the titanium dioxide particles is usually from 1 to 12, in particular 2 to 6 g inorganic oxides and 0.5 to 3, especially 0.7 to 1.5 g of organic compound, based on 100 g titanium dioxide particles. The coating is applied to the particles in aqueous suspension. The inorganic oxides are precipitated from water-soluble compounds, for example alkali metal, especially sodium aluminate, aluminum hydroxide, aluminum sulfate, aluminum nitrate, Natrumsilikat (water glass) or silica precipitated in the aqueous suspension.

p0008Inorganic oxides such as Al₂O₃ and SiO₂ and the hydroxides and their various stages of dehydration, eg oxide hydrates are to be understood, without ever knowing their exact composition and structure. be on the TiO₂ pigment after ignition and grinding in aqueous suspension, the hydrates, for example of aluminum and / or silicon precipitated, then washed and dried pigments. This precipitation may therefore take place directly in a suspension, as obtained in the manufacturing process after annealing and the subsequent wet grinding. The precipitation of the oxides and / or oxide hydrates of the respective metals from the water-soluble metal salts in the known pH range for the aluminum, aluminum sulfate is used for example in aqueous solution (pH less than 4) and between by adding aqueous ammonia solution or sodium hydroxide solution in the pH range 5 and 9, preferably between 7 and 8.5, the oxide hydrate precipitated. Assuming a water glass solution or alkali metal from the pH of the submitted TiO₂ suspension in the strongly alkaline range should be (pH greater than 8). The precipitation is then carried out by addition of mineral acid such as sulfuric acid in the pH range of 5 to 8. After the precipitation of the metal oxides, the suspension is stirred for a further 2 min to about 15 h, wherein the the precipitated layers undergo aging. The coated product is separated from the aqueous dispersion and after washing at elevated temperature, especially at 70 to 110 ° C, dried.

p0009The absolute particle size of titanium dioxide and calcium carbonate should not exceed 10 microns, since larger particles deteriorate the optical properties of the film as well as the process security in the production of the film. The calcium carbonate particles are usually employed as compared to the titanium dioxide particles with a larger average diameter that is 0.7 to 3.0, in particular 0.8 to 1.2 micrometers. The proportion of calcium carbonate particles is advantageously from 4 to 6 wt .-%, based on the carrier layer, wherein a total amount of titanium dioxide and calcium carbonate particles of 10 wt .-%, based on the carrier layer, for a high whiteness and high opacity is sufficient.

p0010The calcium carbonate particles consist of a mixture of natural rock powders, such as calcite, limestone, calcite or chalk, as long as they are not heavily contaminated with other substances. It can also be used by precipitation generated carbonate particles.

p0011Precipitated calcium carbonate can be prepared by various processes. For this purpose, usually natural rock powders based on calcium carbonate, in particular natural limestone decomposes above 900 ° C in calcium oxide and carbon dioxide. After clearing the burnt lime with water is carbonated with purified carbon dioxide. The calcium carbonate particles obtained in aqueous suspension.

p0012The calcium carbonate particles preferably have a coating based on hydrophobic compounds, in particular compounds containing hydrocarbon chains having at least 8, preferably at least 12, especially at least 14 C-atoms. Particularly preferred are saturated organic fatty acids having 8 to 30, especially 14 to 24 carbon atoms, their sodium or potassium salts or their esters, in particular having 1 to 18 carbon atoms, as well as natural or synthetic waxes based on hydrocarbons, acids, esters , soaps or acid amides, polypropylene waxes or polyethylene waxes. The fatty acids, its salts and esters are obtained in particular from animal or vegetable fats or oils, for example soy oil, palm oil, coconut oil or peanut oil, and are therefore present as a mixture of various higher fatty acids. Particularly preferred are palmitic acid, stearic acid and hydroxystearic acid, their alkali metal salts or alkyl esters having 1 to 18 C-atoms, eg methyl, butyl or decyl stearate, are used. It is also possible to use mixtures of these compounds as a coating material. As the emulsifier, the conventional ionic or nonionic surfactant can be used. In general, an amount of 0.5 to 5, especially 0.7 to 1.2 g based on 100 g of calcium carbonate particles, these compounds sufficiently.

p0013These compounds are added in emulsified form to the slurry in aqueous suspension carbonate particles, or passing CO₂ into the aqueous solution of calcium ions in the presence of the emulsified hydrophobic compounds. Then the water is removed.

p0014The one or both sides applied to the backing layers can be both hot- and cold-sealable layers. It can also be non-sealable layers consist for example of the same polymer as the carrier layer. All these layers may be one or both sides. The double-sided outer layers may consist of the same or different materials. A heat seal layer has a softening point that is at least 10 ° C lower than the softening point of the support layer. The sealing layer preferably consists of an ethylene homopolymer (of high density polyethylene or low density polyethylene), a LLDPE, a copolymer of propylene as main component and ethylene, preferably having an ethylene content of 4 to 10 wt .-% (relative to the copolymer ), a copolymer of propylene as main component, and butene (1), preferably with a butene content of 10 to 15 wt .-% (based on the copolymer), a terpolymer of propylene, ethylene and an alpha-olefin having from 4 to 10 carbon atoms, preferably one of from 93.2 to 99.0 wt .-% of propylene, from 0.5 to 1.9 wt .-% of ethylene and 0.5 to 4.9 wt .-% an alpha-olefin having 4 to 10 carbon atoms or a mixture of these polymers. The comonomers are statistically distributed in the polymers substantially. A particularly preferred sealing layers statistical alpha-olefin copolymers are used in which the ratio of weight average value M_W the molecular weight to the number average value M_N the molecular weight distribution between 2.5 and 5.5 is.

p0015The outer layer or the outer layers may also consist of a propylene homopolymer, wherein the melt index of the polymer used for the top layer a preferably should have about 0.5 to 1.0 g / 10 min higher than the melt index of the polymer used for the base layer ( measured at 230 ° C and 2.16 kg load in accordance with DIN 53 735).

p0016Suitable cold seal layers consist of polymers on the basis of natural or synthetic rubber. Suitable outer cover layers consist of acrylates, such as EP-A-0214790, EP-A-0254417 and CH-PS 632 777 are described, or of vinylidene chloride copolymers as described in EP-A- are called 0 088 535th In a particular embodiment, a cover layer of acrylate and the other outer layer of vinylidene chloride copolymer. These compounds can be anchored, if required with an adhesion promoting layer on the underlying layer of polyolefin. Suitable primers are also listed in the documents mentioned. During the cold-sealing layers are applied only in a separate process step (off-line), the acrylates or vinylidene copolymers can also be applied in-line prior to the longitudinal stretching. The in-line process is described for example in EP-A 0 214 790th

p0017When using the film as a packaging material or layers to improve the slippage in order to ensure satisfactory machinability and with regard to smooth Einfül len of the packaged item packaged in bags, a lubricant is added. As the lubricant is suitably a polydialkylsiloxane, preferably used with 1 to 4 C atoms in the alkyl group, in which polydimethylsiloxane is particularly preferred. The amount used of the polydialkylsiloxane in the top layer or layers is from 0.2 to 2.5 wt .-%, preferably 0.5 to 1.6 wt .-%, based on the weight of the outer layers.

p0018The total thickness of the film is 25 to 120, preferably 30 to 60 micrometers, wherein the thickness of the outer layer (s) in each case 0.1 to 5, preferably 0.6 to 2 micrometers.

p0019In order to improve certain properties of the inventive film further, both the base layer and the outer layer (s) may contain appropriate additives in the particular effective amount, preferably are antistatic agents, such as N, N-Bisethoxyalkylamin and nucleating agents, eg sorbitols called. On the positive side, in particular, the addition of processing stabilizers for the core layer has been found. These are compounds which are intended to prevent the thermal degradation of the polymers in the manufacture of the film. Such compounds include phosphite stabilizers and / or Diphosphitstabilisatoren such as tris-2,4-ditert.butylphenyl) phosphite and bis (2,4-ditert.butylphenyl) pentaerythritol diphosphite in an amount from 0.05 to 0.3 by weight %, based on the core layer. Particularly advantageous further has the addition of optical brightener, eg bisbenzoxazoles, bis (styryl) biphenyls, triazine phenylcumarins or Benzotriazolphenylcumarine, proved that, in an amount of 100 to 500, especially 150 to 250 ppm, based on the core layer is used. In particular, to improve the convertibility, the cover layer or the outer layers 0.1 to 1 wt .-%, can be added especially 0.2 to 0.5 wt .-%, of an organic or inorganic antiblocking agent. Suitable antiblocking agents include organic polymers such as polyamide, polyester, polycarbonates and the like, which are incompatible with the polymer in the top layer and be in the form of particles, or inorganic particles, in particular silicon dioxide.

p0020The film may be prepared by conventional processing technologies for the production of multilayer films.

p0021Thus, the film can be produced by extrusion of the support layer or by coextrusion of the individual layers forming the molten polymer materials through a flat-quenching and solidification of the melt film, then biaxially stretch orientation in longitudinal and transverse directions and finally heat setting. For biaxial stretching, the film is heated and stretched first longitudinally and anschießend transversely or first transversely and then longitudinally. The longitudinal stretching is carried out, for example, at 120 to 130 ° C in a ratio of 5: 1 to 7: 1 and subsequent transverse stretching at 160 to 170 ° C in a ratio of 8: 1 to 10: 1, the heat-setting at 150 to 160 ° C for about 0, 5 to 10 seconds. The surfaces of the outer layers may if necessary be subjected to impart the film for the application of further coatings improved absorption capacity and to improve its adhesiveness and laminating properties or printability or metallization after heat fixation nor a surface treatment.

p0022The support layer is preferably produced via master batch technique, ie according to the desired content of titanium dioxide and calcium carbonate particles polypropylene raw material, titanium dioxide masterbatch and calcium carbonate masterbatch are added to an extruder to form the support layer. The term masterbatch is known to understand a stock mixture, i nsbesondere a granular dust-free concentrate of plastic raw material with high amounts of fillers used in the mass processing as an intermediate, such as an addition to any fillers containing granules before extrusion, to derive certain quantities manufacture of filler moldings containing. The masterbatch used in the invention for the addition of TiO₂ or CaCO₃ is characterized in that it more than 30 wt .-%, preferably 40 to 75 wt .-%, based on the total weight of polymers plus fillers, or of TiO₂ particles . CaCO₃ particles and in that it is present as a cylindrical or spherical pellets. It is particularly advantageous to provide a combined master batch with a content of 40 to 75 wt .-%, preferably employ from 40 to 70 wt .-%, of TiO₂- and CaCO₃ particles. The carrier polymer for the masterbatch polyolefins, olefinic copolymers having 2 to 8 carbon atoms are each olefin or mixtures of these compounds. The preferred carrier polymer is polypropy len, polyethylene or a mixture of polypropylene and polyethylene. Through the preferred masterbatch methods are abrasion phenomena that occur during extrusion processing of TiO₂-containing polyolefin, significantly reduced.

p0023By the longitudinal stretching, the backing layer is replaced by a plurality of micro cavities (vacuoles). During stretching operation, the polymer matrix is ​​torn open at the grain boundaries of the TiO₂- and CaCO₃-particles, resulting in the creation of these cavities. Films of this type have a pearlescent luster and a lower density, which is in this case from 0.55 to 0.80, especially 0.65 to 0.75 g / cm³.

p0024The bulk density of the masterbatches is 700-1500 g / l, preferably 950-1250 g / l. Determination of bulk density is carried out in accordance with DIN 53 466. The ratio of the bulk density of the polypropylene raw material for bulk density of the respective masterbatches is less than 0.80, preferably less than 0.65.

p0025The inventive film is suitable for their beneficial properties, in particular because of their high and lasting whiteness as packaging material and for technical purposes, such as laminating, label film, release film or tape carrier film.

p0026The invention is further illustrated by the following examples.

example 1

p0027It is a polypropylene film with heat sealable surface layers present on both sides of an ethylene-propylene copolymer (C₂-content 4.5 wt .-%), the ratio of weight average value M_W (200,000 g / mol) to number average value M_N (50,000 g / mol) of the molecular weight is 4, coextruded through a flat die at about 270 ° C. The mixture (melt) the core layer of the film consists of 91.6 wt .-% polypropylene, 4.8 wt .-% CaCO₃ an average particle size of 1 micrometer and 3.6 wt .-% TiO₂ (rutile) with an average particle size of 0.2 microns, which has an inorganic coating of Al₂O₃ (4 g / 100 g TiO₂) as well as an organic coating of stearic acid (1 g / 100 g TiO₂), the mixture of 82 wt .-% polypropylene, 10 wt. -% of a masterbatch of 48 parts by weight CaCO₃ and 52 parts by weight Polypropylene and 8 wt .-% of a masterbatch consisting of 45 parts by weight TiO₂ and 55 parts by weight Polypropylene is formed. The respective bulk density is 530 g / l for polypropylene, 720 g / l for the masterbatch with CaCO₃ and 750 g / l for the masterbatch with TiO₂.

p0028The film is, as usual, stretched longitudinally and then transversely. The biaxially oriented film has a film thickness of 50 microns, with the sealing layers are each 0.8 microns thick. The film density is 0.72 g / cm³. The surface gloss measurement angle at 20 ° is 25 divisions (measured according to DIN 67 530 with gauge Dr. Lange, Germany, type RL 3).

example 1a

p0029In an analogous manner as in Example 1, a polypropylene film is produced with a one-sided existing surface layer of the same C₂ / C₃ copolymer and an opposed 3 microns thick surface layer of propylene homopolymer. On this surface, the film is at a measurement angle of 20 ° gloss of 63 scale units (DIN 67 530, gauge Dr. Lange, Type RL 3).

example 1b

p0030Example 1a is repeated using the 3 micron thick PP-surface layer of Example 1A is additionally provided with an approximately 0.4 micron thick layer of random C₂ / C₃ copolymer. The C₂ / C₃ copolymer has a C₂ content of 4.5 wt .-%, the ratio of weight average M_W (200,000 g / mol) to number average value M_N the molecular weight distribution (50,000 g / mol) is 4. In order to improve the printability of this surface layer to a corona treatment is subjected. The surface tension is 40 mN / m. The film shows on this surface, particularly at low angle of measurement, a high gloss (measuring angle 20 °, gloss 72 divisions, DIN 67 530, gauge Dr. Lange, Type RL 3), which has a positive effect even after printing of the film.

example 2

p0031Example 1 is repeated, but wherein wt CaCO₃- and TiO₂ content above an addition of 12 wt .-% of a combined masterbatch of 30 wt .-% TiO₂, 40 wt .-% CaCO₃ and 30 wt .-% polypropylene to 88th -% polypropylene he follows. The bulk density of the combined master batch is 1050 g / l.

example 3

p0032Example 2 is repeated, the TiO₂ particles are coated in an analogous manner with Al₂O₃. Instead of stearic acid polydimethylsiloxane (1.5 g / 100 g TiO₂) used in the additional coating of the TiO₂ particles.

Comparative Example 1

p0033Example 2 is repeated, the TiO₂ particles are coated only with Al₂O₃ (4 g / 100 g TiO₂).

Comparative Example 2

p0034Example 2 is repeated, the TiO₂ particles are coated with stearic acid (1 g / 100 g TiO₂).

Comparative Example 3

p0035Example 2 is repeated, the TiO₂ particles remain uncoated.

p0036The opacity, whiteness and yellowing tendency of the film obtained are summarized in the following table.

p0037The opacity and whiteness is performed using the electric remission photometer "ELREPHO" Zeiss, Oberkochen (DE), standard illuminant C, 2 ° standard observer. Opacity is determined to DIN 53 146th The whiteness is defined as W = RY + 3RZ - 3RX.

p0038W = whiteness, RY, RZ and RX = corresponding reflection factors when the Y, Z and X color measurement. The white standard barium sulfate compact (DIN 5033, part 9) is used. A film sample (size A4) is in a device of the type suntest CPS Heraeus, Hanau (DE), irradiated for several days, irradiance 765 W / m², and compared with the naked eye with a non-irradiated sample.<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="4" colsep="0" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" rowsep="0" /><entry namest="col2" nameend="col2" align="center">opacity</entry><entry namest="col3" nameend="col3" align="center">whiteness</entry><entry namest="col4" nameend="col4" rowsep="0" align="center">Yellowing tendency</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="center">%</entry><entry namest="col3" nameend="col3" align="center">%</entry><entry namest="col4" nameend="col4" /></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Example 1 / 1a / 1b</entry><entry namest="col2" nameend="col2" align="right">88</entry><entry namest="col3" nameend="col3" align="right">90</entry><entry namest="col4" nameend="col4" align="left">very low</entry></row><row><entry namest="col1" nameend="col1" align="left">example 2</entry><entry namest="col2" nameend="col2" align="right">92</entry><entry namest="col3" nameend="col3" align="right">93</entry><entry namest="col4" nameend="col4" align="left">very low</entry></row><row><entry namest="col1" nameend="col1" align="left">example 3</entry><entry namest="col2" nameend="col2" align="right">93</entry><entry namest="col3" nameend="col3" align="right">93</entry><entry namest="col4" nameend="col4" align="left">very low</entry></row><row><entry namest="col1" nameend="col1" align="left">Comparative Example 1</entry><entry namest="col2" nameend="col2" align="right">76</entry><entry namest="col3" nameend="col3" align="right">78</entry><entry namest="col4" nameend="col4" align="left">low</entry></row><row><entry namest="col1" nameend="col1" align="left">Comparative Example 2</entry><entry namest="col2" nameend="col2" align="right">82</entry><entry namest="col3" nameend="col3" align="right">76</entry><entry namest="col4" nameend="col4" align="left">clearly</entry></row><row><entry namest="col1" nameend="col1" align="left">Comparative Example 3</entry><entry namest="col2" nameend="col2" align="right">76</entry><entry namest="col3" nameend="col3" align="right">74</entry><entry namest="col4" nameend="col4" align="left">clearly</entry></row></tbody></tgroup></table></tables>