MXPA00003291A - Polypropylene composition useful for making solid state oriented film - Google Patents

Abstract

Disclosed is a polypropylene composition useful for making solid state oriented film. The composition comprises two predominantly isotactic polypropylenes with the melting point temperature of one of the polypropylenes being substantially lower than the melting point temperature of the other. In one embodiment of the composition the higher melting point polypropylene is made by Ziegler-Natta catalysis, while the lower melting point polypropylene is made by metallocene catalysis. Compared to polypropylene with similar melt flow rate and made by Ziegler-Natta catalysis, and solid state oriented film thereof, the composition provides a better balance of elevated temperature draw characteristics and physical properties of oriented film made therefrom. Thus, at the same or lower elevated temperature yield stress, oriented film of the composition exhibits improved properties such as stiffness, elongation-at-break, dimensional stability, and oxygen barrier. Also disclosed is a method for the manufacture of oriented, flexible packaging film. In the method a cast sheet is melt extruded from the composition, and, after cooling until it is solid, the cast sheet is stretched to the desired extent of orientation at a temperature which the sheet does not break while being stretched, but below the Tm. of the composition. The resulting film then is cooled to at least the crystallization temperature of the lower melting point polypropylene.

Description

COMPOSITION OF POLYPROPYLENE. USEFUL TO OBTAIN MOVIES SOLID STATE ORIENTED Field of the Invention This invention relates to chemical techniques. More particularly, it relates to the chemical technique related to synthetic resins derived from 1-olefins, also known as alpha-olefins. Specifically, it refers to the synthetic resins formed by the polymerization of propylene, and to the films obtained therefrom.

BACKGROUND OF THE INVENTION Synthetic resin, formed by the polymerization of propylene, as the sole monomer, is termed as polypropylene. While "polypropylene" has been used occasionally in the art to include a synthetic resin formed by the copolymerization of propylene and a minor amount of another monomer, such as ethylene, the term is not used here. The resin is generally a mixture of propylene homopolymers of different chain lengths and thus different molecular weights. However, it is usually referred to as a "polymer", in simple form.

Well-known commercial polypropylene is a predominantly crystalline and isotactic thermoplastic polymer. It is formed by the polymerization of propylene, by Ziegler-Natta catalysis. As is well known, in such catalysts, the polymerization catalyst comprises the reaction product of an organic compound of a metal of Groups I-III of the Periodic Table (for example, an aluminum alkyl) and an inorganic compound of a transition metal of Groups IV-VIII of the Periodic Table (for example, a titanium halide). The polymerization conditions are also generally well known. Such polypropylene has been used in large quantities in many applications, one of which is the manufacture of films, especially biaxially oriented films, used in the packaging of, for example, food snacks, such as potato flakes and the like. In truth, such a movie has come to be known as the movie "BIPP". The oriented polypropylene film is generally obtained by a method comprising the solid state orientation. One such method is that of tubular puff or double bubbles. Another is the laying frame method.

In one embodiment of the tubular or double bubble blowing method, a composition, consisting essentially of polypropylene, is extruded in molten form through an annular die, to form a molten tube, which is brought into contact with an internal mandrel of cooling, where it begins to solidify. The tube is also cooled in a water bath and pulled through a pair of rollers, which compress it strongly. After passing through the rollers, the cooled, solidified tube is "softened" by reheating with, for example, radiant heaters, at a temperature below the melting point of the polypropylene, but high enough to enable the tube to be expanded by the internal air pressure. The tube is blown by air under pressure. This causes the tube to enlarge and form a controlled bubble, then the tube wall is stretched in all directions, and thus biaxially oriented. The bubble is then crushed through the frame rollers, between a pair of compression rollers, and passed through a router, where the flattened bubble is divided into one or more strips, which are wound into one or more Take reels. In the laying frame method, a composition, consisting essentially of polypropylene, is extruded in molten form through a slot die, to form a molten sheet. This sheet is solidified by cooling on a molding roll. In the on-line orientation practice of this method, the resulting molded sheet is immediately reheated to a temperature below the melting point of the polypropylene, but high enough to enable the composition to be pulled or stretched. In the case of sequential orientation, the "softened" molded sheet is stretched by rollers that rotate at different rotation speeds, so the sheet is stretched with the desired ratio in the longitudinal direction (machine direction). The uniaxially oriented sheet is then fastened at its side edges by chain fasteners and transported on a clothesline, where it is again reheated to such temperature and placed in the direction transverse to the desired stretch ratio, by means of fasteners. chain, which diverge in their forward journey. The resulting film, biaxially oriented, is released from the fasteners, trimmed at the edge and rolled onto a take-up reel. According to the development of recent years, the polymerization of the 1-olefins is carried out by the metallocene catalysis. In fact, this development has become the stage in which at least one polypropylene producer has obtained commercially available samples of predominantly isotactic polypropylene, produced by the polymerization, catalyzed by metallocene, of propylene. In such catalysis, the polymerization catalyst comprises the reaction product of a metallocene and a compound such as alumoxane, trialkyl aluminum, or an ionic activator. A metallocene is a compound with at least one part of cyclopentadienyl, in combination with a transition metal of Groups IV-VIII of the Periodic Table.

Description of the Related Art U.S. Patent No. 5,530,065 to Farley, et al. (see also WO 95/13321), discloses a mixture of (a) a metallocene catalyzed polymer, with a narrow molecular weight distribution and narrow distribution of the composition and (b) a conventional polymer catalyzed with Ziegler-Natta catalysts. , with a broad distribution of molecular weight and composition distribution. The mixture generally describes obtaining an unoriented film, extruding the mixture in a molten state through a flat die and cooling. Describes obtaining a film oriented by the blown film method, that is, extruding the mixture in the molten state through an annular die, blowing and cooling to form a tubular film and then dividing it axially and unfolding it to form a flat film . Although the patent discloses that polymers can be formed by polymerizing or copolymerizing one or more C2-C12 alpha-olefins, examples of which include propylene, more specific descriptions focus on polyethylene and ethylene copolymers. The patent does not disclose the properties of the propylene homopolymers, obtained by the metallocene catalysis, the properties of the propylene homopolymers obtained by conventional Ziegler-Natta catalysis, and the more specific temperature conditions in the disclosed forms of obtaining the movie. The published PCT application, WO 96/11218, whose date of publication is April 18, 1996, discloses a multi-step process for the polymerization of one or more of olefins of the formula CH 2 = CHR, wherein R can be an alkyl with 1-10 carbon atoms. In the first polymerization step, one or more such olefins are polymerized by the Ziegler-Natta catalysis, to form particles of a first polymer. In the next polymerization step, a polymer of one or more such olefins is formed by the catalysis of the metallocene on or within the particles of the first polymer. However, except for the final products of the Examples of the application, these final products are named impact copolymers of propylene, and are obtained by the sequential polymerization of propylene and a mixture of propylene and ethylene, the application does not reveals the properties of the products of the process. Published European Patent Applications, EP 0 841 371 A2, EP 0 841 372 A2 and EP 0 841 373 A2, from Mitsui Chemicals, Inc., disclose a propylene polymer composition, the flow regime of the melt (" MFR ") of which is 5-50 g / 10 minutes, comprising a propylene polymer (A3), prepared by the Ziegler-Natta catalysis, having a desired MFR of 0.01-30 g / 10 min, and a desired concentration of 10 to 90% by weight, and a propylene polymer (A2) prepared by metallocene catalysis, having an MFR of 30-1,000 g / 10 min, and a desired concentration of 10 to 90% by weight . This composition is said to be excellent not only in its resistance to heat, rigidity and elongation of tension to rupture, but also in its molding capacity.

Technical Definitions "Film" means a sheet with a thickness of 154 microns or less.

"MFR" means the flow regime of the melt in decigrams per minute ("dg / min") or grams in ten minutes ("g / 10 min"), as measured in accordance with Method of ASTM D 1238, Condition L. "% mmmm" means the percent by weight of isotactic pentad units, in which the methyls are in stereoscopic alignment, as determined by agronomic-nuclear resonance ("NMR") measurements. It is a measure of isotacticity. The higher the percentage, the greater the isotacticity. "Pl" means the polydispersity index, determined rheological entity, is a measure of the molecular weight distribution. The higher the value, the wider the distribution. "iPP" means a predominantly isotactic polypropylene. "Predominantly isotactic polypropylene" means a polypropylene in which the propylene monomer units in each polymer chain are arranged spatially, so that their methyl groups are projected from the chain in the same stereoscopic ratio, as determined by the NMR , with enough regularity for crystallization. "m-iPP" means an iPP that has been obtained by metallocene catalysis. "Tc" is the temperature in degrees Celsius ("se"), as determined by differential scanning calorimetry, in which a crystallizable molten polymer reaches maximum crystallization when the cooled unit solidifies. "Tn" is the temperature in degrees Centigrade ("SC"), as determined by differential scanning calorimetry, in which the crystalline fraction is a thermoplastic polymer, normally solid, with a crystallinity, or a co-crystallized fraction of a Thermoplastic polymer mixture, normally solid, with crystallinity, becomes completely melted when the solid polymer or mixture is heated. "XI" means the fraction of a polymer, which is insoluble in xylene at 252C. It was measured by dissolving a heavy amount of a polymer in boiling xylene, cooling the solution to that temperature and weighing the solid precipitate free of xylene. It is expressed as a percentage of the weight of the polymer. It is a measure of the crystallinity. The higher the percentage, the greater the crystallinity.

"ZN-iPP" means an iPP that has been obtained by the Ziegler-Natta catalysis.

BRIEF SUMMARY OF THE INVENTION The invention is based on the discovery that, compared to a biaxially oriented film, obtained from a ZN-iPP, a film, biaxially oriented, obtained under substantially the same conditions as a polypropylene composition, with substantially no same MFR as ZN-iPP, and consisting essentially of a ZN-iPP and an m-iPP, has certain significantly improved properties. While the two polypropylenes of the composition are predominantly isotactic, they differ in that the Tm of the m-iPP is substantially less than the Tm of the ZN-iPP. The composition allows a balance of the stretching characteristics at high temperatures and the final properties of the film, this balance is substantially improved compared to the equilibrium obtained with the first mentioned ZN-iPP. Compared to the first mentioned ZN-iPP, the composition exhibits a expanded film process window. Compared with the oriented film obtained from the first mentioned ZN-iPP, the oriented film obtained from the composition at the same elevated temperature, provides improved properties exhibited in the tension. These improved properties include rigidity, elongation at break, dimensional stability and oxygen barrier. These improved properties make the oriented film of the composition especially useful in packaging a flexible film, particularly in oxygen-sensitive and medical-type food packages. In summary, the invention, in one aspect, refers to a composition comprising a mixture of two iPPs with the Tm of one of the iPPs being substantially the Tm of the other. One embodiment of the composition comprises a molten mixture, consisting essentially of two iPPs. One of the utilities of the composition is in the manufacture of the oriented film. Accordingly, the invention, in another aspect, comprises sheets obtained from said composition. The modes of the sheets include the oriented film and the molded sheet, useful for obtaining a oriented film. This oriented film includes a uniaxially oriented film, as well as a biaxially oriented film. Also, this invention, in another aspect, comprises a method for manufacturing the film oriented by the laying frame method. In the method, a molded sheet is extruded in molten form from the composition and then cooled to solid, this sheet molded, at a temperature at which the sheet does not break, while being stretched, but below the Tm of The composition is stretched to the desired extent of orientation and thickness of the film. The resulting oriented film is then cooled to at least the Tc of the composition.

DETAILED DESCRIPTION OF THE INVENTION In a preferred embodiment of the composition of the invention, one of the IPPs of the composition, this iPP is referred to hereinafter as the first iPP, it is a Zn-iPP. In other embodiments, the first iPP is one obtained by the polymerization of propylene by another of the Ziegle-Natta catalysis, for example by the metallocene catalysis. However, in all the modes intended for use in the manufacture of the oriented film, based on the solid state orientation method, the first iPP preferably has these properties: MFR = 0.5-10 dg / min% mmmm = 85 -100 Tm = 160-167SC Numerical values outside these minimum and maximum numerical values are within the broader scope of this invention, provided that the basic concepts of the invention are met.

As indicated above, the other iPP in all modalities of the composition, which is referred to occasionally as the second iPP, is characterized by its Tm substantially smaller than that of the first iPP. In the preferred embodiment, mentioned above, it is an m-iPP. In other embodiments, it is an iPP obtained by another of the metallocene catalysis, for example, the Ziegler-Natta catalysis, and having such a Tm. In all the compositional modalities intended for use in the manufacture of the film oriented by the solid state orientation method, the second iPP preferably has these properties: MFR = 0.5-200 dg / min% mmmm = 75-98 Tm = 110-160SC Numerical values outside these minimum and maximum numerical values are within the broader scope of this invention, provided that the basic concepts of the invention are met. For use in the manufacture of the film oriented by the laying frame method, the MFR of the composition is preferably 1-10 dg / min. Therefore, the first iPP and the second iPP and their relative concentrations are preferably selected to result in this MFR. In general, the weight ratio of the second iPP to the first iPP in the composition is in the approximate range of 5:95 to 95: 5. Minor and greater operable weight ratios of these minimum and maximum ratios are within the broader scope of the invention. More specifically, the selection of the relative weight ratio depends on the second iPP, Tm and the isotacticity of the first iPP. In a given isotacticity of the first iPP, the closer the Tm of the second PP is to the Tm of the first iPP, the higher its concentration in relation to that of the first iPP, to achieve the benefits of this invention. The greater the isotacticity of the first iPP in relation to the second iPP, the lower weight ratio of the second iPP to the first iPP may be lower. In some cases, each of the two essential polypropylenes used to obtain the composition of this invention will comprise at least one antioxidant, generally at a conventional concentration effective to minimize the oxidative degradation of the polypropylene. In some embodiments, one or both of the essential polypropylenes comprise another stabilizing material, selected from the group consisting of antioxidants, heat stabilizers, ultraviolet light inhibitors, and the like. Likewise, they can each comprise other additives, for example, a nucleating agent material, filler material, extension material, coloring material, antacid material and material that improves the properties. In addition to one or more additives included with the essential polypropylenes, the composition may contain other additives, such as, for example, an additional stabilizer material, antacid material, coloring material and nucleating agent material. The composition of this invention is obtained by conventional means and means. In one way, the particles of each iPP, when in the form of pellets, granules, flakes or the like, and with or without any additive, are mixed together to form a simple mixture. In another way, the particles of each iPP, with or without additives, are mixed together under the melting conditions. The molten mixture, thus formed, can be used in its melted condition, as in the manufacture of the film, or extruded through a die to form beads of thickness of the pellet or a cord with thickness of the pellet which is cooled to solidify, and then cut into pellets for future use.

The composition can also be obtained by the multi-step process of the WO patent application. 96/11218 of the PCT, published. In such a case, the product of the process is in the form of particles consisting essentially of both polymers. As indicated above, one utility of the composition of the invention is in the manufacture of a film oriented by the solid state orientation method, and of the molded sheet for use in making the film oriented by the frame method. of laying. In such manufacture, the molded sheet is formed of the composition by conventional shapes and means. In most forms, the composition is extruded in molten form through a sheet die, to form a cast sheet of the selected thickness. This thickness is such that in the subsequent pulling or stretching of the molded sheet, an oriented film of the desired thickness is obtained. The melted sheet is cooled until solidified. If it is to be oriented at a later time, the molded, solid sheet is also cooled to room temperature (usually 20-252C) and stored for future orientation.

In any case, for the orientation of the solid sheet it is reheated to a temperature at which this sheet does not break, while stretching, but below the Tm of the composition (now a solid molten mixture). In this aspect, due to the effects of co-crystallization, the Tm of each polymer component is not actually observed in the solid molten mixture. The sheet at such a temperature is stretched in one direction, transversely to that direction, or both (in sequence or at the same time), in the desired extent of orientation and thickness of the film. The resulting film is then cooled until its temperature is, and preferably below, the Tc of the composition. The biaxially oriented film by the solid state orientation method from the composition of this invention, with the MFR in the range of 1-10 dg / min, results in substantial improvements in the process, and has a substantially improved process equilibrium and properties, compared to the biaxially oriented film obtained under substantially the same set of ZN-iPP conditions, with substantially the same MFR, and the biaxially oriented film obtained under substantially the same set of m-iPP conditions, with substantially the same MFR . The composition exhibits an expanded process window, and a reduced biaxial elastic tension. For the same or lower biaxial elastic stress at elevated temperatures, the film of the composition will exhibit improved properties. These properties include the voltage module at room temperature and elongation at break, dimensional stability and improved barrier properties to oxygen and water vapor. The best mode now considered for carrying out this invention is illustrated by the following Examples.

Example 1 This example illustrates a specific embodiment of the composition of this invention. The formulation of this modality is: Components Parts in Weight ZN-iPP 60 m-iPP 40 Material stabilizer 0.22 The ZN-iPP in this modality is Pro-fax resin 6701, a commercial pellet polypropylene, obtained by the Ziegler-Natta catalysis. It is available from Montell USA Inc. It contains a conventional phenolic antioxidant at approximately 0.1% by weight of the product. The typical properties are: MFR = 0.83 dg / min Tm = 162-164 se (reheat) Tc = 101-104 SC (reheat)% mmmm = 86-89 XI = 95.5% by weight Pl = 3.95 The m-iPP is EX3825 resin, a pelletized propylene homopolymer product, produced by Exxon Chemical Company. The homopolymer is obtained by the metallocene catalysis. It contains a stabilizing material in a small but effective concentration. The typical properties are: MFR = 28 dg / min Tm = 148.3 se (reheat) Tc = 92.12C (reheat)% mmmm = 92.3 XI = 99.5% by weight Pl = 1.87 The stabilizer material is any conventional stabilizer composition useful for stabilizing iPP compositions for the manufacture of a flexible packaging film.

The composition in this example can be in the form of a simple mixture of solid particles (powder, as it is produced in the polymerization process, also known as chips, pellets, granules, spheres and the like). In such a case, the composition is obtained by conventional physical mixing techniques and equipment. The composition may also be in the form of particles (pellets and the like) of a molten mixture of the individual iPPs and the stabilizer material. In this case, the composition is obtained by the conventional mixture of a melt and conventional techniques and equipment for the formation of pellets or other particles. Typically, the Tm and Tc of the molten mixture are 159.3 and 100.5SC.

Example 2 This example illustrates a specific embodiment of a biaxially oriented film of this invention and a method for obtaining it. Pellets of the composition of Example 1 were molded into a sheet with a thickness of 635 microns with a Killion extruder (3.8 cm screw, L / D = 30), equipped with a 25.4 cm flat die for vertical extrusion.

The sheet was cooled on a roller cooled with water, adjusted to a drawing speed of 2.5 m / min. The conditions of the process are: Temp. of extruder cylinders: 230-250-250-250-250-250SC Screen packing: 0.25 / 0.15 / 0.25 mm Screw speed: 50 rpm Die opening: 1 mm Cold roll temperatures: 35-40SC All four sides of a square section of the molded sheet were fastened on a TM Long tensioner. After heating for 45 seconds at a temperature in the 140-155SC range, the section was biaxially stretched by the machine, at a stretch rate of 9,000% per minute, at a stretch ratio of 6: 1 in both directions. The resulting film, biaxially oriented, was cooled to room temperature (20-25SC). The film is typically 25.4 microns thick. The typical properties of the biaxially oriented film are indicated in the following Table 1 of the actual data obtained in the biaxially oriented film of this invention. Such a film was obtained from 5.1 cx 5.1 cm specimens of a cast sheet obtained according to this Example 2. The specimens were biaxially stretched as before at different temperatures in a TM Long laboratory tensor. The tensioner equipment includes a transducer for high temperature operation, which is mounted on the fastening system and interfaces with a computer for strain / strain measurements. Stretched specimens were aged at 23 e and 50% relative humidity for one week, and then the following physical properties were measured by the following identified procedures: Tensile strength (tensile strength) ASTM D-882 Elasticity (secant modulus) at 1% and elongation at break): ASTM D-882 Moisture vapor transmission rate ("MVTR"): ASTM F1249-90 (at 38SC and 100% relative humidity) Oxygen transmission rate (I2TR ") : ASTM D3985-81 (a 23. 32C and dry condition) Fog and Transmission: measured by a Gardner turbidity meter.

For comparison, the Table includes data obtained in a biaxially oriented film of 25.4 microns thick, for a ZN-iPP, which has these properties: MFR = 3.8 dg / min Tm = 162.5sc (reheat) Tc = 99.9SC (reheat)% mmmm = 87.4 XI = 94.3% by weight Pl = 4.39 Such a film was obtained from 5.1 x 5.1 cm specimens of a molded sheet, obtained according to this Example 2, and of ZN-iPP pellets. The specimens were biaxially stretched as before at different temperatures in a TM Long laboratory tensor. The movie was tested as before. Table 1 nm = not measured These data show that for the same biaxial elastic stress or lower, at elevated temperatures, the biaxially oriented film of the composition of this invention exhibited improved properties compared to a biaxially oriented film of an MFR ZN-iPP similar to that of the composition, both films were obtained by the laying frame method TM Long, under substantially the same conditions. These improved properties make the film of this invention particularly suitable for use as a packaging film for oxygen sensitive foods and the like. Additional data was obtained in the test work with a line of commercially relevant scale frame BOPP film. The specific embodiment of the composition of this invention is included in the work. It was a molten mixture consisting essentially of 68 parts by weight of a ZN-iPP having these properties: MFR = 1.2 dg / min Tm = 167.3 se (reheating) Tc = 101.5sc (reheating) XI = 99.0% by weight Pl = 4.8 and 32 parts by weight of an m-iPP, which has the following properties: MFR = 32 dg / min Tm = 146.6sc (reheat) Tc = 101.9sc (reheat) XI = 98.4% by weight Pl = 1.7 The MFR of the mixture was 2.7. The Tm of the mixture was 163.4 and its Tc was 106.5SC. Also included in the test work was a commercial grade film Zn-iPP, the MFR which was narrow to that of the mix. The ZN-iPP had the following properties: MFR = 3 dg / min Tm = 161.4se (reheating) Tc = 99.8SC (reheating) XI = 95.7% by weight Pl = 4.8 The line of laying frame BOPP film comprises a unit of flat die extrusion; a cooling unit for cold rolls, for sheet molding; an orienting unit in the direction of the machine (MDO unit) in which the sheet overheats and, by means of rollers, is oriented in the direction of the machine; an orientation unit in the transverse direction (TDO), where the sheet is reheated in a pre-stretch heating section, stretched in a stretch section in the transverse direction by means of a diverging fastener system which bears a rail and annealed in a tempering section; a beta scanner for film thickness control, and a film winding unit. In the test work, the film was obtained from the mixture and the Zn-iPP, at different temperatures, in the pre-stretching heating section, to determine the stretching temperature operating windows of the mixture and the ZN-iPP. In this regard, the temperature at which the composition forming the film in the pre-stretching heating section is heated is especially critical. It controls the stretching capacity of the composition and the uniformity of the thickness of the film. For any composition, too low a temperature leads to the rupture of the film, while too high a temperature results in partial melting and recrystallization of the composition and, consequently, a cloudy film. In this test work, the operating conditions of the film line, in addition to the PHT, were kept constant, except for the cooling roller and the line speeds, where they were automatically adjusted to maintain the same thickness of the film when It changes from one composition to another. The operating conditions are: Extrusion Temperature: 240-260SC Screw speed: 160 rpm Cooling roll temperature 30se Temperature in the MDO unit: preheat rolls: 120Sc (X2) - 125Sc (X2) -130SC (X2) Stretch rolls: 110SC (x2 ) - 125ßC (x2) - 125SC (x2) Annealing rollers: 130sc (x4) Stretch ratio in machine direction (MDSR): 5: 1 Temperatures in the TDO unit: Heating or pre-stretching section: variable Section Stretch: 160SC Tempering section: 1602C Stretch ratio in transverse direction (TDSR): 8: 1 - 9: 1 Stretch ratio of total area (MDSR x TDSR): 40 -45 Film thickness: 20 microns.

The film samples obtained in the test work were aged at 23 hours and 50% relative humidity for one week, before measuring their physical properties, which are listed in the following Table II. The property of the Young's modulus was measured by the procedure described in ASTM D-882. The other physical properties listed in Table II, except shrinkage of the film, were measured by the procedures identified above, with respect to the properties listed in Table 1. The method for measuring shrinkage of the film comprises placing specimens of the same. cm, in free state, of the film samples on a poly (tetrafluoroethylene) cushion in a forced air oven for 5 minutes at the temperatures indicated in Table II and measuring the extent of shrinkage in the machine directions and cross. In Table II, "MD" means in the machine direction and "TD11 means in the transverse direction." Also "PHT" in Table II means the air temperature in the pre-stretch section of the TDO unit. the travel speed of the direction of the sheet oriented in the direction of the machine through this section, the sheet did not arrive at the PHT, but remained below its Tm.

Table II Mix ZN-ÍPP MFR (dg / min) 2T7 3 Process Variables: PHT (° C) - minimum 170 164 PHT (° C) - optimum 175 175 PHT (° C) - optimum 185 185 PHT (° C) - maximum 195 197 Roller speed cold (rpm) 8 8 Amperage of the MDO unit 10. 5 11 Amperage of the unit TDO 9. 9 10. 6 Line speed (m / min, at the end of 45 45 output of the film of the TDO unit) Film thickness variation 2 sigma 0. 89 0. 53 Properties of the Film: Tension MD 170 150 TD 260 260 Youngs Module (MPa) MD 2640 2290 TD 4600 4150 1% Secant Module (MPA) MD TD 2410 2070 3760 3360 Elongation at break (%) MD TD 158 110 38 37 Shrink film at 120 ° C MD 3.8 4.7 5.7 5.7 TD Shrink film at 135 ° C MD 4.7 5.7 7.6 7.6 TD Film fogging (%) 0.17 0.3 O2TR (cc / m2 / day) Optimal PHT 1560 1600 O2TR (CC / n / day) - PHT minimum 1455 nm MVTR (g / n / day) 5.5 6 MVTR (g / m2 / day) - PHT minimum 5.4 nm nm = not measured In Table II above, the two values of the optimum PHT define the temperature window in which a film of good clarity with a low variation of 2 sigma in thickness (between 0.5 and 1 miera) was obtained. The minimum PHT in each case is the lowest PHT in which a non-uniform stretch appears. as is evident from the white bands or marks of type 'shark skin1. The maximum PHT in each case is the highest PHT at which the film becomes visibly cloudy with a drastic variation in the thickness of the film.

The amperages of the MDO and TDO are proportional to the tensions needed to orient the blade in the machine and transverse directions. The lower values of the shrinkage of the film indicate a better stability of this film. This parameter is important because the BOPP film is usually subjected to numerous procedures after manufacture, where heat is involved and the integrity of the film is essential. From the above data, it can be seen that while the mixture and the ZN-iPP have the same window of operating temperature, the film obtained from the mixture has substantially improved physical properties compared to the film obtained from ZN-iPP. These data also demonstrate that the orientation stresses for biaxially orienting the molded sheet of the mixture were significantly less than the orientation stresses to biaxially orient the molded sheet of a ZN-iPP or MFR similar to that of the mixture. Thus, the composition of this invention, compared to ZN-iPP, of substantially the same MFR, provides a substantially improved equilibrium of high temperature stretching characteristics and physical properties of the oriented film obtained therefrom.

Other Definitions The term "comprises" or similar, in this specification, means "includes without limitation". It is open-ended, it includes the object or grammatical objects indicated later, without excluding another or other grammatical objects. The expression "consists essentially of", in this specification, excludes a substance not designated in a concentration sufficient to substantially adversely affect the essential properties and characteristics of the composition of matter being defined, while allowing the presence of one or more substances Not reported with a concentration or concentrations insufficient to substantially adversely affect the essential properties and characteristics. Other embodiments and features and advantages of this invention will become apparent to those skilled in the art upon reading the foregoing descriptions. Such modalities are within the spirit and scope of the subject matter, unless it is expressly excluded from it by the language of the claims or is not within the doctrine of equivalents.

Claims (10)

1. CLAIMS 1. A composition, useful for the manufacture of a polypropylene film with solid state orientation, this composition comprises a first isotactic polypropylene (iPP) and a second iPP, with the melting temperature (Tm) of the second iPP being substantially smaller that the Tm of the first iPP, the weight ratio of the second iPP to the first iPP is in the approximate range of 5:95 to 95: 5, and the melt flow rate (MFR) of the composition being approximately 1 to 10 dg / min. A composition, according to claim 1, wherein the first iPP is an iPP obtained by the Ziegler-Natta (ZN-iPP) catalysis, and the second iPP is an iPP obtained by the metallocene catalysis. 3. A composition, according to claim 2, wherein the ZN-iPP has the following properties: MFR = 0.5 to 10 dg / min Tm = 160 to 1672C% mmmm = 85 to 100. 4. A composition, according to claim 3, wherein the m-iPP has the following properties: MFR = 0.5 to 200 dg / min Tm 110 to 1602C% mmmm = 75 to 98. 5. An oriented film of the composition of the claim 1. 6. A biaxially oriented film, according to claim 5. 7. A molded sheet, useful for the manufacture of a polypropylene film with solid state orientation, this sheet is composed of the composition of claim 1. 8. A method for obtaining a oriented polypropylene packaging film, this method comprises: forming a molded film of the composition of claim 1; stretching the sheet at a temperature at which this sheet does not break while being stretched, but below the Tm of the composition, in at least one direction, at the desired extent of orientation; and cooling the resulting film until its temperature is at least the crystallization temperature (Tc) of the second iPP. 9. A method, according to claim 8, wherein the stretching is in transverse directions, whereby the film is biaxially oriented. 10. A method, according to claim 9, wherein the stretching is carried out by the laying frame method. SUMMARY OF THE INVENTION A polypropylene composition is disclosed, useful for obtaining a solid state oriented film. The composition comprises two predominantly isotactic polypropylenes, with the temperature of the melting point of one of the polypropylenes being substantially lower than the temperature of the melting point of the other. In one embodiment of the composition, the higher melting point polypropylene is obtained by the Ziegler-Natta catalysis, while the lower melting polypropylene is obtained by the metallocene catalysis. Compared to the polypropylene with similar flow rate of the melt mass and obtained by Ziegler-Natta catalysis, and its oriented solid-state film, the composition provides a better balance of the drawing characteristics at high temperature and physical properties of the oriented film obtained from it. Thus, at the same elastic tension or lower at elevated temperature, the oriented film of the composition exhibits improved properties, such as rigidity, elongation at break, dimensional stability and oxygen barrier. Also disclosed is a method for manufacturing an oriented, flexible, packaging film. In the method, a molded sheet is extruded in molten form from the composition, and, after cooling to a solid, the molded film is stretched to the desired extent of orientation at a temperature at which the sheet does not break while stretches, but below the melting temperature (Tm) of the composition. The resulting film is then cooled to at least the crystallization temperature of the lower melting point propylene.