Field of invention

The present invention relates to polypropylene for the preparation of films, in particular biaxially oriented polypropylene (BOPP) films. Structural characteristics of the claimed polypropylene allow to increase the speed in the process of preparing BOPP films up to 450 m/min or higher without the use of gliding additives (for example, metal stearates), which are known to facilitate the process of film production. The invention also relates to BOPP films, at least one layer of which contains polypropylene. Films according to the invention are suitable for producing packaging, including packaging for food products, adhesive tapes, labels, etc.

Background of the invention

Due to the possibility of a wide variation of polypropylene films properties depending on the composition and method of production, polypropylene films are a popular material for the manufacture of high-quality flexible packaging. In particular, biaxially oriented polypropylene (BOPP) films are used for the manufacture of food and non-food products packaging, for individual or multiple-unit packaging, wherein the packaging can be made transparent or metallized, matte, colored (depending on the filler color). Besides, BOPP films are used to make labels, adhesive tapes etc.

As a rule, the major component of BOPP films is isotactic polypropylene with a degree of isotacticity of 87-89% (isotacticity is determined by the content of isotactic pentads [mmmm] by C ¹³ NMR analysis). The presence of a certain number of defects in the structure of isotactic macromolecules contributes to a higher orientation ability of the polymer. The higher orientation ability of polypropylene, in turn, allows increasing the speed of processing and increasing the volume of film production.

From the document EP2143116 a heat-resistant film is known, at least one layer of such film contains polypropylene having a melt flow rate from 0.5 to 15 g/10 min and xylene cold soluble fraction value of less than 3.5 wt.%; the degree of crystallinity determined by X-ray diffraction ranging from 0.5 to 0.85 and xylene and heptane insoluble fraction content of more than 94% by weight. The film additionally contains at least a second layer containing a heterophasic statistical copolymer (impact copolymer), where the heterophasic statistical copolymer has a melt flow rate of from 0.5 to 15.0 g/10 min and a melting point of from 120 to 170°C. The film is characterized by high tear strength, impact strength and puncture resistance due to the low content of the xylene cold soluble (XCS) fraction. The disadvantage of the solution proposed in EP2143116 is the slow processing speed of polypropylene due to the low content of xylene cold soluble fraction.

From EP0925912 a high metal adhesive film and metallized film are known, the core layer of which is made of isotactic polypropylene having an isotactic pentad [mmmm] content of 88% or more, preferably 90% or more, Mw/Mn is from 2 to 6. However, at the same time, other characteristics of isotactic polypropylene (the content of fractions having certain molecular weights and fractions having a solubility in xylene and heptane) have not been disclosed. The processing speed is also not specified.

Also, from EP0831994, a method of producing a BOPP film is known, where the core layer of the film contains either polypropylene with atacticity of at least 10% or a polymer composition which comprises a mixture of an isotactic polymer with a degree of atacticity of less than 5% and atactic polypropylene, syndiotactic polypropylene, ethylene-propylene copolymers, propylene terpolymers, polybutene or linear low density polyethylene. The use of polypropylene or polymer composition with a degree of atacticity of 10% or more allows obtaining a BOPP film with high tear resistance. There are no other requirements to the characteristics of isotactic polypropylene. Consequently, the content of fractions having certain molecular weights and fractions having a certain solubility in xylene and heptane is not considered as a criterion for determining suitability of the polypropylene for use in film processing.

Thus, the complete combination of structural characteristics of polypropylene for use in BOPP films which allows processing at a speed of at least 450 m/min while maintaining the required level of film strength characteristics in accordance with regulatory documents is not known now.

Summary of the Invention

An object of the present invention is to determine the combination of structural characteristics of polypropylene that allows to increase the efficiency of the process of biaxial orientation of polypropylene to obtain a BOPP film.

The technical result of the present invention resides in the increase of the productivity of the process of obtaining BOPP films by using the polypropylene in the film composition, the structural characteristics of the polypropylene making it possible to process the film at the speed of at least 450 m/min.

An additional technical result resides in the possibility of obtaining a wide range of films: transparent, matte, filled, general purpose, etc. as well as the possibility of applying layer coatings, for example, a metallized layer, printing, etc.

The object of the present invention is solved and the technical result is achieved by introducing a polypropylene into the film, which polypropylene has the following characteristics:

- the amount of the xylene cold soluble fraction (XCS) falls within the range from 2.5 to 4.0 wt.%, wherein the molecular weight of the fraction is mainly within the range of from 1,500 to 50,000 g/mol the proportion of the fraction with molecular weight exceeding 50000 g/mol does not exceed 30 wt.%;

- the amount of the heptane soluble (HS) fraction in the polymer falls within the range from 2.5 to 3.5 wt.%, wherein the proportion of macromolecules having a molecular weight of from 400,000 to 3,000,000 g/mol is 10 wt.% or less and the proportion of components with a molecular weight from 1,500 to 50,000 g/mol is at least 60 wt.%.

The inventors found out that a certain content of xylene-soluble and heptane- soluble fractions characterized by certain values of molecular weights allows to increase the speed of the process of obtaining biaxially oriented films by improving the polypropylene orientation. While not wishing to be bound by any particular theory, the inventors believe that a high (60 wt.% or more) content of low molecular weight (from 1,500 to 50,000 g/mol) heptane-soluble fractions improves the orientation of polymer macromolecules during processing, which, in turn, allows to increase the speed of processing up to 450 m/min and above.

Detailed Description of the Invention

The following is a detailed description of various aspects and embodiments of the present invention.

The polypropylene to be used in the film preparation is characterized by the following:

- the content of xylene cold soluble (hereinafter referred to as XCS) fraction is in the range of from 2.5 to 4.0 wt.% preferably in the range from 3.0 to 3.8 wt.%; - the content of heptane soluble HS fractions (hereinafter referred to as HS), is in the range of from 2.5 to 3.5 wt.%.

A distinctive feature of the claimed polypropylene to be used in the manufacture of BOPP films is the molecular weight of XCS fraction being mainly in the range of from 1,500 to 50,000 g/mol, whereas the amount of XCS with a molecular weight of more than 50,000 g/mol does not exceed 30 wt.%, preferably does not exceed 15 wt.%, more preferably does not exceed 5 wt.%.

Another essential feature of the claimed polypropylene to be used in the manufacture of BOPP films is that the amount of HS fraction with a molecular weight of from 400,000 to 3,000,000 g/mol does not exceed 10 wt.%, preferably is not more than 7 wt.%, most preferably is not more than 5 wt.%, and the amount of HS with a molecular weight of from 1,500 to 50,000 g/mol is at least 60% by weight, preferably at least 70 wt.%, most preferably at least 75 wt.%.

When the amount of the heptane-soluble fraction is below 2.5 wt.%, or the composition of the HS fraction changes toward reducing the proportion of the low molecular weight component (having molecular weights of from 1,500 to 50,000 g/mol) and/or increasing the proportion of the high molecular weight component (having molecular weights of from 400,000 to 3,000,000 g/mol) at the total content of the HS fraction staying in the specified range of 2.5 to 3.5 wt.%, this results in a decrease of the polymer processability.

The amount of the heptane soluble fraction of more than 3.5 wt.% leads to deterioration of physico-mechanical properties of the film, as well as to an increased formation of carbon on the spinneret during processing.

The processability of the film and its properties are also influenced by changes in the amount and composition of XCS fraction.

Thus, when the amount of XCS fraction is below 2.5 wt.%, then the processability of the film decreases, which is due to degradation of the orientation ability of the macromolecules of the used polymer regardless of its molecular weight characteristics.

When using a polymer having an amount of XCS fraction of more than 4.0 wt.% for the preparation of the film, a decrease in its physico-mechanical properties is observed, as well as an increase in the formation of carbon deposit on the spinneret during the polymer-to-film processing.

Preferably the molecular weight distribution of the polypropylene is of from 5 to 7, more preferably from 6 to 7.

Preferably the content of the isotactic fraction of the polymer [mmmm] is of from 87 to 89%;

The present invention provides a polymer for the preparation of BOPP films at high processing speed (at least 450 m/min), which can be obtained using supported Ziegler-Natta titanium-magnesium catalysts of the general formula MgCl2/TiCl4/D1/D2/TEA, where titanium tetrahalides are supported on magnesium halides, D1 is an internal electron donor, D2 is an external electron donor, triethylaluminum (TEA) is a co-catalyst. In particular, Di may be selected from the following of compounds:

Group of alcohols: preferred alcohols are compounds of the formula R'OH, where the R ¹ group is represented by C1-C20 hydrocarbon group. In a more preferred embodiment, R1 is a C1-C20 alkyl group. Specific examples are methanol, ethanol, isopropanol and butanol.

Group of amines: preferred amines are compounds of the formula NR ² 3, where the R ² groups, independently of one another, are hydrogen or C1-C20 hydrocarbon group, provided that not all of them are simultaneously hydrogens. In a more preferred embodiment, R ² is a C1-C20 alkyl group. Specific examples are diethylamine, diisopropylamine and triethylamine.

Group of amides: preferred amides are represented by the formula R ³ CONR ⁴ 2, where R ³ and R ⁴ , independently of one another, are hydrogen or C1-C20 hydrocarbon group. Specific examples are formamide and acetamide.

Group of esters: preferred esters can be selected from aromatic carboxylic acid monoesters such as benzoates, in particular, C1-C20 alkyl benzoates, and monoesters of aliphatic carboxylic acids, such as Ci-Cs alkyl esters of aliphatic monocarboxylic acids, and also 1,8-naphthyl diesters.

Another group comprises C1-C20 alkyl esters of aromatic dicarboxylic acids, such as phthalates, and C1-C20 alkyl esters of aliphatic dicarboxylic acids, such as malonates, succinates and glutarates. In addition, diesters of diols can also be used.

As D2, the organosilicon compound represented by the following formula R _n Si(OR)4-n can be used, where R is a hydrocarbon radical, OR is an alkoxy group, R is a hydrocarbon group, and n is an integer of 0 < n < 4. Examples of the organosilicon compounds represented by this formula which may be used in the present invention include the following: diisopropyldimethoxysilane, tert-butylmethyldimethoxysilane, tert-butylmethyldiethoxysilane tert amylmethyldiethoxysilane,, dicyclohexyldimethoxysilane, cyclohexylmethyldimethoxysilane, cyclohexylmethyldiethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, tert- butyltriethoxysilane, phenyltriethoxysilane, cyclohexyltrimethoxysilane, cyclopentyltrimethoxysilane, 2-methylcyclopentyl trimethoxysilane, cyclopentyltriethoxysilane, n-propyltrimethoxysilane, dicyclopentyldimethoxysilane, dicyclopentyldiethoxysilane, tricyclopentylmethoxysilane, dicyclopentylmethylmethoxysilane, dicyclopentylethylmethoxysilane, and cyclopentyldimethylethoxysilane.

More specific examples of the described catalysts and methods for preparation thereof are disclosed in EP2221320, EP2638080, EP2951215 documents.

The polypropylene of the present invention can be obtained by gas phase or suspension polymerization of propylene in the presence of the above catalysts. In any of the polymerization processes used, the components of the catalytic system (catalyst, co catalyst and optionally an external electron donor) can be brought into contact with each other before adding them to the polymerization reactor. As the co-catalyst, an alkyl- A1 compound can be used, for example, triethyl aluminum, diethyl aluminum chloride, triisobutyl aluminum, tri-n-butyl aluminum, tri-n-hexylamine aluminum, tri-n-octyl aluminum.

Polymerization using a Ziegler-Natta catalyst is generally carried out at a temperature in the range of 50-80°C, preferably in the range of 65-75°C, and an operating pressure in the range of from 0.1 to 5.0 MPa, preferably in the range of from 0.5 to 3,5 MPa.

Preferably, in order to preserve the physico-mechanical properties of the produced film, the melt flow index (MFl230ºC/2.16 _kg ) of the used polymers determined according to ASTM D 1238, should be at least 3 g/10 min.

In particular, polypropylene can be obtained according to the processes described, for example, in the documents US8178633, EP2726517, etc. These methods for producing polypropylene, given as examples, allow to achieve the above characteristics, however, these methods are not exclusive. Polypropylene with the specified properties can be obtained in other ways, known from the prior art.

Preferably, the polypropylene contains stabilizers, which comprise at least a mixture of antioxidants and acid absorbers. Any antioxidants known from the prior art can be used as antioxidants, preferably a mixture of phosphite and phenol antioxidants is used. As an acid absorber any acid absorber known from prior art, except for the absorbers from the class of metal stearates, can be used. Besides, other known additives can be additionally used to allow polypropylene to retain its properties during processing and operation. The amount of stabilizers added to the polypropylene is from 1 to 3 kg per ton of polypropylene, preferably from 1.2 to 2.5 kg per ton of polypropylene, more preferably from 1.2 to 1.5 kg per ton of polypropylene.

The polypropylene according to the invention is substantially free from calcium stearate. Calcium stearate transforms into stearic acid when interacting with acids, which readily migrates to the surface of the film making it impossible to produce a metallized film. The term“substantially free” means a content of less than 0.010% by weight, preferably less than 0.005% by weight, most preferably less than 0.001% by weight.

The specified polypropylene can be used both as a primary and as an additional component of any film layer of any formulations known from the prior art. The exact structure of the film (the number and order of layers) and the content of the layers depend on the requirements for the film and articles made thereof.

In addition to the polypropylene of the invention, the film may contain other polyolefins as well as functional polymers. Copolymers and terpolymers of a-olefins are used as polyolefins, specifically, copolymers of propylene and ethylene, propylene and butene, terpolymers of propylene, ethylene and butene, etc. The following compounds are used as functional polymers: copolymers of ethylene with vinyl alcohol, polyvinylidene chloride, copolymers of vinylidene chloride, polyesters, polyamides (to impart gas and/or aroma-proof properties to the film), interpolymers of ethylene and a- olefin (for adhesive-free paper-to-film lamination), maleic anhydride homopolypropylene (adhesive layer in coextrusion films), etc. In the first embodiment of the present invention, the film is intended for the preparation of wrapping paper, as the base for adhesive tapes, etc. and is a multi-layer polypropylene film, each layer of which consists of polypropylene that has the above characteristics, as well as of antistatic substances and/or anti-caking substances and/or antioxidant. The antiblocking agents are silicon dioxide (SiO ₂ ), polymethyl methacrylate (PMMA), and the like, and the antioxidant is a phenolic antioxidant.

In the second embodiment of the invention, the film is intended for adhesive- free film-to-paper lamination and includes at least the following: a core layer containing polypropylene having the above characteristics, and a functional layer comprising: ethylene-butene copolymer, ethylene-octene copolymer, ethylene-butene-octene terpolymer, ethylene-butene copolymer modified with grafted maleic anhydride, ethylene-octene copolymer modified with grafted maleic anhydride, ethylene-butene- octene terpolymer modified by grafted maleic anhydride or mixtures thereof, or mixtures formed from any of the listed copolymers and/or terpolymers, modified copolymers and/or terpolymers, and blends with hydrogenated petroleum resin.

In the third embodiment of the invention, the film is a barrier film and contains at least the following layers: one layer of propylene polymer that has the above characteristics, and a layer of gas barrier material represented by polyamide, a copolymer of ethylene and vinyl alcohol, etc. A layer of modified polyolefin, for example, a layer of propylene polymer modified with maleic anhydride may optionally be used to improve the adhesion between the layer of propylene polymer meeting the above characteristics and the layer of gas barrier material.

Other embodiments of the present invention are clear to a person skilled in the art and are not disclosed in detail in the description of the invention. For example, additives can be included in the formulation in effective amounts, i.e. in quantities that provide the desired functional properties or improve the parameters and/or functionality of the finished film. In particular, examples of additives include, but are not limited to the following: additives that offer anti-block, anti-static and slip effects, as well as antioxidants and neutralizers, technological additives, nucleating agents, additives that reduce the effect of UV radiation, and UV-absorbers, dyes, fillers, thickeners, modifiers based on hydrocarbon resins, etc.

The total thickness of the films varies over wide range depending on the intended purpose of its use. In the preferred embodiments, the film has a total thickness of from 2 to 100 mm, preferably from 5 to 50 pm, more preferably from 10 to 30 mm.

The film of the present invention is obtained by coextrusion of polypropylene with other polymers followed by biaxial orientation. The orientation value of the film in the Machine Direction is from 4.5 to 5.5, in the Transverse Direction - up to 10.

Films prepared according to the present invention after the drawing can be subjected to processing by the following procedures:

1. embossing on the surface of the film;

2. film surface activation/modification to make it suitable for printing (for example, by plasma treatment, corona treatment, and flame treatment);

3. laminating the prepared film on a woven or non- woven material, in particular, lamination on a paper, foil or other films;

4. depositing a metallic layer (for example, depositing aluminum layer by vacuum technology that is based on the condensation of a metal vapor);

5. applying an adhesive layer onto one or both surfaces of the film, thus obtaining an adhesive film.

Optionally, the treated film is used to make the final product such as packaging, metallized packaging, label, bag, adhesive tape and other articles using the BOPP films according to the invention. Specific examples include, but are not limited to: making a label as specified in EP2197669, WO2017077184, making a bag as disclosed in particular in US9108391document, preparing food packaging as indicated, for example, in WO2016205381, producing a metallized film as described in EP0925912.

Embodiments of the invention

Test methods

1. The melt flow index (MFI) was determined in accordance with ASTM

D1238.

2. The mass fraction of the soluble fraction in boiling heptane/isotactic fraction was determined according to National State Standard 26996-86.

3. The mass fraction of the fraction soluble in xylene was determined according to ISO 16152.

4. Determination of molecular weight distribution (MWD) of polypropylene samples and statistical copolymer of propylene and ethylene was performed by Gel Permeation Chromatography (GPC) on the Agilent PL-GPC 220 system according to ISO 16014-4-2012: High-temperature method. The dissolution temperature was 150°C, the solvent was 1,2,4-trichlorobenzene.

5. Determination of molecular weight properties of xylene-soluble (XCS) and heptane-soluble (HS) substances from the samples was carried out by the method of low-temperature GPC on an Agilent 1200 liquid chromatograph, (Agilent) according to ISO 16014-4-2012: Low-temperature method. The measured dissolution temperature was 40 °C, the solvent was tetrahydrofuran.

6. The microstructure, the degree of isotacticity, and the pentad ratio of polypropylene samples were determined by the method of high resolution nuclear magnetic resonance spectroscopy on carbon nuclei ( ¹³ C NMR) on Bruker Avance III 400 MHz NMR spectrometer. For research purposes, a 250 mg of the sample were dissolved in 2.5 ml of trichlorobenzene with heating to 140°C. The number of scans on ¹³ C nuclei is 16,000. The temperature of measurements was 140°C.

The invention is described in more detail by the examples below. Examples are provided to illustrate the present invention and do not limit the scope thereof.

Example 1.

Propylene polymer having MFI 3.0 g/10 min (at 230°C and 2.16 kg) was obtained by gas-phase technology at a temperature of 65-75°C and a working pressure of 2.2 MPa. A catalyst prepared in a similar way as in example 1 of US9284392 was used as a catalyst, except that diisobutyl dimethoxysilane was used as an external electron donor.

The BOPP film was a five-layer film, with each layer thereof consisting of the above polypropylene and antioxidants such as the hindered phenolic antioxidant, pentaerythritol tetraoxy (3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate) (Irganox 1010), and phosphorus-containing antioxidant, tris (2,4-di-tert-butylphenyl) phosphite (Irgafos 168), and also hydrotalcite as an acid absorber, in the amount of 1.35 kg of stabilizers per ton of polypropylene. The film was prepared by continuous extrusion of polypropylene, by a stepwise orientation, heat setting and cooling of the film slab. Molten materials plasticized in each of the extruders (five extruders corresponding to the number of layers) were combined in a die (head) and flowed confluent from a slot die onto a cooling drum (shaft) in the form of a slab, then the slab was cooled down by passing through a water bath. Next, the film slab was reheated to 100-115°C on heating cylinders and stretched up to 6 times its original length in the machine direction on the rollers. The film stretched in the machine direction was placed into an oven, where it was heated once again by air up to 170-180°C and stretched up in the transverse direction 10 times its original width. The total film thickness was 20 mm, while the outer layers were 0.9 pm thick, the intermediate layers were 2.5 pm thick, the core layer was 13.2 pm thick. The characteristics of the used polypropylene and the processing speed of the film are shown in Table 1.

Example 2

BOPP film was prepared as in Example 1, except that polypropylene had the characteristics shown in Table 1. The speed of film processing is shown in Table 1.

Example 3

The BOPP film was prepared as in Example 1, except for using polypropylene with the characteristics shown in Table 1. The speed of processing of the film is presented in Table 1.

Example 4 (Comparative)

The BOPP film was prepared as in Example 1, except for using polypropylene obtained using a Ziegler-Natta catalyst with the internal electron donor such as dibutyl phthalate (described in EPO 193281), and wherein the polypropylene has the characteristics shown in Table 1. The processing speed of the film is shown in Table 1.

Example 5 (Comparative)

The BOPP film was prepared as in Example 1, except that polypropylene was obtained using a Ziegler-Natta catalyst with the internal electron donor such as 9,9-bis- methoxymethyl-fluorene (described in US8003559), and wherein the polypropylene has the characteristics shown in Table 1. The processing speed of the film is shown in Table 1.

Example 6 (Comparative)

A BOPP film was prepared as in Example 1, except that polypropylene was obtained using a Ziegler-Natta catalyst with the multicomponent internal electron donor such as (3,3-bis (methoxymethyl) -2,6-dimethylheptane and diethyl 2,3 -diisopropyl succinate) (as described in US20160102159), and wherein the polypropylene has the characteristics listed in Table 1. The processing speed of the film is shown in Table 1. Example 7 (Comparative)

A BOPP film was prepared as in Example 1, except that polypropylene was obtained using a Ziegler-Natta catalyst with the multicomponent internal electron donor such as (9,9-bis-methoxymethyl-fluorene and diethyl 2,3 -diisopropyl succinate) (described in US8003559), and wherein the polypropylene has characteristics listed in Table 1. The processing speed of the film is shown in Table 1.

Table 1. Characteristics of polypropylene used in the preparation of films, and the speed of processing

It can be seen from the Table that the processing speed of polypropylene depends on the totality of its structural characteristics. The increased amount (in comparison with polypropylene according to the invention) of xylene soluble and heptane soluble polypropylene fractions with high amount of macromolecules having high molecular weight (more than 50,000 g/mol for XCS and from 400,000 to 3,000,000 g/mol for HS) (as in comparative Examples 4, 6 and 7) does not allow to increase the processing speed up to 450 m/min. In case of using polypropylene having the amount of the XCS fraction identical to the polypropylene according to the invention, but having the amount of the heptane soluble fraction not falling within the range disclosed in the invention (Example 5), the processing speed of 450 m/min is also not achieved due to the excess of high molecular weight components in the heptane soluble fraction. Considering the results obtained for Examples 1, 2, 3, wherein the values of heptane- and xylene soluble fractions with the indicated molecular weights are within the ranges disclosed in the invention, the achievement of high speeds during processing as a result of conforming to the required totality of the features can be mentioned.

Thus, to achieve a processing speed of 450 m/min or higher, the following totality of the features has to be observed:

- the amount of the xylene soluble fraction (XCS) polymer falls within the range from 2.5 to 4.0 wt.%, wherein the molecular weight of the macromolecules is mainly within the range from 1,500 to 50,000 g/mol and the proportion of components with molecular weight exceeding 50,000 g/mol does not exceed 30 wt.%;

- the amount of the heptane soluble fraction (HS) in the polymer falls within the range from 2.5 to 3.5 wt.%, wherein the proportion of macromolecules with a molecular weight from 400,000 to 3,000,000 g/mol is 10 wt.% or less and the proportion of components with a molecular weight from 1,500 to 50,000 g/mol is at least 60 wt.%.