Patent Description:
In the prior art there are known materials based on high-density, medium-density and low-density polyethylenes (PE) produced by various methods, both by a radical mechanism under pressure and temperature (LDPE (HPPE)) and by a catalytic method under low pressure and temperature (homo- and co-polymers (HDPE (LPPE)) and copolymers of ethylene with higher α-olefins, such as linear low-density PE (LLDPE) and medium-density PE (MDPE)). Also known are compositions produced on the basis of these individual types of PE aimed at use in certain narrow fields of technology due to a limited complex of their properties.

Thus, LDPE-based compositions are known for use in the cable industry. For example, LDPE-based compositions for the sheath of electrical cables of <NUM>-<NUM> grade have the following main characteristics: a tensile yield strength (σy) of not less than <NUM> MPa; a tensile strength (σ) of not less than <NUM> MPa; a relative elongation at break (εr) of not less than <NUM>%; a resistance to cracking of not less than <NUM> hours, and an MFR(<NUM>/<NUM>) of not less than <NUM>/<NUM>. However, even a slight increase in the MFR of such a composition to increase the productivity of cable lines leads to a sharp drop in its resistance to cracking. Other disadvantages of the presented composition are insufficient strength, heat resistance (Vicat heat resistance, 10N of not more than <NUM>), low surface hardness, which gives it a low wear resistance, thereby narrowing possible applications of such a composition and reducing its use in both cable and pipe industries and in other industrial sectors.

Improved deformation-strength characteristics, including at low temperature, are shown for compositions based on low-density LPE and medium-density PE (LLDPE and MDPE). Thus, patent <CIT> provides an LLDPE-based composition that is characterized by an MFR of <NUM>-<NUM>/<NUM> and a broad molecular weight distribution (MWD). A peculiarity of such a composition is its relative elongation at break (εr) determined at -<NUM>, which is not less than <NUM>%. The disclosed properties of the composition permit its use in the pipe industry as an anticorrosive coating for steel pipes intended for use, in particular, in cold climate regions.

Application <CIT> describes a composition based on monomodal LPEs of low and medium density with an expanded spectrum of applications provided by an increase in the melt flow rate (MFR, from <NUM> to <NUM>/<NUM>) and a narrowing of the MWD (Mw/Mn, from <NUM> to <NUM>).

However, the main disadvantages of the LPE-based composition are insufficient heat resistance, processing difficulties (pulsation and low melt strength), low barrier properties, and a number of others, which also narrows possible applications of such compositions.

These disadvantages can be eliminated by using in the composition of polyethylene its homo- and copolymer of high density, HDPE. The main advantage of this material over all types of polyethylene is a significant amount of a crystalline phase in its structure, which makes it superior primarily in strength characteristics, heat resistance and barrier properties. However, on the other hand, an increased crystallinity also entails a number of significant drawbacks: reduced resistance to cracking of easily flowable compositions (at an MFR(<NUM>/<NUM>) of ><NUM>/<NUM>, its resistance to cracking is less than <NUM>) and hardness (flexural modulus is higher than <NUM> MPa). In addition, because of a rate of crystallization, HDPE is characterized by unsatisfactory optical properties; in particular, the haze of a film made of HDPE is more than <NUM>%.

A disadvantage, such as low resistance to cracking of easily flowable compositions based on HDPE, can be eliminated by using bimodal HDPE in the composition, which comprises two fractions with different molecular weight characteristics. The low molecular weight part ensures its good processibility, and the and even ultra high molecular weight fraction solves the problem of providing such a composition with a required level of strength characteristics, heat resistance and resistance to cracking.

<CIT> provides a bimodal HDPE composition. According to this invention, the melt flow rate of the low molecular weight fraction of the PE is from <NUM> to <NUM>/<NUM>. The melt flow rate of the high molecular weight fraction provides the total MFR of the composition of from <NUM> to <NUM>/<NUM>. According to <CIT>, the molecular weight fraction of bimodal HDPE can also be copolymer.

A trimodal PE molding composition for coating steel pipes, having a high resistance to cracking is known from <CIT>. The density of the composition is from <NUM> to <NUM>/cm<NUM>. The composition comprises <NUM>-<NUM> wt. % of low molecular weight PE, <NUM>-<NUM> wt. % of high molecular weight copolymer of ethylene with higher C<NUM>-<NUM> α-olefins, and <NUM>-<NUM> wt. % of ultrahigh molecular weight copolymer of ethylene with C<NUM>-<NUM> α-olefins. The MFR(<NUM>/<NUM>) of the composition is from <NUM> to <NUM>/<NUM>.

Patents <CIT>, <CIT>, and <CIT>provide a multimodal composition with parameters and properties varying in wide ranges. The most balanced composition of a bi- and multimodal copolymer intended for external insulation of steel pipes operating at low temperatures (down to -<NUM>) is provided in application <CIT>. A peculiarity of the polymer contained in the composition is a narrow range of permissible values of density (from <NUM> to <NUM>/cm<NUM>), which provides an advantage of this material in the parameter of relative elongation at break at a temperature of -<NUM> (up to <NUM>-<NUM>%).

However, multimodal HDPE-based compositions have low values of hardness, elasticity and impact resistance at low and ultralow temperatures due to a high crystallinity of HDPE. They also have unsatisfactory optical properties, which in turn excludes their use in the production of film materials.

Possible ways of solving the problem of improving the balance between low temperature properties and heat resistance of polyethylene compositions are available from the prior art. These approaches are based on the use of a binary combination of HDPE and LLDPE in a composition. Thus, <CIT> has succeeded in improving simultaneously the resistance to penetration at temperatures of up to <NUM> and high values of low-temperature characteristics of polyethylene pipe insulation, by using a mixture comprising <NUM> to <NUM> wt. % of LLDPE and <NUM> to <NUM> wt. The relative elongation at break (εr) determined for this composition at a temperature of -<NUM> is more than <NUM>%. A disadvantage of the composition is its high viscosity, an MFR(<NUM>/<NUM>) of less than <NUM>/<NUM>, limiting its wide application.

The invention according to <CIT>relates to a composition for coating steel pipes, wherein the composition is based on a mixture of LPE and HDPE and is characterized by the content of HDPE of <NUM> to <NUM> wt. % and LLDPE of <NUM> to <NUM> wt. The HDPE component comprises a mixture of grades for different purposes: <NUM> pipe grade, <NUM> hollow grades, <NUM> film grade, and <NUM> wiredrawing grade, in a certain ratio. In addition, this composition includes <NUM> to <NUM> wt. part of an antioxidant, <NUM> to <NUM> wt. parts of a processing additive, and <NUM> to <NUM> wt. parts of carbon black masterbatch. The proposed material is characterized by a balance of strength of from <NUM> to <NUM> MPa, heat resistance of from <NUM> to <NUM>, and impact strength at low temperatures. A disadvantage is the composition complexity and a low MFR(<NUM>/<NUM>) of <NUM> to <NUM>/<NUM>, which limits its processibility.

Other combinations of polyethylenes with different structural organization of macrochains are also known from the prior art. These compositions are binary mixtures composed of LLDPE and LDPE (HPPE). The properties of such a composition include a combination of properties of individual components constituting this composition. Thus, <CIT> shows that compositions based on a mixture of LLDPE and LDPE demonstrate a higher durability of the stabilizing action of phenolic antioxidants with a MW of not less than <NUM> c. , after exposure of the material to hot water for at least one month. The ratio of LLDPE to LDPE can vary in a wide range: from <NUM> to <NUM> wt. % for LLDPE and from <NUM> to <NUM> wt. % for LDPE. The proposed material is used for coating steel pipelines. Its disadvantage is a low heat resistance; thus, TVicat, <NUM> is less than <NUM>.

Patent <CIT> provides a composition for power cables, with improved processing and dielectric properties achieved due to addition of <NUM> to <NUM> wt. % of LDPE to the basic LLDPE. In order to improve resistance to water tree, the composition can further comprise <NUM> to <NUM> wt. % of polyethylene having <NUM> to <NUM> wt. % of grafted polar groups. A disadvantage of this composition, similarly to the previous one, is a low heat resistance (TVicat, <NUM> of less than <NUM>) and strength parameters (tensile strength of less than <NUM> MPa).

The composition disclosed in <CIT> is based on LLDPE and LDPE in order to expand the applications of the materials that can be used not only for insulation of steel pipes but also for lamination of various solid surfaces. This composition comprises <NUM> to <NUM> wt. % of LDPE and <NUM> to <NUM> wt. % of LLDPE. The LDPE used in the composition is characterized by an MFR(<NUM>/<NUM>) of from <NUM> to <NUM> and a density of from <NUM> to <NUM>/cm<NUM>. LLDPE has the following characteristics: its MFR(<NUM>/<NUM>) is from <NUM> to <NUM> and the density is from <NUM> to <NUM>/cm<NUM>. The composition demonstrates anticorrosion resistance, abrasive wear resistance, resistance to chemicals, a good processability, a high hardness, and an improved impact resistance at low temperatures. Its disadvantage, similarly to the previous analogous binary systems, is low heat resistance, strength and resistance to penetration at elevated temperatures, insufficient for a number of applications.

A binary mixture of multi-(bi)modal HDPE and LDPE is also known from the prior art. The main purpose of this combination is to eliminate or minimize drawbacks in the rheology of the melt of materials based on multi-(bi)modal HDPE, which are low values of melt strength (MS), draw resonance, and neck-in of the melt of multi-(bi)modal HDPE extruded from a die. Similar problems are also characteristic for LLDPE. They result from the linear nature of the macrochains of LLDPE, as well as in HDPE. Thus, a method of combining polyethylenes having linear macrochains with high and long-chain-branched LDPE makes it possible to solve similar problems. However, a binary combination of multi-(bi)modal HDPE and LDPE is more advantageous compared with a mixture of LLDPE and LDPE due to its obvious advantages in heat resistance, strength, and barrier properties.

<CIT> provides a mixture of <NUM> to <NUM>% (preferably from <NUM> to <NUM> wt. %) of multi-(bi)modal HDPE with a density of from <NUM> to <NUM>/cm<NUM>, consisting of at least two fractions: low molecular weight homopolymer of ethylene with a density of <NUM>/cm<NUM> and an MRF(<NUM>/<NUM>) of from <NUM> to <NUM>/<NUM> and high molecular weight copolymer with a comonomer content of from <NUM> to <NUM> mol% at the total MRF of the multi-(bi)modal HDPE of from <NUM> to <NUM>/<NUM>, with <NUM>-<NUM>% (preferably from <NUM> to <NUM> wt. %) LDPE with a density preferably of from <NUM> to <NUM>/cm<NUM> and an MFR(<NUM>/<NUM>) of from <NUM> to <NUM>/<NUM>, wherein the LDPE has highly branched macrochains: at least <NUM> branches per molecule with a preferable length of the branches of not less than <NUM> carbon atoms. It should be noted that the binary composition based on multi-(bi)modal HDPE and LDPE is intended for a narrow field of applications, in particular, only for laminating and coating solid surfaces with a thin polymer barrier layer, due to high MFR values of both polymer components composing the composition: the MFR of multi-(bi)modal HDPE is not less than <NUM>/<NUM> and the MFR of LDPE is not less than <NUM>/<NUM>. It is evident that a consequence of such a high flow rate of the present composition will be a significant drop in such parameters as resistance to cracking and melt strength. In addition, unavoidable drawbacks of the binary mixture of multi-(bi)modal HDPE and LDPE remain the level of low-temperature properties which is insufficient in comparison with LLDPE, relatively low deformation-strength characteristics, such as strength and relative elongation at break, poor optical properties, and a number of others. In addition, significant problems arising with a binary mixture of high-crystalline multi-(bi)modal HDPE and highly branched LDPE are due to thermodynamic incompatibility of these polymer matrices, which is manifested in particular at a low rate or uneven cooling of the melt of this composition. This processing mode causes a noticeable loss of all properties and the surface quality of a product.

It is possible to achieve high values of difficultly combined characteristics, such as tensile strength, heat resistance, surface hardness, elasticity, and extensibility at low and ultralow temperatures, as well as a high stress-cracking resistance and melt strength, which would make it possible to expand the spectrum of applications of such compositions if at least three polymers with different structural organization of macrochains are included in one composition. However, frequently, the main disadvantage of such compositions is the thermodynamic and composite incompatibility of their components.

A composition comprising three components (<CIT>) is closest to the claimed composition. This composition is intended for the production of extrusion coating and films.

According to the embodiments, this composition is produced by dry blending the components. This composition is characterized by the following characteristics: tensile strength at break of <NUM>-<NUM> kPa, elongation of <NUM>-<NUM>%, and puncture resistance of <NUM>-<NUM> n/cm<NUM>, which allow the composition to be used for the production of films. However, a low resistance to cracking (<NUM> hours) does not allow expanding the spectrum of its applications.

Thus, there is still a need for a composition with a unique combination of properties, allowing significantly broadening the spectrum of its applications.

The object of the present invention is to provide a composition characterized by a combination of the following features: high values of tensile strength, heat resistance, surface hardness, elasticity, and extensibility at low and ultralow temperatures, as well as high stress-cracking resistance and melt strength.

The object is addressed and the technical result is achieved by using in a composition several polyethylenes with structurally different macrochains, in particular, low-density polyethylene (LDPE), linear polyethylene (LPE), and high-density polyethylene (HDPE), as well as nucleating agents that control the crystallization rate of macromolecules.

The composition according to the present invention makes it possible to increase the tensile strength up to <NUM> MPa, is characterized by a Vicat (<NUM>) heat resistance of up to <NUM>, Shor D surface hardness of up to <NUM> c. , high elasticity, in particular relative elongation at break of up to <NUM>%, and an extensibility (relative elongation at break) at low and ultralow temperatures (-<NUM>) of up to <NUM>%. In addition, the composition according to the invention has a high stress-cracking resistance of not less than <NUM> and a melt strength of up to <NUM> cN, which is important in a number of applications. The films obtained on the basis of the composition according to the invention are characterized by a high tensile strength, as well as by a high relative elongation at break in the longitudinal and transverse directions. In addition, the composition according to the invention is characterized by satisfactory optical properties (transparency) and surface quality (smoothness, uniformity).

The preparation of a composition with such combination of properties allows its universal application.

In particular, the preparation of the composition characterized by: an MFR(<NUM>/<NUM>) of more than <NUM>/<NUM>, a resistance to cracking of more than <NUM>, a tensile strength of more than <NUM> MPa, a relative elongation at break of more than <NUM>%, a Shore hardness of more than <NUM> c. , a Vicat heat resistance of above <NUM>, and a good surface quality (smoothness and homogeneity of the surface) allows the composition to be used as the outer sheath of fiber-optic cables, as well as for insulation of steel pipes.

For the production of film materials, the following key properties must be satisfied: a tensile strength of <NUM> MPa or more, an elongation at break of not less than <NUM>%, a longitudinal/transverse strength of film of greater than <NUM>/15MPa, a longitudinal/transverse elongation of film of more than <NUM>/<NUM>%, a film tear resistance in the longitudinal/transverse direction of more than <NUM>/<NUM> kgf/cm, transparency, gloss, and smoothness of the surface. In addition, an important property is the melt strength of a composition (not less than <NUM> cN).

Further, the composition, which has a melt strength of greater than <NUM> cN and more, and a Vicat heat resistance of <NUM> or more, allows its use for the production of foamed materials.

Key requirements for the properties of a composition, which allow the use of the composition for insulation of electrical cables, are: a specific volumetric electrical resistivity of <NUM><NUM> Ω·cm or more, an electrical strength of more than <NUM> kV/mm, and a brittleness temperature of not less than -<NUM>.

A contribution of various polyethylenes to the final properties of compositions is specific. Thus, the presence of HDPE as the most highly crystalline component in a composition improves the strength characteristics of the composition, surface hardness and heat resistance. An increase in the relative content of LPE in the composition improves elasticity (relative elongation at break), especially at low and ultralow temperatures, resistance to cracking, and optical properties of the films produced on its basis. The contribution of LDPE is a reduction in the viscosity and an increase in the melt strength, which improves processibility of the composition, reduces technological and energy consumption, and provides new products that cannot be produced from HDPE/LPE binary compositions: foamed materials, blow-molded films and containers, etc. In addition, the presence of LDPE in the composition reduces negative effects during lamination: drow-resonance and neck-in for linear polymers (HDPE and LPE).

However, the use of a mixture of polyethylenes with certain properties is known to be insufficient to obtain a high-quality composition characterized by high physicochemical properties. In general, it is explained by lack of compositional compatibility between polyethylenes with different structures. The last circumstance is one of the main reasons for phase separation and significant deterioration of the characteristics of the composition, in particular its strength characteristics. A reason for the compositional incompatibility in the composition may consist in that the polyethylenes (HDPE, LDPE, LPE) comprise macromolecules with different degrees of branching and different molecular weights and are characterized by different crystallization rates. This results in phase separation during crystallization of polyethylenes, causing the formation of crystalline and amorphous regions. The compatibility between crystalline and amorphous regions depends on what, how and in which way the interfacial region in said polyethylenes is formed.

In this connection, due to different structures of polyethylenes used in the composition and different crystallization rates there is a need to provide their compositional compatibility.

The inventors have unexpectedly found a significant improvement of the compositional compatibility and structural homogeneity of the composition consisting of at least three polyethylenes of different nature, while significantly improving the quality of such systems, which are provided by organic and/or inorganic nucleating agents used as a controller of the crystallization rate of macromolecules.

In general, nucleating agents are known to be introduced into polymer melt to control the crystallization process, artificially generating crystallization grains [<NPL>].

The rate and degree of crystallization of polymers, as well as the size of formed crystalline grains, are determined both by crystallization conditions (temperature and pressure) and by molecular weight characteristics (MW, MWD) and structural organization of polymer molecules. All recited factors have a direct effect on physical and mechanical, optical and other properties of polymers resulted from crystallization and products on their basis.

When using nucleating agents, the crystallization process of polymer begins and occurs on the surface of a new phase (i.e. on the surface of the agent) and differs by a high homogeneity throughout the polymer volume. The use of nucleating agents allows control of the crystallization parameters, thereby influencing the properties of crystalline polymer, which makes it possible to produce polymers with high crystallinity and low-sized crystalline grains similar in form, providing good optical and physical properties of such materials. Thus, nucleating agents are widely used in the crystallization process of polymers such as polypropylene, polyesters (polyethylene terephthalate), polyamides and the like.

However, there are polymers whose polymerization process does not depend on nucleating agents; such polymers are characterized by very high or very low crystal growth rates. For example, for a fast crystallizing polymer such as polyethylene, in particular high-density polyethylene (HDPE), the use of nucleating agents is unreasonable since they are either ineffective or weakly effective (http://plastinfo. ru/information/articles/<NUM>/).

Thus, the use of nucleating agents for the production of materials with a required set of operational and technological properties is widely described in literature, but there is no information on their use as additives that improve the compatibility between polyethylenes of different classes in their multicomponent compositions.

Given the fact that the crystallization process of phases of HDPE and LPE occurs separately and the crystallization of HDPE is ahead of the LPE crystallization, it can be assumed that the formed spatial structure of the HDPE crystalline phase inhibits the process of the LPE macromolecules capable of crystallization due to the uniformly distributed centers of the nucleating agent, thereby creating spatial barriers for them and increasing viscosity of the system as a whole. This inhibitory effect of the HDPE crystalline phase on the crystallization process of LPE macromolecules leads to that the LPE phase has insufficient time to reach a critical size of its domains and, thus, separate from the LDPE phase combined with it, including because the whole composition already has cooled down to this time and reached the viscosity value that prevents the translational flow of macromolecules relative to each other. Thus, the use of nucleating agents in a composition based on polyethylenes of different structural organization makes it possible to significantly improve their compositional compatibility and to provide a composition that is characterized by a set of properties that are absent in separate components.

The term "compositional compatibility" in the present invention means the mutual "miscibility" of polymers, which provides a composition with high physical and mechanical, and operational properties.

Thus, according to the present invention, a composition is provided that includes the following components:.

The LDPE used herein is a polyethylene produced by mechanism of radical-chain polymerization of ethylene under high pressure (up to <NUM> atm or more) in tubular or autoclave reactors. The LDPE used in the composition has an MFR(<NUM>/<NUM>) of from <NUM> to <NUM>/<NUM>, preferably from <NUM> to <NUM>/<NUM>, more preferably from <NUM> to <NUM>/<NUM>, a density of from <NUM> to <NUM>/cm<NUM>, preferably from <NUM> to <NUM>/cm<NUM>, more preferably from <NUM> to <NUM>/cm<NUM>, and a molecular weight in the range of from <NUM><NUM> to <NUM><NUM>, preferably from <NUM><NUM> to <NUM><NUM>.

The molecular weight according to the present invention means an average molecular weight, unless otherwise noted.

The content of LDPE in the composition is from <NUM> to <NUM> wt. %, preferably from <NUM> to <NUM> wt.

Any polyethylene of known trademarks or mixtures thereof can also be used as LDPE. For example, trademarks of LDPE that can be used include PE <NUM>-<NUM>, PE <NUM>-<NUM>, PE <NUM>-<NUM>, PE <NUM>-<NUM>, PE <NUM>-<NUM>, Novex 20P730, LDPE 19N430, CA <NUM>, MA <NUM> and others.

The LPE used herein is a polyethylene produced by a method of anionic-coordination copolymerization of ethylene and higher C<NUM>-<NUM> α-olefins under low pressure on Ziegler-Natta catalysts, according to standard industrial techniques. The LDPE used in the composition has an MFR(<NUM>/<NUM>) of from <NUM> to <NUM>/<NUM>, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>/<NUM>, a density of from <NUM> to <NUM>/cm<NUM>, preferably from <NUM> to <NUM>/cm<NUM>, and a molecular weight in the range of from <NUM><NUM> to <NUM><NUM>, preferably from <NUM><NUM> to <NUM><NUM>, and most preferably from <NUM><NUM> to <NUM><NUM>.

It is preferable to use LPE of low or medium density.

The content of LPE in the composition is from <NUM> to <NUM> wt. %, preferably from <NUM> to <NUM> wt.

Any polyethylene of known trademarks or mixtures thereof can also be used as LPE. For example, trade marks of LPE that can be used include XP <NUM><NUM>, XP <NUM>, XP <NUM>, 3306WC4, PE 5118Q, UF414C4, <NUM>, SABIC LLDPE 318B, SABIC LLDPE <NUM> BE, SABIC LLDPE R500035 and others.

The HDPE used herein is a polyethylene produced by a method of anionic-coordination copolymerization of ethylene and higher C<NUM>-<NUM> α-olefins under low pressure on Ziegler-Natta catalysts, according to standard industrial techniques, and/or a multi-(bi)modal polyethylene (multi-(bi)modal HDPE) produced according to a two-reactor scheme by a method of anionic-coordination homo- or copolymerization of ethylene and higher C<NUM>-<NUM> α-olefins under low pressure on Ziegler-Natta catalysts, according to standard industrial techniques.

The composition according to the invention can comprise both HDPE and multi-(bi)modal HDPE, and a mixture thereof.

HDPE used in the composition are characterized by an MFR(<NUM>/<NUM>) of from <NUM> to <NUM>/<NUM>, more preferably from <NUM> to <NUM>/<NUM>, and have a density of from <NUM> to <NUM>/cm<NUM>, preferably from <NUM> to <NUM>/cm<NUM>. The molecular weight of HDPE is from <NUM><NUM> to <NUM><NUM>, preferably from <NUM><NUM> to <NUM><NUM>, and multi-(bi)modal HDPE comprises a low molecular weight fraction with a molecular weight of from <NUM><NUM> to <NUM><NUM> and a high molecular weight fraction with a molecular weight of from <NUM><NUM> to <NUM><NUM>.

The content of HDPE and/or multi-(bi)modal HDPE in the composition is from <NUM> to <NUM> wt. %, preferably from <NUM> to <NUM> wt. % and from <NUM> to <NUM> wt. %, preferably from <NUM> to <NUM> wt. %, and more preferably from <NUM> to <NUM> wt. %, respectively.

Any polyethylene of known trademarks or mixtures thereof can be used as HDPE. For example, trademarks of HDPE that can be used include PE 6948C, PND-<NUM>-<NUM>, PE2NT22-<NUM>, PND <NUM>-<NUM>, SABIC HDPE B5205, SABIC HDPE B5429, SABIC HDPE F04660, PND PE30T-<NUM>, Yuzex <NUM>, P601 KU, H1000P and others.

The nucleating agent used herein can be an inorganic and/or organic compound known in the art.

Inorganic nucleating agents include carbon black (soot), talc, titanium dioxide, chalk, kaolin, silicon dioxide, preferably carbon black (soot) and talc.

Organic nucleating agents include dibenzylidene sorbitol derivatives, in particular, dibenzylidene sorbitol derivatives with a melting point of less than <NUM>, for example, <NUM>,<NUM>-dimethyl dibenzylidene sorbitol, <NUM>,<NUM>,<NUM>-trideoxy-<NUM>,<NUM>:<NUM>,<NUM>-bis-O-[(<NUM>-propylphenyl)methylene]nonitol sorbitol, salts of carboxylic, phosphonic and other organic acids of aliphatic, cycloaliphatic, aromatic, heterocyclic nature and others, for example, zinc stearate, and <NUM>,<NUM>-cyclohexane dicarboxylic acid calcium salt. It is preferable to use <NUM>,<NUM>-dimethyl dibenzylidene sorbitol, zinc stearate, and <NUM>,<NUM>-cyclohexane dicarboxylic acid calcium salt.

A mixture of an inorganic and an organic nucleating agent is possible as well.

The content of the nucleating agent in the composition is from <NUM> to <NUM>%, preferably from <NUM> to <NUM> wt.

It is preferable to use a dispersed nucleating agent with a particle size of more than <NUM>, preferably from <NUM> to <NUM>. In addition, a particle aspect ratio of <NUM>:<NUM> to <NUM>:<NUM>, preferably from <NUM>:<NUM> to <NUM>:<NUM>, is preferable. The term "aspect ratio" as used herein means a ratio of particle length to diameter.

In addition, the composition can comprise other additives such as antioxidants, heat stabilizers, light stabilizers, or mixtures thereof, and others. Such additives may be sulfur-containing antioxidants, phenolic or phosphite antioxidants, for example pentaerythritol ester of <NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxy-phenylpropionic acid (trademark, Irganox <NUM>), tri-(phenyl-<NUM>,<NUM>-di-tert-butyl)phosphite (trademark, Irgafos <NUM>), and/or similar heat stabilizers of other trademarks, and amine light stabilizers and stabilizers of other types, or synergistic mixtures of stabilizers of trademarks such as Irganox B225, Irganox B215 and others.

The content of other additives in the composition is from <NUM> to <NUM>%, preferably from <NUM> to <NUM> wt.

The composition according to the invention can be prepared by any method known in the art.

The composition can be prepared by mixing components in any order at a temperature higher than the melting points of the polyethylene components composing the composition and lower than their decomposition temperature.

One embodiment of the method for preparing the composition includes a step of previously preparing a mixture (a) comprising a nucleating agent and one of the polyethylenes used in the composition, preferably LPE or HDPE. When preparing the mixture (a), the whole amount of the nucleating agent comprised in the composition is used. The amount of the nucleating agent in the previously prepared mixture (a) is from <NUM> to <NUM> wt. %, preferably from <NUM> to <NUM> wt. The mixture (a) also can comprise an additive, for example an antioxidant, if used. Then, the mixture (a) is mixed with the remaining components of the composition.

When the composition comprises other additives, one embodiment of the method for preparing the composition includes a step of previously preparing a mixture (b) comprising another additive, for example an antioxidant, and one of the polyethylenes used in the composition, preferably LDPE. When preparing the mixture (b), the whole amount of the additive(s) comprised in the composition is used. The amount of said another additive in the previously prepared mixture (b) is from <NUM> to <NUM>%, preferably from <NUM> to <NUM> wt. Then, the mixture (b) is mixed with the remaining components of the composition.

In addition, in one embodiment of the method for preparing the composition, it is preferable to prepare previously a mixture (c) of HDPE and/or LDPE with LPE at a ratio of <NUM>:<NUM> to <NUM>:<NUM>, preferably <NUM>:<NUM>, which improves distribution of polymers throughout the composition. Preferably, the mixture (c) comprises HDPE and LPE. Then, the prepared mixture (c) is mixed with the remaining components of the composition.

The most preferable method for preparing the composition includes steps of preparing intermediate mixtures: a) a mixture of a nucleating agent and one of the polyethylenes used in the composition; b) a mixture of additives, if used, and one of the polyethylenes used in the composition; and c) a mixture of HDPE and/or LDPE with LPE at a ratio of <NUM>:<NUM> to <NUM>:<NUM>, preferably <NUM>:<NUM>. After that, the prepared intermediate mixtures (a), (b), and (c) are mixed with each other and with HDPE and/or LDPE and/or LPE, if necessary, so as to achieve in the composition a required content of each polyethylene, followed by processing in an extruder.

The components are mixed at a temperature higher than the melting points of the polyethylenes composing the composition and lower than their decomposition temperatures. Preferably, the temperature of mixing the components is from <NUM> to <NUM>, more preferably from <NUM> to <NUM>, and most preferably from <NUM> to <NUM>.

The components are mixed in any mixing equipment, in particular rotary blade mixers, rake mixers, screw plasticizers, planetary mixers, belt-screw mixers, pneumatic mixers, as well as blade mixers, propeller mixers, turbine mixers, anchor mixers, gyro mixers, and others.

Processing modes do not differ from standard ones used in each particular case, depending on rheological characteristics of polyethylenes. The most preferable method for processing is extrusion of PE melt. The resulting composition is extruded at a temperature of from <NUM> to <NUM>.

The compositions prepared by the method according to the invention can be used as full-value raw material for the production of articles such as outer sheaths of fiber optic cables, insulation of electrical cables, insulation of steel pipes, tubular film materials, foams, etc..

The following materials were used as initial components of the composition:
Polyethylenes:.

The melt flow rate was determined at a temperature of <NUM> and a load of <NUM> N, according to GOST <NUM>.

The tensile yield strength, tear strength and relative elongation at break were determined according to GOST <NUM> at a speed of testing of <NUM>/min.

The flexural modulus was determined according to ASTMD <NUM>; the type of testing was a three-point bend test at a speed of testing of <NUM>/min.

The Shore D/<NUM> hardness was determined according to GOST <NUM>.

The Vicat (<NUM>) heat resistance test was carried out according to ASTM <NUM>.

The resistance to cracking was determined according to GOST <NUM>.

The melt strength was measured with a capillary rheometer Smart RHEO <NUM>. The melt was forced through the capillary, filled into a pulling device and pulled with a constant acceleration. Once reaching a specific pulling speed, the pulled thread broke. The force fixed on a tensometer at the time of breaking the thread was considered as the melt strength. The strength of polyethylene melt was measured by using a capillary with a diameter of <NUM> and at a temperature of <NUM>.

The strength of films in the longitudinal and transverse directions, as well as relative elongation at break in the longitudinal and transverse directions, were determined according to GOST <NUM>-<NUM>, using a testing machine Zwick Z2. <NUM> (Zwick/Roell, Germany). Samples were conditioned before testing and then tested at room temperature and non-specified humidity.

The specific volumetric and the specific surface electrical resistivity were determined according to GOST <NUM>-<NUM>. The electrical resistivity at constant voltage was measured by using a device Teraohmeter (Ceast, Italy). Samples were conditioned before testing and then tested at a temperature of <NUM>(+/-<NUM>)°C and a humidity of <NUM>(+/- <NUM>)%.

The electrical strength (breakdown point) was measured according to GOST <NUM>-<NUM>. The electrical strength at variable (frequency <NUM>) and constant voltage was measured by using a device Dielectric Rigidity (Ceast, Italy). Samples were conditioned before testing and then tested at a temperature of <NUM>(+/-<NUM>)°C and a humidity of <NUM>(+/-<NUM>)%.

PE compositions were prepared by using steps of previously preparing mixtures of a nucleating agent, an antioxidant and any one of the used PE.

The previously prepared mixtures were mixtures of the nucleating agent with LPE or HDPE, mixtures of the antioxidant (if used) with LDPE, and mixtures of HDPE and/or LDPE with LPE. Said mixtures were prepared in a mixer of Brabender type at a temperature of <NUM> to <NUM>.

Then the mixtures were mixed in a blade mixer to prepare a composition.

The resulting composition was processed in an extrusion line. The maximum melt temperature during the extrusion process was <NUM>.

Samples for physical and mechanical, thermal physical and other tests were prepared by a hot-pressing method under standard conditions at a temperature of <NUM> to <NUM>.

Compositions and results of their studies are given in Tables <NUM>-<NUM> for Examples <NUM>-<NUM>. Examples <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> relate to compositions according to the invention. These examples are given only as illustration of the present invention and are not intended to limit its scope.

The results of the experiments show an advantage of the compositions consisting of three components: LDPE, LPE, and HDPE. Said compositions (examples <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) demonstrate improved properties over the compositions consisting of two components: LDPE and LPE (Example <NUM>), LDPE and HDPE (Example <NUM>), LPE and HDPE (Example <NUM>).

It is also shown that in the absence of a nucleating agent (Example <NUM>), the properties of the compositions consisting of three components are significantly deteriorated, in particular, the following parameters are reduced: tensile strength, relative elongation at break at <NUM> and -<NUM>, Vicat heat resistance, and melt strength; the surface quality of the extruded threads is deteriorated as well.

The results of the above-disclosed experiments showed that to obtain a desired good combination of surface properties, fluidity and physical and mechanical characteristics of PE compositions, a composition must comprise simultaneously a mixture of LDPE, LPE, HDPE and a nucleating agent.

The properties of the composition prepared according to the invention allows its use in various fields, including:.

Thus, for example, the compositions of Examples <NUM>, <NUM>, <NUM>, and <NUM> are characterized by key properties allowing compositions to be used as the outer sheath of fiber optic cables.

The composition of Example <NUM> satisfies, in particular, key requirements for compositions intended for use as insulation of electric cables. In order to confirm realization of this intended purpose, said composition was additionally studied with respect to its dielectric characteristics. According to the conducted studies, the composition is characterized by a specific volumetric electrical resistivity of <NUM>·<NUM><NUM> Ω·cm, an electric strength of <NUM> kV/mm, and a brittleness point of less than -<NUM>.

The composition of Example <NUM> satisfies, in particular, key requirements for the compositions intended for use as insulation of steel pipes.

The compositions of Examples <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> satisfy the key requirements for the compositions intended for use in the manufacture of foamed materials.

The composition according to Example <NUM> with a melt strength of <NUM> cN satisfies, in particular, key requirements for tubular PE films. The tubular film with a thickness of <NUM>, prepared by a standard method in a laboratory line applied to an extruder Collin (Germany) showed the following test results: a transverse strength of <NUM> MPa, a longitudinal strength of <NUM> MPa, a longitudinal relative elongation of <NUM>%, a transverse relative elongation of <NUM>%, a longitudinal tear resistance of <NUM>/cm, a transverse tear resistance of <NUM>/cm, and a tension modulus of <NUM> MPa. It should be noted that such practically important parameter as tear resistance obtained for the film made of the composition according to Example <NUM> is in several times higher than a standard level of this parameter in films made of HDPE and corresponds only to the level of composite (multilayer) PE films. In addition, Table <NUM> demonstrates properties of films of the compositions prepared according to the invention. The thickness of the films is <NUM>-<NUM>. The nucleating agents used in these examples are not a transparent agent and, therefore, optical indices (haze) of the films practically are not changed in the presence of HPN-20E, while in the presence of talc, the haze naturally increases due to dispersed particles of the mineral filler.

Claim 1:
A polyethylene composition for the manufacture of an article, comprising the following components:
- <NUM> to <NUM> wt.% of low-density polyethylene (LDPE);
- <NUM> to <NUM> wt.% of linear polyethylene (LPE);
- <NUM> to <NUM> wt.% of high-density polyethylene (HDPE);
- <NUM> to <NUM> wt.% of a nucleating agent; and
- <NUM> to <NUM> wt.% of an optional other additive,
wherein the HDPE is characterized by an MFR(<NUM>/c,<NUM>) of from <NUM> to <NUM>/<NUM>.