MXPA98000336A - Composition of environment for ca - Google Patents

Abstract

The present invention relates to a cable sheath composition, further characterized in that it consists of a multimodal olefin polymer mixture obtained by the polymerization of at least one alpha-olefin in more than one step and having a density of about 0.915. -0.955 g / cm3 and a molten bath flow rate of about 0.1-3.0 g / 10 min, said olefin polymer mixture comprises at least a first and second olefin polymer, of which the former has a density and a molten bath flow rate selected from (a) about 0.930-0.975 g / cm3 and about 50-2000 g / 10 min and (b) about 0.88.0.93 g / cm3 and about 0.01-0.8 g / 10 m

Description

COMPOSITION OF CABLE ENVELOPE DESCRIPTIVE MEMORY The present invention relates to a cable sheathing composition, as well as to the use thereof as an outer sheath for power cable or a communication cable. The cables, which are intended to mean power cables for high voltage, medium voltage and low voltage - and communication cables such as optical cables, coaxial cables and pair cables, generally comprise a center surrounded by a sheath consisting of a or more layers. The outermost layer is called the outer sheath or layer of sheathing and is made to date of a polymer material, preferably ethylene plastic. The highly diverse fields of application for various types of cables, such as telecommunication cables including conventional copper cables and fiber optic cables, as well as power cables, imply that the sheathing material has to satisfy a number of property requirements that in some respects they are contradictory. In this way, important properties of the wire sheathing materials are good processing capacity, that is, the material should be easy to process within a wide range of temperatures, low shrinkage, high mechanical strength, high surface finish, as well as high resistance to environmental stress cracking (ESCR). Since up to now it has been difficult or even impossible to satisfy all of these property requirements, the prior art sheathing materials have been the result of compromise, so good properties have been obtained in one aspect with the cost of most properties. deficient in some other aspect. In this way, it would be highly advantageous if this compromise that refers to the properties of the cable sheathing materials were reduced or even eliminated. In particular, it would be advantageous if the ESCR of the material could be improved and the age of shrinkage reduced in a given processing capacity. The present invention accomplishes this objective by means of a cable sheath composition which, instead of the unimodal polyethylene plastic used in conventional cable sheath compositions, consists of a multimodal olefin polymer mixture having certain given density and of flow velocity of the molten bath, both in regard to the polymer mixture with respect to the polymers that are part of it. The present invention thus provides a cable sheath composition which is further characterized in that it consists of a multimodal olefin polymer mixture having a density of about 0.915-0.955 g / cm3 and a molten bath flow rate of about 0.1. -3.0 g / min, said olefin polymer mixture comprises at least one first and second olefin polymer, of which the first has a density and a molten bath flow rate selected from (a) about 0.930- 0.975 g / cm3 and approximately 50-2000 g / 10 min and (b) approximately 0.88-0.93 g / cm3 and approximately 0.01-0.8 g / 10 min. The invention further relates to the use of this cable sheathing composition as an outer sheath for a power cable or a communication cable. The additional distinguishing features and advantages of the invention will be apparent from the following description and the appended claims. However, before describing the invention in more detail some key expressions will be defined. By the "modality" of a polymer is meant the structure of the molecular weight distribution of the polymer, that is, the appearance of the curve indicating the number of molecules as a function of molecular weight. If the curve exhibits a maximum, the polymer is called "unimodal", whereas if the curve exhibits a very broad maximum or two or more maximums and the polymer consists of two or more fractions, the polymer is called "bimodal", "multimodal" ", etc. Next, all polymers whose molecular weight distribution curve is very wide or have more than one maximum will be jointly called "multimodal".

The "molten bath flow rate" (MFR) of a polymer is determined in accordance with ISO 1133, condition 4, and is equivalent to the term "molten bath index" previously used. The flow rate of the molten bath, which is indicated in g / 10 min, is an indication of the flowability and hence the processing capacity of the polymer. The higher the flow rate of the molten bath, the lower the viscosity of the polymer. The term "resistance to environmental stress cracking" (ESCR) is intended to mean the resistance of the polymer to the formation of cracks under the action of mechanical stress and a reagent in the form of a surfactant. The ESCR is determined in accordance with ASTM D 1693 A, and the reagent used is Igepal CO-630. By the term "ethylene plastic" is meant a plastic based on polyethylene or ethylene copolymers, the ethylene monomer constituting the majority of the mass. As indicated above, the wire sheath composition according to the invention is distinguished by the fact that it consists of a multimodal olefin polymer mixture of specific melt flow density and flow velocity. It has previously been known to produce ultimodal olefin polymers, in particular bimodal polymers, preferably multimodal ethylene plastics, in two or more reactors connected in series. As examples of this prior art, EP 040 992, EP 041 796, EP 022 376 and WO 92/12182 can be mentioned, which are incorporated herein by reference in the production of multimodal polymers. According to these references, each of the polymerization steps can be carried out in liquid, suspension or gas phase. In accordance with the present invention, the main polymerization steps are preferably carried out as a combination of suspension polymerization / gas phase polymerization or gas phase polymerization / gas phase polymerization. The suspension polymerization is preferably carried out in a so-called loop reactor. The use of the suspension polymerization in a stirred tank reactor is not preferred in the present invention, since said method is not flexible enough for the production of the inventive composition and includes solubility problems. To produce the inventive composition of improved properties a flexible method is required. For this reason, it is preferred that the composition be produced in two main stages of polymerization in a combination of loop reactor / gas phase reactor or gas phase reactor / gas phase reactor. It is especially preferred that the composition be produced in two main stages of polymerization, in which case the first stage is carried out as suspension polymerization in a loop reactor and the second stage is carried out as gas phase polymerization in a gas phase reactor.

Optionally, the main polymerization steps can be preceded by a prepolymerization, in which case up to 20% by weight, preferably 1-10% by weight of the total amount of the polymers is produced. Generally, this technique results in a multimodal polymer mixture through polymerization with the aid of a chromium, metallocene or Ziegler-Natta catalyst in several successive polymerization reactors. In the production of, for example, a bimodal ethylene plastic which according to the invention is the preferred polymer, a first ethylene polymer is produced in a first reactor under certain conditions with respect to the composition of the monomer, the gas pressure hydrogen, temperature, pressure and more. After polymerization in the first reactor, the reaction mixture including the produced polymer is fed to a second reactor, where further polymerization takes place under different conditions. Usually, a first polymer of high melt flow rate (low molecular weight) and with a moderate or small comonomer addition, or no such addition at all, is produced in the first reactor, while in the second reactor a second polymer of low speed of molten bath flow (high molecular weight) and with a greater addition of comonomer. As the comonomer, other olefins having up to 12 carbon atoms are commonly used, such as α-olefins having 3-12 carbon atoms, e.g., propene, butene, 4-methyl-1-pentene, hexene, octene, decene, etc., in the copolymerization of ethylene. The resulting final product consists of an intimate mixture of the polymers that come from the two reactors, forming the different molecular weight distribution curves of these polymers together a molecular weight distribution curve that has a broad maximum or two maximums, ie , the final product is a bimodal polymer mixture. Since the multimodal and especially bimodal polymers, preferably the ethylene polymers and the production thereof belong to the prior art, a detailed description will not be made here, but reference has been made to the above descriptions. It should be noted here that, in the production of two or more polymer components in a corresponding number of reactors connected in series, it is only in the case of the component produced in the reactor of the first stage and in the case of the final product that the speed of molten bath flow, density and other properties can be measured directly on the material removed. The corresponding properties of the polymer components produced in the reactor stages following the first stage can only be determined indirectly on the basis of the corresponding values of the materials introduced and discharged from the respective reactor stages. Even though the multimodal polymers and their production are known per se, it has not previously known to use such multimodal polymer blends in wire sheath compositions. Above all, it has not previously been known to use in this context multimodal polymer mixtures having the specific values of density and flow rate of the molten bath as required in the present invention. As noted above, it is preferred that the multimodal olefin polymer mixture in the wire sheath composition according to the invention be a bimodal polymer blend. It is also preferred that this bimodal polymer blend has been produced by polymerization as indicated above under different polymerization conditions in two or more polymerization reactors connected in series. Due to the flexibility with respect to the reaction conditions obtained in this way, it is preferred that the polymerization be carried out in a loop reactor / a gas phase reactor, a gas phase reactor / a phase reactor, gas or a loop reactor / loop reactor as the polymerization of one, two or more olefin monomers, the different polymerization steps having variable comonomer contents. Preferably, the polymerization conditions in the preferred two-stage method are chosen in this manner so that a comparatively low molecular weight polymer having a moderate, low comonomer content is produced in one step, preferably in the first step. , which is preferred, which does not contain comonomer, due to a high contente and a chain transfer agent (hydrogen gas), while a high molecular weight polymer having a higher comonomer content is produced in another stage, preferably the second stage. However, the order of these stages can be reversed. Preferably, the multimodal olefin polymer mixture according to the invention is a mixture of propylene plastics or, which is what is most preferred, of ethylene plastics. The comonomer or comonomers of the present invention are chosen from the group consisting of α-olefins having up to 12 carbon atoms, which in the case of ethylene plastic means that the comonomer or comonomers are chosen from α-olefins which have 3-12 carbon atoms. Especially preferred comonomers are butene, 4-methyl-1-pentene, 1-hexene and 1-octene. In view of the foregoing, a preferred ethylene plastic blend according to the invention consists of a low molecular weight ethylene homopolymer blended with a copolymer of high molecular weight ethylene and butene, 4-methyl-1-pentene, 1 -hexene or 1-octene. The properties of the individual polymers in the olefin polymer mixture according to the invention should be chosen such that the final olefin polymer mixture has a density of about 0.915-0.955 g / cm3, preferably about 0.920-0.950 g / cm3 and a molten bath flow rate of about 0.1-3.0 g / 10 min, preferably about 0.2-2.0 g / 10 min. According to the invention, this is preferably achieved by the olefin polymer mixture comprising a first olefin polymer having a density of about 0.930-0.975 g / cm3, preferably about 0.955-0.975 g / cm3 and a flow rate of molten bath of about 50-2000 g / 10 min, preferably about 100-1000 g / 10 min and most preferably about 200-600 g / 10 min, and at least one second olefin polymer having a density and a speed of molten bath flow such that the olefin polymer mixture obtains the density and flow rate of molten bath indicated above. If the multimodal olefin polymer mixture is bimodal, ie, it is a mixture of two olefin polymers (a first olefin polymer and a second olefin polymer), the first olefin polymer that is produced in the first reactor and that has the density and the molten bath flow rate indicated above, the density and flow rate of molten bath of the second olefin polymer, which is produced in the second stage reactor, can be determined indirectly, as indicated above, based on the values of the materials supplied and discharged from the second stage reactor. In case the olefin polymer mixture and the first olefin polymer have the above density and melt flow rate values, a calculation indicates that the second olefin polymer produced in the second stage should have a density in the order of about 0.88-0.93 g / cm3, preferably 0.91-0.93 g / cm3, and a molten bath flow rate in the order of about 0.01-0.8 g / 10 min, preferably about 0.05-0.3 g / 10 min. As indicated in the above, the order of the stages can be reversed, which would mean that if the final olefin polymer mixture has a density of about 0.915-0.955 g / cm3, preferably about 0.920-0.950 g / cm3, and a molten bath flow rate of about 0.1-3.0 g / 10 min, preferably about 0.2-2.0 g / 10 min, and the first olefin polymer produced in the first stage has a density of about 0.88-0.93 g / cm3, preferably about 0.91-0.93 g / cm3 and a molten bath flow rate of 0.01-0.8 g / 10 min, preferably about 0.05-0.3 g / 10 min, then the second olefin polymer produced in the second step of a method of two stages, according to the above calculations, has a density in the order of approximately 0.93-0.975 g / cm3, preferably approximately 0.955-0.975 g / cm3, and a molten bath flow rate of 50-2000 g / 10 min , preferably e about 100-1000 g / 10 min, and most preferably about 200-600 g / 10 min. Nevertheless, this order of the stages is less preferred in the production of the olefin polymer mixture according to the invention. To optimize the properties of the wire sheathing composition according to the invention, the individual polymers in the olefin polymer mixture must be present in a weight ratio such that the desired properties contributed by the individual polymers are also achieved in the final olefin polymer mixture. As a result, the individual polymers should not be present in small amounts such as about 10% by weight or less, so as not to affect the properties of the olefin polymer mixture. To be more specific, it is preferred that the amount of olefin polymer having a high molten bath (low molecular weight) flow rate constitute at least 25% by weight, but not more than 75% by weight of the total polymer, preferably 35-55% by weight of the total polymer, thereby optimizing the properties of the final product. The use of multimodal olefin polymer blends of the type described above, results in wire sheath compositions of the invention having much better properties than conventional wire sheath compositions, especially in shrinkage, ESCR and processing capacity. . In particular, the reduced shrinkage of the cable sheath composition of the invention is a great advantage. As indicated above, the wire sheathing composition according to the invention can be used to produce outer sheaths for cables, including power cables, as well as communication cables. Among the power cables, whose outer sheaths can advantageously be produced from the cable sheathing composition according to the invention, high voltage cables, medium voltage cables and low voltage cables can be mentioned. Among the communication cables whose outer sheaths can be advantageously made from the cable sheathing composition according to the invention, there can be mentioned the pairs cables, coaxial cables and optical cables. Some non-restrictive examples designed to further illustrate the invention and its advantages are shown below.

EXAMPLE 1 In a polymerization plant consisting of a loop reactor connected in series to a gas phase reactor and including the use of a Ziegler-Natta catalyst, a bimodal ethylene plastic was polymerized under the following conditions.

The first reactor (loop reactor) In this reactor a first polymer (Polymer 1) was produced by the polymerization of ethylene in the presence of hydrogen (molar ratio of hydrogen to ethylene = 0.38: 1). The resulting ethylene homopolymer had an MFR value of 492 g / 10 min and a density of 0.975 g / cm 3. "5 The second reactor (gas phase reactor) In this reactor a second polymer (Polymer 2) was produced by the polymerization of ethylene and butene (molar ratio in the gas phase of butene to ethylene = 0.22: 1, hydrogen to ethylene = 0.03: 1). The resulting ethylene butene copolymer was present in the form of an intimate mixture with the ethylene homopolymer of the first reactor, the weight ratio of Polymer 1 to Polymer 2 was 45:55. The bimodal mezcal of Polymer 1 and Polymer 2 had a density of 0.941 g / cm3 and an MFR value of 0.4 g / 10 min. After mixing with carbon black, a final product containing 2.5% by weight thereof was obtained, resulting in a final density of 0.951 g / cm3. This final product will be called in the following bimodal ethylene plastic 1. 20 The bimodal ethylene plastic 1 was used as the wire sheath composition and the properties of this composition were determined and compared with those of a conventional wire sheath composition. of unimodal ethylene plastic (Reference 1). Reference 1 had a density of 0.941 g / cm 3 (after mixing to a carbon black content of 2.5% by weight and a density of 0.951 g / cm 3) and an MFR value of 0.24 g / 10 min. In this example, as well as in the following examples, the shrinkage of the composition produced was determined according to a method (in the subsequent called UNI-5079) that had been developed to be able to evaluate the shrinkage tendency of the sheathing materials. The shrinkage is determined in the following manner. Cable samples are extruded for evaluation as follows.

Conductor: solid 3.0 mm Al conductor Wall thickness: 1.0 mm Temperature, given: + 210 ° C or + 180 ° C Distance between the die and the water bath: 35 cm Temperature, water bath: +23"C Line speed: 75 m / min Die type: Half tube Nipple: 3.65 mm Die: 5.9 mm Screw design: Elise Break plate The percentage shrinkage is measured after 24 hours in a room with constant temperature (+ 23 ° C) ), as well as after 24 hours at a temperature of + 100 ° C.

Cable samples measuring approximately 40 cm were measured. Conveniently, the cable sample is marked in such a way that the measurement after conditioning can be carried out at the same point on the cable sample. If the sample shrinks during the measurement, marks of approximately 40 cm must first be made. Afterwards, the length is cut and remedied. Double samples are taken from each cable that will be analyzed. The samples are placed in the room at a constant temperature for 24 hours, after which they are measured and the shrinkage value in percentage is calculated. All samples are then placed on a talc bed at + 100 ° C for 24 hours. The samples are then measured and the total shrink value in percent is calculated based on the initial length. The results of the measurement are indicated in Table 1 below.

TABLE 1 Properties of Bimodal material 1 Reference 1 Resistance to stress rupture (MPa) i 34 38 Elongation at break (%) 800 900 ESCR2 0/2000 h F20 / 550 h Shrinkage (%) at 23 ° C / 24 h3 0.0 0.7 23 ° C / 24 h * 0.0 0.7 Shrinkage (%) at 100 ° C / 24 h3 1.0 2.0 1000C / 24 h < 0.9 2.3 Surface finish5 After extrusion at 180 ° C at 15 m / min 0-1 or 35 m / min 0-1 or 75 m / min or 140 m / min or After extrusion at 210 ° C at 15 m / min min or 35 m / min 0-1 or 75 m / min 0-1 or 140 m / min 0.1 1: Determined according to ISO 527-2 1993 / 5A in cable samples. 2: Determined in accordance with ASTM D 1693 / A, Igepal at 10%. The results are indicated as the percentage of sample bars cracked in a given time. F20 means that 20% of the sample bars were cracked after the indicated time. 3: Determined in accordance with UNI-5079 after extrusion at 180 ° C. 4: Determined in accordance with UNI-5079 after extrusion at 210 ° C. 5: Classification: 0 = excellent at 4 = very uneven. It is evident from the values indicated in Table 1, that the sheathing material of the invention exhibits improved properties in terms of shrinkage, especially at room temperature, and resistance to environmental stress cracking (ESCR). The tensile strength properties of the sheathing material according to the invention are on a level with those of reference 1. Similarly, the processing capacity that can be derived from the MFR value of the sheathing material in accordance with the invention is as good as that of reference 1. It should be emphasized that, while the sheathing material of reference 1 has good processing properties obtained at a cost of poor shrinkage properties, especially at room temperature, the material of The sheathing according to the invention has good processing properties, as well as good (low) shrinkage properties. This is a considerable advantage, which is enhanced by the improved ESCR properties of the sheathing material according to the invention.

EXAMPLE 2 In the polymerization plant of Example 1 a bimodal ethylene plastic was produced under the following conditions.

The first reactor (loop reactor) In this reactor a first polymer was produced (Polymer 1) by the polymerization of ethylene in the presence of hydrogen (molar ratio of hydrogen to ethylene = 0.38: 1).

The resulting ethylene homopolymer had an MFR value of 444 g / 10 min and a density of 0.975 g / cm3.

The second reactor (gas phase reactor) In this reactor a second polymer was produced (Polymer 2) by the polymerization of ethylene and butene (mole ratio of butene to ethylene = 0.23: 1, molar ratio of hydrogen to ethylene = 0.09: 1). The resulting ethylene butene copolymer was present in the form of an intimate mixture with the ethylene homopolymer of the first reactor, the weight ratio of Polymer 1 to Polymer 2 being 40:60. The bimodal mixture of Polymer 1 and Polymer 2, which was the final product, has a density of 0.941 g / cm3 (after an addition of 2.5% by weight of carbon black, 0.951 g / cm3) and an MFR value of 1.4 g / 10 min. In the subsequent, this final product will be called bimodal ethylene plastic 2.

Similarly, another bimodal ethylene plastic was produced (hereinafter referred to as bimodal ethylene plastic 3), the molar ratio of hydrogen to ethylene in the first rector is 0.39: 1 and the resulting ethylene homopolymer (Polymer 1) in the first reactor it has an MFR value of 468 g / 10 min and a density of 0.962 g / cm3. A copolymer of ethylene and butene (Polymer 2) was produced in the second reactor, the molar ratio of butene to ethylene being 0.24: 1, and the molar ratio of hydrogen to ethylene of 0.07: 1. The weight ratio of Polymer 1 to Polymer 2 was 45:55.

The final product (bimodal ethylene plastic 4) had a density of 0.941 g / cm 3 (after mixing with 2.5 wt% of carbon black, 0.951 g / cm 3) and an MFR value of 1.3 g / 10 min. The bimodal ethylene plastic 2 and the bimodal ethylene plastic 3 were used as cable sheath compositions, and the properties of these compositions were determined and compared with those of a prior art sheath composition (reference 2). Reference 2 was a special composition designed for use in cases where a particularly low shrinkage age is required, such as fiber optic applications, and this composition consisted of a melt mixture of a polyethylene fraction having a density of 0.960. g / cm3 and an MFR value of 3.0 g / 10 min, and another polyethylene fraction having a density of 0.920 g / cm3 and an MFR value of 1.0 g / 10 min. This resulted in a final product having a density of 0.943 g / cm 3 (after an addition of 2.5 wt% of carbon black, 0.953 g / cm 3) and an MFR value of 1.7 g / 10 min. The results of the measurements of the properties of the three wire sheath compositions are indicated in Table 2 below.

TABLE 2 Material properties Plastic Reference 1 bimodal ethylene 2 3 Resistance to stress rupture (MPa) 1 32 30 32 Elongation at break (%) * 900 890 1150 ESCR2 0/2000 h 0/2000 h F20 / 550 h Shrinkage (%) at 23 ° C / 24 h «0.0 0.0 0.1 Shrinkage (%) at 100ßC / 24 h * 0.8 1.0 0.8 Surface finish5 After extrusion at 210 ° C at 15 m / min 2 2 3 35 m / min 1-2 1 4 75 m / min 0-1 0 4 140 m / min 0-1 0 4 1: Determined in accordance with ISO 527-2 1993 / 5A. 2: Determined in accordance with ASTM D 1693 / A, Igepal at 10%. The results are indicated as the percentage of sample bars cracked in a given time. F20 means that 20% of the sample bars were cracked after the given time. 3: Determined in accordance with UNI-5079 after extrusion at 210 ° C. 4: Classification: 0 = excellent at 4 = very uneven. As is evident from Table 2, the special sheathing material of the prior art (Reference 2) has good shrinkage properties at room temperature. Nevertheless, the shrinkage properties of Reference 2 have been achieved with the cost of poor processing properties, as is apparent from, among others, the deficient surface finish values. Generally, the sheathing material of Reference 2 can only be processed within a narrow "processing window", that is, with narrow scales in terms of processing parameters. In contrast to Reference 2, the sheathing materials according to the invention (bimodal ethylene plastic 2 and 3) exhibit shrinkage properties as good as those of Reference 2, while exhibiting better processing properties (processing window wider) that include a better surface finish of the cable sheath. However, the sheathing materials according to the invention exhibit much better resistance to environmental stress cracking (ESCR), and also exhibit good resistance to stress rupture.

EXAMPLE 3 In the polymerization plant used in Examples 1 and 2, a bimodal polyethylene plastic (ethylene 4 plastic) was produced under the following conditions. The first reactor (loop reactor) In this reactor a first polymer was produced (Polymer 1) by the polymerization of ethylene in the presence of l-butene and hydrogen gas (molar ratio l-butene: hydrogen gas: ethylene = 1.74: 0.22: 1). Polymer 1 had an MFR value of 310 g / 10 min. and a density of 0.939 g / cm3.

The second reactor (gas phase reactor) The polymer of the loop reactor was transferred to the gas phase reactor, where the further polymerization of ethylene with l-butene in the presence of hydrogen gas was carried out (molar ratio of 1-butene: hydrogen: ethylene = 0.80: 0.02: 1), resulting in a new polymer component (Polymer 2). The weight ratio of Polymer 1 to Polymer 2 was 42:58. The MFR value of the resulting final product was 0.3 g / 10 min., And the density was 0.922 g / cm3. Also excellent mechanical properties and good ESCR were achieved in this case, as well as satisfactory shrink properties, where both polymer components contained l-butene as a comonomer, as is evident from Table 3 below. TABLE 3 Properties of the material Ethylene plastic 4 Resistance to rupture by tension 25.9 MPa Elongation at break 905% ESCR 0/2000 h Shrinkage% 23 ° C / 24 h 0% 100ßC / 24 h 0%

Claims (12)

NOVELTY OF THE INVENTION CLAIMS

1. - A wire sheath composition, further characterized in that it consists of a multimodal olefin polymer mixture obtained by the polymerization of at least one α-olefin in more than one step and having a density of about 0.915-0.955 g / cm3 and a molten bath flow rate of about 0.1-3.0 g / 10 min, said olefin polymer blend comprises at least a first and second olefin polymer, of which the former has a density and a flow rate of molten bath selected from (a) about 0.930-0.975 g / cm3 and about 50-2000 g / 10 min and (b) about 0.88-0.93 g / cm3 and about 0.01-0.8 g / 10 min.

2. A wire sheathing composition according to claim 1, further characterized in that the first olefin polymer has a density of about 0.930-0.975 g / cm3 and a molten bath flow rate of about 50-2000 g / 10 minutes.

3. A wire sheathing composition according to claim 1 or 2, further characterized in that the olefin polymer mixture has a density of about 0.920-0.950 g / cm3 and a molten bath flow rate of about 0.2- 2.0 g / 10 min, and in that the first olefin polymer has a density of about 0.955-0.975 g / cm3 and a molten bath flow rate of about 100-1000 g / 10 min.

4. A cable sheath composition according to any of claims 1-3, further characterized in that the olefin polymer mixture is a mixture of ethylene plastics.

5. A cable sheath composition according to any of claims 1-4, further characterized in that it has been obtained by polymerization of catalyzed in coordination in at least two stages of ethylene and in at least one stage, was used an α-olefin comonomer having 3-12 carbon atoms.

6. A cable sheath composition according to claim 5, further characterized in that the polymerization steps were carried out as suspension polymerization, gas phase polymerization or a combination thereof.

7. A cable sheath composition according to claim 6, further characterized in that the suspension polymerization was carried out in a loop reactor.

8. A cable sheath composition according to claim 7, further characterized in that the polymerization was carried out in a loop reactor / gas phase reactor process in at least one loop reactor followed at least by a gas phase reactor.

9. A cable sheath composition according to any of the preceding claims, further characterized in that it is a bimodal mixture of ethylene plastics.

10. A cable sheath composition according to claim 9, further characterized in that the first ethylene plastic constitutes 25-75% by weight of the total amount of polymers in the composition.

11. The use of the cable sheath composition according to any of the preceding claims as an outer sheath for an energy cable.

12. The use of the cable sheath composition according to any of claims 1-11 as an outer sheath for a communication cable.