Fibre optic cable

Abstract

The present invention relates to a cable comprising a polyolefin layer, preferably a fibre optic cable, wherein the polyolefin layer comprises a multimodal polyolefin composition, wherein the multimodal polyolefin has a density of 0.910-0.960 g / cm3 as measured according to ISO 1183-1 / method A and an MFR2 of 0.2-6.0 g / 10 min as measured according to ISO 1133 at 190 °C and a load of 2,1 kg, wherein the composition is obtained by a process comprising the steps of a) melting and / or melt mixing a multimodal polyolefin with optional additives, thereby obtaining a polymer melt A; b) filtering said polymer melt A through a melt filter with 40-250 μm filter openings, followed by pelletization thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining the multimodal polyolefin composition in the form of pellets; wherein the multimodal polyolefin composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

Description

[0001] FIBRE OPTIC CABLE

[0002] The invention relates to a clean polymer composition for fibre optic cables, a fibre optic cable comprising this composition and methods of reducing the number of contaminants in polyolefins, in particular polyethylenes, for use in said polymer compositions for fibre optic cables.

[0003] Background

[0004] Cables, which include power cables for high, medium or low voltage, and communication cables, generally comprise a core of one or more communication or power transmission media surrounded by a sheath consisting of one or more layers. These layers can comprise an insulation layer and the outermost layer is often referred to as jacket or jacketing layer. Nowadays these layers are made of polymer material, conventionally a polyolefin such as polyethylene.

[0005] The insulation and jacket material has to meet a number of requirements, which vary depending on the intended application of the cable.

[0006] There are for example several types of communication cables: transmission lines when electric signals are transmitted; waveguides when electromagnetic waves are transmitted and optical fibres when light signals are transmitted.

[0007] Fibre optical communication cables, also known as optical-fibre cables, have been used for many years to transmit information. The fibre optical cables includes hair-thin optical fibres as the transmission medium. Consequently, the insulation and jacketing layers have to be very thin as well which puts high demands on material requirements such as the smoothness.

[0008] Furthermore, in a fibre optic cable network installation, one of two methods is used to lay the cable into place: blowing or pulling. In fibre optic cable blowing, high speed air flow is combined with a mechanical pushing force to produce the installation known as blowing or jetting. This is the preferred method used to push fibre optic cable through a preinstalled conduit wherein several hundred meters of cable are pushed into the duct before compressed air is injected into the duct inlet. The fibre optic cable is attached to a blowing head, which has two seals to grip the cable tightly. As the air pressure builds up, the jetting motion pushes the fibre optic cable through the conduit.

[0009] To allow for a smooth installation by blowing, it is very important that there is no or very low friction between the cable outer surface and the duct inner wall, hence for these thin fibre optic cables (FOC), it is very important to have a high cable jacket smoothness. This will reduce the amount of air required to push the cable through the conduit and reduce the need to use blowing lubricants such as oil.

[0010] The fibre optics cables are often installed in small and confined spaces, such as preinstalled ducts with a diameter as low as 8 mm, and the cable jackets also need to protect against sudden impacts during installation so apart from an excellent smoothness to protect against frictional forces, the cable jacket also needs to have a good flexibility, a good abrasion resistance and a good impact strength.

[0011] The inventors have found that many of the above mentioned properties are affected by the presence of contaminants, in particular metal impurities. Contaminant free resins are desirable in many applications. For example, in film blowing, contaminant may generate holes in the bubble, which then disturbs the blowing process and use of the film in the final application. Contaminants also generate weak points in a pipe, which may then cause premature pipe rupture in the end use application and they are weakening electrical properties in wire and cable application. However, contaminant free resins are especially important in the case of fibre optic cables. The cable jacketing layers for this application need to be extremely thin in view of the very small dimensions of the cable’s constructional design. Furthermore, deformations in the jacketing of a fibre optic cable caused by such contaminants can also cause micro-bending which influences fibre attenuation.

[0012] As smaller and smaller constructions are sought, the requirements on the above mentioned properties become even more stringent. Hence, there is an ongoing need in the art for improved polymer materials for fibre optical cables, and more particularly for cable insulation and jacketing material wherein high smoothness, high flexibility, high abrasion resistance and low shrinkage are combined.

[0013] Summary of Invention

[0014] It is an object of the present invention to eliminate or alleviate the above-mentioned need and to provide a superclean fibre optical cable insulation or jacketing composition having that has excellent characteristics and allows a smooth installation by blowing.

[0015] Viewed from one aspect, the invention provides a cable comprising a polyolefin layer, preferably a fibre optic cable, wherein the polyolefin layer comprises a multimodal polyolefin composition, wherein the multimodal polyolefin has a density of 0.910-0.960 g / cm3and an MFR2 of 0.2-6.0 g / 10 min, wherein the composition is obtainable and / or obtained by a process comprising: a) melting and / or melt mixing a multimodal polyolefin with additives, thereby obtaining a polymer melt A; b) filtering said polymer melt A through a melt filter with 40-250 pm filter openings, followed by a pelletization step thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining the multimodal polyolefin composition in the form of pellets; wherein the obtained multimodal polyolefin composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

[0016] The use of a melt filtration step is known in the prior art and is effective to remove all kind of contaminants from polyolefin compositions. However, in the subsequent process steps, the process parts or equipment used may create additional contaminants. Furthermore, the inventors have found that contaminants may also enter the process with added additives, fillers, pigments, masterbatches etc. and brittle particles, e.g. brittle metal particles such as rust, may break into smaller fragments at the melt filter and permeate it. Hence, in case only melts screens or melt filters are used, the resulting polyolefin compositions may still contain contaminants that could be detrimental to the properties of these compositions, especially in the case of fibre optic cable layers and in particular FOC jacketing layers.

[0017] The prior art describes the use of melt screens with a very fine mesh size to alleviate this problem, however their use can be problematic due to generation of a high pressure drop over the screen which could also cause a breakage of the fine screen wires (i.e. causing a leakage whereby the screening / filtering function is effectively lost) in case of fine mesh size screen since wire diameter is very tiny in this kind of screens.

[0018] The inventors have found that by implementing a three-step purification protocol (i.e. steps b to d), the obtained polyolefin compositions are ultraclean. In this context, ultraclean is intended to refer to a polyolefin composition which is free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition, preferably the polyolefin composition is essentially free of contaminants.

[0019] Viewed from another aspect, the invention provides a fibre optic cable insulation or jacketing composition, preferably a jacketing composition, having an excellent smoothness and comprising a multimodal polyolefin with a density of 0.910-0.960 g / cm3and an MFR2 of 0.2-6.0 g / 10 min, wherein the composition is obtainable and / or obtained by: a) melting and / or melt mixing a multimodal polyolefin with additives, thereby obtaining a polymer melt A ; b) filtering said polymer melt A through a melt filter with 40-250 pm filter openings, followed by a pelletization step thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining the multimodal polyolefin composition in the form of pellets; wherein the obtained fibre optic cable composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

[0020] Viewed from yet another aspect, the invention provides a process for producing a polyolefin composition comprising the steps of : al) providing a polyolefin or polymerisation of at least one olefin, thereby obtaining a polyolefin; a) melting and / or melt mixing the polyolefin with additives, thereby obtaining a polymer melt Al; b) filtering said polymer melt Al through a melt filter with 40-250 pm filter openings, followed by a pelletization step thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining a cleaned polyolefin composition in the form of pellets; wherein the obtained polyolefin composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

[0021] Further distinctive features and advantages of the present invention will appear from the following description and the appended claims.

[0022] Detailed description of the invention

[0023] The present invention provides a clean polymer composition for fibre optic cables, in particular a clean insulation or jacketing composition for fibre optic cables, a fibre optic cable comprising this composition and a method of reducing the number of contaminants in polyolefin compositions, in particular polyethylene compositions, for use in said polymer compositions for fibre optic cables. In order to facilitate the understanding of the present invention a detailed description will be given below. First, though, some terms and expression used in the specification and claims will be defined.

[0024] By the "modality" of a polymer is meant the structure of the molecular-weight distribution of the polymer, i.e. the appearance of the curve indicating the number of molecules as a function of the molecular weight. If the curve exhibits one maximum, the polymer is referred to as "unimodal", whereas if the curve exhibits a very broad maximum or two or more maxima and the polymer consists of two or more fractions, the polymer is referred to as "bimodal" , "multimodal" etc. In the following, all polymers whose molecular- weight-distribution curve is very broad or has more than one maximum are jointly referred to as "multimodal" .

[0025] By "polyolefin" is meant an olefin homopolymer or copolymer. The olefin monomer is preferably selected from ethylene or propylene. The comonomer is preferably selected from a-olefins having 3-12 carbon atoms, more preferably 1-butene, 1-hexene, 4-methyl-l- pentene, and 1 -octene, when the olefin monomer is ethylene. When the olefin monomer is propylene the comonomer is preferably selected from ethylene and a-olefins having 4-12 carbon atoms, more preferably ethylene, 1-butene, 1-hexene, 4-methyl-l- pentene, and 1- octene.

[0026] By "polyethylene" or "ethylene (co) polymer" is meant an ethylene homopolymer or copolymer. Similarly, by "polypropylene" or "propylene (co) polymer" is meant a propylene homopolymer or copolymer.

[0027] It will be appreciated that the term “homopolymers” generally refers to polymers consisting essentially of one monomer. Ideally therefore an ethylene homopolymer is free of comonomer. However, small amounts of comonomers, different from ethylene, that do not materially affect the properties may be tolerated as such a material is in essence still a homopolymer. In this regard, a small amount of comonomer may be understood as less than 3 wt%, such as less than 1 wt%, less than 0.5 wt% or less than 0.1 wt% of comonomers, based on the total weight of the homopolymer.

[0028] Fiber Optic Cable

[0029] The fibre optic cable according to the present invention comprises a polyolefin layer, wherein the polyolefin layer comprises a multimodal polyolefin composition, wherein the multimodal polyolefin has a density of 0.910-0.960 g / cm3and an MFR2 of 0.2-6.0 g / 10 min, wherein the composition is obtainable and / or obtained by a process comprising: a) melting and / or melt mixing a multimodal polyolefin with optional additives, thereby obtaining a polymer melt A; b) filtering said polymer melt A through a melt filter with 40-250 pm filter openings, followed by a pelletization step thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining a cleaned polyolefin composition in the form of pellets; wherein the obtained multimodal polyolefin composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

[0030] It is previously known to produce multimodal, in particular bimodal, polyolefins, preferably multimodal polyethylene, in two or more reactors connected in series. As instances of this prior art, mention may be made of EP 040 992, EP 041 796, EP 022 376 and

[0031] WO 92 / 12182, which are hereby incorporated by way of reference as regards the production of multimodal polymers . According to these references, each and every one of the polymerisation stages can be carried out in liquid phase, slurry or gas phase.

[0032] According to the present invention, the main polymerisation stages are preferably carried out as a combination of slurry polymerisation / gas-phase polymerisation or gas-phase polymerisation / gas-phase polymerisation. The slurry polymerisation is preferably performed in a so- called loop reactor. The use of slurry polymerisation in a stirred-tank reactor is not preferred in the present invention, since such a method is not sufficiently flexible for the production of the inventive composition and involves solubility problems. In order to produce the inventive composition of improved properties, a flexible method is required. For this reason, it is preferred that the composition is produced in at least two main polymerisation stages in a combination of loop reactor / gas- phase reactor or gas-phase reactor / gas-phase reactor. It is especially preferred that the composition is produced in two main polymerisation stages, in which case the first stage is performed as slurry polymerisation in a loop reactor and the second stage is performed as gas- phase polymerisation in a gas-phase reactor. Optionally, the main polymerisation stages may be preceded by a prepolymerisation, in which case up to 20 wt%, preferably 1-10 wt%, of the total amount of polymers is produced. Generally, this technique results in a multimodal polymer mixture through polymerisation with the aid of a Single Site or Ziegler-Natta catalyst in several successive polymerisation reactors. The expression "coordination catalyst" or “coordination catalysed polymerization” encompasses catalysts or polymerization using catalysts of the Ziegler-Natta type or Single Site catalysts, such as metallocene catalysts, or a blend thereof. In the production of, say, a bimodal polyethylene, which according to the invention is the preferred polymer, a first polyethylene fraction is produced in a first reactor under certain conditions with respect to monomer composition, hydrogen-gas pressure, temperature, pressure, and so forth. After the polymerisation in the first reactor, the reaction mixture including the polymer produced is fed to a second reactor, where further polymerisation takes place under other conditions. Usually, a first polymer fraction of high melt flow rate (low molecular weight) and with a moderate or small addition of comonomer, or no such addition at all, is produced in the first reactor, whereas a second polymer fraction of low melt flow rate (high molecular weight) and with a greater addition of comonomer is produced in the second reactor.

[0033] As comonomer, use is commonly made of other olefins having up to 12 carbon atoms, such as a-olefins having 3-12 carbon atoms, e.g. propene, butene, 4-methyl- 1- pentene, hexene, octene, decene, etc., in the copolymerisation of ethylene. The resulting end product consists of an intimate mixture of the polymer fractions from the two reactors, the different molecular-weight- distribution curves of these polymer fractions together forming a molecular-weight-distribution curve having a broad maximum or two maxima, i.e. the end product is a bimodal polymer mixture. Since multimodal, and especially bimodal, polymers, preferably ethylene polymers, and the production thereof belong to the prior art, no detailed description is called for here, but reference is had to the above specifications.

[0034] It should be pointed out that, in the production of two or more polymer fractions in a corresponding number of reactors connected in series, it is only in the case of the fraction produced in the first reactor stage and in the case of the end product that the melt flow rate, the density and the other properties can be measured directly on the material removed. The corresponding properties of the polymer fractions produced in reactor stages following the first stage can only be indirectly determined on the basis of the corresponding values of the materials introduced into and discharged from the respective reactor stages.

[0035] Even though multimodal polymers and their production are known per se, it is not, however, previously known to use multimodal polyolefins having the specific values of density, melt flow rate and cleanliness as are required in the present invention in a fibre optic cable, e.g. as the composition of a jacketing layer of a fibre optic cable.

[0036] As hinted at above, it is preferred that the multimodal polyolefin in the polyolefin layer composition in the fibre optic cable according to the invention is a bimodal polyolefin.

[0037] It is also preferred that this bimodal polyolefin has been produced by polymerisation as above under different polymerisation conditions in two or more polymerisation reactors connected in series. Owing to the flexibility with respect to reaction conditions thus obtained, it is most preferred that the polymerisation is carried out in a loop reactor / a gas-phase reactor, a gas-phase reactor / a gas-phase reactor or a loop reactor / a loop reactor as the polymerisation of one, two or more olefin monomers, the different polymerisation stages having varying comonomer contents. Preferably, the polymerisation conditions in the preferred two-stage method are so chosen that a comparatively low-molecular polymer fraction having a moderate, low or, which is preferred, no content of comonomer is produced in one stage, preferably the first stage, owing to a high content of chain-transfer agent, whereas a high- molecular polymer fraction having a higher content of comonomer is produced in another stage, preferably the second stage. The order of these stages may, however, be reversed. Preferably, the multimodal polyolefin in accordance with the invention is a multimodal polypropylene or, which is most preferred, a multimodal polyethylene.

[0038] In view of the above, a preferred multimodal polyethylene for use in the polyolefin layer composition in the fibre optic cable according to the invention consists of a low- molecular ethylene homopolymer mixed with a high- molecular copolymer of ethylene and butene, 4-methyl-l- pentene, 1 -hexene or 1 -octene. It is particularly preferred that the properties of the individual polymers in the multimodal polyolefin according to the invention should be so chosen that the final multimodal polyolefin has a density of about 0.915- 0.955 g / cm3, preferably about 0.920-0.950 g / cm3, and a melt flow rate MFR2 of about 0.2-3.0 g / 10 min, preferably about 0.2-2.0 g / 10 min.

[0039] According to the invention, this is preferably achieved by the multimodal polyolefin comprising a first polyolefin fraction having a density of about 0.930-0.975 g / cm3, preferably about 0.955-0.975 g / cm3, and a melt flow rate MFR2 of about 50-2000 g / 10 min, preferably about 100-1000 g / 10 min, and most preferred about 200-600 g / 10 min, and at least a second polyolefin fraction having such a density and such a melt flow rate that the multimodal polyolefin obtains the density and the melt flow rate indicated above. If the multimodal polyolefin is bimodal, i.e. is a mixture of two polyolefin fractions (a first olefin polymer and a second olefin polymer) , the first polyolefin fraction being produced in the first reactor and having the density and the melt flow rate indicated above, the density and the melt flow rate of the second polyolefin fraction, which is produced in the second reactor stage, may, as indicated in the foregoing, be indirectly determined on the basis of the values of the materials supplied to and discharged from the second reactor stage.

[0040] In the event that the multimodal polyolefin and the first polyolefin fraction have the above values of density and melt flow rate, a calculation indicates that the second polyolefin fraction produced in the second stage should have a density in the order of about 0.880-0.930 g / cm3, preferably 0.910-0.930 g / cm3, and a melt flow rate MFR2 in the order of about 0.01- 0.8 g / 10 min, preferably about 0.05-0.3 g / 10 min.

[0041] As indicated in the foregoing, the order of the stages may be reversed, which would mean that, if the final multimodal polyolefin has a density of about 0.915-0.955 g / cm3, preferably about 0.920-0.950 g / cm3, and a melt flow rate MFR2 of about 0.2-3.0 g / 10 min, preferably about 0.2-2.0 g / 10 min, and the first polyolefin fraction produced in the first stage has a density of about 0.880-0.930 g / cm3, preferably about 0.910-0.930 g / cm3, and a melt flow rate MFR2 of 0.01-0.8 g / 10 min, preferably about 0.05-0.3 g / 10 min, then the second polyolefin fraction produced in the second stage of a two-stage method should, according to calculations as above, have a density in the order of about 0.930-0.975 g / cm3, preferably about 0.955-0.975 g / cm3, and a melt flow rate MFR2 of 50-2000 g / 10 min, preferably about 100-1000 g / 10 min, and most preferred about 200-600 g / 10 min. This order of the stages in the production of the olefin polymer mixture according to the invention is, however, less preferred.

[0042] It is understood that the density of the multimodal polyolefin and the final multimodal polyolefin composition is not necessarily the same. Additives, fillers or colour masterbatches, in particular those comprising carbon black, may influence the density of the final multimodal polyolefin composition. For example, where the multimodal polyolefin preferably has a density of 0.910-0.960 g / cm3, a final multimodal polyolefin composition comprising carbon black preferably has a density of 0.910-0.975 g / cm3.

[0043] In order to optimise the properties of the polyolefin layer composition in the fibre optic cable according to the invention, the individual polymer fractions in the multimodal polyolefin should be present in such a weight ratio that the aimed-at properties contributed by the individual polymer fractions are also achieved in the final multimodal polyolefin. As a result, the individual polymer fractions should not be present in such small amounts, such as about 10wt% or below, that they do not affect the properties of the multimodal polyolefin. To be more specific, it is preferred that the amount of polyolefin fraction having a high melt flow rate (low- molecular weight) makes up at least 25wt% but no more than 75wt% of the multimodal polyolefin, preferably 35-55wt% of the multimodal polyolefin, thereby to optimise the properties of the end product.

[0044] An important characteristic of the polyolefin composition in the fibre optic cable, in particular of the insulation and jacketing layers, and more particularly the multimodal polyolefin comprised therein according to the present invention is its high cleanliness.

[0045] A high cleanliness also contributes to good electric properties when the cleaned polyolefin composition is used in insulation or combined jacketing / insulation layers of cables such that it can withstand a high operating stress in terms of electric field before electrical breakdown occurs. It is contemplated that the maximum electrical field level could be increased with 50% or more with the cleaned polyolefin material of the present invention.

[0046] In one embodiment of the present invention, the polyolefin layer is an insulation layer, a jacketing layer or a combination of an insulation / jacketing layer, preferably a jacketing layer.

[0047] As stated earlier, the insulation and / or jacketing layer of the fibre optics cable according to the present invention comprises the above defined multimodal polyolefin. This means that the insulation and / or jacketing layer composition in the fibre optic cable of the present invention is substantially made up of the multimodal polyolefin. Preferably, the insulation and / or jacketing layer composition consists of the multimodal polyolefin.

[0048] In a preferred embodiment of the present invention, the cable is a fibre optic cable comprising a jacketing layer which comprises the above defined multimodal polyolefin and has a thickness of equal to or less than 1.0 mm, preferably equal to or less than 0.75 mm, more preferably equal to or less than 0.50 mm such as 0.45 mm or equal to or less than 0.30 mm or equal to or less than 0.20 mm or equal to or less than 0.10 mm.

[0049] In any case the cleanliness of the multimodal polyolefin is decisive for the cleanliness of the polyolefin layer composition.

[0050] The cleanliness of the (multimodal) polyolefin material or jacketing composition of the present invention is a critical characteristic and is defined in terms of lack of contaminants or particles in the material. A contaminant is a particle with any dimension larger than 0.1 mm not inherent in the product formulation. Also dark and / or oxidised polyolefin particles larger than 0.1 mm are considered contaminants if they show a sharp edge to the surroundings. As mentioned earlier, the multimodal polyolefin should be essentially free of particles larger than 0.1 mm in a 1 kg sample of material.

[0051] It is understood that the term “essentially free of particles” is intended to refer to less than 5 particles per kilogram sample of material, preferably less than 2 particles per kilogram sample of material and most preferably 0 particles per kilogram sample of material wherein the particles are larger than 0.1mm, and the amount is determined via the “tape method” described in the example section below for a 1 kg sample of material. The sample of material may be a sample of the (multimodal) polyolefin or the jacketing composition of the present invention.

[0052] The determination of the cleanliness of the multimodal polyolefin can be made by extruding a tape of the multimodal polyolefin and examining the tape with an automatic contamination detector based on a light source and a sensitive detector.

[0053] The required cleanliness of the multimodal polyolefin in the polyolefin layer composition in the fibre optic cable is obtained by a three-step purification sequence, steps b) to d), implemented after melt mixing of the polyolefin in step a).

[0054] The multimodal polyolefin may contain optional additives, such as conventional stabilizers, antioxidants, light stabilisers and the like. These are melt mixed or compounded into the polyolefin in a step a), thereby obtaining a polymer melt A.

[0055] Melt mixing may be carried out by conventional melt mixing techniques well known in the art

[0056] During melt mixing step a), the polyolefin and the optional additives can be blended, e.g. melt mixed in an extruder or compounding equipment. Typically, the process involves heating to a temperature of at least 150 °C, preferably at least 160 °C, such as at least 170 °C. The process will generally involve heating to 300 °C or less, such as 250 °C or less. Typically, co- or counterrotating twin screw extruders are used such as those available from Farrel, JSW, Kobe, Coperion or Werner & Pfleiderer. Alternatively, internal mixers such as Banbury or Bolling or continuous single screws such as BUSS kneaders or mixers may be used.

[0057] Preferably, the melt mixing step a) is effected in an extruder.

[0058] Optional additives in the multimodal polyolefin composition are each added in conventional amounts of at most about 10 wt%, preferably at most about 5 wt%, more preferably at most about 2.5 wt%, even more preferably at most about 1.0 wt%, yet even more preferably at most about 0.5 wt% of the multimodal polyolefin.

[0059] After the melt mixing step a), the obtained polymer melt A is filtered in a step b). Step b) is achieved by melting the polymer melt A and passing the melt through a filter with 40-250 pm, preferably 40-225 pm, more preferably 40-200 pm or 60-150 pm filter openings.

[0060] Preferably, the filtering is preferably carried out by melting and extruding the polymer melt A through an extruder with an appropriate melt filter attached to the die. The filter may be of a fixed type, i.e. permanently secured to the extruder die, or of a changing type, i.e. two alternating filters, a filter that moves continuously past the die, housed in a breaker plate or any other type of commercial filter.

[0061] It will be understood that the term “melt filter” includes a melt filter comprising a pack with one or more filters having the same or varying filter openings (e.g. where the filters are arranged by beginning with a larger micron retention size, proceeding to a smaller micron retention size, and ending with a larger micron retention size) and also includes screens, sieves, and other filter media, and other equivalents for which the micron retention size can be determined. A description of various filter media can be found in, for example, FILTRATION OF POLYMER MELTS (1988).

[0062] It is further understood by the person skilled in the art that the melt mixing step a) and the subsequent filtering / pelletization step b) may be performed in different parts of the same extruder, in subsequent extruders or in a combination of mixer s / extruders.

[0063] It is understood that the filtering of the multimodal polyolefin of the invention is facilitated by the good processability thereof. While a conventional unimodal polyethylene for a jacketing layer has a processability of about 20 kg / h at an extruder pressure of about 25 MPa, a preferred multimodal polyethylene for a jacketing layer in a fibre optic cable of the present invention having an MFR2 of 1.7 g / 10 min and a density of 0.942 g / cm3has an output of about 60 kg / h at an extruder pressure of about 25 MPa. With a view to achieve good processability it is preferred that the multimodal polyolefin has an MFR2 of at least 0.5 g / 10 min, more preferably at least 1.0 g / 10 min and most preferably 1.5 g / 10 min. This also facilitates the filtering of the multimodal polyolefin described above.

[0064] As part of the cleanliness of the multimodal polyolefin of the invention is the fact that, except for conventional stabilisers such as antioxidants and light stabilisers, it does not contain many additives. The stabilisers in the multimodal polyolefin of the invention are each added in conventional amounts of at most about 1 wt%, preferably at most about 0.75 wt%, and most preferred at most about 0.5 wt% of the multimodal polyolefin.

[0065] It is understood that at the end of the filtering / pelletization step b), pellets are obtained by pelletization. Generally, the pellets are obtained at the end of the extruder.

[0066] After the pelletization, the obtained pellets are dried and optionally classified by size in a step c).

[0067] After the drying step c), the pellets are pneumatically conveyed and passed through a magnetic grid and / or through a metal separator, preferably through a magnetic grid and a metal separator in step d). After step d), the pellets may be collected in final product silos.

[0068] In a preferred embodiment of the present invention, the pellets are conveyed and first passed through a magnetic grid before being passed through a metal separator second in step d).

[0069] The inventors have found that the metal separator is preferably positioned as one of the last steps, right before transferring the pellets to the product silos, since it has the capability to catch metal contaminants originating from possible equipment failure (including non-magnetic metal particles).

[0070] Optionally, after the drying step c), the pellets are passed into blending silos in step cl), from the blending silos, the pellets go to step d) and are optionally transferred to final product silos.

[0071] In an alternative embodiment of the present invention, after the drying step c), the pellets are passed through a magnetic grid and then into blending silos in step cl), from the blending silos, the pellets are passed through a metal separator in step dl) and optionally transferred to product silos.

[0072] It is understood that the pellets in the product silos contain the cleaned multimodal polyolefin composition which is comprised within the polyolefin layer composition of the fibre optic cable of the present invention.

[0073] It is further understood that the pellets obtained after step b) are dried and can be passed through a magnetic grid and a metal separator without collection in blending silos in between.

[0074] In a preferred embodiment of the present invention, the magnetic grid is equipped with permanent finger magnets having a magnetic induction of more than 5000 G, preferably more than 7500 G, even more preferably more than 10000 G. For example, ferrite or rare- earth NdFeB magnets, preferably NdFeB magnets, can be used. Furthermore, the magnetic grid is preferably arranged in at least two stages above each other to ensure that all impurities come in contact or at least in close proximity to the magnetic fingers.

[0075] An example of a suitable magnetic grid includes, but is not limited to, the magnetic grids available from Magsy, such as the Magsy MSV 350.

[0076] The inventors have found that the magnetic grid as defined above, is capable of catching even very small magnetic particles and even not fully magnetic particles and hence removes any ferromagnetic impurities from the pellets.

[0077] In a preferred embodiment of the present invention, the metal separator is equipped with a metal detector and a diverter system such as a diverter valve, swivel arms, air-blast nozzles, pushers, rejection flaps, swivel hoppers or retraction belts.

[0078] The inventors have found that a metal separator as defined above, is capable of detecting all metal pieces by means of the metal detector, also non-magnetic metals such as stainless steel and aluminum, and will remove them from the pellets by means of a diverter system which turns to a reject line once metal is detected until metal is no longer detected. The detection abilities are not limited to iron or magnetic materials, but rather include every type of metal using the transmitter / receiver method based on electromagnetic induction. While in operation, the metal detector generates a constant electromagnetic field and when a piece of metal passes through the detector, it disturbs the field. The signal subsequently received registers as the presence of metal.

[0079] Examples of suitable metal separators include, but are not limited to, the metal separators available from Mettler Toledo such as the Mettler Toledo GF300.

[0080] Jacketing layer composition

[0081] It is understood that all definitions and preferences, as described above, equally apply for all further embodiments, as described below.

[0082] The fibre optic cable jacketing composition according to the present invention comprises a multimodal polyolefin with a density of 0.910-0.960 g / cm3and an MFR2 of 0.2-6.0 g / 10 min, wherein the composition is obtainable and / or obtained by a process comprising: a) mixing and / or melt mixing a multimodal polyolefin with additives, thereby obtaining a polymer melt A; b) filtering said polymer melt A through a melt filter with 40-250 pm filter openings, followed by pelletization thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining the multimodal polyolefin composition in the form of pellets; wherein the obtained jacketing composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

[0083] Process

[0084] As said, contaminant free resins are desirable in many applications. The inventors have found that by using the process according to the present invention, the obtained polyolefin compositions are suitable for use in pipe applications, wire and cable applications and in particular fibre optic cable applications.

[0085] It is understood that all definitions and preferences, as described above, equally apply for all further embodiments, as described below.

[0086] The process according to the present invention provides a cleaned polyolefin composition and comprises the steps of : al) providing a polyolefin or polymerisation of at least one olefin, thereby obtaining a polyolefin; a) melting and / or melt mixing the polyolefin with optional additives, thereby obtaining a polymer melt Al; b) filtering said polymer melt Al through a melt filter with 40-250 pm filter openings, followed by pelletization thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining a cleaned polyolefin composition in the form of pellets; wherein the obtained polyolefin composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

[0087] The inventors found that when the cleaned polyolefin compositions prepared according to the process of the present invention are used in pipe applications, the chances of pipe rupture in the end use application decreases. Furthermore, the cleaned polyolefin compositions have improved electrical properties when used in wire and cable applications, in particular the electrical breakdown strength (e.g. characterized by the Weibull distribution) will improve.

[0088] Cables should generally be designed for a service life of more than 20 years. This poses a technological challenge in that a single rupture of the jacketing layer causes malfunction of the whole length. Hence, especially in the case of fibre optics cable, using the cleaned polyolefin compositions prepared according to the process of the present invention offer a significant advantage. The insulation and / or jacketing layers for this application are extremely thin and even minor contaminations may disturb the smoothness of these layers or could create weak points which in turn could cause (micro)cracking.

[0089] Furthermore, deformations in the jacketing of a fibre optic cable caused by such contaminants can also cause micro-bending which influences fibre attenuation.

[0090] Optional additives

[0091] In addition to the multimodal polyolefin, the multimodal polyolefin composition according to the present invention may comprise additional components such as a filler and other optional additives for utilization with polyolefins, such as pigments (e.g. carbon black), stabilizers (e.g. antioxidant agents), and / or UV absorbers, antistatic agents and utilization agents (e.g. processing aid agents including calcium stearate) may be present in the multimodal polyethylene composition used in the present invention.

[0092] Preferably, the total amount of these additives is 10 wt.-% or below, more preferably 8 wt.-% or below, most preferably 5 wt.-% or below, based on the total weight of the multimodal polyolefin composition.

[0093] The UV stabilizer may preferably be a hindered amine light-stabilizer (HALS), selected from Chimassorb® 944, Tinuvin® 622, Tinuvin® 770 and mixtures thereof. Above that, any of the UV stabilizers, preferably HALS compounds recited in EP 3 622 541 Al may be used. Two or more UV stabilizers may be combined. Any of the above mentioned UV stabilizers may also be combined with one or more UV absorbers, such as benzotriazoles, benzophenones, triazines, or phenolic compounds. Any of the UV absorbers recited in EP 19 191 837.4 may be used.

[0094] The UV stabilizer may be contained in an amount of from 0.01 to 2 wt.-%, more preferably 0.05 to 1.5 wt.-%, even more preferably 0.1 to 1 wt.-%., yet even more preferably 0.1 to 0.5 wt.-%, based on the total weight of the multimodal polyolefin composition. An antioxidant may be selected e.g. from sterically hindered phenols (e.g. Irganox® 1010 (CAS no 128-37-0), Irganox® 1076 (CAS no 2082-79-3), Irganox® 1035 (Cas no 41484-359), sulfur-containing antioxidants (e.g. Irganox® PS 802 (Cas no 693-36-7), Irganox® PS 800 (Cas no 2500-88-1), phosphites / phosphonites (e.g. Irgafos® 168 (Cas no 31570-04-4), Irgafos® P-EPQ (Cas no 38613-77-3), nitrogen-containing antioxidants (e.g. Naugard® 445 (Cas 10081-67-1), Vulcanox® HS / LG (cas no 26780-96-1). A combination of two or more may be used.

[0095] The antioxidant may be contained in an amount of from 0.01 to 2 wt.-%, more preferably 0.05 to 1 wt.-%, even more preferably 0.1 to 0.75 wt.-%, yet even more preferably 0.1 to 0.5 wt.-%, based on the total weight of the multimodal polyolefin composition.

[0096] A pigment may be selected e.g. from blue, green or orange pigments. A particularly preferred pigment is titan dioxide. Preferably, the total amount of pigments different form carbon black present in the polyethylene composition may be 1 wt.-% or below, further preferred 0.001 to 2 wt.%, of the total multimodal polyolefin composition

[0097] A preferred pigment is carbon black. In case of carbon black, it may be contained in an amount of from 0.01 wt.-% to not more than 5.0 wt.-%, preferably from 0.1 wt.-% to not more than 4.0 wt.-%, even more preferably from 0.15 wt.-% to not more than 3.5 wt.-%, even more preferably from 0.2 wt.-% to not more than 3.0 wt.-% of the total multimodal polyolefin composition. Thus, carbon black may be present or absent in the multimodal polyethylene composition used in the present invention. The carbon black may be added to the multimodal polyolefin composition in form of a carbon black masterbatch in an amount of from 0.1 wt.-% to not more than 10.0 wt.-%, preferably from 0.2 wt.-% to not more than 9.0 wt.-%, even more preferably from 0.5 wt.-% to not more than 8.0 wt.-% of the total multimodal polyolefin composition.

[0098] The multimodal polyolefin composition used in the present invention may be crosslinked in the presence of a crosslinking agent or in the presence of moisture and a condensation catalyst. Known technologies for moisture curing in the presence of a condensation catalyst are described in the references cited above. The multimodal polyolefin composition used in the invention may, in some embodiments, be crosslinked in the presence of a crosslinking agent, such as a peroxide crosslinking agent, present in an amount of from 0.1 to 2.0 wt.-%, preferably from 0.1 to 1.0 wt%, more preferably from 0.1 to 0.5 wt%. This can be done e.g. by soaking the pellets of the polymer composition or the reactants in the melt mixture with the crosslinking agent. Use

[0099] It is understood that all definitions and preferences, as described above, equally apply for all further embodiments, as described below.

[0100] The present invention also provides a use of a contamination reduction sequence to obtain a cleaned polyolefin composition essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition wherein the contamination reduction sequence comprises the steps of b) filtering a molten polyolefin composition through a melt filter with 40-250 pm filter openings, followed by pelletization thereby obtaining pellets; c) drying said pellets; d) passing said pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining the cleaned polyolefin composition, which is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition, in the form of pellets.

[0101] The invention will now be described with reference to the following non-limiting examples.

[0102] Examples

[0103] Determination methods

[0104] Unless otherwise stated in the description or experimental part the following methods were used for the property determinations. wt%: % by weight

[0105] Melt Flow Rate

[0106] The melt flow rate (MFR) was determined according to ISO 1133 and is indicated in g / 10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is determined at 190 °C for polyethylenes and at a loading of 2,1 kg (MFR2; ISO 1133, condition D).

[0107] The processability is defined herein in terms of the extruder output in kg / h at a given pressure in MPa. The extruder used is a single screw one of type Nokia- Maillefer with an L / D ratio of 24 / 1 and diameter 60 mm, run at 180°C. It is an advantage if the output is as high as possible at a given extruder pressure.

[0108] Density

[0109] The density was measured according to ISO 1183-1 / method A. Sample preparation is done by compression moulding in accordance with ISO 17855-2 :2016.

[0110] Determination of contaminant particles - tape method

[0111] Pellets of the multimodal polyolefin composition were fed either manually or automatically into an extruder to produce a cast film or tape. A stable film or tape was produced with a constant thickness of around 70 pm + / - 10 pm and constant take off speed. The cast film or tape was analysed using an optical detection system (comprising a camera and a halogen or programmed LED light source) and the defects were separated into different classes and size classes using the FSA100 software to determine the cleanliness. The camera works in transmission mode with a constant grey level. Particles in or on the film matrix, visible as significantly less transparent areas of the film compared to the matrix when evaluated in transmission mode, are classified as contaminants. The longest dimension of each contaminant was determined from digital image analysis of the geometric configuration of pixels comprising the contaminant. Each inspected tape volume had a weight of at least 1 kg and / or a volume higher than 10 m2

[0112] Example 1:

[0113] A melt screen, magnetic grid and a metal separator were installed in the production line. These also served as analytic tools for this example. As the total contaminant count on industrial production volumes is very small, it is difficult to measure on small batch samples. Therefore, the improvement of the quality of the final product is shown here by analysing how many contaminants are captured by the magnetic grid and the metal separator, which otherwise would ended up in the final product.

[0114] Bimodal polyethylene was produced and this polymer, together with antioxidants, Ca- stearate and a carbon black masterbatch, was fed to an extruder equipped with a melt screen. The mesh size of the melt screen was 150 pm. The melt screen showed about 10% pressure drop increase after 5 days of operation, then it was changed to a new one. The used melt screen contained carbon black agglomerates and small rusty metal pieces. The magnetic grid, which was installed downstream of extruder, was checked twice per day during this 5 days operating period and 0-5 pieces of magnetic pellets were found per check, with a total of 17 pieces. These magnetic pellets were ashed (polymer removed by burning it) and studied under a microscope. This confirmed that the withheld pellets effectively contained small metal pieces.

[0115] Example 2:

[0116] During operation with 1) melt screen, 2) magnetic grid and 3) metal separator placed in the production line, the count of metallic findings in the metal separator suddenly increased remarkably (with no or very low count of metallic findings in normal operation). These metallic findings were identified as small Al-alloy pieces. The pelletization process was stopped and the equipment was checked. It was found that the origin of Al-alloy pieces was an internal blending pipe of the blending silo which was fractured. This example shows that using only a melt screen is not sufficient. Without the presence of the metal separator, the Al-alloy pieces would have remained undetected and would have ended up in the final product. Especially for fibre optic cable applications, this is not acceptable.

[0117] Example 3:

[0118] Multimodal PE products were produced by using a 1) melt screen, 2) magnetic grid and 3) metal separator combination in the production line. Product was collected to the blending silo and blended (=circulated / mixed) there to level out possible variations in the product during the production of the batch. A sample of 1 kg, corresponding to about 30 000 pellets, was taken after blending and transferring to product silo. This sample was analysed according to the “Determination of contaminants” method described above.

[0119] When testing preferred multimodal polyethylene for a fibre optic cable jacketing composition of the present invention the tapes showed the following cleanliness:

Claims

Claims:

1. A cable comprising a polyolefin layer, preferably a fibre optic cable, wherein the polyolefin layer comprises a multimodal polyolefin composition, wherein the multimodal polyolefin has a density of 0.910-0.960 g / cm3as measured according to ISO 1183-1 / method A and an MFR2 of 0.2-6.0 g / 10 min as measured according to ISO 1133 at 190 °C and a load of 2.1 kg, wherein the composition is obtained by a process comprising: a) melting and / or melt mixing a multimodal polyolefin with optional additives, thereby obtaining a polymer melt A; b) filtering said polymer melt A through a melt filter with 40-250 pm filter openings, followed by pelletization thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining the multimodal polyolefin composition in the form of pellets; wherein the multimodal polyolefin composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

2. The cable according to claim 1, wherein in step d) the pellets obtained in step c) are passed through a magnetic grid and a metal separator.

3. The cable according to claim 1 or 2, wherein the composition is obtained by a process further comprising a step al) preceding step a): al) polymerisation of at least one olefin in at least two stages, thereby obtaining a multimodal polyolefin.

4. The cable according to any one of claims 1-3, wherein the multimodal polyolefin is bimodal or trimodal, preferably bimodal and / or comprises polyolefins selected from the group consisting of ethylene and propylene (co)polymers.

5. The cable according to any one of claims 1-4, wherein the multimodal polyolefin has a density of 0.915-0.955 g / cm3as measured according to ISO 1183-1 / method A and a melt flow rate (MFR2) of 0.2-3.0 g / 10 min as measured according to ISO 1133 at 190 °C and a load of 2.1 kg, and comprises at least a first and a second polyolefin fraction, of which the first fraction has either (a) a density of 0.930-0.975 g / cm3as measured according to ISO 1183-1 / method A and a melt flow rate (MFR2) of 50-2000 g / 10 minas measured according to ISO 1133 at 190 °C and a load of 2.1 kg, or (b) a density of 0.880-0.930 g / cm3as measured according to ISO 1183-1 / method A and a melt flow rate (MFR2) of 0.01-0.8 g / 10 min as measured according to ISO 1133 at 190 °C and a load of 2.1 kg.

6. The cable according to any one of claims 3-5, wherein step al) comprises a coordination- catalysed polymerisation in at least two stages of ethylene together with at least one a- olefin comonomer having 3-12 carbon atoms in at least one of the stages, and wherein the polymerisation stages have preferably been carried out as slurry polymerisation, gasphase polymerisation, or a combination thereof and / or the polymerization has been carried out in a loop-reactor / gas-phase-reactor process in at least one loop reactor followed by at least one gas-phase reactor.

7. The cable according to claim 6, wherein the comonomer is selected from the group consisting of 1-butene, 1-hexene, 4-methyl-l -pentene, and 1-octene.

8. The cable according to any one of the preceding claims, wherein the polyolefin layer is an insulation layer, a jacketing layer or a combination of an insulation / jacketing layer, preferably a jacketing layer.

9. The cable according to claim 8 wherein the polyolefin layer is a jacketing layer consisting of the multimodal polyolefin composition.

10. The cable according to claim 8 or 9, being a fibre optic cable wherein the jacketing layer has a thickness of equal to or less than 1.0 mm, preferably equal to or less than 0.75 mm, more preferably equal to or less than 0.50 mm such as 0.45 mm.

11. A cable jacketing composition comprising a multimodal polyolefin with a density of 0.910-0.960 g / cm3as measured according to ISO 1183-1 / method A and an MFR2 of 0.2-6.0 g / 10 min as measured according to ISO 1133 at 190 °C and a load of 2.1 kg, wherein the composition is obtained by a process comprising: a) melting and / or melt mixing a multimodal polyolefin with optional additives, thereby obtaining a polymer melt A; b) filtering said polymer melt A through a melt filter with 40-250 pm filter openings, followed by pelletization thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining the multimodal polyolefin composition in the form of pellets;wherein the jacketing composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

12. The cable composition according to claim 11, wherein in step d) the pellets obtained in step c) are passed through a magnetic grid and a metal separator.

13. A process for producing a polyolefin composition comprising the steps of : al) providing a polyolefin or polymerisation of at least one olefin, thereby obtaining a polyolefin; a) melting and / or melt mixing the polyolefin with optional additives, thereby obtaining a polymer melt Al; b) filtering said polymer melt Al through a melt filter with 40-250 pm filter openings, followed by pelletization thereby obtaining pellets; c) drying said pellets; d) passing the pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining a cleaned polyolefin composition in the form of pellets; wherein the obtained polyolefin composition is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition.

14. The process according to claim 13, wherein in step d) the pellets obtained in step c) are passed through a magnetic grid and a metal separator.

15. The process according to claim 13 or 14, wherein the polyolefin is a multimodal polyolefin with a density of 0.910-0.960 g / cm3as measured according to ISO 1183-1 / method A and an MFR2 of 0.2-6.0 g / 10 min as measured according to ISO 1133 at 190 °C and a load of 2,1 kg.

16. The process according to any of claims 13-15, wherein step al) comprises a coordination-catalysed polymerisation in at least two stages of ethylene together with at least one a-olefin comonomer having 3-12 carbon atoms in at least one of the stages, and wherein the polymerisation stages have preferably been carried out as slurry polymerisation, gas-phase polymerisation, or a combination thereof and / or the polymerization has been carried out in a loop-reactor / gas-phase-reactor process in at least one loop reactor followed by at least one gas-phase reactor.

17. The process according to claim 16, wherein the at least one a-olefin comonomer is selected from the group consisting of 1-butene, 1-hexene, 4-methyl-l -pentene, and 1- octene.

18. Use of a contamination reduction sequence to obtain a cleaned polyolefin composition essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition wherein the contamination reduction sequence comprises the steps of: b) filtering a molten polyolefin composition through a melt filter with 40-250 pm filter openings, followed by pelletization thereby obtaining pellets; c) drying said pellets; d) passing said pellets obtained in step c) through a magnetic grid and / or a metal separator, preferably a magnetic grid and a metal separator, thereby obtaining the cleaned polyolefin composition, which is essentially free from particles with a dimension larger than 0.1 mm in a 1 kg sample of said composition, in the form of pellets.

19. The use according to claim 18, wherein in step d) the pellets obtained in step c) are passed through a magnetic grid and a metal separator.

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