IMPROVED FOAMING BEHAVIOR OF POLYMER COMPOSITIONS USING PASSIVE NUCLEATION

Abstract

The present invention relates to a foamable polymer composition and a foamed polymer composition obtained by foaming the foamable polymer composition. The invention also relates to a cable comprising at least one layer comprising either the foamable polymer composition or the foamed polymer composition. The invention further provides a process for producing a foamed polymer composition.

Description

IMPROVED FOAMING BEHAVIOR OF POLYMERIC COMPOSITIONS USING PASSIVE NUCLEATION Qzocnn / Lznz / E / YiAi Field of Invention The present invention relates to a foamable polymer composition and to a foamed polymer composition obtained by foaming this foamable polymer composition. The invention also relates to a cable comprising at least one layer comprising the foamable polymer composition or the foamed polymer composition. The invention further provides a process for producing a foamed polymer composition. Background of the Invention Communication cables are used to transmit high-frequency (HF) signals, either through electromagnetic waves or as light pulses, as in fiber optic cables. Coaxial cables are an example of communication cables. Coaxial cables consist of two parallel conductors laid concentrically along the same axis and separated by an insulating dielectric. Foamed LDPE, for example, alone or blended with another polymer, is routinely used to insulate communication cables, such as coaxial and / or radio frequency cables. The foaming of the polymer composition can be achieved using Ref. 317686 chemical or physical blowing agents, or a combination of both. Chemical blowing agents are substances that release an expanding gas through thermal decomposition reactions, and the chemical blowing agent is consumed in the foaming reaction. Examples of such substances include hydrazine, hydrazide, azodicarbonamide (ADCA), or those based on combinations of solid organic acids (or a metal salt thereof) and alkali metal carbonate(s) or alkali metal bicarbonate(s), such as a combination of citric acid / citric acid derivative and sodium bicarbonate. Physical blowing agents are gases injected directly into the molten polymer. In such processes, chemical blowing agents are commonly used as cell nucleators, since the gas formed by the blowing agent reaction serves as lower-energy nucleation sites for bubble formation. Examples of gases used as physical blowing agents include N₂ or CO₂. In the physical processes of foam formation, a nucleating agent, also known as a nucleating agent or stimulant, is typically used. The nucleating agent provides points in the insulation where the energy required for bubble formation is lower. These nucleating agents can be passive or active. An active nucleating agent is a substance that decomposes into gaseous products, i.e., a chemical blowing agent, while passive nucleating agents are particles that only provide localized points of lower energy where bubble formation is more likely to occur. In general, passive nucleators, i.e., nucleating agents that do not undergo a chemical reaction to form gas, are considered less effective than active nucleating agents, such as ADCA. Both physical and chemical foam extrusion processes are used for the extrusion of foamed communication cable insulation. For communication cables, a good cell structure within the foam insulation is crucial for achieving isotropic electrical properties. A cell structure with many small cells evenly distributed throughout the insulation is desirable. Cell structure is also important for mechanical properties. Many small, well-distributed cells provide better crush resistance compared to a structure with larger, unevenly distributed cells, which will create weak points in the insulation. The main blowing agent used in the cable industry is azodicarbonamide (ADCA), which has a decomposition temperature range that fits well within the processing window of polyolefins, such as Qzocnn / Lznz / E / YiAi, like polyethylene, provides a fine foam structure, which is a key requirement for cable applications. Due to ADCA's inclusion on the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) Candidate List and the risk of future inclusion on the Authorisation List, significant efforts have been made to find an alternative solution. ADCA was identified as a Substance of Very High Concern (SVHC) and included on the Candidate List because it has been identified as a respiratory sensitizer, with known cases of asthma in workers exposed to ADCA in dust form. Another problem with azodicarbonamide is that ammonia is released from the decomposition reactions of the blowing agent. This disrupts the work environment for cable manufacturers, as it produces an unpleasant odor. Alternatives to ADCA that are compatible with the polyethylene processing window are endothermic blowing agents. Endothermic blowing agents are conventionally combinations of sodium bicarbonate and citric acid or a citric acid derivative. These blowing agents are typically added directly to the extruder hopper or dry-mixed with polyolefins before extrusion. In high-speed extrusion processes such as cable extrusion, this method of adding the blowing agent does not provide sufficient homogenization of the material. Qzocnn / Lznz / E / YiAi expanding agent in the polymer melt and this results in foamed insulation with a poor cell structure and a poor surface. Another drawback of endothermic blowing agents is that they produce polar decomposition products, such as water. These polar decomposition products can negatively impact the electrical properties of the insulation at high frequencies, such as the dissipation factor. This is especially disadvantageous for high-frequency applications, such as coaxial cables. WO 2004 / 094526 refers to foam compositions and cables having a low-loss foam layer. A blowing co-agent selected from hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and perfluorocarbons (PFCs) is used to obtain the low-loss foam. However, such compounds used as a blowing co-agent have a negative impact on the environment. Detailed Description of the Invention It is an object of the invention to provide a foamable polymer composition that overcomes the problems mentioned above. Another object of the invention is to replace hydrazine, hydrazide, or azodicarbonamide (ADCA) in a Qzocnn / Lznz / E / YiAi foamable polymer composition, while maintaining an improved cell structure, i.e., a small and uniform distribution of cells in the foam, in the foamed product. Another object of the invention is to provide a foamable polymer composition that is not foamed using hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and perfluorocarbons. Another object of the invention is to provide a foamed polymer composition having a reduced density and at the same time a high cell density, small cells distributed evenly in the foamed polymer and maintaining an improved, i.e., low, dissipation factor. The present invention is based on the surprising discovery that all the aforementioned objects can be resolved using a mineral nucleating agent, such as talc, in a foamable polymer composition. Therefore, the present invention provides a foamable polymer composition comprising a first polyolefin polymer, a second polyolefin polymer having an MFR2(2.16 kg; 190 °C) of between 2 and 15 g / 10 min measured according to ISO 1133-1, and a mineral nucleating agent in an amount of 0.5 to 6% by weight based on the total foamable polymer composition, wherein the first polymer Qzocnn / Lznz / E / YiAi polyolefin has a higher density than the second polyolefin polymer. The present invention further provides a foamed polymer composition obtained by foaming the foamable polymer composition according to the invention using an expanding agent. The present invention also provides a cable comprising at least one layer comprising the foamable polymer composition according to the invention or the foamed polymer composition according to the invention. The present invention has several remarkable advantages. The foamable polymer composition of the invention can be made to produce foam in a foamed polymer composition without using harmful blowing agents such as ADCA and halogenated hydrocarbons, while at the same time maintaining a good dissipation factor, particularly in comparison with polymer compositions that foam, for example, with ADCA. The foamed polymer composition obtained by foaming the foamable polymer composition exhibits a foam density comparable to that of foams using ADCA but has a comparable or even smaller cell size and a comparable or even higher cell density in the foam compared to foams expanded with ADCA. Qzocnn / Lznz / E / YiAi The present invention uses a mineral nucleating agent (C) as a passive nucleating agent. The mineral nucleating agent is preferably a magnesium-containing compound, a calcium-containing compound, a silicon-containing compound, or mixtures thereof. Preferably, the mineral nucleating agent comprises one or more selected from the group consisting of talc, clay, mica, calcium carbonate, and silica. Among the aforementioned mineral nucleating agents, talc is preferred. Even more preferably, the mineral nucleating agent consists of talc. Talc has the advantage of not containing water and not releasing water during foaming. To obtain a uniform distribution of the mineral nucleating agent in the foamable polymer composition, the mineral nucleating agent is added, preferably in combination, to the foamable polymer composition in powder form, i.e., in the form of small particles, or as a masterbatch. The average particle size is typically on the order of 0.1 µm to 50 µm. Since the nucleating agent is a mineral nucleating agent, its thermal decomposition temperature is high. This has the advantage that the mineral nucleating agent of the invention does not thermally decompose during, for example, the melting and extrusion of the polymer composition. Preferably, the mineral nucleating agent does not decompose. Qzocnn / Lznz / E / YiAi thermally below 275 °C, more preferably not below 300 °C, even more preferably not below 350 °C, even more preferably not below 400 °C, even more preferably not below 500 °C, even more preferably not below 600 °C. The foamable polymer composition according to the invention comprises a first polyolefin polymer (A) and a second polyolefin polymer (B). The first polyolefin polymer (A) has an MFR2 (2.16 kg; 190 °C) preferably of 0.1 to 20 g / 10 min, more preferably of 1 to 17 g / 10 min, more preferably of 2 to 14 g / 10 min, more preferably of 4 to 14 g / 10 min, and even more preferably of 6 to 10 g / 10 min, measured according to ISO 1133-1. The second polyolefin polymer (B) has an MFR2 (2.16 kg; 190 °C) of 2 to 15 g / 10 min measured according to ISO 1133-1. Preferably, the second polyolefin polymer (B) has an MFR2 (2.16 kg; 190 °C) of 2.5 to 12 g / 10 min, more preferably 3 to 10 g / 10 min, more preferably 3.5 to 8 g / 10 min, and even more preferably 4 to 6 g / 10 min measured according to ISO 1133-1. The first polyolefin polymer (A) is preferably present in an amount of 20 to 95% by weight, more preferably in an amount of 40 to 90% by weight, more preferably in an amount of 50 to 85% by weight and even more preferably in an amount of 60 to 80% by weight, based on the total foamable polymer composition, and the second polyolefin polymer (B) is preferably present in an amount of 5 to 80% by weight, more preferably in an amount of 10 to 70% by weight, more preferably in an amount of 15 to 60% by weight and more preferably in an amount of 20 to 40% by weight, based on the total foamable polymer composition. The first polyolefin polymer (A) is preferably an ethylene homopolymer or copolymer or a propylene homopolymer or copolymer, more preferably an ethylene copolymer, and the second polyolefin polymer (B) is preferably an ethylene homopolymer or copolymer or a propylene homopolymer or copolymer, more preferably an ethylene homopolymer. The first polyolefin polymer (A) is preferably a high-density polyethylene homopolymer or copolymer having a density of 935 to 970 kg / m3 measured according to ISO 1183-1 and the second polyolefin polymer (B) is preferably a low-density polyethylene homopolymer or copolymer having a density of 880 to 930 kg / m3 measured according to ISO 1183-1. More preferably, high-density polyethylene (HDPE) is a copolymer and low-density polyethylene (LDPE) is a homopolymer. Homopolymer means that low-density polyethylene (LDPE) comprises at least 90% by weight of ethylene monomer, preferably at least 95% by weight of ethylene monomer, and even more preferably at least 99% by weight of ethylene monomer. In the case of high-density polyethylene (HDPE) being a copolymer, the copolymer comprises an ethylene monomer, preferably in an amount of at least 50% by weight based on the total copolymer, and one or more comonomer(s). The comonomer may be alpha-olefins having from 3 to 12 carbon atoms, for example propene, butene, hexene, octene, decene. Low-density polyethylene (LDPE) is preferably a homopolymer. In the case of foamed polyethylene used in communication cables, both electrical and mechanical properties are important. HDPE has a lower dielectric constant and a lower loss factor than LDPE, as well as greater strength and hardness. High-density polyethylene (HDPE) polymer is polymerized in a low-pressure process and is, for example, an optional HDPE homopolymer or optional HDPE copolymer of ethylene with one or more comonomer(s) as described above. Furthermore, HDPE is polymerized in a low-pressure polymerization process in the presence of a catalyst. The catalyst can be, for example, a Phillips catalyst, a metallocene catalyst, or a Ziegler-Natta catalyst. Polymerization can be, for example, either gas-phase polymerization, suspension polymerization, or a combination of suspension / gas-phase polymerization or gas-phase / gas-phase polymerization. Polymerization can also be solution polymerization. To form a foam from a foamable polymer composition, the composition must have good melt strength, as insufficient melt strength results in a collapsed cell structure that is detrimental to the mechanical and electrical properties of the cable layer, typically the insulation layer. Melt strength can be improved by blending LDPE into the foamable polymer composition to enhance melt strength and ensure a foamed layer with a closed-cell structure and homogeneous cell distribution. Low-density polyethylene (LDPE) polymer is polymerized in a high-pressure radical polymerization process. Furthermore, LDPE is polymerized in this high-pressure process in the presence of an initiator or initiators and chain transfer agents, such as propane, propene, propionaldehyde, and methyl ethyl ketone, to control the molecular frequency response (MFR). LDPE can be produced, for example, in a tubular polymerization reactor, or in an autoclave polymerization reactor. The dissipation factor, also known as tan δ, is a measure of the degree of power dissipation in a dielectric material, that is, a measure of the amount of electrical energy that is transformed into heat in the dielectric material. The foamable polymer composition of the invention preferably has a dissipation factor at 1.9 GHz of 80-10⁻⁶ to 270-10⁻⁶, more preferably of 120-10⁻⁶ to 260-10⁻⁶, and even more preferably of 130-10⁻⁶ to 240-10⁻⁶. The foamable polymer composition preferably comprises an antioxidant. The antioxidant is preferably a phenolic antioxidant, a phosphorus-containing antioxidant, or mixtures thereof, more preferably a mixture thereof. The phenolic antioxidant is preferably a mixture of pentaerythrityl-tetrakis(3(3',5'-di-tert-butyl-4-hydroxyphenyl)-propionate (CAS number 6683-19-8; commercially available from BASF under the trade name Irganox 1010) and tris-(2,4-di-tert-butylphenyl)phosphite (CAS number 31570-04-4; commercially available from BASF under the trade name Irgafos 168). This mixture of antioxidants is commercially available from BASF as Irganox B561. Qzocnn / Lznz / E / YiAi The antioxidant is preferably present in an amount of 0.01% by weight to 2% by weight, more preferably in an amount of 0.04% by weight to 1% by weight, and even more preferably in an amount of 0.08% by weight to 0.5% by weight, based on the total foamable polymer composition. The foamable polymer composition preferably comprises an acid scavenger. The acid scavenger is preferably calcium stearate, sodium stearate, zinc stearate, or mixtures thereof, more preferably zinc stearate. The amount of acid scavenger is preferably from 0.01% by weight to 2% by weight, more preferably from 0.02% by weight to 1% by weight, and even more preferably from 0.04% by weight to 0.08% by weight, based on the total foamable polymer composition. A foamed polymer composition can be obtained by foaming the foamable polymer composition according to the invention using an expanding agent (D). Foaming is a physical foaming process, meaning that the blowing agent (D) is injected into or mixed with the foamable polymer composition. The mineral nucleating agent particles present in the foamable polymer composition act as localized points of lower energy where bubble formation is most likely to occur. The blowing agent (D) is injected into or mixed with the foamable polymer composition, preferably Qzocnn / Lznz / E / YiAi during the extrusion of the foamable polymer composition in an extruder. During extrusion, the blowing agent (D) is mixed in a molten state with the molten polymer composition, and the molten polymer composition is allowed to expand at the extruder nozzle outlet. The temperature during extrusion is preferably between 130 °C and 240 °C. Extrusion is preferably carried out on a gas-injection foaming line. The blowing agent (D) preferably comprises a gas, and the gas comprises N2, CO, CO2, Ar, or mixtures thereof. More preferably, the blowing agent (D) comprises N2 and / or CO2. The blowing agent (D) is preferably used in an amount of 0.01% by weight to 5% by weight, more preferably from 0.015% by weight to 2.5% by weight, more preferably in an amount of 0.02% by weight to 0.2% by weight, more preferably in an amount of 0.03% by weight to 0.1% by weight, based on the foamable polymer composition. Preferably, the blowing agent (D) does not comprise a hydrocarbon, a halogenated hydrocarbon, citric acid or citric acid derivatives, azodicarbonamide, or mixtures thereof. Halogenated hydrocarbons include, for example, hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and perfluorocarbons (PFCs). Qzocnn / Lznz / E / YiAi The foamed polyolefin composition has a mean cell diameter preferably from 5 pm to 500 pm, more preferably from 50 pm to 400 pm, more preferably from 100 pm to 300 pm, more preferably from 150 pm to 275 pm, more preferably from 155 pm to 260 pm and even more preferably from 160 pm to 250 pm. The foamed polymer composition has a density preferably of 85 kg / m3 to 870 kg / m3, more preferably of 150 kg / m3 to 860 kg / m3, more preferably of 300 kg / m3 to 850 kg / m3, more preferably of 350 kg / m3 to 800 kg / m3, and even more preferably of 400 kg / m3 to 775 kg / m3. The invention provides a cable comprising at least one layer comprising the foamable polymer composition according to the invention or the foamed polymer composition according to the invention. Therefore, the cable comprises at least one layer comprising the foamable polymer composition according to either of the embodiments described above, or the cable comprises at least one layer comprising the foamed polymer composition according to either of the embodiments described above. Preferably, the cadle is a communications cadle, preferably a coaxial cadle or a twisted-pair cadle. At least one layer is preferably an insulation layer of the cable. The insulation layer is a layer that surrounds the innermost conductor wire, which is usually made of copper. Preferably, a cladding layer is placed between the innermost conductor wire and the insulation layer. The typical thickness of the insulation layer is 0.01 mm to 80 mm for coaxial cables and 0.1 mm to 2 mm for twisted-pair cables. The foamed polymer composition according to the invention can preferably be produced by a process for producing a foamed polymer composition, the method comprising the steps of: a) providing a foamable polymer composition according to the invention, b) molten mixing of the foamable polymer composition with an expanding agent (D) at a temperature of 130 °C to 240 °C to obtain a molten polymer composition, and c) cause the molten polymer composition to foam. Preferably, the molten mixing of step b) takes place in an extruder, and the foaming of the molten polymer composition obtained in step c) occurs after the molten polymer composition exits an extruder nozzle. In the extruder, the foamable polymer composition is melted and mixed with the blowing agent (D) to obtain a molten ozocnn / Lznz / E / YiAi polymer composition. The temperature in step b) is preferably between 140 °C and 230 °C. Preferably, the extruder can be any extruder known in the art to be suitable for molten mixing of a molten polymer with a blowing agent. All the above-described embodiments of the foamable polymer composition according to the invention are also preferred embodiments of the foamable polymer composition used in the process to produce a foamed polymer composition. All preferred embodiments of the blowing agent (D) as described above are preferred embodiments of the blowing agent (D) used in the process to produce a foamed polymer composition. EXAMPLES 1. Measurement methods a) Flow index The melt flow rate (MFR) is determined according to ISO 1133-1 and is expressed in g / 10 min. The MFR is an indication of the polymer's flowability and, therefore, its processability. The higher the melt flow rate, the lower the polymer's viscosity. The MFR2 of polyethylene (co-)polymers is measured at a temperature of 190 °C and with a load of 2.16 kg. The MFR2 of polypropylene (co-)polymers is measured at a Qzocnn / Lznz / E / YiAi temperature of 230 °C and with a load of 2.16 kg. b) Density of the solid material The method for determining the density of foamable polymer compositions follows ISO 178552 for sample preparation and ISO 1183-1 / method A for density measurement. Compression molding is performed in a controlled-cooling press, with a molding temperature of 180 °C for polyethylene and a cooling rate of 15 °C / min. Samples are conditioned at 23 ± 2 °C for a minimum of 16 hours. Density is determined at 23 ± 0.1 °C using isododecane as the immersion liquid and without buoyancy correction. c) Density of the foamed material To determine the density of the foamed samples, the weight of each sample was measured in an atmosphere of air (wl in g) as well as in a medium at a known temperature (ww in g). All measurements were performed at 22 °C in distilled water with 3 drops of a wetting agent added. The following equation was used to calculate the density: wLp=a—(pw -p¿+p^ where p = density in g / cm3 Pw = density of water in g / cm3 at the temperature of Qzocnn / Lznz / E / YiAi measurement Pl = air density (0.0012 g / cm3) d) Calculated cell density. The cell density (Nb / cm3) of the foamed polymer compositions has been calculated as follows: 1ozocnn / Lznz / E / YiAi where Pf = density of the foamed sample in g / cm3pm = density of the polymer matrix in g / cm3 D = average cell diameter in cm. e) Density reduction calculation The reduction in density (X) in percentage is calculated with the following formula: / df\ X = 1 --L * 100 V DSJ where Ds= density of the solid material in kg / m3 Df = density of the foamed material in kg / m3 f) Determination of the average cell diameter. To determine the mean cell diameter, the cross-sectional area of ​​approximately 60 cells (if available) was measured. The cells were then manually labeled in the Alicona system's image analysis software. The mean cell diameters were calculated assuming the bubbles have a circular cross-section. This method facilitates comparison of foam morphologies across different samples, as the geometry of most cells deviates from the ideal round shape, making a direct comparison of diameters impossible. Using the following equation and subsequently averaging the calculated values ​​of each bubble diameter, the average diameter was determined. Qzocnn / Lznz / E / YiAi where Dz = diameter of a foam cell under the assumption of a circular cross-section in pm Az = cross-section of a foam bubble in pm2. g) Microscopic analysis of foamed polymer compositions All samples were examined for foam density and morphology. For this reason, cell size was measured using Alicona's optical microscope. InfiniteFocus (Alicona Imaging GmbH, Austria). Density was determined using a high-precision balance (Excellence XS Analyze Waage, Mettler Toledo AG, Switzerland) equipped with a density measurement kit (Density Kit, Mettler Toledo AG, Switzerland). 2. Dielectric properties (value of the dielectric loss tangent (tan δ) - dissipation factor) a) Preparation of the plates: The polymer compounds were compression molded at 140 °C in a frame to produce plates 4 mm thick, 80 mm wide, and 130 mm long. The pressure was set high enough to obtain a smooth plate surface. A visual inspection of the plates revealed no inclusions such as trapped air or any other visible contamination. b) Characterization of the plates by their dielectric properties: A split-pole dielectric resonator (tan δ) was used in conjunction with a network analyzer (Rodhe & Schwarz ZVL6) to measure the dielectric constant and tangent delta (tan δ) of the materials. The technique measures the complex permittivity of the dielectric laminar sample (plates) in the frequency range of 1 to 10 GHz. The test is performed at 23 °C. The split-post dielectric resonator (SPDR) was developed by Krupka and his collaborators [see: J. ozocnn / Lznz / E / YiAi Krupka, RG Geyer, J. Baker-Jarvis and J. Ceremuga, Measurements of the complex permittivity of microwave circuit board substrates using a split dielectric resonator and re-entrant cavity techniques, Proceedings of the Conference on Dielectric Materials, Measurements and Applications - DMMA '96, Bath, UK, published by the IEEE, London, 1996.] and is one of the easiest and most convenient techniques to use for measuring microwave dielectric properties. Two identical dielectric resonators are placed coaxially along the z-axis so that there is a small laminar gap between them in which the sample to be measured can be placed. By choosing suitable dielectric materials, the resonant frequency and Q factor of the SPDR can be made to be temperature stable.Once a resonator is fully characterized, only three parameters need to be measured to determine the complex permittivity of the sample: its thickness and the changes in the resonant frequency, Af, and in the Q factor, AQ, obtained when it is placed in the resonator. Samples 4 mm thick were prepared by compression molding as described above and measured at a high frequency of 1.9 GHz. A complete review of the method can be found in J. Krupka, RN Clarke, OC Rochard and AP Gregory, Split-Post Qzocnn / Lznz / E / YiAi Dielectric Resonator technique for precise measurements of laminar dielectric specimens - measurement uncertainties in Proceedings of the XIII Int. Conference MIKON'2000, Wroclaw, Poland, pages 305-308, 2000. 3. Materials As the HDPE component, a unimodal Ziegler-Natta catalyzed HDPE copolymer with butene as a comonomer was used, with an MFR2 of 8 g / 10 min and a density of 9.63 kg / m3. LDPE is an autoclavable LDPE homopolymer that has an MFR2 of 4.5 g / 10 min and a density of 923 kg / m3. nCore 7155-M1-300 is a master lot of Americhem's commercially available azodicarbonamide-based blowing agent (ADCA). It contains 15% active blowing agent. Hydrocerol NUC 5155 is a nucleation masterbatch containing 50% talc in a polyethylene carrier. It is commercially available from Clariant. Irganox B561 is a commercially available antioxidant blend from BASF. Zincum TX is an acidic zinc stearate eliminator, commercially available from Baerlocher. Mistrocell M90 is a talc, commercially available from Imerys Tale. The particle size is d50 = 3.4 pm and BET is 11.0 m² / g. Qzocnn / Lznz / E / YiAi 4. Preparing examples 4.1 Mixing of materials The examples in Tables 1 and 3 were combined in a BUSS MDK46 continuous extruder (year of manufacture 1985). The line is a single-screw extruder with a screw diameter of 46 mm and an L / D of 11. ozocnn / Lznz / E / YiAi Table 1: Composition of comparative (CE) and inventive (IE) examples for foam formation, quantities are given in % by weight Material CE1 IE1-1 IE1-2 HDPE, % weight 69.6 69.6 69.6 LDPE, % weight 29.8 28.25 26.25 Irganox B561, % weight 0.1 0.1 0.1 Zincum TX, % weight 0.05 0.05 0.05 NCore 7155-M1-300, % weight 0.45 Hydrocerol NUC 5515, % weight 2 4 Density, kg / m3 947.4 958.9 966.3 4.2 Extrusion and foaming The polymer granules of the compositions in Table 1 were extruded on a Rosendahl RE45 extrusion line with a 45 mm diameter screw. The extruder has a total length of 32D, including an 8D long oil-quenched barrel extension used for better control of the polymer melt temperature. To achieve longer residence time and improved homogenization, a static mixer (Sulzer SMB-R type, Switzerland) with a length of 4D is mounted between the barrel extension and the extrusion nozzle. A 4.0 mm diameter round die was used. The extruder had 10 temperature zones, and nitrogen (N2) gas was injected between zones 7 and 8. The temperature settings in °C were as follows (the bar indicates different temperature zones): Ti: 40 / 150 / 160 / 160 / 165 / 170 / 190 / 190 / 170 / 170 / 170 / 170 / 170°C The results are given in Table 2 below. The density of the foam is measured at 22 °C. Table 2: Properties of the foamed compositions obtained ozocnn / Lznz / E / YiAi E.g. Nitrogen quantity % weight Temperature adjustment / temp. Melting point (°C) Average cell diameter pm Density kg / m3 Density reduction % Calculated cell density Nb / cm3 CE1 0.04 Ti / 186 191 678 28.4 7.71 104 CE1 0.05 Ti / l 86 167 470 50.4 2.07· 105 IE1-1 0.04 Ti / 188 242 591 38.4 5.05·104 IE1-1 0.05 Ti / 188 212 460 52.0 1.03 105 IE1-2 0.04 Ti / 188 198 549 43.2 1.03 105 IE1-2 0.05 Ti / 189 182 438 54.7 1.71 105 The electrical loss factor (dissipation factor) was measured in Comparative Example 2 and Examples of the Invention IE2-1 to IE2-3. The composition of all examples is given in Table 3 below. After mixing as described above, all materials were compression molded at the foaming temperature (140 °C) as described above. These plates were then subjected to loss factor measurement at 1.9 GHz (tan δ). Table 3: Composition of comparative (CE) and inventive (IE) examples with quantities given in % by weight, and the results of the dissipation factor measurement before foaming. ozocnn / Lznz / E / YiAi Material CE2 IE2-1 IE2-2 IE2-3 HDPE / % weight 69.6 69.6 69.6 69.6 LDPE / % weight 29.95 28.4 26.4 29.4 NCore 7155-M1- 300 / % weight 0.45 Hydrocerol NUC 5515 / % weight 2.0 4.0 Mistrocell M90 / % weight 1.0 Dissipation factor at 1.9 GHz (10·6) 140 141 135 172 As can be seen in Table 3, the use of talc as a passive nucleating agent provides a dissipation factor comparable to or even better than CE2 using ADCA, i.e., a chemical expanding agent. It is hereby stated that, as of this date, the best method known to the applicant for putting the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

1. A foamable polymer composition, characterized in that it comprises: (A) a first polyolefin polymer, (B) a second polyolefin polymer having an MFR2 (2.16 kg; 190 °C) of 2 to 15 g / 10 min measured according to ISO 1133-1, and (C) a mineral nucleating agent in an amount of 0.5 to 6% by weight based on the total foamable polymer composition, wherein the first polyolefin polymer (A) has a higher density than the second polyolefin polymer (B).

2. The foamable polymer composition according to claim 1, characterized in that the mineral nucleating agent (C) comprises one or more selected from the group consisting of talc, clay, mica, calcium carbonate and silica.

3. The foamable polymer composition according to any of the preceding claims, characterized in that the first polyolefin polymer (A) has an MFR2 (2.16 kg; 190 °C) of 0.1 to 20 g / 10 min measured according to ISO 1133-1. Qzocnn / Lznz / E / YiAi 4. The foamable polymer composition according to any of the preceding claims, characterized in that the first polyolefin polymer (A) is present in an amount of 20 to 95% by weight based on the total foamable polymer composition and in that the second polyolefin polymer (B) is present in an amount of 5 to 80% by weight based on the total foamable polymer composition.

5. The foamable polymer composition according to any of the preceding claims, characterized in that the first polyolefin polymer (A) is a high-density polyethylene homopolymer or copolymer having a density of 935 to 970 kg / m3 measured according to ISO 1183-1 and the second polyolefin polymer (B) is a low-density polyethylene homopolymer or copolymer having a density of 880 to 930 kg / m3 measured according to ISO 1183-1.

6. The foamable polymer composition according to any of the preceding claims, characterized in that it has a dissipation factor at 1.9 GHz of between 80-1CU6 and 270-10~6.

7. A foamed polymer composition, characterized in that it is obtained by foaming the foamable polymer composition according to any of claims 1 to 6 in the presence of an expansion agent (D).

8. The foamed polymer composition according to claim 7, characterized in that the blowing agent (D) comprises N2, CO, CO2, Ar or mixtures thereof.

9. The foamed polymer composition according to claims 7 or 8, characterized in that the blowing agent (D) is used in an amount of 0.01% by weight to 5% by weight, based on the foamable polymer composition.

10. The foamed polymer composition according to any of claims 7 to 9, characterized in that the blowing agent (D) does not comprise a hydrocarbon, a halogenated hydrocarbon, citric acid or citric acid derivatives, azodicarbonamide or mixtures thereof.

11. The foamed polymer composition according to any of claims 7 to 10, characterized in that the foamed polyolefin composition has a mean cell diameter of between 5 pm and 500 pm.

12. The foamed polymer composition according to any of claims 7 to 11, characterized in that it has a density of 85 to 870 kg / m3.

13. A cable, characterized in that it comprises at least one layer comprising the foamable polymer composition according to any of claims 1 to 6, or comprises the foamed polymer composition according to ozocnn / Lznz / E / YiAi with any of claims 7 to 12.

14. The cable according to claim 13, characterized in that at least one layer is an insulation layer.

15. The cable according to claim 13 or 14, characterized in that it is a coaxial cable or a twisted-pair cable.