Improved foaming behavior of polymer compositions using inert 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 the foamable polymer composition or the foamed polymer composition. The invention further provides a method of producing the foamed polymer composition.

Description

[0001] This application is a divisional application of the invention patent application with application number 201980075728.1 (international application number: PCT / EP2019 / 083556), application date December 3, 2019, and invention name “Improved foaming behavior of polymer compositions using inert nucleation”. Technical Field

[0002] The present invention relates to a foamable polymer composition and a foamed polymer composition obtained by foaming the foamable polymer composition. The present invention also relates to a cable comprising at least one layer comprising the foamable polymer composition or the foamed polymer composition. The present invention further provides a method for producing the foamed polymer composition. Background Art

[0003] Communication cables are used to transmit high-frequency (HF) signals via electromagnetic waves or as light pulses in fiber optic cables. An example of a communication cable is a coaxial cable. A coaxial cable consists of two separate parallel conductors arranged concentrically along the same axis and separated by an insulating dielectric. For example, foamed LDPE, alone or in a blend with another polymer, is commonly used to insulate communication cables, such as coaxial cables and / or radio frequency cables. The foaming of the polymer composition can be accomplished using chemical or physical foaming agents, or a combination of the two.

[0004] Chemical blowing agents are substances that release a foaming gas by a thermal decomposition reaction and are consumed in the foaming reaction. Examples of such substances are hydrazine, hydrazide, azodicarbonamide (ADCA), or combinations of solid organic acids (or their metal salts) and alkali metal carbonates or alkali metal bicarbonates, such as a combination of citric acid / citric acid derivatives and sodium bicarbonate.

[0005] Physical foaming agents are gases injected directly into the polymer melt. In such processes, chemical foaming agents are often used as cell nucleating agents because the gas formed by the foaming agent reaction serves as a relatively low-energy nucleation site for bubble formation. Gases used as physical foaming agents can be, for example, N2 or CO2.

[0006] Nucleating agents (nucleating agents, nucleators or kickers) are commonly used in physical foaming processes. Nucleating agents provide points in the insulation layer where the energy required for bubble formation is relatively low. These nucleating agents can be inert or active. Active nucleating agents are substances that decompose into gaseous products, i.e., chemical foaming agents, while inert nucleating agents are particles that only provide localized points of lower energy where bubble formation is more likely to occur. Generally, inert nucleating agents, i.e., nucleating agents that do not undergo a chemical reaction to form a gas, are considered less effective than active nucleating agents (such as ADCA).

[0007] Both chemical and physical foaming extrusion processes are used to extrude foamed communication cable insulation. For communication cables, a good cell structure within the foam insulation is crucial for isotropic electrical properties. It is desirable for the insulation layer to have a cell structure with many small cells evenly distributed. Cell structure is also important for mechanical properties. Having many well-distributed small cells will provide better compressive strength than a structure with unevenly distributed larger cells, which can create weak areas in the insulation layer.

[0008] The primary blowing agent used in the cable industry is azodicarbonamide (ADCA), which has a decomposition temperature range that is well-suited to the processing window of polyolefins (such as polyethylene) and imparts a good foam structure, 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, efforts are underway to find alternative solutions. ADCA was identified as an SVHC (Substance of Very High Concern) and included on the Candidate List because it has been identified as a respiratory sensitizer, with known cases of asthma in workers exposed to powdered ADCA.

[0009] Another problem with azodicarbonamide is that ammonia is released from the decomposition reaction of the blowing agent, which can disturb the working environment of cable manufacturers because of its unpleasant odor.

[0010] An alternative to ADCA that is compatible with polyethylene's processing window is an endothermic blowing agent. These are typically a combination of sodium bicarbonate and citric acid or a citric acid derivative. These blowing agents are typically added directly to the extruder hopper or dry-blended with the polyolefin prior to extrusion. In high-speed extrusion processes such as cable extrusion, this method of adding the blowing agent does not allow for adequate homogenization of the blowing agent within the polymer melt, resulting in a foamed insulation layer with poor cell structure and a poor surface finish.

[0011] Another disadvantage of endothermic blowing agents is that they produce polar decomposition products, such as water. These polar decomposition products can negatively impact the high-frequency electrical properties of the insulation, such as the dissipation factor. This is particularly detrimental in high-frequency applications such as coaxial cable.

[0012] WO 2004 / 094526 relates to foam compositions and cables having a low-loss foam layer. Co-blowing agents selected from hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and perfluorocarbons (PFCs) are used to obtain the low-loss foam. However, such compounds used as co-blowing agents do have negative environmental impacts. Summary of the Invention

[0013] It is an object of the present invention to provide a foamable polymer composition which overcomes the above-mentioned problems.

[0014] Another object of the present invention is to replace hydrazine, hydrazide or azodicarbonamide (ADCA) in foamable polymer compositions while maintaining an improved cell structure in the foamed product, ie, small and uniform distribution of cells in the foam.

[0015] Yet another object of the present invention is to provide a foamable polymer composition that is not foamed by using hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs) and perfluorocarbons.

[0016] Yet another object of the present invention is to provide a foamed polymer composition having reduced density while having a high cell density, small cells uniformly distributed throughout the foamed polymer, and maintaining an improved (ie, low) dissipation factor.

[0017] The present invention is based on the surprising finding that all of the above-mentioned problems can be solved by using an inorganic nucleating agent, such as talc, in the foamable polymer composition.

[0018] The present invention therefore provides a foamable polymer composition comprising: a first polyolefin polymer; a second polyolefin polymer having an MFR2 (2.16 kg; 190°C) of 2 to 15 g / 10min, measured according to ISO 1133-1; and an inorganic nucleating agent in an amount of 0.5 to 6 wt%, based on the total foamable polymer composition, wherein the density of the first polyolefin polymer is higher than the density of the second polyolefin polymer.

[0019] The present invention further provides a foamed polymer composition which is obtained by foaming the foamable polymer composition according to the invention using a blowing agent.

[0020] The invention also provides a cable comprising at least one layer comprising a foamable polymer composition according to the invention or a foamed polymer composition according to the invention.

[0021] The present invention has several surprising advantages. The foamable polymer compositions of the present invention can be foamed into foamed polymer compositions without the use of harmful blowing agents such as ADCA and halogenated hydrocarbons, while maintaining a good dissipation factor, especially compared to polymer compositions foamed by, for example, ADCA.

[0022] 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 relative to ADCA foamed foam.

[0023] The present invention uses an inorganic nucleating agent (C) as an inert nucleating agent. The inorganic nucleating agent is preferably a magnesium-containing compound, a calcium-containing compound, a silicon-containing compound or a mixture thereof.

[0024] Preferably, the inorganic nucleating agent comprises one or more selected from talc, clay, mica, calcium carbonate and silicon dioxide. Among the above-mentioned inorganic nucleating agents, talc is preferred. Even more preferably, the inorganic nucleating agent consists of talc. The advantage of talc is that it does not contain water and does not release water during the foaming process. In order to uniformly distribute the inorganic nucleating agent in the foamable polymer composition, the inorganic nucleating agent is added (preferably mixed) to the foamable polymer composition in the form of a powder, i.e., in the form of small particles, or in the form of a masterbatch. The average particle size is generally on the order of 0.1 μm to 50 μm.

[0025] Since the nucleating agent is an inorganic nucleating agent, its thermal decomposition temperature is high. This has the advantage that the inorganic nucleating agent of the present invention does not thermally decompose, for example, during melting and extrusion of the polymer composition. Preferably, the inorganic nucleating agent does not thermally decompose at temperatures 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.

[0026] The foamable polymer composition according to the present invention comprises a first polyolefin polymer (A) and a second polyolefin polymer (B). The first polyolefin polymer (A) preferably has an MFR2 (2.16 kg; 190° C.) measured according to ISO 1133-1 of 0.1 to 20 g / 10 min, more preferably 1 to 17 g / 10 min, more preferably 2 to 14 g / 10 min, more preferably 4 to 14 g / 10 min, and most preferably 6 to 10 g / 10 min.

[0027] The second polyolefin polymer (B) has an MFR2 (2.16 kg; 190° C.) measured according to ISO 1133-1 of 2 to 15 g / 10 min. Preferably, the second polyolefin polymer (B) has an MFR2 (2.16 kg; 190° C.) measured according to ISO 1133-1 of 2.5 to 12 g / 10 min, more preferably of 3 to 10 g / 10 min, more preferably of 3.5 to 8 g / 10 min, most preferably of 4 to 6 g / 10 min.

[0028] The first polyolefin polymer (A) is preferably present in an amount of 20 to 95 wt%, more preferably 40 to 90 wt%, more preferably 50 to 85 wt%, most preferably 60 to 80 wt%, based on the total foamable polymer composition, and the second polyolefin polymer (B) is preferably present in an amount of 5 to 80 wt%, more preferably 10 to 70 wt%, more preferably 15 to 60 wt%, most preferably 20 to 40 wt%, based on the total foamable polymer composition.

[0029] 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.

[0030] The first polyolefin polymer (A) preferably has a density of 935 to 970 kg / m 3 The second polyolefin polymer (B) is preferably a high density polyethylene homopolymer or copolymer of 880 to 930 kg / m 3 Low density polyethylene homopolymer or copolymer.

[0031] More preferably, high density polyethylene (HDPE) is a copolymer and low density polyethylene (LDPE) is a homopolymer. Homopolymer means that low density polyethylene (LDPE) contains at least 90% by weight of ethylene monomers, preferably at least 95% by weight of ethylene monomers, most preferably at least 99% by weight of ethylene monomers.

[0032] In the case where the high density polyethylene (HDPE) is a copolymer, the copolymer comprises ethylene monomer and one or more comonomers, the amount of ethylene monomer being preferably at least 50 wt % based on the total copolymer. The comonomer can be an α-olefin having 3 to 12 carbon atoms, such as propylene, butene, hexene, octene, decene.

[0033] The low density polyethylene (LDPE) is preferably a homopolymer.

[0034] For foamed polyethylene used in communication cables, both electrical and mechanical properties are important. HDPE has a lower dielectric constant and lower dissipation factor than LDPE, as well as higher strength and stiffness.

[0035] High-density polyethylene (HDPE) polymers are polymerized in a low-pressure process and are, for example, optional HDPE homopolymers or optional HDPE copolymers of ethylene and one or more of the above-mentioned comonomers. 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. The polymerization can be, for example, gas phase polymerization, slurry polymerization, or a combination of slurry polymerization / gas phase polymerization or gas phase polymerization / gas phase polymerization. The polymerization can also be solution polymerization.

[0036] In order to foam the foamable polymer composition, it is necessary to have good melt strength, because poor melt strength will lead to collapse of the cell structure, which is detrimental to the mechanical or electrical properties of the cable layer (usually the insulation layer). The melt strength can be improved by incorporating LDPE into the foamable polymer composition to improve the melt strength and ensure that the foamed layer has a closed-cell structure and uniform cell distribution.

[0037] Low density polyethylene (LDPE) polymer is polymerized in a high pressure free radical polymerization process. In addition, LDPE is polymerized in a high pressure polymerization process in the presence of an initiator and a chain transfer agent (such as propane, propylene, propionaldehyde and methyl ethyl ketone) to control the MFR.

[0038] LDPE can be produced, for example, in tubular polymerisation reactors or in autoclave polymerisation reactors.

[0039] The dissipation factor, also known as tan δ, is a measure of the degree of power dissipation in a dielectric material, that is, how much electrical energy in the dielectric material is converted into heat. The dissipation factor of the foamable polymer composition of the present invention at 1.9 GHz is preferably 80.10 -6 To 270.10 -6 , more preferably 120·10 -6 To 260.10 -6 , the most preferred is 130·10 -6 To 240.10 -6 .

[0040] The foamable polymer composition preferably comprises an antioxidant. The antioxidant is preferably a phenolic antioxidant, a phosphorus-containing antioxidant, or a mixture thereof, more preferably a mixture thereof. The phenolic antioxidant is preferably a mixture of pentaerythritol tetrakis(3-(3',5'-di-tert-butyl-4-hydroxyphenyl)propionate (CAS No. 6683-19-8; commercially available from BASF as Irganox 1010) and tris(2,4-di-tert-butylphenyl)phosphite (CAS No. 31570-04-4; commercially available from BASF as Irgafos 168). The antioxidant mixture is commercially available from BASF as Irganox B561.

[0041] The antioxidant is preferably present in an amount of 0.01 to 2 weight percent, more preferably 0.04 to 1 weight percent, and most preferably 0.08 to 0.5 weight percent, based on the total foamable polymer composition.

[0042] The foamable polymer composition preferably comprises an acid scavenger. The acid scavenger is preferably calcium stearate, sodium stearate, zinc stearate or a mixture thereof, more preferably zinc stearate.

[0043] The amount of acid scavenger is preferably 0.01 to 2 wt%, more preferably 0.02 to 1 wt%, most preferably 0.04 to 0.08 wt%, based on the total foamable polymer composition.

[0044] The foamed polymer composition can be obtained by foaming the foamable polymer composition according to the present invention using a foaming agent (D).

[0045] Foaming is a physical foaming process, which means that the blowing agent (D) is injected into or mixed with the foamable polymer composition. The particles of the inorganic nucleating agent present in the foamable polymer composition act as local points of lower energy where bubbles are more likely to occur. Preferably, the blowing agent (D) is injected into or mixed with the foamable polymer composition during the process of extruding the foamable polymer composition in an extruder. During the extrusion process, the blowing agent (D) is melt-mixed with the molten polymer composition and the molten polymer composition is expanded at the outlet of the extruder die. The temperature during extrusion is preferably 130°C to 240°C. Extrusion is preferably carried out in a gas injection foaming line.

[0046] The blowing agent (D) preferably comprises a gas, and the gas comprises N2, CO, CO2, Ar or a mixture thereof. More preferably, the blowing agent (D) comprises N2 and / or CO2, and more preferably, the blowing agent (D) consists of N2 and / or CO2.

[0047] The amount of blowing agent (D) used is preferably 0.01 to 5 wt %, more preferably 0.015 to 2.5 wt %, more preferably 0.02 to 0.2 wt %, more preferably 0.03 to 0.1 wt %, based on the foamable polymer composition.

[0048] Preferably, the blowing agent (D) does not comprise hydrocarbons, halogenated hydrocarbons, citric acid or derivatives of citric acid, azodicarbonamide or mixtures thereof. For example, halogenated hydrocarbons are hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs) and perfluorocarbons (PFCs).

[0049] The average cell diameter of the foamed polyolefin composition is preferably 5 to 500 μm, more preferably 50 to 400 μm, more preferably 100 to 300 μm, more preferably 150 to 275 μm, more preferably 155 to 260 μm, most preferably 160 to 250 μm.

[0050] The density of the foamed polymer composition is preferably 85 kg / m 3 Up to 870kg / m 3 , more preferably 150kg / m 3 Up to 860kg / m 3 , more preferably 300kg / m 3 Up to 850kg / m 3 , more preferably 350kg / m 3 Up to 800kg / m 3 , most preferably 400kg / m 3 Up to 775kg / m 3 .

[0051] The present invention provides a cable comprising at least one layer comprising a foamable polymer composition according to the invention or a foamed polymer composition according to the invention. Thus, the cable comprises at least one layer comprising a foamable polymer composition according to any of the above embodiments, or the cable comprises at least one layer comprising a foamed polymer composition according to any of the above embodiments.

[0052] Preferably, the cable is a communications cable, preferably a coaxial cable or a twisted pair cable.

[0053] The at least one layer is preferably the insulation layer of the cable. The insulation layer is a layer surrounding the innermost conductor, which is typically made of copper. Preferably, a skin layer is arranged between the innermost conductor and the insulation layer. For coaxial cables, the insulation layer typically has a thickness of 0.01 mm to 80 mm, and for twisted-pair cables, the insulation layer typically has a thickness of 0.1 mm to 2 mm.

[0054] The foamed polymer composition according to the present invention can preferably be produced by a method for producing a foamed polymer composition, said method comprising the following steps:

[0055] a) providing a foamable polymer composition according to the invention,

[0056] b) melt mixing the foamable polymer composition with a foaming agent (D) at a temperature of 130° C. to 240° C. to obtain a molten polymer composition, and

[0057] c) foaming the molten polymer composition.

[0058] Preferably, the melt mixing in step b) is performed in an extruder, and the foaming of the molten polymer composition in step c) is performed after the molten polymer composition leaves the die of the extruder. In the extruder, the foamable polymer composition is melted and mixed with the blowing agent (D) to obtain a molten polymer composition. The temperature in step b) is preferably 140°C to 230°C.

[0059] Preferably, the extruder can be any extruder known in the art suitable for melt mixing a polymer melt with a blowing agent.

[0060] All above-mentioned embodiments of the foamable polymer composition according to the invention are also preferred embodiments of the foamable polymer composition for use in the process for producing a foamable polymer composition.

[0061] All preferred embodiments of the blowing agent (D) described above are preferred embodiments of the blowing agent (D) used in the process for producing a foamed polymer composition. DETAILED DESCRIPTION

[0062] Example

[0063] 1. Measurement method

[0064] a) Melt flow rate

[0065] The melt flow rate (MFR) is determined according to ISO 1133-1 and is expressed in g / 10 min. The MFR is an indicator of the flowability of the polymer and, therefore, of the processability of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer.

[0066] The MFR2 of polyethylene (co)polymers was measured at a temperature of 190°C and a load of 2.16 kg. The MFR2 of polypropylene (co)polymers was measured at a temperature of 230°C and a load of 2.16 kg.

[0067] b) Density of solid materials

[0068] The density of the foamable polymer composition is determined by following ISO 17855-2 for sample preparation and ISO 1183-1 / Method A for density measurement.

[0069] Compression molding is accomplished in a controlled cold press using a molding temperature of 180°C and a cooling rate of 15°C / min for polyethylene. The specimens 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.

[0070] c) Density of foamed material

[0071] In order to determine the density of the foamed sample, the L The weight of each sample is measured in a medium (Ww, in g) and at a known temperature. All measurements are carried out at 22°C in distilled water with three drops of wetting agent added. For the calculation of density, the following formula is used:

[0072]

[0073] in,

[0074] ρ = density in g / cm 3 :

[0075] ρ w = density of water at the measurement temperature, in g / cm 3 ﹔

[0076] ρ L = Density of air (0.0012 g / cm 3 ).

[0077] d) Calculation of cell density

[0078] The cell density of the foamed polymer composition (N b / cm 3 ) is calculated as follows:

[0079]

[0080] in

[0081] ρ F = density of the foamed sample, in g / cm 3

[0082] ρ m = density of the polymer matrix in g / cm 3

[0083] D = average cell diameter, in cm

[0084] e) Calculation of density drop

[0085] The percentage of density reduction (X) is calculated as follows:

[0086]

[0087] in

[0088] Ds = density of solid material in kg / m 3

[0089] D F = density of foam material, unit: kg / m 3

[0090] f) Determination of average cell diameter

[0091] To determine the average cell diameter, the cross-sectional area of ​​approximately 60 cells (if available) is measured. For this purpose, the cells are manually marked in the Alicona system's image analysis software. The average cell diameter is calculated assuming the cells have a circular cross-section. This method facilitates comparisons of foam morphology across different samples, as most cell geometries deviate from the ideal circular shape, making it impossible to make a reasonable comparison of directly measured diameters.

[0092] The average diameter was determined by using the following formula and then averaging the calculated values ​​for the individual bubble diameters.

[0093]

[0094] in,

[0095] Dz = diameter of a cell assuming a circular cross section, in μm

[0096] A Z = cross section of a foam bubble, in μm 2 .

[0097] g) Microscopic analysis of foamed polymer compositions

[0098] The density and foam morphology of all samples were examined. For this purpose, the cell size was measured using an optical microscope, Alicona Infinite Focus (Alicona Imaging GmbH, Austria). Density was determined using a high-precision balance (Excellence XS Analyse Waage, Mettler Toledo AG, Switzerland) equipped with a density measurement kit (Density Kit, Mettler Toledo AG, Switzerland).

[0099] 2. Dielectric properties (dielectric loss tangent (tanδ) - dissipation factor)

[0100] a) Preparation of plaque:

[0101] The polymer mix was compression molded in a frame at 140° C. to give a 4 mm thick, 80 mm wide, and 130 mm long plate. The pressure was adjusted to be high enough to obtain a smooth surface of the plate. Visual inspection of the plate showed no inclusions such as trapped air or any other visible contaminants.

[0102] b) Characterization of dielectric properties of the sample:

[0103] To measure the dielectric constant and tangent delta (tan δ) of a material, a split-post dielectric resonator was used with a network analyzer (Rodhe & Schwarz ZVL6). This technique measures the complex dielectric constant of dielectric layered specimens (samples) in the 1 GHz to 10 GHz frequency range. Tests were performed at 23°C.

[0104] The split-pillar dielectric resonator (SPDR) was developed by Krupka and his collaborators (see: J. Krupka, R.G. 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 IEE, London, 1996) and is one of the simplest and most convenient techniques for measuring microwave dielectric properties. Two identical dielectric resonators are coaxially placed along the z-axis, with a small interlayer gap between them. The sample to be measured can be placed in this gap for measurement. By choosing the right dielectric material, the resonant frequency and quality factor (Q-factor) of the SPDR can be made temperature-stable. Once the resonator is fully characterized, only three parameters need to be measured to determine the complex dielectric constant of the sample: its thickness, and the change in resonant frequency, Af, and the change in quality factor, AQ, obtained when it is placed in the resonator.

[0105] A 4 mm thick test specimen was prepared by compression molding as described above and measured at a high frequency of 1.9 GHz.

[0106] This method is reviewed in J. Krupka, RN Clake, O.C. Crochard and A.P. Gregory, “Split-Post Dielectric Resonator technique for precise measurements of laminar dielectric specimens—measurement uncertainties” in Proceedings of the XIII Int. Conference MIKON '2000, Wroclaw, Poland, pp. 305-308, 2000.

[0107] 3. Materials

[0108] A unimodal Ziegler-Natta catalyzed HDPE copolymer with butene as a comonomer was used as the HDPE component, which had an MFR2 of 8 g / 10 min and a 3 density.

[0109] LDPE is an autoclave LDPE homopolymer with an MFR2 of 4.5 g / 10 min and a 3 density.

[0110] nCore 7155-M1-300 is an azodicarbonamide (ADCA) based blowing agent masterbatch commercially available from Americhem that contains 15% active blowing agent.

[0111] Hydrocerol NUC 5155 is a nucleating masterbatch containing 50% talc in a polyethylene carrier, commercially available from Clariant.

[0112] Irganox B561 is an antioxidant mixture commercially available from BASF.

[0113] Zincum TX is a zinc stearate acid scavenger commercially available from Baerlocher.

[0114] Mistrocell M90 is a talc commercially available from Imerys Talc with a particle size of d50 = 3.4 μm and a BET (specific surface area) of 11.0 m2 / g.

[0115] 4. Preparation of Examples

[0116] 4.1 Mixing of materials

[0117] The examples in Tables 1 and 3 were compounded on a BUSS MDK46 continuous extruder (manufactured in 1985). This line is a single-screw kneader with a screw diameter of 46 mm and an L / D ratio of 11.

[0118] Table 1: Compositions of comparative examples (CE) and inventive examples (IE) used for foaming, amounts given in wt %

[0119] Material CE1 IE1-1 IE1-2 HDPE, wt% 69.6 69.6 69.6 LDPE, wt% 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 <![CDATA[Density, kg / m 3 > 947.4 958.9 966.3

[0120] 4.2 Extrusion and foaming

[0121] The polymer pellets of the composition of Table 1 were extruded on a Rosendahl RE45 extruder with a 45 mm diameter screw. The total length of the extruder was 32D, including an 8D long oil-tempered cylindrical elongated structure for better control of the polymer melt temperature. To achieve longer residence time and better homogenization, a 4D long static mixer (SMB-R type, from Sulzer, Switzerland) was installed between the cylindrical elongated structure and the extrusion die. A 4.0 mm circular die was used. The extruder had 10 temperature zones, and gas (N2) was injected between zones 7 and 8.

[0122] The temperature settings in °C are as follows (slashes indicate different temperature zones):

[0123] T1: 40 / 150 / 160 / 160 / 165 / 170 / 190 / 190 / 170 / 170 / 170 / 170 / 170℃

[0124] The results are given below in Table 2. The density of the foams was measured at 22°C.

[0125] Table 2: Properties of the resulting foaming compositions

[0126]

[0127] The electrical loss factor (dissipation factor) was measured for Comparative Example 2 and Inventive Examples IE2-1 to IE2-3. The compositions of all examples are given in Table 3 below. After mixing as described above, all materials were compression molded at the foaming temperature (140°C) as described above. The loss factor (tan δ) of these samples was measured at 1.9 GHz.

[0128] Table 3: Compositions of comparative examples (CE) and inventive examples (IE) (amounts given in wt%) and results of dissipation factor measurements before foaming

[0129] Material CE2 IE2-1 IE2-2 IE2-3 HDPE / wt% 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 MistrocellM90 / weight% 1.0 <![CDATA[Dissipation factor at 1.9 GHz (·10 -6 )]]> 140 141 135 172

[0130] As can be seen from Table 3, the use of talc as an inert nucleating agent has a comparable or even better dissipation factor than CE2 using ADCA (ie, a chemical blowing agent).

Claims

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

2. The foamable polymer composition according to claim 1, wherein The inorganic nucleating agent (C) includes one or more selected from talc, clay, mica, calcium carbonate and silica.

3. A foamable polymer composition according to any one of the preceding claims, wherein The first polyolefin polymer (A) has an MFR2 (2.16 kg; 190° C.) measured according to ISO 1133 of 0.1 to 20 g / 10 min.

4. A foamable polymer composition according to any one of the preceding claims, wherein The first polyolefin polymer (A) is present in an amount of 20 to 95 wt. % based on the total foamable polymer composition, and wherein the second polyolefin polymer (B) is present in an amount of 5 to 80 wt. % based on the total foamable polymer composition.

5. A foamable polymer composition according to any one of the preceding claims, wherein The first polyolefin polymer (A) is a high density polyethylene homopolymer or copolymer having a density of 935 to 970 kg / m 3 , 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 3 .

6. A foamable polymer composition according to any one of the preceding claims, wherein The dissipation factor of the foamable polymer composition at 1.9 GHz is 80.10 -6 To 270.10 -6 .

7. A foamed polymer composition obtained by foaming the foamable polymer composition according to any one of claims 1 to 6 in the presence of a foaming agent (D).

8. The foamed polymer composition according to claim 7, wherein The foaming agent (D) includes N2, CO, CO2, Ar or a mixture thereof.

9. The foamed polymer composition according to claim 7 or 8, wherein The blowing agent (D) is used in an amount of 0.01 to 5 wt % based on the foamable polymer composition.

10. The foamed polymer composition according to any one of claims 7 to 9, wherein The blowing agent (D) does not include hydrocarbons, halogenated hydrocarbons, citric acid or citric acid derivatives, azodicarbonamide or mixtures thereof.

11. The foamed polymer composition according to any one of claims 7 to 10, wherein The foamed polyolefin composition has an average cell diameter of 5 μm to 500 μm.

12. The foamed polymer composition according to any one of claims 7 to 11, wherein The density of the foamed polymer composition is 85 to 870 kg / m 3 .

13. Cable comprising at least one layer comprising the foamable polymer composition according to any one of claims 1 to 6 or comprising the foamed polymer composition according to any one of claims 7 to 12.

14. The cable according to claim 13, wherein The at least one layer is an insulating layer.

15. The cable according to claim 13 or 14, wherein The cable is a coaxial cable or a twisted pair cable.

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