Improved foaming behavior of polymer compositions using blowing agents and nucleating agents

Abstract

The present invention relates to a foamable polymer composition comprising a blowing agent and a foamed polymer composition obtained by foaming the foamable polymer composition. The invention further relates to a cable comprising at least one layer comprising a foamable polymer composition or a foamed polymer composition.

Description

[0001] This application is a divisional application of the invention patent application "Improved Foaming Behavior of Polymer Compositions Using Blowing Agents and Nucleating Agents", the national application number of the parent case is "201980083580.6", the PCT international filing date is December 3, 2019, and the PCT international application number is PCT / EP2019 / 083559. Technical Field

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

[0003] Communication cables are used to transmit high-frequency signals, such as light pulses in fiber optic cables.

[0004] Electrical communication cables for telephone and data transmission are usually twisted pair cables. The assembly of twisted pair cables involves passing insulated conductors through machinery, which can cause wear or deformation. The twisting process is extremely delicate because the back tension greatly affects the tightness and thus the separation of the conductors. Excessive tension during subsequent sheathing and installation will similarly affect the separation of the conductors, resulting in crushing. The key parameters affecting the crushing performance are the tensile strength and hardness, which must be maximized to obtain the best results.

[0005] Another type of electrical communication cable is the coaxial cable structure. Coaxial cables are used to transmit radio, cable television, and data signals. Coaxial cables consist of two separate parallel conductors separated by an insulating dielectric.

[0006] The conductors in communication cables are metals, and the insulating materials can be made of different materials, suitably polyolefins, such as polyethylene. As the frequency of the transmitted signal increases, the relative permittivity of the insulating material becomes more important. The relative permittivity is a property of the material, which is lower relative to air than relative to polyolefins (such as polyethylene). By foaming the polyolefin composition insulating material, gas is introduced, and the relative permittivity decreases linearly with the degree of expansion. The relative permittivity is usually also referred to as the dielectric constant, and these terms can be used interchangeably.

[0007] The foaming of polyolefin compositions can be accomplished using chemical or physical blowing agents or a combination of both. Chemical blowing agents are substances that release a foaming gas through a thermal decomposition reaction and the chemical blowing agent is consumed in the foaming reaction. Examples of such substances are hydrazine, hydrazides or azodicarbonamide (ADCA), citric acid or citric acid derivatives or those based on a combination of a solid organic acid (or its metal salt) with an alkali metal carbonate or an alkali metal bicarbonate, such as a combination of citric acid / citric acid derivative and sodium bicarbonate.

[0008] Physical blowing agents are gases that are directly injected into the polymer melt. In such a process, chemical blowing agents are typically used as cell nucleating agents since the gas formed by the reaction of the blowing agent serves as a lower energy nucleation site for bubble formation. Gases used as physical blowing agents can be, for example, N 2 or CO 2 . Both chemical and physical foaming extrusion processes are used for the extrusion of foamed communication cable insulation. In chemical foaming, all of the foaming gas comes from the decomposition of the chemical blowing agent.

[0009] Nucleating agents (nucleating agent, nucleator or kicker) are typically used in the physical foaming process. Nucleating agents provide lower energy sites in the insulation where bubble formation can occur. These nucleating agents can be inert or active. Active nucleating agents are substances that decompose into gaseous products, i.e., chemical blowing agents, while inert nucleating agents are particles that only provide lower energy local sites where bubble formation is more likely to occur.

[0010] In many foamed polymer applications, the blowing agent masterbatch is added to the polymer composition by direct feeding into the extruder or by dry blending the blowing agent masterbatch with the polymer composition prior to the extrusion process. Depending on the type and length of the extruder, this can result in poor mixing of the blowing agent masterbatch with the polymer composition. Other influencing factors are the rheological differences between the polymer composition and the blowing agent masterbatch, the extrusion speed and the extrusion temperature. In the cable extrusion process, extruders are generally not designed for mixing polymer compositions and communication cables are typically extruded at high line speeds for good productivity. Poor mixing of the blowing agent masterbatch and the polymer composition results in a poor cell structure since the gas and solid nucleating agents are not evenly distributed in the polymer composition, leading to non-uniform bubble nucleation and growth and large areas of solid material without any cells.

[0011] The distribution of the blowing agent in the polymer composition can be improved by melt-blending the blowing agent masterbatch into the polymer composition by kneading before the extrusion process. Since the gas and the solid particles serving as bubble nucleation sites are better distributed in the polymer melt, the cell structure can be improved. For communication cables, it is crucial to have a good cell structure in the foamed insulation to have improved electrical properties. A cell structure with many small and uniformly distributed cells in the insulation is desired. The cell structure is also important for mechanical properties. A structure with many well-distributed small cells will provide better compressive strength compared to a structure with larger and non-uniformly distributed cells (since this will create weak parts in the insulation).

[0012] The main blowing agent used in the cable industry is azodicarbonamide (ADCA), whose decomposition temperature range is very suitable for the processing window of polyolefins (such as polyethylene) and has a fine foam structure, which is a key requirement for cable applications. Due to the fact that ADCA is included in the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) candidate list and the risk of being included in the authorisation list in the future, much effort has been made to find alternative solutions. The reason why ADCA has been identified as a SVHC (Substance of Very High Concern) and included in the candidate list is that it has been identified as a respiratory sensitiser, and there are known cases of workers exposed to powdered ADCA suffering from asthma.

[0013] Another problem with azodicarbonamide is that ammonia is released from the blowing agent decomposition reaction. Since it emits an unpleasant odour, this can interfere with the working environment in a cable manufacturing plant.

[0014] ADCA alternatives that are compatible with the processing window of polyethylene are endothermic blowing agents. Endothermic blowing agents are usually a combination of sodium bicarbonate and citric acid or citric acid derivatives. These blowing agents are usually added directly to the extruder hopper or dry-blended with the polyolefin before extrusion. In a high-speed extrusion process such as cable extrusion, this way of adding the blowing agent does not allow the blowing agent to be fully homogenised in the polymer melt, and this results in a foamed insulation with a poor cell structure and a bad surface.

[0015] The main problem with endothermic blowing agents for cable applications is that sodium bicarbonate starts to decompose below 100 °C and the maximum reaction rate is between 130 and 140 °C, which is generally considered too close to the melting temperature of high-density polyethylene, making it impossible to carry out kneading without completely decomposing the sodium bicarbonate blowing agent. One object of the present invention is to increase the process window, i.e. to allow a higher kneading temperature.

[0016] The blowing agent sodium bicarbonate gradually decomposes into sodium carbonate, water, and carbon dioxide. The decomposition reaction is endothermic and requires heat to occur. The reaction already starts at temperatures below 100 °C, but the reaction rate is very low at these temperatures. As the temperature increases, the reaction increases rapidly, and the maximum reaction rate is between 130 °C and 140 °C. See Hartman et al., Ind. Eng. Chem. Res. 2013, 52, 10619 - 10626. The melting temperature of high-density polyethylene is typically about 130 °C, and it is generally considered impossible to compound sodium bicarbonate into high-density polyethylene because the melting point of the polymer is very close to the maximum decomposition rate of the sodium bicarbonate blowing agent.

[0017] WO 2014 / 018768 provides an additive composition for a polymeric base material, which is particularly useful in applications preferably having a high dielectric constant, a low loss factor, and a low density, such as applications involving wire and cable dielectric materials. The disclosed additive composition consists of a thermoplastic polymer carrier, a chemical blowing agent, and a filler. Summary of the Invention

[0018] An object of the present invention is to provide a foamable polymer composition that overcomes the above problems.

[0019] Another object of the present invention is to replace hydrazine, hydrazide, or azodicarbonamide (ADCA) in the foamable polymer composition while maintaining an improved cell structure in the foamed product, i.e., small and uniformly distributed cells in the foam.

[0020] Yet another object of the present invention is to provide a foamable polymer composition that does not contain halogenated hydrocarbons (such as hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and perfluorocarbons) or fluororesins (such as polytetrafluoroethylene (PTFE)).

[0021] Still another object of the present invention is to provide a foamable polymer composition that has a reduced density, while having a high cell density, small cells uniformly distributed in the foamed polymer, and maintaining an improved (i.e., low) dissipation factor.

[0022] Another object of the present invention is to avoid exothermic blowing agent decomposition, where the heat released from the decomposition reaction initiates a chain reaction, resulting in uncontrolled reaction of the blowing agent during any extrusion process such as compounding.

[0023] Yet another object of the present invention is to increase the decomposition temperature of the blowing agent in the foamable polyolefin polymer composition, thereby avoiding any compounds containing sodium bicarbonate.

[0024] Another object of the present invention is to replace hydrazine, hydrazide or azodicarbonamide while maintaining the same cell structure (small and uniformly distributed cells), while maintaining the production line speed, while maintaining processability, and having the same output from a cable extruder.

[0025] Another object of the present invention is to be able to use a conventional cable extruder, i.e., compared with conventional hydrazine, hydrazide or azodicarbonamide compounds (such as azodicarbonamide (ADCA)), there is no need to replace the equipment, nor to modify the current equipment. The extrusion temperature of the present invention is generally higher, but still within the process window of a conventional cable extruder.

[0026] Another object of the present invention is to increase the process window, which means less degradation risk in compounding, and thus better consistency and better quality. Using the present invention means an increase in the difference between the decomposition temperature of the blowing agent and the melting point of the foamable polymer composition.

[0027] The higher decomposition temperature allows for better dispersion of the blowing agent, nucleating agent and other additives (such as antioxidants) in the foamable polyolefin polymer composition by allowing a higher energy input into the polyolefin polymer composition during mixing, without the risk of loss of the blowing agent by starting the decomposition reaction during the mixing process.

[0028] Therefore, an object of the present invention is to design a blend that produces homogeneous foam that can be extruded at high speed in any cable extruder while having a good cell structure in the foamed layer of the cable.

[0029] Another object of the present invention is to have good distribution of gas nucleation sites in the foamable polyolefin polymer composition.

[0030] The present invention is based on the surprising discovery that all of the above objects can be solved by using a blowing agent containing citric acid and / or citric acid derivatives, preferably in combination with an inorganic nucleating agent, in the foamable polymer composition.

[0031] Accordingly, the present invention provides a foamable polymer composition comprising

[0032] (A) a first polyolefin polymer,

[0033] (B) a second polyolefin polymer having an MFR 2 (2.16 kg; 190 °C) of 2 to 15 g / 10 min, and

[0034] (C) a blowing agent in an amount of 0.01 wt% to 2 wt% based on the total foamable polymer composition,

[0035] Among them, based on the total blowing agent (C), the blowing agent (C) comprises more than 90% by weight of citric acid and / or citric acid derivatives, and among them, the first polyolefin polymer (A) has a higher density than the second polyolefin polymer (B).

[0036] The present invention further provides a foamed polymer composition obtained by foaming the foamable polymer composition according to the present invention.

[0037] There is also provided a cable comprising at least one layer, the layer comprising the foamable polymer composition according to the present invention or the foamed polymer composition according to the present invention.

[0038] Foamable means that at least one layer can be produced in a cable from a composition that can be foamed, for example, in an extrusion process.

[0039] The present invention has several advantages. The foamable polymer composition of the present invention can be foamed into a foamed polymer composition without using harmful blowing agents such as ADCA and halogenated hydrocarbons.

[0040] Therefore, the present invention also avoids the use of exothermic blowing agents such as ADCA, but uses endothermic blowing agents, namely citric acid and / or citric acid derivatives. Endothermic blowing agents are easier to control because a continuous heat supply is required to keep the reaction going. This also enables better control of the gas released from the blowing agent, especially during a continuous process such as extrusion.

[0041] Another advantage of citric acid or citric acid derivatives is that during decomposition, they release CO 2 as the main foaming gas. Compared with N 2 released from hydrazine, hydrazide or azodicarbonamide (ADCA), CO 2 has better solubility in the foamable polymer composition.

[0042] The foamed polymer composition obtained by foaming the foamable polymer composition shows a foam density comparable to that of the foam using ADCA, but has comparable or even smaller cell sizes and comparable or even higher cell densities in the foam compared to the ADCA - foamed foam.

[0043] A blowing agent is a substance capable of generating a cell structure in a foamable polymer composition through a foaming process. Blowing agents are usually used when the polymer is molten. The cell structure in the polymer matrix reduces the density and relative permittivity of the foamable polymer composition.

[0044] Citric acid and citric acid derivatives decompose into water, carbon dioxide and solid decomposition products. The decomposition reaction is endothermic, which means that a continuous supply of heat energy is required for it to occur. The temperature at which the decomposition reaction occurs rapidly depends on the chemical substance (citric acid or citric acid derivative), but is usually about 200 °C. The decomposition temperature of citric acid and citric acid derivatives is much higher than the melting point of polyolefin polymers, so they can be kneaded into polyolefin polymers in the mixing step without pre-decomposition before cable extrusion.

[0045] Preferably, the blowing agent (C) contains more than 92% by weight of citric acid or citric acid derivatives, more preferably more than 95% by weight, or most preferably, the blowing agent (C) consists of citric acid or citric acid derivatives. The advantage of using only "one" blowing agent (C) is that the foaming process can be better controlled because only one decomposition temperature interval needs to be considered during processing, which reduces the complexity of the process.

[0046] Based on the total foamable polymer composition, the amount of the blowing agent (C) is preferably 0.02% to 1.7% by weight, more preferably 0.03% to 1.4% by weight, even more preferably 0.04% to 1.2% by weight, more preferably 0.1% to 1% by weight, and most preferably 0.15% to 0.9% by weight.

[0047] Preferably, the citric acid derivatives include alkali metal salts of citric acid, esters of citric acid, or mixtures thereof. The alkali metal salts of citric acid preferably contain one or more selected from sodium citrate monohydrate, disodium citrate, trisodium citrate, potassium citrate monohydrate, dipotassium citrate, and tripotassium citrate. Among the above alkali metal salts, sodium citrate monohydrate is most preferred.

[0048] Preferably, the blowing agent (C) does not contain halogenated hydrocarbons and / or fluororesins. Halogenated hydrocarbons are, for example, hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and perfluorocarbons (PFCs). Fluororesins are resins containing fluorocarbon bonds, for example, polytetrafluoroethylene (PTFE).

[0049] The foamable polymer composition preferably further contains an inorganic nucleating agent (E). The inorganic nucleating agent (E) is usually a mineral with a high surface area. The interface between the inorganic nucleating agent (E) and the polymer composition melt will act as a nucleation site for bubble formation during the foaming process because the energy required to form bubbles in this interface is lower than that in the bulk polymer melt. The inorganic nucleating agent (E) preferably contains a magnesium-containing compound, a calcium-containing compound, a silicon-containing compound, or a mixture thereof. The inorganic nucleating agent can be any inorganic filler, such as silica, talc, calcium carbonate, kaolin, dolomite, zeolite, mica, wollastonite, or clay minerals.

[0050] In order to achieve a uniform distribution of the inorganic nucleating agent (E) in the foamable polymer composition, the inorganic nucleating agent (E) is added to the foamable polymer composition, preferably kneaded or melt-mixed therewith. The inorganic nucleating agent (E) preferably has a powder form, i.e., a small particle form. The average particle size is typically on the order of 0.1 μm to 50 μm.

[0051] Preferably, the blowing agent masterbatch (BAMB) comprises the blowing agent (C) and the inorganic nucleating agent (E) as described in all of the above embodiments. More preferably, the blowing agent masterbatch consists of the blowing agent (C) and the nucleating agent (E) as described in all of the above embodiments. The blowing agent masterbatch is added to the foamable polymer composition, preferably kneaded or melt-mixed therewith. The blowing agent masterbatch may further comprise a polymer carrier, such as a polyethylene carrier.

[0052] The distribution of the blowing agent (C) and the nucleating agent (E) in the polymer composition can be improved by melt-mixing the blowing agent masterbatch into the foamable polymer composition, preferably by kneading in an extruder before extrusion of the foamable polymer composition. This results in an improved cell structure because the gas released by the decomposition of the blowing agent (C) and the solid particles of the inorganic nucleating agent (E) acting as foaming nucleation sites are better distributed in the polyolefin polymer melt. For communication cables, it is crucial to have a good cell structure within the foamed insulation to have isotropic electrical properties. A cell structure with many small, uniformly distributed cells in the insulation is desired. The cell structure is also important for mechanical properties. A structure with many well-distributed small cells will provide better compressive resistance compared to a structure with larger, non-uniformly distributed cells (as this would create weak parts in the insulation).

[0053] The foamable polymer composition according to the invention comprises a first polyolefin polymer (A) and a second polyolefin polymer (B). The MFR of the first polyolefin polymer (A) measured according to ISO 1133-1 2 (2.16 kg; 190 °C) is preferably 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.

[0054] The MFR of the second polyolefin polymer (B) measured according to ISO 1133-1 2 (2.16 kg; 190 °C) is 2 to 15 g / 10 min. Preferably, the MFR of the second polyolefin polymer (B) 2 (2.16 kg; 190 °C) is 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 most preferably 4 to 6 g / 10 min.

[0055] Based on the total foamable polymer composition, 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 most preferably in an amount of 60 to 80% by weight. And based on the total foamable polymer composition, 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 most preferably in an amount of 20 to 40% by weight.

[0056] The first polyolefin polymer (A) is preferably a homopolymer or copolymer of ethylene or a homopolymer or copolymer of propylene, more preferably a copolymer of ethylene. And the second polyolefin polymer (B) is preferably a homopolymer or copolymer of ethylene or a homopolymer or copolymer of propylene, more preferably a homopolymer of ethylene.

[0057] The first polyolefin polymer (A) is preferably a high-density polyethylene homopolymer or copolymer having a density of 935 to 970 kg / m 3 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 kg / m 3 to 930 kg / m 3 measured according to ISO 1183-1.

[0058] More preferably, the high-density polyethylene (HDPE) is a copolymer, and the low-density polyethylene (LDPE) is a homopolymer. A homopolymer means that the low-density polyethylene (LDPE) contains at least 90% by weight of ethylene monomers, preferably at least 95% by weight of ethylene monomers, and most preferably at least 99% by weight of ethylene monomers.

[0059] In the case where the high-density polyethylene (HDPE) is a copolymer, the copolymer contains ethylene monomers and one or more comonomers. The content of the ethylene monomers is preferably at least 50% by weight 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.

[0060] The low-density polyethylene (LDPE) is preferably a homopolymer.

[0061] For foamed polyethylene used in communication cables, both electrical and mechanical properties are important. HDPE has a lower dielectric constant and lower loss factor than LDPE, as well as higher strength and hardness.

[0062] High-density polyethylene (HDPE) polymers are polymerized in a low-pressure process and are, for example, optionally HDPE homopolymers or optionally HDPE copolymers of ethylene with one or more of the comonomers described above. In addition, HDPE is polymerized in the presence of a catalyst during a low-pressure polymerization process. 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.

[0063] In order to foam a foamable polymer composition, the foamable polymer composition must have good melt strength, because too poor melt strength will cause the cell structure to collapse, which is not conducive to the mechanical or electrical properties of the cable layer (usually the insulating layer). 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 a uniform cell distribution, the melt strength can be improved.

[0064] Low-density polyethylene (LDPE) polymers are polymerized in a high-pressure free-radical polymerization process. In addition, LDPE is polymerized during a high-pressure polymerization process in the presence of an initiator and a chain transfer agent (such as propane, propionaldehyde, and methyl ethyl ketone) to control the MFR.

[0065] LDPE can be produced, for example, in a tubular polymerization reactor or in an autoclave polymerization reactor.

[0066] The foamable polymer composition preferably contains an antioxidant. The antioxidant is preferably a phenolic antioxidant, a phosphorus-containing antioxidant, or a mixture thereof. The phenolic antioxidant is preferably a blend of pentaerythritol-tetra(3-(3’,5’-di-tert-butyl-4-hydroxyphenyl)-propionate (CAS No.: 6683-19-8; commercially available from BASF under the trade name Irganox 1010); and tris(2,4-di-tert-butylphenyl) phosphite (CAS No.: 31570-04-4; commercially available from BASF under the trade name Irgafos 168). This antioxidant blend can be purchased from BASF as Irganox B561.

[0067] Based on the total foamable polymer composition, the antioxidant is preferably present in an amount of 0.01 wt% to 2 wt%, more preferably in an amount of 0.04 wt% to 1 wt%, and most preferably in an amount of 0.08 wt% to 0.5 wt%.

[0068] The foamable polymer composition preferably contains an acid scavenger. The acid scavenger is preferably calcium stearate, sodium stearate, zinc stearate, or a mixture thereof, more preferably zinc stearate.

[0069] Based on the total foamable polymer composition, the amount of the acid scavenger is preferably from 0.01% to 2% by weight, more preferably from 0.02% to 1% by weight, and most preferably from 0.04% to 0.08% by weight.

[0070] The dissipation factor, also known as tan δ, is a measure of the degree of power dissipation in a dielectric material, i.e., a measure of how much electrical energy is converted into heat in the dielectric material. The dissipation factor of the foamable polymer composition at 1.9 GHz is preferably 80·10 -6 to 160·10 -6 more preferably 110·10 -6 to 150·10 -6 more preferably 115·10 -6 to 140·10 -6 and most preferably 120·10 -6 to 135·10 -6 .

[0071] The dissipation factor of the foamable polymer composition at 1 MHz is preferably 20·10 -6 to 140·10 -6 more preferably 22.5·10 -6 to 100·10 -6 more preferably 25·10 -6 to 50·10 -6 and most preferably 30·10 -6 to 45·10 -6 .

[0072] The foamable polymer composition can be obtained by foaming the foamable polymer composition according to the present invention.

[0073] Foaming is carried out by heating the foamable polymer composition to a temperature preferably from 130 °C to 240 °C, more preferably from 130 °C to 230 °C, and even more preferably from 130 °C to 220 °C. In this temperature range, as described above, the blowing agent (C) present in the foamable polymer composition thermally decomposes into gaseous products, thereby obtaining the foamable polymer composition. The heating is preferably carried out in an extruder. After leaving the die of the extruder, the gaseous products can expand, thereby forming the foamable polymer composition.

[0074] Preferably, during the extrusion of the foamable polymer composition in an extruder, the co-blowing agent (D) is added to the foamable polymer composition. During the extrusion process, the co-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, thereby obtaining the foamable polymer composition.

[0075] It is particularly advantageous to use the co - blowing agent (D) and the blowing agent (C) together, compared to using the blowing agent (C) alone, in order to obtain a foamed polymer composition with a higher degree of foaming.

[0076] The extrusion is preferably carried out in an injection - gas - foaming production line.

[0077] The co - blowing agent (D) preferably contains a gas, and the gas includes N 2 、CO、CO 2 、Ar or a mixture thereof. More preferably, the co - blowing agent (D) includes N 2 and / or CO 2 ; more preferably, the co - blowing agent (D) consists of N 2 and / or CO 2 .

[0078] Based on the total foaming polymer composition, the co - blowing agent (D) is preferably used in an amount of 0.01 wt% to 5 wt%, more preferably in an amount of 0.015 wt% to 2.5 wt%, more preferably in an amount of 0.02 wt% to 0.2 wt%, and more preferably in an amount of 0.03 wt% to 0.1 wt%.

[0079] The average cell diameter of the foamed polymer composition is preferably 5 μm to 250 μm, more preferably 50 μm to 100 μm, more preferably 60 μm to 95 μm, and most preferably 65 μm to 90 μm.

[0080] The density of the foamed polymer composition is preferably 95 kg / m 3 to 860 kg / m 3 , more preferably 150 kg / m 3 to 800 kg / m 3 , more preferably 400 kg / m 3 to 750 kg / m 3 , more preferably 420 kg / m 3 to 730 kg / m 3 , more preferably 440 kg / m 3 to 710 kg / m 3 , most preferably 460 kg / m 3 to 690 kg / m 3 .

[0081] The present invention also provides a cable, which includes at least one layer, and the layer contains the foaming polymer composition according to the present invention or the layer contains the foamed polymer composition according to the present invention. Therefore, the cable includes at least one layer, and the layer contains the foaming polymer composition according to any of the above - mentioned embodiments, or the cable includes at least one layer, and the layer contains the foamed polymer composition according to any of the above - mentioned embodiments.

[0082] Preferably, the cable is a communication cable, preferably a coaxial cable or a twisted pair cable.

[0083] The at least one layer is preferably an insulating layer of the cable. The insulating layer is the layer that surrounds the innermost conductor, which is typically made of copper. In a preferred embodiment, the insulating layer comprising the foamable polymer composition of the present invention or the foam polymer composition of the present invention is in direct contact with the conductor. For coaxial cables, the typical thickness of the insulating layer is from 0.01 mm to 80 mm, and for data cables, the thickness of the insulating layer is generally from 0.1 mm to 2 mm.

[0084] The foam polymer composition according to the present invention can preferably be produced by a method for producing a foam polymer composition, which method comprises the following steps:

[0085] a) providing a foamable polymer composition according to the present invention,

[0086] b) heating the foamable polymer to a temperature of 150 °C to 240 °C to obtain a molten polymer composition, and

[0087] c) foaming the molten polymer composition.

[0088] Preferably, the heating in step b) is carried out in an extruder, and the foaming of the molten polymer composition obtained in step c) is carried out after the molten polymer composition exits the die of the extruder.

[0089] Preferably, a co-foaming agent (D) is added to the foamable polymer composition in step b). The addition is preferably carried out by injecting the co-foaming agent (D) into the foamable polymer composition or by melt-mixing the co-foaming agent (D) with the foamable polymer composition. The melt-mixing is preferably carried out in an extruder.

[0090] Preferably, the extruder can be any extruder known in the art suitable for melt-mixing a polymer melt with a foaming agent.

[0091] All of the above-described embodiments of the foamable polymer composition according to the present invention are also preferred embodiments of the foamable polymer composition used in the method for producing a foam polymer composition.

[0092] All of the preferred embodiments of the foaming agent (C) as described above are preferred embodiments of the foaming agent (C) used in the method for producing a foam polymer composition.

[0093] All of the preferred embodiments of the co-foaming agent (D) as described above are preferred embodiments of the co-foaming agent (D) used in the method for producing a foam polymer composition. Detailed Description

[0094] Example

[0095] 1. Measuring method

[0096] a) Melt flow rate

[0097] The melt flow rate (MFR) is determined according to ISO 1133-1 and expressed in g / 10 min. The MFR indicates the fluidity of the polymer and thus its processability. The higher the melt flow rate, the lower the viscosity of the polymer.

[0098] MFR of polyethylene (co)polymers 2 is measured at a temperature of 190 °C and a load of 2.16 kg. MFR of polypropylene (co)polymers 2 is measured at a temperature of 230 °C and a load of 2.16 kg.

[0099] b) Density of solid materials

[0100] The method for determining the density of the foamable polymer composition follows ISO 17855-2 for sample preparation and ISO 1183-1 / Method A for density measurement.

[0101] Compression molding is carried out in a controlled cooling press. The molding temperature for polyethylene is 180 °C and the cooling rate is 15 °C / min. The samples are placed at 23 ± 2 °C for at least 16 hours. The density is determined at 23 ± 0.1 °C using isododecane as the immersion liquid without buoyancy correction.

[0102] c) Density of foamed materials

[0103] To determine the density of the foamed samples, the weight of each sample is measured in an air atmosphere (w L , in g) and in a medium with a known temperature (w W , in g). All measurements are carried out at 22 °C in distilled water with 3 drops of wetting agent added. To calculate the density, the following formula is used:

[0104]

[0105] where

[0106] ρ = density, in g / cm 3

[0107] ρ W = density of water at the measurement temperature, in g / cm 3

[0108] ρ L= air density (0.0012 g / cm 3 )

[0109] d) Calculated cell density

[0110] The cell density (N b / cm 3 ) of the foamed polymer composition is calculated as follows:

[0111]

[0112] where

[0113] ρ F = density of the foamed sample, in g / cm 3

[0114] ρ m = density of the polymer matrix, in g / cm 3

[0115] D = average cell diameter, in cm

[0116] e) Calculation of density drop

[0117] The formula for calculating the density drop (X) as a percentage is as follows:

[0118]

[0119] where

[0120] D S = density of the solid material, in kg / m 3

[0121] D F = density of the foamed material, in kg / m 3

[0122] f) Determination of average cell diameter

[0123] To determine the average cell diameter, the cross-sectional areas of approximately 60 cells (if any) were measured. Thus, the cells were manually marked in the image analysis software of the Alicona system. The average diameter of the cells was calculated assuming a circular cross-section for the bubbles. This method helps to compare the foam morphology of different samples because the geometry of most cells is different from the ideal circle, so a reasonable comparison cannot be made for the directly measured diameters.

[0124] The average diameter was determined by using the following formula and then averaging the calculated values of each bubble diameter.

[0125]

[0126] wherein

[0127] DZ = diameter of a foam cell under the assumption of a circular cross-section, in μm

[0128] AZ = cross-section of a foam bubble, in μm 2

[0129] g) Microscopic analysis of the foamed polymer composition

[0130] The density and foam morphology of all samples were examined. Thus, the cell size was measured using an optical microscope Alicona InfiniteFocus (Alicona Imaging GmbH, Austria). A high-precision balance (Excellence XS Analyze Waage, Mettler Toledo AG, Switzerland) equipped with a density measurement kit (density kit, Mettler Toledo AG, Switzerland) was used to determine the density.

[0131] 2. Dielectric properties (dielectric loss tangent (tanδ) - dissipation factor at 1.9 GHz)

[0132] a) Preparation of plaques:

[0133] The polymer blend was compression molded in a frame at 140 °C to obtain a flat plate 4 mm thick, 80 mm wide and 130 mm long. The pressure was adjusted high enough to obtain a smooth surface of the flat plate. Visual inspection of the flat plate revealed no inclusions such as trapped air or any other visible contaminants.

[0134] b) Characterization of the dielectric properties of plaques:

[0135] To measure the dielectric constant and tangent δ (tanδ) of the material, a split-post dielectric resonator was used together with a network analyzer (Rodhe & Schwarz ZVL6). This technique measures the complex permittivity of dielectric layered samples (plaques) in the frequency range of 1 - 10 GHz. The test was carried out at 23 °C.

[0136] The Split Post Dielectric Resonator (SPDR) was developed by Krupka and his co-workers [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 placed coaxially along the z-axis such that there is a small laminated gap between them in which the sample to be measured can be placed. By choosing a suitable dielectric material, the resonant frequency and 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 permittivity of the sample: its thickness and the changes in the resonant frequency Δf and Q-factor ΔQ obtained when it is placed in the resonator.

[0137] As described above, samples with a thickness of 4 mm were prepared by compression molding and measured at a high frequency of 1.9 GHz.

[0138] A review of this method can be found in J. Krupka, R. N. Clarke, O. C. Rochard and A. P. Gregory, “Split-Post Dielectric Resonator technique for precise measurements of laminar dielectric specimens - measurement uncertainties” in Proceedings of the XIIIInt. Conference MIKON'2000, Wroclaw, Poland, pp. 305 - 308, 2000.

[0139] 3. Dielectric properties (dielectric loss tangent (tanδ) - dissipation factor at 1 MHz)

[0140] a) Preparation of the test specimens

[0141] The polymer blend was compression molded in a frame in a stamping machine at 140 °C to obtain a flat plate with a thickness of 1.12 mm, a width of 210 mm, and a length of 210 mm. The pressure applied using an aluminum plate was 25 bar (2.5 minutes) and 190 bar (4 minutes). Cooling was carried out at a rate of 15 °C / min.

[0142] b) Characterization of the dielectric properties of the specimens

[0143] The method used in this article is a modification of ASTM D150 (2018) (AC loss characteristics and permittivity (dielectric constant) of solid electrical insulating materials).

[0144] The method in this article determines the electrical properties of a polymer composition or polymer blend based on the dissipation factor at a frequency of 1 MHz. The electrical properties are determined using a Q-meter by comparison with a stable reference fluid (silicone oil). The silicone oil used in the examples is Dow Corning 200.

[0145] The specimens obtained as described above were cut to obtain two sample specimens, each with dimensions of 1.12 mm thick, 66.5 mm wide, and 101 mm long, and the samples were placed in a two-terminal liquid displacement cell connected to a resonant circuit. The samples were analyzed in a Q-meter at a frequency of 1 MHz in a stable reference liquid (silicone oil). The dissipation factor was measured with and without the sample specimen in the reference liquid, and the difference with and without the sample specimen represents the reported result. This test was carried out at 23 °C.

[0146] The dissipation factor (DF) of the sample can be measured in a resonant circuit as follows:

[0147] DF = Δ(1 / Q) × CT / CS × (M out / T s +M out -M in )

[0148] where

[0149] Q = the maximum voltage obtained from the Q-meter at resonance

[0150] ΔQ = the voltage difference between the sample in the cell and no sample in the cell

[0151] CT = the total capacitance of the measurement circuit

[0152] CS = the capacitance of the sample

[0153] M out = the micrometer setting at resonance without the sample

[0154] M in = the micrometer setting at resonance with the sample

[0155] Ts = sample thickness

[0156] 4. Materials

[0157] As the HDPE component, a single - peak Ziegler - Natta catalyzed HDPE copolymer with butene as comonomer is used, having an MFR 2 of 8 g / 10 min and a density of 963 kg / m 3 .

[0158] LDPE is an autoclave LDPE having an MFR of 4.5 g / 10 min 2 and a density of 923 kg / m 3 . CA7230 is available from Borealis AG

[0159] nCore 7155 - M1 - 300 is a blowing agent masterbatch based on azodicarbonamide (ADCA) commercially available from Americhem. It contains 15% active blowing agent

[0160] Irganox B561 is an antioxidant blend commercially available from BASF

[0161] Zincum TX is a zinc stearate acid scavenger commercially available from Baerlocher

[0162] PO 2217: An endothermic blowing agent masterbatch commercially available from Tramaco GmbH, which contains a citric acid derivative in an amount of 100 wt% of the added blowing agent. The masterbatch also contains an inorganic nucleating agent

[0163] 5. Preparation of Examples

[0164] 5.1 Material mixing

[0165] The examples in Tables 1 and 3 were compounded on a BUSS MDK46 continuous extruder (built in 1985). This production line is a single - screw kneader with a screw diameter of 46 mm and an L / D of 11

[0166] Table 1: Compositions of comparative examples (CE) and inventive examples (IE) for foaming, amounts are given in wt%

[0167] Material CE1 IE1 IE2 HDPE / wt% 69.6 69.6 69.6 LDPE / wt% 29.8 29.9125 29.575 Irganox B561 / wt% 0.1 0.1 0.1 Zincum TX / wt% 0.05 0.05 0.05 nCore 7155 - M1 - 300 / wt% 0.45 Tracel PO 2217 / wt% 0.3375 0.675

[0168] 5.2 Extrusion and foaming

[0169] Polymer pellets of the composition in Table 1 were extruded on a Rosendahl RE45 extrusion production line with a screw having a diameter of 45 mm. The total length of the extruder is 32D, including an 8D long oil-tempered cylindrical elongated structure for better control of the polymer melt temperature. To achieve a longer residence time and better homogenization, a static mixer (type SMB-R from Sulzer, Switzerland) with a length of 4D was installed between the cylindrical elongated structure and the extrusion die. A circular die with a diameter of 4.0 mm was used. The extruder has 10 temperature zones, and a gas (N 2 ) was injected between zone 7 and zone 8 as a co-foaming agent (D). The temperature settings (°C) are as follows (the slashes indicate different temperature zones):

[0170] T 1 : 40 / 150 / 160 / 160 / 165 / 170 / 190 / 190 / 170 / 170 / 170 / 170 / 170 °C

[0171] The results are shown in Table 2 below. The foam density was measured at 22 °C.

[0172] Table 2: Properties of the obtained foamed compositions

[0173]

[0174] Two dosages of the foaming agent masterbatch were tested. The amount of the foaming agent masterbatch in IE2-1 was slightly lower than the amount of ADCA, while the amount of the foaming agent masterbatch in IE2-2 was almost twice the amount of ADCA. It is known in the literature that the gas yield of ADCA (an exothermic foaming agent) is 220 cm 3 / g, while a blend of citric acid and sodium bicarbonate (an endothermic foaming agent) produces only 120 cm 3 / g of gas. Therefore, the amount of the endothermic foaming agent usually needs to be increased compared to the exothermic foaming agent.

[0175] The electrical loss factor (dissipation factor) has been measured in Comparative Example 2 and Inventive Examples IE2-1 to IE2-3. Table 3 below gives the compositions of all the examples. After compounding as described above, all the materials were compression-molded at the foaming temperature (140 °C) as described above. As described above, the specimens were subjected to dissipation factor measurements (tan δ) at 1.9 GHz and 1 MHz respectively.

[0176] Table 3: Compositions of the comparative examples (CE) and inventive examples (IE), the dosages are given in weight %, and the measurement results of the dissipation factor before foaming.

[0177] Material CE2 IE2 - 1 IE2 - 2 HDPE / wt% 69.60 69.60 69.60 LDPE / wt% 29.95 30.20 29.90 nCore 7155 - M1 - 300 / wt% 0.45 Tracel PO 2217 / wt% 0.2 0.5 <![CDATA[Dissipation factor at 1.9 GHz (·10 -6 )]]> 140 128 128 <![CDATA[Dissipation factor at 1 MHz (·10 -6 )]]> 31 36 39

[0178] Surprisingly, invention examples IE2-1 and IE2-2 in Table 3 show that, compared with the insulating formulation containing ADCA (CE2), the use of an endothermic blowing agent can achieve a similar low dissipation factor at 1.9 GHz. The same is true for the dissipation factor at 1 MHz.

Claims

1. A foamable polymer composition comprising (A) a first polyolefin polymer, (B) A second polyolefin polymer having an MFR 2 measured according to ISO 1133 (2.16 kg; 190 °C) of from 2 to 15 g / 10 min, and (C) a foaming agent in an amount of 0.01% to 2% by weight based on the total foamable polymer composition, wherein based on the total foaming agent (C), the foaming agent (C) comprises citric acid and / or a citric acid derivative in an amount greater than 90% by weight, wherein the first polyolefin polymer (A) has a higher density than the second polyolefin polymer (B), and wherein the foamable polymer composition does not contain halogenated hydrocarbons such as hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs) and perfluorocarbons, or fluororesins such as polytetrafluoroethylene (PTFE).

2. The foamable polymer composition according to the claim, wherein the citric acid derivative includes an alkali metal salt of citric acid, an ester of citric acid, or a mixture thereof.

3. The foamable polymer composition according to any one of the preceding claims, wherein the foamable polymer composition further comprises an inorganic nucleating agent (E).

4. The foamable polymer composition according to any one of the preceding claims, wherein According to the measurement of ISO 1133, the MFR of the first polyolefin polymer (A) 2 (2.16 kg; 190 °C) is from 0.1 to 20 g / 10 min.

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

6. The 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 kg / m 3 to 970 kg / m 3 as measured according to ISO 1183-1, and the second polyolefin polymer (B) is a low-density polyethylene homopolymer having a density of 880 kg / m 3 to 930 kg / m 3 as measured according to ISO 1183-1.

7. The foamable polymer composition according to any one of the preceding claims, wherein The dissipative factor of the foamable polymer composition at 1.9 GHz is 80·10 -6 to 160·10 -6 and / or the dissipative factor at 1 MHz is 20·10 -6 to 140·10 -6 .

8. A foamed polymer composition obtained by foaming the foamable polymer composition according to any one of claims 1 to 7.

9. The foamed polymer composition according to claim 8, wherein the foaming is accomplished by heating the foamable polymer composition to a temperature of 150°C to 240°C.

10. The foamed polymer composition according to claim 8 or 9, wherein a co-foaming agent (D) is added to the foamable polymer composition.

11. The foamed polymer composition according to claim 10, wherein The co-foaming agent (D) includes N 2 , CO, CO 2 , Ar or a mixture thereof.

12. The foamed polymer composition according to any one of claims 10 or 11, wherein based on the total foamable polymer composition, the co-foaming agent (D) is used in an amount of 0.01% to 0.5% by weight.

13. The foamed polymer composition according to any one of claims 8 to 12, wherein the foamed polyolefin composition has an average cell diameter of 5 μm to 250 μm.

14. The foamed polymer composition according to any one of claims 8 to 13, wherein The density of the foamed polymer composition is from 95 kg / m 3 to 860 kg / m 3 .

15. A cable comprising at least one layer, the layer comprising the foamable polymer composition according to any one of claims 1 to 7, or the layer comprising the foamed polymer composition according to claims 8 to 14.

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