Polymer Composition

Abstract

The present invention relates to a polymer composition comprising:a)         at least one polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s);b)         at least one antioxidant;c)         at least one crosslinking agent; andd)         2,4-diphenyl-4-methyl-1-pentene;wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer; andwherein the crosslinking agent is an organic peroxide having a structural formula in accordance with Formula (I):wherein R1, R2 and R3 are each independently selected from linear and branched C1-C6 alkyl groups; andR is the rest of the organic peroxide.

Description

Polymer Composition Field of the InventionThe present invention relates to a polymer composition, for example a power cable insulation polymer composition. The present invention further relates to a process for producing a polymer composition, a crosslinked polymer composition obtainable by crosslinking a polymer composition, a power cable, a process for producing a power cable, and the use of a combination of a certain crosslinking agent and a particular additive (2,4-diphenyl-4-methyl-1-pentene) to reduce exudation of an antioxidant from a polymer composition.  BackgroundPolyethylenes produced in a high pressure (HP) process are widely used in demanding polymer applications wherein the polymers must meet high mechanical and / or electrical requirements. For instance in wire and cable (W&C) applications, for example power cable applications, particularly in medium voltage (MV) and especially in high voltage (HV) and extra-high voltage (EHV) power cable applications, the electrical properties of the polymer composition have a significant importance. Furthermore, the electrical properties, which are of importance, may differ in different cable applications, as is the case between alternating current (AC) and direct current (DC) cable applications.Crosslinking of polymer compositions, e.g. polyethylenes, is widely known, especially in the afore-mentioned W&C applications. In the crosslinking reaction of a polymer, interpolymer crosslinks (bridges) are primarily formed. Crosslinking typically improves the heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance of a polymer composition. Crosslinking is generally performed with a crosslinking agent, e.g. a peroxide, which decomposes under crosslinking conditions to generate free radicals. The crosslinking agent is typically incorporated into the polymer composition before the polymer composition is moulded or otherwise formed into an article, e.g. before extrusion of a layer of the polymer composition onto one or more conductors to form a cable, and before the polymer composition is crosslinked. Such a polymer composition, which comprises a crosslinking agent and thus has the potential to undergo crosslinking but has not yet been crosslinked, is termed “crosslinkable”. Crosslinkable polymer compositions are often subjected to extended periods of storage; both by the manufacturer before they are shipped to the customer and by the customer before they are used. During these periods of storage, it has been observed that antioxidants, which are typically added to the polymer compositions to protect them from the oxidising effects of the environment such as the air, migrate to and accumulate at the surface of the polymer compositions (which are usually in the form of pellets). This migration (which is also termed “exudation”) may result in a depletion of the antioxidant content in the bulk of the polymer composition, which can have a serious adverse effect on the lifetime and stability of the polymer composition. There remains a need therefore for a polymer composition which has improved storage stability. In particular, there is a need for a crosslinkable antioxidant-containing polymer composition which exhibits very little or practically no exudation of antioxidant.  Summary of the InventionIn one aspect, the present invention provides a polymer composition comprising:a) at least one polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s);b) at least one antioxidant;c) at least one crosslinking agent; andd) 2,4-diphenyl-4-methyl-1-pentene;wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer; and wherein the crosslinking agent is an organic peroxide having a structural formula in accordance with Formula (I):wherein R1, R2 and R3 are each independently selected from a linear and branched C1-C6 alkyl groups; and R is the rest of the organic peroxide. Preferably the polymer composition is suitable for use in the insulation layer of a power cable. In another aspect, the present invention provides a process for producing a polymer composition as defined in any preceding aspect herein, the process comprising blending the polyethylene copolymer(s) with the antioxidant(s), the crosslinking agent(s) and 2,4-diphenyl-4-methyl-1-pentene.In yet another aspect, the present invention provides a crosslinked polymer composition obtainable, preferably obtained, by crosslinking a polymer composition as defined in any preceding aspect herein.In yet another aspect, the present invention provides a power cable comprising one or more conductor(s) surrounded by at least one layer, preferably an insulation layer, comprising, preferably consisting of, a polymer composition as defined in any preceding aspect herein or a crosslinked polymer composition as defined in any preceding aspect herein. In yet another aspect, the present invention provides a process for producing a power cable, the process comprising:(i) applying on one or more conductor(s), preferably by (co)extrusion, at least one layer, preferably an insulation layer, comprising, preferably consisting of, a polymer composition as defined in any preceding aspect herein; and(ii) optionally crosslinking the polymer composition.In yet another aspect, the present invention provides a use of a combination of a crosslinking agent and 2,4-diphenyl-4-methyl-1-pentene in a polymer composition further comprising (i) a polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s), and (ii) an antioxidant, to reduce exudation of the antioxidant from the polymer composition; wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer; and wherein the crosslinking agent is an organic peroxide having a structural formula in accordance with Formula (I):wherein R1, R2 and R3 are each independently selected from linear and branched C1-C6 alkyl groups; and R is the rest of the organic peroxide. Detailed Description of the InventionThe present invention relates in one aspect to a polymer composition which has improved storage stability. In particular, by using a combination of a particular crosslinking agent (which is an organic peroxide having a structural formula in accordance with Formula (I)) and a particular additive (2,4-diphenyl-4-methyl-1-pentene) in a polymer composition which further comprises a certain polyethylene copolymer comprising a certain content of polar comonomer(s) and an antioxidant, the present inventors have established that the exudation of the antioxidant from the polymer composition is unexpectedly reduced relative to the case where no such combination is used. Component a) – Polyethylenecopolymer(s)The polymer composition of the present invention comprises at least one polyethylene copolymer, i.e. one or more polyethylene copolymer(s). It is possible to use a mixture of polyethylene copolymers in the polymer composition of the present invention although it is preferred if a single polyethylene copolymer is used. The polyethylene copolymer a) of the polymer composition of the present invention is a polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s), wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer. It will be understood that the one or more polar comonomer(s) are incorporated in the polyethylene copolymer a), i.e. rather than being present as a separate component of the polymer composition. In a preferred embodiment, the third monomer is present and hence the polyethylene copolymer of the invention becomes a polyethylene terpolymer of ethylene, one or more polar comonomer(s) and one or more other non-polar comonomer(s), wherein the content of the one or more polar comonomer(s) in the polyethylene terpolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene terpolymer. The term “copolymer” as used herein therefore covers those polyethylenes which comprise repeating units derived from two or more different monomers.The term “terpolymer” as used herein covers both those polyethylenes which comprise repeating units derived from three different monomers and those polyethylenes which comprise repeating units derived from more than three (e.g. 4 or 5) different monomers. In a preferred embodiment however, the polyethylene copolymer a) of the polymer composition of the present invention comprises repeating units derived from strictly three comonomers (including ethylene), i.e. is a polyethylene terpolymer of ethylene, one polar comonomer and one other non-polar comonomer. As is well known, the term “comonomer” refers to copolymerisable comonomer units. The person skilled in the art understands that the term “non-polar comonomer(s)” as used herein refers to those comonomers which are hydrocarbons, e.g. which consist of carbon and hydrogen atoms only. In contrast, the term “polar comonomer(s)” refers to those comonomers which are not non-polar comonomer(s). Typically the polar comonomer(s) are oxygen-containing comonomers, e.g. comprising a C-O and / or C=O bond. The term “other” in the phrase “one or more other non-polar comonomer(s)” is used to distinguish the non-polar comonomer(s) in the polyethylene copolymer a) from ethylene, i.e. the one or more other non-polar comonomer(s) are non-polar comonomer(s) other than ethylene.The polyethylene copolymer a) can generally be any polyethylene copolymer as defined above, such as any polyethylene copolymer a) which is conventionally used in a layer, for example an insulation layer, of an electrical cable, e.g. of a power cable. The polyethylene copolymer a) can be for example a commercially available polyethylene copolymer or can be prepared according to or analogously to known polymerisation processes in the chemical literature.In a preferred embodiment, the polyethylene copolymer is produced in a high pressure (HP) process, e.g. is a low density polyethylene (LDPE) copolymer. The meaning of LDPE is well known and documented in the literature. In particular, the term “LDPE” is used to distinguish a polyethylene produced in a high pressure process from a polyethylene produced in the presence of an olefin polymerisation catalyst. Furthermore, LDPEs have certain typical features, such as a different branching architecture, compared to polyethylene produced in the presence of an olefin polymerisation catalyst. Typically, the polymerisation of ethylene and further comonomer(s) in a high pressure process is carried out in the presence of an initiator(s).In one embodiment, the polyethylene copolymer is selected from the group of saturated polyethylene copolymers and unsaturated polyethylene copolymers, preferably from the group of unsaturated polyethylene copolymers. In one embodiment, the polyethylene copolymer is an unsaturated LDPE copolymer. In one embodiment, the polar comonomer(s) of the polyethylene copolymer(s) are selected from the group of comonomer(s) containing hydroxyl group(s), alkoxy group(s), carbonyl group(s), carboxyl group(s), ether group(s) or ester group(s), and mixtures thereof. Preferably, comonomer(s) containing carboxyl and / or ester group(s) are used. More preferably, the polar comonomer(s) of said polyethylene copolymer are selected from the group of acrylate(s), methacrylate(s), acetate(s), and mixtures thereof. In a preferred embodiment, the polar comonomer(s) of the polyethylene copolymer(s) is selected from the group of alkyl acrylates, alkyl methacrylates, vinyl acetate, and mixtures thereof. More preferably, said polar comonomer(s) are selected from the group of C1 to C6 alkyl acrylates, C1 to C6 alkyl methacrylates, vinyl acetate, and mixtures thereof. In one embodiment, the polar comonomer(s) is selected from the group of C1 to C6 alkyl acrylates, such as methyl, ethyl, propyl or butyl acrylates, or any mixture thereof.Surprisingly, the present inventors have established that when a polyethylene copolymer a) as defined above (i.e. comprising one or more polar comonomer(s)) is used in the polymer composition of the present invention, the reduction in exudation of antioxidant is more pronounced than when a polyethylene copolymer is used which does not comprise a polar comonomer. When present, the non-polar comonomer(s) may be selected from the group of monounsaturated (= one double bond) comonomers, for example olefins, for example alpha-olefins, for example C3 to C10 alpha-olefins, such as propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, styrene, 1-octene or 1-nonene. Alternatively, the non-polar comonomer(s) is selected from the group of polyunsaturated (= more than one double bond) comonomers. Suitable polyunsaturated comonomers are further described below.The content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer. In a preferred embodiment, the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.05 to 4.5 wt%, more preferably in the range of 0.1 to 4.0 wt%, relative to the total weight of the polyethylene copolymer. In a still further embodiment, the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.05 to 3.0 wt%, 0.75 to 2.5 wt%, or 0.1 to 2.0 wt% such as 0.1 to 1.75 wt%, especially 0.1 to 1.5 wt%, 0.1 to 1.25 wt% or 0.1 to 1.0 wt%, relative to the total weight of the polyethylene copolymer. Where the polyethylene copolymer comprises more than one polar comonomer, the above ranges refer to the total amount of polar comonomer in the polyethylene copolymer, i.e. the sum of the amounts of the individual polar comonomers in the polyethylene copolymer. The content of ethylene in the polyethylene copolymer is not particularly limited but is generally in the range of at least 50 wt%, preferably at least 60 wt%, more preferably at least 70 wt%, most preferably at least 80 wt%, relative to the total weight of the polyethylene copolymer. In one embodiment, the ethylene content of the polyethylene copolymer is 99.9 wt% or less, such 99.0 wt% or less, relative to the total weight of the polyethylene copolymer, such as 90 to 99.0 wt%. The polyethylene copolymer, e.g. the LDPE copolymer, may optionally be unsaturated, i.e. may comprise carbon-carbon double bonds (-C=C-). Preferred unsaturated polyethylene copolymers, e.g. unsaturated LDPE copolymers, contain carbon-carbon double bonds / 1000 carbon atoms in a total amount of at least 0.10 / 1000 carbon atoms, preferably at least 0.20 / 1000 carbon atoms, more preferably at least 0.30 / 1000 carbon atoms, most preferably at least 0.40 / 1000 carbon atoms. The upper limit of the amount of carbon-carbon double bonds present in the unsaturated polyethylene copolymer, e.g. unsaturated LDPE copolymer, is not particularly limited and may for example be less than 5.0 / 1000 carbon atoms, such as less than 3.0 / 1000 carbon atoms. Alternatively, the polyethylene copolymer, e.g. LDPE copolymer, may be saturated. Preferably the saturated polyethylene copolymer, e.g. saturated LDPE copolymer, has a -C=C- content of less than 0.10 / 1000 carbon atoms.As is well known, the (optional) unsaturation can be provided to the polyethylene copolymer, e.g. LDPE copolymer, by means of the comonomer(s), a low molecular weight (Mw) additive compound, such as a crosslinking booster, chain transfer agent (CTA) or scorch retarder additive, or any combinations thereof. The total amount of double bonds means herein double bonds added by any means. If two or more above sources of double bonds are chosen to be used for providing the unsaturation, then the total amount of double bonds in the polyethylene copolymer means the sum of the double bonds present. Any double bond measurements are carried out prior to any crosslinking. In one embodiment, the term "total amount of carbon-carbon double bonds" refers to the combined amount of double bonds which originate from vinyl groups, vinylidene groups and trans-vinylene groups, if present.The crosslinking booster may be a compound containing at least 2 unsaturated groups, such as an aliphatic or aromatic compound, an ester, an ether, an amine, or a ketone, which contains at least 2 unsaturated group(s), such as a cyanurate, an isocyanurate, a phosphate, an ortho formate, an aliphatic or aromatic ether, or an allyl ester of benzene tricarboxylic acid. Examples of suitable esters, ethers, amines and ketones are compounds selected from general groups of diacrylates, triacrylates, tetraacrylates, triallylcyanurate, triallylisocyanurate, 3,9-divinyl-2,4,8,10-tetra-oxaspiro[5,5]-undecane (DVS), triallyl trimellitate (TATM) or N,N,N',N',N",N"-hexaallyl-1,3,5-triazine-2,4,6-triamine (HATATA), or any mixtures thereof. The crosslinking booster can for example be added in an amount of less than 2.0 wt%, for example, less than 1.5 wt%, e.g. less than 1.0 wt%, for example, less than 0.75 wt%, e.g. less than 0.5 wt%, and the lower limit thereof is, for example, at least 0.05 wt%, e.g., at least 0.1 wt%, based on the total weight of the polymer composition.In one embodiment, the optional unsaturation is provided by one or more of the following means: by a chain transfer agent (CTA), by one or more polyunsaturated comonomer(s) or by polymerisation conditions. It is well known that selected polymerisation conditions such as peak temperatures and pressure, can have an influence on the unsaturation level. In a preferred embodiment, the polyethylene copolymer is a polyethylene terpolymer which is a terpolymer of ethylene, one or more polar comonomer(s) and one or more polyunsaturated comonomer(s), i.e. the one or more other non-polar comonomer(s) is present and is selected from the group of polyunsaturated comonomers.Suitable polyunsaturated comonomers for the polyethylene terpolymer include those which have a straight carbon chain with at least 8 carbon atoms and at least 4 carbons between non-conjugated double bonds, of which at least one is terminal. For example, in one embodiment the polyunsaturated comonomer is a diene, preferably a diene which has a straight chain with at least eight carbon atoms and at least 4 carbon atoms between non-conjugated double bonds, at least one of which is terminal. Preferred dienes are selected from C8 to C14 non-conjugated dienes or mixtures thereof, more preferably selected from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, 7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene, or mixtures thereof. Even more preferably, the diene is selected from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, or any mixture thereof.In an especially preferred embodiment therefore, the polyethylene terpolymer is a terpolymer (most preferably an LDPE terpolymer) of ethylene, one or more polar comonomer(s) selected from C1-C6 alkyl acrylates, C1-C6 alkyl methacrylates, and vinyl acetate, and one or more C8 to C14 non-conjugated diene(s). In a preferred embodiment, the content of the one or more polyunsaturated comonomer(s) in the polyethylene copolymer is in the range of 0.05 to 4.5 wt%, more preferably in the range of 0.1 to 4.0 wt%, relative to the total weight of the polyethylene copolymer.It is well known that e.g. propylene can be used as a comonomer or as a chain transfer agent (CTA), or both, whereby it can contribute to the total amount of the carbon-carbon double bonds, preferably to the total amount of the vinyl groups. Herein, when a compound which can also act as comonomer, such as propylene, is used as CTA for providing double bonds, then said copolymerisable comonomer is not calculated to the comonomer content.If the polyethylene copolymer, e.g. LDPE copolymer, is unsaturated, then it preferably has a total amount of carbon-carbon double bonds, which originate from vinyl groups, vinylidene groups and trans-vinylene groups, if present, of at least 0.10 / 1000 carbon atoms, such as at least 0.20 / 1000 carbon atoms, such as at least 0.30 / 1000 carbon atoms, such as at least 0.40 / 1000 carbon atoms, such as at least 0.50 / 1000 carbon atoms. The maximum amount of carbon-carbon double bonds in the unsaturated polyethylene copolymer, e.g. unsaturated LDPE copolymer, which originate from vinyl groups, vinylidene groups and trans-vinylene groups, if present, is not particularly limited and may for example be less than 5.0 / 1000 carbon atoms, such as less than 3.0 / 1000 carbon atoms.In some embodiments, e.g. wherein a high crosslinking level with a low peroxide content is desired, the total amount of carbon-carbon double bonds, which originate from vinyl groups, vinylidene groups and trans-vinylene groups, if present, in the unsaturated polyethylene copolymer, is higher than 0.40 / 1000 carbon atoms, preferably higher than 0.50 / 1000 carbon atoms, more preferably higher than 0.60 / 1000 carbon atoms.If the polyethylene copolymer is an unsaturated polyethylene copolymer, it preferably contains at least vinyl groups and the total amount of vinyl groups is preferably higher than 0.05 / 1000 carbon atoms, still more preferably higher than 0.08 / 1000 carbon atoms, and most preferably higher than 0.11 / 1000 carbon atoms. Preferably, the total amount of vinyl groups is lower than 4.0 / 1000 carbon atoms. Preferably, the polyethylene copolymer, e.g. LDPE copolymer, contains vinyl groups in a total amount of at least 0.20 / 1000 carbon atoms, more preferably of at least 0.30 / 1000 carbon atoms, most preferably of at least 0.40 / 1000 carbon atoms.In an alternative embodiment, the polyethylene copolymer is not unsaturated and possesses less than 0.2 C=C / 1000 C atoms, preferably less than 0.1 C=C / 1000 C atoms. In one embodiment, the polyethylene copolymer is saturated and contains no carbon-carbon double bonds. As the polymer composition of the invention is crosslinkable however, the presence of unsaturation within the polyethylene copolymer, e.g. LDPE copolymer, is preferred.The polyethylene copolymer, e.g. LDPE copolymer, typically has a density higher than 860 kg / m3. The density of the polyethylene copolymer, e.g. LDPE copolymer, is typically not higher than 960 kg / m3. In one embodiment, the density of the polyethylene copolymer, e.g. LDPE copolymer, is in the range of 900 to 945 kg / m3 such as 915 to 935 kg / m3. The melt flow rate (MFR2) of the polyethylene copolymer, e.g. LDPE copolymer, is typically in the range of 0.01 to 50 g / 10min, such as 0.1 to 20 g / 10min, such as 0.2 to 10 g / 10min.The polyethylene copolymer, e.g. LDPE copolymer, is typically produced at high pressure by free radical initiated polymerisation (referred to as high pressure (HP) radical polymerization). The HP reactor can be e.g. a well-known tubular or autoclave reactor or a mixture thereof, preferably a tubular reactor. The high pressure (HP) polymerisation and the adjustment of process conditions for further tailoring the other properties of the polyethylene copolymer depending on the desired end application are well known and described in the literature, and can readily be used by a skilled person. Suitable polymerisation temperatures range up to 400 ºC, preferably from 80 to 350ºC and pressure from 70 MPa, preferably 100 to 400 MPa, more preferably from 100 to 350 MPa. Pressure can be measured at least after compression stage and / or after the tubular reactor. Temperature can be measured at several points during all steps.After the separation the obtained polyethylene copolymer is typically in a form of a polymer melt which is normally mixed and pelletized in a pelletising section, such as pelletising extruder, arranged in connection to the HP reactor system. Optionally, additive(s), such as the antioxidant(s), can be added in this mixer in a known manner. When an unsaturated polyethylene copolymer is prepared, then, as well known, the carbon-carbon double bond content can be adjusted by polymerising the ethylene e.g. in the presence of one or more polyunsaturated comonomer(s), chain transfer agent(s), or both, using the desired feed ratio between monomer, preferably ethylene, and polyunsaturated comonomer and / or chain transfer agent, depending on the nature and amount of C-C double bonds desired for the unsaturated LDPE copolymer. For example, WO 9308222 describes a high pressure radical polymerisation of ethylene with polyunsaturated monomers. As a result, the unsaturation can be uniformly distributed along the polymer chain in random copolymerisation manner.The polymer composition of the invention preferably comprises at least 85.0 wt% of the polyethylene copolymer(s), preferably at least 90.0 wt%, more preferably at least 92.5 wt%, even more preferably at least 93.0 wt%, relative to the total weight of the polymer composition. In a preferred embodiment, the polymer composition comprises at least 93.5 wt% of the polyethylene copolymer, preferably at least 95.0 wt%, most preferably at least 95.5 wt%, relative to the total weight of the polymer composition. In one embodiment, the polyethylene copolymer forms up to 99 wt% of the polymer composition, such as up to 98.5 wt%, such as up to 98.0 wt% of the polymer composition. In a preferred embodiment, the polymer composition comprises 85.0 to 99 wt% of the polyethylene copolymer, for example of the LDPE copolymer, relative to the total weight of the polymer composition. Preferably, the composition comprises the polyethylene copolymer in an amount of 90.0 to 99 wt%, such as 92.5 to 99 wt%, more preferably 93.0 to 98.5 wt%, especially 93.5 to 98.5 wt%, more especially 95.0 to 98.0 wt%, most especially 95.5 to 98.0 wt%, relative to the total weight of the polymer composition. It will be understood that the amount of polyethylene copolymer in the polymer composition can be calculated by knowing the amount of polyethylene copolymer added during preparation of the polymer composition (e.g. during blending of the components of the polymer composition) relative to the total amount of all other components added. When multiple polyethylene copolymers are used in the polymer composition, it will be appreciated that the amounts above refer to the total amount of polyethylene copolymer, i.e. the sum of the amount of individual polyethylene copolymers, in the polymer composition.  Component b) – Antioxidant(s)The polymer composition comprises at least one, i.e. one or more, antioxidant(s). It is possible to use a mixture of antioxidants in the polymer composition of the present invention although it is preferred if a single antioxidant is used.The antioxidant is not particularly limited and can be for example any conventional antioxidant, which is suitable for use in a polymer composition, for example in a power cable polymer composition, for example in a power cable insulation polymer composition. The antioxidant can be for example a commercially available antioxidant or can be prepared according to or analogously to known processes in the chemical literature.In one embodiment, the antioxidant is selected from the group consisting of sterically hindered or semi-hindered phenols, aromatic amines, aliphatic sterically hindered amines, thio compounds, and mixtures thereof. Preferably the antioxidant is a sulfur-containing phenolic antioxidant; more preferably a thiobisphenol such as 4,4'-thiobis (2-tertbutyl-5-methylphenol), 2,2'-thiobis (6-t-butyl-4-methylphenol), 4,4'-thiobis (2-methyl-6-t-butylphenol), thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 4,6-bis (octylthiomethyl)-o-cresol, or mixtures thereof. 4,4'-thiobis (2-tertbutyl-5-methylphenol) is especially preferred. Other thio compounds like di-stearyl-thio-dipropionate or similar compounds with various length on the carbon chains; or mixtures thereof, may also be used.In one embodiment, the polymer composition does not comprise, e.g. is free from, nitrogen-containing antioxidants, such as hindered amine light stabilisers (HALS). For example, in one embodiment the polymer composition comprises one or more sulfur-containing phenolic antioxidant(s), preferably wherein said sulfur-containing phenolic antioxidants (e.g. thiobisphenol antioxidant(s)) are the sole antioxidant(s) in the polymer composition. The amount of antioxidant in the polymer composition is not particularly limited and may be used in any conventional amount as is well known to the skilled person. In one embodiment, the polymer composition comprises the antioxidant(s) in an amount of at least 0.005 wt%, preferably at least 0.01 wt%, more preferably at least 0.02 wt%, most preferably at least 0.04 wt%, relative to the total weight of the polymer composition (100 wt%). In another embodiment, the polymer composition comprises the antioxidant(s) in an amount of 0.50 wt% or less, preferably 0.30 wt% of less, more preferably 0.20 wt% or less, most preferably 0.15 wt% or less, relative to the total weight of the polymer composition (100 wt%). In a preferred embodiment, the polymer composition comprises the antioxidant(s) in an amount of 0.005 to 0.50 wt%, preferably 0.01 to 0.30 wt%, more preferably 0.02 to 0.20 wt%, most preferably 0.04 to 0.15 wt%, relative to the total weight of the polymer composition (100 wt%). It will be understood that the amount of antioxidant in the polymer composition can be calculated by knowing the amount of antioxidant added during preparation of the polymer composition (e.g. during blending of the components of the polymer composition) relative to the total amount of all other components added. When multiple antioxidants are used in the polymer composition, it will be appreciated that the amounts above refer to the total amount of antioxidant, i.e. the sum of the amount of individual antioxidants, in the polymer composition. Component c) – Crosslinking agent(s)The polymer composition comprises at least one, i.e. one or more, crosslinking agent which is an organic peroxide having a structural formula in accordance with Formula (I):wherein R1, R2 and R3 are each independently selected from a linear or branched C1-C6 alkyl group; and R is the rest of the organic peroxide. Surprisingly it has been found that the use of this crosslinking agent in combination with a particular additive (2,4-diphenyl-4-methyl-1-pentene) results in a reduction (i.e. a lowering) of the amount of antioxidant that exudes from the polymer composition during storage and thus leads to an improvement in the storage stability of the polymer composition. Whilst it is possible for the polymer composition to comprise more than one crosslinking agent, it is preferred if the polymer composition comprises a single crosslinking agent. For example, it is preferred if the polymer composition does not comprise any crosslinking agents other than those having a structural formula in accordance with Formula (I), e.g. wherein the organic peroxide having a structural formula in accordance with Formula (I), or a mixture of organic peroxides each having a structural formula in accordance with Formula (I), is / are the only crosslinking agents present in the polymer composition. It is especially preferred if the polymer composition does not comprise, i.e. is free from, dicumyl peroxide (DCP) (CAS No. 80-43-3).In a preferred embodiment, R1, R2 and R3 in Formula (I) are each independently selected from a linear or branched C1-C4 alkyl group. More preferably, R1, R2 and R3 (which can be the same or different) are each a methyl group or an ethyl group. In an especially preferred embodiment, R1, R2 and R3 are each a methyl group, i.e. the groups R1, R2 and R3 together with the carbon atom to which they are attached form a tert-butyl group. The rest of the peroxide “R” is not particularly limited and may for example contain a C1-C20 aliphatic group or a C6-C20 aromatic group. Especially preferred is the case where R is a group having the structural formula –C(CH3)2Ar, wherein Ar is an optionally substituted phenyl group. In a preferred embodiment, the group R has the formula:wherein R4, R5 and R6 are each independently a C1-20 hydrocarbon group, preferably a C1-20 aliphatic group or a C6-20 aromatic group, more preferably a C1-20 alkyl group or a C6-20 aryl group. In a preferred embodiment, R4, R5 and R6 are each independently a linear or branched C1-12 alkyl group, preferably a linear or branched C1-6 alkyl group, more preferably a linear or branched C1-4 alkyl group, especially methyl or ethyl; or a C6-10 aryl group, especially phenyl. Optionally, the R group in Formula (I) comprises further peroxy group(s) in addition to the group shown in Formula (I), i.e. further group(s) having the formula -O-O-. For example, the organic peroxide may be a bisperoxide. In a preferred embodiment, the group R has the formula: wherein R4’ and R5’ are each independently a C1-20 hydrocarbon group, preferably a C1-20 aliphatic group or a C6-20 aromatic group, more preferably a C1-20 alkyl group or a C6-20 aryl group; R6’, R7’ and R8’ are each independently a C1-20 hydrocarbon group, preferably a C1-20 aliphatic group or a C6-20 aromatic group, more preferably a C1-20 alkyl group or a C6-20 aryl group; andL is a bivalent linker, preferably a C1-20 hydrocarbon group, preferably a C1-20 alkyl group or a C2-20 alkenyl group or a C2-20 alkynyl group, preferably a C1-12 alkyl group or a C2-12 alkenyl group or a C2-12 alkynyl group. In a preferred embodiment, R4’ and R5’ are each independently a C1-12 alkyl group, preferably a C1-6 alkyl group, more preferably a C1-4 alkyl group, especially methyl or ethyl; or a C6-10 aryl group, especially phenyl.In a preferred embodiment, the crosslinking agent is selected from the group consisting of di-tert-amylperoxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide, di(tert-butyl)peroxide, butyl-4,4-bis(tert-butylperoxy)-valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxybenzoate, bis(tertbutylperoxyisopropyl)benzene, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tertamylperoxy)cyclohexane, and mixtures thereof. More preferably, the crosslinking agent is selected from 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures thereof. In an especially preferred embodiment, the crosslinking agent is tert-butylcumylperoxide.The amount of crosslinking agent in the polymer composition is also not particularly limited and may for example be any conventional amount used in polymer compositions as is well known in the art. In a preferred embodiment, the polymer composition comprises the crosslinking agent in an amount of at least 0.2 wt%, preferably at least 0.3 wt%, more preferably at least 0.4 wt%, most preferably at least 0.5 wt%, such as at least 0.6 wt%, especially at least 0.75 wt%, relative to the total weight of the polymer composition (100 wt%). In another embodiment, the polymer composition comprises the crosslinking agent in an amount of 10.0 wt% or less, preferably 8.0 wt% or less, more preferably 5.0 wt% or less, most preferably 3.0 wt% or less, such as 2.5 wt% or less, especially 2.0 wt% or less, relative to the total weight of the polymer composition (100 wt%). In a preferred embodiment, the polymer composition comprises the crosslinking agent in an amount of 0.2 to 10.0 wt%, preferably 0.3 to 8.0 wt%, more preferably 0.4 to 5.0 wt%, even more preferably 0.5 to 3.0 wt%, such as 0.6 to 2.5 wt%, especially 0.75 to 2.0 wt%, relative to the total weight of the polymer composition (100 wt%). In one embodiment, the polymer composition comprises the crosslinking agent in an amount of about 1.5 wt%, e.g. in the range of 1.35 to 1.65 wt%. In one embodiment, the amount of crosslinking agent in the polymer composition is 1.65 wt% or less, such as 1.5 wt% or less, or even 1.0 wt% or less. It will be understood that the amount of crosslinking agent in the polymer composition can be calculated by knowing the amount of crosslinking agent added during preparation of the polymer composition (e.g. during blending of the components of the polymer composition) relative to the total amount of all other components added.When multiple crosslinking agents of Formula (I) are used in the polymer composition, it will be appreciated that the amounts above refer to the total amount of crosslinking agents of Formula (I), i.e. the sum of the amount of individual crosslinking agents of Formula (I), in the polymer composition. Component d) – Additive(s)The polymer composition further comprises 2,4-diphenyl-4-methyl-1-pentene. Surprisingly, it has been found that the use of this compound in combination with a particular crosslinking agent as defined above results in a reduction (i.e. a lowering) of the amount of antioxidant that exudes from the polymer composition during storage and thus improves the storage stability of the polymer composition.In a preferred embodiment, the polymer composition comprises at least 0.05 wt% 2,4-diphenyl-4-methyl-1-pentene,such as greater than 0.05 wt% 2,4-diphenyl-4-methyl-1-pentene, preferably at least 0.06 wt% 2,4-diphenyl-4-methyl-1-pentene, more preferably at least 0.07 wt% 2,4-diphenyl-4-methyl-1-pentene, most preferably at least 0.08 wt% 2,4-diphenyl-4-methyl-1-pentene, relative to the total weight of the polymer composition (100 wt%). In another embodiment, the polymer composition comprises up to 1.5 wt% 2,4-diphenyl-4-methyl-1-pentene, for example up to 0.75 wt% 2,4-diphenyl-4-methyl-1-pentene, for example up to 0.70 wt% 2,4-diphenyl-4-methyl-1-pentene, such as up to 0.60 wt%, relative to the total weight of the polymer composition. In a preferred embodiment, the polymer composition comprises 2,4-diphenyl-4-methyl-1-pentene in an amount of 0.05 to 1.5 wt% relative to the total weight of the polymer composition, such as 0.06 to 0.75 wt%, more preferably 0.07 to 0.70 wt%, such as 0.08 to 0.60 wt%. In an especially preferred embodiment, the polymer composition comprises 2,4-diphenyl-4-methyl-1-pentene in an amount of 0.09 to 0.50 wt% relative to the total weight of the polymer composition, especially 0.10 to 0.35 wt%. In another preferred embodiment, the polymer composition comprises 2,4-diphenyl-4-methyl-1-pentene in an amount of 0.05 to 0.40 wt% relative to the total weight of the polymer composition. It will be understood that the amount of 2,4-diphenyl-4-methyl-1-pentene in the polymer composition can be calculated by knowing the amount of 2,4-diphenyl-4-methyl-1-pentene added during preparation of the polymer composition (e.g. during blending of the components of the polymer composition) relative to the total amount of all other components added. In addition to the polyethylene copolymer(s), crosslinking agent(s), antioxidant(s), and 2,4-diphenyl-4-methyl-1-pentene, the polymer composition may optionally comprise further component(s), such as further polymer component(s) and / or one or more further additive(s). As optional additives the polymer composition may contain stabiliser(s), water tree retardant additive(s), processing aid(s), additional scorch retarder(s) (other than 2,4-diphenyl-4-methyl-1-pentene), metal deactivator(s), crosslinking booster(s), flame retardant additive(s), acid or ion scavenger(s), inorganic filler(s), voltage stabilizer(s) or any mixtures thereof. Typically, such additives if present are present in a total amount of less than 10 wt% relative to the total weight of the polymer composition, preferably in an amount of less than 5 wt% relative to the total weight of the polymer composition. In one embodiment, the polymer composition consists essentially of, e.g. consists of, the polyethylene copolymer(s), crosslinking agent(s), antioxidant(s) and 2,4-diphenyl-4-methyl-1-pentene. Polymer compositionThe polymer composition of the present invention comprises components a) to d) as hereinbefore defined. The polymer composition is preferably a polyethylene composition, i.e. polyethylene-based, e.g. wherein at least 50 wt%, preferably at least 70 wt%, more preferably at least 90 wt%, such as at least 95 wt%, of the polymeric components in the polymer composition are polyethylene components. In one embodiment, polyethylene(s) is the sole polymeric component in the polymer composition. It will be understood that (before crosslinking) the polymer composition is non-crosslinked. Preferably the polymer composition is non-foamed and / or free of foaming agent(s). The polymer composition of the invention is typically prepared by blending components a) to d). Blending may take place by any known method in the art, such as melt-mixing. Unless stated otherwise, amounts of components a) to d) are given in weight percent (wt%) relative to the total weight of the polymer composition. The amount of each component refers to the amount of component added when formulating the polymer composition, which is known. The amounts can also be measured on the final polymer composition (before crosslinking).The polymer composition of the present invention exhibits low exudation of antioxidant(s) during storage. The polymer composition is typically supplied in the form of pellets. In one embodiment, the amount of antioxidant on the surface of the pellets of the polymer composition, after storage for 4 weeks at 35 °C under the conditions recited under the heading “Determination of additive content, i.e. the antioxidant content, on pellet surface” in the section “Determination Methods”, and measured according to the method recited in that section, is less than 325ppm, preferably less than 300 ppm, more preferably less than 275 ppm, even more preferably less than 250 ppm. In an especially preferred embodiment, the amount of antioxidant on the surface of the pellets after storage for 4 weeks at 35 °C as defined above is less than 225 ppm, such as less than 200 ppm. In one embodiment, the amount of antioxidant on the surface of the pellets after storage for 4 weeks at 35 °C as defined above, is as low as 0.1 ppm, e.g. as low as 0.5 ppm, e.g. as low as 1.0 ppm. In a particularly preferred embodiment, the polymer composition comprises:a) at least one LDPE copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s), wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer;b) at least one sulfur-containing phenolic antioxidant;c) at least one crosslinking agent selected from the group consisting of di-tert-amylperoxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide, di(tert-butyl)peroxide, butyl-4,4-bis(tert-butylperoxy)-valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxybenzoate, bis(tertbutylperoxyisopropyl)benzene, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tertamylperoxy)cyclohexane, and mixtures thereof; andd) at least 0.05 wt% 2,4-diphenyl-4-methyl-1-pentene, relative to the total weight of the polymer composition (100 wt%). In another particularly preferred embodiment, the polymer composition comprises:a) at least 85.0 wt% of at least one polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s), wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer;b) at least 0.005 wt% of at least one antioxidant;c) at least 0.2 wt% of at least one crosslinking agent which is an organic peroxide having a structural formula in accordance with Formula (I) as hereinbefore defined;d) at least 0.05 wt% 2,4-diphenyl-4-methyl-1-pentene, relative to the total weight of the polymer composition (100 wt%). In yet another particularly preferred embodiment, the polymer composition comprises:a) at least 85.0 wt% of at least one low density polyethylene (LDPE) copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s), wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer;b) at least 0.005 wt% of at least one sulfur-containing phenolic antioxidant;c) at least 0.2 wt% of at least one crosslinking agent selected from the group consisting of di-tert-amylperoxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide, di(tert-butyl)peroxide, butyl-4,4-bis(tert-butylperoxy)-valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxybenzoate, bis(tertbutylperoxyisopropyl)benzene, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tertamylperoxy)cyclohexane, and mixtures thereof; andd) at least 0.05 wt% 2,4-diphenyl-4-methyl-1-pentene, relative to the total weight of the polymer composition (100 wt%).In yet another particularly preferred embodiment, the polymer composition comprises:a) at least 85.0 wt% of at least one low density polyethylene (LDPE) terpolymer of ethylene, one or more polar comonomer(s) and one or more other non-polar comonomer(s), wherein the content of the one or more polar comonomer(s) in the polyethylene terpolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene terpolymer;b) at least 0.005 wt% of at least one sulfur-containing phenolic antioxidant;c) at least 0.2 wt% of at least one crosslinking agent selected from the group consisting of di-tert-amylperoxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide, di(tert-butyl)peroxide, butyl-4,4-bis(tert-butylperoxy)-valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxybenzoate, bis(tertbutylperoxyisopropyl)benzene, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tertamylperoxy)cyclohexane, and mixtures thereof; andd) at least 0.05 wt% 2,4-diphenyl-4-methyl-1-pentene, relative to the total weight of the polymer composition (100 wt%). In one aspect, the present invention relates to a crosslinked polymer composition obtainable, preferably obtained, by crosslinking the polymer composition as hereinbefore defined. The person skilled in the art knows of suitable conditions under which said crosslinking may be achieved. The crosslinking may be partial or substantially complete. Process for producing polymer compositionIn one aspect, the present invention provides a process for producing a polymer composition as hereinbefore defined, the process comprising blending the polyethylene copolymer(s) with the antioxidant(s), the crosslinking agent(s) and 2,4-diphenyl-4-methyl-1-pentene. The blending of the components of the polymer composition may be conducted in any order. For example, in one embodiment, the polyethylene copolymer(s) is first blended with the antioxidant(s) (e.g. by compounding via melt-mixing) before the crosslinking agent(s) and 2,4-diphenyl-4-methyl-1-pentene are added (e.g. by distributing the crosslinking agent(s) and 2,4-diphenyl-4-methyl-1-pentene onto optionally preheated pellets comprising the blend of polyethylene copolymer(s) and antioxidant(s)). Alternatively, the crosslinking agent(s) and / or 2,4-diphenyl-4-methyl-1-pentene may be added to the polyethylene copolymer(s) before or at the same time as the antioxidant(s) is added. During manufacture of the composition, the components can be blended and homogenously mixed, e.g. melt mixed in an extruder. Optionally, the polymer composition may be extruded and / or pelletised. ApplicationsThe polymer compositions of the present invention may for example be used in the production of cables, e.g. power cables. In one aspect, the present invention relates to a power cable comprising a conductor surrounded by at least one layer, preferably an insulation layer, comprising, preferably consisting of, the (crosslinkable) polymer composition as hereinbefore defined or the crosslinked polymer composition as hereinbefore defined.In one embodiment, the power cable is a cable comprising at least one cable core, capable of transferring energy at any voltage, preferably capable of operating at voltages higher than 30 kV.The voltage applied to the power cable can be alternating (AC), direct (DC), or transient (impulse). Moreover, the cable according to the present invention is preferably an AC power cable, for example a cable capable of operating at voltages of 1-525 kV, 6-525 kV, 36-275 kV, such as 66-275 kV, 36-220 kV, such as 66-220 kV, 36-150 kV, such as 66-150 kV, and above 66 kV (RMS voltages, voltage between any two conductors in a three phase cable). In one embodiment, the cable is an AC power cable capable of operating at voltages of equal to or higher than 36 kV, preferably equal to or higher than 66 kV. In some embodiments, the cable is an AC power cable operating at voltages of lower than 550 kV, preferably lower than 525 kV, preferably lower than 400 kV, more preferably lower than 380 kV, especially lower than 320 kV.Typically, a power cable, such as a AC power cable, comprises an inner semiconductive layer comprising a first semiconductive composition, an insulation layer comprising the polymer composition of the present invention and an outer semiconductive layer comprising a second semiconductive composition, in that order.The polymer composition of the present invention is preferably used in the insulation layer of the power cable. Ideally, the insulation layer comprises at least 95 wt%, such as at least 98 wt% of the polymer composition of the invention, such as at least 99 wt%. It is preferred therefore if the polymer composition of the invention is the only non-additive component used in the insulation layer of the cables of the invention. Thus, it is preferred if the insulation layer consists essentially of, e.g. consists of, the polymer composition of the invention.The insulation layer is crosslinkable (in the case that the insulation layer comprises a the crosslinkable polymer composition of the present invention) or crosslinked (in case the insulation layer comprises the crosslinked polymer composition of the present invention).The insulation layer may comprise conventionally used additive(s) for W&C applications. Preferably, the insulation layer does not comprise a carbon black. Preferably the polymer composition does not comprise a carbon black. Also preferably, the insulation layer does not comprise flame retarding additive(s), e.g. a metal hydroxide containing additives in flame retarding amounts.The power cable of the invention preferably contains inner and outer semiconductive layers comprising inner and outer semiconductive compositions respectively. These layers can be made of any conventional material suitable for use in these layers. The inner and the outer semiconductive compositions can be different or identical and may comprise a polymer(s) which is preferably a polyolefin or a mixture of polyolefins and a conductive filler, preferably carbon black. Suitable polyolefin(s) are e.g. polyethylene produced in a low pressure process (LLDPE, MDPE, HDPE) or a polyethylene produced in a HP process (LDPE). The carbon black can be any conventional carbon black used in the semiconductive layers of a AC power cable, preferably in the semiconductive layer of a AC power cable. Preferably the carbon black has one or more of the following properties: a) a primary particle size of at least 5 nm which is defined as the number average particle diameter according ASTM D3849-95a, dispersion procedure D b) iodine number of at least 30 mg / g according to ASTM D1510, c) oil absorption number of at least 30 ml / 100g which is measured according to ASTM D2414. Non-limiting examples of carbon blacks are e.g. acetylene carbon black, furnace carbon black and Ketjen carbon black, preferably furnace carbon black and acetylene carbon black. Preferably, the semiconductive composition(s) comprises 10 to 50 wt% carbon black, based on the weight of the semiconductive composition.In a preferred embodiment, the outer semiconductive layer and / or the inner semiconductive layer are cross-linked.The conductor typically comprises one or more wires. Moreover, the power cable may comprise one or more such conductors. Preferably the conductor is an electrical conductor and comprises one or more metal wires or is a solid electrical conductor made of metal. Examples of suitable metals are aluminium and copper.As well known the cable can optionally comprise further layers, e.g. screen(s), a jacketing layer(s), other protective layer(s) or any combinations thereof.In one aspect, the present invention provides a process for producing a power cable, the process comprising:(i) applying on one or more conductor(s), preferably by (co)extrusion, at least one layer, preferably an insulation layer, comprising, preferably consisting of, the (crosslinkable) polymer composition of the present invention; and(ii) optionally crosslinking the polymer composition. In a preferred embodiment, the process for producing a power cable comprises:(i) applying on one or more conductors, preferably by (co)extrusion, an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order, wherein the insulation layer comprises the (crosslinkable) polymer composition of the invention; and(ii) optionally crosslinking the polymer composition. More preferably, a power cable is produced, wherein the process comprises the steps of(ia) - providing and mixing, preferably melt mixing in an extruder,an optionally crosslinkable first semiconductive composition comprising a polymer, a carbon black and optionally further component(s) for the inner semiconductive layer,- providing and mixing, preferably melt mixing in an extruder, the polymer composition of the invention; and- providing and mixing, preferably melt mixing in an extruder, a second semiconductive composition which is optionally crosslinkable and comprises a polymer, a carbon black and optionally further component(s) for the outer semiconductive layer;(ib) applying on one or more conductors, preferably by coextrusion,- a melt mix of the first semiconductive composition obtained from step (ia) to form the inner semiconductive layer,- a meltmix of polymer composition of the invention obtained from step (ia) to form the insulation layer, and- a meltmix of the second semiconductive composition obtained from step (ia) to form the outer semiconductive layer, and(ii) optionally crosslinking at crosslinking conditions any or all of the first semiconductive composition of the inner semiconductive layer, the second semiconductive composition of the outer semiconductive layer, and the polymer composition of the insulation layer, of the obtained cable.Melt mixing generally means mixing above the melting point of at least the major polymer component(s) of the obtained mixture and is carried out for example, without limiting to, in a temperature of at least 15ºC above the melting or softening point of polymer component(s).The term “(co)extrusion” means herein that in case of two or more layers, said layers can be extruded in separate steps, or at least two or all of said layers can be coextruded in a same extrusion step, as well known in the art. The term “(co)extrusion” means herein also that all or part of the layer(s) are formed simultaneously using one or more extrusion heads. For instance a triple extrusion can be used for forming three layers. In case a layer is formed using more than one extrusion heads, then for instance, the layers can be extruded using two extrusion heads, the first one for forming the inner semiconductive layer and the inner part of the insulation layer, and the second head for forming the outer insulation layer and the outer semiconductive layer.As well known, the polymer composition of the invention and the optional and preferred first and second semiconductive compositions can be produced before or during the cable production process.Preferably, the polymers required to manufacture the cable of the invention are provided to the cable production process in form of powder, grain or pellets. Pellets mean herein generally any polymer product which is formed from reactor-made polymer (obtained directly from the reactor) by post-reactor modification to a solid polymer particles. The additive(s) can be added to the polymer composition as such or as a mixture with a carrier polymer, i.e. in a form of so-called master batch.Any crosslinking agent can be added before the cable production process or during the (melt) mixing step (a). For instance, and preferably, the crosslinking agent and also the further component(s), such as antioxidant(s) and 2,4-diphenyl-4-methyl-1-pentene, can already be present in the polymers used. The crosslinking agent is added, preferably impregnated, onto the solid polymer particles, preferably pellets.The crosslinking of layers can be carried out at increased temperature which is chosen, as well known, depending on the type of crosslinking agent. For instance temperatures above 150ºC, such as from 160 to 350ºC, are typical, however without limiting thereto.The processing temperatures and devices are well known in the art, e.g. conventional mixers and extruders, such as single or twin screw extruders, are suitable for the process of the invention. UsesIn one aspect, the present invention provides the use of a combination of a crosslinking agent and 2,4-diphenyl-4-methyl-1-pentene in a polymer composition further comprising (i) at least one polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s), and (ii) an antioxidant, to reduce exudation of the antioxidant from the polymer composition; wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer; and wherein the crosslinking agent is an organic peroxide having a structural formula in accordance with Formula (I):wherein R1, R2 and R3 are each independently selected from linear and branched C1-C6 alkyl groups; and R is the rest of the organic peroxide. Surprisingly it has been found that the use of this combination of crosslinking agent and additive results in a reduction (i.e. a lowering) of the amount of antioxidant that exudes from the polymer composition during storage and thus leads to an improvement in the storage stability of the polymer composition.In one embodiment, the polymer composition exhibits reduced exudation of the antioxidant relative to a polymer composition which does not comprise an organic peroxide of Formula (I) and / or 2,4-diphenyl-4-methyl-1-pentene but which is otherwise identical to the polymer composition of the present invention. The amount of exudation may be measured according to the method described below for the examples (i.e. under the heading: Determination of additive content, i.e. the antioxidant content, on pellet surface). In one embodiment, reduced exudation of the antioxidant is manifested by a lower amount of antioxidant on the surface of the pellets of the polymer composition, after storage for 4 weeks at 35 °C under the conditions recited under the heading “Determination of additive content, i.e. the antioxidant content, on pellet surface” in the section “Determination Methods”, compared to a comparative polymer composition stored under the same conditions which does not comprise an organic peroxide of Formula (I) and / or 2,4-diphenyl-4-methyl-1-pentene but which is otherwise identical to the polymer composition of the present invention. For example, the polymer composition of the present invention may exhibit 10% less antioxidant content on the pellet surface after storage under the afore-mentioned conditions relative to said comparative polymer composition, preferably 20% less antioxidant content, more preferably 30% less antioxidant content. It will be appreciated that the polymer composition is preferably the polymer composition of the present invention as hereinbefore defined. It will thus be understood that the embodiments (including the preferred embodiments) described in relation to the aspect of the invention relating to the polymer composition also apply to this aspect of the invention. For example, the preferred nature and amount of components a)-d) described in relation to the polymer composition of the present invention are also applicable to the polymer composition in the use of the present invention.Viewed from another aspect, the present invention provides a method of reducing the exudation of an antioxidant from a polymer composition comprising said antioxidant, the method comprising:formulating the polymer composition by combining at least one polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s), at least one antioxidant, at least one crosslinking agent, and 2,4-diphenyl-4-methyl-1-pentene; wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer; andwherein the crosslinking agent is an organic peroxide having a structural formula in accordance with Formula (I):wherein R1, R2 and R3 are each independently selected from a linear or branched C1-C6 alkyl group; and R is the rest of the organic peroxide. Preferably the amount of 2,4-diphenyl-4-methyl-1-pentene in the formulated polymer composition is at least 0.05 wt% relative to the total weight of the polymer composition.In another aspect, the present invention provides the use of a combination of a crosslinking agent of Formula (I) (preferably tert-butylcumylperoxide) and 2,4-diphenyl-4-methyl-1-pentene in a polymer composition comprising an antioxidant. The polymer composition is preferably a polymer composition as described in any preceding aspect. The invention will now be further described with reference to the following non-limiting examples Determination MethodsUnless otherwise stated in the description or experimental part, the following methods were used for the property determinations. wt%: % by weight DensityThe density was measured according to ISO 1183-1 / method A. Sample preparation is done by compression moulding in accordance with ISO 17855-2:2016.  Melt flow rateThe melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g / 10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer, i.e. the polyethylene. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is determined at 190 °C for polyethylenes and may be determined at different loadings such as 2.16 kg (MFR2) or 21.6 kg (MFR21). Methods ASTM D3124-98, and ASTM D6248-98, to determine amount of double bonds in the polymer, i.e. the polyethylenecopolymerThe methods ASTM D3124-98 and ASTM D6248-98 apply for determination of double bonds in the polyethylene copolymer component (a). The polyethylene copolymer component (a) in this method description is hereinafter referred to as “the polymer”.The methodsASTM D3124-98, and ASTM D6248-98, include on the one hand a procedure for the determination of the amount of double bonds / 1000 C-atoms which is based upon the ASTM D3124-98 method. In the ASTM D3124-98 method, a detailed description for the determination of vinylidene groups / 1000 C-atoms is given based on 2,3-dimethyl-1,3-butadiene. In the ASTM D6248-98 method, detailed descriptions for the determination of vinyl and trans-vinylene groups / 1000 C-atoms are given based on 1-octene and trans-3-hexene, respectively. The described sample preparation procedures therein have here been applied for the determination of vinyl groups / 1000 C-atoms, vinylidene groups / 1000 C-atoms and trans-vinylene groups / 1000 C-atoms in the present invention. The ASTM D6248-98 method suggests possible inclusion of the bromination procedure of the ASTM D3124-98 method but the samples with regard to the present invention were not brominated. For the determination of the extinction coefficient for these three types of double bonds, the following three compounds have been used: 1-decene for vinyl, 2-methyl-1-heptene for vinylidene and trans-4-decene for trans-vinylene and the procedures as described in ASTM D3124-98 and ASTM-D6248-98 were followed with the above-mentioned exception.The total amount of vinyl bonds, vinylidene bonds and trans-vinylene double bonds of “the polymer” was analysed by means of IR spectrometry and given as the amount of vinyl bonds, vinylidene bonds and trans-vinylene bonds per 1000 carbon atoms.The polymers to be analysed were pressed to thin films with a thickness of 0.5-1.0 mm. The actual thickness was measured. FT-IR analysis was performed on a Perkin Elmer Spectrum One. Two scans were recorded with a resolution of 4 cm-1. 1) Polymer compositions comprising polyethylene copolymers comprising < 0.4 wt% polar comonomer For polyethylenes copolymers with < 0.4 wt% polar comonomer(s), three types of C=C containing functional groups were quantified, each with a characteristic absorption and each calibrated to a different model compound resulting in individual extinction coefficients: vinyl (R-CH=CH2) via 910 cm-1 based on 1-decene [dec-1-ene] giving E = 13.13 l·mol-1·mm-1vinylidene (RR’C=CH2) via 888 cm-1 based on 2-methyl-1-heptene [2-methyhept-1-ene] giving E = 18.24 l·mol-1·mm-1trans-vinylene (R-CH=CH-R’) via 965 cm-1 based on trans-4-decene [(E)-dec-4-ene] giving E = 15.14 l·mol-1·mm-1. For polyethylene copolymers with < 0.4 wt% of polar comonomer linear baseline correction was applied between approximately 980 and 840 cm-1. 2) Polymer compositions comprising polyethylene copolymers with ≥ 0.4 wt% polar comonomer For polyethylene copolymers with ≥ 0.4 wt% of polar comonomer two types of C=C containing functional groups were quantified, each with a characteristic absorption and each calibrated to a different model compound resulting in individual extinction coefficients: vinyl (R-CH=CH2) via 910 cm-1 based on 1-decene [dec-1-ene] giving E = 13.13 l·mol-1·mm-1vinylidene (RR’C=CH2) via 888 cm-1 based on 2-methyl-1-heptene [2-methyhept-1-ene] giving E = 18.24 l·mol-1·mm-1. For ethylene butyl acrylate systems linear baseline correction was applied between approximately 920 and 870 cm-1.For ethylene ethyl acrylate systems linear baseline correction was applied between approximately 920 and 825 cm-1.For ethylene methyl acrylate systems linear baseline correction was applied between approximately 930 and 870 cm-1. The methods ASTM D3124-98, and ASTM D6248-98, include on the other hand also a procedure to determine the molar extinction coefficient.  At least three 0.18 mol•l-1 solutions in carbon disulphide (CS2) were used and the mean value of the molar extinction coefficient used.The amount of vinyl groups originating from the polyunsaturated comonomer per 1000 carbon atoms was determined and calculated as follows: The polymer to be analyzed and a reference polymer have been produced on the same reactor, basically using the same conditions, i.e. similar peak temperatures, pressures and production rate, but with the only difference that the polyunsaturated comonomer is added during polymerization of the polymer to be analyzed and not added during the polymerization of the reference polymer. The total amount of vinyl groups of each polymer was determined by FT-IR measurements, as described herein. A base level of vinyl groups, formed naturally by the process and from chain transfer agents resulting in vinyl groups (if present), is assumed to be the same for the reference polymer and the polymer to be analyzed. This base level is then subtracted from the measured amount of vinyl groups in the polymer to be analyzed, thereby resulting in the amount of vinyl groups / 1000 C- atoms, which result from the polyunsaturated comonomer. Comonomer Content a) Quantification of alpha-olefin content in low density polyethylenes by NMR spectroscopy:The comonomer content was determined by quantitative 13C nuclear magnetic resonance (NMR) spectroscopy after basic assignment (J. Randall JMS - Rev. Macromol. Chem. Phys., C29(2&3), 201-317 (1989)). Experimental parameters were adjusted to ensure measurement of quantitative spectra for this specific task.Specifically solution-state NMR spectroscopy was employed using a Bruker AvanceIII 400 spectrometer. Homogeneous samples were prepared by dissolving approximately 0.200 g of polymer in 2.5 ml of deuterated-tetrachloroethene in 10 mm sample tubes utilising a heat block and rotating tube oven at 140 °C. Proton decoupled 13C single pulse NMR spectra with NOE (powergated) were recorded using the following acquisition parameters: a flip-angle of 90 degrees, 4 dummy scans, 4096 transients an acquisition time of 1.6s, a spectral width of 20kHz, a temperature of 125 °C, a bilevel WALTZ proton decoupling scheme and a relaxation delay of 3.0 s. The resulting FID was processed using the following processing parameters: zero-filling to 32k data points and apodisation using a gaussian window function; automatic zeroth and first order phase correction and automatic baseline correction using a fifth order polynomial restricted to the region of interest.Quantities were calculated using simple corrected ratios of the signal integrals of representative sites based upon methods well known in the art. b) Determination of Comonomer content of polar comonomers in low density polyethyleneComonomer content (wt%) was determined in a known manner based on Fourier transform infrared spectroscopy (FTIR) determination calibrated with quantitative nuclear magnetic resonance (NMR) spectroscopy.Films were pressed using a Specac film press at 150°C, approximately at 5 tons, 1-2 minutes, and then cooled with cold water in a not controlled manner. The accurate thickness of the obtained film samples was measured.After the analysis with FTIR, base lines in absorbance mode were drawn for the peaks to be analysed. The absorbance peak for the comonomer was normalised with the absorbance peak of polyethylene. An FTIR peak height ratio was correlated to the polar comonomer content by reference materials determined by NMR. The NMR spectroscopy calibration procedure was undertaken in the conventional manner which is well documented in the literature. Quantification of polar comonomer content in polymers by NMR spectroscopyThe polar comonomer content was determined by quantitative nuclear magnetic resonance (NMR) spectroscopy after basic assignment (e.g. “NMR Spectra of Polymers and Polymer Additives”, A. J. Brandolini and D. D. Hills, 2000, Marcel Dekker, Inc. New York). Experimental parameters were adjusted to ensure measurement of quantitative spectra for this specific task (e.g. “200 and More NMR Experiments: A Practical Course”, S. Berger and S. Braun, 2004, Wiley-VCH, Weinheim). Quantities were calculated using simple corrected ratios of the signal integrals of representative sites in a manner known in the art.Below is exemplified the determination of the polar comonomer content of ethylene ethyl acrylate, ethylene butyl acrylate and ethylene methyl acrylate.The weight-% can be converted to mol-% by calculation. It is well documented in the literature. (1) Ethylene copolymers containing butyl acrylateFilm samples of the polymers were prepared for the FTIR measurement: 0.5-0.7 mm thickness was used for ethylene butyl acrylate >6 wt% butyl acrylate content and 0.1 to 0.4 mm thickness was used for ethylene butyl acrylate <6 wt% butyl acrylate content.After the FT-IR analysis the maximum absorbance for the peak for the butyl acrylate >6 wt% at 3450 cm-1 was subtracted with the absorbance value for the base line at 3510 cm-1 (Abutyl acrylate – A3510).  Then the maximum absorbance peak for the polyethylene peak at 2020 cm-1 was subtracted with the absorbance value for the base line at 2120 cm-1 (A2020 –A2120). The ratio between (Abutyl acrylate-A3510) and (A2020-A2120) was then calculated in the conventional manner, which is well documented in the literature.The maximum absorbance for the peak for the comonomer butyl acrylate <6 wt% at 1735 cm-1 was subtracted with the absorbance value for the base line at 1850 cm-1 (Abutyl acrylate - A1850). Then the maximum absorbance peak for polyethylene peak at 2660 cm-1 was subtracted with the absorbance value for the base line at 1850 cm-1 (A2660 - A1850). The ratio between (Abutyl acrylate-A1850) and (A2660-A1850) was then calculated. (2) Ethylene copolymers containing ethyl acrylateFilm samples of the polymers were prepared for the FTIR measurement: 0.5 mm thickness was used for ethylene ethyl acrylate.After the FT-IR analysis the maximum absorbance for the peak for the ethyl acrylate at 3450 cm-1 with linear baseline correction applied between approximately 3205 and 3295 cm-1 (Aethyl acrylate) was determined. Then the maximum absorbance peak for the polyethylene peak at 2020 cm-1 with linear baseline correction applied between approximately 1975 and 2120 cm-1 was determined (A2020). The ratio between (Aethyl acrylate) and (A2020) was then calculated in the conventional manner, which is well documented in the literature. (3) Ethylene copolymers containing methyl acrylateFilm samples of the polymers were prepared for the FTIR measurement: 0.1 mm thickness was used for ethylene methyl acrylate >8 wt% methyl acrylate content and 0.05 mm thickness was used for ethylene methyl acrylate <8 wt% methyl acrylate content.After the analysis the maximum absorbance for the peak for the methyl acrylate >8 wt% at 3455 cm-1 was subtracted with the absorbance value for the base line at 3510 cm-1 (Amethyl acrylate – A3510). Then the maximum absorbance peak for the polyethylene peak at 2675 cm-1 was subtracted with the absorbance value for the base line at 2450 cm-1 (A2675 –A2450). The ratio between (Amethyl acrylate-A3510) and (A2675-A2450) was then calculated in the conventional manner which is well documented in the literature.The maximum absorbance for the peak for the comonomer methyl acrylate <8 wt% at 1164 cm-1 was subtracted with the absorbance value for the base line at 1850 cm-1 (Amethyl acrylate - A1850). Then the maximum absorbance peak for polyethylene peak at 2665 cm-1 was subtracted with the absorbance value for the base line at 1850 cm-1 (A2665 - A1850). The ratio between (Amethyl acrylate-A1850) and (A2665-A1850) was then calculated.  Determination of additive content, i.e. the antioxidant content,in thebulk of the pelletX-ray fluorescence (XRF) was used to measure the antioxidant content in the bulk of the pellet, i.e. in the polymer composition as a whole. As is well known by the person skilled in the art, XRF involves measuring the emission of characteristic X-rays from a material excited by primary X-rays or gamma rays. Different chemical elements emit X-rays with characteristic energies, and therefore the spectral profile of an XRF response from a material is indicative of the chemical elements within. The amount of each element included in the material can be determined using calibration with known standards. XRF analysis was done on a Malvern Panalytical Zetium instrument. The sulfur (S) content was determined by XRF calibrated with standards from Malvern Panalytical covering the measured range. For XRF analysis the composition was melt pressed into 3 mm plaques using a Collin press at 160°C for 2 minutes at 200 bar. The plaques were cooled in the press at 15 °C per minute. The antioxidant content was calculated from the S content in the conventional manner using the antioxidant molecular weight and composition. Determination of additive content, i.e. the antioxidant content, on pellet surfaceFor the storage study, 100 g of pellets of the inventive and comparative compositions were placed in aluminium bags that were sealed. One bag of pellets was prepared for each sample take out. The bags containing the pellets were stored for 0, 2, and 4 weeks at 35 °C. The content of the components on the pellet surface was determined by placing 100 g of pellets in an 800 ml beaker with magnetic stirrer. 100 ml of methanol was added and then the pellets were stirred in the methanol for 5 minutes. A 5 ml sample was taken from the solvent with a syringe. When the solution was transferred from the syringe to the vial to be used for the HPLC analysis the solution was passed through a 0.45 µm PTFE filter placed at the tip of the syringe. The contents of the components (TBCP, DCP, antioxidant and MSD) in the solution were determined via HPLC analysis. This test is done on the ‘zero’ sample at the start of the storage test as well as after storing the pellets for 2 and 4 weeks at 35 °C. The HPLC analyses were done using a EC-C18 (150 x 4.6 mm) column such as Poroshell. The injection volume of the methanol solution was 5 µl. The gradient elution at 1 ml / min was done with a methanol water mixture of 85:15% by volume during test time 0-5 min, 11-15 min and 100% methanol during test time 6-15 min at 50 °C. Experimental Part Components of the polymer compositions of the examples of the present invention and of the comparative examplesAll polymers were low density polyethylenes produced in a high pressure reactor.  LDPE1: a terpolymer of ethylene, butylacrylate (polar comonomer) and 1,7-octadiene (polyunsaturated comonomer) with typical MFR2 ~1.9 g / 10 min, ~0.50 vinyl / 1000 C produced in a high pressure tubular reactor. Content of polar comonomer = 0.36 wt%.  LDPE2: a copolymer of ethylene and 1,7-octadiene (polyunsaturated comonomer) with MFR2 ~1.7 g / 10 min, ~0.60 vinyl / 1000 C produced in a high pressure tubular reactor (comparative). Antioxidant (AO): 4,4′-thiobis(2-tert-butyl-5-methylphenol) (CAS no. 96-69-5), commercially available. Crosslinking agent: tert-butylcumylperoxide (TBCP) (CAS no. 3457-61-2) or dicumyl peroxide (DCP) (CAS no.80-43-3), both commercially available. 2,4-Diphenyl-4-methyl-1-pentene (MSD): CAS no. 6362-80-7, commercially available. Preparation of the polymer compositions of the examples of the present invention and of the comparative examplesIn the examples below, the antioxidant was added to the polyethylene (supplied in the form of pellets) by compounding via melt mixing followed by re-pelletising the prepared composition to form pellets. Thereafter, the crosslinking agent was added to the composition by distributing the crosslinking agent (crosslinking agent is in liquid form) onto the pellets which were preheated for 12 hours at 80 °C. The pellets and the crosslinking agent were stirred for 45 minutes and then continuously heated at 80 °C until the pellets became dry. In cases where 2,4-diphenyl-4-methyl-1-pentene has been used in the compositions, it was added to the pre-heated pellets together with the crosslinking agent under the same conditions as described above for the crosslinking agent. The components and the results of the storage study of the polymer compositions of inventive examples 1-4 (IE1-IE4) and the comparative examples 1-3 (CE1, CE2, and CE3) are given in Table 1.The amount of antioxidant (AO), crosslinking agent (TBCP) or (DCP), and (if present) 2,4-Diphenyl-4-methyl-1-pentene (MSD) is given in wt% relative to the total weight of the polymer composition.  Table 1: Components and storage study results of the polymer compositions of the inventive examples 1-4 and of the comparative examples 1-3 ComponentsIE1IE2IE3IE4CE1CE2CE3PolyethyleneLDPE1LDPE1LDPE1LDPE1LDPE1LDPE1LDPE2AO (wt%)0.110.110.110.110.110.110.09TBCP (wt%)1.351.651.651.651.35-0.80DCP (wt%)-----1.35-MSD (wt%)0.350.100.150.20-0.350.15Amount of AO on pellet surface(ppm)       0 weeks<5<5<5<5<5<5762 weeks<561<514247373844 weeks7187<53238491497 As shown in Table 1, the polymer compositions of the inventive examples comprising both a peroxide according to Formula (I) as hereinbefore defined and 2,4-diphenyl-4-methyl-1-pentene exhibited remarkably low exudation of antioxidant after storage, especially when compared with the polymer composition of the comparative examples 1 and 2 which did not comprise this combination of peroxide and 2,4-diphenyl-4-methyl-1-pentene. The use of a different LDPE (LDPE2) without the polar comonomer also did not result in the same reduction of antioxidant exudation. The polymer compositions of the present invention thus have improved storage stability.

Claims

1. A polymer composition comprising:a) at least one polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s); b) at least one antioxidant; c) at least one crosslinking agent; andd) 2,4-diphenyl-4-methyl-1-pentene;wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer; andwherein the crosslinking agent is an organic peroxide having a structural formula in accordance with Formula (I):  wherein R1, R2 and R3 are each independently selected from linear and branched C1-C6 alkyl groups; and R is the rest of the organic peroxide. 2. A polymer composition as claimed in claim 1, wherein the polyethylene copolymer is a low density polyethylene (LDPE) copolymer, preferably an unsaturated LDPE copolymer; and / or wherein the polyethylene copolymer comprises vinyl groups in a total amount of at least 0.20 / 1000 carbon atoms; preferably at least 0.30 / 1000 carbon atoms; more preferably at least 0.40 / 1000 carbon atoms. 3. A polymer composition as claimed in claim 1, wherein the one or more polar comonomer(s) are selected from acrylates, methacrylates, acetates, or mixtures thereof, preferably from C1 to C6 alkyl acrylates, C1 to C6 alkyl methacrylates, vinyl acetate, or mixtures thereof.  4. A polymer composition as claimed in claim 1, wherein the one or more other non-polar comonomer(s) are present and are preferably selected from polyunsaturated comonomers having a straight carbon chain with at least 8 carbon atoms and of which there are at least 4 carbon atoms between non-conjugated double bonds, at least one of said double bonds is terminal; for example a diene such as a C8 to C14 non-conjugated diene, e.g. selected from 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, or mixtures thereof.  5. A polymer composition as claimed in claim 1, wherein the antioxidant is present in an amount of at least 0.005 wt%, preferably at least 0.01 wt%, more preferably at least 0.02 wt%, most preferably at least 0.04 wt%, relative to the total weight of the polymer composition (100 wt%).  6. A polymer composition as claimed in claim 1, wherein the antioxidant is selected from the group consisting of sterically hindered or semi-hindered phenols, aromatic amines, aliphatic sterically hindered amines, thio compounds, and mixtures thereof; preferably wherein the antioxidant is a sulfur-containing phenolic antioxidant; more preferably a thiobisphenol such as 4,4'-thiobis (2-tertbutyl-5-methylphenol), 2,2'-thiobis (6-t-butyl-4-methylphenol), 4,4'-thiobis (2-methyl-6-t-butylphenol), thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 4,6-bis (octylthiomethyl)-o-cresol, or mixtures thereof.  7. A polymer composition as claimed in claim 1, wherein the polymer composition comprises the crosslinking agent in an amount of at least 0.2 wt%, preferably at least 0.3 wt%, more preferably at least 0.4 wt%, most preferably at least 0.5 wt%, such as at least 0.6 wt%, especially at least 0.75 wt%, relative to the total weight of the polymer composition (100 wt%).  8. A polymer composition as claimed in claim 1, wherein R1, R2 and R3 are each independently selected from linear and branched C1-C4 alkyl groups, preferably wherein R1, R2 and R3 are the same or different and are each a methyl group or an ethyl group, especially a methyl group.  9. A polymer composition as claimed in claim 1, wherein the crosslinking agent is selected from the group consisting of di-tert-amylperoxide, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide, di(tert-butyl)peroxide, butyl-4,4-bis(tert-butylperoxy)-valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxybenzoate, bis(tertbutylperoxyisopropyl)benzene, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tertamylperoxy)cyclohexane, and mixtures thereof; preferably 2,5-di(tert-butylperoxy)-2,5-dimethylhexane, 2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne, tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures thereof; especially tert-butylcumylperoxide. 10. A polymer composition as claimed in claim 1, wherein the polymer composition comprises at least 0.05 wt% 2,4-diphenyl-4-methyl-1-pentene, preferably greater than 0.05 wt% 2,4-diphenyl-4-methyl-1-pentene, more preferably at least 0.06 wt% 2,4-diphenyl-4-methyl-1-pentene, even more preferably at least 0.07 wt% 2,4-diphenyl-4-methyl-1-pentene, most preferably at least 0.08 wt% 2,4-diphenyl-4-methyl-1-pentene, such as within the range of 0.08 and 1.5 wt%, relative to the total weight of the polymer composition (100 wt%). 11. A process for producing a polymer composition according to any of claims 1 to 10, the process comprising blending the polyethylene copolymer(s) with the antioxidant(s), the crosslinking agent(s) and 2,4-diphenyl-4-methyl-1-pentene. 12. A crosslinked polymer composition obtainable, preferably obtained, by crosslinking the polymer composition according to any of claims 1 to 10. 13. A power cable comprising a conductor surrounded by at least one layer, preferably an insulation layer, comprising, preferably consisting of, the polymer composition according to any of claims 1 to 10 or the crosslinked polymer composition according to claim 12.  14. A process for producing a power cable, the process comprising:(i) applying on one or more conductor(s), preferably by (co)extrusion, at least one layer, preferably an insulation layer, comprising, preferably consisting of, the polymer composition according to any of claims 1 to 10; and(ii) optionally crosslinking the polymer composition. 15. Method of using a combination of a crosslinking agent and 2,4-diphenyl-4-methyl-1-pentene in a polymer composition further comprising (i) at least one polyethylene copolymer of ethylene, one or more polar comonomer(s) and optionally one or more other non-polar comonomer(s), and (ii) an antioxidant, to reduce exudation of the antioxidant from the polymer composition; wherein the content of the one or more polar comonomer(s) in the polyethylene copolymer is in the range of 0.001 to 5.0 wt%, relative to the total weight of the polyethylene copolymer; and wherein the crosslinking agent is an organic peroxide having a structural formula in accordance with Formula (I):   wherein R1, R2 and R3 are each independently selected from linear and branched C1-C6 alkyl groups; and R is the rest of the organic peroxide. 

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