WO2025114582A1 - Process for producing a polymer for a cable - Google Patents

Abstract

A process for the preparation of a low density polyethylene (LDPE) homopolymer or LDPE copolymer by radical polymerisation, said process comprising polymerizing ethylene and optionally at least one comonomer in the presence of at least two chain transfer agents: - a C_1-8-alkanol, and - a non-polar chain transfer agent (CTA).

Description

Process for producing a polymer for a cable

Field of invention

The invention relates to a process for producing a low density polyethylene (LDPE) polymer in a high pressure process. In particular, the invention relates to the use of a combination of chain transfer agents to produce a polymer with low dielectric losses (as measured by tan delta) with reduced reactor fouling.

The invention also relates to a polymer produced according to the process of the invention and to a cable comprising said polymer.

Background art

Chain Transfer Agents (CTA) are used during high pressure polymerisation of ethylene to form low density polyethylene (LDPE). These CTAs are primarily used to control the molecular weight and thus indirectly melt flow rate (MFR) of the formed polymer. Their mechanism of action is based on hydrogen atoms that are easy to abstract. Typical CTAs are methyl ethyl ketone, propionaldehyde, and propylene. In EP- A- 3181599, various chain transfer agents are suggested in paras 0050-0051 although the CTA used in the examples is not named. EP-A-3031833 generically mentions the option of using two chain transfer agents but claims the use of a saturated hydrocarbon as the CTA.

CTAs of use in the manufacture of LDPE include therefore a variety of polar and non-polar compounds. Propylene (C3) is an example of a non-polar CTA. This is assumed to provide good electrical properties due to its non-polar structure, especially when it comes to dielectric losses. Another benefit when using propylene as CTA is that vinyl groups are introduced into the polyethylene chain giving improved peroxide crosslinking properties.

Frequently used polar CTAs include propionaldehyde (PA) and methyl ethyl ketone (MEK). Polar CTAs tend to be more reactive than non-polar CTAs and are important therefore in the control of the polymerisation process, e.g. in manipulating the polymer chain length. Due to its polar structure it is assumed not to be beneficial to use this CTA in applications where low dielectric losses are a requirement but their use is important for controlling molecular weight and hence processes in which a combination of a polar and non-polar CTAs are well known such as EP-A- 2310426. For a given MFR, a combination of two CTAs such as an aldehyde (e.g. propionaldehyde) with an olefin (e.g. propylene) leads to a good balance between MFR control and electrical properties.

WO2011/057927 discloses a polymer composition suitable for use in a layer of a power cable. The polymer composition is one that has a low DC electrical conductivity and comprises an LDPE made using a compressor lubricant comprising a mineral oil.

WO2012/150287 describes a power composition suitable for use in a layer of a power cable. The polymer composition is one that comprises LDPE, a peroxide and an ion exchange additive.

WO2015/172009 describes a process for making an ethylene based polymer using a hydrocarbyl free radical initiator.

EP-B-3717526 describes a polyethylene with high vinyl group content. When designing a LDPE polymer for use in a cable, in particular in the insulation layer of a cable, the tan delta of the polymer is an important consideration.

The tan 8 and thus the dielectric losses (which are linearly proportional to the tan 8) shall be as low as possible for both technical and economic reasons:

• Low losses means that low amount of transmitted electric energy is lost as thermal energy inside the cable insulation. These losses will mean economic losses for the power line operator.

• Low losses will reduce the risk for thermal runaway, i.e. an unstable situation where the temperature of the insulation will increase due to the tan 8. When the temperature is increased, normally the tan 8 will also increase. This will further increase the dielectric losses, and thus the temperature. The results will be a dielectric failure of the cable that needs to be replaced. There remains a need to prepare LDPEs in which the tan delta is minimized.

It would also be desirable to decouple the MFR control and the electrical properties to some extent thereby broadening the process window to allow more flexibility to the process. It would also be useful to minimise reactor fouling. In any polymerisation process but particularly in high pressure processes, there is the possibility that the reactor will become fouled.

Fouling problems arise as a result of the highly exothermic nature of the low- density polyethylene (LDPE) polymerization process and the heating-cooling utilities in tubular reactors (TRs). There can therefore be an accumulation of unwanted material on the reactor jacket interior surface, i.e. fouling. The problem of reactor fouling is especially pronounced during polymerization of ethylene with polar comonomers and silane comonomers. The fouling typically manifests itself as poor heat transfer, unstable and inhomogeneous production, with formation of gels and potential build-up of polymer deposits on the inner surfaces of the reactor over time. This may make it difficult to produce a polymer with a consistent and reproducible quality.

The science behind reactor fouling is complex but the inventors have found that problem of fouling appears to be exacerbated when propionaldehyde and methyl ethyl ketone are used as CTAs.

The present inventors have surprisingly found that LDPE polymers can be prepared having low tan delta and in a process which minimises reactor fouling when a combination of an alkanol and non-polar CTA are used as CTAs. Without wishing to be limited by theory, the inventors theorise that the carbonyl motif that derives from polar CTAs such as propionaldehyde and methyl ethyl ketone cause higher tan delta values than the -OH motif. Moreover, the presence of alkanols in the reactor is believed to improve heat transfer at the reactor walls by reducing or removing the residual polymer layer that may be bonded to the reactor walls. In this way, reactor fouling is reduced. This, in turn, improves product output, process control, product quality and reproducibility.

Summary of Invention

Viewed from one aspect the invention provides a process for the preparation of a low density polyethylene (LDPE) homopolymer or LDPE copolymer by radical polymerisation, said process comprising polymerizing ethylene and optionally at least one comonomer in the presence of at least two chain transfer agents, - a Ci-s-alkanol, and

- a non-polar chain transfer agent.

Viewed from another aspect the invention relates to the use of at least two chain transfer agents,

- a Cl-8 alkanol; and

- a non-polar CTA; to reduce fouling in a reactor during the preparation of an LDPE homopolymer or LDPE copolymer by radical polymerisation.

Viewed from another aspect the invention provides a LDPE homopolymer or LDPE copolymer with at least one polyunsaturated comonomer having an MFR2 in the range of 0.25 to 10 g/lOmin determined by ISO1133 at 190°C and 2.16 kg and a vinyl content of at least 0.25 vinyl groups per 1000 C atoms determined by ASTM D3124-98/ASTM D6248-98 and a tan delta measured at room temperature (nondegassed, non-crosslinked) of 2.5 x 10'⁴ or less (determined according to tan delta test A in the determination methods below) and which is obtained by the process as defined in any of the preceding claims.

Viewed from another aspect, the invention provides an LDPE homopolymer or LDPE copolymer with at least one polyunsaturated comonomer, said homopolymer or copolymer having an MFR2 in the range of 0.25 to 10 g/lOmin determined by ISO1133 at 190°C and 2.16 kg and a vinyl content of at least 0.25 vinyl groups per 1000 C atoms determined by ASTM D3124-98/ASTM D6248-98 and a tan delta measured at room temperature (non-degassed, non-crosslinked) of 2.5 x 10'⁴ or less (determined according to tan delta test A in the determination methods below); and wherein said LDPE homopolymer or LDPE copolymer contains at least 1.0 alkoxy groups per 10,000 carbon atoms, for example 1.0 to 20 alkoxy groups per 10,000 carbon atoms, or 1.0 to 10 alkoxy groups per 10,000 carbon atoms, such as 1.0 to 5.0 alkoxy groups per 10,000 carbon atoms; and at least 3.0 hydroxyl groups per 10,000 carbon atoms, for example 3.0 to 20 hydroxyl groups per 10,000 carbon atoms or 3.0 to 15 hydroxyl groups per 10,000 carbon atoms, such as 3.0 to 10.0 hydroxyl groups per 10,000 carbon atoms. Definitions

The term LDPE copolymer used herein implies the presence of ethylene and at least one comonomer such as one or two comonomers. A terpolymer is therefore a type of copolymer herein. If there are only two monomers present (i.e. ethylene and one comonomer) this is a binary copolymer herein. The LDPE copolymer of the invention can therefore be a terpolymer (i.e. with two comonomers) or a binary copolymer.

An LDPE homopolymer is prepared via the polymerisation of ethylene as the only monomer. No comonomer is present.

The term LDPE homo or copolymer of the invention refers herein to a polymer that can be produced by the process of the invention. The LDPE homo or copolymer has properties as defined herein.

Description of the invention

The present invention relates to a process for the preparation of an LDPE homopolymer or copolymer using a blend of two specific chain transfer agents. Surprisingly, this blend of CT As leads to polymers with low tan delta values and minimized reactor fouling.

LDPE homopolymer or LDPE copolymer

The process of the invention targets the preparation of an LDPE homopolymer or LDPE copolymer. LDPE homopolymers are based on the polymerisation of ethylene only.

LDPE copolymers are prepared from the copolymerization of ethylene and at least one comonomer, such as one or two comonomers.

The comonomers that can be used in this aspect of the invention are preferably polar comonomers or non-polar comonomers. The term non-polar is used herein to require that the comonomer comprises C and H atoms only. Polar comonomers comprise at least one heteroatom, such as oxygen, in addition to C and H atoms.

Polar comonomers of interest include alkyl acrylates, alkyl methacrylates, vinyl acetate, (meth)acrylic acid, or a mixture thereof. Further preferably, said polar comonomers are selected from Ci- to Ce-alkyl acrylates, Ci- to Ce-alkyl methacrylates or vinyl acetate. Still more preferably, said polar LDPE copolymer is a copolymer of ethylene with Ci- to C4-alkyl acrylate, such as methyl, ethyl, propyl or butyl acrylate, a copolymer of ethylene with Ci- to C4-alkyl methacrylate or vinyl acetate, or any mixture thereof. The use of n-butyl acrylate is especially preferred.

If a polar comonomer is present, the polar comonomer content is preferably in the range of 0.2 to 0.6 wt% of the polymer, such as 0.25 to 0.4 wt%.

Non-polar comonomer(s) for the LDPE copolymer are preferably polyunsaturated (= more than one double bond) comono mer(s). The polyunsaturated comonomers preferably consist of a straight carbon chain with at least 8 carbon atoms and at least 4 carbons between the non-conjugated double bonds, of which at least one is terminal, more preferably, said polyunsaturated comonomer is a diene. Preferably the diene comprises at least eight carbon atoms, the first carbon-carbon double bond being terminal and the second carbon-carbon double bond being nonconjugated to the first one. Preferred dienes are selected from Cs to C14 nonconjugated dienes or mixtures thereof, more preferably selected from 1,7-octadiene, 1,9-decadiene, 1,11 -dodecadiene, 1,13 -tetradecadiene, 7-methyl-l,6-octadiene, 9- methyl-l,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.

If a non-polar comonomer is present, the non-polar comonomer content is preferably in the range of aat least 0.1 wt%, preferably at least 0.3 wt%, such as at least 0.5 wt%, especially at least 0.7 wt%. The upper limit for non-polar comonomer content might be 1.5 wt%.

It is particularly preferred if the LDPE copolymer of the invention comprises a polyunsaturated comonomer and optionally a polar comonomer such as an alkyl (meth)acrylate.

The LDPE copolymer is preferably a copolymer of ethylene and 1,7- octadiene and optionally an alkyl (meth)acrylate such as n-butyl acrylate.

Preferably, the content of the ethylene in the LDPE copolymer may be between 60 and 99.9 wt.-%, more preferably between 70 and 99.5 wt.-%, even more preferably between 80 and 99.5 wt.-% of the ethylene copolymer. It is preferred if the LDPE homopolymer or copolymer of the invention has an a density in the range of 900 to 940 kg/m³, such as 905 to 935 kg/cm³.

The MFR2 (2.16 kg, 190 °C) of the LDPE homo- or copolymer as said preferred polymer is preferably from 0.1 to 10 g/lOmin, more preferably is from 0.25 to 5.0 g/lOmin, and most preferably is from 1.0 to 3.0 g/lOmin.

It is preferred if the LDPE homo or copolymer has a vinyl group content of at least 0.25/1000 C atoms, especially at least 0.35/1000 C atoms. The upper limit of the amount of vinyl bonds present in the LDPE homo or copolymer is not limited and may preferably be less than 5.0/1000 carbon atoms, preferably less than 3.0/1000 carbon atoms.

The LDPE homopolymer or LDPE copolymer preferably has an MFR2 of 0.25 to 10 g/lOmin and/or a vinyl group content of at least 0.25 vinyl groups per 1000 C atoms.

Unexpectedly, the LDPE homo or copolymer produced using the process of the invention may have a tan 8 when measured on non-degassed, non-crosslinked plaques according to the method described in “Test for tan 8” under Determination methods of 2.5 x 10'⁴or less, such as 0.1 x 10'⁴ to 2.0 x 10'⁴. It is notable that the tan delta values achieved herein are similar too or lower than those achieved using a CTA blend based on a carbonyl containing polar CTA and a non-polar CTA. Moreover, methanol is a cheaper compound than carbonyl containing CTAs such as propionaldehyde and methyl ethyl ketone. On an industrial scale where large amounts of CTA are used, this reduction in price can be significant. The fact therefore that similar or better tan delta properties can be obtained with a much cheaper CTA is advantageous.

Another benefit of the use of alkanols is that these compounds have a lower heat radiation than polar CTAs such as MEK. This increases process safety.

The LDPE homo or copolymer of the invention is prepared in the presence of a CTA blend comprising an alkanol and a non-polar CTA such as propylene. The LDPE homo or copolymer of the invention may therefore comprise certain functional groups that indicate that such a blend was used, as opposed to a CTA blend comprising a carbonyl containing group. If a carbonyl containing group is present in the CTA used, the resulting LDPE homo or copolymer is likely to contain a comparatively higher number of C=O groups which are readily detectable in FTIR. If an alkanol CTA is used however, the LDPE homo or copolymer can be expected to contain a comparatively higher amount of -OH groups or -O- groups (such as in the form of alkoxy groups) which are readily detected with NMR. It is preferred therefore if the LDPE homo or copolymer of the invention contains -OH and/or -O- (alkoxy) groups detectable with proton NMR.

In one embodiment, the LDPE homo or copolymer of the invention comprises at least 1.0 alkoxy groups per 10,000 carbon atoms such as 1.0 to 5.0 alkoxy groups per 10,000 carbon atoms.

In one embodiment, the LDPE homo or copolymer of the invention comprises at least 1.0 methoxy groups per 10,000 carbon atoms such as 1.0 to 5.0 methoxy groups per 10,000 carbon atoms. In one embodiment, the LDPE homo or copolymer of the invention comprises at least 3.0 hydroxyl groups per 10,000 carbon atoms such as 3.0 to 20.0 hydroxyl groups per 10,000 carbon atoms, such as 5.0 to 10.0 hydroxyl groups per 10,000 carbon atoms.

Preferably the LDPE homo or copolymer of the invention has both alkoxy (such as methoxy) and hydroxyl groups as defined above.

The number of alkoxy and hydroxyl groups can be determined by NMR. A detailed protocol for this measurement is provided in the examples section below.

Viewed from another aspect, the invention provides a LDPE homopolymer or LDPE copolymer with at least one polyunsaturated comonomer, said homopolymer or copolymer having an MFR2 in the range of 0.25 to 10 g/lOmin determined by ISO1133 at 190°C and 2.16 kg and a vinyl content of at least 0.25 vinyl groups per 1000 C atoms determined by ASTM D3124-98/ASTM D6248-98 and a tan delta measured at room temperature (non-degassed, non-crosslinked) of 2.5 x 10'⁴ or less; and wherein said LDPE homopolymer or LDPE copolymer contains at least 1.0 alkoxy groups per 10,000 carbon atoms such as 1.0 to 5.0 alkoxy groups per 10,000 carbon atoms; and at least 3.0 hydroxyl groups per 10,000 carbon atoms such as 3.0 to 20.0 hydroxyl groups per 10,000 carbon atoms. Preferably, the LDPE homo or copolymer of the invention as defined above is obtainable by the process of the invention. The use of CTA-blend of the invention preferably provides the desirable electric properties as defined above.

Process

The LDPE homo or copolymer is produced in high pressure process by radical polymerization.

CTAs

The invention relies on the use of a two CTAs. The CTAs can be supplied to the polymerisation process as a mixture or in separate feeds. Within the reactor, the CTAs will obviously mix. For convenience, the two CTAs will be referred to herein as CTA-Mixture or CTA-blend but as noted above, this does not imply that the two CTAs must be supplied to the reactor together. The CTA-mixture can be used in a conventional manner adjusting the molecular weight and thus melt flow rate (MFR) of the produced polymer.

The first CTA used is a Cns-alkanol, preferably a Ci-4-alkanol. The alkyl chain in this molecule can be linear or branched, especially linear. The alkanol is preferably primary or secondary, especially primary. The use of n-propanol, i- propanol, n-butanol, isobutanol, octanol, ethanol or methanol is preferred. It is especially preferred if the alkanol is methanol. Ethanol is also attractive as it has reduced toxicity and flammability.

The second CTA is a non-polar CTA. The term non-polar is used herein to require that the CTA comprises C and H atoms only.

Preferably, the non-polar CTA is selected from a straight chain, branched or cyclic non-aromatic hydrocarbyl. More preferably, the non-polar CTA is selected from one or more of an cyclic, non-aromatic, alpha-olefin of 5 to 12 carbons or from a straight or branched chain alpha-olefin of 3 to 8 carbon atoms, more preferably a straight or branched chain alpha-olefin of 3 to 6 carbon atoms. The preferred nonpolar CTA is propylene.

The combination of propylene and methanol is especially preferred. The ratio of non-polar CTA to the alkanol can be carefully controlled by the skilled person. It is generally preferred if there is an excess of the non-polar comonomer. For example the weight ratio of non-polar CTA to alkanol CTA may be 1 : 1 to 50: 1. In weight percent terms therefore that corresponds to 2-50 wt% alkanol and 98-50 wt% non-polar CTA based on the weight of the blend of CTAs.

It is preferred if the weight ratio of non-polar CTA to alkanol CTA is 3 : 1 to 30:1. In weight percent terms therefore that corresponds to 3-25 wt% alkanol to 97- 75 wt% non-polar CTA based on the weight of the blend of CTAs.

It is preferred if the weight ratio of non-polar CTA to alkanol CTA is 4: 1 to 20: 1. In weight percent terms therefore that corresponds to 5-20 wt% alkanol, 95- 80 wt % non-polar CTA based on the weight of the blend of CTAs.

It is preferred if the weight ratio of non-polar CTA to alkanol CTA is 5: 1 to 15:1. In weight percent terms therefore that corresponds to 6-17 wt% alkanol, 94-83 wt% non-polar CTA based on the weight of the blend of CTAs.

It is preferred if the weight ratio of non-polar CTA to alkanol CTA is 7: 1 to 14:1. In weight percent terms therefore that corresponds to 7-13 wt% alkanol, 93-87 wt% non-polar CTA based on the weight of the blend of CTAs.

It is preferred if the only CTAs used in the process of the invention are the alkanol and non-polar CTAs described herein. Whilst it is possible to use two or more alkanols as described herein or two or more non-polar CTAs as described herein it is preferred if only one alkanol and one non-polar CTA is used.

The process of the invention takes place under high pressure via radical polymerisation. Such a process is well known and documented in the literature.

The ratio of CTA’s and the monomer feeds used can be readily adjusted to tailor the electrical and physical properties of the target LDPE.

The process can be effected in any conventional LDPE polymerisation equipment and conventional process conditions and control means can be used for adjusting the polymer properties, such as MFR, density, optional unsaturation etc in order to achieve the desired polymer properties.

High pressure polymerisation can be effected in a tubular reactor or an autoclave reactor, preferably in a tubular reactor. In one preferable process, the ethylene is polymerized, optionally together with one or more comonomer(s), in the presence of the mixture of chain transfer agents of the invention, preferably in a tubular reactor to obtain a LDPE homopolymer or copolymer with good electrical properties as defined above.

In a typical process, ethylene is fed to a compressor mainly to enable handling of high amounts of ethylene at controlled temperature. The compressors are usually a piston compressor or diaphragm compressors. The compressor is usually a series of compressors that can work in series or in parallel. Most common is 2-5 compression steps. Recycled ethylene and comonomers can be added at feasible points depending on the pressure. Temperature is typically low, usually in the range of less than 200°C or less than 100°C.

Tubular reactor:

The reaction mixture is fed to the tubular reactor. First part of the tube is to adjust the temperature of the feed; usual temperature is 150-170°C. Then the radical initiator is added. As the radical initiator, any compound or a mixture thereof that decomposes to radicals at an elevated temperature can be used. Usable radical initiators are commercially available. Typical copolymerization initiators which can be used include peroxide compounds such as lauryl peroxide, tert-butyl peracetate, tert-butyl peroxypivalate, di-tert-butyl peroxide, di(sec-butyl) peroxydicarbonate, as well as tert-butyl peroctoate. Other free radical initiators which can be used include azo bis compounds such as, e.g., azobisisobutyronitrile. The choice of initiator will depend on the polymerization temperature since each initiator has its own decomposition temperature, at which it efficiently generates free radicals. In general, the initiator is introduced in amounts from about 0.01 to 1 kg per produced ton of polymer. The copolymerization temperature is preferably maintained at about from 120 °C to 320 °C, and more preferably about from 140 °C to 300 °C. The pressure is preferably at least 80 MPa and is preferably maintained at about 130 to 350 MPa, and more preferably within the range of about from 250 to 300 MPa.

The polymerization reaction is exothermic. There can be several radical initiator injections points, e.g. 1-5 points, usually provided with separate injection pumps. The addition of the non-polar and polar CT As is not limited and can be tailored by a skilled person within the limits of the invention depending on the desired end properties of Polymer.

Accordingly, the two chain transfer agents can be added in any injection point to the polymer mixture and in any addition order jointly or separately. The addition of one or two CTAs can be effected from one or more injection point(s) at any time during the polymerization. Also ethylene and optional comonomer(s) can be added at any time of the process, at any zone of the tubular reactor and from one or more injection point(s), e.g. 1-5 point(s), with or without separate compressors. The reactor is continuously cooled e.g. by water or steam. The highest temperature is called peak temperature and the lowest temperature is called radical initiator temperature.

Pressure can be measured at least in the compression stage and after the tube. Temperature can measured at several points during all steps. High temperature and high pressure generally increase output. Using various temperature profiles selected by a person skilled in the art will allow control of structure of polymer chain, i.e. Long Chain Branching and/or Short Chain branching, density, branching factor, distribution of comonomers, MFR, viscosity, Molecular Weight Distribution etc.

The reactor ends conventionally with a valve. The valve regulates reactor pressure and depressurizes the reaction mixture from reaction pressure to separation pressure.

To separate the product the pressure is typically reduced to approx. 30 to 45 MPa. The polymer is separated from the unreacted gaseous products, such as monomer or the optional comonomer, and most of the unreacted products are recovered. Normally low molecular compounds, i.e. wax, are removed from the gas. The pressure can further be lowered to recover and recycle the unused gaseous products, such as ethylene. The gas is usually cooled and cleaned before recycling.

Then the obtained polymer melt is normally mixed and pelletized. Preferably additives can be added in the mixer. Further details of the production of ethylene (co)polymers by high pressure radical polymerization can be found in the Encyclopedia of Polymer Science and Engineering, Vol. 6 (1986), pp 383-410.

The use of alkanols as CTAs in the production of LDPE is also believed to reduce reactor fouling. An industrial tubular reactor will be used to prepare a variety of different polymer products. It is unsurprising that amounts of residual polymer become adhered to the reactor walls during these large scale processes. This is one manifestation therefore of reactor fouling. Reactor fouling may be particularly pronounced after synthesis of copolymers of ethylene and certain polar comonomers which may interact with or even bond to the reactor inner surfaces. Such comonomers include silanes which are capable of forming covalent siloxane bonds to the metal.

It has been found that the presence of alkanols within the reactor improves heat transfer at the reactor walls and a residual polymer layer that is adhered to the reactor wall is reduced or removed. This effect appears particularly pronounced when a residual silane containing polymer layer is present.

Polymer composition

The LDPE homo or copolymer of the invention (which term shall also mean the LDPE homo or copolymer obtained by the process of the invention) may be combined with further polymer components to make valuable articles. LDPE homo or copolymer may also be combined with additives, such as antioxidant(s), free radical generating agent(s), such as crosslinking agent(s), e.g. organic peroxides, scorch retarder(s) (SR), crosslinking booster(s), stabiliser(s), processing aid(s), flame retardant additive(s), water tree retardant additive(s), acid scavenger(s), inorganic filler(s) and voltage stabilizer(s), as known in the polymer field.

It is particularly preferred if the LDPE homo or copolymer of the invention is crosslinked and hence the LDPE homo or copolymer is combined with a crosslinking agent, especially a peroxide. Any peroxide can be used, conveniently dicumyl peroxide. The invention also relates therefore to a LDPE homopolymer or LDPE copolymer that is crosslinked, e.g. crosslinked using a peroxide.

The LDPE homo or copolymer of the invention is useful in a variety of end applications. The preferred use of the LDPE homo or copolymer is in a cable.

It is an object of the invention to provide a cable, preferably a power cable, comprising a conductor surrounded by one or more layers, wherein at least one of said layer(s) comprises a LDPE homo or copolymer of the invention. It is preferable if the LDPE homo or copolymer is used in a crosslinked cable. Crosslinking can be effected by radical reaction using radiation or free radical generating agents.

A crosslinked polymer of the invention might also have a very low tan delta. The tan delta of a crosslinked polymer of the invention measured on a non-degassed, crosslinked plaque at room temperature (following the determination method in the description herein) may be 2.5 x 10'⁴ or less.

In one preferable embodiment of the cable of the invention at least one layer is an insulation layer which comprises said LDPE homo or copolymer of the invention such as a crosslinked LDPE homo or copolymer of the invention.

More preferably, the cable of the invention is a power cable which comprises at least an inner semiconductive layer, an insulation layer and an outer semi conductive layer, in that order, optionally surrounded by a jacketing layer, wherein at least one of said layers, preferably at least the insulation layer, comprises said LDPE homo or copolymer, preferably a crosslinked LDPE homo or copolymer of the invention.

A power cable is defined to be a cable capable of transferring energy operating at any voltage, typically operating at voltages higher than 1 kV. The voltage applied to the power cable can be alternating (AC), direct (DC), or transient (impulse). Cables of the invention are ideally AC cables. The polymer of the invention is very suitable for power cables, especially for power cables operating at voltages higher than 6 kV and are known i.a. as medium voltage (MV), high voltage (HV) and extra high voltage (EHV) power cables, which terms have well known meaning and indicate the operating level of such cable.

Cables can be prepared by applying, preferably by (co)extrusion, one or more layers on a conductor, which layers comprise a polymer, wherein at least one layer comprises said LDPE homo or copolymer of the invention.

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.

Determination methods Addition of crosslinking agent

The crosslinking agent was added to the polyethylene base resin pellets by distributing the crosslinking agent (crosslinking agent is in liquid form) at 70 °C onto preheated pellets for 12 h at 80 °C. The pellets and the crosslinking agent were mixed for 45 min and then continuously heated at 80 °C until the pellets became dry. The amount of crosslinking agent used to crosslink the different compositions are presented in the descriptions of the inventive and comparative compositions.

Compression moulding of PE plaques for tan delta measurement (method A)

Thermoplastic plaques for tan delta measurements are prepared from pellets of inventive polymer and comparative polymer, which are compression moulded using the following method:

First the polymer is melted at 180 °C for 5 min. Then the pressure is increased to 503 N/cm² and maintained for 5 min. Next the plaque i.e. the polymer according to the inventive and comparative polymer is cooled to room temperature with a cooling rate of 15 °C/min still under pressure. At room temperature the pressure is maintained at 503 N/cm² for 4 minutes before it is released. The plaques have a final thickness of 1 ± 0.1 mm and a diameter of 95 mm.

Crosslinking of plaques for tan delta

Pellets of the polymer to be tested with crosslinking agent (CAS 80-43-3) have been prepared as described under ‘Addition of crosslinking’ agent. The crosslinked plaque with the crosslinking agent is prepared from pellets of the test polymer composition, i.e. a polymer composition comprising the polymer according to the present invention with crosslinking agent and a polymer composition comprising a comparative polymer with crosslinking agent, which are compression moulded using the following conditions: First the pellets are melted at 120 °C for 1 min under a pressure of 60 N/cm². Then the temperature is increased to 180 °C at a rate of 18 °C/min and at the same time the pressure is increased to 613 N/cm². The temperature is maintained at 180 °C for 8 min. The total crosslinking time is 12 minutes which includes the time for increasing the temperature from 120 °C to 180 °C. After completed crosslinking the crosslinked plaques, i.e. the crosslinked polymer according to the present invention and the crosslinked comparative polymer, are cooled to room temperature with a cooling rate of 15 °C/min still under pressure. When room temperature is reached the pressure is released after 1 min and the final thickness of the crosslinked plaques are 3 ± 0.1 mm thickness and a diameter of 95 mm.

Tan delta measurement

Tan delta is determined on plaques with the inventive and comparative polymer respectively and crosslinked inventive and comparative polymer respectively. The description of compression moulding of plaques is described in section “Compression moulding of PE plaques for tan delta” and “Crosslinking of plaques for tan delta”. The inventive and comparative polymer plaques or crosslinked polymer plaques are tested at room temperature at 500 V and 50 Hz with an electrode pressure of 6 N/cm² in test cell TETTEX 2914. The test cell is connected to the control unit and power supply TETTEX 2830 + 2831. The tan delta is also determined for crosslinked polymer plaques at 60, 90, and 130 °C by the following procedure. Firstly the electrodes are preheated until a stable temperature is reached. The plaque is then placed inside the test cell with an electrode pressure of 1 N/cm² for 20 ± 5 min. The electrode pressure is then increased to 6 N/cm² and the tan delta is determined at 50 Hz and 500 V. The inventive and comparative polymer crosslinked with the chemical CAS 80-43-3 are tested non-degassed and as well degassed for 72 ± 4 h at 90 °C before tan delta measurement.

Density

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

Melt flow rate

The 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. 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).

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 Avancelll 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 polyethylene Comonomer 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 fdm 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 fdm 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 in polymers by NMR spectroscopy The 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 acrylate

Film 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 (Abutyi 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 (Abutyi 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 (Abutyi 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 - Ai85o). The ratio between (Abutyi acrylate- A1850) and (A2660-A1850) was then calculated.

(2) Ethylene copolymers containing ethyl acrylate

Film 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 (A_eth_yi 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 (Aethyi acrylate) and (A2020) was then calculated in the conventional manner, which is well documented in the literature.

(3) Ethylene copolymers containing methyl acrylate

Film 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 (Amethyi 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 (Amethyi 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 (Amethyi 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 (Amethyi acrylate- Ai 850) and (A2665-A1850) was then calculated.

Methods ASTM D3124-98, and ASTM D6248-98, to determine amount of double bonds in the polymer, i.e. the polyethylene

The methods ASTM D3124-98 and ASTM D6248-98 apply for determination of double bonds in the LDPE component (i). The LDPE component (i) in this method description, referred to as “the polymer”.

The methods ASTM D3124-98, and ASTM D6248-98, include on 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 /raw.s-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 /raws- 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-l -heptene for vinylidene and /raw.s-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 /raw.s-vinylene bonds per 1000 carbon atoms.

The polymer 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 homopolymers and copolymers or and with > 0.4 wt% polar comonomer

For polyethylenes 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-l-ene] giving E = 13.13

I mol ' mm'¹

• vinylidene (RR’C=CH2) via 888 cm'¹ based on 2-methyl-l -heptene [2-methyhept-l- ene] giving E = 18.24 I mol '-mm ¹

• trans-vinylene (R-CH=CH-R’) via 965 cm'¹ based on trans-4-decene [(E)-dec-4-ene] giving E = 15.14 I mol '-mm ¹

For polyethylene homopolymers or copolymers with < 0.4 wt% of polar comonomer linear baseline correction was applied between approximately 980 and 840 cm'¹. 2) Polymer

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-l-ene] giving E = 13.13

I mol' mm ¹

• vinylidene (RR’C=CH2) via 888 cm'¹ based on 2-methyl-l -heptene [2-methyhept-l- ene] giving E = 18.24 I mol '-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 mold'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.

Quantification of 1-hydroxy and 1-methoxy end groups by ¹ H NMR spectroscopy

Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the content of 1-hydroxy and 1-methoxy end groups present in the polymers.

Quantitative ^JH NMR spectra recorded in the solution- state using a Bruker AVNEO 400 NMR spectrometer operating at 400.15 MHz. All spectra were recorded using a ¹³C optimised 10 mm selective excitation probehead at 125°C using nitrogen gas for all pneumatics. Approximately 400 mg of material was dissolved in 7, 7,2,2- tetrachloroethane-<72 (TCE-c6) using approximately 3 mg of 3 mg of 2,6-di-tert- buthyl-4-methylphenol (CAS 128-37-0) as stabiliser. Standard single-pulse excitation was employed utilising a 30 degree pulse, a relaxation delay of 1 s and 10 Hz sample rotation (T.D.W. Claridge, “High-Resolution NMR Techniques in Organic Chemistry”, Elsevier, 1999). A total of 128 transients were acquired per spectra using 4 dummy scans.

Quantitative 'H spectra were processed applying 0.3Hz exponential line broadening, 32K datapoints and spectral width of 20 ppm (T.D.W. Claridge, “High-Resolution NMR Techniques in Organic Chemistry”, Elsevier, 1999). Quantitative properties were determined from the integrals of the respective signals. Validity of signal assignments was verified by ¹³C NMR spectroscopy, 2D ^JH-¹³C HSQC DEPT135 edited spectroscopy (S. Berger, S. Braun, “200 and more NMR experiments”, Wiley-VCH, 2004), comparison of chemical shifts of similar small molecules and plausibility check of multiplicity of proton signal. All chemical shifts were indirectly referenced to TMS at 0.00 ppm using the signal resulting from the residual protonated solvent at 5.95 ppm. Example spectra with respective assignment is shown in Figure 1.

Characteristic signal corresponding to the presence of 1 -hydroxy chain end groups (OH-CH2-CH2-CH2-... ) were observed and the amount quantified using the integral of the triplet at 3.63 ppm (ICH2,-OH) accounting for the number of reporting sites per functional group: fhydroxy ^— IcH2,-OH / 2

Characteristic signal corresponding to the presence of 1 -methoxy chain end groups (CH3-O-CH2-CH2-CH2-... ) were observed and the amount quantified using the average of the integral of the triplet at 3.38 ppm (ICH2,-O-) and the singlet at 3.32 ppm (ICH3,-O-) both accounting for the number of reporting sites per functional group: fmethoxy = (( IcH2,-O- / 2) + (IcH3,-O- / 3)) / 2

The 2,6-di-tert-buthyl-4-methylphenol was considered for later compensation of the bulk signal using the integral of the singlet from the aromatic protons at 6.95 ppm (IBHT) and accounting for the number of reporting sites per molecule:

BHT = IBHT / 2

As typical for quantifications in polyolefins the amount of e.g. unsaturation’s or stabilisers is determined with respect to total carbon atoms, even though quantified by ¹ H NMR spectroscopy.

The total amount of carbon atoms was calculated from integral of the bulk aliphatic signals between 2.5 and 0 ppm (Ibuik) with compensation for the stabiliser:

Ibulkcomp— (Ibulk - (BHT * 21)) / 2 The amount of 1 -hydroxy chain end groups (Ci-hydroxy) was calculated as the number of 1 -hydroxy groups (fhydroxy) in the polymer per 10⁵ total carbons:

Ci -hydroxy [ /10⁵C] - fhydroxy * 100000 / Ibulkcomp

The amount of 1 -methoxy chain end groups (Ci-methoxy) was calculated as the number of 1 -methoxy groups (fmethoxy) in the polymer per 10⁵ total carbons:

Ci -methoxy [ /10⁵C] = frnethoxy * 100000 / Ibulkcomp

Inventive polymer 1

Fresh and recycled ethylene, chain transfer agents and comonomer were compressed in multiple stages with intermediate cooling to reach a pressure of 2850 bar. As chain transfer agent(s), fresh propylene and methanol were continuously added at a ratio by weight of 14: 1 propylene: methanol in amounts sufficient to reach an MFR of 1.84 g/10 min. Fresh comonomer (1,7-octadiene) was added in amount sufficient to reach an average vinyl content of 0.47/1000C in the final polymer. The compressed mixture was heated to approximately 152 °C in a preheating section of a front feed four-zone tubular reactor with an aspect ratio L/D of ca 30,000. Commercially available radical initiators (peroxides) dissolved in essentially inert hydrocarbon solvent were injected directly after the preheater and at three additional points along the reactor in amounts sufficient to reach peak temperatures of 292, 300, 290 and 285 °C. The reaction mixture was depressurized by a pressure control valve, cooled and polymer was separated from unreacted components such as monomers and CTAs.

Comparative polymer 1

Fresh and recycled ethylene, chain transfer agents and comonomer were compressed in multiple stages with intermediate cooling to reach a pressure of 2850 bar. As chain transfer agent(s), fresh propylene and methyl ethyl ketone were continuously added at a ratio by weight of 16: 1 propylene: methyl ethyl ketone in amounts sufficient to reach an MFR of 1.88 g/10 min. Fresh comonomer (1,7- octadiene) was added in amount sufficient to reach an average vinyl content of 0.48/1000C in the final polymer. The compressed mixture was heated to approximately 152 °C in a preheating section of a front feed four-zone tubular reactor with an aspect ratio L/D of ca 30000. Commercially available radical initiators (peroxides) dissolved in essentially inert hydrocarbon solvent were injected directly after the preheater and at three additional points along the reactor in amounts sufficient to reach peak temperatures of 292, 300, 290 and 285 °C. The reaction mixture was depressurized by a pressure control valve, cooled and polymer was separated from unreacted components such as monomers and CTAs.

Inventive Polymer 2

Fresh and recycled ethylene, chain transfer agents and comonomers were compressed in multiple stages with intermediate cooling to reach a pressure of 2850 bar. As chain transfer agent(s), fresh propylene and methanol were continuously added at a ratio by weight of 12: 1 propylene:methanol in amounts sufficient to reach an MFR2 of 1.81 g/10 min. Fresh comonomer n-butyl acrylate was added in amount sufficient to reach 0.33 % by weight in the final polymer. Fresh comonomer (1,7- octadiene) was added in amount sufficient to reach an average vinyl content of 0.42/1000C in the final polymer. The compressed mixture was heated to approximately 155 °C in a preheating section of a front feed four-zone tubular reactor with an aspect ratio L/D of ca 30000. Commercially available radical initiators (peroxides) dissolved in essentially inert hydrocarbon solvent were injected directly after the preheater and at three additional points along the reactor in amounts sufficient to reach peak temperatures of 292, 300, 290 and 285 °C. The reaction mixture was depressurized by a pressure control valve, cooled and polymer was separated from unreacted components such as monomers and CTAs.

Comparative Polymer 2

Fresh and recycled ethylene, chain transfer agents and comonomers were compressed in multiple stages with intermediate cooling to reach a pressure of 2850 bar. As chain transfer agent(s), fresh propylene and methyl ethyl ketone were continuously added at a ratio by weight of 13: 1 propylene: methyl ethyl ketone in amounts sufficient to reach an MFR2 of 1.92 g/10 min. Fresh comonomer n-butyl acrylate was added in amount sufficient to reach 0.35 % by weight in the final polymer. Fresh comonomer (1,7-octadiene) was added in amount sufficient to reach an average vinyl content of 0.43/1000C in the final polymer. The compressed mixture was heated to approximately 155 °C in a preheating section of a front feed four-zone tubular reactor with an aspect ratio L/D of ca 30000. Commercially available radical initiators (peroxides) dissolved in essentially inert hydrocarbon solvent were injected directly after the preheater and at three additional points along the reactor in amounts sufficient to reach peak temperatures of 292, 300, 290 and 285 °C. The reaction mixture was depressurized by a pressure control valve, cooled and polymer was separated from unreacted components such as monomers and CTAs.

Inventive Polymer 3

A set-up consists of a multi-stage compressor, a continuously stirred tank reactor (CSTR), and a valve to control the pressure. The inner volume of the reactor, equipped with a stirrer, is approximately 100 ml. Electrical heating coils allow for heating of the reactor walls to a desired temperature prior to each experiment. Conversion is calculated as the average weight of polymer formed per time unit divided by the feed rates of the reactants.

The reactor is preheated to a temperature of approximately 213 °C. A flow of 2000 g ethylene per hour is added together with propylene and methanol with a molar ratio of 9:1 propylene:methanol in amounts sufficient to reach an MFR2 of 1-10 g/10 min. Comonomer, 1,7-octadiene, is added in amount sufficient to reach an average vinyl content of 0.35-0.60/1000C in the final polymer. If needed, a solvent e.g. Heptane, is used for diluting chain transfer agents and/or monomers. An initiator (Tert-Butyl peroxy-2-ethylhexanoate) is then introduced in sufficient amounts into the reactor until stable conditions are reached at a pressure of -200 MPa and an average reactor temperature of -210-245 °C. Depending on the reactivity, the temperature in the reactor may increase. Conversion is calculated after obtaining steady state conditions in the reactor, approximately 2-15%.

Inventive Polymer 4

A set-up consists of a multi-stage compressor, a continuously stirred tank reactor (CSTR), and a valve to control the pressure. The inner volume of the reactor, equipped with a stirrer, is approximately 100 ml. Electrical heating coils allow for heating of the reactor walls to a desired temperature prior to each experiment. Conversion is calculated as the average weight of polymer formed per time unit divided by the feed rates of the reactants.

The reactor is preheated to a temperature of approximately 213 °C. A flow of 2000 g ethylene per hour is added together with propylene and ethanol with a molar ratio of 11:1 propylene: ethanol in amounts sufficient to reach an MFR2 of 1-10 g/10 min. Comonomer, 1,7-octadiene, is added in amount sufficient to reach an average vinyl content of 0.35-0.60/1000C in the final polymer. If needed, a solvent e.g. Heptane, is used for diluting chain transfer agents and/or monomers. An initiator (Tert-Butyl peroxy-2-ethylhexanoate) is then introduced in sufficient amounts into the reactor until stable conditions are reached at a pressure of -200 MPa and an average reactor temperature of -210-245 °C.

Depending on the reactivity, the temperature in the reactor may increase. Conversion is calculated after obtaining steady state conditions in the reactor, approximately 2-15%.

The properties of the polymers of examples 1 and 2 are summarized in Table 1 Table 1

Inventive example 1 and 2 and comparative example 1 and 2

For inventive and comparative examples 1 and 2, thermoplastic plaques were prepared from the inventive and comparative examples as described above in section “Compression moulding of PE plaques for tan delta”. The thermoplastic plaques were used to determine the tan delta as described above in section “Tan delta measurement”. The results are presented in the Table 2.

Table 2: Tan delta measured on thermoplastic polymer samples

As can be seen in the Table 2 the inventive example results have a lower tan delta than the corresponding comparative example.

Inventive example 3 and 4

Inventive polymer 1 and 2 have been used to prepare inventive example 3 and 4 and comparative polymer 1 and 2 have been used to prepare comparative example 3 and 4, respectively. To the mentioned polymers 1.4 wt% dicumyl peroxide (CAS:80-43-3) has been added according to the method described above in “Addition of crosslinking agent”. Crosslinked plaques were prepared as described above in section “Crosslinking of plaques for tan delta measurement”. Tan delta on the crosslinked plaques were determined as described above in section “Tan delta measurements”. The results are presented in Table 3.

Table 3: Tan delta measured on crosslinked polymer samples

The results in table 3 demonstrate that there is an improvement or at least similar tan delta values when using methanol as CT A compared to using MEK as CTA. In addition, methanol is effective as it reduces fouling which decreases the down-time of the reactor and a more efficient production.

Example 5

After the production of a silane-containing copolymer rendering the reactor in a fouled state, LDPE homopolymer with MFR2= 2 g/10 min and a density of -921 kg/m³ was produced in the fouled tubular reactor. The peak reaction temperatures were in the range of 280-300 °C, the reactor pressure around 280 MPa and suitable feeds of propylene and methanol chain transfer agents were used to obtain the desired polymer properties. Ca 75 kg/h of methanol was injected during a period of 12 hours during which the average heat transfer coefficients increased by -46% (from 368 W/m2»K to 543 W/m2»K). The increase in average heat transfer coefficient is evidence that there is a substantial decrease in siloxane-bonded polymer layer adhered to the reactor walls.

Example 6

The LDPE polymer IP1, CPI, IP2 and CP2 were subjected to NMR analysis to identify the presence of hydroxyl and methoxy containing groups within the polymer architecture.

As can be observed in the table above there is a detectable amount of 1- methoxy and 1 -hydroxy groups in our inventive polymers but no such groups are detected in the comparative examples. Polymers therefore in which a Ci-s alkanol, e.g. methanol or ethanol, is used as the CTA have characteristic, detectable functional groups in their polymer architecture.

Claims

Claims:

1. A process for the preparation of a low density polyethylene (LDPE) homopolymer or LDPE copolymer by radical polymerisation, said process comprising polymerizing ethylene and optionally at least one comonomer in the presence of at least two chain transfer agents:

- a Ci-s-alkanol, and

- a non-polar chain transfer agent (CTA).

2. The process of claim 1 wherein the LDPE homopolymer or LDPE copolymer is produced in a high pressure process, such as at a pressure of at least 80 MPa.

3. The process according to any of the preceding claims, wherein the weight ratio of non-polar CTA to Ci-8-alkanol is 1:1 to 50:1, preferably 3:1 to 30:1.

4. The process according to any of the preceding claims, wherein the chain transfer agents are fed to the radical polymerisation together or separately and the feed ratio is

-2 to 50 wt% of Ci-8-alkanol and

-50 to 98 wt % of non-polar chain transfer agent (CTA), based on the combined amount of the feed of the Ci-8-alkanol and the non-polar chain transfer agent (CTA).

5. The process according to any of the preceding claims wherein the Ci-8- alkanol is a Ci-4-alkanol, such as a linear Ci-4-alkanol.

6. The process according to any of the preceding claims wherein the Ci-8- alkanol is primary or secondary.

7. The process according to any of the preceding claims wherein the Ci-s- alkanol is selected from n-propanol, i-propanol, n-butanol, isobutanol, octanol, ethanol and methanol, preferably methanol.

8. The process according to any of the preceding claims, wherein the non-polar chain transfer agent (CTA) is a straight chain, branched or cyclic, non-aromatic, hydrocarbyl.

9. The process according to any of the preceding claims, wherein the non-polar chain transfer agent (CTA) is selected from a cyclic, non-aromatic, alpha-olefin of 5 to 12 carbon atoms or a straight chain or branched chain alpha-olefin of 3 to 8 carbon atoms.

10. The process according to any of the preceding claims wherein the non-polar CTA is a straight or branched chain alpha-olefin of 3 to 6 carbon atoms, preferably propylene.

11. The process according to any of the preceding claims, wherein the non-polar chain transfer agent (CTA) is propylene and the alkanol is methanol.

12. The process according to any of the preceding claims, wherein the LDPE homopolymer or LDPE copolymer has a MFR2 of 0.25 to 10 g/lOmin determined by ISO1133 at 190°C and 2.16 kg and/or a vinyl group content of at least 0.25 vinyl groups per 1000 C atoms determined by ASTM D3124-98/ASTM D6248-98.

13. The process according to any of the preceding claims wherein the LDPE copolymer comprises at least one polyunsaturated comonomer wherein the polyunsaturated comonomer is a straight chain olefin with at least 8 carbon atoms, at least two non-conjugated double bonds, and wherein at least one of said double bonds is terminal, and there are at least 4 carbon atoms between the non-conjugated double bonds.

14. The process of any preceding claim wherein the LDPE copolymer is a LDPE copolymer of ethylene and 1,7-octadiene and optionally at least one further comonomer, such as an alkyl (meth)acrylate especially n-butyl acrylate.

15. The combined use of at least two chain transfer agents:

- a Cl -8 alkanol; and

- a non-polar chain transfer agent (CTA); to reduce fouling in a reactor during the preparation of an LDPE homopolymer or LDPE copolymer by radical polymerisation.

16. A LDPE homopolymer or LDPE copolymer with at least one polyunsaturated comonomer obtained by the process as defined in any of the preceding claims, said homopolymer or copolymer having an MFR2 in the range of 0.25 to 10 g/lOmin determined by ISO1133 at 190°C and 2.16 kg and a vinyl content of at least 0.25 vinyl groups per 1000 C atoms determined by ASTM D3124- 98/ASTM D6248-98 and a tan delta measured at room temperature (non-degassed, non-crosslinked) of 2.5 x 1 O'⁴ or less.

17. An LDPE homopolymer or LDPE copolymer with at least one polyunsaturated comonomer, said homopolymer or copolymer having an MFR2 in the range of 0.25 to 10 g/lOmin determined by ISO1133 at 190°C and 2.16 kg and a vinyl content of at least 0.25 vinyl groups per 1000 C atoms determined by ASTM D3124-98/ASTM D6248-98 and a tan delta measured at room temperature (nondegassed, non-crosslinked) of 2.5 x 10'⁴ or less; and wherein said LDPE homopolymer or LDPE copolymer contains at least 1.0 alkoxy groups per 10,000 carbon atoms, e.g.1.0 to 20 alkoxy groups per 10,000 carbon atoms, or 1.0 to 10 alkoxy groups per 10,000 carbon atoms, such as 1.0 to 5.0 alkoxy groups per 10,000 carbon atoms measured using proton NMR; and at least 3.0 hydroxyl groups per 10,000 carbon atoms, e.g. 3.0 to 20 hydroxyl groups per 10,000 carbon atoms or 3.0 to 15 hydroxyl groups per 10,000 carbon atoms, such as 3.0 to 10.0 hydroxyl groups per 10,000 carbon atoms measured using proton NMR.

18. A composition comprising the LDPE homopolymer or LDPE copolymer as claimed in claim 16 or 17 and a peroxide.

19. A cable, preferably power cable, comprising a conductor surrounded by one or more layers wherein at least one of said layer(s) comprises an LDPE homopolymer or LDPE copolymer as claimed in any of claims 16 to 17, wherein said layer is optionally crosslinked.