The field of the invention

The present invention refers to insulation material for use in MV or HV power transmission systems comprising a Ci_i ₀ -olefin polymer, a crosslink- er and a crosslinkable organic ion scavenger. The present invention also refers to use of a crosslinkable organic ion scavenger of formula I in insu- lation material for MV or HV power transmission systems and to the use of said insulation material. The invention further relates to a process for preparing said insulation material.

Background of the invention and prior art

Insulation for transmission systems, such as power cables, cable joints, bushes and the like, is important for the reliability of a transmission system. The reliability depends on the insulation material used for covering the conductor or conductor layers. Insulation materials for direct or alternating current (DC or AC) power cables may be exposed to high stresses. This is especially true for insulation materials used in medium (MV) and high voltage (HV) and extra/ultra high voltage (E/U HV) (hereinafter collectively referred to as MV and HV) systems. These insulation materials require a good combination of electrical, thermal and mechanical properties to provide a system having an optimal power transmission capacity.

Cross-linked low-density polyethylene (LDPE), abbreviated with XLPE or PEX, is widely used as insulation material for MV and HV cables. Cross- linking of polyethylene is usually carried out during extrusion in the manufacturing process by reaction of the polyethylene with a crosslinking agent such as dicumyl peroxide (DCP). This modification is required for improving thermo-mechanical properties since LDPE has a rather low melting point of around 115°C. With XLPE, it is possible to achieve a rated maxi- mum conductor temperature of 90°C and a 250°C short circuit rating as required for standard HVAC applications. Crosslinker concentrations of more than 1 wt%, usually between 1.0 and 1.5 wt%, are used in order to achieve a high degree of cross-linking as measured by a gel content of > 70%. Such high gel content is required for achieving the thermo-mechanical stability in the above mentioned MV and HV cable insulation systems.

The drawback of the use of crosslinked olefin polymers, such as DCP cross- linked polyethylene, is the generation of polar volatile by-products. These by-products and its changing concentration and concentration distribution over time may impair electrical insulation properties, such as space charge behavior and electrical conductivity at very high voltages, and limit the use of the material at such high voltages. Especially for DC applications for > 320 kV, low electrical conductivity is crucial in order to avoid a thermal runaway effect and pre-mature breakdown.

Aromatic structures such as so-called voltage stabilizers are known to improve electrical insulation performance (Thomas Hjertberg, V.E., Voltage stabilizers for XLPE cable insulation; 8th International Conference on Insu- lated Power Cables, Jicable 2011, and Timothy Person, B.N., Voltage stabilizing additive assessment in polyethylene insulation; 2011 8th International Conference on Insulated Power Cables, Jicable 2011). However, one drawback with such stabilizers is that these additives are not bonded to the polymer and therefore suffer from migration processes.

Recent work has been published on styrene based additives for XLPE with improved space charge performance (D. van der Born, et al., Evaluation of space charge accumulation processes in small size polymeric cable models, Annual Report Conference on Electrical Insulation and Dielectric Phenom- ena (CEIDP), 2012, pp. 669-672). However, it is characteristic for such styrene compounds to be monofunctional and allowing bonding to the polymer chain via only one reactive moiety.

JPH05314822 discloses an electrical wire used in a nuclear power plant containing a specific amount of highly cross-linked tetrafluoroethylene- ethylene copolymer as insulation material. A cross-linking assistant, such as triallyl cyanurate (TAC), is added to the tetrafluoroethylene-ethylene copolymer to improve the heat resistance and the radiation resistance of the electrical wire. JPS59199739 discloses an insulation tape comprising an insulation layer in which a specific thiobisphenol compound is added as an antioxidant to a composition containing a polyolefin and a peroxide-based crosslinker or crosslinking agent in an amount of 0.5 to 4 wt%. A compound such as tri- aryl cyanurate (TAC) or trimethylolpropane triacrylate (TMPT) in an amount of 0.5 to 3 wt% may be used as a crosslinking assistant in combination with the crosslinking agent.

WO 2011122742 discloses insulation material for HVDC cables comprising low density polyethylene crosslinked with dicumyl peroxide, magnesium oxide and an ion scavenger. The ion scavenger used is an aryl based silane, which does not have any crosslinking functionality.

WO 2012150285 discloses insulation material for HVDC cables comprising an inorganic ion scavenger that does not have crosslinking functionality.

Many known transmission systems have a limited power transmission capacity due to limiting voltage that can be used in the transmission system and limiting working temperature. An increase in working temperature impairs the insulation material in the insulation layer, which impacts the durability of the transmission system. Repair of cables and the like, especially cables at the bottom of a sea or ocean, is costly and should preferably be prevented.

Although many improvements have been made to insulation materials for transmission systems, there is still a need for improving the electrical performance of transmission systems that can be used for high voltages above 320 kV. The use of halogen products should preferably be prevented. Migration problems caused by substances such as inorganic ion scavengers or magnesium oxide should also be prevented. Summary of the invention

The object of the present invention is to provide a transmission system having insulation material in an insulation layer that overcomes the prob- lems mentioned above. One object is to provide a transmission system comprising insulation material that can be used in MV and HV systems in order to transmit power with high capacity over long distances. Another object is to improve the reliability of transmission systems and to decrease aging and manufacturing costs for insulated transmission systems. A further object is to provide insulation material that can handle a higher working temperature, for example a temperature of up to about 80°C. One object is to provide a transmission system that has an improved power transmission capacity, whereby beside the higher working temperature, also the breakdown strength and electrical field stress distribution of the insulation material can be improved.

The objects are achieved by an insulation material for use in MV and HV power transmission systems comprising

- at least one Ci_i ₀ -olefin polymer in an amount of up to 99 wt% of the total weight of the insulation material,

- a crosslinker in an amount between 0.01 and 0.6 wt% of the total weight of the insulation material, and

- a crosslinkable organic ion scavenger of formula I

wherein:

A is an aromatic cycle or aromatic heterocycle comprising one or more rings, whereby the heterocycle comprises one or more atoms selected from O, N and S,

n represents the number of substituents on cycle A, wherein n is 1 to 10, L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o- ₂ , -(CH ₂ )o- ₂ -S-(CH ₂ )o- ₂ , -(CH ₂ ) _0-2 -N-(CH ₂ ) _0-2 , -(CH ₂ ) _0-2 - C(0)-(CH ₂ )o-2, -(CH ₂ )o- ₂ -0-C(0)-(CH ₂ )o-2 and -(CH ₂ ) ₀ - ₂ -C(O)-O-(CH ₂ ) ₀ - ₂ , and

t is 0 to 10,

in an amount between 1 and 30 wt% of the total weight of the insulation material, and

whereby the weight ratio between the crosslinker : the crosslinkable organic ion scavenger is between 1 : 7 and 1 : 300.

The objects are also achieved by the insulation material for use in MV and HV power transmission systems comprising

- at least one Ci_i ₀ -olefin polymer in an amount of up to 99 wt% of the total weight of the insulation material,

- a crosslinker in an amount between 0 and 0.6 wt% of the total weight of the insulation material, and

- a crosslinkable organic ion scavenger of formula I

wherein:

A is an aromatic cycle or aromatic heterocycle comprising one or more rings, whereby the heterocycle comprises one or more atoms selected from O, N and S,

n represents the number of substituents on cycle A, wherein n is 1 to 10,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o-2, -(CH ₂ )o- ₂ -S-(CH ₂ )o- ₂ , -(CH ₂ ) ₀ - ₂ -N-(CH ₂ ) ₀ - ₂ , -(CH ₂ ) _0-2 - C(0)-(CH ₂ )o- ₂ , -(CH ₂ )o _-2 -0-C(0)-(CH ₂ ) _0-2 and -(CH ₂ ) _0-2 -C(O)-O-(CH ₂ ) _0-2 , and

t is 0 to 10, in an amount between 1 and 30 wt% of the total weight of the insulation material, and

whereby the at least one Ci_i ₀ -olefin polymer is free of fluoro and the insulation material is free of antioxidant, or

whereby the at least one Ci_i ₀ -olefin polymer is free of fluoro and the crosslinkable organic ion scavenger is not triaryl cyanurate (TAC) or trime- thylolpropane triacrylate (TMPT).

In one embodiment the insulation material comprises

- at least one Ci_i ₀ -olefin polymer in an amount of up to 99 wt% of the total weight of the insulation material,

- a crosslinker in an amount between 0.01 and 0.6 wt% of the total weight of the insulation material, and

- a crosslinkable organic ion scavenger of formula I

wherein:

A is benzyl or an aromatic heterocycle comprising one or more atoms selected from O, N and S,

n represents the number of substituents on cycle A, wherein n is 1, 2 or 3,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o-2, -(CH ₂ )o- ₂ -S-(CH ₂ )o- ₂ , -(CH ₂ ) ₀ - ₂ -N-(CH ₂ ) ₀ - ₂ , -(CH ₂ ) _0-2 - C(0)-(CH ₂ )o- ₂ , -(CH ₂ )o- ₂ -0-C(0)-(CH ₂ )o-2 and -(CH ₂ ) ₀ - ₂ -C(O)-O-(CH ₂ ) ₀ - ₂ , and

t is 0, 1 or 2,

in an amount between 1 and 30 wt% of the total weight of the insulation material.

In one embodiment the use of the insulation material is limited to high voltage or ultra high voltage transmission systems. The insulation material of the invention has improved electrical insulation properties including lower electrical conductivity and improved space charge behavior compared to known MV and HV insulation materials. This new insulation material is produced with a minimum amount of cross- linker. The crosslinker is used to initiate the forming of free radicals.

The crosslinkable organic ion or electron scavenger (XIES) of formula I has crosslinking functionality with multifunctional moieties containing termi- nal reactive double bonds. Such terminal double bonds enable XIES to function as additional cross-linking points in the formed crosslinked olefin polymer, such as XLPE, and therefore increases the cross-linking density. The polymer chains become interconnected. Therefore, despite the use of low concentration of the crosslinker, high degrees of crosslinking with a gel content (> 70 %) are achieved. The new insulation material has less polar by-products and the electrical insulation properties are improved. The crosslinkable organic ion scavenger of formula I provides excellent electrical insulation properties. Besides the benefit of reduced crosslinker concentration, the specific structure of the XIES of formula I with the ar- omatic core as "electron scavenger unit" allows further property enhancement. Because XIES also scavenges ions, the addition of an antioxidant, such as thiobisphenol, is no longer needed. This also makes the insulation material more environmental friendly and decreases the time and costs for manufacturing the insulation material.

Therefore, XIES of formula I cover several technical aspects of improving the insulation material in terms of space charge behavior and electrical conductivity, i.e. increase of cross-linking density via multiple reactive sides of XIES for the olefin polymer chains, reduction of concentration of cross-linker and therefore reduction of generated (polar) by-products, improving electrical properties, and local bonding by incorporation of electron scavenger units in the olefin polymer network and thus preventing migration processes. The new insulation material can be used at voltages over 300kV. The Ci-io-olefin polymer does not, or does preferably not contain halogen atoms, such as fluoro. This improves the electrical, thermal and mechanical properties of the insulation material and prevents pollution of the environment with halogens.

In another embodiment the Ci_i ₀ -olefin polymer is a Ci_ ₄ -olefin polymer. In one embodiment the Ci_ ₄ -olefin polymer is polyethylene selected from very low density polyethylene, low density polyethylene, medium density polyethylene and high density polyethylene. In a further embodiment the Ci_ ₄ -olefin polymer is low density polyethylene.

In one embodiment the crosslinker is an organic peroxide-based crosslinker of formula R-O-O-R' or R-(C=0)-0-0-C(=0)-R', wherein R and R' independently of each other are selected from the group comprising C ₆ -io-aryl, C ₆ -io-cycloalkyl, Ci_i ₀ -alkyl and hydrogen.

In another embodiment the organic peroxide-based crosslinker is selected from the group comprising C ₆ -io-diaryl peroxide, Ci_i ₂ -diacyl peroxide, Ci_ i2-dialkyl peroxide, peroxy Ci_i ₂ -dialkylester and peroxy Ci_i ₂ -dialkylketal. In a further embodiment the crosslinker is dicumyl peroxide.

In another embodiment the Ci_i ₀ -olefin polymer is low density polyethylene in an amount up to 99 wt% of the total weight of the insulation material,

- the crosslinker is dicumyl peroxide in an amount between 0.01 and

0.6 wt% of the total weight of the insulation material, and - the crosslinkable organic ion scavenger is a compound of formula I

wherein: A is benzyl or an aromatic heterocycle comprising one or more atoms selected from 0, N and S,

n represents the number of substituents on cycle A, wherein n is 1, 2 or 3,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o- ₂ , -(CH ₂ )o- ₂ -S-(CH ₂ )o- ₂ , -(CH ₂ ) ₀ - ₂ -N-(CH ₂ ) ₀ - ₂ , -(CH ₂ ) _0-2 - C(0)-(CH ₂ )o-2, -(CH ₂ )o- ₂ -0-C(0)-(CH ₂ )o-2 and -(CH ₂ ) ₀ - ₂ -C(O)-O-(CH ₂ ) ₀ - ₂ , and

t is 0, 1 or 2,

in an amount between 1 and 30 wt% of the total weight of the insulation material.

In one embodiment the crosslinkable organic ion scavenger is a compound of formula I, wherein:

A is a di- or tri-substituted benzyl or triazinyl,

L is -0-CH ₂ or -C(0)-0-CH ₂ , and

t is 0 or 1.

In another embodiment t is 1. In a further embodiment t is 0. In one embodiment A is benzyl, n is 2 or 3, L is -C(0)-0-CH ₂ and t is 1. In a further embodiment A is benzyl, n is 2, L is-0-CH ₂ and t is 1. In yet another further embodiment A is benzyl, n is 2, L is none (t is 0). In yet a further embodiment A is triazinyl, n is 3, L is-0-CH ₂ and t is 1.

In another embodiment the crosslinkable organic ion scavenger is a com- pound of formula la

or a compound of formula lb

or a compound of formula Ic

or a compound of formula Id

or a compound of formula le

In a further embodiment the Ci_i ₀ -olefin polymer is low density polyethylene in an amount up to 99 wt% of the total weight of the insulation material,

- the crosslinker is dicumyl peroxide in an amount between 0.01 and 0.6 wt% of the total weight of the insulation material, and

n scavenger is a compound of formula la

in an amount between 1 and 30 wt% of the total weight of the insulation material, and whereby the weight ratio between the crosslinker : the crosslinkable organic ion scavenger is between 1 : 7 and 1 : 300.

In a further embodiment the Ci_i ₀ -olefin polymer is low density polyeth- ylene in an amount up to 99 wt% of the total weight of the insulation material,

- the crosslinker is dicumyl peroxide in an amount between 0 and 0.6 wt% of the total weight of the insulation material, and

ion scavenger is a compound of formula la

in an amount between 1 and 30 wt% of the total weight of the insulation material, and

whereby the at least one Ci_i ₀ -olefin polymer is free of fluoro and the insulation material is free of antioxidant. In one embodiment the at least one Ci-io-olefin polymer is free of fluoro and the insulation material is free of thiobisphenol.

In one embodiment the amount of crosslinker is between 0.01 and 0.2 wt% of the total weight of the insulation material. In a further embodi- ment the amount of crosslinker is between 0.01 and 0.1 wt%. In yet another embodiment the amount of crosslinker is about 0 wt%.

Only low amounts of crosslinker need to be used to initiate the crosslink- ing. This improves the electrical, thermal and mechanical properties of the insulation material.

In another embodiment the amount of crosslinkable organic ion scavenger is between 1.5 and 2.5 wt% of the total weight of the insulation material. In one embodiment the amount of crosslinkable organic ion scavenger is between 1.8 and 2.2 wt%. In a further embodiment the amount of cross- linkable organic ion scavenger is about 2 wt%. The crosslinkable organic ion scavenger can also be used in relative low amounts to obtain a dense crosslinked olefin polymer. This improves the costs for the manufacturing process and makes the insulation material more environmentally friendly.

In one embodiment the weight ratio between the crosslinker : the cross- linkable organic ion scavenger is between 1 : 7 and 1 : 12. In a further embodiment the weight ratio is about 1 : 10.

Low concentrations of crosslinker and crosslinkable organic ion scavenger can be used when these are used in the defined ratio. This provides for insulation material with improves electrical, thermal and mechanical properties. This also reduced the manufacturing costs.

In another embodiment the MV and HV power transmission systems is selected from the group comprising cables, joints, bushings, insulated bushes, bus bars and (cable) terminations. The invention also relates to a semiconductive material, which comprises the insulation material as defined above, and which further comprises one or more conductive fillers, and whereby the semiconductive material exhibits a conductivity of more than 10 ⁵ S/m at 25 °C. In another embodiment the semiconductive material exhibits a conductivity of more than 10 ⁶ S/m at 25 °C. In one embodiment the conductive filler is carbon black. In one embodiment, the amount of filler is between 1 and 10 wt% of the total weight of the insulation material.

The invention also relates to a use of the crosslinkable organic ion scaven- ger, as defined above, in insulation material for MV and HV power transmission systems.

One embodiment relates to a use of the insulation material, as defined above, in MV and HV power transmission systems. A further embodiment relates to a use of the semiconductive material, as defined above, in MV and HV power transmission systems. Another object of the invention is to provide a process for preparing insulation material for MV and HV power transmission systems, as defined above, whereby the process comprises the following steps:

a) feeding a mixture of the Ci_i ₀ -olefin polymer, the crosslinker and the crosslinkable organic ion scavenger, as defined above, in an extruder at a feeding speed between 50 and 800 kg/h, and at a temperature above a melting temperature of the Ci_i ₀ -olefin polymer;

b) molding the mixture circumferentially on an object selected from a conductor, an inner screening semiconducting layer, insulation material and an outer screening semiconducting layer;

c) cooling the molded material at a temperature below the melting temperature of the Ci-io-olefin polymer; and

d) collecting the obtained product at a collecting speed between 1 and 30 m/min.

In another embodiment further additives may be added to the mixture in step a). In one embodiment of the process, the process comprises preparation steps prior to feeding step a), comprising;

1) premixing the Ci_i ₀ -olefin polymer, and optionally the crosslinker and the crosslinkable organic ion scavenger, as defined above, in a compounder extruder at a temperature above the melting temperature of the Ci_i ₀ - olefin polymer;

2) cooling and pelletizing the obtained mixture; and

3) drying the mixture at a temperature below the melting temperature of the Ci-io-olefin polymer, optionally at reduced atmospheric pressure. In another embodiment further additives may be added to the premixture of step 1).

One advantage of the use of only low concentrations of crosslinker is that an additional de-gassing step is no longer needed. This improves the time and costs for the manufacturing process. In another embodiment of the process the Ci_i ₀ -olefin polymer is low density polyethylene.

The invention also relates to insulation material for MV and HV power transmission systems prepared by the process defined above.

Furthermore, the invention relates to a MV and HV power cable comprising concentrically arranged:

- an elongate conductor,

- a first semiconducting layer circumferentially covering the conductor,

- a first layer of insulation material, as defined above, circumferentially covering the first semiconducting layer, and

- a second semiconducting layer circumferentially covering the first layer of insulation material,

- a second layer of insulation material, optionally as defined above, circumferentially covering the second semiconducting layer, and

- optionally a jacketing layer and armor covering the outer wall of the second layer of insulation material. Breakdown strength is improved by increasing the density of the insulation material. This further improves the reliability of the transmission system and prevents space charging and aging.

The new transmission system is more reliable and robust and therefore expected to last longer than the transmission systems used today.

Brief description of the drawings The invention will now be explained more closely by means of a description of various embodiments and with reference to the drawings attached hereto.

Fig 1 shows a flow scheme of the preparation process.

Fig 2 shows an illustration of an MV and HV cable. Detailed description of various embodiments of the invention

The insulation material of the present invention may be used in any direct or alternating current (DC or AC) power transmission system or system components such as a power cable. Other transmission systems or system components may be a cable joint. The transmission systems may also be bushings, insulated bushes, bus bars and cable terminations. Further transmission systems or system components may be any electrical device that has insulation. The insulation material of the present invention is es- pecially suitable for use in medium, high and ultra high voltage DC (MVDC and HVDC) transmission systems, preferably high voltage or ultra high voltage DC transmission systems or system components. The high voltage may be a voltage of 300kV or more. A typical transmission system as shown in Fig 2 comprises a conductor 1 or a bundle of conductors extending along a longitudinal axis, which is cir- cumferentially covered by an insulation layer 3 comprising insulation material. The insulation layer 3 may be covered by a sheath 5. For some transmission systems, such as HVDC cables, the conductor 1 may be circumferentially covered by an inner or first semiconductive layer 2, which layer is then covered by the insulation layer 3. The insulation layer 3 may be circumferentially covered by an outer or second semiconductive layer 4. The outer semiconductive layer 4 may be covered by a sheath 5, which may be lead or another metal. This sheath may be further covered by a protection layer 6 that may also have insulation and mechanical properties such as a plastic or rubber material.

The new insulation material comprises at least one Ci_i ₀ -olefin polymer, a crosslinker and a crosslinkable organic ion scavenger. In one embodiment the insulation material consists at least one Ci_i ₀ -olefin polymer, a cross- linker and a crosslinkable organic ion scavenger.

The new insulation material may further comprise additives such as anti- oxidants, filler and the like. Examples may be generic antioxidants with a primary and secondary antioxidant function, such as hindered phenols. In one embodiment no antioxidants are used. In another embodiment the insulation material does not comprise thiobisphenol. Examples of other additives may be stabilizers, ion scavengers, lubricants, scorch retarding agents, fillers and the like. The filler may be micro- or nano-fillers, i.e. fillers with an average particle diameter in nano-meters or micrometers. Examples of such fillers are polyhedral oligomeric silsesquioxanes (POSS), or metal oxides such as oxides, dioxides or trioxides of calcium, zinc, silicon, aluminium, magnesium and titanium. Other filler are CaC0 ₃ and nanoclay. Mixtures of one or more fillers may also be used. Preferred fill- ers are polyhedral oligomeric silsequioxanes (POSS ), MgO, SiOi_ ₂ , Al ₂ 0 ₃ , Ti0 ₂ , CaO, CaC0 ₃ and nanoclay, or mixtures thereof. Another preferred filler is silicon dioxide. The fillers may be crystalline or amorphous or mixtures thereof. Preferably, the fillers are amorphous. For the preparation of semiconductive material a filler such as carbon black is added.

One or more Ci_i ₀ -olefin polymer may be used. The denotation 1-10 includes methylene, ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene and mixtures thereof. One, two or three Ci_i ₀ -olefin polymer may be used. Examples may be selected from polypropylene (PP), biaxially oriented polypropylene (BOPP), high density polyethylene (HDPE), cross-linked polyethylene (XLPE), moderate density PE (MDPE) and low density polyethylene (LDPE), or mixtures thereof. The preferred olefin is polyethylene selected from very low density polyethylene, low density polyethylene, medium density polyethylene and high density polyethylene. The most preferred Ci_i ₀ -olefin polymer is low density polyethylene. The Ci_i ₀ -olefin polymer preferably does not contain halogen atoms, such as fluoro.

The crosslinker may be an azo-based crosslinker. The crosslinker may also be a peroxide-based crosslinker. The crosslinker may be an organic peroxide-based crosslinker of formula R-O-O-R' or R-(C=0)-0-0-C(=0)-R'. R and R' may be the same or different. Examples of R and R' groups may be C _6- io- aryl, such as benzyl or naphthalene, C ₆ -io-cycloalkyl, such as cyclohexane or cyclohexanone, Ci_i ₀ -alkyl, such as ethylene, n-propylene, i-propylene, n-butylene, sec-butylene. R and R' groups may be hydrogen. The organic peroxide-based crosslinker may be selected from the group comprising C ₆ -io-diaryl peroxide, Ci_i ₀ -diacyl peroxide, Ci_i ₀ -dialkyl peroxide, peroxy Ci_i ₀ -dialkylester and peroxy Ci_i ₀ -dialkylketal. Specific examples may be dicumyl peroxide (DCP), acetyl acetone peroxide, benzoyl peroxide, cyclohexanone peroxide and dipropyl peroxide.

The crosslinker may be used in an amount between 0 and 0.2 wt% of the total weight of the insulation material. Preferably the minimum amount needed to initiate crosslinking is used. The amount of crosslinker may be about 0 wt%, or between 0.001 and 0.19 wt%, or between 0.015 and 0.15 wt%, or between 0.05 and 0.199 wt%, or between 0.09 and 0.19 wt%, or about 0.2 wt%, or less then 0.19 wt%.

The crosslinkable organic ion scavenger has crosslinking functionality. The crosslinkable organic ion scavenger may a compound of formula I

wherein:

A is an aromatic cycle or aromatic heterocycle comprising one or more rings, whereby the heterocycle comprises one or more atoms selected from O, N and S,

n represents the number of substituents on cycle A, wherein n is 1 to 10,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o-2, -(CH ₂ )o- ₂ -S-(CH ₂ )o- ₂ , -(CH ₂ ) ₀ - ₂ -N-(CH ₂ ) ₀ - ₂ , -(CH ₂ ) _0-2 -

C(0)-(CH ₂ )o- ₂ , -(CH ₂ )o- ₂ -0-C(0)-(CH ₂ )o-2 and -(CH ₂ ) ₀ - ₂ -C(O)-O-(CH ₂ ) ₀ - ₂ , and

t is 0 to 10. represents the number of substituents on cycle A. Depending on the mber of rings comprised in A, n may be 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10. Preferably n is 1, 2, 3 or 4. More preferably n is 1, 2 or 3. Even more preferably n is 2 or 3.

A is an aromatic cycle or an aromatic cyclic ring comprising at least five atoms selected from C, 0, N and/or S and A contains conjugated planar ring systems with delocalised pi electron clouds instead of discrete alternating single and double bonds. Typical aromatic compounds are benzene and toluene. A may be a monocycle or a polycycle, i.e. A may comprise 1, 2, 3, 4 or 5 rings, whereby none, some or all rings comprise heteroatoms. Examples of A are, but not limited thereto, naphthalene, benzyl, triazinyl, aziridinyl, pyrrolidinyl, pyrrolidino, piperidinyl, piperidino, piperazinyl, piperazino, morpholinyl, morpholino, thiomorpholinyl, thiomorpholino, tetrahydrofuranyl, tetrahydrothiofuranyl, tetrahydropyranyl, and pyranyl. When A is a single ring, A may be di-substituted at the ortho- or para- position, or A may be tri-substituted at the meta-position. In one embodiment A is a di or tri-substituted aromatic heterocycle and comprises one or three N atoms together with the spacer L defined above. In another embodiment A is a di or tri-substituted benzyl together with the spacer L defined above.

A preferred spacer L may be selected from the group comprising -CH ₂ -, C ₂ H ₄ -, -0-(CH ₂ )o- ₂ , -C(0)-(CH ₂ )o- ₂ , -O-C(O)-(CH ₂ ) _0-2 and -C(O)-O-(CH ₂ ) _0-2 , whereby the denotation 0-2 means that no methyl-group or 1 or 2 methyl- group is present in the spacer. t may be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10. Preferably t is 0, 1, 2 or 3. More preferably t is 0 or 1. In one embodiment the crosslinkable organic ion scavenger does not have a spacer because t is 0.

The present invention includes all combinations of all possible substitu- ents for compounds of formula I that fall within the definition. Examples of combinations, but not limited thereto, may be a compound of formula I wherein:

A is benzyl or an aromatic heterocycle comprising one or more atoms selected from O and N, n is 2 or 3,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o- ₂ , -(CH ₂ )o- ₂ -C(0)-(CH ₂ )o- ₂ , -(CH ₂ ) ₀ - ₂ -O-C(O)-(CH ₂ ) ₀ - ₂ and -(CH ₂ )o-2-C(0)-0-(CH ₂ )o-2, and

t is 0 or 1,

A is benzyl or an aromatic heterocycle comprising one or more atoms selected from N,

n is 2 or 3,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o- ₂ , -(CH ₂ )o- ₂ -C(0)-(CH ₂ )o- ₂ , -(CH ₂ ) ₀ - ₂ -O-C(O)-(CH ₂ ) ₀ - ₂ and -(CH ₂ )o-2-C(0)-0-(CH ₂ )o-2, and

t is 0 or 1,

A is benzyl,

n is 2 or 3,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o- ₂ , -(CH ₂ )o- ₂ -C(0)-(CH ₂ )o-2, -(CH ₂ ) ₀ - ₂ -O-C(O)-(CH ₂ ) ₀ - ₂ and -(CH ₂ )o-2-C(0)-0-(CH ₂ )o-2, and

t is 0 or 1,

A is an aromatic heterocycle comprising one or more atoms selected from N,

n is 2 or 3,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o- ₂ , -(CH ₂ )o- ₂ -C(0)-(CH ₂ )o- ₂ , -(CH ₂ ) ₀ - ₂ -O-C(O)-(CH ₂ ) ₀ - ₂ and -(CH ₂ )o-2-C(0)-0-(CH ₂ )o-2, and

t is 0 or 1,

A is di- or tri-substituted benzyl or triazinyl,

L is a spacer selected from the group comprising -CH ₂ -, C ₂ H ₄ -, - (CH ₂ )o- ₂ -0-(CH ₂ )o-2, -(CH ₂ )o- ₂ -C(0)-(CH ₂ )o-2, -(CH ₂ ) ₀ - ₂ -O-C(O)-(CH ₂ ) ₀ - ₂ and -(CH ₂ )o-2-C(0)-0-(CH ₂ )o-2, and

t is 0 or 1,

A is a di- or tri-substituted benzyl or triazinyl, L is -O-CH2 or -C(0)-0-CH ₂ , and

t is 0 or 1.

cavenger may be a compound of formula la nurate (TAC).

The crosslinkable organic ion scavenger may be a compound of formula lb

The crosslinkable organic ion scavenger may be a compound of form venger may be a compound of form

The crosslinkable organic ion scavenger may be a compound of formula le Any one of the crosslinkable organic ion scavengers defined above may be used in insulation material for MV and HV power transmission systems. Any of these crosslinkable organic ion scavengers mentioned above may be used in combination with any Ci_i ₀ -olefin polymer and any crosslinker. Examples may be a combination of any of these crosslinkable organic ion scavengers in combination with LDPE and DCP. In one embodiment the crosslinkable organic ion scavenger is triallyl cyanurate (TAC).

In another embodiment the insulation material does not comprise an inorganic crosslinkable organic ion scavenger. In another embodiment no magnesium oxide is used. In a further embodiment the crosslinkable organic ion scavengers is not triaryl cyanurate (TAC) or trimethylolpropane triacrylate (TMPT).

The amount of crosslinkable organic ion scavenger may be between 1 and 30 wt% of the total weight of the insulation material. The amount must be enough to provide for a crosslinked polymer with a desired density that allows the insulation material to be used in MV and HV transmission systems. The amount of crosslinkable organic ion scavenger may be between 1 and 15 wt%, or between 0.1 and 10 wt%, or between 1 and 10 wt%, or between 1 and 5 wt%, or between 1.5 and 3 wt%, or about 2 wt%. The weight ratio between the crosslinker and the crosslinkable organic ion scavenger may be important to further optimize the crosslinking of the olefin polymer. The weight ratio of crosslinker : crosslinkable organic ion scavenger is between 1:7 and 1:300, or between 1:7 and 1:100, or between 1:6.5 and 1:25, or between 1:7 and 1:15, or between 1:8 and 1:12, or about 1:10.

Although the amount of 0.01 and 0.6 wt% crosslinker together with an amount of 1 to 30 wt% ion scavenger may comprise other weight ratios as well, one embodiment of the invention is limited to the weight ratios mention above. The insulation material may be prepared by extruding the material.

A first step in such a process may be feeding a mixture of the Ci_i ₀ -olefin polymer, the crosslinker and the crosslinkable organic ion scavenger in an extruder. The feeding speed may be between 50 and 800 kg/h, or between 200 and 500 kg/h. The temperature of the mixture may be between 40°C below or 60°C above the melting temperature of the mixture. The temperature may be above a melting temperature of the Ci_i ₀ -olefin polymer. The exact temperature will depend on the specific ingredients used. Examples of suitable temperatures may be a temperature between 70 and 220°C. The cross-linker starts reacting at a potential mixing temperature.

One or more additives may be added to the mixture. The extruder barrel may for example have a temperature between 70 and 220°C.

Any combination of any olefin polymer and any crosslinker and any cross- linkable organic ion scavenger in any of the amounts mentioned above may be used in the process.

The melted mixture is moulded in a next process step. Hereby, the moulded material is extruded to circumferentially cover an object such as a conductor, an inner screening semiconducting layer, an outer screening semi- conducting layer, or other insulation material.

The molded material can be cooled in the subsequent process step at a temperature below the melting temperature of the Ci_i ₀ -olefin polymer and -20°C. Examples of cooling temperatures may be a temperature be- tween 15 and 100°C. The pressure in the extrusion tube may be between 100 and 1000 kPa.

The extruded and cooled product may be collected at a speed between 1 and 30 m/min, or between 5 and 20 m/min. The collecting speed may de- pend on the thickness of the insulation material. In a next step, the insulation layer may be circumferentially covered by a sheath, or optionally a semiconductive layer and then a sheath.

The process may start with a pre-preparation step of premixing the Ci_i ₀ - olefin polymer, the crosslinker and the crosslinkable organic ion scavenger, and optionally additives in a compounder extruder. The temperature of the extruder may be between 40°C below or 60°C above the melting temperature of the mixture. The exact temperature will depend on the specific ingredients used. Examples of suitable temperatures may be a temperature between 70 and 220°C.

In a next preparation step, the melted mixture is cooled to a temperature below the melting temperature of mixture. Examples of cooling temperatures may be a temperature between 80 and -20°C.

The cooled material can subsequently be pelletized and dried at a temperature below the melting temperature of the mixture. Examples of drying temperatures may be a temperature between 40 and 80°C. Drying may be performed under reduced atmospheric pressure, i.e. below about 100 kPa, in order to accelerate the drying step.

Alternatively, only the Ci_i ₀ -olefin polymer is pelletized, or only the Ci_i ₀ - olefin polymer and the crosslinkable organic ion scavenger are pelletized. Another example of a preparation process may be

1) mixing Ci_i ₀ -olefin polymer and a crosslinkable organic ion scavenger and optionally additives in a compounder extruder as described above,

2) cooling and pelletizing the obtained mixture, as described above,

3) drying the pellets as described above, and

4) diffusion a cross-linker during extrusion of the mixture at a temperature between 60 and 90°C, optionally at increased pressure,

a) via a gas phase, or

b) via a liquid phase using a solvent and then drying the obtained product.

Yet another example of a preparation process may be 1) mixing Ci_i ₀ -olefin polymer and optionally additives in a compounder extruder as described above,

2) cooling and pelletizing the obtained mixture, as described above,

3) drying the pellets as described above, and

4) diffusion a cross-linker and a crosslinkable organic ion scavenger into the polymer pellets during extrusion at a temperature between 60 and 90°C, optionally at increased pressure,

a) via a gas phase, or

b) via a liquid phase using a solvent and then drying the obtained prod- uct.

Examples of solvents are organic solvents such as propylene, hexane, heptane and the like. The increased pressure may a pressure over 150 kPa, or a pressure between 150 and 2000 kPa.

The insulation material according to the invention is also suitable for use as a base for the manufacture of semiconductive material such as material used in the semiconductive layers of a transmission system 2, 4. This semiconductive material can be prepared by the addition of a semiconductive filler, such as carbon black, to the mixture in the first process step in any of the processes described above.

Experiments

LDPE powder (0-300 μιη particle size) was mixed vigorously with DCP and TAC powder (various concentrations). The mixture (ca. 30 g) was then placed in a hot press using steel mold in order to manufacture a plate of 180 x 180 x 1 mm plate. The material was pressed at 180 °C with approximately 180 kPa (180 bar) for 15 minutes.

The gel content - reflecting the degree of cross-linking density - was de- termined by Soxhlet extraction in para-xylene for 14 hours and determination a sample of 1 g (cut from the plate).

DC conductivity was determined at 70 °C and 20 kV/mm on 1 mm thick plates with measurements taken after 24 and 100 hours. The results are shown in the table below. Table 1. DC conductivity

The results from XPLEl clearly show that the conductivity decreases when the crosslinker and the organic ion exchanger are added in the amounts specified in the new insulation material of the invention.

The term "conductor" as used herein, means a conductor or a superconductor, which may be one or more conductors bundled together.

The wording "between" as used herein includes the mentioned values and all values in between these values. Thus, a value between 1 and 2 mm includes 1 mm, 1.654 mm and 2 mm.

The wording "low density", as in low density polyethylene, as used herein means densities between 0.80 and 0.97 g/cm ³ , preferably between 0.90 and 0.93 g/cm ³ .

The wording "medium density", as in medium density polyethylene, as used herein means densities between 0.85 and 0.91 g/cm ³ , preferably above 0.92 and below 0.94g/cm ³ .

The wording "high density", as in high density polyethylene, as used herein means densities between 0.90 and 0.999 g/cm ³ , preferably above 0.935 and below 0.97 g/cm ³ .

The wording "transmission system" as used herein is meant to include all and any applications for insulation materials such as for example cables, joints, terminations, bushings, insulated bushes, bus bars and in semiconducting screening material together with acetylene carbon black.

The wording "MV and HV" as used herein is meant to include medium, high voltage and ultra high voltage in direct current or alternating current systems. The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims.