WO2014117809A1

WO2014117809A1 - New insulation materials for use in high voltage power applications and a new process for preparing said insulation material

Info

Publication number: WO2014117809A1
Application number: PCT/EP2013/051651
Authority: WO
Inventors: Gustavo Dominguez; Andreas FRIBERG; Anneli JEDENMALM
Original assignee: Abb Research Ltd
Priority date: 2013-01-29
Filing date: 2013-01-29
Publication date: 2014-08-07

Abstract

The present invention relates to insulation material for use in HV power applications comprising a mixture of amount between 0 and 95 wt% of a first component comprising a Ci-8 olefin polymer, and between 5 and 100 wt% of a second component comprising an ionomer The ionomer is a copolymer of a C_1-8 olefin polymer and a partially neutralized acrylic acid and/or methacrylic acid co-monomer, or anhydride form thereof, comprising between 1 and 20 wt% co-monomer, a multivalent metal salt and a neutralization degree between 1 and 100%. The total amount of C_1-8 olefin polymer from the first and second component is between 80 and 99.5 wt% of the total weight of the insulation material. The insulation material has a thickness of at least 3 mm. The invention also relates to preparation processes and insulation material, HV or extra HV power applications prepared by said process.

Description

Title : New insulation materials for use in high voltage power applications and a new process for preparing said insu lation material.

THE FI ELD OF THE I NVENTION

The present i nvention relates to insulation material suitable for use in high voltage or extra high voltage direct or alternating current (DC or AC) power applications according to claim 1 . The invention also relates to a process for preparing insulation material for high voltage (HV) or extra high voltage (EHV) power applications according to claim 12.

BACKGROU ND OF THE I NVENTION AND PRIOR ART

Insulation material for power applications is exposed to high stresses. This is especially true for insulation material used in high voltage and extra high voltage (hereinafter collectively referred to as HV) power systems. This insulation material requires a good combination of electrical , thermal and mechanical properties. Preferably, the material has low conductivity, low space charge distribution and strong breakdown strength .

Insulation materials are generally prepared by extruding one or more polymers together with additives such as crosslinkers in an extruder at or above a melting temperature of the polymers. The extruded product is subsequently cooled to room temperature. De-gassing and heat treatment must often be performed during extrusion to prevent or remove by-products from the i nsulation material .

Much research has been performed to improve the quality of insulation material and to improve the preparation process. US 4,956,414 descri bes a process for preparation of neutralized ethylene/carboxylic acid copolymers in a high pressure extruder in the absence of a solvent. The product comprises 10-90% neutralized copolymer of crosslinked ethylene/carboxylic acid that is neutralized with a metal salt, such as zinc salt. The metal salt is added as a metal salt of carboxyl-contai ning ethylene copolymer, ethylene graft copolymer or ethylene terpolymer during a neutralization step in the process. The aim of the new process is to prepare i nsulation material without the use of a solvent in order to prevent corrosion problems i n the extruder. The material is obtained as a film , which is speck-free and has good strength properties, transparency and adhesion . US 8,212, 148 describes insulation materials for power cables whereby one of the layers of insulation material comprising a blend of at least two copolymers, whereby both copolymers comprise a blend of an apolar polymer and a polar monomer. The cable in which the insulation material is to be used comprises a separate water-blocking layer.

US 5,283, 1 20 describes insulation material for non-high voltage power cables comprising a polyethylene (PE) copolymer, or a combination of a PE/polypropylene (PP) copolymer, with an ionomer having a C3-C6-alkene(di)carboxylic acid . The acid is at least partly converted into ammonium salts of tertiary amines. The aim is to increase resistance to growth of water tree in the insulation material and improve aging resistance and processability of the material .

JP 2001 -319527 describes the use of the Silane crosslinking agent to improve the manufacturing process of the insulation material . Disclosed is insulation material for use in non-high voltage power cables comprising a PE polymer resin together with a copolymer of PE/ionomer and a Silane cross linking agent. The aim is to improve the water resistance and other mechanical properties of the insulation material that covers a bunch of conductors. This insulation material is peripherally covered by a second insulation layer of crosslinked PE . The carboxylic acid of the ionomers may be neutralized with monovalent metal salts such as potassium or sodi um salt. The insulation material is immersed i n warm water to remove any gaps between the conductor and the insulation material , before being covered by the second insulation layer. US 6,239,377 describes foamed i nsulation material for use in non-high voltage power cables comprising a blend of PP, PE (low density (LD) or high density (HD)) and a copolymer of PE/ionomer, whereby the ionomer may be neutralized with zinc salt. Zinc salt is used to decrease the water moisture absorption , while having little change in electric characteristics. The foamed insulation layer is covered by a polyolefin-based insulation layer.

JP 2012-069523 describes a solution for a problem in processing crosslinked polylactic acid . The insulation material comprises an outer layer comprising the polylactic acid and an inner layer covering the conductor. This inner layer comprises a PE polymer resin , which may comprise ethylene-acrylate copolymerized resi n , or an ionomer resin . The inner layer may have a thickness of 0.01 to 0.5 mm and is not suitable for use in HV power cables.

JP 07-050107 describes a non-high voltage power cable comprising a conductor, covered by a semi-conductor layer, an inside ion trap layer, an insulation layer, an outside ion strap layer and an outer semi-conductive layer and a jacket. The ion trap layers comprise insulation material comprising copolymers of olefin (C2-C4) with carboxylic acid monomers and a metal salt, such as zi nc salt. The ion trap layers prevent ions from entering the insulation layer and thereby decrease the generation of water trees in the layer. EP 323 581 describes a non-high voltage power cable comprising an i nsulation layer that comprises a polymer of ethylene with a copolymer of ethylene-ionomers. The ionomer comprises C3-C6-alkenecarboxylic- or C4-C6-alkenedicarboxylic acids that may be neutralized with a metal salt to a neutralization degree between 2 and 50%. This i nsulation material has an improved resistance to the generation of water trees i n the insulation material .

JP 2009-032662 describes insulation material for coaxial wires in electronic apparatus, whereby the insulation material comprises an ethylene-ionomer resin together with organic clay to improve the resistance of the insulation material .

Most of the insulation materials described above are not suitable for use in HV power applications. There is a need for improved insulation material , especially for use in HV power applications. Especially, there is a need for improved crosslinked insulation material . There is a need for insulation material with improved mechanical , electrical and thermal properties. Preferably, the water resistance of the material is improved and the conductivity of the material does not depend on the humidity of the atmosphere surrounding the material or on the thickness of the material . There is a need for insulation material having improved breakdown strength , improved space charge distribution and a decreased conductivity and electric field dependency of the conductivity. Preferably, the quality of the mechanical , electrical and thermal properties of the i nsulation material are good enough to use only one layer of i nsulation material in the HV power applications.

There is also a need for an improved process for the preparation of insulation material suitable for use in HV power applications, whereby degassing or heat treatment to reduce impurities in the insulation material is no longer needed . There is a need for a faster process, whereby the mechanical , electrical and thermal properties of the material are improved or at least not negatively affected by the process. Preferably, the process has a wider processing window compared to existing processes.

SUMMARY OF THE I NVENTION

The object of the present invention is to provide insulation material for use in HV power applications that overcomes the problems mentioned above.

The object is achieved by insulation material for use in HV power applications comprising a mixture of

- a first component comprising at least one Ci_-8 olefin polymer in an amount between 0 and 95 wt% of a total weight of the insulation material , and

- a second component comprising an ionomer in an amount between 5 and 100 wt% of the total weight of the insulation material , whereby the ionomer is a copolymer comprising a Ci_-8 olefin polymer and an at least partially neutralized acrylic acid and/or methacrylic acid co-monomer, or an anhydride form thereof, comprising between 1 and 20 wt% co-monomer of the total weight of the ionomer,

a multivalent metal salt for neutralizing said acid , and

whereby a neutralization degree of the ionomer is between 1 and 1 00%, and

whereby the total amount of Ci_-8 olefin polymer from the first component and the second component is between 80 and 99.5 wt% of the total weight of the insulation material , and

whereby the insulation material has a thickness of at least 3 mm.

In one embodiment, the i nsulation material for use in HV power applications comprising a mixture of - a first component consisting at least one Ci_-8 olefin polymer in an amount between 0 and 95 wt% of a total weight of the insulation material , and

- a second component consisting an ionomer in an amount between 5 and 100 wt% of the total weight of the insulation material , whereby the ionomer is a copolymer comprising a Ci_-8 olefin polymer and an at least partially neutralized acrylic acid and/or methacrylic acid co-monomer, or an anhydride form thereof, comprising between 1 and 20 wt% co-monomer of the total weight of the ionomer,

a multivalent metal salt for neutralizing said acid , and

whereby a neutralization degree of the ionomer is between 1 and 1 00%, and

whereby the insulation material has a thickness of at least 3 mm. In one embodiment, the insulation material is substantially free of impurities, and free of ammonium salt, Silane crosslinking agent and foaming material .

The insulation material according to the present i nvention has good or improved mechanical , electrical and thermal properties. The material shows a decreased conductivity and electric field dependencies. I nsulation material comprising less ionic co-units from the acid performs better in terms of conductivity compared to material comprising more ionic co-units. The conductivity of the insulation material is substantially independent of the humidity of the atmosphere surrounding the material . The conductivity is preferably less dependent on the thickness of the material . Only one layer of insulation material may be used in a HV power application to provide sufficient insulation . No additional water-resistant layer or ion-trappi ng layer needs to be added around the power applications to improve the quality of insulation .

In another embodiment, the metal salt is selected from a salt of Zn , Mg , Ca and Al .

These bivalent or trivalent metal salts provide for strong ionic clusters in the insulation material . The bond energy and bond strength of the metal-oxygen bonds in the polymer are potentially more than 100 times higher than the energy needed to move a metal ion from the polymer moiety (E . A. Amin , D. G . Truhlar, J. Chem. Theory Comput. 2008, 4, 75-85). Si nce the difference between potential energy from the electric field and bond energy is high , the salts are not expected to dissociate. Thereby, no free charge carriers will be generated under an electric field .

In a further embodiment, the metal salt is a Zn salt, whereby the Zn content is between 0.1 and 5 wt % of the total weight of the ionomer.

The use of zinc salt makes the insulation material less sensitive to water absorption . In one embodiment, the first component is low density polyethylene (LDPE).

In another embodiment, the LDPE is crosslinkable with a crosslinking agent.

Polyethylene has successfully been used in high voltage power applications. Due to the addition of the ionomer in the insulation material , especially in the amounts mentioned above, the insulation material is substantially free of impurities. The combination of LDPE and ionomer also provides for i nsulation material having good or improved mechanical , electrical and thermal properties.

In a further embodiment, the first component is high density polyethylene (HDPE).

High density PE decreases the conductivity of insulation material even further and thereby further improves the quality of the insulation material .

In one embodiment, the ratio between the first component and the second component is between 4: 1 and 1 :2. I n a further embodiment, the first component is an at least one Ci_-8 olefin polymer and the second component is an ionomer.

The quality of the i nsulation material may already improve by the use of only 25 wt% of the second component in combination with the first component comprising at least one Ci_-8 olefin polymer. The total amount of Ci_-8 olefin polymer from the first component and the second component is preferably more than 85 wt%, or 93 wt%. This saves costs and improves the processability of the ingredients.

In another embodiment, the second component is a copolymer of polyethylene copolymerized with methylmetacrylic acid . In a further embodiment, the second component is an ionomer consisting a copolymer of polyethylene copolymerized with methylmetacrylic acid . These ingredients are commercially available at relative low cost. Methylmetacrylic acid (MMA) provides unique ionomer phases in moulded and solid state, which provide for insulation material having good or improved mechanical , electrical and thermal properties. In a further embodiment, the i nsulation material has a thickness between 5 and 35 mm.

The i nsulation material of the present invention , especially with the ingredients used in the ratios mentioned above, can be used to make thick i nsulation material having uniform morphology. The conductivity preferably is independent of the thickness of the material . The quality of the new material is sufficient to be used as a si ngle layer of insulation material circumferentially covering the (semi-)conductors. This will save production time, materials and thus overall costs for the production of the insulation material and insulated power applications.

The objects are also achieved by a use of insulation material descri bed above as insulation material in HV power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) termi nations.

These objects are also achieved by a process for preparing insulation material for HV power applications as described above, whereby the process comprises the following steps:

a) feedi ng the first component and the second component i n an extruder at a feeding speed between 50 and 800 kg/h , and a temperature between Tm minus 40°C and Tm pl us 60°C, whereby Tm is a melting temperature of a polymer of the first component or the second component with highest melting temperature;

b) moulding the obtained material circumferentially on an object selected from a conductor, an i nner screeni ng semiconducting layer, insulation material and an outer screening semiconducting layer;

c) cooling the moulded material at a temperature between Tz and -20°C, whereby Tz is a melting temperature of the polymer of the first component or the second component that has the lowest melting temperature; and d) collecting the material at a collecting speed between 1 and 30 m/min .

In one embodiment, the process comprises preparation steps prior to feeding step a), comprising ;

1 ) premixi ng the first component or the second component in a compounder extruder at a temperature between Tm minus 40°C and Tm plus 60°C;

2) cooling and pelletizing the obtai ned mixture; and

3) drying the mixture at a temperature between Tm minus 50°C and Tm minus 80°C, optionally at reduced atmospheric pressure.

The new process provides for a product that is free of or substantially free of impurities. One advantage of the new process according to the invention is that a degassing or heat treatment step to remove impurities is no longer needed . A neutralization step is neither needed in the process. The process according to the invention is faster and may also allow for both an increased feeding speed and an increased collecting speed compared to conventional processes. These advantages save time, material and costs related to the preparation of insulation material . The process window of the new process is wider, which i n turn allows for improved control of the process and thus improved quality of the material obtained by this process.

In one embodiment of the process, the first component is LDPE , which may be crosslinkable with a crosslinking agent.

Especially, insulation material from crosslinkable LDPE may contai n impurities. These impurities impair the quality of the insulation material . Due to the addition of the ionomer, especially in the amounts used in the present invention , substantially no impurities are present in the insulation material obtained by the new. In another embodiment of the process, the first component is HDPE. The objects are further achieved by an insulation material for high voltage or extra high voltage power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) terminations, prepared by the process described above. And by high voltage or extra high voltage power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) termi nations, prepared by the process described above.

The advantages of the new process are apparent from the discussion herei nabove with reference to the proposed insulation material .

A further embodiment relates to a HV power application comprising concentrically arranged :

- an elongate conductor,

- a first screeni ng semiconducting layer circumferentially covering the conductor,

- a first layer of insulation materials as described above circumferentially covering the first semiconducting layer, and - a second screening semiconducting layer circumferentially covering the first layer of insulation materials,

- a second layer of insulation materials as described above circumferentially covering the second semiconducting layer, and

- optionally a jacketing layer and armor covering the outer wall of the second layer of insulation materials.

BRI EF DESCRI PTI ON OF TH E DRAWI NGS Fig 1 shows the 24 h value DC conductivity at 70°C and 30 kV/mm Grade A material , Grade B material , ionomer β, and ionomer a.

Fig 2 shows electric field dependence of the conductivity at

70°C for ionomer a.

Fig 3 shows DC conductivity at 70°C and 30 kV/mm of ionomer a, a LDPE:ionomer a 50:50 blend , and a LDPE:ionomer a 75:25 blend .

Fig 4 shows electric field dependency of LDPE, ionomer a, and a LDPE:ionomer a 50:50 blend .

Fig 5 shows DC conductivity at 30kV/mm, 70°C on ionomer a neat and pre-exposed to a 100 %RH , 70°C, for 24h . Fig 6 shows DC conductivity at 70°C, 30 kV/mm for

HDPE:ionomer a 50:50 blend and as comparison a LDPE: ionomer a 50:50 blend and ionomer a.

Fig 7 shows DC conductivity dependence with the electric field for HDPE:ionomer a 50:50 blend , LDPE: ionomer a 50:50, ionomer a and LDPE.

Fig 8 shows a flow chart of a process accordi ng to the invention .

Fig 9 shows a Wei bull plot of the breakdown strength .

DETAI LED DESCRI PTI ON OF VARIOUS EMBODI MENTS THE I NVENTION

Insulation material

Insulation material accordi ng to the present invention can be used i n high voltage or extra high voltage direct or alternating current (collectively referred to as HV) power applications. The material can be used in other applications such as cables, joints that connect power cables, terminations at the end of the power cables, and semiconducting screening material comprising said insulation material together with acetylene carbon black. Other applications may be bushings, insulated buses and bus bars. The material is especially suitable for use in HVDC power cables. The insulation material is preferably substantially free of impurities. Further, the insulation material is preferably not foaming.

The insulation material comprises a mixture of a first component comprising at least one Ci_-8 olefin polymer and a second component comprising an ionomer. In one embodiment, the first component is an at least one Ci_-8 olefin polymer and the second component is an ionomer. Examples of a Ci_-8olefin polymer may be ethylene, propylene, butylene, pentene, hexane, heptene or octane, or mixtures thereof, in any isomeric or stereoisomeric form. The polymers may be selected from the group comprising low density polypropylene (LDPE), crosslinkable LDPE, high density polyethylene (HDPE), isotactic polypropylene (iPP), co- polypropylene (cPP), PP based elastomer, and a copolymer of a Ci_-8olefin polymer. In one embodiment the Ci_-8olefin polymer is selected from the group comprising LDPE, crosslinkable LDPE and HDPE, or mixtures thereof. In another embodiment, one polymer is used. Preferably, HDPE used is bimodal HDPE. LDPE may be crosslinked using a crosslinking agent such as peroxide or azo compounds. One example of a crosslinking agent may be dicumylperoxide. Silane crosslinking agents are preferably not used in the insulation material of the present invention.

The insulation material also comprises a second component comprising an ionomer. The ionomer is a copolymer comprising a Ci_-8 olefin polymer and an at least partially neutralized acrylic acid and/or methacrylic acid, or an anhydride form thereof.

Examples of ionomers may be combinations of a Ci_-8olefin polymer, such as any of the Ci_-8olefin polymer mentioned above, with a C_3-6alkenecarboxylic acid or C_3-6alkenedicarboxylic acid. Examples of ionomers may be ethylene/acrylic acid, ethylene/ethyl acetate, ethylene/methyl acrylate, ethylene/ethyl acrylate, ethylene/butyl acrylate, ethylene/isobutyl acrylate, ethylene/methyl metracrylic acid (MMA), ethylene/isobutyl acrylate/methacrylic acid , ethylene/methyl acrylate/maleic anhydride, or mixtures thereof. I n one embodiment the second component comprises an ionomer comprising a copolymer of polyethylene copolymerized with methylmetacrylic acid (MMA).

The amount of acrylic and/or methacrylic co-monomer is between 1 to 20 wt% of the total weight of the second component or the ionomer. The amount of co-monomer may also be between 1 and 15 wt% , or between 5 and 15 wt% , or between 8 and 14 wt% , or below 1 5 wt%.

Monovalent or multivalent cations may be used for the neutralization of the ionomer. Metal salts such as lithium , potassi um or sodium may be used . Preferably, a multivalent metal salt is used . Metal salts selected from the group calcium (Ca), magnesium (Mg ), al umi nium (Al ) and zi nc (Zn) may be used . Ammonium salts are preferably not used in the insulation material of the present i nvention . The metal salts are preferably metal salts of the acid present in the ionomer.

The degree of neutralization of the negative ions of the acid or acids may be between 1 and 100% . Insulation material havi ng good mechanical and electrical properties was obtained using an ionomer with a neutralization degree between 1 0 and 50%, or between 15 and 40%, or between 20 and 30%, or about 25%. Other insulation material havi ng good mechanical and electrical properties was obtained using an ionomer with a neutralization degree between 51 and 100%, or between 75 and 99%, or between 75 and 85%, or over 75%, or about 80%.

The amount of metal salt used may be between 0.1 and 5 wt %, or between 0.5 and 4 wt%, or between 2 and 3 wt% , of the total weight of the ionomer. I n one embodiment, the amount of metal salt in the ionomer is between 0.5 and 5 wt%, or between 0.5 and 1 .5 wt% , or between 2.5 and 3 wt%.

The amount of the first component in the insulation material may be between 0 and 95 wt% , whereby wt% are percentages of a total weight of the insulation material , or between 20 and 80 wt%, or between 40 and 60 wt% . The amount of the second component may be between 5 and 100 wt%, or between 10 and 90 wt%, or between 25 and 75 wt%, or between 40 and 60 wt%, whereby wt% are percentages of a total weight of the insulation material . In one embodiment, the first component is an at least one Ci_-8 olefin polymer and the second component is an ionomer. The first component and the second component together form substantially 100 wt% of the insulation material . In addition to the first component and the second component, additives such as anti-oxidants and the like, may be present i n the material as well .

The ratio between the first component and the second component in the insulation material may be between 4: 1 and 1 :4, or between 3: 1 and 1 :3, or between 3: 1 and 1 :2, or between 3: 1 and 1 : 1 , or 1 : 1 , or 2: 1 , or 3: 1 . Insulation material havi ng good mechanical and electrical properties was obtained using between 75 and 50 wt% of the first component and between 25 and 50 wt% of the second component. In one embodiment, the ratio of HDPE or LDPE to the second component is 1 : 1 . In another embodiment the ratio of HDPE or LDPE to the second component is 3: 1 . In one embodiment, the first component is an at least one Ci_-8 olefin polymer and the second component is an ionomer.

The total amount of Ci_-8 olefin polymer in the insulation material from the first component and the second component together may be between 80 and 99.5 wt% , or between 90 and 99 wt% , or between 93 and 99 wt%, or between 95 and 98 wt% , whereby wt% are percentages of a total weight of the insulation material .

The thickness of the insulation material according to the present invention may be at least 3 mm , or at least 5 mm , or between 5 and 50 mm, or between 5 and 35 mm, or between 10 and 25 mm.

It is to be understood that the insulation material according to the present i nvention may comprise any combination of any ingredient in any amount and ratio mentioned above. All and any such combi nations of ingredients are included in the presentation of the present invention . An example of such combination of ingredients may be insulation material for use in HV power applications comprising a mixture of

- a first component comprising at least one Ci_-4 olefin polymer in an amount between 20 and 80 wt% of a total weight of the insulation material , and

- a second component comprising an ionomer in an amount between 20 and 80 wt% of a total weight of the i nsulation material , whereby the ionomer is a copolymer comprising a C-i-4 olefin polymer and an at least partially neutralized acrylic acid and/or methacrylic acid co-monomer, or an anhydride form thereof, comprising between 1 and 15 wt% co-monomer of a total weight of the ionomer,

a multivalent metal salt for neutralizing said acid , and

whereby a neutralization degree of the ionomer is between 15 and 30%, or between 75 and 100%, and

whereby the total amount of Ci_-4 olefin polymer from the first component and the second component is between 90 and 99.5 wt of a total weight of the insulation material .

In one embodiment, the first component is an at least one Ci_-8 olefin polymer and the second component is an ionomer. Specific examples of insulation materials are insulation material comprising a mixture of

- a first component comprising at least one low or high density C-i -2 olefin polymer in an amount between 50 and 75 wt% of a total weight of the insulation material , and

- a second component comprising an ionomer in an amount between 25 and 50 wt% of a total weight of the insulation material , whereby the ionomer is a copolymer comprising a Ci_-2 olefin polymer and an at least partially neutralized acrylic acid and/or methacrylic acid co-monomer, or an anhydride form thereof, comprising between 5 and 15 wt% co-monomer of a total weight of the ionomer,

a multivalent metal salt for neutralizing said acid , and ,

whereby the total amount of Ci_-2 olefin polymer from the first component and the second component is between 93 and 99.5 wt of a total weight of the insulation material .

In one embodiment, the first component is an at least one Ci_-8 olefin polymer and the second component is an ionomer.

Zinc salt may be used as the preferred metal salt in these embodiments. The zinc content in the ionomer may be between 0 and 3 wt% , or between 0.5 and 1 .5 wt% , and between 2.80 and 2.90 wt% . MMA and LDPE, crosslinkable LDPE may be used in these embodiments as well . MMA and HDPE or bimodal HDPE may also be used in these embodiments.

The insulation material accordi ng to the present invention may have a DC conductivity below 1000 fS/m, more preferably below 100 fS/m, more preferably below 50 fS/m, at 70°C and 30 kV/mm.

Other additives such as anti-oxidants and the like may also be present in the insulation material according to the invention . Examples may be generic antioxidants with a primary and secondary antioxidant function , such as hindred phenols.

Preparation process

Fig . 8 shows a flow chart of a process for preparing insulation material for HV power applications.

The process may start with a preparation step of mixing and melting the first component and the second component in a compounder extruder. The first component, the second component and possi ble additives are as descri bed above. The temperature of the extruder may be between 40°C below or 60°C above the melting temperature of the ingredient or polymer in the mixture from the ionomer or olefin polymer that has the highest melting temperature, i .e. between Tm minus 40°C and Tm plus 60°C. The exact temperature will depend on the specific ingredients used . Examples of suitable temperatures may be a temperature between 70 and 180°C.

In a next preparation step, the melted mixture is cooled to a temperature below the melting temperature of the ingredient or polymer that has the lowest melting temperature (Tz) and room temperature. Examples of cooling temperatures may be a temperature between 80 and -20°C.

The cooled material can su bsequently be pelletized and dried at a temperature between Tm minus 50°C and Tm minus 80°C. Examples of drying temperatures may be a temperature between 40 and 80°C. Drying may be performed under reduced atmospheric pressure, i .e. below 101 .325 kPa, i n order to accelerate the drying step.

The dried material may than be fed into an extruder. The temperature of the mixture may be between Tm minus 40°C and Tm plus 60°C The extruder barrel may for example have a temperature between 70 and 180°C.

The feeding speed of the ingredients into the extruder may be between 50 and 800 kg/h , or between 200 and 500 kg/h .

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 screeni ng semiconducting layer, an outer screening semiconducting layer, or other insulation material .

The extruded product is than cooled to a temperature between Tz and -20°C. In one embodiment, the product is cooled to room temperature.

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 depend on the thickness of the insulation material .

The invention also relates to an HV power application , such as e.g . a cable, which comprises an elongate conductor that is circumferentially covered by a first screeni ng semiconducting layer. This first semiconducting layer is in turn circumferentially covered by a first layer of insulation materials descri bed below. A second screeni ng semiconducting layer subsequently covers the first layer of insulation materials. This second screening semiconducting layer is also circumferentially covered by a second layer of insulation materials described below. Optionally, the outer wall of the second layer of insulation materials may be covered by a jacketing layer and armor.

Experi ments

Materials In the experiments described below a commercially available ionomer a was used , which comprises a polyethylene base with MMA copolymer, which has a degree of neutralization of about 80% with Zn salt. The content of (MMA)Zn i n the ionomer is about 6.66 wt%, and the content of MMAH (acid form) in the ionomer is about 1 .9 wt %.

Also used was ionomer β, which comprises a polyethylene base with MMA copolymer, and which has a degree of neutralization of about 23% with Zn salt.

LDPE used was of electrical grade LDPE and with a typical MFR (melt flow rate) value of 2 g/10 min at 1 90^°C/2.16 kg .

Dicumylperoxide was used as a crosslinking agent in crosslinked LDPE.

HDPE used is a commercially available electrical grade bimodal HDPE. The HDPE used had a typical MFR range of 1 .2 - 2.0 g/10 min at 190°C/2.16 kg .

The pure ionomer showed very good electrical properties, which were improved when diluted with LDPE or HDPE. The dilutions tested comprised from 25 to 100 wt% ionomer. Table 1 . The compositions in wt and mol % for the ionomer a and β.

Table 2. The compositions in wt % of the insulation material comprising a blend of 50:50 or 75:50 of the first component to ionomer a as the second component. Ethylene MMA Zn

50:50 Wt% 93.75 4.8 1 .43

75:25 Wt% 96.75 3 0.25

Hereby, the expressions 50:50 and 75:25 express the weight of the first and second component, respectively, as percentages of the total weight of the compositions. Process

The materials were prepared first in small scale and scaled up to an experimental extruder compounder.

Results

The first test i ntended to show the potential use of ionomers as insulation material . Fig .1 shows the 24 h val ues of the DC conductivity measured at 70°C and 30 kV/mm of ionomer a and ionomer β. For comparison two other materials used as reference and compounded with peroxide (Grade A and Grade B) were included in the plot. Both Grades A and B comprise low density polyethylene, antioxidants and a crosslinking agent, whereby the amount of crosslinking agent in Grade B is reduced in comparison to Grade A. Both grades of ionomers showed better conductivity than the best Grades A or B , and the most neutralized material (ionomer a) showed the best results.

The conductivity dependence with the electric field was determi ned for the most promising ionomer. Fig . 2 shows the field dependency of ionomer a. The conductivity can be fitted with an exponential function having an exponential factor of 1 e-7 m/V.

Blends of ionomer a and LDPE were prepared to show the effect of an ionomer havi ng less ionic groups, and to show the possibility to modify commercial insulation materials with the ionomer. Fig . 3 shows the conductivity of ionomer a compared with two blends at 70°C and 30 kV/mm. It can be seen that the blends of ionomer a and LDPE showed even better results in terms of conductivity than the pure ionomer a. It seems that an insulation material comprising less ionic co-units performs better in terms of conductivity compared to material comprising more ionic co-units. At the same time is shown that a blend of a commercial available HVDC material with ionomer is a suitable material for insulation .

Apparatus and method

DC conductivity was performed usi ng equipment constructed in a house as described by Olsson et al (Proceedings of Nord IS 2009, Experimental determination of DC conductivity for XLPE insulation , Olsson , CO . ; Kallstrand , B . ; Ritums, J . ; Jeroense, M . , 2009). The field dependence of the conductivity for the blend LDPE/ionomer a 50:50 was determined by measuring the conductivity at 20, 30, and 40 kV/mm and 70°C. The corresponding results are shown in Fig . 4. As a comparison , the field dependency of LDPE and the pure ionomer a are shown . It can be observed that the conductivity dependence with the field for the blend is weaker (lowest line) than for the pure ionomer a or pure LDPE . Weak field dependency allows materials to withstand high electric stress. One of the concerns using ionomers is the water intake of the material and the effect of moisture on the conductivity. In order to verify the effect of water intake, a sample of 1 mm thickness was located in an environment with 100% RH , 70°C for 24 h . After the treatment, DC conductivity on the sample was measured . The results are shown in the Fig . 5. As a comparison , the results of a sample that had not been exposed to moisture are also shown . The moisture slightly increases the conductivity. However, the effect is expected to be reduced usi ng blends or ionomers with lower ionic co-units, because of the reduction of the amount of ionic units in the insulation material . HDPE shows very low values of conductivity and high val ues of breakdown strength . However, the stiffness of the polymer makes it difficult for applications in e.g . power cables. Blends of HDPE and LDPE have been reported , but the conductivity of the blends generally is determined by the LDPE content. Blends of HDPE and ionomer a were tested in a ratio of 50:50. The function of the ionomer a is to decrease the stiffness and at the same time mai ntai n the good electrical performance of HDPE . Fig . 6 shows the DC conductivity of a blend of HDPE:ionomer a 50:50.

The electric field dependency of the HDPE :ionomer a blend was determi ned by conductivity measurements at 20, 30 and 40 kV/mm. The corresponding results are shown i n Fig . 7. As a comparison , the field dependency of other blends is shown . The HDPE :ionomer a shows the weakest dependency of the conductivity with the electric field .

Lightni ng impulse breakdown test was performed in a stepwise way on about 100 μιτι thick samples. The test was done in a custom made open air cell . The tests were done at negative polarity and the impulse characteristics were 1 .2 s transient and 50 s decay time. Samples were fixed between the electrodes and impulse generator, which was discharged at the specific voltage. The stepwise increase was done in 5 kV steps until material breakdown (BD). The set up is descri bed by Liu et al (Liu , R. ; Dominguez, G. Farkas, A. , Impulse Breakdown of Extruded Cable Insulation Materials. 201 1 Annual Report Conference on Electrical Insulation and Dielectric Phenomena , 518-521 ).

The Weibull representation of the breakdown test is shown in Fig . 9. Definitions The wordi ng "between" as used herein includes the mentioned values and all values in between these values. Thus, a val ue between 1 and 2 mm includes 1 mm, 1 .654 mm and 2 mm.

The wordi ng "low density" as used herein means densities between 0.90 and 0.93 g/cc.

The wordi ng "high density" as used herein means densities above 0.935 and below 0.95.

The wording "room temperature" as used herein means a temperature between 14°C and 28°C.

The wordi ng "reduced atmospheric" as used herein means a pressure below atmospheric pressure.

The wordi ng "power application" as used herein includes applications for insulation materials selected from cables, joints, bushings, insulated buses, bus bars and (cable) terminations. The wording "high voltage or HV" as used herein is meant to include high voltage and extra high voltage (EHV) 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.

Claims

CLAI MS

1 . An insulation material for use in HV power applications comprising a mixture of

a multivalent metal salt for neutralizing said acid , and

whereby a neutralization degree of the ionomer is between 1 and 1 00%, and

whereby the insulation material has a thickness of at least 3 mm.

2. The i nsulation material accordi ng to claim 1 , whereby the insulation material is substantially free of impurities, and free of ammoni um salt, Silane crosslinking agent and foaming material .

3. The insulation material according to claim 1 or 2, whereby the metal salt is selected from a salt of Zn, Mg , Ca and Al .

4. The insulation material according to any one of the preceding claims, whereby the metal salt is a Zn salt, whereby the Zn content is between 0.1 and 5 wt % of the total weight of the ionomer.

5. The insulation material according to any one of the preceding claims, whereby the first component is LDPE.

6. The i nsulation material accordi ng to claim 5, whereby the LDPE is crosslinkable with a crosslinking agent.

7. The insulation material according to claims 1 to 4, whereby the first component is HDPE.

8. The insulation material according to any one of the preceding claims, whereby the ratio between the first component and the second component is between 4: 1 and 1 :2.

9. The insulation material according to any one of the preceding claims, whereby the second component is a copolymer of polyethylene copolymerized with methylmetacrylic acid .

10. The i nsulation material according to any one of the precedi ng claims, whereby the insulation material has a thickness between 5 and 35 mm.

1 1 . Use of i nsulation material according to any one of the precedi ng claims as i nsulation material in high voltage or extra high voltage power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) terminations.

12. A process for preparing insulation material for HV power applications according to any one of claims 1 to 10, whereby the process comprises the following steps:

a) feedi ng the first component and the second component i n an extruder at a feeding speed between 50 and 800 kg/h , and a temperature between Tm minus 40°C and Tm pl us 60°C, whereby Tm is a melting temperature of a polymer of the first component or the second component with highest melting temperature; b) moulding the obtained material circumferentially on an object selected from a conductor, an i nner screeni ng semiconducting layer, insulation material and an outer screening semiconducting layer;

c) cooling the moulded material at a temperature between Tz and -20°C, whereby Tz is a melting temperature of the polymer of the first component or the second component that has the lowest melting temperature; and

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

13. A process according with the claim 12, whereby the process comprises preparation steps prior to feeding step a), comprising ;

2) cooling and pelletizing the obtai ned mixture; and

14. A process according to claims 12 or 13, whereby the first component is LDPE, which may be crosslinkable with a crosslinking agent.

15. A process according to any one of claims 12 to 14, whereby the first component is HDPE.

16. I nsulation material for high voltage or extra high voltage power applications selected from cables, joints, bushi ngs, insulated buses, bus bars and (cable) terminations, prepared by the process according to any one of claims 12 to 15.

1 7. High voltage or extra high voltage power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) terminations, prepared by the process according to any one of claims 12 to 1 5.

18. A HV power application comprising concentrically arranged : - an elongate conductor,

- a first layer of insulation materials according to any one of claims 1 to 1 0 circumferentially covering the first semiconducting layer, and

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

- a second layer of insulation materials accordi ng to any one of claims 1 to 10 circumferentially covering the second semiconducting layer, and