WO2014206437A1

WO2014206437A1 - A new process for preparing insulation materials for high voltage power applications and new insulation materials

Info

Publication number: WO2014206437A1
Application number: PCT/EP2013/063106
Authority: WO
Inventors: Gustavo Dominguez; Andreas Farkas; Chau-Hon HO; Alun VAUGHAN; Amy PYE; Gary Stevens; Ian HOSIER; Sari Laihonen; Nan Li; Lejun QI; Huigang Sun
Original assignee: Abb Technology Ltd
Priority date: 2013-06-24
Filing date: 2013-06-24
Publication date: 2014-12-31

Abstract

The present invention relates to a process for preparing insulation materials for high voltage power applications and new insulation materials. The materials comprise a mixture of at least one C2- 8olefin polymer and an a-nucleating agent and/or β-nucleating agent. The process comprises the steps a) mixing and melting ingredients at a temperature between Tm and Tm plus 150°C, whereby Tm is the melting temperature of the polymer; b) cooling and pelletizing the mixture; c) feeding the pelletized mixture in an extruder at a feeding speed between 100 and 800 kg/h, whereby the extruder barrel temperature is between Tm minus 50 and Tm plus 150°C; d) cooling the extruded materials in a single step between room temperature and below Tm; and e) optionally collecting the extruded materials at a collecting speed between 10 and 40 m/min. The invention also relates to new HV power applications comprising the new insulation materials.

Description

Title: A new process for preparing insulation materials for high voltage power applications and new insulation materials.

THE FIELD OF THE INVENTION

The present invention relates to a process for preparing insulation materials for high voltage or extra high voltage direct or alternating current (DC or AC) power applications according to claim 1 . The invention also relates to insulation materials suitable for use in high voltage (HV) or extra high voltage (EHV) power applications according to claims 8 to 1 7.

BACKGROUND OF THE INVENTION AND PRIOR ART

Insulation materials for power applications, such as cables, are exposed to high stresses. This is especially true for insulation materials used in high voltage and extra high voltage (hereinafter collectively referred to as HV) systems. These insulation materials require a good combination of electrical , thermal and mechanical properties. Preferably, the materials have low conductivity, low space charge distribution and high breakdown strength. The properties of the materials should preferably not change depending on the temperature of the materials.

Insulation materials are prepared by extruding one or more compounds or polymers together with possible additives in an extruder at or above melting temperatures of the polymers. The extruded product is subsequently cooled to room temperature. Degassing is performed in conjunction with the extrusion process to prevent or remove by-products. Time and money can be saved by increasing the speed of the manufacturing process, for example by increasing the feeding speed or the cooling rate. However, increasing the speed of the overall process often results in insulation materials having impaired qualities, such as decreased breakdown strength or increased space charge distribution . Especially a change in cooling rate can have a negative impact on the quality of the thick power cables used for HV cables. Due to the thickness of these materials, there is a distance between the inner diameter and the outer diameter of the extruded insulation materials. The material cools first at the outer diameter, while the material at the inner diameter cools later. These different cooling rates within the materials may result in differences in morphology of the insulation materials, and thus in differences in mechanical , electrical and thermal properties, depending on the distance of the materials from the outer diameter.

Much research has been performed to improve the quality of insulation materials and to improve the preparation process. GB 1 564 990 describes insulation materials comprising low density polyethylene or crosslinkable polyethylene in combination with an alcohol. The alcohol may be a monohydric or aliphatic alcohol and is added as a tree-growth-inhibitor, i.e. to improve the breakdown strength of the insulation materials.

US 4 520 230 describes insulation materials for power cables comprising crosslinkable polyethylene and 0.3 to 1 .0 part by weight of dibenzylidene-D-sorbitol . The sorbitol is added to improve the breakdown strength of the insulation materials. A manufacturing process is described , whereby the breakdown strength is further improved by controlling the cooling rate of the materials at a temperature near the crystallization temperature of the cross-linked polyethylene at 10°C/min or less. Extrusion is performed at a temperature of 120°C, the coating cable is passed at 2 m/min through a long die heated to 250°C for heating and crosslinking , and then cooled in a cooling tube divided in three temperature zones. Nitrogen gas is passed through the zones during cooling.

US 5 286 924 describes foamed insulation materials comprising olefin polymers, a mineral oil, an antioxidant and a nucleating agent such as dibenzylidene sorbitol . The sorbitol is added to improve the crush resistance of the insulation materials. Preferably, the materials have a density below 0.3 gm/cm². The insulation materials (polypropylene) are prepared by extruding the mixture of components at a temperature profile from 266°C to 166°C at a rate of 20.5 kg/h, while the barrel is maintained at 65.6°C. The materials are collected at a speed of 1 .98 m/min and subsequently stretched to obtain a film having a thickness of 0.24 mm.

JP 05-128915 describes insulation materials comprising polypropylene, or a combination of polypropylene and low density polyethylene, and a sorbitol derivative as a nucleating agent. The insulation materials are characterized by the diameter of spherulites present in the materials. The insulation materials are prepared by melting the mixture of components at a temperature of 170°C to 200°C for 10 to 20 min . Isothermal crystallization is performed at 132 to 138°C for 30 to 120 min . Cooling was performed at a temperature of 10°C to 40°C at a cooling rate of 120 to 200°C/min.

WO 2012/016964 describes a capacitor film comprising at least 70 wt% isotactic polypropylene and optionally a comonomer such as ethylene or a C₄-C₂o a-olefin, and a nucleating agent such as a sorbitol derivative. The nucleating agent is added to improve the breakdown strength. The film is prepared by extrusion at a temperature of 230°C with a barrel having a temperature of 90°C to obtain a film having a thickness of 0.5 mm.

Most of the insulation materials described above are not suitable for use in HV power applications. There is a need for an improved process for the preparation of insulation materials suitable for use in HV power applications that allows for an increased feeding speed, cooling rate and/or an increased collecting speed, without impairing the quality of the materials. There is a need for a process, whereby degassing during the process is no longer needed. There is a need for a process, whereby the mechanical, electrical and thermal properties of the materials do not depend on the cooling rate used in the process and a process that allows for an improved control of these properties. Preferably, the process has a wider processing window compared to existing processes. There is also a need for improved insulation materials, especially for use in HV power applications. Preferably, the morphology within the materials is independent of the distance of the materials from an outer diameter of the material . Most preferably, the morphology and crystallinity of the materials, as well as mechanical, electrical and thermal properties of the materials are substantially uniform within the insulation materials.

SUMMARY OF THE INVENTION

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

The object is achieved by a process for preparing insulation materials for HV power applications as defined in claim 1 , whereby the insulation materials comprise

- at least one C_2-80lefin polymer, and

- one or two nucleating agent in an amount of 0.1 to 3 wt%, selected from an a-nucleating agent of formula I

(I)

wherein X and X₂ are independently selected from H, C-_Moalkyl and C _{- 0}alkoxy, and

a β-nucleating agent of formula I I

H H

(I I)

wherein R is an aliphatic cyclic or non-cyclic hydrocarbon chain, and Ar is an aromatic group selected from phenyl, diphenyl , naphthalene, indolene and pentalene, and

whereby the process comprises the steps of;

a) mixing and melting the ingredients at a temperature between Tm and Tm plus 150°C, whereby Tm is the melting temperature of the polymer;

b) cooling and pelletizing the mixture;

c) feeding the pelletized mixture in an extruder at a feeding speed between 100 and 800 kg/h, whereby the extruder barrel has a temperature between Tm minus 50 and Tm plus 150°C;

d) cooling the extruded materials in a single step at a temperature between room temperature and below Tm; and

e) optionally collecting the extruded materials at a collecting speed between 10 and 40 m/min.

In one embodiment, the temperature in steps a) is between Tm and Tm plus 60°C, and the temperature in step c) is between Tm minus 50 and Tm plus 60°C.

The process according to the invention allows for both an increased feeding speed and an increased collecting speed. Another advantage is that degassing is no longer needed. These advantages save time, materials and overall costs related to the preparation of insulation materials. The process window of the new process is wider, which in turn allows for improved control of the process and thus improved quality of the materials obtained by this process. In one embodiment, the temperature in step a) is between Ts and Ts plus 30°C, whereby Ts is the melting temperature of the one or more nucleating agent, and whereby the temperature of the extruder barrel in step c) is between Tm minus 50 and Ts plus 30°C.

Allowing the temperatures of the melt to be above the melting temperature of the nucleating agent(s) has the benefit of a complete melting of the nucleating agent. This improves the distribution of the nucleating agent in the polymer melt.

In another embodiment, the a-nucleating agent is bis(4- propylbenzylidene)propyl sorbitol, and the β-nucleating agent is N',N'-dicyclohexyl-2,6-naphthalene-dicarboxiamide.

The nucleating agents are commercially available and economically attractive. These nucleating agents can be used together with a great variety of polymers or polymer mixtures. In a further embodiment, the at least one C_2-80lefin polymer is selected from the group comprising LDPE, crosslinkable LDPE, HDPE, iPP, cPP, PP based elastomer and a copolymer of a C_2- solefin polymer. In one embodiment, the insulation materials comprise a combination of two polymers selected from a combination of 10 to 30 wt% HDPE and 70 to 90 wt% LPDE, or a combination of 10 to 30 wt% HDPE and 70 to 90 wt% crosslinkable LDPE, or a combination of 40 to 60 wt% iPP and 40 to 60 wt% cPP, or a combination of 60 to 80 wt% iPP and 20 to 40 wt% PP based elastomer.

The insulation materials obtained from these combinations of polymers show a uniform morphology, low conductivity, low space charge distribution and high breakdown strength.

The present invention also relates to a process for preparing a HV power cable comprising one or more conductors circumferentially isolated by insulation materials that comprises the ingredients as defined above, whereby the process comprises the steps of;

a) feeding the ingredients in an extruder at a feeding speed between 100 and 800 kg/h, whereby the extruder barrel has a temperature between Tm minus 50 and Ts plus 50°C;

b) extruding the ingredients on one or more conductors;

c) cooling the extruded HV power cable at a temperature between room temperature and below Tm; and

d) optionally collecting the extruded power cable at a collecting speed between 10 and 40 m/min.

The advantages of this process are apparent from the discussion hereinabove with reference to the proposed process. Another object is to provide improved insulation materials suitable for use in high voltage or extra high voltage power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) terminations and semiconducting screening materials together with acetylene carbon black.

This object is achieved by insulation materials prepared by the process described above.

The object is also achieved by the insulation materials suitable for use in HV power applications as defined in claim 8, comprising

- at least one polymer selected from the group comprising HDPE, cPP, PP based elastomer, or

a combination of such polymer with a polymer selected from LDPE, crosslinkable LDPE, and a copolymer of a C_2-80lefin polymer, and

H C - O H

C H ₂ - O H (I)

wherein X1 and X2 are independently selected from H , C-_Moalkyl and C _{- 0}alkoxy, and

a β-nucleating agent of formula I I

H H

(l l)

wherein R is an aliphatic cyclic or non-cyclic hydrocarbon chain, and

Ar is an aromatic group selected from phenyl , diphenyl, naphthalene, indolene and pentalene, and

whereby the insulation materials is free of spheroids, mineral oil and isotactic polypropylene.

The high crystallization temperature (solidification at high temperature) of the mixture of ingredients allows for a faster preparation process compared to conventional processes. This improved production speed decreases production costs. The new insulation materials have a lower variation in morphology, or a substantially uniform morphology. The crystallization of the insulation materials is substantially independent of the cooling rate. The mechanical, electrical and thermal properties of the new insulation materials are more uniform and substantially independent of the cooling rate. The properties of the materials are improved , such as a decrease in space charge accumulation and a faster decay of space charges. The insulation materials allows for an increase of the collecting speed.

In one embodiment, the insulation materials comprise a combination of two polymers selected from a combination of HDPE and LPDE, or a combination of HDPE and crosslinkable LDPE, and whereby a ratio of the polymers is between 10:90 and 50:50 wt%.

In another embodiment, the insulation materials comprise a combination of between 10 to 50 wt% HDPE and 50 to 90 wt% LPDE.

These combinations of polymers have the advantage of providing flexible insulation materials for the applications due to the presence of LDPE, while at the same time providing insulation materials having an increased thermal resistance due to the presence of HDPE. This combination provides insulation materials having a low conductivity and increased breakdown strength. These combinations of polymers allow for an optimized production speed.

In a further embodiment, the insulation materials comprise a combination of between 0 to 50 wt% HDPE and 50 to 100 wt% crosslinkable LDPE, and a crosslinking agent in an amount of 0.1 to 2 wt%.

The combination provides materials having the advantageous properties of crosslinkable LDPE.

In one embodiment, the a-nucleating agent is bis(4- propylbenzylidene)propyl sorbitol, and the β-nucleating agent is N',N'-dicyclohexyl-2,6-naphthalene-dicarboxiamide.

In another embodiment, the a-nucleating agent and/or nucleating agent is present in an amount of 0.1 to 0.8 wt%. The advantage of the combination of two different nucleating agents is an increase of the breakdown strength of the resulting insulation materials, which is a desired characteristic for a HV application . A further embodiment relates to insulation materials suitable for use in HV power applications comprising a combination of two polymers selected from 30 to 90 wt% homo or copolymer of polypropylene and 10 to 90 wt% polypropylene based elastomer, whereby the elastomer has a phase separation below 100 nm, when analyzed by scanning electron microscopy, and

at least one nucleating agent selected from bis(4- propylbenzylidene)propyl sorbitol and N',N'-dicyclohexyl-2,6- naphthalene-dicarboxiamide, whereby the at least one nucleating agent is present in an amount of 0.1 to 3 wt%, and

whereby the insulation materials are free of spherulitic crystals and mineral oil .

In another embodiment, the polypropylene is isotactic polypropylene.

An advantage of this embodiment is the excellent electrical properties of the isotactic polypropylene when combined with a nucleating agent. The change in morphology from spherulitic to shish kebab might explain these properties.

In one embodiment, insulation materials suitable for use in HV power applications comprise a combination of two polymers selected from 40 to 60 wt% isotactic polypropylene and 40 to 60 wt% copolymer of polypropylene, and

one or two nucleating agent in an amount of 0.1 to 3 wt%, selected from an a-nucleating agent of formula I

H C - O H

C H ₂ - O H (I)

a β-nucleating agent of formula I I

H H

(I I)

wherein R is an aliphatic cyclic or non-cyclic hydrocarbon chain, and

whereby the insulation materials are free of spherulitic crystals and mineral oil.

In a further embodiment, insulation materials defined above comprise a shish kebab structure when visualised with scanning electron microscopy.

One possible mechanism explaining breakdown in insulation materials is that breakdown follows the shorter path between the crystalline and amorphous phase of the insulation materials. The formation of shish kebab instead of the classical spherulites increases the length of the path. The advantage of that is an improved electrical insulation property and a uniformity of the morphology. The properties are independent of the cooling rate used in the preparation process.

The objects are also achieved by a use of insulation materials defined above in high voltage or extra high voltage power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) terminations and semiconducting screening materials together with acetylene carbon black. A further embodiment relates to a HV power application comprising concentrically arranged:

- an elongate conductor;

- a first screening 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.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig . 1 shows a flow scheme of the process.

Fig . 2 shows DSC scans of polymeric blends with and without an a-nucleating agent.

Fig . 3 shows a SEM Images of polymeric blends with and without an a-nucleating agent.

Fig . 4 shows conductivity as a function of the electric field for different temperatures and cooling rate on polymeric blends with and without an α-nucleating agent. Fig . 5 shows breakdown strength of polymeric blends with and without an α-nucleating agent. Fig . 6 Weibull parameters from DC breakdown test on of polymeric blends with and without an a-nucleating agent.

Fig. 7,8 shows space charge measurements of polymeric blends with and without an a-nucleating agent.

Fig . 9 shows a schematic overview of the apparatus used to perform conductivity measurements.

Fig. 10 shows a schematic overview of the system used to perform DC breakdown measurements.

DETAI LED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

Preparation process

Fig . 1 shows a flowchart of a process for preparing insulation materials for HV power applications. The insulation materials comprise a mixture of one or more polymers and at least one nucleating agent such as an α-nucleating agent and/or a β- nucleating agent. Examples of ingredients of the insulation materials are specified below.

In one embodiment, the ingredients are mixed in the first step of the process. The mixture of ingredients is heated to melt the ingredients. Mixing and melting can be performed simultaneously.

The melting temperature may be a temperature between the melting temperature of the polymer (Tm) and about 150°C above Tm (Tm plus 150°C). The melting temperature may also be between Tm and 50°C to 100°C above Tm, or between Tm and 60°C above Tm. The melting temperature may be between 100 and 300°C. For insulation materials comprising mainly polyethylene, the melting temperature may be between 100 and 170°C. For insulation materials comprising mainly polypropylene, the melting temperature may be between 190 and 250°C. Alternatively, the melting temperature may be a temperature between the melting temperature of the nucleating agent (Ts) and about 30°C above Ts (Ts plus 30°C). The melting temperature may also be between Ts and 20 to 40°C above Ts. The melting temperature may be between 150 and 290°C. The melting temperature may be between 210 and 250°C. Preferably, the melting temperature of the nucleating agent (Ts) falls within the range of a temperature between Tm and Tm plus 150°C. In a second step of the process, the melted mixture of ingredients is cooled to a temperature between room temperature and a temperature of solidification of the mixture. The temperature by which the mixture solidifies may differ and depend on the specific ingredients present in the mixture.

After solidification , the obtained materials may be cut into smaller pieces. For example, the materials may be pelletized.

In the next step of the process, the pelletized materials are fed into an extruder. The feeding speed may be between 100 and 800 kg/h , or between 250 and 750 kg/h , or between 300 and 500 kg/h, or above 300 kg/h, or about 400 kg/h .

The extruder barrel may have a temperature that allows the materials to flow through the extruder. This temperature may be between Tm minus 50°C and Tm plus 150°C, or between Tm minus 30°C and Tm plus 60°C, and will differ and depend on the specific ingredients present in the mixture. Examples of temperature ranges may be between 50°C and 250°C, or 90°C and 140°C, or 130°C and 210°C.

Alternatively, the extruder barrel may have a temperature between Tm minus 50°C and Ts plus 30°C. Examples of temperature ranges may be between 50°C and 250°C, or 90°C and 230°C, or 120°C and 300°C. The product obtained from the extruder is cooled in a next process step. Cooling is preferably performed in a single step procedure, at a temperature between room temperature and a temperature below Tm. Examples of temperature ranges may be between -50°C and 150°C, or -20°C and 80°C, or -10°C and 100°C.

The cooling rate may vary. The mechanical , electrical and thermal properties of the obtained cooled materials are preferably independent of the cooling rate used in the process step. The cooling rate may be between 5°C/min and 200°C/min .

The thickness of the obtained materials may vary. In one embodiment, the thickness of the obtained materials is more than 0.5 mm, or 0.8 mm. In another embodiment, the thickness is 12 mm or more, or between 5 and 30 mm, or between 15 and 25 mm.

In a next process step, the extruded materials may be collected at a collecting speed between 1 and 50 m/min, or between 10 and 40 m/min, or between 20 and 50 m/min, or between 5 and 20 m/min . The collecting speed may depend on the thickness of the insulation materials.

The present invention also relates to a process for preparing an insulation cable that can be used in HV power cables. Said cable comprises one or more conductors that are circumferentially isolated by the insulation materials comprising the ingredients as defined below.

In a first step of preparing the HV power cables an extruder is fed with the mixture of ingredients. The ingredients may have been melted prior to feeding at the temperatures mentioned above. The feeding speed may be the same as the feeding speed mentioned above. The temperature of the extruder barrel may be between Tm minus 50 and Ts plus 50°C. Examples of temperature ranges may be between 50°C and 250°C, or 90°C and 140°C, or 120°C and 300°C. In the next process step, the ingredients are extruded on the cable and cooled at a temperature between room temperature and a temperature below Tm. Examples of temperature ranges may be between -50°C and 150°C, or -20°C and 80°C, or -20°C and 100°C.

The thickness of the obtained materials may be the same as mentioned above. In a next process step, the extruded materials may be collected at a collecting speed between 1 and 50 m/min, or between 10 and 40 m/min, or between 20 and 50 m/min, or between 5 and 20 m/min . The collecting speed may depend on the thickness of the insulation materials.

This same or similar process for preparing a HV power cable may be used for preparing other HV power applications as mentioned below. The invention also relates to an HV power application, which comprises an elongate conductor that is circumferentially covered by a first screening semiconducting layer. This first semiconducting layer is in turn circumferentially covered by a first layer of insulation materials described below. A second screening semiconducting layer subsequently covers the first layer of insulation materials. This second screening semiconducting layer is 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 armour.

Insulation materials

The invention also relates to insulation materials prepared by the process described above and to high voltage or extra high voltage direct or alternating current (collectively referred to as HV) power applications comprising said insulation materials. The invention further relates to HV power applications prepared according to the process outlined above. The HV power applications may be selected from joints that connect power cables, terminations at the end of the power cables, bushings, insulated buses, bus bars and semiconducting screening materials comprising said insulation material together with acetylene carbon black. The material is especially suitable for use in HVDC power cables.

The novel insulation materials may be characterized by their mechanical, electrical and thermal properties. The insulation materials are free or substantially free of spherulites. Rather, "shish kebab" objects are present in the insulation materials according to the present invention . Preferably, the insulation materials are free or substantially free of mineral oil. In one embodiment, the insulation materials are free of isotactic polypropylene.

The breakdown strength and space charge density of the insulation materials are independent of the cooling rate used in the preparation process. In one embodiment, the breakdown strength has a characteristic Weibull Eo parameter of 430±50 kV/mm, when measured in 100 μιτι layers using breakdown test as described below.

In another embodiment, the space charge distribution does not show any presence of homocharges.

The decay of the accumulated charge occurs in the first 2-5 min. I n samples without the nucleating agent this decay occurs later. The insulation materials may have a thickness of more than 0.5 mm, or more than 0.8 mm. The thickness may be 12 mm or more, or between 5 and 30 mm, or between 15 and 25 mm.

The insulation materials according to the present invention comprise one or more polymers. In one embodiment, the combination comprises one polymer. In another embodiment, the combination comprises two polymers. The polymers may be polyethylene or polypropylene based materials. 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) and PP based elastomer. The elastomer has a phase separation (between a crystalline and an amorphous phase) below 150 nm, or below 125 nm, or below 100 nm, when observed in a scanning electron microscope.

The polymers may also be a copolymer of a C_2-80lefin polymer. Examples of C_2-80lefin polymer may be ethylene, propylene, butylene, pentene, hexane, heptene or octane, or mixtures thereof, in any isomeric or stereoisomeric form.

In one embodiment, the insulation comprises one or more polymers selected from the group comprising HDPE, non-isostatic PP or PP based elastomer. In another embodiment, the insulation materials comprise a combination of at least two polymers selected from the group comprising LDPE, crosslinkable LDPE, HDPE, cPP, PP based elastomer and a copolymer of a C_2-80lefin polymer, thereby disclaiming isostatic PP.

Examples of specific combinations of polymers may be HDPE and LPDE, which may be crosslinkable, HDPE and iPP, a combination of HDPE and cPP, a combination of HDPE and PP based elastomer, a combination of LPDE, which may be crosslinkable, and iPP, a combination of LPDE, which may be crosslinkable, and cPP, a combination of LPDE, which may be crosslinkable, and PP based elastomer, a combination of iPP and cPP, a combination of iPP and PP based elastomer, and a combination of cPP and PP based elastomer. The improved insulation materials according to the present invention may be obtained by mixing any of these combinations of polymers together with any nucleating agent(s) mentioned below. The polymers may be combined in a ratio between 10:90 and 50:50 weight percentage of total weight of the polymer mixture (wt%). Examples of ratios in a combination of polymers may be 10 to 50 wt% HDPE and 50 to 90 wt% LPDE, or 10 to 30 wt% HDPE and 70 to 90 wt% LPDE. Another example may be a combination of 10 to 50 wt% HDPE and 50 to 90 wt% crosslinkable LDPE, or 10 to 30 wt% HDPE and 70 to 90 wt% crosslinkable LDPE, together with a crosslinking agent in an amount of 0.1 to 2 wt%. Further examples may be a combination of 40 to 60 wt% iPP and 40 to 60 wt% cPP, or a combination of 60 to 80 wt% iPP and 20 to 40 wt% PP based elastomer.

More specific combinations of polymers may comprise 20 wt% HDPE and 80 wt% LPDE, or a combination of 20 wt% HDPE and 80 wt% crosslinkable LDPE. Other specific combinations may be 50 wt% iPP and 50 wt% cPP, or a combination of 70 wt% iPP and 30 wt% PP based elastomer. The improved insulation materials according to the present invention may be obtained by mixing any of these combinations of polymers together with any nucleating agent mentioned below.

The crosslinking agent may be selected from the group comprising organic peroxides, azo compounds, silanes. One example of a crosslinking agent may be dicumyl peroxide.

The crosslinking agent may be used in an amount of 0.1 to 2 by weight of the total weight of polymer (wt%), or 0.5 to 1 .5 wt%. In one embodiment, dicumyl peroxide is used as a crosslinking agent in an amount of 0.6 to 1 .2 wt %.

The at least one nucleating agent may be an a-nucleating agent and/or a β-nucleating agent.

The α-nucleating agent may be a sorbitol derivative. For example, the derivatives described in WO2007/127067 may be used in the insulation materials of the present invention. Other suitable sorbitol derivatives may be any compound of formula

(I)

wherein X and X₂ are independently selected from H, C-_Moalkyl and Ci-ioalkoxy. In one embodiment, X and X₂ are i ndependently selected from H, methyl, ethyl, n-propyl, i-propyl, n-butyl and sec- butyl. In another embodiment, X and X₂ are independently selected from H, n-propyl and i-propyl. In a further embodiment, X and X₂ are independently selected from methoxy, ethoxy, n-propoxy, i- propoxy, n-butoxy and sec-butoxy. In yet a further embodiment, X and X₂ are the same. In one embodiment, the α-nucleating agent is dibenzylidene sorbitol. In another embodiment, the a-nucleating agent is bis(4-propylbenzylidene)propyl sorbitol.

Another example of an α-nucleating agent may be a compound of formula I I I

The β-nucleating agent may be any compound of formula I I

(l l)

wherein R is an aliphatic hydrocarbon cyclic or non-cyclic chain, and

Ar is an aromatic group selected from phenyl, diphenyl , naphthalene, indolene and pentalene.

Examples of R may be cyclopentyl, cyclohexyl, cycloheptyl or cyclooctyl . In one embodiment, R is cyclohexane. Other examples of R may be methyl, ethyl , propyl, butyl, pentyl, hexyl, heptyl or octyl, in any isomeric or stereomeric form.

In one embodiment, Ar is naphthalene.

An example of a β-nucleating agent is N' ,N'-dicyclohexyl-2,6- nap

The one or two nucleating agent may be used in an amount of 0.01 to 10% by weight of the total weight of polymer (wt%). The nucleating agent may be used in an amount of 0.01 to 3 wt%, or 0.01 to 1 wt%, or 0.05 to 1 wt%, or 0.1 to 1 wt%, or 0.1 to 0.8 wt%, or 0.6 to 0.8 wt%, or about 0.5 wt%.

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

Experiments

Materials

LDPE used was of electrical grade LDPE and with a typical MFR (melt flow rate) value of 2 g/10 min at 190^°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.

Bis(4-propylbenzylidene) propyl sorbitol (DBS) as a-nucleating agent. Blends/mixtures containing a combination of about 80 wt% of LDPE and about 20 wt% of HDPE (LDPE/HDPE 80/20), or a combination of about 50 wt% of iPP and about 50 wt% of cPP (iPP/cPP 50/50) were prepared. Two batches of each combination of polymers were prepared, whereby one batch also comprised about 0.5 wt% of DBS. The other batch did not comprise DBS. In the following a comparison of the different properties is shown as examples.

Process The materials were prepared first in small scale with melt blending , and scaled up to an experimental extruder compounder. The characterization of the samples was done in order to ensure the reproducibility from upscaling. Results

Differential scanning calorimetry (DSC). Fig . 2a shows that the blend of LDPE/HDPE 80/20 (left) and the blend of LDPE/HDPE 80/20 + 0.5wt% of DBS (right) each exhibit two transitions on melting or crystallization. The higher transition being associated with the HDPE component and the lower transition with the LDPE component. When comparing the two systems, it is clear that the transition associated with the HDPE component is broader in the sample containing DBS, but that the melting occurs at the same temperature. A significant difference between the blends is that the HDPE crystallizes at 1 17°C (i.e. crystallization of the HDPE component occurs earlier in the sample containing DBS, which indicates a nucleating effect of the DBS). In the fast cooled sample the crystallization appears between the melting associated with LDPE and the melting associated with HDPE. In both blends the total enthalpy associated with melting or crystallization is about 130 J/g and is slightly lower for fast cooled samples. Fig . 2b shows similar results for a mixture comprising the blend of iPP/cPP 50/50 (left) and a blend of iPP/cPP 50/50 + 0.5 wt% of DBS (right). Comparison between the batches shows that the samples containing DBS are more independent of the cooling rate.

Apparatus, method

A pair of 3 mm thick steel mould plates (about 15 x 15 cm) and a heated hydraulic laboratory press were used to prepare film and plaque samples for subsequent testing. Initial material stock was pressed into about 2 mm thick plaques at 150 °C to prevent any oxidation at this stage using a pressure of 3 tons/2 minutes between sheets of Melanex. Plaques were then allowed to cool to room temperature, cut and re-pressed as many times as necessary to remove any included air bubbles. Subsequent samples (10 x 10 cm) for analysis were prepared using spacers; a 2 mm thick spacer was manufactured from steel and equivalent 0.3 mm and 0.15 mm thickness spacers were manufactured from Melanex. An appropriate weight of polymer (assuming a density of 1 g/cm3) was used. To produce about 0.1 mm thickness samples for DC breakdown testing, a 0.5 g sample and no spacer was used instead. In all cases, the inside of the steel mould plates was lined with 0.1 mm thick Melanex sheets to prevent sticking. The pressing was performed at 180°C and the polymer was allowed to melt for 5 minutes. Pressure of 3 ton (30 bar or 30 Pa) for 0.3 and 0.15 mm samples or 5 ton (50 bar) for 0.1 mm samples was then applied and maintained for a further 5 minutes, after which the polymer was permitted to relax for a further 5 minutes.

The resulting plaques were then subjected to non-isothermal crystallization conditions as follows. Fast cooling was performed by plunging samples directly into tap water at 25°C, whilst a medium cooling rate (about 5 K/min over about 60 min) was achieved by placing the mould assembly onto a heatproof brick and allowing it to cool to room temperature without interfering. Slow cooling (about 1 K/min over about 3 hours) was achieved by leaving the mould in situ in the press and simply turning the heaters off.

Differential Scanning Calorimetry (DSC) was performed using 2 to 5 mg samples enclosed in standard aluminum DSC cans in a Perkin Elmer DSC-7 instrument. Before use, the instrument was calibrated at various scan rates with high purity indium allowing melting and cooling scans to be performed at various rates. Crystallization was performed by cooling at constant rates from 1 to 10 K/min, while subsequent melting scans were performed by heating the samples at 10 K/min. Scanning microscopy

Fig. 3 shows a selection of SEM images. In Fig. 3a the blend of LDPE/HDPE 80/20 is characterized by a morphology of banded spherulites, which are about 10 μιτι in diameter. The objects become larger and more sheaf-like under slow cooling. In contrast, "shish kebab" objects are formed in the DBS-containing blend and are indicative of epitaxial crystallization of HDPE on DBS fibrils as discussed in G. Gherbaz, et al., In 2008 Ann. Rep. Conf. Electr. Insul. Diel. Phen., pp. 161 -164, 2008. The slow cooling in the DBS containing blend appears to show evidence of etched DBS fibrils (top centre). Fig. 3b shows similar results for a mixture comprising the blend of iPP/cPP 50/50 (left) and a blend of iPP/cPP 50/50 + 0.5 wt% of DBS (right). Apparatus, method

Samples were initially chemically etched to provide contrast in the SEM. Etching was performed using a standard permanganic reagent composed of 1 % potassium permanganate in an acid mixture composed of 1 part water, 2 parts phosphoric acid and 5 parts sulphuric acid. The etching was performed for 4 hours under vigorous shaking after which the etchant was quenched in a mixture of 1 part hydrogen peroxide in 4 parts of a 2:7 mixture of sulphuric acid and water. Samples were recovered, washed twice in distilled water and twice in acetone and left to dry. The samples were mounted onto standard aluminum SEM stubs using double sided sticky tape and then sputter coated with gold.

SEM studies were performed using a Jeol JSM5910 instrument operated at 15 kV using the secondary electron mode with a spot size of 25 nm. Images were obtained and stored electronically using the supplied software.

Conductivity

Field dependency on the electrical conductivity was determined for three different temperatures and for three different cooling rates. Fig . 4a shows the plot for the blend of LDPE/HDPE 80/20 and the blend of LDPE/HDPE 80/20 (left) + 0.5 wt% of DBS (right). Fig . 4b shows the plot for blend of iPP/cPP 50/50 (left) and a blend of iPP/cPP 50/50 + 0.5 wt% of DBS. From the plot it is clear that the conductivity does not significantly change by the addition of DBS.

Apparatus, method

DC conductivity was performed using equipment constructed in a house as shown schematically in Fig 9. Measurements were made on gold coated 0.3 mm thick samples at 25, 60 and 90°C (measured using the sample holder). Half an hour was allowed for each temperature to stabilize prior to taking measurements. Standard measurements were performed from 0.2 to 12 kV (0.7 to 40 kV/mm) in steps of 0.2 kV. 10 seconds settling time was allowed at each voltage before making a current measurement. Application of higher fields was avoided due to increased risk of sample breakdown and possible damage to the picoammeter. Breakdown strength

The Weibull plots are shown in Fig. 5 and the associated Weibull parameters are listed in table of Fig. 6. The typical uncertainty in the breakdown values (Eo) is ± 10 kV/mm. Previously, in the absence of DBS, slow or medium cooling resulted in enhanced breakdown strength, whereas with DBS present, the breakdown strength was not strongly dependent on thermal processing as evident in Fig. 5. While the shape parameter (β values) are consistent (Fig. 6) and the same trends are maintained with cooling rate independent of the presence of DBS, the melt DBS containing blends exhibit an about 10% increase in breakdown strength compared to the non-DBS containing blends. Whether this is due to morphological or chemical effects is unclear, however the current data certainly does not indicate that melt DBS containing blending is detrimental to electrical breakdown strength and therefore represents a valid way of preparing insulation materials.

Apparatus, method

The apparatus was based upon the standard ASTM D 149-87 for the ramp testing of solids and equipment was constructed based around a Spellman SL150 100 kV DC power supply (Fig 10). The test cell was composed of a Perspex frame incorporating two spring loaded opposing 6.25 mm steel ball bearings immersed in silicone oil and 0.1 mm thick sheets were tested. A minimum distance of 10 mm between adjacent breakdown holes was necessary to avoid flashovers and the ball bearings were changed after every 10 breakdowns as stipulated in the standard. The CCU ("custom control unit") was built in house and generates a rising control voltage at a built in house and performing three functions; (a) generates a rising control voltage at a constant ramp rate to provide an increasing voltage at the sample, (b) detects a current of > 0.3 mA indicating that a breakdown has occurred, (c) safely shuts down the high voltage power supply in the event of a breakdown event. Additional protection to the DC supply in the event of a breakdown is afforded by a series protection resistor. The external meter (Maplin WG020) linked to the power supply and run in maximum hold mode permits the maximum voltage (i.e. the breakdown voltage) to be held for subsequent recording. An increasing DC voltage at a rate of 100 V/s was applied to the test specimen until failure. Twenty tests were processed using Weibull statistics to yield the DC breakdown strength and shape parameter.

Space charge density

Space charge results indicate that DBS containing blends and non- DBS containing blend are well behaved systems and display only minor accumulations of homocharge under the electrodes. There was no change in the behaviour depending on cooling conditions and therefore only results for medium cooled samples are shown here. Initial measurements (Fig. 7a1 ) indicate homocharge accumulation (arrowed) with clear changes still evident after 60 min. Charge decay measurements (Fig. 7a1 right) indicate a negative charge (and a corresponding positive image charge on the electrode). The opposite effect occurs at the anode. The measurements confirm the formation of homocharge at both electrodes with no significant charge accumulation in the bulk of the sample.

Fig. 7a2 shows analogous data for the blend containing DBS, whilst the type of charge is identical, the magnitude of stored charge is significantly reduced by the presence of DBS. This reduction is particularly evident in charge decay measurements (Fig. 7a2 right), which do not indicate any stored charge, the singular peaks, which do not change over time, arise solely due to the applied DC pulse. Fig . 8b1 , 8b2 shows similar results for a mixture comprising the blend of iPP/cPP 50/50 (left) and a blend of iPP/cPP 50/50 + 0.5 wt% of DBS (right).

Apparatus, method

Space charge measurements were performed on 0.15 mm (nominal) thickness samples using a PEANUTS pulsed electro-acoustic system (5- Lab). This system has the facility of applying both a constant DC voltage to the sample as well as a 400 Hz, 600 V signal for the measurement of space charge and employs a PVDF sensor. Data were collected using a Tektronix storage scope at 2 GHz sample rate and then de-convoluted using standard techniques [1 , 2] implemented in a Labview program. After performing a calibration at 1 kV (about 7 kV/mm, a field low enough not to introduce any stored charge), a constant DC voltage was applied to the samples (6 kV (about 40 kV/mm)) and measurements of space charge were then performed after 2, 5, 10, 20, 40 and 60 minutes. Finally, space charge decay measurements were performed by repeating the above protocol but with the DC power supply switched off over a period of 40 minutes.

1. T. Maeno, T. Futami, H. Kushibe, T. Takada, "Measurement of Spatial Charge Distribution in Thick Dielectrics Using the Pulsed Electroacoustic

Method", IEEE Trans. Electr. Insul., 23, 3, pp. 433-439, 1988.

2. Y. Li, M. Yasuda, T. Takada, "Pulsed Electroacoustic Method Measurement of Charge Accumulation in Solid Dielectrics", IEEE Trans. Diel. Electr. Insul., 1 , 2, pp. 188-195, 1994.

Definitions

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 used herein means densities between 0.90 and 0.93 g/cc.

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

The wording "fast cooling" as used herein means cooling under water at 25°C from melt.

The wording "medium cooling" as used herein means about 5 K/min during 60 min.

The wording "slow cooling" as used herein means about 1 K/min over 3 h.

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

The wording "power application" as used herein includes applications for insulation material selected from high voltage or extra high voltage power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) terminations and semiconducting screening materials together with acetylene carbon black.

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

. A process for preparing insulation materials for HV powe pplications, whereby the insulation materials comprise

- at least one C_2-80lefin polymer, and

one or two nucleating agent in an amount of 0.1 to 3 wt% selected from an a-nucleating agent of formula I

a β-nucleating agent of formula I I

H H

(l l)

whereby the process comprises the steps of;

b) cooling and pelletizing the mixture;

c) feeding the pelletized mixture in an extruder at a feeding speed between 100 and 800 kg/h, whereby the extruder barrel has a temperature between Tm minus 50 and Tm plus 150°C; d) cooling the extruded materials in a single step at a temperature between room temperature and below Tm; and

2. A process according to claim 1 , whereby the temperature in step a) is between Ts and Ts plus 30°C, whereby Ts is the melting temperature of the one or more nucleating agent, and whereby the temperature of the extruder barrel in step c) is between Tm minus 50 and Ts plus 30°C.

3. The process according to claim 1 or 2, whereby the a-nucleating agent is bis(4-propylbenzylidene)propyl sorbitol, and the β- nucleating agent is N',N'-dicyclohexyl-2,6-naphthalene- dicarboxiamide.

4. The process according to any one of claims 1 to 3, whereby the at least one C_2-80lefin polymer is selected from the group comprising LDPE, crosslinkable LDPE, HDPE, iPP, cPP, PP based elastomer and a copolymer of a C_2-80lefin polymer.

5. The process according to any one of claims 1 to 4, whereby the insulation materials comprise a combination of two polymers selected from a combination of 10 to 30 wt% HDPE and 70 to 90 wt% LPDE, or a combination of 10 to 30 wt% HDPE and 70 to 90 wt% crosslinkable LDPE, or a combination of 40 to 60 wt% iPP and 40 to 60 wt% cPP, or a combination of 60 to 80 wt% iPP and 20 to 40 wt% PP based elastomer.

6. A process for preparing a HV power cable comprising one or more conductors circumferentially isolated by insulation materials that comprise the ingredients as defined in any one of claims 1 , 3, 4 or 5, whereby the process comprises the steps of;

b) extruding the ingredients on one or more conductors; c) cooling the extruded HV power cable at a temperature between room temperature and below Tm; and

7. Insulation materials prepared by the process according to any one of claims 1 to 5.

8. An insulation materials suitable for use in HV power applications comprising

(I)

wherein X1 and X2 are independently selected from H, Ci and C _{- 0}alkoxy, and

a β-nucleating agent of formula II

H H

(l l) wherein R is an aliphatic cyclic or non-cyclic hydrocarbon chain, and

whereby the insulation materials are free of spheroids, mineral oil and isotactic polypropylene.

9. The insulation materials according to claim 8, whereby the insulation materials comprise a combination of two polymers selected from a combination of HDPE and LPDE, or a combination of HDPE and crosslinkable LDPE, and whereby a ratio of the polymers is between 10:90 and 50:50 wt%.

10. The insulation materials according to claim 8, whereby the insulation materials comprise a combination of between 10 to 50 wt% HDPE and 50 to 90 wt% LPDE.

1 1 . The insulation materials according to claim 8, whereby the insulation materials comprise a combination of between 0 to 50 wt% HDPE and 50 to 100 wt% crosslinkable LDPE, and a crosslinking agent in an amount of 0.1 to 2 wt%.

12. The insulation mateirial according to any one of claims 8 to 1 1 , whereby the a-nucleating agent is bis(4-propylbenzylidene)propyl sorbitol, and the β-nucleating agent is N',N'-dicyclohexyl-2,6- naphthalene-dicarboxiamide.

13. The insulation materials according to any one of claims 8 to 12, whereby the α-nucleating agent and/or the β-nucleating agent are present in an amount of 0.1 to 0.8 wt%.

14. Insulation materials suitable for use in HV power applications comprising a combination of two polymers selected from 30 to 90 wt% homo or copolymer of polypropylene and 10 to 90 wt% polypropylene based elastomer, whereby the elastomer has a phase separation below 100 nm, when analyzed by scanning electron microscopy, and at least one nucleating agent selected from bis(4- propylbenzylidene)propyl sorbitol and N',N'-dicyclohexyl-2,6- naphthalene-dicarboxiamide, whereby the at least one nucleating agent is present in an amount of 0.1 to 3 wt%, and

whereby the insulation materials are free of spherulitic crystals oids and mineral oil.

15. The insulation materials according to claim 14, whereby the polypropylene is isotactic polypropylene.

16. Insulation materials suitable for use in HV power applications comprising a combination of two polymers selected from 40 to 60 wt% isotactic polypropylene and 40 to 60 wt% copolymer of polypropylene, and

(I)

wherein X1 and X2 are independently selected from H , Ci and C _{- 0}alkoxy, and

a β-nucleating agent of formula I I

H H

(l l) wherein R is an aliphatic hydrocarbon cyclic or non-cyclic chain, and

whereby the insulation materials are free of spheroids and mineral oil.

17. Insulation materials according to any one of claims 7 to 16, wherein the insulation materials comprise a shish kebab structure when visualised with scanning electron microscopy.

18. Use of insulation materials according to any one of claims 7 to 18 as insulation materials in high voltage or extra high voltage power applications selected from cables, joints, bushings, insulated buses, bus bars and (cable) terminations and in semiconducting screening materials together with acetylene carbon black.

19. A HV power application comprising concentrically arranged:

- an elongate conductor;

- a first layer of insulation materials according to any one of claims 7 to 18 circumferentially covering the first semiconducting layer, and

- a second layer of insulation materials according to any one of claims 7 to 18 circumferentially covering the second semiconducting layer, and