The present invention relates to an electrically insulating composite material, a method of making the material and electrical cables comprising the material.

Electrical and thermal ratings for medium-voltage and high-voltage solid-insulated alternating (AC) and direct current (DC) power cables are relevant to all power networks. In particular, these factors strongly influence distribution efficiency and infrastructure durability in hot climates in developing countries. They are also important considerations in networks where renewables generation sources are significant leading to widely varying network loading. Accordingly, cable

improvements are anticipated to particularly impact the scope of renewable energy connectivity in developing economies. The drive for improved insulation properties derives from the projected expansions in regional transmission networks and increased renewables connections to be undertaken around the globe. For instance, it is projected that by 2020 there will be a power cable demand of 490,000km p.a., including 1250km p.a. of offshore cables. Improved insulation properties will also enable milestones towards fulfilment of the Global Energy Interconnection vision that targets maximum global exploitation of renewable resources. For instance, the short-term need for 70GW additional generation capacity in sub-Saharan Africa has stimulated cross border transmission upgrade schemes where the increased supply is projected to comprise a majority of renewable sources. Additionally, growing demand in south-east Asia has been projected to be met in significant part by renewables including hydroelectric power in Cambodia, Lao PDR and Myanmar, geothermal in Indonesia and solar in Australia. Meanwhile, the Asian super grid plans interconnections for up to 10GW transmission of renewably generated power in north-east Asia.

The expansion presents opportunities for higher extra high voltage (EHV) capacity cables rated from sookV to 8ookV and above. This will require materials with greater resilience and durability to ensure low failure frequency and reduced transmission losses through superior heat rejection. Equally, medium and high voltage underground and submarine connections are proliferating to facilitate delivery of newly installed renewable energy to the grid. This requires cable deployment locations with challenging access for maintenance or replacement, increasing the value of improved resistance to failure, as well as locations where enhanced heat rejection will meaningfully impact transmission efficiency and insulation stability. Both AC and DC connections are required, with precise performance requirements dependent on transmission mode and power rating.

The present invention arises from the inventors’ work in attempting to overcome the problems with the prior art. In accordance with a first aspect of the invention, there is provided an electrically insulating composite material comprising a thermoplastic or thermoset and hexagonal boron nitride (h-BN), characterised in that the weight ratio of the thermoplastic or thermoset to h-BN is between 80:20 and 99.5:0.5. Advantageously, the addition of h-BN to a polymer matrix improves the electrical breakdown strength and resistivity of the material.

In a first embodiment, the weight ratio of the thermoplastic or thermoset to h-BN may be between 90:10 and 99.5:0.5, more preferably between 92:8 and 99:1, and most preferably between 94:6 and 98:2. The inventors have shown that compositions which fall within this embodiment exhibit the most improved electrical breakdown performance.

In a second embodiment, the weight ratio of the thermoplastic or thermoset to h-BN maybe between 80:20 and 90:10, between 82:18 and 88:12, or between 84:16 and 86:14.

In may be appreciated that the ratios in the first and second embodiment may be combined. For instance, in a third embodiment, the weight ratio of the thermoplastic or thermoset to h-BN maybe between 82:18 and 99.5:0.5, more preferably between 82:18 and 99:1, and most preferably between 84:16 and 98:2. By varying the ratio of the thermoplastic or thermoset to h-BN of the properties of the electrically insulating material can be adapted to ensure that the material has the required electrical and thermal properties for a given application. The electrically insulating composite material may comprise an antioxidant. The antioxidant may be selected from the group consisting of a hydroxyaryl (phenolic) compound, an aminoarene, a diaminoarene, a dialkylhydroxylamine, a

triarylphosphite, a mono- or di-aryloligo(alkyl)phosphite, a thiodipropionate dialkyl ester, a thiodipropionate dialkyl polyester; an alkyl disulfide and a

thiobis(hydroxyarene). The hydroxyaryl compound maybe an oligohydroxyaryl compound. Preferably, the electrically insulating composite material comprises less than 5 wt% antioxidant, more preferably less than 4 wt%, less than 3 wt%, less than 2 wt% or less than 1 wt%, and most preferably less than 0.5 wt%, less than 0.4 wt%, less than 0.3 wt% or less than 0.2 wt%. Preferably, the electrically insulating composite material comprises at least 0.01 wt% antioxidant, more preferably at least 0.02 wt%, at least 0.04 wt%, at least 0.06 wt% or at least 0.08 wt%, and most preferably at least 0.1 wt%. In a preferred embodiment, the electrically insulating composite material comprises between 0.1 wt% and 0.15 wt%.

In one embodiment, the electrically insulating composite material comprises a thermoset and h-BN. The thermoset may be a crosslinked polyolefin or a crosslinked polysiloxane. The crosslinked polyolefin maybe crosslinked polyethylene (XLPE). In a preferred embodiment, the electrically insulating composite material comprises a thermoplastic and h-BN. The thermoplastic may be a crosslinkable thermoplastic or thermoplastic elastomer. The thermoplastic may be a polyamide; a polyester; a polyolefin, poly( vinyl chloride), polytetrafluoroethylene, poly(vinyl acetate), a polyacrylate, polystyrene or a compatible mixture and/ or a copolymer thereof. The polyester may be poly( ethylene terephthalate), poly(butylene terephthalate) or polyflactic acid). The polyolefin may be polyethylene, polypropylene, polybutene, polyhexene, polyoctene, polybutadiene or a copolymer thereof. In a preferred embodiment, the thermoplastic is a polyolefin, more preferably polyethylene or polypropylene, and most preferably polypropylene.

Preferably, the h-BN comprises particles disposed within a matrix defined by the thermoplastic or thermoset.

Preferably, the h-BN particles have a mean largest dimension which is less than 50 pm, less than 30 pm, less than 10 pm, more preferably less than 5 pm, less than 4 pm, less than 3 pm or less than 2 pm. The largest dimension may be less than 1 pm. At least 95% of the h-BN particles may have a largest dimension which is less than to pm, less than 5 pm, less than 4 pm, less than 3 pm, less than 2 pm or less than 1 pm. At least 99% of the h-BN particles may have a largest dimension which is less than 10 pm, less than 5 pm, less than 4 pm, less than 3 pm, less than 2 pm or less than 1 pm. The electrically insulating composite material may not comprise any h-BN particles with a largest dimension of at least 10 pm, more preferably at least 5 pm, at least 4 pm, at least 3 pm or at least 2 pm, and most preferably is at least 1 pm. As discussed in the examples, the inventors assessed size of h-BN particles within the composite material using scanning electron microscopy (SEM). This method could be used to determine the mean largest dimension of h-BN particles within the composite material and/or that the composite material did not comprise any h-BN particles above a certain size. Alternatively, the size of the h-BN particles may be assessed prior to inclusion within the composition. For instance, the particle size may be determined using dynamic light scattering (DLS).

The electrically insulating composite material comprises less than 0.3 wt% of a molecular sieve, more preferably less than 0.1 wt% or less than 0.01 wt% of a molecular sieve. The electrically insulating composite material may comprise less than 1 wt% dioxtyl adipate, more preferably less than 0.1 wt% or less than 0.01 wt% dioxtyl adipate. The electrically insulating composite material may comprise less than 5 wt% or less than 1 wt% of a metal and/ or metal oxide, more preferably less than 0.1 wt% or less than 0.01 wt% of a metal and/or metal oxide. The electrically insulating composite material may comprise less than 5 wt% or less than 1 wt% carbon fibres and/or carbon nanotubes, more preferably less than 0.1 wt% or less than 0.01 wt% carbon fibres and/ or carbon nanotubes. The electrically insulating composite material may comprise less than 1 wt% rubber, more preferably less than 0.1 wt% or less than 0.01 wt% rubber.

Preferably, the electrically insulating composite material does not comprise a molecular sieve, dioxtyl adipate, a metal, a metal oxide, carbon fibres, carbon nanotubes and/or rubber.

The electrically insulating composite material comprises the thermoplastic or thermoset and h-BN, and optionally an antioxidant, which may amount to at least 98%, preferably at least 99% and more preferably at least 99.5% of the total composite mass. In one embodiment, the electrically insulating composite material consists of the thermoplastic or thermoset and h-BN, and optionally an antioxidant.

The h-BN particles may comprise one or more surface functional groups. The surface function groups may be a hydroxyl (OH) group or an amine (NH or NH ₂ ) group.

The h-BN particles may comprise one or more surface-associated species. The surface- associated species maybe present at exterior surfaces as a surfactant, or at interior surfaces as intercalated species. The surface-associated species may comprise a carboxylic acid or inorganic acid or a salt thereof; boric acid or a salt thereof; boron; water; a common organic dispersant; an amine or its ammonium salt; and/or an organic or inorganic halogen compound. The surface-associated species combine to at most 5% of h-BN mass, preferably at most 2.5% of h-BN mass, or at most 0.5% of the total mass of the composite.

The carboxylic acid maybe a C ₁ -C ₁₂ carboxylic acid, more preferably a C -C ₆ carboxylic acid. The inorganic acid may be a C ₁ -C ₁₂ sulfonic acid, sulfuric acid, chlorosulfonic acid, nitric acid, phosphoric acid or permanganic acid. The salt of the carboxylic acid, inorganic acid or boric acid may be an alkali metal or alkaline earth metal salt.

The common organic dispersant maybe a C -C ₆ alcohol or diol, a C -C ₆ ketone, a C -C ₆ ether or diether, a C -C ₆ ester, a C -C ₆ amide, a C ₁ -C ₁₀ aliphatic alkane or alkene, a C ₃ - C ₁₀ cyclic alkane or alkene, a C ₆ -C ₁₂ aromatic hydrocarbon or a 5 to 12 membered heterocyclic compound.

The amine may have formula NR ₃ and the ammonium slat may have formula [NR ₄ ]X, where each R is independently a hydrogen or a C ₁ -C ₁₀ alkyl or alkenyl and X is an anion. X may be a halogen.

Preferably, the electrically insulating composite material has a resistivity of at least lxio ¹² hm at 6o°C. More preferably, the electrically insulating composite material has a resistivity of at least 2x1o ¹² ilm, 4x1o ¹² ilm, 6xio ¹² hm or 8xio ¹² hm at 6o°C, and most preferably has a resistivity of at least 1x1o ¹³ ilm, 2x1o ¹³ ilm, 3x1o ¹³ ilm, 4x1o ¹³ ilm or 5x1o ¹³ ilm at 6o°C. Preferably, the resistivity is determined by determining the DC electrical conductivity on a sample with a thickness of 0.2 mm and then calculating the resistivity as the inverse of the conductivity. Preferably, the DC electrical conductivity was determined using a test cell containing opposing 20 mm diameter electrodes. Preferably, the DC electrical conductivity was determined using a constant field of 10 kV/mm and by measuring the current over 30 minutes.

Preferably, the electrically insulating composite material has an AC electrical breakdown strength of at least 150 kV/mm at 25°C, and more preferably at least 160 kV/mm, at least 170 kV/mm or at least 180 kV/mm at 25°C, and most preferably at least 190 kV/mm or 200 kV/mm at 25°C. Preferably, the AC electrical breakdown strength is determined by the method described in ASTM Di49-97a. Preferably, the AC electrical breakdown strength is determined using an increasing 50 Hz AC field (voltage ramp 500 V/s) applied until failure. Preferably, the electrically insulating composite material has a tensile strength of at least 10 MPa at 23°C, more preferably at least 11 MPa or at least 12 MPa at 23°C, and most preferably at least 12.5 MPa at 23°C. Preferably, the electrically insulating composite material has an elongation-at-break of at least 300% at 23°C, more preferably at least 350% at 23°C, and most preferably at least 400% at 23°C. The tensile strength and/ or elongation-at-break may be determined by the method described in ASTM D638. The specimen geometry used may be“Type IV”.

Preferably, the electrically insulating composite material has thermal conductivity of at least 0.35 W/mK at 6o°C, more preferably at least 0.36 W/mK at 6o°C, and most preferably at least 0.37 W/mK at 6o°C.

In a preferred embodiment, the electrically insulating composite material is formed by extrusion. In accordance with a second aspect, there is provided a method of producing an electrically insulating composite material, the method comprising contacting a thermoplastic, and optionally a crosslinking agent, with hexagonal boron nitride (h- BN), wherein the weight ratio of the thermoplastic, and optionally the crosslinking agent, to h-BN is between 80:20 and 99.5:0.5, and thereby producing an electrically insulating composite material. Preferably, the method of the second aspect provides the t electrically insulating composite material of the first aspect.

The electrically insulating composite material, the thermoplastic or thermoset and/or h-BN may be as defined in relation to the first aspect.

Preferably, the thermoplastic or thermoset is a thermoplastic.

The method may comprise:

- contacting h-BN with a first quantity of a thermoplastic material, and optionally a crosslinking agent, to provide a first composition;

contacting the first composition with a further quantity of a thermoplastic material, and optionally a crosslinking agent, to provide a further composition. The further composition may be the electrically insulating composite material.

Alternatively, the method may comprise contacting the further composition with a further quantity of a thermoplastic material, and optionally a crosslinking agent, to provide a yet further composition. This process may be repeated until the yet further composition possesses the desired ratio of thermoplastic and h-BN.

In a first embodiment, the method may comprise:

dissolving the thermoplastic, and optionally the crosslinking agent, in a first solvent;

- dispersing the h-BN in a second solvent;

combining the first solvent comprising the dissolved thermoplastic, and optionally the crosslinking agent, and the second solvent comprising the dispersed h-BN, thereby contacting the thermoplastic with h-BN; and removing the first and second solvent to produce the electrically insulating composite material or the first composition.

The first solvent may be selected to provide full solubility for the thermoplastic, to have an appropriate liquid temperature range, to be miscible with the second solvent and/or to evaporate readily. The first solvent may have a melting point which is less than 25°C, more preferably less than 20°C or less than 15°C, and most preferably less than io°C. The first solvent may have a melting point which is between -200°C and 25°C, more preferably between -150°C and 20°C or between -ioo°C and 15°C, and most preferably between -50°C and io°C. The first solvent may have a boiling point which is at least 8o°C, more preferably at least 90°C or at least ioo°C, and most preferably at least no°C. The first solvent may have a boiling point which is between 8o°C and 200°C, more preferably between 90°C and i8o°C or between ioo°C and i6o°C, and most preferably between no°C and 150°C.

A first solvent which meets the above criteria can be selected by the skilled person. For instance, the first solvent may comprise an aromatic compound, an alkane and/or an alkene. The aromatic compound, alkane and/or alkene may optionally be substituted with a halogen. The halogen maybe fluorine or chlorine. The first solvent may comprise xylene, toluene, tetrachloroethylene, chlorobenzene or heptane. The xylene maybe o-xylene, m-xylene, p-xylene or a combination thereof. In a preferred embodiment, the first solvent is a combination of o-xylene and p-xylene.

The thermoplastic may be dissolved in the first solvent at an elevated temperature. The elevated temperature may be at least 50°C, more preferably at least 8o°C, at least 90°C, at least ioo°C, at least no°C or at least 120°C, and most preferably at least 130°C or at least 140°C. The elevated temperature maybe between 50°C and 250°C, more preferably between 8o°C and 2io°C, between 90°C and 200°C, between ioo°C and 190°C, between no°C and i8o°C or between 120°C and 170°C, and most preferably between 130°C and i6o°C or between 140°C and 150°C.

The h-BN may be dispersed in the second solvent using sonication or high-shear mixing.

The second solvent may be selected to stably disperse the h-BN, to not dissolve the thermoplastic, to have an appropriate liquid temperature range, to be miscible with the first solvent and/ or to evaporate readily. The second solvent may have a melting point which is less than io°C, more preferably less than o°C or less than -25°C, and most preferably less than -50°C. The second solvent may have a melting point which is between -250°C and io°C, more preferably between -200°C and o°C or between -175°C and -25°C, and most preferably between -150°C and -50°C. The second solvent may have a boiling point which is at least 40°C, more preferably at least 50°C or at least 6o°C, and most preferably at least 65°C. The second solvent may have a boiling point which is between 40°C and 150°C, more preferably between 50°C and 125°C or between 6o°C and ioo°C, and most preferably between 05°C and 85°C.

A second solvent which meets the above criteria can be selected by the skilled person. For instance, the second solvent may comprise an alcohol, a ketone, an aldehyde or an ether. The alcohol maybe a C -C ₆ alcohol, more preferably a C -C ₃ alcohol. The alcohol maybe methanol, ethanol, i-propanol or 2-propanol. The ketone maybe a C ₃ -C ₆ ketone. The ketone may be acetone or butanone. The aldehyde may be a C -C ₆ aldehyde. The ether may be a C -C ₆ ether. The ether may a cyclic ether. The ether may be tetrahydrofuran (THF).

Subsequent to combining the first solvent comprising the dissolved thermoplastic and the second solvent comprising the dispersed h-BN, the method may comprise allowing the mixture to cool to a temperature below ioo°C, more preferably below 8o°C, 6o°C or 40°C, and most preferably below 30°C. While the mixture is allowed to cool, the method may comprise stirring the mixture.

The first and second solvents may be removed by evaporation, preferably under reduced pressure.

In a second embodiment, the method may comprise contacting thermoplastic and h- BN, wherein the thermoplastic is not dissolved in a first solvent and the h-BN is not dispersed in a second solvent. The thermoplastic and h-BN may be contacted at an elevated temperature. The elevated temperature may at least 50°C, more preferably at least 6o°C, at least 70°C, at least 8o°C, at least 90°C or at least ioo°C, and most preferably at least no°C, at least 120°C or at least 130°C.

The thermoplastic and h-BN may be mixed at a speed of at least 1 RPM, more preferably at least 2 RPM, at least 3 RPM or at least 4 RPM, and most preferably at least 5 RPM. The method may comprise melting the thermoplastic and then mixing the thermoplastic and h-BN at a speed of at least 10 RPM, more preferably at least 20 or 30 RPM and most preferably at least 40 RPM.

In embodiments where the method has produced a first composition, the method may comprise melt compounding the first composition with a further quantity of a thermoplastic material, and optionally a crosslinking agent, to provide a further composition.

The melt compounding maybe conducted at a temperature of at least 50°C, more preferably at least 6o°C, at least 70°C, at least 8o°C, at least 90°C or at least ioo°C, and most preferably at least no°C, at least 120°C or at least 130°C. The first composition and the further quantity of a thermoplastic material, and optionally a crosslinking agent, may be mixed at a speed of at least l RPM, more preferably at least 2 RPM, at least 3 RPM or at least 4 RPM, and most preferably at least 5 RPM. The method may comprise melting the first composition and the further quantity of a thermoplastic material and then mixing the first composition and the further quantity of a

thermoplastic material, and optionally a crosslinking agent, at a speed of at least 10 RPM, more preferably at least 20 or 30 RPM and most preferably at least 40 RPM. Alternatively, in embodiments where the method has produced a first composition, the method may involve contacting the first composition with a further quantity of a thermoplastic material, and optionally a crosslinking agent, and extruding the resultant mixture, to provide a further composition. To ensure homogenous mixing, the resulting mixture may be extruded multiple times. In a preferred embodiment, the resulting mixture is extruded at least three times, to provide a further composition. Preferably, after the resultant mixture has been extruded it is pelletised prior to being extruded again. The resultant mixture may be extruded at a temperature of at least 50°C, more preferably at least 6o°C, at least 70°C, at least 8o°C, at least 90°C or at least ioo°C, and most preferably at least no°C, at least 120°C or at least 130°C.

Preferably, the resultant mixture is extruded at a speed of at least 5 RPM, more preferably at least 10 RPM or at least 15 RPM, and most preferably at least 20 RPM.

In embodiments where the method comprises contacting a thermoplastic and a crosslinking agent with h-BN, the method preferably comprises providing a sufficient quantity of the crosslinking agent to provide a composition comprising between 0.5 wt% and 5 wt% of the crosslinking agent, more preferably between 1 wt% and 4 wt % of -lithe crosslinking agent, and most preferably between 2 wt% and 3 wt% of the crosslinking agent.

The crosslinking agent may be a peroxide, silane or sulfur. The peroxide may be a branched alkyl peroxide such as dicumyl peroxide (DCP) or an acyl peroxide such as benzoyl peroxide. The silane may be a mono, di, tri- or tetra- alkyl, alkenyl or aryl silane. In a preferred embodiment the crosslinking agent is dicumyl peroxide.

The method may comprise activating a crosslinking reaction and thereby produce an electrically insulating composite material comprising a crosslinked polymer.

Preferably, the crosslinking reaction is activated after the thermoplastic and crosslinking agent are contacted with h-BN. The method may comprise heating the composition comprising the h-BN, thermoplastic and crosslinking agent to activate the crosslinking reaction. Preferably, the composition is heated to at least ioo°C, more preferably at least 120°C, at least 140°C or at least i6o°C, and most preferably at least i8o°C or at least 200°C. Preferably, the mixture is heated to between ioo°C and 300°C, more preferably between 120°C and 28o°C, between 140°C and 26o°C or between i6o°C and 240°C, and most preferably between i8o°C and 220°C. Preferably, while the composition is heated it is subjected to a pressure of at least 10 bar, more preferably at least 20 bar, at least 30 bar or at least 40 bar, and most preferably at least 45 bar. Preferably, while the composition is heated it is subjected to a pressure of between 10 bar and too bar, more preferably between 20 bar and 80 bar, between 30 bar and 70 bar or between 40 bar and 60 bar, and most preferably between 45 bar and 55 bar. Advantageously, the high pressure prevents bubble formation within the material.

Preferably, the electrically insulating composite material comprising a crosslinked polymer is then degassed under vacuum, preferably under dynamic vacuum.

Preferably, the material is degassed at a temperature between 20°C and ioo°C, more preferably between 30°C and 90°C or between 40°C and 8o°C, and most preferably between 50°C and 70°C. Preferably, the material is degassed for at least 1 hour, more preferably at least 5 hours, at least 10 hours, at least 15 hours, or at least 20 hours, and most preferably at least 40 hours or at least 60 hours. Prior to contacting the thermoplastic or thermoset with h-BN, the method may comprise chemically and/or thermally exfoliating the h-BN.

The method may comprise thermally exfoliating the h-BN. The method may comprise holding the h-BN at an elevated temperature. The elevated temperature may be at least 200°C, at least 300°C, at least 400°C, at least 500°C, at least 6oo°C or at least 700°C or at least 8oo°C. The elevated temperature may be between 200°C and 1500°C, between 300°C and 1400°C, between 400°C and 1300°C, between 500°C and 1200°C, between 6oo°C and noo°C, between 700°C and iooo°C or between 8oo°C and 900°C. The temperature may be between 320°C and 6oo°C, between 350°C and 550°C or between 400°C and 500°C. The h-BN may be held at the elevated temperature for at least 5 minutes, more preferably at least 15 minutes, at least 30 minutes or at least 45 minutes, and most preferably at least 1 hour. The h-BN maybe held at the elevated temperature for between 5 minutes and 24 hours, more preferably between 15 minutes and 12 hours, between 30 minutes and 10 hours or between 45 minutes and 9 hours, and most preferably between 1 hour and 8 hours.

Subsequent to being held at an elevated temperature, the h-BN may be quenched by immersing the h-BN in a liquid. The liquid preferably has a mass at least 5 times greater than the mass of the h-BN, more preferably at least 10 times, at least 20 times, at least 30 time or at least 40 time greater than the mass of the h-BN, and most preferably at least 50 times greater than the mass of the h-BN. The liquid may have a pre-immersion temperature of less than 50°C, more preferably less than 40°C or less than 30°C, and most preferably less than 25°C. In a preferred embodiment, the liquid has a pre-immersion temperature of between -196 °C and 25 °C. The liquid may be water, liquid nitrogen or a volatile organic liquid.

The method may comprise allowing the liquid to reach ambient temperature. The method may comprise drying the h-BN under a vacuum to remove the liquid therefrom.

Alternatively, or additionally, the method may comprise chemically exfoliating the h- BN. The method may comprise:

dispersing the h-BN in a liquid to form a dispersion; and

contacting the dispersion with an acid to chemically or structurally modify the h-BN; or contacting the dispersion with an acid and hydrogen peroxide to create a reaction solution and thereby chemically or structurally modify the h-BN.

The liquid may comprise an alcohol, and the alcohol may comprise 2-propanol.

The dispersion may comprise between 1 wt% and 50 wt% h-BN, more preferably between 3 wt% and 40 wt% h-BN or between 4 wt% and 30 wt% h-BN, and most preferably between 5 wt% and 25 wt% h-BN. The acid may comprise a Bronsted acid. The acid may comprise a small molecule.

Each molecule of the acid may comprise 1, 2, 3 or more acidic proton sites. The amount of acid used may be sufficient to form a reaction solution comprising between 0.1 and 20 mol/dms, more preferably between 0.5 and 10 mol/dm ³ , and most preferably between 1 and 5 mol/dm ³ .

The amount of hydrogen peroxide used maybe sufficient to form a reaction solution comprising between 0.05 and 10 mol/dm ³ , more preferably between 0.25 and 5 mol/dm ³ , and most preferably between 0.5 and 2.5 mol/dm ³ . Preferably, the concentration of hydrogen peroxide in the reaction solution is half the concentration of the acid.

Preferably, the method comprises heating the reaction solution to an elevated temperature. The elevated temperature maybe between 50°C and 200°C, more preferably between 6o°C and 150°C or between 70°C and ioo°C, and most preferably between 8o°C and 90°C. The method may comprise heating the reaction solution to the elevated temperature for at least 1 hour, more preferably at least 5 hours, at least 10 hours, at least 15 hours or at least 20 hours, and most preferably at least 24 hours.

The method may comprise collecting the chemically modified h-BN. The chemically modified h-BN may be collected using centrifugation or filtration. The method may comprise washing the chemically modified h-BN. The chemically modified h-BN may be washed using water and/or an alcohol and/or a volatile organic solvent. The method may comprise drying the chemically modified h-BN. The chemically modified h-BN maybe dried under vacuum and/or at a temperature of at least 30°C, at least 40°C or at least 50°C. In one embodiment, the method may comprise chemically exfoliating the h-BN and then thermally exfoliating the h-BN. The h-BN may be chemically and thermally exfoliated as described above. The electrically insulating composite material of the first aspect may be used in an electrical cable.

Accordingly, in accordance with a third aspect, there is provided an electrical cable comprising the electrically insulating composite material of the first aspect.

The cable may be a medium voltage (MY), a high voltage (HV) or an extra high voltage (EHV) cable according to the voltage categories defined by IEC60038. Under this categorisation an MV cable may be suitable for use between 1 kV and 35 kV, an HV cable may be suitable for use between 35 kV and 230 kV, and an EHV cable may be suitable for use at greater than 230 kV. The cable may be an alternating current (AC) or direct current (DC) cable. An alternating current may be alternating at a frequency of between 10 Hz and 90 Hz, more preferably a between 20 Hz and 80 Hz or between 30 Hz and 70 Hz, and most preferably between 40 Hz and 60 Hz. In a preferred embodiment, the alternating current is alternating at a frequency of about 50 Hz.

All features described herein (including any accompanying claims, abstract and drawings), and/ or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/ or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 is a structural representation of hexagonal boron nitride (h-BN);

Figure 2 shows scanning electron microscope (SEM) images of h-BN in a polyethylene (PE) or polypropylene blend (PP) matrix, the scale bar is 10 pm, and the percentage given to the left of the images indicates the h-BN mass fraction for the composites shown in that row;

Figure 3 is a graph showing the conductivity of h-BN composites in LDPE at 6o°C, · corresponds to LDPE, while x, + and o denote conductivity of LDPE composites with h- BN mass fraction of 2 wt%, 5 wt% and 10 wt% respectively; Figure 4 shows SEM images of composites with an LDPE matrix and 2% mass fraction of: A) h-BN; B) CEh-BN; C) SMh-BN; and D) CE-SMh-BN, the scale bar represents lopm;

Figure 5 shows graphs of DC electrical conductivity for LDPE composites of A) CEh- BN and B) CE-SMh-BN against time. Marker type denotes filler mass fraction: · is LDPE only (o wt%), x is 0.5 wt%, + is 1 wt%, o is 2 wt%, A is 5 wt%, and o is 10 wt%; Figure 6 shows graphs of DC electrical conductivity of composites of CEh-BN in A) XLPE and B) PP against time. Marker type denotes CEh-BN mass fraction: · is matrix (XLPE or PP) only (o wt%), x is 0.5 wt%, + is 1 wt%, is 2 wt%, A is 5 wt%, and o is 10 wt%;

Figure 7 is a graph showing stress vs strain for LDPE composites prepared by melt extrusion, specimen curves presented are those most representative of the mean for each composition;

Figure 8 is a graph showing DC electrical conductivity of melt extruded LDPE composites with 2% filler mass fraction, and an XLPE composite of 2% mass fraction CEh-BN against time. · is LDPE only, x is LDPE composite with 2 wt% h-BN, + is LDPE composite with 2 wt% CEh-BN, is LDPE composite with 2 wt% SMh-BN, A is LDPE composite with 2 wt% CE-SMh-BN, o is XLPE composite with 2 wt% CEh-BN; and

Figure 9 is a graph showing thermal conductivity of matrix polymers and composites at 50-90°C in io°C intervals. Circles correspond to LDPE - o for LDPE only, · for a 10% h-BN composite of LDPE, * for a 10% CEh-BN composite of LDPE. Triangles correspond to XLPE - D for XLPE only, A for a 10% CEh-BN composite of XLPE. Crosses correspond to PP - x for PP only, + for a 5% CEh-BN composite of PP, grey struck-through cross for a 10% CEh-BN composite of PP.

Example 1: Impact of h-BN (untreated) on composite properties

Preparation

Materials

The hexagonal boron nitride (h-BN) selected for development was Momentive NXi, with a nominal particle size of 600 nm.

Hexagonal boron nitride denotes a substance of simple formula BN: comprising boron and nitrogen atoms in 1:1 atom ratio, whereby the bonds between those atoms form hexagonal repeat units, extended in two dimensions to form sheets, see Figure 1. H-BN particles may be formed of single layer sheets, or aggregates of two or more layers to form stacks in a third dimension. H-BN particles may also be formed from

agglomerated aggregates and/or single layer sheets.

Exxon Mobil was selected as a supplier for current generation low-density polyethylene (LDPE) commonly used to prepare insulation-grade crosslinked polyethylene (XLPE). This class of material can be readily crosslinked through the addition of dicumyl peroxide (DCP) followed by high temperature treatment.

A series of polypropylene blends was assessed to combine the strong dielectric performance of isotactic polypropylene with the mechanical flexibility of a copolymer. The blend components were selected from:

• Isotactic polypropylenes (iPP) with high tensile modulus OioooMPa);

• Propylene copolymers, including block copolymers, with low-to-medium tensile modulus (<ioooMPa). The comonomer in each material was one or more olefin, selected from the range including ethylene and C4-C12 a-olefms.

Following a screening process based on AC electrical breakdown strength, the polypropylene blend matrix herein referred to as PP was established as a blend of 50 wt% of a semicrystalline isotactic polypropylene with tensile modulus of 1560 MPa, with 50 wt% of a fully amorphous ethylene/propylene copolymer.

Polymer composites of untreated h-BN were prepared by a method involving masterbatching by a gelation method and composition modification by compounding, or through a compounding process only, by the following typical procedures.

To masterbatch by gelation

The matrix polymer or one of the matrix polymers (sufficient to prepare 25g of composite - 15 g or 20 g) of an intended polymer blend was dissolved in xylenes (200ml) at 145°C. Separately, hexagonal boron nitride (h-BN; 1 part per 4 parts matrix polymer, or 2 parts per 3 parts matrix polymer) was dispersed in 2-propanol (IPA; 10 ml per g h-BN) by sonication with a probe sonicator for 30 minutes using a Hielscher UP200S probe sonicator, with stirring halfway. The polymer/xylene solution was removed from the heat to initiate gelation, then the h-BN/IPA dispersion was immediately added, maintaining constant stirring until the mixture had cooled to a waxy solid. Following gelation, all volatiles were removed by evaporation under reduced pressure, to produce a polymer composite masterbatch of 20 wt% or 40 wt% h- BN and 80 wt% or 60 wt% polymer.

To produce h-BN polymer composites of between 0.5% and 10% by weight h-BN from masterbatches

To provide a range of composites with variable boron nitride content from a single gelation masterbatch precursor, a series of successive dilutions was carried out by melt compounding. LDPE composites were compounded at 130 °C whilst PP blend composites were compounded at 170 °C. In this procedure, 25 g of master batch was introduced into the mixer (HAAKE Polylab QC) along with a further 25 g of the matrix polymer, with the added matrix polymer formulated such that the target composition of the matrix polymer is achieved in the case of polymer blend composites. For the formation of XLPE composites, dicumyl peroxide (DCP) was introduced with the matrix polymer in a quantity sufficient to produce a composite comprising 2 wt% DCP. The contents of the mixer were compounded at 40 RPM for 20 minutes, after which the torque had reached a stable value. After completion of the compounding program, a composite with half the original h-BN concentration was produced. A fraction of that composite was used as a material for testing, whilst a fraction was used for further compounding to prepare h-BN composites with lower h-BN mass fraction. The further compounding steps were carried out using the same compounding program, with addition of matrix polymer(s) (and in the case of XLPE composites, DCP) to achieve the mass fractions required of the target composite. Further repetition of the compounding sequence delivered a series of composites with h-BN mass fraction at 0.5%, 1%, 2%, 5%, 10% and 20%.

For the preparation of XLPE composites, compounding resulted in LDPE composite materials loaded at 2% by weight with DCP. The crosslinking reaction was activated by thermal treatment during specimen formation as described below. Specimen formation

A laboratory hydraulic press (JBT Engineering, Micro-Mould) with heated platens was used to prepare samples for testing. A pair of 10 cm square sheet steel plates (3 mm thickness) was lined with sheets of Melanex (0.1 mm thickness) to prevent sticking. From the h-BN thermoplastic composites prepared, initially a i-2mm thickness sheet was hot pressed under an applied pressure of 3obar to eliminate any included air bubbles. LDPE composites, including those to be converted to XLPE composites, were pressed at 130 °C whilst (co)polymer blend composites containing isotactic

polypropylene (PP) were pressed at 170 °C.

To provide 0.2 mm thickness samples, 1 g of material was placed into each of four 5 x 5 cm compartments of a 0.2 mm thick Melanex mould, which was then pressed at 30 bar for 5 minutes to yield 4 samples. To provide 0.1 mm thickness sheets for AC electrical breakdown testing, 0.5 g of material was placed directly between the liner sheets and pressed at 50 bar for 5 minutes. After pressing the samples were rapidly cooled on a cold metal sheet.

XLPE composites were formed from DCP-loaded LDPE composites by a crosslinking reaction. Sheets of DCP-loaded LDPE composites prepared by the above methods were heated in their respective moulds at 200 °C for 10 minutes under sobar pressure to prevent bubble formation within the material. Specimens were then degassed at 6o°C under dynamic vacuum for 3 days.

Prior to electrical testing, material specimens were conditioned according to one of three protocols. Ambient conditioned samples were left out on trays to equilibrate with the laboratory atmosphere for at least 2 weeks. Wet conditioned samples were placed into polyethylene bags containing distilled water for at least 2 weeks; the air was removed ensuring complete immersion of the samples. Dry conditioned samples were stored in a vacuum dessicator with silica gel as a dessicant for at least 2 weeks.

Specimen testing

Thermogravimetric analysis (TGA)

5 mg samples were heated in air at a rate of 20 K/min using a Perkin Elmer Pyris 1 TGA and the weight loss was recorded from room temperature to 6oo°C. The instrument was calibrated for temperature using various metals (Curie point determination) and for weight with a standard calibration mass.

Differential scanning calorimetry (DSC)

Heating and cooling scans were performed at 10 K/min on 5 mg samples using a Perkin Elmer DSC-7. The instrument was calibrated for both temperature and heat flow using high purity indium. Scanning electron microscopy (SEM)

Samples for SEM examination were first etched for 4 hours using a standard permanganic reagent according to published techniques, mounted onto standard aluminium SEM stubs, gold coated and then examined at 15 kV in a Philips XL30 microscope.

AC electrical breakdown testing

AC electrical breakdown testing was performed using a D149 automated test set (Phenix Technologies) according to ASTM Di49-97a. Tests were performed with ambient temperature at 25°C. A test cell was used which was composed of opposing 6.3 mm steel ball bearings held together with a weight (too g) immersed in a tank of silicone fluid (Dow Corning 200/20 CS). After measuring the local thickness, the sample was placed between the ball bearings and an increasing 50 Hz AC field (voltage ramp 500 V/s) was then applied until failure. Twenty such tests were performed on separate areas of the sample at least 10 mm apart and the resulting data were processed using Weibull statistics to yield the breakdown strength (E) and shape parameter (b). The ball bearings were changed after every 20 tests.

DC Electrical conductivity measurements

DC Electrical conductivity measurements on 0.2 mm thickness samples were performed using equipment built in house. An automated test set comprising a Keithley 6485 Picoammeter and a Spellman high voltage supply equipped with a test cell containing opposing 20 mm diameter electrodes was used. The test cell was

incorporated within an oven to facilitate heating. A constant field of 10 kV/mm was applied and the current was then measured over 30 minutes (1 reading every 10 seconds) and from this and the sample geometry, the conductivity was calculated. Resistivity is calculated as the inverse of conductivity, therefore, for the purpose of assessing the performance of electrical insulation materials, reduction in conductivity is used as a proxy for enhancement in resistivity, with statistical treatment of conductivity valid for calculated resistivity from each dataset. Numerical conductivity and resistivity values presented are derived from the mean of measurements taken between 1600- 1800 s; uncertainty is estimated for the measurement range above using the coefficient of variation (Cv), which is the standard deviation divided by the mean expressed as a percentage. Resistivity ratio is a measure of the enhancement due to filler loading in the resistivity of a composite, calculated by the resistivity of a composite divided by the resistivity of the relevant matrix polymer alone - applicable when measured in the same series and conditions.

Tensometry measurements

Tensometry measurements were made using l mm thick specimens. Sheets of the analyte material were first pressed to form l mm thick sheets or (for XLPE composites) imm sheets of the precursor material were heated to 200°C for to minutes under pressure to carry out the crosslinking reaction. Subsequently, specimens were cut using a cutting press fitted with an ASTM D638 Type IV die: 115 mm long, 65 mm neck length and 6 mm neck width. Tensile testing was carried out on these samples using a

Zwick/Roell 5 kN Universal Testing Machine operating with a 5kN load cell. Test protocols followed the ASTM D638 standard at 23°C, with o.iMPa preload, 50mm/min extension rate and contact extensometer measurement (25 mm gauge length) only for elongation. Between four and ten specimens were tested for each material. Data recovered from the tests focused on the metrics commonly included in cable material standard documents: ultimate tensile strength (OM) and elongation-at-break ( B). The mean of each parameter from all tested specimens is presented.

Thermal conductivity measurements

Thermal conductivity measurements were made on plaque samples 2mm thick and 50mm in diameter using a Fox 50 thermal analysis system. Samples were subjected to heating differentials of 20°C and the thermal conductivity measured at io°C intervals between 50 and 90°C. The values of thermal conductivity were compared to a polycarbonate calibration standard.

Characterisation and analysis

Scanning electron microscopy

Scanning electron microscopy (Figure 2) revealed the visibly homogeneous distribution of h-BN particles within LDPE and polypropylene blend matrices at h-BN mass fractions between 2% and 20%. The particle largest dimensions are typically on the lpm scale, with some larger aggregates not exceeding lopm in their largest dimension increasing in population with increasing h-BN loading.

Mechanical analysis

To assess the impact of h-BN content in LDPE composites, tensile testing was undertaken. The key mechanical property metrics, as defined for cable insulation materials in medium-voltage and high-voltage cables, are ultimate tensile strength and elongation-at-break. Comparable mechanical properties of example matrix materials, including LDPE, were able to be sustained upon addition of h-BN at loadings between 2 wt% and 5 wt%. At a loading of 10 wt% h-BN tensile strength was enhanced but elongation-at-break was decreased by 38% when compared to the matrix material, see

Table 1.

Table 1: Tensometrv results for h-BN composites of LDPE

AC Electrical breakdown testing

The loading of thermoplastic matrices with h-BN has the effect of producing a composite material with enhanced AC electrical breakdown strength compared with the thermoplastic material, as shown in Table 2. At 2wt%-5wt% h-BN in LDPE, AC electrical breakdown strength is enhanced by 5%, with loading at iowt% and 20wt% providing enhancements of 16% and 20% respectively. In a PP blend matrix, enhancements in breakdown strength of 6%, 10% and 17% are attained at 2wt%, 5wt% and iowt% h-BN mass fraction, with no further enhancement at 20wt% mass fraction.

Table 2: AC electrical breakdown tests for h-BN composites of LDPE and PP

DC Electrical conductivity

The DC electrical conductivity was recorded at 25°C and at 6o°C, a temperature representative of the insulation operating temperature in MY and HV cables during normal service. H-BN composites have generally decreasing conductivity, thus increasing resistivity with increasing h-BN mass fraction, see Table 3. Composites of LDPE with 2, 5 and 10 wt% h-BN had resistivity of 1.5x1o ¹³ , 4.1x1o ¹³ and 1.1x1o ¹⁴ Wih respectively at 6o°C (equivalent to enhancement of 60, 350 and 1200% compared to LDPE), as illustrated in Figure 3.

Table 2: Conductivity, resistivity and relative resistivity at 6o°C of h-BN composites of

LDPE

Example 2: h-BN composites prepared using chemically and thermally exfoliated h-BN

Preparation

A series of techniques was assessed for treatment of h-BN to introduce surface functional groups or to enhance the dispersion of h-BN filler within the composites produced by further exfoliation of bulk h-BN or stabilisation of an exfoliated state. The structure of h-BN particles is that of aggregated thin plates (in this study, of supplier- determined length and width of average size 6oonm), which may further agglomerate regularly or irregularly into particles with maximum dimension >ipm, and in some cases >5 pm. Exfoliation is undertaken to eliminate agglomerated material and reduce aggregation such that the largest dimension for any particle is the plate length, with thickness reduced to <ioonm where possible.

Four techniques were assessed, namely:

1) a high-temperature thermal treatment was used to deliver surface-modified h- BN (SMh-BN), with surface functional groups being hydroxyl (OH), as identified by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA);

2) a chemical treatment was used to enhance the exfoliation of h-BN to deliver chemically exfoliated h-BN (CEh-BN) no particles have a largest dimension >2pm when dispersed in a polymer matrix, as verified following composite formation by SEM;

3) a combination of the above chemical and high-temperature thermal treatments was used sequentially to deliver chemically exfoliated surface-modified h-BN (CE-SMh- BN), with surface functional groups being hydroxyl (OH), where no particles have a largest dimension >2pm when dispersed in a polymer matrix; and

4) a chemical treatment combined with a lower-temperature thermal treatment to further promote exfoliation and efficiently remove chemical treatment agents to deliver chemically exfoliated h-BN where the chemical treatment agents had been removed (CETh-BN).

High temperature thermal treatment

h-BN powder was incubated in a high temperature furnace with an air atmosphere at a temperature of 800-900 °C for a period of 1-8 hours. The incubation temperature and period selected depended upon the degree of surface modification desired - longer incubation or higher temperature leads to a greater degree of surface modification.

Immediately following the incubation period, the h-BN was thermally quenched by immersion in a large quantity of liquid (>50 times the mass of h-BN used) at a pre- immersion temperature of between -196 °C and 25 °C. The liquid used may be liquid nitrogen, water or a volatile organic liquid with a very high autoignition point. The dispersion was allowed to reach ambient temperature (typically 25 °C) and the liquid allowed to evaporate to deliver a dry, white solid material. Where the quenching liquid used is not liquid nitrogen, the solid may need to be dried under vacuum.

Low temperature thermal treatment

The low temperature treatment was as described above, except the h-BN powder was incubated in the furnace at a lower temperature of 400 °C for the period of 1-8 hours.

Chemical treatment

h-BN powder was first dispersed in a suitable dispersant (typically an alcohol, such as 2-propanol) at a weight fraction of between 5% and 25%. The dispersal process can be accelerated and/ or enhanced using either probe (vessel interior) sonication, bath (vessel exterior) sonication or high-shear mixing (blade or planetary).

To the resultant h-BN dispersion was then added an acid and optionally hydrogen peroxide (H ₂ 0 ₂ ). The acid was a small molecule Bronsted acid with 1, 2, 3 or more acidic proton sites, and was added in an amount sufficient to generate a concentration within the reaction mixture of between 1 and 5 moldnrs. The H ₂ 0 ₂ , when it was used, was provided as a solution of 8 moldnrs in water and was added in an amount sufficient to deliver a concentration within the reaction mixture of half that of the acid.

The reaction mixture was heated to its reflux temperature (typically 80-90 °C) and stirred for 24 hours. The reaction mixture was then cooled and the solid material component recovered by one of two methods: centrifugation at 4000G, or filtration, dependent on the h-BN particle behaviour. The solids recovered were washed with one or more alcohols to remove non-bound acidic species. Where centrifugation was used, the centrifugation, recovery and washing of the solid material was repeated at least three times using one or more alcohols, and a further two times using a low-boiling volatile organic solvent. The solid material was then dried under vacuum or at an elevated temperature (50-150 °C). The solid material collected can have a light grey appearance, similar to that of commercial highly exfoliated materials - thought to derive from morphological changes or from surface functionalisation or surface associated species.

Chemical and thermal treatments

For the preparation of CE-SMh-BN, the chemical treatment step was followed by the high temperature thermal treatment step, each step as described above. The material recovered following the thermal treatment step was a white solid.

For the preparation of CETh-BN, the chemical treatment step was followed by the low temperature thermal treatment step, each step as described above. The material recovered following the thermal treatment step was a white solid.

Composites of SMh-BN, CEh-BN and CE-SMh-BN were used to prepare composite materials by the same methods used to prepare h-BN composite materials described in Example 1. Characterisation and analysis

SEM

SEM analysis of LDPE composites with 2 wt% h-BN following various treatments illustrated the effect of each treatment in exfoliation of h-BN and thus their influence on dispersion within the composite structure, see Figure 4. The removal of

agglomerated material with largest dimension > >ipm is considered key to achieving an optimal balance of properties, in particular mechanical and electrical properties. Composites of 2 wt% h-BN in LDPE, shown in Figure 4A, feature a moderate population of filler agglomerate particles of largest dimension >ipm, with some of largest dimension >5pm. An analogous composite of SMh-BN, shown in Figure 4C, also featured agglomerate particles with largest dimension >5 pm. An analogous composite of CEh-BN, shown in Figure 4B, featured no agglomerate particles with largest dimension >5 pm and no particles of largest dimension >2 pm. An analogous composite of CE-SMh-BN, shown in Figure 4D, featured no agglomerate particles with largest dimension >5 pm, and some particles of largest dimension >2 pm. Tensometry

The tensile test results for composites of CEh-BN and CE-SMh-BN in LDPE showed little effect of the h-BN treatment method on the composite mechanical properties.

The LDPE composites sustained the tensile strength and elongation-at-break performance of the matrix polymer up to 5 wt% CEh-BN or CE-SMh-BN compared to composites containing up to 5 wt% untreated h-BN. Composites containing 10 wt% CEh-BN or CE-SMh-BN in LDPE were found to be more brittle than composites containing 10 wt% h-BN. Table 4: Tensometrv results for LDPE and XLPE composites

In XLPE composites of CE-h-BN, analogous to the materials currently deployed as insulation in MY and HV power cables, high tensile strength and elongation-at-break were also sustained at 2 wt% and 5 wt% h-BN. Furthermore, at 10 wt% h-BN, the mechanical performance parameters were sustained above the requirements of MY and HV cable insulation standards.

AC electrical breakdown strength

The AC electrical breakdown strength of LDPE composites with CEh-BN and CE-SMh- BN was assessed and a strong influence of the h-BN treatment method on AC electrical breakdown performance was established.

As shown in table 5, composites of LDPE with CEh-BN deliver enhancement in AC electrical breakdown strength of 2okV/mm (13%) over LDPE at only 0.5 wt% CEh-BN, increasing to 44kV/mm (28%) and 46kV/mm (29%) enhancement at 2 wt% and 5 wt% CEh-BN, respectively. No further enhancement was observed at 10 wt% CEh-BN.

Table 5 also shows that composites of LDPE with CE-SMh-BN deliver AC electrical breakdown performance enhancement of nkV/mm (7%) at 0.5 wt% mass fraction, i6kV/mm (10%), 23kV/mm (14%) and 3ikV/mm (19%) at mass fractions of 1 wt%, 2 wt% and 5 wt% CE-SMh-BN, respectively. Further enhancement (4ikV/mm, 26%) is observed at 10 wt% CE-SMh-BN. Table 5: AC electrical breakdown results of LDPE composites of treated h-BN

Due to the high enhancement of AC electrical breakdown performance at very low mass fraction of filler, CEh-BN was selected to be tested in composite where the matrix was XLPE or PP. XLPE typically has a lower AC electrical breakdown strength than the LDPE from which it is prepared as a result of the presence of side products from the crosslinking process. PP typically has a higher AC electrical breakdown strength than LDPE. As shown in table 6, enhancement of AC electrical breakdown strength of XLPE composites with CEh-BN was observed to be i9kV/mm (13%) at only 0.5 wt% CEh-BN. The pattern observed for the analogous LDPE composites was reflected in XLPE composites. In particular, substantial enhancement was observed for 1 wt% and 2 wt% CEh-BN, namely 27kV/mm (18%) and 38kV/mm (25%), respectively. Slight further enhancement was observed at 5 wt% CEh-BN, namely 39kV/mm (26%), and no further enhancement was observed at 10 wt% CEh-BN.

Table 6 also shows that AC electrical breakdown testing of composites of PP with CEh- BN resulted in steadily increased enhancement up to 5 wt% CEh-BN, which showed an enhancement of 29kV/mm (16%), with no further enhancement at 10 wt% CEh-BN.

Table 6: AC electrical breakdown results for XLPE and PP composites of CEh-BN

DC electrical conductivity

For LDPE composites of CEh-BN and CE-SMh-BN, DC electrical conductivity was measured at 25°C and 6o°C, a representative service temperature for MV and HV power cable insulation. At 25°C, DC conductivity for each composite matched that of LDPE. At 6o°C, DC conductivity of LDPE composites decreased with increasing mass fraction of both CEh-BN and CE-SMh-BN, as shown in Figures 5A and 5B, respectively. At 2 wt% to 10 wt% CEh-BN or 1 wt% to 10 wt% CE-SMh-BN, conductivity of less than 2.5x1o ¹⁶ S/cm was recorded, corresponding to resistivity of at least 34xio ¹ 3 ilm.

Accordingly, the resistivity enhancement compared to LDPE for those composites is at least 700%, with the highest resistivity of 1.2x1o ¹⁴ Wih recorded for a 5% CE-SMh-BN composite of LDPE, an enhancement of 1800%. Table 7: Conductivity, resistivity and relative resistivity at 6o°C of CEh-BN and CE-

SMh-BN composites of LDPE at filler mass fraction 0.5-10%

a) To ensure ambient conditions were precisely accounted for, an ΌRE matrix reference measurement was taken for each composite series.

XLPE and PP composites of CEh-BN also were found to have decreased DC electrical conductivity at 6o°C in comparison to the respective matrix polymer, as shown in Figure 6. As shown in Figure 6A, XLPE composites had incrementally decreasing conductivity with increasing CEh-BN mass fraction, reaching 9.2x1o ¹⁶ S/cm at 2 wt% CEh-BN (i.e. a resistivity of 1.1x1o ¹³ Wih) and 1.4x1o ¹⁶ S/cm at 10 wt% CEh-BN (i.e. a resistivity of 7.1x1o ¹³ Om). This equates to resistivity enhancement of 380% for 2 wt% CEh-BN and 3000% for 10 wt% CEh-BN, when compared to XLPE.

As shown in Figure 6B, PP composites of CEh-BN had very low DC electrical conductivity, with 1-10 wt% CEh-BN composites each resulting in conductivity <ixio ¹⁶ S/cm. Resistivity calculated for those composites (with the very high values giving rise to high coefficients of variation) were between 2.2x1o ¹⁴ and 5.1x1o ¹⁴ ilm, enhancements of 1500-3600% respectively when compared to PP. Table 8: Conductivity, resistivity and relative resistivity at 6o°C of CEh-BN composites of XLPE and PP at filler mass fraction 0.5-10%

Thermal conductivity

Thermal conductivity (Tc) measurements of LDPE, XLPE and PP composites were undertaken to assess the effect of highly thermally conductive fillers on the Tc of polyolefin matrices, see Figure 9. The temperature range of 50-90°C encompasses the anticipated service temperature of MV and HV cables, with representative numerical values taken at 6o°C. Composites with filler mass fractions <5% showed no clear difference in thermal conductivity from that of their respective matrix polymers.

Composites of 10% h-BN and 10% CEh-BN in LDPE elevated the thermal conductivity of LDPE by 10% and 12% to 0.359 and 0.364 Wm^K ¹ , respectively, at 6o°C. A composite of 10% CEh-BN in XLPE enhanced the Tc of XLPE by 8% to 0.374 WnVK ^-1 at 6o°C, while PP composite Tc are elevated by similar absolute values that represent greater proportional enhancement of the lower Tc PP matrix: 5% CEh-BN in PP delivers 10% enhancement to 0.236 WnVK ¹ , while 10% CEh-BN in PP delivers 21%

enhancement to 0.260 WnVK ^-1 at 6o°C. Example 2: h-BN composites prepared bv alternative processing routes

Dry compounded or compounded/extrusion preparation

H-BN composite preparation methods were evaluated for selected composites in order to assess the impact of the process type, and to demonstrate and validate scalable production. Melt extrusion processing in particular was undertaken to verify the capability for composite preparation through processes representative of those deployed for MV or HV power cable insulation production.

Dry compounding

A dry compounding method was developed to provide composites with no exposure to solvent. This procedure was similar to the one used to dilute gelation-masterbatched materials, described in example 1, and used the same mixer and temperature programs. Dry h-BN, CEh-BN, CETh-BN, SMh-BN or CE-SMh-BN was introduced into the pre- heated mixer either before the matrix polymer or polymer blend, or following initial softening of the matrix polymer or polymer blend. Mixing was started at 5 RPM until the host polymer was observed to be completely melted at which time the speed was increased to 40 RPM and mixing proceeded as described above. As a safeguard against hazardous nanoparticle exposure, either continuous monitoring of airborne particle levels or system closure with localised air extraction was employed.

This above dry compounding procedure was used to produce polymer or polymer blend composite masterbatches with 20wt% h-BN, CEh-BN, SMh-BN or CE-SMh-BN, for extrusion processing to lower weight % composites. Melt extrusion

A masterbatch, produced as described above, was removed from the compounder and pressed immediately to a imm sheet. The sheet was shredded, then the resultant shreds washed with water, rinsed with acetone and dried under vacuum. Extrusion blending was carried out using a Brabender single screw extruder with a high-mixing waveform screw and single-entry hopper loading. Masterbatch shreds were dispersed at the appropriate loading in vacuum-dried matrix polymer pellets to deliver a composite with 2 wt% filler, and the components added to the extruder feed together. For rod die extrusion, water cooling was used and the rod was pelletised (nominal rod diameter = 4mm, pellet length = 5mm). The following parameters were used:

• Zone temperatures: 1 (feed) = i6o°C; 2 (mixing 1) = 170°C; 3 (mixing 2) =

170°C; 4 (pre-die) = 170°C; 5 (die head) = i8o°C;

· Extrusion speed: 20rpm initial, ramped to 8orpm; and

• Pelletiser speed: i.6m/min initial, ramped to 6.4m/min.

Following the extrusion blending process, pellets were dried at 50°C under vacuum for i8h prior to repeating the extrusion cycle until satisfactory filler distribution was achieved - assessed macroscopically by multipoint thermogravimetric analysis (% residue at 6oo°C taken to represent h-BN content).

The formation of 2 wt% h-BN composites in LDPE by the fully dry process was efficient, with good distribution of the h-BN fillers in the masterbatch from compounding and high extrusion speeds deployable to minimise extrusion cycle time. Three extrusion cycles were required to ensure homogeneous distribution of h-BN fillers in the 2 wt% composites, as verified by multipoint TGA to 2% +/- 0.2%.

Formation of XLPE composites of 2 wt% filler was undertaken by further compounding of the analogous LDPE composite with DCP then crosslinking, using the processes described in Example 1.

Composites produced by dry compounding or melt extrusion were assessed for mechanical properties by tensometry, for AC electrical breakdown strength and for DC electrical conductivity.

For tensometry specimens, further extrusion of thermoplastic composites to produce a ribbon of thickness between tm and 3mm was followed by specimen sampling where the length dimension of the ASTM D638 type 4 specimen is parallel to the axis of extrusion. Testing was then carried out as described in Example 1.

Specimens for AC electrical breakdown and DC electrical conductivity testing were prepared, conditioned and tested as described in Example 1. Tensometry

Tensile testing of extruded LDPE revealed a strengthening effect and toughening effect in the axis of extrusion when compared with analogous non-extruded composites, see Figure 7. That effect was replicated for each composite of LDPE with 2 wt% h-BN, CEh- BN, SMh-BN or CE-SMh-BN. Tensile strength (OM) and elongation-at-break ( B) are comparable for each composite, see Figure 7, with no notable effect of h-BN treatment method or consequent dispersion within the matrix.

Table Q: Tensometry results for LDPE composites prepared bv melt extrusion

AC electrical breakdown

To assess the influence of process method on the AC electrical breakdown performance, LDPE composites with 2wt% filler were compared, see Table 10. The relative breakdown strength enhancement hierarchy of CEh-BN> CE-SMh-BN> h-BN > matrix only, that was observed for the composites prepared by gelation masterbatch methods in Example 2, was sustained for melt extruded composite materials. Each composite tested recorded AC electrical breakdown performance comparable to their gelation masterbatch prepared analogous composite. Table 10: AC electrical breakdown results for melt extruded LDPE composites with 2 wt% filler, and an XLPE composite of 2 wt% CEh-BN

The LDPE composite of 2 wt% SMh-BN also provides an enhancement in AC electrical breakdown performance (8 kV/mm, 5%) but less so than h-BN, consistent with the presence of large agglomerate particles present as illustrated by SEM in Example 2. Greatest enhancement in AC electrical breakdown performance over that of LDPE was delivered by CEh-BN (31 kV/mm, 19%). An XLPE composite of 2% mass fraction CEh-BN, prepared from a melt extruded LDPE composite, delivers an enhancement of 29 kV/mm (19%) over that of XLPE.

DC electrical conductivity

For each of the LDPE composites with 2 wt% filler prepared by melt extrusion, DC electrical conductivity was recorded. Treatment methods for the h-BN filler has little influence on DC electrical conductivity of the resultant composites - 2 wt% h-BN, CEh- BN, SMh-BN and CE-SMh-BN composites each had conductivity between 1.9x1o ¹⁶ and 2.2x1o ¹⁶ S/cm (corresponding to resistivity of between 4.6x1o ¹³ and 5.4x1o ¹³ Wih and enhancements of 710-840% with respect to LDPE). Each composite recorded conductivity approximately a tenth of the conductivity observed for LDPE, see Figure 8.

Table 11: Conductivity, resistivity and relative resistivity at 6o°C of 2% h-BN, CEh-BN. SMh-BN and CE-SMh-BN composites of LDPE produced bv extrusion: and a 2% CE-h- BN composite of XLPE prepared from the analogous extruded LDPE composite

An XLPE composite of 2 wt% CEh-BN, prepared using a melt extruded LDPE composite, sustained comparably low conductivity (2.7x1o ¹⁶ S/cm) to the LDPE precursor, despite the crosslinking process. The resistivity was thus elevated by 550% from that observed for LDPE and >1000% of the resistivity previously observed for XLPE. Example 4: Larger particle size h-BN composites

The applicability of the composite preparation methods and properties to h-BN with larger particle sizes was investigated. Of particular interest was the impact of differing particle sizes on composite mechanical properties,

Preparation

Materials

To assess the scope of the preparation processes for h-BN of different particle size, Henze Hebofill 501, was used for these experiments (denoted h-BN2), with a median particle size of 45 pm.

The polypropylene blend matrix herein referred to as PP2 was established as a blend of 50 wt% of a semicrystalline isotactic polypropylene with tensile modulus of 1560 MPa, with 50 wt% of a semicrystalline ethylene/propylene copolymer. h-BN Treatment

h-BN2, with larger particle size, was used following either:

1) Drying of as-received materials at 120°C

2) Chemical and lower-temperature thermal treatment using the process described in Example 1. The larger particle size of h-BN2 enables the use of filtration during the treatment procedure.

Composite preparation

Composites were prepared using h-BN (i.e. h-BN with a nominal particle size of 600 nm, as described in example 1), h-BN2 (i.e. h-BN with a median particle size of 45 pm) and CETh-BN2 (i.e. h-BN with a median particle size of 45 pm which has been chemically and thermally treated using the methods described in example 2), each loaded at 2 wt% in LDPE and PP2 matrices. The composites were formed using the dry compounding method described in Example 3

Tensile testing

Tensile testing specimens were cut from 1 mm pressed sheets, formed as described in Example 1. Tensile testing of composites in LDPE reveal approximate retention of tensile strength and elongation-at-break of the matrix polymer when either h-BN or h-BN2 are loaded at 2 wt% (Table 12), with very similar, marginal impact for either particle size. The LDPE composite of 2 wt% CETh-BN has slightly reduced tensile strength and elongation-at-break; however, those values remain within 7% and 8% of the respective matrix values.

Table 12: Tensometrv results for thermoplastic h-BN and h-BN2 composites

The sensitivity of PP2 blend matrices to hller particle size was significantly more pronounced. Whereas PP2 composites of 2 wt% h-BN (6oonm nominal particle largest dimension) had tensile strength and elongation-at-break very similar to the matrix polymer, PP2 composites of 2 wt% h-BN2 (45 pm median particle largest dimension) had substantially reduced tensile properties; O _M =20.9 MPa, 8 _B = 620%, 28% and 19% lower than the respective matrix values. The tensile properties of PP2 composites of 2 wt% CETh-BN2 highlight the positive impact of treatment of h-BN2 on the tensile strength and elongation-at-break of the resultant composites: O _M =25.4 MPa, 8 _B = 710%, still 12% and 7% lower than the respective matrix values, but 22% and 15% higher than the values for composites prepared with untreated h-BN2

Conclusions

The inventors have shown that the addition of h-BN to a polymer matrix improves the AC electrical breakdown strength and resistivity of the material. The inventors also show that this effect can be improved if the h-BN is exfoliated to control the size of the h-BN particles, and that exfoliation of the h-BN can enhance composite strength and toughness. Finally, the inventors show that composites which have been extruded exhibit improved strength and toughness in the axis of extrusion.