Crosslinked polymer composition, structured layer and cable
The present invention relates to a crosslinked polymer composition comprising a crosslinked polyolefin, wherein the polymer composition comprises, prior to crosslinking, a polyolefin and peroxide which is in an amount of less than 35 mmol —O—O-/kg polymer composition, characterized in that the crosslinked polymer composition has been in a direct contact with a semiconductive composition for 24 h at 70° C., and that the crosslinked polymer composition thereafter has an electrical DC-conductivity of 150 fS/m or less, wherein the electrical DC-conductivity is measured in accordance with “DC conductivity method”, as described under “Determination methods”, on a plaque of the crosslinked polymer composition at 70° C. and 30 kV/mm mean electric field from a non-degassed and 1 mm thick plaque sample of the crosslinked polymer composition; a layered structure, cable, e.g. a power cable, use of the crosslinked polymer composition and the structured layer, both, for producing a crosslinked power cable, e.g., a cross linked direct current (DC) power cable; and a process for producing a cable.
1. A cable comprising a polymer layer and at least one semiconductive layer adjacent to the polymer layer,
wherein the polymer layer comprises a crosslinked polymer composition, wherein the polymer composition comprises, prior to crosslinking,
a first low density polyethylene (LDPE),
an antioxidant,
a peroxide in an amount of less than 35 mmol —O—O-/kg polymer composition, and
no 2,4-diphenyl-4-methyl-1-pentene;
wherein the semiconductive layer comprises a second low density polyethylene (LDPE) and carbon black;
characterized in that when the crosslinked polymer composition has been in a direct contact with a semiconductive composition for 24 h at 70° C., the crosslinked polymer composition thereafter has an electrical DC-conductivity of 130 fS/m or less;
wherein the electrical DC-conductivity is measured in accordance with “DC conductivity method”, as described under “Determination methods”, on a plaque of the crosslinked polymer composition at 70° C. and 30 kV/mm mean electric field from a non-degassed and 1 mm thick plaque sample of the crosslinked polymer composition.
2. The cable according to claim 1 , wherein the polymer composition, prior to crosslinking, comprises the peroxide in an amount of 34 mmol —O—O-/kg polymer composition or less.
3. The cable according to claim 1 , wherein the first LDPE and/or the second LDPE is an optionally unsaturated LDPE homopolymer or an optionally unsaturated LDPE copolymer of ethylene with one or more comonomer(s).
4. The cable according to claim 1 , wherein the first LDPE and/or the second LDPE is an unsaturated LDPE copolymer of ethylene with at least one polyunsaturated comonomer and optionally with one or more other comonomer(s),
wherein the at least one polyunsaturated comonomer consists of a straight carbon chain with at least 8 carbon atoms and at least 4 carbons between the non-conjugated double bonds, of which at least one is terminal, and
wherein the first LDPE and/or the second LDPE contains carbon-carbon double bonds/1000 carbon atoms in a total amount of at least 0.1/1000 carbon atoms.
5. The cable according to claim 1 , wherein the first LDPE contains, prior to the crosslinking, at least vinyl groups and the total amount of vinyl groups is higher than 0.01/1000 carbon atoms.
6. The cable according to claim 1 , wherein the first LDPE and/or the second LDPE is produced in a high pressure process, wherein the high pressure process comprises a step of compressing one or more monomer(s) under pressure in a compressor, wherein a compressor lubricant comprising a mineral oil is used for lubrication in the compressor.
7. A method of making the cable according to claim 1 , the method comprising the steps, in any order, of
exposing a polymer layer composition comprising the first low density polyethylene, the peroxide in an amount of less than 35 mmol —O—O-/kg polymer composition, and no 2,4-diphenyl-4-methyl-1-pentene to a curing procedure during which the maximum temperature of the polymer layer composition is above 150° C., whereby the polymer layer composition is crosslinked to form the crosslinked polymer composition; and
providing the at least one semiconductive layer comprising the second LDPE and carbon black adjacent to said polymer layer.
8. The cable according to claim 4 , wherein said at least one polyunsaturated comonomer is a diene which comprises at least eight carbon atoms, a first carbon-carbon double bond, and a second carbon-carbon double bond, the first carbon-carbon double bond being terminal and the second carbon-carbon double bond being non-conjugated to the first carbon-carbon double bond.
9. The cable of claim 8 , wherein the diene is selected from the group consisting of C 8 - to C 14 -non-conjugated dienes and mixtures thereof.
10. The cable of claim 9 , wherein the diene is selected from the group consisting of 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, 7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene, and mixtures thereof.