Power cable polymer composition comprising thermoplastic and having advantageous properties

Abstract

The invention relates to power cable polymer composition which comprises a thermoplastic polyethylene having a chlorine content which is less than X, wherein X is 10 ppm, a power cable, for example, a high voltage direct current (HV DC), a power cable polymer insulation, use of a polymer composition for producing a layer of a power cable, and a process for producing a power cable.

Claims

1. A power cable insulation polymer composition comprising a single site thermoplastic polyethylene having a chlorine content which is less than X, wherein X is 10 ppm; wherein the thermoplastic polyethylene is the sole polymer component, is multimodal and is a medium density ethylene copolymer of ethylene and one or more C.sub.3-C.sub.20 alpha-olefins; or a high density ethylene homopolymer or copolymer of ethylene and one or more C.sub.3-C.sub.20 alpha-olefins; wherein the power cable insulation polymer composition does not comprise an acid scavenger, and wherein the power cable insulation polymer composition has a DC electrical conductivity of 4.0 fS/m or less.

2. A power cable insulation polymer composition according to claim 1, wherein the power cable insulation polymer composition is a high voltage (HV) and/or extra-high voltage (EHV) power cable insulation polymer composition.

3. A power cable insulation polymer composition according to claim 1, wherein the power cable insulation polymer composition is a high voltage direct current (HV DC) and/or an extra high voltage direct current (EHV DC) power cable insulation polymer composition.

4. A power cable insulation polymer composition according to claim 1, wherein X is 5.0 ppm.

5. A power cable insulation polymer composition according to claim 1, wherein the power cable insulation polymer composition comprises a chromium catalyst polyethylene with a density of 930 to 949 kg/m.sup.3.

6. A power cable which power cable comprises a polymer composition being, or comprising, the power cable insulation polymer composition according to claim 1.

7. A method of producing a power cable with the insulation polymer composition of claim 1 comprising: producing at least one layer of a power cable with the composition, said power cable comprising a conductor surrounded by at least an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order.

8. The method according to claim 7, wherein the power cable is a direct current (DC) power cable.

9. A power cable insulation polymer composition according to claim 1, wherein the power cable insulation polymer composition does not comprise an ion scavenger.

Description

(1) FIG. 1 shows a schematic picture of the measurement setup used in the DC conductivity method as described under Determination methods. Explanation of the numbered parts 1-6: 1 Connection to high voltage; 2 Measuring electrode; 3 Electrometer/Pico Ammeter; 4 Brass electrode; 5 Test sample, i.e. polymer composition; 6 Si-rubber.

EXPERIMENTAL PART

(2) Determination Methods

(3) Unless otherwise stated in the description or experimental part the following methods were used for the property determinations.

(4) wt %: % by weight

(5) Melt Flow Rate

(6) The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is determined at 190 C. for polyethylene. MFR may be determined at different loadings such as 2.16 kg (MFR.sub.2) or 21.6 kg (MFR.sub.21).

(7) Density

(8) Low density polyethylene (LDPE): The density was measured according to ISO 1183-2. The sample preparation was executed according to ISO 1872-2 Table 3 Q (compression moulding).

(9) The single site polyethylene (SSPE) in Examples 2 and 3 (low pressure process polyethylene), and Example 4, Example 5, chromium catalyst polyethylene (low pressure process polyethylene): Density of the Examples 2, 3, 4 and 5 were measured according to ISO 1183/1872-2B.

(10) Chlorine Content Calculation

(11) The chlorine content has been estimated by theoretical calculation. For the comparative example, the chlorine content can either be measured via analytical techniques, well described in the literature, or can be calculated based on the amount of Ti and Mg present in the catalyst. For the inventive examples 2 and 3, the amount of chlorine is calculated based on the amount of metal present in the catalyst formulation. Further, in relation to the inventive example 4 and example 5, the chromium catalyst does not contain any chlorine.

(12) Furthermore, by knowing the amount of chlorine present in the catalyst and the productivity, i.e. the amount of polymer produced by amount of catalyst, the residual amount of chlorine in the final polymer composition can be easily calculated.

(13) DC Conductivity Method

(14) Electrical conductivity measured at 70 C. and 30 kV/mm mean electric field on a 1 mm plaque sample consisting of the polymer composition.

(15) Plaque Sample Preparation:

(16) The plaques are compression moulded from pellets of the test polymer composition. The final plaques have a thickness of 1 mm and a diameter of 260 mm.

(17) The plaques are compression moulded at 130 C. for 600 s at a pressure of 2.6 MPa.

(18) Thereafter the temperature is increased and reaches 180 C. after 5 min. The temperature is then kept constant at 180 C. for 1000 s. Finally the temperature is decreased at a cooling rate 15 C./min until room temperature is reached when the pressure is released.

(19) Measurement Procedure:

(20) A high voltage source is connected to the upper electrode in order to apply voltage over the test sample. The resulting current through the sample is measured with an electrometer/picoammeter. The measurement cell is a three electrodes system with brass electrodes placed in a heated oven circulated with dried compressed air to maintain constant humidity level. The diameter of the measurement electrode is 100 mm.

(21) The applied voltage is 30 kV DC meaning a mean electric field of 30 kV/mm. The temperature is 70 C. and the whole experiments last for 24 hours including heating to the test temperature. The current through the plaque is logged. The current after 24 hours is used to calculate the conductivity of the test sample i.e. the plaque consisting of the polymer composition to be tested.

(22) Polymer Compositions:

Example 1 (Comparative)

(23) A commercially available HDPE, i.e. HE6068, supplied by Borealis Finland, which is a bimodal high density polyethylene having an MFR.sub.2 of 1.7 g/10 min and a density of 944 kg/m.sup.3.

Example 2 (Inventive)

(24) A polyethylene polymerised by means of a single site catalyst in solution, i.e. a thermoplastic polyethylene having a chlorine content which is less than X, in accordance with the present invention, is commercially available from Borealis Plastomers (NL) under the tradename QUEO8230. The QUEO8230 is a very low density polyethylene (1-octene as the comonomer), has an MFR.sub.2 of 30 g/10 min (190 C./2.16 kg) and a density of 882 kg/m.sup.3.

Example 3 (Inventive)

(25) A single site polyethylene, i.e. a thermoplastic polyethylene having a chlorine content which is less than X, in accordance with the present invention, was prepared as described below.

(26) Single Site Catalyst

(27) Possible Preparation of the Single Site Catalyst

(28) 16.4 kg methylalumoxane in toluene (30 weight-%, supplied by Albemarle) was mixed with 8.5 kg dry toluene and added to 0.46 kg di(n-benzyl)di(n-butylcyclopentadienyl)hafnium in toluene (67.9 wt %) and stirred at room temperature for 20 min. The obtained solution was added dropwise during 45 min to 10 kg activated silica (commercial silica carrier, XPO2485A, having an average particle size 20 m, supplier: Grace) and stirred at room temperature for 3 hours. The solvents were evaporated off under nitrogen flow at 50 C. to obtain the supported catalyst. The obtained catalyst had an Al/Hf ratio of 200 mol/mol; a Hf-concentration of 0.42 wt % and an Al-concentration of 14.0 wt %.

(29) Preparation of Single Site Polyethylene

(30) A single site polyethylene i.e. a thermoplastic polyethylene having a chlorine content which is less than X, in accordance with the present invention, was prepared using in addition to prepolymerisation reactor, a slurry-loop reactor as well as a gas phase reactor. The prepolymerisation stage was carried out in slurry in a 50 dm.sup.3 loop reactor under conditions and using feeds of catalyst (as prepared above), monomers, antistatic agent and diluent (propane (C3)) as disclosed in Table 1. The obtained slurry together with the prepolymerised catalyst was introduced into a 500 dm.sup.3 loop reactor to carry out the actual polymerisation. The polymer slurry was withdrawn from the loop reactor and transferred into a flash vessel operated at 3 bar pressure and 70 C. temperature where the hydrocarbons were substantially removed from the polymer. The polymer was then introduced into a gas phase reactor. The conditions and feeds/feed ratio in loop and gas phase polymerisation steps are disclosed in Table 2 and 3

(31) TABLE-US-00001 TABLE 1 Process conditions in the Prepolymerisation (inventive example 3) Example 3 (Inventive) Temperature [ C.] 60 Pressure [bar] 62 Catalyst Feed [g/h] 38 Antistatic feed [g/h] 7 C2 feed [kg/h] 2 C4 feed [g/h] 50 C3 feed [kg/h] 32

(32) TABLE-US-00002 TABLE 2 Process condition in the Loop reactor and properties (inventive example 3) Example 3 (Inventive) Temperature [ C.] 85 Pressure [bar] 58 C2 feed [kg/h] 33 H2 feed [g/h] 7.9 C4 feed [kg/h] 1.7 C3 feed [kg/h] 71 H2/C2 ratio [mol/kmol] 0.55 C4/C2 ratio [mol/kmol] 91 C4/C2 feed ratio [g/kg] 0.05 Production rate [kg/h] 29.9 Split [wt %] 49 MFR.sub.2 [g/10 min] 100 Density [kg/m3] 939

(33) TABLE-US-00003 TABLE 3 Process conditions in the Gas phase reactor and properties (inventive example 3) Example 3 (Inventive) Temperature [ C.] 80 Pressure [bar] 20 C2 feed [kg/h] 39.3 H2 feed [g/h] 0 C4 feed [kg/h] 0 C6 feed [kg/h] 1.2 C2 concentration [mol %] 21.7 H2/C2 ratio [mol/kmol] 0 C4/C2 ratio [mol/kmol] 0 C6/C2 ratio [mol/kmol] C4/C2 feed ratio [g/kg] 0 C6/C2 feed ratio [g/kg] Production rate [kg/h] 32 Split [wt %] 51 MFR.sub.2 [g/10 min] 2.5 Density [kg/m3] 934

(34) For all the tables:

(35) C2: ethylene

(36) C3: propane

(37) H2: hydrogen

(38) C4: 1-butene

(39) C6: 1-hexene

(40) The powder produced after the multistage polymerisation was compounded and pelletised using an extruder and the medium density polyethylene obtained had a MFR.sub.2=2.2 dg/10 min and a density=935.8 kg/m.sup.3.

Example 4 (Inventive) and Example 5 (Inventive)

(41) A chromium catalyst polyethylene, i.e. a thermoplastic polyethylene having a chlorine content which is less than X, in accordance with the present invention, was prepared as described below.

(42) Chromium Catalyst

(43) Commercially available chromium catalyst BCF03E supplied from Grace Catalyst AB. was used for Example 4 (Inventive).

(44) Commercially available chromium catalyst BCF01E supplied from Grace Catalyst AB. was used for Example 5 (Inventive)

(45) Chromium Catalyst Polyethylene:

(46) The chromium catalyst polyethylene (1-hexene as the comonomer) for example 4 and the chromium catalyst polyethylene (1-butene as the comonomer) for example 5 was prepared with the above chromium catalysts and as described below.

(47) Fluid Bed Gas Phase Example

(48) The following provide a fluid bed gas phase example for producing the Example 4 (Inventive), i.e. a thermoplastic polyethylene having a chlorine content which is less than X, where the conditions for the polymerisation of Table 4 may be used. In a fluidized bed gas phase reactor an olefin is polymerised in the presence of a polymerisation catalyst in an upwards moving gas stream. The reactor typically contains a fluidized bed comprising the growing polymer particles containing the active catalyst located above a fluidization grid. The polymer bed is fluidized with the help of the fluidization gas comprising the olefin monomer, eventual comonomer(s), eventual chain growth controllers or chain transfer agents, such as hydrogen, and eventual inert gas. The fluidization gas is introduced into an inlet chamber at the bottom of the reactor. One or more of the above-mentioned components may be continuously added into the fluidization gas to compensate for losses caused, among other, by reaction or product withdrawal. From the inlet chamber the gas flow is passed upwards through a fluidization grid into the fluidized bed. The fluidization gas passes through the fluidized bed. The superficial velocity of the fluidization gas must be higher than the minimum fluidization velocity of the particles contained in the fluidized bed, as otherwise no fluidization would occur. On the other hand, the velocity of the gas should be lower than the onset velocity of pneumatic transport, as otherwise the whole bed would be entrained with the fluidization gas.

(49) When the fluidization gas is contacted with the bed containing the active catalyst, the reactive components of the gas, such as monomers and chain transfer agents, react in the presence of the catalyst to produce the polymer product, i.e. the chromium catalyst polyethylene. At the same time the gas is heated by the reaction heat. The unreacted fluidization gas is removed from the top of the reactor and cooled in a heat exchanger to remove the heat of reaction. The gas is cooled to a temperature which is lower than that of the bed to prevent the bed from heating because of the reaction. It is possible to cool the gas to a temperature where a part of it condenses. When the liquid droplets enter the reaction zone they are vaporised. The vaporisation heat then contributes to the removal of the reaction heat. The condensing agents are non-polymerisable components, such as n-pentane, isopentane, n-butane or isobutane, which are at least partially condensed in the cooler. The gas is then compressed and recycled into the inlet chamber of the reactor. Prior to the entry into the reactor fresh reactants are introduced into the fluidization gas stream to compensate for the losses caused by the reaction and product withdrawal. It is generally known how to analyze the composition of the fluidization gas and to introduce the gas components to keep the composition constant. The actual composition is determined by the desired properties of the product and the catalyst used in the polymerisation.

(50) The catalyst may be introduced into the reactor in various ways, either continuously or intermittently. The polymeric product may be withdrawn from the gas phase reactor either continuously or intermittently. Combinations of these methods may also be used. Typically the fluidized bed polymerisation reactor is operated at a temperature within the range of from 50 to 110 C., preferably from 65 to 110 C. The pressure is suitably from 10 to 40 bar, preferably from 15 to 30 bar.

(51) TABLE-US-00004 TABLE 4 Process conditions for fluid bed gas phase polymerisation (Example 4 (inventive)) and Example 5 (Inventive) Example 4 Example 5 (Inventive) (Inventive) Temperature [ C.] 103 106 Ethylene partial [bar] 10 6.6 pressure H2/C2 [mol/kmol] 30 30 C6/C2 [mol/kmol] 6 0 C4/C2 [mol/kmol] 0 2 MFR.sub.2 [g/10 min] 1.0 0.6 MFR.sub.21 [g/10 min] 62 11.5 Density [kg/m.sup.3] 946 951.3

(52) For the table 4:

(53) C2: ethylene

(54) H2: hydrogen

(55) C6: 1-hexene

(56) C4: 1-butene

(57) TABLE-US-00005 TABLE 5 Polymer compositions and the DC electrical conductivity results: Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 compar- inven- inven- inven- Inven- Components ative tive tive tive tive HDPE (HE6068) 100 QUEO8230, VLDPE, 100 wt %* Single site 100 polyethylene, wt %* Ex 4 Chromium catalyst 100 polyethylene, wt %* Ex 5 Chromium catalyst 100 polyethylene, wt %* Chlorine content, ~12 <10 <0.5 <0.5 <0.5 ppm DC conductivity, 12.6 7 0.2 4.3 7.55 fS/m Ref is HDPE, i.e. HE6068 *The amounts of polymer composition components in table 5 are based on the combined amount of the used polymer composition components. The amount 100 wt % of polymer in table 5 means that this polymer is the sole polymer component.

(58) As can be seen form table 5, the thermoplastic polyethylenes, having a chlorine content which is less than X, of inventive examples 2, 3, 4 and 5, show excellent low DC conductivity. From inventive example 4 and 5 is it evident that a moderate decrease in density lowers the DC conductivity more than moderate. Inventive example 3 shows even lower DC conductivity compared to example 2 and 4, 5. It has surprisingly been found that a multistage process for polymerisation of polyethylene, suitably with a single site polyethylene polymerisation, gives even lower DC conductivity. The multistage process for polymerisation of polyethylene is suitably a combination of slurry reactor and gas phase reactor, suitably as described in example 3.

(59) The thermoplastic polyethylenes, having a chlorine content which is less than X, in accordance with the present invention, are suitable in the power cable polymer compositions of the invention, e.g. in DC power cables, for example, in HV DC power cables.