POLYMER BLEND COMPOSITION FOR WIRE AND CABLE APPLICATIONS WITH ADVANTAGEOUS ELECTRICAL PROPERTIES

Abstract

The invention provides a polymer composition comprising a low density polyethylene (LDPE); polypropylene (PP) and a compatibiliser. The invention also relates to cables comprising said polymer composition and the use of the polymer composition in the manufacture of an insulation layer of cable.

Claims

1-17. (canceled)

18. A power cable comprising one or more conductors surrounded by at least an insulation layer, wherein said insulation layer comprises a polymer composition comprising: a) a low density polyethylene (LDPE); b) polypropylene (PP) and; c) a compatibilizer selected from a linear low density polyethylene (LLDPE).

19. The power cable as claimed in claim 1, wherein the conductivity of the polymer composition is 40 fS/m or less, when measured at 30 kV/mm according to DC conductivity method as described under Determination Methods.

20. The power cable as claimed in claim 1, wherein the conductivity of the polymer composition is 70 fS/m or less, when measured at 45 kV/mm according to DC conductivity method as described under Determination Methods.

21. The power cable as claimed in claim 1, wherein the conductivity of the polymer composition is 80 fS/m or less, when measured at 60 kV/mm according to DC conductivity method as described under Determination Methods.

22. The power cable as claimed in claim 1, wherein the conductivity of the polymer composition is at least 10% lower than the conductivity of an otherwise identical polymer composition comprising neither the PP nor the compatibilizer, when measured according to DC conductivity method as described under Determination Methods at an electrical stress which is 30 kV/mm or more.

23. The power cable as claimed in claim 1, wherein the LDPE is an LDPE homopolymer or a LDPE copolymer with at least one non-polar comonomer.

24. The power cable as claimed in claim 1, wherein the PP is a propylene copolymer with one or more comonomers.

25. The power cable as claimed in claim 1, wherein: the LDPE is present in an amount of at least 55 wt % relative to the total weight of the polymer composition as a whole.

26. The power cable as claimed in claim 1, wherein the PP b) is present in an amount of from 0.01 to 40 wt % relative to the total weight of the polymer composition as a whole.

27. The power cable as claimed in claim 1, wherein the compatibilizer c) is present in an amount of from 0.01 to 10 wt % relative to the total weight of the polymer composition as a whole.

28. The power cable as claimed in claim 1, wherein: the PP is a random copolymer of propylene or a heterophasic copolymer of propylene.

29. The power cable as claimed in claim 1, wherein the PP is a heterophasic copolymer comprising a propylene random copolymer as a matrix phase (RAHECO) or a heterophasic copolymer having a propylene homopolymer as a matrix phase (HECO).

30. The power cable as claimed in claim 1, wherein the compatibilizer is a multimodal LLDPE.

31. The power cable as claimed in claim 1, wherein the melting point (DMTA) of the polymer composition, which is defined as the temperature where the storage shear modulus, G, is 2 MPa, is at least 118 C.

32. The power cable as claimed in claim 1, wherein the polymer composition is non crosslinked.

33. The power cable as claimed in claim 15, wherein the non-crosslinked polymer composition comprises: a) at least 55 wt % of the LDPE, wherein the LDPE is a low density polyethylene (LDPE) homopolymer or an LDPE copolymer with a non polar comonomer; b) 1.0 to 40 wt % of the PP, wherein the PP is a polypropylene copolymer (PP) and; c) 0.25 to 10 wt % of the compatibilizer.

34. The power cable as claimed in claim 15, wherein the non-crosslinked polymer composition comprises: a) at least 55 wt % of the LDPE, wherein the LDPE is a low density polyethylene (LDPE) homopolymer or an LDPE copolymer with a non polar comonomer; b) 1.0 to 30 wt % of the PP, wherein the PP is a polypropylene copolymer (PP) and; c) 0.25 to 5.0 wt % of the compatibilizer

35. The power cable as claimed in claim 15, wherein the non-crosslinked polymer composition comprises: a) at least 55 wt % of the LDPE, wherein the LDPE is a low density polyethylene (LDPE) homopolymer; b) 1.0 to 40 wt % of the PP, wherein the PP is a polypropylene copolymer (PP) and; c) 0.25 to 10 wt % of the compatibilizer.

36. The power cable as claimed in claim 15, wherein the non-crosslinked polymer composition comprises: a) at least 55 wt % of the LDPE, wherein the LDPE is a low density polyethylene (LDPE) homopolymer; b) 1 to 40 wt % of the PP, wherein the PP is a heterophasic polypropylene copolymer or random heterophasic polypropylene copolymer (PP) and; c) 0.25 to 10 wt % of the compatibilizer.

37. A process for producing the power cable as claimed in claim 1, comprising the step of: applying on a conductor at least an insulation layer comprising a polymer composition comprising: a) a low density polyethylene (LDPE); b) polypropylene (PP) and; c) a compatibilizer selected from a linear low density polyethylene (LLDPE).

38. A power cable comprising one or more conductors surrounded by at least an insulation layer, wherein said insulation layer comprises a thermoplastic polymer composition comprising: a) a low density polyethylene (LDPE); b) polypropylene (PP) and; c) a compatibilizer selected from a linear low density polyethylene (LLDPE) or an olefin block copolymer.

39. A process for producing the power cable as claimed in claim 21, comprising the step of: applying on a conductor at least an insulation layer comprising a thermoplastic polymer composition comprising: a) a low density polyethylene (LDPE); b) polypropylene (PP) and; c) a compatibilizer selected from a linear low density polyethylene (LLDPE) or an olefin block copolymer.

Description

[0223] The invention will now be described with reference to the following non limiting examples and FIGURE.

[0224] FIG. 1 (and Table 2) shows the conductivity results for Inventive compositions (IE 1) and (IE 2) and Comparative composition (CE 1).

DETERMINATION METHODS

[0225] Unless otherwise stated in the description or experimental part the following methods were used for the property determinations. [0226] Wt %: % by weight

[0227] Melt Flow Rate

[0228] 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 and at 230 C. for polypropylene. MFR may be determined at different loadings such as 2.16 kg (MFR.sub.2), 5.0 kg (MFR.sub.5) or 21.6 kg (MFR.sub.21).

[0229] Molecular Weight

[0230] Mz, Mw, Mn, and MWD are measured by Gel Permeation Chromatography (GPC) according to the following method:

[0231] The weight average molecular weight Mw and the molecular weight distribution (MWD=Mw/Mn wherein Mn is the number average molecular weight and Mw is the weight average molecular weight; Mz is the z-average molecular weight) is measured according to ISO 16014-4:2003 and ASTM D 6474-99. A Waters GPCV2000 instrument, equipped with refractive index detector and online viscosimeter was used with 2GMHXL-HT and 1G7000HXL-HT TSK-gel columns from Tosoh Bioscience and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert-butyl-4-methyl-phenol) as solvent at 140 C. and at a constant flow rate of 1 mL/min. 209.5 L of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 1 kg/mol to 12 000 kg/mol. Mark Houwink constants were used as given in ASTM D 6474-99. All samples were prepared by dissolving 0.5-4.0 mg of polymer in 4 mL (at 140 C.) of stabilized TCB (same as mobile phase) and keeping for max. 3 hours at a maximum temperature of 160 C. with continuous gentle shaking prior sampling in into the GPC instrument.

[0232] Density

[0233] 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).

[0234] Xylene Solubles (XS)

[0235] Xylene solubles were determined at 23 C. according ISO 6427.

[0236] DC Conductivity Method

[0237] Circular plaques were compression moulded from pellets of the test polymer composition, i.e. the polymer composition of the present invention, using a 0.5 mm thick stainless steel frame with a 330 mm circular hole. The final plaques therefore had a thickness of 0.5 mm and a diameter of 330 mm. Mylar films were placed between the polymer and the press surfaces.

[0238] The test polymer composition was then press-moulded at 130 C. for 60 s at a pressure of 2 MPa. Thereafter the pressure was increased to 20 MPa while the temperature gradually increased to reach 180 C. after 200 s. The temperature was kept constant at 180 C. for 340 s during which the plaque became fully crosslinked by means of the peroxide, if present in the test polymer composition. Finally, the temperature decreased with a cooling rate of 15 C./min until room temperature was reached and the pressure was released.

[0239] A high voltage source was connected to the upper electrode, to apply voltage over the test sample. The resulting current through the sample, i.e. sample of the polymer composition of the present invention, was measured with an electrometer. The measurement cell was a three electrodes system with brass electrodes placed in an oven at 70 C. The diameter of the measurement electrode was 100 mm.

[0240] A DC voltage of 15 kV was applied leading to a mean electric field of 30 kV/mm. The current through the plaque, i.e. the polymer composition of the present invention, was logged for 11 hours and the current at that time was used to calculate the conductivity of the insulation at 30 kV/mm. The voltage was then increased to 22.5 kV leading to a mean electric field of 45 kV/mm. The current through the plaque, i.e. the polymer composition of the present invention, was logged for 6 hours and the current at that time was used to calculate the conductivity of the insulation at 45 kV/mm. The voltage was then increased to 30 kV leading to a mean electric field of 60 kV/mm. The current through the plaque, i.e. the polymer composition of the present invention, was logged for 6 hours and the current at that time was used to calculate the conductivity of the insulation at 60 kV/mm.

[0241] Dynamic Mechanical Thermal Analysis (DMTA)

[0242] Dynamic Mechanical Thermal Analysis (DMTA) was performed using the torsion rectangular setup according to ISO 6721-7. Temperature was increased at a rate of 2 K/min from 50 C. until melting was reached. The frequency was 1 Hz.

[0243] Experimental Part

[0244] The following materials were used:

[0245] Low density polyethylene, i.e. LDPE polymerA commercially available low density polyethylene (LDPE930), i.e. Bormed LE6609-PH with density 930 kg/m.sup.3, available from Borealis Polyolefine GmbH (Austria), is included in all Inventive Examples 1 and 2 and Comparative Example (i.e. IE 1, IE 2 and CE 1).

[0246] Polypropylene (PP) A commercially available heterophasic copolymers comprising a propylene random copolymer as matrix phase (RAHECO), i.e. Bormed SC876CF available from Borealis Polyolefine GmbH (Austria), is included in both Inventive Examples 1 and 2 (IE 1 and IE 2).

[0247] Compatibiliser

[0248] A commercially available multimodal ethylene copolymer (LLDPE), i.e. Anteo FK1820A available from Borouge Pte Ltd (Singapore), is included in Inventive Example (IE 1).

[0249] A commercially available olefin block copolymer (Copolymer) INFUSE 9807 available from DowDuPont (Olefin Block Copolymer) having a density of 866 kg/m.sup.3, is included in Inventive Example (IE 2).

[0250] The low density polyethylene, i.e. LDPE polymer: Here the commercially available LDPE polymer (LDPE homopolymer, LDPE.sub.930) has the properties of Table 1:

TABLE-US-00001 TABLE 1 Polymer properties of LDPE (LDPE homopolymer, LDPE.sub.930) Base Resin Properties LDPE MFR2, 190 C. [g/10 min ] 0.3 Density [kg/m.sup.3] 930 Tensile modulus 350 MPa Flex Modulus 330 MPa

TABLE-US-00002 TABLE 2 Polymer compositions of the invention (IE 1 and IE 2) and reference (comparative) compositions (CE 1), and also the electrical conductivity results of the polymer compositions: CE 1 IE 1 IE 2 LDPE930 [wt. %] 100 94 94 LLDPE [wt. % ] 0 1 0 Copolymer [wt. %] 0 0 1 PP [wt. %] 0 5 5 DC conductivity at 90 C. [fS/m] 30 kV/mm 41.9 23.2 19.5 45 kV/mm 72.8 26.2 23.1 60 kV/mm 87.4 27.6 25.4 Melting point DMTA [ C.] 116 120 120

[0251] Melting point .sub.DMTA is defined as the temperature where the storage shear modulus, G is 2 MPa.

Examples

[0252] As can be seen from Table 2, polymer compositions of inventive examples (IE 1 and IE 2) show excellent low DC conductivity. Furthermore, the DC conductivity drops significantly when the pure LDPE is enhanced in accordance with the invention. The polymer compositions of the invention are particularly useful in DC power cables, preferably in HV DC power cables.

[0253] Further, the higher thermomechanical performance is illustrated in Table 2 herein. As can be seen the effective melting point of the LDPE/PP blends, i.e. of the polymer compositions of the invention, is about 4 C. degrees higher than for same LDPE without PP. This, may not be regarded to be a huge difference, however for HVDC cables, where the maximum overload temperature is 105 C., such an improvement can be of significant importance. This may offer the possibility to use thermoplastic LDPE based blends for future HVDC insulation.