PROCESS FOR MANUFACTURING A POWER CABLE AND POWER CABLE OBTAINABLE THEREOF

Abstract

The present invention relates to a process for manufacturing a power cable, which power cable comprises at least one core comprising a conductor and an expanded and crosslinked insulation layer surrounding said conductor, wherein said process comprises the steps a) to d): a) providing a blend of: a polymer composition comprising a polyolefin material, which polyolefin material bears silane moieties; a catalyst and a foaming system, wherein the provided blend will comprise at least 0.1% by weight of a foaming agent, with respect to the total weight of the polyolefin material; b) extruding a blend, as described in step a), on the conductor to form an insulation layer; c) foaming the insulation layer; and d) crosslinking the insulation layer; a power cable which is obtainable by the process, and use of the power cable.

Claims

1. Process for manufacturing a power cable, which power cable comprises at least one core comprising a conductor and an expanded and crosslinked insulation layer surrounding said conductor, wherein said process comprises the steps a) to d): a) providing a blend of: a polymer composition comprising a polyolefin material, which polyolefin material bears silane moieties, a catalyst and a foaming system, wherein the provided blend will comprise at least 0.1% by weight of a foaming agent, with respect to the total weight of the polyolefin material; b) extruding a blend, as described in step a), on the conductor to form an insulation layer; c) foaming the insulation layer; and d) crosslinking the insulation layer.

2. A process according to claim 1, wherein the foaming system comprises 0.1 to 0.7% by weight of the foaming agent.

3. A process according to claim 1, wherein the polymer material comprises a low density polyethylene with hydrolysable silane groups.

4. The process according to claim 1, wherein the polyolefin material comprises 0.001 to 15% by weight of silane compound.

5. A power cable which is defined as in, and is obtainable by the process according to claim 1.

6. The power cable according to claim 5, wherein the insulation layer has an expansion degree of 3 to 40%.

7. The power cable according to claim 5, wherein the insulation layer is in contact with the conductor.

8. The power cable according to claim 5, which is a low voltage cable or a medium voltage cable.

9. The power cable according to claim 5, which comprises three cores each comprising a conductor.

10. The power cable according to claim 5, wherein the insulation layer has an expansion degree of from 5% to 30%.

11. The power cable according to claim 5, wherein the insulation layer has an expansion degree of from 5% to 25%.

12. The power cable according to claim 5, wherein the insulation layer has an average cell size equal to or lower than 150 m.

13. The power cable according to claim 5, wherein the insulation layer has an average cell size equal to or lower than 100 m.

14. (canceled)

Description

LEGEND OF FIGURE

[0089] FIG. 1 shows breaking stress as a function of expansion degree

EXAMPLES

1. Methods

[0090] a. Hot Set Elongation

[0091] The crosslinking of the polymer composition, i.e. the insulation layer of the present invention, was determined according to IEC-60811-2-1 (hot set method and permanent set) by measuring the thermal deformation at 200 C. and a load of 0.2 MPa. The conductor was removed from the insulation, i.e. from the insulation layer of the present invention, resulting in a tubular insulation specimen. Reference lines were marked 20 mm apart on the tubular test specimen and the inner and outer diameters were measured. The test samples, i.e. originating from the insulation layer of the present invention, were fixed vertically from upper end thereof in an oven heated to 200 C. and a load of 0.2 MPa was attached to the lower end of the test samples. The distance between the upper and lower clamps holding a sample and the load, respectively, was 5 mm. After 15 min in the oven, the distance between the pre-marked lines was measured and the percentage of hot set elongation calculated, hot set elongation %. For permanent deformation %, sometimes also referred to as permanent set, the tensile force (weight) was removed from the test sample and after recovery in 200 C. for 5 minutes the sample was taken out of the oven and let to cool in room temperature to ambient temperature. The permanent set % was calculated from the distance between the marked lines after cooling. The reported values are the average values from three tests.

b. Tensile Testing According to EN 60811-100

[0092] The tensile strength and elongation at break of 150 mm long tubular insulation test specimen from stripped cable samples, i.e. originating from the insulation layer of the present invention, were measured in accordance with ISO 527-1:1993 at 23 C. and 50% relative humidity on a Doli-Alwetron TCT 25 tensile tester at a speed of 250 mm/min. A digital extensiometer with a starting distance of 50 mm was used for determination of the elongation at break. The starting distance between the clamps of the tensile tester was 115 mm. A 1 kilo Newton load cell was used for the measurements. The samples were conditioned for minimum 16 hours at 23+/2 C. and 50% relative humidity prior testing. The average value out of 6-10 samples is reported herein.

c. MFR

[0093] The melt flow rate MFR2 was measured in accordance with ISO 1.133 at 190 C. and a load of 2.16 kg.

d. Density

[0094] The density was measured according to ISO 1183A on samples prepared according to ISO1872-2.

2. Materials

[0095] The polyolefin material which bears silane moieties and is comprised in the polymer composition used in the process according to the present invention, is in the examples herein an ethylene vinylsilane copolymer Visico LE4423, i.e. a crosslinkable polyolefin with hydrolysable silane groups, supplied by Borealis having a density of 922.5 kg/m.sup.3, an MFR.sub.2.16 of 1.0 g/10 min and VTMS comonomer content is 1.1% by weight.

[0096] The catalysts used in the process according to the present invention are comprised in catalyst masterbatches, which are in the examples herein:

the commercially available master batch of an organotin catalyst, i.e. LE4438, which catalyses silane crosslinking reactions, supplied by Borealis, and
the commercially available master batch of an silane condensation catalyst, i.e. LE4476, wherein the active catalyst component is based on sulfonic acid, supplied by Borealis.

[0097] The foaming system used in the process according to the present invention is in the examples herein: an azodicarbonamide (ADC) masterbatch containing 15% by weight ADC in low density polyethylene. Available commercially as nCore 7155-M1-300 from the supplier Americhem.

3. Sample Preparation

[0098] Prior to testing, the foaming system used in the inventive examples was compounded into a polyolefin material bearing silane moieties using a BUSS AG co-kneader type PR46B-11D/H1 (50 mm screw). Compounding is a type of melt mixing of polymers where one or more polymers and/or additives are mixed in molten state. It is often used for dispersion and distribution of additives and fillers in a polymer melt. 2% by weight of the foaming system nCore 7155-M1-300 was mixed into 98% by weight LE4423-SE05 in the compounding process.

[0099] The blends used for producing the exemplified foamed insulation samples were obtained by taking the mix containing the polyolefin material bearing silane moieties and the foaming system and dry blending said mix with 5% by weight of a catalyst masterbatch containing silane crosslinking catalyst and other additives. The blends were then extruded on 1.5 mm.sup.2 solid Cu conductor preheated to 110 C. on a Nokia-Maillefer 60 mm extruder. The extruded cable was cooled in a 50 C. water bath positioned 60 cm from the die exit. Temperature settings, insulation thickness, die size and line speed for each sample can be seen in

Table 1. The cable samples were after extrusion crosslinked for 24 hours in a 90 C. water bath, resulting in the insulation layer of the present invention, prior to hot set and mechanical testing.

[0100] The comparative example CE1 was produced in the same way, except that no foaming system was added to the polyolefin material bearing silane moieties, nor was any compounding performed.

TABLE-US-00001 TABLE 1 Sample CE1 IE1 IE2 IE3 IE4 Insulation LE4423-SE05 LE4423-SE05 LE4423-SE05 LE4423-SE05 LE4423-SE05 material containing 2% containing 2% containing 2% containing 2% by weight by weight by weight by weight nCore nCore nCore nCore 7155-M1-300 7155-M1-300 7155-M1-300 7155-M1-300 Catalyst LE4438 LE4438 LE4438 LE4438 LE4476 masterbatch (5% by weight added to the insulation material) Insulation 0.72 0.73 0.46 0.46 0.49 thickness (mm) Expansion (%) 0 10 19 5 12 Die (mm) 2.8 2.8 2.3 2.3 2.3 Line speed 75 75 75 110 75 (m/min) Temp settings 150/160/170/ 190/200/210/ 190/200/210/ 190/200/210/ 190/200/210/ Z1/Z2/Z3/Z4/Z5, 170/170, 210/210, 210/210, 210/210, 210/210, H1/H2/H3 ( C.) 170/170/170 210/210/210 210/210/210 210/210/210 210/210/210 Hot set elongation 41.4 41.9 42.5 41.9 Fail (%) Permanent 2.3 1.7 2.5 2 Fail deformation (%) Stress at break 23.4 18.8 14.7 21.1 15.0 (MPa) Strain at break 417 401 345 403 479 (%) Average number 0 8 per 0.73 mm 3 per 0.46 mm 8 per 0.46 mm 5 per 0.49 mm of cells in insulation cross section Average cell size 35 100 25 60 (m)

[0101] Table 1 shows inventive examples, IE1-IE4, i.e. the insulation layer of the present invention, compared to solid EVS copolymer insulation, i.e. the comparative example, CE1. Hot set was tested according to EN 60811-50 and tensile testing according to EN 60811-100. Results from these tests are presented in

Table 1.

[0102] The foamed insulation can be seen as a composite material consisting of gas cells in a polymeric matrix. One would expect that hot set would be higher for foamed samples as foaming results in less polymeric material in the insulation that can bear the load during the measurement. It is thus very surprising to see that the hot set values for the inventive samples IE1, IE2 and IE3, which were crosslinked with an organotin catalyst, are equal to the solid CE1 crosslinked with the same catalyst under the same conditions. The foamed inventive example IE4 containing a sulphonic acid catalyst gives a strain break during hot set testing. This result was expected as decomposition of the blowing agent azodicarbonamide results in alkaline decomposition products and it is known that the activity of sulphonic acid catalysts are reduced in presence of alkaline substances.

[0103] The average cell size and the number of cells per insulation cross-section can also be seen in

Table 1. There is a large variation in cell size and number of cells between the different insulation samples: IE2 contains few large cells while IE1 and IE3 contain more and smaller cells. Surprisingly, the cell size and number of cells does not seem to have any impact on tensile properties, it is only the degree of expansion that seem to be important. FIG. 1 shows breaking stress as a function of expansion degree, and it can here be seen that stress at break decreases linearly with expansion degree seemingly independent of how the air is divided inside the insulation.

[0104] Thus, it has, accordingly, hereby been shown that an expanded and crosslinked insulation layer comprising an expanded and crosslinked blend, in accordance with the present invention as described herein, meets the general hot set and tensile requirements for cable insulation materials.