Semiconductive polymer composition for electric power cables

Abstract

The invention provides a novel semiconductive polymer composition with improved smoothness and dispersibility of carbon black when compounding the polymer composition and feasible balance with other properties such as volume resistivity. The semiconductive polymer composition comprises (a) from 30 to 90 wt % of a polymer component, (b) from 10 to 70 wt % of carbon black and the carbon black (b) has a mass pellet strength (MPS) according to ASTM D1937-13 of from 50 to 250 N. The invention further relates to a process for preparing the semiconductive polymer composition comprising the steps of: i) introducing 30-90 wt % of a polymer component as defined above and 0-8 wt % additives in a mixer device and mixing the polymer component and additives at elevated temperature such that a polymer melt is obtained; ii) adding 10-70 wt % of a carbon black as defined above to the polymer melt and further mixing of the polymer melt.

Claims

1. A process for preparing a semiconductive polymer composition, comprising the steps of: i) introducing 30-90 wt % of a polymer component (a) and 0-8 wt % additives in a mixer device and mixing the polymer component and additives at elevated temperature such that a polymer melt is obtained; ii) adding 30-70 wt % of a carbon black (b) having a mass pellet strength (MPS) according to ASTM D1937-13 of from 50 to 180 N and further mixing of the polymer melt to obtain a semiconductive polymer mixture, wherein the carbon black (b) has an average pellet size according to ASTM D1511-12 of from 0.1 to 5 mm, an average value of individual pellet hardness for the 5 hardest pellets (M5H) according to ASTM D5230-13 of from 15 to 35 cN, an average value of individual pellet hardness (CSAV) according to ASTM D5230-13 of from 10 to 30 cN and an iodine adsorption number measured according to ASTM D1510-13, method A of from 38 to 48 g/kg; and iii) extruding and pelletising the obtained polymer mixture.

2. The process for preparing a semiconductive polymer composition according to claim 1, wherein the carbon black (b) has one or more of the following characteristics: a BET surface area (STSA value), measured by nitrogen adsorption according to ASTM D 6556-10 of from 20 to 60 m.sup.2/g; a DBP oil absorption number measured according to ASTM D2414-13 of from 100 to 150 cm.sup.3/100 g.

3. The process for preparing a semiconductive polymer composition according to claim 1, wherein the composition has a surface smoothness measured according to the surface smoothness analysis using a tape sample as described herein of not more than 200 particles/m.sup.2 having a width of larger than 150 m, and/or not more than 9 particles/m.sup.2 having a width of larger than 200 m.

4. The process for preparing a semiconductive polymer composition according to claim 1, wherein said carbon black (b) is furnace carbon black.

5. The process for preparing a semiconductive polymer composition according to claim 1, wherein said polymer component (a) comprises an alpha-olefin polymer.

6. The process for preparing a semiconductive polymer composition according to claim 1, wherein said polymer component (a) comprises a homopolymer of a C.sub.2-12 alphaolefin or a copolymer of a C.sub.2-8 alpha-olefin with one or more comonomers of an C.sub.3-30 alpha-olefin.

7. The process for preparing a semiconductive polymer composition according to claim 1, wherein polymer component (a) is selected from a branched ethylene homo- or copolymer and a linear ethylene homo- or copolymer.

8. The process for preparing a semiconductive polymer composition according to claim 1, wherein said polymer component (a) comprises at least one polyunsaturated comonomer.

9. The process for preparing a semiconductive polymer composition according to claim 1, wherein said polymer component (a) comprises at least one polar comonomer.

10. The process for preparing a semiconductive polymer composition according to claim 9, wherein said polar comonomer is selected from the group consisting of: vinyl carboxylate esters, (meth)acrylates, olefinically unsaturated carboxylic acids, (meth)acrylic acid derivatives, and vinyl ethers.

11. The process for preparing a semiconductive polymer composition according to claim 9, wherein the content of polar comonomer in said polymer component(a) is from 0.5 to 35 wt %, based on the total amount of said polymer component (a).

12. The process for preparing a semiconductive polymer composition according to claim 1, wherein the composition has a MFR.sub.21 of from 1.0 g/10 min to 15 g/10 min, measured according to ISO 1133 at 125 C. and a load of 21.6 kg.

13. The process for preparing a semiconductive polymer composition according to claim 9, wherein the composition is cross-linkable via radical reaction or via silane groups.

Description

Example 1

(1) Semiconductive Polymer Composition of the Invention.

(2) 61.4 wt % of conventional ethylene butyl acrylate (EBA) copolymer produced via radical polymerisation in a high pressure tubular reactor, and the copolymer having the following properties: MFR.sub.2 of 18 g/10 min (ISO 1133, load 21.6, 190 C.), butyl acrylate (BA) comonomer content of 14 wt %, melt temperature of 110 C., density 924 kg/m.sup.3 (ASTM D792), was fed together with 0.4 wt % of commercially available antioxidant (4,4-bis(1,1-dimethylbenzyl)diphenylamine) to the first hopper of a Buss mixer, MDK/E 200, (commercially available from Buss with a reciprocating co-kneader). The polymer component was mixed under heating to a molten stage. The temperature profile in said mixer for this test was as follows measured from the molten polymer mixture: first section 104 C. second section 117 C., third section 159 C., fourth section 201 C. and fifth section 208 C. The carbon black (b) used for preparing the semiconductive composition of this example had the properties as shown in Table 1. The carbon black (CB1) (modified N550 grade, (Birla carbon)) was added in two stages. The first part of 27.5 wt % of the carbon black was fed to mixer before said second section of 117 C. and the rest, second part, of said carbon black 10.7 wt % before said third section of 159 C. The total content of added carbon black was 38.2 wt %. The total throughput of the mixer was 1200 kg/hrs and the screw speed of the mixer was set at 121 rpm. The molten polymer mixture obtained from the mixer was then transferred to a commercial extruder, available from Berstorff, which was operating as an integrated unit with said mixer to provide 150 bar pressure for filtering the molten polymer through a 150 m mesh filter in a known manner. The operating temperature of said extruder was approximately 220 C. After the filtration the polymer was pressed thorough an extrusion plate forforming pellets thereof in conventional manner. After pelletisation the pellets are dried and some of about 4 kg pellets are taken out for tape sample preparation for the SSA-analysis as defined above under Determination Methods in order to determine the surface smoothness of the obtained material.

Comparative Example 1

(3) Comparative Semiconductive Polymer Composition

(4) The preparation for producing this composition was effected in the same way as described in Example 1 except that the type of carbon black (CB2) was modified as specified in Table 1.

Example 2

(5) Semiconductive Polymer Composition of the Invention

(6) The preparation for producing this composition was effected in the same way as described in Example 1 except that the type of carbon black (CB3) was modified as specified in Table 1.

Example 3

(7) Semiconductive Polymer Composition of the Invention

(8) The preparation for producing this composition was effected in the same way as described in Example 1 except that the type of carbon black (CB4) was modified as specified in Table 1. The carbon black was N550 grade, supplier: Orion carbon, former: Evonik.

Comparative Example 2

(9) Comparative Semiconductive Polymer Composition

(10) The preparation for producing this composition was effected in the same way as described in Example 3 except that the type of carbon black (CB5) was modified as specified in Table 1.

(11) TABLE-US-00001 Example Example Example Comparative Comparative 1 2 3 Example 1 Example 2 Unit CB1 CB3 CB4 CB2 CB5 CB properties Type N550 N550 N550 N550 N550 STSA m.sup.2/g 41 40 38 42 39 Iodone adsorption number g/kg 45 45 41 45 41 DBP adsorption number cm.sup.3/100g 122 122 123 122 122 CB pellet MPS N 156.91 137.29 78.45 284.39 313.81 properties Toughness Individual CSAV (average) cN 12.75 11.77 22.56 16.67 22.56 of CB pellet M5H (average for the cN 20.59 20.59 29.42 N/A 46.09 pellets hardness 5 hardest pellets) CB Pellet size mm 0.76 0.55 1.2 0.81 0.78 Compound Surface roughness Pips > 150 m pcs/m.sup.2 10 40 9 226 500 Properties Pips > 200 m pcs/m.sup.2 2 6 1 10 70 VR at room temp Ohm-cm 5 5 4 5 N/A CB Content % 38.2 38.2 38.2 38.2 38.2

(12) The samples were found to give different dispersion level of carbon black in the compounds. CB1 (inventive sample) achieved the highest dispersion level of carbon black which results in the lowest surface protuberance. This is due to the improved pellet parameters as also reported in Table 1. CB2 (comparative sample) gave no preferable (or too high) level of surface protuberance for semiconductive applications due to too high value of mass pellet strength (MPS). CB3 (inventive sample) gave much better carbon black dispersion level than CB2. The surface smoothness level is acceptable but not as high as that of the composition of Example 1.

(13) CB4 gives excellent dispersion level of carbon black which results in very good surface smoothness, and yet the volume resistivity value is the lowest. This is due to the appropriate pellet parameters and also the combination of low mass pellet strength (MPS) and large pellet size. On the other hand, CB5 gives no preferable (or the highest) level of surface protuberance for semiconductive applications. This is due to too high values of MPS and M5H.

(14) At the same time the semiconductive polymer compositions of the present invention provide excellent surface smoothness according to the above SSA test. Example 1 and Example 3 provided compositions with excellent surface smoothness as the respective carbon blacks CB1 and CB4 met the specifications of the invention with respect to pellet crush resistance. CB 2 had an excessive mass pellet strength so that dispersion in the polymer resin were poor. As a result, a large number of particles with a size of >150 m and >200 m were detected in a tape sample according to the SSA test. The composition of Example 2 has a fair compromise between surface smoothness and volume resistivity. Due to a rather small average pellet size, the surface smoothness is not as high as with CB1 in Example 1.

(15) A comparison between Example 3 and Comparative Example 2 shows that CB5 has excessively high pellet crush resistance (MPS and M5H) and thus the surface smoothness was poor due to a large number of particles with a size of >150 m and >200 m detected in a tape sample according to the SSA test.

(16) The above examples show that a semiconductive polymer composition containing a specific, modified carbon black meeting the specifications of the invention regarding pellet crush resistance lead to a composition having a superior overall performance regarding surface smoothness, dispersibility of the carbon black in the polymer composition and electric resistance (volume resistivity) which is important in semiconductive applications, especially if the compositions are used for the production of a semiconductive layer of an electric power cable.