Semiconductive polymer composition for electric power cables
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.
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 2 /g;
a DBP oil absorption number measured according to ASTM D2414-13 of from 100 to 150 cm 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 2 having a width of larger than 150 μm, and/or
not more than 9 particles/m 2 having a width of larger than 200 μm.
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 2-12 alphaolefin or a copolymer of a C 2-8 alpha-olefin with one or more comonomers of an C 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 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.