Use Of A Silicone Rubber Composition For The Manufacture Of An Insulator For High Voltage Direct Current Applications

Abstract

The invention relates to a silicone rubber composition having specific dielectric properties which can be used as insulator material in high voltage direct current applications and a method for the manufacture of cable accessories like cable joints. The invention comprises as well a method for the determination of the optimum dielectric properties and the related amount of dielectric active additives.

Claims

1-36. (canceled)

37: A silicone composition, obtained by curing a composition comprising: a) 100 pt. wt. of at least one polyorganopolysiloxane having alkenyl groups, b) 0-100 pt. wt. of a crosslinker component comprising one or more polyorganohydrogensiloxanes, c) 0-100 pt. wt. of a filler component comprising one or more reinforcing silicas or resins, d) >0.1-2 pt. wt. of at least one dielectric active compound, e) a curing catalyst selected from the group consisting of 0-1000 ppm of a compound enabling hydrosilylation and 0.1 to 2 wt.-% of an organic peroxide each related to the sum of the amounts of the components a) to d), and f) 0-50 pt. wt. of one or more auxiliary additives, said silicone composition having a temperature dependency of the volume resistivity in the range of 25 to 90° C. at an electric field of 10 kV/mm to 30 kV/mm, such that the ratio of the maximum volume resistivity and the minimum volume resistivity is <10, and the volume resistivity in the range of 25 to 90° C. at an electric field of 10 kV/mm to 30 kV/mm is between 1*10.sup.11 and 1*10.sup.16 Ohm*cm.

38: The silicone composition according to claim 37, comprising as component a) a polyorganosiloxane having organic substituents R selected from the group consisting of alkyl, phenyl, and trifluoropropyl groups and substituents R.sup.1 selected form the group consisting of alkenyl, such as vinyl groups, and an average degree of polymerization P.sub.n between 100 to 12000 siloxy units.

39: The silicone composition according to claim 38, wherein the crosslinker component b) is selected from the group consisting of polyorganohydrogensiloxane comprising units of the formula RHSiO and R.sub.2HSiO.sub.0.5 and having a concentration of SiH units of 1 to 100 mol. % related to all siloxane units of the polyorganohydrogensiloxane of component b.

40: The silicone composition according to claim 39, wherein the filler component c) is selected from fumed silicas having a surface area according to BET of 50 to 400 m.sup.2/g.

41: The silicone composition according to claim 40, wherein the dielectric active compound d) is selected from the group consisting of conductive fillers and semi-conductive fillers.

42: The silicone composition according to claim 40, wherein the dielectric active compound d) is selected from the group consisting of carbon black, graphite, graphenes, fullerenes, carbon nanotubes; oxides, carbides, ferrites or spinels of Ti, Al, Zn, Fe, Mn, Mo, Ag, Bi, Zr, Ta, B, Sr, Ba, Ca, Mg, Na, K, and Si and inorganic salts of the foregoing; and ionic liquids and ionic polymers.

43: The silicone composition according to claim 40, wherein the dielectric active compound d) is a conductive filler having a BET surface of 30 to 1500 m.sup.2/g and an average particle size of D.sub.50 between 0.001 to 50 μm.

44: The silicone composition according to claim 40, wherein the dielectric active compound d) is a conductive carbon black having a BET surface of >30 m.sup.2/g and an average particle size of D.sub.50 between 5 to 500 nm.

45: The silicone composition according to claim 37, wherein the dielectric active compound d) is a conductive carbon black having at least one of the following properties: i) a BET surface area of >100 to 1500 m.sup.2/g, ii) a particle size of D.sub.50 between 5 to 500 nm, iii) a DBP pore volume 300-600 ml/100 g, iv) iodine adsorption 700-1200 mg/g, v) pH 8-11, vi) metal content <50 ppm, vii) sulfur content <150 ppm, and/or water content <0.5 wt. %, viii) volatiles content <1 wt. %, ix) fines <125 micron in pellets <10 wt.-%, x) grit content: <50 mg/kg, xi) ash content: <0.1 wt.-%.

46: The silicone composition according to claim 40, wherein the dielectric active compound d) is an ionic polymer or ionic liquid selected from the group consisting of organic compounds or polymers comprising ammonium, phosphonium, carboxylic, phosphate or sulfonate groups.

47: The silicone composition according to claim 37, wherein the curing catalyst e) is a hydrosilylation catalyst selected from the group consisting of metals or metal compounds of Pt, Pd, Rh, Co, Ni, Ir or Ru.

48: The silicone composition according to claim 37, wherein the curing catalyst e) is an organic peroxide selected from the group consisting of substituted or unsubstitued dialkyl-, alkylaroyl-, diaroyl-peroxides.

49: A method for the manufacture of an insulator or a field grading assembly, comprising said insulator, for high voltage direct current applications, comprising the steps of A) shaping the silicone composition as defined in claim 36 by extrusion through a nozzle or by a mould and B) curing the shaped composition by heat or light to form a shaped insulator or a field grading assembly, comprising said insulator.

50: An insulator or a field grading assembly comprising said insulator for high voltage direct current application which is obtained by curing the composition according to claim 37.

51: A method for the manufacture of a cable joint or a cable termination comprising the steps of: A1) providing a conductive shaped silicone composition which differs from the silicone composition according to claim 38, which is optionally cured, and B1) encapsulating at least a part of the surface of the composition of step A1) with a composition according to claim 1 in a mold to form and cure a cable joint or cable termination.

52: A method for sealing and/or insulating connected cables or closing cable ends by the use of the cable joint according to claim 51 comprising the steps of j) providing an insulated wire having a thermoplastic or elastomer multi-layered sheath appropriate for direct current insulation and naked wire or connectors, jj) encapsulating naked wire or connectors by putting over onto the surface of the insulating sheath of j) the holes of a tube-like previously moulded and cured cable joint according to claim 15 under mechanical extension of the joint in such a way that an overlap between the shaped silicone cable joint and the sheath onto the wire insulation of about more than 0.5 cm is achieved whereby the silicone cable joint seals the sheathed insulation of the insulated wire by mechanical pressure of the relaxed joint forming an encapsulating insulation also for the naked wire and connectors.

53: A method for the determination of the optimum amount of a dielectric active compound in a cured silicone composition for the use as high voltage direct current insulator comprising the steps i) measuring the temperature dependency of the volume resistivity between 25 to 90° C. in an interval of the electric field of between 10 kV/mm to 30 kV/mm for said cured silicone composition, ii) adjusting the concentration of a dielectric active compound in said cured silicon composition such that the ratio of the maximum volume resistivity and the minimum volume resistivity in said range of 25 to 90° C. at an electric field of 10 kV/mm to 30 kV/mm, is at least <10, and that the volume resistivity in the range of 25 to 90° C. at an electric field of 10 kV/mm to 30 kV/mm is between 1*10.sup.11 and 1*10.sup.16 Ohm*cm.

54: The method according to claim 53, wherein the concentration of a dielectric active compound in said cured silicon composition is adjusted such that the ratio of the maximum volume resistivity and the minimum volume resistivity becomes minimal at a given voltage.

Description

EXAMPLES

Example 1: Preparation of a Master Batch Comprising the Dielectric Active Compound d)

[0389] In order to improve the dispersion quality of the carbon black a master batch was produced as follows:

[0390] 100 kg of a vinyl terminated linear polydimethylsiloxane as component a) having a viscosity of 10 Pa.Math.s at 20° C. was placed in a planetary mixer and mixed with 12.7 kg carbon black Ketjenblack EC 300 J (Akzo) having BET surface 800 m.sup.2/g (350 DBP pore volume ml/100 g) with a primary particle size of 40 nm. This mixture was stirred in a twin blade kneader till a homogeneous mixture was obtained after 45 min.

[0391] The homogeneous mixture was then further dispersed over 30 min on a three-roll mill to obtain a much better dispersion of the carbon black. After this treatment all particles in the filler batch show a particle size of smaller than 10 μm by a grindometer evaluation and the mixture had a viscosity at 20° C. of 550-700 Pa.Math.s at a shear rate D=10.sup.1 s.sup.−1.

TABLE-US-00001 TABLE 1 Master batch incl. component d) pt. wt. Vinyl terminated PDMS 10 Pa .Math. s (U10) M.sup.Vi.sub.2D.sub.40 88.7 Ketjen Black EC300J-batch (11.3% carbon in U10) carbon black 11.3 sum 100.0

Example 2

2a

[0392] A transparent catalyst base compound was produced as follows: 11.8 kg of a vinyl terminated linear polydimethylsiloxane (U10) component a) with a viscosity of 10 Pa.Math.s at 20° C., and 21.3 kg of a vinyl terminated linear polydimethylsiloxane component a) with a viscosity of 65 Pa.Math.s at 20° C. were placed in a twin blade kneader and mixed with 3.4 kg of hexamethyldisilazane, 0.03 kg 1,3-divinyltetramethylsilazane, and 1.4 kg of water. Then 17 kg of fumed silica with a BET surface of 300 m.sup.2/g component c) was gradually added at 25-40° C. and mixed in and dispersed under reflux until a uniform mixture was obtained. This mixture was stirred and heated to reflux for 30 minutes. The volatiles were then distilled off at 100° C., then 150° C. for 1 h and by applying subsequently vacuum of 20 mbar pulled for 30 minutes.

[0393] The mixture was diluted with 34.4 kg of the above polydimethylsiloxane of 10 Pa.Math.s and 4.2 kg of a linear vinyl terminated polydimethylsiloxane component a2) having vinyl side groups and vinyl concentration of 2 mmol/g having a viscosity of 0.2 Pa.Math.s component a2). Finally 0.21 kg of a platinum vinylsiloxane complex component e) solution of the Karstedt type in vinyl terminated polydimethylsiloxane (1.47% Pt) was admixed. 2b

[0394] A transparent crosslinker base compound was produced as follows: 11.9 kg of a vinyl terminated linear polydimethylsiloxane with a viscosity of 10 Pas at 20° C., 21.6 kg of a vinyl terminated linear polydimethylsiloxane with a viscosity of 65 Pa.Math.s at 20° C. were placed in a twin blade kneader and mixed with 3.4 kg of hexamethyldisilazane, 0.03 kg 1,3-divinyltetramethylsilazane, and 1.4 kg of water.

[0395] Then 23.2 kg of fumed silica with a BET surface of 300 m.sup.2/g were gradually added at 25-40° C. mixed in and dispersed under reflux until a uniform mixture was obtained. This mixture was stirred and heated to reflux for 30 minutes. The volatiles were then distilled off at 100° C., heated to 150° C. for 1 h and by subsequently applying vacuum up to 20 mbar pulled for 30 minutes.

[0396] The mixture was diluted with 27.4 kg of the above polydimethylsiloxane of 10 Pa.Math.s, 0.13 kg of an inhibitor 1-ethinyl-2-cyclohexanol (ECH) as component f) and then the mixture was completed with 5.9 kg of M2D.sub.20D.sup.H.sub.10a linear polydimethylhydrogenmethylsiloxane component b) and 9.5 kg M2D.sub.100D.sup.H.sub.20 of a linear polydimethylhydrogenmethylsiloxane at the end of the mixing process.

Example 2c

[0397] Compounds of 2a and 2b are mixed in a ratio of 1:1 to form the mixture of example 2c.

[0398] The mixture has the overall composition after evaporation of the volatiles as shown in table 2.

Example 2d

[0399] The inventive composition example 2d is made by addition of 5 pt. wt of example 1 to 95 pt. wt. of example 2c

[0400] The example 2d comprises therefore about 18 wt. % of silyated SiO.sub.2 filler, 0.5 wt. % of carbon black admixed e.g. via a batch with predispersed carbon black as shown in example 1 for a master batch. The molar ratio of SiH:SiVi is 3.3:1. The mixture contains 15.7 ppm platinum and 650 ppm of the inhibitor 1-ethinyl-2-cyclohexanol (ECH). The mixture has a viscosity of 50 Pa.Math.s at a shear rate of D=10 s.sup.−1. The material has a liquid to pasty consistency. The mechanical properties are reported in table 3.

Example 3

[0401] Example 3 was prepared according to the procedure of example 2d. The composition after evaporation of the volatiles is shown in table 2.

Comparative Example 4

[0402] Comparative example 4 was prepared according to the procedure of example 2d. The composition after evaporation of the volatiles is shown in table 2.

[0403] Test Results

[0404] Table 3 is showing all of the test results of mechanical and electrical properties measured for the cured silicone compositions of the examples and comparative example. The compositions were cured in a mould for 10 min at 175° C. in order to achieve test sheets of different thicknesses between 1.5, 2 and 6 mm; the evaluations followed the test standards defined above.

[0405] The inventive composition of ex. 2d showed a very small temperature dependency of the volume resistivity in the applied voltage range, which is favourable in particular for the high voltage direct current application in cable joints. It also passed the test in a real cable joint. In particular, the temperature dependency of the volume resistivity in the range of 25 to 90° C. at an electric field of 10 kV/mm to 30 kV/mm, i.e. the ratio of the maximum volume resistivity and the minimum volume resistivity in said range is 4.8×10.sup.15/1.1×10.sup.15=4.36 (example 2d A stage).

[0406] Example 3 showed still an acceptable temperature dependency of the volume resistivity in the applied voltage range, but the temperature dependency of the volume resistivity in the range of 25 to 90° C. at an electric field of 10 kV/mm to 30 kV/mm, i.e. the ratio of the maximum volume resistivity and the minimum volume resistivity in said range is 7.2×10.sup.15/8.4×10.sup.1′=8.57. Example 3 in some instance could already show failures due to disruptive discharge and a loss of tracking resistance.

[0407] The volume resistivity values show that the temperature dependency of example 2d is smaller than in example 3 which is favourable for the high voltage application in cable joints. Additionally, the volume resistivity in example 2d (A stage) decreases with increasing applied voltages whereas in example 3 the volume resistivity increases with increasing voltages at all particular given temperatures.

[0408] Comparative example 4 does not provide a level of volume resistivity high enough for an appropriate insulator. Accordingly the maximum measurable voltage was only up to 0.3 kV for 25, 60 and 90° C. Higher voltages lead to complete material breakdown.

TABLE-US-00002 TABLE 2 composition of example 2c, 2d and 3, and comparative example 4 (all percentages are weight percent) Example Example Comp. 2c 2d Ex. 3 Ex. 4 Vinyl terminated PDMS 10 Pa .Math. s M.sup.V.sub.i2D.sub.540 50.55 50.05 50.05 49.05 Vinyl terminated PDMS 65 Pa .Math. s M.sup.Vi.sub.2D.sub.900 21.5 21.5 21.5 21.5 Silylated SiO.sub.2 .sup.1) 300 m.sup.2/g 18 18 18.4 16.5 PDMS vinyl side & end groups 0.2 Pa .Math. s M.sup.Vi.sub.2D.sub.75D.sup.vi.sub.10 2.1 2.1 2.1 2.1 SiH PDMS 4.3 mmol/g SiH M.sub.2D.sub.20D.sup.H.sub.10 4.74 4.74 4.74 4.74 SiH PDMS 2.3 mmol/g SiH M.sub.2D.sub.100D.sup.H.sub.20 2.94 2.94 2.94 2.94 1-ethinyl-2-cyclohexanol ECH 0.06 0.06 0.06 0.06 Pt-cat. 1.47% Pt in PDMS 10 Pa .Math. s vinyl term. Pt-0-M.sup.Vi.sub.2-complex 0.11 0.11 0.11 0.11 Ketjen black EC300J component d) carbon black — 0.5 0.1 3 sum 100.00 100.00 100.00 100.00 .sup.1) silylated SiO2 incl. parts of silazane reaction products

TABLE-US-00003 TABLE 3 Mechanical and electrical properties of the examples 2d, example 3 and comparative example 4 ex. 2d ex. 2d ex. 3 Comp. ex. 4 Examples A-stage ).sup.2 B-stage).sup.3 A-stage ).sup.2 A stage ).sup.2 Example 2a + 2b 1:1 pt. wt. 100 100 100 100 Carbon black % 0.5 0.5 0.1 3 Viscosity @25° C. before cure Pa .Math. s 51 51 not measured Hardness °Shore 32 29 33 31 Tensile strength MPa 5.6 5.1 5.9 4.6 Elongation % 555 518 396 458 Modulus 50% MPa 0.38 0.36 0.62 0.43 Modulus 100% MPa 0.65 0.68 1.19 0.75 Modulus 200% MPa 1.4 1.56 2.66 1.53 Modulus 200% MPa 2.32 2.58 4.25 2.52 Tear resistance ASTM D 624 die B N/mm 22 23 28 8 Surface resistivity Ohm 2.3*10.sup.16 3.8*10.sup.16 2.6*10.sup.16 1.3*10.sup.6 Tracking resistance 4.5 kV )* passed passed passed failed Volume resistivity Vol. res. 10 KV @ 25° C. Ohm*cm 4.8*10.sup.15 5.5*10.sup.15 6.5*10.sup.15 failure Vol. res. 10 KV @ 60° C. Ohm*cm 3.8*10.sup.15 4.8*10.sup.15 1.8*10.sup.15 failure Vol. res. 10 KV @ 90° C. Ohm*cm 2.5*10.sup.15 2.4*10.sup.15 8.4*10.sup.14 failure Vol. res. 20 KV @ 25° C. Ohm*cm 3.5*10.sup.15 4.7*10.sup.15 7.2*10.sup.15 failure Vol. res. 20 KV @ 60° C. Ohm*cm 2.4*10.sup.15 3.4*10.sup.15 2.1*10.sup.15 failure Vol. res. 20 KV @ 90° C. Ohm*cm 1.3*10.sup.15 2.3*10.sup.15 8.5*10.sup.14 failure Vol. res. 30 KV @ 25° C. Ohm*cm 2.1*10.sup.15 3.3*10.sup.15 7.3*10.sup.15 failure Vol. res. 30 KV @ 60° C. Ohm*cm 1.5*10.sup.15 2.6*10.sup.15 2.3*10.sup.15 failure Vol. res. 30 KV @ 90° C. Ohm*cm 1.1*10.sup.15 2.2*10.sup.15 8.8*10.sup.14 failure ).sup.2 10 min 175° C. ).sup.3post cured 4 h 200° C. )* IEC 60587