Silicone Elastomers for HVDC

Abstract

The present invention relates to crosslinked silicone elastomers, their production method, and their use in high voltage direct current (HVDC) applications.

Description

[Technical field]

[0001] The present invention relates to a crosslinked silicone elastomer, its production process and its use in HVDC (High Voltage Direct Current) systems. [Background technology]

[0002] Electricity can be transmitted substantially more cost-effectively over long distances by HVDC than by HVAC (High Voltage Alternating Current) systems, because of the smaller electrical losses involved with HVDC. Especially for long distance underground HVDC, cable connections must be used at a high rate, i.e. every 1-2 km.

[0003] However, insulating materials used in high voltage AC systems typically cannot be utilized in HVDC systems because the electrical stresses for AC and DC conditions can be significantly different.

[0004] The local distribution of the electric field in HVDC applications is determined by the volume resistivity of the electrical insulating material used. Thus, in the prior art, these cable connections mainly use EPDM (Ethylene-Propylene-Diene Rubber) because the resistance of that material is below that of the polyolefin-based insulating materials for the cables.

[0005] EPDM is rigid and uses many fillers (impurities) which often result in unfavorable behavior in assembly and handling situations.

[0006] Silicone elastomers have not been used so far for HVDC applications because their resistance is too high compared to cable insulation materials.

[0007] Therefore, in the prior art, conductive fillers (such as metal powder, metal flakes, carbon black or carbon nanotubes) are used to adjust the resistance of cured silicone elastomers. These types of fillers can bring about further problems because they are present in very small amounts, making it very impossible to distribute these fillers evenly in the material during the mixing process and then provide uniform electrical properties in the cured silicone elastomer. They also cause the physical properties of the cured silicone elastomer to deteriorate and the dielectric strength to deteriorate.

[0008] WO2021195038A1 discloses a silicone composition containing a silica filler partially surface-treated with a fluorinated hydrophobizing agent, where the high cost of raw materials is a significant drawback.

[0009] The obvious drawbacks of the systems known from the prior art can be summarized as follows:

[0010] The disadvantages of the filled systems lie firstly in their reproducibility (e.g. the sharp drop in resistance in the region of the percolation limit) and secondly in their possible anisotropic effects (metal oxides on platelet-like carrier systems) and their dependence on humidity.

[0011] Fluorinated systems are fundamentally very cost intensive and halogenated polymers should essentially be avoided wherever possible for environmental reasons.

[0012] EPDM based mixtures are stiff and difficult to handle and install. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] International Publication No. 2021 / 195038 Summary of the Invention [Problem to be solved by the invention]

[0014] There is therefore a great need for a silicone composition for producing silicone elastomers for HVDC applications that does not exhibit the above-mentioned drawbacks of the prior art. [Means for solving the problem]

[0015] Surprisingly, it has been found that the present crosslinked silicone elastomers of the present invention durably exhibit the required reduction in electrical resistance.

[0016] The present invention therefore provides a crosslinked silicone elastomer having a volume resistivity adjusted to the volume resistivity of the adjacent cable insulation, This volume resistivity is determined according to standard IEC 62631-3-1 in a heatable guard ring arrangement with a field strength of 1 kV / mm for a crosslinked silicone elastomer of 0.5 mm thickness and meets the following values ​​after application of the test voltage: - After 10000 minutes: <1.0×10 16 Ω*cm The crosslinked silicone elastomer is (A) 50% to 99% by weight of at least one diorganopolysiloxane having at least two crosslinkable groups per molecule; (B) 0.5% to 5% by weight of at least one peroxide; (C) 0% to 50% by weight of at least one reinforcing filler; and (X) does not contain any conductive or semi-conductive additives; The amounts of all components may be obtained by crosslinking a base composition which adds up to 100% by weight; This base composition - Applying to a substrate or filling into a mold, - in a first step, the crosslinking is carried out by heating to a temperature of at least the 10h HLT (= 10h half-life temperature) of the peroxide (B), the heating duration corresponding to at least 0.2 of the HL (= half-life) of the peroxide (B) at the chosen crosslinking temperature, In a second step, the heat treatment is carried out above a temperature of 10 hours HLT of the peroxide (B), the heat treatment duration corresponding to at least 1 HL of the peroxide (B) at the selected heat treatment temperature. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In order not to overly broaden the page count of the present description, only preferred embodiments of individual features are described below, but the trained reader will explicitly understand this nature of the present disclosure, as any combination of different preference levels is also explicitly disclosed and explicitly desired.

[0018] The volume resistivities of these crosslinked silicone elastomers of the present invention are as follows: - After 1 minute: <1.0×10 15 Ω*cm, preferably <8.0×10 14 Ω*cm, more preferably <5.0×10 14 Ω*cm, - After 15 minutes: <3.0×10 15 Ω*cm, preferably <2.0×10 15 Ω*cm, more preferably <8.0×10 14 Ω*cm, - After 10000 minutes: <1.0×10 16 Ω*cm, preferably <8.0×10 15 Ω*cm, more preferably <6.0×10 15 Ω*cm.

[0019] The energy source used for crosslinking and heat treatment is preferably an oven, such as a forced air drying cabinet, a heated tunnel, a heated roll, a heated plate, a heated mold, or thermal radiation in the infrared range.

[0020] It has become clear that the volume resistivity of the cured silicone elastomers of the present invention can be reliably adjusted only by the amount of peroxide (B) according to the present invention in combination with the crosslinking and heat treatment conditions of the present invention, despite the elimination of the use of conductive or semiconductive additives (X). Flexible silicone elastomers can be produced with advantageous properties already established in AC voltage applications compared to other materials (resistance to electrical degradation, gas permeability, light transparency, elasticity over a wide temperature range). This possibility of the present invention to reliably adjust the volume resistivity now makes it possible to utilize these advantages also in DC applications.

[0021] <Volume Resistivity Measurement Method> The measurements are performed in a heatable guard ring configuration with a field strength of 1 kV / mm on 0.5 mm thick crosslinked silicone elastomer in accordance with standard IEC 62631-3-1 "Guidelines for the determination of dielectric and resistive properties of solid insulating materials - Part 3-1: Determination of resistive properties (DC Methods) - Volume resistance and volume resistivity, general method".

[0022] The measuring device used is a guard ring measuring cell "Solid Test Cell 2914" from Tettex Instruments equipped with an "Eaton Sefelec 1500-M" or Sefelec M1501P megohmmeter.

[0023] The volume resistance was measured and the volume resistivity was calculated.

[0024] The heating duration of the crosslinking in the first step is preferably carried out for at least 1 HL of peroxide (B) at the selected crosslinking temperature, more preferably for at least 2 HL of peroxide (B) at the selected crosslinking temperature.

[0025] In one preferred embodiment, the crosslinking in the first step is carried out at a temperature of at least 10 hours HLT (= 10-hour half-life temperature) to a maximum of 1 minute HLT (= 1-minute half-life temperature) of the peroxide (B) used, more preferably at a temperature of at least 10 hours HLT to 1 minute HLT, at most 10 °C lower, of the peroxide (B) used.

[0026] <Component (A)> The component (A) of the composition of the present invention is a diorganopolysiloxane of the general formula (1) or a mixture of diorganopolysiloxanes. R 1 a R 2 b SiO (4-a-b) / 2 (1)

[0027] R 1 is a substituted or unsubstituted monovalent hydrocarbon group containing no aliphatic unsaturated group. R 2 is a substituted or unsubstituted monovalent hydrocarbon group that is aliphatically unsaturated.

[0028] The subscripts a and b are positive numbers within the range of 1 ≤ a ≤ 3, 0 ≤ b ≤ 1, and 1 < a + b ≤ 3.

[0029] In one preferred embodiment, each molecule contains on average at least two unsaturated groups R 2 bonded to silicon atoms.

[0030] In particular, R 1 is a monovalent SiC-bonded optionally substituted hydrocarbon group having 1 to 18 carbon atoms and containing no aliphatic carbon-carbon multiple bond.

[0031] Group R 1Examples of the alkyl group include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, 1-n-butyl, 2-n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl, tert-pentyl groups, hexyl groups such as n-hexyl groups, heptyl groups such as n-heptyl groups, octyl groups such as n-octyl groups and isooctyl groups such as 2,2,4-trimethylpentyl groups, nonyl groups such as n-nonyl groups, and decyl groups such as n-decyl groups. groups, dodecyl groups such as n-dodecyl groups, and octadecyl groups such as n-octadecyl groups, cycloalkyl groups such as cyclopentyl, cyclohexyl, cycloheptyl and methylcyclohexyl groups, aryl groups such as phenyl, naphthyl, anthryl and phenanthryl groups, alkaryl groups such as o-, m-, p-tolyl, xylyl and ethylphenyl groups, and aralkyl groups such as benzyl, α- and β-phenylethyl groups.

[0032] Substituted group R 1 Examples of R are haloalkyl groups such as 3,3,3-trifluoro-n-propyl, 2,2,2,2',2',2'-hexafluoroisopropyl, heptafluoroisopropyl, and haloaryl groups such as o-, m- and p-chlorophenyl groups, and preferably all of the groups mentioned above for R, which can be substituted by mercapto groups, epoxy functions, carboxyl groups, keto groups, enamine groups, amino groups, aminoethylamino groups, isocyanato groups, aryloxy groups, acryloyloxy groups, methacryloyloxy groups, hydroxyl groups and halogen groups.

[0033] base R 1 is preferably a monovalent hydrocarbon group having 1 to 6 carbon atoms, more preferably a methyl group.

[0034] R 2 is, in particular, a monovalent SiC-bonded hydrocarbon radical having an aliphatic carbon-carbon multiple bond.

[0035] base R 2Examples are alkenyl groups such as the vinyl, 5-hexenyl, cyclohexenyl, 1-propenyl, allyl, 3-butenyl and 4-pentenyl groups, and alkynyl groups such as the ethynyl, propargyl and 1-propynyl groups.

[0036] base R 2 is preferably an alkenyl group, more preferably a vinyl group.

[0037] In a preferred embodiment, R 1 is a methyl group, and R 2 are vinyl groups. The structure of the diorganopolysiloxane (A) may be linear or branched, with linear structures being preferred. The viscosity of the diorganopolysiloxane (A) at 25° C. (determined according to DIN 53018) is between 1000 mPa·s and 50,000,000 mPa·s. In a preferred embodiment, the viscosity of the diorganopolysiloxane (A) is between 500,000 and 40,000,000 mPa·s, more preferably even between 2,000,000 and 30,000,000 mPa·s, and is therefore in the region of polysiloxanes typically used for high temperature vulcanized (i.e. crosslinked, HTV) rubbers.

[0038] In another embodiment, the viscosity of the diorganopolysiloxane (A) at 25° C. (determined according to DIN 53018) is preferably between 1000 mPa·s and 100000 mPa·s, more preferably between 5000 and 50000 mPa·s. Polysiloxanes in this viscosity range are typically used for liquid silicone rubbers (LSR).

[0039] Diorganopolysiloxane (A) may be, for example, vinyl terminated polydimethylsiloxane, vinyl terminated polydimethyl-polymethylvinyl-siloxane, or trimethylsilyl terminated polydimethyl-polymethylvinyl-siloxane. Component (A) may consist of a single diorganopolysiloxane or a mixture of two or more diorganopolysiloxanes.

[0040] (A) is used in an amount of 50% to 99% by weight, preferably 55% to 85% by weight, and more specifically 60% to 80% by weight.

[0041] <Ingredient (B)> The crosslinking agents used are peroxides, which act as a source of free radicals. They are selected from the group of alkyl peroxides, diaryl peroxides, alkylaryl peroxides, aralkyl peroxides and hydroperoxides. A single peroxide or hydroperoxide can be used as component (B) or a combination of different peroxides or peroxides and hydroperoxides.

[0042] Examples of organic peroxides are acyl peroxides, such as dibenzoyl peroxide, bis(4-chlorobenzoyl)peroxide, bis(2,4-dichlorobenzoyl)peroxide and bis(4-methylbenzoyl)peroxide, alkyl and aryl peroxides, such as di-tert-butylperoxide, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane, dicumyl peroxide and 1,3-bis(tert-butylperoxyisopropyl)benzene, perketals, such as 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, peresters, such as diacetylperoxydicarbonate, tert-butylperbenzoate, tert-butylperoxyisopropylcarbonate, tert-butylperoxyisononanoate, dicyclohexylperoxydicarbonate and 2,5-dimethylhexane-2,5-diperbenzoate.

[0043] It is known in the prior art that peroxides can be distinguished as vinyl-specific and non-vinyl-specific peroxides, see for example the text SILICONES by Pachaly et al., WILEY-CH, ISBN-10: 3-527-30770-2, ISBN-13: 978-3527-30770-8, page 41 ff.

[0044] (B) is used in an amount of 0.5% to 5% by weight, preferably 1% to 4% by weight, particularly preferably 1.5% to 3% by weight.

[0045] The peroxide has the following properties.

[0046] Half-life (HL): The HL of the peroxide at a specified temperature indicates the time after half of the amount of the peroxide has decayed.

[0047] Data on the half-life can be found in the literature and are provided by peroxide producers. The values between individual data points can be extrapolated by the Arrhenius kinetics.

[0048] 10-hour HLT: The 10-hour half-life temperature is the temperature at which half of the amount of the peroxide decays within 10 hours.

[0049] 1-minute HLT: The 1-minute half-life temperature is the temperature at which half of the amount of the peroxide decays within 1 minute.

[0050] In a preferred embodiment, a vinyl-containing diorganopolysiloxane is used as component (A) and a vinyl-specific peroxide is used as component (B).

[0051] <Reinforcing filler (C)> The reinforcing filler (C) that can be used is fumed silica or precipitated silica having a BET specific surface area of at least 50 m 2 / g.

[0052] The above-mentioned positively reinforcing silica filler (C) may be hydrophilic or may be hydrophobized by known methods.

[0053] Precipitated and fumed silicas and mixtures thereof are preferred. Particularly preferred is fumed silica surface-treated with a silylating agent. Hydrophobization techniques have long been known to the skilled person in the prior art. Silica can be hydrophobized by in situ methods, either before incorporation into the polyorganosiloxane or in the presence of the polyorganosiloxane. Both methods can be carried out either as batch operations or continuously. The silylating agents used can be all hydrophobizing agents known to the skilled person. These are preferably silazanes, more particularly hexamethyldisilazane and / or 1,3-divinyl-1,1,3,3-tetramethyldisilazane, and / or polysilazanes, in which case water can also be used in addition. In addition, it is also possible to use other silylating agents as hydrophobizing agents, for example SiOH- and / or SiCl- and / or alkoxy-functional silanes and / or siloxanes. It is also possible to use cyclic, linear or branched non-functional organosiloxanes as silylating agents, such as octamethylcyclotetrasiloxane or polydimethylsiloxane, in each case by itself or in addition to silazane.To accelerate hydrophobization, a further possibility is to add catalytically active additives, such as hydroxides.Hydrophobization can be carried out in one step with one or more hydrophobizing agents, or in multiple steps with one or more hydrophobizing agents.

[0054] Precipitated silica or fumed silica is preferred. More preferably, the BET specific surface area is 80 to 400. 2 / g, more preferably 100 to 400m 2 / g of silica.

[0055] The actively reinforcing silica fillers (C) can be used individually or in mixtures.

[0056] The amount of the reinforcing filler (C) is in the range of 0 to 50% by weight, preferably 15 to 45% by weight, and more preferably 20 to 40% by weight.

[0057] <Further Components (D)> Further components that can be used in the context of the compositions of the invention are known to the skilled artisan for many years from the prior art: non-limiting examples include non-reinforcing fillers, plasticizers, adhesion promoters, soluble dyes, inorganic and organic pigments, solvents, fungicides, fragrances, dispersing aids, rheological additives, corrosion inhibitors, antioxidants, light stabilizers, heat stabilizers, flame retardants.

[0058] <Component (X)> The base composition of the present invention does not contain conductive or semiconductive additives (X). However, by free, it is meant that such additives may be present up to the typical indications of impurities. These types of additives have been known to those skilled in the art for a long time. Examples are carbon black, metals, metal oxides, semiconductors (e.g. SiC, Si) in the form of nanoparticles.

[0059] The present invention further provides a method for producing the crosslinked silicone elastomer of the present invention, the volume resistivity of which is adjusted to the volume resistivity of the adjacent cable insulation, This volume resistivity is determined according to standard IEC 62631-3-1, in a heatable guard ring configuration with a field strength of 1 kV / mm, for a crosslinked silicone elastomer of 0.5 mm thickness, and meets the following values ​​after application of the test voltage: - After 10000 minutes: <1.0×10 16 Ω*cm The crosslinked silicone elastomer is (A) 50% to 99% by weight of at least one diorganopolysiloxane having at least two crosslinkable groups per molecule; (B) 0.5% to 5% by weight of at least one peroxide; (C) 0% to 50% by weight of at least one reinforcing filler; and (X) does not contain any conductive or semi-conductive additives; The amounts of all components may be obtained by crosslinking a base composition which adds up to 100% by weight; This base composition - Applying to a substrate or filling into a mold, - in a first step, the crosslinking is carried out by heating to a temperature of at least the 10h HLT (= 10h half-life temperature) of the peroxide (B), the heating duration corresponding to at least 0.2 of the HL (= half-life) of the peroxide (B) at the chosen crosslinking temperature, In a second step, the heat treatment is carried out above a temperature of the 10 hour HLT of the peroxide (B), the heat treatment duration corresponding to at least 1 HL of the peroxide (B) at the selected heat treatment temperature.

[0060] Further provided by the present invention is the use of crosslinked silicone elastomers for insulation applications, more particularly for HVDC applications such as HVDC fittings. EXAMPLES

[0061] The following examples illustrate basic possibilities for carrying out the invention, but are not intended to limit the invention to the content disclosed herein.

[0062] In the following examples, all figures relating to parts and percentages are by weight unless otherwise indicated. Unless otherwise indicated, the following examples are carried out at the pressure of the ambient atmosphere, i.e. at about 1000 hPa, at room temperature, i.e. at about 20° C., or at the temperature that occurs when the reactants are mixed at room temperature without additional heating or cooling.

[0063] The compositions of the present invention can be prepared by simple mixing of the components in a mixing assembly typically used for silicone rubber compositions (cross-arm stirrer, paddle stirrer, compounder, extruder, two-roll mill).

[0064] In the text and tables that follow: E13=10 13 E14=10 14 E15=10 15 E16=10 16 HL=half life

[0065] <Peroxide = Crosslinking agent>

[0066] Crosslinker 1: Crosslinker 1 is dicumyl peroxide and has the following properties: 10 hours HLT: 111℃ 1 min HLT: 168℃ Half-life at 120℃: approx. 5.3 hours Half-life at 130℃: approx. 1.6 hours Half-life at 140℃: approx. 0.5 hours Half-life at 165°C: approx. 1.5 minutes

[0067] Crosslinker 2: Crosslinker 2 is a 50% paste of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane in silicone oil and has the following properties: 10 hours HLT: 116℃ 1 min HLT: 176℃

[0068] Crosslinker 3: Crosslinker 3 is a 50% paste of bis(4-methylbenzoyl) peroxide in silicone oil. 10 hours HLT: 70℃ 1 min HLT: 130℃

[0069] Crosslinker 4: Crosslinker 4 is a 50% paste of bis(2,4-dichlorobenzoyl) peroxide with the following properties: 10 hours HLT: 51℃ 1 min HLT: 119℃

[0070] <Base composition 1> In a laboratory compounder, 750 g of vinyldimethylsiloxy-terminated polydimethylsiloxane (PDMS) with a viscosity of 20,000 mPas (25°C) was initially introduced and heated to 150°C, resulting in a BET specific surface area of ​​300 m 2550 g of hydrophobic fumed silica having 1.0 g / g and a carbon content of 3.9 wt.% was mixed. This gave a highly viscous composition, which was subsequently diluted with 300 g of vinyldimethylsiloxy-terminated polydimethylsiloxane having a viscosity of 20000 mPas (25°C). Mixing was carried out under reduced pressure (10 mbar) at 150°C for 1 hour to remove volatile constituents.

[0071] <Base composition 2> In a compounder, 100 parts of a dimethylvinylsilyloxy-terminated dimethylsiloxane-methylvinylsiloxane copolymer containing 99.94 mole % dimethylsiloxy units and 0.06 mole % methylvinylsiloxy units and having a degree of polymerization of about 6000 siloxy units and a specific surface area measured according to the BET technique of 300 m2 are mixed until the mixture becomes homogeneous. 2 / g and 7 parts of a dimethylhydroxysiloxy-terminated dimethylsiloxane oligomer having a viscosity of 40 mPa·s, followed by heating at 170° C. for 2 hours.

[0072] [Example 1 (not in accordance with the present invention)] 80.0 g of base composition 1 was mixed with 18.7 g of vinyldimethylsiloxy-terminated polydimethylsiloxane having a viscosity of 20000 mPa·s (25° C.) at 25° C., 0.1 g of ethynylcyclohexanol, 1.9 g of a copolymer of dimethylsiloxy, methylhydrogensiloxy and trimethylsiloxy units having a viscosity of 300 mPa·s at 25° C. and a SiH content of 0.47%, and 0.1 g of a solution containing platinum-sym-divinyltetramethyldisiloxane complex and 1 wt. % platinum. The composition was mixed homogeneously with a paddle stirrer and then degassed in a desiccator (approx. 10 mbar for 10 min).

[0073] The silicone composition thus prepared was subsequently crosslinked in a hydraulic press according to the times and temperatures shown in the table. After demolding, the 0.5 mm thick silicone elastomer film was heat treated in a forced air oven according to the conditions shown in the table. The volume resistivity was then determined according to the technique described.

[0074] [Table 1]

[0075] [Example 2] 80.0 g of base composition 1 was mixed at 25° C. with 20.0 g of a vinyldimethylsiloxy-terminated polydimethylsiloxane having a viscosity of 20000 mPa·s (25° C.) and the amount of peroxide shown in Table 2. The composition was mixed homogeneously with a paddle stirrer and then degassed in a desiccator (approximately 10 mbar for 10 min).

[0076] The silicone compositions thus produced were then crosslinked in a hydraulic press according to the times and temperatures shown in Table 2. After demolding, the 0.5 mm thick silicone elastomer films were heat treated in a forced air oven according to the conditions shown in the table. The volume resistivity was then determined according to the technique described.

[0077] [Table 2]

[0078] [Example 3] 80.0 g of base composition 1 was mixed at 25° C. with 20.0 g of a vinyldimethylsiloxy-terminated polydimethylsiloxane having a viscosity of 20000 mPa·s (25° C.) and the amount of peroxide shown in Table 3. The composition was mixed homogeneously with a paddle stirrer and then degassed in a desiccator (approximately 10 mbar for 10 min).

[0079] The silicone compositions thus produced were then crosslinked in a hydraulic press according to the times and temperatures shown in Table 3. After demolding, the 0.5 mm thick silicone elastomer films were heat treated in a forced air oven according to the conditions shown in the table. The volume resistivity was then determined according to the technique described.

[0080] [Table 3]

[0081] [Example 4] The components shown in Table 4 were mixed homogeneously with a paddle stirrer and then degassed in a desiccator (approximately 10 mbar for 10 minutes).

[0082] The silicone compositions thus produced were then crosslinked in a hydraulic press according to the times and temperatures shown in Table 4. After demolding, the 0.5 mm thick silicone elastomer films were heat treated in a forced air oven according to the conditions shown in Table 4. The volume resistivity was then determined.

[0083] [Table 4]

[0084] [Example 5] A mixture of 100 g of base composition 2 and 4.0 g of crosslinker 2 was prepared on a roll. The silicone composition thus prepared was subsequently crosslinked in a hydraulic press at 165° C. for 15 minutes. After demolding, a 0.5 mm thick silicone elastomer film was heat treated at 200° C. for 4 hours in a forced air oven. The volume resistivity was then determined according to the technique described.

[0085] [Example 6] A mixture of 100 g of polydimethylsiloxane with a degree of polymerization of about 6000 siloxy units and 1.5 g of crosslinker 4 was prepared on a roll. The silicone composition thus prepared was subsequently crosslinked in a hydraulic press at 165° C. for 15 minutes. After demolding, a 0.5 mm thick silicone elastomer film was heat treated in a forced air oven at 200° C. for 8 hours. The volume resistivity was then determined according to the technique described.

[0086] [Table 5]

[0087] [Example 7] 80.0 g of Base Composition 1 was mixed at 25° C. with 20.0 g of vinyldimethylsiloxy-terminated polydimethylsiloxane having a viscosity of 20000 mPa·s (25° C.) and 2.0 g of Crosslinker 1. The composition was mixed homogeneously with a paddle stirrer and then degassed in a desiccator (approximately 10 mbar for 10 min).

[0088] The silicone composition thus prepared was subsequently crosslinked in a hydraulic press according to the times and temperatures shown in the table. After demolding, the 0.5 mm thick silicone elastomer film was heat treated in a forced air oven according to the conditions shown in the table. The volume resistivity was then determined according to the technique described.

[0089] [Table 6]

[0090] [Example 8] After the volume resistivity determination, the test plates of Example 7c) were tightly packed in aluminum foil and stored at 80° C. After the storage times shown in the table, the test plates were removed from the oven and the volume resistivity values ​​(measured for 15 min) were noted. The plates were then packed again in aluminum foil and further stored at 80° C.

[0091] [Table 7]

[0092] [Example 9] The test panels from 3a were left in the measuring cell for the volume resistivity measurements and the volume resistivity was measured (at 1 kV / mm and 90° C.) after the times shown in Table 9.

[0093] [Table 8]

[0094] [Table 9]

[0095] For selected examples, test plates were produced according to the conditions given in the examples for the preparation of mechanical test specimens. The measurement results are summarized in Table 11. The mechanical properties were determined by standard measurement techniques.

[0096] [Table 10]

Claims

1. A crosslinked silicone elastomer having a volume resistivity adjusted to the volume resistivity of the adjacent cable insulation, This volume resistivity is determined according to standard IEC 62631-3-1, in a heatable guard ring configuration with a field strength of 1 kV / mm, for a crosslinked silicone elastomer of 0.5 mm thickness, and meets the following values ​​after application of the test voltage: - After 10,000 minutes: < 1.0×10 １６ Ω*cm The crosslinked silicone elastomer is (A) 50% to 99% by weight of at least one diorganopolysiloxane having at least two crosslinkable groups per molecule; (B) 0.5% to 5% by weight of at least one peroxide; (C) 0% to 50% by weight of at least one reinforcing filler; and (X) does not contain conductive or semiconductive additives; It can be obtained by crosslinking a base composition in which the amounts of all components add up to 100% by weight, This base composition - Applying to a substrate or filling into a mould, in a first step, crosslinking is carried out by heating to a temperature of at least the 10 hour HLT (= 10 hour half-life temperature) of the peroxide (B), the heating duration corresponding to at least 0.2 of the HL (= half-life) of the peroxide (B) at the chosen crosslinking temperature, In a second step, the heat treatment is carried out above a temperature of 10 hours HLT of the peroxide (B), the heat treatment duration corresponding to at least 1 HL of the peroxide (B) at the selected heat treatment temperature, of the crosslinked silicone elastomer.

2. 2. Crosslinked silicone elastomer according to claim 1, characterized in that the heating duration of crosslinking in the first step corresponds to at least 1 HL of peroxide (B) at the selected crosslinking temperature.

3. 2. Crosslinked silicone elastomer according to claim 1, characterized in that the heating duration of crosslinking in the first step corresponds to at least 2HL of peroxide (B) at the selected crosslinking temperature.

4. Crosslinked silicone elastomer according to any one of claims 1 to 3, characterized in that the crosslinking in the first step is carried out at a temperature of at least 10 hours HLT and at most 1 minute HLT (= 1 minute half-life temperature) of the peroxide (B).

5. Crosslinked silicone elastomer according to any one of claims 1 to 3, characterized in that the crosslinking in the first step is carried out at a temperature of at least 10 hours HLT and up to 10°C lower than the 1 minute HLT of the peroxide (B).

6. The base composition comprises at least 50 ml of (C). ２ 6. Crosslinked silicone elastomer according to claim 1, characterized in that it contains from 15% to 45% by weight of at least one fumed or precipitated silica having a BET specific surface area of ​​1.0 to 1.0 g / g.

7. Crosslinked silicone elastomer according to any one of claims 1 to 6, characterized in that it comprises from 1% to 4% by weight of at least one peroxide (B).

8. 8. The crosslinked silicone elastomer according to claim 1, wherein component (A) is a vinyl-containing diorganopolysiloxane, and component (B) is a vinyl-specific peroxide.

9. The volume resistivity is 8.0×10 after 10,000 minutes. １５ 9. Crosslinked silicone elastomer according to claim 1, characterized in that it has a modulus of less than Ω*cm.

10. The volume resistivity is 6.0×10 after 10,000 minutes. １５ 9. Crosslinked silicone elastomer according to claim 1, characterized in that it has a modulus of less than Ω*cm.

11. 1. A method for producing a crosslinked silicone elastomer having a volume resistivity adjusted to the volume resistivity of an adjacent cable insulation, comprising: This volume resistivity is determined according to standard IEC 62631-3-1, in a heatable guard ring configuration with a field strength of 1 kV / mm, for a crosslinked silicone elastomer of 0.5 mm thickness, and meets the following values ​​after application of the test voltage: - After 10,000 minutes: < 1.0×10 １６ Ω*cm The crosslinked silicone elastomer is (A) 50% to 99% by weight of at least one diorganopolysiloxane having at least two crosslinkable groups per molecule; (B) 0.5% to 5% by weight of at least one peroxide; (C) 0% to 50% by weight of at least one reinforcing filler; and (X) does not contain conductive or semiconductive additives; It can be obtained by crosslinking a base composition in which the amounts of all components add up to 100% by weight, This base composition - Applying to a substrate or filling into a mould, in a first step, crosslinking is carried out by heating to a temperature of at least the 10 hour HLT (= 10 hour half-life temperature) of the peroxide (B), the heating duration corresponding to at least 0.2 of the HL (= half-life) of the peroxide (B) at the chosen crosslinking temperature, A method of manufacture in which in a second step the heat treatment is carried out above a temperature of 10 hours HLT of the peroxide (B), the heat treatment duration corresponding to at least 1 HL of the peroxide (B) at the selected heat treatment temperature.

12. Use of the crosslinked silicone elastomers according to claims 1 to 10 for insulating applications.

13. 13. The use according to claim 12 for HVDC applications.

14. 13. Use according to claim 12 for HVDC fittings.

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