SILICONE RUBBER COMPOSITION

Abstract

A curable silicone elastomer composition is disclosed. The composition comprises one or more non-fluorinated polydiorganosiloxane polymers and silica filler. The silica filler is at least partially treated with a fluorinated hydrophobing treating agent. Also provided is a method of making the composition and its use in the manufacture of insulators for high voltage applications, especially high voltage direct current (HVDC) applications and accessories such as cable joints, cable terminal applications, and connectors. The fluorinated hydrophobing treating agents are selected from one or more silanol terminated fluorinated siloxane oligomer(s) having from 2 to 20 siloxane units, and/or one or more fluorinated silane diol(s), and/or one or more fluorinated trialkoxy silane(s), and/or one or more fluorinated silazane(s), or a mixture thereof.

Claims

1. A curable silicone elastomer composition comprising: (A) at least one non-fluorinated polydiorganosiloxane; (B) at least one reinforcing silica filler which is at least partially hydrophobically treated with a fluorinated hydrophobing treating agent selected from; one or more silanol terminated fluorinated siloxane oligomer(s) having from 2 to 20 siloxane units, and/or one or more fluorinated silane diol(s), and/or one or more fluorinated trialkoxy silane(s), and/or one or more fluorinated silazane(s), or a mixture thereof; and at least one of component (C) or component (D): (C) at least one organohydrogenpolysiloxane (C)(i), at least one hydrosilylation catalyst (C)(ii) and optionally at least one cure inhibitor (C)(iii); (D) at least one peroxide catalyst.

2. The curable silicone elastomer composition in accordance with claim 1, wherein the composition contains ≤0.1 wt. % of the composition of electrically conductive filler or electrically semi-conductive filler or a mixture thereof.

3. The curable silicone elastomer composition in accordance with claim 1, wherein when component (C) is present in the composition, component (A) contains at least two alkenyl or alkynyl groups per molecule, and when component (D) is the sole catalyst in the composition, the presence of at least two alkenyl or alkynyl groups per molecule in component (A) is optional.

4. The curable silicone elastomer composition in accordance with claim 1, wherein component (B) is at least partially treated with one or more fluorinated hydrophobing treating agents selected from the group of: trifluoropropyltrimethoxysilane and trifluoropropyltriethoxysilane; silanol terminated trifluoropropylalkyl siloxanes having from 2 to 20 siloxane repeating units, where the alkyl groups have 1 to 6 carbons; and bis(trifluoropropyldialkyl)silazanes, where each alkyl group has 1 to 6 carbons; to render component (B) hydrophobic.

5. The curable silicone elastomer composition in accordance with claim 1, wherein component (C)(i) is present and selected from one or more of: (i) trimethylsiloxy-terminated methylhydrogenpolysiloxanes, (ii) trimethylsiloxy-terminated polydimethylsiloxane-methylhydrogensiloxanes, (iii) dimethylhydrogensiloxy-terminated dimethylsiloxane-methylhydrogensiloxane copolymers, (iv) dimethylsiloxane-methylhydrogensiloxane cyclic copolymers, (v) copolymers composed of (CH.sub.3).sub.2HSiO.sub.1/2 units and SiO.sub.4/2 units, and (vi) copolymers composed of (CH.sub.3).sub.3SiO.sub.1/2 units, (CH.sub.3).sub.2HSiO.sub.1/2 units, and SiO.sub.4/2 units.

6. The A-curable silicone elastomer composition in accordance with claim 1, further comprising one or more ingredients selected from: thermally conductive fillers, non-conductive fillers, pot life extenders, flame retardants, lubricants, non-reinforcing fillers, pigments, coloring agents, adhesion promoters, chain extenders, silicone polyethers, mold release agents, diluents, solvents, UV light stabilizers, bactericides, wetting agents, heat stabilizers, compression set additives, plasticizers, and mixtures thereof.

7. The curable silicone elastomer composition in accordance with claim 1, wherein the composition is stored in two parts prior to use, a Part A containing components (A), (B) and (C)(ii), and a part B containing components (A), (B), (C)(i) and (C)(iii).

8. A high voltage insulator comprising an elastomeric product of the curable silicone elastomer composition in accordance with claim 1.

9. A high voltage insulator comprising an elastomeric product obtained or obtainable by curing the curable silicone elastomer composition in accordance with claim 1.

10. (canceled)

11. The high voltage insulator in accordance with claim 8, used as an insulator adapted to reduce electrical stress in high voltage direct current (HVDC) applications.

12. The high voltage insulator in accordance with claim 8, used alone or as part of an article or assembly, optionally wherein the article or assembly is a cable accessory, a cable joint or cable termination materials, boots, sleeves, and/or other fittings in high voltage direct current (HVDC) applications.

13. (canceled)

14. (canceled)

15. (canceled)

16. (canceled)

17. A method of preparing the curable silicone elastomer composition in accordance with claim 1, the method comprising: (i) making a silicone base composition by mixing component (A) with at least one reinforcing silica filler; and (ii) introducing component (C), or component (D), or a mixture of components (C) and (D), and storing the resulting composition; wherein when component (C) is present, the composition is stored in two or more parts with components (C)(i) and (C)(ii) being kept in separate parts; and wherein the at least one reinforcing silica filler is either at least partially treated with a fluorinated hydrophobing treating agent prior to or during step (i).

18. The method in accordance with claim 17, wherein the reinforcing silica filler is treated with treating agent prior to step (i) and all the reinforcing silica filler is treated with a fluorinated hydrophobing treating agent prior to step (i) or alternatively the reinforcing silica filler is partially treated with a fluorinated hydrophobing treating agent prior to step (i) and the remainder is treated with a non-fluorinated treating agent prior to step (i).

19. The method in accordance with claim 17, wherein component (A) is divided into multiple predetermined aliquots with each aliquot being mixed with a predetermined amount of reinforcing silica filler and a fluorinated hydrophobing treating agent or non-fluorinated treating agent in situ such that multiple partial bases are prepared with the reinforcing silica filler being treated in situ and then subsequently the multiple partial bases are mixed together to obtain the final product of step (i).

20. The method in accordance with claim 17, wherein component (A) is mixed with the reinforcing silica filler and the fluorinated hydrophobing treating agent in situ such that the reinforcing silica filler is treated in situ to obtain the final product of step (i).

21. The method in accordance with claim 17, wherein component (A) is mixed with the reinforcing silica filler and a mixture of fluorinated hydrophobing treating agent and non-fluorinated treating agent in situ such that the reinforcing silica filler is treated in situ to obtain the final product of step (i).

22. The method in accordance with claim 17, in which the composition is in two or more parts, and wherein the parts are mixed together in a multi-part mixing system prior to cure.

23. The method in accordance with claim 17, in which the composition is further processed by injection moulding, encapsulation moulding, press moulding, dispenser moulding, extrusion moulding, transfer moulding, press vulcanization, centrifugal casting, calendering, bead application or blow moulding.

24. The method in accordance with claim 17, further defined as a method for the manufacture of a high voltage direct current insulator, wherein the composition is introduced into a mold prior to cure to form a moulded silicone article.

25. The method for the manufacture of a high voltage direct current insulator in accordance with claim 24, wherein the composition is either injection moulded to form an article or overmolded by injection moulding around an article.

26. (canceled)

27. (canceled)

28. (canceled)

Description

EXAMPLES

[0164] In the following examples and compositions, all viscosities are given at 25° C. and were determined relying on the cup/spindle method of ASTM D 1084 Method B, using the most appropriate spindle from the Brookfield® RV or LV range for the viscosity range unless otherwise indicated. Williams plasticity values are provided in accordance with ASTM D-926-08. Vinyl content and Si—H content of polymers was determined by quantitative IR in accordance with ASTM E168.

Example 1

[0165] Liquid Silicone Rubber compositions using non-fluorinated polydiorganosiloxane polymer were prepared as LSR Base 1 and LSR Base 2 as shown in Table 1a below. The fumed silica was treated in situ during preparation of the respective LSR Base.

TABLE-US-00001 TABLE 1a Composition of LSR Base 1 and LSR Base 2 (wt. %) LSR Base 1 LSR Base 2 Dimethylvinyl-terminated dimethyl siloxane 73.9% 71.0% viscosity approximately 55 Pa .Math. s Trimethyl silyl treated fumed silica having 26.1% surface area ~300 m.sup.2/g (BET) Trifluoropropylmethylsiloxy treated fumed 29.0% silica having surface area ~300 m.sup.2/g (BET)

[0166] LSR Base 1 and LSR Base 2 were mixed together in the ratios as shown for examples 1.1 to 1.8 in Table 1.b

TABLE-US-00002 TABLE 1b amounts of LSR Base 2 and LSR Base 1 in compositions Proportion of each LSR base present (wt. %) Example LSR Base 2 LSR base 1 1.1 Comparative 0 100 1.2 12.5 87.5 1.3 25 75 1.4 50 50 1.5 62.5 37.5 1.6 75 25 1.7 87.5 12.5 1.8 100 0

[0167] This mixture of the two bases as depicted in Table 1b was then mixed with the other ingredients to give a series of curable compositions which incorporate varying amount of fluorinated filler treated silica.as shown in table 1c.

TABLE-US-00003 TABLE 1c Curable Formulations Curable Formulation (wt. %) Mix of LSR Base 1 and LSR Base 2 96.74 Dimethylvinyl-terminated dimethyl methylvinyl 2.311 siloxane -viscosity 370 mPa .Math. s, 1.16% vinyl 1-Ethynylcyclohexanol 0.039 Karstedt's catalyst (Platinum, 1,3-diethenyl- 0.155 1,1,3,3-tetramethyldisiloxane complexes) diluted in dimethyl vinyl terminated siloxanes to give approximately 0.54 wt. % of Pt Dimethyl, methylhydrogen siloxane having 0.69 0.755 wt. % H as SiH and a viscosity of 43 mPa .Math. s

[0168] Given the samples were tested immediately a two-part composition was not required and the ingredients typically in the part B composition of a two-part composition as discussed above were mixed into each alternative mixture of LSR Base 1 and LSR Base 2 direct in accordance with Table 1c above.

[0169] The different samples produced were prepared as curable sheets were press cured for 10 mins @ 120° C. to form 0.5 mm thick cured sheets. Volume Resistivity was measured at room temperature with a polarization voltage of 1000 V and a polarization time of 60 s. It will be noted that 1.1 is deemed a comparative example as it is the only example in Table 1b which did not contain silica treated with the fluorinated treating agent.

[0170] Once cured Volume resistivity was measured in accordance with ASTM D257-14 Standard Test Methods for DC Resistance or Conductance of Insulating Materials on cured sheets ranging in thickness from 0.5 to 2 mm using a Keithley® 8009 test cell coupled with a Keithley® 5½-digit Model 6517B Electrometer/High Resistance Meter, controlled with Model 6524 High Resistance Measurement Software: D257.

[0171] Within the Model 6524 High Resistance Measurement Software an alternating polarity test was implemented as an “Hi-R” test to minimise the effects of background currents. This is described in detail in Keithley White Paper “Improving the Repeatability of Ultra-High Resistance and Resistivity Measurements” by Adam Daire.

[0172] The Hi-R alternating polarity test was used to minimise effects of background current. This method is designed to improve high resistance/resistivity measurements which are prone to large errors due to background currents.

[0173] An Alternating Polarity stimulus voltage was used with a view to isolating stimulated currents from background currents. When the Alternating Polarity method is used, the Voltage Source output of the electrometer alternates between two voltages: Offset Voltage+Alternating V, and Offset Voltage−Alternating V, at timed intervals (the Measure Time).

[0174] A current measurement (Imeas) is performed at the end of each alternation. After four Imeas values are collected, a current reading is calculated (Icalc). Icalc is the binomially weighted average of the last four current measurements (Imeas1 through Imeas4):

Icalc=(1*Imeas1−3*Imeas2+3*Imeas3−1*Imeas4)/8

The signs used for the four terms are the polarities of the alternating portion of the voltages generating the respective currents. This calculation of the stimulated current is unaffected by background current level, slope, or curvature, effectively isolating the stimulated current from the background current. The result is a repeatable value for the stimulated current and resistance or resistivity that are calculated from it. The time dependence of the stimulated current is a material property. That is, different results will be obtained when using different Measure Times, due to material characteristics.

[0175] A Measure Time of 60 seconds was used with 3 voltage cycles typically of +1000V then −1000V. From the 6 resulting measured currents the software obtains 3 Icalc values, the 1.sup.st of these are rejected and then the subsequent 2 values used to calculate volume resistivity (VR) from

Volume Resistivity=(V.sub.max−V.sub.min)×area/(2×Icalc×Sample Thickness)

The two resulting volume resistivity values were averaged to give a final value. The results for each combination are depicted in Table id below.

TABLE-US-00004 TABLE 1d Volume Resistivity Results Volume Resistivity (ohm-cm) Example 1000 V, 60 s, Room Temperature (RT) 1.1 Comparative 7.57 × 10.sup.14 1.2 3.27 × 10.sup.14 1.3 2.35 × 10.sup.14 1.4 1.89 × 10.sup.14 1.5 1.19 × 10.sup.14 1.6 4.92 × 10.sup.13 1.7 1.92 × 10.sup.13 1.8 4.93 × 10.sup.12

[0176] Here examples 1.2 to 1.8 with different levels silica filler treated with the fluorinated treating agent show lower volume resistivity than 1.1 comparative which has no silica filler treated with the fluorinated treating agent.

[0177] Further samples of 1.2, 1.3, 1.4, 1.6 and 1.8 were prepared and in this instance they were post cured, for 4 hours (4 h) @ 200° C., and further volume resistivity testing was carried out at higher polarization voltages, longer polarization times and higher temperatures and these results are provided in Table 1e below.

TABLE-US-00005 TABLE 1e Volume Resistivity Results at higher electrical fields and longer polarization times. Volume Resistivity (ohm-cm) Temper- Polarization Voltage, Polarization Time Example ature 4,000 V, 2 h 6,000 V 4 h 8,000 V 12 h 1.1 Comparative 90° C. 6.94 × 10.sup.15 1.30 × 10.sup.16 1.75 × 10.sup.16 1.3 90° C. 1.00 × 10.sup.15 1.06 × 10.sup.15 1.15 × 10.sup.15 1.4 90° C. 1.79 × 10.sup.14 2.03 × 10.sup.14 2.37 × 10.sup.14 1.6 90° C. 2.27 × 10.sup.13 3.09 × 10.sup.13 5.81 × 10.sup.13 1.6 70° C. 3.45 × 10.sup.13 4.47 × 10.sup.13 8.55 × 10.sup.13 1.6 40° C. 6.49 × 10.sup.13 7.69 × 10.sup.13 1.08 × 10.sup.14 1.8 90° C. 9.97 × 10.sup.12 2.68 × 10.sup.13 5.01 × 10.sup.13

[0178] Here examples 1.4, 1.6 and 1.8 continue to show lower volume resistivity than 1.1 comparative under a range of polarization voltages, polarization times and temperatures.

Example 2 High Consistency Rubber Examples

[0179] High consistency rubber bases were prepared. Blends of HCR 1 and HCR 2 with the compositions shown in Table 2a were prepared at various ratios as shown in Table 2b to provide examples 2.1 to 2.8. The base compositions utilised are described in Table 2a below.

TABLE-US-00006 TABLE 2a Composition of HCR 1, HCR 2 and HCR 3 HCR 1 HCR 2 HCR 3 (wt. %) (wt. %) (wt. %) Dimethylvinyl-terminated dimethyl 45.47 44.35 48.1 Siloxane gum having Williams plasticity of about 154 mm/100 having a vinyl content of 0.014 wt. % Dimethylvinyl-terminated dimethyl 16.53 16.15 17.5 methylvinyl Siloxane gum having Williams plasticity of about 155 mm/100 having a vinyl content of 0.067 wt. % Trimethyl silyl treated fumed silica with 31.37 16.8 surface area ~300 m.sup.2/g (BET) - vinyl functionalization of about 0.051 mmol/g Trifluoropropylmethylsiloxy treated fumed 32.87 17.6 silica with surface area ~300 m.sup.2/g (BET) - vinyl functionalization of about 0.051 mmol/g XIAMETER ™ RBM-9202 Catalyst 0.47 0.47 XIAMETER ™ RBM-9200 Inhibitor 1.49 1.49 XIAMETER ™ RBM-9201 Crosslinker 4.67 4.67

[0180] In HCR 1 and HCR 2 rather than the standard peroxide curing agent a commercially available hydrosilylation cure catalyst package (XIAMETER™ Addition Cure package from Dow Silicones Corporation of Midland Mich. USA) was used it comprises a platinum catalyst, an Si—H containing cross-linker and a cure inhibitor. These ingredients may be added in any suitable order for example, XIAMETER™ RBM-9200 Inhibitor, may be first added, followed by XIAMETER™ RBM-9202 Catalyst and lastly XIAMETER™ RBM-9201 Crosslinker. These are added in sequence with the inhibitor added first (when present). This should be well dispersed before Catalyst is added. The cross-linker was added last.

[0181] In the case of HCR 3 which was cured using a peroxide catalyst 100 parts by weight of the composition indicated in Table 2a above was mixed with 1 part by weight of which was a 45% paste of 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane in silicone. This is available commercially under a range of trade names such as DHBP-45-PSI (United Initiators). The volume resistivity results for HCR 3 were measured in the same manner as described above and were found to be 8.12×10.sup.13 ohm-cm.

Cured Sheets

[0182] Cured sheets were prepared at 0.5 mm thickness using compression molds, a hydraulic press set at 300 psi (2.17 MPa) and a temperature of 120° C. for 10 minutes, cured sheets were suspended in vented ovens and post cured for up to 4 hours at 200° C. The volume resistivity results of the HCR1 and HCR 2 sheets were determined as described previously and are tabulated in Table 2b below.

TABLE-US-00007 TABLE 2b Volume Resistivity Results Average Volume Example % HCR 1 % HCR 2 Resistivity (ohm-cm) 2.1 Comparative 100.00 0.00 1.03 × 10.sup.15 2.2 Comparative 100.00 0.00 1.06 × 10.sup.15 2.3 75.00 25.00 5.09 × 10.sup.14 2.4 50.01 49.99 2.36 × 10.sup.14 2.5 50.00 50.00 2.18 × 10.sup.14 2.6 24.98 75.02 4.71 × 10.sup.13 2.7 0.00 100.00 2.26 × 10.sup.12 2.8 0.00 100.00 1.90 × 10.sup.12

[0183] Here examples 2.3 to 2.8 with variable levels of silica filler treated with the fluorinated treating agent show lower volume resistivity than 2.1 or 2.2 comparatives which have no fluorinated siloxane treated silica.

[0184] Further samples of 2.1, 2.6 and 2.7 were prepared and in this instance they were post cured for up to 4 h @200° C., and further volume resistivity testing was carried out at higher polarization voltages, longer polarization times and higher temperatures and these results are provided in Table 2c below.

TABLE-US-00008 TABLE 2c Volume Resistivity Results at higher electrical fields and longer polarization times. Volume Resistivity (ohm-cm) Temper- Polarization Voltage, Polarization Time Example ature 4,000 V, 2 h 6,000 V 1 h 8,000 V 1 h 2.1 Comparative 90° C. 3.28 × 10.sup.15 3.92 × 10.sup.15 4.43 × 10.sup.15 2.6 90° C. 6.08 × 10.sup.12 6.85 × 10.sup.12 5.68 × 10.sup.12 2.7 90° C. 2.75 × 10.sup.12 3.97 × 10.sup.12 5.36 × 10.sup.12

[0185] Examples 2.6 and 2.7 continue to show lower volume resistivity than 2.1 comparative under a range of polarization voltages and polarization times.

Example 3

[0186] Liquid silicone rubber compositions using non-fluorinated polydiorganosiloxane polymer were prepared as LSR Base 3 and LSR Base 4 as shown in Table 3a below. The fumed silica was treated in situ during preparation of the LSR Base or HCR.

TABLE-US-00009 TABLE 3a Composition of LSR Base 3 and LSR Base 4 LSR Base 3 LSR Base 4 Dimethylvinyl-terminated dimethyl siloxane 72.4% 75.3% viscosity approximately 55 Pa .Math. s 50 mol % Trimethylsilyl and 50 mol % 27.6% Trifluoropropylmethylsiloxy treated fumed silica having surface area ~300 m.sup.2/g (BET) 50 mol % Trimethylsilyl and 50 mol % 24.7% Trifluoropropyldimethylsilyl treated fumed silica having surface area ~300 m.sup.2/g (BET)

[0187] Materials were cured using the same formulation as shown in table 1b except that the mix of LSR Base 1 and LSR Base 2 was replaced with either LSR base 3 or LSR base 4. Sheets of 0.5 mm thickness were cured for 10 mins @ 120° C. and were then measured for volume resistivity in the manner described above and the results are depicted in Table 3c below.

TABLE-US-00010 TABLE 3c Volume Resistivity Results Average Volume Example LSR Base Resistivity (ohm-cm) 1.1 Comparative 100% LSR Base 1 7.57 × 10.sup.14 1.4 50% LSR Base 1, 1.89 × 10.sup.14 50% LSR Base 2 2.3 LSR base 3 9.24 × 10.sup.13 2.4 LSR Base 4 1.87 × 10.sup.12

[0188] Thus, for these examples where the fumed silica is treated with a mixture of fluorinated and non-fluorinated treating agents show a lower volume resistivity than obtained for Example 1.4 where the LSR Base 1 and 2 are blended after silica treatment. All examples show reduced volume resistivity compared to comparative 1.1 regardless of method for preparing the mixture of fluorinated and non-fluorinated treating agents.