LIQUID SILICONE RUBBER COMPOSITION

Abstract

Addition curing electrically conductive liquid silicone rubber (LSR) compositions, their methods of preparation, and cured elastomeric products made from the compositions are provided. The compositions are cured to form elastomeric products suitable for high voltage applications such as cable joints, cable terminal applications, cable accessories and connectors. In general, the composition comprises: (a) at least one polydiorganosiloxane having at least two alkenyl groups per molecule: (b) at least one organohydrogenpolysiloxane: (c) at least one reinforcing filler: (d) at least one hydrosilylation catalyst: and (e) an electrically conductive filler. Component (e) comprises: (i) extra conductive carbon black: and (ii) single walled carbon nanotubes.

Claims

1. A conductive liquid silicone rubber composition, the composition comprising: (a) at least one polydiorganosiloxane having at least two alkenyl groups per molecule; (b) at least one organohydrogenpolysiloxane; (c) at least one reinforcing filler present in an amount of from 10 to 25% by weight of the composition; (d) at least one hydrosilylation catalyst; and (e) an electrically conductive filler comprising; (i) extra conductive carbon black present in an amount of from 1.5 to 5.5% by weight of the composition, and (ii) single walled carbon nanotubes present in an amount of from 0.05 to 1% by weight of the composition.

2. The conductive liquid silicone rubber composition in accordance with claim 1, comprising at least two polydiorganosiloxanes having at least two alkenyl groups per molecule, each having a viscosity of from 150 mPa.Math.s to 150,000 mPa.Math.s at 25° C., measured using a Brookfield DV-III Ultra Programmable Rheometer for viscosities ≥50,000 mPa.Math.s, and a Brookfield DV 3T Rheometer for viscosities less than 50,000 mPa.Math.s.

3. The conductive liquid silicone rubber composition in accordance with claim 1, comprising: a first polydiorganosiloxane having at least two alkenyl groups per molecule and a viscosity in the range of from 50,000 to 100,000 mPa.Math.s at 25° C., measured using a Brookfield DV-III Ultra Programmable Rheometer; a second polydiorganosiloxane having at least two alkenyl groups per molecule and a viscosity in the range of from 5,000 to 20,000 mPa.Math.s at 25° C., measured using a Brookfield DV 3T Rheometer; and a third polydiorganosiloxane having at least two alkenyl groups per molecule and a viscosity in the range of from 150 to 1,000 mPa.Math.s at 25° C., measured using a Brookfield DV 3T Rheometer.

4. The conductive liquid silicone rubber composition in accordance with claim 1, wherein the reinforcing filler (c) is a fumed silica having a surface area of from 100 to 600 m.sup.2/g using the BET method.

5. The conductive liquid silicone rubber composition in accordance with claim 1, wherein the extra conductive carbon black (e)(i) has at least one of the following properties: (i) a BET surface area of at least 500 m.sup.2/g, optionally of from 500 to 1,600 m.sup.2/g, optionally of from 500 to 1,500 m.sup.2/g, optionally of from 600 to 1,300 m.sup.2/g, optionally of from 750 to 1,250 m.sup.2/g, as determined by ASTM D 6556; and/or (ii) a D50 aggregate particle size of between 5 to 500 nm, optionally of from 10 to 200 nm, as measured by using disc centrifuge photosedimentometry (DCP) according to ISO 15825:2017; and/or (iii) a dibutyl phthalate (DBP) pore volume of from 300 to 600 ml/100 g, optionally of from 300 to 550 ml/100 g, optionally of from 300 to 400 ml/100 g, as measured using ASTM D-2414.

6. The conductive liquid silicone rubber composition in accordance with claim 1, wherein the composition is stored before use in two parts, Part A and part B, to keep components (b) and (d) separate to avoid premature cure of the composition.

7. The conductive liquid silicone rubber composition in accordance with claim 6, wherein Part A includes components (a), (c), and (d), and wherein Part B includes components (a), (b), and (c).

8. The conductive liquid silicone rubber composition in accordance with claim 7, wherein part B also includes a cure inhibitor.

9. The conductive liquid silicone rubber composition in accordance with claim 1, further comprising at least one additive selected from the group consisting of: (i) at least one inhibitor present in an amount of from 1 to 500 moles of inhibitor per mole of component (d); (ii) at least one mold release agent present in an amount of from 0.1 to 5% by weight of the composition; (iii) at least one chain extender present in an amount of from 0.1 to 3% by weight of the composition; (iv) at least one heat stabilizer present in an amount of from 0.01 to 1.0% weight of the composition; (v) at least one flame retardant present in an amount of from 0.1 to 5% by weight of the composition; (vi) at least one pigment and/or at least one colouring agent; and (vii) combinations thereof.

10. A process for the manufacture of the composition in accordance with claim 1, the process comprising the steps of: I) preparing a conductive filler masterbatch by mixing together; i) a silicone rubber base material comprising a first dimethyl vinyl terminated polydimethylsiloxane (a) and component (c); ii) component (e); and iii) at least one dimethyl vinyl terminated polydimethylsiloxane (a); II) introducing the conductive filler masterbatch of step I) into a Part A composition including a dimethyl vinyl terminated polydimethylsiloxane (a), component (c), and component (d), and/or into a Part B composition including a dimethyl vinyl terminated polydimethylsiloxane (a), component (c), and component (b), wherein Part A contains no component (b) and Part B contains no component (d); and III) mixing Part A and Part B together.

11. The process in accordance with claim 10, wherein the first dimethyl vinyl terminated polydimethylsiloxane (a) of step I) has a viscosity of from 50,000 to 150,000 mPa.Math.s at 25° C., measured using a Brookfield DV-III Ultra Programmable Rheometer, and wherein the at least one dimethyl vinyl terminated polydimethylsiloxane (a) of step I) is a mixture of second and third dimethyl vinyl terminated polydimethylsiloxanes, with the second dimethyl vinyl terminated polydimethylsiloxane having a viscosity of from 5,000 to 25,000 mPa.Math.s at 25° C., measured using a Brookfield DV 3T Rheometer, and the third dimethyl vinyl terminated polydimethylsiloxanes having a viscosity of from 150 to 1,000 mPa.Math.s at 25° C. measured using a Brookfield DV 3T Rheometer.

12. The process in accordance with claim 10, wherein step I) comprises pre-mixing the components in a mixer to form an initial mixture and then mixing the resulting initial mixture on a three-roll mill.

13. An article cured from the conductive liquid curable silicone rubber composition in accordance with claim 1, the article selected from the group consisting of cable joints, cable terminal applications, cable accessories, spark-plug connectors, electrical insulators, single-wire seals, plug connector seals, tubing and valves, automobile components, connector seals, spark plug boots, electric and electronic parts, rolls for copy machines, and packing for microwave ovens.

14. (canceled)

Description

EXAMPLES

[0118] All viscosities were measured at 25° C. unless otherwise indicated. Viscosities of individual ingredients in the following examples were measured using a Brookfield DV-III Ultra Programmable Rheometer for viscosities ≥50,000 mPa.Math.s, and a Brookfield DV 3T Rheometer for viscosities less than 50,000 mPa.Math.s, unless otherwise indicated.

[0119] A liquid silicone rubber base as disclosed in Table 1 was used in the following working examples. As an initial step a silicone base composition, having the composition as listed in Table 1 below, was prepared in a kneader mixer

TABLE-US-00001 TABLE 1 Silicone base composition (containing optional additives): Weight % Fumed silica surface area of about 300 m.sup.2/g (ISO 9277: 2010) 28.15 dimethylvinyl-terminated polydimethylsiloxane (1) having a 65.0 viscosity of 65,000 mPa .Math. s Hexamethyldisilazane 5.0 Dimethylhydroxy terminated vinylmethyl dimethyl 0.15 polysiloxane having a viscosity of about 30 mPa .Math. s Water 1.7

[0120] The base composition as prepared in accordance with the composition indicated in Table 1 was then used in the preparation of a conductive filler masterbatch composition in accordance with Table 2.

TABLE-US-00002 TABLE 2 Conductive Filler masterbatch Composition Weight % Extra conductive carbon black having a BET surface 8.76 area of 1000 m.sup.2/g (ASTM D 6556) Single wall carbon nanotube in trimethylterminated 3.44 polydimethylsiloxane, (10% active) dimethylvinyl-terminated polydimethylsiloxane (2) 48.2 having a viscosity of 12,000 mPa .Math. s dimethylvinyl-terminated polydimethylsiloxane (3) 6.26 having a viscosity of 300 mPa .Math. s Silicone base material (Table 1) 33.34

[0121] The ingredients of the conductive filler masterbatch composition were pre-mixed together in a planetary mixer and then were further mixed using a 3-roll mill.

[0122] As previously indicated rather than making the final composition, to avoid premature cure, three compositions as hereinbefore described were prepared, each having a Part A composition (Table 3) containing the hydrosilylation catalyst (Pt) and excluding cross-linker and a Part B composition (Table 4) containing cross-linker and no catalyst were individually prepared using the compositions detailed in Table 3 and 4 below.

TABLE-US-00003 TABLE 3 LSR Part A Compositions Inventive Inventive Inventive Example 1 Example 2 Example 3 Formulation of LSR Part A (wt. %) (wt. %) (wt. %) Conductive masterbatch (Table 2) 39.75 39.75 20.6 Silicone base composition (Table 1) 53.85 53.85 57.50 Single wall carbon nanotube 2.70 masterbatch (10% active) dimethylvinyl-terminated 18.10 polydimethylsiloxane (1) dimethylvinyl-terminated 5.00 5.00 polydimethylsiloxane (2) Dimethylhydroxy terminated 0.45 0.45 0.45 polydimethylsiloxane having a viscosity of 15 mPa .Math. s Platinum catalyst solution 0.95 0.95 0.65

[0123] The platinum catalyst solution is a platinum catalyst/vinyl polymer dispersion having about 1.43 wt. % divinyltetramethyldisiloxane complex of platinum(CAS #68478-92-2) or about 0.7 wt. % platinum group metal.

TABLE-US-00004 TABLE 4 LSR Part B Compositions Inventive Inventive Inventive Example 1 Example 2 Example 3 Formulation LSR Part B (wt. %) (wt. %) (wt. %) Conductive masterbatch (Table 2) 55.50 49.50 27.4 Silicone base composition (Table 1) 31.15 37.15 48.45 Single wall carbon nanotube 2.70 masterbatch (10% active) dimethylvinyl-terminated 13.30 polydimethylsiloxane (1) dimethylvinyl-terminated 4.50 4.50 polydimethylsiloxane(2) Dimethylhydroxy terminated 0.45 0.45 0.45 polydimethylsiloxane having a viscosity of 15 mPa .Math. s Inhibitor/vinyl polydimethyl- 4.00 4.00 4.00 siloxane dispersion containing 3% by weight ETCH Trimethylsiloxy-terminated 2.65 2.65 2.40 polydimethyl methylhydrogen- siloxane having about 0.7% wt. % H as SiH and a viscosity of about 40 mPa .Math. s Dimethylhydrogensiloxy- 1.75 1.75 1.30 terminated polydimethylsiloxane having about 0.15% wt. % H as SiH and a viscosity of about 10 mPa .Math. s

[0124] Table 5 below shows the total filler content in wt % in the above 3 examples as hereinbefore described.

TABLE-US-00005 TABLE 5 Total Amount of each Filler in the combination of Part A and Part B Inventive Inventive Inventive Example 1 Example 2 Example 3 Total filler Content (wt. %) (wt. %) (wt. %) Extra conductive 4.18 3.91 2.00 carbon black Single wall carbon 0.164 0.153 0.35 nanotube Fumed silica 16.43 17.07 17.11

[0125] The resulting Part A and Part B compositions were mixed together in a suitable mixer and the resulting final composition was cured for a period of 10 minutes at a temperature of 120° C. and post cured at a temperature of 150° C. for 4 hours. After mixing and/or cure the physical properties of the three examples were assessed and are tabulated in Table 6 below

TABLE-US-00006 TABLE 6 Physical Properties of Final Composition and Cured Elastomer Products Inventive Inventive Inventive Performance testing: Example 1 Example 2 Example 3 Viscosity- (Pa .Math. s), at a shear rate 260 230 240 of 10 s.sup.−1, measured using a AR2000 EX plate-plate rheometer from TA Instruments of Delaware USA Volume resistance (Ω .Math. cm) - 28 31.3 43 GB/T 3048.3-2007 Hardness (Shore A) - ASTM D2204 41 43 43 Tensile strength (MPa) - ASTM 7.11 7.49 6.77 D412 Elongation (%) - ASTM D412 568 606 560 Tear (Die B, kN/m)- ASTM D624B 29.4 31.3 33.1 Bond with insulation LSR (N/25 mm) 54 50 55

[0126] The Bond with insulation LSR test was undertaken by subjecting samples to a 1800 peel test using an Inston tensiometer at a crosshead speed of 50 mm/min.

[0127] It can be seen the examples above all showed characteristics sought, not least low viscosity, stable volume resistivity, higher elongation and acceptable bond strength compared to the comparatives.

[0128] Three comparative compositions were also provided using either extra conductive carbon black or single walled carbon nanotubes as the conductive fillers. As was the case with the examples 1-3 above the three comparative examples were made initially into two parts. Each comparative composition having a Part A composition (Table 7) containing the hydrosilylation catalyst (Pt) and excluding cross-linker and a Part B composition (Table 8) containing cross-linker and no catalyst were individually prepared using the compositions detailed in Table 7 and 8 below.

TABLE-US-00007 TABLE 7 Part A Composition of Comparative examples Comp. Comp. Comp. Example 1 Example 2 Example 3 Formulation of LSR Part A (wt. %) (wt. %) (wt. %) Extra conductive carbon black 6.00 6.00 Silicone base composition (Table 1) 39.9 44.3 58.00 Single wall carbon nanotube 6.00 masterbatch (10% active) dimethylvinyl-terminated 47.35 39.01 34.95 polydimethylsiloxane (1) dimethylvinyl-terminated 4.5 8.8 polydimethylsiloxane (3) Dimethylhydroxy terminated 0.45 0.45 0.45 polydimethylsiloxane having a viscosity of 15 mPa .Math. s Platinum catalyst solution (as 1.80 1.44 0.60 above)

TABLE-US-00008 TABLE 8 Part B Composition of Comparative Examples Comp. Comp. Comp. Example 1 Example 2 Example 3 Formulation of LSR Part B (wt. %) (wt. %) (wt. %) Extra conductive carbon black 6.00 6.00 Silicone base composition (Table 1) 39.9 44.3 78.65 Conductive masterbatch (Table 2) 6.00 dimethylvinyl-terminated 40.53 32.75 7.50 polydimethylsiloxane (1) dimethylvinyl-terminated 4.5 8.8 polydimethylsiloxane (3) Dimethylhydroxy terminated 0.45 0.45 0.45 polydimethylsiloxane having a viscosity of 15 mPa .Math. s Inhibitor/vinyl polydimethyl- 4.50 4.50 4.00 siloxane dispersion containing 3% by weight ETCH Trimethylsiloxy-terminated 2.53 3.20 1.90 polydimethyl methylhydrogen- siloxane having about 0.7% wt. % H as SiH and a viscosity of about 40 mPa .Math. s Dimethylhydrogensiloxy- 1.59 1.00 terminated polydimethylsiloxane having about 0.15% wt. % H as SiH and a viscosity of about 10 mPa .Math. s

[0129] Table 9 below shows the total filler content in wt. % in the above 3 examples as hereinbefore described.

TABLE-US-00009 TABLE 9 Total Amount of Each Filler in the Combination of Part A and Part B for the Comparative Examples Comparative Comparative Comparative Total filler Content Example 1 Example 2 Example 3 Extra conductive carbon black 6.00 6.00 Single wall carbon nanotube 0.60 Fumed silica 11.30 12.5 19.60

[0130] The resulting Part A and Part B compositions of the comparative examples were mixed together in a suitable planetary mixer and the resulting final composition was cured for a period of 10 minutes at a temperature of 120° C. and post cured at a temperature of 150° C. for 4 hours. After mixing and/or cure the physical properties of the three examples were assessed and are tabulated in Table 10 below

TABLE-US-00010 TABLE 10 Physical Properties of final Composition and Cured Elastomer Products of the Comparative Examples Compar- Compar- Compar- ative ative ative Performance testing: Example 1 Example 2 Example 3 Viscosity- (Pa .Math. s)), at a shear rate 380 400 220 of 10 s.sup.−1, measured using a AR2000 EX plate-plate rheometer from TA Instruments of Delaware USA Volume resistance(Ω .Math. cm) - 58 56 48 GB/T 3048.3-2007 Hardness (Shore A) - ASTM D2204 34.5 40 51 Tensile strength (MPa) - ASTM 6.44 6.0 7.17 D412 Elongation (%) - ASTM D412 626 400 595 Tear (Die B, KN/m)- ASTM D624B 27.1 30 28 Bond with insulation LSR (N/25 mm) 39.86 39.86 39.9

[0131] It will be seen that the comparatives containing extra conductive carbon black as the conductive filler had viscosities >350,000 mPa.Math.s, much higher volume resistivity results than the examples above and variable elongation values and poorer bond with insulation results. The comparative composition relying on single walled carbon nanotubes had low viscosity values but had pretty high Shore A hardness and volume resistivity results including high cost. None provided all the characteristics sought unlike the examples above.