WO2021195037A1

WO2021195037A1 - Silicone rubber composition

Info

Publication number: WO2021195037A1
Application number: PCT/US2021/023613
Authority: WO
Inventors: Robert A. Drake; Beatriz C. MA
Original assignee: Dow Silicones Corporation
Priority date: 2020-03-25
Filing date: 2021-03-23
Publication date: 2021-09-30

Abstract

The present disclosure relates to a silicone based composition comprising one or more fluorinated polydiorganosiloxane polymers and silica filler which silica filler is at least partially treated with a fluorinated hydrophobing treating agent, a method of making same 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 wherein the 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.

Description

SILICONE RUBBER COMPOSITION

[0001] The present disclosure relates to a silicone based composition comprising one or more fluorinated polydiorganosiloxane polymers and silica filler, which silica filler is at least partially treated with a fluorinated hydrophobing treating agent, a method of making same 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.

[0002] Whilst, in most instances, alternating current (AC) is preferred for the supply of electricity to end users, long distance power transmission for distances e.g. > 1000 km, may be undertaken using high voltage direct current (HVDC) systems because it involves lower electrical loss and therefore can be less expensive. Long distance HVDC transmission is generally undertaken in three ways, overhead e.g. via pylons; through underground systems and where necessary via “submarine” systems for transportation under the sea etc. It is probably fair to say that underground systems are significantly more aesthetically pleasing to the general public than pylons as the latter, whilst practical, can be considered an eyesore. However, underground HVDC transmission is the most challenging for the supplier as it generally involves the use of multiple lengths of cable joined together through cable joints every 1 to 2 km compared to overhead and submarine systems. Hence, whilst cable joints are required for any sort of HVDC transmission, the requirement is particularly acute in the case of underground systems.

[0003] However, the insulating materials utilised with respect to AC current transmission systems are not always transferrable to direct current transmission systems because electrical stress is significantly different for AC and DC conditions, not least because the insulating material is exposed to a higher continuous electrical stress under DC conditions which can lead to a dielectric breakdown of materials. Dealing with such matters is becoming particularly important today given the HVDC voltage requirements for new cables and cable accessories keep increasing and can now be greater than (>) 500kV or even >800 kV.

[0004] In the transmission of direct current, a power cable sy stem has resistive electric field distribution characteristics, with the electric field distribution depending on volume resistivity. In contrast for joints to be used in high voltage alternating current applications it is important to minimise any difference in permittivity between the cable insulation and joint insulation to obtain the desired performance.

[0005] Hence, for HVDC systems in e.g joint boxes for HVDC power cables the cable is surrounded by an inner layer of “cable insulation” which may be made from a suitable material such as cross- linked polyethylene (XL, PE) and said cable insulation is surrounded by a layer of further insulation, typically referred to as “joint insulation” which is often provided in the form of ethylene propylene diene monomer rubber (EPDM) or a silicone rubber elastomeric material. In high voltage direct current applications therefore, it is important to minimise any difference in volume resi stivity between the cable insulation and joint insulation in order to ensure the electric field is uniformly distributed at the interface between them in order to avoid e.g. dielectric breakdown. Hence, it is desired that the joint insulation and cable insulation materials are designed to have volume resistivity values as close to each other as possible.

[0006] Current approaches using a combination of crosslinked polyethylene (XLPE) as the cable insulation and silicone rubber based elastomeric materials as the joint insulation are seeing stability issues due to differences in their electrical properties. Typically, unmodified silicone rubber elastomeric materials are too insulating in comparison to XLPE under the same electrical field strength. The silicone rubber-based materials are excellent electrical insulators once cured into a final product, typically they have a volume resistivity of less than or equal to (≥ 10¹⁵) ohm-cm depending on sample preparation and measurement methods, but this is much greater than the typical volume resistivity of XLPE. [0007] Historically, the industry solution has been the introduction of electrically conductive fillers (e.g. metal powder, metal flakes, carbon blacks or carbon nanotubes) or electrically semi-conductive fillers into the silicone rubber compositions to render the silicone rubber elastomeric materials produced therefrom sufficiently conductive to enable the distribution of local DC loadings through a marginally conductive silicone elastomeric product made from a conductive LSR composition providing a bulk resistivity in the range 10 to 10 ohm-cm or alternatively

10 to 10 ohm-cm.

[0008] However, whilst the use of these electrically conductive fillers and / or electrically semi-conductive fillers is able to solve the distribution of local DC loadings, the introduction of such fillers can create further issues, not least an inability to control and/or obtain uniform electrical properties within a silicone elastomer, worsening of physical properties and reduced dielectric strength.

[0009] It has recently beet· found that an elastomeric material made from compositions comprising a mixture of fluorinated polydiorganosiloxane polymer(s) and non-fluorinated polydiorganosiloxane polymer(s) prepared by mixing fluorinated polydiorganosiloxane polymer base(s) and non-fluorinated polydiorganosiloxane polymer base(s), wherein the respective bases comprise the polymer and a reinforcing filler, is able to provide an insulating material with a volume resistivity closer to that of the cross-linked polyethylene without the need for electrically conductive fillers or electrically semi-conductive fillers. However, the use of such a mixture, whilst a significant improvement on the use of compositions using only non-fluorinated polydiorganosiloxane polymers filled with electrically conductive fillers or electrically semi-conductive fillers has potential disadvantages in that mixtures of fluorinated polydiorganosiloxane polymers and non-fluorinated polydiorganosiloxane polymers can phase separate and therefore may require compatiblising agents.

[0010] Hence, there remains a need to develop silicone rubber insulation materials capable of withstanding the high electrical stresses applied on cable insulation and cable joint insulation in high voltage direct current (HVDC) systems and high voltage alternating current (HVAC) systems. In the case of HVDC systems it is desirable to provide silicone-based insulators having electrical properties to match the range of XLPE volume resistivity values which could be advantageous for applications in high voltage direct current (HVDC) such as cables and cable joints. [0011] It has now been determined that the need to utilise either elastomers comprising mixtures of fluorinated polydiorganosiloxane polymers and non-fluorinated polydiorganosiloxane polymers or silicone rubber compositions containing electrically conductive and/or electrically semi-conductive fillers can be avoided by using one or more fluorinated polydiorganosiloxane polymers with finely divided reinforcing silica filler which is at least partially treated with a fluorinated hydrophobing treating agent.

[0012] There is provided a curable silicone elastomer composition comprising:

(A) at least one alkyl, alkenyl alkynyl and/or silanol terminated fluorinated polydiorganosiloxane polymer having a minimum viscosity of 1000mPa.s at 25°C;

(B) at least one reinforcing silica filler wherein said filler 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 (C) or (D) wherein (C) is at least one organohydrogenpolysiloxane (C)(i), at least one hydrosilylation catalyst (C)(ii) and optionally at least one cure inhibitor (C)(iii); or

(D) at least one peroxide catalyst.

[0013] There is also provided a use of curable silicone elastomer composition comprising:

(B) at least one reinforcing silica filler wherein said filler 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 (C) or (D) wherein

(C) is at least one organohydrogenpolysiloxane (C)(i), at least one hydrosilylation catalyst (C)(ii) and optionally at least one cure inhibitor (C)(iii); and

(D) is at least one peroxide catalyst; which composition contains < 0.1 wt. % (% by weight) of the composition of electrically conductive filler or electrically semi-conductive filler or a mixture thereof; in or as a high voltage direct current insulator.

[0014] There is also provided a high voltage direct current insulator comprising an elastomeric product of a curable silicone elastomer composition comprising:

(D) is at least one peroxide catalyst.

[0015] In a still further embodiment there is provided a high voltage direct current insulator comprising an elastomeric product obtained or obtainable by curing silicone elastomer composition comprising

(B) at least one reinforcing silica filler wherein at least one reinforcing silica filler wherein said filler 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 (C) or (D) wherein

(D) is at least one peroxide catalyst.

[0016] There is also provided a method of preparing a curable silicone elastomer composition comprising:

(B) at least one reinforcing silica filler wherein at least one reinforcing silica filler wherein said filler 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 (C) and optionally (D) wherein

(C) is a hydrosilylation cure package comprising at least one organohydrogenpolysiloxane (C)(i), at least one hydrosilylation catalyst (C)(ii) and optionally at least one cure inhibitor (C)(iii); and

(D) is at least one peroxide catalyst; by

(i) making a silicone base composition by mixing fluorinated polydiorganosiloxane (A) with at least one reinforcing silica filler and

(ii) introducing components (C), or a mixture of component (C) and component (D) and storing the resulting composition; wherein when the composition contains hydrosilylation cure package (C) the composition is stored in two or more parts with components (C)(i) and (C) (ii) being kept in separate parts; characterised in that the at least one reinforcing silica filler is at least partially treated with a fluorinated treating agent prior to or during step (i).

[0017] The composition as herein before described is free from non-fluorinated polydiorganosiloxane polymers containing silanol groups and having greater than (>) 20 repeating siloxane units.

[0018] For the sake of this application the term “free from” shall be understood to mean does not contain said non-fluorinated polydiorganosiloxanes other than trace impurity amounts.

[0019] Preferably the composition contains < 0.1 wt. % (% by weight) of the composition of electrically conductive filler or electrically semi-conductive filler or a mixture thereof and in one embodiment the composition described above contains 0 (zero) wt. % (% by weight) of electrically conductive filler or electrically semi-conductive filler. [0020] When (C), a hydrosilylation cure package, is present in the composition, component (A) must contain at least one, alternatively at least two unsaturated groups such as alkenyl or alkynyl groups per molecule. However, when component (D) is the only means of catalysis for the cure process the presence of at least one alkenyl or alkynyl group per molecule alternatively at least two alkenyl or alkynyl groups per molecule in component (A) is preferred but is not essential.

[0021] For the purpose of this application “Substituted” means one or more hydrogen atoms in a hydrocarbon group has been replaced with another substituent. Examples of such substituents include, but are not limited to, halogen atoms such as chlorine, bromine, and iodine; halogen atom containing groups (other than fluoro) such as chloromethyl; oxygen atoms; oxygen atom containing groups such as (meth)acrylic and carboxyl; nitrogen atoms; nitrogen atom containing groups such as amino-functional groups, amido-functional groups, and cyano-functional groups; sulphur atoms; and sulphur atom containing groups such as mercapto groups.

[0022] It has been found herein that silicone rubber compositions containing fluorinated silicone polymers (A) and reinforcing silica fillers, where at least a part of the silica is 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, are showing greater effect in changing the electrical properties of silicone rubber into a conductive range matching the desired XLPE volume resistivity values. The modified silica treatment using said fluorinated treating agent acts in reducing the volume resistivity of silicone rubber to at most 60% of the volume resistivity for a comparable silicone rubber prepared without the modified silica treatment, which is the desired targeted range which equates to the typical volume resistivity of the XLPE material used as the cable insulation. It has surprisingly been identified that been identified that using silica at least partially treated with a fluorinated treating agent such as a fluorinated silanediol or siloxanediol can potentially be used in combination with different silicone rubbers such as liquid silicone rubber (LSR) and high consistency rubber (HCR) to target the electrical properties of these materials to desired ranges by varying the content of silica treated with a fluorinated treating agent within the range of treated silica required in the composition.

[0023] Surprisingly it has been identified that no non-fluorinated polydiorganosiloxane polymers, electrically conductive fillers or electrically semi-conductive fillers are necessary in the silicone rubber compositions herein to obtain silicone elastomers having a desired volume resistivity if one varies the content of silica treated with a fluorinated treating agent within the amount of treated silica in the current compositions. It will also be shown that the silicone rubber formulations which can be utilised may include liquid silicone rubber compositions or high consistency silicone rubber-based materials utilising polydiorganosiloxane polymer gums. By using the silica treated with a fluorinated treating agent as the only fluorinated part of the composition it has been identified that we are able to efficiently access a wide range of volume resistivities whilst only changing the loading of the silica treated with a fluorinated treating agent as shown in the examples, which is then much more economic than previous solutions and has the advantage of avoiding the previously mentioned compatibility etc. type issues previously encountered with previous solutions to this problem.

[0024] The at least one alkyl, alkenyl alkynyl and/or silanol terminated fluorinated polydiorganosiloxane polymer having a minimum viscosity of 1000mPa.s at 25°C, (A), comprises units having the formula

wherein each R² may be the same or different and denotes a branched or linear fluoroalkyl group having from 1 to 8 carbon atoms; each Z may be the same or different and denotes a divalent alkylene group containing at least two carbon atoms, a hydrocarbon ether or a hydrocarbon thioether. Each R² group is linked to a silicon atom via a Z group, each R³ is the same or different and denotes an optionally substituted saturated or unsaturated silicon-bonded, monovalent hydrocarbon group. It will be appreciated that the value of d + e is a maximum of 4 but is 3 for M type groups and is 2 for D type groups wherein typically d = 0 to 2, e= 0 to 2, preferably the average value of d is about 1 and the average value of e is about 1.

[0025] Examples of suitable saturated R³ groups include alkyl groups, such as methyl, ethyl, propyl, isopropyl, butyl, hexyl, 2-ethylhexyl, octyl, isooctyl and decyl. Preferably, when e is >0 at least 90 percent, and more preferably with the exception of alkenyl groups, all of the R³ groups in the fluorosilicone polymer are methyl groups. Preferably when d is 0, on average about at least one R³ per unit contains at least one carbon -fluorine bond alternatively when e is 0, at least one R² per unit is CF -.

[0026] Preferably R² denotes a fluoroalkyl group having at least one carbon atom, alternatively having from 1 to carbon atoms, over the complete range of from 5 to 100 mol% fluorinated siloxane units. Each fluoroalkyl group present has at least one -C-F bond. The R² groups can be identical or different and can have a normal or a branched structure. Preferably at least some, most preferably greater than 50 mol% of the fluoroalkyl groups are perfluoroalkyl groups. Examples thereof include CF -, C F -, C F -, such as CF CF CF - or (CF ) CF-, C F -, such as CF CF CF CF -, (CF ) CFCF -, (CF ) C- and CF CF (CF )CF-; C F such as CF CF CF CF CF -, CeFis-, such as CF (CF₂) CF -; C F -, such as CF (CF₂CF ) -; and C_SF_I7.

[0027] Each perfluoroalkyl group is bonded to a silicon atom by way of Z, a divalent spacing group containing carbon, hydrogen and, optionally, oxygen and/or sulphur atoms which are present as ether and thioether linkages, respectively. The sulphur and oxygen atoms, if present, must be bonded to only carbon atoms.

[0028] Each Z radical can have any structure containing the elements listed but is preferably an alkylene radical (i.e. an acyclic, branched or unbranched, saturated divalent hydrocarbon group). Examples of suitable alkylene radicals include -CH₂CH₂-, -CH₂CH₂CH₂-, -CH(CH₃)CH₂-, -(CH₂CH₂)₂- and -CH(CH₃)CH₂CH₂-. In one embodiment each fluorinated radical, R²Z, preferably has the formula R²CH CH -, i.e. Z is an ethylene group.

[0029] As previously indicated alkenyl and/or alkynyl groups are optional but preferred in component (A) when component (C) is involved in the cure process but are optional if the sole catalyst for the cure process in component (D). Hence, when present, suitable alkenyl groups in component (A) typically contain from 2 to 10 carbon atoms, preferred examples include vinyl, isopropenyl, allyl, and 5-hexenyl. They may be present as terminal groups or pendant on the polymer chain.

[0030] The at least one alkyl, alkenyl alkynyl and/or silanol terminated fluorinated polydiorganosiloxane polymer having a minimum viscosity of 1000mPa.s at 25°C, (A) may additionally comprise a proportion of up to about 90%, alternatively up to about 80% of the total number of units per molecule of non-fluorinated siloxane units having the formula

(R⁴)cSiO(4-0/2 wherein R⁴ denotes an optionally substituted saturated or unsaturated silicon-bonded, monovalent hydrocarbon group, wherein c = 0 to 3 but preferably the average value of c is about 2. Each R⁴ contains no fluorine (and therefore R⁴ cannot contain any of the fluoro containing substituents previously identified.

[0031] As previously indicated R⁴ denotes an optionally substituted saturated or unsaturated silicon-bonded, monovalent hydrocarbon group. Preferably each R⁴ may be the same or different and is selected from Cj to Cjo alkyl groups; alkenyl groups such as vinyl or allyl groups; and/or aryl groups such as such as phenyl, tolyl, benzyl, beta-phenylethyl, and styryl. Preferably at least two R⁴ substituents per molecule are alkenyl or alkynyl groups, alternatively alkenyl groups. When present, each alkenyl group will have from 2 to 8 carbon atoms, alternatively each alkenyl group is a vinyl group. [0032] Examples of Component (A) include polymers of (3,3,3-trifluoropropyl) methylsiloxy units; polymers of (3,3,3-trifluoropropyl) ethylsiloxy units; copolymers of dimethylsiloxy units and (3,3,3-trifluoropropyl) methylsiloxy units; copolymers of dimethylsiloxy units, (3,3,3-trifluoropropyl)methylsiloxy units, and vinylmethylsiloxy units; copolymers of (3,3,3-trifluoropropyl)methylsiloxy units and vinylmethylsiloxy units; and poly(3,3,3-trifluoropropyl)methylsiloxane. Component (A) is trialkyl, alternatively trimethyl terminated, vinyldimethyl terminated, dimethylhydroxy terminated (3,3,3-trifluoropropyl)dimethyl terminated (3,3,3- trifluoropropyl)diethyl terminated (3,3,3-trifluoropropyl)alkenylalkyl terminated, (3,3,3- trifluoropropyl)alkynylalkyl terminated but may comprise some (3,3,3-trifluoropropyl)alkylhydroxy terminated polymers e.g. (3,3,3-trifluoropropyl)methylhydroxy terminated polymers and , (3,3,3-trifluoropropyl)ethylhydroxy terminated polymers providing each polymer contains at least two unsaturated groups selected from alkenyl and alkynyl groups per molecule.

[0033] The molecular structure of component (A) is also typically linear, however, there can be some branching due to the presence of T units (as defined above) within the molecule. To achieve a useful level of physical properties in the elastomer prepared by curing the composition as hereinbefore described, the molecular weight of component (A) should be sufficient so that it achieves a viscosity of at least 1000 mPa.s at 25°C 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. The upper limit for the molecular weight of component (A) is also not specifically restricted. In the case when the composition is designed to make a liquid fluorosilicone rubber (F-LSR) composition the final composition is typically limited only by the processability of the F-LSR composition of the present.

[0034] However, the at least one alkyl, alkenyl alkynyl and/or silanol terminated fluorinated polydiorganosiloxane polymer having a minimum viscosity of 1000mPa.s at 25°C, (A) may also be a gum. As previously indicated gums typically have a viscosity of at least 1,000,000 mPa.s at 25°C. Given the difficulty in measuring viscosity above these values, they tend to be described by way of their Williams plasticity values in accordance with ASTM D-926- 08 as opposed to by viscosity. When component (A) is a gum, it preferably has a viscosity resulting in a Williams’s plasticity of at least 30mm/100 measured in accordance with ASTM D-926-08, alternatively at least 50mm/100 measured in accordance with ASTM D-926-08, alternatively at least lOOmm/100 measured in accordance with ASTM D-926-08, alternatively from lOOmm/100 to 400mm/100 in accordance with ASTM D-926-08.

[0035] The alkenyl and alkynyl group content of component (A) is determined using quantitative infra-red analysis in accordance with ASTM E168.

(B) Reinforcing silica filler

[0036] Component B is reinforcing silica filler which is at least partially hydrophobically treated with a fluorinated treating agent to achieve high level of physical properties that characterize some types of cured silicone elastomer that can be prepared using the composition herein, a reinforcing silica filler (B), such as a finely divided silica filler.

[0037] The finely divided forms of silica may be, for example, selected from fumed silica, precipitated silica and/or colloidal silica. They are particularly preferred because of their relatively high surface area, which is typically at least 50 m²/g. Fillers having surface areas of from 100 to 600 m²/g measured in accordance with the BET method, alternatively of from 100 to 500 m²/g (using the BET method in accordance with ISO 9277: 2010), alternatively of from 200 to 400 m²/g (using the BET method in accordance with ISO 9277: 2010), are typically used. [0038] When component B, the reinforcing silica filler(s), are naturally hydrophilic (e.g. untreated silica fillers) they are often surface treated with one or more known filler treating agents to prevent a phenomenon referred to as "creping" or "crepe hardening" during processing of the curable composition.

[0039] The reinforcing silica filler(s) may be treated prior to introduction in the composition or in situ (i.e. in the presence of at least a portion of the other components of the composition as hereinbefore described by blending these components together until the filler is completely surface treated and uniformly dispersed to form a homogeneous material). In one embodiment, untreated filler (B) is treated in situ with a treating agent in the presence of component (A).

[0040] The fluorinated treating agent may comprise a silanol terminated fluorinated siloxane oligomer comprising from 2 to 20 siloxane units having the formula

(R⁶Z)_h(R⁷)_jSiO_(4-h-j)/2

Wherein each R⁶ may be the same or different and may be the same as R² a defined above i.e., a branched or linear fluoroalkyl group having from 1 to 8 carbon atoms; each Z is the same as defined above; with each R⁶ group being linked to a silicon atom via a Z group, each R⁷ is the same or different and is defined as R³, i.e. an alkyl group having from 1 to 10 carbons, h may be from 1 to 3 and j may be from 0 to 3 with (h + j) being from 1 to 3.

[0041] Examples of suitable saturated R groups include alkyl groups, such as methyl, ethyl, propyl, isopropyl, n-butyl, t-butyl, hexyl, 2-ethylhexyl, octyl, isooctyl and decyl, alternatively having from 1 to 6 carbons, alternatively methyl, ethyl, propyl, isopropyl, n-butyl, or t-butyl, alternatively methyl or ethyl, alternatively methyl; [0042] Preferably R denotes a fluoroalkyl group having at least one carbon atom, alternatively having from 1 to 8 carbon atoms, over the complete range of from 5 to 100 mol% fluorinated siloxane units. Each fluoroalkyl group present has at least one -C-F bond. The R groups can be identical or different and can have a normal or a branched structure. Preferably at least some, most preferably greater than 50 mol % of the fluoroalkyl groups are perfluoroalkyl groups. Examples thereof include CF -, C F -, C F -, such as CF CF CF - or (CF ) CF-, C F -, such as CF CF CF CF -, (CF ) CFCF -, (CF ) C- and CF CF (CF )CF-; C F such as CF CF CF CF CF -, C6F13-, such as CF (CF ) CF -; C F -, such as CF (CF CF ) -; and C_SF_I7.

[0043] The viscosity of the oligomer is from about lOmPa.s to 300mPa.s, alternatively from about lOmPa.s to 200mPa.s at 25°C 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.

[0044] Each perfluoroalkyl group is bonded to a silicon atom by way of Z, a divalent spacing group containing carbon, hydrogen and, optionally, oxygen and/or sulphur atoms which are present as ether and thioether linkages, respectively. The sulphur and oxygen atoms, if present, must be bonded to only carbon atoms.

[0045] Each Z radical can have any structure containing the elements listed but is preferably an alkylene radical (i.e. an acyclic, branched or unbranched, saturated divalent hydrocarbon group). Examples of suitable alkylene radicals include -CH₂CH₂-, -CH₂CH₂CH₂-, -CH(CH₃)CH₂-, -(CH₂CH₂)₂- and -CH(CH₃)CH₂CH₂-. In one embodiment each fluorinated radical, R⁶Z, preferably has the formula R⁶CH CH -, i.e. Z is an ethylene group. [0046] As indicated above, h may be from 1 to 3 and j may be from 0 to 3 with (h + j) being from 1 to 3, alternatively (h + j) is 2 or 3, alternatively (h + j) is 2 with h = 1 and j = 1. Preferably, when j is >0 at least 90 percent, and more preferably all of the R⁷ groups are methyl groups.

[0047] The fluorinated siloxane oligomer may additionally comprise a proportion of up to about 90%, alternatively up to about 80% of the total number of units per molecule of non-fluorinated siloxane units having the formula (R⁴)cSiO(4-c)/2 wherein R⁴ and c are as defined above. Each R⁴ contains no fluorine (and therefore R⁴ cannot contain any of the fluoro containing substituents previously identified.

[0048] As previously indicated R⁴ denotes an optionally substituted saturated or unsaturated silicon-bonded, monovalent hydrocarbon group. Preferably each R⁴ may be the same or different and is selected from Cj to Cjo alkyl groups; alkenyl groups such as vinyl or allyl groups; and/or aryl groups such as such as phenyl, tolyl, benzyl, beta-phenylethyl, and styryl. When present, each alkenyl group will have from 2 to 8 carbon atoms, alternatively each alkenyl group is a vinyl group.

[0049] The fluorinated siloxane oligomer having from 2 to 20 siloxane units maybe exemplified through the following formula

H-O-[(R⁶Z)_h(R⁷)j Si-O]_k-H wherein R⁶, Z, h, R⁷, and j are as defined above and k is from 2 to 20.

[0050] The fluorinated silane diol maybe exemplified by the formula

(HO)2 Si (R⁶Z)(R⁷) wherein R⁶, Z and R⁷are each as defined above.

[0051] The fluorinated trialkoxy silane maybe exemplified by the formula R⁶Z- Si (R^g)₃ wherein R⁶ is as defined above and each R^g may be the same or different and is an alkoxy group having from 1 to 6 carbons, alternatively and alkoxy group having from 1 to 4 carbons, alternatively is a t-butoxy, ethoxy or methoxy group.

[0052] The fluorinated silazane maybe exemplified by the formula ((R⁶Z)(R⁷)₂-Si)₂-NH wherein R⁶, Z and R⁷ are each as defined above. In one alternative each R⁷ has from 1 to 6 carbons, alternatively 1 to 3 carbons, alternatively is ethyl or methyl.

[0053] The fluorinated treating agents may for example be selected from the group of trifluoropropyltrialkoxysilanes, such as trifluoropropyltrimethoxysilane and trifluoropropyltriethoxysilane; silanol terminated trifluoropropylalkyl siloxanes having from 2 to 20 siloxane repeating units and wherein the alkyl groups have 1 to 6 carbons such as silanol terminated trifluoropropylmethyl siloxane having from 2 to 20 siloxane repeating units and silanol terminated trifluoropropylethyl siloxane having from 2 to 20 siloxane repeating units and bis(trifluoropropyldialkyl)silazanes where each alkyl group has 1 to 6 carbons, alternatively 1 to 3 carbons, alternatively is a methyl or ethyl group.

[0054] The treating agents are used primarily provided to render the filler(s) hydrophobic and therefore easier to handle and obtain a homogeneous mixture with the other components, although as previously discussed it has been determined here that by varying the amount of reinforcing filler treated with one or more of the fluorinated treating agents described above the volume resistivity of the resulting elastomeric material may be varied.

[0055] The remainder of reinforcing silica (B) (if any) is treated using a non-fluorinated hydrophobing treating agent such as, for the sake of example, organosilanes, polydiorganosiloxanes, or organosilazanes, hexaalkyl disilazane, short chain siloxane diols, a fatty acid or a fatty acid ester such as a stearate; all of which are non-fluorinated. Again, this is to render the residual reinforcing silica (B) fillers(s) hydrophobic and therefore easier to handle and obtain a homogeneous mixture with the other ingredients. Specific examples include but are not limited to non-fluorinated liquid hydroxyl- terminated polydiorganosiloxane containing an average from 2 to 20 repeating units of diorganosiloxane, hexaorganodisiloxane, hexaorganodisilazane, and the like. [0056] In either treatment case described above, a small amount of water or ammonium hydroxide maybe added together with the silica treating agent(s) as processing aid. The surface treatment of the fillers makes them easily wetted by the polymers of component (A). These surface modified fillers do not clump and can be homogeneously incorporated into component (A) resulting in improved room temperature mechanical properties of the uncured compositions.

[0057] The amount of silica reinforcing filler used in the compositions described herein is typically from about 1 to 40 wt. % (weight%) of the composition, alternatively 5 to 35 wt. % of the composition, alternatively from 10 to 35 wt. % of the composition alternatively from 15 to 35 wt. % of the composition. The treating agent utilised is typically added in an amount of from 1 to 10 wt. % of the total composition (i.e. after mixing part A and part B when stored as a multipart composition, alternatively from 1 to 10 wt. % of the total composition.

[0058] In one embodiment, at least 20 wt. % of the total silica surface is treated using a fluorinated hydrophobing treating agent. This fluorinated hydrophobing treating agent may be uniformly distributed across all silica surface, when a mixture of said fluorinated treating agents and non-fluorinated treating agents are applied onto the silica simultaneously, sequentially, or concentrated on certain portions of the silica surface where treated separately or sequentially, as desired using a suitable treating process or treating the silica in situ alternatively, at least 30 wt. % of the total weight of reinforcing silica (B); alternatively, at least 40 wt. % of the total weight of reinforcing silica

(B); alternatively, at least 50 wt. % of the total weight of reinforcing silica (B); alternatively, at least 60 wt. % of the total weight of reinforcing silica (B); alternatively, at least 80 wt. % of the total weight of reinforcing silica (B) or alternatively, 100 wt. % of the total weight of reinforcing silica (B). In the above the maximum in each instance is 100 wt. % of the total weight of reinforcing silica (B).

[0059] The composition is cured using a curing package of at least one of components (C), (D) or a mixture of components (C) and (D) wherein

(D) is at least one peroxide catalyst.

(C)(i) Organohydrogenpolysiloxane

[0060] When present, component (C)(i) is an organohydrogenpolysiloxane, which operates as a cross-linker for curing component (A), by the addition reaction of the silicon-bonded hydrogen atoms in component (C)(i) with the alkenyl groups in component (A) under the catalytic activity of component (C)(ii). Component (C)(i) normally contains 3 or more silicon-bonded hydrogen atoms so that the hydrogen atoms of this component can sufficiently react with the alkenyl groups of component (A) to form a network structure therewith and thereby cure the composition. Some or all of Component (C)(i) may alternatively have 2 silicon bonded hydrogen atoms per molecule when component (A) has > 2 alkenyl or alkynyl, alternatively alkenyl groups per molecule.

[0061] The molecular configuration of component (C)(i) is not specifically restricted, and it can be straight chain, branch-containing straight chain, or cyclic. While the viscosity of this component is not specifically restricted, it may typically be from 0.001 to 50 Pa.s at 25°C 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, in order to obtain a good miscibility with component (A).

[0062] Component (C)(i) is typically added in an amount such that the molar ratio of the total number of the silicon-bonded hydrogen atoms in component (C)(i) to the total number of all alkenyl and alkynyl groups, alternatively alkenyl groups in component (A) is from 0.5:1 to 20:1. When this ratio is less than 0.5:1, a well-cured composition will not be obtained. When the ratio exceeds 20: 1 , there is a tendency for the hardness of the cured composition to increase when heated. The silicon-bonded hydrogen (Si-H) content of organohydrogenpolysiloxane (C)(i) is determined using quantitative infra-red analysis in accordance with ASTM E168.

[0063] Examples of component (C)(i) include but are not limited to:

(i) trimethylsiloxy-terminated methylhydrogenpolysiloxane,

(ii) trimethylsiloxy-terminated polydimethylsiloxane-methylhydrogensiloxane,

(iii) dimethylhydrogensiloxy-terminated dimethylsiloxane-methylhydrogensiloxane copolymers,

(iv) dimethylsiloxane-methylhydrogensiloxane cyclic copolymers,

(v) copolymers composed of (CH₃)₂HSiO_1/2 units and SiO_4/2 units, and

(vi) copolymers composed of (CH₃)₃SiO_1/2 units, (CH₃)₂HSiO_1/2 units, and S1O_4/2 units.

[0064] Typically component (C)(i) is present in the composition in an amount of from 0.5 to 10 wt.% of the total composition which amount is determined dependent on the required molar ratio of the total number of the silicon- bonded hydrogen atoms in component (C)(i) to the total number of all alkenyl and alkynyl groups as previously discussed.

(C)(ii) Hvdrosilylation catalyst

[0065] When present, hydrosilylation catalyst (C)(ii) is one of the platinum metals (platinum, ruthenium, osmium, rhodium, iridium and palladium), or a compound of one or more of such metals. Platinum and platinum compounds are preferred due to the high activity level of these catalysts in hydrosilylation reactions.

[0066] Example of preferred hydrosilylation catalysts (C)(ii) include but are not limited to platinum black, platinum on various solid supports, chloroplatinic acids, alcohol solutions of chloroplatinic acid, and complexes of chloroplatinic acid with ethylenically unsaturated compounds such as olefins and organosiloxanes containing ethylenically unsaturated silicon-bonded hydrocarbon groups. The catalyst (C)(ii) can be platinum metal, platinum metal deposited on a carrier, such as silica gel or powdered charcoal, or a compound or complex of a platinum group metal.

[0067] Examples of suitable platinum-based catalysts include

(i) complexes of chloroplatinic acid with organosiloxanes containing ethylenically unsaturated hydrocarbon groups are described in US 3,419,593;

(ii) chloroplatinic acid, either in hexahydrate form or anhydrous form;

(iii) a platinum-containing catalyst which is obtained by a method comprising reacting chloroplatinic acid with an aliphatically unsaturated organosilicon compound, such as divinyltetramethyldisiloxane;

(iv) alkene-platinum-silyl complexes as described in US Pat. No. 6,605,734 such as (COD)Pt(SiMeCl

where

“COD” is 1,5-cyclooctadiene; and/or

(v) Karstedf s catalyst, a platinum divinyl tetramethyl disiloxane complex typically containing about 1 wt.% of platinum in a solvent, such as toluene may be used. These are described in US3, 715,334 and US3, 814,730.

[0068] The hydrosilylation catalyst (C)(ii) when present, is present in the total composition in a catalytic amount, i.e. an amount or quantity sufficient to promote a reaction or curing thereof at desired conditions. Varying levels of the hydrosilylation catalyst (C)(ii) can be used to tailor reaction rate and cure kinetics. The catalytic amount of the hydrosilylation catalyst (C)(ii) is generally between 0.01 ppm, and 10,000 parts by weight of platinum-group metal, per million parts (ppm), based on the combined weight of the composition components (a) and (b); alternatively between 0.01 and 5000 ppm; alternatively between 0.01 and 3,000 ppm, and alternatively between 0.01 and 1,000 ppm. In specific embodiments, the catalytic amount of the catalyst may range from 0.01 to 1,000 ppm, alternatively 0.01 to 750 ppm, alternatively 0.01 to 500 ppm and alternatively 0.01 to 100 ppm of metal based on the weight of the composition. The ranges may relate solely to the metal content within the catalyst or to the catalyst altogether (including its ligands) as specified, but typically these ranges relate solely to the metal content within the catalyst. The catalyst may be added as a single species or as a mixture of two or more different species. Typically, dependent on the form/concentration in which the catalyst package is provided the amount of catalyst present will be within the range of from 0.001 to 3.0 wt. % of the composition.

Inhibitor (C)(iii)

[0069] Compositions of the aforementioned components (A), (C)(i), and (C)(ii) may begin to cure at ambient temperature. To obtain a longer working time or pot life of a hydrosilylation cured composition when (C)(i) and (C)(ii) are present a suitable hydrosilylation reaction inhibitor (C)(iii) may also be used in order to retard or suppress the activity of the catalyst. Hydrosilylation reaction inhibitors are well known in the art and include hydrazines, triazoles, phosphines, mercaptans, organic nitrogen compounds, acetylenic alcohols, silylated acetylenic alcohols, maleates, fumarates, ethylenically or aromatically unsaturated amides, ethylenically unsaturated isocyanates, olefinic siloxanes, unsaturated hydrocarbon monoesters and diesters, conjugated ene-ynes, hydroperoxides, nitriles, and diaziridines.

[0070] One class of known hydrosilylation reaction inhibitor includes the acetylenic compounds disclosed in US 3,445,420. Acetylenic alcohols such as 2-methyl-3-butyn-2-ol constitute a preferred class of inhibitors that will suppress the activity of a platinum-containing catalyst at 25 °C. Compositions containing these inhibitors typically require heating at temperature of 70 °C or above to cure at a practical rate.

[0071] Examples of acetylenic alcohols and their derivatives include 1-ethynyl-l-cyclohexanol (ETCH), 2-methyl- 3-butyn-2-ol, 3-butyn-l-ol, 3-butyn-2-ol, propargylalcohol, l-phenyl-2-propyn-l-ol, 3,5-dimethyl-l-hexyn-3-ol, 1- ethynylcyclopentanol, 3-methyl-l-penten-4-yn-3-ol, and mixtures thereof. Derivatives of acetylenic alcohol may include those compounds having at least one silicon atom.

[0072] When present, inhibitor concentrations as low as 1 mole of inhibitor per mole of the metal of catalyst

(C)(ii) will in some instances impart satisfactory storage stability and cure rate. In other instances, inhibitor concentrations of up to 500 moles of inhibitor per mole of the metal of catalyst (C)(ii) are required. The optimum concentration for a given inhibitor in a given composition is readily determined by routine experimentation. Dependent on the concentration and form in which the inhibitor selected is provided/available commercially, when present in the composition, the inhibitor is typically present in an amount of from 0.0125 to 10 wt. % of the composition.

[0073] When component C is relied upon to cure the composition, typically the composition will be stored in two parts, often referred to as Part A and Part B with a view to separating components (C)(i) and (C)(ii) prior to cure. Typically, when present, component (C)(iii) is present in the same part as the cross-linker (C)(i). Such 2-part compositions are designed to enable easy mixing immediately prior to use and are typically in a weight ratio of Part A : Part B of from 15:1 to 1:1.

(D) Peroxide Catalyst

[0074] The composition as described herein may alternatively or additionally be cured with a peroxide catalyst (D) or mixtures of different types of peroxide catalysts.

[0075] The peroxide catalyst may be any of the well-known commercial peroxides used to cure fluorosilicone elastomer compositions. The amount of organic peroxide used is determined by the nature of the curing process, the organic peroxide used, and the composition used. Typically, the amount of peroxide catalyst utilised in a composition as described herein is from 0.2 to 3 wt. %, alternatively 0.2 to 2 wt. % in each case based on the weight of the composition. [0076] Suitable organic peroxides are substituted or unsubstituted dialkyl-, alkylaroyl-, diaroyl-peroxides, e.g. benzoyl peroxide and 2,4-dichlorobenzoyl peroxide, ditertiarybutyl peroxide, dicumyl peroxide, t- butyl cumyl peroxide, bis(t-butylperoxyisopropyl) benzene bis(t-butylperoxy)-2, 5-dimethyl hexyne 2,4-dimethyl-2,5-di(t- butylperoxy) hexane, di-t-butyl peroxide and 2,5-bis(tert-butyl peroxy)-2,5-dimethylhexane. Mixtures of the above may also be used.

Optional additional ingredients

[0077] Additional optional ingredients may be present in the silicone rubber composition depending on the intended use thereof. Examples of such optional ingredients include thermally conductive fillers, non-conductive fillers, pot life extenders, flame retardants, lubricants, non-reinforcing fillers, compression set additives, 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.

[0078] Pot life extenders, such as triazole, may be used, but are not considered necessary in the scope of the present invention. The liquid curable silicone rubber composition may thus be free of pot life extender.

[0079] Examples of flame retardants include aluminium trihydrate, chlorinated paraffins, hexabromocyclododecane, triphenyl phosphate, dimethyl methylphosphonate, tris(2,3-dibromopropyl) phosphate (brominated tris), and mixtures or derivatives thereof.

[0080] Examples of lubricants include graphite, talc, boron nitride, molybdenum disulfide, and mixtures or derivatives thereof.

[0081] Further additives include silicone fluids, such as trimethyl terminated or dime thy lhydroxy terminated siloxanes. typically have a viscosity < 150 mPa.s at 25°C 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. When present such silicone fluid may be present in the liquid curable silicone rubber composition in an amount ranging of from 0.1 to 5% by weight (% wt.), based on the total weight of the composition.

[0082] Examples of pigments include titanium dioxide, chromium oxide, bismuth vanadium oxide, iron oxides and mixtures thereof.

[0083] Examples of adhesion promoters include alkoxysilane containing methacrylic groups or acrylic groups such as methacryloxymethyl-trimethoxysilane, 3-methacryloxypropyl-tirmethoxysilane, 3-methacryloxypropyl- methyldimethoxysilane, 3-methacryloxypropyl-dimethylmethoxysilane, 3-methacryloxypropyl-triethoxysilane, 3- methacryloxypropyl-methyldiethoxysilane, 3-methacryloxyisobutyl-trimethoxysilane, or a similar methacryloxy- substituted alkoxysilane; 3-acryloxypropyl-trimethoxysilane, 3-acryloxypropyl-methyldimethoxysilane, 3- acryloxypropyl-dimethyl-methoxysilane, 3-acryloxypropyl-triethoxysilane, or a similar acryloxy-substituted alkyl- containing alkoxysilane; zirconium chelate compound such as zirconium (IV) tetraacetyl acetonate, zirconium (IV) hexafluoracetyl acetonate, zirconium (IV) trifluoroacetyl acetonate, tetrakis (ethyltrifluoroacetyl acetonate) zirconium, tetrakis (2,2,6,6-tetramethyl-heptanethionate) zirconium, zirconium (IV) dibutoxy bis(ethylacetonate ), diisopropoxy bis (2,2,6,6-tetramethyl-heptanethionate) zirconium, or similar zirconium complexes having b- diketones (including alkyl-substituted and fluoro-substituted forms thereof) and epoxy-containing alkoxysilanes such as 3-glycidoxypropyl trimethoxysilane, 3-glycidoxypropyl triethoxysilane, 3-glycidoxypropyl methyldimethoxysilane, 4-glycidoxybutyl trimethoxysilane, 5,6-epoxyhexyl triethoxysilane, 2-(3,4- epoxycyclohexyl) ethyltrimethoxysilane, or 2-(3,4-epoxycyclohexyl) ethyltriethoxysilane.

[0084] Examples of chain extenders include disiloxane or a low molecular weight polyorganosiloxane containing two silicon-bonded hydrogen atoms in terminal positions. The chain extender typically reacts with alkenyl groups of component (A), thereby linking two or more molecules of component (A) together and increasing its effective molecular weight and the distance between potential cross-linking sites.

[0085] A disiloxane is typically represented by the general formula (HR^SthO. When the chain extender is a polyorganosiloxane, it has terminal units of the general formula HR¾SiOi and non-terminal units of the formula R^b SiO. In these formulae, R^a and R^b individually represent unsubstituted or substituted monovalent hydrocarbon groups that are free of ethylenic unsaturation and fluoro content, which include, but are not limited to alkyl groups containing from 1 to 10 carbon atoms, substituted alkyl groups containing from 1 to 10 carbon atoms such as chloromethyl, cycloalkyl groups containing from 3 to 10 carbon atoms, aryl containing 6 to 10 carbon atoms, alkaryl groups containing 7 to 10 carbon atoms, such as tolyl and xylyl, and aralkyl groups containing 7 to 10 carbon atoms, such as benzyl.

[0086] Further examples of chain extenders include tetramethyldihydrogendisiloxane or dimethylhydrogen- terminated polydimethylsiloxane.

[0087] A chain extender may be added in an amount from 1 to 10 parts by weight, based on the weight of component (A), typically 1 to 10 parts per 100 parts of component (A).

[0088] Examples of heat stabilizers include metal compounds such as red iron oxide, yellow iron oxide, ferric hydroxide, cerium oxide, cerium hydroxide, lanthanum oxide, copper phthalocyanine. Aluminum hydroxide, fumed titanium dioxide, iron naphthenate, cerium naphthenate, cerium dimethylpolysilanolate and acetylacetone salts of a metal chosen from copper, zinc, aluminum, iron, cerium, zirconium, titanium and the like. The amount of heat stabilizer present in a composition may range from 0.01 to 1.0 wt. % of the total composition.

[0089] The present invention thus provides a silicone rubber composition, which comprises:

Component (A) in an amount of from 40 to 95 wt. % of the composition

Component (B) in an amount of from 5 to 60 wt. % of the composition. The total weight % of the composition being 100 wt. % for any composition.

[0090] When the composition is cured via hydrosilylation the composition may comprise 0.5 to 10 wt. % of component (C)(i), 0.01 to 1% wt. % component (C)(ii) and from 0 to 1 wt. % of component (C)(iii). The total weight % of the composition being 100 wt. % for any composition. In such cases the composition will be stored prior to use in two parts Part A and Part B. Typically, Part A will contain some of Component (A), some of Component (B) and Component (C)(ii) and part B will contain the remainder of components (A) and (B) together with components (C)(i). Optional inhibitor (C)(iii) when present may be present in either or both of the part (A) or part (B) compositions. The optional ingredients present in the composition may introduced into either or both the part A composition or the part B composition as desired providing they do not cause any negative effect to the respective part. The two-part composition may be designed to be mixed together in any suitable ratio e.g. from 15 : 1 to 1 : 1, but typically is mixed in a ratio of part A : part B of 1 to 1. In cases where the ratio is 15 : 1 or greater part B may comprise only cross-linker (C)(i) and optionally inhibitor (C)(iii).

[0091] The curable silicone elastomer composition described above may be prepared by

(i) making a silicone base composition by mixing the at least one alkyl, alkenyl alkynyl and/or silanol terminated fluorinated polydiorganosiloxane polymer having a minimum viscosity of 1000mPa.s at 25°C, (A) with at least one reinforcing silica filler and

(ii) introducing components (C), component (D) or a mixture of component (C) and component (D) and storing the resulting composition; wherein when the composition contains hydrosilylation cure package (C) the composition is stored in two or more parts with components (C)(i) and (C)(ii) being kept in separate parts; characterised in that the at least one reinforcing silica filler is at least partially treated with a fluorinated treating agent either prior to or during step (i).

[0092] In one embodiment the reinforcing silica filler treated with treating agent prior to step (i) of the process. In this embodiment all the reinforcing silica filler is treated with a fluorinated treating agent prior to step (i) or alternatively the reinforcing silica filler is partially treated with a fluorinated treating agent prior to step (i) and the remainder is treated with a non-fluorinated treating agent prior to step (i). In this embodiment once the reinforcing silica filler has been treated prior to step (i) the treated reinforcing silica filler is mixed with component (A) to form the base resulting from step (i) of the process. In a further alternative the silica may be treated by a mixture of fluorinated treating agent(s) and non-fluorinated treating agents simultaneously or sequentially.

[0093] In an alternative embodiment the at least one alkyl, alkenyl alkynyl and/or silanol terminated fluorinated polydiorganosiloxane polymer having a minimum viscosity of 1000mPa.s at 25°C, (A) is mixed with one or multiple aliquots of the reinforcing silica filler and one or multiple aliquots of the fluorinated treating agent in situ such that the reinforcing silica filler is treated in situ to obtain the final product of step (i).

[0094] In an alternative embodiment the at least one alkyl, alkenyl alkynyl and/or silanol terminated fluorinated polydiorganosiloxane polymer having a minimum viscosity of 1000mPa.s at 25 °C, (A) is mixed with one or multiple aliquots of the reinforcing silica filler and one or multiple aliquots of a mixture of, from 0 to 100 wt. % fluorinated treating agent and from 0 to 100 wt. % non-fluorinated treating agent in situ such that the reinforcing silica filler is treated in situ to obtain the final product of step (i).

[0095] In an alternative embodiment component (A) is mixed with the reinforcing silica filler and a mixture of fluorinated 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).

[0096] The resulting product of step (i) may be divided for use as a base for part A and as a base for part B . Alternatively, when the composition is being hydrosilylation cured two separate bases may be prepared at the end of step (i). These may have the same or a different composition. For example, the base for part A composition may have utilised a fluorinated treating agent, a non-fluorinated treating agent or a mixture of fluorinated treating agent and non-fluorinated treating agents. Likewise, the base for part B composition may have utilised a fluorinated treating agent, a non-fluorinated treating agent or a mixture of fluorinated treating agent and non-fluorinated treating agents. In each instance, at least the base for part A composition or the base for part B composition must comprise reinforcing silica filler at least partially treated with fluorinated treating agent.

[0097] Irrespective of the method for achieving the above the amount of amount of reinforcing silica filler treated with a fluorinated treating agent present in the composition is designed to provide an elastomeric product upon cure with a volume resistivity within a predefined range so as to be compatible with the volume resistivity of neighbouring cable insulation or the like, for example cross-linked polyethylene.

[0098] Any mixing techniques and devices described in the prior art can be used for this purpose. The particular device to be used will be determined by the viscosities of individual components and the final curable coating composition. Suitable mixers include but are not limited to paddle type mixers and kneader type mixers. Cooling of components during mixing may be desirable to avoid premature curing of the composition.

[0099] When the composition herein is designed to be an LSR composition, the viscosity of the composition ranges of from 10 to 1,000 Pa.s, alternatively of from 10 to 500 Pa.s, alternatively of from 100 to 500 Pa.s in each case at 25°C measured using a cone and plate rheometer at 10 's or relying on Williams plasticity measurements for the most viscous materials where the at least one alkyl, alkenyl alkynyl and/or silanol terminated fluorinated polydiorganosiloxane polymer having a minimum viscosity of 1000mPa.s at 25 °C, (A) comprises at least one gum. [0100] The present silicone rubber composition may alternatively be 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.

[0101] Curing of the curable silicone rubber composition may be carried out as required by the type of silicone rubber utilized. Typical curing temperatures may range of from 80 to 200°C, alternatively of from 100 to 170°C. The time for the cure will depend on the cure temperature and method chosen but will typically be approximately from 5 minutes to 1 hour. Furthermore, if required the resulting cured elastomers may be post cured. Any suitable post cure may be undertaken if desired. For example, the cured elastomer may be post cured in an oven at a temperature of from 150 to 250°C, alternatively of from 170°C to 230°C for a pre-determined period of time e.g. 2 to 10 hours as required.

[0102] Curing can for example take place in a mold to form a moulded silicone article. The composition may for example be injection moulded to form an article, or the composition can be overmolded by injection moulding around an article or over a substrate.

[0103] There is also provided herein a high voltage insulator, alternatively a high voltage direct current insulator comprising an elastomeric product of a curable silicone elastomer composition described herein and/or a high voltage direct current insulator comprising an elastomeric product obtained by curing a silicone elastomer composition as described herein. Typically, the composition contains less than or equal to (<) 0.1 wt. % of the composition of electrically conductive filler or electrically semi-conductive filler or a mixture thereof and in one embodiment the composition described above contains 0 (zero) wt. % of electrically conductive filler.

[0104] The cured product of the above described composition may be used as a high voltage direct current or high voltage alternating current insulator but may be particularly adapted to reduce electrical stress in high voltage direct current (HVDC) applications, i.e. power cable systems or the like. As previously indicated, there is provided a high voltage insulator, which is preferably a high voltage direct current insulator comprising an elastomeric product of a silicone elastomer composition described herein. The high voltage insulator, which is preferably a high voltage direct current insulator may be used alone or may form part of an article or assembly e.g. a composite part of an assembly such as in cable accessories, as cable joint or cable termination materials, boots, sleeves and or other fittings in high voltage insulator applications, alternatively high voltage direct current applications, in field grading assemblies as a suitable insulating layer and in other suited cable accessories and connectors.

[0105] In a further embodiment there is provided a method for the manufacture of an high voltage insulator or a field grading assembly, comprising said high voltage insulator, which is preferably a high voltage direct current (HVDC) for use in a high voltage insulator applications, preferably high voltage direct current insulator applications, comprising the steps of: i) shaping a suitable amount of the silicone composition as hereinbefore described by an appropriate means e.g. for the sake of example by way of extrusion or using a mold and ii) curing the shaped composition to form a shaped insulator or a field grading assembly, comprising said insulator.

[0106] The high voltage direct current insulator described above may be a part of a cable accessory for high voltage direct current applications such as a cable joint, cable termination or cable connector which can e.g. seals the ends of cables having a thermoplastic or rubber cable insulation.

[0107] The present invention further provides a method for sealing and / or insulating connected cables or closing cable ends by the use of the cable joint as described before, comprising the steps of (i) providing an insulated wire having a thermoplastic or elastomer multi-layered sheath appropriate for direct current insulation and naked wire or connectors, and (ii) encapsulating naked wire or connectors by putting over onto the surface of the insulating sheath of (i) the holes of a tube-like previously molded and cured cable joint as described before 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.

[0108] The composition as described herein may be used for the manufacture of a cable joint intended for sealing cable ends of one or more cables having a thermoplastic polyolefin or rubber cable insulation, wherein the cable joint seals cable ends of one or more cables having a thermoplastic polyolefin or rubber cable insulation.

[0109] The composition as hereinbefore described may be used in the manufacture of cable accessories, as cable joint or cable termination material in high voltage applications, especially high voltage direct current applications like for high- voltage direct current power cable applications. The cured silicone composition in accordance with the present invention can be used in the construction of all kinds of field grading assemblies, like geometric, capacitive, refractive, resistive or non-linear field grading assemblies for high voltage direct current (HVDC) applications. The cured silicone composition can be also used in field grading assemblies for high voltage direct current (HVDC) applications, where it essentially or exclusively acts in insulating layers as insulator which further contribute to electrical stress reduction in addition to the field grading materials. In certain cases, it may act also as field grading material, in particular, in resistive field grading assemblies cable joints, cable terminal applications, cable accessories and connectors.

[0110] For example in the case of a high voltage direct current cable joint there may be provided a cable joint for connecting a pair of high voltage direct current power cables comprising a means for receiving and connecting a pair of high voltage direct current cables, layer of cable insulation adapted to surround the high voltage direct current cables when in said cable joint and a layer of silicone rubber joint insulation surrounding the cable insulation in said cable joint which silicone rubber joint insulation is as hereinbefore described and which is adapted to have a volume resistivity within a predefined range of the volume resistivity of the cable insulation. Preferably the cable insulation is made from cross- linked polyethylene. During assembly of the cable joint the volume resistivity of the cable insulation is determined e.g. in accordance with ASTM D257-14, a standard test method for DC Resistance or Conductance of Insulating Materials and then a suitable silicone rubber joint insulation material is prepared as described herein designed to have a similar volume resistivity in accordance with ASTM D257-14.

EXAMPLES

[0111] 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

[0112] Several compositions using fluorinated polydiorganosiloxane polymer were prepared as depicted in Table la (excluding peroxide curing agent). They are identified as Ex. 1, Ex. 2 and Comp. 1. Table la Compositions of Ex. 1, Ex. 2 and Comp. 1

[0113] All compositions were cured using a peroxide curing agent by mixing 100 parts of the composition depicted in Table la with 1 part of a peroxide masterbatch which was a 45 wt. % 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)).

[0114] Catalyzed materials were press cured for 10 minutes at 170 °C then post cured for 4h at 200 °C.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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).

[0119] 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 (Imeas 1 through Imeas4):

Icalc = (l*Imeas1 - 3*Imeas2 + 3*Imeas3 - l*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.

[0120] 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^st of these are rejected and then the subsequent 2 values used to calculate VR from Volume Resistivity (VR) = (V_max - V_mi„) x area / (2 x Icalc x Sample Thickness)

The two resulting Volume Resistivity values were averaged to give a final value. The results for each combination are depicted in Table lb below.

Table lb Volume Resistivity results

[0121] It can be seen that in Ex. 1 and Ex. 2, in which the silica is only treated with a hydroxyl terminated trifluoropropylmethylsiloxane shows substantially reduced volume resistivity, compared to Comp. 1 where the silica is treated with dimethylsiloxane species.

[0122] Liquid Silicone Rubber compositions using a fluorinated polydiorganosiloxane polymer were prepared as Ex. 2 parts A & B and Comp. 4 parts A and B as shown in Table 2a below. The fumed silica was treated in situ during preparation of the respective LSR Base.

Table 2a Wt. % of Each Component in Ex. 3 and 4

Table 2b Wt. % of Each Component in Comp. 4

[0123] The compositions were mixed together in a 1 : 1 wt. ratio. Cured Sheets

[0124] Cured sheets were prepared at 0.5mm thickness using compression molds, a hydraulic press set at 300 psi (2.17MPa) and a temperature of 120°C for 10 minutes. The volume resistivity results of the sheets were determined as described previously and are tabulated in Table 2c below. Table 2c

[0125] Thus, the presence of silica in Ex. 3, treated with an hydroxyl terminated trifluoropropylmethylsiloxane, viscosity @ 25°C 110 mPa.s in accordance with the disclosure herein, results in a considerable decrease in volume resistivity compared to Comp. 2 which does not contain such treated silica. Ex 4 with a mixture of fluoro and non- fluoro treatment shows considerably decreased volume resistivity compared to Comp 2.

Claims

WHAT IS CLAIMED IS:

1. A curable silicone elastomer composition comprising

(B) at least one reinforcing silica filler 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 (C) or (D) wherein

(C) is at least one organohydrogenpolysiloxane (C)(i), at least one hydrosilylation catalyst (C)(ii) and optionally at least one cure inhibitor (C)(iii); or

(D) at least one peroxide catalyst.

2. A curable silicone elastomer composition in accordance with claim 1 characterised in that the composition < 0.1% by weight of the composition of electrically conductive filler or electrically semi-conductive filler or a mixture thereof.

3. A curable silicone elastomer composition in accordance with any preceding claim characterised in that when (C) the hydrosilylation cure package is present in the composition (A) must contain at least two alkenyl or alkynyl groups per molecule and when component (D) is the sole catalyst the presence of at least two alkenyl or alkynyl groups per molecule in (A) is optional.

4. A curable silicone elastomer composition in accordance with any preceding claim characterised in that filler (B) is at least partially treated with one or more fluorinated treating agents selected from the group of trifluoropropyltrimethoxysilane and trifluoropropyltriethoxysilane; silanol terminated trifluoropropylalkyl siloxanes having from 2 to 20 siloxane repeating units and wherein the alkyl groups have 1 to 6 carbons such as silanol terminated trifluoropropylmethyl siloxane having from 2 to 20 siloxane repeating units and silanol terminated trifluoropropylethyl siloxane having from 2 to 20 siloxane repeating units and bis(trifluoropropyldialkyl)silazanes where each alkyl group has 1 to 6 carbons to render the said filler(s) hydrophobic.

5. A curable silicone elastomer composition in accordance with any preceding claim characterised in that when present (C)(i) may be selected from one or more of:

(i) trimethylsiloxy-terminated methylhydrogenpolysiloxane,

(ii) trimethylsiloxy-terminated polydimethylsiloxane-methylhydrogensiloxane,

(iv) dimethylsiloxane-methylhydrogensiloxane cyclic copolymers,

(v) copolymers composed of (CH ) HSiO_1/2 units and S O units, and

(vi) copolymers composed of (CH₃)₃SiO_1/2 units, (CH₃)₂HSiO_1/2 units, and

units.

6. A curable silicone elastomer composition in accordance with any preceding claim characterised in that the composition may additionally comprise one or more optional 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. A curable silicone elastomer composition in accordance with any preceding claim characterised in that when component (C) is present the composition is stored prior to use, in two parts, 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 a curable silicone elastomer composition in accordance with any one of claims 1 to 7.

9. A high voltage insulator comprising an elastomeric product obtained or obtainable by curing silicone elastomer composition in accordance with any one of claims 1 to 7.

10. A high voltage insulator in accordance with any one of claims 8 or 9 characterised in that when (C) the hydrosilylation cure package is present in the composition (A) must contain at least two alkenyl or alkynyl groups per molecule and when component (D) is the sole catalyst the presence of at least two alkenyl or alkynyl groups per molecule in (A) is optional.

11. A high voltage insulator in accordance with any one of claims 8, 9 or 10 used as an insulator adapted to reduce electrical stress in high voltage direct current (HVDC) applications.

12. A high voltage insulator in accordance with any one of claims 8, 9, 10 or 11 used alone or as part of an article or assembly.

13. A high voltage insulator in accordance with claim 12 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 applications.

14. A high voltage insulator in accordance with claim 8, 9, 10, 11, 12 or 13 characterised in that when (C) the hydrosilylation cure package is present in the composition (A) must contain at least two alkenyl or alkynyl groups per molecule and when component (D) is the sole catalyst the presence of at least two alkenyl or alkynyl groups per molecule in (A) is optional.

15. A high voltage insulator in accordance with claim 8, 9, 10, 11, 12, 13 or 14 characterised in a cable accessory.

16. A high voltage insulator in accordance with claim 8 to 15 wherein the high voltage insulator is a high voltage direct current insulator.

17. A method of preparing a curable silicone elastomer composition in accordance with any one of claims 1 to 7 by

(i) making a silicone base composition by mixing component (A) with at least one reinforcing silica filler and

(ii) introducing components (C), or a mixture of component (C) and component (D) and storing the resulting composition; wherein when the composition contains hydrosilylation cure package (C) the composition is stored in two or more parts with components (C)(i) and (C) (ii) being kept in separate parts; characterised in that the at least one reinforcing silica filler is either at least partially treated with a fluorinated treating agent prior to or during step (i).

18. A method of preparing a curable silicone elastomer composition in accordance with claim 17 characterised in that the reinforcing silica filler is treated with treating agent prior to step (i) and all the reinforcing silica filler is treated with a fluorinated treating agent prior to step (i) or alternatively the reinforcing silica filler is partially treated with a fluorinated treating agent prior to step (i) and the remainder is treated with a non-fluorinated treating agent prior to step (i).

19. A method of preparing a curable silicone elastomer composition in accordance with claim 17 characterised in that component (A) may be divided into multiple predetermined aliquots with each aliquot being mixed with a predetermined amount of reinforcing silica filler and a fluorinated 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. A method of preparing a curable silicone elastomer composition in accordance with claim 17 characterised in that the fluorinated polydiorganosiloxane (A) is mixed with the reinforcing silica filler and the fluorinated treating agent in situ such that the reinforcing silica filler is treated in situ to obtain the final product of step (i).

21. A method of preparing a curable silicone elastomer composition in accordance with claim 17 characterised in that component (A) is mixed with the reinforcing silica filler and a mixture of fluorinated 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. A method in accordance with any one of claims 17 to 21 in which the composition is in two or more parts the parts are mixed together in a multi-part mixing system prior to cure.

23. A method in accordance with any one of claims 17 to 22 in which the silicone rubber composition may alternatively be 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. A method for the manufacture of a high voltage direct current insulator in accordance with any one of claims 17 to 23 wherein the composition is introduced into a mold prior to cure to form a moulded silicone article.

25. A method for the manufacture of a high voltage direct current insulator in accordance with claims 17 to

24 wherein the composition is either injection moulded to form an article or overmolded by injection moulding around an article.

26. Use of curable silicone elastomer composition in accordance with any one of claims 1 to 7 in or as a high voltage direct current insulator.

27. Use of a silicone composition according to claim 26 for the reduction of the electrical stress in high voltage direct current (HVDC) applications.

28. Use of a silicone composition according to claim 26 or 27 as an insulator for high voltage direct current (HVDC) applications.