Silicone rubber composition

Abstract

The present disclosure relates to hydrosilylation (addition) curable flame-retardant silicone rubber compositions, flame retardant silicone rubber elastomers made upon the cure of said compositions and their applications and uses.

Description

[0001] SILICONE RUBBER COMPOSITION The present disclosure relates to hydrosilylation (addition) curable flame-retardant silicone rubber compositions, flame retardant silicone rubber elastomers made upon the cure of said compositions and their applications and uses. Hydrosilylation curable silicone rubber compositions that contain organopolysiloxane polymers having unsaturated (alkenyl and / or alkynyl) groups and compounds containing silicon-bonded hydrogen atoms curable by a hydrosilylation reaction in the presence of a hydrosilylation catalyst are known in the art. Whilst hydrosilylation curable liquid silicone rubber compositions containing siloxane polymers having a viscosity of up to about 750,000mPa.s at 25oC are usually stored prior to use in two or more parts to prevent premature cure, whilst higher viscosity siloxane polymers often referred to in the industry as silicone gums having a viscosity of greater than, often much greater than, 1,000,000mPa.s at 25oC are most commonly cured via a free radical process utilising peroxide catalysts. However, using hydrosilylation (addition) cure systems for curing compositions comprising siloxane gums in what are often referred as high consistency rubber (HCR) compositions have the following advantages over radical (i.e., peroxide) reaction curable silicone rubber compositions usually used when the silicone polymer is a high viscosity silicone gum: • they can cure faster and at lower temperatures; • the cure process is substantially odorless; • the cure process does not usually require post cure treatments; and • the resulting elastomers usually have a higher tear strength. Whilst silicone elastomers are well known for providing excellent heat resistance at elevated temperatures, e.g., up to 250oC or even higher, regrettably they do not have as good a level of flame retardancy as is desired for some of the applications for which they are utilised unless provided with flame retardant additives. For the avoidance of doubt by flame retardancy additives we mean additives introduced into silicone rubber compositions which when cured to silicone elastomers provide the elastomeric product with the ability to slow down the burning process and promote self- extinguishment when a flame source is removed. Furthermore, it is to be understood that while the addition of flame-retardant additives into liquid silicone rubber compositions provide a degree of protection against fire hazards for the resulting silicone elastomers after the composition has been cured, their performance is generally lower than that of inherently flame-resistant materials. When exposed to flames or intense heat, flame-retardant materials may initially resist ignition, but they can eventually catch fire if the exposure is sustained. High consistency rubbers exposed to flames can form problematic levels of volatile organic compounds (VOCs) that evaporate and ignite in air / oxygen atmospheres in presence of a spark and / or flame (fire triangle). HCRs, without additive systems, are hardly self-extinguishing under vertical flame test conditions and regularly burn under vertical test conditions, sometimes even completely up to the clamp gripper. For electrical supply insulation applications such as high voltage insulation, an Underwriters Laboratories of US (UL) standard test UL94 (The Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing) a UL94 V0 performance of the HCR elastomer is highly favorable, if not mandatory, at a sample / coating thickness of 3 mm due to original equipment manufacturers (OEM) requirements. UL 94 has been harmonized with other standards i.e., IEC 60695-11-10, IEC 60695-11-20, ISO9772 and ISO9773. Hence, several flame retardant additives have been proposed to be used in silicone elastomeric materials to enhance their flame-resistant properties. Examples of flame-retardant additives proposed for use in peroxide cured silicone rubber elastomers to provide improved flame retardancy include platinum materials such as platinum and platinum materials as described in US3514424A and US3635874A. Combinations of such platinum materials in combination with other additives including combinations such as titanium dioxide, carbon black, Group II metal oxides, rare earth metal oxides and rare earth metal hydroxides, and an aromatic acid selected from the group consisting of mononuclear aromatic acids and halogenated mononuclear aromatic acids. There remains an ongoing need for the development of further fire-retardant silicone rubber elastomers especially in view of the ever increasingly complex environmental, health and safety requirements. There is provided herein a hydrosilylation curable flame-retardant silicone rubber composition, which comprises the following components: a) an organopolysiloxane polymer having a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08; and at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups; b) reinforcing silica filler; c) an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule, d) a flame-retardant additive comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5- Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, alkyl 3-(3,5-di-tert-butyl-4- hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'- Methylene-bis(6-tert-butyl-p-cresol); and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminium oxide, zinc oxide and / or magnesium compounds selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, in an amount of from 0.05 to 2.0 wt. % of the composition; n and e) a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof. Compositions of the above type are commonly known in the industry as high consistency silicone rubbers often referred to as HCRs because of the extremely high viscosity of the polymer (a). Hence, for the sake of this example such a composition may be referred to as an HCR composition. There is also provided a silicone elastomeric material which is the cured product of the hydrosilylation curable silicone rubber composition in accordance with any preceding claim, which silicone elastomeric material has a UL94 V-0 performance at a sample thickness of 3 mm measured in accordance with IEC 6069511-10 test. There is also a process described herein for the preparation of a hydrosilylation cured flame retardant silicone rubber comprising the steps of (i) preparing a silicone rubber base composition comprising a) an organopolysiloxane polymer having a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08; and at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups; b) reinforcing silica filler; (ii) mixing into the silicone rubber base of step (i) c) an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule, d) a flame-retardant additive comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, , alkyl 3-(3,5-di- tert-butyl-4-hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'-Methylene-bis(6-tert-butyl-p-cresol); and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminium oxide, zinc oxide and / or magnesium compounds selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, 0.05 to 2.0 wt. % of the composition; and simultaneously or subsequently e) a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof; and (iii) curing the composition. There is also provided a silicone elastomeric material obtained or obtainable from the process above. There is also provided herein a use of an additive (d) comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, alkyl 3- (3,5-di-tert-butyl-4-hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'-Methylene-bis(6-tert-butyl-p-cresol); and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminium oxide, zinc oxide and / or magnesium compounds selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, 0.05 to 2.0 wt. % of the composition as a flame-retardant additive in a composition otherwise comprising the following components: a) an organopolysiloxane polymer having a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08; and at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups; b) reinforcing silica filler; c) an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule, and e) a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof. There is also provided herein an electrical power cable comprising: o an electrically conductive core; o one or more layers of insulation around said core and o an outer sheath made from the flame-retardant silicone rubber being the cured product of the above composition. There is also provided herein the use of flame-retardant silicone rubber being the cured product of the above composition as a means of electrical insulation on a part of an electrical power supply means, for example as an outer sheath for an electrical power cable insulation, grid line insulation applications and busbar insulation. There is also provided herein a method for preparing a part of an electrical power supply means such as an electrical power cable or busbar comprising the step of applying and curing an outer sheath around the part of an electrical power supply means wherein said outer sheath is the cured product of the above composition hereinbefore described. In each case the total weight (wt.) % of each composition shown herein is 100 wt. %. Surprisingly, it has been determined that the addition of low amounts (0.5 to 2.5 wt.%) of phenolic antioxidants, especially sterically hindered phenolic antioxidants can significantly reduce flame times of, platinum cured, silicone elastomers prepared by curing HCR compositions containing silicone gums as described above and can surprisingly turn the flammability performance of this HCR from completely failing UL94 testing or having V-1 rating to having a significantly improved UL94 V-0 rating. Even more unexpected was the fact that these properties can be further enhanced with the addition of similar amounts of magnesium oxide or a magnesium carbonate enabling compositions as described herein. The components of the composition are as follows: Component (a) Component (a) is an organopolysiloxane polymer having a William’s plasticity of at least. 100mm / 100 measured in accordance with ASTM D-926-08; and at least two unsaturated groups per molecule. The unsaturated groups are selected from alkenyl and / or alkynyl groups. Each organopolysiloxane polymer of component (a) comprises multiple siloxy units, of formula (I): R’aSiO(4-a) / 2 (I) The subscript “a” is 0, 1, 2 or 3. Siloxy units may be described by a shorthand (abbreviated) nomenclature, namely - "M," "D," "T," and "Q", when R’ is as described above, alternatively an alkyl group, typically a methyl group. The M unit corresponds to a siloxy unit where a = 3, that is R’3SiO1 / 2; the D unit corresponds to a siloxy unit where a = 2, namely R’2SiO2 / 2; the T unit corresponds to a siloxy unit where a = 1, namely R’1SiO3 / 2; the Q unit corresponds to a siloxy unit where a = 0, namely SiO4 / 2. The organopolysiloxane polymer of component (a) is substantially linear but may contain a proportion of branching due to the presence of T units (as previously described) within the molecule, hence the average value of a in structure (I) is about 2. The unsaturated groups of component (a) may be positioned either terminally or pendently on the organopolysiloxane polymer, or in both locations. The unsaturated groups of component (a) may be alkenyl groups or alkynyl groups as described above. Each alkenyl group, when present, may comprise for example from 2 to 30, alternatively 2 to 24, alternatively 2 to 20, alternatively 2 to 12, alternatively 2 to 10, and alternatively 2 to 6 carbon atoms. When present the alkenyl groups may be exemplified by, but not limited to, vinyl, allyl, methallyl, isopropenyl, propenyl, and hexenyl and cyclohexenyl groups. Each alkynyl group, when present, may also have 2 to 30, alternatively 2 to 24, alternatively 2 to 20, alternatively 2 to 12, alternatively 2 to 10, and alternatively 2 to 6 carbon atoms. Examples of alkynyl groups may be exemplified by, but not limited to, ethynyl, propynyl, and butynyl groups. Preferred examples of the unsaturated groups of component (a) include vinyl, propenyl, isopropenyl, butenyl, allyl, and 5-hexenyl. In formula (I), each R’, other than the unsaturated groups described above, is independently selected from an aliphatic hydrocarbyl group, a substituted aliphatic hydrocarbyl group, an aromatic group or a substituted aromatic group. Each aliphatic hydrocarbyl group may be exemplified by, but not limited to, alkyl groups having from 1 to 20 carbons per group, alternatively 1 to 15 carbons per group, alternatively 1 to 12 carbons per group, alternatively 1 to 10 carbons per group, alternatively 1 to 6 carbons per group or cycloalkyl groups such as cyclohexyl. Specific examples of alkyl groups may include methyl, ethyl, propyl, pentyl, octyl, undecyl, and octadecyl groups, alternatively methyl and ethyl groups. Substituted aliphatic hydrocarbyl group are preferably non-halogenated substituted alkyl groups. The aliphatic non-halogenated organyl groups are exemplified by, but not limited to alkyl groups as described above with a substituted group such as oxygen containing groups such as polyoxyalkylene groups, carbonyl groups, alkoxy groups and hydroxyl groups. Further organyl groups may include boron containing groups. Examples of aromatic groups or substituted aromatic groups are phenyl groups and substituted phenyl groups with substituted groups as described above. Component (a) may, for example, be selected from polydimethylsiloxanes, alkylmethylpolysiloxanes, alkylarylpolysiloxanes or copolymers thereof (where reference to alkyl means any suitable alkyl group, alternatively an alkyl group having two or more carbons) providing each polymer contains at least two unsaturated groups, typically alkenyl groups as described above and has a degree of polymerisation of at least 2,500. They may for example be trialkyl terminated, alkenyldialkyl terminated alkynyldialkyl terminated or may be terminated with any other suitable terminal group combination providing each polymer has a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08 and at least two unsaturated groups. Hence component (a) may, for the sake of example, be: a dialkylalkenyl terminated polydimethylsiloxane, e.g., dimethylvinyl terminated polydimethylsiloxane; a dialkylalkenyl terminated dimethylmethylphenylsiloxane, e.g., dimethylvinyl terminated dimethylmethylphenylsiloxane; a trialkyl terminated dimethylmethylvinyl polysiloxane; a dialkylvinyl terminated dimethylmethylvinyl polysiloxane copolymer; a dialkylvinyl terminated methylphenylpolysiloxane, a dialkylalkenyl terminated methylvinylmethylphenylsiloxane; a dialkylalkenyl terminated methylvinyldiphenylsiloxane; a dialkylalkenyl terminated methylvinyl methylphenyl dimethylsiloxane; a trimethyl terminated methylvinyl methylphenylsiloxane; a trimethyl terminated methylvinyl diphenylsiloxane; or a trimethyl terminated methylvinyl methylphenyl dimethylsiloxane. In each case component (a) has a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08. Organopolysiloxane polymers of this magnitude are generally referred to in the industry as organopolysiloxane polymer gums, siloxane gums or silicone gums (hereafter referred to as a silicone gum) because of their very high viscosity (at least 1,000,000 mPa. s at 25oC, often many millions mPa. s at 25oC) and high molecular weight. Because of the difficulty in measuring the viscosity of such highly viscous fluids silicone gums, tend to be defined by way of their William’s plasticity values as opposed to by viscosity. Component (a) is a silicone gum and has a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08, alternatively at least 125mm / 100 measured in accordance with ASTM D-926-08, alternatively at least 140mm / 100 measured in accordance with ASTM D-926-08. Typically, silicone gums have a William’s plasticity of from about 100mm / 100 to 450mm / 100 measured in accordance with ASTM D-926-08. The number average molecular weight (Mn) and weight average molecular weight (Mw) of such polymers are typically determined by gel permeation chromatography using polystyrene standards. If where shown or required, the present disclosure the number average molecular weight and weight average molecular weight values of the silicone gums used as component (a) herein were determined using a Waters 2695 Separations Module equipped with a vacuum degasser, and a Waters 2414 refractive index detector (Waters Corporation of MA, USA). The analyses were performed using certified grade toluene flowing at 1.0 mL / min as the eluent. Data collection and analyses were performed using Waters Empower GPC software. The degree of polymerisation of the polymer was approximately the number average molecular weight of the polymer divided by 74 (the molecular weight of one component (I) depicted above). Typically, the alkenyl and / or alkynyl content, e.g., vinyl content of the polymer is from 0.01 to 3 wt. % for each organopolysiloxane polymer containing at least two silicon-bonded alkenyl groups per molecule of component (a), alternatively from 0.01 to 2.5 wt. % of component (a), alternatively from 0.01 to 2.0 wt. %, alternatively from 0.01 to 1.5 wt. % of component (a) of the or each organopolysiloxane polymer containing at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups per molecule of component (a). The alkenyl / alkynyl content of component (a) is determined using quantitative infra-red analysis in accordance with ASTM E168. Component (a) may be present in the composition in an amount of from 40 wt. % to about 90 wt. % of the composition, alternatively from 45 to 85 wt. % of the composition, alternatively from 50 to 80 wt. % of the composition. Typically, component (a) is present in an amount which is the difference between 100 wt. % and the cumulative wt. % of the other components / ingredients of the composition. Component (b) Component (b) is at least one reinforcing silica filler. Preferably said reinforcing silica fillers are in a finely divided form. The reinforcing silica fillers (b) may be exemplified by fumed silica, colloidal silicas and / or a precipitated silica. Precipitated silica, fumed silica and / or colloidal silicas are particularly preferred because of their relatively high surface area, which is typically at least 50 m² / g (BET method in accordance with ISO 9277: 2010); alternatively, having surface areas of from 50 to 450 m² / g (BET method in accordance with ISO 9277: 2010), alternatively having surface areas of from 50 to 300 m² / g (BET method in accordance with ISO 9277: 2010), are typically used. All these types of silica are commercially available. The reinforcing silica filler(s) of component (b) are naturally hydrophilic and are treated with one or more treating agents (c) to render them hydrophobic. These surface modified reinforcing fillers of component (b) do not clump and can be homogeneously incorporated into organopolysiloxane polymer (a), described below, as the surface treatment makes the fillers easily wetted by organopolysiloxane polymer (a). Component (b) is present in an amount of up to 50 wt. % of the composition, alternatively from 1.0 to 50 wt. % of the composition, alternatively of from 5.0 to 45 wt. % of the composition, alternatively of from 10.0 to 40 wt. % of the composition. The reinforcing silica fillers when used are naturally hydrophilic which renders them difficult to inter-mix with the polydiorganosiloxane polymer(s) and as such said fillers are usually either pre- treated with a treating agent to render them hydrophobic, or alternatively are provided in a hydrophilic form. In the latter case a hydrophobic treating agent is usually (but not always) provided to treat the silica in situ during the base mixing process. i.e., incorporation and dispersion of silica in polymer is done in presence of a treating agent. The product of this mixing step is a silicone rubber base composition. This may be provided in a form suitable for mixing with other ingredients as discussed below. Alternatively, it may be in the form of a concentrate (often referred to by the industry as a “masterbatch” (MB)) which is typically diluted with further polydiorganosiloxane polymer(s) before use. A variety of treating agents may be utilised to render the filler hydrophobic. The treating agents reacts with OH-groups on the silica filler surface resulting in a reduced number of free OH-groups on the silica and as such rendering the silica surface increasingly hydrophobic. Typically, the one or more fillers (b) is or are surface treated with a suitable low molecular weight organosilicon compounds disclosed in the art applicable to prevent creping of organosiloxane compositions during processing. For example, organosilanes, polydiorganosiloxanes, or organosilazanes e.g., hexaalkyl disilazane, short chain siloxane diols or fatty acids or fatty acid esters such as stearates may be used to render the filler(s) hydrophobic and therefore easier to handle and obtain a homogeneous mixture with the other ingredients. Optionally additional hydrophobing agents may be utilised, for example, organosilanes, or organosilazanes e.g., hexaalkyl disilazane and short chain methyl vinyl siloxane diols. Specific examples include, but are not restricted to, silanol terminated trifluoropropylmethylsiloxane, silanol terminated vinyl methyl (ViMe) siloxane, hexaorganodisiloxanes, such as hexamethyldisiloxane, divinyltetramethyldisiloxane; hexaorganodisilazanes, such as hexamethyldisilazane (HMDZ), divinyltetramethyldisilazane and tetramethyldi(trifluoropropyl)disilazane; hydroxyldimethyl terminated polydimethylmethylvinyl siloxane, octamethyl cyclotetrasiloxane, and silanes including but not limited to methyltrimethoxysilane, dimethyldimethoxysilane, vinyltrimethoxysilane, methyltriethoxysilane, vinyltriethoxysilane, chlorotrimethyl silane, dichlorodimethyl silane, trichloromethyl silane. In one embodiment, the treating agent may be selected from silanol terminated vinyl methyl (ViMe) siloxane, hexaorganodisiloxanes, such as hexamethyldisiloxane, divinyltetramethyldisiloxane; hexaorganodisilazanes, such as hexamethyldisilazane (HMDZ), divinyltetramethyldisilazane and; hydroxyldimethyl terminated polydimethylmethylvinyl siloxane, octamethyl cyclotetrasiloxane, and silanes including but not limited to, methyltriethoxysilane, dimethyldiethoxysilane and / or vinyltriethoxysilane. A small amount of water can be added together with the silica treating agent(s) as processing aid. Component (c) Component (c) functions as a cross-linker and is provided in the form of an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule. Component (c) normally contains three or more silicon-bonded hydrogen atoms so that the hydrogen atoms can react with the unsaturated alkenyl and / or alkynyl groups of component (a) to form a network structure therewith and thereby cure the composition. Some or all of Component (c) may alternatively have two silicon bonded hydrogen atoms per molecule when polymer (a) has greater than two unsaturated groups per molecule. The molecular configuration of the organosilicon compound having at least two, alternatively at least three Si-H groups per molecule (c) is not specifically restricted, and it can be a straight chain, branched (a straight chain with some branching through the presence of T groups), cyclic or silicone resin based. While the molecular weight of component (c) is not specifically restricted, the viscosity is typically from 5 to 50,000 mPa.s at 25ºC relying as measured using TA instruments AR2000 Rheometer in plate-plate model at shear rate 10s-1, in order to obtain a good miscibility with polymer (a). Silicon-bonded organic groups used in component (c) may be exemplified by alkyl groups such as methyl, ethyl, propyl, n-butyl, t-butyl, pentyl, hexyl; aryl groups such as phenyl tolyl, xylyl, or similar aryl groups; 3-chloropropyl, 3,3,3-trifluoropropyl, or similar halogenated alkyl group, preferred alkyl groups having from 1 to 6 carbons, especially methyl ethyl or propyl groups or phenyl groups. Preferably the silicon-bonded organic groups used in component (b) are alkyl groups, alternatively methyl, ethyl or propyl groups. Examples of the organosilicon compound having at least two, alternatively at least three Si-H groups per molecule (c) include but are not limited to: (a) trimethylsiloxy-terminated methylhydrogenpolysiloxane, (b) trimethylsiloxy-terminated polydimethylsiloxane-methylhydrogensiloxane, (c) dimethylhydrogensiloxy-terminated dimethylsiloxane-methylhydrogensiloxane copolymers, (d) dimethylsiloxane-methylhydrogensiloxane cyclic copolymers, (e) copolymers and / or silicon resins consisting of (CH3)2HSiO1 / 2 units, (CH3)3SiO1 / 2 units and SiO4 / 2units, (f) copolymers and / or silicone resins consisting of (CH3)2HSiO1 / 2units and SiO4 / 2units, (g) Methylhydrogensiloxane cyclic homopolymers having between 3 and 10 silicon atoms per molecule; alternatively, component (c), the cross-linker, may be a filler, e.g., silica treated with one of the above, and mixtures thereof. In one embodiment the Component (c) is selected from a methylhydrogenpolysiloxane capped at both molecular terminals with trimethylsiloxy groups; a copolymer of a methylhydrogensiloxane and a dimethylsiloxane capped at both molecular terminals with trimethylsiloxy groups; dimethylsiloxane capped at both molecular terminals with dimethylhydrogensiloxy groups; a copolymer of a methylhydrogensiloxane and a dimethylsiloxane capped at both molecular terminals with dimethylhydrogensiloxy groups. The cross-linker (c) is generally present in the hydrosilylation curable flame retardant silicone rubber composition such that the molar ratio of the total number of the silicon-bonded hydrogen atoms in component (c) to the total number of alkenyl and / or alkynyl groups in polymer (a) or in the composition if different 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. Preferably in an amount such that the molar ratio of silicon-bonded hydrogen atoms of component (c) to alkenyl / alkynyl groups, alternatively alkenyl groups of component (a) or in the composition ranges from 0.7 : 1.0 to 5.0 : 1.0, preferably from 0.9 : 1.0 to 2.5 : 1.0, and most preferably from 0.9 : 1.0 to 2.0 : 1.0. The silicon-bonded hydrogen (Si-H) content of component (c) is determined using quantitative infra-red analysis in accordance with ASTM E168. In the present instance the silicon-bonded hydrogen to alkenyl (vinyl) and / or alkynyl ratio is important when relying on a hydrosilylation cure process. Generally, this is determined by calculating the total weight % of alkenyl groups in the composition, e.g., vinyl [V] and the total weight % of silicon bonded hydrogen [H] in the composition and given the molecular weight of hydrogen is 1 and of vinyl is 27 the molar ratio of silicon bonded hydrogen to vinyl is 27[H] / [V]. Typically, dependent on the number of unsaturated groups in component (a) as well as the number of Si-H groups in component (c), component (c) will be present in an amount of from 0.1 to 10 wt. % of the hydrosilylation curable flame retardant silicone rubber composition, alternatively 0.1 to 7.5wt. % of the hydrosilylation curable flame retardant silicone rubber composition, alternatively 0.5 to 7.5wt. %, further alternatively from 0.5% to 5 wt. % of the hydrosilylation curable flame retardant silicone rubber composition. It may be introduced into the composition in a masterbatch with a vinylsiloxane polymer or gum. Component (d) Component (d) is a flame-retardant additive comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, alkyl 3-(3,5-di- tert-butyl-4-hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'-Methylene-bis(6-tert-butyl-p-cresol); and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminium oxide, zinc oxide and / or magnesium compounds selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, 0.05 to 2.0 wt. % of the composition; The flame-retardant additive of component (d) comprises one or more phenolic antioxidants elected from the group of an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, alternatively 1 to 18 carbons, alternatively 1 to 16 carbons e.g., stearyl 3,4,5- trihydroxybenzoate and dodecyl 3,4,5-trihydroxybenzoate, the latter of which is often referred to as dodecyl gallate having the structure: phenolic antioxidants: tannic acid tannic acid (chemical description (1,2,3,4,6-pentakis[3,4-dihydroxy-5-[(3,4,5- trihydroxybenzoyl)oxy]benzoate]beta-D-glucopyranose) antioxidant 1 in the following examples) of the structure

[0002] IrganoxTM1330, sold by BASF which is 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4- hydroxybenzyl) benzene or is alternatively known as 3,3',3'',5,5',5''-hexa-tert-butyl-α,α',α''- (mesitylene-2,4,6-triyl)tri-p-cresol) (antioxidant 2 in the following examples) and which has the structure: from the IrganoxTMrange including an alkyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate wherein the alkyl group has 1 to 20 carbons, alternatively 1 to 18 carbons, an example being IrganoxTM1076 sold by BASF which is named octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate and has the following structure: which may be used herein include for the sake of example the following: methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate which has the structure: cresol) or 2,2 -Methylene-bis(6-tert-butyl-4-methylphenol) which has the structure: The phenolic antioxidants of component (d) may be present in the composition in an amount of from 0.001 to 4wt. % of the composition, alternatively of from 0.01 to 3.5wt. % of the composition, alternatively 0.01 to 3wt. % of the composition, alternatively 0.01 to 2.5 wt. % of the composition, alternatively 0.01 to 2.0 wt. % of the composition. The optional one or more inorganic oxides of component (d) when present, is one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminum oxide or zinc oxide. The optional magnesium compound of component (d), when present is selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, with magnesium oxide, magnesium carbonates and magnesium hydroxycarbonates particularly preferred. The magnesium carbonates and magnesium hydrogen carbonates may be selected from magnesite (MgCO3), barringtonite (MgCO3.2H2O), nesquihonite (MgCO3.3H2O), lansfordite (MgCO3.5H2O); pokrovskite (Mg2(CO3)(OH)2.0.5H2O), artinite (Mg2(CO3)(OH)2.3H2O), hydromagnesite (Mg5(CO3)4(OH)2.4H2O) which is sometimes referred to as light magnesium carbonate, dypingite (Mg5(CO3)4(OH)2.5H2O) which is sometimes referred to as heavy magnesium carbonate, giorgiosite (Mg5(CO3)4(OH)2.5-6H2O) and shelkovite (Mg7(CO3)5(OH)4.24H2O). The optional one or more inorganic oxides of component (d), or magnesium compound when either are present, is / are present in the composition in an amount of from 0.05 to 2.0 wt. % of the composition, alternatively from 0.1 to 1.5 wt. % of the composition, alternatively from 0.1 to 1.25 wt. % of the composition alternatively from 0.15 to 1.25 wt. % of the composition. Component (e) Component (e) of the hydrosilylation curable flame-retardant silicone rubber composition, is a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof. These are usually selected from catalysts of the platinum group of metals (platinum, ruthenium, osmium, rhodium, iridium and palladium), or a compound of one or more of such metals. Alternatively, platinum and rhodium compounds are preferred due to the high activity level of these catalysts in hydrosilylation reactions, with platinum compounds most preferred. In a hydrosilylation (or addition) reaction, a hydrosilylation catalyst such as component (e) herein catalyses the reaction between an unsaturated group, usually an alkenyl group e.g., vinyl with Si-H groups. The hydrosilylation catalyst of component (e) can be a platinum group metal, a platinum group metal deposited on a carrier, such as activated carbon, metal oxides, such as aluminum oxide or silicon dioxide, silica gel or powdered charcoal, or a compound or complex of a platinum group metal. Preferably the platinum group metal is platinum. Examples of preferred hydrosilylation catalysts of component (e) are platinum based catalysts, for example, platinum black, platinum oxide (Adams catalyst), platinum on various solid supports, chloroplatinic acids, e.g., hexachloroplatinic acid (Pt oxidation state IV) (Speier catalyst), chloroplatinic acid in solutions of alcohols e.g., isooctanol or amyl alcohol (Lamoreaux catalyst), and complexes of chloroplatinic acid with ethylenically unsaturated compounds such as olefins and organosiloxanes containing ethylenically unsaturated silicon-bonded hydrocarbon groups, e.g., tetra- vinyl-tetramethylcyclotetrasiloxane-platinum complex (Ashby catalyst). Soluble platinum compounds that can be used include, for example, the platinum-olefin complexes of the formulae (PtCl2.(olefin)2 and H(PtCl3.olefin), preference being given in this context to the use of alkenes having 2 to 8 carbon atoms, such as ethylene, propylene, isomers of butene and of octene, or cycloalkanes having 5 to 7 carbon atoms, such as cyclopentene, cyclohexene, and cycloheptene. Other soluble platinum catalysts are, for the sake of example a platinum-cyclopropane complex of the formula (PtCl2C3H6)2, the reaction products of hexachloroplatinic acid with alcohols, ethers, and aldehydes or mixtures thereof, or the reaction product of hexachloroplatinic acid and / or its conversion products with vinyl-containing siloxanes such as methylvinylcyclotetrasiloxane in the presence of sodium bicarbonate in ethanolic solution –. Platinum catalysts with phosphorus, sulfur, and amine ligands can be used as well, e.g., (Ph3P)2PtCl2; and complexes of platinum with vinylsiloxanes, such as sym-divinyltetramethyldisiloxane. Hence, specific examples of suitable platinum-based catalysts of component (e) 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(SiMeCl2)2 where “COD” is 1,5-cyclooctadiene; and / or (v) Karstedt's catalyst, is a Pt2(divinyl tetramethyl disiloxane)3 complex typically containing from 30 to 50 wt. % platinum metal in the complex. It is typically introduced into a silicone rubber composition in a premix with a vinyl siloxane polymer. The combination being from about from 0.25 to 2.0 wt. % of the catalyst complex in 99.75 wt. % to 98 wt. % of the vinyl siloxane polymer which usually has a viscosity of from about 200 to 750 mPa.s Solvents such as toluene and the like organic solvents have been used historically as alternatives but the use of vinyl siloxane polymers by far the preferred choice. These are described in US3,715,334 and US3,814,730. In one preferred embodiment component (e) may be selected from co-ordination compounds of platinum. In one embodiment hexachloroplatinic acid and its conversion products with vinyl-containing siloxanes, Karstedt's catalysts and Speier catalysts are preferred. The catalytic amount of the hydrosilylation catalyst is generally between 0.01 ppm, and 10,000 parts by weight of platinum-group metal, per million parts (ppm), based on the weight of the composition; alternatively, between 0.01 and 5000ppm; 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 is provided e.g., in a polymer or solvent, the amount of component (e) present will be within the range of from 0.001 to 3.0 wt. % of the composition, alternatively from 0.001 to 2.5 wt. % of the composition, alternatively 0.01 to 2.0 wt. %, of the hydrosilylation curable flame-retardant silicone rubber composition. Additional optional components Additional optional components may be present in the hydrosilylation curable flame-retardant silicone rubber composition as hereinbefore described depending on the intended final use thereof. Examples of such optional components include cure inhibitors, compression set additives, pigments and / or coloring agents, non-reinforcing fillers and other additional additives such as pot life extenders, mold release agents, UV light stabilizers, bactericides, and mixtures thereof. Optional hydrosilylation reaction inhibitors The hydrosilylation curable flame-retardant silicone rubber composition as described herein may also comprise one or more optional hydrosilylation reaction inhibitors. Hydrosilylation reaction inhibitors are used, when required, to prevent or delay the hydrosilylation reaction inhibitors curing process especially during storage. The optional hydrosilylation reaction inhibitors of platinum- based catalysts 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. Alkenyl-substituted siloxanes as described in US3989667 may be used, of which cyclic methylvinylsiloxanes are preferred. One class of known hydrosilylation reaction inhibitors are the acetylenic compounds disclosed in US3445420. 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 ºC. Compositions containing these inhibitors typically require heating at temperature of 70 ºC or above to cure at a practical rate. Examples of acetylenic alcohols and their derivatives include 1-ethynyl-1-cyclohexanol (ETCH), 2- methyl-3-butyn-2-ol, 3-butyn-1-ol, 3-butyn-2-ol, propargyl alcohol, 1-phenyl-2-propyn-1-ol, 3,5- dimethyl-1-hexyn-3-ol, 1-ethynylcyclopentanol, 3-methyl-1-penten-4-yn-3-ol, and mixtures thereof. Derivatives of acetylenic alcohol may include those compounds having at least one silicon atom. When present, hydrosilylation reaction inhibitor concentrations may be as low as 1 mole of hydrosilylation reaction inhibitor per mole of the metal of catalyst (e) will, in some instances, still impart satisfactory storage stability and cure rate. In other instances, hydrosilylation reaction inhibitor concentrations of up to 500 moles of inhibitor per mole of the metal of catalyst are required. The optimum concentration for a given hydrosilylation reaction inhibitor in a given composition is readily determined by routine experimentation. Dependent on the concentration and form in which the hydrosilylation reaction inhibitor selected is provided / available commercially, when present in the composition, the inhibitor is typically present in an amount of from 0.0125 to 10wt. % of the composition. When present the reaction inhibitor may be introduced into the composition is a masterbatch. In one embodiment the inhibitor, when present, is selected from 1-ethynyl-1-cyclohexanol (ETCH) and / or 2-methyl-3-butyn-2-ol and is present in an amount of greater than zero to 0.1 wt. % of the composition. Optional Pigments / Colorants The hydrosilylation curable flame-retardant silicone rubber composition as described herein may further comprise one or more pigments and / or colorants which may be added if desired. The pigments and / or colorants may be coloured, white, black, metal effect, and luminescent e.g., fluorescent and phosphorescent. Suitable white pigments and / or colorants include titanium dioxide, zinc oxide, lead oxide, zinc sulfide, lithophone, zirconium oxide, and antimony oxide. Suitable non-white inorganic pigments and / or colorants include, but are not limited to, iron oxide pigments such as goethite, lepidocrocite, hematite, maghemite, and magnetite black iron oxide, yellow iron oxide, brown iron oxide, and red iron oxide; blue iron pigments; chromium oxide pigments; cadmium pigments such as cadmium yellow, cadmium red, and cadmium cinnabar; bismuth pigments such as bismuth vanadate and bismuth vanadate molybdate; mixed metal oxide pigments such as cobalt titanate green; chromate and molybdate pigments such as chromium yellow, molybdate red, and molybdate orange; ultramarine pigments; cobalt oxide pigments; nickel antimony titanates; lead chrome; carbon black; lampblack, and metal effect pigments such as aluminium, copper, copper oxide, bronze, stainless steel, nickel, zinc, and brass. Suitable organic non-white pigments and / or colorants include phthalocyanine pigments, e.g., phthalocyanine blue and phthalocyanine green; monoarylide yellow, diarylide yellow, benzimidazolone yellow, heterocyclic yellow, DAN orange, quinacridone pigments, e.g., quinacridone magenta and quinacridone violet; organic reds, including metallized azo reds and nonmetallized azo reds and other azo pigments, monoazo pigments, diazo pigments, azo pigment lakes, β-naphthol pigments, naphthol AS pigments, benzimidazolone pigments, diazo condensation pigment, isoindolinone, and isoindoline pigments, polycyclic pigments, perylene and perinone pigments, thioindigo pigments, anthrapyrimidone pigments, flavanthrone pigments, anthanthrone pigments, dioxazine pigments, triarylcarbonium pigments, quinophthalone pigments, and diketopyrrolo pyrrole pigments. The pigments and / or colorants, when present, are present in the range of from 2 wt. %, alternatively from 3 wt. %, alternatively from 5 wt. % of the composition to 15 wt. % of the composition, alternatively to 10 wt. % of the composition. Non-Reinforcing Fillers Non-reinforcing fillers may include crushed quartz, diatomaceous earths, barium sulphate, iron oxide, titanium dioxide and carbon black, talc, wollastonite. Other fillers which might be used alone or in addition to the above include aluminite, calcium sulphate (anhydrite), gypsum, calcium sulphate, clays such as kaolin, magnesium hydroxide e.g., brucite, graphite, copper carbonate, e.g., malachite, nickel carbonate, e.g., zarachite, barium carbonate, e.g., witherite and / or strontium carbonate e.g., strontianite. Other fillers may include, aluminium oxide, silicates from the group consisting of olivine group; garnet group; aluminosilicates; ring silicates; chain silicates; and sheet silicates. The olivine group comprises silicate minerals, such as but not limited to, forsterite and Mg2SiO4. The garnet group comprises ground silicate minerals, such as but not limited to, pyrope; Mg3Al2Si3O12; grossular; and Ca2Al2Si3O12. Aluminosilicates comprise ground silicate minerals, such as but not limited to, sillimanite; Al2SiO5; mullite; 3Al2O3.2SiO2; kyanite; and Al2SiO5. Ring silicates may be utilized as non-reinforcing fillers, these include silicate minerals, such as but not limited to, cordierite and Al3(Mg,Fe)2[Si4AlO18]. The chain silicates group comprises ground silicate minerals, such as but not limited to, wollastonite and Ca[SiO3]. Sheet silicates may alternatively or additionally be used as non-reinforcing fillers where appropriate group comprises silicate minerals, such as but not limited to, mica; K2AI14[Si6Al2O20](OH)4; pyrophyllite; Al4[Si8O20](OH)4; talc; Mg6[Si8O20](OH)4; serpentine for example, asbestos; Kaolinite; Al4[Si4O10](OH)8; and vermiculite. Another optional additive herein may include pot life extenders, such as triazole, may be used, but are not considered necessary in the scope of the present invention. The hydrosilylation curable flame-retardant silicone rubber composition may thus be free of pot life extender. Hence, in one alternative, the present disclosure thus provides a hydrosilylation curable flame-retardant silicone rubber composition, which comprises: a) an organopolysiloxane polymer having a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08, alternatively at least 125mm / 100 measured in accordance with ASTM D-926-08, alternatively at least 140mm / 100 measured in accordance with ASTM D-926-08, with a maximum William’s plasticity of from about 300mm / 100 to 450mm / 100 measured in accordance with ASTM D-926-08, and at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups present in the composition in an amount of from 40 wt. % to about 90 wt. % of the composition, alternatively from 45 to 85 wt. % of the composition, alternatively from 50 to 80 wt. % of the composition; b) reinforcing silica filler; alternatively, fumed silica, colloidal silicas and / or a precipitated silica having a BET surface area of at least 50 m² / g (ISO 9277: 2010); alternatively, having surface areas of from 50 to 450 m² / g (ISO 9277: 2010), alternatively having surface areas of from 50 to 300 m² / g (BET method in accordance with ISO 9277: 2010), are typically used and are present in an amount of up to 50 wt. % of the composition, alternatively from 1.0 to 50wt. % of the composition, alternatively of from 5.0 to 45wt. % of the composition, alternatively of from 10.0 to 40wt. % of the composition. c) an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule, component (c)is present in an amount of from 0.1 to 10 wt. % of the hydrosilylation curable flame-retardant silicone rubber composition, alternatively 0.1 to 7.5wt. % of the hydrosilylation curable flame-retardant silicone rubber composition, alternatively 0.5 to 7.5wt. %, further alternatively from 0.5% to 5 wt. % of the hydrosilylation curable flame retardant silicone rubber composition, Component (d) a flame-retardant additive comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, alkyl 3-(3,5-di- tert-butyl-4-hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'-Methylene-bis(6-tert-butyl-p-cresol); the phenolic antioxidants of component (d) may be present in the composition in an amount of from 0.001 to 5wt. % of the composition, alternatively of from 0.01 to 5wt. % of the composition, alternatively 0.01 to 3wt. % of the composition and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminum oxide, zinc oxide may be present and / or a magnesium compound selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, in an amount of present in the composition in an amount of from 0.05 to 2.0 wt. % of the composition, alternatively from 0.1 to 1.5 wt. % of the composition, alternatively from 0.1 to 1.25 wt. % of the composition alternatively from 0.15 to 1.25 wt. % of the composition. ; and e) a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof, in an amount dependent on the form / concentration in which the catalyst is provided, within the range of from 0.001 to 3.0 wt. % of the composition, alternatively from 0.001 to 2.5 wt. % of the composition, alternatively 0.01 to 2.0 wt. %, of the hydrosilylation curable flame-retardant silicone rubber composition. The total wt. % of any combination of the components in the above composition is 100 wt. %. The composition may also contain one or more of the above optional additives in amounts indicated again providing the total wt. % of the composition is 100 wt. %. Mixtures of the aforementioned components (a) the polymer, (c) the cross-linker, and (e) the hydrosilylation catalyst, may begin to cure at ambient temperature. Hence, the hydrosilylation curable flame-retardant silicone rubber compositions as hereinbefore described may be stored in two parts which are mixed together immediately before use when the composition is not prepared for immediate use. In such a case, the two parts are generally referred to as Part (A) and Part (B) and are designed to keep components (c) the cross-linker(s) and (e) the catalyst(s) apart to avoid premature cure. Typically, in such cases a Part A composition will comprise components (a), (b), and (e) and Part B will comprise components (a), (b) and (c) and when present, hydrosilylation reaction inhibitors. When in two parts component (d) may be in part (A), Part (B) or may be divided between both Part A and Part B. Alternatively, when in two parts component (d) is present in the Part (B) composition. Other optional additives, when present in the composition, may be in either Part A or Part B providing they do not negatively affect the properties of any other component (e.g., catalyst inactivation). The part A and part B of a hydrosilylation curable flame-retardant silicone rubber composition are mixed together shortly prior to use to initiate cure of the full composition into a silicone elastomeric material. The compositions can be designed to be mixed in any suitable weight ratio e.g., part A : part B may be mixed together in any suitable weight ratios. Typically, the part A and part B compositions are mixed together using a two-roll mill or kneader mixer. However, given component (a) is a silicone gum, generally the composition is prepared by combining all of components together at ambient temperature into a one-part composition in cases where the composition is to be used immediately as described above. In such a case the preparation of the hydrosilylation cured flame retardant silicone rubber comprising the steps of (i) preparing a silicone rubber base composition comprising a) an organopolysiloxane polymer having a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08; and at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups; b) reinforcing silica filler; (ii) mixing into the silicone rubber base of step (i) c) an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule, d) a flame-retardant additive comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, alkyl 3-(3,5-di- tert-butyl-4-hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'-Methylene-bis(6-tert-butyl-p-cresol); and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminum oxide, zinc oxide and / or magnesium compounds selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, in an amount 0.05 to 2.0 wt. % of the composition; and; and simultaneously or subsequently e) a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof; and (iii) curing the composition. Typically, a base is prepared first to enable the reinforcing silica fillers to be treated in-situ with a hydrophobing treating agent and then the remaining ingredients can be introduced into the mixture in any suitable order. The other ingredients are often provided in the form of masterbatches or concentrates for ease of mixing into, for example, a vinylsiloxane polymer such as component (a). This may include a reaction inhibitor masterbatch when present in the composition. 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 components and the final curable coating composition. Suitable mixers include but are not limited to paddle type mixers e.g., planetary mixers and kneader type mixers. However, when component (a) is a gum mixing is preferably undertaken, as previously indicated using a kneader mixer. Cooling of components during mixing may be desirable to avoid premature curing of the composition. Step (i) may be achieved by mixing together components (a) and (b) together with the optional treating agent at a temperature in the range of from 80oC to 250oC, alternatively from 100oC to 220oC, alternatively 120oC to 200oC for a period of from 30 minutes to 2 hours, alternatively 40 minutes to 2 hours, alternatively of from 45 minutes to 90 minutes, to ensure the reinforcing silica filler is in-situ treated by the optional treating agent and thoroughly mixed into component (a). The resulting base may then be cooled to approximately room temperature (23oC to 25oC). Components (c), (d) and simultaneously or subsequently component (e) the catalyst (catalyst composition e.g., Karstedt’s catalyst) are then added as well as optional inhibitor (e.g., Ethynyl Cyclohexanol (ETCH)) when desired and any other optional additives in any suitable order, or simultaneously and mixing to homogeneity. Once prepared because of the reactivity of the components (a), (c) and (e) the composition will cure. Typically, cure will take place at a temperature between 80oC and 180oC, alternatively between 100oC and 170oC, alternatively between 120oC and 170oC. This may take place in any suitable manner for example, the composition may be introduced into a mold and is then press cured for a suitable period of time, e.g., from 2 to 10 minutes or as otherwise desired or required. The present hydrosilylation curable flame-retardant 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, calendaring, bead application or blow moulding. As and when required samples may be additionally post-cured by heating to a temperature of 130oC to 200oC for up to 4 hours. In the case of a process for the manufacture of a two-part hydrosilylation curable flame-retardant silicone rubber composition as hereinbefore described the process may comprise the steps (i) the same as step (i) for the preparation of the one-part composition above, (ii) dividing the resulting mixture into two parts, part A and part B and introducing the catalyst (e) into part A and the cross-linker and inhibitor (if present) in the part B composition. (iii) Introducing any other optional additives into either or both part A and part B; (iv) Storing the part A and part B compositions separately. Typically, when utilised the part A and part B compositions are thoroughly mixed in a suitable weight ratio as described above, e.g., in a weight ratio of about 1 : 100 immediately before use in order to avoid premature cure. Cure is then undertaken as described above for the one-part composition. Components in each of Part A and / or Part B may be mixed together individually or may be introduced into the composition in pre-prepared in combinations for, e.g., ease of mixing the final composition. For example, components (a) and (b) may be mixed together to form a base composition usually optional treating agent is introduced into the mixture so that the reinforcing silica filler (b) can be treated in-situ. Alternatively, the reinforcing silica filler (b) may be pre- treated with optional treating agent although this is not preferred. The resulting base material can be split into two or more parts, typically part A and part B and appropriate additional components and additives may be added, if and when required into Part (A) and Part (B) with catalyst (e) usually in Part (A) and cross-linker (c) typically in Part (B). This enables such compositions and the cured silicone rubber materials resulting therefrom in the Auto and cable markets as flame resistant rubber materials, for e.g., use in electrical insulation but also in the manufacture of automotive parts, cable accessories; electrical and electronic parts; packaging parts; construction parts; household parts; and gasket sealants. The cable accessories may bee are electrical connectors, electrical terminations and wire seals. In one embodiment there is also provided herein the use of flame-retardant silicone rubber material being the cured product of the hydrosilylation curable flame-retardant silicone rubber composition described above as a means of electrical insulation on a part of an electrical power supply means, for example as a means of electrical insulation of an electrical power insulator selected from a suspension insulator, a tension insulator, a post insulator a railway insulator a hollow core insulator and / or as insulation for an electrical power cable, a busbar or a surge arrestor. There is also provided herein an electrical power cable comprising: o an electrically conductive core; o one or more layers of insulation around said core and o an outer sheath made from the flame-retardant silicone rubber being the cured product of the above composition. The outer sheath of such electric cables e.g., may be used for example high voltage power cable in electrical vehicles and high-speed trains, in high heat resistant rubber for turbo charger hoses but are mainly intended for electrical power supply. The outer sheath may be applied by any suitable method, for example by extrusion. Furthermore, the composition herein may be used in or for the manufacture of automotive parts, cable accessories; electrical and electronic parts; packaging parts; construction parts; household parts; and gasket sealants. Examples All plasticity measurements provided were measured in accordance with ASTM D-926-08. In the examples herein a base composition was prepared by mixing 39 wt. % of a hydrophobically treated fumed silica filler having a surface area of approximately 300m2 / g, 35 wt. % of a dimethylvinylsiloxy terminated polydimethylsiloxane having a plasticity 140-165 mm / 100; and 26 wt. % of a dimethylvinylsiloxy terminated poly-dimethyl-methylvinyl-polysiloxane copolymer also having a plasticity 140-165 mm / 100. Once the base was prepared an addition cure reaction inhibitor masterbatch, a cross-linker masterbatch and a Karstedt’s catalyst masterbatch were introduced and mixed into the base. The addition cure reaction inhibitor masterbatch was a silicone gum base composition containing 10 wt. % of 1-ethynyl-1-cyclohexanol (ETCH). Hence, as there is 2.5 wt. % of the cure reaction inhibitor masterbatch in the composition the amount of ETCH in the total composition is 0.25 wt. % i.e., 2500 ppm. The cross-linker masterbatch was a silicone gum base composition containing 19 wt. % of a dimethylhydrogen unit bearing methylsilsesquioxane resin having a capillary viscosity of 15 mm2 / s (cSt) measured in accordance with ASTM D-445 and a hydrogen content as SiH of 0.8 wt. %. The Karstedt catalyst was provided in a silicone gum base masterbatch containing 22.2 wt. % of a Karstedt catalyst premix. The Karstedt catalyst premix contained 1.27 wt. % of Pt2(divinyl tetramethyl disiloxane)3 complex diluted in 98.73 of a vinyl terminal poly(dimethylsiloxane-co- methylvinylsiloxane) having a viscosity of 450 mPa.s at 25°C using a BrookfieldTMrotational viscometer with a cone plate arrangement with cone CP-52 at 12rpm. Examples 1, 2 and 3 (Ex.1, Ex.2 and Ex.3) and a comparative example (C.1) were prepared using the components described above with the compositions provided in Table 1 below. Table 1: The compositions of Ex.1, Ex.2, Ex.3 and C.1 (wt. %) C.1 Ex.1 Ex.2 Ex.3 HCR Base 91.60 90.60 90.60 90.20 Antioxidant 1 was tannic acid; and Antioxidant 2 was 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene. The composition was mixed using a two-roll mill from Vacuum Vulk having a roll diameter of 200 mm and roll length of 600 mm length. Once thoroughly mixed, each composition was cured at 120°C for 10 minutes and subsequently the resulting elastomers were given UL94 V0 tests. Test plates for 3mm UL94 V0 tests were prepared and the compositions thereon were cured at 135°C for 10 minutes. The resulting test plates with cured silicone elastomeric material thereon were then stored in accordance with the UL94 requirements for 48hours (h) at room temperature and 50% relative humidity. Subsequent to the required storage period, each sample was evaluated for their flame retardant behavior via the flammability test in accordance with the International Electrotechnical Commission (IEC) 6069511-10 test i.e., the UL 94 test utilised for vertical burning. Individual test specimens were cut to dimensions of 13 mm x125 mm x 3 mm. Table 2: 3mm UL94 V0 tests in accordance with (IEC) 6069511-10 test of Ex.1, Ex.2, Ex.3 and C.1 C.1 Ex.1 Ex.2 Ex.3 In the above: t1 is the “Afterflame time” after a first flame application in accordance with the test method; t2, is the Afterflame time after the second flame application, t2 in accordance with the test method; and t3 is the afterglow time after said second flame application in accordance with the test method; Wherein an “afterflame” is defined as a flame which persists after the ignition source has been removed. The afterflame Time is the length of time during which an afterflame persists under the test conditions. Afterglow is the persistence of glowing combustion after both removal of the ignition source and the cessation of any flaming and the afterglow Time is the length of time during which an afterglow persists under test conditions. It can be seen that the examples gave significantly better results than C.1 which contained no antioxidant as hereinbefore described. It will be particularly noted that the t2 + t3 result of Ex.3 which contained magnesium oxide gave the best results and even achieved t1 of 0 seconds.

Claims

WHAT IS CLAIMED IS:

1. A hydrosilylation curable flame-retardant silicone rubber composition, which comprises the following components: a) an organopolysiloxane polymer having a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08; and at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups; b) reinforcing silica filler; c) an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule, d) a flame-retardant additive comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5- Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, alkyl 3-(3,5-di-tert-butyl-4- hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'- Methylene-bis(6-tert-butyl-p-cresol); and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminium oxide, zinc oxide; and / or magnesium compounds selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, in an amount 0.05 to 2.0 wt. % of the composition and e) a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof.

2. A hydrosilylation curable flame-retardant silicone rubber composition in accordance with claim 1 wherein the phenolic antioxidant in component (d) is selected from a dodecyl 3,4,5- trihydroxybenzoate; tannic acid; 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene; octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate; a methyl 3-(3,5-di-tert-butyl- 4-hydroxyphenyl)propanoate; and 2,2'-Methylene-bis(6-tert-butyl-p-cresol).

3. A hydrosilylation curable flame-retardant silicone rubber composition in accordance with claim 1 which component (d) comprises a magnesium compound selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture.

4. A hydrosilylation curable flame-retardant silicone rubber composition in accordance with claim 3 wherein the magnesium carbonate, magnesium hydroxy carbonate or mixture thereof selected from MgCO3, MgCO3.2H2O, MgCO3.3H2O, MgCO3.5H2O, Mg2(CO3)(OH)2.0.5H2O, Mg2(CO3)(OH)2.3H2O, Mg5(CO3)4(OH)2.4H2O, Mg5(CO3)4(OH)2.5H2O, Mg5(CO3)4(OH)2.5-6H2O and Mg7(CO3)5(OH)4.24H2O.

5. A hydrosilylation curable flame-retardant silicone rubber composition in accordance with any preceding claim wherein the composition additionally comprises one or more additives selectedfrom cure inhibitors, compression set additives, pigments and / or coloring agents, non-reinforcing fillers, pot life extenders, mold release agents, UV light stabilizers and bactericides.

6. A silicone elastomeric material which is the cured product of the hydrosilylation curable silicone rubber composition in accordance with any preceding claim . which silicone elastomeric material has a UL94 V-0 performance at a sample thickness of 3 mm measured in accordance with IEC 6069511-10 test.

7. A process for the preparation of a hydrosilylation cured flame retardant silicone rubber comprising the steps of (i) preparing a silicone rubber base composition comprising a) an organopolysiloxane polymer having a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08; and at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups; b) reinforcing silica filler; (ii) mixing into the silicone rubber base of step (i) c) an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule, d) a flame-retardant additive comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, alkyl 3-(3,5-di- tert-butyl-4-hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'-Methylene-bis(6-tert-butyl-p-cresol); and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminium oxide, zinc oxide and / or magnesium compounds selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, in an amount 0.05 to 2.0 wt. % of the composition; and simultaneously or subsequently e) a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof; and (iii) curing the composition.

8. A process for the preparation of a hydrosilylation cured flame retardant silicone rubber in accordance with claim 7 comprising magnesium oxide, magnesium carbonate, a magnesium hydroxy carbonate or a mixture thereof selected from MgCO3, MgCO3.2H2O, MgCO3.3H2O, MgCO3.5H2O, Mg2(CO3)(OH)2.0.5H2O, Mg2(CO3)(OH)2.3H2O, Mg5(CO3)4(OH)2.4H2O, Mg5(CO3)4(OH)2.5H2O, Mg5(CO3)4(OH)2.5-6H2O and Mg7(CO3)5(OH)4.24H2O.

9. A process for the preparation of a hydrosilylation cured flame retardant silicone rubber in accordance with claim 7 or 8 wherein components (c) and (e) are added into the base in the form of masterbatches.

10. A silicone elastomeric material obtained or obtainable from any one oof claims 7, 8 or 9.

11. Use of an additive (d) comprising one or more phenolic antioxidants selected from an alkyl 3,4,5-trihydroxybenzoate wherein the alkyl group contains from 1 to 20 carbons, tannic acid, 1,3,5- Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, alkyl 3-(3,5-di-tert-butyl-4- hydroxyphenyl)propanoate wherein the alkyl group contains from 1 to 20 carbons and 2,2'- Methylene-bis(6-tert-butyl-p-cresol); and optionally one or more inorganic oxides selected from titanium dioxide, zirconium oxide, aluminium oxide, zinc oxide and / or magnesium compounds selected from magnesium oxide, a magnesium carbonate, a magnesium hydrogen carbonate; or a mixture thereof, in an amount 0.05 to 2.0 wt. % of the composition as a flame-retardant additive in a composition otherwise comprising the following components: a) an organopolysiloxane polymer having a William’s plasticity of at least 100mm / 100 measured in accordance with ASTM D-926-08; and at least two unsaturated groups per molecule, which unsaturated groups are selected from alkenyl or alkynyl groups; b) reinforcing silica filler; c) an organosilicon compound having at least two, alternatively at least three Si-H groups per molecule, and e) a hydrosilylation catalyst comprising or consisting of a platinum group metal or a compound thereof.

12. Use of an additive (d) in accordance with claim 11 wherein said additive (d) comprising magnesium oxide, magnesium carbonate, a magnesium hydroxy carbonate or a mixture thereof selected from MgCO3, MgCO3.2H2O, MgCO3.3H2O, MgCO3.5H2O, Mg2(CO3)(OH)2.0.5H2O, Mg2(CO3)(OH) Mg7(CO3)5(OH) 13. An electricalo an electrically conductive core; o one or more layers of insulation around said core and o an outer sheath made from the flame-retardant silicone rubber being the cured product of any one of claims 1 to 5.

14. Use of flame-retardant silicone rubber being the cured product of the composition in accordance with any one of claims 1 to 5, as a means of electrical insulation.

15. Use of flame-retardant silicone rubber in accordance with claim 14, as a means of electrical insulation of an electrical power insulator selected from a suspension insulator, a tension insulator, a post insulator a railway insulator a hollow core insulator and / or as insulation for an electrical power cable, a busbar or a surge arrestor.

Images