Heat curable silicone rubber compositions

Abstract: The present invention provides an improved heat curable silicone composition, having increased tear strength, oil resistance, and reduced compression set, comprising (A) a blend containing (1) a vinyl-containing organopolysiloxane or vinyl-containing organopolysiloxane blend having a viscosity of about 5,000 to about 500,000 centipoise at 25°C, and (2) a vinyl-containing organopolysiloxane or vinyl-containing organopolysiloxane blend having a viscosity of about 8,000,000 to about 100,000,000 centipoise at 25°C; (B) an organopolysiloxane resin copolymer; (C) an organic peroxide catalyst; (D) a filler; and (E) an organohydrogenpolysiloxane.

Description: The present invention relates to heat curable silicone rubber compositions. More particularly, this invention relates to heat curable silicone rubber compositions having improved tear strength, oil resistance and compression set properties. Heat curable silicone rubber compositions are known in the art. Such compositions are known for their ability to resist change at elevated temperature and exposure to adverse weather conditions over extended periods of time. In particular applications, however, such as automobile seals and gaskets, materials are needed which have not only the outstanding properties of organopolysiloxane elastomers but also have a satisfactory degree of toughness. Toughness can be measured by tear strength kNm -1 (pounds per inch, i.e. "pi"), which is the ability of the rubber to withstand continual abrasion without breakdown in mechanical structure. In addition to toughness, elastomeric materials for use in automobiles must have a satisfactory degree of resiliency or ability to recover shape after deformation especially by compressive forces. Efforts have been undertaken in the past to produce heat curable silicone elastomer compositions having increased tear strength and reduced compression set properties. For example, U.S. Patent No. 4,539,357 (Bobear '357) discloses a heat-curable silicone composition comprising a vinyl-terminated linear diorganopolysiloxane gum having a viscosity in the range of 1x10 6 to 20x10 7 mPa.s (centipoise) at 25°C, a vinyl-containing diorganopolysiloxane gum having a viscosity varying from 1x10 6 to 20x10 7 mPa.s (centipoise) at 25°C, a filler, a hydride-containing polysiloxane, and an organic peroxide or organic hydroperoxide curing agent. The composition may further comprise a vinyl-containing organopolysiloxane resin copolymer. It is stated in Bobear '357 at column 6, lines 22-29 that there should be no vinyl-containing fluid in the composition since it has been found that compositions containing vinyl-containing fluids of a viscosity of 500,000mPa.s (centipoise) or less result in elastomers with good tear strength initially, but after the composition has been subjected to post-cure, its tear strength properties degrade dramatically. After post-cure, the Bobear '357 composition has a tear strength of above 35KNm -1 (200 pi). U.S. Patent No. 3,652,475 to Wada et al. (Wada) discloses heat curable elastomeric silicone compositions reportedly having high tear strength, excellent compression set and resiliency, comprising two vinyl-unsaturated polydiorganosiloxanes each having a degree of polymerization of at least 3000; a vinyl-unsaturated polydiorganosiloxane having a degree of polymerization of from 10 to 1000; a silica filler; and an organic peroxide catalyst. The samples prepared in the Wada examples had tear strength values of about 40(KNm -1 ) i.e., about 225 pi. U.S. Patent No. 3,671,480 to Wada et al. (Wada) discloses a heat curable elastomeric silicone composition having improved tear strength and superior compression set, comprising a mixture of two vinyl-unsaturated polydiorganosiloxanes, one of which appears to have a high molecular weight and the other of which appears to have a low molecular weight; a polyorganohydrogensiloxane; silica filler; and a platinum compound. The examples in Wada report tear strength values of about 50 KNm -1 , i.e., about 280 pi. Compression set values are not given. U.S. Patent No. 4,061,609 to Bobear discloses a platinum catalyzed silicone rubber composition comprising a vinyl-containing polysiloxane or a blend of such polysiloxanes; platinum; and a hydrogen-containing polysiloxane. Preferably, the vinyl-containing polysiloxane has a viscosity in the range of 1000 to 300,000,000 mPa.s (centipoise). A low viscosity vinyl-containing polysiloxane may be added to the basic composition as a reinforcing agent to give the final composition good physical strength. Claim 4 in Bobear recites a first polysiloxane having a viscosity of 1,000,000 to 200,000,000 mPa.s (centipoise) at 25°C and a second polysiloxane having a viscosity of 50,000 to 500,000 mPa.s (centipoise) at 25°C. The Bobear patent is directed to improving the work life of silicone rubber compositions, rather than tear strength. In the examples, tear strength values of greater than 35KNm -1 (200 pi) were obtained, but the compositions having these values contained two high viscosity vinylpolysiloxanes rather than one high viscosity vinylpolysiloxane and one low viscosity vinylpolysiloxane. U.S. Patent No. 3,660,345 to Bobear discloses organopolysiloxane compositions which are convertible to elastomers having high tear strength and resiliency, comprising a two component blend of vinyl-containing organopolysiloxanes; silica filler, and a process aid which can be, for example, a methoxy-terminated polysiloxane, a silanol-terminated polydimethylsiloxane, or hexamethyldisilazane. A peroxide is used as the catalyst. The viscosity of one of the two vinyl-containing organopolysiloxanes can be in the range of 1 to 1 billion mPa.s (centipoise) at 25°C, while the viscosity of the other can be between 100,000 to 2,000,000,000 mPa.s (centipoise) at 25°C. The examples in Bobear report tear strength values of greater than 35KNm -1 (200 pi). U.S. Patent No. 3,696,068 to Creamer discloses heat-curable silicone gumstock compositions having high die "B" tear strength as well as low compression set values, comprising a vinyl-containing diorganopolysiloxane gum and a linear vinyl-containing fluid having a viscosity of from 10 to 150,000 centistokes at 25°C. The crosslinkable vinyl siloxane gum has a viscosity in excess of 200,000 centistokes at 25°C. The Creamer composition is cured by a peroxide catalyst. Some of the samples prepared in Creamer had tear strength values in excess of 26KNm -1 (150 pi). Tear strength after post cure was not measured in any of the examples, which is significant since, as pointed out previously herein, it has been found that, in compositions containing low viscosity vinyl-containing fluids, tear strength drops dramatically after post cure. U.S. Patent No. 3,884,866 to Jeram et al. (Jeram) discloses a high strength organopolysiloxane composition suited for low pressure injection molding, comprising (A) a first component containing (i) a vinyl-containing high viscosity organopolysiloxane having a viscosity of 5000 to 1,000,000 mPa.s (centipoise) at 25°C; and (ii) a low viscosity vinyl-containing organopolysiloxane having a viscosity of 50 to 5000 mPa.s (centipoise) at 25°C; (iii) a filler; and (iv) a platinum catalyst; and (B) a hydrogen silicone composition. The highest tear strength reported in the examples was 44KNm -1 (250 pi). Although heat curable silicone rubber compositions having good tear strength and low compression set properties are known in the art, it is continually desirable to provide heat curable silicone rubber compositions having improved tear strength and compression set properties. Therefore, it is an object of the present invention to provide a heat curable silicone rubber composition with higher tear strength and lower compression set than the silicone rubber compositions described above. It is a further object of the present invention to provide an oil resistant heat curable silicone rubber composition having high tensile strength and elongation properties in addition to high tear strength and reduced compression set. These objects are achieved in the present invention. Summary of the Invention The present invention is based on the discovery that the combination of a vinyl-containing organopolysiloxane resin of the type described hereinafter, a vinyl-containing diorganopolysiloxane fluid having a viscosity of from 5,000 to 500,000 mPa.s (centipoise) at 25°C, and a vinyl-containing diorganopolysiloxane gum having a viscosity of from 8,000,000 to 100,000,000 mPa.s (centipoise) at 25°C, will cure in the presence of an organic peroxide catalyst and a linear organohydrogenpolysiloxane to form a heat cured silicone elastomer having high tear strength, i.e. greater than 44KNm -1 (250 pi), even after post-cure baking, and low compression set properties, i.e. 15% or less. The present invention provides a heat curable silicone rubber composition, comprising by weight: (A) 100 parts of a blend comprising by weight: (1) from 5 to 50% of a vinyl-containing diorganopolysiloxane or a vinyl-containing diorganopolysiloxane blend, wherein the vinyl-containing diorganopolysiloxane or the vinyl-containing diorganopolysiloxane blend has a viscosity of 4,000 to 500,000 mPa.s (centipoise) at 25°C; and (2) from 50 to 95% of a vinyl-containing diorganopolysiloxane or a vinyl-containing diorganopolysiloxane blend wherein the vinyl-containing diorganopolysiloxane or the vinyl-containing diorganopolysiloxane blend has a viscosity of 8,000,000 to 100,000,000 mPa.s (centipoise) at 25°C; (B) from 1 to 30 parts based on component (A) of an organopolysiloxane resin copolymer comprising (1) R 3 SiO 1/2 units and SiO 2 units where R is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation with a ratio of R 3 SiO 1/2 units to SiO 2 units ranging from 0.5:1 to 1.5:1, wherein the copolymer contains from 0.5 to 10 weight percent vinyl groups; or (2) R 3 SiO 1/2 units, SiO 2 units, and R 2 / 2SiO 2/2 units, where R i as defined above, where the ratio of R 3 SiO 1/2 units to SiO 2 units is from 0.5:1 to 1.5:1, and the R 2 SiO 2/2 units are present in an amount of from 1 to 70 mole percent based on the total number of moles of siloxy units in the copolymer, and where the resinous copolymer contains from 0.5 to 10.0 weight percent vinyl groups, or (3) mixtures of the units of (1) and (2); (C) from 0.1 to 5.0 parts based on component (A) of an organic peroxide free radical curing catalyst, (D) from 5.0 to 200 parts based on component (A) of a reinforcing filler; and (E) from 0.1 to 10 parts based on component (A), of an organohydrogensiloxane having a hydrogen content ranging from 0.05 to 2.0 percent by weight and having a viscosity ranging from 5 to 500 mPa.s (centipoise) at 25°C. The heat curable compositions of this invention have good tear die B and compression set properties without a sacrifice in other beneficial physical properties, e.g., oil resistance, tensile strength, hardness, and elongation. Detailed Description of the Invention The present invention is directed to a heat curable silicone composition having improved compression set and tear strength properties, containing (A) a blend containing (1) a vinyl-containing diorganopolysiloxane or a vinyl-containing diorganopolysiloxane blend, wherein the vinyl-containing diorganopolysiloxane or a vinyl-containing diorganopolysiloxane blend has a viscosity of from 4000 to 500,000 mPa.s (centipoise) at 25°C and (2) a vinyl-containing diorganopolysiloxane or a vinyl-containing diorganopolysiloxane blend, wherein the vinyl-containing diorganopolysiloxane or vinyl-containing diorganopolysiloxane blend has a viscosity of from 8,000,000 to 100,000,000 mPa.s (centipoise) at 25°C; (B) an organopolysiloxane resin copolymer; (C) an organic peroxide catalyst; (D) filler; and (E) an organohydrogenpolysiloxane. The composition of this invention contains 100 parts of a blend comprising (1) from 5 to 50, preferably from 10 to 40, and most preferably from 15 to 25 parts by weight of (A) of a vinyl-containing diorganopolysiloxane or a vinyl-containing diorganopolysiloxane blend having a viscosity of from 5000 to 500,000 centipoise at 25°C and (2) from 50 to 95, preferably from 55 to 90, and most preferably from 60 to 80, parts by weight of a vinyl-containing diorganopolysiloxane or a vinyl-containing diorganopolysiloxane blend having a viscosity of from 8,000,000 to 100,000,000 mPa.s (centipoise) at 25°C. Component (A)(1) is a vinyl-containing diorganopolysiloxane or a vinyl-containing diorganopolysiloxane blend having a viscosity of from 5000 to 500,000, preferably from 40,000 to 500,000, and most preferably from 60,000 to 150,000 mPa.s (centipoise) at 25°C. The vinyl-containing polymer or blend of component (A)(1) has a vinyl concentration in the range of from 0.001 to 10.0, preferably from 0.01 to 1.0, and most preferably from 0.04 to 0.10, weight percent. The organo groups in the vinyl polymer or blend of polymers are monovalent hydrocarbon radicals. The vinyl polymer or polymers of component (A)(1) can be vinyl-terminated and can contain vinyl-on-chain units. In preferred embodiments, however, the vinyl polymer has no vinyl-on-chain units. Preferably, the vinyl polymer of component (A)(1) has the formula: wherein Vi is vinyl, R 1 is selected from the class consisting of vinyl, phenyl, alkyl radicals of 1 to 8 carbon atoms, and fluoroalkyl radicals of 3 to 10 carbon atoms and mixtures thereof, and R 2 is an alkyl radical of 1 to 8 carbon atoms or a fluoroalkyl radical of 3 to 10 carbon atoms or mixtures thereof, w and z vary such that the viscosity of (A)(1) ranges from 4,000 to 500,000 mPa.s (centipoise) at 25°C, the vinyl concentration being in the range of from 0.001 to 10 weight percent. Examples of alkyl radicals which can be represented by R 1 and R 2 include methyl, ethyl, and propyl radicals. An example of a fluoroalkyl radical is 3,3,3-trifluoropropyl. Preferably, R 1 is either vinyl or methyl, and R 2 is methyl. Component (A)(2) is a vinyl-containing diorganopolysiloxane or a vinyl-containing diorganouoiysiloxane blend wherein the vinyl-containing diorganopolysiloxane or blend has a viscosity of 8,000,000 to 100,000,000, preferably from 20,000,000 to 35,000,000, and most preferably from 25,000,000 to 30,000,000 mPa.s (centipoise) at 25°C. Component (A)(2) has a vinyl concentration in the range of from 0.001 to 0.1, preferably from 0.002 to 0.008, and most preferably from 0.003 to 0.006, weight percent. The organo groups in the vinyl polymer or polymers of component (A)(2) are monovalent hydrocarbon radicals. The vinyl polymer or polymers of Component (A) (2) may contain vinyl-on-chain groups and vinyl end-groups. Preferably, the polymer is just vinyl-terminated. In preferred embodiments, the vinyl polymer of Component (A)(2) has the formula: wherein Vi is vinyl, R 4 is a C 1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, R 5 is a C 1-8 monovalent hydrocarbon radical, y and t vary such that the viscosity of component (A)(2) ranges from 8,000,000 to 100,000,000 mPa.s (centipoise) at 25°C and having a vinyl concentration in the range of from 0.001 to 0.1 weight percent. The radical R 4 can be any of the radicals given for radical R 2 in formula (2) above. The radical R 5 can be any of the radicals given for radicals R 1 and R 3 in formula (2) above. The vinyl-containing polymers of Components (A)(1) and (A)(2) can be made by a process well known in the art such as, for instance, by reacting cyclotetrasiloxanes in the presence of low molecular weight linear vinyl chain stoppers at high temperatures in the presence of basic catalysts so as to yield the polymer of the desired molecular weight. When the reaction is over, the catalyst is neutralized and the excess cyclics vented off to result in the desired polymer. By controlling the amount of chain stopper and the temperature of reaction, the molecular weight of the desired vinyl-containing polymer end product can be controlled. For more details of the process by which such vinyl-containing polymers are produced, reference is made, for example, to U.S. Patent No. 3,660,345. Component (A)(2) is present in the blend of component (A) in an amount within the range of from 50 to 95, preferably from 55 to 90, and most preferably from 60 to 80, parts by weight. Component (B) is an organopolysiloxane resin copolymer. The resin copolymer may contain (1) R 3 SiO 1/2 monofunctional units and SiO 2 tetrafunctional units, where R is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation with a ratio of R 3 SiO 1/2 units to SiO 2 units ranging from 0.5:1 to 1.5:1, wherein 0.5 to 10 weight percent of the silicon atoms contain silicon bonded vinyl groups; or the organopolysiloxane resin copolymer may contain (2) R 3 SiO 1/2 monofunctional units, SiO 2 tetrafunctional units and R 2 SiO 2/2 difunctional units where R is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation, where the ratio of R 3 SiO 1/2 units to SiO 2 units is from 0.5:1 to 1.5:1, and the R 2 SiO 2/2 units are present in an amount of from 1 to 70 mole percent based on the total number of moles of siloxy units in the copolymer, the resinous copolymer containing from 0.5 to 10.0 weight percent vinyl groups; or the organopolysiloxane resin copolymer may contain (c) a mixture of (1) and (2). The organopolysiloxane resin copolymers present in the composition of this invention are known and are produced by methods well known in the art. Such resins are usually produced by hydrolysis of the appropriate chlorosilanes in a method well known in the art. For further information as to such resins, reference is made, for example, to U.S. Patent No. 3,436,366 (Modic). Component (B) is present in the composition of this invention in an amount ranging from 1.0 to 30, preferably from 1.0 to 20, and most preferably from 2.5 to 10, parts by weight based on (A). Component (C) is an organic peroxide free radical initiator curing catalyst. The preferred peroxide curing agents are organic peroxides conventionally used to cure silicone elastomers. Examples of suitable organic peroxide catalysts for use in the present invention are disclosed, for example, in U.S. Patent No. 4,539,357 to Bobear. Suitable peroxide catalysts include dialkyl peroxides, such as di-tertiary-butyl peroxide, tertiary-butyl-triethylmethyl peroxide, tertiary-butyl-tertiary-butyl-tertiary-triphenyl peroxide, t-butyl perbenzoate and a di-tertiary alkyl peroxide such as dicumyl peroxide. The preferred dialkyl peroxide is 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane. The most preferred peroxides are the vinyl specific peroxides and the ones that have been found to give the best curing tear strength composition are, for instance, 2,5 dimethyl-2,5-di(t-butylperoxy)hexane, 2,5 dimethyl-2,5-di(t-butylperoxy)hexyne-3, di-t-butylperoxide, 5-butylcumylperoxide, alpha, alpha prime bis(t-butylperoxy)di-isopropylbenzene. In the molding of typical silicone compositions, molding temperatures are usually about 204°C (400°F). Under such conditions a single peroxide catalyst is used. However, molding cycle times necessary to achieve final cure in the mold are considered to be excessive. To decrease cycle times, therefore, molders will remove the composition from the mold prior to complete cure, necessitating a post bake step to achieve final cure. This post bake step is necessary because residual peroxide catalyst in the material will continue to catalyze the reaction until the peroxide catalyst has decomposed to noncatalytic products, as determined by the temperature "halflife" of the peroxide. In order to eliminate the post bake step, it has been found preferable to use a combination of peroxide catalysts comprising a high temperature peroxide catalyst and a low temperature peroxide catalyst. The function of the low temperature peroxide catalyst is to quickly catalyze the reaction at the molding temperature which is as low as 160°C (320°F) and allow the material to achieve a degree of crosslinking such that the material retains its shape after removal from the mold. Thus, the low temperature peroxide achieves partial crosslinking at the mold temperature in a very short period of time. The high temperature peroxide catalyst uses the residual heat of the silicone elastomer after it has been removed from the mold to continue crosslinking to final cure. Thus, since the material continues to crosslink at low temperatures, i.e. lower than molding temperatures, the post bake step becomes unnecessary. By the terms "low temperature peroxide" and "high temperature peroxide", reference is being drawn to the temperature half life of the peroxide. It is preferable that the high temperature peroxide have a ten hour half life at greater than 110°C. It is also preferable that the low temperature peroxide have a ten hour half life at 110°C or less. It is more preferable that the high temperature peroxide have a ten hour half life at 115°C or greater and the low temperature peroxide have a ten hour half life at 100°C or less. It has been found that using a combination of a low and a high temperature peroxide catalysts imparts unusually low compression set values to the cured silicone elastomer without sacrifice of durometer, tensile, or elongation. It has also been found that molding temperatures may be decreased if the low temperature peroxide catalyst itself activates at lower temperatures. That is, molding has been accomplished at a temperature as low as 160°C (320°F) while maintaining a low compression set. There is no reason that molding may not be accomplished at still lower temperatures as long as the proper high and low temperature peroxides are chosen. It is preferable that the peroxides chosen do not decompose into acids. Acid decomposition products have been found to adversely affect compression set. It has been found that 1,1-bis(t-butyl peroxy)-3,3,5-trimethylcyclohexane works well as the low temperature peroxide catalyst. It has also been found that 2,5-dimethyl-2,5,-di(t-butyl peroxy) hexane works well as the high temperature peroxide catalyst. Component (D) of the composition of this invention comprises from 5 to 200, preferably from 10 to 100, and most preferably from 30 to 70, parts based on (A) of a reinforcing filler. A reinforcing filler is needed in the composition if the composition is to have high tear strength. Examples of reinforcing fillers that can be used include, for instance, fumed silica and precipitated silica, with fumed silica being preferred. The fumed silica or preciptitated silica can be treated with various agents so as to prevent the uncured composition from structuring, for example, cyclopolysiloxanes as disclosed in U.S. Patent No. 2,938,009 to Lucas and silazanes as disclosed in U.S. Patent No. 3,535,743 to Smith. Extending fillers can be used in combination with the reinforcing fillers, treated or untreated, to get the proper balance in final physical properties in the silicone elastomer. These include, for example, titanium dioxide, lithopone, zinc oxide, zirconium silicate, silica aerogel, iron oxide, diatomaceous earth, calcium carbonate, glass fibers, magnesium oxide, chromic oxide, zirconium oxide, aluminum oxide, alpha quartz, calcined clay, carbon, graphite, cotton, synthetic fibers, aluminum silicate, and calcium silicate. Component (E) is an organohydrogenpolysiloxane present in an amount ranging from 0.1 to 10.0, preferably from 0.5 to 8.0, and most preferably from 0.8 to 5.0, percent by weight and having a viscosity ranging from 5 to 500, preferably from 10 to 100, and most preferably from 10 to 50, mPa.s (centipoises) at 25°C. Preferably, the hydride polysiloxane is a linear diorganopolysiloxane polymer having the formula wherein R 6 is hydrogen or a C 1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, p and q vary such that the viscosity of the organohydrogenpolysiloxane varies from 10 to 100 mPa.s (centipoise) at 25°C and the hydrogen content of the polymer varies from 0.05 to 2.0 percent by weight. The linear hydride described above can be made by many procedures which are known in the art, and particularly by the hydrolysis of the appropriate chlorosilanes. For more information as to the production of such hydrides, reference is made, for example, to U.S. Patent No. 4,041,010, Another type of hydride cross-linking agent that can be used in this invention is a hydride resin containing monovalent HR 7 / 2SiO 1/2 and tetravalent SiO 2 units wherein R 7 is a C 1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, the R 7 +H to Si ratio varies from 1.0 to 2.7, the hydrogen content of the resin varies from 0.05 to 2.0 weight percent. It should be noted that the R 7 radical can be any of the radicals previously given for the R 6 radical in formula (3) above. The resin of formula (4) can be made by hydrolysis of the appropriate chlorosilanes and purification thereof. Another type of hydride resin is one having difunctional units. This resin contains HR 7 SiO 1/2 monofunctional units, SiO 2 tetrafunctional units, and R 7 / 2SiO 2/2 difunctional units, wherein the R 7 +H to Si ratio varies from 1.2 to 2.7 and R 7 is as defined above, and the hydrogen concentration varies from 0.05 to 2.0 percent by weight. The hydride resin containing difunctional siloxy units can be produced by methods well known in the art as described in U.S. Patent No. 4,041,010. Component (E) is present in the composition of the present invention in an amount ranging from 0.1 to 10, preferably from 0.5 to 8, and most preferably from 0.8 to 5, parts by weight based on (A). The composition may further comprise from about 0.5 to 20, preferably from 1 to 10, and most preferably from 1 to 4, parts by weight based on (A) of (F) magnesium oxide, which is used to impart oil resistance to the elastomer. It should be noted that, since magnesium oxide is a known filler, if added in amounts greater than 10 parts it will act more like a filler than an oil resistant additive. The composition of this invention may further contain 0.01 to 20, preferably from 0.5 to 5, and most preferably from 0.5 to 2, parts by weight of (G) a cerium-containing compound. The cerium-containing compound acts tends to reduce the compression set of the elastomer. Even though the use of a dual peroxide catalytic system imparts low compression set, the use of magnesium oxide will adversely affect this and cause an increase in compression set. It has been found, however, that the use of magnesium oxide may be offset by using a cerium containing compound. It is theorized that cerium will complex with the silicone polymer, the filler, or both to improve the compression set of the silicone elastomer. What has been found to be particularly useful in this regard has been cerium hydroxide (Ce(OH) 4 ). In order to work, however, it is necessary for the cerium hydroxide to undergo a heating step in order to drive off water and allow the cerium to complex with the silicone. There is no limit to the amount of cerium hydroxide that may be added to lower compression set. An excess will not negatively affect compression set values. It should be noted, however, that if added in amounts greater than 10 parts the cerium hydroxide will act more like a filler than a low compression set additive. An alternative to using cerium hydroxide involves the use of tetramethyldivinyldisilazane to impart low compression set. In order to impart low compression set, there should be added from 0.01 to 10 parts by weight tetramethyldivinyldisilazane per 100 parts components (A) and (B). Preferably, there is added from 0.01 to 1 part by weight tetramethyldivinyldisilazane per 100 parts components (A) and (B). In order to demonstrate various features of this invention, the following examples are submitted. They are for illustrative purposes and are not intended to limit in any way the scope of this invention. EXPERIMENTAL In the tables below, the term "Polymer/Resin #1" refers to a resin/polymer blend containing 75 parts by weight of a vinyl-terminated polydimethylsiloxane polymer having a viscosity of 85,000 mPa.s (centipoises) at 25°C. and a vinyl content of 0.055 weight percent, and 25 parts by weight of a vinyl-containing "MQ" resin ("M" refers to monofunctional (CH 3 ) 3 SiO 1/2 units and "Q" refers to tetrafunctional SiO 4/2 units). The term "Polymer/Resin #2" refers to a resin/polymer blend containing 75 parts by weight of vinyldimethyl-stopped polydimethylsiloxane having a viscosity of 4000 mPa.s (centipoises) at 25°C and a vinyl content of 0.15 weight percent, and 25 parts by weight of a vinyl-containing MQ resin. The term "LVVP" refers to a low viscosity vinyl-stopped dimethyl polymers of the type used in Resin/Polymer #1 and Resin/Polymer #2. It is to be understood that the polymer represented by the term "LVVP" is a separate and distinct polymer from those used in Resin/Polymer #1 and #2. The term "HVVP" refers to a high viscosity vinyl-terminated dimethylpolysiloxane gum having a viscosity of 30,000,000 mPa.s (centipoises) at 25°C. The catalyst used in the examples was 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane. The SiH-crosslinker was a hydride linear polysiloxane having hydride terminal groups and hydride on-chain groups and a viscosity of 50 mPa.s (centipoise) at 25°C. The filler used in the examples below was fumed silica treated with octamethylcyclotetrasiloxane (D 4 ). The term "Resin" refers to a vinyl-containing MQ resin of the type used in Resin/Polymer #1 and Resin/Polymer #2. It is to be understood that the resin represented by the term "Resin" is a separate and distinct resin from those used in Resin/Polymer #1 and #2. Examples 1-5 Five samples were prepared having the formulations set forth in Table 1 below. The compositions were prepared by mixing the ingredients and then molding the mixtures for 10 minutes at 177°C (350°F). All parts are by weight. Examples 1-5: Composition Example No. 1 2 3 4 5 Resin/ Polymer #1 20 0 0 0 0 Resin/ Polymer #2 0 20 0 0 0 HVVP 80 80 80 80 95 LVVP 0 0 15 20 0 Resin 0 0 5 0 5 Catalyst 0.4 0.4 0.4 0.4 0.4 SiH Crosslinker 1.8 1.8 1.8 1.8 1.8 Filler 26 26 26 26 26 The tensile strength ("tensile"), modulus ("100% Mod.), elongation, hardness, specific gravity ("SQ"), tear die B, and compression set properties of the samples prepared in Examples 1-5 were measured and the results presented in Table 2 below. Examples 1-5: Properties Example No. 1 2 3 4 5 Tensile (psi) (1389) (1250) (1350) (1300) (1297) KPa 9584 8625 9315 8970 8949 100% Mod. 143 150 120 85 92 Elongation (%) 856 730 860 900 945 Hardness, Durometer 44 46 40 35 35 SQ 1.14 1.14 1.14 1.13 1.14 Die B, Tear Strength (pi) (280) (270) (260) (130) (130) KNm -1 49 47 46 23 23 Compression Set(%) 13% 13% 13% 18% 22% The data presented in Table 2 above indicates that tear die B is significantly higher and the compression set is lower for the samples containing the combination of resin, high viscosity vinyl polymer, and low viscosity vinyl polymer (Examples 1-3) than for samples containing the polymer blend alone and no resin (Example 4) or samples containing resin and high viscosity vinyl polymer but no low viscosity vinyl polymer (Example 5). Thus, it can be seen from the data summarized in Table 2 above that the combination of the resinous copolymer and the high viscosity and low viscosity vinyl polymers improves tear die B and compression set properties without sacrificing other beneficial physical properties. Examples 6-10 Five samples having the compositions set forth in Table 3 below were prepared according to the method used in Examples 1-5 above. The "silanol-stopped fluid" referred to in Table 3 below is a silanol-stopped polydiorganosiloxane fluid having a viscosity of between 25-40 centistokes at 25°C and a silanol content of between approximately 5.5% to 14.1%. All parts are by weight. Examples 6-10: Composition Example No. Ingredient 6 7 8 9 10 HVVP 80.0 80.0 80.0 80.0 70.0 Resin/Polymer #1 15.0 15.0 15.0 15.0 22.5 Fumed silica (D 4 ) 26.0 26.0 35.0 26.0 40.0 Si-H crosslinker 1.2 1.2 1.2 1.8 1.2 tetramethyldivinylsilazane 0.1 0.1 0.5 0.1 3.0 silanol-stopped fluid 2.0 2.0 2.0 3.0 2.0 Ce(OH) 4 1.0 1.0 1.0 1.0 1.0 2,5 dimethyl-2,5-bis(t-butylperoxy)hexane 0.4 0.4 0.4 0.4 0.4 MgO 2.5 2.5 2.5 2.5 2.5 Extending filler 0.0 6.0 6.0 6.0 0.0 The samples prepared in Examples 6-10 were molded at 177°C (350°F) for 10 minutes. A second sample of Example 7 was prepared and post-baked for 4 hours at 204°C (400°F). This sample is represented as Example 11 in Table 4 below. Table 4 summarizes the physical properties of the samples prepared in Examples 6-11. Examples 6-11: Physical Properties Example No. Property 6 7 8 9 10 11 Compression set, % 8.0 13.0 10.2 12.0 15.0 10.2 Tear Die B,(pi) (300.0) (305.0) (312.0) (315.0) (320.0) (315) KNm -1 53 54 55 55 56 55 Tensile,(psi) (1200.0) (1170.0) (1390.0) (1350.0) (1380.0) (1340) KPa 8280 8073 9591 9315 9522 9246 Shore A 42.0 41.0 56.0 45.0 61.0 47 Elongation 850.0 860.0 750.0 840.0 650.0 806 Modulus,(psi) (116.0) (120.0) (195.0) (140.0) (170.0) (147) KPa 800.4 828 1345.5 966 1173 1014.3 The data shown in Table 4 indicates that good compression set and tear die B properties are obtained using the combination of the resin and the high viscosity and low viscosity vinyl polymers, with tear strength not decreasing with post-bake. After curing the elastomers of Examples 6, 7, and 8, the elastomer of Examples 6 and 8 were immersed for 70 hours at 149°C (300°F) in #1 oil and #2 oil, and the elastomer of Example 7 was immersed for 168 hours at 149°C (300°F) in #1 oil and #3 oil. Tables 5-7 summarize the changes in physical properties for each of the examples. Changes in Physical Properties After Immersion in #1 Oil Example Test 6 7 8 Tensile (% change) -12 -11 -12 Elongation (% change) -9 -7 -5 Hardness (change) -5 -7 -6 % Volume Swell + 4 +5 + 4 Changes in Physical Properties After Immersion in #2 Oil Test Example 8 Tensile (% change) -19 Elongation (% change) -10 Hardness (change) - 9 % Volume Swell +10 Changes in Physical Properties After Immersion in #3 Oil Test Example 7 Tensile (% change) -58 Elongation (% change) -31 Hardness (change) -20 % Volume Swell +39%

Claims: A heat curable silicone composition comprising by weight: (A) 100 parts of a blend comprising by weight: (1) from 5 to 50% of a vinyl-containing diorganopolysiloxane or vinyl-containing diorganopolysiloxane blend, wherein the vinyl-containing diorganopolysiloxane or vinyl-containing diorganopolysiloxane blend has a viscosity of 4,000 to 500,000 mPa.s (centipoise) at 25°C; and (2) from 50 to 95% of a vinyl-containing diorganopolysiloxane or vinyl-containing diorganopolysiloxane blend, wherein the vinyl-containing diorganopolysiloxane or vinyl-containing diorganopolysiloxane blend has a viscosity of 8,000,000 to 100,000,000 mPa.s (centipoise) at 25°C; (B) from 1 to 30 parts based on component (A) of an organopolysiloxane resin copolymer comprising (1) R 3 SiO 1/2 units and SiO 2 units where R is selected from the group consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation with a ratio of R 3 SiO 1/2 units to SiO 2 units ranging from 0.5:1 to 1.5:1, wherein the copolymer contains from 0.5 to 10 weight percent vinyl groups; or (2) R 3 SiO 1/2 units, SiO 2 units, and R 2 SiO 2/2 units, where R is as defined above, where the ratio of R 3 SiO 1/2 units to SiO 2 units is from 0.5:1 to 1.5:1, and the R 2 SiO 2/2 units are present in an amount of from 1 to 70 mole percent based on the total number of moles of siloxy units in the copolymer, and where the resinous copolymer contains from 0.5 to 10.0 weight percent vinyl groups, or (3) mixtures of the units of (1) and (2); (C) from 0.1 to 5.0 parts based on component (A) of an organic peroxide free radical curing catalyst, (D) from 5.0 to 200 parts based on component (A) of a reinforcing filler; and (E) from 0.1 to 10 parts based on component (A), of an organohydrogensiloxane having a hydrogen content ranging from 0.05 to 2.0 percent by weight and having a viscosity ranging from 5 to 500 mPa.s (centipoise) at 25°C. The composition of claim 1 wherein component (A)(1) has a viscosity of from 40,000 to 500,000 mPa.s (centipoise) at 25°C. The composition of claim 2 wherein component (A)(1) has a viscosity of from 60,000 to 150,000 mPa.s (centipoise) at 25°C. The composition of claim 1 wherein component (A)(1) has the formula: wherein Vi is vinyl, R 1 is selected from the class consisting of vinyl, phenyl, alkyl radicals of 1 to 8 carbon atoms, and fluoroalkyl radicals of 3 to 10 carbon atoms and mixtures thereof, and R 2 is an alkyl radical of 1 to 8 carbon atoms or a fluoroalkyl radical of 3 to 10 carbon atoms or mixtures thereof, w and z vary such that the viscosity of (A)(1) ranges from 4,000 to 500,000 mPa.s (centipoise) at 25°C, the vinyl concentration being in the range of from 0.001 to 10 weight percent. The composition of claim 1 wherein component (A)(I) is present in an amount ranging from 10 to 40 parts by weight based on (A). The composition of claim 5 wherein component (A)(1) is present in an amount ranging from 15 to 25 parts by weight based on (A). The composition of claim I wherein component (A)(2) has a viscosity of from 20,000,000 to 35,000,000 mPa.s (centipoise) at 25°C. The composition of claim 7 wherein component (A)(2) has a viscosity of from 25,000,000 to 30,000,000 mPa.s (centipoise) at 25°C. The composition of claim 1 wherein component (A)(2) has the formula : wherein Vi is vinyl, R 4 is a C 1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, R 5 is a C 1-8 monovalent hydrocarbon radical, y and t vary such that the viscosity of component (A)(2) ranges from 8,000,000 to 100,000,000 mPa.s (centipoise) at 25°C and having a vinyl concentration in the range of from 0.001 to 0.1 weight percent. The composition of claim 1 wherein component (A)(2) is present in an amount ranging from 55 to 90 parts by weight based on (A). The composition of claim 10 wherein component (A)(2) is present in an amount ranging from 60 to 80 parts by weight based on (A). The composition of claim 1 wherein component (B) is present in an amount ranging from 1.0 to 20 parts by weight based on (A). The composition of claim 12 wherein component (B) is present in an amount ranging from 2.5 to 10 parts by weight based on (A). The composition of claim 1 wherein the peroxide catalyst is a combination of peroxide catalysts comprising a high temperature peroxide catalyst and a low temperature peroxide catalyst. The composition of claim 14 wherein the high temperature catalyst is 2,5-dimethyl-2,5-di(t-butyl peroxy) hexane and the low temperature catalyst is 1,1-bis(t-butyl peroxy)-3,3,5-trimethylcyclohexane. The composition of claim 1 wherein the organic peroxide catalyst is selected from the group consisting of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di-t-butylperoxide, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3. The composition of claim 16 wherein the organic peroxide catalyst is 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane. A composition according to claim 1 wherein the filler is selected from the group consisting of fumed silica and precipitated silica. The composition of claim 1 wherein the filler is present in an amount ranging from 10 to 100 parts by weight based on (A). The composition of claim 19 wherein the filler is present in an amount ranging from 30 to 70 parts by weight based on (A). The composition of claim 1 wherein the hydride polysiloxane is a linear diorganopolysiloxane polymer having the formula wherein R 6 is hydrogen or a C 1-8 monovalent hydrocarbon radical free of alipahtic unsaturation, p and q vary such that the viscosity of the organohydrogenpolysiloxane varies from 10 to 100 mPa.s (centipoise) at 25°C, and the hydrogen content of the polymer varies from 0.05 to 2.0 percent by weight. The composition according to claim 1 wherein the organohydrogenpolysiloxane is a hydride resin containing monovalent HR 7 / 2SiO 1/2 units and tetravalent SiO 2 units wherein R 7 is a C 1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, the R 7 +H to Si ratio varies from 1.0 to 2.7, and the hydride content of the resin varies from 0.05 to 2.0 weight percent. The composition of claim 1 wherein the hydride-containing polysiloxane is a hydride resin containing HR 7 / 2SiO 1/2 units, divalent (R 7 ) 2 SiO 2/2 units, and SiO 2 units, wherein R 7 is a C 1-8 monovalent hydrocarbon radical free of aliphatic unsaturation, the R 7 +H to Si ratio varies from 1.2 to 2.7, and the hydride content of the resin varies from 0.05 to 2.0 weight percent. The composition of claim 1 wherein component (E) is present in an amount ranging from 0.5 to 8 parts by weight based on (A). The composition of claim 24 wherein component (E) is present in an amount ranging from 0.8 to 5 parts by weight based on (A). A composition according to claim 1 further comprising by weight (F) from 0.5 to 20 parts by weight based on (A) of magnesium oxide. The composition of claim 26 wherein the magnesium oxide is present in an amount within the range of 1 to 10 parts by weight based on (A). A composition according to claim 1 further comprising by weight (G) from 0.01 to 20 parts by weight based on (A) of (G) a cerium compound or from 0.01 to 10 parts by weight of tetramethyldivinyldisilazane. The composition of claim 28 wherein the cerium compound is cerium hydroxide.

Inventors: JERAM EDWARD MATTHEW, WARD BRIAN JOHN, JERAM, EDWARD MATTHEW, WARD, BRIAN JOHN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1991
Main classification: C08L83
IPC codes: C08K5, C08L83