Heat curing silicone rubber composition

Abstract: A silicone rubber composition is disclosed having improved mold release properties comprising (A) silicone rubber base compound, (B) organopolysiloxane having no aliphatic unsaturated groups, (C) higher fatty acid or higher fatty acid metal salt, (D) water, and (E) curing agent.

Description: The present invention concerns a heat curing silicone rubber composition which can be used as a wire harness sealer and connector. In more detail, the present invention is a heat curing silicone rubber composition having excellent mold release properties which is suitable for use in a metallic mold having an intricate structure. It is known that silicone rubber compositicns lack mold release properties. Therefore, the rubber tears when it is removed from the mold, and the mold becomes contaminated over an extended period of time. A method has been proposed as a means to solve this problem in which an internal mold release agent, such as stearic acid, zinc stearate, or calcium stearate is added to the silicone rubber composition JP-A 49-124151. However, even in silicone rubber compositions in which an internal mold release agent is added, the mold release properties are not sufficient for obtaining silicone rubber molded products having a protruding and intricate structure, such as a hollow curved structure. The existing silicone rubber composition of the art are not adequate, particularly for the molding in a single mold of multiple silicone rubber products having intricate structures. For example, with a mold designed to yield several hundred products per operating run (when molding a waterproof seal packing for a wire harness connector), a tremendous amount of time is required to remove the product from the mold using existing silicone rubber compositions. Therefore, poor performance in production efficiency results in an increase in cost. Heat-curable organosiloxane compositions are disclosed in EP-A-0302802, EP-A-0590595, EP-A-0459464 and in EP-A-0440168. The described compositions exhibit improved physical characteristics, for example oil-resistance, or reduced mold staining and scorching. EP-A-065020 discloses heat-curing silicone rubber compositions with hitherto-known mold-release properties. US 5,066,714 discloses a curable organopolysiloxane putty-like composition with conventional mold-release properties. As a result of a search for a solution for the aforementioned problems, we have determined that a silicone rubber composition, to which water is added and then mixed, has excellent mold release properties JP-A 7-126592. We have now found that the aforementioned silicone rubber composition, when combined with a specific compound, has superior mold release properties as claimed herein. Specifically, the objective of our invention is to offer a silicone rubber composition having excellent mold release properties which greatly exceed the mold release properties of existing silicone rubber compositions. The present invention is a silicone rubber composition having improved mold release properties. The composition comprises (A) silicone rubber base compound, (B) organopolysiloxane having no aliphatic unsaturated groups, (C) higher fatty acid or higher fatty acid metal salt, (D) water, and (E) curing agent. The present invention introduces a heat curing silicone rubber composition comprising: (A) 100 parts by weight of a silicone rubber base compound comprising (a) 100 parts by weight of a first organopolysiloxane described by average unit formula R a SiO (4-a)/2 where R is a substituted or unsubstituted monovalent hydrocarbon group and a is a value of 1.95 to 2.05 and (b) 10 to 100 parts by weight of reinforcing filler, (B) 1 to 20 parts by weight of a second organopolysiloxane described by formula R 1 3 SiO(R 2 2 SiO) n SiR 1 3 where R 1 and R 2 are identical or different substituted or unsubstituted monovalent hydrocarbon groups, not including aliphatic unsaturated groups, and n is an integer with a value of 0 to 1000, (C) 0.025 to 5 parts by weight of a higher fatty acid or higher fatty acid metal salt, (D) 0.05 to 5 parts by weight of water, and (E) a curing agent in an amount sufficient for curing the composition. To further explain, the silicone rubber base compound comprising component (A) of the present invention is the main component of our silicone rubber composition. The first organopolysiloxane (component (a)) which is used in our silicone rubber base contains substituents R, where R can be a substituted or unsubstituted monovalent hydrocarbon group. R is, for example, alkyl groups such as methyl, ethyl, and propyl; alkenyl groups, such as vinyl and allyl; cycloalkyl groups such as cyclohexyl; aralkyl groups such as p-phenylethyl; aryl groups such as phenyl; and alkyl halide groups such as chloromethyl, 3-chloropropyl, and 3,3,3-trichloropropyl. For the first organopolysiloxane, a is 1.95 to 2.05. The molecular weight of the first organopolysiloxane is not particularly limited and can be those used in the industry for such applications. Generally, those organopolysiloxanes having a viscosity above 107 mPa·s at 25°C or greater and a number average molecular weight of 25 x 10 4 or greater are ideal for our use. Examples of component (a) include dimethylsiloxane/methyl vinylsiloxane copolymer with both chain ends blocked by trimethylsiloxy groups, dimethylsiloxane/methylvinylsiloxane copolymer with both chain ends blocked by dimethylvinylsiloxy groups, and dimethylpolysiloxane with both chain ends blocked by dimethylvinylsiloxy groups. The reinforcing filler, component (b), is used in existing silicone rubber base compounds, and examples include reinforcing silica and carbon black. Fumed silica and precipitated silica are examples of reinforcing powder silica. From these, fine powders having a particle diameter of 50 mµ or less and a relative surface area of 100 m 2 /g or greater is preferred. Also, preferred are the reinforcing silicas whose surface is pretreated with, for example, organosilane, organodisilazane, or diorganocyclopolysiloxane. Carbon black can also be used to provide the silicone rubber with conductivity, and examples include acetylene black, Ketjen Black, furnace black, and thermal black. The second organopolysiloxane (component (B)) of the present invention is a component for providing a silicone rubber molded product with lubricating ability. This component also provides mold release properties. In the second organopolysiloxane formula, R 1 and R 2 are substituted or unsubstituted monovalent hydrocarbon groups not having aliphatic unsaturated bonds. Specific examples of R 1 and R 2 include alkyl groups such as methyl, ethyl, and propyl; cycloalkyl groups such as cyclohexyl; aralkyl groups such as β -phenylethyl; aryl groups such as phenyl; and alkyl halide groups, such as chloromethyl, 3-chloropropyl, and 3,3,3-trifluoropropyl. In the present invention, it is preferred that R 2 is either an alkyl group or phenyl group, and at least 5 mole% of R 2 is a phenyl group, and more preferred is when at least 20 mole% of R 2 is a phenyl group. Examples of component (B) are dimethylpolysiloxane with both chain ends blocked by trimethylsiloxy groups, dimethylsiloxane/phenylmethylsiloxane copolymer with both chain ends blocked by trimethylsiloxy groups, and dimethylsiloxane/diphenylsiloxane copolymer with both chain ends blocked by trimethylsiloxy groups. The second organopolysiloxane, component (B), is directly added and mixed into the silicone rubber base compound comprising component (A). The mixing amount of component (B) is from 1 to 20 parts by weight per 100 parts by weight of component (A). This is because the adhesion of the silicone rubber composition increases and the roll-mill processibility decreases when the mixing amount exceeds 20 parts by weight of component (B). Conversely, the bleeding amount of the second organopolysiloxane is not sufficient after molding, and it is difficult to obtain the lubricating ability when it is less than 1 part by weight. The higher fatty acid or higher fatty acid metal salt, component (C), used in the present invention is a necessary component for providing the mold release properties. Its effect is further improved by a synergetic effect through a combined use with water as component (D). Specific examples of the higher fatty acids are lauric acid, stearic acid, palmitic acid, oleic acid, and arachic acid. Examples of the higher fatty acid metal salts are magnesium stearate, calcium stearate, zinc stearate, cobalt stearate, aluminum stearate, barium stearate, zinc laurate, barium laurate, magnesium laurate, calcium laurate, zinc oleate, magnesium oleate, and manganese oleate. The mixing amount of component (C) added to our composition is within a range of 0.025 to 5 parts by weight of the silicone rubber base compound (component (A)). Properties of the silicone rubber (particularly heat resistance and compression set) are lowered when the amount of component (C) is too high, and desired mold release properties cannot be obtained when it is too low. Water, component (D), used in the present invention is considered necessary for providing mold release properties. This component greatly improves the mold release properties in combination with the aforementioned component (B) and component (C). The mixing amount of component (D) is 0.05 to 5 parts by weight per 100 parts by weight of component (A). This is because voids are easily formed within the silicone rubber molded product when the mixing amount exceeds 5 parts by weight, and a significant improvement of the mold release properties is not obtained when it is less than 0.05 part by weight. The curing agent, component (E), of the present invention cures our composition when heated into a silicone rubber. Organic peroxides are generally used as the curing agent. Also, when the organopolysiloxane of component (A) includes alkenyl groups, it is possible to use an organohydrogenpolysiloxane in which there is silicon-hydrogen bonding, as a crosslinker, in combination with a platinum catalyst to effect curing of the composition. The mixing amount of the curing agent is an amount sufficient for curing the composition of the present invention. Examples of useful organic peroxide curing agents include dicumyl peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-di(benzoylperoxy), 2,5-dimethyl-2,5-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, and benzoyl peroxide. The mixing amount of the organic peroxide is generally within the range of 0.1 to 10 parts by weight per 100 parts by weight of component (A). Examples of the organohydrogenpolysiloxane crosslinker include methylhydrogenpolysiloxane with both chain ends blocked by trimethylsiloxy groups, dimethylsiloxane/methylhydrogensiloxane copolymer with both chain ends blocked by trimethylsiloxy groups, and dimethylsiloxane/methylhydrogensiloxane copolymer with both chain ends blocked by dimethylhydrogensiloxy groups. Examples of useful platinum catalysts are chloroplatinic acid, an alcohol solution of chloroplatinic acid, a complex of chloroplatinic acid and divinyltetramethylsiloxane, platinum black, and a thermoplastic resin encapsulated catalyst including a platinum compound. The mixing amount of the organohydrogenpolysiloxane is an amount such that the ratio between the number of moles of silicon-hydrogen bonds included within the organohydrogenpolysiloxane and the number of moles of alkenyl groups within component (A) is established at 0.5:1 to 20:1. The mixing amount of the platinum catalyst is within the range of 0.1 to 500 parts by weight of platinum per 1,000,000 parts by weight of organopolysiloxane present in the claimed invention. The silicone rubber composition of our invention is made of the aforementioned components (A) through (E). However, it is acceptable to add and mix conventional additives, that have been used in conventional silicone rubber compositions, including inorganic fillers such as diatomaceous earth, quartz, calcium carbonate, mica, aluminum oxide, aluminum hydroxide, zinc oxide, magnesium oxide, and titanium oxide and heat resistant agents such as rare-earth oxides, rare-earth hydroxides, cerium silanolate; and cerium fatty acid salts, as long as the purpose of the present invention is not lost. The silicone rubber composition of this invention is easily obtained by simply mixing components (A) through (E). Particularly, it is desirable to mix 100 parts by weight of a first organopolysiloxane as component (a) into 10 to 100 parts by weight of reinforcing filler as component (b) to form a silicone rubber base compound designated as our component (A). Then it is desirable to mix sequentially into each 100 parts of component (A): 1 to 20 parts by weight of a second organopolysiloxane as component (B), 0.025 to 5 parts by weight of a higher fatty acid or a higher fatty acid metal salt as component (C), 0.05 to 5 parts by weight of water as component (D) and 0.1 to 10 parts by weight of a curing agent as component (E). In some cases, it is difficult to mix water as component (D) into the silicone rubber composition. In such cases, a mixture of water and silica is prepared beforehand, and this is preferrably added and mixed into a mixture of components (A) through (C). The mixing ratio of water and silica is desirably 1 to 100 parts by weight of silica per 100 parts by weight of water, and it is most preferable at 5 to 20 parts by weight of silica per 100 parts of water. The silicone rubber composition of the present invention has excellent mold release properties. Therefore, our silicone rubber molded product does not chip or tear during removal from the mold, even when it is applied to a metallic mold having an intricate structure. Also, the productivity of the molding process increases greatly due to easy removal of our moldings from the mold. Accordingly, the present composition is very useful as a silicone rubber composition for press molding, transfer molding, and injection molding, for example, in those applications where metallic molds having an intricate structure are used. The present composition is particularly effective for molding a connector and sealer, such as a wire harness sealer and connector. The present invention is further explained by reference to the following examples. In the examples, parts refer to parts by weight, and the viscosity indicates a value measured at 25°C. The mold release properties in the examples are evaluated in the manner described below. Figure 1 is a schematic cross-sectional diagram of a metallic mold that is used to evaluate the mold release properties in the examples of the present invention. Figure 2 is an enlarged schematic cross-sectional diagram of the projecting parts of the middle of the metallic mold in Figure 1. Figure 3 is a schematic diagram of a metallic mold which is used to evaluate the mold release properties in the examples of this invention. In the Figures, element 1 is the upper mold, element 2 is the middle mold, element 3 is a cavity, and element 4 is the lower mold. Evaluation Method 1. A silicone rubber composition was pressure molded using a metallic mold having projections (the number of projections was 77) as illustrated in Figure 1. The silicone rubber molded product obtained was pulled out by force after molding, and the number of projections tearing from the molded product and remaining in the mold were counted. The number of projections remaining in the mold are reported as a ratio to the total number of projections (i.e. 77) in Tables I, II, and III under the heading "Defect Ratio". Evaluation Method 2. A silicone rubber composition was pressure molded using a metallic mold as illustrated in Figure 3. The upper and lower metallic mold were removed after molding, and the molded product within the middle mold was removed by extrusion using a cylindrical shaped (diameter of 5 mm) metallic bar. The load required for this removal from the mold was read using a digital measuring device and is shown as the mold removing load (kg) in Tables I, II, III under the heading "Removing Load". Application Example 1. 100 parts of an organopolysiloxane with both chains ends blocked by dimethylvinylsiloxy groups (degree of polymerization of 5,000) consisting of 99.85 mole% dimethylsiloxy units and 0.15 mole% methylvinylsiloxy units, 5.0 parts of a dimethylsiloxane with both chain ends blocked by silanol groups having a viscosity of 60 mPa . s, and 40 parts by weight of wet process silica with a relative surface area of 130 m 2 /g were placed into a mixer and mixed with heating to prepare a silicone rubber base compound (denoted in Table I as "base". After cooling, 8 parts of dimethyl siloxane/phenylmethylsiloxane copolymer with both chain ends blocked by trimethylsiloxy groups (the copolymer comprised 50 mole% phenylmethylsiloxy units and had a viscosity of 128 mPa . s) and 0.2 part of zinc stearate were mixed into 100 parts of this silicone rubber base compound using a two-roll mill. Water in the amount indicated in Table I was added and mixed into the composition. Here, water was added and mixed under conditions in which 10 parts of dry process silica with a relative surface area of 200 m 2 /g were added in paste form into 90 parts of water. Then, 0.4 part of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane was added and mixed to prepare a heat curing silicone rubber composition. The mold release properties of these heat curing silicone rubber compositions were measured, and the results are shown in Table I. For comparison, a heat setting silicone rubber composition was prepared in the same manner as above, except that water was not added (Comparative Example 1). Also, a heat setting silicone rubber composition was prepared in the same manner as above, except that the dimethylsiloxane/phenylmethylsiloxane copolymer was not added (Comparative Example 2). Furthermore, another heat setting silicone rubber composition was prepared in the same manner as above, except that zinc stearate was not added (Comparative Example 3). The mold release properties of these compositions were measured, and the results are also shown in Table I. Application Example 2. A heat setting silicone rubber composition was prepared in the same manner as in Application Example 1, except that the amount of the dimethylsiloxane, with both chain ends blocked by silanol groups as described in Application Example 1, was 10.0 parts, and 40 parts of dry process silica with a relative surface area of 200 m 2 /g were mixed instead of 40 parts of wet process silica. The mold release properties of this composition were measured, and the results are shown in Table II. For comparison, a heat setting silicone rubber composition was prepared in the same manner as above, except that water was not added and mixed. The mold release properties of this composition were measured, and the results are also shown in Table II. Application Example 3. A silicone rubber base compound was prepared by mixing 50 parts of the silicone rubber base compound obtained in Application Example 1 into 50 parts of the silicone rubber base compound obtained in Application Example 2 on a two-roll mill. 8 parts of the dimethylsiloxane/phenylmethylsiloxane copolymer with both chain ends blocked by trimethylsiloxy groups (the copolymer comprised 50 mole% phenylmethylsiloxy units, and the viscosity was 128 mPa . s) and 0.2 part of zinc stearate were mixed into 100 parts of this silicone rubber compound on a two-roll mill. Water was added and mixed into this composition in the same manner as in Application Example 1. Then, 0.4 part of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane was added and mixed to prepare a heat cured silicone rubber composition. The mold release properties of these compositions were measured, and the results are shown in Table III.

Claims: A heat curing silicone rubber composition which comprises: (A) 100 parts by weight of a silicone rubber base compound comprising (a) 100 parts by weight of a first organopolysiloxane described by average unit formula R a SiO (4-a)/2 where R is a substituted or unsubstituted monovalent hydrocarbon group and a is a value of 1.95 to 2.05 and (b) from 10 to 100 parts by weight of reinforcing filler, (B) from 1 to 20 parts by weight of a second organopolysiloxane described by formula R 1 3 SiO(R 2 2 SiO) n SiR 1 3 , where R 1 and R 2 are the same or different substituted or unsubstituted monovalent hydrocarbon groups not containing aliphatic unsaturated bonds, and n is an integer with a value of 0 to 1,000, (C) from 0.025 to 5 parts by weight of a higher fatty acid or higher fatty acid metal salt where said fatty acid contains at least 12 carbon atoms, (D) from 0.05 to 5 parts by weight of water, and (E) a curing agent in an amount sufficient for curing the composition selected from organic peroxides or, when (A) includes alkenyl groups, organohydrogensiloxanes in combination with a platinum catalyst. A heat curing silicone rubber composition according to Claim 1, where the first organopolysiloxane has a viscosity above 107 mPa.s at 25°C. A heat curing silicone rubber composition according to Claim 1 or 2, where the reinforcing silica filler is a silica having a particle diameter of 50 µm or less and a relative surface area of 100 m 2 /g or greater. A heat curing silicone rubber composition according to Claim 3, where the reinforcing silica filler is pretreated with a treating agent selected from organosilane, organodisilazane, and diorganocyclopolysiloxane. A heat curing silicone rubber composition according to any of Claims 1 to 4, where R 1 is an alkyl group and R 2 is an alkyl group or a phenyl group with the proviso that at least 5 - 20 mole percent of R 2 is a phenyl group. A heat curing silicone rubber composition according to any of Claims 1 to 5, where component (C) is zinc stearate. A heat curing silicone rubber composition according to any of Claims 1 to 6, where the curing agent is 2,5-dimethyl-2,5-di(t-butylperoxy)hexane. A heat curing silicone rubber composition according to Claim 7 comprising 0.1 to 10 parts by weight of the curing agent per 100 parts by weight of component (A). A heat curing silicone rubber composition according to any of Claims 1 to 6, where the curing agent is an organohydrogensiloxane in combination with a platinum catalyst. A heat curing silicone rubber composition according to Claim 9, in which the platinum catalyst is selected from chloroplatinic acid, an alcohol solution of chloroplatinic acid, a complex of chloroplatinic acid and divinyltetramethylsiloxane, platinum black, and a thermoplastic resin encapsulated catalyst comprising a platinum compound.

Inventors: HONMA HIROSHI, MATSUSHITA TAKAO, OTA KENJI, HONMA, HIROSHI, MATSUSHITA, TAKAO, OTA, KENJI, Honma, Hiroshi, Dow Corning Toray, Matsushita, Takao, Dow Corning Toray, Ota, Kenji, Dow Corning Toray
Applicants: DOW CORNING TORAY SILICONE, DOW CORNING TORAY SILICONE COMPANY LTD.
Application year: 1996
Main classification: C08L83
IPC codes: C08K5, C08L83, C08K3