Mother mold-forming silicone rubber composition and mother mold obtainable therefrom

Abstract: In a silicone rubber composition comprising an organopolysiloxane as a base polymer, 0.1-50 parts by weight of a modified silicone fluid having a melting point of up to 100°C and a refractive index difference of 0.01-0.05 from the base polymer is blended per 100 parts by weight of the composition. The composition is cast and cured to form a mother mold which maintains parting properties over a long term.

Description: This invention relates to a silicone rubber composition for forming mother molds (master negatives) and a mother mold. A process of producing a plurality of parts by using a silicone rubber mold as a master negative, casting a resin material in the mold, and curing the material therein has been widely employed in the industry covering from the manufacture of hobby miniature models to the manufacture of industrial trial models. The silicone rubber used for that purpose has the advantage that a master negative can be formed with ease because it is liquid prior to curing and can be cured simply by mixing with a curing agent and holding at room temperature or heating. Nevertheless, the resin cast and molded in the silicone rubber mold can tightly stick to the mold and is difficult to remove from the mold because the mold is of rubber nature. This problem is commonly solved by using a parting agent (releasing agent). The application of a parting agent is a consideration when it is desired to reduce the process time. One solution is disclosed in JP-A 7-118534 wherein a crosslinkable base polymer is compounded with a non-crosslinkable silicone polymer having a greater chain length than the base polymer, achieving an improvement in parting properties (releasing properties). However, this approach still suffers from several problems. Since the silicone fluid added as the parting agent has a greater viscosity than the base polymer, the composition has a higher viscosity than ordinary compositions. Some blending methods can restrain the migration of the non-reactive silicone fluid (parting agent) to the surface. Only a small amount of the parting agent migrates to the surface at the start of molding. JP-A-7 316 517 discloses an electron beam curable release agent for an adhesive tape or label release paper based on a (meth)acryloylalkyl-group-containing polysiloxane and a polydialkylsiloxane. Preferred embodiments of the invention may provide a silicone rubber composition which forms mother molds exhibiting good parting properties from the initial and has a low viscosity, and a mother mold manufactured therefrom. It has been found that when a suitable organopolysiloxane base polymer from which a silicone rubber mother mold is to be formed is compounded with a non-crosslinkable oily substance having a refractive index difference of at least 0.01 from the base polymer and a melting point of up to 20°C, the substance migrates to the mold surface, which eliminates a need for parting agent or significantly reduces the number of applications of parting agent. The initial migration of the substance to the mold surface is improved. Since the oily substance need not have a greater molecular weight than the base polymer, the composition can have a low viscosity. Accordingly the invention provides a silicone rubber composition for forming mother molds comprising an organopolysiloxane (defined below) as a base polymer wherein up to 50 parts by weight of an oily substance (defined below) is blended per 100 parts by weight of the composition, together with a specified curing/crosslinking agent and a specified catalyst, also defined below. Also provided is a mother mold obtained by casting and curing the composition. The base polymer of the silicone rubber composition for forming mother molds according to the invention is a linear dimethylpolysiloxane of the following average compositional formula (1) containing on the average at least two alkenyl radicals in a molecule. R 1 a SiO (4-a)/2 Herein R 1 is a methyl, vinyl or allyl radical, 0.01 to 10 mol% of R 1 being alkenyl (vinyl or allyl) radicals, and "a" is a positive number of 1.9 to 2.4. The viscosity is 500 to 1,000,000 mPa·s at 25°C. Preferably 0.02 to 5 mol% of the entire R 1 radicals are alkenyl radicals. If the content of alkenyl radicals is too low, the resulting composition may become less curable. If the content of alkenyl radicals is too high, cured products may have poor physical properties including tensile strength, tear strength and elongation. The alkenyl radical may attach to either a silicon atom at the end of the molecular chain or a silicon atom intermediate of the molecular chain or both. The preferred alkenyl radical is vinyl. In formula (1), "a" is a positive number of 1.9 to 2.4, and preferably 1.95 to 2.05. Also preferably there are alkenyl radicals at both ends of the molecular chain. Further preferably, the organopolysiloxane is end-blocked with triorganosilyl radicals such as trivinylsilyl, methyldivinylsilyl, dimethylvinylsilyl or trimethylsilyl. The organopolysiloxane has a wide choice of molecular weight. It has a viscosity of 500 to 1,000,000 mPa·s (centipoise), and especially 1,000 to 100,000 mPa·s (centipoise) at 25°C because it will cure into a rubbery elastomer and gives a liquid silicone rubber composition. Illustrative examples of the alkenyl-containing organopolysiloxane are diorganopolysiloxanes shown by the following general formulae. Herein, R is methyl, n is a positive number inclusive of 0, m is such a positive number that the sum of CH=CH 2 radicals in the molecule may be two or more, and n or n+m is a number that satisfies the above viscosity range. An inorganic filler may be blended in the silicone rubber composition of the invention. Typical inorganic fillers are silica fillers in microparticulate form including dry silicas such as fumed silica and wet silicas such as precipitated silica. The filler is used in order to impart a sufficient strength to the cured composition, that is, as a reinforcing filler. Preferably the filler has a specific surface area of at least 10 m 2 /g, more preferably 50 to 500 m 2 /g and most preferably 150 to 350 m 2 /g, as measured by the BET method. Commercially available silicas may be used as the filler. Of the dry silicas, hydrophilic silica is available under the trade name of Aerosil 200 and 300, and hydrophobic silica is available under the trade name of R972, R974 and R976 which are silicas whose surface having methyl, ethyl or vinyl radicals is treated with organosilane or organosilazane compounds having the above radicals (i.e., methyl, ethyl or vinyl radicals) as a substituent, all from Nippon Aerosil K.K. An example of wet silica is Nipsil LP from Nippon Silica K.K. From the reinforcement standpoint, an appropriate amount of the silica filler blended is about 5 to 60 parts, more preferably about 10 to 40 parts by weight per 100 parts by weight of the alkenyl-containing organopolysiloxane. Besides the silica fillers, other fillers may be added for reinforcing or extending purposes. Such other fillers are ground silica or quartz flour such as crystalline silica (e.g., crystallite and Min-U-Sil®), fused silica, spherical silica such as silica prepared by the sol-gel method, metal oxides such as alumina, and carbon black. It will be appreciated that in mixing the organopolysiloxane with the filler, heat mixing of the components along with a silazane compound, silanol-containing organosilane or siloxane compound added as the additive is advantageous because of improvements in flow and parting properties. For the purpose of improving the physical properties (e.g., tensile strength and tear strength) of cured products, it is also effective to use in the composition an organopolysiloxane resin of a three-dimensional network structure which consists essentially of R 1 SiO 3/2 units and/or SiO 4/2 units and optionally, at least one of R 1 3 SiO 1/2 units and R 1 2 SiO 2/2 units wherein R 1 is as defined above, contains alkenyl and/or silanol radicals, and is soluble in toluene. An appropriate amount of the organopolysiloxane resin blended is 0 to about 200 parts, more preferably about 5 to 100 parts by weight per 100 parts by weight of the alkenyl-containing organopolysiloxane as the base polymer. In the composition of the invention, an organohydrogenpolysiloxane is blended as a curing or crosslinking agent for the alkenyl-containing organopolysiloxane (base polymer). This organohydrogenpolysiloxane serves as a crosslinking agent such that hydrosilylation addition reaction takes place between alkenyl radicals attached to silicon atoms in the base polymer and hydrogen atoms attached to silicon atoms (that is, SiH radicals) in the organohydrogenpolysiloxane in the presence of a catalyst, thereby accomplishing crosslinking and curing. The organohydrogenpolysiloxane has at least two, more preferably at least three hydrogen atoms each attached to a silicon atom (that is, SiH radicals) in a molecule and is represented by the following average compositional formula (2): R 2 b H c SiO (4-b-c)/2 wherein R 2 is a substituted or unsubstituted monovalent hydrocarbon radical of 1 to 10 carbon atoms, b is a positive number of 0.7 to 2.1, c is a positive number of 0.001 to 1.0, and the sum of b and c is from 0.8 to 3.0. Further preferably it is liquid at room temperature (25°C). Herein, R 2 is a substituted or unsubstituted monovalent hydrocarbon radical of 1 to 10 carbon atoms, and especially 1 to 8 carbon atoms. Preferred radicals for R 2 are those free of aliphatic unsaturation, specifically, alkyl, aryl, aralkyl, and substituted alkyl radicals, with methyl, ethyl, propyl, phenyl and 3,3,3-trifluoropropyl being especially preferred. The letter b is a positive number of 0.7 to 2.1, preferably 1 to 2, c is a positive number of 0.001 to 1.0, preferably 0.01 to 1.0, and b+c is from 0.8 to 3.0, preferably from 1 to 2.4. The molecular structure may be linear, cyclic, branched or three-dimensional network. The SiH radical may attach at the end or intermediate of a molecular chain or both. The molecular weight is not critical. It should have a viscosity of 1 to 1,000 mPa·s, and preferably 1 to 500 mPa·s, at 25°C. Illustrative examples of the organohydrogenpolysiloxane include 1,1,3,3-tetramethyldisiloxane, methylhydrogen cyclic polysiloxane, methylhydrogensiloxane-dimethylsiloxane cyclic copolymers, both end trimethylsiloxy-blocked methylhydrogenpolysiloxane, both end trimethylsiloxy-blocked dimethylsiloxane-methylhydrogensiloxane copolymers, both end dimethylhydrogen-siloxy-blocked dimethylpolysiloxane, both end dimethyl-hydrogensiloxy-blocked dimethylsiloxane-methylhydrogensiloxane copolymers, both end trimethylsiloxy-blocked methylhydrogensiloxane-diphenylsiloxane copolymers, both end trimethylsiloxy-blocked methylhydrogensiloxane-diphenylsiloxane-dimethylsiloxane copolymers, copolymers of (CH 3 ) 2 HSiO 1/2 units and SiO 4/2 units, copolymers of (CH 3 ) 2 HSiO 1/2 units, (CH 3 ) 3 SiO 1/2 units and SiO 4/2 units, and copolymers of (CH 3 ) 2 HSiO 1/2 units, SiO 4/2 units and (C 6 H 5 ) 3 SiO 1/2 units. An appropriate amount of the organohydrogenpolysiloxane blended is about 0.1 to 200 parts, and especially about 0.3 to 50 parts by weight per 100 parts by weight of the alkenyl-containing organopolysiloxane (base polymer). Differently stated, the organohydrogenpolysiloxane is used in such amounts that the ratio of the number of silicon-bonded hydrogen atoms (i.e., SiH radicals) in the organohydrogenpolysiloxane to the number of silicon-bonded alkenyl radicals in the base polymer may range from 0.4:1 to 4:1, and more preferably from 0.8:1 to 3:1. Also blended in the composition of the invention is an addition reaction catalyst selected from platinum group metals, and compounds and complexes thereof. Examples include platinum black, platinic chloride, chloroplatinic acid, reaction products of chloroplatinic acid with monohydric alcohols, complexes of chloroplatinic acid with olefins, platinum catalysts such as platinum bisacetoacetate, palladium catalysts such as tetrakis(triphenylphosphine)palladium and dichlorobis-(triphenylphosphine)palladium, and rhodium catalysts such as chlorotris(triphenylphosphine)rhodium and tetrakis-(triphenylphosphine)rhodium. The addition reaction catalyst is used in a catalytic amount selected from the range 0.1 to 1,000 ppm, and especially 1 to 200 ppm of platinum group metal based on the sum of the alkenyl-containing organopolysiloxane and organohydrogenpolysiloxane. Less than 0.1 ppm of platinum group metal often fails to promote curing of the composition whereas more than 1,000 ppm is uneconomical. In the composition of the invention, there may be compounded various other components commonly used in conventional compositions of this type. Such other components are, for example, cure retarders or addition reaction inhibitors as typified by acetylene alcohol, and silane and siloxane derivatives of acetylene alcohol and thixotropic agents such as polyethers. According to the invention, an oily substance is added to the silicone rubber composition comprising the above-described components. The oily substance used herein has a melting point of up to 100°C and has a refractive index at 25°C which differs by at least 0.01, preferably at least 0.015, and more preferably at least 0.02 from the refractive index at 25°C of the base polymer (that is, alkenyl-containing organopolysiloxane). The oily substance used herein should be semi-compatible with the base polymer rather than fully compatible with the base polymer. The degree of compatibility can be expressed in terms of the difference of refractive index between the oily substance and the base polymer. The oily substance fail to exert its effect if its refractive index difference from the base polymer is less than 0.01. The upper limit of the refractive index difference is preferably 0.05. If the refractive index difference is greater than 0.05, the oily substance is so poorly compatible that the outer appearance may become white turbid, rendering inefficient the operation of cutting out a master model. Namely, if the oily substance used lacks compatibility, a greater than necessity amount of the parting component migrates to the surface of a mold to exacerbate the surface state of resin molded therein, or the composition becomes too thixotropic, inviting the drawbacks of disturbed flow, inefficient deaeration and poor self-leveling during casting to form a rubber mold. The oily substance is a modified silicone fluids which is liquid at 20°C. The modified silicone fluids are silicone fluids, typically dimethylsilicone fluids (polydimethylsiloxane), which are modified by replacing some radicals bonded to silicon atoms by various substituents to produce polyether-modified, alkyl-modified (modified with long-chain alkyl of 6 to 30 carbon atoms, preferably about 12 to 20 carbon atoms), higher fatty acid ester-modified, fluorine-modified, and phenyl-modified silicone fluids. Specifically, phenyl-modified dimethylsilicone fluids are suitable since they are inexpensive and commercially used in large amounts. These modified silicone fluids are preferred because of ease of mixing and transparency of the resulting system. The modified silicone fluids should have a viscosity of 30 to 1,000 mPa·s and especially 30 to 500 mPa·s at 25°C. The oily substance used herein may have a lower viscosity than the base polymer. This eliminates the problem that the prior art composition is increased in viscosity by adding the parting agent thereto, and allows the composition to be adjusted to any desired viscosity. The oily substance is blended in an amount of 0.1 to 50 parts, and preferably 0.1 to 20 parts by weight per 100 parts by weight of the silicone rubber composition (exclusive of the oily substance). An excess of the oily substance may augment the migration of the substance to the surface too much and detract from mechanical properties. If desired, various well-known components are added to the composition of the invention. The invention may be combined with the technique of adding a silicone fluid having a higher viscosity than the alkenyl-containing polysiloxane as the base polymer as disclosed in JP-A 7-118534. According to the invention, a mother mold is obtainable by casting the silicone rubber composition in a predetermined mold, followed by curing, in a conventional manner. With respect to the curing conditions of the silicone rubber composition, curing is generally effected at or near the temperature at which a resin material for forming duplicate parts in the mother mold is cured, for the purpose of mitigating the shrinkage of the resin material upon curing. Typically, the silicone rubber composition is cured at a temperature of about 40 to 60°C for about 2 to 20 hours. The mother mold thus obtained is effective in molding epoxy resins or urethane resins into articles. EXAMPLE Examples of the invention are given below by way of illustration and not by way of limitation. EXAMPLES General Procedure Into a kneader were admitted 800 parts by weight of a dimethylpolysiloxane blocked with a vinyldimethylsilyl radical at each end of its molecular chain and having a viscosity of about 5,000 mPa·s at 25°C and a refractive index of 1.404 at 25°C as a base polymer and 200 parts by weight of Aerosil R976 having a specific surface area of about 240 m 2 /g by the BET method (Nippon Aerosil K.K.). After thorough agitation, 80 parts by weight of hexamethyldisilazane and 10 parts by weight of water were added. Cold blending was effected for one hour without application of heat. The mixture was heated to 150°C, mixed for two hours at the temperature, then cooled down to room temperature. To 500 parts by weight of the thus obtained mixture were added an oily substance, 25 parts by weight of methylhydrogenpolysiloxane represented by the average formula shown below, and 100 ppm calculated as platinum atom of chloroplatinic acid-octyl alcohol complex as a catalyst. After thorough agitation, the composition was deaerated in vacuum, cast into a mold, and cured at 60°C for 2 hours to form a recessed mother mold. Example 1 A sample was prepared by the above procedure, using 2 parts by weight of phenyl-modified polydimethylsiloxane KF50 (phenyl modification 5 mol%, viscosity 100 centistokes at 25°C, refractive index 1.428 at 25°C, by Shin-Etsu Chemical Co., Ltd.) as the oily substance. Using this sample, a mother mold was formed. Example 2 A sample was prepared as in Example 1 except that 2 parts by weight of long-chain alkyl-modified polydimethylsiloxane KF414 (viscosity 100 centistokes at 25°C, refractive index 1.427 at 25°C, by Shin-Etsu Chemical Co., Ltd.) was added as the oily substance. Using this sample, a mother mold was formed. Example 3 A sample was prepared as in Example 1 except that 0.5 parts by weight of phenyl-modified polydimethylsiloxane KF50 (phenyl modification 5 mol%, viscosity 100 centistokes at 25°C, refractive index 1.428 at 25°C, by Shin-Etsu Chemical Co., Ltd.) and 5 parts by weight of both end trimethylsilyl-blocked polydimethylsiloxane KF96 (viscosity 100,000 centistokes at 25°C, refractive index 1.403 at 25°C, by Shin-Etsu Chemical Co., Ltd.) were added as the oily substance. Using this sample, a mother mold was formed. Comparative Example 1 A sample was prepared as in Example 1 except that the oily substance was omitted. Using this sample, a mother mold was formed. Comparative Example 2 A sample was prepared as in Example 1 except that 5 parts by weight of both end trimethylsilyl-blocked polydimethylsiloxane KF96 (viscosity 100 centistokes at 25°C, refractive index 1.403 at 25°C, by Shin-Etsu Chemical Co., Ltd.) was added as the oily substance. Using this sample, a mother mold was formed. Comparative Example 3 A sample was prepared as in Example 1 except that 5 parts by weight of both end trimethylsilyl-blocked polydimethylsiloxane KF96 (viscosity 100,000 centistokes at 25°C, refractive index 1.403 at 25°C, by Shin-Etsu Chemical Co., Ltd.) was added instead as the oily substance. Using this sample, a mother mold was formed. Comparative Example 4 A sample was prepared as in Example 1 except that the oily substance was 10 parts by weight of pareffinic process oil PW 32 having a refractive index difference from the base polymer of 0.02-0.05 at 25°C and a kinematic viscosity of 30.85 x 10 -6 m 2 s -1 at 40°C. A mother mold was formed. Using each of the thus obtained mother molds, a procedure of casting urethane resin Hicast 3017 (H&K Co.) into the recess, curing at 60°C for one hour, and taking out the urethane article from the mother mold was repeated for 30 cycles. At suitable intervals, the force required to remove the urethane article was measured and rated according to the following criterion. Exc: smooth removal Good: a light force Fair: a strong force Poor: a very strong force or stuck At the same time, the surface of the urethane article which had been in contact with the mother mold was measured for gloss using a gloss meter IG-310 by Horiba K.K. The results are shown in Table 1. Example Comparative Example E1 E2 E3 CE1 CE2 CE3 CE4 Gloss 1st 92 95 97 97 97 95 97 5th 95 89 98 95 95 96 95 10th 89 92 95 92 95 92 97 20th 92 94 99 88 82 94 95 25th 89 92 97 85 79 93 92 30th 92 89 98 72 75 89 90 Release Exc Exc Exc Poor Poor Fair Good There has been described a silicone rubber composition comprising an organopolysiloxane base polymer to which an oily substance having a specific refractive index difference is added. The composition is effective for forming a mother mold capable of maintaining parting properties over a long period of time.

Claims: A silicone rubber composition for forming mother molds comprising: (i) a linear dimethylpolysiloxane of the following average compositional formula (1) containing on the average at least two alkenyl radicals in a molecule: R 1 a SiO (4-a)/2 wherein R 1 is a methyl or an alkenyl radical selected from vinyl and allyl radicals, 0.01 to 10 mol% of R 1 being alkenyl radicals, and "a" is a positive number of 1.9 to 2.4, and having a viscosity of 500 to 1,000,000 mPa·s at 25°C, as a base polymer; (ii) an organohydrogenpolysiloxane of the following average compositional formula (2): R 2 b H c SiO (4-b-c)/2 having, in a molecule, at least 2 hydrogen atoms attached to silicon atoms, wherein R 2 is a substituted or unsubstituted monovalent hydrocarbon radical of 1 to 10 carbon atoms, b is a positive number of 0.7 to 2.1, c is a positive number of 0.001 to 1.0, and the sum of b and c is from 0.8 to 3.0, and having a viscosity of 1 to 1,000 mPa·s at 25°C, as a curing or crosslinking agent, in such an amount that the ratio of the number SiH radicals in the organohydrogenpolysiloxane to the number of silicon-bonded alkenyl radicals in the base polymer may range from 0.4:1 to 4:1; (iii) an addition reaction catalyst selected from platinum group metals, and compounds and complexes thereof in an amount of 0.1 to 1,000 ppm of platinum group metal based on the sum of the linear dimethylpolysiloxane and organohydrogenpolysiloxane, and (iv) a polyether-modified, C 6 -C 30 alkyl-modified, higher fatty acid ester-modified, fluorine-modified or phenyl-modified silicone fluid being liquid at 20°C, and having a viscosity of 30 to 1,000 mPa·s at 25°C, and a refractive index which differs by at least 0.01 from the refractive index of the base polymer (i), in an amount of 0.1 to 50 parts by weight per 100 parts by weight of the composition exclusive of component (iv). The composition of claim 1 wherein the polyether-modified, C 6 -C 30 alkyl-modified, higher fatty acid ester-modified, fluorine-modified or phenyl-modified silicone has a refractive index which differs by a least 0.01 up to 0.05 from the refractive index of the base polymer. A mother mold comprising a cured product of the silicone rubber composition of claims 1 or 2. Use of the composition of claims 1 or 2 for forming a mother mold.

Inventors: MIYOSHI KEI, TANAKA KOUICHI, MIYOSHI, KEI, TANAKA, KOUICHI
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 2000
Main classification: C08L83
IPC codes: B29C33, C08L83