Room temperature vulcanizing fluorosilicone compositions

Abstract: A method for continuously producing a room temperature vulcanizable composition is disclosed. A base mixture of a silanol stopped fluorosilicone polymer, a silica filler and iron oxide are processed in an extruder at an elevated temperature between about 50 and 200°C. Thereafter, a tin based catalyst, an acetoxy cross-linking agent and an optional adhesion promoter are added to the extruder near the exit port. The resulting material has improved storage stability.

Description: BACKGROUND OF THE INVENTION The present invention relates to one-part room temperature vulcanizing (RTV) rubber sealant compositions. More particularly, the invention relates to RTV fluorosilicone rubber sealant compositions being continuously processed in an extruder at high temperatures and having improved storage stability. RTV fluorosilicone rubber sealant compositions are known in the art. Such compositions are known for their ability to resist change at elevated temperatures and exposure to adverse conditions over extended periods of time. In general, materials are needed which have the outstanding properties such as good tensile strength and also retain such properties after prolonged storage prior to cure. Efforts have been made in the past to produce RTV silicone elastomer compositions having increased strength and adhesion. Exemplary RTV elastomers are disclosed in US-A-4,514,529. Although one-part RTV compositions having reasonably good tensile physical properties are known in the art, it is desirable to produce on a continuous basis, as opposed to batch process, RTV compositions having good physical properties and cure characteristics after prolonged storage. SUMMARY OF THE INVENTION The present invention is based upon the discovery that a base polymer comprising a blend of a silanol stopped fluorosilicone polymer, a treated silica filler, and iron oxide may be continuously processed in an extruder at an elevated temperature followed by the addition of an acetoxy crosslinker, a tin based catalyst and an optional adhesion promoter further down the extruder to produce an RTV sealant having good room temperature adhesion and tensile strength and retainable cure properties after prolonged storage. In a particular embodiment, the invention comprises (A) a base composition including (A1) 100 parts by weight of a silanol stopped fluorosilicone polymer such as poly(methyltrifluoropropyl) siloxane having a viscosity ranging from 600 to 300,000 cps at 25°C; (A2) from 3 to 25 parts by weight based upon (A1) of a treated silica filler; (A3) up to 200 parts by weight based upon (A1) of finely divided Fe 2 O 3 having a particle size ranging from 0.05 to 5 µm (microns). In addition, the RTV composition also includes (B) a catalyst solution comprising (B1) up to 0.5 parts by weight based upon (A) of a tin based catalyst; (B2) from 2 to 15 parts by weight based upon (A) of an acetoxy cross-linking agent comprising (a) a methyltriacetoxy silane or (b) ethyltriacetoxysilane or mixtures of (a) and (b); and (B3) from 0.1 to 4 parts by weight based upon (A) of diacetoxydibutoxysilane adhesion promoter. The base composition is continuously processable at an elevated temperature ranging from 50 to 200°C and the catalyst solution (B) is added at near room temperature after processing (A). Preferably the base polymer A is processed at a temperature ranging from 75 to 150°C and is processed for up to 5 minutes before adding the catalyst solution B. The RTV compositions of the invention cure well and importantly retain curable properties after prolonged storage in the uncured state. DETAILED DESCRIPTION OF THE INVENTION : The present invention is directed to an RTV curable fluorosilicone sealant composition which may be produced on a continuous basis and which has good cured properties after prolonged storage in the uncured state. The composition comprises (A) a base composition formed of (A1) a fluorosilicone polymer; (A2) a silica filler; and (A3) Fe 2 O 3 . The base composition (A) is combined with (B) a catalyst solution including (B1) a tin based catalyst; (B2) an acetoxy cross-linking agent; and (B3) diacetoxydibutoxysilane as an adhesion promoter. In accordance with the invention, (A1) is a silanol terminated fluorosilicone having a viscosity ranging from 600 to 300,000, preferably from 2,000 to 200,000 and more preferably from 3,000 to 150,000 cps at 25°C. In the examples below, (A1) is a silanol stopped poly(methyltrifluoropropyl) siloxane having the formula: HO(RR 1 SiO) x OH where R is independently selected from monovalent hydrocarbon radicals, free of aliphatic unsaturation containing from 1 to 8 carbon atoms, R 1 is independently selected from fluorinated monovalent hydrocarbon radicals free of aliphatic unsaturation containing from 1 to 8 carbon atoms and x varies so that (A1) has a viscosity ranging from 600 to 300,000 cps at 25°C. In the examples below, x is selected so that (A1) has a viscosity of 15,000 cps at 25°C. (A2) comprises from 3 to 25, preferably from 3 to 20 and more preferably from 5 to 15 parts by weight based upon (A1) of a reinforcing filler such as fumed silica (SiO 2 ). The filler preferably has a surface area of between 100 and 300 m 2 /gm, more preferably, the surface area is 200 m 2 /gm in the untreated state. The filler may be treated with various agents so as to prevent the composition from structuring, for example, cyclopolysiloxanes as disclosed in US-A-2,938,009 to Lucas and silazanes as disclosed in US-A-3,635,743 to Smith, or both. The cyclopolysiloxanes may be, for example, octamethylcyclotetrasiloxane, present in an amount ranging from 15 to 20 weight percent of the filler. In the examples below, (A2) is a trimethylsilyl treated fumed silica having a surface area in the untreated state of about 200 m 2 /gm. (A3) is a finely divided Fe 2 O 3 iron oxide powder having an average particle size ranging from 0.05 to 5 µm (microns). (A3) is present in an amount up to 200 parts, preferably up to 60 parts and more preferably 30 parts by weight based upon 100 parts of (A1). In the examples below, (A3) is a commercially available product sold by Pfizer under the trade designation R2200 having an average particle size of about 0.25 µm (microns). (B1) is a tin based catalyst which may be present in an amount up to 0.50 parts by weight based upon 100 parts of the base composition (A). Preferably, the catalyst is present in an amount from 0.01 to 0.4 parts by weight based upon (A), and more preferably (B1) is present in an amount ranging from 0.05 to 0.30 parts based upon (A). In the examples below, (B1)(a) is dibutyltindilaurate and (B1) (b) is dibutyltindiacetate. (B2) is an acetoxy silane cross-linking agent. Preferably, (B2) comprises (a) methyltriacetoxy silane or (b) ethyltriacetoxysilane or mixtures of (a) and (b). The cross-linking agent is present in an amount from 2 to 15 parts by weight based upon 100 parts of the base composition (A), preferably from 2 to 10 parts by weight based upon (A), and more preferably from 3 to 7 parts by weight based upon (A). (B3) is an adhesion promoter which may be present in an amount from 0.1 to 4 parts by weight based upon 100 parts of the base composition (A), preferably from 0.2 to 2 parts by weight based upon (A) and more preferably ranging from 0.3 to 1.5 parts by weight based upon (A). (B3) is diacetoxydibutoxysilane. The following examples demonstrate the effect of the present invention. Examples 1 and 2 set forth the preparation and testing of sealants in which the base polymer (A) is prepared by combining Components (A1), (A2) and (A3) in a first supply port of an extruder. In Examples 1 and 3, (A1), (A2) and (A3) are continuously processed at a low temperature. In Examples 2 and 4, (A1), (A2) and (A3) are continuously processed at a high temperature. Components (B1), (B2) and (B3) are added at a supply port between the first supply port and the extruder exit port. Example 1 An extruder was set up to combine the following ingredients in the following proportions: (A) base composition: (A1) 100 parts - silanol stopped fluorosilicone polymer (approximately 150,000 cps at 25°C) poly(methyltrifluoropropyl) siloxane; (A2) 9 parts by weight based upon (A1) - trimethyl silyl treated fumed silica, 200 m 2 /gm in the untreated state; (A3) 30 parts by weight based upon (A1) - Fe 2 O 3 . The above materials were added to the extruder to constitute the base composition and were continuously processed at 50°C for about 1 min. Thereafter, 5.79 parts by weight based upon the sum of (A1), (A2) and (A3) of a catalyst solution (B) was added to the end of the extruder: (B) Catalyst solution: (B1)(b) dibutyltindiacetate - 1.2 parts; (B2)(a) methyltriacetoxy silane - 80 parts; (B3) diacetoxydibutoxy silane - 20 parts; Shortly after manufacture, the material was tested for physical properties after curing 7 days at 75°C at 50% relative humidity. The results are listed on Table 1-A. Initial Shore A 32 Tensile Strength (psi) kPa (526) 3627 Elongation % 312 Mod. at 50% ext. (psi) (64) 441 Samples of the uncured sealant were stored at room temperature for one year and tested in a similar fashion. The results are listed in Table 1B. Initial Shore A 10 Tensile Strength (psi) kPa (124) 855 Elongation % 365 Mod at 50% ext. (psi) kPa (23) 159 After storage, the sealant cured to an elastomer which was 22 Shore A points lower and retained only about 24% of its initial tensile strength. Example 2 The materials were combined as set forth in Example 1 except that processing temperature of the base polymer (A) was maintained at 100°C. Initial properties and resulting properties after one year are set forth in Tables 2A and 2B, respectively. Initial Shore A 34 Tensile Strength (psi) kPa (533) 3675 Elongation % 333 Mod at 50% ext. (psi) kPa (68) 469 Initial Shore A 22 Tensile Strength (psi) kPa (428) 2951 Elongation % 438 Mod at 50% ext. (psi) kPa (40) 276 After one year in storage, the material cured to an elastomer which was only 12 Shore A points lower and which retained approximately 80% of its initial tensile strength, demonstrating improved shelf life at elevated processing temperatures. Example 3 An extruder was set up to combine the following ingredients in the following proportions: (A1) 100 parts (A2) 9 parts (A3) 30 parts The above materials were added to the first section of the extruder and processed at 50°C to constitute the base portion of the RTV sealant. The catalyst solution (B) (5.79 parts per 100 parts of base (A)) was added to the later section of the extruder as described in Example 1. Catalyst solution (B): (B1)(a) dibutyltindilaurate 0.6 parts (B2)(a) methyltriacetoxysilane 80 parts (B3) diacetoxydibutoxysilane 30 parts The sealant was tested as set forth in Example 1 for initial properties and properties after one year storage in the uncured state. The initial properties are set forth in Table 3A and the properties after storage in the uncured state after one year are set forth in Table 3B. Initial Shore A 32 Tensile Strength (psi) kPa (583) 4020 Elongation % 332 Mod at 50% ext. (psi) kPa (66) 455 Initial Shore A 19 Tensile Strength (psi) kPa (360) 2482 Elongation % 431 Mod at 50% ext. (psi) kPa (32) 221 After storage, the sealant cured to an elastomer which was 13 Shore A points lower and retained only 62% of the initial tensile strength. Example 4 The materials were combined as in Example 3 except that the basic polymer (A) was processed at 100°C. Initial properties and properties after one year are set forth below in Tables 4A and 4B, respectively. Initial Shore A 33 Tensile Strength (psi) kPa (514) 3544 Elongation % 320 Mod at 50% ext. (psi) kPa (71) 490 Initial Shore A 27 Tensile Strength (psi) kPa (583) 4020 Elongation % 450 Mod at 50% ext. (psi) kPa (50) 345 The examples demonstrate that continuously produced fluorosilicone RTV sealants, when processed at elevated temperatures, exhibit improved storage stability.

Claims: A method for continuously producing a shelf stable room-temperature vulcanizable composition from an extruder having a first supply port, an exit port, and a second supply port located along said extruder between said first supply port and said exit port, said method comprising the steps of: (1) supplying a base composition (A) to said extruder at said first supply port and maintaining the temperature of the base composition between 50 and 200°C, said base composition (A) comprising: (A1) 100 parts by weight of a silanol terminated fluorosilicone having the formula HO(RR 1 SiO) x OH, wherein each R is a monovalent hydrocarbon radical free of aliphatic unsaturation containing from 1 to 8 carbon atoms, each R 1 is a fluorinated monovalent hydrocarbon free of aliphatic unsaturation having from 1 to 8 carbon atoms and x varies so that (A1) has a viscosity of 600 to 300,000 cps at 25°C; (A2) from 3 to 25 parts by weight based upon (A1) of a treated silica filler having a surface area of 100 to 300 m 2 /gm in an untreated state, (A3) up to 200 parts by weight based upon (A1) of finely divided Fe 2 O 3 having an average particle size ranging from 0.05 to 5 µm (microns); (2) adding a catalyst solution (B) to said extruder at said second supply port, said catalyst solution (B) comprising: (B1) up to 0.5 parts by weight based upon 100 parts of the base composition (A) of a tin based catalyst; (B2) from 2 to 15 parts by weight based upon 100 parts of the sum of (A1), (A2) and (A3) of at least one acetoxy cross-linking agent; and (B3) diacetoxydibutoxysilane. The method as defined in claim 1, wherein the amount of (B3) is up to 4 parts by weight based upon 100 parts of the sum of (A1), (A2) and (A3) of diacetoxydibutoxysilane. The method as defined in claim 1, wherein (A2) is trimethyl silyl treated. The method as defined in claim 1, wherein (B1) comprises at least one of (a) dibutyltindilaurate and (b) dibutyltindiacetate. The method as defined in claim 1, wherein (B2) comprises at least one of (a) methyltriacetoxy silane and (b) ethyltriacetoxysilane. The method as defined in claim 1, wherein said base polymer (A) is processed at a temperature ranging from 75 to 150°C. The method as defined in claim 1, wherein said base polymer (A) is processed for up to 5 minutes before adding the catalyst solution (B). A shelf stable room temperature vulcanizable composition having been continuously produced in an extruder having an exit port, a first supply port upstream of said exit port, and a second supply port at a location along said extruder between said first supply port and said exit port, said composition comprising: a silanol stopped fluorosilicone polymer, a silica filler and a ferrous oxide which are supplied to said extruder at said first supply port and which are processed at a temperature ranging from 50 to 200°C to form a base composition; and a catalyst solution including a tin based catalyst, an acetoxy cross linking agent and a diacetoxydibutoxysilane added to the base composition in the extruder at said second supply port between the first supply port and the exit port. A room temperature vulcanizable composition as in claim 8, wherein said fluorosilicone polymer comprises: (A1) 100 parts by weight of a silanol terminated fluoro polymer having the formula OH(RR 1 SiO) x OH, wherein each R is a monovalent hydrocarbon radical free of aliphatic unsaturation containing from 1 to 8 carbon atoms, each R 1 is a fluorinated monovalent hydrocarbon free of aliphatic unsaturation containing from 1 to 8 carbon atoms and x varies so that (A1) has a viscosity of 600 to 300,000 cps at 25°C; said silica filler comprises: (A2) from 3 to 25 parts by weight based upon (A1) of a treated silica filler having a surface area of 100 to 300 m 2 /gm in an untreated state; (A3) up to 200 parts by weight based upon (A1) of finely divided Fe 2 O 3 having an average particle size ranging from 0.05 to 5 µm (microns); said cross-linking agent comprises: (B2) from 2 to 15 parts by weight based upon 100 parts of the sum of (A1), (A2) and (A3) of at least one acetoxy cross-linking agent selected from the group consisting of methyltriacetoxy silane and ethyltriacetoxysilane; and said tin based catalyst is present in an amount up to 0.5 parts by weight based upon 100 parts of the sum of (A1), (A2) and (A3). A room temperature vulcanizable composition as in claim 9, wherein said diacetoxydibutoxysilane is present in an amount of from 0.1 to 4 parts by weight based on (A1).

Inventors: DZIARK JOHN JOSEPH, DZIARK, JOHN JOSEPH
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1993
Main classification: C08L83
IPC codes: C08K3, C09K3, B29C31, C08L83, B29B7, C08J3, B29L31, B29B11, C08K5, B29K83, B29K105