Method for preparing an in-situ reinforced silicone elastomer

Abstract: A method is described for preparing in situ reinforced silicone elastomers and includes the compositions prepared by the method. The method comprises forming a mixture comprising an end-functional diorganopolysiloxane; a stoichiometric excess of a hydrolyzable siloxane having hydrolyzable functionality described by formula -R 2 SiQ 3-b R b , where each R is independently selected from a group consisting of alkyls comprising one to six carbon atoms, alkenyls comprising two to six carbon atoms and aryls, R 2 is a divalent hydrocarbon radical comprising one to 12 carbon atoms, each Q is independently selected from a group of radicals consisting of hydroxy, acyloxy comprising one to six carbon atoms and substituted or unsubstituted alkoxy comprising one to six carbon atoms; and b=0 to 2; and a tin salt catalyst, followed by contacting the mixture with water to effect curing. The resulting silicone elastomers are in situ reinforced to provide high strength.

Description: The present invention is a method for preparing in situ reinforced silicone elastomers and includes the compositions prepared by this method. The method constitutes forming a mixture comprising an end-functional diorganopolysiloxane; a stoichiometric excess of a hydrolyzable siloxane having hydrolyzable functionality described by formula -R²SiQ 3-b R b , where each R is independently selected from a group consisting of alkyls comprising one to six carbon atoms, alkenyls comprising two to six carbon atoms and aryls, R² is a divalent hydrocarbon radical comprising one to 12 carbon atoms, each Q radical is independently selected from a group consisting of hydroxy, acyloxy comprising one to six carbon atoms, substituted and unsubstituted alkoxy comprising one to six carbon atoms, b=0 to 2; and a tin salt catalyst followed by contacting the mixture with water to effect curing. The resulting silicone elastomers are in situ reinforced to provide high strength. It is known that crosslinked diorganopolysiloxanes must be reinforced with suitable fillers to obtain silicone elastomers with desirable mechanical properties such as high tensile strength and elongation. A typical filler used for this purpose is pyrogenic silica. The manufacture of pyrogenic silica for this purpose is an energy intensive process resulting in expenses that are reflected in the cost of the product silicone elastomer. In addition, the mixing of silica into viscous polymers, such as for the preparation of high-consistency elastomers, requires significant energy and time which yields products with properties that are highly dependent on the mixing process. In our claimed method, the reinforcing filler is generated in situ, thus solving the problems of the expense of externally forming the reinforcing filler or of the expense, time and variability associated with mixing the reinforcing filler into the diorganopolysiloxanes. Use of the hydrolyzable siloxane compositions described herein to in situ reinforce silicone elastomers provides a method of controlling the tensile strength, elongation and durometer properties of the elastomer without the separate addition of reinforcing filler. Mark et al., Makromol. Chem. Rapid Commun. 3: 681-685 (1982), describe a two-step process where silanol end-blocked polydimethylsiloxane (PDMS) or vinyl end-blocked PDMS are end-linked to form elastomeric matrices. The end-linked elastomeric matrices are then swollen with tetraethoxysilane (TEOS) and the absorbed TEOS is hydrolyzed in place by immersion of the sample into glacial acetic acid. Jaing et al., Colloid & Polymer Sci. 262: 758-760 (1984), used the same two-step process as described by Mark but they used a constant, relative-humidity chamber to cure the TEOS in situ rather than hydrolyzing TEOS in acetic acid. Subsequently, Mark et al., Macromolecules 17: 2613-2616 (1984), disclosed a one-step or simultaneous curing in an in situ filling process. In this process, silanol end-blocked PDMS, TEOS and tin catalyst are mixed at various TEOS to PDMS ratios and cured using atmospheric moisture. The tin catalysts employed were dibutyltin diacetate and stannous (II) ethylhexanoate. Tang et al., Polymer Engineering and Science 25: 29-31 (1985), introduced a one-step simultaneous curing and filling method where a bimodal mixture of silanol end-terminated PDMS, including one long and one short chain polymer fraction, was combined with stannous(II) ethylhexanoate and various amounts of TEOS and was then cured using atmospheric moisture. Ning et al., Polymer Bulletin 13: 155-161 (1985), reported using trifunctional silanes to form resinous reinforcing phases in a two-step process. In this work, a vinyl-terminated PDMS was tetrafunctionally end-linked and then swollen with various amounts of TEOS, ethyltriethoxysilane, diethyldiethoxysilane or any combination of these alkoxysilanes. Precipitation of the alkoxysilanes to form the silica or silicate resin was done by immersion of test samples into an aqueous solution of ethyl amine. Mark et al., Polymer Bulletin 14: 325-329 (1985), dealt with using trifunctional silanes as precursors to form trifunctional silicate resins in a simultaneous one-step curing and filling process. They mixed silanol end-terminated PDMS and stannous(II) ethylhexanoate with either vinyltriethoxysilane, methyltriethoxysilane or phenyltriethoxysilane at various alkoxysilane to PDMS ratios and later cured their samples under atmospheric conditions. Sur et al., Makromol. Chem. 187: 2861-2866 (1986), teaches a two-step process where silanol-terminated PDMS is end-linked and then swollen with either tetramethoxysilane, TEOS, tetrapropoxysilane or tetrabutoxysilane. The samples were cured by immersion in aqueous solutions containing one of a variety of amine catalysts. U.S. Patent 4,341,842 claims a silicone rubber composition having (A) 100 parts of a silanol end-stopped diorganopolysiloxane polymer with a viscosity varying from 100 to 500,000 centipoise (mPa·s) at 25°C., (B) from 0.1 to 15 parts by weight of an alkyl silicate or partial hydrolysis product of the silicate, and (C) from 0.01 to 5 parts by weight of a metal salt of a carboxylic acid, where parts by weight of a metal salt of a carboxylic acid, where the composition is cured at room temperature to a silicone elastomer. The present invention provides a method for preparing an in situ reinforced silicone elastomer comprising: (A) forming a mixture comprising (i) a diorganosiloxane polymer having the formula Q 3-a R a Si(OSiR₂) x OSiR a Q 3-a , (ii) a 5 to 10 time stoichiometric excess of a hydrolyzable siloxane independently selected from hydrolyzable siloxanes described by formulae (a) R₃Si(OSiQ¹R) m (OSiRR¹) n OSiR₃ , (b) (RQ¹SiO) y , and (c) Si(OSiR₂Q¹)₄ (iii) a tin salt catalyst; and then (B) contacting said mixture with water to effect formation of an in situ reinforced silicone elastomer; where each R is independently selected from a group consisting of (i) alkyls comprising one to six carbon atoms, (ii) alkenyls comprising two to six carbon atoms and (iii) aryls; where each R¹ is independently selected from hydrogen atom or R; where each Q is independently selected from a group of radicals consisting of (i) hydroxy, (ii) acyloxy comprising two to six carbon atoms and (iii) substituted or unsubstituted alkoxy comprising one to six carbon atoms and where each Q¹ is described by formula -R²SiQ 3-b R b , wherein R² is a divalent hydrocarbon radical comprising one to 12 carbon atoms, R and Q are as previously described, and b=0 to 2; a=0 to 2, x=0 to 1000, m=3 to 500, n=0 to 497, m+n=3 to 500 and y=3 to 10. The method of forming our mixture comprising the diorganosiloxane polymer, the hydrolyzable siloxane and a tin salt catalyst is not critical to the present process and is any appropriate process known to those skilled in the art. For example, the mixture can be formed in a standard mixer or blender. This mixture can then be extruded, molded, dip-coated, wiped or sprayed onto a substrate. The present method requires a diorganosiloxane polymer as described by formula (1). The diorganosiloxane polymer contains substituent R, where each R is independently selected from a group consisting of alkyls comprising one to six carbon atoms, alkenyls comprising two to six carbon atoms and aryls. The substituent R is, for example, methyl, ethyl, propyl, tert-butyl, hexyl, 1,1,1-trifluoropropyl, perfluoropropyl, phenyl, xylyl and naphthyl. Preferred is where R is methyl. The diorganosiloxane polymer also contains on each end, at least one, two or three substituents Q, where each Q is a reactive functionality independently selected from a group of radicals consisting of hydroxy, acyloxy comprising one to six carbon atoms and unsubstituted or substituted alkoxy comprising one to six carbon atoms. The substituent Q is, for example, hydroxy, methoxy, ethoxy, butoxy, acetoxy and 1,1,1-trifluoropropoxy. Preferred is when Q is hydroxy. More preferred is when Q is hydroxy and each a equals two. The diorganosiloxane polymer comprises x number of repeating units of formula -(OSiR₂)-, where R is as previously described and x=0 to 1000. Preferred is where x is a value within a range of 10 to 1000. Most preferred is where x is a value within a range of 20 to 800. The diorganosiloxane polymer is mixed with a stoichiometric excess of a hydrolyzable siloxane described by formulas (2), (3) and (4), or their partial hydrolyzates. The hydrolyzable siloxane serves as a crosslinker for the diorganosiloxane polymer and as a source material for the in situ formation of filler within the matrices of our crosslinked diorganosiloxane polymer. The hydrolyzable siloxanes have substituents R, where R is as previously described. Preferred is when R is methyl. The hydrolyzable siloxanes have substituent Q¹, where each Q¹ is described by formula (5). The substituent R² is a divalent hydrocarbon radical comprising one to 12 carbon atoms. Preferred is when R² is an alkylene radical comprising two to six carbon atoms. Most preferred is when R² is ethylene. In formula (5), the substituents Q and R are as previously described and b=0, 1 or 2. It is preferred that Q be an alkoxy comprising one to six carbon atoms. More preferred is when Q is methoxy. In formula (5), it is preferred that b=0. Hydrolyzable siloxanes described by formula (2) have m number of repeating units described by formula - (OSiQ¹R)-, where Q¹ and R are as previously described and m is an integer within a range of three to 500. These hydrolyzable siloxanes have n number of repeating units described by formula -(OSiRR¹)- where R is as previously described, each R¹ is independently selected from R or hydrogen atom and n is an integer within a range of zero to 497. In formula (2), the sum of m plus n equals three to 500. Preferred is when each R¹ is independently selected from methyl or hydrogen atom. Formula (3) describes hydrolyzable cyclosiloxanes having substituents R and Q¹ as previously described and y number of repeating units, where y is an integer within a range of 3 to 10. The preferred cyclosiloxane is one where y is within a range of four to six. In formula (4), the R and Q¹ substituents of the hydrolyzable siloxane are as previously described. By "stoichiometric excess" of hydrolyzable siloxane, it is meant that the number of Q substituents provided to our method by addition of the hydrolyzable siloxane is greater than the number of Q substituents provided to the method by the diorganosiloxane polymer. It is preferred that the hydrolyzable siloxane be added to our method at a concentration where the amount of Q substituents provided by the hydrolyzable siloxane is five to ten times stoichiometric excess in relation to the Q substituents added to the method by the diorganosiloxane polymer. The hydrolyzable siloxanes useful in the present method are prepared, for example, by the hydrosilation of organohydrogensiloxane polymers, oligomers, cyclic or branched compounds with alkene-substituted hydrolyzable silanes such as vinyltrimethoxysilane. This hydrosilation process is conducted in the presence of a platinum or platinum complex catalyst. A typical hydrosilation catalyst is, for example, a complex of platinum with symdivinyltetramethylsiloxane. Our mixture comprising the diorganosiloxane polymer and the hydrolyzable siloxane also comprises a tin salt catalyst. The tin salt catalyst is any such catalyst which facilitates the crosslinking of the diorganopolysiloxane polymer by the hydrolyzable siloxane, facilitates the condensing of the hydrolyzable siloxane to form an in situ filler, or facilitates both. The tin salt catalyst is, for example, dibutyltin dilaurate, dibutyltin dibenzoate, dibutyltin diacetate or stannous(II) ethylhexanoate. A preferred tin salt catalyst is dibutyltin dilaurate. The concentration of tin salt catalyst in the present method is within a range of 0.005 to 10 weight percent of the combined weight of the catalyst, the diorganosiloxane polymer and the hydrolyzable siloxane added to our method. Preferred is when the concentration of the tin salt catalyst is within a range of 0.01 to 2 weight percent of the combined weight as described. Our mixture comprising the diorganosiloxane, the hydrolyzable siloxane and the tin salt catalyst is contacted with water to effect curing of the mixture to an in situ reinforced silicone elastomer. The present method requires the presence of water to effect crosslinking of the diorganosiloxane polymers by the hydrolyzable siloxane and to effect in situ filler formation by stoichiometric excess hydrolyzable siloxane. The water can be provided to the method in liquid or vapor form. For example, the mixture comprising the diorganosiloxane polymer, the hydrolyzable siloxane and the tin salt catalyst can be cured in an environment having a relative humidity within a range of 20 to 100 percent. Preferred is when the mixture is cured in an environment having a relative humidity within a range of 30 to 70 percent. The silicone elastomers prepared by the present method cure readily at room temperature. However, any temperature within a range of 0°C. to 100°C. is suitable for conducting our method. Preferred is when our method is conducted at a temperature within a range of 20°C. to 30°C. The silicon elastomers prepared by the present method are unique elastomers having high tensile strength, typically above 500 psi (3.45 MPa). This high tensile strength is achieved without the addition of externally generated reinforcing fillers. In addition, the type of hydrolyzable siloxane can also be varied as a means of altering such physical properties as elongation and durometer while still maintaining tensile strength. Therefore, also claimed herein are silicone elastomer compositions prepared by the present method. Example 1 An in situ filled silicone elastomer was prepared using methyl(trimethoxysilylethyl)cyclosiloxane as a precursor for in situ filler formation within the elastomer. A mixture of three silanol end-terminated polydimethylsiloxane polymers comprising by weight 25 percent polymer of Mn=2,700, 25 percent polymer of Mn=38,000 and 50 percent of polymer of Mn=58,000 was formed. This mixture was further mixed with 10.6 mole percent of methyl(trimethoxysilylethyl)cyclosiloxanes described by formula {Me{(MeO)₃SiCH₂CH₂}SiO}₄₋₆ and 0.2 weight percent of dibutyltin dilaurate. The mixture was poured into 15 x 100 mm chases and placed in a constant relative humidity chamber at 50 percent relative humidity to effect curing. The cured samples were tested for tensile strength and elongation according to ASTM Standard D12-87. The Shore A durometer reading was determined by standard techniques. The cured silicone elastomer was determined to have a tensile strength of 579 psi (3.99 MPa), elongation of 70 percent and Shore (A) durometer of 60. Example 2 An in situ filled silicone elastomer was prepared as described in Example 1 using 5.3 mole percent of methyl(trimethoxysilylethyl)cyclosiloxane. Physical properties of the cured elastomer were determined by the methods described in Example 1. The cured elastomer had a tensile strength of 845 psi (5.83 MPa), elongation of 253 percent and Shore (A) durometer of 37. Example 3 An in situ filled silicone elastomer was prepared as described in Example using 0.08 weight percent tin octoate. Physical properties of the cured elastomer were determined by the methods described in Example 1. The cured elastomer had a tensile strength of 285 psi (1.96 MPa), elongation of 41 percent and Shore (A) durometer of 64. Example 4 An in situ filled silicone elastomer was prepared using a methyl(trimethoxysilylethyl)/methylhydrogen linear siloxane polymer as precursor for in situ filler formation in the elastomer. A diorganosiloxane polymer mixture similar to that described in Example 1 was formed. Into the diorganosiloxane polymer mixture was mixed 13.2 weight percent of a methyl(trimethoxysilylethyl)/methylhydrogen linear siloxane polymer described by the following formula: Me₃SiO{Me{(MeO)₃SiCH₂CH₂}SiO}₃₅(MeHSiO}₃₅SiMe₃. Then, 0.2 weight percent of dibutyltin dilaurate was stirred into the siloxane mixture and the mixture cured as described in Example 1 to form an in situ reinforced silicone elastomer. Physical properties of the silicone elastomer were determined by the methods described in Example 1. The silicone elastomer had a tensile strength of 848 psi (5.85 MPa), elongation of 203 percent and Shore (A) durometer of 39. Example 5 An in situ filled silicon elastomer was prepared using a branched methyl(trimethoxysilylethyl)siloxane as a precursor for in situ filler formation in the elastomer. A diorganosiloxane polymer mixture similar to that described in Example 1 was formed. Into the diorganosiloxane polymer mixture was mixed 13.2 weight percent of a branched methyl(trimethoxysilylethyl)siloxane described by the following formula: Si{OSiMe₂{CH₂CH₂Si(MeO)₃}}₄. The resulting mixture was catalyzed, cured and physical properties determined as described in Example 1. The silicone elastomer had a tensile strength of 876 psi (6.04 MPa), elongation of 86 percent and Shore (A) durometer of 58. Example 6 A composition similar to that described in Example 5 was cured using 0.2 weight percent tin octoate as catalyst. Physical properties of the resulting silicone elastomer were determined as described in Example 1. The silicone elastomer had a tensile strength of 677 psi (4.67 MPa), elongation of 53 percent and Shore (A) durometer of 57.

Claims: A method for preparing an in situ reinforced silicone elastomer comprising: (A) forming a mixture comprising (i) a diorganosiloxane polymer having the formula: Q3-aRaSi(OSiR₂) xOSiRaQ3-a , (ii) a 5 to 10 time stoichiometric excess of a hydrolyzable siloxane independently selected from hydrolyzable siloxanes having the formulae: (a) R₃Si(OSiQ¹R)m(OSiRR¹)nOSiR₃ , (b) (RQ¹SiO)y , and (c) Si(OSiR₂Q¹)₄ (iii) a tin salt catalyst; and then (B) contacting said mixture with water to effect formation of an in situ reinforced silicone elastomer; where each R is independently selected from a group consisting of (i) alkyls comprising one to six carbon atoms, (ii) alkenyls comprising two to six carbon atoms and (iii) aryls; where each R¹ is independently selected from hydrogen atom or R; where each Q is independently selected from a group of radicals consisting of (i) hydroxy, (ii) acyloxy comprising two to six carbon atoms and (iii) substituted or unsubstituted alkoxy comprising one to six carbon atoms and where each Q¹ is described by the formula: -R²SiQ3-bRb , wherein R² is a divalent hydrocarbon radical comprising one to 12 carbon atoms, R and Q are as previously described, and b=0 to 2; a=0 to 2, x=0 to 1000, m=3 to 500, n=0 to 497, m+n=3 to 500 and y=3 to 10. A method according to claim 1 where each R substituent of the diorganosiloxane polymer is methyl, x=10 to 1000, each Q substituent of the diorganosiloxane polymer is hydroxy and a equals two. A method according to claim 1, where each R¹ is independently selected from a methyl group or hydrogen atom. A method according to claim 1 where the hydrolyzable siloxane is a hydrolyzable cyclosiloxane and y is within a range of four to six. A method according to claim 1 where the hydrolyzable siloxane is added to the method at a concentration where the amount of Q substituents provided to the process by the hydrolyzable siloxane is five to ten times stoichiometric excess in relation to the Q substituents added to the method by the diorganosiloxane. A method according to claim 1 where the concentration of the tin salt catalyst is within a range of 0.005 to 10 weight percent of the combined weight of the catalyst, the diorganosiloxane polymer and the hydrolyzable siloxane. A method according to claim 1 where the mixture is cured in an environment having a relative humidity within a range of 20 percent to 100 percent. An in situ reinforced silicone elastomer obtainable by the method of claim 1.

Inventors: BERGSTROM DEBORA FRANCES, BURNS GARY THOMAS, GIWA-AGBOMEIRELE PATRICIA ANN, BERGSTROM, DEBORA FRANCES, BURNS, GARY THOMAS, GIWA-AGBOMEIRELE, PATRICIA ANN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1995
Main classification: C08L83
IPC codes: C08G77, C08L83, C08K5