Precured silicone emulsion

Abstract: A shelf stable aqueous silicone emulsion which yields an elastomer upon removal of the water is produced by combining a hydroxyl endblocked polydiorganosiloxane present as an emulsion of dispersed particles in water, a crosslinker and a tin catalyst, the tin catalyst being in the form of a divalent tin atom combined with organic radicals. The emulsion, after crosslinking of the polydiorganosiloxane, can be reinforced with colloidal silica without affecting the shelf life of the reinforced emulsion.

Description: This invention encompasses a process for producing an aqueous silicone emulsion which gives an elastomer upon removal of the water which can be reinforced with a silica filler and still retain it's properties upon aging, the shelf stable aqueous silicone emulsion resulting from the process and the elastomer produced by removal of the water from the emulsion. The process combines an anionically stabilized, hydroxyl endblocked polydiorganosiloxane; crosslinker selected from silanes (a) of the formula R a Si(OR') 4-a where R is hydrogen, monovalent hydrocarbon radical or substituted hydrocarbon radical of from 1 to 6 carbon atoms inclusive, functionalized hydrocarbon radicals or amine containing radicals, with the proviso that R be chosen such that the crosslinker must be capable of migrating from the aqueous phase into the emulsion micelle of (1), R' is a lower alkyl radical of from 1 to 6 carbon atoms inclusive or a radical of the formula -N=CR"₂ where R" is hydrogen or monovalent hydrocarbon radical of from 1 to 6 carbon atoms inclusive, a is 0 or 1, (b) siloxanes having at least 3 reactive groups bonded to silicon per molecule, the reactive group being selected from the group consisting of hydrogen and reactive group of the formula OR' where R' is as defined above, and (c) silazanes of the formula R a Si(NR"₂) 4-a where R, R" and a are as defined above; and a tin (II) catalyst, such as stannous octoate. The emulsion can be reinforced with colloidal silica, without loss of shelf life. The use of the divalent tin (II) as catalyst is a crucial part of the invention. A first aspect of the invention consists in an aqueous silicone emulsion having an improved shelf life and which yields an elastomer upon removal of the water under ambient conditions comprising the product obtainable by: (A) mixing: (1) 100 parts by weight of an hydroxyl-endblocked polydiorganosiloxane stabilized in emulsion with an anionic surfactant, and having a weight average molecular weight of above 50,000, the organic radicals being monovalent hydrocarbon radicals containing less than 7 carbon atoms per radical or 2-(perfluoroalkyl)ethyl radicals having less than 7 carbon atoms per radical, said emulsion being an emulsion of dispersed particles in water, and having a pH of from 4 to 10.5, (2) from 0.1 to 1.0 parts by weight of tin (II) (stannous) catalyst, and (3) from 0.1 to 5 parts by weight of crosslinker selected from (a) silanes of the formula R a Si(OR') 4-a where R is hydrogen, monovalent hydrocarbon radical or substituted hydrocarbon radical of from 1 to 6 carbon atoms inclusive, functionalized hydrocarbon radicals or amine containing radicals, with the proviso that R be chosen such that the crosslinker must be capable of migrating from the aqueous phase into the emulsion micelle of (1), R' is a lower alkyl radical of from 1 to 6 carbon atoms inclusive or a radical of the formula -N=CR"₂ where R" is hydrogen or monovalent hydrocarbon radical of from 1 to 6 carbon atoms inclusive, a is 0 or 1; (b) siloxanes having at least 3 reactive groups bonded to silicon per molecule, the reactive group being selected from hydrogen, reactive groups of the formula OR', where R' is as defined above, and amine radicals of the formula -NR"₂, where R" is as defined above; and (c) silazanes of the formula R a Si(NR"₂) 4-a , where R, R" and a are as defined above, the tin (II) catalyst (2) being added before or simultaneously with the crosslinker, then (B) aging at room temperature for sufficient time to allow crosslinking of the polydiorganosiloxane, to give an emulsion of a polydiorganosiloxane which yields an elastomer upon removal of the water, and also inactivation of said catalyst, whereby said aqueous emulsion can be optionally reinforced with colloidal silica filler without loss of shelf life. A second aspect of the invention consists in a process for producing an aqueous silicone emulsion having an improved shelf life and which yields an elastomer upon removal of the water under ambient conditions comprising: (A) mixing: (1) 100 parts by weight of an hydroxyl-endblocked polydiorganosiloxane stabilized in emulsion with an anionic surfactant, and having a weight average molecular weight of above 50,000, the organic radicals being monovalent hydrocarbon radicals containing less than 7 carbon atoms per radical or 2-(perfluoroalkyl)ethyl radicals having less than 7 carbon atoms per radical, said emulsion being an emulsion of dispersed particles in water, and having a pH of from 4 to 10.5, (2) from 0.1 to 1.0 parts by weight of tin (II) (stannous) catalyst, and (3) from 0.1 to 5 parts by weight of crosslinker selected from (a) silanes of the formula R a Si(OR') 4-a , where R is hydrogen, monovalent hydrocarbon radical or substituted hydrocarbon radical of from 1 to 6 carbon atoms inclusive, functionalized hydrocarbon radicals or amine containing radicals, with the proviso that R be chosen such that the crosslinker must be capable of migrating from the aqueous phase into the emulsion micelle of (1), R' is a lower alkyl radical of from 1 to 6 carbon atoms inclusive or a radical of the formula -N=CR"₂ where R" is hydrogen or monovalent hydrocarbon radical of from 1 to 6 carbon atoms inclusive, a is 0 or 1; (b) siloxanes having at - least 3 reactive groups bonded to silicon per molecule, the reactive group being selected from hydrogen, reactive groups of the formula OR', where R' is as defined above and amine radical of the formula -NR"₂, where R" is as defined above; and (c) silazanes of the formula R a Si(NR"₂) 4-a , where R, R" and a are as defined above, the tin (II) catalyst (2) being added before or simultaneously with the crosslinker, (B) aging at room temperature for sufficient time to allow crosslinking of the polydiorganosiloxane, to give an emulsion of a polydiorganosiloxane which yields an elastomer upon removal of the water, and also inactivation of said catalyst, whereby said aqueous emulsion can be optionally reinforced with colloidal silica filler without loss of shelf life. Hydroxyl-endblocked polydiorganosiloxane in emulsion form can be crosslinked to form what is known in the art as precrosslinked or precured silicone emulsions. Removal of water from these precured emulsions produces silicone elastomers. The precured silicone elastomer emulsions described by Johnson et al. in U.S. Patent No. 4,221,688 are of this type. The useful life (shelf life) of the emulsions of Johnson et al. is limited due to a reaction of the hydroxyl-endblocked polydiorganosiloxane and the silica filler in the presence of tetrafunctional tin catalyst. This invention encompasses a process for producing an aqueous silicone emulsion that is precured, but has improved shelf life. Crosslinking is accomplished in the emulsion by reaction of the hydroxyl-endblocked polydiorganosiloxane and the crosslinker in the presence of a divalent tin (tin (II)) catalyst. The anionically stabilized, hydroxyl endblocked polydiorganosiloxane used in this invention is now well known in the art. The hydroxyl-endblocked polydiorganosiloxane (A) is one which can be emulsified, and, when crosslinked, imparts elastomeric properties to the product obtained after the removal of the water from the emulsion, and which is anionically stabilized. Tensile strengths and elongations at break improve with increasing weight average molecular weight (Mw), with suitable tensile strengths and elongations obtained above 50,000 Mw. The maximum weight average molecular weight is one corresponding to a polymer which can be emulsified and which will give elastomeric properties to the product obtained after the water is removed from the emulsion. Weight average molecular weights up to about 1,000,000 for the hydroxyl-endblocked polydiorganosiloxane are expected to be practical for this invention. The preferred Mw for the hydroxyl-endblocked polydiorganosiloxanes are in the range of 200,000 to 700,000. The organic radicals of the hydroxyl-endblocked polydiorganosiloxane can be monovalent hydrocarbon radicals containing fewer than seven carbon atoms per radical and 2-(per-fluoroalkyl)ethyl radicals including 3,3,3-trifluoropropyl and 2-(perfluorobutyl)ethyl. The hydroxyl-endblocked polydiorganosiloxanes preferably contain organic radicals in which at least 50 percent are methyl. The hydroxyl-endblocked polydiorganosiloxanes are essentially linear polymers containing two organic groups per silicon atom but may include trace amounts of monoorganosiloxane or triorganosiloxy units present as impurities of the manufacturing process. The preferred hydroxyl-endblocked polydiorganosiloxanes are the hydroxyl-endblocked polydimethylsiloxanes. The preferred anionically stabilized, hydroxyl-endblocked polydiorganosiloxanes are those prepared by the method of anionic emulsion polymerization described by Findlay et al. in U.S Patent No. 3,294,725, issued December 27, 1966, which show the methods of -polymerization, the ingredients used, and the hydroxyl-endblocked polydiorganosiloxane obtained in an emulsion. Another method of preparing the anionically stabilized, hydroxyl-endblocked polydiorganosiloxane is described by Hyde et al. in U.S. Patent No. 2,891,920, issued June 23, 1959, which show the hydroxyl-endblocked polydiorganosiloxanes, the ingredients used, and their method of preparation. These methods and others are known in the art. The hydroxyl-endblocked polydiorganosiloxanes used in this invention are those which are anionically stabilized. For the purpose of this invention "anionically stabilized" means the hydroxyl-endblocked polydiorganosiloxane is stabilized in emulsion with an anionic surfactant. An anionic surfactant is a surface-active material consisting of a hydrophobic segment joined to an hydrophilic segment. The hydrophilic segment is an anion such as a sulphate, sulfonate, or phosphate. The hydrophobic segment is a long chain, linear or branched, hydrocarbon radical such as octadecyl or dodecyl. The anion portion is balanced by a cation such as sodium. This silicone emulsion is in the form of an oil-in-water emulsion, i.e., the polydiorganosiloxane is a dispersed phase of particles in a continuous phase of water. The shelf stability of the aqueous silicone emulsion of this invention is the result of the use of divalent tin as the tin catalyst, rather than the tetravalent tin which has been used in the past in aqueous silicone emulsions of this type. The divalent, stannous form of tin does not cause a reaction between the hydroxyl-endblocked polydiorganosiloxane and silica, as is caused when the tetravalent, stannic form of tin is used as the catalyst. Therefore, when stannous catalyst is combined with hydroxyl-endblocked polydiorganosiloxane and the crosslinker of this invention, crosslinking occurs readily; furthermore, the crosslinked polymer in emulsion can be reinforced with silica without a loss of shelf life. It is desirable that the tin (II) (stannous) catalyst be compatible in the polymer. The tin (II) (stannous) catalyst used in this invention can be any of the organic stannous carboxylates, such as stannous oleate, stannous neodecanoate, or stannous octoate. The preferred stannous catalyst is stannous octoate (stannous bis(2-ethylhexanoate)). The polydiorganosiloxane in the emulsion of this invention is crosslinked by the use of a crosslinker selected from (a) silanes of the formula R a Si(OR') 4-a where R is hydrogen, monovalent hydrocarbon radical or substituted hydrocarbon radical of from 1 to 6 carbon atoms, inclusive, functionalized hydrocarbon radicals, or amine-containing radicals, with the proviso that R be chosen such that the crosslinker must be capable of migrating from the aqueous phase into the emulsion micelle of (1), R' is a lower alkyl radical of from 1 to 6 carbon atoms, inclusive, or a radical of the formula -N=CR"₂ where R" is hydrogen or monovalent hydrocarbon radical of from to 6 carbon atoms, inclusive, a is 0 or 1; (b) siloxanes having at least 3 reactive groups bonded to silicon per molecule, the reactive group being selected from hydrogen, reactive groups of the formula OR', where R' is as defined above, and amine radicals of the formula -NR"₂, where R" is as defined above; and (c) silazanes of the formula R a Si(NR"₂) 4-a , where R, R", and a are as defined above. The crosslinker must be capable of migrating from the aqueous phase into the emulsion micelle of (1). Whether or not crosslinkers will adaquately perform in the process of this invention is determined experimentally. Crosslinker and tin (II) catalyst are added with stirring to a sample of the hydroxyl-endblocked polydiorganosiloxane emulsion, and the mixture is allowed to remain undisturbed for 24 hours to complete the crosslinking process. Usually 1 percent crosslinker and 0.5 percent tin (II) catalyst (based upon polymer weight) are sufficient. After crosslinking, films of the emulsion are cast, and, after drying, they are examined. If the films are tack free and elastomeric, the crosslinker is suitable; but if they are non-elastic, the crosslinker is unsuitable. If they are slightly elastomeric and tacky, crosslinking occurred to some extent, i.e., the crosslinker may be suitable if more is used. For example, silanes of the formula (CH₃O)₃SiCH₂CH₂NH₂ or (CH₃O)₃Si(CH₂)₃NH(CH₂)₂NH₂ did not function due to the fact that they remained in the aqueous phase when added to the emulsion and did not migrate into the micelle, so that the reaction with the hydroxyl-endblocked polymer could take place. Other crosslinkers which were found unsuitable included (CH₃O) ₃Si(CH₂)₂C₆H₄SO₂N₃ and (CH₃CH₂O)₃Si(CH₂)₃N=C=O. A silane of the formula (CH₃O)₃SiCH₂CH₂NHC₆H₅ did provide a crosslinked emulsion polymer since this silane does migrate into the micelle. R can be hydrogen; a monovalent hydrocarbon radical or substituted hydrocarbon radical such as an alkyl radical, for example, methyl, ethyl, propyl, isopropyl or butyl, an alkenyl radical such as vinyl or allyl, an halogenated hydrocarbon such as chloropropyl or trifluoropropyl, an aryl radical such as phenyl, functionalized hydrocarbon radicals such as glycidoxy, acryl or methacryl, and mercapto, and amine-containing radicals such as phenylaminopropyl but not aminopropyl or aminoethylaminopropyl, for the reasons given above. R' is chosen from methyl, ethyl, propyl, isopropyl, and butyl. Suitable silanes include ethylorthosilicate, normal propylorthosilicate, mercaptopropyltrimethoxysilane, methyltrimethoxysilane, phenyltrimethoxysilane, chloropropyltrimethoxysilane, trifluoropropyltrimethoxysilane, ethyltrimethoxysilane, triethoxysilane, and vinyltrimethoxysilane. Preferred silanes are vinyltrimethoxysilane and normal propylorthosilicate. The crosslinker can also be a ketoxime functional silane where the functional radical is of the formula -O-N=CR"₂, where R" is hydrogen or monovalent hydrocarbon radical of from 1 to 6 carbon atoms, inclusive. Examples of such silanes include methyltris(methylethylketoxime)silane, methyltris(dimethylketoxime)silane, methyltris(diethylketoxime)silane, and vinyltris(methylethylketoxime)silane. A preferred ketoximo silane is methyltris(methylethylketoxime)silane. The crosslinker can also be a siloxane having at least 3 reactive groups bonded to silicon per molecule, the reactive group being selected from hydrogen, OR', where R' is a lower alkyl radical of from 1 to 4 carbon atoms inclusive, ketoximo radicals of the formula -O-N=CR"₂, and an amine radical of the formula -NR"₂. Such crosslinkers are illustrated by low molecular weight organosilicon hydrides such as polymethylhydrogensiloxane, low molecular weight copolymers containing methylhydrogensiloxy and dimethylsiloxy groups, -(OSi(OEt)₂)-, (ethylpolysilicate), (OSiMeC₂H₄Si(OMe)₃)₄, and(OSiMeON=CR"₂)₄, where Me is the methyl radical and Et is the ethyl radical. Silicon compounds other than siloxanes having at least 3 reactive groups per molecule, such as (MeO)₃SiC₂H₄Si(OMe)₃ are also suitable, provided they move from the aqueous phase into the micelle so that they can react with the polymer. It is believed that the non-reactive radicals in the siloxane can be any group provided it does not inhibit the migration of the siloxane crosslinker into the micelle. The crosslinked emulsion produced by the method of claim 4 can be reinforced and/or extended through the addition of filler. Any of the common fillers which do not react with the emulsion are suitable. Inert fillers suitable for use in anionic silicone emulsions are well known. The fillers have -an average particle size below 10 micrometers, preferably below 2 micrometers. Examples of fillers include carbon blacks, titanium dioxide, aluminum oxide, calcium carbonate, zinc oxide, mica, and various pigments. Titanium oxide has been found to be particularly useful as an ultraviolet light screening agent. The crosslinked emulsion of this invention is particularly useful because it can be reinforced with colloidal silica. Although the common fumed silica can be used as reinforcement, preferred is a colloidal silica dispersion. There are commercially available aqueous dispersions of fumed colloidal silica. A preferred silica dispersion is a sol of colloidal silica such as the commercially available Nalcoag™ silicas from Nalco Chemical Company. The process of this invention begins by mixing the emulsion of hydroxyl-endblocked polydiorganosiloxane (1), tin (II) catalyst (2), and crosslinker (3). For each 100 parts by weight of hydroxyl-endblocked polydiorganosiloxane, there is from 0.1 to 1.0 part by weight of tin (II) catalyst. The amount of tin catalyst used has an effect upon how long it takes for the polydiorganosiloxane in the emulsion to react with the crosslinker to provide a crosslinked polymer within the emulsion micelles. A preferred amount of tin (II) catalyst is from 0.25 to 1.0 part by weight. The crosslinker is present in an amount of from 0.1 to 5 parts by weight. The rate of crosslinking and the modulus of the crosslinked elastomer formed by drying the emulsion can be varied by varying the amount of crosslinker used. As more crosslinker is used, the modulus is raised. It is not necessary to add more crosslinker than is required to react with the hydroxyl present on the polydiorganosiloxane. A preferred amount of crosslinker is from 0.5 to 2 parts by weight. When the crosslinker is a tetrafunctional material, a is 0, while a is 1 for a trifunctional crosslinker. During the process of mixing, preferably, the tin (II) catalyst is added to the emulsion of polydiorganosiloxane first, followed by the crosslinker. The order of mixing is not critical as the crosslinker can be added first, followed by the catalyst, or the crosslinker and catalyst can be mixed together and then added to the polydiorganosiloxane emulsion, provided there is no significant delay between the additions. As soon as the ingredients are mixed, the reaction between the hydroxyl-endblocked polydiorganosiloxane and the crosslinker will begin, resulting in the production of crosslinked polymer. The reaction is allowed to proceed until the emulsion gives an elastomer upon removal of the water. This point can be monitored by simply testing a portion of the mixture during the crosslinking process. The pH of the emulsion during crosslinking should be between 4 and 10.5, the preferred range being between 5 and 10.5. Outside of this range, crosslinking does not occur, presumably due to hydrolysis of both the crosslinker and the catalyst. Different crosslinkers give different rates of reaction. For example, vinyltrimethoxysilane requires approximately 1/2 hour to crosslink, while normal orthopropylsilicate requires approximately 2 hours to complete the crosslinking process. In both cases, the tin II level was at 0.5 pph. If the crosslinked emulsion is then allowed to shelf age, the catalyst will gradually become inactive, due to hydrolysis. The rate of hydrolysis of the catalyst is dependant upon the pH of the emulsion. It is preferred that the pH of the emulsion be raised a value above 11, at which point the catalyst hydrolyzes quickly and is no longer active. It is preferred that the emulsion be at a pH of from 5 to 10.5 during the crosslinking step. Once the catalyst is inactivated, the pH can be adjusted to any desired level. However, if the pH is below 3, polymerization will begin again. At this point in the process, the emulsion can be used to give a coating of a crosslinked polydiorganosiloxane. Such an emulsion may be used for the treatment of paper or fabrics, for example, by coating the substrate, then removing the water. Because the tin (II) catalyst is no longer active, the emulsion can be reinforced with colloidal silica, without causing shelf life problems. The colloidal silica is added to the emulsion after it is crosslinked and the catalyst is inactivated. If the silica is present before the polydiorganosiloxane is crosslinked, the tin (II) catalyst will not catalyze the reaction and the polydiorganosiloxane will not properly crosslink. It is preferred that the colloidal silica used for reinforcement be in the form of a suspension, and that the pH of the colloidal silica suspension be about that of the crosslinked polydiorganosiloxane emulsion to which it is being added. Extending filler can also be added to the crosslinked emulsion. Extending fillers are used to provide opacity, color, higher specific gravity, and reduced cost. Additional ingredients which can be added to the composition include such things as antifoam to aid during the mixing process, stabilizers such as organic amines, preferably, 2-amino-2-methyl-1-propanol, and pigments such as titanium dioxide and carbon black, to provide opaqueness or color to the composition. The emulsions of this invention can be used as coatings, and, when filled, to give a paste consistency, as sealants and adhesives. When the emulsions of this invention are dried by removal of the water, an elastomer results. Because colloidal silica can be used as a reinforcement without problems of shelf life or useful life, excellent physical properties for the elastomer are possible, regardless of the age of the emulsion. The following examples are included for illustrative purposes only and should not be construed as limiting the invention which is properly set forth in the appended claims. Example 1 To 200 g of an anionically emulsified polydimethylsiloxane emulsion having a solids content of about 70 percent by weight, the emulsion particles being less than 0.6 micrometers in average diameter and the polymer having a molecular weight average of greater than 240,000, and having a pH of 10.3, was added 0.35 g of stannous octoate (stannous bis(2-ethylhexanoate)) and the emulsion was stirred for 5 minutes. Next, 1.05 g of vinyltrimethoxysilane (VTM) was added dropwise to the emulsion with stirring. Following addition of VTM, the emulsion was stirred an additional 3 minutes, the container (one pint jar) was capped and the emulsion was allowed to remain undisturbed at room temperature for 1 hour. Following this period, the emulsion was again stirred while 5 g of 2-amino-2-methyl-1-propanol (AMP) was added dropwise. After all of the AMP was added, stirring was continued while 93 g of a 15 percent solids (by weight) sol of colloidal silica (Nalcoag™ 1115) was slowly added to the emulsion in a thin stream. Following addition of the colloidal silica, several drops of an antifoam, Nalco™ 2311, were added. Stirring was stopped and a portion of the emulsion was transferred to several 56.83 ml (2 ounce) vials which were then centrifuged lightly to exclude air bubbles in the emulsion. Emulsion was poured into several 100 mm diameter Petri dishes (8 g emulsion per dish) and the dishes were left uncovered and undisturbed for 24 hours. The resulting films were elastomeric and after 1 week of aging, mechanical properties were determined: 2.413 MPa (350 psi) -ultimate tensile strength, 1000 % ultimate elongation. The total solids content of this emulsion was approximately 50 percent by weight. It consisted of a precured emulsion containing 10 parts of silica per 100 parts of polymer (10 pph silica). In addition, it contained 0.25 pph tin (II) catalyst and 0.75 pph VTM, both based on polymer weight. Example 2 Either 2 normal HCl or 20 percent aqueous diethylamine was added to the anionically emulsified polydimethylsiloxane emulsion of Example 1 so as to produce a series of 7 emulsions having a pH range of from 2.6 to 11.0, inclusive. Each emulsion polymer was crosslinked by adding 0.25 pph VTM and 0.75 pph stannous octoate (both based on polymer by weight) following the procedure of Example 1. Next, each emulsion was filled with 10 pph (solids, based on polymer weight) Nalcoag™ 1115 colloidal silica and films were cast into Petri dishes, also following the procedure of Example 1. After aging the films for 1 week, mechanical properties were determined, the results are presented in Table I. pH Tensile Strength 2.6 no cure 3.1 no cure 4.6 260 psi (1.793 MPa) 6.8 280 psi (1.931 MPa) 9.1 250 psi (1.724 MPa) 10.3 290 psi (2 MPa) 11.0 no cure Example 3 A series of emulsion polymers was prepared using a procedure similar to that of Example 1 except that VTM, the crosslinker, was added to the emulsion before the addition of tin catalyst. In this series, time between addition of crosslinker and catalyst was varied from zero (both VTM and catalyst were mixed together before addition to emulsion) to 60 minutes. In all of the samples, crosslinker and catalyst were added dropwise to the emulsion with stirring. After addition of catalyst, each emulsion was stirred for 5 minutes longer, allowed to stand undisturbed for 1 hour and filled with 10 pph Nalco™ 1115 colloidal silica as described in Example 1. Also using the procedure of Example 1, films from each emulsion were cast, aged for 4 days and mechanical properties were determined, the results are given in Table II. Composition of these emulsions was the same as the one described in Example 1. Time Between Addition of VTM & Sn (II) octoate Tensile Strength 0 240 psi (1.655 MPa) 1 minute 260 psi (1.793 MPa) 15 minutes no cure 30 minutes no cure 60 minutes no cure The same experiment was repeated except that tin catalyst was added to the emulsions prior to addition of crosslinker. Composition of these emulsion was the same as those in Table II. Results of this are given in Table III. Time Between Addition of Sn (II) octoate & VTM Tensile Strength 0 240 psi (1.655 MPa) 1 minute 250 psi (1.724 MPa) 15 minutes 255 psi (1.758 MPa) 30 minutes 250 psi (1.724 MPa) Another series of emulsions was prepared using a procedure similar to the one of Example 1. In this series, time allowed for crosslinking was varied. Crosslinking was terminated by raising pH of the emulsions to 11-12 by addition of 2-amino-2-methyl-1-propanol (AMP). Thus the same procedure as that of Example 1 was used to prepare this series of emulsions except that the time between addition of VTM and AMP was changed. Results from this series are given in table IV. Compositions of emulsions in Table IV are the same as those in Tables I, II, and III and Example 1. Time for X-linking (time between VTM & AMP addition) Tensile Strength Elongation 5 minutes no cure 30 minutes 295 psi (2.034 MPa) 1200 % 1 hour 305 psi (2.103 MPa) 1080 % 2 hour 290 psi (2 MPa) 980 % 3 hour 310 psi (2.137 MPa) 970 % 20 hour 280 psi (1.931 MPa) 1000 % Example 4 A series of emulsions was prepared using the same procedure as that described in Example 1, except the amount of crosslinker, VTM, was changed. Elastomeric films were prepared also according to the procedure of Example 1 and their mechanical properties were determined, see Table V. Excluding the amount of VTM, composition of emulsions used to prepare elastomers in Table V was the same as the emulsion of Example 1. VTM (pph based on polymer, by wt.) Tensile Strength Elongation 0.25 120 psi (827 kPa) 400 % 0.50 300 psi (2.069 MPa) 620 % 0.75 380 psi (2.620 MPa) 920 % Another series of emulsions was prepared, also using the procedure described in Example 1. In this case, the crosslinker was a mixture of VTM and its partial hydrolysis products. Crosslinker consisted of approximately 50 percent VTM; the remainder was a mixture of siloxanes such as divinyltetramethoxydisiloxane, trivinylpentamethoxytrisiloxane and higher homologs up to and including the pentasiloxane. In this series, the composition of the emulsions was the same as for the emulsion of Example 1 except for the amount and type of crosslinker and the amount of colloidal silica. Elastomeric films-from these emulsions were prepared using the procedure of Example 1 and their mechanical properties were determined. The results are summarized in Table VI. VTM (pph) Nalco™ 1115 (pph) Tensile Strength Elongation 0.25 10 300 psi (2.069 MPa) 1000 % 0.125 15 (no cure) < 30 psi (207 kPa) < 50 % 0.25 15 390 psi (2.689 MPa) 970 % 0.375 15 370 psi (2.551 MPa) 950 % 0.50 15 315 psi (2.172 MPa) 720 % 0.75 15 325 psi (2.240 MPa) 650 % Example 5 Following the procedure of example 1, a series of emulsions was prepared using various alkoxysilanes as crosslinker. Except for the crosslinker, emulsions in this series had the same composition as the emulsion of Example 1. Mechanical properties of elastomers derived from these emulsions are given in Table VII. Crosslinker Tensile Strength Elongation 2 pph A 250 psi (1.724 MPa) 520 % 2 pph B 290 psi (2 MPa) 620 % 1 pph C 210 psi (1.448 MPa) 380 % 1 pph D 300 psi (2.069 MPa) 450 % 0.5 pph D 270 psi (1.862 MPa) 570 % 0.5 pph E 230 psi (1.586 MPa) 470 % 0.75 pph E 200 psi (1.379 MPa) 440 % 1.0 pph E 200 psi (1.379 MPa) 400 % 2.0 pph E 290 psi (2 MPa) 540 % 0.75 pph F 380 psi (2.620 MPa) 920 % 2.0 pph G 600 psi (4.137 MPa) 910 % A = Si(OEt)₄ (EOS) B = -(OSi(OEt)₂)- (EPS) C = (OSiMeC₂H₄Si(OMe)₃)₄ D = (MeO)₃SiC₂H₄Si(OMe)₃ E = MeSi(OMe)₃ (MTM) F = CH₂CHSi(OMe)₃ (VTM) G = Si(OC₃H₇)₄ (NPOS) Example 6 To 1 kilogram of the anionically emulsified polydimethylsiloxane emulsion of Example 1 in an one-half gallon jar was added dropwise with stirring enough 2 normal HCl to lower the pH of the emulsion to from 5 to 8. Next, stirring was continued while 5.52 g of VTM were added dropwise to the emulsion. After stirring for 5 minutes, 3.5 g of stannous octoate were added dropwise to the emulsion, also -with stirring. After addition of tin catalyst was complete, the emulsion was stirred for 5 minutes longer, the container was capped and the emulsion was allowed to remain undisturbed for 16 hours at room temperature. Next, the emulsion was stirred while 20 g of AMP were added dropwise. Two 100 g portions of the crosslinked emulsion polymer were transferred to smaller containers and to one of them were added, with stirring, 21 g of Nalcoag™ 1050 colloidal silica, a 50 percent solids (by weight) sol of colloidal silica. To the other emulsion were added, also with stirring, 58.3 g of Cabosperse™ SC-2. an aqueous, dispersed, fumed silica. These two samples consisted of emulsion polymer, 0.75 pph VTM, 0.5 pph Sn (II) catalyst and 15 pph silica. Two more emulsions were prepared which had compositions corresponding to the above-described emulsions except that a Sn (IV) catalyst was used instead of the Sn (II) catalyst: To 500 g of the anionically emulsified polydimethylsiloxane emulsion of Example 1 were added dropwise with stirring, 2.63 g of VTM followed by 3.5 g of 50 percent solids by weight emulsion of dioctyltindilaurate (Sn (IV) catalyst), also with stirring. Stirring was continued while 10 g of AMP were added dropwise, after which stirring was stopped, the container was closed and the emulsion was allowed to remain undisturbed for 72 hours at room temperature. Next, the emulsion was divided into two 200 g portions, and, to one, 42 g of Nalcoag™ 1050 colloidal silica were added with stirring and to the other, 117 g of Cabosperse™ SC-2 aqueous, dispersed, fumed silica were added, also with stirring. These two samples consisted of emulsion polymer, 0.75 pph VTM, 0.5 pph Sn (IV) catalyst and 15 pph silica. Elastomeric films were made from these four emulsions using the procedure described in Example 1, and their mechanical properties were determined. The emulsions were stored in closed containers and, periodically, samples were removed, elastomeric films were cast and their mechanical properties were determined. The results are given in Table VIII. Shelf age Catalyst Silica Tensile psi Elongation percent 0 Sn (II) Nalco™ 1050 100 (689.5 kPa) 850 1.5 mo Sn (II) Nalco™ 1050 110 (758 kPa) 1050 2 mo Sn (II) Nalco™ 1050 105 (724 kPa) 1030 3 mo Sn (II) Nalco™ 1050 110 (758 kPa) 1100 6 mo Sn (II) Nalco™ 1050 110 (758 kPa) 1070 0 Sn (IV) Nalco™ 1050 100 (689.5 kPa) 670 1.5 mo Sn (IV) Nalco™ 1050 80 (552 kPa) 560 2 mo Sn (IV) Nalco™ 1050 70 (483 kPa) 520 3 mo Sn (IV) Nalco™ 1050 65 (448 kPa) 400 5 mo Sn (IV) Nalco™ 1050 60 (414 kPa) 290 6 mo Sn (IV) Nalco™ 1050 55 (379 kPa) 230 0 Sn (II) Cabot™ SC-2 190 (1.310 MPa) 600 1.5 mo Sn (II) Cabot™ SC-2 185 (1.276 MPa) 800 2 mo Sn (II) Cabot™ SC-2 185 (1.278 MPa) 810 3 mo Sn (II) Cabot™ SC-2 190 (1.310 MPa) 820 6 mo Sn (II) Cabot™ SC-2 210 (1.448 MPa) 780 0 Sn (IV) Cabot™ SC-2 350 (2.413 MPa) 960 1.5 mo Sn (IV) Cabot™ SC-2 210 (1.448 MPa) 730 2 mo Sn (IV) Cabot™ SC-2 200 (1.379 MPa) 730 3 mo Sn (IV) Cabot™ SC-2 160 (1.103 MPa) 540 5 mo Sn (IV) Cabot™ SC-2 145 (1 MPa) 420 6 mo Sn (IV) Cabot™ SC-2 130 (0.896 MPa) 360 Sn (II) = stannous octoate; Sn (IV) = dioctyltindilaurate Example 7 To 200 g of the anionically emulsified polydimethylsiloxane emulsion of Example 1 were added with stirring enough 2N HCl to lower the pH of the emulsion to from 5 to 8. Next, stirring was continued while 1.05 of VTM were added dropwise, followed immediately by the dropwise addition of 0.70 g of stannous octoate. Stirring was stopped, the container was closed and the emulsion was allowed to remain undisturbed for -about 18 hours. Stirring was started and 4 g of AMP were added dropwise, followed by the addition of 140 g of Nalco™ 1115 colloidal silica (poured into the stirred emulsion as a thin stream). This emulsion consisted of emulsion polymer, 0.5 pph Sn (II) octoate, 0.75 pph VTM and 15 pph colloidal silica. A 20 g portion of the emulsion was transferred to a small vial, and was lightly centrifuged to exclude air bubbles. The emulsion was poured into 2 100 mm diameter plastics Petri dishes and allowed to remain uncovered and undisturbed for 1 week under ambient conditions. The mechanical properties of the resulting elastomeric film were determined. The remainder of the emulsion was stored in a closed container for 6 months under ambient conditions, and, at 2 months' intervals, portions were removed and cast into films, and the mechanical properties of the resulting films were determined. The results from this shelf age study are given in Table IX. Sheft Age Tensile Strength Elongation percent 0 420 psi (2.896 MPa) 760 2 months 400 psi (2.758 MPa) 780 4 months 430 psi (2.965 MPa) 770 6 months 450 psi (3.103 MPa) 840 10 months 430 psi (2.965 MPa)8 850 Example 8 To 200 g of the polydimethylsiloxane emulsion of Example 1, having a pH of 8.3, was added 0.70 g of stannous octoate (stannous bis(2-ethylhexanoate)) and the emulsion was stirred for 5 minutes. Next, 1.40 g of methyltris(ethylmethylketoxime)silane (MTO) were added dropwise to the emulsion with stirring. Following the addition of the MTO, the emulsion was stirred an additional 3 minutes, the container [0.568 litre (one pint) jar] was capped, and the emulsion was allowed to remain undisturbed for 20 hours at room temperature. Following this period, the emulsion was again stirred while 4 g of 2-amino-2-methyl-1-propanol (AMP) were added dropwise. After all of the AMP had been added, stirring was continued while 93 g of the sol of colloidal silica of Example 1 were slowly added to the emulsion in a thin stream. Following the addition of the colloidal silica, several drops of an antifoam, Nalco™ 2311, were added. Stirring was stopped and a portion of the emulsion was transferred to several 56.83 ml (2 ounce) vials, which were then centrifuged lightly to exclude air bubbles in the emulsion. The emulsion was poured into several 100 mm diameter Petri dishes (8 g emulsion per dish), and the dishes were left uncovered and undisturbed for 24 hours. The resulting films were elastomeric, and, after 1 week of aging, their mechanical properties were determined to be: 2.413 MPa (350 psi) ultimate tensile strength, and 850 % ultimate elongation. The total solids content of this emulsion was approximately 50 percent by weight. It consisted of a precured emulsion containing 10 parts of silica per 100 parts of polymer (10 pph silica). In addition, it contained 0.50 pph stannous catalyst and 2 pph MTO, both based on polymer weight. Example 9 This is a comparative example. To 681.3 g of an hydroxyl-endblocked polydimethylsiloxane fluid with a degree of polymerization (DP) of about 35 were added 25 g of a 30 % aqueous solution of sodium laurylsulfate, and the resulting mixture was stirred while 426 g of aqueous, colloidal silica (Nalco™ 1115) having a solids content of 15 percent were slowly added. Next, the mixture was homogenized using a laboratory single stage homogenizer at 51.71 MPa (7500 psi) to produce an oil-in-water emulsion also containing a dispersion of silica. The emulsion was polymerized by adding 5 g of 4N hydrochloric acid and stirring the emulsion for several minutes. The emulsion was allowed to stand undisturbed for 17 hrs at room temperature in a closed container. At the end of this period, the emulsion had thickened to a paste-like consistency, but it had not inverted or coagulated. Polymerization was terminated by adding sufficient 2-amino-2-methyl-propan-1-ol (AMP) with stirring to raise the pH of the emulsion to 7.5-9.0. This mixture consisted of an oil-in-water emulsion of (PDMS) polydimethylsiloxane also containing a dispersion of colloidal silica. The PDMS content of this emulsion was approximately 58 percent by weight, its silica content was approximately 10 pph (based on the weight of PDMS) and the total solids content of the mixture was approximately 63 percent. An attempt to cross-link the above emulsion polymer was made by adding 0.29 g of stannous octoate to an 100 g portion of the emulsion, with stirring, followed by the addition of 0.45 g of vinyltrimethoxysilane (VTM), also with stirring. Stirring was continued for about 5 minutes after the addition of VTM, and the emulsion was allowed to stand undisturbed for 16 hours at room temperature. A film was cast from this emulsion by pouring 8.0 g of emulsion into an 100 mm dia. plastics Petri dish, and allowing it to dry under ambient conditions for 20 hours. The resulting film was not elastomeric. Instead, it was polymer thickened with silica. Another film of the same emulsion was cast after allowing the emulsion to stand for 72 hours at room temperature. This film was dried for 16 hours and, upon examination, it too was found to be not elastomeric. To another 100 g portion of the emulsion polymer was added 0.58 g of dibutyltindilaurate followed by 0.45 g of VTM, both with stirring. A small amount of AMP was added, also with stirring, so as to raise the pH of the emulsion to about 10. After this mixture had been allowed to stand undisturbed for 24 hours at room temperature, a film was cast by pouring 8.0 g of emulsion into an 100 mm dia. petri dish. After the film had dried for 20 hours at room temperature it was examined and found to be elastomeric. Example 10 This is a comparative example. To 200 g of the polydimethylsiloxane emulsion of Example 1, having a pH of 9-10, were added, with stirring, 77.8 g of aqueous, dispersed, fumed silica (Cabot™ SC-2) having a solids content of 18 percent. Stirring was continued while 0.35 g of stannous octoate (stannous bis(2-ethylhexanoate)) was added and 5 minutes later, 1.05 g of vinyltrimethoxysilane (VTM) were added dropwise. Following the addition of VTM, the emulsion was stirred for an additional 3 minutes, the container [0.568 litre (one pint jar)] was capped, and the emulsion was allowed to remain undisturbed at room temperature for 2 hours. Following this period, a small portion (10 g) of the emulsion was poured into a Petri dish and the emulsion was allowed to dry at room temperature for about 16 hours. Upon examination, the dried film was not elastomeric. It consisted of thickened polymer. Example 11 To 200 g of an anionically emulsified polydimethylsiloxane emulsion having a solids content of about 70 percent by weight and a pH of about 8.3, the emulsion particles being less than 0.6 micrometer in average diameter and the polymer having a molecular weight average greater than 240,000, was added 0.70 g of stannous octoate (stannous bis(2-ethylhexanoate)), and the emulsion was stirred for 5 minutes. Next, 1.40 g of trimethylsiloxy-endblocked polydiorganosiloxane having an average of five methylhydrogensiloxane units and three dimethylsiloxane units per molecule with a silicon-bonded hydrogen atom content in the range of about 0.7 to 0.8 weight percent were added dropwise to the emulsion with stirring. Following the addition of the organosilicon hydride, the emulsion was stirred for an additional 3 minutes, the container [0.568 litre ((one pint jar)] was capped, and the emulsion was allowed to remain undisturbed for 20 hours at room temperature. Hydrogen evolution was noted by a slight pressurization of the container as well as significant bubble formation in the emulsion. The emulsion was again stirred while 4 g of 2-amino-2-methyl-1-propanol (AMP) were added dropwise. After all of the AMP were added, stirring was continued while 93 g of a 15 percent solids (by weight) sol of aqueous, colloidal silica (Nalcoag™ 1115) were slowly added to the emulsion in a thin stream. Following addition of the colloidal silica, several drops of an antifoam, Nalco™ 2311, were added. Stirring was stopped and a portion of the emulsion was transferred to several 56.83 ml (2 ounce) vials, which were then centrifuged lightly to remove bubbles from the emulsion. Emulsion was poured into several 100 mm diameter Petri dishes (8 g emulsion per dish), and the dishes were left -uncovered and undisturbed for 24 hours. The resulting films were elastomeric, and, after 1 week of aging under ambient conditions, their mechanical properties were determined to be 1.103 MPa (160 pounds per square inch) ultimate tensile strength and 480 percent ultimate elongation. The total solids content of this emulsion was approximately 50 percent by weight. It consisted of a precured emulsion containing 10 parts of silica per 100 parts of polymer (10 pph silica). In addition, it contained 0.50 pph stannous catalyst and 1 pph organosilicon hydride crosslinker, both based on polymer weight. Example 12 This is a comparative example. To 200 g of the emulsion of Example 11 was added 0.70 g of stannous octoate (stannous bis(2-ethylhexanoate)) and the emulsion was stirred for 5 minutes. Next, 1.40 g of 3-(2-aminoethylamino)propyltrimethoxysilane were added dropwise to the emulsion with stirring. Following the addition of the aminofunctional silane, the emulsion was stirred for an additional 3 minutes, the container [0.568 litre (one pint jar)] was capped, and the emulsion was allowed to remain undisturbed for 20 hours at room temperature. A portion of the emulsion was transferred to a small vial and the vial was centrifuged lightly to remove air bubbles from the emulsion. 10 g of emulsion were poured into an 100 mm diameter Petri dish, and the dish was left uncovered and undisturbed for 24 hours. The resulting film was not elastomeric, indicating that the emulsion polymer had not been crosslinked. The same experiment was repeated except that 2.8 g of the aminofunctional silane were used. The dried film from this experiment was only very slightly elastomeric and it was extremely tacky. Nalco™ 1115 colloidal silica was added to this emulsion so as to yield an emulsion having 10 parts by weight of (dry) silica per 100 parts of polymer. A film cast from this emulsion and dried for 20 hours was not elastomeric. The same results were obtained when the experiment was repeated using 4.2 g of the aminofunctional silane as crosslinker. The same results were obtained when 1.4 g of amonopropyltriethoxysilane were used as a crosslinker.

Claims: An aqueous silicone emulsion having an improved shelf life and which yields an elastomer upon removal of the water under ambient conditions comprising the product obtainable by: (A) mixing: (1) 100 parts by weight of an hydroxyl-endblocked polydiorganosiloxane stabilized in emulsion with an anionic surfactant, and having a weight average molecular weight of above 50,000, the organic radicals being monovalent hydrocarbon radicals containing less than 7 carbon atoms per radical or 2-(perfluoroalkyl)ethyl radicals having less than 7 carbon atoms per radical, said emulsion being an emulsion of dispersed particles in water, and having a pH of from 4 to 10.5, (2) from 0.1 to 1.0 parts by weight of tin (II) (stannous) catalyst, and (3) from 0.1 to 5 parts by weight of crosslinker selected from (a) silanes of the formula R a Si(OR')₄₋ a where R is hydrogen, monovalent hydrocarbon radical or substituted hydrocarbon radical of from 1 to 6 carbon atoms inclusive, functionalized hydrocarbon radicals or amine containing radicals, with the proviso that R be chosen such that the crosslinker must be capable of migrating from the aqueous phase into the emulsion micelle of (1), R' is a lower alkyl radical of from 1 to 6 carbon atoms inclusive or a radical of the formula -N=CR"₂ where R" is hydrogen or monovalent hydrocarbon radical of from 1 to 6 carbon atoms inclusive, a is 0 or 1; (b) siloxanes having at least 3 reactive groups bonded to silicon per molecule, the reactive group being selected from hydrogen, reactive groups of the formula OR', where R' is as defined above, and amine radicals of the formula -NR"₂, where R" is as defined above; and (c) silazanes of the formula R a Si(NR"₂) 4-a , where R, R" and a are as defined above, the tin (II) catalyst (2) being added before or simultaneously with the crosslinker, then (B) aging at room temperature for sufficient time to allow crosslinking of the polydiorganosiloxane, to give an emulsion of a polydiorganosiloxane which yields an elastomer upon removal of the water, and also inactivation of said catalyst, whereby said aqueous emulsion can be optionally reinforced with colloidal silica filler without loss of shelf life. The emulsion of Claim 1, in which, during inactivation of the catalyst by shelf aging, the pH of the emulsion is raised to above 11, at which point said catalyst (2) is no longer active. The emulsion of Claim 1 or Claim 2, in which there is also added a colloidal filler after step (B). A process for producing an aqueous silicone emulsion having an improved shelf life and which yields an elastomer upon removal of the water under ambient conditions comprising: (A) mixing: (1) 100 parts by weight of an hydroxyl-endblocked polydiorganosiloxane stabilized in emulsion with an anionic surfactant, and having a weight average molecular weight of above 50,000, the organic radicals being monovalent hydrocarbon radicals containing less than 7 carbon atoms per radical or 2-(perfluoroalkyl)ethyl radicals having less than 7 carbon atoms per radical, said emulsion being an emulsion of dispersed particles in water, and having a pH of from 4 to 10.5, (2) from 0.1 to 1.0 parts by weight of tin (II) (stannous) catalyst, and (3) from 0.1 to 5 parts by weight of crosslinker selected from (a) silanes of the formula R a Si(OR')₄₋ a , where R is hydrogen, monovalent hydrocarbon radical or substituted hydrocarbon radical of from 1 to 6 carbon atoms inclusive, functionalized hydrocarbon radicals or amine containing radicals, with the proviso that R be chosen such that the crosslinker must be capable of migrating from the aqueous phase into the emulsion micelle of (1), R' is a lower alkyl radical of from 1 to 6 carbon atoms inclusive or a radical of the formula -N=CR"₂ where R" is hydrogen or monovalent hydrocarbon radical of from 1 to 6 carbon atoms inclusive, a is 0 or 1; (b) siloxanes having at least 3 reactive groups bonded to silicon per molecule, the reactive group being selected from hydrogen, reactive groups of the formula OR', where R' is as defined above and amine radical of the formula -NR"₂, where R" is as defined above; and (c) silazanes of the formula R a Si(NR"₂)₄₋ a , where R, R" and a are as defined above, the tin (II) catalyst (2) being added before or simultaneously with the crosslinker, (B) aging at room temperature for sufficient time to allow crosslinking of the polydiorganosiloxane, to give an emulsion of a polydiorganosiloxane which yields an elastomer upon removal of the water, and also inactivation of said catalyst, whereby said aqueous emulsion can be optionally reinforced with colloidal silica filler without loss of shelf life. The process of Claim 4, in which there is added step (C) of raising the pH of the emulsion to a value greater than 11 to destroy the activity of the catalyst (2). The process of Claim 4, in which there is added step (D) of the addition of colloidal silica filler. A reinforced elastomer obtainable from an aqueous silicone emulsion according to any of claims 1 to 3 by the addition of colloidal silica as a filler after the inactivation of the catalyst (2) in said emulsion, the addition of said colloidal silica filler being followed by the removal of water from the resulting mixture to yield said reinforced elastomer.

Inventors: LILES DONALD TAYLOR, LILES, DONALD TAYLOR
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1990
Main classification: C08L83
IPC codes: C08J3, C08K5, C08L83, C08K3, B01J31