Moisture curable organosiloxane compositions exhibiting extended workability

Abstract: Low concentrations of a liquid silanol terminated polydiorganosiloxane, with a number average molecular weight less than 1000, extend the working time of tin-catalyzed, moisture-curable organosiloxane compositions without substantially decreasing the composition cure rate or adversely affecting the physical properties of cured materials, particularly elastomers, prepared using these compositions.

Description: This invention relates to moisture curable organosiloxane compositions. More particularly, it provides additives for organosiloxane compositions that cure by the reaction of silanol-containing polyorganosiloxanes with organosilicon compounds containing silicon-bonded hydrolyzable groups. Preferred additives impart increased working time to these compositions without substantial reduction in cure rates. Various proposals have been made regarding curable silicone masses. For example, DE3001127 discloses moisture curable compositions which rely on use of silicone resin. These materials are complex and non linear and generally speaking are significantly cross-linked structures which lead to different products from those achieved with silanols which are linear or branched. These resins frequently are not liquids and its is necessary therefore to employ them as solutions in organic solvents. The formulations referred to in the specification do not contain reinforcing silica in "filler quantities". This specification does not appear to contain any suggestion that one may use a silanol terminated polydiorganosiloxane of low molecular weight to provide a working time extender. The prior art also describes organosiloxane compositions which cure to yield crosslinked elastomers and other useful products in the presence of moisture by the reaction of polyorganosiloxanes containing at least two silanol groups per molecule with organosilicon compounds containing at least three silicon-bonded alkoxy or other hydrolyzable groups, per molecule as the curing agent. This reaction is typically catalyzed by tin compounds or organotitanium compounds. This prior art is represented by US-A 3,607,801. When the curing agent is an alkoxy silane and the curing catalyst is a tin compound, the reactive ingredients of these compositions are typically packaged in two containers with the curing agent and curing catalyst in one container and the silanol-containing polyorganosiloxane in a second container. The curing reaction occurs when the contents of the two containers are combined in the presence of atmospheric moisture. The time interval over which the composition can be fabricated by pouring, molding, extrusion, calendaring or other known techniques is referred to in the art as the "working time" of the composition. GB 2,076,841 is concerned with modifying the cure rate of platinum catalysed addition cured silicone rubber. These compounds require presence of a hydrogen bearing silicone and of a silicone bearing unsaturated groups and they require a platinum catalyst. It proposes to employ a so called silanol containing material to regulate the cure rate of these compounds. The examples employ either dimethyl silanol chain stopped polydimethyl siloxane fluid or untreated silica filler for regulating the cure. It is important to note that the curing system involved in the compounds of GB 2,076,841 is different from the curing system employed in the compounds of the claimed invention. In particular the condensation of silanol with curing agent in presence of moisture as referred to in the subject application can occur readily at room temperature. The addition cure reation referred to in the specification of GB 2,076,841 appears to be contemplated primarily for heat cure processes. The length of time over which the portion of the composition containing the silanol-terminated polyorganosiloxane can be stored without exhibiting a substantial increase in viscosity is referred to in the art as "storage stability". For certain applications, such as mold making, it is desirable to prepare relatively large batches of curable material containing all of the reactive ingredients, including the curing agent and curing catalyst. Then, it is necessary to have the material remain workable over a relatively long time period, by delaying the initiation of the curing reaction, without substantially decreasing the rate of this reaction once it begins. We have found that a small amount of unreacted liquid, silanol-terminated polydiorganosiloxane extends the working time of tin-catalyzed, moisture curable organosiloxane compositions without substantially decreasing the cure rate of the composition or adversely affecting the physical properties of cured materials, particularly elastomers, prepared using these compositions. This invention provides a method of preparing a moisture curable filled silicone composition having an extended working time which comprises mixing components consisting essentially of (A) a liquid curable linear or branched polydiorganosiloxane containing at least two silanol groups per molecule and having a number average molecular weight greater than 20,000; (B) as the curing agent, an organosilicon compound containing at least three silicon-bonded hydrolysable gbroups per molecule, wherein the concentration of said curing agent is sufficient to cure said polyorganosiloxane in the presence of moisture; (C) as the curing catalyst, a tin compound at a concentration of 0.5 to 5 weight percent based on the weight of (A) to promote curing of said composition in the presence of moisture, (E) reinforcing filler and (F) optionally conventional adjuvants for example liquid diluent, non-reinforcing filler, surfactant, stabiliser and colouring matter, which method comprises incorporating into said mixture a silanol-terminated polydiorganosiloxane having a number average molecular weight (Mn) less than 1000 at a concentration of at least 0.6 weight percent, based on the total weight of said composition as a working time extender. This invention also provides a method for extending the working time of these compositions that cure by the reaction of a polyorganosiloxane, an organosilicon compound, moisture and a tin compound catalyst. Our method comprises including as an ingredient of the curable composition at least 0.6 weight percent of a silanol-functional polydiorganosiloxane exhibiting a Mn of less than 1000. The presence of at least 0.6 weight percent, based on the total weight of said composition, of a low-molecular weight, liquid, silanol-functional polydiorganosiloxane, ingredient D, extends the working time of our curable compositions. This ingredient also lengthens the time period during which a mixture of the curable polyorganosiloxane, ingredient A, and a reinforcing filler, typically a finely divided form of silica, can be stored without exhibiting any substantial increase in viscosity resulting from an interaction between ingredient A and the reinforcing filler, frequently referred to as "creping" or "crepe hardening". This phenomenon is indicated by a substantial viscosity increase in silica-filled, organosiloxane compositions following blending. It is believed due to interaction between hydroxyl groups present on the filler and curable hydroxyl-substituted polyorganosiloxanes. The Mn of ingredient D is less than 1000, preferably from 500 to 1000. This molecular weight is too low to form a cured material with any useful level of physical properties if it were to be reacted with the curing agent portion of the present compositions as the only silanol-functional polyorganosiloxane present in the composition. Liquid silanol terminated polydiorganosiloxanes of ingredient D are often used as filler treating agents for curable organosiloxane compositions to prevent or minimize creping. The organic groups bonded to the silicon atoms of ingredient D are monovalent unsubstituted or substituted hydrocarbon radicals that preferably contain from 1 to 6 carbon atoms. Polydiorganosiloxanes suitable for use as ingredient D are represented by the formula HO[(R 1 ) 2 SiO] n H. The two hydrocarbon radicals represented by R 1 are individually selected from monovalent substituted and unsubstituted hydrocarbon radicals and n represents a degree of polymerization equivalent to a Mn of less than 1000. Hydrocarbon radicals represented by R 1 include alkyl, such as methyl and ethyl; substituted alkyl, such as chloromethyl or 3,3,3-trifluoropropyl; cycloalkyl, such as cyclohexyl; aryl such as phenyl and aralkyl such as benzyl. Preferably, at least one of the two R 1 radicals is methyl and the remaining radical is methyl, phenyl or 3,3,3-trifluoropropyl. Preferably, at least 80 percent of the R 1 radicals are the same as the hydrocarbon radicals present in ingredient A to ensure compatibility of these ingredients. These hydrocarbon radicals are preferably methyl. The concentration of ingredient D is typically from 0.6 to 10 weight percent, preferably from 1 to 5 percent, based on the total weight of our curable organosiloxane compositions. Organosiloxane compositions suitable for use with our working time extenders will cure in the presence of atmospheric moisture by the reaction of a polyorganosiloxane containing at least two silanol groups with an organosilicon compound containing at least three silicon-bonded alkoxy or other hydrolyzable groups. The curable polyorganosiloxanes of ingredient A contain at least two silanol groups per molecule and can exhibit a linear or branched structure. Preferred polyorganosiloxanes are liquids at 25°C. The repeating units of ingredient A can be represented by the general formula R 2 b SiO (4-b)/2 , where R 2 represents an unsubstituted or substituted monovalent hydrocarbon radical and b is 1 or 2. When b is 2, the hydrocarbon radicals of R 2 can be identical or different. This ingredient can contain one, two or more types of repeating units. To achieve a useful level of tensile strength and other physical properties in our cured materials, the Mn of ingredient A should be at least 20,000, preferably from 25,000 to 75,000. The viscosity of polyorganosiloxanes with Mn above 100,000 are typically too viscous for convenient and ready processing, particularly when blending with the other ingredients of this composition using conventional mixing equipment. Curing agents for ingredient A are typically organosilicon compounds containing an average of at least three silicon-bonded hydrolyzable groups per molecule (ingredient B). Preferred hydrolyzable groups are alkoxy containing from 1 to 4 carbon atoms, acetoxy and ketoximo, such as methylethylketoximo. The curing agent can be a silane, disiloxane or a polyorganosiloxane. Silanes are generally preferred, based on their cost and availability. Preferred silanes are methyltrimethoxysilane, phenyltrimethoxysilane, methyltriethoxysilane, methyltriacetoxysilane, methyltri(methylethylketoximo)silane, alkyl orthosilicates such as tetraethyl orthosilicate and condensation products of these orthosilicates, which are typically referred to as alkyl polysilicates. The concentration of curing agent is sufficient to convert the composition to an elastomeric or resinous material exhibiting the desired physical properties in the presence of moisture. Typical moisture curable compositions contain from 0.5 to 6 weight percent of alkoxy, or other hydrolyzable group, based on the total weight of the curable composition. In addition to curable polyorganosiloxane (A) and curing agent (B), our curable compositions include a tin-containing catalyst (C) to promote hydrolysis of the hydrolyzable groups present on the curing agent in the presence of moisture. Useful catalysts are divalent tin salts of carboxylic acids such as stannous acetate or stannous octoate and organotin compounds such as dimethyltin dineodecanoate, dibutyltin diacetate, dibutyltin dilaurate and dioctyltin diacetate. The carboxylic acid portion of these tin compounds contain from 1 to 20 carbon atoms. The concentration of curing catalyst appears critical to achieving a practical cure rate following the inhibition period resulting from the presence of our working time extender. In preferred compositions, this concentration is from 0.5 to 5 weight percent, based on the weight of the curable polyorganosiloxane (ingredient A). A particular advantage of our working time extenders is their ability to increase the concentration, and/or the activity, of the catalyst to achieve a more rapid cure rate and achieve the desired working time before initiation of the curing reaction without adverse affect on the storage stability of the composition. In addition to ingredients A, B, C and D, our compositions can contain additional ingredients to modify the properties of the curable composition or the cured materials prepared using this composition. These additional ingredients include liquid diluents; reinforcing fillers, such as finely divided silicas of the fume or precipitated type; non-reinforcing fillers, such as quartz or calcium carbonate; water to accelerate curing; surfactants; stabilizers to inhibit degradation by heat or ultraviolet light; anti-oxidants; dyes; pigments and flame retardants. Non-reactive polyorganosiloxanes are a preferred class of diluents. The curable compositions of this invention are prepared by blending all of the ingredients together. Curing begins when the composition is exposed to moisture, and requires from several minutes to several hours, depending upon the relative humidity in the curing environment; the temperature or the type and concentration of curing catalyst and the working time extender. Using preferred working time extenders, the initiation of the curing reaction can be delayed up to 8 hours while still retaining commercially useful curing times. While longer working times are achieved by increasing the extender concentration, the accompanying increase in cure time becomes so large as to make the compositions impractical for many commercial applications. When it is desired to prepare our compositions more than one or two days before curing them, the compositions are preferably stored in two containers, one of which contains ingredients A, D and any water. The curing agent and curing catalyst are packaged in a separate container. Fillers and other optional ingredients can be packaged in either or both containers, so long as these are substantially free of water. The elastomers prepared using our compositions are suitable for use in a variety of end use applications, including sealants, coating materials and potting compounds or encapsulants for electrical and electronic devices. These compositions are particularly useful for preparing molds. The following examples describe preferred curable compositions of the present invention and should not be interpreted as limiting the invention of the claims. Unless otherwise indicated, all parts and percentages in the examples are by weight and viscosities are the values measured at 25°C. Example 1 This example demonstrates the increase in working time achieved by using a working time extender of the present invention. The base portion of a curable organosiloxane composition was prepared by blending the following ingredients to homogeneity: 42 parts of a silanol-terminated polydimethylsiloxane exhibiting a viscosity of 50 Pa·s; 21 parts of a trimethylsiloxy-terminated polydimethyl-siloxane exhibiting a viscosity of 0.325 Pa·s; 3 parts,of hexamethyldisilazane; 21 parts of a precipitated silica exhibiting an average particle size of 4 micrometers; 1 part of water; 11 parts of zirconium silicate exhibiting an average particle size of 5 micrometers; 42 parts of a silanol-terminated polydimethylsiloxane exhibiting a viscosity of 10 Pa·s; 0.8 part of an emulsion prepared by blending 35 parts of water, 60 parts of a liquid trimethylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of 0.35 Pa·s, 4 parts of a non-ionic surfactant, (2,6,8-trimethyl-4-nonyloxypolyethyleneoxyethanol); and 1 part of sodium polyethylene oxide octylphenoxy sulfonate. A curing agent/catalyst mixture was prepared by blending 7 parts of a trimethylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of 0.05 Pa·s, 2 parts of ethyl polysilicate, 0.6 part of dibutyltin dilaurate and 0.5 part of a green pigment. Curable compositions were prepared by adding 1 part of this mixture to the base portion of the compositions, which is equivalent to 0.6 weight percent of the catalyst based on the weight of the curable composition. The silanol-terminated polydimethylsiloxane used to extend the working time of our compositions exhibited an Mn of 850. This polymer will be referred to hereinafter as the "working time extender". The working time extender was added to the other ingredients of the curable composition as a 15 weight percent solution in a 1:1 weight ratio mixture of two trimethylsiloxy-terminated polydimethylsiloxanes, one of which had a viscosity of 0.05 Pa·s and the second having a viscosity of 0.02 Pa·s. Curable compositions of this invention were prepared by blending to homogeneity 10 parts of the base with 1 part of the curing agent/catalyst mixture and then adding the amounts of working time extender solution reported in Table 1. The viscosity of the initial compositions were measured using a Brookfield model HAF viscometer equipped with a number 6 spindle. The compositions were allowed to stand at a temperature of 25°C. and a relative humidity of 50% ± 5%. The viscosity measurement was repeated at the time intervals reported in Table 1. The time intervals are measured from the preparation of the curable composition The time interval required for the viscosity of each of the compositions to reach a viscosity 1.5 Pa·s, a practical upper limit for workability, is also reported in Table 1. To determine the effect of concentration of working time extender on the physical properties of cured elastomers prepared from our curable compositions, each of the compositions was allowed to cure as a 0.15 cm. slab for two weeks at a temperature of 25°C. and a relative humidity of 50 percent. The specimens required for the ASTM test procedures for measuring tensile strength, tear strength and durometer hardness were cut from each of the cured slabs. These properties of the elastomers are summarized in Table 2. Viscosity (Pa·s) Working Time Extender (Wt%) After X Minutes, X= 0 0.6 1.2 1.8 0 23 17 19 9 7 23 17 19 9 30 34 20 20 10 75 133 26 23 11 120 Gel 39 28 12 190 400 400 15 225 Gel 62 18 285 150 21 340 Gel 71 370 250 375 - 4000 Additional Working Time (Minutes) 0 90 170 275 Working Time Extender (Wt%)* Tensile Strength (Mpa) Tear Strength Kn/m Durometer Shore A Scale 0 2.28 21.5 23 0.6 2.44 21.5 21 1.2 2.31 17.3 17 1.8 2.13 13.3 12 To demonstrate the effect of a low catalyst concentration on the working time and the cure rate of an organosiloxane composition of the present invention, a comparison curable composition containing 0.31 weight percent of dibutyltin dilaurate was prepared and evaluated. This concentration is below the preferred lower catalyst limit of 0.5 percent, based on the weight of the curable composition. The base portion was the same as described in the preceding example and the curing agent/catalyst mixture contained the following ingredients: 76 parts of a trimethylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of 0.05 Pa·s; 20 parts of tetraethyl orthosilicate; 3.4 parts of dibutyltin dilaurate; and 0.25 part of a red dye. A mixture of 10 parts of the base with 1 part of this curing agent/catalyst mixture contained 0.31 weight percent of the catalyst. This comparison curable composition was combined with 3, 6 or 9 weight percent of the 15 weight percent solution of the working life extender also described in the preceding example. The resultant mixture was blended to homogeneity. The time required for the composition to achieve a viscosity of 150 Pa·s at a temperature of 25°C. and a relative humidity of 50 ± 5 % is recorded in Table 3. Working Time Extender (%)* Time To Achieve 150 Pa·s Viscosity (Minutes) 0 71 0.45 164 0.9 304 1.35 1550 The compositions required from 1-3 days to develop a dry, non-tacky surface. They also needed from 4-10 days to cure completely, which would not be acceptable for a commercial scale operation. Example 2 This example demonstrates the increase in storage stability and working time for compositions of different initial viscosities that can be achieved using a working time extender of this invention. Two batches of a base portion of a curable organosiloxane composition containing types and amounts of ingredients described in Example 1 were prepared without the addition of a working time extender. These will be referred to hereinafter as batches 1 and 2. Four months after being prepared the viscosity of each batch of base was measured as also described in Example 1. The viscosity of batch 1 was 53 Pa·s and the viscosity of batch 2 was 44.5 Pa·s. At that time, 2 weight percent of a working time extender was added to a portion of each batch and the four compositions were stored under ambient conditions for an additional 6 months, at which time the viscosity measurments were repeated. The viscosities are recorded in Table 4. The working time extender was a silanol-terminated polydimethylsiloxane exhibiting an Mn of 850. Thirty-five days following addition of the working time extender, 10 parts of each of the four base compositions were blended with 1 part of a curing agent/catalyst mixture prepared by blending 7 parts of a trimethylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of 0.05 Pa·s; 2 parts of ethyl polysilicate; 0.6 part of dibutyltin dilaurate and 0.5 part of a green pigment. The curable compositions contained 1.4 weight percent of the catalyst based on the total weight of the curable composition. The working time of each of the curable compositions was determined by measuring the time required for the viscosity of the compositions to reach a value of 50 Pa·s. This time interval is recorded in Table 5. Storage Stability of Base Portions Composition 1 Composition 2 Initial Viscosity 53.0 Pa·s 44.5 Pa·s Final Viscosity 0% Extender 130 Pa·s 90 Pa·s 2% Extender 51 Pa·s 40 Pa·s Working Time Extender (%) Working Time (Minutes) Composition 1 Composition 2 0 40 50 2 180 180

Claims: A method of preparing a moisture curable filled silicone composition having an extended working time which comprises mixing components consisting essentially of (A) a liquid curable linear or branched polydiorganosiloxane containing at least two silanol groups per molecule and having a number average molecular weight greater than 20,000; (B) as the curing agent, an organosilicon compound containing at least three silicon-bonded hydrolysable groups per molecule, wherein the concentration of said curing agent is sufficient to cure said polyorganosiloxane in the presence of moisture; (C) as the curing catalyst, a tin compound at a concentration of 0.5 to 5 weight percent based on the weight of (A) to promote curing of said composition in the presence of moisture, (E) reinforcing filler and (F) optionally conventional adjuvants for example liquid diluent, non-reinforcing filler, surfactant, stabiliser and colouring matter, which method comprises incorporating into said mixture a silanol-terminated polydiorganosiloxane having a number average molecular weight (Mn) less than 1000 at a concentration of at least 0.6 weight percent, based on the total weight of said composition as a working time extender. A method according to Claim 1 wherein said working time extender has the general formula HO[(R 1 ) 2 SiO] n H, the two R 1 substituents on each silicon atom are individually selected from monovalent substituted and unsubstituted hydrocarbon radicals and n represents a degree of polymerization equivalent to a number average molecular weight of less than 1000; the concentration of said working time extender is from 0.6 to 10 weight percent, based on the total weight of said composition; the repeating units of said curable polyorganosiloxane correspond to the formula R 2 b SiO (4-b)/2 , wherein R 2 represents an unsubstituted or substituted monovalent hydrocarbon radical and b is 1 or 2. A method according to Claim 2 wherein the number average molecular weight of said working time extender is from 500 to 1000; at least one of the two hydrocarbon radicals bonded to each silicon atom of said working time extender and said curable polyorganosiloxane is methyl and the remaining hydrocarbon radicals are selected from methyl, phenyl or 3,3,3-trifluoropropyl; the concentration of said working time extender is from 1 to 5 weight percent, based on the weight of said composition; the number average molecular weight of said curable polyorganosiloxane is at least 20,000; the hydrolyzable groups present on said curing agent contain from 1 to 4 carbon atoms and are selected from alkoxy, acetoxy and ketoximo; the concentration of hydrolyzable groups in said curing agent constitute from 0.5 to 6 weight percent, based on the weight of said composition; said curing catalyst is selected from divalent tin salts of carboxylic acids or organotin compounds and constitutes from 0.5 to 5 weight percent of said composition. A method according to Claim 1 wherein the number average molecular weight of said curable polyorganosiloxane (A) is from 25,000 to 75,000; said curing agent is selected from alkoxysilanes or alkyl polysilicates and said curing catalyst is a dialkyltin salt of a carboxylic acid.

Inventors: DONATELLI JOAN MARIE, LARSON KENT ROBERT, MCMAHON DANIEL FRANCIS, DONATELLI, JOAN MARIE, LARSON, KENT ROBERT, MCMAHON, DANIEL FRANCIS
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1995
Main classification: C08L83
IPC codes: C08K5, C08L83