HEAT-CURABLE SILICONE COMPOSITIONS, USE THEREOF AND STABILIZER THEREFOR

Abstract: Organosilicon compositions which cure by way of a metal-catalyzed reaction of silicon-bonded hydroxyl radicals and/or silicon-bonded olefinic hydrocarbon radicals with silicon-bonded hydrogen atoms are stabilized for hours at room temperature by the incorporation of a hydrocarbon­ oxy-alkyl maleate. However, the stablized compositions are rapidly curable at low elevated temperature. Liquid organo­ polysiloxane compositions of this invention are particularly useful for coating operations, particularly paper coating operations to prepare an adhesive-releasing layer.

Description: The present invention relates to compositions which cure by way of a platinum group metal catalyzed reaction of silicon-bonded hydroxyl radicals and/or silicon-bonded olefinic hydrocarbon radicals with silicon-bonded hydrogen atoms. More specifically, the present invention relates to such curable compositions wherein the room temperature catalytic activity of the platinum group metal catalyst has been greatly inhibited by the presence of an inhibitor component. Organosilicon compositions in which a platinum group metal catalyst is inhibited in its cure-promoting activity at room temperature by the presence of a catalyst inhibitor are well known in the organosilicon art. Examples of various classes of such metal catalyst inhibitors include unsaturated organic compounds such as ethylenically or aromatically unsaturated amides, U.S. Patent No. 4,337,332; acetylenic compounds, U.S. Patent No. 3,445,420; ethylenically unsaturated isocyanates, U.S. Patent No. 3,882,083; olefinic siloxanes, U.S. Patent No. 3,989,667; unsaturated hydrocarbon diesters, U.S. Patent No. 4,256,870; and conjugated eneynes, U.S. Patent Nos. 4,465,818 and 4,472,563; other organic compounds such as hydroperoxides, sulfoxides, amines, phosphines, phosphites and nitriles; and various metal salts. While the known platinum group metal catalyst inhibitors are effective for delaying or preventing the room temperature cure of organosilicon compositions which cure by way of a platinum group metal catalyzed reaction, a long-standing problem with the use of such inhibitors still remains. A continuing problem with inhibited, platinum group metal catalyzed organosilicon compositions is that the cure time and/or the cure temperature of the composition is undesirably increased by the use of an inhibitor. While it is desirable to inhibit or prevent the room temperature cure of such a composition, it is rarely desirable to inhibit the cure of the composition at elevated temperature. This problem is of particular significance for applications where the organosilicon composition is used to rapidly coat a substrate, such as is practised in the adhesive release coating art. In the coatings art, such as the paper coating art, the coating composition that is used to coat a substrate should not gel before it has been applied to the substrate; however, it should rapidly cure thereafter, preferably with only a moderate amount of added energy. This means that the coating compositions preferably should not react at ambient temperature for as long as eight hours but should fully cure in less than a few minutes when heated. The present invention provides organosilicon compositions which have improved room temperature stability and elevated temperature cure rate and which are particularly useful in the coatings art. An inhibitor having unusually stable performance is also provided. It is an object of this invention to provide improved curable organosilicon compositions. It is also an object of this invention to provide organopolysiloxane compositions which do not cure at room temperature for long periods of time but which cure rapidly when formed into a desired shape and heated to low elevated temperatures. It is a particular object of this invention to provide liquid organopolysiloxane coating compositions which remain liquid for hours at temperatures up to 104°F (40°C) but which cure within 90 seconds when coated onto a substrate and heated to a temperature of as low as 180°F (82°C). It is a further object of this invention to provide an inhibitor for a platinum group metal catalyzed organopolysiloxane composition which will provide for the composition both room temperature cure inhibition and a short elevated temperature cure time which does not drift, i.e., change, as the curable composition ages. These objects, and others which will occur to one of ordinary skill in the curable organosilicon composition art upon considering the following disclosure and appended claims, are obtained by the compounds, composition of the present invention which, briefly stated, involves incorporating into a curable organosilicon composition comprising silicon-bonded hydrogen atoms and silicon-bonded radicals reactive therewith, in the presence of a platinum group metal catalyst, an effective amount of bis(2-methoxyisopropyl) maleate as an inhibitor component. The present invention provides a curable composition obtained by homogeneously mixing components comprising: (A) an organosilicon component having a viscosity from 100 milli Pascal-seconds to 100 kilo Pascal-seconds at 25°C, and an average of from one to three silicon-bonded monovalent radicals per silicon atom selected from the group consisting of hydroxyl radicals, hydrocarbon radicals, aliphatically saturated halohydrocarbon radicals and cyanoalkyl radicals, there being an average of at least two monovalent radicals per molecule of Component (A), selected from the group consisting of hydroxyl radicals and olefinic hydrocarbon radicals, the remaining silicon valences thereof being satisfied by divalent radicals, free of aliphatic unsaturation, selected from the group consisting of oxygen atoms, hydrocarbon radicals, hydrocarbon ether radicals, halohydrocarbon ether radicals and halohydrocarbon radicals, said divalent radicals linking silicon atoms, (B) an organosilicon component having a viscosity from 1 to 100 milli Pascal-seconds at 25°C, containing at least two silicon-bonded hydrogen atoms per molecule of Component (B) and an average of from one to two silicon-bonded monovalent radicals per silicon atom selected from the group consisting of cyanoalkyl radicals, hydroxyl radicals and aliphatically saturated hydrocarbon and halohydrocarbon radicals, the remaining silicon valences thereof being satisfied by divalent radicals, free of aliphatic unsaturation, selected from the group consisting of oxygen atoms, hydrocarbon radicals, hydrocarbon ether radicals, halohydrocarbon ether radicals and halohydrocarbon radicals, said divalent radicals linking silicon atoms, (C) an amount of a platinum group metal-containing catalyst component corresponding to at least one part by weight of metal for every one million parts of components (A) and (B) taken together, and (D) an amount of bis(2-methoxyisopropyl) maleate having the formula: sufficient to provide at least one molecule of said maleate for each platinum group metal atom in the composition, the amounts of Components (A) and (B) being sufficient to provide a ratio of the number of silicon-bonded hydrogen atoms to the number of silicon-bonded olefinic hydrocarbon radicals plus silicon-bonded hydroxyl radicals of from 1/100 to 100/1. Herein the term "curable", as applied to compositions, denotes a chemical change which leads to an increase in the molecular weight of the composition. Said increase in molecular weight, typically, is accompanied by an increase in the viscosity of the curable composition. For most uses of the compositions of this invention the term curable denotes a change from the liquid or formable state to the solid or gelled state for the composition. The curing of the compositions of this invention is accomplished by a platinum group metal catalyzed reaction between silicon-bonded hydroxyl radicals of Component (A) and silicon-bonded hydrogen atoms of Component (B) and/or between silicon-bonded olefinic hydrocarbon radicals of Component (A) and silicon-bonded hydrogen atoms of Component (B), the former reaction being a condensation reaction leading to the formation of siloxane bonds and hydrogen gas and the latter reaction being an addition reaction leading to silcarbane bonds. Component (A) of the compositions of this invention can be any organosilicon compound containing two or more silicon atoms linked by divalent radicals and containing an average of from 1 to 3 silicon-bonded monovalent radicals per silicon, with the proviso that the organosilicon compound contains at least two silicon-bonded radicals selected from hydroxyl radicals and olefinic hydrocarbon radicals. This component can be a solid or a liquid, freely flowing or gum-like. Examples of said divalent radicals linking silicon atoms in Component (A) include oxygen atoms, which provide siloxane bonds, and aliphatically saturated hydrocarbon, hydrocarbon ether, halohydrocarbon ether and halohydrocarbon radicals which provide silcarbane bonds. The divalent radicals can be the same or different, as desired. Examples of suitable divalent hydrocarbon radicals include alkylene radicals, such as -CH₂-, -CH₂CH₂-, -CH₂CHCH₃, -(CH₂)₄-, and -(CH₂)₁₈-; cycloalkylene radicals, such as arylene radicals, such as and and combinations of hydrocarbon radicals, such as and Examples of suitable divalent halohydrocarbon radicals include any divalent hydrocarbon radical wherein one or more hydrogen atoms have been replaced by halogen, such as fluorine, chlorine or bromine. Preferably, the halogen atom is not located on an aliphatic carbon atom which is adjacent to, or 1 carbon removed from, silicon. Preferable divalent halohydrocarbon radicals have the formula -CH₂CH₂C n F 2n CH₂CH₂-wherein n has a value of from 1 to 10 such as, for example, -CH₂CH₂CF₂CF₂CH₂CH₂-. Examples of suitable divalent hydrocarbon ether radicals and halohydrocarbon ether radicals include -CH₂CH₂OCH₂CH₂-, -CH₂CH₂CF₂OCF₂CH₂CH₂-, and -CH₂CH₂CH₂OCH₂CH₂CH₂-. Examples of said monovalent radicals in Component (A) include hydroxyl radicals, hydrocarbon radicals, aliphatically saturated halohydrocarbon radicals and cyanoalkyl radicals. Examples of suitable monovalent hydrocarbon radicals include alkyl radicals, such as CH₃-, CH₃CH₂-, CH₃CHCH₃, C₈H₁₇-, C₁₀H₂₁- and C₂₀H₄₁-; cycloaliphatic radicals, such as cyclohexyl; aryl radicals, such as phenyl, tolyl, xylyl, anthracyl and xenyl; aralkyl radicals, such as benzyl and 2-phenylethyl; and olefinic hydrocarbon radicals, such as vinyl, allyl, methallyl, butenyl, hexenyl, octenyl, cyclohexenyl and styryl. Alkenyl radicals are preferably terminally unsaturated. Typical monovalent hydrocarbon radicals are methyl, phenyl and vinyl. Examples of suitable aliphatically saturated monovalent halohydrocarbon radicals include any monovalent hydrocarbon radical which is free of aliphatic unsaturation and has at least one of its hydrogen atoms replaced with halogen, such as fluorine, chlorine or bromine. Preferably, the halogen atom is not located in an aliphatic carbon atom which is adjacent to, or 1 carbon removed from, silicon. Preferable monovalent halohydrocarbon radicals have the formula C n F 2n+1 CH₂CH₂- wherein n has a value of from 1 to 10, such as, for example, CF₃CH₂CH₂-. Examples of suitable cyanoalkyl radicals include NCCH₂CH₂- and NCCH₂CH₂CH₂-. It is preferred that Component (A) of the compositions of this invention possess flowability at low temperature and degradative stability at high temperature. Therefore, it is preferred that Component (A) have at least 50 percent siloxane structure, i.e., at least 50 percent of said divalent radicals linking silicon atoms are oxygen atoms. It is highly preferable that all of the divalent radicals linking silicon atoms are oxygen atoms, thereby providing a highly preferred Component (A), i.e., organopolysiloxanes. Organopolysiloxanes suitable for use as Component (A) have the average unit formula R '' / cSiO (4-c)/2 wherein R'' denotes said monovalent radicals noted above and c has a value of from greater than 0 to 3. Suitable siloxane units in the organopolysiloxanes having the above general formula are siloxane units having the formulae R '' / 3SiO 1/2 , R '' / 2SiO 2/2 , R''SiO 3/2 and SiO 4/2 . Said siloxane units can be combined in any molecular arrangement such as linear, branched, cyclic and combinations thereof, to provide organopolysiloxanes that are useful as Component (A). A preferred organopolysiloxane Component (A) for the composition of this invention is a substantially linear organopolysiloxane having the formula X₂RSiO(XRSiO) x SiRX₂. By substantially linear, it is meant that the component contains, at most, only trace amounts of silicon atoms bearing 3 or 4 siloxane linkages. It is to be understood that the term substantially linear also encompasses organopolysiloxanes which can contain up to about 15 percent by weight cyclopolysiloxanes which are frequently coproduced with the linear organopolysiloxanes. In the formula shown immediately above, each R denotes an aliphatically saturated monovalent hydrocarbon or halohydrocarbon radical having from 1 to 20 carbon atoms and being exemplified above. The several R radicals can be the same or different, as desired. Additionally, each X denotes a hydroxyl radical, an R radical or an olefinic hydrocarbon radical, preferably having from 2 to 8 carbon atoms and being exemplified above. Of course, at least 2 X radicals are either olefinic hydrocarbon radicals, preferably vinyl, or hydroxyl radicals. To ensure linearity of an organopolysiloxane having the above formula, no more than two silicon-bonded hydroxyl radicals should be present in the molecule. Finally, the value of the subscript x is such that the organopolysiloxane Component (A) has a viscosity at 25°C of from 100 millipascal-seconds (100 centipoises) to 100 kilopascal-seconds (100,000,000 centipoises and more). The exact value of x needed to provide a viscosity value falling within said limits depends upon the identity of the X and R radicals; however, for hydroxyl-terminated and/or hydrocarbyl-terminated polydimethylsiloxane, x will have a value of from 60 to 10,000. In terms of preferred monovalent hydrocarbon radicals, noted above, examples of linear organopolysiloxanes of the above formula which are suitable as Component (A) for the composition of this invention include HOMe₂SiO(Me₂SiO) x SiMe₂OH, PhMeViSiO(Me₂SiO) x SiphMeVi, HOMe₂SiO(Me₂SiO) 0.9x (MeViSiO) 0.1x SiMe₂OH, HOMe(CF₃CH₂CH₂)SiO(Me(CF₃CH₂CH₂)SiO) x SiMe(CF₃CH₂CH₂)OH, ViMe₂SiO(Me₂SiO) 0.95x (MeViSiO) 0.05x SiMe₂Vi, Me₃SiO(Me₂SiO) 0.9x (MeViSiO) 0.1x SiMe₃, PhMeViSiO(Me₂SiO) 0.8x (MePhSiO) 0.1x (Ph₂SiO) 0.1x SiPhMeVi and ViMe₂SiO(Me₂SiO) x SiMe₂Vi wherein Me, Vi and Ph denote methyl, vinyl and phenyl, respectively. Highly preferred linear organopolysiloxanes (A) for the composition of this invention have the formula X₂RSiO(Me₂SiO) b (MeViSiO) d SiRX₂ wherein R and X are as noted above and the sum of b plus d is equal to x , also noted above. The values of the subscripts b and d can be zero or greater; however, the sum of b plus d has a value of from about 60 to about 10,000 and the value of b is typically greater than the value of d . In a preferred embodiment of the present invention, wherein the curable composition, preferably solventless, is used to coat a solid substrate, such as paper, with an adhesive-releasing coating, Component (A) has the formula shown immediately above wherein the value of b plus d is sufficient to provide a viscosity at 25°C for the Component (A) of from 100 mPa·s, to 100 Pa·s, preferably from 100 mPa·s to 10 Pa·s and, most preferably, from 100 mPa·s to 5 Pa·s; said viscosities corresponding approximately to values of b + d of from 60 to 1000, preferably from 60 to 520 and, most preferably, from 60 to 420. In addition, the value of subscript d is preferably limited to less than 0.1 b such as zero, 0.02 b or 0.08 b . Component (B) of the compositions of this invention can be any organosilicon compound containing two or more silicon atoms linked by divalent radicals and containing an average of from one to two silicon-bonded monovalent radicals per silicon atom and an average of at least two, and preferably three or more, silicon-bonded hydrogen atoms per molecule thereof. Examples of said divalent radicals linking silicon atoms in Component (B) are as delineated above for Component (A), including preferred examples. As with Component (A), the divalent radicals within Component (B) can be the same or different, as desired. Furthermore, the divalent radicals that are present in Component (B) can, but need not, be the same as the divalent radicals that are present in Component (A). Examples of said monovalent radicals in Component (B) include hydroxyl radicals, cyanoalkyl radicals and aliphatically saturated hydrocarbon and halohydrocarbon radicals, as delineated above for Component (A), including preferred examples. The monovalent radicals that are present in Component (B) can, but need not, be the same as the monovalent radicals that are present in Component (A). Component (B) must contain an average of at least two silicon-bonded hydrogen atoms per molecule thereof. Preferably Component (B) contains an average of three or more silicon-bonded hydrogen atoms such as, for example, 5, 10, 20, 40 and more. As with Component (A), Component (B) preferably contains at least 50 percent, and most preferably 100 percent, siloxane structure, thereby providing a highly preferred Component (B), i.e., organopolysiloxanes. Organopolysiloxanes suitable for use as Component (B) have the average unit formula R '' / eH f SiO (4-e-f)/2 wherein R'' denotes said monovalent radicals and the sum of e plus f has a value of from greater than 0 to 3. Preferably the value of f does not exceed 1. Suitable siloxane units in the organopolysiloxanes having the above general formula are siloxane units having the formulae R '' / 3SiO 1/2 , R '' / 2HSiO 1/2 , R '' / 2SiO 2/2 , R''HSiO 2/2 , R''SiO 3/2 , HSiO 3/2 and SiO 4/2 . Said siloxane units can be combined in any molecular arrangement such as linear, branched, cyclic and combinations thereof, to provide organopolysiloxanes that are useful as Component (B). A preferred organopolysiloxane Component (B) for the compositions of this invention is a substantially linear organopolysiloxane having the formula YR₂SiO(YRSiO) y SiR₂Y wherein each R denotes an aliphatically saturated monovalent hydrocarbon or halohydrocarbon radical having from 1 to 20 carbon atoms and being exemplified above. The several R radicals can be the same or different, as desired. Additionally each Y denotes a hydrogen atom or an R radical. Of course, at least 2 Y radicals must be hydrogen atoms. Finally, the value of the subscript y is such that the organopolysiloxane Component (B) has a viscosity at 25°C of from 1 to 100 millipascal-seconds. The exact value of y needed to provide a viscosity value falling within said limits depends upon the number and identity of the R radicals; however, for organopolysiloxanes containing only methyl radicals as R radicals y will have a value of from 1 to 100. In terms of preferred monovalent hydrocarbon radicals, noted above, examples of linear organopolysiloxanes of the above formula which are suitable as Component (B) for the compositions of this invention include HMe₂SiO(Me₂SiO) y SiMe₂H, (CF₃CH₂CH₂)MeHSiO(Me(CF₃CH₂CH₂)SiO) y -SiHMe(CH₂CH₂CF₃), Me₃SiO(MeHSiO) y SiMe₃, HMe₂SiO(Me₂SiO) 0.5y -(MeHSiO) 0.5y SiMe₂H, HMe₂SiO(Me₂SiO) 0.5y (MePhsiO) 0.1y -(MeHSiO) 0.4y SiMe₂H, Me₃SiO(Me₂SiO) 0.4y (MeHSiO) 0.6y SiMe₃, (MeHSiO) y , (HMe₂SiO)₄Si and MeSiO(SiMe₂H)₃. Highly preferred linear organopolysiloxane (B) for the compositions of this invention have the formula YR₂SiO(Me₂SiO) p (MeHSiO) q SiR₂Y wherein Y and R have the meaning noted and exemplified above and the sum of p plus q is equal to y , also noted above. The values of p and q can be zero or greater; however, the sum of p plus q has a value of from 1 to about 100 and the value of q is typically greater than the value of p . The amounts of Components (A) and (B) that are used in the compositions of this invention are not narrowly limited. Said amounts, typically expressed in terms of the ratio of the number of silicon-bonded hydrogen atoms of Component (B) to the number of silicon-bonded hydroxyl and/or olefinic hydrocarbon radicals of Component (A), are sufficient to provide a value of from 1/100 to 100/1 for said ratio. Of course, the extent of molecular weight increase is directly related to the extent that the value of said ratio approaches 1/1. For the liquid coating compositions of this invention which are to be used in the coating method of this invention, hereinbelow delineated, the value of said ratio should have a value of from 1/2 to 1.5/1, and preferably about 1/1. Organosilicon polymers are, of course, well known in the organosilicon art. Organopolysiloxanes are clearly the most significant and most widely used form of organosilicon polymers, in the art and in this invention; many are commercially prepared. The preparation of the organosilicon components that are used in the compositions of this invention is well documented and needs no intensive delineation herein. Briefly, organopolysiloxanes are typically prepared by way of hydrolysis and condensation of hydrolyzable silanes such as Me₂SiCl₂, Me₃SiCl, MeSiCl₃, SiCl₄, Me₂Si(OMe)₂, MeSi(OMe)₃ and Si(OCH₂CH₃)₄ or by way of acid- or alkali-catalyzed siloxane equilibration of suitable siloxane precursors such as (Me₂SiO)₄ and Me₃SiOSiMe₃, which themselves are prepared by way of said hydrolysis and condensation reaction. Organopolysiloxane Component (A) can be prepared as noted above with the proviso that a silane or siloxane containing at least one silicon-bonded olefinic hydrocarbon radical is used, alone or in conjunction with other silanes or siloxanes, in sufficient amount to provide the necessary number of olefinic hydrocarbon radicals in the organopolysiloxane. Examples of olefinic hydrocarbon radical-containing silanes or siloxanes include ViMe₂SiCl, MeViSiCl₂, ViSiCl₃, (MeViSiO)₄ and ViMe₂SiOSiMe₂Vi. Organopolysiloxane Component (B) can be prepared as noted above with the proviso that a silane or siloxane containing at least one silicon-bonded hydrogen atom, instead of olefinic hydrocarbon radical, is used, alone or in combination with other silanes or siloxanes, in sufficient amount to provide the necessary number of silicon-bonded hydrogen atoms in the organopolysiloxane. Examples of hydrogen atom-containing silanes or siloxanes include HMe₂SiCl, HMeSiCl₂, HSiCl₃, HMe₂SiOSiMe₂H and (MeHSiO)₄. Component (B) is preferably prepared under nonalkaline conditions. Organosilicon polymers having silcarbane and siloxane structures can be prepared, for example, from monomeric species that have nonoxygen divalent radicals, such as or ClMe₂SiC₆H₄SiMe₂Cl, using standard siloxane bond-forming chemistry and incorporating one or more of the olefinic hydrocarbon radicals or hydrogen atom-containing silanes or siloxanes noted above, and other silanes or siloxanes, as desired. Organosilicon polymers which contain no siloxane bonds can be prepared, for example, by a hydrosilylation reaction between silanes or silcarbanes bearing silicon-bonded olefinically unsaturated hydrocarbon radicals, such as Vi₂SiMe₂ or ViMe₂SiC₆H₄SiMe₂Vi and silanes or silcarbanes bearing silicon-bonded hydrogen atoms, such as H₂SiMe₂ or HMe₂SiC₆H₄SiMe₂H. Other suitable methods for preparing the organosilicon components that are used in the compositions of this invention also occur in the organosilicon art. Component (C) of the composition of this invention is a catalyst component which facilitates the reaction of the silicon-bonded hydrogen atoms of Component (B) with the silicon-bonded hydroxyl and/or silicon-bonded olefinic hydrocarbon radicals of Component (A) and can be any platinum group metal-containing catalyst component. By platinum group it is meant herein ruthenium, rhodium, palladium, osmium, iridium and platinum. Component (C) can be a platinum group metal; a carrier, such as silica gel or powdered charcoal, bearing a platinum group metal; or a compound or complex of a platinum group metal. Component (C) is preferably a platinum-containing catalyst component since they are most widely used and available and because they provide a more favorable effect for the compositions of this invention in terms of pot life and cure time, as hereinafter discussed. A preferred platinum-containing catalyst component in the compositions of this invention is a form of chloroplatinic acid, either as the commonly available hexahydrate form or as the anhydrous form, because of its easy dispersibility in organosilicon systems. A particularly useful form of chloroplatinic acid is that composition obtained when it is reacted with an aliphatically unsaturated organosilicon compound such as divinyltetramethyldisiloxane, as disclosed by U.S. Patent No. 3,419,593. The amount of platinum group metal-containing catalyst component that is used in the compositions of this invention is not narrowly limited as long as there is a sufficient amount to accelerate a room temperature reaction between the silicon-bonded hydrogen atoms of Component (B) with the silicon-bonded hydroxyl and/or olefinic hydrocarbon radicals of Component (A). The exact necessary amount of said catalyst component will depend upon the particular catalyst and is not easily predictable. However, for chloroplatinic acid said amount can be as low as one part by weight of platinum for every one million parts by weight of organosilicon Components (A) plus (B). Preferably, said amount is at least 10 parts by weight, on the same basis. For compositions of this invention which are to be used in the coating method of this invention the amount of platinum-containing catalyst component to be used is sufficient to provide from 10 to 500 parts by weight platinum per one million parts by weight of organopolysiloxane Components (A) plus (B). Component (D) of the compositions of this invention is bis-(2-methoxyisopropyl) maleate having the formula: We have shown that this hydrocarbonoxyalkyl maleate, namely, bis-(2-methoxyisopropyl) maleate, is very effective as a cure inhibitor, i.e., viscosity stabilizer, at room temperature in the composition of this invention while allowing the rapid curing of said compositions at low elevated temperatures. Thus, bis(2-methoxyisopropyl) maleate, has been found to have the desirable ability to provide a commercially useful inhibiting effect at room temperature for certain coating compositions of this invention without increasing, i.e., inhibiting, the cure time of said coating composition at low elevated temperature. Bis(2-methoxyisopropyl maleates can be prepared by any known method, for example, by the full esterification of maleic acid, maleic anhydride or maleyl chloride with isopropyl alcohol. The amount of the maleate to be used in the compositions of this invention is not critical and can be any amount that will retard the above-described catalyzed reaction at room temperature while not preventing said reaction at low elevated temperature. While not wishing to be limited by any theory, we believe that there should be at least one molecule of bis-(2-methoxyisopropyl) maleate for each platinum group metal atom in the composition, to form a room temperature stable complex therebetween. Preferably, a large excess of maleate molecules compared to platinum group metal atoms is used. In the liquid organopolysiloxane compositions that are used in the coating method of this invention the amount of bis(2-methoxyisopropyl) maleate is typically sufficient to provide from 25 to 50 molecules thereof for every platinum atom in the composition. The addition of the Component (D) to a composition comprising (A), (B) and (C) slows down the rate of cure at room temperature over long periods of time, but at temperatures in excess of 70°C the inhibiting effect of the maleate observed at room temperature disappears and a faster curing rate is realized. the cure of the curable composition can be retarded at room temperature for short periods of time or for very long periods of time by the proper amount of bis-(2-methoxyisopropyl) maleate. No exact amount of the maleate can be suggested to give a specified storage life at room temperature. The rate of cure will depend upon the ratio of maleate to platinum, the form of the platinum catalyst, the nature and amounts of Components (A) and (B) and the presence or absence of other nonessential ingredients. Bis-(2-methoxyisopropyl) maleate added in small amounts such as 0.1 weight percent based on the weight of the curable composition provides increased pot life in all systems, but, in most cases, does not fully retard the reaction at room temperature. In larger amounts such as 3 weight percent, bis-(2-methoxyisopropyl) maleate, provides completely inhibited cures at room temperature. However, some systems are inhibited at room temperature at one mol of bis-(2-methoxyisopropyl) maleate to one mol of platinum while others may require 10, 20, 50 or 1000 moles of the maleate per one mole of platinum to inhibit the system at room temperature. The amount of bis-(2-methoxyisopropyl) maleate is therefore dependent upon the desired use, and the nature of the system. The skilled worker should, therefore, determine the optimum level for each system. The composition of this invention can contain any of the optional components commonly used in platinum group metal-catalyzed organosilicon compositions, such as fillers, solvents, surfactants, colorants, stabilizers and physical property modifiers. Examples of fillers useful in the compositions of this invention include reinforcing fillers and extending fillers. Examples of reinforcing fillers include: silica, such as fume silica and precipitated silica; and treated silica, such as fume or precipitated silica that has been reacted with e.g., an organohalosilane, a disiloxane, or a disilazane. Examples of extending fillers include crushed quartz, aluminum oxide, aluminum silicate, zirconium silicate, magnesium oxide, zinc oxide, talc, diatomaceous earth, iron oxide, calcium carbonate, clay, titania, zirconia, mica, glass, such as ground glass or glass fiber, sand, carbon black, graphite, barium sulfate, zinc sulfate, wood flour, cork, fluorocarbon polymer powder, rice hulls, ground peanut shells. Examples of said solvents include aliphatic hydrocarbons, such as pentane, hexane, heptane, octane, nonane and the like; aromatic hydrocarbons such as benzene, toluene and xylene; alcohols such as methanol, ethanol, and butanol; ketones such as acetone, methylethyl ketone and methyl-isobutyl ketone; and halogenated solvents such as fluorine-, chlorine-, and bromine-substituted aliphatic or aromatic hydrocarbons, such as trichloroethane, perchloroethylene, bromobenzene. Two or more solvents may be used together. Examples of stabilizers include antimicrobial preparations, mildewcides, antioxidants, flame retardants and ultraviolet radiation stabilizers. Examples of physical property modifiers include adhesion promoters, crosslinking agents and controlled release additives, such as the siloxane resins disclosed in U.S. Patent No. 3,527,659. The compositions of this invention are made by homogeneously mixing Components (A), (B), (C) and (D), and any optional components, using suitable mixing means, such as a spatula, a drum roller, a mechanical stirrer, a three-roll mill, a sigma blade mixer, a bread dough mixer, and a two-roll mill. The order of mixing Components (A) to (D) is not critical; however, it is preferred that Components (B) and (C) be brought together in the presence of Component (D), most preferably in a final mixing step. Thus, it is possible to mix all components in one mixing step immediately prior to the intended use of the curable composition. Alternatively, certain components can be premixed to form two or more packages which can be stored, if desired, and then mixed in a final step immediately prior to the intended use thereof. It is preferred to mix Components (C), (D) and a portion of Component (A), along with certain optional components such as fillers and solvents, to provide a first package and Component (B), along with the remaining portion of Component (A), if any, to provide a second package. These two packages can then be stored until the composition of this invention is desired and then homogeneously mixed. It is also possible to place Components (B), (C) and (D) in three separate packages and to place Component (A) in one or more of said separate packages and the three packages stored until needed. The compositions of this invention are useful as formable compositions to provide organosilicon articles such as O-rings, tubing, wire-coating and gaskets; as encapsulant and sealant compositions; and as coating compositions, among others. Specifically, the compositions of the present invention may be used to render a solid surface less adherent to materials which normally adhere thereto. The liquid curable organopolysiloxane compositions of the invention may be coated onto a solid substrate, preferably at room temperature, and thereafter heated to effect a cure of the coating. The coating step may be accomplished in any suitable manner known in the art, such as by spreading, brushing, extruding, spraying and rolling. A significant characteristic of the liquid curable compositions of this invention is their long pot life. Thus, the viscosity does not double in value over a period of several hours, thereby allowing an extended application time. In a preferred embodiment of the instant process, the solid substrate is paper. Other suitable solid substrates which can be coated by the process of this invention include other cellulosic materials such as wood, cardboard and cotton; metallic materials such as aluminum, copper, steel and silver; siliceous materials such as glass and stone; and synthetic polymer materials such as polyolefins, polyamides, polyesters and polyacrylates. As to form, the solid substrate can be sheetlike, such as adhesive release liner, textiles and foil; or substantially three-dimensional in form. After the liquid curable composition has been coated onto a substrate, it is heated to convert the liquid coating to the non-liquid state. Another significant characteristic of the compositions of this invention is the rapid curing which occurs when the coated composition is heated to low elevated temperatures, such as 70°C. Typically, the coated composition will cure fully when heated, for example, at 82°C for 90 seconds. Higher heating temperatures, such as up to 160°C, will provide correspondingly shorter curing times. In a particular embodiment, a flexible sheet material, such as paper, metal foil or tapestock, is coated with a thin coating of the liquid curable composition, preferably in a continuous manner, and the thus-coated material is then heated so as rapidly to cure the coating whereby a sheet-like material is produced bearing on at least one surface thereof an adhesive-release coating. The adhesive-release coating is subsequently brought into contact with a pressure-sensitive adhesive, preferably in a continuous manner, to form an article having a peelable adhesive/coating interface. Examples of such an article include adhesive labels having a peelable backing, adhesive tape in roll form and adhesive package in a strippable container. The composition of this invention is applicable to adhesive materials other than pressure sensitive adhesives. Examples of such adhesive materials include foods, asphalt and gum polymers. The following examples are disclosed to further teach, but not limit, the invention which is properly delineated by the appended claims. All amounts (part and percentages) are by weight unless otherwise indicated. Viscosities were measured with a rotating spindle viscometer. Bath life of a composition means the time interval required for the viscosity of the composition to reach a value of twice the room temperature viscosity of the freshly prepared composition. Cure time for composition means the time interval required for the composition, when coated onto S2S kraft paper, at the thickness of 0.454 kg (1 pound) per ream, to attain the no smear, no migration, no rub-off condition. The no smear condition was determined by lightly streaking the coating with a finger and observing for the absence of haze in the streaked area. The no migration condition was determined by firmly adhering a common, pressure-sensitive adhesive tape to the coating, removing the tape and folding the removed tape together, adhesive surfaces to each other. Absence of migration of the coating to the tape was indicated by noting that the double tape was as difficult to separate as unused tape so doubled. The no rub-off condition was determined by vigorously rubbing the coating with the index finger and noting that the coating could not be removed from the paper. Bis(2-methoxyisopropyl) maleate was prepared from 23 parts of maleic acid, 60 parts of 1-methoxy-2-propanol, 100 parts of toluene and a trace of concentrated sulfuric acid. The esterification reaction was conducted at reflux under a Dean-Stark water trap for 8 hours. The bis-(2-methoxyisopropyl) maleate was isolated by vacuum distillation in 73% yield. B.p. = 127-130°C/120 Pa (0.09 Torr). In the following examples, bis (2-methoxyisopropyl) maleate illustrates the invention. References to other maleates are included only by way of comparison. Specifically, Examples 4 and 12-18 illustrate the invention, whilst Examples 1-3, 5-7 and 8-11 are included in order to give preparative details and for comparison. Examples 1 to 7 Two hundred fifty parts of a dimethylvinylsiloxy-terminated polydimethyl-co-methylvinylsiloxane having a viscosity of 300 mPa·s (300 cP) and a vinyl content of about 1.2% were mixed with 4.75 parts of a platinum-containing catalyst containing 0.6% Pt (114 ppm Pt, based on 300 mPa·s polymer) and consisting of H₂PtCl₆·6H₂O dissolved in tetramethyldivinyldisiloxane 1.25 parts of bis(2-methoxyethyl) maleate and 11.75 parts of an organohydrogenpolysiloxane crosslinker containing (CH₃)₃SiO 1/2 units, (CH₃)₂SiO 2/2 units and CH₃(H)SiO 2/2 units and having a silicon-bonded hydrogen content of about 1.1%. The bath life of the resulting composition at room temperature was 118 hours. The bath life of the same composition, except not containing an inhibitor, was less than 30 minutes at room temperature. This experiment was repeated with six other hydrocarbonoxyalkyl maleates having the general formula cis-R'O(DO) a DO₂CCH=CHCO₂D(OD) a OR'. The results are summarized in Table I. Examples 8-11 The experiment of Example 1 was repeated except the amount of the bis(2-methoxyethyl) maleate was varied. The resulting compositions of this invention were tested for bath life at 40°C and for cure tame at 180°F (82°C), initially and at full bath age. The results are summarized in Table II. These examples illustrate the effect of the amount of oxyalkylene maleate on the bath life and cure time of the composition. Example Maleate, parts Bath Life Hours at 40°C Cure Time, seconds at 82°C (180°F) Initial Aged 8 1.25 >10 70 90 9 1.125 7.5 50 65 10 1.0 7 40 55 11 0.875 4.5 35 55 Example 12 A mixture of 250 parts of the vinyl-containing polymer having a viscosity of 300 mPa·s, 4 parts of the platinum-containing catalyst, both described in Examples 1-7, 2.5 parts of cyclopolymethylvinylsiloxane, 16.25 parts of a liquid silicone resin crosslinker prepared by the method of U.S. Patent No. 4,310,678 and having an SiH content of 0.77% and an SiOH content of 1.36% and 2.25 parts of bis(2-methoxyisopropyl) maleate was prepared. The resulting composition of this invention had a bath life at room temperature of greater than 24 hours. The same composition, but without the bis(2-methoxyisopropyl) maleate, gelled in 10 minutes at room temperature. This example illustrates the compositions of this invention comprising a linear vinyl-containing siloxane fluid and a resinous organohydrogenpolysiloxane crosslinker. Example 13 A mixture of 250 parts of a vinyldimethylsiloxy-terminated polydimethylsiloxane having a viscosity of 2 Pa·s (2,000 cP) and a vinyl content of about 0.25%, 0.3 part of the platinum-containing catalyst of Examples 1-7, 3.85 parts of Me₃SiO(Me₂SiO)₃(MeHSiO)₅SiMe₃ crosslinker, 3.2 parts of bis(2-methoxyisopropyl) maleate and 98.8 parts of trimethylsiloxy-treated silica filler was prepared. The composition of this invention was stable at 50°C for 3 months and cured to an elastomeric composition in 10 minutes when heated to 150°C. The identical composition, but without the bis(2-methoxyisopropyl) maleate, was not stable and gelled within 3 minutes at 50°C. This example illustrates a composition of this invention comprising a particulate filler. Example 14 A mixture of 250 parts of a blend of a silanol-terminated polydimethylsiloxane gum having a number average molecular weight of about 270,000 and a silanol-terminated polydimethylsiloxane fluid having a silanol content of 1.4%, 3.4 parts of the platinum-containing catalyst of Examples 1-7, 11.9 parts of Me₃SiO(MeHSiO)₃₅SiMe₃ crosslinker, 591 parts of xylene and 17.1 parts of bis(2-methoxyisopropyl) maleate was prepared. The resulting composition of this invention experienced no change in viscosity at room temperature for 7 days and no gel formation after 2.5 months. The same composition, but without the bis(2-methoxyisopropyl) maleate, gelled within 2 minutes at room temperature. The above composition of this invention, 200 parts, was diluted with 800 parts of toluene and the diluted solution was coated onto S2S kraft paper. When heated to 300°F (149°C), the coating cured to a no smear, no migration, no rub-off condition in 70 seconds. This example illustrates a composition of this invention comprising a silanol-containing organopolysiloxane and an organohydrogenpolysiloxane crosslinker. Examples 15-17 The experiment of Example 4 was repeated except the amount of bis(2-methoxyisopropyl) maleate was varied. The viscosity, initially and after 8 hours at 40°C, and the 180°F cure time, initially and after 8 hours at 40°C, were measured for the resulting compositions of this invention. The results, shown in Table III, illustrate the stable 180°F (82°C) cure time of the compositions of this invention which contain the novel bis(2-methoxyisopropyl) maleate after being held at 40°C for 8 hours. A comparison of Examples 15, 16 and 17 with comparison Examples 11, 10 and 8, respectively, shows that the use of bis(2-methoxyethyl) maleate provides a shorter cure time than the use of bis(2-methoxyisopropyl) maleate; however, the latter hydrocarbonoxyalkyl maleate provides a stable cure rate throughout the life of the bath whereas the former hydrocarbonoxyalkyl maleate does not provide this stability. Example 18 A coating bath containing 551 parts of toluene, 3226 parts of heptane, 250 parts of a dimethylvinylsiloxy-terminated polydimethyl-co-methylvinylsiloxane gum containing 2 mol percent vinyl radicals and having a number average molecular weight of about 260,000, 2.9 parts of the platinum-containing catalyst described in Example 1, 2.4 parts of cyclopolymethylvinylsiloxane, 4 parts of the organohydrogenpolysiloxane crosslinker described in Example 15 and 2.4 parts of bis(2-methoxyisopropyl) maleate was prepared and a portion thereof immediately coated onto S2S kraft paper using No. 12 Mayer Rod. The immediately coated paper was placed in a forced-air oven at 77°C (170°F) for 80 seconds to cure the coated composition to a no smear, no migration, no rub-off condition. The coating bath was allowed to stand for 17 hours at room temperature and the paper coating process was repeated. The delay-coated composition cured to the same condition in 60 seconds in a 93°C (200°F) forced-air oven. When the above paper coating process was repeated with the same composition, except containing no hydrocarbonoxyethyl maleate the immediately coated composition cured in 40 seconds at 77°C (170°F); however, the delay-coated composition would not cure to a no rub-off condition when heated for 120 seconds at 93°C (200°F). The ability of the composition of this invention to cure to the no rub-off condition after being aged at room temperature for a period indicates bath stability thereof.

Claims: A curable composition obtained by homogeneously mixing components comprising (A) an organosilicon component having a viscosity from 100 milli Pascal-seconds to 100 kilo Pascal-seconds at 25°C, and an average of from one to three silicon-bonded monovalent radicals per silicon atom selected from the group consisting of hydroxyl radicals, hydrocarbon radicals, aliphatically saturated halohydrocarbon radicals and cyanoalkyl radicals, there being an average of at least two monovalent radicals per molecule of Component (A), selected from the group consisting of hydroxyl radicals and olefinic hydrocarbon radicals, the remaining silicon valences thereof being satisfied by divalent radicals, free of aliphatic unsaturation, selected from the group consisting of oxygen atoms, hydrocarbon radicals, hydrocarbon ether radicals, halohydrocarbon ether radicals and halohydrocarbon radicals, said divalent radicals linking silicon atoms, (B) an organosilicon component having a viscosity from 1 to 100 milli Pascal-seconds at 25°C, containing at least two silicon-bonded hydrogen atoms per molecule of Component (B) and an average of from one to two silicon-bonded monovalent radicals per silicon atom selected from the group consisting of cyanoalkyl radicals, hydroxyl radicals and aliphatically saturated hydrocarbon and halohydrocarbon radicals, the remaining silicon valences thereof being satisfied by divalent radicals, free of aliphatic unsaturation, selected from the group consisting of oxygen atoms, hydrocarbon radicals, hydrocarbon ether radicals, halohydrocarbon ether radicals and halohydrocarbon radicals, said divalent radicals linking silicon atoms, (C) an amount of a platinum group metal-containing catalyst component corresponding to at least one part by weight of metal for every one million parts of components (A) and (B) taken together, and (D) an amount of bis(2-methoxyisopropyl) maleate having the formula: sufficient to provide at least one molecule of said maleate for each platinum group metal atom in the composition, the amounts of Components (A) and (B) being sufficient to provide a ratio of the number of silicon-bonded hydrogen atoms to the number of silicon-bonded olefinic hydrocarbon radicals plus silicon-bonded hydroxyl radicals of from 1/100 to 100/1.

Inventors: KWAI LO, PETER YIN, THAYER, LEROY ELTON, WRIGHT, ANTHONY POPE
Applicants: DOW CORNING CORPORATION
Application year: 1985
Main classification: C08L83
IPC codes: C08L83, C07F7, C09J7, D21H19, C08K5, C07C69, C07F9