Organohydrogenpolysiloxane crosslinking agent for silicone pressure-sensitive adhesives

Abstract: A silicone pressure-sensitive adhesive composition is disclosed that is based on a siloxane resin, a polydiorganosiloxane polymer with unsaturated groups in its molecule, a silicon hydride-functional crosslinking agent and a hydrosilation catalyst, wherein the crosslinking agent is a reaction product of a cyclic or linear organohydrogenpolysiloxane and an unsaturated organic compound selected from alpha-alkenes having 6 to 28 carbon atoms or aromatic compounds having the formula Ph-R⁴, wherein Ph represents a phenyl radical and R⁴ is a terminally unsaturated monovalent hydrocarbon group having 2 to 6 carbon atoms.

Description: The present invention relates to a silicone pressure-sensitive adhesive which comprises a resin component, a polymer component having unsaturated groups in its molecule, a silicon hydride-functional crosslinking agent and a catalyst. More particularly, the silicone pressure-sensitive adhesive of this invention is characterized by a crosslinking agent which is a reaction product of a organohydrogenpolysiloxane and an unsaturated organic compound. Silicone pressure-sensitive adhesives (hereinafter SPSAs) typically contain at least two primary components, namely, a linear siloxane polymer and a tackifier resin comprised of triorganosiloxane (M) units (R 3 SiO 1/2 units, where R denotes a monovalent organic group) and silicate (Q) units (SiO 4/2 units). In addition, SPSA compositions are generally provided with crosslinkers in order to optimize various properties of the final adhesive product. In view of the high viscosity imparted by the polymer component, these SPSA compositions are typically dispersed in an organic solvent for ease of application. United States Patent No. 3,929,704 teaches a SPSA comprising an MQ resin, silicone gum and a cross linking agent, wherein the crosslinking agent is a peroxide admixed with a plasticizer and an extender. Alternatively, when the silicone polymer contains unsaturated groups in its molecule, such SPSA compositions can be cured by incorporating an organohydrogenpolysiloxane and a hydrosilation catalyst. European Patent Publication No. 0 355 991 is illustrative of such compositions. There, the polymer component comprises a diorganoalkenylsiloxy-endblocked polydiorganosiloxane and a diorganohydrogensiloxy-endblocked polydiorganosiloxane which allow the formation of a relatively low viscosity SPSA composition without the need for a solvent. The use of certain SiH-functional compounds in conjunction with non-silicone pressure sensitive adhesives (PSAs) is also known. Japanese Patent Publication (Kokai) Hei 4(1992)-145188 to Kanefuchi Chemical Industry Co., Ltd. discloses organic PSAs which are a blend of (A) an alkenyl group-containing organic compound or organic polymer, (B) an organic compound containing at least two SiH groups in its molecule, (C) a hydrosilylating catalyst and (D) an organic adhesive resin. Additionally, United States Patent No. 4,831,080, demonstrates a method for improving the bond strength of a pressure-sensitive adhesive by contacting two PSA components to form a PSA construction. Therein, the first component comprises a conventional PSA and a liquid organohydrogenpolysiloxane. The second component comprises the same PSA and a cure agent for the liquid organohydrogenpolysiloxane. A similar two-component PSA system is described in United States Patent No. 5,037,886. There, the organohydrogenpolysiloxane is reacted with a siloxane resin before being incorporated into the first PSA component and a cure agent for this reaction product is mixed with the second component. Although SPSAs of the prior art do offer the industry considerable advantage over organic counterparts, there remains room for improvement. Thus, in a silicone pressure-sensitive tape which comprises a SPSA layer coated onto a plastic film (a self-wound tape), it is highly desirable to achieve the simultaneous properties of high adhesion, high tack and smooth adhesive release from substrates at high rates of peel. This latter characteristic is difficult to achieve with some SPSAs and an annoying noise, referred to as "chatter" herein, is noted when such a tape is unwound from a roll at relatively high speed. Aside from presenting a nuisance in numerous industrial operations, this "chatter" is associated with the unwanted transfer of adhesive from the tape to the substrate contacted therewith (e.g., the back side of the tape) during a rapid peel operation. Furthermore, the uneven adhesive release can be detrimental to process machinery since it can induce vibrations therein, especially when a wide roll of tape (one to two meters) is being unwound. None of the above prior art, nor any other art known to applicants, suggest the preparation of SPSAs based on a siloxane resin component, a siloxane polymer component having unsaturated groups in its molecule, a silicon hydride-functional crosslinking agent and a hydrosilation catalyst, characterized by said crosslinking agent being a reaction product of an organohydrogenpolysiloxane and an unsaturated organic compound. The silicone adhesives of the present invention exhibit good adhesion to metal and other substrates and have high tack surfaces. Moreover, it has surprisingly been found that, in addition to the above mentioned desirable characteristics, SPSAs of the present invention provide a smooth adhesive release from substrates when delaminated therefrom at a high rate of peel. Thus, little or no adhesive is transferred to the substrate under these high peel rate conditions and the aforementioned "chatter" associated with prior art systems can be reduced or eliminated. The present invention therefore relates to a silicone pressure-sensitive adhesive comprising a siloxane polymer having at least two olefinically unsaturated groups in its molecule, a siloxane resin, an SiH-functional crosslinking agent and a catalytic amount of a platinum group metal-containing catalyst, wherein said crosslinking agent is a hydrosilation reaction product of (I) an organohydrogenpolysiloxane selected from a cyclic siloxane having the formula a linear siloxane having the formula and a linear siloxane having the formula wherein R 3 is independently selected from alkyl radicals having 1 to 4 carbon atoms, a phenyl radical, a trifluoropropyl radical and a chloropropyl group, x has an average value of 4 to 8, y has an average value of 5 to 25 and z has an average value of 1 to 20, with the proviso that y + z ≦ 25 ; and (II) an unsaturated organic compound selected from an alpha-alkene having 6 to 28 carbon atoms and an aromatic compound having the formula Ph-R 4 , wherein Ph represents a phenyl radical and R 4 is a terminally unsaturated monovalent hydrocarbon group having 2 to 6 carbon atoms. Component (A) of the present invention is a soluble organopolysiloxane resin consisting essentially of R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units. By the term "soluble", it is meant that the organopolysiloxane can be dissolved, substantially completely, in either a hydrocarbon liquid such as benzene, toluene, xylene and heptane or in a silicone liquid such as cyclic or linear polydiorganosiloxanes. Preferably the resin is soluble in component (B), delineated below. In the formula for component (A), R denotes a monovalent radical selected from hydrocarbon and halogenated hydrocarbon radicals, having less than 20 carbon atoms and preferably having from 1 to 10 carbon atoms. Examples of suitable R radicals include alkyl radicals, such as methyl, ethyl, propyl, pentyl, octyl, undecyl and octadecyl; cycloaliphatic radicals, such as cyclohexyl; aryl radicals such as phenyl, tolyl, xylyl, benzyl, alpha-methyl styryl and 2-phenylethyl; alkenyl radicals such as vinyl; and chlorinated hydrocarbon radicals such as 3-chloropropyl and dichlorophenyl. To enhance the solubility of component (A) in component (B), it is desirable to select the predominant organic radicals of the former to match the predominant organic radicals of the latter. Preferably, at least one-third and, more preferably, substantially all R radical in the formula for component (A), are methyl radicals. The methyl radicals can be distributed in any desired arrangement among the R 3 SiO 1/2 siloxane units; however, it is preferred that each R 3 SiO 1/2 siloxane unit contain at least one, and more preferably at least two, methyl radicals. Examples of preferred R 3 SiO 1/2 siloxane units include Me 3 SiO 1/2 , PhMe 2 SiO 1/2 and Ph 2 MeSiO 1/2 where Me denotes methyl and Ph denotes phenyl. Component (A) includes a resinous portion wherein the R 3 SiO 1/2 siloxane units (M units) are bonded to the SiO 4/2 siloxane units (Q units), each of which is bonded to at least one other SiO 4/2 siloxane unit. Some SiO 4/2 siloxane units are bonded to hydroxyl radicals resulting in HOSiO 3/2 units (TOH units), thereby accounting for the silicon-bonded hydroxyl content of the organopolysiloxane and some are bonded only to other SiO 4/2 siloxane units. In addition to the resinous portion, component (A) can contain a small amount of a low molecular weight material comprised substantially of a neopentamer organopolysiloxane having the formula (R 3 SiO) 4 Si, the latter material being a byproduct in the preparation of the resin according to the methods of U.S. Patent No. 2,676,182. For the purposes of the present invention, component (A) consists essentially of R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units in a molar ratio of 0.6 to 1.6, respectively. It is preferred that the mole ratio of the total M siloxane units to total Q siloxane units of (A) be between 1.1 and 1.4, most preferably about 1.1 to 1.3. The above M/Q mole ratios can be easily obtained by 29 Si nuclear magnetic resonance, this technique being capable of a quantitative determination of the molar contents of: M (resin), M(neopentamer), Q (resin), Q(neopentamer) and TOH. For the purposes of the present invention, as implicitly stated supra, the M/Q ratio {M(resin) + M(neopentamer)}/{Q(resin) + Q(neopentamer)} represents the ratio of the total number of triorganosiloxy groups of the resinous and neopentamer portions of (A) to the total number of silicate groups of the resinous and neopentamer portions of (A). It is further preferred that the resinous portion of component (A) have a number average molecular weight (M n ) of 2,300 to 3,000 when measured by gel permeation chromatography (GPC), the neopentamer peak being excluded from the measurement. In this molecular weight determination, narrow fractions of MQ resins are used to calibrate the GPC equipment, the absolute molecular weights of the fractions being first ascertained by a technique such as vapor phase osmometry. Component (A) can be prepared by any method providing said method yields a soluble organopolysiloxane consisting essentially of R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units. It is preferably prepared by the silica hydrosol capping process of U.S. Patent No. 2,676,182; as modified by U.S. Patent No. 3,627,851 and U.S. Patent No. 3,772,247. These patents teach how to prepare soluble organopolysiloxanes which meet the requirements of the present invention. Briefly stated, the combined effect of the teachings of these patents will yield a component (A) for this invention having a limited concentration of sodium silicate solution, a lower silicon-to-sodium ratio in the sodium silicate and/or a reduced time before capping the neutralized sodium silicate solution. These effects will prevent excessive growth of the silica particles and produce a soluble organopolysiloxane having the preferred M n in comparison to U.S. Patent No. 2,676,182 above. Thus, the preferred silicate concentration is generally limited to a value of 40 to 120, preferably 60 to 100 and, most preferably, 75 grams/liter. The neutralized silica hydrosol is preferably stabilized with an alcohol, such as isopropanol, and capped with R 3 SiO 1/2 siloxane units as soon as possible, preferably within 30 seconds, after being neutralized. The sodium silicate employed preferably has the formula Na 2 O xSiO 2 , wherein x has a value of 2 to 3.5. The level of the silicon bonded hydroxyl groups on the organopolysiloxane component (A), may be reduced, preferably to 1 weight percent or less. This may be accomplished by reacting hexamethyldisilazane with the organopolysiloxane. Such a reaction may be catalyzed, for example, with trifluoroacetic acid. Alternatively, trimethylchlorosilane or trimethylsilylacetamide may be reacted with the organopolysiloxane, a catalyst not being necessary in this case. Component (B) of the present invention is a polydiorganosiloxane having at least two olefinically unsaturated groups in its molecule. Preferably, this component is a polymer or copolymer having the general formula R 1 R 2 2 SiO(R 2 2 SiO) n SiR 2 2 R 1 wherein each R 2 is a monovalent radical independently selected from hydrocarbon and halogenated hydrocarbon radicals and each R 1 denotes a radical selected from R 2 radicals and an OH radical. Examples of R 2 radicals include the hydrocarbon and halogenated hydrocarbon radicals delineated above for R, as well as monovalent hydrocarbon radicals which contain olefinic unsaturation, such as alkenyl and cycloalkenyl radicals, such as vinyl, allyl, butenyl, hexenyl, cyclohexenyl and beta-cyclohexenylethyl. For the purposes of the present invention, at least two R 2 radicals of the above formula must contain olefinic unsaturation, preferably in the form of alkenyl radicals having 2 to 14 carbon atoms. Component (B) can be comprised of a single polydiorganosiloxane or a mixture of two or more different polydiorganosiloxanes. It is preferred that at least 50% and, more preferably, at least 85%, of the R 2 radicals of component (B) are methyl radicals, which radicals can be distributed in any manner in the polydiorganosiloxane. Further, component (B) can comprise trace amounts of siloxane branching sites provided that it remains flowable. Most preferably, component (B) has the general formula R''R' 2 SiO(R' 2 SiO) n SiR' 2 R'' wherein each R' independently denotes a methyl or phenyl radical, at least 85 percent of which radicals are methyl and each R'' denotes an alkenyl radical having 2 to 14 carbon atoms. Preferably, R'' is a vinyl radical. The average values of the subscript n in the above formulas depend on the type of SPSA being prepared. For formulations which include sufficient solvent to impart a practical working viscosity to the final SPSA composition, the average value of n is typically 200 or greater. When SPSA compositions having 2 weight percent or less of volatile compounds are desired (solventless systems), the average value of n should be about 50 to about 200 in order to produce a final SPSA having a practical working viscosity. For the purposes of the present invention, the volatile content of such a solventless system is determined by heating a two gram sample of the material to 150°C. for one hour and noting the weight of the residue. The average value of n used should provide an organopolysiloxane component (B) having a viscosity at 25°C. of about 50 millipascal-seconds (50 centipoise) to about 100 kilopascal-seconds (100,000,000 centipoise), the viscosity being a function of the R 2 radicals on this polymer. For the preferred solventless systems, described infra, component (B) should have a viscosity of about 50 to 800 mPa·s (centipoise), preferably 200 to 500 mPa·s, when measured at 25°C. Examples of polydiorganosiloxanes which are suitable for use as component (B) in the compositions of this invention include ViMe 2 SiO(Me 2 SiO) n SiMe 2 Vi, ViMe 2 SiO(Me 2 SiO) 0.95n (MePhSiO) 0.05n SiMe 2 Vi, ViMe 2 SiO(Me 2 SiO) 0.98n (MeViSiO) 0.02n SiMe 2 Vi, Me 3 SiO(Me 2 SiO) 0.95n (MeViSiO) 0.05n SiMe 3 , PhMeViSiO(Me 2 SiO) n SiPhMeVi and PhMeViSiO(Me 2 SiO) 0.8n (MePhSiO) 0.1n (Ph 2 SiO) 0.1n SiPhMeVi, wherein Vi denotes a vinyl radical. Crosslinking agent (C) of the present invention is a reaction product of an organohydrogenpolysiloxane (I) and an unsaturated organic compound (II) and represents the major departure from conventional cure systems for SPSAs. The organohydrogenpolysiloxane (I) used to prepare component (C) is either a cyclic siloxane having the formula or a linear siloxane having the formula or wherein R 3 is independently selected from alkyl radicals having 1 to 4 carbon atoms, such as methyl, ethyl and propyl; a phenyl radical; a trifluoropropyl radical; and a chloropropyl group. In formulas (i), (ii) and (iii), x has an average value of 4 to 8, y has an average value of 5 to 20 and z has an average value of 1 to 20, with the proviso that y + z ≤ 25. In preferred cyclic organohydrogenpolysiloxanes, each R 3 is matched with the organic radicals of components (A) and (B) and the average value of x is 5 to 7, most preferably 5. The preferred linear organohydrogenpolysiloxanes are represented by formula (ii) wherein y is 5 to 10 and each R 3 is similarly matched. Thus, in highly preferred SPSAs wherein the orgainc groups of both components (A) and (B) are predominantly methyl, R 3 is also methyl. The above described organohydrogenpolysiloxanes of the invention are well known in the art and many of these compounds are available commercially. The unsaturated organic compound (II) used in the preparation of component (C) can be a linear or branched alpha-alkene having 6 to 28 carbon atoms. This compound is preferably selected from linear alkenes having the formula CH 2 =CH 2 (CH 2 ) m CH 3 wherein m is 5 to 25, more preferably 9 to 25 and most preferably 9 to 15. When there are fewer than 6 carbons in the above alkene (when m = 3), the improved smooth adhesive release of the SPSA is not observed whereas, when there are more than about 28 carbon atoms in the alkene (when m = 25), the amount of crosslinking agent needed to cure the PSA composition becomes too large in proportion to the components (A) and (B). However, even though unbranched alkenes having a total of 24 to 28 carbons result in crosslinking agents which can provide good adhesives, their generally waxy consistency is less desired since this imparts a greater viscosity to the final SPSA formulation. Specific examples of preferred alpha-alkenes include 1-decene, 1-dodecene and 1-octadecene. Alternatively, (II) can be an aromatic compound having the formula Ph-R 4 , wherein R 4 is a terminally unsaturated monovalent hydrocarbon group having 2 to 6 carbon atoms. Preferably, unsaturated organic compound (II) is alpha-methylstyrene. In order to prepare crosslinking component (C) of the present invention, organohydrogenpolysiloxane (I) and unsaturated organic compound (II) are reacted using a platinum metal-type catalyst to promote the hydrosilation addition of the SiH functionality of the former to the unsaturation of the latter. This reaction is well known in the art and is typically conducted at temperatures in the range of 100 to 150°C., either neat or in the presence of an inert organic solvent such as toluene or hexane. For the purposes of the present invention, the relative amounts of components used to form crosslinking agent (C) are such that at least 10 mole percent of the original SiH functionality present in the organohydrogenpolysiloxane (I) is reacted with the unsaturated organic compound (II) and the final reaction product contains 3 to 15 residual SiH groups. If more than about 15 SiH groups are left on the crosslinking agent, a SPSA having low tack and adhesion results. On the other hand, when fewer than about 3 SiH groups remain, ineffective crosslinking ensues and cohesive strength of the SPSA is low. It is preferred that the final crosslinking agent (C) contain 4 to 6 residual SiH groups per molecule and the ratio of (II) to (I) to be used in the above reaction can be calculated to approximate such a result. The optimum relative amounts of components (A) and (B) that are used to form SPSA compositions according to the present invention are best determined by routine experimentation since adhesive performance is at least a function of the molecular weights of resin (A) and polymer (B). Typically, the proportions are such that from 50 to 90 parts by weight of component (A) is used for every 100 parts by weight of components (A) + (B) based on non-volatile content. In general, the crosslinking agent (C) is added in an amount sufficient to provide from 1 to 15 silicon-bonded hydrogen atoms for each unsaturated radical in the composition. The preferred ratio depends on the type of composition. For example, preferred solventless SPSAs of the invention employ a low molecular weight polymer. In this case, the crosslinking agent is added in a sufficient amount to provide from 1 to 4 silicon bonded hydrogen atoms, most preferably 1.5 to 2.0, for every olefinically unsaturated radical in the composition. When the above ratio is below 1, poor crosslinking and cohesive strength result; when the ratio is above 15, the resulting SPSA has low tack and reduced adhesion. The SPSA compositions of the present invention additionally contain a platinum group metal-containing catalyst (D). These catalysts include all of the well known platinum and rhodium catalysts which are effective for catalyzing the reaction of silicon-bonded hydrogen atoms with silicon-bonded alkenyl radicals or hydroxyl radicals. In addition, complexes of the metals ruthenium, palladium, osmium and irridium can be utilized. A preferred platinum-containing catalyst is a chloroplatinic acid-vinylsiloxane complex disclosed in U.S. Patent No. 3,419,593. Such platinum group metal-containing catalysts accelerate the reaction of component (B) with the crosslinking agent (C) and permit room temperature or low temperature curing of the composition. The platinum group metal-containing catalyst is added in an amount sufficient to provide 0.1 to 1,000, preferably 1 to 500 and most preferably 10 to 300, parts by weight of platinum group metal for each one million weight parts of the composition. It is recommended that the instant SPSA compositions also include a catalyst inhibitor for platinum group metal-containing catalysts that is known in the art. Preferred inhibitors include various "ene-yne" systems, such as 3-methyl-3-pentene-1-yne and 3,5-dimethyl-3-hexene-1-yne; acetylenic alcohols, such as 3-methyl-1-butyne-3-ol, 3,5-dimethyl-1-hexyne-3-ol, 3-methyl-1-pentyne-3-ol and phenylbutynol; maleates and fumarates, such as the well-known dialkyl, dialkenyl and dialkoxyalkyl fumarates and maleates; cyclovinylsiloxanes and benzyl alcohol. The catalyst inhibitor can be used in any amount that will retard the above-described catalyzed addition reaction at room temperature while not preventing said reaction at elevated temperature. The proportion of this ingredient to be used may be readily determined by routine experimentation. Curing of the compositions of this invention can be accomplished by heating at temperatures of up to 300°C., preferably at 80 to 150°C., for a suitable length of time. In general, compositions of this invention are made by homogeneously mixing 50 to 90 parts by weight of component (A) and 10 to 50 parts by weight of component (B) in the presence of a non-reactive solvent. Solvents useful in the present invention include hydrocarbons, such as toluene, xylene, heptane and mineral spirits; volatile siloxanes, such as octamethylcyclotetrasiloxane and hexamethyldisiloxane; halohydrocarbons, alcohols, esters, ketones and combinations of these solvents. The amount of solvent required depends on the viscosity of the polydiorganosiloxane, component (B). Higher viscosity polydiorganosiloxane polymers require more solvent than lower viscosity polydiorganosiloxane polymers to facilitate the preparation, handling and applying of the compositions. Suitable mixing means include 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. Component (C) may be mixed with (A) and (B) and the catalyst inhibitor can be incorporated in this mixture. The platinum group metal-containing catalyst is preferably added last or just prior to use. The preferred solventless SPSA compositions of the present invention may also be prepared by mixing the components in a solvent. Typically, component (A) is dissolved in 5 to 15 weight percent of an organic solvent to facilitate mixing and component (B) is then added. When the viscosity of component (B) is low enough, mixing can be accomplished without the aid of a solvent (i.e., an essentially 100% solids SPSA composition is also contemplated herein). To obtain a solventless composition of the present invention, the mixture of components (A), (B) and solvent is typically devolatilized under conditions equivalent to vacuum stripping at 130°C. and 133-266 Pa (1-2 millimeters of mercury) in a batch operation. The removal of solvent can also be accomplished by any of the known techniques such as contact with a stream of inert gas, evaporation, distillation, thin film stripping and the like. Excessively high temperatures should be avoided when components (A) and (B) or their mixtures are being devolatilized. A temperature of 200°C. and preferably 150°C. should not be exceeded. The crosslinking agent and catalyst are then added to the devolatilized mixture of (A) and (B) to complete the composition. In general, small amounts of additional ingredients may be added to the compositions of this invention. For example, antioxidants, pigments, stabilizers, fillers, etc., may be added as long as they do not materially reduce the pressure sensitive adhesive properties of these compositions. The compositions of this invention find utility as SPSAs and will readily stick to a solid support, whether flexible or rigid. These compositions may be applied to a surface by any suitable means such as rolling, spreading, spraying, etc. and then cured. The surface of the support and the substrate to which the support is adhered may be any known solid material such as metals, including aluminum, silver, copper, iron and their alloys; porous materials such as paper, wood, leather and fabrics; organic polymeric materials such as polyolefins, like polyethylene and polypropylene; fluorocarbon polymers such as polytetrafluoroethylene and polyvinylfluoride; polystyrene; polyamides such as nylons; polyesters and acrylic polymers; painted surfaces; siliceous materials such as concrete, bricks, cinderblocks and glass or glass cloth. Porous materials such as glass cloth are often impregnated with a substance that will prevent migration of the SPSA from one surface to another surface of the support. In this regard, it is also well known to chemically treat the surface of a flourocarbon polymer support to enhance the adhesion of the silicone pressure sensitive adhesive to said surface. Useful articles which can be prepared with the SPSAs of this invention include pressure sensitive tapes, labels, emblems and other decorative or informational signs. An especially useful article is one comprising a flexible or rigid support that can withstand extreme temperatures, hot or cold, and carrying on at least one surface thereof the SPSAs of this invention. Such an article makes full use of the stability at high temperatures and the flexibility at low temperatures that the SPSAs of this invention possess. The following examples are presented to further illustrate the compositions of this invention. All parts and percentages in the examples are on a weight basis and all measurements were obtained at 25°C., unless indicated to the contrary. Each of the adhesive formulations described infra was cast as a 1.5-2.0 mil (0.038 - 0.051 mm) film on 2.0 mil (0.051 mm) thick MYLAR™ (polyester) sheets, the adhesive was cured by heating the coated sheets at 130°C. for 6 minutes and the cured SPSA was evaluated according to the following procedures. Adhesion (A) Adhesion was measured by cutting the adhesive-coated sheets into one inch (25.4 mm) wide tape, rolling this pressure-sensitive tape onto a stainless steel panel with a five pound (2,270 g) roller and then pulling the tape from the panel at a 180 degree angle at a rate of 12 inches per minute (305 mm/min). The average force required to peel each film from the panel is reported in Newtons/meter (N/m). Percent Adhesive Failure (AF) When the above described adhesion test resulted in some adhesive transfer from the tape to the steel panel (some cohesive failure), the AF value represents the percent of the adhesive film remaining on the tape (i.e., AF = 100% when no adhesive transferred to the panel; AF = 0% when all of the adhesive transfers to the steel panel.) Intermediate values between these extremes (0 - 100%) were determined visually. When a tape sample had an AF value of 100% by the above adhesion test method, the procedure was repeated at a high peel rate by manually pulling the tape off the panel as rapidly as possible. Although the actual pull rate was not determined, this procedure provided reproducible results. In this case, the AF value was determined as follows. The amount of adhesive failure was noted and given a value from 0 to 10 (i.e., 10 when there was no adhesive transfer and 0 when there was complete transfer to the steel panel. This adhesive failure value was then added to 100 to give the final AF value for a test sample. Thus, if there was no cohesive failure, a value of 10 was added to the original 100 to give a final AF value of 110. If the adhesive failed entirely in a cohesive manner during the high speed peel, a value of zero was added to the original 100 to give a final AF value of 100. Intermediate values of cohesive failure were treated in a prorated fashion. According to this rapid peel procedure, the adhesive/cohesive failure was observed to take on one of four categories. The first three were cohesive failure type modes of varying degrees. The last category was no cohesive failure and only an adhesive failure mode. (1) The first failure type resulted in at least some adhesive transfer to the steel panel (cohesive failure), with 50 to 100 percent of the tape areas being so affected. (2) The second type of failure produced "chatter" lines which were about 1/16 inch (1.6 mm) wide and were distributed substantially uniformly along the length of the peeled tape. Usually such samples exhibited an AF value of 109 or, much less frequently, 108. (3) The final failure type resulted in a single thin line at the very end of the tape after it was peeled from the steel panel. These samples were given a value of 109.5 in recognition of the very low level of cohesive failure. (4) There was no adhesive transfer to the steel panel; AF = 110. Finger Tack (FT) Finger tack is a relatively subjective evaluation and was obtained by touching the surface of the adhesive to determine the degree of "stickyness" thereof. Descriptive terms such as high (H), low (L) and average (A) are reported in the tables below. Highly preferred formulations of the present invention simultaneously exhibit adhesion values of 50 oz/in ( 547 N/m) and high FT values, as required of conventional SPSAs used in commercial applications and, at the same time, have AF values of at least 109.5. Example 1 An adhesive master batch was prepared by thoroughly mixing 201 g of a vinyl-terminated polydimethylsiloxane fluid having a number average molecular weight of 11,000 with 600 g of a xylene solution of a soluble silicate resin. The resin, which was capped with hexamethyldisilazane, had a residual hydroxyl content of less than 1/4 weight percent, a number average molecular weight of 2,600 and consisted essentially of Me 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units in a molar ratio of 1.3:1, respectively. The molecular weight of the resin was obtained by gel permeation chromatography (GPC) using Varian TSK 4000 + 2500 columns at 35°C. and chloroform mobile phase at 1 mL/min. Narrow molecular weight fractions of a similar resin were used to calibrate the system. The above resin solution had a solids content of 90.5%, as determined by heating 1.0 g of the solution in an air oven at 150°C. for 1 hour. The mixture was then devolatilized at 150°C. and at 665 Pa (5 mm Hg) pressure for about one hour. Example 2 A crosslinking agent of the present invention was prepared by charging a 500 mL, 3-necked flask fitted with a mechanical stirrer, thermometer/temperature controller and an addition funnel with 100 g (0.333 mole) of a cyclic siloxane having the formula (MeHSiO) 5 . The addition funnel was filled with a mixture of 41.86 g (0.355 mole) of alpha-methylstyrene (AMS) and 1.16 g of a platinum complex. This catalyst was a chloroplatinic acid complex of divinyltetramethyldisiloxane diluted with dimethylvinylsiloxy endblocked polydimethylsiloxane to provide 0.65 weight percent platinum prepared according to Example 1 of U.S. Patent No. 3,419,593. The cyclic siloxane was heated to 120°C., at which point heating was discontinued and the dropwise addition of the funnel contents was started. The AMS was added at such a rate that the reaction exotherm maintained the temperature of the flask contents at 120°C. This addition step lasted 30 minutes, after which the reaction mixture was heated at 120°C. for an additional hour and was subsequently filtered. The resulting yellow liquid had a viscosity of 65 centistokes (65 mm 2 /s). Example 3 Seven gram portions of the adhesive master batch prepared in Example 1 were weighed into separate vials and additional vinyl-terminated polydimethylsiloxane fluid (or additional resin) as well as the crosslinking agent of Example 2 were blended therewith to achieve the resin/polymer (R/P) weight ratios and silicon hydride/vinyl on silicon (SiH/SiVi) molar ratios reported in the first and second columns of Table 1, respectively. Additionally, 0.01 g of a bis-(methoxymethyl)ethyl maleate inhibitor and 0.06 g of the above described platinum complex catalyst solution were thoroughly mixed into each of these vials. Each such adhesive formulation was applied to sheets of polyester, cured and tested according to the above described methods, the results are shown in Table 1. R/P SiH/SiVi A AF FT 69 1.8 503 109.5 HIGH LINE AT END 70 1.8 613 109 HIGH 71 1.8 711 102 HIGH 69 1.2 481 110 HIGH 70 1.2 569 109.5 HIGH LINE AT END 71 1.2 613 108 HIGH 70 1.5 569 109 HIGH VERY FEW CHATTER LINES 70 1.5 569 109 HIGH VERY FEW CHATTER LINES 70 1.5 569 109 HIGH VERY FEW CHATTER LINES Example 4 A series of crosslinking agents was prepared according to the method of Example 2, but on a smaller scale wherein the reaction was carried out in a one ounce (30 mL) vial. The mole ratio of AMS to (MeHSiO) 5 was varied as shown in the last column of Table 2. These crosslinking agents were used to prepare adhesives as in Example 3 wherein the R/P ratio was 70/30 and the SiH/SiVi ratio was 1.3. After being cured on polyester films, the adhesives were tested and the results are reported in Table 2. A AF FT AMS/(MeHSiO) 5 624 110 HIGH 1.0/1.0 678 108 HIGH 1.2/1.0 635 108 HIGH 1.4/1.0 678 105 HIGH 1.6/1.0 656 102 HIGH 1.8/1.0 503 100 HIGH 2.0/1.0 The above results indicate that for this particular system (70/30 R/P ratio) the preferred AMS/cyclic siloxane ratio is between 1.0 and 2.0. Example 5 A crosslinking agent was prepared as in Example 2 wherein 30.0 grams (0.1 mole) of (MeHSiO) 4 was reacted with 12.4 grams (1.05 mole) of AMS. Adhesives were prepared according to the methods of Example 3 using this crosslinking agent and these were tested as before. The results are presented in Table 3. R/P SiH/SiVi A AF FT 68 1.8 394 104 HIGH 69 1.8 558 99 HIGH SPOTS 70 1.8 678 99 HIGH SPOTS 68 1.2 459 109.5 HIGH LINE AT END 69 1.2 602 107 HIGH 70 1.2 678 99 HIGH SPOTS 69 1.5 591 107 HIGH 69 1.5 613 105 HIGH 69 1.5 569 107 HIGH Example 6 A crosslinking agent was prepared by reacting AMS and a linear siloxane having the structure Me 3 SiO(MeHSiO) n SiMe 3 , wherein n had an average value of 8, as described in Example 2. The proportions of the reactants used were: 50 gms of the linear siloxane (0.615 mole SiH); 38 g AMS (0.322 mole); and 0.2 g of the above described platinum catalyst. The solution exothermed within five minutes. This crosslinker was used to prepare and test adhesives as described in Example 3. The results are shown in Table 4. R/P SiH/SiVi A AF FT 69 1.8 503 109 HIGH VERY FEW CHATTER LINES 70 1.8 602 109 HIGH CHATTER LINES 71 1.8 678 102 HIGH 69 1.2 481 110 HIGH 70 1.2 624 109 HIGH CHATTER LINES 71 1.2 700 104 HIGH 70 1.5 580 109 HIGH CHATTER LINES 70 1.5 569 109 HIGH CHATTER LINES 70 1.5 547 109 HIGH CHATTER LINES Example 7 A crosslinking agent was prepared according to the methods of Example 2 wherein 30 g of (MeHSiO) 5 (0.10 moles) were reacted with 17.64 g of 1-dodecene (0.105 moles) using 2 drops of platinum catalyst complex. An exotherm was observed within one minute of combining the reactants. After the temperature began to drop, the product was placed in a 70°C. oven for an hour. This crosslinker was a colorless, clear liquid having a viscosity of about 10-15 centistokes (10-15 mm 2 /s). The above crosslinking agent was used to prepare adhesives according to the methods of Example 3 and the ratios of resin/polymer and molar ratio of SiH/SiVi are presented in Table 5, along with corresponding test results. R/P SiH/SiVi A AF FT 69 1.8 536 110 H NO CHATTER LINES 70 1.8 602 109.5 H LINE AT END, NO CHATTER LINES 71 1.8 656 102 H SPOTS, CHATTER LINES 69 1.2 558 109.5 H LINE AT END, NO CHATTER LINES 70 1.2 635 109.5 H LINE AT END, SLIGHT CHATTER 71 1.2 700 109.5 H LINE AT END, SLIGHT CHATTER 70 1.5 602 109.5 H LINE AT END, NO CHATTER LINES 70 1.5 602 109.5 H LINE AT END, NO CHATTER LINES 70 1.5 580 109.5 H LINE AT END, NO CHATTER LINES Example 8 The procedures of Example 7 were repeated using a crosslinking agent prepared by reacting 15.12 g of 1-dodecene (0.09 mole) with 16.67 g (0.205 equivalents of SiH) of a siloxane having the structure Me 3 SiO(MeHSiO) n SiMe 3 , wherein n had an average value of 8, using 0.03 g of the platinum complex catalyst. The crosslinking agent was capped in a bottle and heated at 75°C./18 hours. The adhesive formulation parameters and test results appear in Table 6. R/P SiH/SiVi A AF FT 69 1.8 427 110 HIGH NO CHATTER LINES 70 1.8 536 109 HIGH CHATTER LINES 71 1.8 569 109 HIGH CHATTER LINES 69 1.2 405 110 HIGH NO CHATTER LINES 70 1.2 492 110 HIGH NO CHATTER LINES 71 1.2 558 109 HIGH FEW CHATTER LINES 70 1.5 481 109 HIGH FEW CHATTER LINES 70 1.5 492 109 HIGH FEW CHATTER LINES 70 1.5 492 109 HIGH FEW CHATTER LINES Example 9 A crosslinking agent was prepared according to the methods of Example 2 wherein 30 g of (MeHSiO) 5 (0.1 mole) were reacted with 26.46 g of 1-octadecene (0.105 mole) using 0.03 g of the platinum complex catalyst. A rapid exotherm was observed wherein the temperature rose to 100°C. This reaction product was heated in a capped bottle (75°C./3 hours) and then used to formulate the adhesives shown in Table 7 according to the method outlined in Example 3. Test results, obtained as described above, are also presented in Table 7. SiH/SiVi R/P A AF FT 1.8 69 492 109.5 H NO CHATTER; LINE AT END 1.8 70 591 107 H SPOTS; CHATTER 1.8 71 656 107 H SPOTS; CHATTER 1.2 69 481 110 H 1.2 70 580 109.5 H SLIGHT CHATTER; LINE AT END 1.2 71 678 109.5 H SLIGHT CHATTER; LINE AT END 1.5 70 558 109.5 H NO CHATTER; LINE AT END 1.5 70 569 109.5 H NO CHATTER; LINE AT END 1.5 70 558 109.5 H NO CHATTER; LINE AT END Example 10 The procedure of Example 9 was followed wherein the crosslinking agent was a reaction product of 30 g of (MeHSiO) 5 (0.1 mole) and 8.84 g of 1-hexene (0.105 mole) using 0.03 g of the platinum complex catalyst. An exotherm was observed after several minutes of mixing the reactants and the product was heated in a capped bottle (75°C./2 hours) and then used to formulate adhesives, as described above. The results are presented in Table 8. SiH/SIVi R/P A AF FT 1.8 69 492 105 H NO CHATTER; ADHESIVE ISLANDS 1.8 70 503 109.5 H NO CHATTER; LINE AT END 1.8 71 591 25 H 1.2 69 470 109 H CHATTER; CHATTER LINES 1.2 70 569 85 H CHATTER 1.2 71 678 40 H CHATTER 1.5 70 580 105 H CHATTER 1.5 70 580 105 H CHATTER 1.5 70 591 109 H SLIGHT CHATTER; LINES Example 11 The procedure of Example 9 was followed wherein the crosslinking agent was a reaction product of 30 g of (MeHSiO) 5 (0.1 mole) and 32 g of a mixture of C20-C24 alpha-alkenes (approximately 0.105 mole) using 0.03 g of the platinum complex catalyst. A mild exotherm was observed 20 minutes after mixing the reactants and the product (a white wax) was heated in a capped bottle (75°C./18 hours) and then used to formulate adhesives, as described above. The results are presented in Table 9. SiH/SiVi R/P A AF FT 1.8 69 536 109.5 H LINE AT END 1.8 70 602 109.5 H LINE AT END 1.8 71 645 108 H CHATTER LINES 1.2 69 263 108 H SPOTS 1.2 70 558 109.5 H LINE AT END 1.2 71 624 109 H CHATTER LINES 1.5 70 613 109 H VERY FEW CHATTER LINES 1.5 70 613 109 H VERY FEW CHATTER LINES 1.5 70 591 109 H VERY FEW CHATTER LINES Example 12 The procedure of Example 9 was followed wherein the crosslinking agent was a reaction product of 15 g of (MeHSiO) 5 (0.05 mole) and 5.88 g of 1-octene (0.0525 mole) using 0.03 g of the platinum complex catalyst. An exotherm was observed within 5 minutes and the product was heated in a capped bottle at 75°C. and then used to formulate adhesives, as described above. The results are presented in Table 10. SiH/SiVi R/P A AF FT 1.8 69 503 109.5 H LINE AT END 1.8 70 536 109 H CHATTER LINES 1.8 71 624 107 H SPOTS 1.2 69 492 109.5 H LINE AT END 1.2 70 569 109.5 H LINE AT END 1.2 71 613 107 H SPOTS 1.5 70 602 109 H CHATTER LINES 1.5 70 602 109 H CHATTER LINES 1.5 70 613 109 H CHATTER LINES (Comparative) Example 13 Adhesives were prepared and tested according to the methods of Example 3 using a conventional crosslinking agent consisting essentially of a trimethylsiloxy-terminated polymethylhydrogensiloxane having an average degree of polymerization of 80. The formulation parameters and test results are presented in Table 11. R/P SiH/SiVi A AF FT 69 1.8 438 100 VERY LOW 70 1.8 317 100 VERY LOW 71 1.8 284 100 NONE 69 1.2 383 40 HIGH 70 1.2 514 90 AVG 71 1.2 372 0 HIGH 70 1.5 525 90 VERY LOW 70 1.5 481 100 VERY LOW 70 1.5 536 90 VERY LOW (Comparative) Example 14 Adhesives were prepared and tested according to the methods of Example 3 using a conventional crosslinking agent consisting essentially of a polymethylhydrogensiloxane having the formula Me 3 SiO(MeHSiO) n SiMe 3 wherein n had an average value of 8. The formulation parameters and test results are presented in Table 12. R/P SiH/SiVi A AF FT 69 1.8 449 109 AVG CHATTER LINES 70 1.8 503 109 AVG CHATTER LINES 71 1.8 569 109 LOW CHATTER LINES 69 1.2 438 100 HIGH 70 1.2 514 100 HIGH 71 1.2 602 100 AVG 70 1.5 525 109 AVG CHATTER LINES 70 1.5 503 109 AVG CHATTER LINES 70 1.5 525 109 AVG CHATTER LINES

Claims: A silicone pressure-sensitive adhesive composition comprising a siloxane polymer having at least two olefinically unsaturated groups in its molecule, a siloxane resin, an SiH-functional crosslinking agent and a catalytic amount of a platinum group metal-containing catalyst, characterised in that the crosslinking agent is a hydrosilation reaction product of (I) an organohydrogenpolysiloxane selected from a cyclic siloxane having the formula a linear siloxane having the formula and a linear siloxane having the formula wherein R 3 is independently selected from alkyl radicals having 1 to 4 carbon atoms, a phenyl radical, a trifluoro-propyl radical and a chloropropyl group, x has an average value of 4 to 8, y has an average value of 5 to 25 and z has an average value of 1 to 20, with the proviso that y + z ≤ 25; and (II) an unsaturated organic compound selected from an alpha-alkene having 6 to 28 carbon atoms and an aromatic compound having the formula Ph-R 4 , wherein Ph represents a phenyl radical and R 4 is a terminally unsaturated monovalent hydrocarbon group having 2 to 6 carbon atoms. A composition according to Claim 1 wherein, (A) the siloxane resin is present in an amount of from 50 to 90 parts by weight and comprises a soluble organopolysiloxane consisting essentially of R 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units wherein each R denotes a monovalent radical selected from hydrocarbon and halogenated hydrocarbon radicals at least 1/3 of all R radicals being methyl and the mole ratio of R 3 SiO 1/2 siloxane units to SiO 4/2 siloxane units in the soluble organopolysiloxane having a value of from 0.6 to 1.6; (B) the siloxane polymer having at least two olefinically unsaturated groups in its molecule is present in an amount of from 10 to 50 parts by weight and comprises a polydiorganosiloxane having the general formula R 1 R 2 2 SiO(R 2 2 SiO) n SiR 2 2 R 1 wherein each R 2 denotes a monovalent radical independently selected from hydrocarbon and halogenated hydrocarbon radicals, at least 1/2 of all R 2 radicals being methyl, each R 1 denotes a radical selected from R 2 radicals and OH radicals, the subscript n has an average value of at least 50 and wherein at least two R 2 radicals contain olefinic unsaturation, the sum of components (A) and (B) being 100 parts by weight; and the crosslinking agent is present in the composition in an amount sufficient to provide from 1 to 15 SiH groups for each olefinic radical in said composition. The composition according to claim 2, wherein component (A) consists essentially of (CH 3 ) 3 SiO 1/2 siloxane units and SiO 4/2 siloxane units, R 1 and R 2 of component (B) are independently selected from methyl radical, phenyl radical and alkenyl radicals in each molecule of said component (B), the subscript n of said component (B) is a number having an average value of 50 and wherein each R 3 of said organohydrogenpolysiloxane (I) of said component (C) is a methyl radical. The composition according to claim 3, wherein said organohydrogenpolysiloxane (I) is represented by the formula wherein Me denotes a methyl radical, x has an average value of 5 to 7 and said unsaturated organic compound (II) is selected from alpha-methylstyrene and an alkene represented by the formula CH 2 =CH 2 (CH 2 ) m CH 3 where m is an integer from 5 to 25. The composition according to claim 3, wherein said organohydrogenpolysiloxane (I) is represented by the formula wherein Me denotes a methyl radical, y has an average value of 5 to 10 and said unsaturated organic compound (II) is selected from alpha-methylstyrene and an alkene represented by the formula CH 2 =CH 2 (CH 2 ) m CH 3 where m is an integer from 5 to 25. The composition according to claims 4 or 5, wherein said soluble organopolysiloxane (A) has a silicon-bonded hydroxyl content of 1 percent by weight or less, based on the weight of soluble organopolysiloxane. An article having on at least one surface thereof the cured composition of claim 2. A pressure-sensitive adhesive tape comprising a flexible support having on at least one surface thereof the cured composition of claim 2.

Inventors: BRADY WILLIAM PATRICK, VINCENT GARY ALLEN, BRADY, WILLIAM PATRICK, VINCENT, GARY ALLEN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1993
Main classification: C09J7
IPC codes: C09J7, C09J183