SILICONE ADHESIVE FORMULATIONS

Abstract: Physical properties, especially tack characteristics, of sili­ cone adhesives comprising a silicone polymer and a silicone resin are improved by incorporation, prior to curing, of certain silicone cluster compounds. One preferred cluster compound has the formula RSi [OSi(OR′) 3|3 wherein R is a methyl group and R′ is a secondary butyl group.

Description: SILICONE ADHESIVE FOWSULATIONS The present invention relates generally to silicone adhesives and more particularly to improvements in physical properties, especially tack characteristics, of pressure sensitive silicone adhesives. Silicone pressure sensitive adhesives are generally provided in commerce in a form comprising an admixture of a silicone polymer, a silicone resin, a suitable solvent and, optionally, additives such as viscosity stabilizers. Curing of such mixtures is by thermal and/or catalytic means. Among the more common silicone polymers employed are hydroxyl-endblocked siloxane gums containing dimethylsiloxane, diphenylsiloxane, methylvinylsiloxane and/or phenylmethylsiloxane units. By way of example, these gums can be formed by the reaction of dimethylsiloxane cyclic compounds with relatively low viscosity hydroxyl-endblocked polydimethyl 5 iloxanes in the presence of a suitable catalyst such as potassium silanolate. Resins typically employed in silicone pressure sensitive adhesive formulations include trimethylsiloxy endblocked silicates providing about 2 percent silanol groups and having a ratio of trimethylsiloxy groups to SiC2 groups of from about 0.4 to 1 to about 1.2 to 1 and preferably about 0.6 to 1. Solvents suitable for use in silicone pressure sensitive adhesives include aromatic solvents generally and preferably xylene. Additives for viscosity stabilization include isopropanol. Formulàtions as described above provide a two phase system including a continuous, non-polar phase of polymer containing dissolved resin and a disperse, polar phase of resin containing dissolved polymer. The character of the continuous phase is generally acknowledged to provide for the cohesive properties of the adhesive while the disperse phase characteristics provide for the tack properties of the adhesive. Adhesive formulations, therefore, are selected to provide ratios of polymer to resin such as will generate acceptable balances between desired characteristics of cohesiveness and tack in the end product. Put another way, adhesive strength of the blends of polymer and resin forming a silicone pressure sensitive adhesive may be increased by increasing the resin content, but at the loss of tack properties. By way of example, silicone pressure sensitive adhesive formulations generally are based upon a total combined weight of resin and polymer equalling 100 parts in a suitable solvent carrier, plus any of the conventional additives. In adhesive formulations projected for an end use wherein high tack characteristics are required, a relatively low weight ratio of resin to polymer is employed--on the order of 45 parts. resin to 55 parts polymer. Formulations projected for end uses wherein exceptionally high adhesion characteristics are required will incorporate a relatively high ratio of resin to polymer--on the order of 75 parts resin to 25 parts polymer. The iatter formulations are acknowledged to have rather poor tack characteristics when cured. There exists in the art, therefore, a need for silicone pressure sensitive adhesive formulations that provide both the superior tack properties ordinarily associated only with those formulations having relatively low resin to polymer ratios as well as the strong adhesive characteristics ordinarily associated with formulations having relatively high resin to polymer ratios. According to the present invention tack characteristics of a cured silicone pressure sensitive adhesive are enhanced by incorporating into adhesive formulations, prior to curing, a silicone cluster compound. This invention specifically relates to an improvement in a silicone pressure sensitive adhesive composition comprising 45 to 75 parts by weight of a silicone resin and 25 to 55 parts by weight of a silicone polymer, the improvement comprising adding to said composition 0.5 to 10 parts by weight, based on a total of 100 parts by weight of the resin and polymer, of a cluster compound selected from the group consisting of (A) RSiCOSi(OR')3)3, (B) MOSitOSi(OR')3)3a, (C) M < OSiR{OSi(OR')3}2)a, R" R" and (D) f (R'O) 3SiO}2Si(OSiR' ''2) (OSiR' ' F2)nOSitOSi(OR1 )32 wherein R is hydrogen, an alkyl, alkenyl, aryl or aralkyl group, R' is hydrogen, an alkyl, alkenyl, aryl or aralkyl group, with the proviso that at least a majority of the R' radicals are strically hindered alkyl groups having at least 3 carbon atoms, M is a substituted or unsubstituted branched or straight chain hydrocarbon radical, a is 2, 3, or 4, n is an integer from 0 to 300, R" is hydrogen, an alkyl, alkenyl, aryl, aralkyl or -OSi(OR')3 group and R"' is hydrogen, an alkyl, alkenyl, aryl, aralkyl, hydroalkyl, and halo or cyano substituted alkyl, alkenyl, aryl, aralkyl and hydroalkyl. More specifically, the invention preferably provides for enhancing tack properties of pressure sensitive adhesives based on relatively high resin to polymer ratio formulations by. adding to such formulations from 0.5 to .10 and preferably 1 to 5 parts by weight (based on a total of 100 parts by weight of resin and polymer) of a silicone cluster compound as described above. Also provided according to the invention are formulations providing adhesives with good adhesive strength as well as good tack properties after heat aging by adding to the silicone pressure sensitive adhesive formulations such silicone cluster compounds. Preferred among the cluster compounds useful in practice of the invention are those alkoxysilicone cluster compounds of the above formula wherein R is methyl and each R' secondary butyl. The silicone cluster compounds suitable for use in practice of the invention include those described'in, and prepared by the methods set forth in U.S. Letters Patent Nos. 3,965,136 issued June 22, 1976; 3,992,429 issued November 16, 1976; 4,058,546 issued November 15, 1977; and 4,077,993 issued March 7, 1978, all to Knollmeuller. The disclosures of these patents are hereby incorporated by reference. Such cluster compounds have the general formulae RSitOSi(OR')3}31 MOSi{OSi(OR')3}3a, MOSiR{OSi(OR' )3}2a' and R" R " ' ' {(R'O)3SiO}2Si(OSiR1,,2)(OSiR'''2)nOSitOSi(OR')3}2 wherein R is hydrogen, an alkyl, alkenyl, aryl or aralkyl group, R' is hydrogen, an alkyl, alkenyl, aryl or aralkyl group, with the proviso that at least a majority of the R' radicals are sterically hindered alkyl groups having at least 3 carbon atoms, M is a substituted or unsubstituted branched or straight chain hydrocarbon radical, a is 2, 3, or 4, n is an integer from 0 to 300, R" is hydrogen, an alkyl, alkenyl, aryl, aralkyl or -OSi(OR')3 group, and R' " is hydrogen, an alkyl, alkenyl, aryl, aralkyl, hydroalkyl, and halo or cyano substituted alkyl, alkenyl, aryl, aralkyl and hydroalkyl. Preferred cluster compounds for use in the invention are those wherein R is a methyl group and each R' radical is a secondary butyl radical. Such preferred dusted compounds are presently commercially available from Olin Chemicals (Stamford, Connecticut) under the trade designation "Silicate Cluster 102" and "Silicate Cluster 2102". According to one aspect of the invention, cluster compounds are cold mixed into uncured adhesive formulations containing a relatively high ratio of silicone resin to silicone polymer gum and which consequently, would be expected to provide cured adhesives possessing high adhesive strength but rather low tack characteristics, As employed herein, the term "relatively high resin to polymer ratio formulation" shall mean a. silicone pressure sensitive adhesive formulation wherein the silicone resin is present in the formulation in quantity equal to or greater than the quantity of silicone polymer gum. More specifically, the term comprehends those silicone adhesive formulations wherein the silicone resin component comprises from 45 to 75 (preferably 45 to 60) parts by weight and the silicone polymer gum component comprises from 25 to 55 (preferably 40 to 55) parts by weight in a formulation based on a total of 100 parts by weight of the resin plus polymer. It has been found that the addition of from about 0.5 to about 10 parts by weight cluster compound (based on 100 parts of the resin-polymer combinationj to adhesives having a relatively high resin to polymer ratio formulation will allow for development of a cured adhesive material possessing high adhesive strength characteristics and high tack characteristics. The preferred quantity of cluster compound to add to such formulations is from about 1 to about 5 parts by weight and it is noted that additions of quantities in excess of 5 parts by weight provide only minimally greater enhancement of tack properties over and above that obtained by addition of about 5 parts by weight. According to another aspect of the invention1 uncured silicone adhesive formulations having improved tack characteristics after curing and heat aging are prepared wherein 0.5 to 10 parts (based on 100 parts of the resin polymer combination) of the cluster compounds as described above are added to the formulation. The following examples are provided as illustrative of practice of the invention and are not intended to establish limits upon the invention as subsequently claimed. Quantitative tack measurements reported therein are performed through use of a POLYKENS brand Probe Tack Tester (Testing Machines, Inc., Amityville, NY). Briefly summarized, tack measurements, expressed in units of grams of tack, were obtained using a probe velocity of 0.5 cm/sec, a contact pressure of 100 grams/cm2, and contact time of 0.5 seconds. Quantitative adhesion measurements reported therein were obtained through use of a one-half inch wide aluminum or Teflons tape which contained a 1.5 mil layer of cured adhesive. The tape was adhered to a stainless steel panel and stripped at a rate of 12 inches/minute at an angle of 1800, with the results expressed in ounces per inch. The adhesion test corresponds to ASTM D-1000. Example 1 A cold mix of the following general formulation was prepared: 27.4 parts by weight of a polydimethylsiloxane gum (prepared by reaction of 100 parts by weight dimethylsiloxane cyclic trimer with 0.40 parts by weight of a hydroxy-endblocked polydimethylsiloxane fluid having a viscosity of 60-70 centistokes and 0.24 parts by weight of a potassium silanolate catalyst); 29.7 parts by weight of a resinous trimethylsiloxy endblocked silicate providing about 2 percent silanol groups and having a ratio of trimethylsiloxy groups to SiO2 groups of from about 0.4 to 1 to about 1.2 to 1 and incorporated as a 70 percent solids xylene solution; 6.3 parts by weight of a hexamethyldisilazane-treated resinous silicate as previously noted (providing a maximum of about 0.4 percent silanol groups) and incorporated as a 60 percent solids xylene solution; 2.3 parts by weight isopropanol; 34.2 parts by weight xylene; and, 0.14 parts by weight of a reaction product of 115 parts by weight tetramethylguanidine and 144 parts by weight 2-ethylhexanoic acid in 1036 parts by weight xylene; The formulation thus obtained exemplifies an adhesive material having a relatively high resin to polymer ratio. Example 2 Three 157 gram samples of the cold mix formulation of Example 1 were separated for testing. Control, first and second test samples respectively had 0, 1 gram and 5 grams of the silicone cluster compound RSi{OSi(OR')3}3, wherein R is a methyl group and R' is a secondary butyl group, added with mixing prior to mixing with 1 part by weight benzoyl peroxide and coating on aluminum or Teflons strips. The coated compositions were cured by heat treatment at 700C for fifteen minutes and then 1500C for five minutes. Adhesion test measurements with aluminum strips for control, first and second test samples provided values of 89, 83, and 80 respectively. Tack measurements for control, first and second test samples were 236, 353 and 533, respectively. Accelerated aging of the aluminum coating samples was carried out by treatment at 2500C for 24 hours. Adhesion testing results for the control, first and second test samples so aged were 68, 70 and 70, respectively, while tack testiny results for the control and second test samples were 140 and 330, respectively. Tack measurements with Teflons coated strips for the control, first and second test samples were 333, 520 and 580 respectively. Accelerated aging (2500C for 24 hours) of the coated Teflons strips was effected and tack measurements of 180, 500 and 410r respectively, were obtained. The above example indicates not only substantial improvements in tack accompanying the addition of the cluster compound, but some evidence of enhancement of heat stability of the cured adhesive composition as well. Example 3 A test was conducted to ascertain the effects on adhesion over long-term aging of adhesive formulations at room temperature. Four 50 gram samples of an adhesive formulation prepared according to Example 1 were separated for testing. To control, first, second and third samples were added respectively 0, 0.3, 1.5, and 3.6 grams of the preferred methyl, secondary butyl cluster compound used in Example 2. After being mixed with one percent benzoyl peroxide, coated on aluminum or Telfone and cured with heat treatment of fifteen minutes at 70 C and five minutes at 150"C, adhesion values were determined for aluminum and Teflons strips and the strips then stored at room temperature. Adhesion values were also determined at various intervals thereafter. The results of the tests are set forth in Table 1 below and indicate no substantial adverse effects of the cluster compounds on long term adhesion characteristics.. Table 1 Aluminum Strips Aging Adhesion - ounces/inch First Second Third Control Sample Sample Sample 0 (Initial) 88 88 84 64 3 days 94 90 86 62 7 days 96 96 92 62 14 days 100 100 78 66 1 month 96 100 76 60 2 months 96 90 70 57 3 months 100 83 80 59 6 months 84 100 72 66 15 months 76 82 72 59 Teflons Strips Aging Tack - grams First Second Third Control Sample Sample Sample 0 (Initial) 460 667 787 770 3 days 400 740 840 805 7 days 340 687 823 826 14 days 420 750 660 810 1 month 350 740 620 760 2 months 310 770 760 750 3 months 380 750 810 790 6 months 370 580 820 760 15 months 685 713 697 740 Example 4 To a 164 gram sample of the formulation of Example 1 there was added 3 grams of the silicone cluster compound MOSiRfOSi(OR')3}2a, wherein R is a methyl group, R' is a secondary butyl group, a is 2 and M is -CH2C(CH3)2CH2-, with mixing prior to curing with 1% by weight of benzoyl peroxide for fifteen minutes at 700C followed by five minutes at 150QC. In the adhesion test on aluminum a value of 82 was obtained while in the tack test on Teflons a value of 680 was obtained. As noted previously, it has been found that additions of from 0.5 to 10 parts by weight of cluster compound per 100 parts of resin plus polymer in an uncured adhesive formulation result in substantial enhancement of tack characteristics of the cured adhesive. It is preferred, however, that additions of cluster compound be within the range of from about 3 to about 5 parts by weight inasmuch as enhancement of tack ceases to be significant after levels of more than about 5 parts by weight have been reached. It should also be found that additions of the cluster compound do not appear to require any unusual processing steps other than simple mixing and that the usual range of catalyst (e.g., from 0.5 to 4 weight percent benzoyl peroxide based on solids content of the formulation) may be employed satisfactorily to cure the formulations to which the cluster compound has been added. Consistent with the above description of addition of the cluster compounds to existing formulations having a relatively high resin to polymer ratio is that aspect of the invention which provides for the addition of the cluster compound to silicone pressure sensitive adhesive formulations which may be subjected to heat aging to enhance their tack characteristics. It is believed that from about 0.5 to about 10 parts by weight, based on 100 parts of the combined weights of the resin and polymer, of the cluster compounds will result in cured adhesives having enhanced tack as well as good adhesion properties. It is preferred that from about 3 to about 5 parts of the cluster compound be used. It will be apparent from the above that practice of the invention allows for both the development of high tack, highly adhesive silicone pressure sensitive adhesives as well as modification, prior to curing, of existing formulations which would ordinarily be expected to form highly adhesive cured products having low tack characteristics. The latter aspect is of substantial commercial significance because it allows the end user to stock supplies of adhesive formulations generating highly adhesive cured products but to modify such formulations to develop aggressively tacky cured adhesives on a small scale and as desired for a particular end use. Modifications and variations of the invention as above described are expected to occur to those skilled in the art upon consideration of the foregoing description and consequently only such limitations as appear in the appended claims should be placed on the invention.

Claims: Claims 1. in a silicone pressure sensitive adhesive composition comprising 45 to 75 parts by weight of a silicone resin and 25 to 55 parts by weight of a silicone polymer gum, characterized in that it further comprises 6.5 to 10 parts by weight, based on a total of 100 parts by weight of the resin and polymer gum, of a cluster compound selected from the group consisting of (A) RSi{OSi(OR')3}3, (B) MOSifOSi(OR')3}3)a, (C) M < OSiRCOSi(OR')3) 2)ar and R" R" (D) t(R'O)3SiO}2Si(OSiR'''2)(OSiR'''2)nOSitOSi(OR')3}2 wherein R is hydrogen, an alkyl, alkenyl, aryl or aralkyl group, R' is hydrogen, an alkyl, alkenyl, aryl or aralkyl group with the proviso that at least a majority of the R' radicals are sterically hindered alkyl groups having at least 3 carbon atoms, is a substituted or unsubstituted branched or straight chain hydrocarbon radical, a is 2, 3, or 4, n is an integer from 0 to 300, " is hydrogen, an alkyl, alkenyl, aryl, aralkyl or -OSi(OR')3 group, and R'l' is hydrogen, an alkyl, alkenyl, aryl, aralkyl, hydroalkyl, and halo or cyano substituted alkyl, alkenyl, aryl, aralkyl, and hydroalkyl. 2. A pressure sensitive adhesive as defined in claim 1 wherein 1 to 5 parts by weight cf the cluster compound is employed. 3. A pressure sensitive adhesive as defined in claim 2 wherein the cluster compound has formula (A). 4. A pressure sensitive adhesive as defined in claim 3 wherein the formula (A) R is a methyl group and each R' is a secondary butyl group. 5. A pressure sensitive adhesive as defined in claim 3 wherein the silicone resin is 45 to 60 parts by weight and the silicone polymer gum is 40 to 55 parts buy weight. 6. A pressure sensitive adhesive as defined in claim 2 wherein the cluster compound. has formula (B). 7. A pressure sensitive adhesive as. defined in claim 2 wherein the cluster compound has formula (C). 8. A pressure sensitive adhesive as defined in claim 7 wherein in formula (C) R is a methyl group, each R' is a secondary butyl group, a is 2, and M is -CH2C(CH3)CH2- radical. 9. A pressure sensitive adhesive as defined in claim 8 wherein the silicone resin is 45 to 60 parts by weight and the silicone polymer gum is 40 to 55 parts by weight. 10. A pressure sensitive adhesive as defined in claim 2 wherein the cluster compound has formula (D).

Inventors: BLIZZARD, JOHN DONALD
Applicants: DOW CORNING CORPORATION
Application year: 1981
Main classification: C09J3
IPC codes: C09J183, C08L83