Abstract:
Novel mercaptoorganosiloxane compounds of the formula ##STR1## wherein n has a value of from 1 to 3 inclusive, are useful as components of primer compositions for enchancing the adhesion of silicone elastomers to substrates.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to novel sulfur-containing siloxane compounds and to compositions including the compounds which are useful as primers for promoting adhesion of silicone elastomers to substrates. 
     2. Description of the Prior Art 
     Adhesion of silicone elastomers, e.g. sealant compositions, to substrate surfaces has been the subject of substantial research and developmental work in the past. Of particular concern has been the enhancement of adhesion characteristics for elastomers formed by the platinum catalyzed reaction of vinyl endblocked siloxane polymers and .tbd.SiH-group-containing materials which proceeds according to the generalized reaction: ##STR2## 
     SUMMARY OF THE INVENTION 
     The present invention provides compounds of the formula ##STR3## wherein n has a value of from 1 to 3 inclusive, which compounds are formed by the reaction of 1,1,3,3-tetramethyl-1,3-divinyldisiloxane and a selected mercaptoalkyltrimethoxysilane. A preferred compound, 1-(7,7,-dimethoxy-8-oxa-3-thia-7-silanonyl)-1,1,3,3-tetramethyl-3-vinyldisiloxane is formed by the reaction of 1,1,3,3-tetramethyl-1,3-divinyldisiloxane and gamma mercaptopropyltrimethoxy silane. The compounds are employed in the development of primer formulations to enhance adhesion characteristics of silicone elastomer sealant compositions to surfaces of substrates, especially. 
     Incorporated by reference herein is U.S. Patent application Ser. No. 99,300, by Gary R. Homan and Chi-Long Lee, filed concurrently herewith and entitled &#34;Mercaptoorganopolysiloxane and Curable Compositions Including Same&#34;, and U.S. Patent application Ser. No. 99,303, by Gary R. Homan and Jan M. Blevins, filed concurrently herewith and entitled &#34;Novel Mercaptoorganopolysiloxanes&#34;. 
    
    
     DESCRIPTION OF THE INVENTION 
     EXAMPLE 1 
     1-(7,7-dimethoxy-8-oxa-3-thia-7-silanonyl)-1,1,3,3-tetramethyl-3-vinyldisiloxane was prepared according to the following process. Two and one-half moles (490 grams) of gamma mercaptopropyltrimethoxy silane was added to a 1-liter flask. A portion of a total quantity of one mole (186 grams) of 1,1,3,3-tetramethyl-1,3-divinyldisiloxane was added to the flask, resulting in the immediate formation of a bluegreen color and generation of heat. The flask was placed in a hood and stirred. The exotherm was measured at 75° C, subsided to about 55° C. and remained there for 10 minutes. The remaining 1,1,3,3-tetramethyl-1,3-divinyldisiloxane was added with no further exotherm. Gas/liquid chromotography revealed consumption of substantially all of the 1,1,3,3-tetramethyl-1,3-divinyldisiloxane reagent and the presence of mono- and diadduct products. The reaction product was distilled through a Vigreaux column to yield 60 g of approximately 95% pure product having a boiling point of 157° C. at 3.0 mm Hg. 
     Infrared analysis of the product confirmed the structure, i.e., ##STR4## 
     --Si(CH 3 ) 2  OSi(CH 3 ) 2  CH═CH 2  : 1595, 1405, 1250, 1050, 1005, 955, 840, 785 and 520 cm -1   
     A sample of the product was introduced directly into a mass spectrometer and heated to 200° C. at 10 -6  torr and the analysis was also in agreement with the structure, i.e., 
     M +  at m/e 382, m +  --CH 3  at m/e 367, m +  --CH═CH 2  at m/e 355, CH 2  ═CHSi (CH 3 ) 2  OSi(CH 3 ) 2  --at m/e 159 and (CH 3  O) 3  Si--at m/e 121. 
     The H +  nmr spectrum also agrees with the structure. The proton ratio indicated that some disulfide product had been formed. 
     
         ______________________________________       Proton Ratio δValues______________________________________CHCH.sub.2    2.9            5.6-6.2CH.sub.3 O    9.2             3.5SCH.sub.2     3.8            ˜2.5 ##STR5##     2.5            ˜1.6 ##STR6##     4.2            -- ##STR7##     11.4           1.3,0.09______________________________________ 
    
     The foregoing reaction process may suitably be carried out in the presence of ferric 2-ethylhexanoate (octoate) or suitable free radical generating catalysts, as well as with use of radiant energy (e.g., ultraviolet). 
     In a like manner, the foregoing general process may be employed to prepare the corresponding 1-(6,6-dimethoxy-7-oxa-3-thia-6-silaoctyl)-1,1,3,3-tetramethyl-3-vinyldisiloxane and 1-(5,5-dimethoxy-6-oxa-3-thia-5-silaheptyl)-1,1,3,3-tetramethyl-3-vinyldisiloxane compounds through substitution of the appropriate mercaptoethyl and mercaptomethyl substituted silanes as reactants. 
     EXAMPLE 2 
     Two primer formulations were prepared by using the compound of Example 1. The constituents of the formulations in parts by weight were as follows: 
     
         ______________________________________Component          No. 1        No. 2______________________________________Toluene            57 parts     57 partsExample 1 compound 1 part       1 partEthylpolysilicate  1 part       1 partBis (acetylacetonyl)diisopropyltitanate              1 part       1 partPlatinum Complex.sup.(1)              --           0.3 parts______________________________________ 
    
     The two formulations were applied to various substrates and qualitatively graded in terms of capacity to enhance the adhesion of Silastic® E silicone rubber, room temperature vulcanizing sealant. The results of the qualitative grading, represented in terms of percent cohesive failure, are set out in Table 1. 
     
                       TABLE 1______________________________________      Percent Cohesive FailureSubstrate    Formulation No. 1                      Formulation No. 2______________________________________Aluminum     100           50Anodized Aluminum        100           100Stainless Steel        100           100Polycarbonate        0             0______________________________________ 
    
     EXAMPLE 3 
     A lap shear was performed according to ASTM C 273-61. The primer composition prepared according to Formulation No. 1 of Example 2 was wiped onto pre-cleaned aluminum panels and allowed to set for one hour. A Silastic® E silicone rubber sealant was prepared and applied to the primed and unprimed (control) panels. The rubber was cured at 150° C. for one-half hour. Adhesive strengths for five primed and unprimed samples were determined on an Instron tester. The average adhesive strength of the unprimed samples was 172 kilopascals while the average adhesive strength of the primed samples was 1400 kilopascals. 
     Numerous modifications and variations in the foregoing preparative process as well as the foregoing formulation of primer compositions are expected to occur to those of ordinary skill in the art. Consequently only such limitations as appear in the appended claims should be placed on the invention.