RTV SILICONE COMPOSITIONS USING CARBAMATE FUNCTIONAL ALPHA SILANE CROSSLINKERS

FIELD OF THE INVENTION

[0001] The present invention provides silicone adhesives having rapid cure speed and good stability. In particular, the present invention provides moisture curable compositions including alkoxy RTV silicones, which have alpha organo functional groups, and methods for preparing these compositions.

BRIEF DESCRIPTION OF RELATED TECHNOLOGY

[0002] Moisture curable silicone adhesives are used in a broad range of applications including construction, highway, electronic device and package assembly, appliance assembly and consumer uses. Typically, curable adhesives used in these applications have been tailored to provide the strength and toughness required for the application at hand. In addition to these properties, rapid cure speeds and product stability are often desired.

[0003] Alkoxy-terminated polysiloxanes have been used to prepare moisture curable silicone adhesives with desirable properties. These reactive polysiloxanes are prepared by endcapping silanol terminated silicones with alkoxysilane crosslinkers in the presence of a catalyst. The endcapped silanols may then be cured (i.e. cross-linking of reactive silicones) by exposure to ambient conditions in the presence of a catalyst. The moisture in the air hydro lyzes the alkoxy groups on the silicon atom(s) to form through a condensation reaction a siloxane linkage that advances the cure of the silicone material.

[0004] Although effective, these silicone adhesives often exhibit cure speeds that are too slow for certain applications. In particular, in some applications it is desirable to use an adhesive that has a quick tack-free time ("TFT"). "Tack" is the surface stickiness or adhesiveness of the composition. "Tack free" accordingly is the time it takes for the composition to cure to the point that it is no longer sticky to the touch. There are advantages to having a tack-free surface, including the ability to manipulate substrates to which the composition has been applied without

disturbing the bond and the reduction of undesired flow of the composition. As such, it may be desirable in a variety of applications to use adhesive compositions that develop faster TFT than traditional RTV materials, as well as faster full cure times.

[0005] One traditional way of developing faster TFT is to incorporate high levels of moisture-curing catalyst in the composition. The disadvantage to doing this is the compromise in stability and shelf-life of the final product. Premature curing due to the presence of catalyst is a significant concern that must be addressed and properly balanced against the desired cure speed. Sometimes a reduction in catalyst solves the stability issue, but also lowers the TFT and cure speed. Therefore, it is desirable to prepare moisture-curable compositions that require less moisture-curing catalyst without compromising TFT and full cure speed.

SUMMARY OF THE INVENTION

[0006] In some embodiments, there is provided a compound including the structure:

where R, R ¹ , R ² and R ³ are independently selected from C _M alkyl, a is 0 or 1, and n is

[0007] Some embodiments provide a reaction product of:

(a)

where R » 1 , τ R>2~, r R.3 a _ndJ R _π 4 are independently selected from Ci _-4 alkyl and a is 0 or 1; and

(b) a hydroxy-terminated polydiorganosiloxane.

[0008] In some embodiments, there is provided a moisture curable composition including: a) a reactive polyorganosiloxane including the structure:

R ¹

where R, R ¹ , R ² and R ³ are independently selected from Ci _-4 alkyl, a is 0 or 1, and n is from 10 to 3500; b) a moisture cure system for the reactive polyorganosiloxane; and c) optionally a reactive silane.

[0009] In some embodiments, there is provided a moisture curable pre-mix composition including: a) a reactive polyorganosiloxane including the structure:

where R, R ¹ , R ² and R ³ are independently selected from Ci _-4 alkyl, a is 0 or 1, and n is from 10 to 3500; b) at least one dry filler; and c) optionally a reactive silane, wherein the composition is substantially free of added moisture.

[0010] Some embodiments provide a method of preparing a reactive polyorganosiloxane including the step of: reacting a compound including the structure:

where R ¹ , R ² , R ³ and R ⁴ are independently selected from Ci _-4 alkyl and a is 0 or 1, with a hydroxy-terminated polydiorganosiloxane in the presence of a catalyst.

[0011] Some embodiments provide a method of preparing a reactive polyorganosiloxane including the steps of: a) combining a hydroxy-terminated polydimethylsiloxane with at least one filler; b) adding a catalyst and a compound including the structure:

where R ¹ , R ² , R ³ and R ⁴ are independently selected from Ci _-4 alkyl and a is 0 or 1 ; and c) forming in situ a reactive polyorganosiloxane including the structure:

O H OR ³ ₍₂ -a) / R \ OR ³ _(2- a) H O R ¹ O — C N CH ₂ -Si — θj-Si — O-j-Si — CH ₂ — N C — OR ¹

R ² (a) \ R / _n R ² (a)

where R, R ¹ , R ² and R ³ are independently selected from Ci _-4 alkyl, a is O or 1, and n is

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention provides silicone compounds and moisture curable compositions containing these compounds, which produce rapid TFTs and cure rapidly to produce polymeric reaction products having enhanced stability, adhesion and elongation properties. The compounds and compositions include a silicone polymer endcapped with an alpha organofunctional silane group.

[0013] These inventive compounds and compositions are useful in a variety of end-use applications including adhesive, sealant, coating and potting applications, as well as other uses in the electronic, automotive and consumer markets.

[0014] The compounds according to the present invention include reactive polyorganosiloxanes having the following general formula I:

R ¹

[0015] In formula I, desirably, R, R ¹ , R ² and R ³ may be independently selected from Ci _-4 alkyl, a is 0 or 1, and n is from 10 to 3500.

[0016] Formula I may be formed as the reaction product of a hydroxy-terminated polydiorganosiloxane and an alpha organofunctional silane. The alpha organofunctional silane includes those having the following general formula II:

II

[0017] In formula II above, desirably, R ¹ , R ² , R ³ and R ⁴ may be independently selected from Ci _-4 alkyl and a is 0 or 1. More specifically, formula II is a carbamate functional alpha silane. Desirably, R ¹ , R ² , R ³ and R ⁴ are each methyl groups.

[0018] Hydroxy- terminated polydiorganosiloxanes suitable for use in the present invention include those having the following general formula III:

III

[0019] In formula III above, desirably, R ⁵ , R ⁶ , R ⁷ R ⁸ , R ⁹ and R ¹⁰ may be independently selected from Ci _-4 alkyl.

[0020] An example of a commercially available hydroxy-terminated polydiorganosiloxane suitable for reaction with a compound of formula II is polydimethylsiloxane ("PDMS"), as represented by formula IV:

rv

[0021] The number of repeating units, "n" plays a role in determining the molecular weight and hence the viscosity of the composition of the invention, particularly because the alpha organo functional terminated end product of the reaction oftentimes has substantially the same viscosity as the silanol-terminated reactant. Thus, n may be, for example, an integer from about 10 to about 3500. The viscosity may be readily chosen for a particular product application. Viscosities of such hydroxy-terminated polydiorganosiloxanes are often within the range of from about lOcps to about 400,000 cps. Desirably, the viscosity range for those siloxanes used in the present invention may be from about 1000 cps to about 100,000 cps.

[0022] In accordance with the present invention, a carbamate functional alpha silane of formula II may be reacted with a hydroxy-terminated polydiorganosiloxane of formula III in the presence of a catalyst to form a compound of formula I. More specifically, the carbamate functional alpha silane acts to endcap the hydroxy-terminated polydiorganosiloxane thereby forming formula I. Formula I therefore is an alpha organo functional terminated reactive polysiloxane. That is, the carbamate functional group is positioned alpha to the silicone. Due, at least in part to the alpha carbon, the alkoxy groups have enhanced capacity as leaving groups, which in turn accelerates moisture cure.

[0023] The endcapping reaction may occur in the presence of a catalyst. Organo -lithium compounds are particularly suitable for this reaction. Examples of suitable organo-lithium compounds include, but are not limited to: methyl lithium; n-butyl lithium; sec-butyl lithium; t- butyl lithium; n-hexyl lithium; 2-ethylhexyl lithium; n-octyl lithium; phenyl lithium; vinyl lithium; lithium phenylacetylide; lithium (trimethylsilyl) acetylide; lithium dimethylamide; lithium diethylamide; lithium diisopropylamide; lithium dicyclohexylamide; lithium silanolate; lithium siloxanolate; and combinations thereof. Other catalysts known to those skilled in the art may be useful in forming the reactive polyorganosiloxanes of the present invention, but organo- lithium compounds are preferred because of the advantages associated therewith, as described in U.S. Patent Nos. 5,300,608, 5,498,642, 5,516,812 and 5,663,269 (assigned to Henkel Corporation), which are incorporated herein by reference in their entirety.

[0024] Due to the presence of the alkoxy groups, compounds of formula I have the capability of curing by moisture curing mechanisms. Accordingly, the present invention also is directed to moisture curable compositions including the reactive polyorganosiloxane of formula I, as described above, and a moisture cure system. The moisture curable compositions also may include a variety of optional ingredients, including fillers, condensable silanes, curable silicones, and adhesion promoters, among others.

[0025] The cure system used in the moisture curable compositions of the present invention includes, but is not limited to, catalysts or other reagents which act to accelerate or otherwise promote the curing of the composition of the invention. Desirably, the cure system includes a moisture curing, or condensation, catalyst. Any suitable moisture curing catalyst may be used, such as, organic titanium compounds, organic tin compounds, organic amines, combinations thereof, or any other known catalyst for moisture-curing silicones.

[0026] More specifically, suitable moisture curing catalysts include, but are not limited to, tin rv salts of carboxylic acids, such as dibutyltin dilaurate, organotitanium compounds such as tetrabutyl titanate, and partially chelated derivatives of these salts with chelating agents such as acetoacetic acid esters and β-di-ketones. Desirably, titanium alkoxide, dibutyl tin laurate or alkyl tin carboxylate are used. Additionally, organic amines such as tetramethylguandinemaines, diazabicyclo[5.4.0]undec-7-ene (DBU) and triethylamine and the like may be used.

[0027] Moisture curing catalysts may be present in amounts of about 10% or less by weight of the composition, more desirably about 0.01% to about 1.0% by weight of the composition and most desirably about 0.05% to about 0.5% by weight of the composition.

[0028] Fillers optionally may be included in the compositions of the present invention.

Generally, any suitable mineral, carbonaceous, glass, or ceramic filler may be used, including, but not limited to: fumed silica; clay; metal salts of carbonates; sulfates; phosphates; carbon black; metal oxides; titanium dioxide; ferric oxide; aluminum oxide; zinc oxide; quartz; zirconium silicate; gypsum; silicium nitride; boron nitride; zeolite; glass; plastic powder; and combinations thereof. The filler may be present in the composition in any suitable concentration

in the curable silicone composition. Generally, concentrations of from about 5% to about 80 % by weight of the composition are sufficient. However, a more desirable range would be 20-60%.

[0029] Among the more desirable fillers are reinforcing silicas. The silica may be a fumed silica, which may be untreated (hydrophilic) or treated with an adjuvant so as to render it hydrophobic. The fumed silica should be present at a level of at least about 5% by weight of the composition in order to obtain any substantial reinforcing effect. Although optimal silica levels vary depending on the characteristics of the particular silica, it has generally been observed that the thixotropic effects of the silica produce compositions of unpractically high viscosity before maximum reinforcing effect is reached. Hydrophobic silicas tend to display lower thixotropic ratios and therefore greater amounts can be included in a composition of desired consistency, hi choosing the silica level, therefore, desired reinforcement and practical viscosities must be balanced. A hexamethyldisilazane treated silica is particularly desirable (HDK2000 by Wacker- Chemie, Burghausen, Germany).

[0030] In some embodiments of the present invention, it may be desirable to incorporate a dry filler. For example, a moisture curable pre-mix composition may include the reactive polyorganosiloxane of formula I and at least one dry filler. Such dry fillers generally have a water content of less than about 0.5% by weight of the composition. Such compositions desirably are substantially free of added moisture, thereby preventing premature curing of the reactive polyorganosiloxane. The pre-mix compositions also may include additional reactive silanes, adhesion promoters or combinations thereof.

[0031] It also may be desirable to include additional reactive silanes in the moisture curable compositions of the present invention. Additional reactive silanes may be particularly desirable in compositions in which the variable "a" in the polyorganosiloxane of formula I is 1. Suitable condensable silanes generally include, for example, alkoxy silanes, acetoxy silanes, enoxy silanes, oximino silanes, amino silanes and combinations thereof. Other suitable silanes include vinyl trimethoxy silanes and alpha functioiialized silanes.

[0032] Additional reactive silanes may be present in amounts of about 0.5% to about

10% by weight of the composition. A more desirable range would be 0.5-5.0%.

[0033] Adhesion promoters also may be included in the moisture curable compositions.

An adhesion promoter may act to enhance the adhesive character of the moisture curable composition for a specific substrate (i.e., metal, glass, plastics, ceramic, and blends thereof). Any suitable adhesion promoter may be employed for such purpose, depending on the specific substrate elements employed in a given application. Various organosilane compounds, particularly aminofunctional alkoxysilanes, may be desired.

[0034] Suitable organosilane adhesion promoters include, for example, 3- aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3- aminopropylmethyldiethoxysilane, 3-aminopropylmethyldimethoxysilane, methylaminopropyltrimethoxysilane, 1 ,3,5-tris(trimethylsilylpropyl)isocyanurate, 3- glycidoxypropyltrimethoxysilane, 3-glycidoxypropylethyldimethoxysilane, 2- glycidoxyethyltrimethoxysilane, 2-cyanoethyltrimethoxysilane, 3-cyanopropyltriethoxysilane, isocyanatopropyltriethoxysilane, isocyanatopropyltrimethoxysilane, and combinations thereof.

[0035] Adhesion promoters, when present, may be used in amounts of about 0.1% to about 10% by weight of the composition. Desirably, the adhesion promoter is present from about 0.2% to about 2.0% by weight of the composition.

[0036] The compositions also may include any number of optional additives, such as pigments or dyes, plasticizers, alcohol scavengers, stabilizers, anti-oxidants, flame retardants, UV-stabilizers, biocides, fungicides, thermal stabilizing agents, rheological additives, tackifiers, and the like or combinations thereof.

[0037] In the presence of moisture at typical ambient conditions, the compositions of the present invention cure rapidly due, at least in part, to the alpha organofunctional groups. As discussed above, due to the alpha carbon, the alkoxy groups have enhanced capacity as leaving groups, thereby permitting rapid cure. The compositions accordingly have a quick TFT at which

point the mass is no longer sticky to the touch. In particular, upon cure, the compositions have a TFT of about 10 seconds to about one week in typical ambient conditions, more desirably less than about one minute. The compositions also exhibit faster full cure times than traditional RTV materials.

[0038] Because the compositions of the present invention cure rapidly, less moisture- curing catalyst may be used than with traditional RTV materials. Reducing the amount of moisture-curing catalyst may be desirable to prevent shelf-life problems. As described above, the presence of any moisture in the packaging of compositions containing moisture-curing catalysts may lead to premature polymerization. The ability to use less moisture-curing catalyst in the compositions of the present invention therefore provides additional stability without compromising cure speed.

[0039] Upon cure, the compositions of the present invention also exhibit good adhesion and elongation properties. These properties may be modified to some degree by the selection of different moisture-curing catalysts. For instance, organic tin and titanium catalysts may provide materials having a higher modulus, i.e., tougher, than alternative catalysts, such as DBU or Lewis bases. DBU and Lewis bases may cause chain extension to provide more flexible and softer materials, i.e., lower modulus, upon cure. These materials may have higher elongation than those produced with tin and titanium catalysts.

[0040] The present invention also provides methods of preparing a moisture curable composition. In accordance therewith, a compound of formula II, as described above, is reacted with a hydroxy-terminated polydiorganosiloxane, also as described above. The compound of formula II endcaps the hydroxy-terminated polydiorganosiloxane to form a reaction product, i.e., the reactive polyorganosiloxane of formula I, and excess methyl alcohol.

[0041] The components desirably are reacted in the presence of a catalyst. The catalyst may be an organo-lithium compound, as described above.

[0042] The reaction may be performed at reaction temperatures of from about O ⁰ C to about 100 ⁰ C, and preferably at about 50-75 ⁰ C. The reaction may be performed for a time period suitable to permit the siloxanes to be capped. This time period depends, among other things, on the temperature of the reaction mixture, but will generally be within about 1 hour.

[0043] After the endcapping reaction is complete, the reaction mixture is cooled to room temperature.

[0044] Another method of the present invention is directed to preparing a reactive polyorganosiloxane. In accordance therewith, a hydroxy-terminated polydimethylsiloxane is combined with at least one filler. A catalyst and a compound of formula II, as described above, are added to the hydroxy-terminated polydimethylsiloxane and filler(s) combination. The catalyst may be an organo-lithium compound. A reactive polyorganosiloxane of formula I, as described above, is formed in situ once the catalyst and compound of formula II are added to the initial combination of components. In particular, the compound of formula II endcaps the hydroxy-terminated polydimethylsiloxane to form a reaction product, i.e., the reactive polyorganosiloxane of formula I, and excess alcohol.

[0045] The moisture curable compositions may be used, for example, to seal or bond substrates, such as, but not limited to, gaskets. In gasketing applications, the moisture curable composition may be applied to one of the substrates which will form part of the gasket, cured or at least partially cured, and then joined to a second substrate to form the gasket assembly. Such gasketing applications include, for example, form-in-place gaskets. For instance, the compositions may be applied to a substrate and subjected to curing conditions. The compositions may also be used to seal together substrates by applying the composition to at least one of two substrate surfaces, mating the substrate surfaces in an abutting relationship to form an assembly, and exposing the composition to moisture to effect cure. The substrates should be maintained in the abutting relationship for a time sufficient to effect curing.

EXAMPLES

Examples 1-12:

[0046] In Examples 1-12, moisture curable compositions were prepared in accordance with the present invention.

[0047] The components listed in Table 1 below were combined in the indicated amounts for the moisture curable compositions of Examples 1-6. The components listed in Table 2 below were combined in the indicated amounts for the moisture curable compositions of Examples 7- 12.

TABLE l

TABLE 2

[0048] The alpha-organofunctional silane and hydroxy-terminated PDMS for each composition were reacted in the presence of the catalyst to form end-capped reactive polyorganosiloxanes. Accordingly, reactive polyorganosiloxanes of formula I, as described above, were formed. The reactions were carried out at temperatures of about 50-75 ⁰ C for a time period of about one hour.

[0049] The reactive polyorganosiloxanes and remaining components listed in Tables 1-2 above for each Example were combined to form moisture curable compositions. The compositions were subjected to ambient moisture-curing conditions and measured for the time it takes to skin-over the top of the composition and the TFT. More specifically, the compositions were measured at 0 days, after 1 day in an 82°C oven and/or after 3 days in an 82 ⁰ C oven. The results for Examples 1-6 are depicted in Table 3 below. The results for Examples 7-12 are depicted in Table 4 below.

TABLE 3

TABLE 4