Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2001-03-29
2002-12-03
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C528S015000, C528S025000, C528S026000, C528S031000, C528S032000, C525S477000, C525S476000, C427S387000, C156S329000
Reexamination Certificate
active
06489031
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to self-adhesive addition-crosslinking silicone compositions and addition-crosslinked silicone elastomers and composite materials prepared therefrom.
2. Background Art
It is known that the adhesion of addition-crosslinked silicone elastomers to numerous substrates, such as plastics, metals and glasses, is poor, i.e. if an addition-crosslinking silicone elastomer material is applied to a substrate and then crosslinked, the silicone elastomer formed can, as a rule, be peeled off the substrate surface without difficulty, for example by applying only small tensile forces. Frequently, spontaneous delamination of silicone elastomers from the substrate may occur. However, since strong and permanent adhesion of the silicone elastomer to the substrate is of decisive importance in numerous applications, a large number of special measures have been proposed for achieving a strong bond between substrates and silicone elastomers.
In principle, the adhesive strength of the silicone elastomer/substrate composite can be increased by suitably changing the chemical and/or physical characteristics of the substrate, or at least its surface, prior to application of the addition-crosslinking silicone elastomer composition. This can be effected, for example, by pretreating the substrate surface with adhesion-promoting additives, so-called primers; by subjecting the substrate surface to plasma treatment; by mixing special additives into the substrate; by selectively adjusting the morphology of the substrate, for example by increasing the surface roughness, etc. These measures have, inter alia, the disadvantage that additional process steps are required or that the characteristics of the substrate have to meet special requirements.
The adhesive strength of the silicone elastomer/substrate composite can also be increased by selectively changing the chemical and/or physical characteristics of the addition-crosslinking silicone elastomer material. Numerous adhesion-promoting additives are known which promote self-adhesion of the resulting silicone elastomer to various substrates when mixed with the uncrosslinked silicone material. These include compounds which contain highly reactive functional groups, such as alkoxy, epoxy, carboxyl, amino, etc., these groups generally being chosen so that the adhesion promoter is capable of reacting both with the substrate and with a silicone elastomer component. Although incorporation of such adhesion promoters may make it possible to dispense with pretreatment of the substrate, the adhesive strength achieved frequently does not meet minimal requirements, in particular where vulcanizing temperatures are limited to less than 100° C., important for some applications. In addition, further increases in adhesive strength by employing higher contents of these adhesion promoters is possible only to a limited extent, since the highly reactive groups borne by the adhesion promoters have an increasingly disadvantageous effect on performance characteristics such as shelf-life, crosslinking characteristics (inhibition), toxicological safety, etc. For these reasons, interest has been focused on keeping the content of adhesion promoters as low as possible.
The adhesion promoters most frequently used for self-adhesive addition-crosslinking silicone elastomers are epoxy-functional silanes such as glycidyloxypropyltrimethoxysilane; or methacrylate-functional silanes such as methacryloxypropyltrimethoxysilane; or vinyl silanes such as vinyltrimethoxysilane, or combinations thereof. The organic functional groups relevant for the buildup of composite strength, however, are too unreactive at temperatures below 100° C. to bring about sufficient adhesion to the substrate. In the case of many important applications of self-adhesive silicone elastomers, the maximum vulcanizing temperature is limited, one example being the encapsulation of electronic circuits. The temperature-sensitive components such as coils, capacitors, and ICs, permit only very limited thermal loads without risking loss of function. Frequently, only brief heating at temperatures from 50° C. to 80° C. is possible.
U.S. Pat. No. 5,164,461 and European published application EP-A-451 946 describe addition-crosslinking silicone elastomers with intrinsic adhesion to the substrates that are contacted during vulcanization. The most common adhesion promoters here are epoxide- and/or methacrylate-functional alkoxysilanes. The vulcanizing temperatures are required to be at least 130° C. Using the maleate- and fumarate-functional alkoxysilane additives claimed in U.S. Pat. No. 5,164,461, self-adhesion is achieved at crosslinking temperatures of at least 100° C.
U.S. Pat. No. 5,595,826 describes adhesion promoters based on reaction products of aliphatically unsaturated monoalcohols or polyalcohols with organofunctional alkoxysilanes such as methacryloxypropyltrimethoxysilane or glycidyloxypropyltrimethoxysilane. Additives of this kind bring about self-adhesion of addition-crosslinking silicone elastomers at vulcanizing temperatures from 70 to 100° C. However, high adhesive strengths are achieved exclusively with metallic and inorganic substrates, for example steel, aluminum, glass and copper.
U.S. Pat. Nos. 5,416,144 and 5,567,752 describe adhesion promoters based on reaction products of amines or aminoalkoxysilanes with methacryloxypropyl- or glycidyloxypropyltrimethoxysilane. With these additives, self-adhesion is achieved at vulcanizing temperatures from 80 to 100° C. A problem in this case is the strong inhibition on the Pt-catalyzed crosslinking reaction caused by amine compounds, as is well known. Furthermore, contact between amine compounds and SiH components constitutes a potential risk, since formation of hydrogen gas must be expected. Moreover, due to the loss of SiH groups, undervulcanization is expected. A further disadvantage of highly polar additives of this kind is severe thixotroping of the siloxane composition and the associated reduction in fluidity.
EP-A-286 387 describes organosilicon compounds with a &bgr;-keto ester function. In particular, alkoxysilanes having at least one alkyl-bonded &bgr;-keto ester function are described. Numerous applications of such structures are indicated, including applications as constituents of primers for epoxy resin/glass fiber composites. EP-A-295 657 describes metal chelate complexes formed, for example, from titanium and the chelating ligand, trialkoxysilylpropylacetoacetate. Structures of this kind are claimed to be adhesion-promoting additives in epoxy resin formulations.
U.S. Pat. No. 5,041,481 describes adhesion promoters obtained by the reaction of 1,3-diketone compounds with aminoalkylalkoxy silanes. The latter are claimed, inter alia, to be additives in condensation-crosslinking silicone elastomers. It must be noted that all the adhesion-promoting additives described based on 1,3-diketone compounds necessitate complicated reactions with alkoxysilanes for attachment of alkoxysilyl functionality. Disadvantages in this context are that in some cases, highly toxic reactants such as trimethoxysilane are required. Further, owing to keto/enol tautomerism, a difficult reaction regime is created, with numerous secondary reactions such as propene elimination or reduction of the keto group by SiH groups. Distillative purification and isolation of the reaction products are possible only with massive loss of yield, owing to gellation as a result of the reaction of the enol groups of the 1,3-diketone structure with alkoxysilyl groups with elimination of alcohol. Owing to their known effect of strong inhibition of hydrosilylation, adhesion promoters based on amino-functional alkoxysilanes are of only limited usefulness in addition-crosslinking silicone elastomers.
SUMMARY OF THE INVENTION
The present invention pertains to addition crosslinking compositions containing (A), an organopolysiloxane with unsaturated hydrocarbon group functionality; (B), an Si—H functional crosslinker containing compatability decreasing
Eberl Georg
Müller Philipp
Brooks & Kushman P.C.
Dawson Robert
Robertson Jeffrey B.
Wacker-Chemie GmbH
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