Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2001-11-09
2004-05-18
Peng, Kuo-Liang (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S901000, C528S034000, C525S474000, C556S442000
Reexamination Certificate
active
06737494
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to novel polyorganosiloxane cross-linkers or chain extenders for room temperature vulcanizable (RTV) sealants, and to novel RTV sealant compositions containing same. The invention further relates to novel methods of production of the claimed cross-linkers or chain extenders.
BACKGROUND
Several types of RTV sealants are have been previously described as discussed below:
Siloxanol-silicate Type
In the earliest RTV silicone system the major polymer component was a linear or moderately branched silicone with silanol terminal groups. Alkyl silicates or alkylpolysilicates, typically tetraethyl orthosilicate or partially hydrolyzed and condensed products therefrom, were used as crosslinking agents. Catalysts typically employed were tin compounds, e.g., stannous octoate, dibutyltindilaurate. Fillers. color pigments and other ancillary materials were also used. The system was usually prepared in two parts, thereby maintaining separation of the siloxanol polymer and the catalyst. At point of use, the two parts were mixed, thereby initiating crosslinking of the siloxanol. The fluid, or plastic working life, of the material is limited thereafter. Accurately measured proportions and thorough mixing were necessary to produce uniformly cured articles. The need for mixing limits useful compositions to those that were easily stirred and poured, thereby limiting both polymer viscosity and the level of filler loading. In the early development of this system the role of water was not appreciated. Later, it was established that at least catalytic amounts of water were essential and that unless special steps were taken for its rigorous exclusion prior to use, water absorbed on the filler or otherwise present could prematurely catalyze the crosslinking reactions.
Moisture Reactive Types
The next major development in RTV silicones was the one-part system in which a mixture of all components (except water) remained workable in a sealed container until exposed to moisture, such as that present in the atmosphere. The major polymer component was a linear or lightly branched silicone having reactive groups that readily underwent hydrolytic polymerization to form siloxanes. The reactive groups, that readily underwent hydrolytic polymerization could be present in terminal or non-terminal positions or both. A large number of reactive groups were reported in the prior art to be effective. The crosslinking agents usually possessed three or more identical reactive groups are present per molecule. A wide variety of catalysts are used; the choice depending on the nature of the functional group that readily underwent hydrolytic polymerization. Metal and amino carboxylate salts are often useful. While cross-linkers of this type are highly effective, they do possess limitations. For instance, polyfunctional silane monomers represent highly concentrated sites of functionality. They are therefore usually not well suited as means for modifying the properties of the cured sealant. The presence of identical functional groups upon the cross-linker also poses problems concerning workability of the sealant composition since gellation times and curing times vary little. This may be remedied by employing sealants containing mixtures of cross-linkers. However, problems associated with proper distribution of the cross-linkers within the sealant composition then occur.
The utility of the moisture reactive type of RTV silicon is limited by the nature of the by-products HX (X being the functional group that readily underwent hydrolytic polymerization) which can be objectionably acidic. toxic, corrosive, malodorous or, in some other way, undesirable. This limitation has stimulated the search for RTV silicon systems which form only innocuous by-products or even none at all.
Vinyl-hydridosiloxane Type
In this more recent development, crosslinking is accomplished by hydrosilylation. The major polymer components is usually a linear or lightly branched silicone with vinyl or other olefinic groups in terminal or non-terminal positions. The crosslinker is usually a low molecular weight siloxane with three or more hydridosiloxane units per molecule. Catalysts are typically platinum compounds that are effective at parts per million (ppm) levels. Compositions of this type that are active at room temperature are two-part systems. One-part systems are made with inhibited catalysts but require elevated temperatures for at least brief periods to activate the catalyst and are therefore not true RTV systems. A drawback of this kind of crosslinking system is that the platinum catalyst can be poisoned by many substances. This type of curing mechanism has a significant advantage in that no undesirable reaction products are formed.
Oxygen Curable Mercaptoalkyl Type
In this most recently developed type of RTV silicone, crosslinking occurs by oxidation of mercaptoalkyl substituents upon contact with molecular oxygen. The major polymer components is a linear or lightly branched silicone having mercaptoalkyl substituents, such as a 3-mercaptopropyl bonded directly to the silicon. Crosslinker components are optional and are usually low molecular weight silicones having three or more mercaptoalkyl substituents per molecule. Catalysts are organic compounds of pro-oxidant metals such as cobalt.
With respect to limitations imposed by by-products, in this system the major by-product is water which is considered to be relatively innocuous and which can usually be tolerated or, if necessary, removed in many applications. However, under some conditions, side reactions may result in the formation of small amounts of malodorous and toxic hydrogen sulfide. Furthermore, in contact with sensitive surfaces, such as silver or copper, unreacted mercaptoalkyl groups may have undesirable interactions. Also, compositions containing disulfide linkages can degenerate with formation of corrosive sulfur compounds such as sulfurous and sulfuric acids on exposure to moisture and air at elevated temperatures.
RTV sealant compositions often also contain non-reactive silicone oils as viscosity modifiers. However, while these oils do indeed aid in the formulation of the sealant, their presence in the crude product is often undesirable since their non-reactive nature allows them to bleed out of the cured material.
Acetoxysilanes are well known cross-linking agents for one-part room temperature vulcanizable silicone rubber compositions. A common type of such an acetoxysilane cross-linking agent is methyltriacetoxysilane. Methyltriacetoxysilane is known to have a melting point of 40° C., and has a disadvantage in that it must be melted or blended with another silane in order to be able to add it to a continuous extruder.
Blends of methyltriacetoxysilane and ethyltriacetoxysilane have been used as cross-linking agent in acetoxy cure RTVs.
In view of the inadequacies associated with the various RTV compositions discussed above, there is a need for RTV compositions that are not associated with undesirable by-products and which possess more varied physical properties than compositions of the prior art afford. While this latter quality may be partly attainable through the use of a mixture of different cross-linkers within a give sealant composition, this leads to variations in their concentration throughout the composition.
SUMMARY OF THE INVENTION
The present invention provides for a one part moisture curable room temperature vulcanizable silicone composition comprising an alkylalkoxyacyloxysilane selected from the group consisting of
R
1
SiR
2
a
R
3
3-a
and
R
4
R
5
b
R
6
3-b
Si—O—SiR
7
R
8
c
R
9
3-c
and mixtures thereof; where R
1
, R
4
and R
7
are independently selected from the group of alkyl radicals having the formula (CH
2
)
n
X, where n has 1 to 10 carbon atoms, and X is selected from the group consisting of hydrogen, amino, epoxy, cyano, thiocyano, allyl, and vinyl; R
2
R
5
and R
8
are independently selected from the group of acyloxy radicals having the formula OC(O)(CH
2
)
m
H, where m has 1 to 8 carbon atoms; R
3
, R
6
and R
9
a
Peng Kuo-Liang
Wheelock Kenneth S.
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