Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2000-12-14
2002-09-17
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C528S017000, C528S018000, C528S032000, C522S033000, C522S044000, C522S046000, C522S040000, C522S041000, C522S065000, C522S060000
Reexamination Certificate
active
06451870
ABSTRACT:
FIELD OF INVENTION
The present invention relates to dual curing silicone compositions which are capable of cross-linking when subjected to radiation in the ultraviolet (“UV”) or visible (“VIS”) regions of the electromagnetic spectrum, and/or by a moisture condensation method. When cured, these compositions have excellent adhesion to a variety of substrates. They are particularly useful in potting and coating applications.
BRIEF DESCRIPTION OF THE RELATED TECHNOLOGY
Silicone rubber and liquid compositions exist in various forms and their characteristics may be modified to impart specific characteristics such as cure chemistry, viscosity, polymer type and purity. They can be formulated into one-part or two-part systems and a particular silicone composition can be engineered to be cured by more than one mechanism. Moisture-curing mechanisms, heat-curing mechanisms, and photoinitiated curing mechanisms are among those mechanisms used to initiate cure, i.e., cross-linking of reactive silicones. These mechanisms are based on either condensation reactions, whereby moisture hydrolyzes certain groups on the silicone backbone, or addition reactions which can be initiated by a form of energy, such as electromagnetic radiation or heat. For example, reactive polyorganosiloxanes can be cured by heat in the presence of a peroxide, or they can be cured by heat in the presence of a silicon hydride-containing (SiH) compound and a metallic hydrosilylation catalyst, such as an organo-platinum catalyst.
Typically, moisture curable materials are manufactured by endcapping &agr;,&ohgr;-silanol terminated silicones with various crosslinkers such as alkoxysilanes, oximinosilanes, acetoxysilanes, aminosilanes, and other silanes with hydrolyzable groups attached to the silicon atom(s) thereof. The resulting silicone materials are stored in moisture impermeable containers.
During application to a respective substrate, the materials are extruded or otherwise applied and exposed to ambient conditions for curing. The moisture in the air then will hydrolyze the hydrolyzable groups (such as alkoxy, oximino, acetoxy, and amino) on the silicon atom(s) to form silanol, either with or without inclusion of a catalyst. The resulting silanol can then further react with remaining unhydrolyzed groups in a condensation reaction, to form a siloxane linkage resulting in the cure of the silicone material.
Although these materials when cured are very reliable and possess superior properties as coatings, the moisture cure tends to be slow. Cure times of 24 hours or more may often be needed before a full cure can be achieved. Such cure times limit through-put in the manufacture of coated components, since full cure of the coated components is needed before the components can be used in the next step of the manufacture process.
As a result, a third curing mode, ultraviolet light curing, has gained wide acceptance in recent years. The curing is relatively fast, with the cured elastomer showing good adhesion to the substrates. In situations where portions of the coated material are shaded during the UV cure, a secondary cure mode, usually moisture cure can be further incorporated.
Typically, UV cure of silicone compositions can be achieved by either a thiol-ene cure or by an acrylate cure. In the thiol-ene cure, a thiol functional silicone is reacted with a vinyl functional silicone. The cure is fast and the surface dry to the touch upon the completion of the cure. However, it has been reported that in commercial applications the cured product does not heat age well, and the uncured composition tends to lack long-term storage stability.
On the other hand, acrylate functional silicone is usually storage stable and the cured products exhibit excellent high temperature resistance. However, with an acrylate cure in the presence of atmospheric oxygen, the surface cure tends to be incomplete and the cured product often times tends to be tacky.
Dual-curing silicone compositions using UV light and moisture curing mechanisms are disclosed in U.S. Pat. No. 4,528,081 (Lien) and U.S. Pat. No. 4,699,802 (Nakos). These patents disclose compositions particularly useful for conformal coatings in electronic applications where the substrate has shadow areas which are not readily accessible to direct UV light and require moisture cure for cross-linking of those areas. Ordinarily, in addition to the photoinitiator present for radiation polymerization, a moisture curing catalyst such as an organotitanate or organotin is present. Without the moisture curing catalyst, moisture cure does not ordinarily take place with any degree of certainty or in any predictable time frame. Thus, as a practical matter, without the moisture curing catalyst, the moisture curing aspect of these compositions would not be practical for commercial use.
U.S. Pat. No. 4,587,173 (Eckberg) discloses dual curing silicone compositions using heat and UV light as separate cross-linking mechanisms. This patent discloses a reactive polyorganosiloxane which requires direct silicon-bonded hydrogen atoms and direct silicon-bonded alkenyl radicals on the same or different polysiloxane chains. These compositions also contain a photoinitiator and a precious metal/hydrosilation catalyst. The presence of the photoinitiator allows cross-linking of the silicon-bonded hydrogen atoms and silicon-bonded alkenyl radicals. These compositions are said to be able to cross-link at room temperature or at elevated temperatures by the precious metal catalysis of the silicon-bonded hydrogen atoms and silicon-bonded alkenyl radicals. Platinum is among the catalysts used for the thermal hydrosilation cure reaction. Moreover, this patent requires a peroxide, which can decompose over time even at room temperature and thereby limit shelf-life.
U.S. Pat. No. 4,603,168 (Sasaki) discloses a method of curing organopolysiloxane compositions, which requires the use of heat in combination with ultraviolet radiation. These compositions contain an organopolysiloxane having per molecule at least two alkenyl groups bonded directly to the silicone atom. Other organic groups may also be present, such as alkyl groups, halogenated alkyl groups, aryl groups, aralkyl groups, and alkaryl groups on the organopolysiloxane backbone. In addition, an organohydrogenpolysiloxane containing at least two organohydrogensiloxane or hydrogensiloxane units per molecule, a platinum catalyst, an addition-reaction retarder and a photoinitiator are also disclosed. The alkenyl groups must be bonded directly to the silicone atom without an organo group therebetween. Both the '173 and '168 patents are also limited to very thin coatings.
Dual-curing compositions employing UV- and moisture-cure mechanisms have a basic disadvantage in that once exposed to ambient moisture, they begin to cure. In many cases, this results in premature curing and shortened shelf life, as well as pot life in use. The advantage of the moisture cure mechanism is that it provides a means to cure shadow areas blocked from UV light. This is particularly important when high temperature curing is not an acceptable commercial option due, for instance, to the heat sensitivity of the substrate to which the reactive silicone is applied. For example, in conformal coatings where the substrate is an electronic circuit board, high temperature curing systems such as those which use peroxides, are not practical. Conventionally, moisture, UV, heat or combinations thereof have been employed as curing mechanisms for such applications.
More recently, as disclosed in the '173 and '68 patents discussed above, heat and UV curing mechanisms have been combined. While these patents disclose compositions which may be useful for heat sensitive substrates due to the combination of UV and low temperature heat cure, each requires a specific type of organopolysiloxane. In the '173 patent, the polyorganosiloxane backbone contains both a hydrogen atom bonded to a silicon atom in the backbone, as well as an olefinic group bonded to a silicon atom. In the '168 patent, the pol
Chu Hsien-Kun
DeCato Alfred A.
Bauman Steven C.
Henkel Loctite Corporation
Moore Margaret G.
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