Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2002-03-29
2004-10-19
Peng, Kuo-Liang (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C525S477000
Reexamination Certificate
active
06806311
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a mold-release agent which may be used in particular in the processing of thermoset moldings. According to the invention, the mold-release agent brings about the ready separation of the molding from the mold, while at the same time bringing about chemical linkage of short-chain and/or long-chain silicone compounds to the surface of the thermoset molding. This achieves effective hydrophobicization of the surface of the molding.
BACKGROUND OF THE INVENTION
Thermoset materials, in particular epoxy resins, are widely used as electrically insulating materials in medium- and high-voltage technology, for example for the construction of switchgear assemblies. The insulating materials currently known mostly have adequately good electrical properties. The suitability of these insulating materials for use is therefore also based on high requirements for mechanical properties, in particular for mechanical strength and mechanical flexibility, and for behavior in the presence of contaminant layers, and for resistance to the complex electrical and climatic stresses arising at high humidity. Epoxy resins in particular are treated below as a preferred embodiment. Similar considerations apply to other thermoset materials.
Although the properties of epoxy resins are excellent for an insulating material, their use has remained essentially restricted to indoor applications. Insulation materials represent sectors in which epoxy resins are widely used, examples being found in instrument transformers, bushings, power breakers, dry-type transformers, and electrical machinery. Insulation made from epoxy resins is used only rarely in outdoor applications. The causes for this are primarily lack of resistance to UV radiation and behavior in the presence of a contaminant layer. Another aspect here is low resistance to complex electrical and atmospheric stresses. Studies have shown that failure of insulation parts can occur even in indoor systems. If, for example, a deposit of contaminant layers is present on the surface of the molding and there is the possibility of water condensation, some of the stresses which arise are markedly greater than those to which outdoor systems are exposed, and these therefore cause an increased level of failure. Conditions of this type are also termed difficult indoor conditions.
If condensation of water arises on the surfaces under the conditions described, insulating action is substantially dependent on the water-repellency (hydrophobic properties) of the surface. If these surfaces are highly hydrophobic, as is the case with some silicones, deposits of condensed water form as individual droplets separate from one another. However, in the case of hydrophilic surfaces, wetting with a liquid generally gives rise to formation of a coherent film deposit which can lead to short-circuiting of the insulator. A hydrophobic surface is therefore highly advantageous. Epoxy resin surfaces, like many other thermoset resin surfaces, do not have hydrophobic properties, since the epoxy resin matrix has polar character.
Various methods have previously been proposed to improve hydrophobic surface properties. For example, U.S. Pat. No. 4,537,803 claims the addition of a polymerizable silicone oil to the epoxy resin mixture. U.S. Pat. No. 5,306,747 relates to the addition of a modified silicone oil which can react chemically with the resin system. Other proposals likewise include the addition of silicone-based compounds to the epoxy resin mixture. A fundamental disadvantage of all of these methods is that the thermoset matrix to be polymerized is altered by the addition of the silicone compound. For example, the nature of the alterations in physical properties, in reactivity, and in properties relating to flow and to machining may be such as to cause demixing phenomena or electrical discharges in cavities, for example.
SUMMARY OF THE INVENTION
It has been found that the process of the present invention eliminates all of the disadvantages mentioned from addition of the silicone compound to the matrix, or entirely prevents the occurrence of these problems, if it is possible to hydrophobicize only the surface of the molding in an adequate manner. Surprisingly, it has now been found that it is possible to hydrophobicize only the surface of the moldings in the molding procedure, without any additional process step, if the mold-release agent used comprises a mixture which comprises (i) at least one compound with mold-release action, preferably a silicone compound, and also (ii) at least one reactive silane compound which is capable of reacting chemically with the epoxy resin surface which forms during the molding process. The mold-release agent of the invention may comprise (iii) other additives where appropriate, for example aqueous and/or non-aqueous solvents. It is surprising that the component (i), i.e. the silicone compound with mold-release action, can be added to component (ii), i.e. the reactive silane compound, without any noticeable inhibition of the mold-release action of the component (i) by the silane compound. At the same time, the reactive component (ii) is capable, during the molding or the mold-release procedure, of reacting in situ with the molding surface which forms. What is formed here is a stable uniform, thin, hydrophobic, silicone-containing layer on the surface of the molding. The surface of the molding is modified chemically in such a way as to have, under difficult indoor conditions, a markedly higher level of hydrophobic properties and a markedly higher level of hydrophobic stability, with the result that surface-modified moldings of this type can be used with advantage as insulating parts in medium- or high-voltage technology. According to the invention, the molding material systems known per se, and also the procedures for their preparation, may be retained here with no alteration. Their physical properties and their behavior in the molding process are not altered. Starting, therefore, from the highly adhesive properties of epoxy resins, the mold-release agent of the invention is used to permit manufacture of moldings of this type without adhesion within the mold, and at the same time to provide effective in-situ hydrophobicization of the surface of the molding formed, in the same process step.
The present invention has been defined in the claims. In particular, the present invention provides a mold-release agent for the production of thermoset moldings with a hydrophobicized surface, characterized in that this comprises (i) at least one compound with mold-release action, preferably a silicone compound, and also (ii) at least one reactive, where appropriate hydrolyzable, silane compound which is capable of reacting chemically with the uncured or incompletely cured thermoset surface. The mold-release agent of the invention may comprise (iii) other additives where appropriate, preferably aqueous and/or non-aqueous solvents.
The term “uncured or incompletely cured thermoset surface” means the surface in situ in the molding process. This term also includes, for example, the surface of a demolded incompletely cured molding which is subjected to post-curing. This means that the mold-release agent of the invention may be used as mold-release agent and hydrophobicizing agent in the molding process and/or as hydrophobicizing agent in the post-curing process, the mentioned advantageous properties being retained.
The present invention also provides the use of the mold-release agent of the invention in the molding process and/or in the post-curing process for the production of thermoset moldings with a hydrophobicized surface.
The invention further provides a process for producing thermoset moldings with a hydrophobicized surface, which comprises using, in the molding process, a mold-release agent which comprises (i) at least one compound with mold-release action, and also (ii) at least one reactive, where appropriate hydrolyzable, silane compound which is capable of reacting chemically in situ with the uncured or incompletely
ABB Research LTD
Burns Doane Swecker & Mathis L.L.P.
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