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
1996-05-20
2001-11-13
Sergent, Rabon (Department: 1711)
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...
C252S182200, C252S182240, C252S570000, C252S576000, C524S775000, C524S789000, C524S791000, C524S871000, C524S873000, C524S078000, C528S048000, C528S073000, C528S075000, C528S085000, C528S485000, C528S488000, C528S490000
Reexamination Certificate
active
06316542
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a crosslinkable dielectric composition based on polydiene polyol, a polyisocyanate and an inert liquid filler (an insulating liquid) which conserves, before crosslinking, a viscosity which is sufficiently low at a given temperature to allow its use, in particular, for impregnating solid dielectrics such as contained in condensers, cables and measuring transformers.
This composition, which is liquid during the impregnation, becomes an insulating solid after the crosslinking.
The invention also relates to a process for the preparation of the said composition.
The field of application relates most particularly to that of electrical appliances operating by AC or DC voltage and whose insulations are subjected to high electric fields, higher than 10 kV/mm.
For these appliances, one of the main functions of the impregnating liquids is the replacement of all of the air contained in the cavities of the solid insulants.
The presence of air in the solid insulants leads to partial discharges, resulting in the destruction of the dielectric and thus of the appliance.
In certain cases (high-voltage transformers and cables with circulating oil), the insulant also acts as a heat transfer fluid to remove the heat produced in the said appliances by the Joule effect.
However, liquid electric insulants have drawbacks such as the possibility of escaping from the appliances in the case of leaks, with the unfavourable consequences of the risk of the environment being polluted by the liquid and the breakdown of the electrical appliance.
They moreover have the drawback of having a lower breakdown voltage than the solid electric insulants.
Many attempts have been made to impregnate high-voltage insulations with solidifiable liquid systems for all electrical appliances for which the presence of a liquid is not essential in order to remove the heat.
However, these attempts have all been unsuccessful in the field of application concerned, that is to say high-voltage uses with high potential gradient.
As regards solidifiable liquids, two types of compounds have been proposed.
The first type consists of crystallizable organic liquids such as microcrystallized paraffin waxes. For the products of this type, although they possess good dielectric properties, shrinkages are still observed due to the crystallization, which result in the presence of cavities in the impregnated solid insulations.
The second type of solidifiable liquid consists of polycondensable and/or crosslinkable 1- or 2- component systems, filled to a greater or lesser extent, in particular with an inert liquid in order to minimize the shrinkage.
Such systems are described in the Japanese patent application No. 61-197620 published on Sept. 1, 1986.
However, the dielectric properties of these products are mediocre, which prohibits their use in the field of application concerned, namely high-voltage insulations with high potential gradient.
In particular, the angle of loss or dielectric dissipation factor, hereinafter referred to as tangent delta (tan &dgr;), is too high.
Thus, in the abovementioned Japanese patent application, the tan &dgr; of the products claimed is always higher than 5×10
−2
at 20° C., the standard temperature (JIS K-6911).
It may moreover b e noted in this application that the compositions claimed are intended for coating condenser bobbins.
SUMMARY OF THE INVENTION
A crosslinkable impregnating dielectric composition has now been found, this composition comprising at least one polydiene polyol, at least one polyisocyanate and at least one chemically inert liquid filler, characterized in that the functionality of the polyisocyanate is greater than or equal to 2 and preferably between 2.2 and 4, and in that the amount of the chemically inert liquid filler is greater than 90% by weight of the composition is used, in order to maintain a viscosity of less than 50 mPa s at the impregnation temperature for a period at least equal to 1 day and in order to obtain a tangent &dgr; (tan &dgr;) at 20° C. and at 50 Hz, after complete crosslinking, of less than 0.02 and preferably of less than 0.01.
Now, the term impregnation temperature denotes the temperature to which the composition according to the invention is brought in order to impregnate a solid insulation such as a plastic film which can be used, in particular, in condensers.
This impregnation temperature, maintained for a period at least equal to 1 day, is equal to at least 20° C. and is preferably between 40° C. and 80° C.
According to the present invention, during the impregnation period defined above, the viscosity of the composition may change but must remain below about 50 mPa s.
According to the present invention, the amount of the chemically inert liquid filler is by weight greater than 90% and, preferably, between 92% and 96% of the composition.
According to the present invention, the chemically inert liquid filler is an insulating liquid, which may fully solubilize the polydiene polyols and the polyisocyanates, and which is chosen from alkylbenzenes such as decylbenzenes and dodecylbenzenes; dielectric esters which are, for example, products of reaction of polyvalent alcohols such as pentaerythritol with monovalent carboxylic acids such as n-heptanoic acid; alkyl phthalates such as dibutyl phthalate and dioctyl phthalate; alkylpolyaromatic compounds such as monoisopropylbiphenyl (MIPB) and phenylxylylethanes (PXE); mixtures of benzyltoluenes and benzyl-benzyltoluenes such as those described, in particular, in European patent No. 136,230; mixtures of mono- and bis(methylbenzyl)xylenes such as those described in European patent application No. 0,443,899; mixtures of benzyltoluene and diphenylethane; dielectric plant oils such as rapeseed oils and corn oils, as well as a combination of at least two of the insulating liquids mentioned above.
Mixtures of benzyltoluenes and benzyl-benzyltoluenes comprising from 50% to 90% by weight of benzyltoluenes (mixture of the o, m and p isomers) and from 50% to 10% by weight of benzyl-benzyltoluenes, dielectric esters such as dibutyl sebacate and dioctyl adipate, alkyl phthalates such as dioctyl phthalate or dielectric plant oils such as rapeseed oil will preferably be used as the chemically inert liquid filler.
According to the present invention, the chemically inert liquid filler has a viscosity at 20° C. of not more than 100 mPa 8 and preferably of between 4 and 30 mPa s, measured according to ASTM standard D445.
According to the present invention, the polyisocyanate used may be chosen from aromatic, aliphatic and cycloaliphatic polyisocyanates and those which contain an isocyanurate ring in their molecule, having at least two isocyanate functions in their molecule, which are capable of reacting with hydroxyl functions of a polyol in order to form a three-dimensional polyurethane network resulting in gelation of the composition.
As illustrations of aromatic polyisocy-anates which can be used according to the present invention, mention will be made of 4,4′-diphenylmethane diisocyanate (MDI), polymeric MDIs and triphenylmethane triisocyanate.
As an illustration of an aliphatic polyisocyanate which can be used according to the present invention, mention will be made of 1,6-diisocyanatohexane biuret
As illustrations of cycloaliphatic polyisocyanates, mention will be made of isophorone diisocyanate (IPDI), cyclohexyl diisocyanate (CHDI) and 4,4′-dicyclohexylmethane diisocyanate.
As illustrations of polyisocyanates which contain the isocyanurate ring in their molecule, mention will be made of the hexamethylene diisocyanate trimers marketed by the company Rhône-Poulenc under the name Tolonate HDT and tris [1-(isocyanotomethyl)-1,3,3-trimethylcyclohexane] isocyanurate marketed by the company Hüls under the name Vestanat T 1890/100.
The amount of polyisocyanate according to the present invention is chosen such that the NCO/OH molar ratio is about 1 and preferably between 0.85 and 1.15.
According to the present invention, the polydiene polyol is a hydroxytele
Berger Noelle
Jay Pierre
Atofina
Millen White Zelano & Branigan P.C.
Sergent Rabon
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