Title of Invention

PROCESS FOR THE PREPARATION OF A CROSSLINKABLE IMPREGNATING ELECTRIC COMPOSITION

Abstract

(57) Abstract: The invention relates to a process for the preparation of a crosslinkable impregnating dielectric composition and a crosslinlcable impregnating dielectric composition comprising a polydiene polyol, a polyisocyanate and a chemically inert insulating liquid characterized in that a sufficient amount of the said insulating liquid is used in order to 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 tan8 at 20''C and at 50 Hz, after complete crosslinking, of less than 0.02. The crosslinkable impregnating dielectric composition may be used to impregnate solid dielectrics. The composition is liquid during impregnation and becomes an insulating solid after crosslinking. PRICE: THIRTY RUPEES

Full Text

The invention relates to the preparation of a crosslinkable dielectric composition based on polydiene polyol, a polyisocyanate and an inert liquid filler suitable for use, in particular, for impregnating solid dielectrics such as contained in condensers, cables and measuring transformers. Such compositions, which are liquid during the impregnation, become insulating solids after crosslinking. The present invention is particularly applicable to 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 features 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 destruction of the dielectric and thus of the applicance. 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 by the Joule effect in the appliances. However, liquid electric insulants have tailbacks such as the possibility of escaping from the appliances by leaking, 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 electrical appliances for which the presence of a liquid is not essential in order to remove the heat. However, these attempts have been unsuccessful in high-voltage uses with high potential gradient. As regards solidifiable liquids, two types of compositions 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 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 published on 01.09.1986 under No. 61-197620. However, the dielectric properties of these products are mediocre, which prohibits their use in high-voltage insulations with high potential gradient. In particular, the angle of loss or dielectric dissipation factor, hereinafter referred to as tangent delta (tans), is too high. Thus, in the abovementioned Japanese patent application, the tan6 of the products described is higher than 5x10'‘ at 20°C, the standard temperature (JIS K-6911) It may moreover be noted in this application that the compositions are intended for coating condenser bobbins. According to the present invention, there is provided a process for the preparation of a crosslinkable impregnating dielectric composition comprising at least one polydiene polyol, at least one polyisocyanate, and at least one chemically inert insulating liquid, wherein the functionality of the polyisocyanate is greater than or equal to 2, and the composition contains more than 90% by weight of the chemically inert insulating liquid such that the composition retains a viscosity of less than 50 mPa s at the impregnation temperature for a period at least equal to 1 day and has a tangent 5 (tan §) at 20°C and at 50 Hz, after complete crosslinking, of less than 0.02, which process comprises forming a mixture A comprising one or more polyols dissolved in a chemically inert insulating liquid, and a mixture B comprising one or more polyisocyanates dissolved in a chemically inert insulating liquid; contacting mixture A and/or mixture B, separately, with an adsorbent earth at a temperature of between 20°C and 80°C; and subsequently removing the adsorbent earth and contacting the mixtures A and B optionally with addition of further chemically inert insulating liquid. Th composition according to the invention has, 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. By the term "impregnation temperature" is meant the temperature to which, in use, 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. The impregnation temperature, maintained for a period of at least 1 day, is generally 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. The weight amount of chemically inert liquid filler in the composition according to the invention is greater than 90% and, preferably, is between 92% and 96%. The chemically inert liquid filler used in the process of the present invention is a chemically inert insulating liquid. The insulating liquid may fully solubilize the polydiene polyol and the polyisocyanate and which is chosen from alkylbenzenes such as decylbenzenes and dodecylbenzenes; dielectric esters which are, for example, reaction products of polyvalent alcohols such as pentaerythritol with monovalent carboxylic acids such as n-heptanoic acid; alkyl phthalates such as dibutyl phthalate and dioctyl — phthalate; alkyIpolyaromatic compounds such as monoisopropylbiphenyl (MIPB) and phenylxylylethanes (PXE); mixtures of benzyltoluenes and benzyl-benzyltoluenes such as those described, in particular, in European Patent Specification No. 136,230; mixtures of mono- and bis(methylbenzyl)xylenes such as those described in the published Specification of European Patent Application No. 0,443,899; mixtures of benzyltoluene and diphenylethane; dielectric plant oils such as rapeseed oils and corn oils, or a combination of at least two of the insulating liquids mentioned above. Mixtures of benzyltoluenes and benzyl-benzyl toluenes 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-benzyl toluenes, 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 chemically inert liquid filler. The chemically inert liquid filler used according to the present invention suitably, has a viscosity at 20°C of not more than 100 mPa s and preferably of between 4 and 30 mPa s, measured according to ASTM standard D445. The polyisocyanate used according to the present invention, 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 polyisocyanates which can be used according to the present invention, mention will be made of 4,4'-diphenylmethane isocyanates (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 ‘C(0)NH(CH2)6NCO OCN(CH2)6-N’ ‘C(0)NH(CH2)6NCO As illustrations of cycloaliphatic polyisocyanates, mention will be made of isophorone isocyanates (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 isocyanates trimers marketed by the company Rhone-‘ name Tolonate HDT and tris [1-(isocyanotomethyl)-1,3,3-trimethylcyclohexane] isocyanurate marketed by the company Hiils tinder the name Vestanat T 1890/100. The ajno\int of polyisocyanate according to the present invention is chosen such that the NCO/OH molar ratio is in the region of 1 and preferably between 0.85 and 1.15. The polydiene polyol used according to the present invention may be a hydroxytelechelic conjugated diene oligomer which may be obtained by various processes, such as radical polymerization of conjugated diene having from 4 to 20 carbon atoms, in the presence of a polymerization initiator such as hydrogen peroxide or an azo compound such as azo-2,2-bis[2-methyl-N-(2-hydroxyethyl)propionamide] or the anionic polymerization of conjugated diene having from 4 to 20 carbon atoms in the presence of a catalyst such as naph thalenedi 1 i thivun. The conjugated diene of the polydiene polyol may be chosen from the group comprising butadiene, isoprene, chloroprene, 1,3-pentadiene and cyclopentadiene. It would not be departing from the scope of the invention if hydroxytelechelic oligomers of conjugated dienes epoxidized on the chain and hydrogenated hydroxytelechelic oligomers of conjugated dienes were used. The polydiene polyols used according to the present invention suitably have numbler-average molar masses of not more than 7 000 and preferably of between 1000 and 3000. They typically have functionalities ranging from 1 to 5 and preferably ranging from 1.8 to 3 and a dynamic viscosity, measured at 30°C, at least equal to 600 mPa s. As an illustration of polydiene polyols, mention will be made of the hydroxylated polybutadienes marketed by the company Elf Atochem S.A. under the names Poly Bd® 45 HT and Poly Bd® 20 LM. The dielectric composition according to the present invention, may comprise, in addition to the polydiene polyol, one or more polyols of low molar mass. The expression polyol of low molar mass is understood to refer to polyols having molar masses ranging from 50 to 800. As illustrations of such polyols, mention may be made of ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, polyether polyols, butane-1,4-diol, hexane-1,6-diol, 2-ethyl-hexane-l,3-diol, N,N-bis(2-hydroxypropyl)aniline, 3-methyl-1,5-pentanediol, trimethylol propane, pentaerythritol, propoxylated bisphenol A marketed by the company Akzo under the name Dianol 320 and mixtures of at least two abovementioned polyols. In the event that a polyol of low molar mass is used, the NCO/OH molar ratio should be calculated bearing in mind the hydroxyl functions provided by the polyol of low molar mass. Although the use of a catalyst is not essential, one may be used in certain cases, if so desired. The catalyst may be chosen from the group comprising tertiary amines, imidazoles and organometallic compounds. As illustrations of tertiary amines, mention may be made of 1,4-diazabicyclo [2.2.2] octane (DABCO) and N,N,N',N",N" -pentamethyldiethylenetri amine. As illustrations of organometallic compounds, mention may be made of dibutyltin dilaurate, dibutyltin acetate and organobismuth derivatives. The crosslinkable composition of the present invention may be prepared by mixing, at room temperature (about 20°C), the various constituents by any means of sufficient stirring to ensure good dispersion of the constituents. However, the Applicant has observed that by working in this manner, compositions may be obtained which, although admittedly crosslinkable and having viscosities at the time of impregnation which are in accordance with those specified according to the present invention, provide mediocre dielectric properties in certain cases, in particular high tanS values. According to a first aspect of the present invention, the process for preparing crosslinkable compositions, in particular with a view to avoiding the obtaining of mediocre dielectric properties, comprises forming a mixture A comprising one or more polyols dissolved in a chemically inert liquid filler, and a mixture B comprising one or more polyisocyanates dissolved in a chemically inert liquid filler, contacting mixture A and/or mixture B separately; with an adsorbent earth at a temperature of between 20°C and 80°C; and subsequently removing the adsorbent earth and contacting the mixtures A and B optionally with addition of further chemically inert liquid filler. Thus the process may comprise the preparation, firstly and separately, of a mixture A, by dissolving one or more polyols in a chemically inert liquid filler, and of a mixture B, by dissolving one or more polyisocyanates in a chemically inert liquid filler, and then followed by placing in contact, separately, each of the mixtures A and/or B obtained with an adsorbent earth at a temperature of between 20°C and 80°C. Subsequently the adsorbent earth is removed from the mixtures A and B and the treated and any untreated mixtures A and B are brought into contact optionally with addition of fiirther chemically inert liquid filler. This placing in contact with adsorbent earth may take place in two p different ways. According to a first way of working, the adsorbent earth is added to the mixtures A and/or B, the mixtures A and/or B containing the said adsorbent earth are stirred separately for a period at least one hour and the said adsorbent earth is then removed from the mixtures A and/or B. In this way of working, the adsorbent earth is typically used, in the mixtures A and/or B, in a proportion of 0.1 to 5 parts by weight, and preferably of 1 to 3 parts by weight, per 100 parts by weight of mixture. Identical or different amounts of adsorbent earth may be added to the mixtures A and/or B. The adsorbent earth may be removed from the mixtures A and/or B by means known to those skilled in the art, e.g. filtration or centrifugation. The second way of working comprises circulating the mixtures A and/or B in separate coltimns packed with adsorbent earth of suitable particle size. In either variant of the process, the weight amounts of chemically inert liquid filler in the mixtures A and/or B may typically be at least 40 % and preferably between 85 % and 99.5 % of the mixtures A and/or B. As illustrations of adsorbent earths which can be used according to the present invention mention will be made of Fuller's earth, bentonites, diatomaceous earths, attapulgite and activated silica. The efficiency of these processes may be evaluated by measuring the tan6 of the liquid at 20°C and at 50 Hz. It would not constitute departure from the scope of the invention if the process was repeated separately one or more times on either of the mixtures A and B or on both mixtures until a level of dielectric properties which is acceptable for the use envisaged was obtained. As regards the mixture B, it is observed that the content of isocyanate function is retained substantially unchanged. In order to prepare the dielectric composition according to the invention, the treated or untreated mixtures A and B are placed in contact by either variant of the process, partially or totally, optionally with complementary addition of pure chemically inert liquid filler. This placing in contact may be performed by any means of mixing which makes it possible to ensure good homogenization. The mixtures A and B may also contain one or more additives such as antioxidants, epoxides, anthraquinone and derivatives. The weight amounts of the mixtures A and B, as well as of any optionally added of pure chemically inert liquid filler, to be placed in contact during the preparation of the said composition will be determined so as to obtain a final composition comprising at least 90 % by weight of chemically inert liquid filler and suitably an NCO/OH molar ratio of between 0.85 and 1.15. The composition according to the invention can be used in particular as agent for impregnating the solid insulations contained in electrical appliances (condensers, transformers and high-voltage cables). The placing in contact of the composition and the sxibstrate to be impregnated is typically carried out at a temperature of between 20°C and 90°C, generally under reduced pressure in a first step and then at atmospheric pressure. When the impregnation has ended, the temperature may be raised to 100°C, or even higher, for a short period in order to improve the crosslinking and to obtain a solid product. The composition according to the invention has the advantage of leading to insulations possessing an improved lifetime and better breakdown resistance. The composition also has a low tand value, of less than 0.02 measured at 20°C and at 50 Hz, and high breakdown voltages. The Examples which follow further illustrate the invention. In the accompanying drawings: Figure 1 is a graph showing the effect of chemically insert liquid filler content on viscosity according to Example 1, Figure 2 is a graph showing the effect of impregnating temperature on viscosity according to Example 2, Figure 3 is a graph showing the dielectric properties of compositions according to Example 3, and Figures 4 and 5 are grapns snowing cne lifetime of condensers according to Example 9. The compositions were prepared using the following constituents: - PolyBd® 45 HT (referred to hereinbelow as PolyBd): hydroxylated polybutadiene of Mn equal to 2800 (determined by steric exclusion chromatography), having a hydroxyl niimber 1Q’, expressed in milliequivalents per gram (meq/g), equal to 0.83, a viscosity equal to 5000 mPa s at 30°C and a density equal to 0.90, - Jarylec C 100 marketed by the company Elf Atochem S.A. (referred to hereinbelow as Jarylec): chemically inert liquid filler consisting of 75 % by weight of benzyltoluenes and 25 % by weight of benzyl-benzyl toluenes, having a viscosity at 20°C of 6.5 mPa s measured according to ASTM standard D-445, - dioctyl phthalate marketed by the company Elf Atochem (referred to hereinbelow as DOP): chemically inert licjuid filler consisting of diethylhexyl ortho-phthalate, having a viscosity at 20°C equal to 80 mPa s, - dibutyl sebacate (referred to hereinbelow as DBS): having a viscosity at 20°C equal to 9.3 mPa s, - dodecylbenzene (referred to hereinbelow as DDB): having a viscosity at 20°C equal to 10.4 mPa s, - food-grade rapeseed oil having a viscosity at 20°C equal to 69.5 mPa s, - Tolonate HDT/LV (referred to hereinbelow as Tolonate): tris(6-isocyanatohexyl)isocyanurate, having an NCO content equal to 23 %, a functionality of about 3.4 and a viscosity at 25°C equal to 1200 (+/- 300) mPa s, - Voranol CP 455 marketed by the company Dow Chemical (referred to hereinbelow as Voranol): polyether polyol of molar mass equal to 450, having an IQH equal to 6.77 meq/g and a viscosity at 25°C equal to 330 mPa s, - Isonate M 143 (referred to hereinbelow as Isonate) marketed by the company Dow Chemical: polymeric MDI having an NCO content equal to 3 0 %, a functionality eqpaal to 2.2 and a viscosity at 20 °C equal to 130 mPa s, - adsorbent earth: attapulgite preactivated at 150°C. ♦ Preparation of the contpositionB according to the process of the invention Either PolyBd or a mixture of PolyBd and Voranol in proportions of 92.25 % by weight of PolyBd to 7.75 % by weight of Voranol (mixtures A), on the one hand/ and either Isonate or Tolonate (mixtures B), on the other hand, are dissolved separately in Jarylec. The proportions of the various constituents are reported in Table 1. 3 g of attapulgite per 100 g of mixture are added to each of the mixtures A and B, the mixtures are then stirred separately at room temperature (about 20°C) for 3 to 18 hours and they are then filtered under reduced pressure in order to remove the attapulgite. The tan6 at 20°C and at 50 Hz is measured on each of the mixtures A and B before and after treatment with attapulgite. The results are reported in Table 1. COMPOSITIONS {% by weight) (20' TcUiS 'C/50 Hz) Before treatment After treatment Al Jarylec PolyBd + Voranol 88 12 8.3 X 10"‘ 1.2 X 10"' A2 Jarylec PolyBd + Voranol 79 21 8.7 X 10"‘ 0.23 X 10"' A3 Jarylec PolyBd 84 16 3 X lO-" 1.2 X lO-* Bl Jarylec Xsonate 96 4 27.8 X 10- i 0.5 X 10"' B2 Jarylec Tolonate 87.2 12.8 37 X 10'‘ 3.4 X 10-' 33 Jarylec Tolonate 86.5 13.5 34 X 10"' 14 X 10-' TABLE 1 The composition B3 undergoes a second treatment on attapulgite under conditions identical to the first treatment. It is observed that the tanS is equal to 0.4 x 10'‘ (20°C/50 Hz). The NCO contents were monitored before and after treatment and it is observed that they are virtually identical. Next, the mixtures A and B are placed in contact in a glass reactor fitted with a stirrer. The following are determined on the compositions obtained: - the influence of the content of chemically inert liquid filler (Jarylec) on the change in viscosity, - the influence of the impregnating temperature on the change in viscosity, - the dielectric properties of the compositions, by measuring the tan6 and the breakdovm voltage. EXAMPLE 1 ♦ Influence of the Jarylec content on the change in viscosity Compositions are prepared by placing in contact a mixture Al or A2 with the mixture Bl so as to obtain an NCO/OH molar ratio eqpial to 0.915 and weight amounts of Jarylec in the compositions equal to 85 %, 90 % (compositions not in accordance with the invention), 92 % and 94 %. The compositions are then left at room temperature and the viscosity is recorded as a function of time. The results are shown in the graph of Figure 1 of the accompanying drawings. In this graph, the viscosity of the compositions in mPa s has been represented on the y-axis and the time in days on the X-axis. Also, the symbols have the following meanings: -o- the composition containing 85 % by weight of Jarylec, -D- the composition containing 90 % by weight of Jarylec, -A- the composition containing 92 % by weight of Jarylec, -A- the composition containing 94 % by weight of Jarylec. EXAMPLE 2 ♦ Influence of the impregnating temperature on the chamge in viscosity The change in viscosity of a composition obtained by placing in contact mixtures A3 and B2 so as to obtain an NCO/OH molar ratio equal to 0.95 and a weight content of Jarylec equal to 94 % was studied at various temperatures. This composition consists of: 94 % of Jarylec, 5.2 % of PolyBd and 0.8 % of Tolonate. The change in viscosity of this composition as a function of time at various temperatures is shown in the graph of Figure 2 of the accompanying drawings. In this graph, the viscosity in mPa s is represented on the y-axis and the time in days on the x-axis. Also, the symbols have the following meanings: -D- the change at 20°C in viscosity as a function of time, -A- the change at 40°C in viscosity as a function of time, -A- the change at 60°C in viscosity as a function of time, -•- the change at 80°C in viscosity as a function of time. EXAMPLE 3 ♦ Dielectric qualities of the compositions according to the invention The above composition (94 % of Jarylec, 5.2 % of PolyBd and 0.8 % of Tolonate) was poured into a leaktight tan6 cell and then placed in an oven at 100°C and maintained at this temperature for 100 days. The tanS values were measured periodically in the course of the residence time at 100°C. The results are shown in the graph of Figure 3 of the accompanying drawings. In this graph, the tanS at 100°C is represented on the y-axis and the time in days on the X-axis. After one week at 100°C, which corresponds to the time for formation of the gel, the tan6 stabilizes at a very low value, below the values specified for a new insulating oil. This value is stable over time at a temperature of 100°C. EXAMPLE 4 A composition is prepared by placing in contact mixtures Al and Bl so as to obtain an NCO/OH molar ratio equal to 0.91 and a Jarylec content equal to 92 %. This composition consists of: 92 % by weight of Jarylec, 6.3 % by weight of PolyBd, 0.5 % by weight of Voranol and 1.2 % by weight of Isonate. This composition is poured into three cells, with spherical electrodes spaced 1 mm apart, in order to measure the breakdown voltage. The three cells are then placed at 80°C for 8 days for crosslinking. After cooling to room temperature, the breakdown voltage over an AC voltage gradient is measured according to lEC standard 156. Breakdown voltage measurements, carried out under the same conditions, are made using the insulating liquids: Univolt 52 mineral oil and Jarylec, previously degassed and filtered on a Millipore filter of porosity less than 1 mm. The average values of the breakdown voltage results for the gel and the two insulating liquids are as follows: Composition according to the invention 95.2 kV Univolt 52 mineral oil 40.0 kV Jarylec 44.5 kV It is seen that the breakdown voltage between spherical electrodes spaced 1 mm apart is more than 2 times higher for the composition according to the invention than for those obtained with the best insulating liquids used for transformers (mineral oil) and for condensers (Jarylec). EXAMPLES 5 TO 8 Compositions according to the invention containing chemically inert liquid fillers other than Jarylec. EXJ’MPLE 5 A composition is prepared containing DOP as liquid filler. 8.30 g of Tolonate are mixed, under a nitrogen atmosphere, with 940 g of DOP pretreated on attapulgite. 51.7 g of PolyBd and 3 mg/kg of dibutyltin dilaurate, referred to hereinbelow as DBTL, are then added. The composition thus obtained contains 94 % of DOP and the NCO/OH molar ratio is 1.0. The viscosity at 60°C is determined on a portion of this composition. The viscosity goes from 20 mPa s to 75 mPa s over 6 days. After 10 days at 90°C, the composition is totally crosslinked. Another portion of the composition is placed in a leaktight cell for measuring the dissipation factor. After 8 days at 90°C and cooling, the tan6 at 20°C - 50 Hz of the gel obtained is measured. A tan6 = 0.0031 is found. EXAMPLE 6 A composition is prepared containing a mixture of DDB and DOP as liquid filler, in a proportion of 80 % of DDB and 20 % of DOP. 8.41 g of Tolonate are mixed, under a nitrogen atmosphere, with 939.2 g of the mixture DDB + DOP. 52.4 g of PolyBd and 3 mg/kg of DBTL are then added and the resulting mixture is treated on attapulgite. The composition thus obtained contains 93.9 % of inert liquid filler and the NCO/OH molar ratio is 1.0. The viscosity at 60°C is determined on a portion of this composition, this value being 5 mPa s. After 4 days at 60°C, the composition is totally crosslinked. Another portion of the composition is placed in a leaktight cell for measuring the dissipation factor. After 8 days at 90°C and cooling, the tan6 at 20°C - 50 Hz of the gel obtained is measured. A tan6 = 0.00024 is fovmd. EXAMPLE 7 A composition is prepared containing DBS as liquid filler. 8.88 g of Tolonate are mixed, under a nitrogen atmosphere, with 936.2 g of DBS pretreated on attapulgite. 55.4 g of PolyBd and 10 mg/kg of DBTL are then added. The composition thus obtained contains 93.6 % of inert liquid filler and the NCO/OH molar ratio is 1.0. The viscosity at 60°C is determined on a portion of this composition. It goes from 4 mPa s to 46 mPa s over 4 days. After 6 days at 90°C, the composition is totally crosslinked. Another portion of the composition is placed in a leaktight cell for measuring the dissipation factor. After 8 days at 90'C and cooling, the tan6 at 20°C - 50 Hz of the gel obtained is measured. A tanS = 0.0079 is fotmd. EX2’MPLE 8 A composition is prepared containing rapeseed oil as liquid filler. The rapeseed oil is pretreated on attapulgite. 11.3 g of Tolonate are mixed, under a nitrogen atmosphere, with 915 g of rapeseed oil. 73.7 g of PolyBd and 5 mg/kg of DBTL are then added. The composition thus obtained contains 91.5 % of rapeseed oil. The NCO/OH molar ratio is 1.0. The viscosity at 60°C is determined on a portion of this composition. It goes from 34 mPa s to 92 mPa s over 2 days. After 8 days at 90°C, the composition is totally crosslinked. Another portion of the composition is placed in a leaktight cell for measuring the dissipation factor. After 8 days at 90°C and cooling, the tan6 at 2 0°C and 50 Hz of the gel obtained is measured. A tan6 = 0.00029 is found. EXAMPLE 9 Properties of all-propylene-film condensers impregnated with compositions according to the invention In order to illustrate the excellent behaviour of high-voltage insulations impregnated with compositions according to the invention, tests were carried out using condenser bobbins for DC voltage containing two layers of polypropylene films 13.6 fin. in thickness. Three series of 10 condensers were impregnated, on the one hand with Jarylec and on the other hand with two compositions according to the invention having the same NCO/OH molar ratio equal to 0.95 and consisting respectively of: • 94 % of Jarylec, 5.20 % of PolyBd and 0.80 % of Tolonate, referred to hereinbelow as composition X, and of • 96 % of Jarylec, 3.47 % of PolyBd and 0.53 % of Tolonate, referred to hereinbelow as composition Y. The impregnation was performed under vacuum at room temperature. Immediately after impregnation, the experimental models were maintained for 6 days at 40'C for the Jarylec and the composition Y and at 60°C for the composition X. The experimental models were then left for 12 days at 90°C for heat formation. Two tests of lifetime at room temperature were then carried out, at a very high DC potential gradient (390 and 350 V//im DC) . The results obtained are presented in the graphs of Figures 4 and 5 respectively. In these graphs, the number of surviving condensers is represented on the y-axis and the time in hours on the x-axis. Also, the curves represented are: Jarylec composition X .... composition Y The experimental models impregnated with compositions according to the invention led to lifetimes (time for breakdown of half of the condensers) which were from 70 to 240 % higher than those obtained with Juryless alone. WE CLAIM: 1. A process for the preparation of a crosslinkable impregnating dielectric composition comprising at least one polydiene polyol, at least one polyisocyanate and at least one chemically inert insulating liquid, wherein the functionality of the polyisocyanate is greater than or equal to 2, and the composition contains more than 90% by weight of the chemically inert insulating liquid such that the composition retains a viscosity of less than 50 mPa s at the impregnation temperature for a period at least equal to 1 day and has a tangent 8 (tan 5) at 20°C and at 50 Hz, after complete crosslinking, of less than 0.02, which process comprises forming a mixture A comprising one or more polyols dissolved in a chemically inert insulating liquid, and a mixture B comprising one or more polyisocyanates dissolved in a chemically inert insulating liquid; contacting mixture A and/or mixture B, separately, with an adsorbent earth at a temperature of between 20°C and 80°C; and subsequently removing the adsorbent earth and contacting the mixtures A and B optionally with addition of further chemically inert insulating liquid. 2. The process according to claim 1, wherein in the composition, the functionality of the polyisocyanate is between 2.2 and 4. 3. The process according to claim 1 or 2, wherein the composition contains between 92% and 96% by weight chemically inert insulating liquid. 4. The process according to any one of the preceding claims, wherein the impregnation temperature is at least 20°C. 5. The process according to claim 4, wherein the impregnation temperature is between 40°C and 90°C. 6. The process according to any one of the preceding claims, wherein the chemically inert insulating liquid is chosen from alkylbenzenes, dielectric esters, alkylpolyaromatic compounds, alkyl phthalates, mixtures of benzyltoluenes and benzl-benzyltoluenes, mixtures of mono- and bis-(methylbenzyl) xylenes, mixtures of benzyltoluenes and diphenylethane, and plant oils. 7. The process according to any one of claims 1 to 5, wherein the chemically inert insulating liquid is dioctyl phthalate. 8. The process according to any one of claims 1 to 5, wherein the chemically inert insulating liquid is dibutyl sebacate or dioctyl adipate. 9. The process according to any one of claims 1 to 5, wherein the chemically inert insulating liquid is rapeseed oil. 10. The process according to any one of claims 1 to 5, wherein the chemically inert insulating liquid is a mixture 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. 11. The process according to claim 10, wherein the chemically inert insulating liquid is a mixture comprising 75% by weight of benzyltoluenes and 25% by weight of benzyl-benzyltoluenes. 12. The process according to any one of the preceding claims, wherein the chemically inert insulating liquid has a viscosity at 20°C of not more than 100 mPa s measured according to ASTM standard D445. 13. The process according to any one of claims 1 to 11, wherein the chemically inert insulating liquid has a viscosity at 20°C between 4 and 30 mPa s, measured according to ASTM standard D445. 14. The process according to any one of the preceding claims, wherein the polydiene polyol is a hydroxytelechelic conjugated diene oligomer. 15. The process according to claim 14, wherein the conjugated diene is butadiene. 16. The process according to any one of the preceding claims, wherein the polydiene polyol has a number-average molar mass of not more than 7000. 17. The process according to any one of claims 1 to 15, wherein the polydiene diol has a number-average molecular mass between 1000 and 3000. 18. The process according to any one of the preceding claims, wherein the polydiene polyol has a functionality of from 1 to 5. 19. The process according to any one of claims 1 to 17, wherein the polydiene polyol has a functionality of from 1.8 to 3. 20. The process according to any one of the preceding claims, wherein the polyisocyanate contains an isocyanurate ring in its molecule. 21. The process according to claim 20, wherein the polyisocyanate is tris[l-(isocyanoto-methyl)-l,3,3-trimethylcyclohexane] isocyanurate or the trimer of hexamethylene diisocyanate. 22. The process according to any one of claims 1 to 19, wherein the polyisocyanate is an aromatic polyisocyanate. 23. The process according to claim 22, wherein the polyisocyanate is 4,4'-diphenylmethane diisocyanate (MDI) or a polymeric MDI. 24. The process according to any one of claims 1 to 19, wherein the polyisocyanate is a cycloaliphatic polyisocyanate. 25. The process according to claim 24, wherein the polyisocyanate is 4,4'-diphenylmethane diisocyanate. 26. The process according to any one of the preceding claims, wherein one or more polyols of low molar mass are optionally added. 27. The process according to claim 26, wherein the polyol has a molar mass of from 50 to 800. 28. The process according to any one of the preceding claims, wherein the composition has a NCO/OH molar ratio in the region of 1. 29. The process according to any one of claims 1 to 27 wherein the composition has a NCO/OH molar ratio between 0.85 and 1.15. 30. The process according to any one of the preceding claims, wherein a catalyst is optionally added to the composition. 31. The process according to any one of the preceding claims, wherein the treatment of one or both of the mixtures A and/or B, by contacting, separately, with adsorbent earth at a temperature of between 20°C and 80°C, is performed by adding the adsorbent earth to the mixtures A and/or B, the mixtures A and/or B containing the said adsorbent earth are then stirred separately for a period at least equal to 1 hour, the said adsorbent earth is then removed from the mixtures A and/or B and the treated and, if appropriate, untreated mixtures A and B are placed in contact after optional complementary addition of chemically inert insulating liquid. 32. The process according to any one of claims 1 to 30, wherein the mixtures A and/or B are placed in contact with the adsorbent earth by circulating the mixtures A and/or B in separate columns packed with adsorbent earth. 33. The process according to any one of the preceding claims, wherein the mixtures A and/or B contain at least 40% by weight chemically inert insulating liquid. 34. The process according to any one of claims 1 to 32, wherein the mixtures A and/or B contain between 85% and 99.5% by weight chemically inert insulating liquid. 35. The process according to any one of preceding claims, wherein the adsorbent earth is attapulgite. 36. The process according to any one of preceding claims, wherein the adsorbent earth is used, in the mixtures A and/or B, in a proportion of from 0.1 to 5 parts by weight of mixture. 37. The process according to any one of claims 1 to 35, wherein the adsorbent earth is used, in the mixtures A and/or B, in a proportion of from 1 to 3 parts by weight, per 100 parts by weight of mixture. 38. A process for the preparation of a crosslinkable impregnating dielectric composition substantially as herein described with reference to the accompanying drawings.

Documents:

804-mas-1996 abstract.pdf

804-mas-1996 claims.pdf

804-mas-1996 correspondence others.pdf

804-mas-1996 correspondence po.pdf

804-mas-1996 description (complete).pdf

804-mas-1996 drawings.pdf

804-mas-1996 form-2.pdf

804-mas-1996 form-26.pdf

804-mas-1996 form-4.pdf

804-mas-1996 form-6.pdf

804-mas-1996 petition.pdf