Coating processes – Electrical product produced – Coil or winding
Reexamination Certificate
2001-09-14
2003-10-28
Dawson, Robert (Department: 1712)
Coating processes
Electrical product produced
Coil or winding
C523S457000, C523S466000, C528S409000, C528S418000
Reexamination Certificate
active
06638567
ABSTRACT:
The present invention relates to curable compositions, to their use, for example as casting resins in the production of air-cooled transformers and other electrical components, and to the crosslinked products obtainable by curing the compositions, which products are distinguished by the fact that they exhibit simultaneously the features of good flame retardance, high mechanical strength and low dielectric losses at high operating temperatures.
Air-cooled transformers (voltage range up to about 40 kV) are provided with a winding, the sheathing of which consists of an electrically insulating synthetic resin. In addition to providing insulation, the synthetic resin sheathing should also contribute to the mechanical strength of the windings and also have flame-retardant properties.
The critical factors for a sheathing resin for high performance transformers are the oxygen index for combustibility, the temperature at which the dielectric loss factor tan 6 is 25% at 50 Hz and the crack index value achieved, which is a measure of resistance to temperature variation.
Flame-retardant casting resins for potting air-cooled transformers are well known and are generally based on bisphenol A epoxy resins, reinforcing fillers and flame-retardants. For example, U.S. Pat. No. 3,202,947 describes flame-retardant compositions for air-cooled transformers, containing liquid bisphenol A diglycidyl ethers, hexahydrophthalic acid, hydrated alumina and tris(chloroalkyl) phosphates.
Cycloaliphatic resin systems are also known. U.S. Pat. No. 4,009,141 describes electrically insulating curable compositions consisting of selected cycloaliphatic epoxy resins and dicarboxylic anhydrides, which are reinforced with large amounts of zirconium silicate fillers and contain finely divided hydrated alumina as additional second filler. They are suitable for the encapsulating insulation of electrical components, such as, for example, of metal transformer components, or transformer bushings.
Curable, flame-retardant compositions for air-cooled transformers are also described in FR 2 630 578 B1. Those compositions contain at least 20% by weight pretreated aluminium hydroxide, based on the total composition consisting of resin, hardener and reinforcing additives. “Pretreated” in this context means that, by means of heat treatment, water is removed from the aluminium hydroxide in an amount of about from 0.5 to 10% by weight, based on the original weight before removal of the water.
Since in such systems the dielectric loss factor tan &dgr; increases considerably at higher temperature, those systems are not suitable for transformers having high operating temperatures.
There is therefore a need for casting resin formulations that exhibit simultaneously the features of flame retardance, low dielectric losses and good mechanical properties, especially good cracking behaviour.
That problem has now been solved by the use of cycloaliphatic systems comprising core/shell polymers, as described in EP 0 578 613 A2. It has been found that the addition of certain fillers in certain ratios and certain amounts yields potting compounds that are distinguished both by low brittleness and by a low tan &dgr; value and also by good flame retardance.
The present invention accordingly relates to curable compositions comprising
(a) a cycloaliphatic epoxy resin that is liquid at RT and, suspended therein, a core/shell polymer,
(b) a polycarboxylic anhydride and
(c) fillers,
wherein the composition is flame-retardant because two different fillers (c1) and (c2) are present, the nature of filler (C1) being such that, starting at RT, it is able to release water as the temperature rises, the total proportion of fillers (C1) and (c2) is from 58 to 73% by weight, based on the total amount of components (a), (b), (C1) and (c2), and the ratio by weight of the fillers (C1):(c2) is from 1:3 to 1:1.
The compositions according to the invention are resin systems of moderate to relatively high viscosity that can be fully cured by heat. In the cured state they are thermosetting materials of relatively high rigidity having a glass transition temperature of about from 80 to 140° C. The term “cycloaliphatic epoxy resin” in the context of this invention denotes any epoxy resin having cycloaliphatic structural units, that is to say it includes both cycloaliphatic glycidyl compounds and &bgr;-methylglycidyl compounds-as well as epoxy resins based on cycloalkylene oxides. “Liquid at room temperature (RT)” is to be understood as meaning pourable compounds that are liquid at 25° C., i.e. are of low to medium viscosity (viscosity less than about 20 000 mPa·s).
Suitable cycloaliphatic glycidyl compounds and &bgr;-methylglycidyl compounds are the glycidyl esters and &bgr;-methylglycidyl esters of cycloaliphatic polycarboxylic acids, such as tetrahydrophthalic acid, 4-methyltetrahydrophthalic acid, hexahydrophthalic acid, 3-methylhexahydrophthalic acid and 4-methylhexahydrophthalic acid.
Further suitable cycloaliphatic epoxy resins are the diglycidyl ethers and &bgr;-methylglycidyl ethers of cycloaliphatic alcohols, such as 1,2-dihydroxycyclohexane, 1,3-dihydroxycyclohexane and 1,4-dihydroxycyclohexane, 1,4-cyclohexanedimethanol, 1,1-bis(hydroxymethyl)cyclohex-3-ene, bis(4-hydroxycyclohexyl)methane, 2,2-bis(4-hydroxycyclohexyl)propane and bis(4-hydroxycyclohexyl)sulfone.
Examples of epoxy resins having cycloalkylene oxide structures are bis(2,3-epoxycyclopentyl) ether, 2,3-epoxycyclopentylglycidyl ether, 1,2-bis(2,3-epoxycyclopentyl)ethane, vinyl cyclohexene dioxide, 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate, 3,4-epoxy-6-methylcyclohexylmethyl 3′,4′-epoxy-6′-methylcyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl) adipate and bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate.
Preferred cycloaliphatic epoxy resins are bis(4-hydroxycyclohexyl)methanediglycidyl ether, 2,2-bis(4-hydroxycyclohexyl)propanediglycidyl ether, tetrahydrophthalic acid diglycidyl ester, 4-methyltetrahydrophthalic acid diglycidyl ester, 4-methylhexahydrophthalic acid diglycidyl ester, 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate and especially hexahydrophthalic acid diglycidyl ester.
The cycloaliphatic epoxy resins can also be used in combination with aliphatic epoxy resins. As “aliphatic epoxy resins” it is possible to use epoxidation products of unsaturated fatty acid esters. It is preferable to use epoxy-containing compounds derived from mono- and poly-fatty acids having from 12 to 22 carbon atoms and an iodine number of from 30 to 400, for example lauroleic acid, myristoleic acid, palmitoleic acid, oleic acid, gadoleic acid, erucic acid, ricinoleic acid, linoleic acid, linolenic acid, elaidic acid, licanic acid, arachidonic acid and clupanodonic acid.
For example, there are suitable the epoxidation products of soybean oil, linseed oil, perilla oil, tung oil, oiticica oil, safflower oil, poppyseed oil, hemp oil, cottonseed oil, sunflower oil, rapeseed oil, polyunsaturated triglycerides, triglycerides from euphorbia plants, groundnut oil, olive oil, olive kernel oil, almond oil, kapok oil, hazelnut oil, apricot kernel oil, beechnut oil, lupin oil, maize oil, sesame oil, grapeseed oil, lallemantia oil, castor oil, herring oil, sardine oil, menhaden oil, whale oil, tall oil and derivatives thereof.
Also suitable are higher unsaturated derivatives that can be obtained by subsequent dehydrogenation reactions of those oils.
The olefinic double bonds of the unsaturated fatty acid radicals of the above-mentioned compounds can be epoxidised in accordance with known methods, for example by reaction with hydrogen peroxide, optionally in the presence of a catalyst, an alkyl hydroperoxide or a peracid, for example performic acid or peracetic acid. Within the scope of the invention, both the fully epoxidised oils and the partially epoxidised derivatives that still contain free double bonds can be used for component (a).
Preference is given to the use of epoxidised soybean oil and epoxidised linseed oil.
When cycloaliphatic epoxy resins
Aylward D.
Dawson Robert
Proskauer Rose LLP
Vantico Inc.
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