WO1998019313A1

WO1998019313A1 - Dielectric, radiation curable coating compositions and metal conductors coated with such coating

Info

Publication number: WO1998019313A1
Application number: PCT/NL1997/000588
Authority: WO
Inventors: Vadim Krongauz; Stephen Lapin; Antony Tortorello
Original assignee: Dsm N.V.
Priority date: 1996-10-31
Filing date: 1997-10-27
Publication date: 1998-05-07
Also published as: KR20000052972A; TW381111B; EP0935804A1

Abstract

Provided is a metal conductor having a cured coating of about 10 to about 500 micrometer thickness which coating has a dielectric dissipation factor (60Hz, 24 °C) of lower than about 0.05 and is a radiation-cured coating formulated from components comprising: a) an acrylate functional urethane oligomer having a hydrocarbon backbone, b) one or more mono- or polyfunctional diluents, and optionally, and c) a photoinitiator.

Description

DIELECTRIC, RADIATION CURABLE COATING COMPOSITIONS AND METAL CONDUCTORS COATED WITH SUCH COATING

Background of the invention Metal conductors are in general coated with a dielectric coating for insulating the conductor. Such coatings require good insulating properties in a variety of environments and/or under extreme conditions such as in transformer coils in power distribution transformers. According to US-A-4481258 , the prior art used paper as insulating material. Although US-A- 4481258 proposes a coating as insulating material, paper is still in use in the manufacture of transformer coils. The coatings proposed in US-A-4481258 are certain UV-curable materials comprising acrylate-ester adducts, acrylate urethane adducts and acrylate functional diluents. These coatings require both UV cure, and a post-cure at a temperature of 130°C for 4- 17 hr . The process for making transformer coils with these coatings is unattractive in particular because of the post-cure required. Typically insulating coatings for high power transformer coils should exhibit several properties.

- As the metal is coated and thereafter is bent in a required form, the coating should be flexible so that it can withstand bending of the coated conductor as it is wound into a coil.

- The coating should be able to withstand immersion in oil for 28 days at 150°C as described in US-A- 4481258.

- The coating should remain adherant at elevated temperature that is encountered when the transformer is under load. The coating should have a dielectric constant smaller than 5% at 60 Hz (24°C), - The coating should have a dielectric dissipation factor smaller than 0.05 at 24°C before and after hot oil exposure and smaller than 0.2 at 150°C, both at 60 Hz.

It is an object of the invention to provide dielectric radiation-curable coating compositions and metal conductors coated with UV-curable coating compositions which have required properties as insulating materials.

Summary of the invention The invention relates to a metal conductor with a cured coating of about 10-500 μm thickness which coating has a dielectric dissipation factor (60Hz, 24°C) of lower than about 0.05 and is a radiation-cured coating formulated from components comprising: a) an acrylate functional urethane oligomer having a hydrocarbon backbone b) one or more mono- or polyfunctional diluents, and optionally, c) one ore more light sensitive radical generating compounds. Furthermore, the invention relates to a radiation curable coating composition comprising a) an acrylate functional urethane oligomer having a hydrocarbon backbone b) one or more mono- or polyfunctional diluents and optionally, and optionally c) one or more light sensitive radical generating compounds which coating when cured with radiation has a dielectric dissipation factor at 60 Hz at 24°C of lower than about 0.05, a dissipation factor at 60 Hz at 150°C of lower than about 0.2, and an elongation at 25°C of a 25 μm thin coating of at least about 50%. The insulating coating layer on the metal conductor has outstanding insulating properties, both at low and high temperature. The insulating coating layer appears to have a low dielectric constant, e.g. lower than about 5 (60Hz, 24°C) and a good dielectric breakdown value. Furthermore, the coating is flexible as to allow bending of the metal conductor.

The metal conductor preferably is an iron, copper, aluminum or silver conductor. In particular aluminum, copper or silver are preferred. The metal conductor can be in the form of a wire or a strip. The coated metal conductor can be used in capacitors, transformers, motors and the like. The coated metal conductor can be used in hot oil environments because of the outstanding properties of the coating. Hence, the invention is most suitable for coating aluminum or copper strip or wire used in forming power distribution transformer coils. The cross-section of the strips commonly ranges from about 0.1-1.7 mm thick and 7-60 cm wide. The strips are wound into coils which are then assembled with cores to form transformers.

In general, the metal wire or strip is coated as a straight continuous web and the coated metal wire or strip may be wound for storage or for direct use. Hence, the coating when cured, should be cured well at the surface so that no blocking occurs in case the metal conductor is stored. Further, the coating of the present invention is flexible so that winding for either storage, and/or bending of the coil or wire in the manufacture of articles like transformers does not cause damage to the coating. Thus, the coating measured at 25 μm thickness, preferably has an elongation of at least about 50%. In particular, the coating has at least one Tg of below 20°C (as measured by the peak of the tan δ curve in a DSM analysis at 1 Hz).

As the coated metal conductor can be used in a hot oil environment, most preferably the coating has a dissipation factor at 60 Hz at 150°C of lower than about 0.2. Furthermore, the coating preferably has a dissipation factor at 60 Hz at 24°C before and after a hot oil aging test of lower than about 0.05. The coating exhibits its insulating properties even as very thin films. The coating preferably has a thickness of about 10-500 μm, and more in particular between of about 10-100 μm.

The first component of the radiation curable coating is an acrylate functional urethane oligomer (a) having a hydrocarbon backbone. The word backbone is used to denote the oligomer or polymer to which the acrylate urethane groups are attached. This acrylate functional urethane oligomer preferably is used in an amount of about 20-80 wt.% with respect to the total coating composition. More preferably, the amount is about 30-65 wt.%.

The oligomer (a) utilized in the present invention preferably is the reaction product of (i) a hydrocarbon compound with groups reactive with an isocyanate; (ii) a polyisocyanate; and (iii) an hydroxy functional endcapping monomer.

The hydrocarbon compound with groups reactive with an isocyanate (i) is provided by a linear or branched hydrocarbon containing a plurality of said reactive end groups, and providing a hydrocarbon backbone to the oligomer. The isocyanate reactive groups may be thiol, amine or hydroxy. Particularly preferred are hydroxy groups. Because of the amine and thiol groups, the urethane oligomer may comprise urea or thio-urea groups. The hydrocarbon portion is preferably from about 400 to about 4,000 molecular weight. Molecular weight in this case is determined by gel permeation chromotography (GPC), using a methylene chloride solvent, as measured against polystyrene molecular weight standards. By "hydrocarbon" is meant a non-aromatic compound containing a majority of methylene groups (-CH₂-) and which may contain internal unsaturation and/or pendent unsaturation. Fully saturated (i.e., hydrogenated) hydrocarbons are preferred because the electric dissipation factor of the cured coating increases as the degree of unsaturation increases. Suitable hydrocarbon polyols include hydroxyl-terminated, fully or partially hydrogenated 1 , 2-polybutandiene; 1,4- and 1,2- polybutadiene copolymers; 1 , 2-polybutadiene polyol hydrogenated to an iodine number of from 9 to 21; fully or partially hydrogenated polyisobutylene polyol; mixtures thereof, and the like. Preferably, the hydrocarbon polyol is substantially fully hydrogenated, and thus a preferred polyol is hydrogenated 1,2- polybutadiene, and hydrogenated 1 , 4-, 1 ,2-polybutadiene copolymers with about 50-80% 1 , 4-butadiene and 50-20% 1,2-butadiene copolymerized monomers. Suitable hydrocarbon polyamines or polythiols include the above described polyols with thiol or amino groups in stead of the hydroxy groups.

The polyisocyanate component (ii) is aromatic or non-aromatic, and preferably is non-aromatic. A suitable aromatic polyisocyanate is toluene diisocyanate. Non-aromatic polyisocyanates of from 4 to 20 carbon atoms may be employed. Suitable saturated aliphatic polyisocyanates include isophorone diisocyanate; dicyclohexylmethane-4,4 '-diisocyanate; 1 , 4-tetramethylene diisocyanate; 1, 5-pentamethylene diisocyanate; 1 , 7-heptamethylene diisocyanate; 1,8- octamethylene diisocyanate; 1 , -nonamethylene diisocyanate; 1 , 10-decamethylene diisocyanate; 2,2,4- trimethyl-1, 5-pentamethylene diisocyanate; 2,2'- dimethyl-1 , 5-pentamethylene diisocyanate; 3-methoxy- 1, 6-hexamethylene diisocyanate; 3-butoxy-l, 6- hexametalyene diisocyanate; omega, omega '-dipropylether diisocyanate; 1 , 4-cyclohexyl diisocyanate; 1,3- cyclohexyl diisocyanate; trimethylhexamethylene diisocyanate; and mixtures thereof. Isophorone diisocyanate is the preferred aliphatic polyisocyanate.

The reaction rate between a hydroxyl- terminated hydrocarbon and the diisocyanate may be increased by use of a catalyst in the amount of 100 to 200 ppm. Suitable catalyst include dibutyl tin dilaurate, dibutyl tin oxide, dibutyl tin di-2- hexanoate, stannous oleate, stanous octoate, lead octoate, ferrous acetoacetate, and amines such as tr iethylamine, diethylmethlamine, triethylenediamine, dimethyl-ethylamine, morpholine, N-ethyl morpholine, piperazine, N,N-dimethyl benzylamine, N,N-dimethyl laurylamine, and mixtures thereof. A preferred catalyst is dibutyl tin dilaurate. The endcapping monomer (iii) is a hydroxyl- terminated aliphatic acrylate or methacrylate, preferably an alkoxylated (meth)acrylate wherein 1-10 molecules of ethylene, propylene at butylene oxide are reacted with acrylic or methacrylic acid. Suitable hydroxyl-terminated monoacrylates which may be used as the endcapping monomer include hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate and hydroxypropyl methacrylate. Hydroxyethyl acrylate is preferred because it imparts a faster cure rate to the polyurethane oligomer. The molar ratio of the hydrocarbon compound, diisocyanates and endcapping monomer is preferably approximately 1:2:2.

The second component (b) is constituted by one or more mono- or polyfunctional diluents.

Preferably, the diluents are acrylate or methacrylate functional. However, minor amounts of other type of monomer can be used as well. The amount of component (b) preferably is about 20-80 wt.% of the total coating composition, more preferred about 20-70 wt.%.

Particularly preferred is the use of about 10-50 wt.% of monofunctional diluent (s), and 5-40 wt.% of polyfunctional diluent (s).

The second component (b) of the composition comprises preferably a monofunctional alkyl acrylate or methacrylate-based diluent for monomer. The alkyl portion of the monomer has between 6 and 18 carbon atoms, and preferably between 8 and 15, and therefore is hydrocarbon in character. This monomer may be either straight chain, branched or cyclic. This component comprises from about 5 percent to about 50 percent by weight of the composition, based upon the total weight of the coating composition. Preferably, it comprises from about 5 percent to about 50 percent, and more preferably from about 10 percent to about 40 percent by weight of the composition. The monomer is selected to be one that is compatible with the oligomer discussed above. Suitable examples of C₆ to C_ιe alkyl acrylate or methacrylate- based monomers include hexyl acrylate; hexyl methacrylate; cyclohexylacrylate; cyclohexyl- methacrylate; 2-ethylhexyl acrylate; 2-ethylhexyl methacrylate; isooctyl acrylate; isooctyl methacrylate; octyl acrylate; octyl methacrylate; decyl acrylate? decyl methacrylate; isodecyl acrylate; isodecyl methacrylate; isobornylacrylate; isobornylmethacrylate; lauryl acrylate; lauryl methacrylate; stearyl acrylate; stearyl methacrylate.

The second component further comprises preferably an alkylacrylate polyfunctional diluent (or monomer) in an amount of about 5-50 wt.%, preferably about 5-40 wt.%. Suitable examples of these polyfunctional monomers are C₄-C₁₅ hydrocarbon diol acrylates; C₄-C₁₅ hydrocarbon diol methacrylates; and mixtures of the above. The term hydrocarbon includes cycloalkylgroups. Other suitable polyfunctional acrylates are (alkoxylated) polyolpolyacrylates.

Examples of suitable polyfunctional monomers include butanediol dimethyacrylate, butanediol diacrylate, propanediol dimethacrylate, propanediol diacrylate, pentanediol dimethacrylate, pentanediol diacrylate, hexanediol dimethacrylate, hexanediol diacrylate, neopentylglycol dimethacrylate, neopentylglycol diacrylate, tr imethylolpropane triacrylate, tr imethylolpropane tr imethacrylate, polypropylene glycol diacrylate, polypropylene glycol dimethacrylate, cyclohexanedi-methanoldiacrylate or -methacrylate and, tricyclodecane dimethanol di (meth)acrylate. Preferred alkyl acrylate monomers include isobornyl acrylate, 2-ethylhexylacrylate, isooctylacrylate, cyclohexylacr late, hexanedioldiacrylate, tricyclodecanedimethanol- diacrylate. Other diluents may be used in amounts of preferably less than about 10 wt.%. Examples of these diluents are N-vinyl functional or vinylether functional compounds with a molecular weight lower than 500. Examples of these diluents are N-vinyl caprolactam, butyl-vinylether , tr iethyleneglycoldi- vinylether , butanediol-divinylether and the like.

The diluents preferably are used in a quantity sufficient to adjust the total coating composition to a viscosity of lower than about 2000 mPa.s, preferably lower than about 800 mPa.s at 25°C, measured with a couette apparatus (cup-and-bob viscometer at a frequency 100 rpm) .

The coating composition of the present invention preferably does not comprise substantial amounts of monomers with relatively strong dipole moments such as N-vinylpyrrolidone, phenoxyeth lacr late, polyoxyalkylane- alkylphenolacrylate and the like. The coating composition furthermore, preferably does not comprise -in substantial amounts - those monomer for which dipoles can be easily included, such as aromatic group containing acrylates such as phenylacrylates. The man skilled in the art can easily determine the amount allowed in the composition by measuring the dissipation factor .

The coating is radiation curable, and can be cured with electron beam irradiation or with light with a wavelength between about 200-700 mm. In the latter case, the composition comprises a light sensitive radical generating compound or mixture of compounds as photoinitiators. The photoinitiator , when used in a small but effective amount to promote radiation cure, must provide reasonable cure speed without causing premature gelation of the composition.

Suitable photoinitiators include the following: hydroxycyclohexylphenyl ketone; hydroxymethylphenylpropanone; dimethoxyphenyl- acetophenone; 2-methyl-l , [4- (methyl thio)phenyl ]- 2-morpholino-propanone-l ; l-(4-isopropylphenyl )- 2-hydroxy-2-methylpropan-1-one ; 1- ( 4-dodecylphenyl )-2- hydroxy-2-methylpropan-l-one; 4-(2- hyd oxyethoxy)phenyl- (2-hydroxy-2-propyl )ketone; diethoxyacetophenone ; 2 , 2-di-sec-butoxyacetophenone ; diethoxy-phenyl acetophenone; and mixtures of these. The photoinitiator - if used - preferably comprises from about 1.0 percent to about 10.0 percent by weight of the composition, based upon the total composition. Preferably, the amount of photoinitiator is from about 2.0 percent to about 7.0 percent by weight. The photoinitiator should be chosen such that a cure speed, as measured in a dose versus modulus curve, of less than about 2.0 J/cm², and preferably less than about 1.0 J/cm², is required, when the photoinitiator is used in the designated amount.

The composition preferably also contains an adhesion promoter. The adhesion promoter is preferably a compound having a groups participating in the radical curing reaction, and a group that adheres to the metal conductor. The group that participates in the curing reaction can be preferably, vinyl, (meth) acrylate or thiol. The group that adheres to the metal conductor preferably is hydroxy, acid, zirconate, titanate or silane. The acid may be for example carboxylic phosphoric or sulphonic. Most preferred is a (meth)acrylate functionalized carboxylic acid or phosphoric acid. Some examples of suitable adhesion promoters include, but are not limited to, hydroxyethyl (meth)acrylate, hydroxypropyl (meth) acrylate, di- or trialkoxy zirconates or titanates, vinyl tr imethoxysilane, mercaptopropyltr imethoxy silane, acrylic acid, methacrylic acid, β-carboxyethyl acrylate, Ebecryl 170 and Ebercyl 169. The Ebercyl products are acrylate ester derivatives, available from Radcure Specialties in Atlanta, Georgia, and are phosphoric acid based adhesion promoters.

Mono or diester or phosphoric acid having the following formula are also suitable adhesion promoters:

R 0 0

I II II [H₂C = C - C - 0 - A - 0]_m - P - [OR']_x

I

[OH]_p

where m + 1 + p = 3

R = H or CH₃ A = C_nH_2n, and 2 < n < 6

R' = C, to C₁₄ alkyl, aryl, alkaryl, or alkyleneoxy. Representative of the various species of organo-phosphate esters having the above formula include, but are not limited to, (1) methylmethacryloyloxyethyl phosphate, where

(R = CH₃; A = -C₂H₄-; R' = CH₃, m= 1 and P = i);

(2) ethyl methacryloyloxyethyl phosphate, where

(R = CH₃; A = -C₂H₄-; R' = C₂H_S); m = 1 and

P = i); (3) propylacryloyloxyethyl phosphate, where (R = H; A = -C₂H₄-; R' = C₃H₇; m = 1 and p = 1);

(4) methyl acryloyloxyethylphosphate, where (R = H,

A = -C₂H₄-; R' = CH₃, m = 1 and p = 1);

(5) ethylacrylyoyloxyethylphosphate, where (R = H; A = -C₂H₄-; m = 1 and p = 1; R' = C₂H₅);

(6) propylmethacryloyloxy-ethylphosphate, where

(R = CH₃; A = -C₂H₄-; R' = C₃H₇; m = 1 and

P = i);

(7) bis (methacryloxyethyl ) phosphate, where (R = CH₃; A = -C₂H₄-; m = 2; l = 0; p = l); and

(8) bis(acryloxyethyl )phosphate, where (R = H?

A = -C₂H₄-; m = 2; l = 0; p = l). The adhesion promoter helps the coating composition adhere to the metal conductor. The adhesion promoter may be used in an amount in the range of about 0.2 to 5 wt.% of the composition. Care should be exercised, that the amount of adhesion promotor is not so large that insulating properties are decreased below acceptable level. It is an unexpected advantage of the coating composition of the present invention, that an adhesion promotor can be used in effective amounts and that still very good insulating properties are achieved.

In addition to the above components, the composition may also contain other components that are known to those skilled in the art including stabilizers, surfactants, plasticizers, chain transfer agents and the like.

In addition, it may be useful to use a small amount of pigment or dye to color the coating. This allows simple visual control of the coated metal conductor. This is in particular useful, in case the metal conductor is only partly coated. Suitable pigments or dyes are for example copper phthalocyanine blue, crystal violet lactone (blue), crystal malachite green, sheet fed rubine (red). The amount of pigment - if used - will in general be about 0.2-5 wt.% relative to the coating composition.

The coating may be applied on the metal conductor using known coating method, such as spraying, vacuum coatings dipping and doctoring. The coating may be applied under a nitrogen atmosphere to preclude oxygen inhibition, however this is not necessary. If e.g. a relatively large amount of photoinitiator is used, the cure of the surface of the film is adequate as well. The invention will be further elucidated by the following, non limiting examples.

Examples

Preparation of acrylate functional oligomer A.

Isophorone diisocyanate (IPDI 429 g) was dissolved in laurylacrylate (420 g) with 1 g BHT (butylated hydroxy toluene) 0.7 g phenolthiazine and 2 g dibutyltindilaurate. To this mixture, 224 g of hydroxyethylacrylate (HEA) was slowly added, and the temperature was kept below 35°C. To the acrylate- isocyanate adduct, 2318 g of a hydrocarbon diol was added (Nisso PB 2000) and it was allowed to react. About 105 g of laurylacrylate was added and the final NCO content was determined to be below 0.1 %. This oligomer A had a theoretical molecular weight of 3089 and was a clear solution of 85% oligomer in 15% laurylacrylate.

Preparation of acrylate functional oligomer B

In an analogous way as the preparation of oligomer A, an oligomer was made from 400 g IPDI, 139 g HEA, 2876 g Nisso PB 2000 and 380 g laurylacrylate. The theoretical molecular weight of the oligomer is 5733.

Preparation of acrylate functional oligomer C In an analogous way as the preparation of oligomer A, oligomer C was prepared from 81 g IPDI, 42 g HEA, 430 g of Nisso PB 2000 and 140 g isobornylacrylate. The theoretical molecular weight of the oligomer is 3093.

Examples I-VII

Coatings were prepared with the oligomer in 15% diluent mixtures A-C, with further diluents and a photoinitiator as shown in Table 1. The coatings were applied on an aluminum plate and cured with 2 J/cm² light of a fusion D bulb. For measuring the dissipation factor 150 μm thick films were cast on glass plate and cured with 2 J/cm; the dissipation factor was measured at 24°C and 150°C at 60 Hz with standard equipment with stainless steel electrodes. Results are shown in Table 1 as well.

TABLE 1

10

15

Photomer 3016 is: Bisphenol-A-diacrylate

²⁾ SA 1002 is: tr icyclododecanedimethanol diacrylate

³⁾ SR349 is: ethoxylated bis phenol-A-diacrylate

Examples VIII-XVIII

In an analogous way, further coating compositions were made and tested. The oligomer C was used in these examples. Compositions and results are summarized in

Tables 2 and 3. The coatings were spin coated on an aluminiumpanel, and cured with 1 J/cm, resulting in a 12.5 μm film; furthermore, coatings were cast on a glass plate and cured with 2 J/cm².

TABLE 2

10

15

TABLE 3

10

15

Claims

C L A I M S

1. Metal conductor having a cured coating of about 10 to about 500 μm thickness which coating has a dielectric dissipation factor (60Hz, 24°C) of lower than about 0.05 and is a radiation-cured coating formulated from components comprising: a) an acrylate functional urethane oligomer having a hydrocarbon backbone; b) one or more mono- or polyfunctional diluents, and optionally; and c) a photoinitiator.

2. Metal conductor according to claim 1 wherein the metal is iron, copper, aluminum or silver.

3. Metal conductor according to claim 2 wherein the metal is aluminum, copper or silver.

4. Metal conductor with a cured coating according to any one of claims 1-3 wherein the cured coating has an elongation at 25°C of at least about 50% as a 25 μm thin coating.

5. Metal conductor with a cured coating according to any one of claims 1-4, wherein the cured coating has a dissipation factor at 60 Hz at 150°C of lower than about 0.2.

6. Metal conductor with a cured coating according to any one of claims 1-5, wherein the coating is formulated from components further comprising an adhesion promotor.

7. Metal conductor with a cured coating according to any one of claims 1-6, wherein the cured coating is a radiation-cured coating formulated from components comprising: a) about 20 to about 80 wt.% of an acrylate functional urethane oligomer having a hydrocarbon backbone; b) about 20 to about 80% of one or more mono- or polyacrylate functional monomers and optionally; c) about 1 to about 10 wt.% of one or more light sensitive radical generating compounds; and optionally d) about 0.2-5 wt.% of an adhesion promotor.

8. Metal conductor with a cured coating according to any one of claims 1-7, wherein the cured coating is a coating cured by irradiation with light with a wavelength between about 200-700 μm, and in which component c is present in about 1-10 wt.% with respect to the total coating composition.

9. Metal conductor with a cured coating according to any one of claims 1-8, wherein the cured coating is a coating which is formulated from components comprising a) about 30-65 wt.% of an acrylate functional urethane oligomer having a hydrocarbon backbone. b) about 20-70 wt.% of at least two acrylate functional diluents, one is a monoacrylate and one is a polyacrylate functional compound c) about 1-10 wt.% of one or more light sensitive radical generating compounds d) about 0.2-5 wt.% of acid functional adhesion promotor.

10. Metal conductor with a cured coating according to any one of claims 6-9, wherein the adhesion promotor is an acid functional compound.

11. Metal conductor with a cured coating according to any one of claims 1-10 wherein the coating is formulated from components, one of the components is about 0.2-5 wt.% of a pigment or a dye.

12. Metal conductor with a cured coating according to any one of claims 1-11, wherein the coating has a thickness of about 10-100 μm.

13. Metal conductor with a cured coating according to any one of claims 1-12, wherein the coating has a dielectric constant lower than about 5.

14. Metal conductor with a cured coating according to any one of claims 1-13, wherein the coating has a dielectric dissipation factor lower than about 0.05 (60Hz, 24°C) after hot oil exposure (150°C).

15. Metal conductor with a cured coating according to any one of claims 1-14, wherein the coating is formulated from components consisting essentially of a) about 30-65 wt.% of an acrylate functional urethane oligomer having a hydrocarbon backbone bl) about 10-50 wt.% of a mono-acrylate functional diluent b2 ) about 5-40 wt.% of a poly-acrylate functional diluent c) about 2-7 wt.% of at least one photoinitiator d) about 0.2-4 wt.% of adhesion promotor e) about 0.2-2 wt.% of a pigment.

16. Radiation curable coating composition comprising a) an acrylate functional urethane oligomer having a hydrocarbon backbone b) one or more mono- or polyfunctional diluents and optionally c) one or more light sensitive radical generating compounds which coating when cured with radiation has a dielectric dissipation factor at 60 Hz at 24°C of lower than about 0.05, a dissipation factor at 60 Hz at 150°C of lower than about 0.2, and an elongation at 25° of a 25 μm thin coating of at least about 50%.

17. Radiation curable coating according to claim 16 wherein the urethane oligomer is the reaction product of a hydrocarbon polyol, a polyisocyanate and an hydroxyfunctional endcapping monomer.