LAMINATED STRUCTURE WITH LOW DIELECTRIC LOSSES

Abstract: The present invention relates to a laminated structure with low dielectric losses, which structure is characterized in that it comprises two or more layers of cellulose fibre paper separated by spacing layers comprising fibres which by adhesion to one another by means of a suitable surface layer on said fibres of a material which melts at lower temperature than the fibres, form a lattice structure with a fibre content of less than 30 percent by volume.

Description: Laminated structure with low dielectric losses Coarse cables are today insulated with oil impregnated, so- called cable paper, where the cable paper i.a. serves as a carrier of the oil insulation. Cable paper is found partly in the form of cellulose fibre paper, partly in combinations of cellulose fibre, polyolefin fibre and/or film of polyolefins. The purpose of the polyolefin additives, which have lower dielectric losses than cellulose, is to lower the total dielectric losses of the construction by "diluting". SE-B-7705609-1 describes a laminated structure in which a PP-non-woven material is laminated together with paper by pressing at temperatures at which the PP-fibres partly melt to cause bonding on one hand between the PP-fibres and on the other hand between the PP-fibres and the paper fibres. If the temperature is too low no bonding is achieved. If the temperature is too high the fibres melt and the non-woven material is destroyed. The temperature range which must be applied to achieve bonding but not melting of the PP-fibres is very limited and causes problems in industrial production. The present invention relates to a laminated structure compri¬ sing two or more layers of cellulose fibre paper separated by spacing layers (non-woven) built upp of fibres which by -» suitable adhesion to one another form a lattice structure with a fibre content of less than 30 percent by volume. By means of said adhesion one can achieve the same stability, i.e. mechanical strength, thickness tolerances etc. previously achieved with non-bonded fibres with a fibre content of over 40 percent by volume. The laminates produced have dielectric losses which closely follow the mixing rule, i.e. d* ~ t d + (1-t ) d I <--* p p P n wherein d represents the dielectric losses, t represents the total thickness of the layer types used and indexes 1, p and n represent laminate, paper and non-woven, respectively. Since the dielectric losses of the non-woven material are much lower than those of the paper the dielectic losses are reduced approximately proportionally to the reduced amount of paper used. This means, for example, that with a thickness of the layers: paper: 40 μm, non-woven: 100 μm, paper 40 μm, the laminate has dielectric losses of approx. 0.5 of the dielec¬ tric losses of a cable paper of the same thickness produced entirely of the same paper. The lattice fibre structure is built up of fibres with a sheath of, or in another way applied suitable polymer which melts at temperatures lower than the fibres. The sheath can either be continuous and completely surround the fibre core or cover it partially. This sheath can be produced by fibre spinning through a double extruder. By heating a "non-woven"-structure of this fibre to tempera¬ tures at which the low melting sheath sticks and simultaneous¬ ly compressing the non-woven structure and maintaining it pressed together until the temperature has lowered so that the sheathing material has stiffened, the sheathing material at those points where two fibres are pressed against each other serve as joining points whereby a lattice structure with good dimension stability even at relatively low fibre contents is formed.. An example of this type of fibre is ES-fibre from Danaklon A/S, which on heating to 120° C, compressing and cooling to 80° C forms the required structure, cf. the drawing. The lattice structure can alternatively be obtained by providing the joints between the fibres which are not melted at the heating, compression and cooling, by means of the addition of a suitable binding agent which melts at a lower temperature than the fibres. The additive may be supplied in the form of fibres or film of suitable polymer, which melts at. a lower temperature than the fibres forming the spacing layer, whereby on heating to the melting point of the additive this melts and creates the joints. Examples of suitable material for the construction are, for example, different types of polyolefins, polypropylene-fibres (PP-fibre) with polyethylene (PE) or a suitable olefin copolymer with lower melting point than polypropylene as a "binding agent". The invention is illustrated by means of the following com¬ parative examples which, however, are not meant to limit the scope of the invention in any way but are only intended to illustrate it. Example 1 CABLE PAPER LAMINATE BUILT UP OF CABLE PAPER AND NON-WOVEN LAYERS OF PP-FIBRE WITH A SHEATH OF PE Laminates were produced by compressing normal cable paper and non-woven of polypropylene fibre (PP-fibre) with a sheath of polyethylene (PE). Before compression the non-woven material was washed for re¬ moval of any optional substances added during its manufacture. Laminates were produced having different thicknesses: 120-400 μm by combining cable paper and non-woven layers having different thicknesses. All the laminates consisted of three layers: cable paper / non-woven / cable paper. The outer layer of the cable paper was chosen to provide a surface with the same properties as those of normal cable paper. The thickness of the paper layers and the non-woven layers was chosen in relation to the thickness of commercially available cable paper. The laminates were produced by pressing at temperatures above the melting point of the surface layer but below that of the PP-fibres, for example between 130-150°C and at a pressure of between 0.1-5 MPa. The dielectric losses of the laminate followed the same formula as stated above. Example 2 CABLE PAPER LAMINATE BUILT UP OF CABLE PAPER AND LAYERS OF NON-WOVEN OF PP-FIBRES WITH PE-FILM AS BINDING AGENT Laminates were produced by pressing at the same temperature and pressure as in example 1 but with a non-woven layer comprising PP-fibre only. The bonding, n one hand between the PP-fibres and on the other hand between the PP-fibres and the paper fibres, was achieved by placing a thin PE-film on one hand between PP-fibre layers and on the other hand between the outer PP-fibre layers and the paper layers and by melting at the compression. The construction of the laminates was cable paper / thin P PEE--ffiillmm // ((PPPP--nnoonn--wwoo 1 ven/thin PE-film/PP-non-woven) / thin m PE-film/-cable paper. m represents the number of layers laminated in between the cable paper layers. Surfaces of normal cable paper were used to give the laminate the same surface properties as those of normal cable paper. For the thinner laminates m=l was chosen, for the thicker laminates 1< m <10. Laminates with thicknesses between 100 and 400 μm were produced. The thickness of the PE-films was - 20 μm and did not add to the thickness of the laminate because they melted under the compression. The dielectric losses of the laminate followed the same formula as previously stated. Example 3 CABLE PAPER LAMINATE BUILT UP OF CABLE PAPER AND NON-WOVEN LAYERS OF PP-FIBRES AND PE-FIBRES Laminates were produced in the same way as in example 2 but the PE-film was replaced by PE-fibres which either were placed between the different layers in the laminate or mixed in among the PP-fibres in the non-woven material. The dielectric losses of the laminate followed the same approximate formula as previously stated.

Claims: CLAIMS 1. A laminated structure with low dielectric losses, c h a r a c t e r i z e d in that it comprises two or more layers of cellulose fibre paper separated by spacing layers built up of fibres which by adhesion to one another by means of a suitable surface layer on the fibres of a material which melts at lower temperature than the fibres, forms a lattice structure with a fibre content of less than 30 percent by volume. 2. A laminated structure according to claim 1, c h a r a c¬ t e r i z e d in that the surface layer comprises a sheath surrounding the fibres which form the spacing layer. 3. A laminated structure according to claim 1, c a r a c¬ t e r i z e d in that the surface layer is produced by means of a film of polymeric material placed between layers of fibres of which the spacing layer(s) is/are built up whereafter the film is melted by heating to the melting point of the film material. 4. A laminated structure according to claim 1, c h a r a c ¬ t e r i z e d in that the surface layer on the fibres which form the spacing layer is provided by means of fibres of polymeric material mixed in among the fibres of the spacing layer whereafter the mixed in fibres are melted by heating to their melting point. 5. A laminated structure according to claim 1, c h a r a c ¬ t e r i z e d in that the surface layer on the fibres forming the spacing layer is provided by means of layers of fibres of polymeric material with lower melting point than the fibres of the spacing layer, which are alternately layered with layers of spacing fibres, whereafter the lower melting fibres are melted by heating to their melting point. 6. A laminated structure according to any of the previous claims, c h a r a c t e r i z e d in that the spacing layer is comprised of polypropylene fibres. 7. A laminated structure according to any of the previous claims, c h a r a c t e r i z ed in that the material forming surface layer on the fibres of the spacing layer consists of polyethylene. 8. A laminated structure according to any of claims 1-6, c h a r a c t e r i z e d in that the material forming the surface layer on the fibres of the spacing layer consists of an olefin copolymer with lower melting point than poly¬ propylene.

Inventors: JANSSON JAN-FREDRIK, JANSSON, JAN-FREDRIK
Applicants: JANSSON JAN FREDRIK, JANSSON, JAN-FREDRIK
Application year: 1989
Main classification: H01B3
IPC codes: B32B5, B32B29, D04H13, H01B3, H01B17