Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
  • H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
  • H01B3/30—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
  • H01B3/40—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes epoxy resins

Definitions

• the invention relates to an impregnation medium for impregnation of a porous fibrous matrix to achieve an electric insulation material, where the impregnation medium comprises filler particles consisting Of AI 2 O 3 dispersed in a thermosetting resin.
• porous fibrous matrix refers to a fibrous matrix that has such permeability that the impregnation medium is capable of penetrating into the cavities between the fibres of the matrix.
• Mica tape usually comprises two layers, one is called mica paper, which is mica flakes enriched with binder resin, such as epoxy, and the other layer is a support layer.
• the support layer is usually made of a porous structure, such as glass fibres or polymeric fibres.
• Epoxy is a thermosetting epoxide polymer that cures (polymerize and crosslinks) when mixed with a curing agent and a catalyst.
• Epoxies find significant use in many applications including: paints and coatings, adhesives, industrial tooling and compos- ites, electrical systems and electronics because of their good thermal, mechanical and electrical properties, low cost, ease of processing and good chemical resistance.
• most epoxy resins are brittle. Therefore many kinds of micro-sized fillers have been added into epoxy resin to form composites with better combination of mechanical, thermal and electrical properties. Toughening epoxy by introduc- tion of soft particles, such as rubber, has proven to be an effective way and widely used. However, it reduces the stiffness of the epoxy resin.
• Rigid particles have also been used to improve the stiffness of epoxy resin.
• the limitation of such filler particles is that they cause a decrease in ductility and opacity.
• a large window of opportunity has open to overcome these limitations through the use of filler particles with sizes in the nanometre range.
• the interface between the filler particles and the matrix in a polymer nanocomposite constitutes a much greater area within the bulk material, and hence influences the composite's properties to a much greater extent, even at rather low filler loading.
• WO20061 18536 discloses an electric insulation material formed by a porous fibrous matrix that is impregnated with an impregnation medium.
• One of the aims of the invention described in the above mentioned document is to provide an electric insulation material with high thermal conductivity, which is demanded for the specific use described in the document, which is as insulation material in electrical bushings.
• the above mentioned document describes the use of amounts of particle fillers in the impregnation medium, up to 25 vol-%. Nanocomposites with such high concentration of filler particles are usually brittle, although the thermal conductivity usually is very high.
• WO20061 18536 also indicates that a plurality of materials for the filler particles can be used, of which most is not suitable for the purpose of the present invention.
• an impregnation medium for impregnation of a porous fibrous matrix to achieve an electric insulation material, being improved with respect to impregnation mediums already known, for instance by providing an insulation material with improved ductility and increased stiffness without losing strength, thermal conductivity and electrical resistivity, e.g. for the purpose of electrical machine insulation.
• the object of the present invention is to provide an impregnation medium for impregnation of a porous fibrous matrix to achieve an electric insulation material being improved with respect to such impregnation media already known by at least partially addressing said need.
• This object is according to the invention obtained by providing an impregnation medium and an electric insulation material of the type defined in the introduction, in which the impregnation medium comprises filler particles consisting of AI 2 O 3 dispersed in a thermosetting resin, and that the filler particles constitute between 0.1 -10 vol-% of the impregnation medium.
• AI 2 O 3 particles have a surface which to a very high extent is terminated by OH groups. These OH groups can be chemically modified introducing functionalizing groups which bind to a matrix in which the particles are dispersed.
• OH groups can be chemically modified introducing functionalizing groups which bind to a matrix in which the particles are dispersed.
• the toughening is due to higher ductility in the impregnation medium because the filler particles provide for derealization of plastic deformation. Also, the filler particles cause crack deflection in the impregnation medium, which results in difficulties for cracks to travel through the material and cause failure. It has surprisingly been seen that an optimization of the concentration of filler particles within the range described above contributes to give an electric insulation material impregnated with the impregnation medium of the invention a better thermal conductivity as well as mechanical properties well- suited for the use of the electric insulation material in e.g. electrical machines, while the good electrical insulation from the thermosetting resin is maintained.
• the filler particles constitutes between 2-8 vol-%, preferably 3-6 vol-% of the impregnation medium.
• the mechanical properties as well as the thermal conductivity and the electrical resistivity has an optimum for the desired applications when the filler particle concentration is between 2-8 vol-%, preferably 3-6 vol-%
• thermosetting resin comprises an epoxy resin.
• Epoxy resin has the high electrical resistance demanded by the invention. It also has a viscosity, which is low enough for enabling homogenous impregnation of the porous fibrous matrix.
• the epoxy can, after impregnation of the porous fibrous matrix, be the subject of curing to form the electric insulation material of the invention.
• the average size of the filler particles are in the size range of 1 -100 nm, preferably 20-60 nm, most preferred 30-50 nm.
• the filler particles In order to make the impregnation medium to penetrate the porous fibrous matrix completely during the impregnation the filler particles have to be smaller than 100 nm, i.e. substantially smaller than the pores or cavities in the porous fibrous matrix.
• nanoparticles In comparison with micro particles, i.e. particles with sizes in the micrometer region, nanoparticles have a tendency to better remain in dispersion within the impregnation medium than the micro particles, without causing sedimentation and wear of the matrix.
• Particles in the size range of 20-60 nm, preferably 30-50 nm, are especially well-suited to fulfil the requirements during the impregnation of the porous fibrous matrix.
• the filler parti- cles have a narrow size distribution.
• narrow size distribution refers to a size distribution in which more than 90 % of the particles have a particle size in the range of 0.2-2 times the mean particle size.
• a narrow size distribution of the filler particles allows for a homogenous impregnation of the porous fibrous matrix with the impregnation medium comprising the filler particles.
• the sizes of the particles differs a lot, i.e. have a broad size distribution; larger particles can become concentrated in the surface region of the bulk of the porous fibrous matrix, whereas the smaller particles are the only ones reaching all the bulk. This can cause anisotropic behaviour of the mechanical, thermal and electrical properties.
• the filler particles are substantially uniformly dispersed in the impregnation medium.
• a uniform dispersion of the filler particles in the impregnation medium results in isotropic properties like thermal conductivity, mechanical strength, ductility and electrical resistance. This is desired in most electric insulation material, e.g. for use as electric insulation of electrical machines.
• the surfaces of the filler particles are coated with a surface modifier configured to inhibit agglomeration of the filler particles, e.g. by making the surfaces of the filler particles more hydrophobic and/or configured to form covalent bonds with the matrix during cross linking of the thermosetting resin.
• the surface modifier will inhibit agglomeration of the filler particles in the impregnation medium. Agglomeration can cause formation of aggregates several micrometers in size. This can decrease the mechanical strength of the impregnation medium and cause crack formation, since the aggregates can work as crack initiation sites. Also, by introducing surface modifiers containing functional groups which can form covalent bonds to the thermosetting resin during crosslinking the bonds between the filler particles and the thermosetting resin in the impregnation medium can be strengthened and can also promote interfacial debonding which would also contribute to the toughening of the material. According to another embodiment of the invention the surface modifier is an organic compound, a silane or a compound which can be described as a combination of an organic compound and a silane, such as an aminosilane.
• the compounds mentioned above are especially useful as surface modifiers and fulfils the demands mentioned above of the present invention.
• the invention also relates to an electric insulation material comprising a porous fibrous matrix impregnated with an impregnation medium, where the impregnation medium is the impregnation medium according to the invention.
• the porous fibrous matrix comprises mica or mica and at least one in the group consisting of: cellulose fibres, glass fibres and polymeric fibres.
• porous fibrous matrix is in the form of paper, pressboard, laminate, tape, weave or sheets.
• the invention also relates to the use of an electric insulation material according to the invention in a machine insulating system.
• Fig 1 shows representative stress-strain graphs for neat polymer, 3.1 vol-% NT-AI 2 O 3 /epoxy nanocomposite and 3.1 vol-% APTES-AI 2 O 3 /epoxy nanocomposite,
• Fig 2 shows graphs for strain-to-break versus particle concentration for neat polymer, NT-AI 2 O 3 /epoxy nanocomposites and APTES-AI 2 O 3 /epoxy nanocompo- sites,
• Fig 3 shows the fatigue crack growth rate versus applied stress intensity range curve for neat polymer, 3.1 vol- % NT-AI 2 O 3 /epoxy nanocomposite and 3.1 vol-% APTES-AI 2 O 3 /epoxy nanocomposite,
• Fig 4 shows graphs over the loss tangent data at 50 Hz for neat polymer, NT-AI 2 O 3 /epoxy nanocomposites and APTES-AI 2 O 3 /epoxy nanocomposites, at 25°C, 70 0 C, 120 0 C and 155°C,
• Fig 5 shows graphs over the relative change in electric breakdown strength (%) for the NT-AI 2 O 3 /epoxy nanocomposites and APTES-AI 2 O 3 /epoxy nanocomposites compared to the neat polymer as a function of nanoparticle concentration (vol-%), and
• Fig 6 shows graphs over the DC-resistivity at different electric fields (1 kV/mm and 3kV/mm) and temperatures (25°C and 70 0 C) for (a) NT-AI 2 O 3 nanocomposites, and (b) APTES-AI 2 O 3 nanocomposites.
• neat polymer ⁇ epoxy resin which not was added with filler particles, denoted as neat polymer.
• the neat polymer is not a part of the invention but is included in this description for comparison.
• thermosetting matrix polymer including (i) Araldite F - bisphenol A liquid epoxy resin ; (ii) HY905 - modified dicarbox- ylic anhydride hardener; (iii) DY062 - amine catalyst.
• the mixing ratio of epoxy resin to hardener was 1 : 1 by weight.
• NanoTek® Aluminium Oxide (AI 2 O 3 ) was purchased from Nanophase Technologies Corporation, with an average particle size of 45 nm.
• APTES 3-aminopropyltriethoxysilane
• the aluminium oxide nanoparticle surface modification procedure was: (i) addition of 1 g AI 2 O 3 nanoparticles to 50 ml of 95% ethanol; (ii) sonication of the mixture for 5 minutes using a wand; (iii) addition of 1 .5 g APTES and sonication of the mixture for another 10 minutes; (iv) refluxing the mixture for 3 days at 80 0 C in an oil bath; (v) centrifuging and washing the nanoparticles with ethanol and hexane to remove the by-products and extra silane, followed by drying of the nanoparticles in a vacuum oven overnight at room temperature.
• NT-AI 2 O 3 and APTES-AI 2 O 3 /epoxy nanocomposites with 1 .6, 3.1 , 4.6 and 6.1 vol-% nanoparticle concentrations were prepared.
• the NT-AI 2 O 3 nanoparticles were dried in vacuum at 190 0 C overnight. Three steps were used to prepare the nanocomposites.
• the nanoparticles were dispersed in the liquid epoxy resin to prepare a "masterbatch". This was the key step for the whole procedure.
• a well-dispersed masterbatch was achieved by shear mixing using a Hauschild SpeedMixer®. This machine provides high shear stress gradients to disrupt particle aggregates.
• the dispersion was improved by adding 1/8" aluminium oxide balls during mixing. The balls were removed before curing. Second, the hardener and catalyst were added as per composition requirements into the masterbatch; Third, the mixture was mixed, degassed at room temperature, cured at 80 0 C for 6 h, and then post-cured at 135°C for 10 h. Neat polymer samples were also made to compare with the nanocomposites.
• Fracture toughness of the materials was measured on an lnstron 4204 machine based on ASTM Standard E1820 and D5045 with compact tension fracture specimen at a loading speed of 1 mm/min. Then the fracture energy was calculated based on
• TDCB Tapered-double-cantilever-beam
• DSC Differential Scanning Calorimetry
• Tg glass transition temperature
• TA® Instruments was used to check the glass transition temperature (Tg) of the nanocomposites and neat polymer.
• the samples were heated from 25°C to 160 0 C with a constant heating rate of 10°C/min. Data obtained from the second heating run were used, and at least 3 samples were tested for each material. The results are presented in Table 1 .
• the dielectric response, permittivity and loss tangent of neat polymer, NT-AI 2 O 3 /epoxy and APTES-AI 2 O 3 /epoxy nanocompo- sites were performed on a Heraeus V ⁇ tsch machine with an IDA data collection system.
• T 9 One indication of proper curing in epoxies is the glass transition temperature (T 9 ).
• T 9 One indication of proper curing in epoxies is the glass transition temperature (T 9 ).
• T 9 the glass transition temperature
• the glass transition temperature decreased at high filler concentration (1 1 0 C de- crease at 6.1 vol-%), which indicates a change in curing reaction due to the surface modification of AI 2 O 3 nanoparticles. This is likely due to a reaction of the APTES with the epoxy system.
• Figure 1 shows the representative stress-strain curves for the 3.1 vol-% NT-AI 2 O 3 /epoxy nanocomposites and the 3.1 vol-% APTES-AI 2 O 3 /epoxy nanocomposite obtained by tensile tests using ASTM D638-03.
• the mean strain-to-break values are listed in Table 1 . It is clear that the strain-to-break, which indicates the ductility of the material, for the 3.1 vol-% APTES- AI 2 O 3 /epoxy nanocomposite increased significantly (39%) compared to the neat polymer.
• the strain-to-break of the 3.1 vol-% NT-AI 2 O 3 /epoxy nanocomposite did not change compared to the neat polymer.
• the Young's modulus increased for all the nanocomposites compared to the neat polymer.
• strain-to-break as a function of nanoparticle concentration for both of the NT-AI 2 O 3 /epoxy and APTES-AI 2 O 3 /epoxy nanocomposites is presented in Figure 2, and the corresponding data are listed in Table 1 .
• the APTES-AI 2 O 3 /epoxy nanocomposites achieved better properties. All of the strain-to-break values for the APTES- AI 2 O 3 /epoxy nanocomposites are higher than both neat polymer and NT-AI 2 O 3 /epoxy nanocomposites at the same particle loading, except for 6.1 vol-%. An 1 1 °C decrease in glass transition temperature, which indicates incomplete curing, might explain the decreased ductility at 6.1 vol-%.
• the fatigue-crack propagation in neat polymer, 3.1 vol-% NT- AI 2 O 3 /epoxy and 3.1 vol-% APTES-AI 2 O 3 /epoxy nanocomposites were measured.
• the values of the Paris exponent n dropped from 10 for neat polymer to 7 for 3.1 vol-% NT-AI 2 O 3 /epoxy nanocomposites, and further to 5 for 3.1 vol-% APTES- AI 2 O 3 /epoxy nanocomposites (Fig 3).
• the results indicate the retardation of fatigue crack growth rate by adding the nanoparti- cles into the polymer system, especially for the surface modified nanoparticles.
• the loss tangent at 50 Hz increased from 0.18 for neat polymer to 0.39 for 6.1 vol-% APTES-AI 2 O 3 /epoxy nanocomposite and 0.57 for 6.1 vol-% NT- AI 2 O 3 /epoxy nanocomposite.
• All of the APTES-AI 2 O 3 /epoxy nanocomposites had lower loss tangent values compare to the NT-AI 2 O 3 /epoxy nanocomposites at the same filler particle concentration.
• the relative permittivity increased slightly from 7.01 for neat polymer to 8.20 for 6.1 vol-% APTES- AI 2 O 3 /epoxy nanocomposite and 8.38 for 6.1 vol-% NT- AI 2 O 3 /epoxy nanocomposites.

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