POLYMER COMPOSITION COMPRISING A DIELECTRIC LIQUID OF IMPROVED POLARITY

Abstract

A polymer composition has at least one polypropylene-based thermoplastic polymer material and a dielectric liquid. The dielectric liquid has at least one mineral oil and at least one polar compound of the benzophenone type, acetophenone type or a derivative thereof.

Claims

1. A polymer composition comprising: at least one polypropylene-based thermoplastic polymer material and a dielectric liquid, wherein the dielectric liquid comprises at least one mineral oil and at least one polar compound of the benzophenone type, acetophenone type or a derivative thereof.

2. The polymer composition according to claim 1, wherein the dielectric liquid comprises at least 70% by mass of mineral oil, relative to the total mass of the dielectric liquid.

3. The polymer composition according to claim 1, wherein the mineral oil is chosen from naphthenic oils and paraffinic oils.

4. The polymer composition according to claim 1, wherein the polar compound of the benzophenone type, acetophenone type or a derivative thereof represents at least 2.5% by mass relative to the total mass of the dielectric liquid.

5. The polymer composition according to claim 1, wherein the polar compound of the benzophenone type, acetophenone type or a derivative thereof corresponds to formula (I) below: ##STR00002## in which R1 and R2, identical or different, are aryl, alkyl or alkylene-aryl groups, the groups R1 and R2 possibly being linked together via the element A representing a single bond or a group —(CH2)n- with n equal to 1 or 2.

6. The polymer composition according to claim 5, wherein the compound of formula (I) is benzophenone, dibenzosuberone, fluorenone or anthrone.

7. The polymer composition according to any claim 1, wherein the ratio of the number of aromatic carbon atoms to the total number of carbon atoms in the dielectric liquid is less than 0.3.

8. The polymer composition according to claim 1, wherein the dielectric liquid represents from 1% to 20% by mass relative to the total mass of the polymer composition.

9. The polymer composition according to claim 1, wherein the polypropylene-based thermoplastic polymer material comprises at least one homopolymer or one copolymer of propylene P1, and at least one homopolymer or one copolymer of □-olefin P2.

10. The polymer composition according to claim 9, wherein the propylene copolymer P1 is a copolymer of propylene and ethylene.

11. The polymer composition according to claim 9, wherein the propylene homopolymer or copolymer P1 represents from 40% to 70% by mass of the polypropylene-based thermoplastic polymer material.

12. The polymer composition according to claim 9, wherein the □-olefin homopolymer or copolymer P2 is a heterophasic copolymer comprising a thermoplastic phase of propylene type and a thermoplastic elastomer phase of the type copolymer of ethylene and of an □-olefin, a polyethylene or a mixture thereof.

13. The polymer composition according to claim 9, wherein the □-olefin homopolymer or copolymer P2 represents from 30% to 60% by mass of the polypropylene-based thermoplastic polymer material.

14. A process for preparing the polymer composition as defined in claim 1, wherein said process comprises at least one step i) of mixing a polypropylene-based thermoplastic polymer material with said dielectric liquid.

15. The process according to claim 14, wherein the mixing is performed according to the following substeps: i-a) mixing a mineral oil and at least one polar compound of the benzophenone type, acetophenone type or a derivative thereof, so as to form said dielectric liquid, and i-b) mixing said polypropylene-based thermoplastic polymer material with the dielectric liquid as obtained in the preceding substep i-a).

16. The process for manufacturing an electric cable as defined in claim 14, wherein said process comprises at least one step 1) of extruding the polymer composition around an elongated electrically conducting element, to obtain an electrically insulating layer surrounding said elongated electrically conducting element.

17. The process according to claim 16, wherein the polymer composition is prepared according to a process comprising at least one step i) of mixing a polypropylene-based thermoplastic polymer material with said dielectric liquid.

18. The process according to claim 16, wherein the mixing is performed according to the following substeps: i-a) mixing a mineral oil and at least one polar compound of the benzophenone type, acetophenone type or a derivative thereof, so as to form said dielectric liquid, and i-b) mixing said polypropylene-based thermoplastic polymer material with the dielectric liquid as obtained in the preceding substep i-a).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0149] FIG. 1 is a schematic view of an electric cable according to a preferred embodiment in accordance with the invention.

[0150] FIG. 2 shows the curves of the tangent delta (tan(δ)) at 90° C. as a function of the frequency (Hz), for a layer according to the invention and for a comparative layer.

[0151] For the sake of clarity, only the elements essential to the understanding of the invention have been represented schematically, and are not to scale.

DETAILED DESCRIPTION

[0152] The medium-voltage or high-voltage power cable 1, illustrated in FIG. 1, comprises a central elongated electrically conducting element 2, especially made of copper or aluminium. The power cable 1 also comprises several layers arranged successively and coaxially around this central elongated electrically conducting element 2, namely: a first semi-conducting layer 3 known as the “internal semi-conducting layer”, an electrically insulating layer 4, a second semi-conducting layer 5 known as the “external semi-conducting layer”, an earthing and/or protective metal shield 6, and a protective outer sheath 7.

[0153] The electrically insulating layer 4 is a non-crosslinked extruded layer, obtained from the polymer composition according to the invention.

[0154] The semi-conducting layers 3 and 5 are thermoplastic (i.e. non-crosslinked) extruded layers.

[0155] The presence of the metal shield 6 and of the outer protective sheath 7 is preferential, but not essential, this cable structure being well known per se to those skilled in the art.

EXAMPLES

1. Polymer Compositions

[0156] Table 1 below collates polymer compositions in which the amounts of the compounds are expressed as weight percentages relative to the total weight of the polymer composition.

[0157] Compositions C1 and C2 are comparative compositions, and compositions I1 and I2 are in accordance with the invention.

TABLE-US-00001 TABLE 1 Polymer compositions C1 C2 I1 I2 Propylene copolymer 100 100 100 100 Linear low-density polyethylene 50 50 50 0 Heterophasic propylene copolymer 50 50 50 100 Mineral oil 0 12 12 12 Benzophenone 0 0 0.7 0.7 Antioxidant 0.7 0.7 0.7 0.7

[0158] The origin of the compounds in table 1 is as follows: [0159] statistical propylene copolymer sold by the company Borealis under the reference Bormed RB 845 MO; [0160] linear low-density polyethylene sold by the company ExxonMobil Chemicals under the reference LLDPE LL 1002 YB; [0161] heterophasic copolymer sold by the company Basell Polyolefins under the reference Adflex Q 200F; [0162] mineral oil sold by the company Nynas under the reference Nytex 810, the oil comprises 10% of aromatic carbon atoms, 43% of naphthenic carbon atoms and 47% of paraffinic carbon atoms; [0163] antioxidant sold by the company Ciba under the reference Irganox B 225, which comprises an equimolar mixture of Irgafos 168 and Irganox 1010; and [0164] benzophenone sold by the company Sigma-Aldrich under the reference B9300.

2. Preparation of the Non-Crosslinked Layers

[0165] The compositions collated in table 1 are used as follows.

[0166] 102 ml of mineral oil, 6 g of antioxidant and 6 g of benzophenone were mixed with stirring at about 75° C., so as to form a dielectric liquid.

[0167] The dielectric liquid was subsequently mixed with 850 g of propylene copolymer, 425 g of linear low-density polyethylene and 425 g of heterophasic copolymer in a container, and the resulting mixture was then homogenized using a twin-screw extruder (Berstorff twin screw extruder) at a temperature of about 145 to 180° C., and then melted at about 200° C. (screw speed: 80 rpm).

[0168] The molten homogenized mixture was then formed into granules.

[0169] Cables were manufactured with a laboratory extruder and subjected to electrical characterizations. Each of the cables comprised: [0170] an electrically conducting element with a cross section of 1.4 mm, [0171] a first semi-conducting layer surrounding said electrically conducting element, [0172] an electrically insulating layer obtained from the polymer composition of the invention or from a comparative polymer composition surrounding said first semi-conducting layer, and [0173] a second semi-conducting layer surrounding said electrically insulating layer.

[0174] The cables had a total outside diameter of about 6.1 mm and a total length of about 3.64 m. They were stripped of the second semi-conducting layer over a thickness of 150 μm and a length of 87 cm.

[0175] The electrically insulating layer had a thickness of 1.5 mm (internal and external radius of 1.4 mm and 2.9 mm, respectively).

[0176] The semi-conducting layers were thermoplastic layers having the following composition: 48% by mass of statistical propylene copolymer (Bormed RB 845 MO); 20% by mass of a heterophasic copolymer (Adflex Q 200F); 25% by mass of carbon black (Vulcan XC 500); 6.5% by mass of a mineral oil (Nytex 810) and 0.5% by mass of antioxidant.

3. Characterization of the Non-Crosslinked Layers

[0177] The dielectric breakdown strength of the layers was measured by applying on the electrically conducting element of the cable a 50 Hz AC voltage ramp of 2 kV/s. The second semi-conducting layer was connected to the earth. The two ends were immersed in distilled water.

[0178] Statistical analysis (Weibull model) was performed on 10 experimental values of dielectric breakdown strength obtained.

[0179] The tangent delta (tanδ) (or loss factor) at 25° C., 90° C. and 130° C. of the layers as prepared above was measured by dielectric spectroscopy using a machine sold under the trade name Alpha-A by the company Novocontrol Technologies.

[0180] The tangent of the loss angle gives an indication regarding the energy dissipated in a dielectric in the form of heat.

[0181] The tests were performed on samples with a thickness close to 0.5 mm, over a frequency range from 40 to 60 Hz with a 500 V voltage adapted according to the thickness of the test sample, so as to apply an electric field of 1 kV/mm. A temperature of 25° C., 90° C. or 130° C. was applied during the various tests.

[0182] Stress whitening resistance tests were also performed by subjecting samples with a thickness close to 1 mm to a mechanical test such as a test of curvature of the sample at room temperature with a radius of curvature of 5 mm.

4. Results

[0183] The tangent delta results obtained are collated in FIGS. 2A (layer obtained from composition C1) and 2B (layer obtained from composition I1).

[0184] FIG. 2 shows the tangent delta (tan(δ)) (or tangent of the loss angle) at 25° C. (curves with filled diamonds), at 90° C. (curves with filled squares) and at 130° C. (curves with filled triangles) as a function of the frequency (Hz), for the comparative non-crosslinked layer (FIG. 2A) and for the non-crosslinked layer according to the invention (FIG. 2B).

[0185] It is clearly seen that the layer obtained from composition I1 according to the invention has dielectric losses that are very slightly higher than those obtained with the comparative composition C1, especially at 130° C., but which remain very acceptable.

[0186] The results at 50 Hz are collated in table 2 below:

TABLE-US-00002 TABLE 2 Compositions C1 C2 I1 Tangent delta at 25° C. 1.2 × 10.sup.−4  9.4 × 10.sup.−5 1.2 × 10.sup.−4 Tangent delta at 90° C. 7.5 × 10.sup.−5 5.41 × 10.sup.−5 8.2 × 10.sup.−5 Laboratory cable 103 112 137 Dielectric breakdown strength (kV/mm)

[0187] Table 2 shows that the polymer compositions according to the invention have better dielectric properties (i.e. better electrical insulation). In particular, the dielectric breakdown strength of the layer I1 of the invention is improved, relative to that of the comparative compositions C1 and C2.

[0188] The layer obtained from composition I1 moreover showed no stress whitening, whereas the layer obtained from the comparative composition C1 was revealed to be sparingly resistant since a white mark appeared immediately at the bend in to the manual stress applied.