Polymer composition with improved stress whitening resistance

Abstract

A polymer composition is provided with improved stress whitening resistance, having at least one thermoplastic polymer material and a dielectric liquid. A process for preparing the polymer composition, a cable having at least one electrically insulating layer obtained from the polymer composition, and a process for preparing the cable are also provided.

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 compound corresponding to formula (I) below:
R.sup.1-A-R.sup.2  (I) in which R.sup.1 and R.sup.2, identical or different, are unsubstituted aryl groups and the element A represents a single bond or an alkylene group, wherein the polypropylene-based thermoplastic polymer material comprises at least one homopolymer or one copolymer of propylene P.sub.1, and at least one homopolymer or one copolymer of α-olefin P.sub.2.

2. The polymer composition according to claim 1, wherein the aryl group comprises from 5 to 20 carbon atoms.

3. The polymer composition according to claim 1, wherein the element A is an alkylene group containing from 1 to 10 carbon atoms.

4. The polymer composition according to claim 3, wherein the alkylene group is a group —(CH.sub.2).sub.n— with 1≤n≤10; a group —(CHR).sub.n— with 1≤n≤5 and R being an alkyl group; a statistical group —(CHR).sub.p—(CH.sub.2).sub.m—, with 1≤p+m≤9, and R being an alkyl group; or a statistical group —(CHR).sub.p1—(CH.sub.2)m′—(CHR′).sub.p2—, with 1≤p.sub.1+m′+p.sub.2≤8, and R and R′ being different alkyl groups.

5. The polymer composition according to claim 1, wherein at least one of said groups R.sup.1 or R.sup.2 of the compound of formula (I) is a phenyl group.

6. The polymer composition according to claim 1, wherein the compound of formula (I) is diphenylethane, diphenylmethane or 1,2,3,4-tetrahydro(1-phenylethyl)naphthalene.

7. The polymer composition according to claim 1, wherein the dielectric liquid comprises at least 50% by mass of at least one compound of formula (I), relative to the total mass of the dielectric liquid.

8. The polymer composition according to claim 1, wherein the ratio of the number of aromatic carbon atoms to the total number of carbon atoms in the dielectric liquid is greater than or equal to 0.6.

9. 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.

10. The polymer composition according to claim 1, wherein the propylene copolymer P.sub.1 is a copolymer of propylene and ethylene.

11. The polymer composition according to claim 10, wherein the propylene homopolymer or copolymer P.sub.1 represents from 40% to 70% by mass of the polypropylene-based thermoplastic polymer material.

12. The polymer composition according to claim 10, wherein the α-olefin homopolymer or copolymer P.sub.2 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 α-olefin, a polyethylene or a mixture thereof.

13. The polymer composition according to claim 10, wherein the α-olefin homopolymer or copolymer P.sub.2 represents from 30% to 60% by mass of the thermoplastic polymer material.

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

15. A cable comprising at least one elongated electrically conducting element, and at least one electrically insulating layer obtained from a polymer composition as defined in claim 1.

16. The cable according to claim 15, wherein the electrically insulating layer is a non-crosslinked layer.

17. A process for manufacturing an electric cable as defined in claim 15 said process comprising 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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) 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

(3) 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.

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

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

(6) The presence of the metal shield 6 and of the outer protective sheath 7 is preferentially, but not essentially, this cable structure being well known per se to those skilled in the art.

EXAMPLES

(7) 1. Polymer Compositions

(8) 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.

(9) Composition C1 is a comparative composition, and composition I1 is in accordance with the invention.

(10) TABLE-US-00001 TABLE 1 Polymer compositions C1 I1 I2 Propylene copolymer 50.00 50.00 50.00 Linear low-density polyethylene 25.00 25.00 50.00 Heterophasic propylene copolymer 25.00 25.00 0 1,2,3,4-Tetrahydro(1- 0 7.70 7.70 phenylethyl)naphthalene Antioxidant 0.3 0.3 0.3

(11) The origin of the compounds in table 1 is as follows: statistical propylene copolymer sold by the company Borealis under the reference Bormed RB 845 MO; linear low-density polyethylene sold by the company ExxonMobil Chemicals under the reference LLDPE LL 1002 YB; heterophasic copolymer sold by the company Basell Polyolefins under the reference Adflex Q 200F; dielectric liquid constituted of 1,2,3,4-tetrahydro(1-phenylethyl)naphthalene sold by the company Dow under the reference Dowtherm RP; and antioxidant sold by the company Ciba under the reference Irganox B 225, which comprises an equimolar mixture of Irgafos 168 and Irganox 1010.

(12) 2. Preparation of the Non-Crosslinked Layers

(13) The compositions collated in table 1 are used as follows.

(14) 130 g of dielectric liquid and 5 g of antioxidant were mixed in a glass container with stirring.

(15) The resulting mixture 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 polymer composition was then extruded using a twin-screw extruder (Berstorff twin screw extruder) at a temperature of about 200° C.

(16) A comparative layer not in accordance with the invention was prepared as described above, but solely using the mixture of polymers and of oxidant.

(17) 3. Characterization of the Non-Crosslinked Layers

(18) The stress whitening resistance was evaluated manually by bending two layers as prepared above from compositions C1 and I1, respectively.

(19) The dielectric breakdown strength of the layers was measured using a device comprising two stainless-steel hemispherical electrodes about 20 mm in diameter (one electrode under tension and the other connected to earth) and a dielectric oil sold by the company Bluestar Silicones under the reference Rhodorsil 604 V 50. By definition, the dielectric breakdown strength is the ratio between the breakdown voltage and the thickness of the insulator. The breakdown voltage was measured at about 24° C., with a humidity of about 50%, using the stepped voltage climb method. The applied voltage was an alternating voltage with a frequency of about 50 Hz and the voltage step-up rate was about 1 kV's to the point of breakdown. 12 measurements were taken for each non-crosslinked layer.

(20) The tangent delta (tan δ) (or loss factor) 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.

(21) The tangent of the loss angle gives an indication regarding the energy dissipated in a dielectric in the form of heat.

(22) The tests were performed on layers with a thickness close to 0.5 mm at 90° C., at a frequency of 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.

(23) 4. Results

(24) The layer obtained from composition I1 showed no whitening, whereas the layer obtained from the comparative composition C1 was revealed to be sparingly resistant since a white mark at the bend appeared immediately when the manual stress was applied.

(25) The dielectric breakdown strength and loss factor results are presented in table 2 below:

(26) TABLE-US-00002 TABLE 2 Dielectric breakdown strength (kV/mm) Tangent delta at 90° C. C1 129.25 7.5 × 10.sup.−5 I1 127.11 8.2 × 10.sup.−5

(27) Consequently, the polymer compositions according to the invention have better properties in terms of stress whitening resistance while at the same time ensuring good dielectric properties.