Polymer composition for wire and cable applications with advantageous thermomechanical behaviour and electrical properties

Abstract

The invention provides a polymer composition comprising a) a low density polyethylene (LDPE); and b) a conjugated aromatic polymer. The invention also relates to cables comprising said polymer composition and the use of the polymer composition in the manufacture of an insulation layer of cable.

Claims

1. An insulation layer comprising a polymer composition, the polymer composition comprising a blend of: a) a low density polyethylene (LDPE); and b) a conjugated aromatic polymer; wherein the LDPE is present in an amount of 90 to 99.99 wt % relative to the total weight of the polymer composition as a whole; wherein the conjugated aromatic polymer b) is present in an amount of from 0.01 to 10 wt %, relative to the total weight of the polymer composition as a whole; wherein the insulation layer does not comprise a carbon black, wherein the polymer composition is prepared by melt-mixing above the melting point of at least the LPDE and the melt-mixed polymer composition is heat treated by remelting at a temperature above the melting temperature of the conjugated aromatic polymer of from 220° C. to 350° C., wherein the conductivity of the polymer composition is 1.0 E-12 S/cm or less when measured according to DC conductivity method as described under “Determination Methods”, wherein DC conductivity when measured according to DC conductivity method as described under “Determination Methods” of the polymer composition is at least 10% lower than an otherwise identical polymer composition not comprising the conjugated aromatic polymer, and wherein the conjugated aromatic polymer is a polythiophene having the general formula (I): ##STR00003## wherein R.sub.1 is hydrogen and R.sub.2 is hexyl; and wherein n is an integer in the range 2 to 1000.

2. The insulation layer of claim 1, wherein the conjugated aromatic polymer b) is present in an amount of 0.02 to 5 wt %, relative to the total weight of the polymer composition as a whole.

3. The insulation layer of claim 1, wherein the LDPE is selected from a LDPE homopolymer and a LDPE copolymer of ethylene with one or more comonomer(s), wherein the LDPE homopolymer and LDPE copolymer of ethylene with one or more comonomer(s) are optionally unsaturated.

4. The insulation layer of claim 1, wherein the LDPE is an LDPE homopolymer.

5. The insulation layer of claim 1, wherein the polymer composition is non crosslinked.

6. The insulation layer of claim 1, wherein the polymer composition has a strain below 30% after 20 min, when measured in accordance with DMA method A as described under “Determination methods”.

7. A cable comprising one or more conductors surrounded by at least an insulation layer, wherein said insulation layer is as defined in claim 1.

8. The cable of claim 7, wherein the cable is a power cable.

9. A process for producing a cable, comprising: applying on a conductor, at least an insulation layer, wherein the insulation layer is as defined in claim 1.

10. A method of manufacturing an insulation layer in a cable comprising, using an insulation layer as defined in claim 1 to produce the insulation layer in a cable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows creep elongation of neat LDPE, a blend of LDPE with 2 wt % HDPE and pressed and remelted blends of LDPE with 2 wt % P3HT under their own weight, solidified by rapid quenching. Note that samples are false-colored to increase contrast.

(2) FIG. 2 shows the DMA results for neat LDPE and a blend of LDPE with 2 wt % P3HT (comparative example 1 and Inventive example 1) with and without heat treatment. The neat LDPE and non heat treated blend are measured using method A. The heat treated samples are measured using both methods A and B.

(3) FIG. 3 shows the conductivity results for Inventive compositions 2 to 5 and Comparative compositions 3 and 4.

DETERMINATION METHODS

(4) Unless otherwise stated in the description or experimental part the following methods were used for the property determinations.

(5) Wt %: % by weight

(6) Melt Flow Rate

(7) The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is determined at 190° C. for polyethylene and at 230° C. for polypropylene. MFR may be determined at different loadings such as 2.16 kg (MFR.sub.2) or 21.6 kg (MFR.sub.21).

(8) Molecular Weight

(9) Mz, Mw, Mn, and MWD are measured by Gel Permeation Chromatography (GPC) according to the following method:

(10) The weight average molecular weight Mw and the molecular weight distribution (MWD=Mw/Mn wherein Mn is the number average molecular weight and Mw is the weight average molecular weight; Mz is the z-average molecular weight) is measured according to ISO 16014-4:2003 and ASTM D 6474-99. A Waters GPCV2000 instrument, equipped with refractive index detector and online viscosimeter was used with 2×GMHXL-HT and 1× G7000HXL-HT TSK-gel columns from Tosoh Bioscience and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert-butyl-4-methyl-phenol) as solvent at 140° C. and at a constant flow rate of 1 mL/min. 209.5 μL of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 1 kg/mol to 12 000 kg/mol. Mark Houwink constants were used as given in ASTM D 6474-99. All samples were prepared by dissolving 0.5-4.0 mg of polymer in 4 mL (at 140° C.) of stabilized TCB (same as mobile phase) and keeping for max. 3 hours at a maximum temperature of 160° C. with continuous gentle shaking prior sampling in into the GPC instrument.

(11) Comonomer Contents

(12) a) Quantification of Alpha-Olefin Content in Low Density Polyethylenes by NMR Spectroscopy:

(13) The comonomer content was determined by quantitative 13C nuclear magnetic resonance (NMR) spectroscopy after basic assignment (J. Randall JMS—Rev. Macromol. Chem. Phys., C29(2&3), 201-317 (1989). Experimental parameters were adjusted to ensure measurement of quantitative spectra for this specific task.

(14) Specifically solution-state NMR spectroscopy was employed using a Bruker AvanceIII 400 spectrometer. Homogeneous samples were prepared by dissolving approximately 0.200 g of polymer in 2.5 ml of deuterated-tetrachloroethene in 10 mm sample tubes utilising a heat block and rotating tube oven at 140 C. Proton decoupled 13C single pulse NMR spectra with NOE (powergated) were recorded using the following acquisition parameters: a flip-angle of 90 degrees, 4 dummy scans, 4096 transients an acquisition time of 1.6s, a spectral width of 20 kHz, a temperature of 125 C, a bilevel WALTZ proton decoupling scheme and a relaxation delay of 3.0 s. The resulting FID was processed using the following processing parameters: zero-filling to 32 k data points and apodisation using a gaussian window function; automatic zeroth and first order phase correction and automatic baseline correction using a fifth order polynomial restricted to the region of interest.

(15) Quantities were calculated using simple corrected ratios of the signal integrals of representative sites based upon methods well known in the art.

(16) b) Comonomer Content of Polar Comonomers in Low Density Polyethylene

(17) (1) Polymers Containing >6 wt % Polar Comonomer Units

(18) Comonomer content (wt %) was determined in a known manner based on Fourier transform infrared spectroscopy (FTIR) determination calibrated with quantitative nuclear magnetic resonance (NMR) spectroscopy. Below is exemplified the determination of the polar comonomer content of ethylene ethyl acrylate, ethylene butyl acrylate and ethylene methyl acrylate. Film samples of the polymers were prepared for the FTIR measurement: 0.5-0.7 mm thickness was used for ethylene butyl acrylate and ethylene ethyl acrylate and 0.10 mm film thickness for ethylene methyl acrylate in amount of >6 wt %. Films were pressed using a Specac film press at 150° C., approximately at 5 tons, 1-2 minutes, and then cooled with cold water in a not controlled manner. The accurate thickness of the obtained film samples was measured.

(19) After the analysis with FTIR, base lines in absorbance mode were drawn for the peaks to be analysed. The absorbance peak for the comonomer was normalised with the absorbance peak of polyethylene (e.g. the peak height for butyl acrylate or ethyl acrylate at 3450 cm.sup.−1 was divided with the peak height of polyethylene at 2020 cm.sup.−1). The NMR spectroscopy calibration procedure was undertaken in the conventional manner which is well documented in the literature, explained below.

(20) For the determination of the content of methyl acrylate a 0.10 mm thick film sample was prepared. After the analysis the maximum absorbance for the peak for the methylacrylate at 3455 cm.sup.−1 was subtracted with the absorbance value for the base line at 2475 cm.sup.−1 (A.sub.methylacrylate−A.sub.2475). Then the maximum absorbance peak for the polyethylene peak at 2660 cm.sup.−1 was subtracted with the absorbance value for the base line at 2475 cm.sup.−1 (A.sub.2660−A.sub.2475). The ratio between (A.sub.methylacrylate−A.sub.2475) and (A.sub.2660−A.sub.2475) was then calculated in the conventional manner which is well documented in the literature.

(21) The weight-% can be converted to mol-% by calculation. It is well documented in the literature.

(22) Quantification of Copolymer Content in Polymers by NMR Spectroscopy

(23) The comonomer content was determined by quantitative nuclear magnetic resonance (NMR) spectroscopy after basic assignment (e.g. “NMR Spectra of Polymers and Polymer Additives”, A. J. Brandolini and D. D. Hills, 2000, Marcel Dekker, Inc. New York). Experimental parameters were adjusted to ensure measurement of quantitative spectra for this specific task (e.g “200 and More NMR Experiments: A Practical Course”, S. Berger and S. Braun, 2004, Wiley-VCH, Weinheim). Quantities were calculated using simple corrected ratios of the signal integrals of representative sites in a manner known in the art.

(24) (2) Polymers Containing 6 wt. % or Less Polar Comonomer Units

(25) Comonomer content (wt. %) was determined in a known manner based on Fourier transform infrared spectroscopy (FTIR) determination calibrated with quantitative nuclear magnetic resonance (NMR) spectroscopy. Below is exemplified the determination of the polar comonomer content of ethylene butyl acrylate and ethylene methyl acrylate. For the FT-IR measurement a film samples of 0.05 to 0.12 mm thickness were prepared as described above under method 1). The accurate thickness of the obtained film samples was measured.

(26) After the analysis with FT-IR base lines in absorbance mode were drawn for the peaks to be analysed. The maximum absorbance for the peak for the comonomer (e.g. for methylacrylate at 1164 cm.sup.−1 and butylacrylate at 1165 cm.sup.−1) was subtracted with the absorbance value for the base line at 1850 cm.sup.−1 (A.sub.polar comonomer−A.sub.1850). Then the maximum absorbance peak for polyethylene peak at 2660 cm.sup.−1 was subtracted with the absorbance value for the base line at 1850 cm.sup.−1 (A.sub.2660−A.sub.1850). The ratio between (A.sub.comonomer−A.sub.1850) and (A.sub.2660−A.sub.1850) was then calculated. The NMR spectroscopy calibration procedure was undertaken in the conventional manner which is well documented in the literature, as described above under method 1).

(27) The weight-% can be converted to mol-% by calculation. It is well documented in the literature.

(28) Below is exemplified how polar comonomer content obtained from the above method (1) or (2), depending on the amount thereof, can be converted to micromol or mmol per g polar comonomer as used in the definitions in the text and claims:

(29) The millimoles (mmol) and the micro mole calculations have been done as described below.

(30) For example, if 1 g of the poly(ethylene-co-butylacrylate) polymer, which contains 20 wt % butylacrylate, then this material contains 0.20/M.sub.butylacrylate (128 g/mol)=1.56×10.sup.−3 mol. (=1563 micromoles).

(31) The content of polar comonomer units in the polar copolymer C.sub.polar comonomer is expressed in mmol/g (copolymer). For example, a polar poly(ethylene-co-butylacrylate) polymer which contains 20 wt. % butyl acrylate comonomer units has a C.sub.polar comonomer of 1.56 mmol/g. The used molecular weights are: M.sub.butylacrylate=128 g/mole, M.sub.ethylacrylate=100 g/mole, M.sub.methylacrylate=86 g/mole).

(32) Density

(33) Low density polyethylene (LDPE): The density was measured according to ISO 1183-2. The sample preparation was executed according to ISO 1872-2 Table 3 Q (compression moulding).

(34) Xylene Solubles (XS)

(35) Xylene solubles were determined at 23° C. according ISO 6427.

(36) Method for Determination of the Amount of Double Bonds in the Polymer Composition or in the Polymer

(37) A) Quantification of the Amount of Carbon-Carbon Double Bonds by IR Spectroscopy

(38) Quantitative infrared (IR) spectroscopy was used to quantify the amount of carbon-carbon doubles (C═C). Calibration was achieved by prior determination of the molar extinction coefficient of the C═C functional groups in representative low molecular weight model compounds of known structure.

(39) The amount of each of these groups (N) was determined as number of carbon-carbon double bonds per thousand total carbon atoms (C═C/1000C) via:
N=(A×14)/(E×L×D)
were A is the maximum absorbance defined as peak height, E the molar extinction coefficient of the group in question (l.Math.mol.sup.−1.Math.mm.sup.−1), L the film thickness (mm) and D the density of the material (g.Math.cm.sup.−1).

(40) The total amount of C═C bonds per thousand total carbon atoms can be calculated through summation of N for the individual C═C containing components.

(41) For polyethylene samples solid-state infrared spectra were recorded using a FTIR spectrometer (Perkin Elmer 2000) on compression moulded thin (0.5-1.0 mm) films at a resolution of 4 cm.sup.−1 and analysed in absorption mode.

(42) 1) Polymer Compositions Comprising Polyethylene Homopolymers and Copolymers, Except Polyethylene Copolymers with >0.4 wt % Polar Comonomer

(43) For polyethylenes three types of C═C containing functional groups were quantified, each with a characteristic absorption and each calibrated to a different model compound resulting in individual extinction coefficients: vinyl (R—CH═CH2) via 910 cm.sup.−1 based on 1-decene [dec-1-ene] giving E=13.13 l.Math.mol.sup.−1.Math.mm.sup.−1 vinylidene (RR′C═CH2) via 888 cm.sup.−1 based on 2-methyl-1-heptene [2-methyhept-1-ene] giving E=18.24 l.Math.mol.sup.−1.Math.mm.sup.−1 trans-vinylene (R—CH═CH—R′) via 965 cm.sup.−1 based on trans-4-decene [(E)-dec-4-ene] giving E=15.14 l.Math.mol.sup.−1.Math.mm.sup.−1

(44) For polyethylene homopolymers or copolymers with <0.4 wt % of polar comonomer linear baseline correction was applied between approximately 980 and 840 cm.sup.−1.

(45) 2) Polymer Compositions Comprising Polyethylene Copolymers with >0.4 wt % Polar Comonomer

(46) For polyethylene copolymers with >0.4 wt % of polar comonomer two types of C═C containing functional groups were quantified, each with a characteristic absorption and each calibrated to a different model compound resulting in individual extinction coefficients: vinyl (R—CH═CH2) via 910 cm.sup.−1 based on 1-decene [dec-1-ene] giving E=13.13 l.Math.mol.sup.−1.Math.mm.sup.−1 vinylidene (RR′C═CH2) via 888 cm.sup.−1 based on 2-methyl-1-heptene [2-methyl-hept-1-ene] giving E=18.24 l.Math.mol.sup.−1.Math.mm.sup.−1
EBA:

(47) For poly(ethylene-co-butylacrylate) (EBA) systems linear baseline correction was applied between approximately 920 and 870 cm.sup.−1.

(48) EMA:

(49) For poly(ethylene-co-methylacrylate) (EMA) systems linear baseline correction was applied between approximately 930 and 870 cm.sup.−1.

(50) 3) Polymer Compositions Comprising Unsaturated Low Molecular Weight Molecules

(51) For systems containing low molecular weight C═C containing species direct calibration using the molar extinction coefficient of the C═C absorption in the low molecular weight species itself was undertaken.

(52) B) Quantification of Molar Extinction Coefficients by IR Spectroscopy

(53) The molar extinction coefficients were determined according to the procedure given in ASTM D3124-98 and ASTM D6248-98. Solution-state infrared spectra were recorded using a FTIR spectrometer (Perkin Elmer 2000) equipped with a 0.1 mm path length liquid cell at a resolution of 4 cm.sup.−1.

(54) The molar extinction coefficient (E) was determined as l.Math.mol.sup.−1.Math.mm.sup.−1 via:
E=A/(C×L)
where A is the maximum absorbance defined as peak height, C the concentration (mol.Math.l.sup.−1) and L the cell thickness (mm).

(55) At least three 0.18 mol.Math.l.sup.−1 solutions in carbondisulphide (CS.sub.2) were used and the mean value of the molar extinction coefficient determined.

(56) DMA Creep Test—Methods A and B

(57) Melt pressed films with a thickness of 1 mm, width of 4 mm and length of around 10 mm where placed in a TA DMA Q800 using a film tension setup. A stress corresponding to 1 kPa (method A) or 2 kPa (method B) was applied to the films while temperature was increased from 50° C. to 115° C. (method A) or from 50° C. to 125° C. (method B) using a heating rate of 10° C./min. Temperature was held at 115° C. or 125° C. respectively for at least 40 min while still applying a stress of 1 kPa or 2 kPa respectively. Strain was recorded during the experiment and a final strain was noted after 40 min. Heat treated films were heated to 250° C. and then subsequently allowed to cool back to room temperature, prior to carrying out the above method.

(58) Creep Measurements

(59) 1 mm thick dog-bone-shaped pieces, approximately 60 mm in length, 15 mm in width at the wide point and 6 mm at the narrow point, were cut from melt-pressed films described above in connection with the DMA test. The melt pressed film had been cooled at three different rates, i.e. (1) quenched in liquid nitrogen, (2) cooled at ΔT/Δt˜−25° C. min-1 and (3) slowly cooled at ΔT/Δt˜−1° C. min-1. The pieces of melt-pressed film were suspended in an oven preheated to 116° C., i.e. to a temperature above Tm LDPE but below the co-crystal melting peak.

(60) In FIG. 1, the temperature of the dog bone shaped pieces of melt-pressed film is measured at the times indicated using a thermocouple. At 0 minutes therefore, the pieces of melt-pressed film have been heated to around 110° C. After fifty minutes, the pieces of melt-pressed film are essentially at the same temperature as the oven and remain at that temperature.

(61) DC Conductivity Method

(62) Conductivity measurements were obtained by the use of dielectric spectroscopy. All measurements were performed on disk-shaped samples with 40 mm diameter and ˜0.1 mm thickness.

(63) Broadband Dielectric Spectroscopy (BDS) was performed using a Novocontrol alpha spectrometer in a frequency range of 10.sup.−2 to 10.sup.7 Hz, at different temperatures in the range 253-383K with an error of ±0.1K, at atmospheric pressure and under nitrogen atmosphere.

(64) For selected temperatures frequency scans were also performed to investigate the local and ion dynamics. The sample cell consisted of two silver-coated electrodes 40 mm in diameter and the sample with a thickness of about 0.1 mm. The complex dielectric permittivity ε*=ε′−iε″, where ε′ is the real and ε″ is the imaginary part, is generally a function of frequency, ω, temperature T, and pressure P, although here only the frequency and temperature dependencies have been investigated. The complex dielectric conductivity σ* can be also calculated from the complex dielectric function ε* as σ*=iωε.sub.1ε*, (ε.sub.f is the permittivity of free space, 8.854 pF/m) where conductivity can also be analysed in a real and an imaginary part: σ*=σ′+iσ″. This means the conductivity data are effectively an alternative representation of the permittivity, nevertheless focusing on different features of the dielectric behaviour.

(65) Experimental Part

(66) The following materials were used:

(67) Polythiophene: Poly(3-hexylthiophene-2,5-diyl(P3HT), a commercially available regio-regular polythiophene (supplier Solaris Chem Inc)

(68) HDPE: A conventional unimodal high density polyethylene (0.8 mol % 1-butene content, as the comonomer) which is produced in a gas phase reactor. The HDPE has an MFR.sub.2 of 12g/10 min (190° C./2.16 kg) and a density of 962 kg/m.sup.3.

(69) LDPE: LDPE homopolymer having the properties of Table 1:

(70) TABLE-US-00001 TABLE 1 Polymer properties of LDPE Base Resin Properties LDPE MFR.sub.2, 190° C. [g/10 min] 0.3 Density [kg/m.sup.3] 930 Tensile modulus 350 MPa Flex Modulus 330 MPa

Example 1

(71) Several mixtures were prepared by melt mixing at 210° C. for 10 minutes in a Haake mini twin screw extruder followed by hot pressing samples of an area 40 mm×100 mm at 220° C. and 100 kN press force. Spacers with a thickness of 1 mm were used to control thickness. An extra heat treatment was applied on one sample by remelting the pressed sample on a hot plate pre-heated to 250° C. for 3 minutes followed by slow cooling by turning of the hot plate. Sample was removed when temperature reached 200° C. and then finally cooled at room temperature. The compositions studies are shown in Table 2.

(72) TABLE-US-00002 TABLE 2 Polymer compositions of the invention and reference compositions: Inv. Components comp 1 Comparative comp. 1 Comparative comp. 2 LDPE, wt % 98 100 98 P3HT, wt %  2 — — HDPE, wt % — —  2
Creep Measurements

(73) In FIG. 1 a visual representation of the creep measurement results for the compositions of Table 2 can be seen. From these results we can see that the inclusion of P3HT in the LDPE matrix offers significant improvement of the thermomechanical behaviour.

(74) After an initial lag time of about 15 min pure LDPE samples (comparative comp. 1) started to elongate under their own weight and eventually reached the bottom of the oven (See FIG. 1). For comparative comp. 2 with HDPE=2 wt % the sample largely kept its shape even after an extended period of time, indicating excellent form stability at 115° C. Moreover, we observed a minor impact of the cooling rate, which correlates with the degree of co-crystallization on the creep resistance. Rapid quenching maximizes the formation of co-crystals, whereas slow cooling increasingly favours segregation of LDPE and HDPE and thus the formation of pure crystalline domains. For comparative comp. 2 quenching resulted in low creep with a rate of ˜0.2% min.sup.−1, whereas for slower cooling no creep was observed. In the case of P3HT the shape and size of the sample remain almost intact.

(75) For the inventive P3HT based system (inventive example 1) the melting step at 250° C. is advantageous for improving the thermomechanical behaviour. At the compounding temperature of 210° C., P3HT is semi-crystalline (melting temperature typically between 220 to 250° C.), which prevents good homogeneisation during the performed extrusions step. P3HT melts during the hot-pressing step at 220° C. and, due to partial miscibility in polyethylene, diffuses into LDPE, forming a more homogeneous blend. Upon cooling a fine distribution of semi-crystalline P3HT is obtained. The purple colour of the samples indicates that P3HT is semi-crystalline. Pressed sample at 220° C. displays only small improvements in thermomechanical properties at 116° C., however the remelted sample has greater thermomechanical properties at temperatures of nearly 118° C. with almost no elongation at all. When remelting the blend and cooling it slowly P3HT has the possibility to form a network of crystals percolating the material, the network of high melting P3HT crystals can then act as frame that helps the material keeps it dimensional stability. For the sample that was not heat treated the crystalline domains of P3HT were probably not mixed or distributed to the same extent causing inferior thermomechanical properties.

(76) DMA Results

(77) FIG. 2 shows the DMA results for neat LDPE and a blend of LDPE with 2 wt % P3HT (comparative example 1 and Inventive example 1) with and without heat treatment. The neat LDPE and non heat treated blend are measured using method A. The heat treated samples are measured using both methods A and B. In particular, the heat treated samples show excellent results with a strain below 20% for at least up to 50 minutes.

Example 2

(78) Several mixtures were prepared by melt mixing the components at 160° C. for 10 minutes in a Haake mini twin screw extruder followed by hot pressing samples of an area of 40 mm×100 mm at 250° C. and 100 kN press force. Spacers with a thickness of 0.1 mm were used to control thickness. The conductivity results are shown in Table 2 and FIG. 1.

(79) TABLE-US-00003 TABLE 3 Polymer compositions of the invention and reference compositions and the electrical conductivity results: Inv. Inv. Inv Inv Ref Ref Components Comp 2 Comp 3 Comp 4 Comp 5 comp 3 comp 4 LDPE, wt %* 99.9 99.5 99 98.5 100 P3HT, wt %*  0.1  0.5  1  1.5 100 DC conductivity S/cm 1.65E−14 3.72E−14 1.52E−14 2.48E−14 1.64E−12 4.98E−09

(80) As can be seen from Table 3, polymer compositions of inventive examples 2-5 show excellent low DC conductivity. Furthermore, the DC conductivity drops by around 2 orders of magnitude when the pure LDPE is enhanced with the P3HT. The polymer compositions of the invention are particularly useful in DC power cables, preferably in HV DC power cables.