POLYMER COMPOSITION

Abstract

A polymer composition comprising (i) at least 70 wt% of low-density polyethylene (LDPE) homopolymer or copolymer having a density of 905 to 935 kg/m.sup.3 (ISO 1183-2) and an MFR.sub.2 of 0.1 to 10 g/10 min (ISO1133 at 190° C., 2.16 kg); (ii) 0.5 to 20 wt % of a high density polyethylene (HDPE) having a density of 940 kg/m.sup.3 or more and an MFR.sub.2 of 0.1 to 50 g/10 min; and (iii) 0.05 to 10 wt% of an aliphatic, preferably alkyl, functional inorganic nanoparticle filler.

Claims

1. A polymer composition, comprising: (i) at least 70 wt% of a low-density polyethylene (LDPE) homopolymer or copolymer having a density of 905 to 935 kg/m.sup.3 (ISO 1183-2) and an MFR.sub.2 of 0.1 to 10 g/10 min (ISO1133 at 190° C., 2.16 kg); (ii) 0.5 to 20 wt% of a high density polyethylene (HDPE) having a density of 940 kg/m.sup.3 or more and an MFR.sub.2 of 0.1 to 50 g/10 min; and (iii) 0.05 to 10 wt% of an aliphatic functional inorganic nanoparticle filler comprising inorganic nanoparticles each having a surface functionalized by an aliphatic group.

2. The polymer composition of claim 1, wherein the aliphatic functional inorganic nanoparticle filler comprises inorganic oxide nanoparticles.

3. The polymer composition of claim 1, wherein the aliphatic functional inorganic nanoparticle filler comprises aluminium oxide, magnesium oxide or zinc oxide nanoparticles.

4. The polymer composition of claim 1, wherein the aliphatic functional inorganic nanoparticle filler comprises aluminium oxide nanoparticles.

5. The polymer composition of claim 1, wherein the aliphatic group is a C1-20 alkyl group.

6. The polymer composition of claim 1, wherein the alkyl-aliphatic group is a C6-12 alkyl group.

7. The polymer composition of claim 1, wherein the aliphatic functional inorganic nanoparticle filler is functionalized by reaction with an alkylsilane .

8. The polymer composition of claim 1, wherein the LDPE is a low-density polyethylene homopolymer or an unsaturated LDPE copolymer of ethylene with at least one polyunsaturated comonomer .

9. The polymer composition of claim 1, wherein the polymer composition has a DC conductivity of: less than 4.0 x10.sup.-17 S/m when measured at 70° C.; less than 2.5 x10.sup.-17 S/m when measured at 60° C.; less than 3.5 x10.sup.-16 S/m when measured at 90° C.; or a combination thereof.

10. The polymer composition of claim 1, wherein the polymer composition is not crosslinked.

11. The polymer composition of claim 1, comprising (i) at least 75 wt% of the low-density polyethylene (LDPE); (ii) 0.5 to 15 wt% of the high density polyethylene (HDPE); and (iii) 0.5 to 10 wt% of the aliphatic functional inorganic nanoparticle filler.

12. The polymer composition of claim 1, comprising (i) at least 90 wt% of the low-density polyethylene (LDPE); (ii) 1.0 to 8.0 wt% of the high density polyethylene (HDPE); and (iii) 1.0 to 8.0 wt% of the aliphatic functional inorganic nanoparticle filler.

13. The polymer composition of claim 1, wherein the polymer composition has a storage modulus of: at least 1.0x10.sup.5 Pa at 115° C.; at least 5.0x10.sup.4 Pa at 120° C.; at least 3.0x10.sup.4 Pa at 125° C.; or a combination thereof.

14. A process for the preparation of the polymer composition of claim 1, the process comprising blending: (i) at least 70 wt% of the low-density polyethylene (LDPE) homopolymer or copolymer ; (ii) 0.5 to 20 wt% of the high density polyethylene (HDPE) ; and (iii) 0.05 to 10 wt% of the aliphatic functional inorganic nanoparticle filler.

15. A cable comprising a conductor surrounded by one or more layers, wherein one or more of said layers comprises the polymer composition of claim 1 .

16. A power cable comprising a conductor surrounded at least by an inner semiconductive layer, an insulation layer, and an outer semiconductive layer, in that order, wherein at least one of the inner semiconductive layer, the insulation layer, or the outer semiconductive layer comprises the polymer composition of claim 1 .

17. The power cable of claim 16, wherein the power cable is a high voltage (HV) power cable or an ultra-high voltage (UHV) power cable.

18. A method of use of the polymer composition of claim 1, the method comprising using the polymer composition to manufacture of a layer in a cable.

19. The polymer composition of claim 1, wherein the aliphatic group comprises a linear C1-20 alkyl group.

20. The polymer composition of claim 8, wherein the polyunsaturated comonomer comprises a straight carbon chain with at least 8 carbon atoms, wherein at least 4 of the carbon atoms are between two non-conjugated double bonds, wherein at least one of the non-conjugated double bonds is terminal.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0075] FIG. 1 shows the conductivity of the samples disclosed in Table 1 as a function of temperature. The inventive examples comprising LDPE, HDPE and the nanoparticle filler have significantly lower conductivity than the comparative examples consisting of LDPE, LDPE+HDPE, or LDPE+nanoparticle filler alone.

[0076] FIG. 2 shows the storage modulus of the samples disclosed in Table 1 as a function of temperature. The inventive examples comprising LDPE, HDPE and the nanoparticle filler have significantly improved storage modulus than the comparative examples consisting of LDPE or LDPE + nanoparticle filler alone.

DETERMINATION METHODS

[0077] Unless otherwise stated in the description or experimental part the following methods were used for the property determinations:

[0078] (wt% = % by weight)

Density

[0079] The density of the polymer samples was measured according to ISO 1183-2.

Melt Flow Rate (MFR)

[0080] 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. Unless otherwise specified, the term MFR as used herein refers to MFR.sub.2 (190° C., 2.16 kg).

Sample Preparation

[0081] The polymer composition of the present invention and comparative examples, were melt compounded in a Micro 5 cc Twin Screw Compounder (DSM Xplore) at 150° C. for 6 min with a screw speed of 100 rpm. The extruded nanocomposite rods were cut into pellets and compression-moulded under a load of 200 kN into 80 .Math.m thick films using a TP400 laboratory press (Fontijne Grotnes B.V., the Netherlands) at 130° C. for 10 min. The samples were finally cooled to 25° C. at a rate of 20° C./min while maintaining the compressive load.

DC-conductivity Measurement

[0082] The electrical conductivity measurements were performed following standard procedure according to IEC, in Methods of Test for Volume Resistivity and Surface Resistivity of Solid Electrical Insulating Materials, Standard 60093, 1980, applying a direct current (DC) voltage (Glassman FJ60R2) over the film sample, i.e. the polymer composition of the present invention and comparative examples, and measuring the charging current with an electrometer (Keithley 6517A). The current signal was recorded by Lab VIEW software incorporated in a personal computer and stored for further analysis. An oven was used to control temperature, whereas an overvoltage protection secured the electrometer from damaging due to possible overshoots and a low-pass filter removed high frequency disturbances. A stainless steel three-electrode system was used, in which the high voltage electrode was a cylinder with a diameter of 45 mm, the current measuring electrode was 30 mm in diameter, and the guard ring eliminated surface currents. Good contact between the high-voltage electrode and the film sample was achieved by placing an Elastosil R570/70 (Wacker) layer between them. The experiments were conducted on at 60, 70 and 90° C. for 6 h. The applied voltage was 2.6 kV corresponding to an electric field of 30 kV/mm, giving conditions 60-(90° C.) in temperature and electric field resembling the stress conditions in the insulation of a real HVDC cable. The test was repeated twice for each material to assess the reproducibility.

Storage Modulus (G′)

[0083] The storage modulus (G′) of the samples as a function of temperature was measured by dynamic mechanical thermal analysis (DMTA). The storage modulus is indicated in Pa. The characterisation of polymer melts by torsional dynamic mechanical thermal analysis (DMTA) was performed using an Anton Paar MCR702 TwinDrive (Graz, Austria) rheometer operating in the single motor-transducer configuration (stress-controlled). A SCF cylindrical sample fixture was used, with the temperature being controlled via a CTD450 convection oven. The temperature was increased from 30 to 130° C. at a rate of 2° C./min while the samples were subjected to a 1% strain amplitude at a frequency of 0.8 Hz. The test samples were prepared directly from extruded strands (3 mm in diameter) by cutting to a total length of 40 mm, so that the free sample length was ca. 26 mm. The results are shown in FIG. 2.

Experimental Part

Materials

[0084] The materials used in this work are as follows: [0085] LDPE: Density 922 kg/m.sup.3, MFR.sub.2 1.9 g/10 min. [0086] HDPE: (Unimodal Ziegler Natta HDPE, density = 962 kg/m.sup.3, MFR.sub.2 at 190° C. = 12 g/10 min) [0087] C.sub.8—Al.sub.2O.sub.3: Preparation method described below

Preparation of Octyl-Coated Aluminium-oxide Nanoparticles (C.SUB.8.-Al.SUB.2.O.SUB.3.)

[0088] Aluminium oxide nanoparticles (Nanodur from Nanophase Inc, CAS number 1344-28-01, density 3.97 g/cm.sup.3) were coated with n-octyltriethoxysilane (Sigma-Aldrich, CAS-number 3069-42-9). The reactions were conducted in a mixed medium of 2-propanol and water. Ammonia hydroxide (aq. 25 %) was used as a catalyst to promote the hydrolysis and the condensation of the silanes. After surface modification, the nanoparticles were dried for 20 h at 80° C. in a vacuum oven (Fisher Scientific Vacucell, MMT group). The average diameter of the spherical Al.sub.2O.sub.3 nanoparticles was 50 nm, according to TEM images analysis.

Preparation of Polymer Compositions

[0089] Octyl-coated aluminium oxide nanoparticles (C.sub.8—Al.sub.2O.sub.3) were dispersed in n-heptane (0.3 ml n-heptane/1g nanoparticles) and ultrasonicated for 5 minutes, whereafter 0.02 wt.% antioxidant Irganox 1076 (Ciba Speciality Chemicals, CAS number 2082-79-3) was added. Desired proportions of grinded, low-density polyethylene LDPE and high density polyethylene HDPE were added to the nanoparticle suspensions.

[0090] The LDPE/HDPE/C.sub.8—Al.sub.2O.sub.3 slurry was shaken for 1 h with a Vortex Genie 2 shaker (Scientific Instruments Inc) and dried overnight at 80° C. After drying, the powder was shaken for another 30 min and then extruded for 6 min at 150° C. and 100 rpm (Micro 5 cc twin screw compounder, Xplore instruments). The extruded materials were dried overnight at 80° C. in vacuum-oven.

Results

[0091] The composition and properties of samples of the polymer compositions according to the present invention (IE1-3) and samples of comparative compositions (CE1-9) are shown in Table 1. The DC-conductivity of each sample at each temperature is shown graphically in FIG. 1.

TABLE-US-00001 Example LDPE (wt%) HDPE (wt%) C8—Al.sub.2O.sub.3 (wt%) Temperature (°C) Conductivity (S/m) CE1 100 60 9.02E-15 CE2 100 70 1.25E-14 CE3 100 90 1.13E-13 CE4 96 4 60 1.24E-15 CE5 96 4 70 6.82E-15 CE6 96 4 90 2.36E-14 CE7 97 3 60 3.90E-17 CE8 97 3 70 4.86E-17 CE9 97 3 90 5.76E-16 IE1 93 4 3 60 1.94E-17 IE2 93 4 3 70 2.75E-17 IE3 93 4 3 90 1.77E-16

[0092] The inventors have established that the inventive compositions have excellent (i.e. low) DC-conductivity, even at high temperatures (e.g. up to 90° C.). With reference to the data in Table 1 and FIG. 1, it can be seen that the DC-conductivity of the inventive compositions IE1-3 is unexpectedly more than two orders of magnitude lower than the pure LDPE compositions of CE1-3.

[0093] Surprisingly, the conductivity of the inventive examples is also significantly reduced relative to that of blends consisting of LDPE and HDPE (CE4-6) or LDPE and nanoparticle filler (CE7-9) alone. The reduction in DC conductivity may even be synergistic.

[0094] It is surprising that the conductivity of the polymer composition of the invention is so low given that the mechanism of conduction is different for the LDPE/HDPE blends and the LDPE/Al.sub.2O.sub.3 system. There is therefore no expectation that the conductivity of the combined polymer composition should be lower than the comparative examples.

[0095] Furthermore, this reduction in DC conductivity is obtained whilst maintaining or even improving the thermomechanical properties of the polymer composition (e.g. in terms of storage modulus). This is despite the presence of the nanoparticle filler which might be expected to reduce thermomechanical performance. The introduction of HDPE to LDPE creates a system that is melt miscible and phase separates upon crystallisation. This leads to the creation of co-crystals that create a network acting as physical crosslinks and gives the system far better thermomechanical properties. The introduction of the nanoparticles might be expected to disturb this fine balance but surprisingly this is not the case. Analysis of the blends suggests that the thermomechanical properties of the inventive examples are at least maintained or improved relative to the comparative examples (see FIG. 2).

[0096] The low conductivity of the compositions according the present invention makes them particularly suitable for use in applications where low conductivity is essential, such as in the insulation layer of power cables.