Polymer composition for W and C application with advantageous electrical properties

Abstract

The invention relates to a polymer composition with improved DC electrical properties, to the use of the composition for producing a cable layer and to a cable surrounded by at least one layer comprising the polymer composition.

Claims

1. A process for producing a power cable wherein the process comprises the steps of applying on a conductor an inner semiconductive layer comprising a first semiconductive composition, an insulation layer comprising an insulation composition and an outer semiconductive layer comprising a second semiconductive composition, in that order, wherein the composition of at least the insulating layer comprises a polymer composition comprising (a) from 1.0 to 40 wt % of a polyethylene selected from the group consisting of: very low density polyethylene (VLDPE) having a density of from 850 to 909 kg/m.sup.3, linear low density polyethylene (LLDPE) having a density of from 909 to 930 kg/m.sup.3, medium density polyethylene (MDPE) having a density of from 930 to 945 kg/m.sup.3, and high density polyethylene (HDPE) having a density of 945 kg/m.sup.3 or more; (b) from 60 to 99 wt % of a low density polyethylene (LDPE), and (c) dicumyl peroxide in an amount of 0.1 to 110 mmol OO/kg polymer composition.

2. The process according to claim 1, wherein the process comprises a further step of crosslinking at least the insulation composition of said insulation layer.

3. The process according to claim 2, the process further comprising crosslinking at least one of the first semiconductive composition of the inner semiconductive layer and the second semiconductive composition of the outer semiconductive layer, in the presence of a crosslinking agent at crosslinking conditions.

4. A polymer composition comprising (a) from 0.1 to 40 wt % of a polyethylene selected from the group consisting of: very low density polyethylene (VLDPE) having a density of from 850 to 909 kg/m.sup.3, linear low density polyethylene (LLDPE) having a density of from 909 to 930 kg/m.sup.3, medium density polyethylene (MDPE) having a density of from 930 to 945 kg/m.sup.3, and high density polyethylene (HDPE) having a density of 945 kg/m.sup.3 or more; (b) from 60 to 99.9 wt % of a low density polyethylene (LDPE); and (c) dicumyl peroxide in an amount of 0.1 to 110 mmol OO/kg polymer composition.

5. The polymer composition according to claim 4, wherein the polymer composition has an electrical conductivity of 160 fS/m or less, when measured according to DC conductivity method (1) using a 1 mm thick plaque sample as described under Determination Methods.

6. The polymer composition according to claim 4, wherein the amount of the polyethylene (a) is from 1.0 to 30 wt %, based on the combined weight of the polyethylene (a) and the LDPE (b).

7. The polymer composition according to claim 4, wherein the amount of the LDPE (b) is from 70 to 99 wt %, based on the combined weight of the polyethylene (a) and the LDPE (b).

8. The polymer composition according to claim 4, wherein the polyethylene (a) is an ethylene homopolymer or a copolymer of ethylene with one or more comonomer(s).

9. The polymer composition according to claim 4, wherein the LDPE (b) is an LDPE polymer selected from a saturated or an unsaturated LDPE homopolymer or a saturated or an unsaturated LDPE copolymer of ethylene with one or more comonomer(s).

10. The polymer composition according to claim 4, wherein the LDPE (b) is an unsaturated LDPE polymer, which is selected from an unsaturated LDPE homopolymer or an unsaturated LDPE copolymer of ethylene with one or more comonomer(s), and comprises a total amount of carbon-carbon double bonds/1000 carbon atoms of more than 0.4/1000 carbon atoms.

11. The polymer composition according to claim 4, wherein the LDPE (b) is an unsaturated LDPE copolymer of ethylene with at least one polyunsaturated comonomer.

Description

EXPERIMENTAL PART

Preparation of the Components of the Polymer Compositions of the Present Invention and of the References

(1) The polyolefins were low density polyethylenes produced in a high pressure reactor. The production of inventive and reference polymers is described below. As to CTA feeds, e.g. the PA content can be given as liter/hour or kg/h and converted to either units using a density of PA of 0.807 kg/liter for the recalculation.

(2) LDPE1:

(3) Ethylene with recycled CTA was compressed in a 5-stage precompressor and a 2-stage hyper compressor with intermediate cooling to reach initial reaction pressure of ca 2628 bar. The total compressor throughput was ca 30 tons/hour. In the compressor area approximately 4.9 liters/hour of propion aldehyde (PA, CAS number: 123-38-6) was added together with approximately 81 kg propylene/hour as chain transfer agents to maintain an MFR of 1.89 g/10 min. Here also 1,7-octadiene was added to the reactor in amount of 27 kg/h. The compressed mixture was heated to 157 C. in a preheating section of a front feed two-zone tubular reactor. A mixture of commercially available peroxide radical initiators dissolved in isododecane was injected just after the preheater in an amount sufficient for the exothermal polymerisation reaction to reach peak temperatures of ca 275 C. after which it was cooled to approximately 200 C. The subsequent 2nd peak reaction temperatures was 264 C. The reaction mixture was depressurised by a kick valve, cooled and polymer was separated from unreacted gas.

(4) LDPE2:

(5) Purified ethylene was liquefied by compression and cooling to a pressure of 90 bars and a temperature of 30 C. and split up into to two equal streams of roughly 14 tons/hour each. The CTA (methyl ethyl ketone (MEK)), air and a commercial peroxide radical initiator dissolved in a solvent were added to the two liquid ethylene streams in individual amounts. Here also 1,7-octadiene was added to the reactor in amount of 40 kg/h. The two mixtures were separately pumped through an array of 4 intensifiers to reach pressures of 2200-2300 bars and exit temperatures of around 40 C. These two streams were respectively fed to the front (zone 1) (50%) and side (zone 2) (50%) of a split-feed two-zone tubular reactor. MEK was added in amounts of 190 kg/h to the front stream to maintain a MFR.sub.2 of around 2 g/10 min. The front feed stream was passed through a heating section to reach a temperature sufficient for the exothermal polymerization reaction to start. The reaction reached peak temperatures were 251 C. and 290 C. in the first and second zones, respectively. The side feed stream cooled the reaction to an initiation temperature of the second zone of 162 C. Air and peroxide solution was added to the two streams in enough amounts to reach the target peak temperatures. The reaction mixture was depressurized by product valve, cooled and polymer was separated from unreacted gas.

(6) TABLE-US-00002 TABLE 1 Polymer properties of LDPE1 and LDPE2 Base Resin Properties LDPE1 LDPE2 MFR 2.16 kg, at 190 C. [g/10 min] 1.89 1.90 Density [kg/m.sup.3] 923 922 Vinyl [C = C/1000 C] 0.54 0.33 Vinylidene [C = C/1000 C] 0.16 0.27 Trans-vinylene [C = C/1000 C] 0.06 0.07
HDPE:

(7) 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 12 g/10 min (190 C./2.16 kg) and a density of 962 kg/m.sup.3. The same base resin, except that combined with another additive system than specified in table 2, is used in a commercially available grade Bormed HE9621-PH (supplier Borealis).

(8) Compounding of the Polymer Compositions:

(9) Each polymer component of a test polymer composition were added as separate pellets to a pilot scale extruder (Prism TSE 24TC) together with additives, if not present in the pellets, other than the crosslinking agent. The obtained mixture was meltmixed in conditions given in the below table and extruded to pellets in a conventional manner.

(10) TABLE-US-00003 Extruder Out- Set Values Temperatures [ C.] put Pres- Zone Zone Zone Zone Zone Zone [kg/ sure Filter 1 2 3 4 5 6 rpm h] [bar] [mesh] 80 155 165 175 175 180 225 7.5 60 325

(11) The crosslinking agent, herein peroxide, if present, was added on to the pellets and the resulting pellets were used for the experimental part.

(12) The amounts of polymer component(s), peroxide, additives (AO and SR) are given in table 2.

(13) TABLE-US-00004 TABLE 2 Polymer compositions of the invention and reference compositions and the electrical conductivity results: Inv. comp1 Inv. comp2 Inv. comp3 Inv. comp4 Inv. comp5 Ref 1 Ref 2 Components LDPE1, wt %* 95 90 85 80 100 LDPE2, wt %* 80 100 HDPE, wt %* 5 10 15 20 20 AO, wt %** 0.08 0.08 0.08 0.08 0.08 0.08 0.08 SR, wt %** 0.05 0.05 0.05 0.05 0.35 0.05 0.35 Crosslinking agent, 28 (0.75) 28 (0.75) 28 (0.75) 28 (0.75) 50 (1.35) 28 (0.75) 50 (1.35) mmolOO/kg polymer composition (wt %**) Lamella thickness 1.9-14 1.9-15 1.9-16 1.9-16 1.9-16 1.9-8.5 1.8-8.6 [nm] Crystal fraction 5.3 9.9 17.3 20.1 18 0 0 with lamella thickness >10 nm [wt %] Crystallinity [wt %] 44.2 46.1 47.6 47.6 47.5 45.3 40 Weight fraction 2.3 4.6 8.2 9.6 8.5 0 0 of crystals with lamella thickness >10 nm [wt %] Electrical properties DC conductivity Method (1), 1 7.5 3.9 3.9 5.6 47 26 122 mm plaque (fS/m) Method (2), 5.5 15.3 41.6 mm cable (fS/m) Crosslinking agent: Dicumylperoxide (CAS no. 80-43-3) AO: Antioxidant: 4,4-thiobis (2-tertbutyl-5-methylphenol) (CAS no. 96-69-5) SR: Scorch retardant: 2,4-Diphenyl-4-methyl-1-pentene (CAS 6362-80-7) *The amounts of polymer components in table are based on the combined amount of the used polymer components. The amount 100 wt % of polymer component in table 1 means that the polymer is the sole polymer component. **The amounts of peroxide (wt %), AO and SR are based on the final composition. In this context the above used definitions have the following meanings: Lamella thickness = Thickness of crystal lamellas in the material (fractions* <0.1 wt % are ignored). *Refer to crystal fractions of one degree Celsius intervals. Crystal fraction with lamella thickness >10 nm = Fraction of the crystals which have a thickness above 10 nm based on the amount of the crystallised part of the polymer Crystallinity = wt % of the polymer that is crystalline Weight fraction of crystals with lamella thickness >10 nm [wt %] = Crystal fraction with lamella thickness >10 nm Crystallinity.