POLYOLEFIN PIPE RESIN WITH VERY GOOD SAGGING AND SLOW CRACK GROWTH RESISTANCE

Abstract

The invention relates to a polyethylene (PE) composition comprising a base resin which comprises (A) a first ethylene homo- or copolymer fraction, wherein fraction (A) has melt flow rate, MFR2, from 100 to 600 g/10 min; and (B) a second ethylene-1-hexene copolymer fraction, wherein fraction (A) has a lower molecular weight than fraction (B) and wherein fraction (B) is present in an amount of from 50.0 to 58.0 wt.% based on the total weight of the base resin; wherein the base resin has a content of units derived from 1-hexene from 0.44 to 0.79 mol% based on the total amount of base resin; wherein the base resin has a molecular weight distribution, being the ratio of Mw/Mn, from 32 to 40 and the base resin has a Z average molecular weight, Mz, of more than 1,500 kg/mol; wherein the polyethylene composition has a melt flow rate MFR5 from 0.10 to 0.25 g/10 min; and a melt flow rate ratio, FRR21/5, from 30 to 42; and wherein the polyethylene composition has a critical temperature, Tc, in the rapid crack propagation test of -10° C. or lower and not less than -25° C. The invention also relates to a PE composition obtainable by a multistage process, an article comprising a PE composition, a pipe and use of a PE composition comprising a base resin for producing an article.

Claims

1. A polyethylene composition comprising a base resin which comprises (A) a first ethylene homo- or copolymer fraction, wherein fraction (A) has melt flow rate, MFR.sub.2, as measured in accordance with ISO 1133 from 100 to 600 g/10 min; and (B) a second ethylene-1-hexene copolymer fraction, wherein fraction (A) has a lower molecular weight than fraction (B) and wherein fraction (B) is present in an amount of from 50.0 to 58.0 wt.% based on the total weight of the base resin; wherein the base resin has a content of units derived from 1-hexene from 0.44 to 0.79 mol% based on the total amount of base resin; wherein the base resin has a molecular weight distribution, being the ratio of Mw/Mn, from 32 to 40 and the base resin has a Z average molecular weight, Mz, of more than 1,500 kg/mol; wherein the polyethylene composition has a melt flow rate MFR.sub.5 from 0.10 to 0.25 g/10 min; and a melt flow rate ratio, FRR.sub.21/5, from 30 to 42; and wherein the polyethylene composition has a critical temperature, Tc, in the rapid crack propagation test of -10° C. or lower and not less than -25° C.

2. Polyethylene composition according to claim 1, wherein the polyethylene composition has a strain hardening modulus of 75 MPa or higher; or wherein the polyethylene composition has a failure time in accelerated creep test (ACT) of more than 1500 h; or wherein the polyethylene composition has a failure time in the short term pressure resistance (STPR) test at a stress level of 5.4 MPa at 80° C. of at least 200 h; or wherein the polyethylene composition has a failure time in the short term pressure resistance (STPR) test at a stress level of 12.0 MPa at 20° C. of at least 130 h; or wherein the polyethylene composition has a stress at yield at 80° C. from 6.0 to 7.0 MPa; or wherein the base resin has a viscosity at a shear stress of 747 Pa (eta.sub.747) of more than 700 kPa*s.

3. Polyethylene composition according to claim 1, wherein fraction (A) has a melt flow rate, MFR.sub.2, as measured in accordance with ISO 1133 from 100 to 230 g/10 min; or wherein fraction (A) has a melt flow rate, MFR.sub.2, as measured in accordance with ISO 1133 of 231 to 550 g/10 min.

4. Polyethylene composition according to claim 1, wherein fraction (B) of the base resin has a content of units derived from 1-hexene from 0.81 to 1.60 mol%, based on the total amount of fraction (B); or wherein fraction (B) is present in an amount of from 51.0 to 57.0 wt.%, based on the total weight of the base resin.

5. Polyethylene composition according to claim 1, wherein the base resin has a number average molecular weight, Mn, of 7,300 g/mol or higher; or wherein the base resin has a number average molecular weight, Mn, of 9,150 g/mol or lower; or wherein the base resin has a molecular weight distribution, being the ratio of Mw/Mn, from 32.5 to 39.5; or wherein the base resin has a Z average molecular weight, Mz, of more than 1,600 kg/mol.

6. Polyethylene composition according to claim 1, wherein the base resin has a density of at least 945 kg/m.sup.3; or wherein the polyethylene composition has a density of at least 953 kg/m.sup.3; or wherein the base resin has a content of units derived from 1-hexene from 0.45 to 0.78 mol%, based on the total amount of the base resin.

7. Polyethylene composition according to claim 1, wherein the polyethylene composition has a melt flow rate, MFR.sub.5, from 0.12 to 0.22 g/10 min; oe wherein the polyethylene composition has a critical temperature, Tc, in the rapid crack propagation test of -12° C. or lower and/or not less than -23° C.

8. A process for producing a polyethylene composition according to claim 1, wherein the base resin is produced in a multi-stage polymerization process in the presence of a Ziegler-Natta catalyst.

9. A polyethylene composition obtainable by a multistage process, the multistage process comprising the steps of a) polymerizing ethylene in the presence of a catalyst, in one or more loop reactor(s), in the presence of an alkyl aluminium compound and a chain transfer agent for obtaining fraction (A), the fraction (A) having a melt flow rate, MFR.sub.2, from 100 to 600 g/10 min; and b) transferring fraction (A) to a gas phase reactor feeding ethylene and comonomer to the gas phase reactor, further polymerizing to obtain a base resin comprising fraction (A) obtained in step a) and fraction (B) obtained in step b), wherein fraction (B) of the base resin has a content of units derived from 1-hexene from 0.81 to 1.60 mol%, based on the total amount of fraction (B); and wherein fraction (A) has a lower molecular weight than fraction (B) and wherein fraction (B) is present in an amount of from 50.0 to 58.0 wt.%, based on the total weight of the base resin; and c) extruding the base resin into a polyethylene composition having a melt flow rate, MFR.sub.5, from 0.10 to 0.25 g/10 min, and having a melt flow rate ratio, FRR.sub.21/5, from 30 to 42.

10. The polyethylene composition according to claim 9, wherein the process comprises a prepolymerization step before step a); or wherein the base resin has a molecular weight distribution, being the ratio of Mw/Mn, from 32 to 40; or wherein the base resin has a Z average molecular weight, Mz, of more than 1,500 kg/mol; or wherein the base resin has a viscosity at a shear stress of 747 Pa (eta.sub.747) of more than 700 kPa*s, and not more than 1400 kPa*s; or wherein the polyethylene composition has a critical temperature, Tc, in the rapid crack propagation test of -10° C. or lower, and not less than 25° C.

11. The polyethylene composition according to claim 9, wherein the polymerization catalyst is a Ziegler-Natta catalyst.

12. An article comprising the polyethylene composition according to claim 1.

13. The article according to claim 12 being a pipe or pipe fitting.

14. A pipe according to claim 13, wherein the pipe has a critical temperature, Tc, in the rapid crack propagation test of -10° C. or lower and not less than -25° C.; or wherein the pipe has a failure time in the accelerated creep test (ACT) of more than 1500 h; or wherein the pipe has a failure time in the short term pressure resistance (STPR) test at a stress level of 5.4 MPa at 80° C. of at least 200 h; or wherein the pipe has a failure time in the short term pressure resistance (STPR) test at a stress level of 12.0 MPa at 20° C. of at least 130 h.

15. A method for producing an article, comprising extruding the composition of claim 1.

Description

EXAMPLES

Materials

[0208] Comparative Example 5 (CE5) is commercially available black polyethylene compositions Eltex TUB 121 N9000 from INEOS.

[0209] Catalyst component used in (co)polymerization of ethylene in inventive and comparative examples is Lynx 200, which is a commercially available Ziegler-Natta catalyst manufactured and supplied by Grace Catalysts Technologies.

Preparation of Polymers

[0210] Polyethylene base resins and compositions according to the invention (IE1-IE7) and for comparison (CE1-CE4) were produced using Lynx 200 catalyst.

CE1

[0211] A loop reactor having a volume of 50 dm.sup.3 was operated at a temperature of 60° C. and a pressure of 56 bar. Into the reactor were fed ethylene, propane diluent and hydrogen. Also a solid polymerization catalyst component Lynx 200 was introduced into the reactor together with triethylaluminium cocatalyst so that the molar ratio of Al/Ti was about 15 mol/mol. The estimated production split was 2 wt%.

[0212] A stream of slurry was continuously withdrawn and directed to a loop reactor having a volume of 150 dm.sup.3 and which was operated at a temperature of 95° C. and a pressure of 54.5 bar. Into the reactor were further fed additional ethylene, propane diluent and hydrogen so that the ethylene concentration in the fluid mixture was 2.9 % by mole and the hydrogen to ethylene ratio was 268 mol/kmol. The estimated production split was 19 wt%. The ethylene homopolymer withdrawn from the reactor had MFR.sub.2 of 23 g/10 min.

[0213] A stream of slurry from the reactor was withdrawn intermittently and directed into a loop reactor having a volume of 350 dm.sup.3 and which was operated at 95° C. temperature and 53 bar pressure. Into the reactor was further added a fresh propane, ethylene, and hydrogen so that the ethylene concentration in the fluid mixture was 3.5 mol% and the molar ratio of hydrogen to ethylene was 242 mol/kmol. The ethylene homopolymer withdrawn from the reactor had MFR.sub.2 of 20 g/10 min. The estimated production split was 31 wt%.

[0214] The slurry was withdrawn from the loop reactor intermittently and directed to a flash vessel operated at a temperature of 50° C. and a pressure of 3 bar. From there the polymer was directed to a fluidized bed gas phase reactor operated at a pressure of 20 bar and a temperature of 85° C. Additional ethylene and 1-hexene comonomer, nitrogen as inert gas and hydrogen were added so that the molar ratio of hydrogen to ethylene was 1 mol/kmol and the molar ratio of 1-hexene to ethylene was 42 mol/kmol. The estimated production split was 48 wt%. The polymer had a melt flow rate MFR.sub.5 of 0.18 g/10 min and a density of 946 kg/m.sup.3.

IE1 to IE7 and CE2 to CE4

[0215] The procedure of CE1 was repeated by adapting reactor conditions as described in Table 2.

[0216] The polymer powder of each of the samples IE1 to IE7 and CE1 to CE4 was mixed under nitrogen atmosphere with 5.5% of carbon black master-batch (CB content 40%), 2500 ppm of antioxidants and 400 ppm Ca-stearate. Then it was compounded and extruded under nitrogen atmosphere to pellets by using a JSW CIMP90 twin screw extruder with the melt temperature of about 280° C. and SEI between 170-250 kwh/t to obtain the polyethylene compositions.

Pipe Extrusion

[0217] Pipe extrusion of 32×3 and 110×10 mm (outer diameter × wall thickness) pipes was performed on a Krauss-Maffei 45-36D (L/D) single screw extruder. The extruder has a modified PP-barrier screw installed with four heated cylinder zones and five tool zones. The downstream equipment is a 9 m spray- cooling vacuum tank with two chambers and a defined water temperature of 20° C.

[0218] For 32 mm pipe extrusion, the samples have been processed at an output rate of 50 kg/h and a screw speed of the extruder of ~ 57 rpm. The achieved melt temperature was 220-221° C. at a melt pressure of 213-216 bars. All samples have been produced at a constant meter weight of 280 g/m and a line speed of 2.97 m/minute.

[0219] The 110 mm pipes of the samples have been processed at an output rate of 160 kg/h and a screw speed of 183 - 185 rpm. The achieved melt temperature was 209° C. at a melt pressure of 201 - 205 bars. All samples have been produced at a constant meter weight of 3.13 kg/m and 0.85 m/minute.

TABLE-US-00001 Temperature profile for pipe extrusion of 32 mm and 110 mm pipes Temperature profile 32 ×3 mm pipes [°C] Temperature profile 110 ×10 mm pipes [°C] Inlet zone 75 75 Barrel zone 1 220 225 Barrel zone 2 215 220 Barrel zone 3 210 215 Barell zone 4 210 210 Barrel zone 5 200 210 Adaptor 200 200 Tool zone 1 200 200 Tool zone 2 200 200 Tool zone 3 200 200 Tool zone 4 210 210 Tool zone 5 210 210

TABLE-US-00002 Polymerization conditions for IE1 to IE7 and CE1 to CE4 Example IE1 IE2 IE3 IE4 IE5 IE6 IE7 CE1 CE2 CE3 CE4 Prepoly.reactor Temp. (°C) 50 50 50 60 50 50 50 60 60 60 60 Press. (kPa) 5666 5628 5649 5840 5720 5767 5782 5626 5622 5630 5652 Split (wt.%) 2 2 2 2 2 2 2 2 2 2 2 First loop reactor Temp. (°C) 95 95 95 95 95 95 95 95 95 95 95 Press. (kPa) 5484 5508 5513 5533 5545 5537 5551 5452 5452 5467 5475 C2 conc. (mol%) 3.0 3.1 3.0 2.5 3.0 3.1 3.1 2.9 4.4 2.9 3.3 H2/C2 ratio (mol/kmol) 667 672 669 710 598 579 583 268 328 1057 1298 Split % 19 19 19 18 20 20 20 19 18 20 17 MFR.sub.2 (g/10 min) 310 315 310 212 170 128 135 23 27 539 296 Second loop reactor Temp. (°C) 95 95 95 95 95 95 95 95 95 95 95 Press. (kPa) 5321 5276 5302 5213 5390 5437 5445 5298 5296 5302 5303 C2 conc. (mol%) 3.3 3.4 3.4 3.2 3.1 3.0 3.1 3.5 3.9 4.5 3.9 H2/C2 ratio (mol/kmol) 735 738 741 641 588 583 581 242 371 1067 1303 Split % 26 26 26 26 24 24 24 31 20 25 20 Density by balance, plaque 971 971 971 971 971 971 971 -- -- -- -- MFR.sub.2 (g/10 min) 312 320 320 376 180 166 170 20 36 514 420 Temp. (°C) 85 85 85 85 85 85 85 85 85 85 85 Press. (kPa) 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 H2/C2 ratio (mol/kmol) 11 9 9 22 14 11 11 1 29 5 27 C6/C2 ratio (mol/kmol) 50 46 44 28 46 50 49 42 30 40 34 split % 53 53 53 54 54 54 54 48 60 53 61 Density (kg/m.sup.3) 947 947 947 947 947 947 947 946 946 945 946

TABLE-US-00003 Properties of the base resins and the polyethylene compositions for IE1 to IE7 and CE1 to CE5 Example IE1 IE2 IE3 IE4 IE5 IE6 IE7 CE1 CE2 CE3 CE4 CE5 Base resin Density (kg/m.sup.3) 947 947 947 947 947 947 947 946 946 945 946 NA Polyethylene composition MFR.sub.5 [g/10 min] 0.18 0.16 0.15 0.15 0.20 0.16 0.15 0.17 0.16 0.16 0.17 0.22 MFR21 [g/10 min] 6.06 5.82 5.79 4.95 6.08 5.47 5.24 6.10 3.45 6.42 4.99 7.52 FRR.sub.21/5 33.1 36.4 38.6 33.2 30.4 34.2 34.9 35.9 21.6 40.1 29.4 34.2 Mn [g/mol] 8700 8520 8680 8510 8415 8845 8945 11150 12450 7310 8440 9225 Mw [g/mol] 306000 331000 317000 295000 300500 323000 315000 290500 255000 288500 268000 240000 Mz [g/mol] 1815000 1980000 1860000 1720000 1755000 1850000 1800000 1730000 1270000 1685000 1440000 1245000 Mw/Mn 35.2 38.8 36.5 34.7 35.7 36.5 35.2 26.1 20.5 39.5 31.8 26.0 C6 total [mol%] 0.61 0.56 0.55 0.60 0.46 0.52 0.52 0.76 0.48 0.88 0.70 0.46 C6 in HMW fraction [mol%] 1.15 1.06 1.04 1.11 0.85 0.96 0.96 1.58 0.80 1.65 1.15 NA Eta747 [kPa*s] 786.44 1015 966.5 986.22 737 888 986 1383.5 671.49 917.68 701.32 828.12 SH modulus [MPa] 95.9 93.3 90.4 87.6 75.5 84.1 85.1 88.0 68.3 103.2 84.4 87.3 Density [kg/m3] 957.0 958.2 958.8 957.6 959.8 958.4 958.2 956.1 955.8 956 955.7 959.4 SPTR (12.0 MPa/20° C.) [h] 252.3 460.5 420.0 203.7 -- -- -- 100.4 113.4 50.2 102.3 -- SPTR (5.4 MPa/80° C.) [h] 373.0 3636.4 2125.3 487.4 -- -- -- -- -- 3.5 -- -- Tc [°C] -13.9 -16.5 -15.6 -15.7 -- -- -- -16.8 -31.8 -12.4 -17.6 -- CIS (23° C. [kJ/m.sup.2] 40.65 38.58 36.54 -- 32.57 38.36 40.2 -- -- -- -- 35.85 CIS (0° C.) [kJ/m.sup.2] 29.23 28.45 25.97 -- 22.27 27.66 27.93 43.7 42.66 41.25 37.76 22.45 CIS (-20° C.) [kJ/m.sup.2] 17.5 16.99 17.0 -- 14.93 18.05 19.64 24.02 28.51 22.86 22.43 14.63 PLT+, 110 mm pipes [h] >3500 >3500 >3500 -- -- -- -- -- -- -- -- -- ACT [h] >3000 >3000 >3000 -- -- -- -- 3636 -- -- 3111.4 -- NPT [h] >4682 >4682 >4682 -- -- -- -- >12961 6556 361 >12097 -- Meltpressure, bar (110 mm) 205 204 201 225 -- -- -- 198 273 196 231 -- Meltpressure, bar (32 mm) 216 215 213 236 -- -- -- 207 285 208 244 -- Carbon Black (wt.%) 2.2 2.2 2.2 2.3 2.3 2.2 2.2 2.3 2.3 2.6 2.1 -- Stress at yield, MPa, 0.3 mm/min 6.24 6.35 6.36 6.11 -- -- -- 6.18 6.69 5.65 6.03 6.73

[0220] As can be derived from Table 3, IE1 to IE7 demonstrate the combination of excellent slow crack growth resistance together with very good impact resistance and good rapid crack propagation resistance (low critical temperature), while also meeting the requirements of PE100 standard and having very good sagging resistance (as demonstrated e.g. by Mz values). Furthermore, the compositions of IE1 to IE7 also show excellent behavior in the accelerated point loading test and in the ACT test.

[0221] This advantageous combination of properties is achieved by the specific Mz and eta.sub.747 values of the composition, the specific MWD (as demonstrated by Mw/Mn and FRR.sub.21/5 values), the specific amount of 1-hexene in the high molecular weight fraction, the specific amount (weight fraction) of the high molecular weight fraction, the MFR in the defined range and the specific comonomer used (1-hexene).

[0222] Comparative Examples 1 to 5 demonstrate that deviations from the inventive polymer structure yield in polymer compositions having inferior property combinations compared to IE1 to IE7. None of Comparative examples 1 to 5 has the advantageous combination of properties of the inventive polymer compositions.