Polyolefin Composition With Improved Resistance To High Temperature

Abstract

The present invention relates to a polyethylene composition comprising a base resin which comprises a first ethylene homo- or copolymer fraction (A) having a melt flow rate MFR.sub.2 from 1 to 150 g/10 min, preferably from 5 to 100 g/10 min, and a second ethylene copolymer fraction (B) having a content of units derived from a comonomer from 0.30 to 1.00 mol %, preferably of from 0.40 to 0.85 mol % and more preferably of from 0.45 to 0.70 mol %, wherein fraction (A) has a lower molecular weight than fraction (B) and wherein fraction (B) is present in an amount of from 45 to 70 wt. %, preferably 47 to 67 wt. %, more preferably 49 to 65 wt. %, even more preferably 50 to 62 wt. % based on the total weight of the base resin; and wherein the polyethylene composition has a melt flow rate MFR.sub.5 from 0.10 to 0.35 g/10 min, preferably from 0.10 to 0.25 g/10 min and a strain hardening modulus from 50 to 150 MPa.

Claims

1. A polyethylene composition comprising a base resin which comprises (A) a first ethylene homo- or copolymer fraction having a melt flow rate MFR.sub.2 from 1 to 150 g/10 min, and (B) a second ethylene copolymer fraction having a content of units derived from a comonomer from 0.30 to 1.00 mol %, wherein fraction (A) has a lower molecular weight than fraction (B) and wherein fraction (B) is present in an amount of from 45 to 70 wt. %, based on the total weight of the base resin; wherein the polyethylene composition has a melt flow rate MFR.sub.5 from 0.10 to 0.35 g/10 min; and wherein the polyethylene composition has a strain hardening modulus from 50 to 150 MPa.

2. The polyethylene composition according to claim 1, wherein the base resin has a molecular weight distribution, being the ratio of Mw/Mn; and/or wherein the base resin has a polydispersity index (PI) from 1.2 to 3.0 Pa.sup.−1 and/or wherein the base resin has a viscosity at a shear stress of 747 Pa (eta.sub.747) from 200 to 800 kPa*s; and/or wherein the base resin has a density in the range of 940 to 957 kg/m.sup.3.

3. The polyethylene composition according to claim 1, wherein in fraction (B) the units derived from a comonomer are units derived from at least one alpha-olefin comonomer, preferably 1-hexene and/or 1-butene and more preferably 1-hexene; and/or wherein the base resin has a content of units derived from the comonomer of not more than 0.6 mol %, preferably not more than 0.5 mol % based on the base resin.

4. The polyethylene composition according to claim 1, wherein fraction (A) is an ethylene homopolymer; and/or wherein fraction (A) has a melt flow rate MFR.sub.2 from 7.5 to 75 g/10 min.

5. The polyethylene composition according to claim 1, wherein the base resin has been produced in a multistage process; or wherein the base resin has been produced in a multistage process in the presence of a Ziegler-Natta catalyst.

6. The polyethylene composition according to claim 1, wherein the base resin has an average molecular weight, Mn, in the range of from 9.000 to 20.000 g/mol, preferably from 10.000 to 18.000 g/mol.

7. The polyethylene composition according to claim 1, wherein the polyethylene composition has a strain hardening modulus from 55 to 100 MPa; and/or wherein the polyethylene composition has a stress at yield at 80° C. from 6.5 to 7.5 MPa; and/or wherein the polyethylene composition has a failure time in the short term pressure resistance (STPR) test at a stress level of 5.9 MPa at 80° C. of at least 700 h; and/or wherein the polyethylene composition has a failure time in the short term pressure resistance (STPR) test at a stress level of 6.2 MPa at 80° C. of at least 70 h; and/or wherein the polyethylene composition satisfies the following inequation: Stress at yield at 80° C.>9.58-0.92*log (NPT).

8. 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 1 to 150 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) has a content of units derived from the comonomer of 0.30 to 1.00 mol % and wherein fraction (B) is present in an amount of from 45 to 70 wt % based on the total weight of the base resin; c) extruding the base resin into a polyethylene composition having a melt flow rate MFR.sub.5 from 0.10 to 0.35 g/10 min; wherein the polyethylene composition has a strain hardening modulus from 50 to 150 MPa.

9. The polyethylene composition according to claim 8, wherein the process comprises a prepolymerization step before step a).

10. The polyethylene composition according to claim 8, wherein the polymerization catalyst is a Ziegler-Natta catalyst; and/or wherein the fraction (A) obtained in step a) has a melt flow rate from 7.5 to 75 g/10 min.

11. An article comprising a polyethylene composition according to claim 1 or claim 7.

12. An article according to claim 11 wherein the article is a pipe or a fitting.

13. A pipe according to claim 12, wherein the pipe has a resistance to stress cracking measured by the notched pipe test of more than 500 hours; and/or wherein the pipe has a critical temperature, Tc, of −15° C. or lower.

14. A method for producing an article, the method comprising the step of: forming the article from the polyethylene composition of claim 1 or claim 7.

15. A method of producing a pipe or fitting having an improved pressure resistance at a temperature of 30° C. or more of more than 10 MPa, the method comprising the step of forming the pipe or fitting from the polyethylene composition of claim 1 or claim 7.

Description

EXAMPLES

[0190] Polyethylene base resins and compositions according to the invention and for comparison were produced using catalyst A or catalyst B.

Inventive Example 1 (IE1)

[0191] A loop reactor having a volume of 50 dm.sup.3 was operated at a temperature of 60° C. and a pressure of 55 bar. Into the reactor were fed ethylene, propane diluent and hydrogen. Also a solid polymerization catalyst component A produced as described above 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 3 wt %.

[0192] 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 bar. Into the reactor were further fed additional ethylene, propane diluent and hydrogen so that the ethylene concentration in the fluid mixture was 5.2% by mole and the hydrogen to ethylene ratio was 193 mol/kmol. The estimated production split was 20 wt %. The ethylene homopolymer withdrawn from the reactor had MFR.sub.2 of 5 g/10 min.

[0193] 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 52 bar pressure. Into the reactor was further added a fresh propane, ethylene, and hydrogen so that the ethylene concentration in the fluid mixture was 5.2 mol-% and the molar ratio of hydrogen to ethylene was 207 mol/kmol. The ethylene homopolymer withdrawn from the reactor had MFR2 of 13 g/10 min. The estimated production split was 27 wt %.

[0194] 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 30 mol/kmol and the molar ratio of 1-hexene to ethylene was 37 mol/kmol. The estimated production split was 50 wt %. The polymer had a melt flow rate MFR.sub.5 of 0.15 g/10 min and a density of 947.2 kg/m.sup.3.

IE2 to IE11 and CE1 to CE7

[0195] The procedure of IE1 was repeated by changing reactor conditions as described in Tables 1 and 2.

[0196] The polymer powder of each of the samples IE1 to IE 11 and CE1 to CE7 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 250° C. and SEI specified in below tables to obtain the polyethylene compositions.

[0197] Polymerization conditions and properties of the produced base resins and polyethylene compositions of the inventive and comparative examples are shown in Tables 1, 2 and 3.

TABLE-US-00001 TABLE 1 Polymerization conditions for inventive examples IE1 IE2 IE3 IE4 IE5 IE6 IE7 IE8 IE9 IE10 IE11 Catalyst A A A A A A B B B B B Prepoly reactor Temp. (° C.) 60 60 70 70 70 70 60 60 60 60 60 Press. (kPa) 5550 5541 5618 5601 5601 5601 5707 5705 5798 5802 5802 Split (%) 3 3 2 2 2 2 3 3 3 3 3 First loop reactor Temp. (° C.) 95 95 95 95 95 95 95 95 95 95 95 Press. (kPa) 5442 5437 5455 5404 5409 5413 5573 5579 5555 5556 5556 C2 conc. (mol-%) 5.2 3.8 7.0 3.9 5.1 4.7 2.4 3.0 4.1 4.3 4.3 H2/C2 ratio (mol/kmol) 193 444 148 206 149 156 419 426 248 251 258 Split (%) 20 15 14 13 14 13 20 19 20 20 20 MFR2 (g/10 min) 5 45 25 49 32 35 68 50 21 18 22 Second loop reactor Temp. (° C.) 95 95 95 95 95 95 95 95 95 95 95 Press. (kPa) 5211 5207 5291 5389 5388 5388 5481 5492 5393 5394 5395 C2 conc. (mol-%) 5.2 4.8 4.4 3.8 4.2 4.1 2.8 3.3 4.6 4.7 4.8 H2/C2 ratio (mol/kmol) 207 381 171 166 141 138 380 419 250 258 256 Split (%) 27 21 29 30 29 30 26 2 24 24 24 MFR2 (g/10 min) 13 86 41 30 29 24 147 150 27 30 29 Gas phase reactor 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) 30 49 9 5 5 7 34 40 25 25 25 C6/C2 ratio (mol/kmol) 37 36 15 29 25 21 33 33 33 33 33 Split (%) 50 61 55 55 55 55 51 51 53 53 53 Powder Density (kg/m3) 947.2 947.2 949.7 947.4 947.7 948.8 951.1 951.7 950.1 950.2 950.7 MFR5 (g/10 min) 0.15 0.15 0.11 0.15 0.18 0.14 0.20 0.23 0.15 0.15 0.16 Compound MFR5 (g/10 min) 0.14 0.16 0.14 0.15 0.16 0.14 0.21 0.23 0.14 0.15 0.16 MFR21 (g/10 min) 3.01 3.00 3.45 3.73 3.65 3.43 5.71 6.34 3.45 3.73 3.8 Density (kg/m3) 958.3 958.4 962.4 958.9 960.8 961.5 963.0 963.1 960.3 962.0 960.3 PE White spots 5.7 3.0 5.2 6.2 5.6 5.20 5.0 5.5 White Spot Area 0.40 0.02 0.35 0.71 0.60 0.36 SEI (kwh/t) 189.3 197.2 175.4 185.1 178.7 180.5 160.6 133.6 178.7 181.6 181.7 Melt pump power (kW) 8.0 8.2 9.7 9.8 10.1 12.2 8.2 7.2 8.3 8.1 8.3

TABLE-US-00002 TABLE 2 Polymerization conditions for comparative examples CE1 CE2 CE3 CE4 CE5 CE6 CE7 Catalyst A A B B B B B Prepoly reactor Temp. (° C.) 70 70 60 60 60 60 60 Press. (kPa) 5625 5771 5705 5708 5713 5689 5708 Split (%) 2 2 3 3 3 3 3 First loop reactor Temp. (° C.) 95 95 95 95 95 95 95 Press. (kPa) 5453 5536 5585 5584 5587 5586 5582 C2 conc. (mol-%) 7.0 3.4 3.4 3.4 3.0 3.8 3.0 H2/C2 ratio (mol/kmol) 154 415 634 583 599 618 618 Split (%) 14 16 21 19 22 17 22 MFR2 (g/10 min) 29 657 189 144 135 256 168 Second loop reactor Temp. (° C.) 95 95 95 95 95 95 95 Press. (kPa) 5292 5393 5505 5500 5503 5499 5490 C2 conc. (mol-%) 4.4 3.3 4.1 3.5 3.8 3.5 3.5 H2/C2 ratio (mol/kmol) 169 345 649 563 552 647 552 Split (%) 29 27 30 30 32 25 30 MFR2 (g/10 min) 34 350 440 400 237 596 440 GPR Temp. (° C.) 85 80 85 85 85 85 85 Press. (kPa) 2000 2002 2000 2000 2000 2000 2000 H2/C2 ratio (mol/kmol) 14 7 15 49 1 35 52 C6/C2 ratio (mol/kmol) 5 59 14 79 1 7 24 Split (%) 55 55 46 48 43 55 45 Powder Density (kg/m3) 954.5 946.0 956.4 948.7 947.4 956.6 957.0 MFR5 (g/10 min) 0.13 0.20 0.07 0.40 0.13 0.10 0.50 Compound MFR5 (g/10 min) 0.14 0.22 0.08 0.43 0.14 0.10 0.52 MFR21 (g/10 min) 3.18 6.0 3.82 13.20 8.83 3.04 15.40 Density (kg/m3) 964.8 957.0 966.9 960.4 958.2 967.7 968.5 PE White spots 4.9 4.6 14.0 6.1 10.8 12.7 7.8 White Spot Area 0.7 0.32 SEI (kwh/t) 181.8 174.3 157.7 154.8 178.4 165.0 183.2 Melt pump power (kW) 7.8 7.6 7.6 6.7 7.3 9.2 5.3

TABLE-US-00003 TABLE 3 Properties of inventive examples and comparative examples Example IE1 IE2 IE3 IE4 IE5 IE6 IE7 IE8 IE9 IE10 PI [Pa(.sup.−1)] 2.01 1.58 2.26 2.5 2.44 2.38 2.28 2.19 2.21 2.34 Eta747 476 352 675 704 750 764 347 304 502 453 [kPa*s] Mn [g/mol] 17650 14100 12350 13500 13900 14100 10800 10650 15500 15100 Mw [g/mol] 273500 254000 271500 277000 284500 287500 235000 233500 289000 275000 Mz [kg/mol] 1280 1105 1385 1305 1375 1360 1125 1105 1310 1250 Mw/Mn 15.5 18.0 22.0 20.5 20.5 20.4 21.8 21.9 18.6 18.2 SH 68.6 66.6 53 77.6 71.9 68.9 70 68.3 67.6 64.5 modulus [MPa] Stress at 6.99 6.96 7.01 6.93 7 7.07 6.9 6.9 7.09 7.1 yield [MPa] C6 [mol %] 0.33 0.36 0.27 0.38 0.35 0.27 0.32 0.33 0.25 0.25 C6 in HMW 0.66 0.59 0.49 0.69 0.63 0.50 0.63 0.65 0.47 0.47 [mol %] STPR 5.9 5.9 6.0 6.2 6.2 6.2 5.9 5.9 n. m. n. m. Stress level (80° C.) [MPa] STPR 3000 >10963 2224 117 192 414 903 1446 n. m. n. m. Failure time.sup.1 [h] STPR D D D D D D D n. m. n. m. Failure type.sup.2 Tc [° C.] −35.2 −39.7 −37.6 −26.5 −30.5 −24.1 −21.7 n. m. n. m. NPT.sup.3 [h] 5796 7991 656 >6000 4777 2979 1385 1278 n. m. n. m. Tensile 1068 1049 1128 1089 1116 1144 1043 1151 n. m. n. m. modulus [MPa] CIS (23° C.) 83.0 70.7 51.4 n. m. n. m. n. m. 39.7 37.9 n. m. n. m. [KJ/m.sup.2] CIS (0° C.) 49.0 44.7 34.1 35.3 33.2 34.6 28.0 27.5 n. m. n. m. [kJ/m.sup.2] CIS 39.1 35.8 29.3 27.3 26.4 27.8 18.7 17.9 n. m. n. m. (−20° C.) [KJ/m.sup.2] Example IE11 CE1 CE2 CE3 CE4 CE5 CE6 CE7 PI [Pa(.sup.−1)] 2.25 2.21 2.61 2.95 2.7 3.9 1.89 2.7 Eta747 453 617 401 1390 167 1329 729 146 [kPa*s] Mn [g/mol] 15300 11900 10400 8380 8415 8225 8615 8095 Mw [g/mol] 279000 275000 260000 329500 200000 315000 280500 193000 Mz [kg/mol] 1270 1375 1290 1665 1005 1750 1295 1015 Mw/Mn 18.2 23.1 25.0 39.3 23.8 38.3 32.6 23.8 SH 64.1 29.7 91.3 60 90.8 140 46.2 42 modulus [MPa] Stress at 7.09 7.08 6.45 7.27 6.1 5.8 6.9 6.8 yield [MPa] C6 [mol %] 0.26 0.06 0.64 0.12 0.72 0.91 0.08 0.21 C6 in HMW 0.50 0.11 1.16 0.26 1.52 2.10 0.15 0.46 [mol %] STPR n. m. 5.0 5.8 5.8 5.4 5.0 5.4 5.6 Stress level (80° C.) [MPa] STPR n. m. 924 50 1508 90 2110 639 1399 Failure time.sup.1 [h] STPR n. m. B D B D D B B Failure type.sup.2 Tc [° C.] n. m. −39.8 −18.5 −41.7 −7.1 −10.7 −41.8 −16.3 NPT.sup.3 [h] n. m. 29 274 5078 2856 45 76 Tensile n. m. 1284 1043 1350 1061 1003 1344 1330 modulus [MPa] CIS (23° C.) n. m. 52.1 49.2 34.8 43.3 60.9 53.3 18.2 [KJ/m.sup.2] CIS (0° C.) n. m. 35.4 n. m. 27.5 29.7 45.0 38.6 12.6 [kJ/m.sup.2] CIS n. m. 30.6 22.0 23.0 10.7 24.4 30.8 6.8 (−20° C.) [KJ/m.sup.2] .sup.1Geometric average of two measurements. .sup.2D = ductile; B = brittle. .sup.3Geometric average of two measurements. n. m. = not measured

[0198] CEs 1, 3, 6 and 7 all show low NPT values, making these compositions unsuitable for PE100. The NPT is higher for CEs 4 and 5, but at the same time the stress at yield parameter is low. Contrary to this, the inventive examples achieve a good balance between notched pipe test results (measured of the slow crack growth resistance) and the stress at yield. This demonstrates that any deviation from the claimed polymer design yields an inferior balance of the above properties.

[0199] It is evident that all inventive examples have a superior balance between stress at yield and notched pipe test results and they all satisfy the following relationship: stress at yield>9.58-0.92*log(NPT).