Bimodal polyethylene composition and pipe comprising the same

Abstract

The present invention relates to a bimodal polyethylene composition comprising a low molecular weight polyethylene homopolymer fraction and high molecular weight polyethylene copolymer fraction having a C.sub.4 to C.sub.10 -olefin comonomer content of 0.25 to 3% mol with respect to the total monomer comprised in the high molecular weight polyethylene comonomer fraction, wherein the content of the low molecular weight polyethylene is from 50 to 60 wt % with respect to the total weight of the bimodal polyethylene composition; and the bimodal polyethylene composition has a soluble fraction according to Temperature Rising Elution Fractionation in 1,2,4-trichlorobenzene with 300 ppm of butylated hydroxytoluene at 150 C. of less than 6 wt %; and a pipe comprising the same.

Claims

1. A bimodal polyethylene composition comprising a low molecular weight polyethylene homopolymer fraction and high molecular weight polyethylene copolymer fraction having a C.sub.4 to C.sub.10 -olefin comonomer content of 0.25 to 3% mol with respect to the total monomer comprised in the high molecular weight polyethylene comonomer fraction, wherein the content of the low molecular weight polyethylene is from 50 to 60% wt with respect to the total weight of the bimodal polyethylene composition; and the bimodal polyethylene composition has a soluble fraction according to Temperature Rising Elution Fractionation in 1,2,4-trichlorobenzene with 300 ppm of butylated hydroxytoluene at 150 C. of less than 6% wt.

2. The bimodal polyethylene composition according to claim 1, wherein the bimodal polyethylene composition has a density from 0.945 to 0.965 g/cm.sup.3 and a MI.sub.5 of 0.10 to 0.50 g/10 min.

3. The bimodal polyethylene composition according to claim 1, wherein the bimodal polyethylene composition comprises 50 to 60% wt with respect to the total weight of the bimodal polyethylene composition of the low molecular weight polyethylene homopolymer fraction and 40 to 50% wt with respect to the total weight of the bimodal polyethylene composition of the high molecular weight polyethylene copolymer fraction.

4. The bimodal polyethylene composition according to claim 1, wherein the bimodal polyethylene composition has the soluble fraction according to Temperature Rising Elution Fractionation in 1,2,4-trichlorobenzene with 300 ppm of butylated hydroxytoluene at 150 C. of 2 to 5.9% wt.

5. The bimodal polyethylene composition according to claim 1, wherein the bimodal polyethylene composition has the soluble fraction according to Temperature Rising Elution Fractionation in 1,2,4-trichlorobenzene with 300 ppm of butylated hydroxytoluene at 150 C. of 3 to 5.9% wt.

6. The bimodal polyethylene composition according to claim 1, wherein the high molecular weight polyethylene copolymer fraction has a C.sub.4 to C.sub.10 -olefin comonomer content of 0.25 to 2% mol with respect to the total monomer comprised in the high molecular weight polyethylene comonomer fraction.

7. The bimodal polyethylene composition according to claim 1, wherein the high molecular weight polyethylene copolymer fraction has a C.sub.4 to C.sub.10 -olefin comonomer content of 0.5 to 1% mol with respect to the total monomer comprised in the high molecular weight polyethylene comonomer fraction.

8. The bimodal polyethylene composition according to claim 1, wherein the low molecular weight polyethylene is from 50 to 55% wt with respect to the total weight of the bimodal polyethylene composition.

9. The bimodal polyethylene composition according to claim 1, wherein the high molecular weight polyethylene is from 45 to 50% wt with respect to the total weight of the bimodal polyethylene composition.

10. The bimodal polyethylene composition according to claim 1, wherein the C.sub.4 to C.sub.10 -olefin comonomer is selected from the group consisting of 1-butene, 1-hexene, 1-octene, 1-decene and mixtures thereof.

11. Pipe comprising the bimodal polyethylene composition according to claim 1.

12. The bimodal polyethylene composition of claim 2, having a density from 0.952 to 0.965 g/cm.sup.3.

13. The bimodal polyethylene composition of claim 12, having a density from 0.955 to 0.963 g/cm.sup.3.

14. The bimodal polyethylene composition of claim 2, having a MI.sub.5 from 0.15 to 0.35 g/cm.sup.3.

15. The bimodal polyethylene composition of claim 14, having a MI.sub.5 from 0.18 to 0.28 g/cm.sup.3.

16. The bimodal polyethylene composition of claim 10, wherein the C.sub.4 to C.sub.10 -olefin comonomer is 1-hexene.

Description

EXAMPLES

(1) In order to produce comparative (Comp.) and inventive (Inv.) bimodal PE resin, the polymerization process and procedure is typically the same as that of CX slurry process. Also, Ziegler-Natta catalyst is used. The comonomer type was applied by 1-hexene. However, the operating conditions have to optimize with polymer design.

(2) The polymerization catalysts include coordination catalysts of a transition metal called Ziegler-Natta (ZN). A commercial available Ziegler-Natta catalyst was used. Bimodal polyethylene resins, hereinafter base resin, produced in accordance with two-stage cascade slurry polymerization process and having composition ratios of a) low molecular weight (LMW) HDPE having MI.sub.2 in the range of 100 to 700 g/10 min, and density 0.970 g/cm.sup.3 and b) the bimodal high molecular weight (HMW) HDPE containing LMW from 1.sup.st reactor and having MFR.sub.5 0.20-0.340 g/10 min and density 0.945-0.965 g/cm.sup.3. The LMW HDPE resin is a homopolymer polymerized in the first reactor in the absence of comonomer. The HMW PE resin produced in the second reactor is copolymer containing 1-hexene content of 0.5-1.0% mol. The bimodal resin comprises 50 to 60% wt. of the first polyethylene homopolymer fraction and 40 to 50% wt. of a second polyethylene copolymer fraction.

(3) The obtaining bimodal PE product from the second reactor was dried and the resulting powder sent to a finishing operation where it was compounded with carbon black 2-2.5 wt % in extruder at 210 C. under nitrogen atmosphere with 2000 ppm Ca/Zn stearate and 3000 ppm hindered phenol/phosphate stabilizers and, then, pelletized. Density and MI were obtained using the pelletized resins.

(4) Plastic pipe is produced by extruding molten polymer through an annular die. The pipe is formed by passing the molten extrudate through a sizing sleeve and then to a cooling tank where water is sprayed on the outer surface. Solidification proceeds from the outer surface radially inward.

(5) Polymerization conditions and polymer properties are shown in Table 1-2, respectively. Testing results and analysis were applied and recorded on the compound.

(6) TABLE-US-00001 TABLE 1 Polymerization conditions of Comparative example and Inventive example. Sample Comp. Comp. Comp. Property Unit 1 2 3 Inv. 1 Inv. 2 Homopolymer Split ratio % 53-55 53-55 58-60 54-56 51-53 Temperature C. 81-85 81-85 81-85 81-85 81-85 Pressure Bar 6.0-6.5 6.0-6.5 6.0-6.5 7.5-8.0 6.0-6.5 Hexane flow rate L/h 75.79 75.79 77.79 44.8 49.38 Ethylene flow L/h 1507.4 1507.4 1932.5 1243.7 1621.8 rate Hydrogen flow NL/h 1276.24 1276.24 286.64 443 239.4 rate Catalyst flow g/h 4.05 4.05 4.26 3.03 2.68 rate Copolymer Split ratio % 45-46 45-46 39-40 45-46 46-48 Temperature C. 70-75 70-75 68-70 68-70 67-69 Pressure Bar 1.5-3.0 1.5-3.0 1.5-3.0 1.5-3.0 1.5-3.0 Hexane flow rate L/h 71.0 71.0 80.9 88.0 65.6 Ethylene flow L/h 2178.2 2178.2 2282.6 2804.0 3597.0 rate Hydrogen flow NL/h 148.87 148.87 115.01 1.77 1.57 rate Comonomer Kg/h 0.991 0.991 1.25 1.15 2.15 Comonomer type Hx-1 Hx-1 Hx-1 Hx-1 Hx-1

(7) All comparative and inventive examples were produced using different polymerization conditions. The low polymer content was adjusted by changing the conditions of centrifuge separation between polymer powder and diluent in slurry. Inventive example 1 and Inv. 2, show high pressure resistance at 20 C. in various hoop stress from 12.0 to 13.2 MPa. The SCG property shown by accelerated crack test (ACT) results of Comp. 1-3 and Inv. 1-2 is linearly inverse proportional to low polymer content indicated by soluble fraction from TREF measurement. From ACT results by Hessel, Germany are more than 1000 hours which accordance with correlation of Full Notched Creep Test (FNCT) for more than 8760 hours. In certain embodiment of invention, the obtainable invention meets the designation of PE112RC. All the results indicated the distinguish features and advantages of the inventive ethylene copolymer compositions over the prior art.

(8) TABLE-US-00002 TABLE 2 Polymer properties of Example 1, Example 2 and Comparative examples. Sample Property Unit Comp. 1 Comp. 2 Comp. 3 Inv. 1 Inv. 2 Homopolymer MFR.sub.2 g/10 min 670 620 232 556 340 Copolymer Density g/cm3 0.963 0.962 0.963 0.962 0.959 MFR.sub.5 g/10 min 0.24 0.27 0.22 0.2 0.21 1-Hexene Content % mol 0.71 0.7 0.62 0.78 0.77 Crystallinity % 59.2 61.37 60.1 63 59.72 Soluble fraction by % wt 10.3 10 9.1 5.5 3.5 TREF Mw g/mol 231043 225450 233430 263166 267811 Mn g/mol 6860 6937 7991 8679 10136 Mz g/mol 1363644 1294832 1612888 1910970 186469 MWD 33.7 32.5 29.2 30.3 26.4 Eta747 Pa .Math. s 723 716 1211 1017 842 Charpy impact kJ/m.sup.2 21.73 23.453 22.9 30.77 33.69 (23 C.) Charpy impact kJ/m.sup.2 15.586 17.817 13.9 22.47 25.46 (0 C.) Charpy impact kJ/m.sup.2 8.104 7.617 6.5 10.9 11.37 (30 C.) ACT h 84.6 102.2 335.6 1001 1160 Pressure resistance at 20 C. 13.0 MPa h 58 NA 87 627 NA 12.8 MPa h 89 NA 155 766 NA 12.6 MPa h 152 NA 260 1273 NA 12.4 MPa h 203 NA 264 2522 NA 12.0 MPa h 548 NA 728 >4200 NA Estimated LPL at MPa NA NA NA >11.2 NA 50 years

(9) The features disclosed in the foregoing description and in the claims may, both separately and in any combination, be material for realizing the invention in diverse forms thereof.