Bimodal high density polyethylene

Abstract

The invention is directed to bimodal polyethylene having a flow ratio FRR ranging between 30 and 40, a density ranging between 949.0 and 952.0 kg/m3, an MFR.sub.190/5 ranging between 0.1 and 0.2 g/10 min and comprising from 50 to 54% by weight of an ethylene homopolymer A and from 46-50% by weight of an ethylene-butene copolymer B, where all percentages are based on the total weight of the composition and wherein ethylene homopolymer A has a viscosity number 70 and 100 cm3/g and a density between 968 and 972 kg/m3. The polyethylene is suitable to be applied in the production of pipes.

Claims

1. A bimodal polyethylene having a flow ratio FRR ranging between 30 and 40, a density ranging between 949.0 and 952.0 kg/m.sup.3, an MFR.sub.190/5 ranging between 0.1 and 0.2 g/10 min and comprising from 50 to 54% by weight of an ethylene homopolymer A and from 46 to 50% by weight of an ethylene-butene copolymer B, where all percentages are based on the total weight of the composition and wherein ethylene homopolymer A has a viscosity number 70 and 100 cm.sup.3/g and a density between 968 and 972 kg/m.sup.3 and wherein the bimodal polyethylene has impact resistance (according to notched Charpy measurements at 23 C.; ISO 179)30 kJ/m.sup.2 and 40 kJ/m.sup.2, impact resistance (notched Charpy measurements at 30 C.; ISO 179) 15 kJ/m.sup.2 and 20 kJ/m.sup.2, strain hardening (measured according to the strain hardening method, based on the publication by Kurelec, L. et al in Polymer 2005, 46, p 6369-6379) 40 MPa and 50 MPa, and shear thinning index 65 and 80.

2. The bimodal polyethylene according to claim 1 characterized in that the density of ethylene homopolymer A ranges between 969 and 971 kg/m.sup.3.

3. The bimodal polyethylene according to claim 1, characterized in that the viscosity number of ethylene homopolymer A ranges between 77 and 90 cm.sup.3/g.

4. The bimodal polyethylene according to claim 1, characterized in that the density of the bimodal polyethylene ranges between 949.0 and 951.0 kg/m.sup.3.

5. The bimodal polyethylene according to claim 1, wherein the bimodal polyethylene comprises from 50 to 52% by weight of the ethylene homopolymer A and from 48 to 50% by weight of the ethylene-butene copolymer B.

6. The bimodal polyethylene according to claim 1, wherein an amount of butene incorporated in the ethylene-butene copolymer B ranges from 0.1 to 5% by weight.

7. A process for the preparation of bimodal polyethylene according to claim 1 with a two-step slurry polymerisation process in the presence of a catalyst system comprising (I) the solid reaction product obtained from the reaction of: a) a hydrocarbon solution containing 1) an organic oxygen containing magnesium compound or a halogen containing magnesium compound, and 2) an organic oxygen containing titanium compound and b) an aluminium halogenide having the formula AlR.sub.nX.sub.3-n in which R is a hydrocarbon moiety containing 1-10 carbon atoms, X is halogen and 0<n<3, and (II) an aluminium compound having the formula AlR.sub.3 in which R is a hydrocarbon moiety containing 1-10 carbon atoms.

8. An article comprising the bimodal polyethylene according to claim 1.

9. A pipe having a pressure resistance at 20 C., 12 MPa (measured according to ISO 1167 with pipe dimensions 323 mm) >8000 hours comprising the bimodal polyethylene according to claim 1.

Description

EXAMPLES

(1) The solids content in the catalyst suspension was determined in triplo by drying 5 ml of a catalyst suspension under a stream of nitrogen, followed by evacuating for 1 hour and subsequently weighing the obtained amount of dry catalyst.

(2) The density of the polymers is measured according to ISO1183.

(3) The viscosity number is determined according to ISO 1628-3.

(4) The melt-indices MFR.sub.190/5 and MFR.sub.190/21.6 are measured according to method ASTM D-1238 under a load of 5 and 21.6 kg at 190 C.

(5) The Flow Rate Ratio (FRR) being calculated as MFR.sub.190/21.6/MFR.sub.190/5 is indicative for the rheological broadness of the material.

(6) The split of the bimodal polymer is defined as the weight fraction of the lower molecular weight material in the overall polymer. For the semi-batch process as described in the following polymerization examples, this translates into the cumulative ethylene consumption from the first polymerization step compared to the cumulative ethylene consumption in the combined first and second step.

(7) The elemental compositions of the catalysts were analysed using Neutron Activation Analysis.

(8) The alkoxide content in the final catalyst was determined by GC analysis of a water-quenched catalyst sample.

(9) The tensile tests were performed according to ISO 527-2.

(10) Notched Charpy measurements were performed according to ISO 179

(11) The resistance to so called slow crack growth was measured using the strain hardening method, based on the publication by Kurelec, L. et al in Polymer 2005, 46, p 6369-6379.

(12) Shear thinning index SHI(2.7/210) is the ratio of the viscosity of the polyethylene composition at different shear stresses. In the present invention, the shear stresses at 2.7 kPa and 210 kPa are used for calculating the SHI(2.7/210) which may serve as a measure of the broadness of the molecular weight distribution.

(13) Rheological parameters such as Shear Thinning Index SHI and DMS parameters are determined by using a rheometer. The definition and measurement conditions for SHI are described in detail on page 8 line 29 to page 11, line 25 of WO 00/22040.

(14) DMS parameters indicate the frequency () at which a pre-defined/reference complex modulus value is reached for a polymer in a frequency sweep experiment. The DMS parameters are suffixed with a number (e.g. DMSn) where n denotes the reference G* value in the format n*10.sup.4 Pa. Different DMS values as typically calculated at different reference G* are:

(15) DMS2 indicates (rad/s) at G*=2*10.sup.4 Pa

(16) DMS5 indicates (rad/s) at G*=5*10.sup.4 Pa

(17) DMS10 indicates (rad/s) at G*=10*10.sup.4 Pa

(18) DMS21 indicates (rad/s) at G*=21*10.sup.4 Pa

(19) The calculation of DMS values takes into account the entire range of and G* data of a frequency sweep experiment (=100 rad/s to =0.01 rad/s). The plot of log vs log G* is used for DMS calculation. A 3.sup.rd order polynomial is used to fit the data such that the R.sup.2 value of the fit is more than 0.99. The values of log was calculated using the trend line equation at different reference G* values and consequently, values of DMS2, DMS5, DMS10 and DMS21 were evaluated. DMS 21/2 is a ratio of the DMS21 and DMS 2 values.

(20) The molecular weight distribution is measured by using size exclusion chromatography (SEC). In the examples this was done by using a Polymer Laboratories PL-GPC220 instrument equipped with 3 columns (Polymer Laboratories 13 m PLgel Olexis, 3007.5 mm) at an oven temperature of 140 C. A refractive index (RI) detector, a viscosity detector (Polymer Laboratories PL BV-400 viscometer) and an IR detector (Polymer Char IR5) were used. The instrument was calibrated with linear PE standards.

(21) The pressure resistance at all temperatures and hoop stress values is measured according to ISO 1167 with pipe dimensions 323 mm.

Experiment I

(22) Preparation of a Hydrocarbon Solution Comprising the Organic Oxygen Containing Magnesium Compound and the Organic Oxygen Containing Titanium Compound

(23) 100 grams of granular Mg(OC.sub.2H.sub.5).sub.2 and 150 millilitres of Ti(OC.sub.4H.sub.9).sub.4 were brought in a 2 litre round bottomed flask equipped with a reflux condensor and stirrer. While gently stirring, the mixture was heated to 180 C. and subsequently stirred for 1.5 hours. During this, a clear liquid was obtained. The mixture was cooled down to 120 C. and subsequently diluted with 1480 ml of hexane. Upon addition of the hexane, the mixture cooled further down to 67 C. The mixture was kept at this temperature for 2 hours and subsequently cooled down to room temperature. The resulting clear solution was stored under nitrogen atmosphere and was used as obtained. Analyses on the solution showed a titanium concentration of 0.25 mol/I.

Experiment II

(24) Preparation of the Catalyst

(25) In a 0.8 liters glass reactor, equipped with baffles, reflux condenser and stirrer, 424 ml hexanes and 160 ml of the complex from Example I were dosed. The stirrer was set at 1200 RPM. In a separate flask, 100 ml of 50% ethyl aluminum dichloride (EADC) solution was added to 55 mL of hexanes. The resulting EADC solution was dosed into the reactor in 15 minutes using a peristaltic pump. Subsequently, the mixture was refluxed for 2 hours. After cooling down to ambient temperature, the obtained red/brown suspension was transferred to a glass P4 filter and the solids were separated. The solids were washed 3 times using 500 ml of hexanes. The solids were taken up in 0.5 L of hexanes and the resulting slurry was stored under nitrogen. The solid content was 64 g ml.sup.1

(26) Catalyst analysis results:

(27) Ti 10.8 wt %; Mg 11.2 wt %; Al 5.0 wt %; Cl 65 wt %; OEt 3.2 wt % and OBu 2.6 wt %.

Example I and Comparative Examples A and B

(28) The polymerization was carried out in a continuous installation, consisting of 2 equally sized CSTR polymerization reactors in series using hexanes as diluent. The reactors contain a headspace that is continuously analyzed on the composition using an on-line analyzer. In the first reactor, a lower molecular weight polyethylene homopolymer is produced, followed by the production of a high molecular weight ethylene-butene copolymer.

(29) Between the first reactor and the second reactor, a flash step was applied, primarily aimed at removing the hydrogen coming out from the first reactor by means of reducing the pressure.

(30) The applied catalyst was prepared in a 400 L reactor, using a recipe analogous the one described in Experiment II. The catalyst flow to the first reactor was adjusted to reach the desired ethylene partial pressure in this first reactor as is indicated in the Table 1. The applied cocatalst was tri-isobutyl aluminium (TiBAI).

(31) The applied recipes of Example I and Comparative Examples A and B are described in Table 1.

(32) TABLE-US-00001 TABLE 1 Comparative Comparative Example I Example A Example B Reactor 1 Temperature [ C.] 88 88 88 Ethylene partial pressure [bar] 1.7 1.5 1.7 H.sub.2/C.sub.2 ratio in headspace [v/v] 4.1 5.4 3.8 C2/hexane feed [kg/kg] 0.663 0.649 0.648 Viscosity number [cm.sup.3/g] 80 60 90 Density [kg/m.sup.3] 971 973 970 [TiBAI] mmol/L 1.1 1.1 1.1 Split % 51 55 57 Reactor 2 Temperature [ C.] 78 78 78 Ethylene partial pressure [bar] 2.96 2.1 2.1 H2/C2 ratio in headspace [v/v] 0.013 0.010 0.003 C.sub.4/C.sub.2 in headspace [v/v] 0.04 0.02 0.05 C.sub.2/hexane feed [kg/kg] 0.29 0.28 0.27 Bimodal grade (powder) MFI5 (g/10 min) 0.11 0.10 0.10 Density (kg/m.sup.3) 951 956 953
Results Black Compound

(33) The bimodal grade PE powder was stabilised by adding 2000 ppm of calcium stearate, 2000 ppm of Irganox 1010 and 1000 ppm of Irgafos 168. To this, 2-2.5 wt % carbon black (grade Printex PA from Degussa, Germany) was added. The stabilised powder mixture was extruded into pellets using a commercial NT extruder from Coperion, Germany comprising of 2 extruders in series (ZSK 250 and ZSK 350). The pellets were used for the mentioned analyses.

(34) TABLE-US-00002 TABLE 2 Overview of polymer characteristics MFR.sub.5 MFR.sub.21 Density Mn Mw g/10 min g/10 min FRR kg/m.sup.3 (kg/mol) (kg/mol) Mw/Mn Example I 0.11 3.8 34 961 10 350 34.0 Comparative 0.10 5.1 51 965 8 380 47.5 Example A Comparative 0.10 4.6 46 963 8 350 43.7 Example B

(35) TABLE-US-00003 TABLE 3 Overview of mechanical properties Strain Notched Notched hardening Charpy 23 C. Charpy- modulus E-modulus Yield stress (kJ/m.sup.2) 30 C. (kJ/m.sup.2) (MPa) (MPa) (MPa) Example I 32.0 3.1 17.1 0.9 46.2 1.1 1106 52.8 27.1 0.4 Comparative 38.3 2.5 19.1 0.8 38.7 0.9 1152 38.3 29.2 0.3 Example A Comparative 23.7 0.7 12.3 0.8 42.9 0.7 1192 66.0 28.1 0.1 Example B

(36) TABLE-US-00004 TABLE 4 Overview of rheological properties DMS DMS2 DMS5 DMS10 DMS21 21/2 SHI Example I 0.0890 0.4836 2.3779 20.7667 233 73.54 Comparative 0.0507 0.2883 1.5157 16.0856 317 120.73 Example A Comparative 0.0745 0.4389 2.3682 24.3607 326 122.45 Example B
Pipe Tests

(37) The pressure tests have been performed on pipe specimens with the dimension 323.0 mm. Dimensions and tolerances of the specimens comply with ISO 4427-2 provisions. A geometric mean was only calculated when minimum 2 specimens failed. Several different stress levels were selected. The results are summarized in Table 5.

(38) The pressure resistance at all temperatures and hoop stress values is measured according to ISO 1167 with pipe dimensions 323 mm.

(39) TABLE-US-00005 TABLE 5 Pipe tests Required Minimum PE 100 reference according Comparitive Comparitive to DIN Product Unit Example I Example A Example B 8075 Pressure tests at 20 C. Hoop stress 12.0 MPa h >8000 8000 >8000 100 11.7 MPa h >8000 1700 >8000 2500 11.05 MPa h >5000 >5000 >5000 4380 Pressure tests at 80 C. Hoop Stress 4.9 MPa h >8800 1720 5300 2500 5.4 MPa h 4170 270 3800 165 5.7 MPa h 5900 1320 2190 1000

(40) Example I shows significant improvement of all mechanical properties (such as impact and strain hardening modulus) for a rheologically narrower material. The pressure resistance is significantly higher than PE100. Example I shows the best performance regarding the combination of pressure resistance, impact resistance and other mechanical properties.