MULTIMODAL POLYETHYLENE

Abstract

The invention is directed to polyethylene having a multimodal molar mass distribution, having a density in the range from 940 to 948 kg/m.sup.3, having an MFI .sub.190/5 in the range from 1.0 to 3.5 g/10 min and comprising from 45 to 47% by weight of an ethylene copolymer A and from 53 to 55% by weight of an ethylene copolymer B, where all percentages are based on the total weight of the composition wherein ethylene-1-butene copolymer A has a viscosity number in the range between 70 and 110 cm.sup.3/g and a density between 960 and 973 kg/m.sup.3. The polyethylene is suitable to be applied in pipe coating applications.

Claims

1. Polyethylene having a bimodal molar mass distribution, having a density in the range from 940 to 948 kg/m.sup.3, having an MFI .sub.190/5 in the range from 1.0 to 3.5 g/10 min and comprising from 45 to 47% by weight of ethylene-1-butene copolymer A and from 53 to 55% by weight of an ethylene copolymer B, where all percentages are based on the total weight of the composition; wherein ethylene-1-butene copolymer A has a viscosity number in the range between 70 and 110 cm.sup.3/g and a density between 960 and 973 kg/m.sup.3.

2. Polyethylene according to claim 1, characterized in that ethylene-1-butene copolymer A has a viscosity number in the range between 90 and 100 cm.sup.3/g.

3. Polyethylene according to claim 1, characterized in that ethylene-1-butene copolymer A has a density between 963 and 967 kg/m.sup.3.

4. Polyethylene according to claim 1, having a density in the range from 943 to 947 kg/m.sup.3 and having an MFI .sub.190/5 in the range from 2.0 to 2.5 g/10 min.

5. A process for the preparation of 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, 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.

6. A process according to claim 5, characterised in that the organic oxygen containing magnesium compound is a magnesium alkoxide, the organic oxygen containing titanium compound is a titanium alkoxide and the aluminium halogenide is an alkyl aluminium chloride.

7. A process according to claim 5, characterised in that the molar ratio of Al from I b):Ti from I a) 2 ranges between 6:1 and 10:1.

8. Steel pipe coating composition comprising polyethylene according to claim 1, or polyethylene obtained with the process according to any one of claims 5-7 as top layer.

9. A pipe coated with the steel pipe coating composition according to claim 8.

10. Steel pipe coating composition comprising polyethylene obtained with the process according to claim 5 as top layer.

Description

EXAMPLES

[0067] 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.

[0068] The density of the polymers is measured according to ISO1183.

[0069] The viscosity number is determined according to ISO 1628-3.

[0070] The melt-indices MFI .sub.190/1.2, MFI .sub.190/5 and MFI .sub.190/21.6 are measured according to method ASTM D-1238 under a load of 1.2, 5 and 21.6 kg at 190 C.

[0071] The Flow Rate Ratio (FRR) being calculated as MFI .sub.190/21.6/MFI .sub.190/5 is indicative for the rheological broadness of the material.

[0072] 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.

[0073] The elemental compositions of the catalysts were analysed using Neutron Activation Analysis.

[0074] The alkoxide content in the final catalyst was determined by GC analysis of a water-quenched catalyst sample.

[0075] The oxidation state of the catalyst was determined via oxidative titration with ferric sulphate following procedures as published by Garof, T.; Johansson, S.; Pesonen, K.; Waldvogel, P.; Lindgren, D. European Polymer Journal 2002, 38, 121; and Weber, S.; Chiem, J. C. W.; Hu, Y. Transition Met. Organomet. Catal. Olefin Polym. 1988, p 45-53; and Fregonese, D.; Mortara, S.; Bresadola, S. J. Mol. Cat A: Chem. 2001, 172, 89.

[0076] Wax amount in the diluent is determined as follows: After the polymerization, the resulting polymer suspension is cooled down to 30 C. and subsequently transported to a filter. Nitrogen pressure is applied on the filter to facilitate separation of the powder from the diluent. From the diluent, two 100 mL samples are taken. These two solution samples are dried overnight at 50 C. under nitrogen atmosphere. The amount of residue is weighed and this is considered as the amount of waxes, the amount that is soluble in the hexanes at 30 C.

[0077] Hexane extractables from PE powder have been determined in the following way: In a Bchi extraction system B-811, 5 g of PE powder (m0) is put in an extraction thimble (3390 mm) made from a thick filter paper. This is placed in a holder and assembled in the extraction column. Empty weight of the round bottom flask and some boiling chips is noted (m1). 200 mL hexane is filled in the round bottom flask and fitted onto the extraction system. The extraction cycle is started under an inert atmosphere. In total, 40 cycles are performed. After the round bottom flask has cooled, it is taken out from the extraction set up. Hexane is removed under vacuum at 60 C. for 30 minutes. After cooling, the weight of the round bottom flask is noted (m2). The amount of hexane soluble extractables is determined using equation 1:

[00001]$\begin{array}{cc}\mathrm{Weight}.Math.\text{-}.Math.\%.Math..Math.\mathrm{soluble}.Math..Math.\mathrm{part}=\frac{\left(m.Math..Math.2-m.Math..Math.1\right)}{m.Math..Math.0}100& \left(1\right)\end{array}$

where:
m1: weight of the round bottom flask with a few boiling chips (in g)
m2: weight of the round bottom flask with a few boiling chips and hexane soluble components (in g)
m0: dry weight of the PE powder (in g)

[0078] The tensile tests were performed according to ISO 527-2.

[0079] Brittleness temperature has been measured according to ASTM D746-07.

[0080] Bell tests have been performed according to ASTM 1693B.

[0081] Hardness Shore D measurements have been performed according to ISO 868.

[0082] Vicat softening N50 (9.8N) measurements have been performed according to ISO 306.

Experiment I

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

[0083] 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/l.

Experiment II

Preparation of the Catalyst

[0084] 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

Catalyst Analysis Results:

Ti 10.8 wt %; Mg 11.2 wt %; Al 5.0 wt %; CI 65 wt %; OEt 3.2 wt % and OBu 2.6 wt %.

Comparative Example A

Preparation of Bimodal PE Using 2 a Step Batch Polymerization

[0085] The polymerization was carried out in a 20 litres autoclave using 10 litres purified hexanes as a diluent. 8 mmols of tri-isobutylaluminum were added to the 10 litres purified hexanes. In the first stage of the polymerization reaction the mixture was heated to 85 C. and pressurized with 1.2 bars ethylene and a hydrogen to ethylene ratio in the headspace of 4.2 v/v (volume/volume). Subsequently a slurry containing 40 mg of the catalyst obtained in Experiment I was dosed. The temperature was maintained at 85 C. and the pressure was kept constant by feeding ethylene. The amount of ethylene, needed to maintain constant pressure was monitored and is considered to be a direct measure for the amount of polymer produced. The hydrogen to ethylene ratio in the headspace was measured via online-GC and hydrogen was fed to maintain this ratio constant at 4.2 v/v. The first phase of the reaction was stopped after 180 minutes. Stopping was performed by de-pressurizing and cooling down the reactor contents. The second stage of the reactor is started by adding 1-butene to the reactor subsequently raising the temperature to 80 C. and pressurizing the reactor with ethylene and hydrogen. The set partial pressure of ethylene in the second phase is 3.0 bar and the ratios for hydrogen to ethylene and 1-butene to ethylene are respectively 0.075 and 0.140 v/v. The reaction was stopped when a split of 46 had been reached. This split can be calculated directly by comparing the amount of ethylene uptake during the different stages of polymerisation. Stopping was performed by de-pressurizing and cooling down the reactor. The reactor contents were passed through a filter; the polymer powder was collected and subsequently dried.

[0086] An amount of 1231 grams of bimodal HDPE powder was produced.

[0087] The PE powder was stabilised by adding 2000 ppm of calcium stearate, 2000 ppm of Irganox 1010 and 1000 ppm of Irgafos 168. The stabilised powder was extruded into pellets using a lab scale co-rotating twin screw extruder having a L/D of 25.5, throughput of 50 g/min and rpm of 100. The pellets were used for the mentioned analyses. [0088] The polymer had the following characteristics [0089] viscosity number first reactor product 73 cm.sup.3/g [0090] overall density 945 kg/m.sup.3 [0091] overall MFI .sub.190/5 1.31 g/10 min and [0092] FRR 17.

Example I

Polymerization Bimodal HDPE Using the Catalyst of Experiment I

[0093] The polymerization was carried out similarly to the procedure as described in Comparative Example A with the exceptions that 30 mg of the catalyst as prepared in Experiment I was added to the reactor, and using a hydrogen to ethylene ratio of 2.5 v/v and a 1-butene to ethylene ratio of 0.01 v/v was used in the first stage. In the second stage the partial pressure of ethylene is set to 3.0 bar, a hydrogen to ethylene and 1-butene to ethylene ratio of respectively 0.112 and 0.139 were used. 1122 grams of bimodal HDPE powder was produced.

[0094] The PE powder was stabilised by adding 2000 ppm of calcium stearate, 2000 ppm of Irganox 1010 and 1000 ppm of Irgafos 168. The stabilised powder was extruded into pellets using a lab scale co-rotating twin screw extruder having a L/D of 25.5, throughput of 50 g/min and rpm of 100. The pellets were sent for various analyses.

The polymer had the following characteristics: [0095] viscosity number first reactor product 94 cm.sup.3/g [0096] density first reactor product 964 kg/m.sup.3 [0097] overall density 945 kg/m.sup.3 [0098] overall MFI .sub.190/5 2.07 g/10 min and [0099] FRR 16.

[0100] The amount of waxes in the diluent and hexane extractables in the polymer has been summarized in Table 1.

TABLE-US-00001 TABLE 1 Hexane Wax in diluent extractables from Example (g/kg PE) PE powder (%) A 16 2.55 I 10 2.23

[0101] It has been shown that for the same overall density, broadness (from FRR) and even higher MFI, Example I (example with copolymer in first polymerization stage) results in significantly lower waxes in diluent and hexane extractables from PE powder which contributes favorably to the overall economy of a bimodal process. Comparative Example A (example with homopolymer in first polymerization stage) shows a higher wax amount compared to Example I.

[0102] The obtained polymers have been analysed on the mechanical properties such as tensile properties, brittleness temperature, Bell tests, Hardness Shore D and Vicat softening temperature A/50 (9.8N) measurements. The minimum requirements that a PE steel pipe coating material should fulfil as laid down in ISO 21809-1 have also been mentioned. The results are summarized in Table 2.

TABLE-US-00002 TABLE 2 Mechanical properties Hard- Vicat Strain Brittleness ness softening Yield at temper- Bell Shore temperature stress break ature tests D A/50 (9.8N) experiment MPa % ( C.) (h) () ( C.) Requirement 15 600 1000 55 110 ISO 21809-1 A 21 1600 <90 >1000 58 122 I 21 1700 <90 >1000 58 123

[0103] Example I (with copolymer A) shows better mechanical properties and a lower wax amount compared to Comparative Example A (with homopolymer A).