POLYETHYLENE HOMO- OR COPOLYMER HAVING IMPROVED WEAR PROPERTIES

Abstract

The present invention relates to a polyethylene homo- or copolymer having improved wear properties. In particular, the invention relates to an ultra-high molecular weight polyethylene having improved wear properties prepared using a heterogeneous Ziegler catalyst system. Said polyethylene homo- or copolymer is characterized in that the abrasion index of said polyethylene is related to the elongational stress according to the formula (I): in which ES=elongational stress as measured according to ISO 11542-2:1998 AI=abrasion index, as measured according to ISO 15527:2010 where the reference material according to ISO 15527:2010 is set to 100; β<1.8; and −0.015<a<−0.017.

[00001]$\begin{array}{cc}\mathrm{AI}<\frac{\mathrm{ES}-\beta }{\alpha }& \left(I\right)\end{array}$

Claims

1. Polyethylene homo- or copolymer characterized in that the abrasion index of said polyethylene is related to the elongational stress according to the formula: $\mathrm{AI}<\frac{\mathrm{ES}-\beta }{\alpha }$ in which ES=elongational stress as measured according to ISO 11542-2:1998 AI=abrasion index, as measured according to ISO 15527:2010 where the reference material according to ISO 15527:2010 is set to 100; β<1.8; and −0.015<α<−0.017.

2. Polyethylene according to claim 1 in which the polyethylene is produced by reacting ethylene with optionally one of more other α-olefin monomer in the presence of a Ziegler catalyst.

3. Polyethylene according to claim 1 in which the polyethylene has a molecular weight of more than 500,000 g/mol.

4. Polyethylene according to claim 1 in which the elongational stress is less than 0.5 MPa.

5. Polyethylene according to claim 1 in which the abrasion index is less than 80, said abrasion index measured according to ISO 15527:2010 where the reference material according to ISO 15527:2010 is set to 100.

6. Polyethylene homo- or copolymer, in which said polyethylene is a polyethylene powder produced by polymerizing ethylene in the presence of a catalyst composition comprising I) the product obtained by combining: a) a hydrocarbon solution comprising: i) a magnesium-containing compound selected from an organic oxygen-containing magnesium compound and a halogen-containing magnesium compound; and ii) an organic oxygen-containing titanium compound; b) a solution comprising: i) a metal-containing compound having the formula (I):
MeR.sub.nX.sub.3-n  (I) in which X is a halogen, Me is a metal of group III of Mendelejev's Periodic Table of Elements, R is a hydrocarbon moiety comprising 1-10 carbon atoms, and n is 1≦n<3, or a dimer of a compound of formula (I); and ii) a silicon-containing compound of formula R′.sub.mSiCl.sub.4-m in which 0≦m≦2 and R′ is a hydrocarbon moiety comprising at least one carbon atom; in which the combination of solutions a) and b) results in a suspension of solid particles; II) an organo aluminium compound having the formula AlR′.sub.3 in which R′ is a hydrocarbon moiety containing 1-10 carbon atoms, and III) one or more of an external electron donor selected from the group of 1,2-dialkoxy hydrocarbon compounds; characterized in that the molar ratio of the external electron donor III) to the titanium present in I) is between 0.5 and 5.5 the span of the polyethylene powder obtained from the process as measured according to ISO-13320:2009 is between 0.9 and 1.3

7. Polyethylene according to claim 6 in which the abrasion index of said polyethylene is related to the elongational stress according to the formula: $\mathrm{AI}<\frac{\mathrm{ES}-\beta }{\alpha }$ in which ES=elongational stress as measured according to ISO 11542-2:1998 AI=abrasion index, as measured according to ISO 15527:2010 where the reference material according to ISO 15527:2010 is set to 100; β<1.8; and −0.015<α<−0.017.

8. Polyethylene according to claim 6 in which the polyethylene has a molecular weight of more than 500,000 g/mole.

9. Polyethylene according to claim 6 in which the elongational stress is less than 0.5 MPa.

10. Polyethylene according to claim 6 in which the abrasion index is less than 80, said abrasion index measured according to ISO 15527:2010 where the reference material according to ISO 15527:2010 is set to 100.

11. Process for the production of polyethylene according to claim 1 in which the process comprises the polymerization of ethylene in the presence of a catalyst composition comprising I) the product obtained by combining: a) a hydrocarbon solution comprising: i) a magnesium-containing compound selected from an organic oxygen-containing magnesium compound and a halogen-containing magnesium compound; and ii) an organic oxygen-containing titanium compound; b) a solution comprising: i) a metal-containing compound having the formula (I):
MeR.sub.nX.sub.3-n  (I) in which X is a halogen, Me is a metal of group III of Mendelejev's Periodic Table of Elements, R is a hydrocarbon moiety comprising 1-10 carbon atoms, and n is 1≦n<3, or a dimer of a compound of formula (I); and ii) a silicon-containing compound of formula R′.sub.mSiCl.sub.4-m in which 0≦m≦2 and R′ is a hydrocarbon moiety comprising at least one carbon atom; in which the combination of solutions a) and b) results in a suspension of solid particles; II) an organo aluminium compound having the formula AlR′.sub.3 in which R′ is a hydrocarbon moiety containing 1-10 carbon atoms, and III) one or more of an external electron donor selected from the group of 1,2-dialkoxy hydrocarbon compounds to obtain a polyethylene powder.

12. Process according to claim 11 in which the molar ratio of the external electron donor III) to the titanium present in I) is between 0.5 and 5.5, and the span of the polyethylene powder obtained from the process as measured according to ISO-13320:2009 is between 0.9 and 1.3.

13. Process according to any claim 11 in which: the metal-containing compound is selected from n-butyl aluminium dichloride, isobutyl aluminium dichloride, diisobutyl aluminium chloride, di-n-butyl aluminium chloride, sesquiisobutyl aluminium chloride, ethyl aluminium dibromide, ethyl aluminium dichloride, propyl aluminium dichloride, diethyl aluminium chloride, diisobutyl aluminium chloride, or mixtures thereof; the silicon-containing compound is SiCl.sub.4; and the external electron donor is one or more selected from 1,2-dimethoxybenzene, 1,2,4-trimethoxybenzene, 1,2-diethoxybenzene, 2,3-dimethoxytoluene, 1-allyl-3,4-dimethoxybenzene, 1,2-dimethoxyethane, 1,2-dimethoxy cyclohexane, 1,2-dimethoxypropane, 1,2-dimethoxybutane, 2,3-dimethoxybutane and/or mixtures thereof.

14. Process according to claim 11 characterized in that the process is a continuous process.

15. Articles of UHMWPE obtained via a process according to claim 11.

Description

COMPARATIVE EXAMPLES

[0116] Several commercially available UHMWPE grades were used as comparative examples. In example C-1, Stamylan UH610 (available from DSM) was used; in example C-2, GUR 4150 (available from Celanese) was used; in example C-3, GUR 4120 (available from Celanese) was used.

TABLE-US-00001 TABLE 1 Ethylene Hydrogen Ratio Bulk Elongational Abrasion pressure feed Yield donor: density D.sub.50 Stress Index Example (bar) (mg/h) kg/g Ti (kg/m.sup.3) (μm) Span (MPa) (%) IIIA 5.65 10 8.6 — 426 155 1.3 0.245 75 IIIB 5.70 14 8.3 — 413 146 1.2 0.217 92 IIIC 6.50 14 9.6 — 400 160 1.3 0.295 80 IIID 10.60 20 25.0 3.9 460 143 1.2 0.205 80 IIIE 10.70 20 21.0 4.4 470 140 1.3 0.276 67 IIIF 11.30 10 25.0 5.3 474 146 1.0 0.461 65 C-1 544 143 0.8 0.543 80 C-2 495 120 1.1 0.485 79 C-3 456 128 1.0 0.266 100

[0117] The yield is defined as the quantity of polyethylene in kilograms produced per quantity of catalyst in grams that is introduced into the reactor.

[0118] The ratio donor: Ti is defined as the molar ratio of the external donor to the titanium present in I) of the catalyst.

[0119] The bulk density was measured according to ISO 60:1977.

[0120] The average particle size D.sub.50 is the average particle size of the polymer particles measured according to ISO-13320:2009. The span is calculated as (D.sub.90−D.sub.10)/D.sub.50. D.sub.90 and D.sub.10 were measured according to ISO-13320:2009.

[0121] Elongational stress was measured according to ISO 11542-2:1998 at 150° C. over a 10 minute period. For measurement of the elongational stress, the polyethylene powders obtained from Experiment III, examples IIIA to IIIF and the Comparative Examples C-1 to C-3 were shaped into test specimens by compression moulding. The thus obtained specimens were tested according to Annex A of ISO 11542-2:1998.

[0122] Abrasion resistance was measured according to ISO 15527:2010. For measurement of abrasion resistance, the polyethylene powders obtained from Experiment III, examples IIIA to IIIF and the Comparative Examples C-1 to C-3 were shaped into test specimens by compression moulding. The thus obtained specimens were tested according to Annex B of ISO 15527:2010. The reference material used in determination of the abrasion index was Ticona GUR 4120.

[0123] The above presented examples clearly show that the polyethylenes according to the present invention have a superior balance of processability and wear characteristics.