ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE POWDER HAVING IMPROVED SWELLING PERFORMANCE

Abstract

The present invention relates to an ultra-high molecular weight polyethylene (UHMWPE) powder having a BET specific surface area of ≥0.50 m.sup.2/g as determined in accordance with ISO 9277 (2010). Such UHMWPE powder allows for preparation of a gel solution comprising the powder to a desired swelling ratio at moderate temperatures within a reduced swelling period.

Claims

1. An ultra-high molecular weight polyethylene (UHMWPE) powder having a BET specific surface area of ≥0.50 m.sup.2/g as determined in accordance with ISO 9277 (2010).

2. The UHMWPE powder according to claim 1 wherein the UHMWPE has an intrinsic viscosity (IV) of ≥10.0 dl/g as determined in accordance with ISO 1628-3 (2010).

3. The UHMWPE powder according to claim 1, wherein the UHMWPE has a viscosity-average molecular weight (M.sub.v) of ≥2.0.Math.10.sup.6 g/mol, as determined in accordance with ASTM D4020 (2011).

4. The UHMWPE powder according to claim 1, wherein the UHMWPE powder has an average particle size D.sub.50 of ≤250 μm, as determined in accordance with ISO 13320 (2009).

5. A process for preparation of a UHMWPE gel solution comprising in this order the steps of: (a) providing a quantity of a UHMWPE powder according to claim 1; (b) providing a quantity of a solvent; and (c) performing a swelling step by mixing (a) and (b) at a temperature of between 100 and 150° C. for a period of between 10 and 25 minutes.

6. A gel solution comprising a UHMWPE powder according to claim 1.

7. The gel solution according to claim 6, wherein the gel solution comprises ≥5.0 and ≤20.0 wt % of the UHMWPE powder with regard to the total weight of the gel solution.

8. The gel solution according to claim 6, wherein the solution is a system comprising a solvent selected from tetralin, decalin, kerosene and paraffin oil, or the UHMWPE powder.

9. The gel solution according to claim 6, wherein the UHMWPE powder comprises an absorbed quantity of the solvent such that the powder has a swelling ratio Q of at least 3.0, wherein the swelling ratio represents the ratio of the weight of the UHMWPE powder after having been subjected to a swelling step versus the weight of the UHMWPE powder prior to the swelling step.

10. A process for the production of gel-spun UHMWPE fibres comprising in this order the steps of: (i) processing the gel solution according to claim 6 in an extruder; (ii) extruding the gel solution through a spinneret to obtain spun filaments; (iii) cooling the spun filaments to obtain solvent-containing gel filaments; (iv) removing the solvent from the solvent-containing gel filaments to obtain UHMWPE filaments; and (v) drawing the UHMWPE filaments at a temperature of between 80 and 150° C. to obtain UHMWPE fibres.

11. The process according to claim 10, wherein the drawing step (v) is performed in a single drawing stage, in two consecutive drawing stages, or in three consecutive drawing stages, wherein the temperature is increased in each subsequent stage.

12. The process according to claim 10, wherein the extruder is a twin-screw extruder.

13. An UHMWPE fibre produced using the UHMWPE powder according to any one of claim 1.

14. An article comprising the UHMWPE fibre according to claim 13, wherein the article is selected from fibres, ropes, nets, non-woven textiles, woven textiles, and composites comprising such woven textiles.

15. (canceled)

16. An UHMWPE fibre produced using the gel solution according to claim 6.

17. An UHMWPE fibre produced using the process of claim 10.

Description

[0039] Preparation of UH powders according to the invention

[0040] A catalyst for the manufacture of UH powder was prepared according to the method as set out below.

[0041] To a 3 l round bottom flask equipped with a stirrer, a dropping funnel and a water cooler, 185 g of Mg(OC.sub.2H.sub.5).sub.2 (1.62 mol) as a solid and 275 ml of Ti(OC.sub.4H.sub.9).sub.4 (0.799 mol) as a liquid were added, both at room temperature (20° C.). The dropping funnel was filled with 2792 ml of hexane. The mixture of Mg(OC.sub.2H.sub.5).sub.2 and Ti(OC.sub.4H.sub.9).sub.4 in the round bottom flask was heated to a temperature of 180° C. and stirred at 300 rpm for 1.5 hours. A clear liquid was obtained. The mixture was then cooled down to 120° C. The hexane was added slowly whilst the solution was kept at a temperature of 120° C. After the hexane was added to the solution completely, the solution was cooled down to room temperature. The resulting solution comprising the precursor adduct was stored under nitrogen.

[0042] To a 1 l. baffled glass reactor, 400 ml hexane, 17.3 ml SiCl.sub.4 (151 mmol) and 3.5 ml 50 wt % ethylaluminium dichloride (EADC) (12 mmol) in hexane was added. The reactor was stirred at 1700 rpm. To this mixture, 200 ml of the solution obtained above in hexane (20 wt % precursor in hexane, corresponding to 50 mmol) was added to the reactor during a period of 4 hours, while maintaining the stirring speed at 1700 rpm. Subsequently, the suspension was refluxed at hexane boiling temperature of 69° C. for 2 hours under maintained stirring at 1700 rpm, after which it was cooled to 20° C. under stirring at 250 rpm. The obtained catalyst slurry was filtered through a P4 filter and washed 6 times with each 500 ml of hexane. The resulting catalyst has an average particle size D.sub.50 of 3.80 μm and a span of 1.00.

[0043] Using the catalysts that was prepared as per the above, ethylene polymerisation experiments were conducted to obtain UH powders according to the invention. In a continuously operated 20 l. CSTR reactor filled to 75% of its volume with hexane as diluent, ethylene was polymerised. The concentration in the gas cap of the reactor was monitored by gas chromatography. The level of the liquid/polymer slurry in the reactor was maintained by controlling the discharge quantity and frequency and the supply of make-up reactants. The reactor was heated to a polymerisation temperature as presented in the table below. The polymerisation was initiated by providing ethylene under continuous flow whilst constantly dosing the catalyst. By controlling the quantity of catalyst provided per quantity of reacted monomers, the catalyst yield and the polymer particle size of the obtained UH powder product can be steered. The catalyst was provided in such quantities as to result in a polymer particle size D.sub.50 of ca. 140-160 μm. During the reaction, the contents of the reactor were subjected to stirring at 950 rpm. Triisobutyl aluminium was added to the reactor in such amount that the concentration of aluminium in the outlet slurry of the reactor was kept at 40 ppm. An antifouling agent (Statsafe 6633) was continuously added to the reactor in such an amount that the concentration of the antifouling agent was maintained at 80 ppm in the slurry.

[0044] The characteristics of the polymerisation experiments of the inventive example and of the UH powders produced as a result of these experiments are provided in the following table.

TABLE-US-00001 Example I-1 I-2 I-3 Ethylene pressure (bar) 2.4 1.8 2.2 Polymerisation temperature (° C.) 80 75 75 Catalyst yield (kg polymer/g catalyst) 31 24 18 Hydrogen/ethylene molar ratio 0.001 0 0.002 1-butene/ethylene molar ratio 0.014 0.002 0.001 Density (kg/m.sup.3) 925 927 927 Bulk density of produced UH powder 490 500 509 (kg/m.sup.3) Average particle size D.sub.50 (μm) 149 143 144 Elongational stress (N/mm.sup.2) 0.23 0.42 0.45 Intrinsic viscosity (IV) (dl/g) 21.7 30.5 28.8 BET surface area (m.sup.2/g) 0.78 0.86 0.54

[0045] Wherein:

[0046] The bulk density of the UH powders was determined in accordance with ISO 60 (1977);

[0047] The density is determined in accordance with ASTM D792 (2008);

[0048] The average particle size D.sub.50 is the average particle size of the UH powder particles as determined in accordance with ISO 13320 (2009);

[0049] The elongational stress was measured according to ISO 11542-2 (1998) at 150° C. over a 10 minute period. Elongational Stress is understood as the stress that is necessary to stretch a test rod of the material to be tested by exactly 600% at a temperature of 150° C. in a suitable heat transfer medium within 10 minutes after starting the measurement. For measurement of the elongational stress, the UH powders of each experiment were shaped into test specimens by compression moulding at 210° C. followed by punching out test specimens according to ISO/CD 11542-2.4. The thus obtained specimens were tested according to Annex A of ISO 11542-2 (1998).

[0050] The powders as prepared according to the invention were analysed to identify the intrinsic viscosity and the BET surface area. The intrinsic viscosity (IV) was determined according to the method of ISO 1628-3 (2010). The BET surface area (BET) was determined according to the method of ISO 9277 (2010). Properties of the UH powders of the examples are presented in the table below.

[0051] Further, a number of commercially UH powders were used as comparative examples to demonstrate the effect of the invention, the analysis of which is presented in the table below.

TABLE-US-00002 Sample IV BET C-1 21.0 0.26 C-2 21.5 0.23 C-3 26.4 0.20

[0052] In the table above, C-1 is a sample of grade SLL-4, obtainable from Shanghai Lianle; C-2 is a sample of grade GUR4022, obtainable from Celanese; and C-3 is a sample of grade UH805, obtainable from Jiujiang Xinxing.

[0053] The above samples 11 through I-3 and C-1 through C-3 were used in swelling experiments. The UH powder samples were in each case added to a 250 ml three-necked round-bottomed flask containing 150 ml paraffin oil to reach a concentration of 1 wt % UHMWPE in oil. The grade of paraffin oil used was No. 70.

[0054] The round-bottomed flask was heated in a thermostatic oil bath to a temperature of as indicated in the table below under constant stirring using a mechanical stirrer. The temperature was maintained throughout the swelling period. The homogeneous suspension of the UH powder in oil changed after a period t.sub.1 (expressed in the table below in minutes) into the form of white flocs. t.sub.1 was determined by visual observation of the formation of the flocs. In the context of the present invention, t.sub.1 is reflects the swelling time. The contents of the flask were then poured into a Breitbart funnel to remove extra solvent. The weight of the swollen UHMWPE was then measured as W.sub.1. The solvent that was remaining as absorbed in the swollen UHMWPE was then extracted with dichloromethane under ultrasonic conditions for 30 minutes. After three extractions, the extracted UHMWPE sample was dried in a vacuum oven at 70° C. for 4 hours. The dry UHMWPE sample was then weighed and recorded as W.sub.2. The swelling ratio Q was calculated by the equation Q=W.sub.1/W.sub.2.

TABLE-US-00003 Swelling Sample temperature (° C.) 128 130 132 134 I-1 t.sub.1 (min) — — — — Q — — — — I-2 t.sub.1 (min) 19.21 17.16 15.47 12.02 Q 1.887 2.161 3.383 3.085 I-3 t.sub.1 (min) 23.01 18.23 13.42 12.19 Q 1.549 2.220 2.932 4.028 C-1 t.sub.1 (min) 26.30 19.50 15.30 14.15 Q 1.429 2.000 2.474 3.533 C-2 t.sub.1 (min) 36.20 22.00 15.30 14.20 Q 1.442 1.444 1.471 3.158 C-3 t.sub.1 (min) 38.59 25.27 18.44 14.44 Q 1.378 1.465 2.308 2.529