Process for the preparation of an UHMWPE homopolymer

Abstract

A process for the preparation of an ultra-high molecular weight ethylene homopolymer having a MFR.sub.21 of 0.01 g/10 min or less, said process comprising: (I) prepolymerising at least ethylene at a temperature of 0 to 90° C. in the presence of a heterogeneous Ziegler Natta catalyst to prepare an ethylene prepolymer having an Mw of 40,000 to 600,000 g/mol; and thereafter in the presence of the prepolymer and said catalyst; (II) polymerising ethylene at a temperature of 55° C. or less, such as 20 to 55° C., to prepare said UHMW ethylene homopolymer; wherein the UHMW ethylene homopolymer comprises up to 8 wt. % of said prepolymer.

Claims

1. A process for the preparation of an ultra-high molecular weight (UHMW) ethylene homopolymer having a MFR.sub.21 of 0.01 g/10 min or less, said process comprising: (I) pre-polymerizing at least ethylene at a temperature of 0 to 90° C. in the presence of a heterogeneous Ziegler Natta catalyst to prepare an ethylene prepolymer having an Mw of 40,000 to 600,000 g/mol; and thereafter in the presence of the prepolymer and said catalyst; (II) polymerizing ethylene at a temperature of 55 ° C. or less to prepare said UHMW ethylene homopolymer; wherein the UHMW ethylene homopolymer comprises up to 8 wt. % of said prepolymer.

2. The process as claimed claim 1, wherein the prepolymer is an ethylene homopolymer.

3. The process as claimed in claim 1, wherein the heterogeneous Ziegler Natta catalyst comprises a support.

4. The process as claimed in claim 1, wherein the pre-polymerization step (I) is carried out in the presence of hydrogen.

5. The process as claimed in claim 1, wherein the pressure of the pre-polymerization step (I) is 20 to 40 bars and/or wherein the pressure of the polymerization step (II) is 10 to 30 bars.

6. The process as claimed in claim 1, wherein the pre-polymerization step (I) is carried out at a temperature of 20 to 80° C. and/or wherein the polymerization step (II) is effected at a temperature of 25 to 50° C.

7. The process as claimed in claim 1, wherein the prepolymer has a Mw of 100,000 to 600,000.

8. The process as claimed in claim 1, wherein hydrogen is present in the pre-polymerization step (I) and the hydrogen is removed from the product of the pre-polymerization step (I) before the subsequent polymerization step (II).

9. The process as claimed in claim 1, wherein the partial pressure of ethylene in the polymerization step (II) is 5 to 7 bars.

10. The process as claimed in claim 1, wherein the polymerization step (II) is carried out at 30° C. and with an ethylene pressure of 6 bar.

11. The process as claimed in claim 1, wherein the polymerization step (II) is effected in the absence of hydrogen.

12. The process as claimed in claim 1, wherein the UHMW polyethylene homopolymer comprises 1.0 to 8.0 wt. % of the prepolymer.

13. The process as claimed in claim 1, wherein the polymerization step (II) is carried out in a slurry loop reactor.

14. The process as claimed in claim 3, wherein the support is a silica support.

15. The process as claimed in claim 3, wherein the support has a particle size in the range of 5 to 30 microns.

16. The process as claimed in claim 1, wherein step (II) is carried out at 20 to 55° C.

17. The process as claimed in claim 1, wherein the prepolymer has a Mw of 150,000 to 550,000 g/mol.

Description

ANALYTICAL TESTS

(1) Melt Flow Rate

(2) The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the melt viscosity of the polymer. The MFR is determined at 190° C. for polyethylene. The load under which the melt flow rate is determined is usually indicated as a subscript, for instance MFR.sub.2 is measured under 2.16 kg load, MFR.sub.5 is measured under 5 kg load or MFR.sub.21 is measured under 21.6 kg load.

(3) Density

(4) Density of the polymer was measured according to ISO 1183/1872-2B.

(5) Thermal Properties

(6) All experiments are performed using a TA Q2000 differential scanning calorimeter. Polyethylene samples are weighted in low mass aluminium Tzero pans and lids on a XS3DU Mettler Toledo precision balance (sensitivity of ±0.001 mg). Between 1-5 mg of sample is used for performing DSC experiments. To avoid any thermal oxidation the experiments are conducted under a nitrogen atmosphere with a flow rate of 50 mL/min. A heating-cooling-heating temperature ramp from 50 to 180° C. is performed at a linear rate of 10° C/min. The heat of fusion and peak melting temperature are determined by integrating the melting peak from 100 to 160° C. using the sigmoidal horizontal baseline integration option in the universal analysis 2000 software.

(7) The crystallinity is determined against the theoretical value of heat of fusion of 100% crystalline PE of 293 J/g.

(8) Molecular Weight

(9) For the pre-polymerised material molecular weight averages, molecular weight distribution (Mn, Mw, Mz MWD)

(10) Molecular weight averages (Mz, Mw and Mn), Molecular weight distribution (MWD) and its broadness, described by polydispersity index, PDI=Mw/Mn (wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) were determined by Gel Permeation Chromatography (GPC) according to ISO 16014-1:2003, ISO 16014-2:2003, ISO 16014-4:2003 and ASTM D 6474-12 using the following formulas:

(11) $\begin{array}{cc}{M}_n=\frac{{.Math.}_{i=1}^N{A}_i}{{.Math.}_{i=1}^N\left(A_i/M_i\right)}& \left(1\right)\\ M_w=\frac{{.Math.}_{i=1}^N\left(A_{i}x{M}_i\right)}{{.Math.}_{i=1}^N{A}_i}& \left(2\right)\\ M_z=\frac{{.Math.}_{i=1}^N\left(A_{i}x{M}_i^2\right)}{{.Math.}_{i=1}^N\left(A_i/M_i\right)}& \left(3\right)\end{array}$

(12) 15

(13) For a constant elution volume interval ΔV.sub.i, where A.sub.i, and M.sub.i are the chromatographic peak slice area and polyolefin molecular weight (MW), respectively associated with the elution volume, V.sub.i, where N is equal to the number of data points obtained from the chromatogram between the integration limits.

(14) A high temperature GPC instrument, equipped with either infrared (IR) detector (IR4 or IRS from PolymerChar (Valencia, Spain) or differential refractometer (RI) from Agilent Technologies, equipped with 3× Agilent-PLgel Olexis and 1× Agilent-PLgel Olexis Guard columns was used. As the solvent and mobile phase 1,2,4-trichlorobenzene (TCB) stabilized with 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) was used. The chromatographic system was operated at 160° C. and at a constant flow rate of 1 mL/min. 200 μL of sample solution was injected per analysis. Data collection was performed using either Agilent Cirrus software version 3.3 or PolymerChar GPC-IR control software.

(15) The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with 19 narrow MWD polystyrene (PS) standards in the range of 0,5 kg/mol to 11 500 kg/mol. The PS standards were dissolved at room temperature over several hours. The conversion of the polystyrene peak molecular weight to polyolefin molecular weights is accomplished by using the Mark Houwink equation and the following Mark Houwink constants:
K.sub.PS=19×10.sup.−3 mL/g, α.sub.PS=0.655
K.sub.PE=39×10.sup.−3 mL/g, α.sub.PE=0.725
K.sub.PP=19×10.sup.−3 mL/g, α.sub.PP=0.725

(16) A third order polynomial fit was used to fit the calibration data. All samples were prepared in the concentration range of 0,5 -1 mg/ml and dissolved at 160° C. for 2.5 hours for PP or 3 hours for PE under continuous gentle shaking.

(17) Uniaxial Solid-State Deformation

(18) A general procedure for the preparation of tapes is as follows: 25 g of polymer powder is poured into a mold with a cavity of 620 mm in length and 30 mm in width and compression-molded at 130 bar for 10 min to form a sheet. The sheet is preheated for at least 1 min and rolled with a Collin calander (diameter rolls: 250 mm, slit distance 0.15 mm, inlet speed 0.5 m/min). The tape is immediately stretched on a roll (speed 2.5 m/min). The rolled and stretched tape is further stretched in two steps on a 50 cm long oil heated hot plate. The tape comes in contact with the hot plate after 20 cm from the entrance of the hot plate. The draw ratio is obtained by dividing specific weight of the sheet prior to deformation by the specific weight of the tape after stretching.

(19) Tensile Testing

(20) Tensile properties are measured using an Instron 5566 tensile tester at room temperature (25_C). To avoid any slippage, the side action grip clamps with flat jaw faces are used. The nominal gauge length of the specimen is 100 mm, and the test is performed at a constant rate of extension (crosshead travel rate) 50 mm/min. The breaking tenacity (or tensile strength) and modulus (segment between 0.3 and 0.4 N/tex) are determined from the force against displacement between the jaws.

EXAMPLES

(21) Catalyst Preparation—ZN1

(22) Complex Preparation:

(23) 87 kg of toluene was added into the reactor. Then 45.5 kg Bomag A (Butyloctyl magnesium) in heptane was also added in the reactor. 161 kg 99.8% 2-ethyl-1-hexanol was then introduced into the reactor at a flow rate of 24-40 kg/h. The molar ratio between BOMAG-A and 2-ethyl-1-hexanol was 1:1.83.

(24) Solid Catalyst Component Preparation:

(25) 330kg silica (calcined silica, Sylopol ® 2100) and pentane (0,12 kg/kg carrier) were charged into a catalyst preparation reactor. Then EADC (Ethylaluminium dichloride) (2.66 mol/kg silica) was added into the reactor at a temperature below 40° C. during two hours and mixing was continued for one hour. The temperature during mixing was 40 -50° C. Then Mg complex prepared as described above was added (2.56 mol Mg/kg silica) at 50° C. during two hours and mixing was continued at 40 -50° C. for one hour. 0.84 kg pentane/kg silica was added into the reactor and the slurry was stirred for 4 hours at the temperature of 40 -50° C. . Finally, TiCl.sub.4 (1,47 mol/kg silica) was added during at least 1 hour at 55° C. to the reactor. The slurry was stirred at 50-60° C. for five hours. The catalyst was then dried by purging with nitrogen. Molar composition of the ready catalyst is: Al/Mg/Ti=1.5/1.4/0.8 (mol/kg silica). This catalyst is called ZN1 herein.

(26) Ethylene Homo-Polymerisation Procedure

(27) The polymerisation experiments were carried out in a 3 L bench scale reactor. About 100 mg of ZN1 catalyst was used in all the polymerisations and tri-ethylaluminium (TEA) was used as co-catalyst with an Al/Ti ratio of 15.

(28) Prepolymerisation Step

(29) To an empty 3.0 L reactor 1250 ml of propane was fed to the polymerisation reactor as a polymerisation medium. After addition of the reaction medium, hydrogen was introduced (0.4-2.5 Bar) after which temperature was increased to 70° C. A batch of ethylene (3.7 bar) was added, then reactor pressure was allowed to be stable at 0.2 bar of overpressure and stirring speed was increased to 450 rpm. Then the catalyst and the co-catalyst were added through automatic feeding using nitrogen and 100 ml of propane. The total reactor pressure was 30.0-33.5 bar depending upon the amount of hydrogen used, which was maintained by continuous ethylene feed. The prepolymerisation was stopped after 1-5 g of ethylene was consumed (prepoly degree=1-6%).

(30) The Mw of the pre-polymerisation fraction and the relative amount of pre-polymerisation fraction is presented in Table 1. Higher hydrogen concentrations lead to lower Mw in the prepolymer. Longer residence time leads to a higher wt. % of the prepolymer in the overall UHMWPE.

(31) Polymerisation Step

(32) After the desired degree of pre-polymerisation was achieved, the volatile components were flushed and the reactor was cooled to 20° C. 1500 ml of propane was fed to the polymerisation reactor as a polymerisation medium. The reactor was warmed to the desired temperature (30-40° C.). Dimethoxydimethylsilane as an external electron donor was added to the polymerisation reactor (mole ratio: Si/Ti=1.25). A batch of ethylene (4-6 bar) was added, then reactor pressure was allowed to be stable at 0.2 bar of overpressure and stirring speed was increased to 450 rpm. The total reactor pressure was 16-21 bar depending upon the polymerisation temperature and ethylene pressure, which was maintained by continuous ethylene feed. Polymerisation time was 60 min after which the polymerisation was stopped by venting off the monomer together with the reaction medium. Activity of the catalyst was measured on the basis of the amount of polymer produced (Activities are in the range of 0.5-2.0 kgPO/g Cat/h). The detailed conditions for the series of experiments are listed in Table 1.

(33) T.sub.onset, Tm, Width, ΔH and crystallinity results are disclosed in Table 2. The MFR.sub.21 value is too low to be measured. The polymers of the inventive examples have densities of 955-957 kg/m.sup.3.

(34) Comparative example 1 corresponds to example 1E3 of WO2015/121162, i.e. prepared without any pre-polymerisation step. This polymer could not be processed in solid-state and was thus not used in further testing.

(35) In comparative example 2 the Mw of the pre-polymer is only 20×10.sup.3 g/mol and the monomer pressure in the main polymerization is 4 bar, i.e. below the preferred minimum pressure of 5 bar. The polymer of comparative example 2 was still processable in solid-state, but the tensile modulus and tensile strength are lower than the inventive polymers for a given draw ratio as can be seen in the FIGS. 1 and 2.

(36) TABLE-US-00001 TABLE 1 Main Prepol. Polym. Monom. Fraction Prepol. Mw Temp. Press. Processing Example (wt. %) (10.sup.3 g/mol) (° C.) (bar) in solid-state Comp. 1 0.0 — 40 4 No Comp. 2 6.0  20 40 4 Yes Inv. 1 3.0 150 30 6 Yes Inv. 2 1.5 500 30 6 Yes Inv. 3* 3.0 150 30 6 Yes *Without External Donor

(37) TABLE-US-00002 TABLE 2 T.sub.onset T.sub.m Width ΔH.sub.melt.sup.1 Crystallinity Experiment (° C.) (° C.) (° C.) J/g % Comp 2 133.7 138.8 4.4 191 65 Inv 1 136.3 139.8 3.1 188 64 Inv 2 137.1 140.5 3.0 186 63 Inv 3 136.1 139.6 3.2 181 62 .sup.1Considering the melting enthalpy of 100% crystalline to be 293 J/g
Uniaxial Solid-State Deformation

(38) After compression, the samples were pre-stretched by rolling below the melting point of the polymer as obtained from the reactor. The calendaring (rolling) step combined by stretching resulted in a draw ratio of 10. The rolling step is applied to achieve homogeneity in the strip and to avoid extrinsic failure during uniaxial drawing. The second stretching step on the rolled and the prestretched tapes was carried out at approx. 148-153° C. Results are given in Table 3.

(39) TABLE-US-00003 TABLE 3 Rolling/Pre- Compression stretching Stretching Experiment ° C. ° C. ° C. Failure Comp 2 133 131/136 148 No Inv 1 135 136/143 151 No Inv 2 136 136/143 152 No Inv 3 134 136/143 151 No