The invention is directed to a catalyst for the gas phase polymerization of ethylene. The production processes of LDPE, HDPE and LLDPE are summarised in

"Handbook of Polyethylene" by Andrew Peacock (2000; Dekker; ISBN 0824795466) at pages 43-66. The catalysts can be divided in three different subclasses including Ziegler Natta catalysts, chromium oxide based catalyst (Phillips catalysts) and single site catalysts. The various processes may be divided into solution polymerization processes employing homogeneous (soluble) catalysts and processes employing supported

(heterogeneous) catalysts. The latter processes include both slurry and gas phase processes.

The production of HDPE with gas phase processes is for example disclosed in US5453471 , EP1285010B1 , EP2322567A, EP1303546B, EP1907430B1 and US5473027.

A chromium oxide based catalyst, which is commonly referred to in the literature as

"the Phillips catalyst", can be obtained by calcining a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere. The chromium oxide catalysis and the ethylene polymerisation with this specific catalyst are disclosed in "Handbook of Polyethylene" by Andrew Peacock at pages 61-64.

Pullukat et al. (Journal of Polymer Science; Polymer chemistry Edition; vol18, 2857-

2866; 1980) discloses thermally activated ethylene polymerisation catalysts which contain chromium and titanium on silica.

Catalyst compositions based on chromium on silica support result in gas phase polymerisations in HDPE with a relatively broad molecular weight distribution (MWD) in the range between for example 10 and 14. With use of these catalysts polymers with different ranges of molecular weight can be produced depending on the temperature of the reactor.

It is a disadvantage of ethylene polymerisation in gas phase with catalyst compositions based on chromium on silica support that there is a limitation in density and high load melt index (HLMI) of the polyethylene. The lowest obtainable density obtained with this type of catalyst in a gas phase polymerisation process having substantially no reactor fouling ranges between about 940 and 945 kg/m ³ . Furthermore, relatively low melt indices for example in the range between 2 and 5 (HLMI 21 ,6kg) cannot be obtained at high production rates.

Low densities in combination with low HLMI are desired to achieve a good balance of mechanical properties, for example high stress crack resistance and high impact strength. It is the object of the present invention to provide a catalyst composition having a high activity in gas phase polymerisation processes while producing polyethylene with a medium or low density below 940 kg/m ³ and a low HLMI to be able to use these polymers in a broader range of applications where a good balance of mechanical properties is needed.

The catalyst composition according to the invention consists of a chromium compound, a vanadium compound and a silica support material, wherein the silica support material has a surface area (SA) > 600 m ² /g and≤ 800 m ² /g, a pore volume (PV)≥ 1.65 and≤ 2.0 cm ³ /g and an average particle size in the range≥ 30 and≤ 40 micrometres.

The use of the catalyst composition according to the invention in a gas phase ethylene polymerization reactor at bed temperatures of≥ 100°C results in polyethylene with a density between≥ 930 and≤ 945 kg/m ³ , more preferably between≥ 934 and≤ 938 kg/m ³ , wherein the density is determined according to IS01 183.

The ethylene copolymer obtained with the catalyst according to the present invention has a high load melt index (HLMI)≥ 2 g/10 min and≤ 10 g/10 min (according to ASTM D-1238 Condition F using a load of 21.6 kg at a temperature of 190°C). Preferably the ethylene copolymer obtained with the catalyst according to the present invention has a high load melt index (HLMI)≥ 2 g/10 min and < 5 g/10 min.

Mw / M _n of the ethylene copolymer obtained with the catalyst according to the present invention ranges between≥ 10 and≤ 30 (according to size exclusion

chromatography (SEC) measurement), more preferably the M _w / M _n of the ethylene copolymer obtained with the catalyst according to the present invention ranges between≥ 15 and < 27.

The Izod impact strength (-30°C) of the ethylene copolymer obtained with the catalyst according to the present invention ranges between≥ 10 KJ/m ² and≤ 50 KJ/m ² (according to ISO 180/A). Preferably, the Izod impact strength (-30°C) of the ethylene copolymer obtained with the catalyst according to the present invention ranges between≥ 15 KJ/m ² and < 40 KJ/m ² .

The strain hardening modulus of the ethylene copolymer obtained with the catalyst according to the present invention ranges between≥ 30 MPa and≤ 55 MPa (ISO/DIS 18488). Preferably, the strain hardening modulus ranges between≥ 40 MPa and≤ 50 MPa. More preferably, the strain hardening modulus ranges between≥ 45 MPa and < 50 MPa.

The resin bulk density of the ethylene copolymer obtained with the catalyst according to the present invention ranges between between≥405 kg/m ³ and≤ 520 The present invention results in a HDPE polymerization system that is also capable of producing medium density polyethylene with enhanced physical and mechanical properties along with the HDPE products.

The catalyst according to the invention is useful for the production of ethylene homo and copolymers or only copolymers.

The catalyst according to the invention results in enhanced co-monomer incorporation. Suitable comonomers include butene and hexene. The preferred co- monomer is 1-hexene.

The chromium compound may be selected from various organic and inorganic forms of chromium. Preferably, the chromium compound is selected from chromium acetate, chromium acetyl acetonate, chromium acetate hydroxide and chromium trioxide. Most preferably chromium acetate or chromium acetyl acetonate is applied.

Preferably, the amount of chromium in the catalyst composition is between 0.20 % by weight and 0.90 % by weight. More preferably, the amount of chromium in the catalyst composition is between 0.23 % by weight and 0.55 % by weight. Preferably, the loading is between 0.23 %by weight and 0.50 % by weight.

The vanadium ⁴⁺ compound can be added to the unactivated chromium on silica catalyst followed by activation in dry air at temperatures in the range of 600-800 °C in order to convert Cr ( ³⁺ ) to Cr ( ⁶⁺ ).

The catalyst composition results in a polyethylene with a relatively low density and with low HLMI. In addition, the catalyst composition according to the invention yields in high catalyst activities, without the need for a co-catalyst. Typical applied co- catalysts are organoaluminium compounds, such as triisobutylaluminium, and triethylboron (TEB).

Preferably, also the gas phase polymerisation process for the production of ethylene copolymers is performed without the presence of a co-catalyst.

Suitable examples of the vanadium compound include organic oxygen containing vanadium compounds.

Suitable examples of organic oxygen containing vanadium compounds include alkoxides, phenoxides, oxyalkoxides, condensed alkoxides, carboxylates and enolates.

A suitable organic oxygen-containing vanadium compound is a vanadium alkoxide.

According to a preferred embodiment of the invention the vanadium

compounds are selected from vanadium(V) oxy triethoxide and vanadium(V) oxy tri- propoxide. The vanadium content of the catalyst composition ranges between 0.5 % by weight and 4.0 % by weight. Preferably, the loading is between 0.9 % by weight and 1.5 % by weight.

In combination with the vanadium compounds with levels of no more than 0.7 % to produce enhanced co-monomer incorporation catalyst. An amount of more than 1 % produces extra broad MWD which may be not desirable.

Preferably, the silicon oxide support material has a surface area (SA) > 630 m ² /g and < 800 m ² /g.

More preferably, the silicon oxide support material has a surface area (SA) > 645 m ² /g and < 800 m ² /g.

Preferably the pore volume (PV) is≥ 1.75 cm ³ /g and and≤ 2.0 cm ³ /g.

Preferably the average particle size is≥ 32 and≤ 40 micrometres.

The invention is also directed to a gas phase polymerisation process for the production of ethylene copolymers in the presence of a catalyst composition based on a chromium compound, a vanadium compound and a silica support material having a surface area (SA) > 600 m ² /g and < 800 m ² /g, a pore volume (PV)≥ 1.65 and < 2.0 cm ³ /g and an average particle size in the range≥ 30 and≤ 40 micrometres.

The surface area and pore volume of the supports can be determined by the BET nitrogen adsorption method. Test Method: ASTM D 1993-03 (2013) Standard Test Method for Precipitated Silica-Surface Area by Multipoint BET Nitrogen Adsorption.

See also references "Adsorption, Surface Area and Porosity" by S.J. Gregg and K.S.W. Sing, Academic Press, London (1982) and "Introduction to Powder Surface Area" by S. Lowell, J. Wiley & Sons, New York, NY, (1979).

The vanadium in combination with the specific silica support results in a lower density and a broadening of the MWD.

The catalyst preparation may take place by adding silica to a round bottom flask, equipped with a condenser and a mechanical stirrer dried under nitrogen purge. Next the chromium compound for example chromium acetate hydroxide is added to the silica and then slurried in for example methanol. Afterwards drying methanol solvent takes place with nitrogen purge. The dried chromium salt on silica powder is cooled down to room temperature and then slurried with for example iso-pentane, which is allowed to mix for for example 30 minutes at 45°C then drying the solvent at 95°C with nitrogen purge. For the chromium catalyst activation the dried catalyst powder was placed in the calciner.

It is possible to add an organic oxygen containing titanium compound to the catalyst system according to the invention. A geomembrane is a very low permeability synthetic membrane liner or barrier used with any geotechnical engineering related material so as to control fluid (or gas) migration in a human-made project, structure, or system. Geomembranes are made from relatively thin continuous polymeric for example HDPE sheets. Continuous polymer sheet geomembranes are, by far, the most common. The main physical properties of geomembranes in the as-manufactured state are thickness (smooth sheet, textured, asperity height), density, melt flow index, mass per unit area (weight) and vapor transmission (water and solvent). There are a number of mechanical tests that have been developed to determine the strength of polymeric sheet materials. Many have been adopted for use in evaluating geomembranes. They represent both quality control and design, i.e., index versus performance tests for example tensile strength and elongation (index, wide width, axisymmetric, and seams), tear resistance, impact resistance, puncture resistance, interface shear strength, anchorage strength and stress cracking (constant load and single point). Geomembranes have been used in the environmental, geotechnical, hydraulic, transportation, and private development applications.

The use of polyethylene in the production of geomembranes is disclosed for example in "Ageing of HDPE geomembranes exposed to air, water and leachate at different temperatures by Rowe et al. (Geotextiles and Geomembranes 27(2009) 137- 151 ; Oxidative resistance of high-density polyethylene geomembranes "by Mueller and Jakob (Polymer Degradation and Stability 79(2003) 161-172 and "Durability of HDPE geomembranes" by Rowe et al. (Geotextiles and Geomembranes 20(2002) 77-95.

The polyethylene obtained with the catalyst system according to the invention provides articles with much higher ESCR values with excellent resistance to wear and tear.

According to a preferred embodiment of the invention the polyethylene obtained with the catalyst composition according to the invention is used to produce a geomembrane or geotextile.

The polyethylene obtained with the catalyst composition according to the invention can also be used to manufacture of other articles such as different types of industrial packaging such as for example large industrial open head drums, tight head drums, jerry cans, fuel tanks and intermediate bulk containers (IBC).

US 2015/065667 and US 2015/080540 disclose a catalyst composition based on a chromium compound, a vanadium compound and an inorganic support material. The support material is modified with titanium and fluorine and is selected from the group consisting of silica, alumina, titania, zirconia, magnesia, calcium oxide and inorganic clays. The inorganic carrier has a surface area from 50 to 500 m ² /g

respectively 600 m ² /g, a pore volume from 0.1 , respectively 0.5 to 5.0 cm ³ /g. The

catalyst is used together with a co-catalyst for polymerisation. The catalyst produces polyethylene resins having the properties of broad molecular weight distribution and improved content and distribution of comonomer.

WO 2013/186025 discloses a process for the gas phase polymerisation of

ethylene in the presence of a catalyst composition comprising a support material

carrying a chromium compound and a magnesium compound. The catalyst

composition may comprise besides the chromium compound and the magnesium

compound, optionally a metal compound. The metal may be titanium, vanadium,

hafnium or zirconium.

The invention will be elucidated by means of the following non-limiting

examples.

Examples:

The properties of the polymers produced in the Examples were determined as follows:

The high load melt index (HLMI) is determined using the procedures of ASTM D-

1238 Condition F using a load of 21.6 kg at a temperature of 190°C.

Bulk density was measured according to ASTM D-1895.

The average particle size was measured according to ASTM D-4464-10.

Polymer molecular weight and its distribution (MWD) were determined by Polymer Labs 220 gel permeation chromatograph (GPC). The chromatograms were run at 150°C using 1 ,2,4- trichlorobenzene as the solvent with a flow rate of 0.9 ml/min. A refractive index detector is used to collect the signal for molecular weights. The software used is Cirrus from PolyLab for molecular weights from GPC. The calibration of the HT-GPC uses a Hamielec type calibration with broad standard and fresh calibration with each sample set.

Example I

Synthesis of catalyst composition

To a three-necked round bottom flask, equipped with a condenser and a mechanical stirrer 50 g of dried silica with average particle size of 33 micrometres, a pore volume (PV) of 1.8 m ² / g and a surface area of 650 m ³ /kg was dried at 150°C for 3 hours. After which 1.1 g of chromium acetate hydroxide were added to the silica then slurried in 250 cm ³ of Methanol (100%), which was stirred at 70°C for 60 minutes. Afterwards, drying ethanol solvent took place at 85°C with nitrogen purge. The dried chromium salt on silica powder was cooled down to room temperature then slurried with 250cm ³ of iso-pentane, to be followed by the addition of 5 cm ³ of 100% VO(OC2H ₅ )3 (Vanadium oxy tri ethoxide) which was allowed to mix for 30 minutes at 45°C then drying the solvent at 75°C with nitrogen purge. For the chromium catalyst activation the dried catalyst powder was placed in the calciner and the following sequence was followed:

Ramp from ambient to 150°C in 3 hours under N2 flow then hold for 10 minutes

- Ramp from 150°C to 450°C in 3 hours

- At 450°C switch from N ₂ to 0 ₂ flow

- Ramp from 450°C to 755°C in 3 hours under 0 ₂

- Hold at 755°C for 4 hours

- Cool to room temperature then switch to N2 purge.

- Elemental analysis: 0.49 wt% Cr and 1.05 wt% V

Ethylene Gas Phase Polymerization

An autoclave with a volume of 1 liters was purged with nitrogen at 130°C for 30 minutes. After cooling the autoclave to 90°C, the reactor was pressurized with 15 bar ethylene simultaneously. Then 0.03 g of the catalyst composition described in the synthesis of catalyst composition was injected into the reactor via catalyst injection pump. 1-hexene addition (6 g of 1-hexene were introduced to the reactor within 60 minutes, feeding about 0.1 g of 1-hexene per minute). The reactor temperature was raised to 103°C. Ethylene polymerization was carried out for 60 minutes, with ethylene supplied on demand to maintain the total reactor pressure at 20 bar (C6/C2= 0.0542. After 1 hour of ethylene polymerization 1 13 grams of polyethylene were recovered giving a catalyst productivity of 3766 g PE/g cat h at 15 bar ethylene.

The characteristics of the obtained polyethylene:

• weight average molecular weight Mw: 789883 g/mol

• number average molecular weight Mn: 35765 g/mol

• molecular weight distribution (Mw/Mn): 22.1

· Density: 0.938 g/cm ³

• Resin bulk density: 407 kg/m ³ .

• Fines level: 0.15 %.

• ESCR Strain Hardening modulus: 45.5 MPa

• HLMI 2.6 g/10min (190 °C, 21.6 kg)