WO2007118863A1

WO2007118863A1 - Chromium-based catalysts

Info

Publication number: WO2007118863A1
Application number: PCT/EP2007/053649
Authority: WO
Inventors: Philippe Bodart; Guy Debras; Mieke Dams
Original assignee: Total Petrochemicals Research Feluy SA
Current assignee: Total Petrochemicals Research Feluy SA
Priority date: 2006-04-13
Filing date: 2007-04-13
Publication date: 2007-10-25
Anticipated expiration: 2008-10-13
Also published as: EP2004704B1; EP1845110A1; JP2009533511A; CN101421315B; KR20080112295A; CN101421315A; ES2355304T3; ATE491730T1; EP2004704B2; EP2004704A1; CN101421314B; EA200802145A1; DE602007011236D1; JP5283129B2; US20100069585A1; EA015825B1; CN101421314A

Abstract

The present invention provides a process for preparing a supported chromium- based catalyst for the production of polyethylene comprising the steps of a) providing a silica-based support having a specific surface area of at least 250 m2/g and of less than 400 m2/g and comprising a chromium compound deposited thereon, the ratio of the specific surface area of the support to chromium content being at least 50000 m2/g Cr; b) dehydrating the product of step a); and c) titanating the product of step b) in an atmosphere of dry and inert gas containing at least one vaporised titanium compound of the general formula selected from RnTi(OR')m and (RO)nTi(OR')m, wherein R and R' are the same or different hydrocarbyl groups containing from 1 to 12 carbon atoms, and wherein n is 0 to 3, m is 1 to 4 and m+n equals 4, to form a titanated chromium-based catalyst having a ratio of specific surface area of the support to titanium content of the titanated catalyst ranging from 5000 to 20000 m2/g Ti.

Description

CHROMIUM-BASED CATALYSTS

FIELD OF THE INVENTION

The present invention relates to a chromium-based catalyst for producing polyethylene and to a method for preparing polyethylene using a chromium- based catalyst. The present invention further relates to a product obtained with said catalyst.

BACKGROUND AND OBJECTS OF THE INVENTION

For polyethylene, and for high-density polyethylene (HDPE) in particular, the molecular weight distribution (MWD) is a fundamental property that partially determines the properties of the polymer, and thus its applications. It is generally recognised in the art that the molecular weight distribution of a polyethylene resin can determine the mechanical properties of the resin and that the provision of different molecular weight polyethylene molecules can significantly affect the rheological properties of the polyethylene as a whole.

In this description, by polyethylene it is meant both homopolymers of ethylene and copolymers of ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms. High-density polyethylene means polyethylene resins that have a density of about 0.941-0.965 g/cm^, and medium-density polyethylene means polyethylene resins that have a density of about 0.926-0.940 g/ctvβ. By polymerisation, it is meant both homo- and copolymerisation of ethylene.

The molecular weight distribution can be defined by means of a curve obtained by gel permeation chromatography. Generally, the molecular weight distribution (MWD) is more simply defined by a parameter, known as the dispersion index D, which is the ratio between the average molecular weight by weight (Mw) and the average molecular weight by number (Mn). The dispersion index constitutes a measure of the width of the molecular weight distribution. Since an increase in the molecular weight normally improves some of the physical properties of polyethylene resins, there is a trend towards polyethylene having high molecular weight. High molecular weight molecules however render the polymer more difficult to process. On the other hand, a broadening in the molecular weight distribution tends to improve the flow of the polymer when it is being processed at high shear rates. Accordingly, in applications requiring a rapid transformation of the material through a die, for example in blowing and extrusion techniques, the broadening of the molecular weight distribution permits an improvement in the processing of polyethylene at high molecular weight (high molecular weight polyethylenes have a low melt index, as is known in the art). It is known that when the polyethylene has a high molecular weight and also a broad molecular weight distribution, the processing of the polyethylene is made easier as a result of the low molecular weight portion while the high molecular weight portion contributes to good mechanical properties for the polyethylene resin. A polyethylene of this type may be processed using less energy with higher processing yields.

As a general rule, a polyethylene having a high density tends to have a high degree of stiffness. In general, however, the environmental stress crack resistance (ESCR) of polyethylene has an inverse relationship with stiffness. In other words, as the stiffness of polyethylene is increased, the environmental stress crack resistance is decreased, and vice versa. This inverse relationship is known in the art as the ESCR-rigidity balance. It is required, for certain applications, to achieve a compromise between the environmental stress crack resistance and the rigidity of the polyethylene.

Polyethylene is well known in the art for use in making various finished goods, especially moulded products, such as bottles or containers.

A variety of catalyst systems are known for the manufacture of polyethylene.

It is known in the art that the mechanical properties of a polyethylene resin vary depending on what catalyst system was employed to produce the polyethylene. One of the reasons is that different catalyst systems tend to yield different molecular weight distributions in the polyethylene produced. Thus for example the properties of a polyethylene resin produced using a chromium oxide-based catalyst (i.e. a catalyst known in the art as a "Phillips- type catalyst") are different from the properties of a product employed using a Ziegler-Natta catalyst.

While chromium-based catalysts have been known since the 1950's, different attempts have been made to improve them. In order to improve either the mechanical properties or the melt index of the polyethylene products, it has been proposed to add titanium as a promoter to a chromium-based catalyst. US 4,184,979 discloses that titanium can be incorporated into a catalytic composition by adding to a chromium-based catalyst, which has been heated in a dry inert gas, a titanium compound such as titanium tetraisopropoxide. The titanated catalyst is then activated at elevated temperature. The ethylene polymers obtained with this process do not however have satisfactory mechanical properties especially with regard to the environmental stress crack resistance (ESCR).

In EP 882 743, a titanated catalyst providing polyethylene is obtained by providing a silica support having a specific surface area of at least 400 m^/g, depositing a chromium compound, dehydrating at a temperature of at least 300°C in an atmosphere of dry inert gas, titanating the chromium-based catalyst at a temperature of at least 300⁰C in an atmosphere of dry and inert gas containing a titanium compound of the general formula selected from Ti(OR)4 to form a titanated chromium-based catalyst having a titanium content of from 1 to 5 wt-%, based on the weight of the titanated catalyst and activating the titanated catalyst at a temperature of from 500 to 900⁰C.

Emphasis was put on the titanation procedure and on the use of a high surface area (of at least 400 m^/g). The exemplified chromium content of the catalyst was typically set at about 1 wt-%.

Although EP 882 743 provides a catalyst to manufacture a resin with good ESCR and/or tear stress compared to other prior art documents, the use of said catalyst however leads to a rather low melt index potential, which results in reduced polymerisation unit operability, and sometimes limits the resin processability. In addition, if such a catalyst is operated in a gas phase process, where lower chromium content has to be used in order to avoid excessive reaction rates and run away during polymerisation, the problem is that the melt index is further reduced, down to an unacceptable level.

There is therefore a need to further improve the catalyst operability while maintaining or improving the resin properties.

The present invention aims at alleviating at least some of these drawbacks while still producing high or medium density polyethylene with good environmental stress crack resistance (ESCR), high impact resistance and good processability.

SUMMARY OF THE INVENTION

The applicants have found that at least some of these drawbacks can be alleviated by combining the use of a catalyst with relatively low specific surface area and chromium content, using titanation under specific conditions to attain a given support surface area/final titanium content ratio and using elevated activation temperatures.

The present invention thus provides a process for preparing a supported chromium-based catalyst for the production of polyethylene comprising the steps of:

a) providing a silica-based support having a specific surface area of at least 250 m^/g and of less than 400 m^/g and comprising a chromium compound deposited thereon, the ratio of the specific surface area of the support to chromium content being at least 50000 m^/g Cr; b) dehydrating the product of step a) c) titanating the product of step b) in an atmosphere of dry and inert gas containing at least one vaporised titanium compound of the general formula selected from R_nTi(OR')_m and (RO)_nTi(OR')_m, wherein R and R' are the same or different hydrocarbyl groups containing from 1 to 12 carbon atoms, and wherein m is 1 to 4 and m+n equals 4, to form a titanated chromium-based catalyst having a ratio of specific surface area of the support to titanium content of the titanated catalyst ranging from 5000 to 20000 m²/g Ti.

The present invention further provides a chromium-based catalyst for the production of polyethylene. Said catalyst is obtainable according to the process of the present invention.

The present invention also provides a polymerisation process for preparing polyethylene by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, in the presence of the chromium-based catalyst obtainable according to the process of the invention. The present invention also provides a polyethylene homopolymer or a copolymer of ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, obtainable according to the polymerisation process of the present invention.

The present invention further provides a use of the chromium-based catalyst obtainable from the process according to the present invention, for producing a polyethylene by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms. The polyethylene obtained has a high environmental stress crack resistance and a low melt fracture index.

The present invention still further provides a use of a polyethylene homopolymer or a copolymer of ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, obtainable from the polymerisation process of the present invention for manufacturing moulded articles. These moulded articles have an increased impact resistance. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for preparing a supported chromium-based catalyst for the production of polyethylene comprising the steps described in claim 1.

It has thus been found that the manufacturing process according to the present invention allows the preparation of a catalyst, which, when used in the polymerisation of ethylene, leads to a polyethylene having an unexpected combination of good properties, especially for blow moulding applications.

Indeed, the inventors have found that, in the production of polyethylene resins, a particular chromium-based catalyst having a moderate specific surface area support, which has been dehydrated and the surface titanated prior to the activation of the catalyst at elevated temperatures, can unexpectedly yield polyethylene having high impact and/or environmental stress crack resistance.

Suitable supports used in this invention are silica-based and comprise amorphous silica having a surface area of at least 250 m^/g, preferably of at least 280 m^/g, and less than 400 m^/g, preferably less than 380 m^/g and more preferably less than 350 m^/g, including said values. The specific surface area is measured by N₂ adsorption using the well-known BET technique. In a previous application, EP 882 743, it had been assumed that a high surface area of at least 400 m^/g was a prerequisite for obtaining polyethylene with good properties.

Silica-based supports comprise at least 50% by weight of amorphous silica. Preferably the support is a silica support or a silica alumina support. In the case of silica alumina supports, the support comprises at most 15% by weight of alumina. The support can have a pore volume of 1 cm^/g to 3 cm^/g. Supports with a pore volume of 1.3 - 2.0 cm^/g are preferred. Pore volume is measured by INb desorption using the BJH method for pores with a diameter of less than 1000A. Supports with too small a porosity result in a loss of melt index potential and in lower activity. Supports with a pore volume of over 2.5 cm^/g or even with a pore volume of over 2.0 cm^/g are less desirable because they require special expensive preparation steps (e.g. azeotropic drying) during their synthesis or subsequent modification with chromium compounds. In addition, because they are usually more sensitive to attrition during catalyst handling, activation or use in polymerisation, these supports often lead to more polymer fines production, which is detrimental in an industrial process.

The silica-based support can be prepared by various known techniques such as but not limited to gelification, precipitation and/or spray-drying. Usually, particle size D50 is from 20 μm, preferably from 30 μm and more preferably from 35 μm, up to 150 μm, preferably up to 100 μm and most preferably up to 70 μm. D50 is defined as a particle diameter, with 50 wt-% of particles having a smaller diameter and 50 wt-% of particles having a larger diameter. Particle size D90 is up to 200 μm, preferably up to 150 μm, most preferably up to 110 μm. D90 is defined as a particle diameter, with 90 wt-% of particles having a smaller diameter and 10 wt-% of particles having a larger diameter. Particle size D10 is at least 5 μm, preferably at least 10 μm. D10 is defined as a particle diameter, with 10 wt-% of particles having a smaller diameter and 90 wt-% of particles having a larger diameter. Particle size distribution is determined using light diffraction granulometry, for example, using the Malvern Mastersizer 2000. The particle morphology is preferably microspheroidal to favour fluidisation and to reduce attrition.

Prior to use for catalyst synthesis, the support is dried by heating or pre- drying under an inert gas, in a manner known to those skilled in the art, e.g. at about 200⁰C for from 8 to 16 hours under nitrogen or other suitable gases. Known chromium-containing compounds capable of reacting with the surface hydroxyl groups of the silica-based supports can be used for deposition of chromium on said support. Examples of such compounds include chromium nitrate, chromium(lll)acetate, chromium(lll)acetylacetonate, chromium trioxide, chromate esters such as t-butyl chromate, silyl chromate esters and phosphorous-containing esters, and mixtures thereof. Preferably, chromium acetate, chromium acetylacetonate or chromium trioxide is used.

The chromium content of the chromium-based catalyst is chosen to get a ratio of the specific surface area of the support to chromium content of at least 50000 m²/g chromium, preferably from 50000 or 55000 m²/g chromium, up to 75000, 100000 or 200000 m²/g chromium. Thus, there is at most 1 g of chromium per 50000 m² of specific surface area of the support.

The chromium-based catalyst can be prepared by dry mixing or non-aqueous impregnation but is preferably prepared by impregnation of silica with an aqueous solution of a soluble chromium compound such as chromium acetate, chromium acetylacetonate or chromium trioxide.

After the chromium compound is deposited on the support, the chromium- based catalyst can be stored under a dry and inert atmosphere, for example, nitrogen, at ambient temperature.

The supported chromium-based catalyst is subjected to a pre-treatment in order to dehydrate it and drive off physically adsorbed water from the silica or silica-based support. The removal of physically adsorbed water can help to avoid the formation of crystalline Tiθ2 as a product from the reaction of water with the titanium compound subsequently introduced during the titanation procedure, as described below. The dehydration step is preferably carried out by heating the catalyst to a temperature of at least 220⁰C, more preferably of at least 250⁰C and most preferably of at least 270°C, in a fluidised bed and in a dry inert atmosphere of, for example, nitrogen. The dehydration step is usually carried out for 0.5 to 2 hours. In a next step, the supported chromium-based catalyst is loaded with one or more titanium compounds. The titanium compounds may be of the formula RnTi(OR')_m, (RO)_n Ti(OR')_m and mixtures thereof, wherein R and R' are the same or different hydrocarbyl groups containing 1 to 12 carbon atoms, and wherein m is 1 , 2, 3 or 4 and m+n equals 4. Preferably, the titanium compounds are titanium tetraalkoxides Ti(OR')4 wherein each R' is the same or different and can be an alkyl or cycloalkyl group each having from 3 to 5 carbon atoms. Mixtures of these compounds can also be used. The titanation is preferably performed by progressively introducing the titanium compound into a stream of a dry and inert non-oxidizing atmosphere, for example, nitrogen. The titanation step is carried out at a temperature so that titanium compound is present in its vaporised form. The temperature is maintained preferably at at least 220⁰C, more preferably at at least 250⁰C and most preferably at at least 270°C. The titanium compound can be pumped as a liquid into the reaction zone where it vaporizes.

This titanation step is controlled so that the ratio of the specific surface area of the support to titanium content of the resultant catalyst is from 5000 to

20000 m²/g Ti, and preferably from 5000, 6500, 7500 or 9000 m²/g Ti, up to 12000, 15000 or 20000 m²/g Ti. Preferably, if the support has a specific surface area of from at least 250 m²/g and of less than 380 m²/g, the ratio of specific surface area of the support to titanium content of the titanated catalyst ranges from 5000 to 20000 m²/g Ti, and if the support has specific surface area of from at least 380 and of less than 400 m²/g, the ratio of specific surface area of the support to titanium content of the titanated catalyst ranges from 5000 to 8000 m²/g Ti. The total amount of titanium compound introduced into the gas stream is calculated in order to obtain the required titanium content in the resultant catalyst and the progressive flow rate of the titanium compound is adjusted in order to provide a titanation reaction period of 0.5 to 2 hours. After the introduction of the titanium compound, the catalyst can be flushed under a gas stream for a period of typically 0.75 to 2 hours. The dehydration and titanation steps are preferably performed in the vapour phase in a fluidised bed.

After titanation the catalyst can be stored under a dry and inert atmosphere, for example, nitrogen, at ambient temperature.

According to an embodiment of the invention, the process further comprises a step d) consisting of activating the titanated product of step c). In order to activate the titanated catalyst, it must be subjected to dry air at an elevated activation temperature for at least 2 hours, preferably for at least 4 hours. The activation temperature can range from 500⁰C or 525 ⁰C, up to 600⁰C, 650°C, 700⁰C, 750°C, 800°C or 850⁰C. The atmosphere is changed from the dry and inert atmosphere, such as nitrogen, to dry air, either progressively or instantly. If after the titanation step, the catalyst is not intended for storage, the temperature can be progressively increased from the titanation temperature to the activation temperature without intermediate cooling.

The present invention also relates to an activated chromium-based catalyst for the production of polyethylene, comprising a silica-based support having a specific surface area of at least 250 m^/g and of less than 400 m^/g, a chromium compound deposited on the support, a ratio of the specific surface area of the support to chromium content of the catalyst of at least 50000 m^/g, and a titanium compound deposited on the support to obtain a ratio of the specific surface area of the support to titanium content of the catalyst of 5000 to 20000 m2/g Ti. Said activated catalyst is obtainable according to a process of the present invention.

The details and embodiments mentioned above in connection with the process for manufacturing the catalyst also apply with respect to the activated catalyst according to the present invention. The invention yet further relates to a method for preparing polyethylene by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, in the presence of an activated chromium-based catalyst comprising a silica-based support having a specific surface area of at least 250 m^/g and lower than 400 m^/g, a chromium compound deposited on the support, a ratio of the specific surface area of the support to chromium content of the catalyst of at least 50000 m^/g, and a titanium compound deposited on the support to obtain a ratio of the specific surface area of the support to the titanium content of the titanated catalyst of 5000 to 20000 m2/g Ti, said activated catalyst being obtainable according to a process of the present invention.

The details and embodiments mentioned above in connection with the process for manufacturing the catalyst also apply with respect to the polymerisation method according to the present invention.

Compared to previous systems, the activated catalyst according to the present invention has an improved melt index potential for the resulting resin although the catalyst's activation temperature is kept below 700⁰C while at the same time retaining sufficient activity. This broadens the polymerisation conditions thus reducing constraints in the production process of polyethylene and keeping an acceptable melt index for processability.

The ethylene polymerisation or copolymerisation method of the present invention is preferably carried out in the liquid phase (slurry process) or in the gas phase.

In a liquid slurry process, the liquid comprises ethylene, and where required one or more alpha-olefinic comonomers comprising from 3 to 10 carbon atoms, in an inert diluent. The comonomer may be selected from 1-butene, 1- hexene, 4-methyl 1-pentene, 1-heptene and 1-octene. The inert diluent is preferably isobutane. The polymerisation process is typically carried out at a polymerisation temperature of from 85 to 110⁰C and at a pressure of at least 20 bars. Preferably, the temperature ranges from 95 to 110°C and the pressure is at least 40 bars, more preferably from 40 to 42 bars to produce polymer resins with high environmental stress crack resistance (ESCR). Other compounds such as a metal alkyl or hydrogen may be introduced into the polymerisation reaction to regulate activity and polymer properties such as melt flow index. In one preferred process of the present invention, the polymerisation or copolymerisation process is carried out in a liquid-full loop reactor.

The method of the invention is particularly suited for gas phase polymerisations. Gas phase polymerisations can be performed in one or more fluidised bed or agitated bed reactors. The gas phase comprises ethylene, if required an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, such as 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene or mixtures thereof and an inert gas such as nitrogen. Optionally a metal alkyl can also be injected in the polymerisation medium as well as one or more other reaction-controlling agents, for example, hydrogen. For medium and even lower density polyethylenes obtained in gas phase polymerisations, the lower the temperature within the reactor and the lower the ratio of the specific surface area to chromium content of the catalyst i.e. the higher the chromium content, the better the processability of the resin will be due to the presence of increased long chain branching. For medium and high density polyethylenes, the higher the temperature and the higher the ratio of the specific surface area to chromium content of the catalyst i.e. the lower the chromium content, the better the mechanical properties of the resin will be. Reactor temperature can be adjusted to a temperature of from 80, 85, 90 or 95°C up to 100, 110, 112 or 115°C {Report 1: Technology and Economic Evaluation, Chem Systems, January 1998). Optionally a hydrocarbon diluent such as pentane, isopentane, hexane, isohexane, cyclohexane or mixtures thereof can be used if the gas phase unit is run in the so-called condensing or super-condensing mode.

The activated and titanated chromium-based catalyst is introduced into the polymerisation reactor. The ethylene monomers, and comonomer if present, are fed into the polymerisation reactor. The polyethylene obtained with the catalyst of this invention has a broad molecular weight distribution (MWD) that is represented by the dispersion index D of typically from 12 to 23, more typically of 14 to 22 and a density typically from 0.930 or from 0.934 g/cm³, up to 0.960 g/crrβ. Although the molecular weight distribution is very broad, the shear resistance (SR) is quite limited (SR is defined as HLMI/MI2 where HLMI is the high load melt index measured at 190⁰C and under a load of 21.6 kg and MI2 is the melt index measured at 190⁰C under a load of 2.16 kg, both with the ASTM D-1238 standard method). Nonetheless, the polyethylene exhibits good resistance to melt fracture when processed into bottles. The polyethylene obtained with the catalyst of this invention has high environmental stress crack resistance (ESCR) and very high impact resistance.

The invention also relates to an ethylene homopolymer or a copolymer of ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, obtainable by polymerising ethylene, or copolymerising ethylene and an alpha-olefin comonomer comprising 3 to 10 carbon atoms, in the presence of an activated catalyst according to the present invention. The polyethylene can be for example high density polyethylene or medium density polyethylene. According to a particular embodiment, the polyethylene is a semi high molecular weight polyethylene, i.e. with an HLMI ranging from 5 to 12 g/10 min. The polyethylene of the present invention can also be a blow moulding grade with an HLMI of 15 to 30 g/10 min. The polyethylene can also be a grade for blown film, more specifically medium density film grade with a density of from 0.934 to 0.945 g/cm³.

According to another embodiment, the polyethylene is a medium or low density resin obtained by polymerising ethylene in the presence of the activated catalyst of the invention in the gas phase. During polymerisation, the lower the ratio of specific surface area to chromium content of the catalyst and the lower the temperature in the gas phase reactor, the higher the degree of long chain branching will be. A polyethylene that is easily processable can thus be obtained.

The present invention also relates to a use of an activated chromium-based catalyst according to the present invention, for producing a polyethylene having high environmental stress crack resistance and low melt fracture index, by polymerising ethylene, or copolymerising ethylene and an alpha- olefinic comonomer comprising 3 to 10 carbon atoms. The polyethylene produced according to the present invention thus has a high environmental stress crack resistance and a low incidence of melt fracture when melted and subjected to rotational shear at varying speeds. It is therefore especially suitable for use in blow moulded bottles or containers, as well as any other use where such properties are required or preferred.

The details and embodiments mentioned above in connection with the homo and copolymers of ethylene also apply with respect to the use of the activated chromium-based catalyst obtainable according to the present invention.

The present invention moreover relates to a use of ethylene homopolymer or a copolymer of ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms obtainable from the polymerisation process of the present invention for manufacturing moulded articles. In particular the ethylene homopolymer or copolymer can be used to manufacture blow moulded articles, films and pipes. By using the polyethylene of the present invention, the impact resistance of the moulded article is increased.

The details and embodiments mentioned above in connection with the process for manufacturing the ethylene polymers also apply with respect to their uses according to the present invention.

The following Examples are given to illustrate the invention without limiting its scope. EXPERIMENTAL PART

Initial chromium-silica catalysts

Catalyst A was obtained by deposition of about 0.51 wt-% chromium (Cr) on a microspheroidal silica support. The chromium source was Cr(lll)acetate. Impregnation with Cr-acetate was performed by incipient wetness impregnation, using an aqueous solution of the Cr-salt. This is typical of all the catalysts below.

This catalyst was thus a Cr-silica catalyst. Main properties were: surface area (SA) = 306 m²/g, pore volume = 1.53 ml/g, Cr content = 0.51 wt-%. The ratio surface area/Cr = 60000 m²/g Cr.

Catalyst B was similar to catalyst A, but its surface area was 315 m²/g, pore volume = 1.44 ml/g, Cr-content = 0.51 wt-%. The ratio surface area/Cr =

61760 m²/g Cr.

Catalyst C was again similar to catalyst A but its surface area was 301 m²/g, pore volume = 1.34 ml/g, Cr-content = 0.23 wt-%. The ratio surface area/Cr =

130870 m²/g Cr.

Catalyst D was a Cr-silica catalyst on a granular silica support. Its surface area was 319 m²/g, pore volume = 1.57 ml/g, Cr-content = 1.06 wt-%. The ratio surface area/Cr = 30000 m²/g Cr. The impregnation was carried out as explained for catalyst A.

Catalyst E was similar to catalyst A, except that the silica support was characterised by a high surface area. The main properties were surface area = 461 m²/g, pore volume = 1.27 ml/g, Cr-content = 0.26 wt-%. The ratio surface area/Cr = 177310 m²/g Cr. Catalyst F was similar to catalyst D, but had a lower Cr loading. Surface area = 466 m^/g, pore volume =1.3 ml/g, Cr-content = 0.41 wt-%. The ratio surface area/Cr = 113660 m²/g Cr.

Catalyst G was also similar to catalyst A, but from another batch. The surface area was 317 m^/g, pore volume = 1.46 ml/g, Cr-content = 0.53 wt-%. The ratio surface area/Cr = 59800 m^/g Cr.

Catalyst H was similar to catalyst A, but from another batch. The surface area was 301 m^/g, pore volume = 1.53 ml/g, Cr-content = 0.56 wt-%. The ratio surface area/Cr = 53750 m²/g Cr. D50 = 46 μm, D10 = 13 μm, D90 = 88 μm. Fines content (d < 31 μm) was about 25 wt%.

Catalyst I was similar to catalyst A, but from another batch. The specific surface area was 319 m^/g and the Cr-content 0.55 wt-%. The ratio specific surface area/Cr was 58000 m^/g Cr.

Catalyst J was similar to catalyst A, but from another batch. The specific surface area was 306 m^/g and the Cr-content 0.53 wt-%.

Catalyst K was similar to catalyst A, but from another batch. The ratio specific surface area/Cr was 51300 m^/g Cr.

Titanation of chromium-silica catalysts

Labscale titanation and activation of chromium catalysts

A series of titanated chromium-silica catalysts (Ti-Cr-Si-catalyst) were prepared using the chromium-silica materials described above and using the following procedure.

The support impregnated with the chromium-compound was introduced in an activator vessel incorporating a fluidised bed, flushed under nitrogen and the temperature was raised from room temperature to 300⁰C. The dehydration step was carried out at this temperature for 2 hours. After this dehydration step, liquid titanium tetraisopropoxide (TYZOR® TPT), stored under anhydrous nitrogen, was progressively introduced in the lower part of the fluidised bed in the activator vessel maintained at 300⁰C so that titanium compound vaporised. The amount of titanium isopropoxide was calculated in order to get the required titanium content in the resultant catalyst. The flow thereof was adjusted to complete the addition in about 30 minutes. After the injection was completed, the catalyst was flushed with nitrogen for about 2 hours. Nitrogen was then replaced by air and the temperature was raised to the desired activation temperature. In the activation step, the titanium containing catalyst was maintained at the desired activation temperature for 6 hours. Then, the temperature was progressively decreased to 350°C. At 350⁰C, the gas flow was switched to nitrogen for further cooling to room temperature. At room temperature, the catalyst was kept under dry inert atmosphere. Two small scale activators were used, one able to handle 1 kg of catalyst powder, and one able to handle 50 g of catalyst powder.

Industrial scale titanation and activation of Cr-catalysts

The starting catalyst was activated in an industrial fluidised bed activator according to the following procedure:

- about 200 kg of the starting solid was introduced in a fluidised bed activator;

- the starting solid was heated up to 120°C and then to 270°C in 3 hours under nitrogen and maintained at this temperature for about 2 hours;

- about 41 - 45 kg of titanium tetraisopropoxide (available under the trade name TYZOR® TPT) were progressively injected in the fluidised bed (over 2 hours whilst maintaining the temperature at 270⁰C so that titanium compound vaporised); - the obtained titanated catalyst was further maintained at 270⁰C under nitrogen flow for 2 hours; - nitrogen was replaced by air and the titanated catalyst was heated up to 550⁰C and maintained at 550⁰C for 6 hours;

- the activated catalyst was cooled down to 350°C under air and then to room temperature under nitrogen; - the activated catalyst was unloaded under nitrogen and kept under inert atmosphere prior to further use in polymerisation.

Bench scale polymerisations

A series of polymerisation experiments were carried out in bench scale to evaluate the potential of the different catalysts.

Testing was performed in suspension in isobutane, in slurry mode. The reactor was a 5-litre volume autoclave type with an agitator and a double wall. Hot water was passed through the double wall to control the internal temperature. The activated catalyst was introduced in the dry, clean autoclave, under nitrogen. 2 litres of liquid isobutane used as diluent was then introduced in the autoclave and the temperature was raised to the desired value. Gaseous ethylene (C2) was introduced into the reaction vessel. Ethylene pressure was adjusted to maintain a constant ethylene concentration in the liquid phase, typically 6 wt-%. Overall pressure was maintained constant by introducing fresh ethylene in the reactor. If required, 1-hexene (C6) comonomer could be used to change the density of the polymer. One shot of 1-hexene was introduced at the start of the reaction. All the polymerisations were carried out under stirring to get a homogeneous mixing. After polymerisation was complete, the reactor was vented off and cooled down to room temperature. The powder was then dried in a vacuum oven to eliminate the residual monomers and isobutane prior to further processing (stabilization, extrusion, characterisation).

Industrial scale polymerisation

Industrial polymerisation trials were performed in a fluidised bed reactor. The polymerisation conditions are detailed in the corresponding examples. Characterisation of the polymers

The polymers obtained in the Examples and Comparative examples were tested with different methods.

The melt index of the polymers was measured according to the standard ASTM D 1238. MI2 corresponds to a measure at 190°C under a load of 2.16 kg. HLMI corresponds to a measure at 190°C under a load of 21.6 kg and the results are given in g/10 minutes. Shear ratio SR2 was calculated as HLMI/MI2.

The density was measured according to the standard ASTM D1505-85 and given in g/cnΑ

The number average molecular weight Mn, the weight average molecular weight Mw and the z-average molecular weight Mz were measured by gel permeation chromatography Waters S.A. GPC2000 gel permeation chromatograph. The chromatograph had been calibrated on a broad standard. Three columns were used, two Shodex AT-806MS columns from Showa Denko and one Styrogel HT6E column from Waters. The injection temperature was 145 ⁰C, the injection volume comprised about 1000 ppm of stabiliser butylhydroxytoluene (BHT). The sample was prepared by mixing 10-15 g of polyethylene with 10-15 ml of 1 ,2,4-trichlorobenzene (TCB) comprising BHT during 1 hour at 155 ⁰C. The mixture was filtered on a membrane of 0.50 μm and the concentration of the solution was 0.1 % in room temperature.

The detector used was refractory indexer and the results were treated with the program Empower of Waters S.A. The results are given in kDa. The molecular weight distribution MWD, or more simply defined by a parameter known as the dispersion index D, was calculated as the ratio of Mw and Mn. A value of SR2/MWD is also given and it gives an estimation of the long chain branching (LCB), that is, a higher SR2/MWD corresponds to a higher long chain branching content. Environmental stress cracking resistance ESCR was measured according to the conditions described in the standard ASTM D 1693, conditions "B", as recommended for PE with densities higher than 0.925g/cm³. The ESCR tests are tests that are currently carried out in order to check the resistance of polyethylene to crack propagation when in contact with various chemical products.

The conditions of test were chosen in order to accelerate the mechanism of crack propagation: the test was carried out at 50⁰C, the surfactant was lgepal CO 630, also named "Antarox", and the surfactant was used in pure form (100 %). The material to be tested was compression moulded into plates, out of which 10 specimens were punched out. The specimens were notched, bent and placed in contact with a surfactant at the test temperature. The test was therefore carried out at constant strain. The samples were visually checked twice a day to detect the appearance of any cracks on the specimens. When cracks had been detected on all specimens, the F50 time was calculated (time after which 50 % of the specimens are considered as "broken"). The results are thus given in hours.

Melt fracture is a flow instability phenomenon occurring during extrusion of thermoplastic polymers at the fabrication surface/polymer melt boundary. The occurrence of melt fracture produces severe surface irregularities in the extrudate as it emerges from the orifice. The naked eye detects this surface roughness in the melt-fractured sample as a frosty appearance or matte finish as opposed to an extrudate without melt fracture that appears clear. In this description, melt fracture was estimated from Gottfert measurement of shear viscosity over a range of shear rates typical of those found in the process die.

The measurements were carried out at a temperature of 210⁰C with a die 10 mm in length and 1 mm in diameter. The shear rates were 750 s-1 , 725 s-1 , 700 s-1 , 650 s-1 , 600 s-1 , 500 s-1 , 400 s-1 and 300 s-1. The evaluation of melt fracture was made visually. The Charpy impact resistance was determined according to the standard ASTM D-5045-91a. The method consisted in determining the resistance to impact by a V-shaped hammer (5.154 kg, Charpy ISO) falling on a normalised test specimen in normalised conditions, from a certain height. The test specimens were notched on Notchvis and with a razor blade a slight pressure was applied at the bottom of the notch in order to create the beginning of a fracture. The Charpy impact resistance was measured at two different temperatures, namely +23°C and -30⁰C. The results are given in average resilience (kJ/m^). The standard deviation (StdDev) of the resilience is also given.

EXAMPLE 1

Catalyst A was used for polymerisation of ethylene in bench scale. Titanation was performed in small scale as stated above (1 kg catalyst powder). Ti concentration was 2.8 wt-%. Final activation step was carried out at a temperature of 550⁰C. The activated catalyst was tested in a bench scale reactor as described above.

The test conditions and the resin properties are presented in Table 1. Target productivity was 1000 g polymer/g catalyst. Runs 1A and 1 B were conducted at 98°C and 102⁰C, respectively.

EXAMPLE 2

In Example 2, two batches of activated catalyst A were prepared. Final Ti- concentration was 3.0 and 2.8 wt-%, respectively, and activation temperature was increased to 575°C and 600⁰C respectively. Activation was run in the same equipment as in Example 1. Polymerisation was performed under the same conditions as in Example 1 , at a temperature of 98°C. The data is shown in Table 1.

EXAMPLE 3

Example 3 was carried out using the same starting material as in Examples 1 and 2. Activation was performed using the same activator as in Examples 1 and 2. However, Ti content of the activated catalyst was raised to 3.7 and 3.8 wt-%, respectively and activation temperature was set at 550⁰C and 600⁰C respectively.

The polymerisation was performed under the same conditions as in Examples 1 and 2, the temperature being fixed at 98°C. The data is presented in Table 1.

Table 1

Comments on data from Examples 1 to 3

The results show that activation at a higher temperature results in higher catalyst activity, in higher resin melt index, but in slightly narrower MWD (from 23 (1A), 19.5 (2A) and 17.6 (2B) respectively at 550, 575 and 600⁰C). However, for a given MI2, the shear response was slightly increased (see 1 B and 2A). This could contribute to a higher content of long chain branching when the catalyst is activated at a higher temperature.

Increasing the target Ti-concentration increases the melt index of the obtained polyethylene, while maintaining the MWD essentially constant (compare 2B and 3B). However, at higher activation temperatures, this results in slightly lower ESCR, showing that the catalyst system is more sensitive to activation at higher Ti-contents.

EXAMPLE 4

Example 4 was carried out using catalyst B as the starting material. Catalyst B was activated in a small scale activator (50 mg catalyst powder), following the above general labscale procedure.

Target titanium content of activated catalyst was 2.8 wt-%. Activation temperature was set at 550⁰C.

Polymerisation was carried out in bench scale following the procedure described above. Target productivity was set at about 2000 g PE/g catalyst. Temperature was varied between 96 and 100°C at varying hexene concentrations to get different densities. The data is presented in Table 2.

EXAMPLE 5

Example 5 was carried out in a similar manner to Example 4, except that the activation temperature was set at 650⁰C. The data is presented in Table 2.

Table 2

Comments on data from Examples 4 and 5

The data of Examples 4 and 5 show that an increase of activation temperature has a significant effect on the MWD and on the mechanical properties of the final PE resin. At 650⁰C, MWD is reduced from 15-20 to 12-14, while the ESCR of polyethylene with densities of 0.956-0.957 g/crrβ is divided by a number ranging from 2 to 3.

EXAMPLE 6

A similar polymerisation was carried out using catalyst C (0.23 wt-% Cr) after titanation (3 wt-% Ti) and activation at 650⁰C (50 mg powder). The results are displayed in Table 3.

COMPARATIVE EXAMPLE 7

Comparative example 7 was carried out using catalyst D (nominal Cr-content: 1 wt-%). Catalyst D was titanated (4 wt-% Ti) and further activated at 650°C in a small scale activator (50 mg catalyst). It was used for copolymerisation of ethylene with 1-hexene. The results are displayed in Table 3.

COMPARATIVE EXAMPLE 8

Catalyst D was activated in a small scale activator at 870⁰C without the titanation step. As such it was used for polymerisation of ethylene in a bench autoclave reactor. The data is presented in Table 3.

Table 3

Comments on data from Example 6 and Comparative examples 7 and 8

Table 3 compares the results obtained with different chromium loadings, including the results with 0.5 wt-% chromium (example 5B). The results show that increasing the chromium concentration to 1 wt-% results in lower mechanical properties. Indeed, although MWD is similar, final ESCR is lower (24 hours vs. 44 and 54 hours). In addition, the shear ratio is higher for a similar MWD. Activation of

Ti-free catalyst at a very high temperature is required to get a sufficient melt index potential that results in high shear response and a quite narrow MWD. This shows that the density of chromium sites on the surface is an important parameter and hence a higher surface area/g Cr is more favourable.

COMPARATIVE EXAMPLE 9

Catalyst E (SA = 461 m²/g, Cr = 0.26 wt-%) was titanated as described above (Ti = 5.15 wt-%) and activated at 550⁰C in a small scale activator (50 mg powder). It was used for a polymerisation trial in bench scale. The data is presented in Table 4.

COMPARATIVE EXAMPLE 10

Catalyst F (SA = 466 m²/g, Cr = 0.41 wt-%,) was titanated and then activated at 550⁰C in a small scale activator (50 mg powder). It was tested on bench scale for further evaluation. The data is presented in Table 4.

Table 4

Comments on results from Comparative examples 9 and 10

Table 2, examples 4A-4D, provide results obtained with a low surface area catalyst titanated and activated at 550⁰C and table 4 provides results with high surface area catalyst titanated and activated at 550⁰C.

The high surface area catalysts have a much lower melt index potential than the lower surface area catalysts as evidenced by the low melt index and/or the at least 4°C higher polymerisation temperature required to get somewhat acceptable melt index potential. The high molecular weight of the polymers obtained results in a very good ESCR/density compromise. However, processing is significantly more difficult as shown by the very rapid onset of melt fracture observed when using polymers obtained with such catalysts.

EXAMPLE 11

Catalyst G was used in an industrial trial. Activation was carried out following the previous procedure described for large-scale activation. Titanium content was 3.1 wt-% and activation temperature was set at 550⁰C.

The titanated and activated catalyst was used in a gas phase fluidised bed reactor for co-polymerisation of ethylene and 1-hexene to produce HDPE. The throughput of the gas phase reactor was 25-30 t/hour PE.

Polymerisation was carried out at a temperature of from 108 to 112°C. Ethylene partial pressure was about 16 bars. Hydrogen H₂ (3 and 25 mol-%) was used to control the melt index of the final polyethylene. 1-hexene feed ratio of 0.038 to 0.500 wt-% was used.

Five lots of HDPE resins with HLMI ranging from 8.3 to 18.4 g/10 min were produced.

The fluff was stabilized with antioxidants, 500 ppm lrgafos-168 and 500 ppm lrganox-1010 prior to extrusion and pelletisation.

The products were characterised for catalyst productivity, flow properties (MI2 and HLMI), density and mechanical properties (ESCR and Charpy impact resistance).

The reaction conditions and some properties of the resulting resins are presented in Table 5. Table 6 presents the ESCR and Charpy impact resistance behaviour as compared to similar commercial resins. Figures 1 and 2 show the ESCR- density compromise and the Charpy impact resistance of the resins according to the present invention vs. commercial Cr-catalysed HDPE materials. It is noteworthy that in Example 11 , reactor conditions were very stable and reactor operability was very good, with e.g. very low electrostatic build up during the gas phase polymerisation. Figure 1 compares the ESCR-rigidity compromise of the experimental resins with commercial materials. In Figure 1 , the density is given on the abscissa and the ESCR in hours on the ordinate. The upper curve shows the behaviour of second generation chromium grades, the lower curve shows the behaviour of first generation chromium grades and the triangles show the behaviour of the materials according to the present invention.

Commercial materials are divided between 1^st generation HDPE resins and 2^nd generation HDPE resins. First generation HDPE resins are known for very easy processability but limited mechanical properties. Representative materials are for instance former Finathene® 5502 and 47100 grades or 5502 grade presently sold by TOTAL Petrochemicals. Second generation HDPE resins are known for improved mechanical properties (i.e. a better density/rigidity - ESCR compromise). Typical resins include e.g. HDPE materials sold under the names 53140, 53080 and 49080 (sold by TOTAL Petrochemicals), or sold under the names Stamylan (D8621 (SABIC) or Polimeri® BC86 (Polimeri Europa).

Figure 1 shows that a catalyst according to the present invention is able to produce polymers having similar or better density-ESCR compromise than previously existing Cr-HDPE resins.

Table 6 and Figure 2 compare the impact resistance of the experimental resins with those of commercial semi-high molecular weight materials. E1 stands for Example 11 B, E2 for Example 11 E and E3 for Example 11 D. C1 stands for a commercial resin 2003SN53, C2 for a commercial resin 53140 and C3 for a commercial resin 53080, all sold by TOTAL Petrochemicals.

The data shows that at low temperature, impact resistance is as good as that of the comparison materials with a lower density. At +23°C, the Charpy impact resistance of the experimental resins is significantly improved (+30 %).

Table 5

Table 6

EXAMPLE 12

Catalyst H was activated following the large scale activation procedure. Target titanium content was set at 4 wt-% and activation temperature at 650⁰C (surface area/titanium=7525 m²/g Ti). The activated catalyst was used for the production of a medium density film grade polyethylene. The polymerisation conditions are presented in Table 7.

The obtained resin (d=0.944 g/crrβ) was blown into a film.

Table 7

EXAMPLE 13

Catalyst I was titanated according to the industrial scale titanation procedure. The target titanium content was set to a target ratio of surface area/titanium of 5942 m^/g Ti. The catalyst was activated at 620⁰C. The activated catalyst was used to polymerise ethylene on a large scale at conditions necessary to obtain a density of 0.954 g/cm3 and an HLMI of 20 dg/min. The conditions and results are provided in Table 8.

COMPARATIVE EXAMPLE 14

Table 8 also provides a comparison of Example 13 presented as Comparative Example 14, which is a polyethylene prepared with a chromium catalyst having a specific surface area to chromium ratio of about 32600 m^/g Cr. The chromium catalyst was not titanated and activated at a temperature of 780°C.

Table 8

As it can be seen in the table above, the catalyst according to the invention provides a polyethylene with an environmental stress crack resistance almost 4 times as long the catalyst according to the prior art.

EXAMPLE 15 and 16

Catalyst J was titanated according to the industrial scale titanation procedure. The target titanium content was set to a target ratio of surface area/titanium of 10200 m^/g Ti. The catalyst was activated at 675 ⁰C. The activated catalyst was used to polymerise ethylene on a large scale at conditions necessary to obtain a density of 0.936 g/cm3 and an HLMI of 15 dg/min. The same activated catalyst J was used to polymerise ethylene on a large scale at conditions necessary to obtain a density of 0.928 g/crrβ and an HLMI of 14 dg/min. The conditions and results are provided in Table 9.

Table 9

The obtained resins were blown into a film. EXAMPLE 17

Catalyst K was titanated according to the industrial scale titanation procedure. The specific surface area/chromium ratio comprises 51300 m^/g Cr, whereas the target titanium content was set to a target ratio of surface area/titanium of 6200 m^/g Ti. The activated catalyst was used to polymerise ethylene on a large scale at conditions necessary to obtain a density of 0.919 g/cm^ and an HLMI of 18.9 dg/min. The conditions and results are provided in Table 10.

Table 10

Claims

1. A process for preparing a supported chromium-based catalyst for the production of polyethylene comprising the steps of

a) providing a silica-based support having a specific surface area of at least 250 m^/g and of less than 400 m^/g and comprising a chromium compound deposited thereon, the ratio of the specific surface area of the support to chromium content being at least 50000 m^/g Cr; b) dehydrating the product of step a); c) titanating the product of step b) in an atmosphere of dry and inert gas containing at least one vaporised titanium compound of the general formula selected from R_nTi(OR')_m and (RO)_nTi(OR')_m, wherein R and R' are the same or different hydrocarbyl groups containing from 1 to 12 carbon atoms, and wherein n is 0 to 3, m is 1 to 4 and m+n equals 4, to form a titanated chromium-based catalyst having a ratio of specific surface area of the support to titanium content of the titanated catalyst ranging from 5000 to 20000 m^/g

Ti.

2. A process according to claim 1 , wherein if the support has a specific surface area of from at least 250 m^/g and of less than 380 m^/g, the ratio of specific surface area of the support to titanium content of the titanated catalyst ranges from 5000 to 20000 m^/g Ti, and if the support has specific surface area of from at least

380 m^/g and of less than 400 m^/g, the ratio of specific surface area of the support to titanium content of the titanated catalyst ranges from 5000 to 8000 m^/g Ti.

3. The process according to claim 1 or claim 2, wherein step b) is carried out at a temperature of at least 220⁰C in an atmosphere of dry and inert gas.

4. The process according to any one of the foregoing claims, wherein step c) is carried out at a temperature of at least 220⁰C.

5. The process according to any one of the foregoing claims, wherein step c) is carried out at a temperature of at least 250⁰C.

6. The process according to any one of the foregoing claims, wherein step c) is carried out a temperature of at least 270⁰C.

7. A process according to any one of the foregoing claims, characterised in that the support has a specific surface area of from 280 to 380 m^/g.

8. A process according to claim 7, characterised in that the support has a specific surface area of from 280 to 350 m^/g.

9. A process according to any one of the foregoing claims, wherein the at least one titanium compound is selected from the group consisting of tetraalkoxides of titanium having the general formula Ti(OR')4 wherein each R' is the same or different and can be an alkyl or cycloalkyl group each having from 3 to 5 carbon atoms, and mixtures thereof.

10. A process according to any one of the foregoing claims, wherein the ratio of the specific surface area of the support to titanium content of the titanated catalyst is from 6500 to 15000 m²/g Ti.

11. A process according to any of the foregoing claims, wherein the ratio of the specific surface area of the support to chromium content ranges from 50000 to 200000 m²/g Cr.

12. A process according to any one of the foregoing claims, characterised in that it further comprises a step d) of activating the titanated chromium-based catalyst of step c) at a temperature of from 500 to 850°C.

13. A process according to claim 12, characterised in that it further comprises a step d) of activating the titanated chromium-based catalyst of step c) at a temperature of from 500 to 700⁰C.

14. An activated chromium-based catalyst for the production of polyethylene obtainable according to claim 12 or 13.

15. A method for preparing polyethylene by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, in the presence of an activated chromium-based catalyst according to claim 14.

16. A method according to claim 15, wherein the polymerisation is carried out in a gas phase process.

17. A method according to claim 16 wherein the polymerisation is carried out in a fluidised bed gas phase reactor.

18. An ethylene homopolymer or a copolymer of ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, obtainable according to the method of claim 16 or 17.

19. Use of an activated chromium-based catalyst according to claim 14, for producing polyethylene, by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms.

20. Use according to claim 19 for producing a semi-high molecular weight polyethylene, with an HLMI ranging from 5 to 12 g/10 min.

21. Use of an ethylene homopolymer or a copolymer of ethylene and an alpha- olefinic comonomer comprising 3 to 10 carbon atoms according to claim 18 to manufacture moulded articles.

21. Use according to claim 21 to manufacture blow moulded articles.

22. Use according to claim 21 to manufacture films.

23. Use according to claim 21 to manufacture pipes.