[0001] The present invention relates to a process for the polymerization of olefins carried out in the presence of a supported antistatic compound. The invention also relates to a supported antistatic compound, to a process for its preparation and to its use in a process for the polymerization of olefins.

[0002] In polymerization processes carried out in continuous, particularly in gas-phase processes for the polymerization of olefins, there is the need to face up to the formation of polymer agglomerates in the polymerization reactor. The polymer agglomerates involves many negative effects. For example, they can disrupt the discharge of polymer from the reactor by plugging the polymer discharge valves. Furthermore, the agglomerates may also partially cover the fluidization grid of the reactor with a loss in the fluidization efficiency.

[0003] It has been found that the presence of fine polymer particles in the polymerization medium favors the formation of polymer agglomerates. These fines may be present as a result of introducing fine catalyst particles or breakage of catalyst and polymer particles within the polymerization medium. The fines are believed to deposit onto and electrostatically adhere to the inner walls of the polymerization reactor and the equipment for recycling the gaseous stream such as, for example, the heat exchanger. If the fines remain active, then the particles will grow in size resulting in the formation of agglomerates, also caused by the partial melting of the polymer itself. These agglomerates when formed within the polymerization reactor tend to be in the form of sheets. Agglomerates can also partially plug the heat exchanger designed to remove the heat of polymerization reaction.

[0004] Several solutions have been proposed to resolve the problem of formation of agglomerates during a gas-phase polymerization process. Solutions include the deactivation of the fine polymer particles, the control of the catalyst activity and, above all, the reduction of the electrostatic charge by introducing antistatic agents inside the reactor.

[0005] EP 0359444 describes the introduction into the polymerization reactor of small amounts of an activity retarder in order to keep substantially constant either the polymerization rate or the content of transition metal in the polymer produced. The process is said to produce a polymer without forming agglomerates.

[0006] US Patents 4,803,251 and 5,391 ,657 describe methods for reducing the polymer sheeting by adding to the reactor additives that generate positive or negative charges depending on whether the electrostatic level detected in the reactor is negative or positive respectively. In US Patent 5,391 ,657, Si02 is mentioned among the possible negative charge generating additives.

[0007] EP 0560035 discloses a polymerization process in which an anti- fouling compound is used to eliminate or reduce the build-up of polymer particles on the walls of a gas-phase polymerization reactor. This anti-fouling compound is preferably selected from alkydiethanolamines, which may be fed at any stage of the gas- phase polymerization process in an amount greater than 100 ppm by weight with respect to the produced (co)polymer. Said anti-fouling compound is capable to selectively inhibit the polymerization on polymer particles smaller than 850 μιη, the latter being responsible for fouling problems and polymer sheeting.

[0008] The prior art literature generally teaches to introduce antistatic/antifouling compounds directly inside the polymerization reactor. Alternative methods for introducing antistatic compounds into a polymerization reactor have been proposed.

[0009] International patent applications WO2007/041810 discloses supporting an olefin polymerization catalysts together with a polysulfone antistatic on a porous metal oxide and using such supported antistatic catalyst in olefin polymerization.

[0010] International patent applications WO2012/041810 and WO2012/04181 1 disclose methods for feeding antistatic components to a polymerization reactor by preparing a catalyst suspension incorporating said antistatic components and successively transferring the catalyst suspension to the polymerization reactor.] A side effect of the use of antistatic compounds in polymerization processes is that they also act as catalyst poison and therefore, even if used in small amounts, depress to a certain extent the polymer yields. None of the methods proposed hitherto has successfully addressed the need of being effective in avoiding the formation of polymer agglomerates in the reactor while at the same time minimizing negative effects on catalyst yield. The applicant has found that this and other advantages can be achieved by the process of the present invention.

] Therefore, according to a first object, the present invention provides an olefin polymerization process comprising the steps of:

(a) contacting an antistatic compound with a support in the absence of a transition-metal-based catalyst component to obtain a supported antistatic compound;

(b) introducing the supported antistatic compound into a polymerization reactor; and then

(c) polymerizing an olefin in the polymerization reactor in the presence of a catalyst.

] The term "antistatic compound", as used in the present description, includes:

- antistatic substances capable of neutralizing the electrostatic charges of the polymer particles; and

- cocatalyst deactivators that partially deactivate the aluminium alkyl co-catalyst used as a component of the olefin polymerization catalyst, provided that they do not substantially inhibit the overall polymerization activity.

] Consequently, an "antistatic compound" according to the invention is any substance that is capable to prevent, eliminate or substantially reduce the formation of build-up of polymer on any equipment of the polymerization plant, including sheeting of reactor walls and deposit of polymer agglomerates onto the gas recycle line.

Suitable antistatic compounds for use in the process according to the invention can be selected from the following classes:

1 ) hydroxyesters with at least two free hydroxyl groups, such as glycerol monostearate (GMS90) and glycerol mono palmitate;

2) alcohols containing up to 7 carbon atoms;

3) ketones containing up to 7 carbon atoms;

4) polyepoxidate oils, such as epoxidized soyabean oil (for example EDENOL D81 ) and epoxidized linseed oil (for example EDENOL D316);

5) polyglycerol esters, such as diglycerol monooleate (for

example GRINSTED PGE O80/D);

6) alkyldiethanolammines of formula R-N(CH2CH20H)2 wherein

R is an alkyl radical comprised between 10 and 20 carbon atoms; and

7) amides of formula R-CONR'R", wherein R, R\ and R" may be the same or different, and are a saturated or unsaturated hydrocarbon radical having 1 to 20 carbon atoms.

[0016] The preferred class of antistatic compounds used in the process of the invention is represented by the hydroxyesters of class 1 ). The most preferred is glycerol monostearate (GMS90), which is added to the dispersion tank of step (a) in form of powder or micro-beads: preferred morphologies are microbeads.

[0017] Among the compounds of class 6) the preferred antistatic is a commercial product sold under the trademark ATMER ^® 163 (synthetic mixture of alkyldiethanolammines of formula R- N(CH2CH20H)2, where R is an alkyl radical containing 13-15 carbon atoms. Also natural-based alkyldiethanolammines may be used, for instance ARMOSTAT ^® 410LM.

[0018] The antistatic compound used in the present invention may be used in a liquid form, or alternatively, in solid form. The physical state of the antistatic depends on melting point of the antistatic and on the selected temperature when supporting it. Use of antistatic compounds in solid form is preferred,

[0019] As support, preference is given to using finely divided supports which can be any organic or inorganic solid. In particular, the support component can be a porous support such as talc, a sheet silicate such as montmorillonite, mica, an inorganic oxide or a finely divided polymer powder (e.g. polyolefin or a polymer having polar functional groups).

[0020] Examples of preferred inorganic supports include silicon dioxide, aluminum oxide and mixed oxides of the elements calcium, aluminum, silicon, magnesium or titanium and also corresponding oxide mixtures. Other inorganic oxides which can be used alone or in combination with the abovementioned preferred oxidic supports are, for example, MgO, CaO, AIP04, Zr02, ΤΊΟ2, B2O3 or mixtures thereof. Still other preferred inorganic support materials are inorganic halides such as MgC or carbonates such as Na2C0 ₃ , K2CO3, CaC0 ₃ , MgC0 ₃ , sulfates such as Na2S0 ₄ , Al2(S0 ₄ ) ₃ , BaS0 ₄ , nitrates such as KNOs, Mg(N0 ₃ ) ₂ or AI(N0 ₃ ) ₃ .

[0021] The inorganic support can be subjected to a thermal treatment that, among other effects, allows to remove adsorbed water. Such a drying treatment is generally carried out at temperatures in the range from 40 to 1000°C, preferably from 50 to 600°C, with drying at from 50 to 200°C preferably being carried out under reduced pressure and/or a blanket of inert gas (e.g. nitrogen), or the inorganic support can be calcined at temperatures of from 200 to 1000°C, if appropriate to produce the desired structure of the solid and/or to set the desired -OH concentration on the surface.

[0022] The support can also be treated chemically using customary desiccants such as metal alkyls, preferably aluminum alkyls, chlorosilanes or SiCI ₄ , or else methylaluminoxane. The treatment of silica gel with NH ₄ SiF6 or other fluorinating agents e.g. leads to fluorination of the silica gel surface, or treatment of silica gels with silanes containing nitrogen-, fluorine- or sulfur-containing groups leads to correspondingly modified silica gel surfaces. Appropriate treatment methods are described, for example, in WO 00/031090.

[0023] Organic support materials such as finely divided polyolefin powders (e.g. polyethylene, polypropylene or polystyrene) can also be used and are preferably likewise freed of adhering moisture, solvent residues or other impurities by appropriate purification and drying operations before use. It is also possible to use functionalized polymer supports, e.g. ones based on polystyrene, polyethylene, polypropylene or polybutylene, via whose functional groups, for example ammonium or hydroxy groups, at least one of the catalyst components can be immobilized. Polymer blends can also be used.

[0024] Particular preference is given to using silica gels as solid support component, since particles whose size and structure make them particularly suitable as supports for olefin polymerization can be produced from this material. In particular spray-dried silica gels comprising spherical agglomerates of smaller granular particles, i.e. primary particles, have been found to be particularly useful. The silica gels can be dried and/or calcined before use.

[0025] The silica gels used are generally used as finely divided powders having a mean particle diameter D50 of from 5 to 200 μιη, preferably from 10 to 150 μιη, particularly preferably from 15 to 100 μιη and in particular from 20 to 70 μιη, and usually have pore volumes of from 0.1 to 10 cm ³ /g, preferably from 0.2 to 5 cm ³ /g, and specific surface areas of from 30 to 1000 m2/g, preferably from 50 to 800 m ² /g and in particular from 100 to 600 m ² /g. Typical silica gels suitable for the process of the present invention are available in the market, e.g. from W. R. Grace & Co, Maryland, USA.

[0026] The support is generally dry, i.e. it is not suspended in a solvent and the residual moisture content is less than 5% by weight, preferably less than 2% by weight and more preferably less than 0.5% by weight, based on moist support. [0027] The antistatic compound is preferably applied in such an amount that the concentration of the antistatic compound is from 10 to 60%wt, preferably from 20 to 50%wt and particularly preferably from 30 to 40%wt, per gram of support component.

[0028] The support component can optionally be brought into contact with a organometallic compound before being brought into contact with the antistatic compound. Preferred organometallic compounds are methyllithium, ethyllithium, n-butyllithium, methylmagnesium chloride, methylmagnesium bromide, ethylmagnesium chloride, ethylmagnesium bromide, butylmagnesium chloride, dimethylmagnesium, diethylmagnesium, dibutylmagnesium, n-butyl- n-octylmagnesium, n-butyl-nheptylmagnesium, in particular n-butyl-n- octylmagnesium, tri-n-hexylaluminum, triiscibutylaluminum, tri-n- butylaluminum, triethylaluminum, dimethylaluminum chloride, dimethylaluminum fluoride, methylaluminum dichloride, methylaluminum sesquichloride, diethylaluminum chloride and trimethylaluminum and mixtures thereof.

[0029] The thus-obtained supported antistatic compound displays very good powder flow.

[0030] Catalyst components that can be used in the polymerization process of the invention are Ziegler-Natta catalyst components comprising a magnesium halide, a titanium compound having at least a Ti-halogen bond, and optionally electron donor compounds. The magnesium halide is preferably MgCl2 in active form which is widely known from the patent literature as a support for Ziegler-Natta catalysts. The preferred titanium compounds are TiCU and TiC . Ti-haloalcoholates of formula Ti(OR) _n - _y Xy, where n is the valence of titanium, y is a number between 1 and n-1 X is halogen and R is a hydrocarbon radical having from 1 to 10 carbon atoms, can also be used.

[0031] Other solid catalyst components which may be used are those based on a chromium oxide supported on a refractory oxide, such as silica, and activated by a heat treatment. Catalysts obtainable from those components consist of chromium (VI) trioxide chemically fixed on silica gel. These catalysts are produced under oxidizing conditions by heating the silica gels that have been doped with chromium(l ll)salts (precursor or precatalyst). During this heat treatment, the chromium(lll) oxidizes to chromium(VI), the chromium(VI) is fixed and the silica gel hydroxyl group is eliminated as water.

[0032] Still other solid catalyst components which may be used are single- site catalysts supported on a carrier, preferably metallocene catalysts comprising:

- at least a transition metal compound containing at least one n bond; and

- at least a cocatalyst selected from an alumoxane or a compound able to form an alkylmetallocene cation.

[0033] The catalysts may be optionally subjected to prepolymerization, preferably in a loop reactor, before being fed to the polymerization reactor. The prepolymerization of the catalyst system is usually carried out at a low temperature in a range from 0°C to 60°C.

[0034] Olefins that can be polymerized in the process according to the invention are a-olefins of formula CH ₂ =CHR, where R is hydrogen or a hydrocarbon radical having from 1 to 12 carbon atoms. Examples of such olefins are ethylene, propylene, 1 -butene, 1 -hexene, 1 - octene. They can be polymerized either alone to form homopolymers, or in combination among them to produce copolymers.

[0035] The process of the invention can be carried out in any polymerization plant comprising one or more liquid-phase and/or gas-phase polymerization reactors. Suitable liquid-phase reactors are loop reactors and continuously stirred tank reactors (CSTR). Suitable gas- phase reactors include fluidized bed reactors, stirred bed reactors and reactors having two interconnected polymerization zones as described in EP 0782587 and EP 1012195.

[0036] The process according to the present invention is very effective in avoiding the formation of polymer agglomerates in the polymerization reactor and, at the same time, presents surprisingly high polymerization activities.

[0037] According to another object, the present invention provides a supported antistatic compound that does not comprise a transition- metal-based catalyst component. Preferred antistatic compounds and supports are those indicated above.

[0038] According to a further object, the present invention provides a process for the preparation of a supported antistatic compound comprising the step of contacting an antistatic compound with a support in the absence of a transition-metal-based catalyst component.

[0039] A still further object of this invention is the use of a supported antistatic compound free of a transition-metal-based catalyst component in a process for the polymerization of olefins.

[0040] Further advantages and characteristics of the present invention will appear clear from the following examples, which are provided for illustrative purposes only and are not intended to limit the scope of the invention.

EXAMPLES

Test Methods

Melt index E (MIE)

[0041] Determined according to ASTM-D 1238, condition E (190°C/2.16 kg).

Poured bulk density (PBD)

[0042] Determined according to DIN-53194.

Particle size distribution (PSD)

[0043] Determined by using a Tyler Testing Sieve Shaker RX-29 Model B available from Combustion Engineering Endecott provided with a set of six sieves, according to ASTM E-1 1-87, of number 5, 7, 10, 18, 35, and 200 respectively. [0044] Example 1

Silica dehydratation

[0045] Silica was loaded into a vessel and was dehydratated at 200°C under vacuum (26mbar) for 8 hours (lab conditions) with a residual water content of between 2000 and 5000 ppm.

Antistatic (GMS) supportation

[0046] Iso-hexane was loaded into a stirred reactor and was heated up to 55°C. GMS was loaded in the reactor up to the amount of 75g/L. After 3 hours, silica support was loaded in the reactor up to the amount of 175g/L with respect to the starting iso-hexane, with a total load of GMS and silica of 250g/L and a ratio between GMS and silica of 30/70 by weight. After 30 minutes of contacting between dissolved GMS and silica, the drying step started. The equipment was placed under vacuum (26 mbar) in order to remove the iso-hexane and the temperature was raised according to a ramp up to 90°C in order to remove all the iso-hexane. When all iso-hexane was removed, the solid (GMS on silica) was cooled to room temperature and unloaded from the vessel.

Preparation of prepolymerized catalyst

[0047] A magnesium chloride and alcohol adduct containing about 3 mols of alcohol was prepared following the method described in example 2 of USP 4,399,054, but working at 2000 RPM instead of 10000 RPM. The adduct were subject to a thermal treatment, under nitrogen stream, over a temperature range of 50-150 °C until a weight content of 25% of alcohol was reached. Into a 2 L four-necked round flask, purged with nitrogen, 1 L of TiCI4 was introduced at 0°C. Then, at the same temperature, 70 g of a spherical MgCI2/EtOH adduct containing 25 %wt of ethanol and prepared as described above were added under stirring. The temperature was raised to 140 °C in 2 h and maintained for 60 min. Then, the stirring was discontinued, the solid product was allowed to settle and the supernatant liquid was siphoned off. The solid residue having average particle size of about 60 micrometers was then washed once with heptane at 80°C and five times with hexane at 25°C and dried under vacuum at 30 °C and analyzed. Into a 260cm3 glass reactor provided with stirrer, 351.5 cm3 of hexane at 20°C and whilst stirring 7 g of the catalyst prepared as above described were introduced at 20°C. Keeping constant the internal temperature, 5.6 cm3 of tri-n-octylaluminum (TNOA) in hexane (about 370 g/l) were slowly introduced into the reactor and the temperature was brought to 10°C. After 10 minutes stirring, 10 g of propylene were carefully introduced into the reactor at the same temperature during a time of 4 hours. The consumption of propylene in the reactor was monitored and the polymerization was discontinued when a theoretical conversion of 1 g of polymer per g of catalyst was deemed to be reached. Then, the whole content was filtered and washed three times with hexane at a temperature of 20°C (50 g/l). After drying the resulting pre-polymerized catalyst was analyzed and found to contain 1.1 g of polypropylene per g of catalyst.

Preparation of catalyst suspension

[0048] A dispersion tank with an internal diameter of 14,5 cm is used. This tank is equipped with a stirrer, an external water jacket for the temperature regulation, a thermometer and a cryostat.

[0049] The following components were used to prepare the catalyst suspension:

- the above indicated Ziegler Natta catalyst powder;

- white oil Winog 70 having a viscosity of 70 cStokes at 40°C.

[0050] 1005 g of white oil Winog 70 were fed into the dispersion tank at room temperature (25°C). Successively, 300 g of catalyst powder were loaded to the tank containing the oil, while continuously maintaining under stirring the dispersion tank.

[0051] Once completed the feed of catalyst, the obtained suspension is maintained under stirring conditions for 30 minutes adjusting the temperature of the dispersion tank at 13°C. The velocity of the stirring device is adjusted to 85 rpm during the mixing of the components of the suspension. [0052] The obtained suspension has a catalyst concentration of about 252 g/l (grams of catalyst for liter of oil). The catalyst suspension contains the antistatic compound in a weight ratio GMS90/catalyst of 0, 13. Preparation of catalyst paste

[0053] 431 g of molten vaseline grease BF (thickening agent) were fed to the dispersion tank containing the catalyst suspension at a feed temperature of 80°C. The molten thickening agent is slowly fed for a time of 3 minutes, while stirring the catalyst suspension in the dispersion tank with a velocity of 85 rpm. The catalyst suspension is maintained at a temperature of 13°C during the addition of the molten vaseline grease: as a consequence, the molten thickening agent solidifies almost instantaneously on contact with the catalyst suspension. After the feed of the molten vaseline, the components of the catalytic paste were always maintained under stirring at 85 rpm for a time of 90 minutes. During this time the temperature is kept at 13°C in the dispersion tank.

[0054] The obtained catalytic paste has a weight ratio grease/oil of about 0,43, while the concentration of the solid (catalyst+antistatic) in the catalytic paste is equal to about 170 g/l.

Catalyst activation

[0055] The obtained catalytic paste is withdrawn by the dispersion tank by a dosing syringe and is then continuously transferred by means of two dosing syringes to a first catalyst activation vessel, then to a second, then to a third. Triisobutyl-aluminium (TIBAL) is used as the cocatalyst with a weight ratio TIBAL/catalyst of 1.5. No external donor is used and propane is fed as a diluent to the activation vessels. The above components were pre-contacted in three activation vessels at a temperature of 40°C for 19, 44 and 25 minutes respectively.

[0056] The activated catalytic paste is discharged from the activation vessel and is continuously fed to a gas-phase fluidized bed reactor for the polymerization of olefins. Polymerization

[0057] The activated catalytic paste and the supported GMS were introduced into the fluidized bed reactor, where ethylene is polymerized to produce high density polyethylene (HDPE). The polymerization is operated in the presence of propane as a polymerization diluent and hydrogen as the molecular weight regulator. The polymerization conditions and the composition of the gaseous reaction mixture were indicated in Table 1. The characterization of the HDPE discharged from the reactor is reported in Table 2.

Example 2C (comparison)

[0058] Example 1 is repeated with the only difference that during the preparation of catalyst suspension 40 g of microbeads of GSM were added to the suspension of catalyst powder in oil, so that the obtained catalytic paste incorporates GMS and that no supported GMS was used. The polymerization conditions and the composition of the gaseous reaction mixture are indicated in Table 1. The characterization of the HDPE discharged from the reactor is reported in Table 2.

Examples 3 and 4

[0059] The procedure set forth in Example 1 was repeated, except that a different amount of GMS was used.

[0060] The polymerization conditions and the composition of the gaseous reaction mixture are indicated in Table 1. The characterization of the

HDPE discharged from the reactor is reported in Table 2.

Example 5C (comparison)

[0061] The procedure set forth in Example 2 was repeated, except that the same catalyst as in Examples 3 and 4 has been used.

[0062] The polymerization conditions and the composition of the gaseous reaction mixture are indicated in Table 1. The characterization of the

HDPE discharged from the reactor is reported in Table 2.

Example 6 [0063] The procedure set forth in Example 1 was repeated, except that the catalyst components were pre-contacted in two activation vessels at a temperature of 40°C for 19 and 44 minutes respectively, followed by a pre-polymerization treatment in a third vessel at a temperature of 40°C for 35 minutes with a ratio ethylene/catalyst of 10 g/g.

[0064] The polymerization conditions and the composition of the gaseous reaction mixture are indicated in Table 1. The characterization of the HDPE discharged from the reactor is reported in Table 2.

[0065] By comparing the data on polymer particle size, it results that in the examples according to the invention the amount of so-called fines were substantially reduced with respect to the comparative examples. This brought about a significant reduction of fouling inside the fluidized bed reactor and in the equipment arranged along the gas recycle line (compressor and heat exchanger). It is also notable that the examples according to the invention show a higher specific mileage.

[0066] Table 1 - Process conditions

2 - HDPE characterization