Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-10-13
2002-10-22
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S078000, C526S079000, C526S348200, C526S348500, C526S348600, C526S129000
Reexamination Certificate
active
06469111
ABSTRACT:
The present invention relates to a process for the polymerization of &agr;-olefins in the gas phase at from 30 to 125° C. and a pressure of from 5 to 80 bar.
The present invention further relates to the use of MgO and/or ZnO as. antistatic agent in this polymerization process.
The polymerization of &agr;-olefins in the gas phase frequently results in formation of deposits on the walls of the reactor and to the formation of lumps. This formation of deposits and lumps is at least partially attributable to electrostatic charging. The formation of deposits leads to blockages in the product discharge system and thereby hinders the continuous operation of such gas-phase plants.
Electrostatic charging is influenced in a complex manner by numerous system parameters in the gas-phase polymerization process, for example by the particle size distribution of the polymer and of the catalyst, the chemical composition of the catalyst, the reaction temperature, the reaction pressure and the composition of the circulating gas.
U.S. Pat. No. 5,391,657 describes a method by which deposit formation in gas-phase polymerizations of &agr;-olefins can be prevented by adding inorganic additives (MgO, ZnO, Al
2
O
3
, CuO and mixtures of these) which generate positive charges or inorganic additives (V
2
O
5
, SiO
2
, TiO
2
, Fe
2
O
3
and mixtures of these) which generate negative charges as a function of the particular electrostatic charging situation in the reactor. However, this method requires the continual measurement of the electrostatic charge and also a metering-in system which is regulated in a complex manner as a function of this measurement, and the method is therefore associated with a considerable outlay in terms of apparatus.
Catalysts or catalyst systems which comprise MgO as support material or as a modifying additive are also already known from the literature. Thus, U.S. Pat. No. 5,408,015 describes a catalyst system comprising a chromium oxide catalyst, a Ziegler catalyst supported on MgO and also from about 1 to 15% by weight of MgO as additives. The ratio of chromium oxide catalyst to MgO-supported Ziegler catalyst is from 6:1 to 100:1, so that the overall mixture contains at least 2% by weight of MgO. The addition of MgO makes it possible to prepare ethylene polymers or copolymers (HDPE) having a broad molecular weight distribution and properties which are particularly advantageous for blow molding, applications.
U.S. Pat. No. 4,946,914 describes a supported catalyst which is produced by combining a chromium-containing catalyst with a modifier, viz. an oxide of an element of group IIa of the Periodic Table of the Elements. MgO is mentioned as an example of a modifier. The modifier is added in order to obtain polymers having a higher high load melt index (HLMI) than those obtained using a catalyst system without this modifier. For this purpose, it is said to be important that the modifier contains at least 0.5% by weight, for best results about 2% by weight, of water.
The previously known methods for preventing electrostatic charging in the gas-phase polymerization of &agr;-olefins still leave something to be desired in respect of their effectiveness or their technical complexity.
It is an object of the present invention to find a process for polymerizing &agr;-olefins in the gas phase in which the formation of deposits on the reactor walls and at the bottom of the reactor can be prevented in a simple and efficient manner.
We have found that this object is achieved by a process for the polymerization of &agr;-olefins in the gas phase at from 30 to 150° C. and a pressure of from 5 to 80 bar, wherein use is made of a catalyst or a catalyst mixture containing as antistatic agent from 0.1 to 5% by weight of ZnO and/or anhydrous MgO, based on the total amount of the catalyst mixture except for a process in which the catalyst mixture comprises a chromium catalyst and an Mgo-supported Ziegler catalyst which is modified with an alkene and with alkylaluminum hydride an also comprises tree MgO and the total amount of the MgO is not less than 2% by weight of the catalyst mixture.
The process of the present invention enables especially ethylene and propylene and in particular ethylene to be homopolymerized or copolymerized. Particularly suitable comonomers are &agr;-olefins having from 3 to 8 carbon atoms. A process in which mixtures of ethylene with C
3
-C
8
-&agr;-olefins are copolymerized is particularly advantageous. C
3
-C
8
-&agr;-olefins which are useful in such a copolymerization are, in particular, propene, butene, pentene, 4-methylpentene, hexene, heptene and octene, and also mixtures of these.
The polymerization process is carried out at from 30 to 125° C., preferably from 80 to 120° C. The pressure is from 5 to 80 bar, preferably from 20 to 60 bar.
The polymerization can be carried out by various gas-phase methods, ie. for example in gas-phase fluidized beds or in stirred gas phases.
The antistatic agent used is ZnO and/or anhydrous MgO. In this context, anhydrous means that the water content of the MgO should be less than 0.5% by weight, preferably less than 0.3% by weight, based on the total mass of the MgO. The ZnO too is preferably used in anhydrous form. The dewatering of the oxides is most simply carried out by heating under reduced pressure, for example to from 150 to 450° C. under reduced pressure. The drying time depends on the temperature selected. Good results are achieved, for example, at 250° C. under reduced pressure for a period of 8 hours.
Among the oxides having an antistatic effect, ZnO is worthy of particular emphasis.
The antistatic agent or the mixture of the two antistatic agents is added to the catalyst or the catalyst mixture in an amount of from 0.1 to 5% by weight, based on the total amount of catalyst or catalyst mixture. The antistatic agent is preferably present in the catalyst or the catalyst mixture in an amount of more than 0.2% by weight and less than 2% by weight.
The oxides which have an antistatic effect can be used in a wide variety of particle sizes. The oxides are particularly effective if they are very fine. Thus, mean particle diameters of from 10 to 200 &mgr;m, in particular from 20 to 100 &mgr;m, have been found to be particularly useful. Also advantageous are particle diameters which are similar to the size of the catalyst particles.
In the process of the present invention it is possible to use various catalysts as are customary for the polymerization of &agr;-olefins. Thus, suitable catalysts are, for example, the supported chromium catalysts also known as Phillips catalysts.
The application of soluble chromium compounds to support materials is generally known. Suitable support materials are especially inorganic compounds, in particular porous oxides such as SiO
2
, Al
2
O
3
, MgO, ZrO
2
, TiO
2
, B
2
O
3
, CaO, ZnO or mixtures of these. The support materials preferably have a particle diameter of from 1 to 300 &mgr;m, in particular from 30 to 70 &mgr;m. Examples of particularly preferred supports are silica gels and aluminosilicate gels, preferably those of the formula SiO
2
.a Al
2
O
3
, where a is a number from 0 to 2, preferably from 0 to 0.5; these are thus aluminosilicates or silicon dioxide. Such products are commercially available, eg. silica gel 332 from Grace.
The doping of the catalyst support with the chromium-containing active component is generally carried out from a solution or, in the case of volatile compounds, from the gas phase. Suitable chromium compounds are chromium(VI) oxide, chromium salts such as chromium(III) nitrate and chromium(III) acetate, complexes such as chromium(III) acetylacetonate or chromium hexacarbonyl, or else organometallic compounds of chromium, eg. bis(cyclopentadienyl)chromium(II), organic esters of chromium(VI) acid or bis(arene)chromium(O).
The active component is generally applied to the support by bringing the support material in a solvent into contact with a chromium compound, removing the solvent and calcining the catalyst at from 400 to 1100° C. For this purpose, the support material can b
Hack Johannes
Lange Arim
Mihan Shahram
Rosendorfer Philipp
BASF - Aktiengesellschaft
Cheung William
Keil & Weinakauf
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