Process of producing &agr;-olefin polymers

Details Process of producing &agr;-olefin polymers Process of producing &agr;-olefin polymers

2001-11-02

2004-02-10

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...

C526S064000, C526S068000, C526S065000, C526S084000, C526S201000

Reexamination Certificate

active

06689846

ABSTRACT:

The present invention relates to a process for producing poly-&agr;-olefin compositions, in particularly polypropylene copolymer compositions having uniform quality with desired stiffness and impact strength properties and being suitable for a wide range of applications including different packaging applications.
It is known that especially polypropylene polymers have suitable resistance to heat and chemicals, and they also have attractive mechanical properties. Further, it is known that desired properties, e.g. stiffness and impact strength properties of polypropylene can be achieved by copolymerizing propylene with ethylene or other alfa-olefin monomers and optionally by adding elastomeric components to the copolymer matrix. Polypropylene copolymers can thus be used as alternatives e.g. for soft poly(vinyl chloride) (PVC). Further, polypropylene homo- or copolymers are very suitable in a wide range of applications, where emissions of chlorinated organic compounds and many other emissions should be strictly restricted.
By “&agr;-olefin monomer” in this connection is meant an &agr;-olefin which is capable of polymerization by the insertion (Ziegler-Natta) mechanism. An &agr;-olefin is a compound having the structure CH
2
═CHR, wherein R is a linear, branched or cyclic alkyl group. In particular R is a linear or branched alkyl group having 1 to 12 carbon atoms or a cyclic alkyl group having 4 to 8 carbon atoms. Typical &agr;-olefin monomers used in the present invention are propylene, 1-butene, 4-methylpentene, 1-hexene and octene. Preferably &agr;-olefin is propylene. Said &agr;-olefins are copolymerized with ethylene and optionally with other &agr;-olefins, such as butene.
Typically &agr;-olefin copolymers are nowadays prepared with a multiphase process comprising one or more bulk and/or gas phase reactors in the presence of Ziegler-Natta catalyst system comprising a catalyst comprising a compound of a transition metal belonging to groups 4 to 6 of the Periodic Table of Elements (IUPAC 1990), and a cocatalyst based on an organic compound of a metal belonging to any of groups 1 to 3 and 13 of said Table. Typical compounds of transition metals are the chlorides, especially the tetrachloride of titanium. Typical organometallic cocatalysts are organoaluminium compounds such as aluminium alkyl compounds and especially trialkyl aluminiums. Further, this kind of catalyst system has been developed by depositing and thus solidifying the transition metal compound on a more or less inert and particulate support and by adding to the catalyst composition in the stages of its preparation several additives, among others internal and external electron donors, which act as stereoregulating agents. A typical support is magnesium chloride, typical internal electron donors are the dialkyl phthalates and typical external electron donors are the alkyl alkoxy silanes. These compounds have improved the polymerization activity, the operating life and other properties of the catalyst system and above all the properties of the polymers which are obtained by means of said catalyst system. In order further to improve the properties of such a catalyst system at least a part of it has been contacted with a small amount of monomer to give a polymer coated, so called prepolymerized catalyst or catalyst system.
The multiphase polymerization process can comprise several polymerization stages. It is common knowledge that the polymer matrix, which comprises homo/homo, homo/random or random/random (co)polymers with a comonomer content needed to obtain the desired properties, can be prepared in the first stage of the polymerization process. The first stage can comprise bulk phase and optionally gas phase reactor(s). Very often the first stage comprises one bulk and one gas phase reactor. Advanced heterophasic copolymers can be obtained, if one or more additional gas phase reactors, which are often called as rubber phase reactors, are used, combined in series with the first stage reactor(s). Copolymerization of ethylene monomer and &agr;-olefins in the presence of the polymer matrix from the first stage is carried out in the rubber phase reactor(s), which step forms the second stage of the polymerization process.
Fouling is a common problem during the olefin polymerization process. Fouling occurs when product from the bulk reactor is transferred forward, e.g. to the flash and to the gas phase reactor for further polymerizing. The polymer product which is to be transferred is “tacky” or “sticky” and adheres to the walls of the reactor and other surfaces in flash and gas phase reactors. Further, in the gas phase reactor fouling occurs due to the static electricity caused by tacking of the charged polymer particles on the walls. Detrimental fouling is caused in the gas phase reactors by the fines. i.e. very small particles containing active catalyst. Such particles are often called “hot” catalyst particles. Especially the fouling caused by the rubbery fines in the rubber phase reactor is very detrimental for the process and the products.
It is known that the degree of fouling can to some extent be restricted by adding various antistatic chemicals to the first stage reactors. Further, addition of catalyst deactivation chemicals, known as catalyst “killers”, results in killing or reducing of the catalyst activity which, in turn, reduces the formation of the tacky material.
EP Patent 669 946 discloses a two stage gas phase process for producing polypropylene copolymer wherein a gel reduction component is introduced into the first stage reactor for preventing fouling in the second stage reactor. Said gel reduction component is an electron donor and acts as a catalyst deactivator. As most preferred deactivators are cited alkylene glycols and derivatives thereof, but also methanol and ethanol are mentioned.
In U.S. Pat. No. 4,182,810 there is disclosed a method of reducing fouling during particle form polymerization of ethylene, carried out in hydrocarbon diluent, and typically in loop reactors. The fouling is caused by the adherence of polymer particles to the walls of the polymerizing reactors and reduced by adding to the reaction medium a composition comprising a mixture of polysulfone copolymer, a polymeric polyamine, an oil-soluble sulfonic acid and toluene. A typical antifouling agent containing this kind of mixture is supplied by DuPont under the trade name Stadis 450.
U.S. Pat. No. 5,026,795 describes a process for preventing fouling in a one stage gas phase copolymerization reactor by admixing an antistatic agent with a liquid carrier comprising comonomers such as 1-hexene, and introducing said mixture to the reactor. Ethylene is used as a monomer. The antistatic agent used comprises Stadis 450 type agents. It should be pointed out that only a very small amount of antistatic agent is necessary, otherwise the antistatic agent would poison or deactivate the catalyst which, according to the cited patent, is not desired.
Further, it is known that tertiary amine compositions and compositions of olefin-acrylonitrile copolymer and polymeric polyamines have been used as antistatic agents for preventing fouling in polymerization processes, especially in ethylene polymerization processes.
As the above discussion shows, a wide range of modification possibilities of polypropylene results in a wide range of applications where polypropylene can be used. However, many applications, such as medical and food packaging applications set limits to the allowable residues and emissions of the used raw materials, additives and adjuvants. Therefore, the use of many known antistatic agents is limited for a wide range of applications.
“Stadis-type” agents, such as Stadis 450, have been used as antistatic agents in polyethylene processes as is shown in the examples of the above U.S. Patents. The antistatic effect of “Stadis-type” agents may, as such, be sufficient to to reduce fouling in polyethylene polymerizing processes. However, when &agr;-olefins are polymerized, more complicated catalyst systems are used, which in turn complicate

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