Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-11-07
2004-06-08
Rabago, Roberto (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S124300, C526S124900, C526S125300, C526S351000, C526S065000, C526S904000, C525S053000
Reexamination Certificate
active
06747103
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to propylene polymers. In particular, the present invention concerns high-stiffness propylene homopolymers and propylene copolymers containing a high-stiffness polypropylene matrix. The invention also relates to a process for preparing propylene homo- and copolymers.
2. Description of Related Art
Propylene (PP) homo- and copolymers have excellent resistance to heat and chemicals as well as attractive mechanical properties, such as stiffness and impact resistance. However, processing of polypropylene by, e.g., injection moulding, thermoforming or blow moulding, to form thin-walled containers has resulted in products having insufficient stiffness, transparency and cycle time. This is caused by the semi-crystalline nature of polypropylene.
In the prior art it has been proposed to improve the stiffness, transparency and cycle time of moulded polypropylene by blending the polymer with various nucleating agents such as dibenzilidene sorbitol (DBS), sodium benzoate or di(alkylbenzilidene)sorbitol. These traditional nucleating agents tend to bleed out from the polymer composition during processing and many of them give rise to fumes with an offensive smell. As a solution to these problems, it has been suggested in the art to use vinyl compounds, such as polymers of vinyl cycloalkanes and 3-methyl-1-butene, as nucleating agents in the form of propylene copolymers or polypropylene compounds, cf. EP Patent Specifications Nos. 0 151 883, 0 152 701, 0 206 515, 0 368 577 0 369 658 and0 417 319. EP Patent No.0 152 701 discloses prepolymerization of Ziegler-Natta catalysts with vinyl cyclohexane to provide a polymer slurry which is washed and distilled to produce a vinyl cyclohexane powder containing the active catalyst. The prepolymerized catalyst composition is then used for polymerization of propylene to form a propylene copolymers with improved stiffness and having a high degree of crystallinity and a high crystallization temperature.
There are some major problems associated with the prior art solutions using polymerized vinyl compounds for nucleation of polypropylene. Thus, the products contain impurities in the form of unreacted monomers and extensive washing of the product has to be carried out before the catalyst can be used. These washing steps will reduce that activity of the catalyst. In fact, the whole work-up of the prepolymerized catalyst, including separation of the catalyst from the polymerization medium, washing and drying, will cause extra costs and impair the activity of the catalyst. Further, if very high stiffness properties are desired, they are not achieved with these polymers without adding specific fillers or additives.
Finally, it should be pointed out that it is known in the art to carry out prepolymerization in a medium comprising a viscous substance and to continue the prepolymerization with vinyl cyclohexane or to carry out the prepolymerisation in the presence of 4-methyl-1-pentene monomer and a weakly coordinating donor, MTBE (cf. Finnish Patent No. 95387). Due to the fact that, e.g., no washing, drying, sieving and transferring steps are needed, the catalyst activity is maintained.
SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the problems relating to the prior art and to provide nucleated propylene homopolymers, propylene copolymers comprising propylene random and heterophasic copolymers.
In particular the present invention aims at providing high-stiffness propylene homo-polymers as well as propylene copolymers.
The present invention also aims at providing a process by which it is possible to prepare modified catalysts providing excellent nucleation of propylene polymers and containing essentially no reactant or solvent residues which would impair the long-term activity of the catalysts. In particular, it is an object of the present invention to provide a process for increasing the stiffness of nucleated polypropylene. Further, in case of propylene homopolymers, as a final product or as polymer matrix, the amount of xylene solubles comprised in the polymer will be reduced.
Further, it is an object of the present invention to provide extruded and moulded products comprising the present propylene homo- and copolymer compositions.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The invention is based on providing a specific kind of catalyst system of Ziegler-Natta type useful for polymerization of propylene, optionally together with comonomers, for producing propylene polymers having high-stiffness properties. The catalyst is modified by polymerizing a vinyl compound in the presence of said catalyst. The modification takes place in a medium which is a liquid or a highly viscous hydrocarbon medium and which does not dissolve the polymerized polymer. Further, the modification is accomplished in such a way that all or practically all vinyl compound used for modification is consumed during polymerization. To reach that aim, the polymerization is continued at elevated temperature until the concentration of the unreacted vinyl compounds in the catalyst composition after polymerization is about 0.5 wt-%, preferably less than 2000 ppm by weight and in particular 1000 ppm by weight or less. A sufficient amount of the initial reactant is a maximum of 10 times, preferably below three times the weight of the catalyst. The modification is carried out before any conventional, usually continuous prepolymerization with an olefinic monomer, to ensure that the polymerization of the vinyl compound is complete.
As a result of these features, the amount of reactant residues in the modified catalyst composition is small, and in the final propylene polymer it is below limits of determination using the Gas Chromatography-Mass Spectrometry (GC-MS) method, which is less than 0.01 ppm by weight. Since the reaction medium contains only very small amounts of unreacted reactant residues or dissolved polymer residues, no washing of the modified catalyst composition is needed before the catalyst is fed to polymerization.
According to the present invention the stiffness of the polymer can further be improved if the above catalyst modification is carried out in the presence of external donor, particularly a strongly coordinating external donor.
Further, according to the invention an improved stiffness/impact strength balance can be obtained for heterophasic polypropylenes. Accordingly, either the stiffness is higher, while the impact strength remains on the desired level, or the impact strength is better, while the stiffness remains on the desired level, or both the stiffness and the impact strength are slightly better, than if the modification were carried out in the absence of the strongly coordinating external donor.
According to another aspect of the present invention, in the case of propylene homopolymers, the amount of xylene solubles formed in the nucleated polypropylene is reduced. Thus, it has been found that the amount of xylene solubles can be radically reduced when the catalyst modification is carried out according to the invention by polymerizing the catalyst with a vinyl compound in the presence of a strongly coordinating donor. As a result, a high-stiffness propylene homopolymer is obtained exhibiting a xylene solubles concentration of less than 1.5%. Propylene homopolymer can be a final product or a polymer matrix, into which other comonomers, such as &agr;-olefins or ethylene, can be combined.
More specifically, the propylene polymer according to the invention is mainly characterized by a nucleated high-stiffness propylene polymer composition obtainable by homopolymerization or copolymerization of propylene in the presence of a catalyst system comprising a catalyst component, a cocatalyst component, and an external donor, the procatalyst component of the catalyst system containin
Leskinen Pauli
Vestberg Torvald
Borealis Technology Oy
Rabago Roberto
LandOfFree
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