Process for the preparation of high performance polypropylene

Abstract

The invention discloses a process for preparing high-performance propylene polymers, the process utilizing a high activity, highly stereoselective Ziegler-Natta catalyst and two or more stages of polymerization carried out under different hydrogen concentrations to prepare propylene polymers having broad molecular weight distribution, wherein non-unifomess of isotacticity of molecular chains of the final propylene polymers is improved by adjusting or controlling stereoselectivity of catalytic active sites under different hydrogen concentrations, namely, making the low molecular weight fraction of the polymers having a higher isotacticity and making the high molecular weight fraction of the polymers having a lower isotacticity, thereby overcoming the drawbacks of the propylene polymers having broad molecular weight distribution known in the art. The resulting final polymers have excellent combined properties, in particular, remarkably improved mechanical properties.

Description

CROSS REFERENCE OF RELATED APPLICATION[0001]The present application claims the priority of the Chinese Patent Application No. 200610076310.7, filed on Apr. 20, 2006, which is incorporated herein by reference in its entirety and for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to a process for preparing high-performance polypropylenes, and more specifically, to a process for preparing polypropylenes having excellent performance and broad molecular weight distributions, of which high molecular weight fraction has a lower isotacticity and low molecular weight fraction has a higher isotacticity.BACKGROUND OF THE INVENTION[0003]In general, polypropylene resins having broad molecular weight distributions (of which polydispersity indexes as measured by a rheological method are typically larger than 4.0) exhibit better performance, because high molecular weight fraction of the resins imparts better mechanical strength, creep resistance, etc. to the resins, while lo...

AI Technical Summary

Benefits of technology

[0026]It is noted that, in techniques as disclosed in literatures for preparing propylene polymers having broad molecular weight distribution through multi-stage polymerization, no means is employed to adjust stereoselectivities of the active sites of the catalysts in different polymerization stages so that the isotacticity of the high molecular weight fraction of the resulting polymers is higher than that of the low molecular weight fraction of the polymers. In contrary, in the process according to the invention, the isotacticity of the low molecular weight fraction of the propylene polymers is made higher than the isotacticity of the high molecular weight fraction of the propylene polymers produced in the first polymerization stage by enhancing stereoselectivities of the active sites of the catalysts in the second polymerization stage for producing the low molecular weight fraction of the polymers. The isotacticity of polypropylenes may be characterized by an isotactic index as measured according to GB 2142-89, or by two parameters as measured by 13C-NMR spectroscopy, i.e., pentad ([mmmm]) content by mole and average length of isotactic sequences. The larger the values measured by these methods, the higher the isotacticity of the polypropylene. Such data can demonstrate the effects of the invention.

[0027]In the process for preparing polypropylenes having broad molecular weight distribution according to the invention, the first and second stages of polymerization are performed under different concentrations of a molecular weight control agent, for example, hydrogen gas, in order to make the final propylene polymers having broadened molecular weight distribution. In general, melt flow rate (MFR) of a final polymer may be controlled depending on the intended use of the polymer, and the MFR of the propylene polymer produced in the first polymerization stage can be so controlled that a ratio of the MFR of the final propylene polymer to the MFR of the propylene polymer produced in the first polymerization stage is in a range of from about 5 to about 15. For example, when the final polymer will be used as a tubing material, the MFR of the propylene polymer produced in the first polymerization stage can be controlled as being in a range of from 0.01 to 0.03 g / 10 min., and the MFR of the final propylene polymer can be controlled as being in a range of from 0.1 to 0.3 g / 10 min.; and when the final polymer will be used as a film material, the MFR of the propylene polymer produced in the first polymerization stage can be controlled as being in a range of from 0.2 to 0.4 g / 10 min., and the MFR of the final propylene polymer can be controlled as being in a range of from 2 to 4 g / 10 min. The MFR values are measured according to ISO1133, at 230° C., under 2.16 kg loading.

[0028]In the process according to the invention, a ratio of the output of the first stage of polymerization to the output of the second stage of polymerization may be in a range of from 30:70 to 70:30, preferably from 35:65 to 55:45.

[0029]The polymerization can be carried out in a liquid phase process, or in a gas phase process, or in a combination process of gas phase and liquid phase. In the case where the polymerization is carried out in liquid phase, polymerization temperature is in a range of from 0° C. to 150° C., and preferably from 40° C. to 100° C., and polymerization pressure is higher than saturated vapor pressure of propylene at the corresponding polymerization temperature. In the case where the polymerization is carried out in gas phase, polymerization temperature is in a range of from 0° C. to 150° C., and preferably from 40° C. to 100° C., and polymerization pressure may be normal pressure or higher, and preferably in a range of from 1.0 to 3.0 MPa (gauge, similarly hereinafter).

[0030]Comonomers which may copolymerize with propylene in the process according to the invention include ethylene and C4-C12 α-olefins, for example, 1-butene, 1-hexene, and 1-octene.

[0031]As mentioned above, the process of the invention may be performed in either a continuous process or a batch process. In the case where a continuous process is employed, the process of the invention may be performed in two or more loop reactors in series, or in two or more tank reactors in series, or in two or more gas phase reactors in series, or in any combination of a loop reactor, a tank reactor, and a gas phase reactor. In the case where the first stage of polymerization is a continuous liquid phase polymerization, it is preferable to subject the catalyst to a continuous or batch prepolymerization. By subjecting the catalyst to propylene prepolymerization, it is possible to control effectively particle morphology of the polymers during the reaction, to reduce breaking of polymer particles, and for the catalyst to exert effectively its catalytic activity. The prepolymerization is generally conducted under mild conditions, wherein polymerization temperature is preferably lower than 30° C., and prepolymerization rate is controlled as being in a range of from 3 to 1000 grams of PP per gram of the catalyst. In the case where the first stage of polymerization is a continuous gas phase polymerization, the catalyst may or may not be subjected to a prepolymerization.

Problems solved by technology

Such polymers may have many defects in the practical applications.

For example, the fractions having low molecular weight and low isotacticities tend to migrate out from the interior of the materials during processing and during long-term use of articles, and thus adversely affect the performance and use of the articles.

And the fractions having high molecular weight and high isotacticities tend to form thick lamellar crystal in the materials, and this is disadvantageous for some applications of propylene polymers.

For example, when such resins are used to high-speed produce BOPP films, film breaking phenomenon occurs likely.

Thus, the current Ziegler-Natta catalyst-based processes for preparing propylene polymers having broad molecular weight distributions will cause the formation of a large amount of low molecular weight, low isotacticity fraction and a large amount of high molecular weight, high isotacticity fraction while broadening molecular weight distribution of the polypropylenes, so that it is impossible to obtain performance-optimized propylene polymers.