GB2076834A - Propylene polymer and process for its preparation - Google Patents

Abstract

A propylene polymer having a meltflow range between 0.2 and 30g/10 min., isotactic index not less than 92%, dimer-trimer content not exceeding 4g/kg of polymer, crystalline melting point of at least 165 DEG C, ratio of weight- average molecular weight to number- average molecular weight at least 7:1, Ti content not exceeding 3 ppm, Mg content not exceeding 40ppm, Cl content not exceeding 100 ppm and total ash content not exceeding 400ppm is prepared in a continuous fashion by feeding propylene monomer and Ziegler catalyst into a reactor, wherein the pressure is suitably elevated to maintain at least some of the propylene in the liquid phase. The product is withdrawn as a slurry in liquid propylene.

Description

SPECIFICATION eropylene polymer and process for its preparation In the past the conventional catalyst system used in propylene polymerization has been an unmodified or an electron donor-modified titanium halide component, activated with an organoaluminum cocatalyst. Typical examples of conventional propylene polymerization catalyst systems include cocrystallized titanium trichloride-aluminum trichloride catalysts of the general formula n.TiC13.AIC13, activated with diethyl aluminum chloride or triethylaluminum. The cocrystallized titanium trichloride-aluminum trichloride can have been subjected to a modification treatment with a suitable electron donor compound to increase its activity or stereospecificity. Such compounds include phosphorus compounds, esters of inorganic and organic acid ethers and numerous other compounds.

Recently new catalysts have been developed which are far more active than the aforementioned conventional catalysts in the polymerization of alpha-olefins. Briefly described, these catalysts are comprised of a titanium halide catalyst component supported on magnesium dihalide and an alkylaluminum compound, which can be present as a complex with an electron donor compound. These catalyst components have been described in the patent literature, e.g. in U.S. Patents No. 3,830,787, No. 3,953,414, No.4,051,313, No.4,115,319 and No.4,149,990.

The advantages of polymerizing propylene in the presence of the magnesium halide supported catalyst described above are the improved yields (polymer/Ti w/w) and stereospecificity (= isotactic index or % heptane insolubles), while the mechanical properties are essentially uneffected.

The object of the present invention is to provide a novel propylene polymer which exhibits improved processability when extruded or injection molded as compared to conventional propylene polymers.

Another object of the present invention is to provide a novel polypropylene which can be processed at lower extrusion or molding temperatures and/or lower extrusion or molding pressures than conventional polypropylene resins of the same meltflows.

Further objects will become apparent from a reading of the specification and appended claims.

The invention In accordance with the present invention there is provided a novel propylene polymer having a meltflow range between about 0.2 and about 309/10 min., isotactic index not less than about 92%, dimer-trimer content not exceeding about 4g/kg of polymer, crystalline melting point of at least about 165"C, ratio of weight-average molecular weight to number-average molecular weight at least about 7, Ti content not exceeding about 3ppm, Mg content not exceeding about 40ppm, Cl content not exceeding about 100ppm and total ash content not exceeding about 400ppm.

The catalyst components used in the process to produce the propylene polymer of the invention can be any one of the recently developed, high activity magnesium halide supported catalyst components and organoaluminum cocatalyst components disclosed e.g. in U.S. Patents No.3,830,787, No. 3,953,414, No.

4,051,313, No. 4,115,319 and No.4,149,990, hereby incorporated in this application by reference.

Typically, such a catalyst composition is a two component composition where the components are introduced separately into the polymerization reactor. Component (a) of such a composition is advan tageously selected from trialkyl aluminums containing from 1 to 8 carbon atoms in the alkyl group, such as triethyl aluminum, trimethyl aluminum, tri-n-butyl aluminum, triisobutyl aluminum, tr-iisohexyl aluminum, tri-n-octyl aluminum and triisooctyl aluminum. Most preferably the trialkyl aluminum is complexed with an electron donor priorto introduction into the polymerization reactor. Best results are achieved when esters of carboxylic acids or diamines, particularly esters of aromatic acids are used as the electron donors.

Some typical examples of such compounds are methyl- and ethylbenzoate, methyl- and ethyl-pmethoxybenzoate, diethyl-carbonate, ethylacetate, dimethylmaleate, triethylborate, ethyl-o-chiorobenzoate, ethylnaphthenate, methyl-p-toluate, ethyltol uate, ethyl-p-butoxy-benzoate, ethyl-cyclohexanoate, ethylpivalate, N,N,N',N'-tetramethylenediamine, 1 ,2,4,-tri-methylpiperazine, 2,5-dimethylpiperazine and the like. The molar ratio of aluminum alkyl to electron donor can range between 1 and 100, preferably between 2 and 5.

Solutions of the electron donor and the trialkyl aluminum compound, neat or in a hydrocarbon such as hexane or heptane, are preferably prereacted for a certain period of time generally less than 1 hour prior to feeding the mixture into the polymerization reaction zone.

The other component of the catalyst composition is either a titanium tri- or tetrahalide supported on magnesium dihalide, or a complex of a titanium tri- or tetrahalide with an electron donor compound supported on magnesium dihalide. The halogen in the respective halides can be chlorine, bromine or iodine, the preferred halogen being chlorine. The electron donor, if it is used in forming a complex, is suitably selected from the esters of inorganic and organic oxygenated acids and the polyamines.Examples of such compounds are the esters of aromatic carboxylic acids, such as benzoic acid, p-methoxybenzoic acid and p-toluic acids and particularly the alkyl esters of said acids; the alkylene diamines, e.g. N',N",N'", N""-tetramethylethylene-diamine. The magnesium to electron donor molar ratio are equal to or higher than 1 and preferably between 2 and 10. Generally the titanium content expressed as titanium metal ranges between 0.1 and 20 wt % in the supported catalyst component and preferably between 1 and 3 wt %.

The preparation of such supported catalyst components have been described in the prior art and are commercially available.

The catalyst components (a) and (b) are fed to the reaction zone in amounts such that the Alibi molar ratio is maintained in the broad range between about 1 and about 10,000 and preferably between about 10 and 200.

It is essential that the polymerization process used to produce the polymer of this invention is one wherein the propylene functions as the liquid diluent as well as feed to the reaction, except for small quantities of inert hydrocarbons, e.g. hexane, mineral oil, petrolatum, etc., that may be used for the introduction of the catalyst components into the reaction zone. The reaction conditions used in the process of this invention generally include polymerization temperatures in the range from about 115 Fto about 175"F and preferably in the range from about 125"F to about 160 F. The pressure should be sufficiently elevated to maintain at least a portion of the propylene in the liquid phase. Suitably pressure of 200 psig and higher are used, e.g. up to about 500 psig.Total solids in the reaction zone, in accordance with this system, are ordinarily in the order of from 15 to 50%, although obviously lower or higher, for example up to 60% polymer solids can be achieved. In order, however, to efficiently handle the slurry, it is preferred to keep the polymerization to the percent solids above indicated. The reaction is continuous and monomer feed and catalyst components are continuously fed to the reactor and a slurry of polymer product and liquid propylene is withdrawn, preferably through a cyclic discharge valve which simulates continuous operation. If desired, any of the known various modifiers such as hydrogen may be added for their intended purpose.

The withdrawn polymer slurry is let down in pressure to, for example 50 psig or less in a low pressure zone (meaning a zone maintained at a pressure lowerthan that in the polymerization reaction) where due to the drop in pressure and the volatile nature of the polymerization ingredients, there is a flashing of these volatiles from the solid polymer. This flashing, which can be aided by heating, results in a polymer powder which is substantially dry and which by this term is to be understood to be a polymer containing 5% or less volatiles. The unreacted monomer stream is taken overhead from this low pressure flashing zone and at least a portion thereof is compressed and condensed and returned to the reator. The polymer is usually passed to a final drying zone to remove residual volatiles.

Alternately the liquid polymerization medium can be filtered or centrifuged under pressure and the liquid propylene can be returned (after suitable purge) to the reactor, thus saving energy in the form of recompression. This method has the advantage that certain soluble impurities (the aluminum alkyls and organic esters) are removed from the polymer. This in turn leads to a polymer product with lower residual impurities content.

Because of the generally high productivity of the supported catalyst system expressed in terms of pounds of polymer produced per pound of titanium metal, there is no need to remove catalyst residues from the polymer in a deashing step as is the case with conventional catalyst.

Various additives can, if desired, be incorporated into the polypropylene resin, such as fibers, fillers, antioxidants, metal deactivating agents, heat and light stabilizers, dyes, pigments,lubricants and the like.

The polymers can be used with advantage in the manufacture of fibers, filaments and films by extrusion; of rigid articles by injection moulding; and of bottles by blow molding techniques.

The advantages of the polymers of this invention compared to polymers produced by conventional catalysts and having similar mechanical properties include wider processability range, lower processing energy requirements, superior ability to fill thin sections and multiple cavity molds, better draw-down, easier drawability and higher processing speed in the continuous filament and staple fiber production.

For example, based on spiral meltflow measurements it was found that polymers of this invention having meltflows (ASTM-1238) in the range of about 2-1 2g/1 Omin. can be processed at 50-300F lower molding temperatures, or 350-150 psi lower molding pressures than conventional polymers of same meltflows (ASTM-1 238).

It is believed that the molecular weight distribution, Mw/Mn is the property that best relates to the improvement in polymer rheological properties and processability. Typically, polymerization with a conventional catalyst system would result in a polymer product having a Mw/Mn ratio of at most 6.5 and generally below 6, while the polymers of this invention have Mw/Mn ratios of at least 7, e.g. between about 7 and about 10.

The following examples further illustrate the advantages obtained by the invention.

Examples 1 and2 Homopolymerizations of propylene were conducted in large scale continuous pilot plant operations, wherein monomer and catalyst components were continuously charged to a stirred reactor, the monomer feed rate corresponding to 2 hours residence time in the reactor. The organoaluminum compound of the catalyst system was a hexane solution of either triisobutyl aluminum (TIBA) or triethyl aluminum (TEA) which had been treated prior to introduction into the reactor with a hexane solution of methyl-p-toluate (MPT), an electron donor compound. The solid supported titanium halide catalyst component was a commercially available catalyst (FT-1) obtained from Montedison, S.p.A., Milan, Italy. The supported catalyst component contained about 1.5 wt % titanium, 20.3 wt % magnesium, 60.0 wt % chlorine and 9.6 wt % hydrocarbon volatiles. Ethylbenzoate had been used in the manufacture of the supported catalyst component. The two catalyst components were added at rates directly proportional to the polypropylene production rates and in amounts sufficient to maintain a polymer solids concentration in the reactor slurry at a nominal value of about 40%. The catalyst productivity (lb polymer/lb of Ti metal) was calculated in each case from the polymer slurry withdrawal rate, solids content in the slurry and the titanium catalyst component addition rate.The pertinent operating conditions and results are shown in Table 1 together with typical (control) product properties of deashed polypropylene obtained with standard catalyst components i.e. cocrystallized 3TiCI3-AICI3 with diethyl aluminum chloride as cocatalyst.

As indicated in the table, standard ASTM test methods were used to determine the majority of the properties of the polymer product.

The Mw/Mn ratio was determined by liquid chromatography using o-dichlorobenzene as solvent.

Isotactic index was obtained by determination of percent heptane insolubles after extraction of the polymer with boiling heptane, and the dimer-trimer content was found by gas chromatographic analysis of the heptane extract at room temperature.

Crystalline melting point was measured by differential scanning calorimetry.

The contents of Ti, Mg and Al were determined by atomic absorption analysis of polymer ash dissolved in hydrochloric acid and the chlorine content by colorimetric determination of combusted polymer sample using a Parr oxygen bomb.

TABLE 1 Example No. Control 1 2 Catalyst 3TiCl3.AlCl3 FT-1 FT-1 Alkyl aluminum DEAC TIBA TEA Trialkyl aluminum/MPT mol ratio 2.8 3.6 Al/Ti-mol ratio 1.8:1 152 100 Reactor Temperature F 155 155 155 Reactor Pressure psig 440 442 468 Residence Time- hrs. 2.0 2.0 2.0 Productivity kg/gTi 8 435 300 Additives: BHT-ppm 500 500 1200 Irganox 1010-ppm 500 500 500 Calcium stearate ppm 100 - 1000 Glycerol monostearate-ppm 500 500 Hydrotalcite-ppm None 1000 1000 Properties: Dimer-trimer content-g/kg polymer 12 3 TABLE 1 (continued) Example No.Control 1 2 Isotactic lndex-% 94 93.4 95.6 Melt Flow g/l0min. (1) 3.3 2.1 4.1 Density gm/cc (2) 0.9040 0.9029 0.9029 Mn 49,000 39,300 36,000 Mw 320,000 323,000 297,000 Mw/Mn 6.5 8.2 8.3 Tensile Strength @ Yield - psi (3) 4900 4750 5250 @ Break- psi (3) - 3020 3140 Elongation at Break-%(3) 350 310 358 Flex Modulus psi x 105 (4) 1.90 1.87 2.22 Tensile Modulus psi x 105(3) 1.90 1.71 2.32 HDT- at 66 psi 0C(5) 101 100 102 Crystalline Melting Point- C 170 168 168 Hardness (Rockwell) (6) 85 92 93 LTB- C (7) 22 20 20 Izod Impact ft Ibs/in (8) 0.6 1.1 1.0 Polymer Impurities: Ash-ppm 110 300 170 Mg-ppm 0 29 39 Ti-ppm 1 3 3 Cl-ppm 18 84 90 Al-ppm 4 135 186 (1)ASTM D1238, Cond. L (2) ASTM D1505 (3) ASTM D638 (4) ASTM D790 (5) ASTM D648 (6) ASTM D785 (7) ASTM D746 (8) ASTM D256

Claims (24)

1. A propylene polymer having a meltflow range between 0.2 and 30 g/l0min., isotactic index not less than 92%, dimer-trimer content not exceeding 4 g/kg, crystalline melting point of at least 1 650C, ratio of weight-average molecular weight to number-average molecular weight (Mw/Mn) at least 7/1, Ti content not exceeding 3ppm, Mg content not exceeding 40ppm, CI content not exceeding 100ppm and total ash content not exceeding 400ppm.

2. A polymer according to claim 1, wherein said Mw/Mn ratio is between 7/1 and 10/1.

3. A polymer according to claim 1 substantially as described in Example 1 or 2.

4. A process for preparing a polymer as claimed in claim 1, which process comprises continuously feeding propylene monomer and catalyst components to a polymerization reactor, polymerizing the propylene at a temperature between 46"C (115 F) and 80"C (175"F) and at a sufficiently elevated pressure to maintain at least a portion of the propylene in the liquid phase and withdrawing product in a substantially continuous fashion as a slurry in liquid propylene, wherein the catalyst components comprise:: (a) an aluminum trialkyl or an aluminum trialkyl at least partially complexed with an electron donor compound, and (b) a titanium tri- ortetrahalide supported on a magnesium dihalide, or a complex of a titanium tri- or tetrahalide with an electron donor compound supported on a magnesium dihalide.

5. A process according to claim 4, wherein the alkyl group of the aluminum trialkyl of component (a) contains from 1 to 8 carbon atoms.

6. A process according to claim 4, wherein the aluminum trialkyl of component (a) is triisobutyl aluminum.

7. A process according to claim 4, 5 or 6, wherein the electron donor of component (a) is an ester of a carboxylic acid or a diamine.

8. A process according to claim 7, wherein said electron donor is an ester of an aromatic acid.

9. A process according to claim 8, wherein the ester is methyl-p-toluate.

10. A process according to any one of claims 4 to 9, wherein component (a) comprises trialkyl aluminum and electron donor in a molar ratio between 1:1 and 100:1.

11. A process according to claim 10, wherein said molar ratio is between 2:1 and 5:1.

12. A process according to any one of claims 4 to 11, wherein component (a) is prepared by prereacting an aluminum trialkyl with an electron donor for less than one hour before the polymerization reaction.

13. A process according to any one of claims 4 to 12, wherein the titanium tri- or tetrahalide of component (b) is titanium trichloride or titanium tetrachloride.

14. A process according to any one of claims 4 to 13, wherein the magnesium dihalide of component (b) is magnesium dichloride.

15. A process according to any one of claims 4 to 14, wherein the electron donor of component (b) is a polyamine or an ester of an inorganic or an organic oxygen-containing acid.

16. A process according to claim 15, wherein said electron donor is an ester of an aromatic carboxylic acid.

17. A process according to claim 16, wherein the ester is ethylbenzoate.

18. A process according to any one of claims 4 to 17, wherein component (b) comprises the magnesium halide and an electron donor in a molar ratio of at least 1:1.

19. A process according to claim 18, wherein said molar ratio is between 2:1 and 10:1.

20. A process according to any one of claims 4 to 19, wherein the titanium content of the catalyst component (b), expressed as titanium metal, is between 0.1 and 20 weight percent.

21. A process according to claim 20, wherein the titanium content is between 1 and 3 weight percent.

22. A process according to any one of claims 4 to 21, wherein catalyst components (a) and (b) are provided to the reaction zone in a molar ratio of Al/Ti of between 1:1 and 10,000:1.

23. A process according to claim 22, wherein said Al/Ti molar ratio is between 10:1 and 200:1.

24. A process according to claim 4 substantially as described with reference to Example 1 or 2.