Two-stage process for producing polypropylene compositions
View Patent ↗A process for polymerizing propylene in the presence of a polymerization catalyst by polymerizing propylene with a comonomer selected from the group of ethylene and C4-C10 alpha-olefins in two polymerization stages where the comonomer is present in at least one of the polymerization stages. The first polymerization stage is conducted in a loop reactor and the second polymerization stage in a gas phase reactor. The polymer produced in first polymerization stage has a higher melt flow rate and a lower content of comonomer units than the final polymer mixture. The process can be operated with a high throughput and catalyst productivity. The polymers are useful for making films.
1. A process for polymerizing propylene in the presence of a polymerization catalyst comprising (I) a solid catalyst component comprising magnesium and titanium compounds and an internal electron donor; and (II) a cocatalyst comprising an aluminum alkyl and optionally an external electron donor, said process comprising the steps of:
(A) continuously polymerizing propylene by introducing streams of propylene, hydrogen and said polymerization catalyst into a loop reactor at a temperature of from 65 to 100° C. and a pressure of from 25 to 100 bar to produce slurry of particles of a first homopolymer of propylene having a melt flow rate MFR 2 of from 3 to 20 g/10 min in a first reaction mixture;
(B) withdrawing a slurry stream from said loop reactor, said slurry stream comprising said first reaction mixture and said particles of the first homopolymer of propylene, said particles further comprising said polymerization catalyst, and passing the slurry stream into a gas phase reactor;
(C) continuously copolymerizing propylene by introducing streams of propylene, a comonomer selected from the group of ethylene and C4-C10 alpha-olefins and optionally hydrogen into said gas phase reactor at a temperature of from 65 to 100° C. and a pressure of from 10 to 40 bar to produce particles comprising a polymer mixture of said first homopolymer of propylene and a second copolymer of propylene, said polymer mixture having a content of comonomer units from 0.1 to 5 mol-% and a melt flow rate MFR 2 of from 1 to 10 g/10 min; wherein said polymer mixture comprises from 35 to 65% by weight of said first homopolymer and from 65 to 35% by weight of said second copolymer, and wherein the melt flow rate MFR2 2 of said polymer mixture is lower than the melt flow rate MFR 2 of said first homopolymer;
(D) withdrawing a stream comprising said polymer mixture from said gas phase reactor;
(E) removing hydrocarbons from said stream withdrawn from said gas phase reactor to produce a polymer stream with reduced content of hydrocarbons and optionally introducing additives to the polymer mixture; and
(F) extruding said polymer mixture into pellets;
wherein the ratio of the melt index MFR 2 of the polymer mixture to the melt index MFR 2 of the first homopolymer is not higher than 0.8.
2. The process according to claim 1 wherein the loop reactor is operated at a temperature within the range of from 70 to 95° C.
3. The process according to claim 1 wherein the melt flow rate MFR 2 of the first homopolymer of propylene is 4 to 15 g/10 min.
4. The process according to claim 1 wherein the gas phase reactor is operated at a temperature within the range of from 75 to 95° C.
5. The process according to claim 1 wherein the melt flow rate MFR 2 of the polymer mixture is from 1 to 7 g/10 min.
6. The process according to claim 1 wherein the content of comonomer units in the polymer mixture is from 0.1 to 3% by mole and the content of propylene units is from 97 to 99.9% by mole.
7. The process according to claim 1 wherein said polymer mixture comprises from 40 to 60% by weight of said first homopolymer and from 60 to 40% by weight of said second copolymer.
8. The process according to claim 1 wherein the ratio of the melt index MFR 2 of the polymer mixture to the melt index MFR 2 of the first homopolymer is not higher than 0.6.
9. The process according to claim 8 wherein the ratio of the melt index MFR 2 of the polymer mixture to the melt index MFR 2 of the first homopolymer is at least 0.1.
10. The process according to claim 1 wherein the comonomer present in the second polymerization stage is ethylene.
11. The process according to claim 1 wherein the content of comonomer units in the second copolymer of propylene is from 0.3 to 1.0% by mole.
12. The process according to claim 11 wherein the absolute value of the difference in the fractions of polymer remaining soluble in xylene at 25° C. after cooling in the first homo- or copolymer and in the polymer mixture is not greater than 30% of the fraction of the polymer remaining soluble in xylene at 25° C. after cooling in the polymer mixture, wherein the fraction of the polymer remaining soluble in xylene at 25° C. after cooling is determined according to ISO 16152.
13. A process for producing a film comprising the steps of:
(1) producing a propylene polymer composition according to claim 1 ; and
(2) extruding said propylene polymer composition into a film.
14. The process according to claim 13 further comprising the step of orienting the film in two directions.
15. The process according to claim 1 wherein the melt flow rate MFR 2 of the first homopolymer of propylene is from 5 to 10 g/10 min.
16. The process according to claim 1 wherein the melt flow rate MFR 2 of the polymer mixture is from 2 to 5 g/10 min.
17. The process according to claim 8 wherein the ratio of the melt index MFR 2 of the polymer mixture to the melt index MFR 2 of the first homopolymer is at least 0.2.
18. The process of claim 13 , wherein the ratio of the melt index MFR 2 of the polymer mixture to the melt index MFR 2 of the first homopolymer is at least 0.2 and not higher than 0.6.
19. A process for polymerizing propylene in the presence of a polymerization catalyst comprising (I) a solid catalyst component comprising magnesium and titanium compounds and an internal electron donor; and (II) a cocatalyst comprising an aluminum alkyl and optionally an external electron donor, said process comprising the steps of:
(A) continuously polymerizing propylene by introducing streams of propylene, hydrogen and said polymerization catalyst into a loop reactor at a temperature of from 65 to 100° C. and a pressure of from 25 to 100 bar to produce slurry of particles of a first homopolymer of propylene having a melt flow rate MFR 2 of from 3 to 20 g/10 min in a first reaction mixture;
(B) withdrawing a slurry stream from said loop reactor, said slurry stream comprising said first reaction mixture and said particles of the first homopolymer of propylene, said particles further comprising said polymerization catalyst, and passing the slurry stream into a gas phase reactor;
(C) continuously copolymerizing propylene by introducing streams of propylene, a comonomer selected from the group of ethylene and C4-C10 alpha-olefins and optionally hydrogen into said gas phase reactor at a temperature of from 65 to 100° C. and a pressure of from 10 to 40 bar to produce particles comprising a polymer mixture of said first homopolymer of propylene and a second copolymer of propylene, said polymer mixture having a content of comonomer units from 0.1 to 5 mol-% and a melt flow rate MFR 2 of from 1 to 10 g/10 min; wherein said polymer mixture comprises from 35 to 65% by weight of said first homopolymer and from 65 to 35% by weight of said second copolymer, and wherein the melt flow rate MFR2 2 of said polymer mixture is lower than the melt flow rate MFR 2 of said first homopolymer;
(D) withdrawing a stream comprising said polymer mixture from said gas phase reactor;
(E) removing hydrocarbons from said stream withdrawn from said gas phase reactor to produce a polymer stream with reduced content of hydrocarbons and optionally introducing additives to the polymer mixture; and
(F) extruding said polymer mixture into pellets;
wherein the content of comonomer units in the second copolymer of propylene is from 0.3 to 1.0% by mole.
20. The process according to claim 19 wherein the absolute value of the difference in the fractions of polymer remaining soluble in xylene at 25° C. after cooling in the first homo- or copolymer and in the polymer mixture is not greater than 30% of the fraction of the polymer remaining soluble in xylene at 25° C. after cooling in the polymer mixture, wherein the fraction of the polymer remaining soluble in xylene at 25° C. after cooling is determined according to ISO 16152.