Polypropylene copolymer composition having subzero impact resistance
Polypropylene polymer compositions are disclosed that can be formulated to have excellent clarity properties in conjunction with excellent impact resistance properties at subzero temperatures. The polypropylene polymer compositions are heterophasic compositions containing a first phase polymer combined with a second phase polymer. The first phase polymer is a polypropylene and alpha-olefin copolymer while the second phase polymer is also a polypropylene and alpha-olefin random copolymer. The second phase polymer contains relatively high amounts of ethylene. The increased amounts of ethylene in the second phase polymer were found to dramatically improve impact resistance at subzero temperatures.
1. A polypropylene composition comprising:
a first polymer phase comprising a polypropylene polymer, the polypropylene polymer having a xylene soluble content of less than about 10% by weight and having a melt flow rate of from about 20 g/10 min to about 50 g/10 min;
a second polymer phase combined with the first polymer phase, the second polymer phase comprising a propylene and alpha olefin copolymer; and
wherein the polypropylene composition has a melt flow rate of 3 g/10 min or greater, and wherein the ratio of the melt flow rate of the first polymer phase to the melt flow rate of the polypropylene composition is greater than or equal to 1, the polypropylene composition having a xylene soluble portion and a xylene insoluble portion, the polypropylene composition having a total xylene soluble content of from about 12% to about 25% by weight, the xylene soluble portion containing the alpha olefin in an amount from about 55% to about 70% by weight, the xylene insoluble portion containing the alpha olefin in an amount from about 15% to about 40% by weight, the polypropylene composition having a Gardner impact strength at −20° C. of greater than about 200 inch-lbs.
2. A polypropylene composition as defined in claim 1 , wherein the composition further contains a clarifier and wherein the polypropylene composition has a haze at 1 mm of less than about 45%.
3. A polypropylene composition as defined in claim 1 , wherein the second polymer phase is in the form of polymer particles dispersed within the first polymer phase, the polymer particles having an average particle size of greater than or equal to about 1 micron.
4. A polypropylene composition as defined in claim 1 , wherein the polypropylene polymer contained in the first polymer phase comprises a polypropylene random copolymer containing ethylene and wherein the second polymer phase comprises a propylene ethylene copolymer.
5. A polypropylene composition as defined in claim 4 , wherein the ethylene content in the xylene soluble portion of the polypropylene composition is from about 60% to about 70% by weight.
6. A polypropylene composition as defined in claim 4 , wherein the ethylene content in the xylene insoluble portion of the polypropylene composition is from about 20% to about 38% by weight.
7. A polypropylene composition as defined in claim 1 , wherein the composition has a flexural modulus of from about 500 MPa to about 1000 MPa.
8. A polypropylene composition as defined in claim 1 , wherein the composition has a Gardner impact strength at −20° ° C. of from about 300 inch-lbs to about 500 inch-lbs.
9. A polypropylene composition as defined in claim 4 , wherein the polypropylene random copolymer in the first polymer phase contains ethylene in an amount from about 1% to about 4% by weight.
10. A polypropylene composition as defined in claim 4 , wherein the propylene ethylene copolymer in the second polymer phase contains ethylene in an amount of greater than about 75% by weight, such as in an amount greater than about 80% by weight.
11. A polypropylene composition as defined in claim 1 , wherein the second polymer phase is present in the polypropylene composition in an amount from about 15% to about 50% by weight.
12. A polypropylene composition as defined in claim 1 , wherein the polypropylene composition has a total xylene soluble content of from about 15% to about 21% by weight.
13. A polypropylene composition as defined in claim 1 , wherein the polypropylene composition has a haze at 1 mm of from about 15% to about 45%.
14. A polypropylene composition as defined in claim 1 , wherein the polypropylene composition has a clarity of greater than about 90%, such as greater than about 92%.
15. A polypropylene composition as defined in claim 1 , wherein the polypropylene composition has a melt flow rate of from about 15 g/10 min to about 30 g/10 min.
16. A polypropylene composition as defined in claim 1 , wherein the second polymer phase is formed in the presence of the first polymer phase.
17. A polypropylene composition as defined in claim 1 , wherein the polypropylene composition exhibits at least three tan delta peaks at three different temperatures.
18. A polypropylene composition as defined in claim 1 , wherein the composition further contains an antacid and an antioxidant.
19. A molded article formed from the polypropylene composition as defined in claim 1 .
20. A molded article as defined in claim 19 , wherein the molded article is an injection molded article.