Methods for improving color stability in polyethylene resins

1. A process for producing a polymer composition with long-term color stability, the process comprising:

(a) contacting a chromium-based catalyst system with ethylene and an optional olefin comonomer under polymerization conditions in a polymerization reactor system to produce an ethylene polymer; and

(b) combining the ethylene polymer with an additive system comprising zinc stearate and/or calcium stearate, a phenolic antioxidant, a monophosphite antioxidant, and a diphosphite antioxidant, to form the polymer composition, wherein the polymer composition comprises:

from about 150 ppm to about 350 ppm of zinc stearate and/or calcium stearate;

from about 50 ppm to about 5000 ppm of the phenolic antioxidant;

from about 200 ppm to about 3000 ppm of the monophosphite antioxidant; and

from about 200 ppm to about 3000 ppm of the diphosphite antioxidant;

wherein the polymer composition has a PE color number of at least 200 after storage for 4 weeks at a temperature of 80° C.; and wherein the polymer composition has a Δcolor/Δtime of less than 40.

2. The process of claim 1 , wherein:

the polymerization reactor system comprises a solution reactor, a gas-phase reactor, a slurry reactor, or a combination thereof;

the ethylene polymer comprises an ethylene homopolymer and/or an ethylene/α-olefin copolymer; and

the step of combining comprises blending and/or melt processing.

3. The process of claim 1 , wherein a weight ratio of the diphosphite antioxidant to the monophosphite antioxidant is in a range from about 5:1 to about 1:5.

4. The process of claim 1 , wherein the polymer composition comprises zinc stearate.

5. The process of claim 1 , wherein the polymer composition comprises calcium stearate.

6. The process of claim 1 , wherein the polymer composition contains from about 200 ppm to about 2500 ppm of the phenolic antioxidant, from about 750 ppm to about 3000 ppm of the monophosphite antioxidant, from about 750 ppm to about 3000 ppm of the diphosphite antioxidant, and from about 150 ppm to about 300 ppm of zinc stearate and/or calcium stearate.

7. The process of claim 1 , wherein the chromium-based catalyst system comprises a titanated chromium catalyst.

8. The process of claim 1 , wherein the ethylene polymer is characterized by:

a density in a range from about 0.92 to about 0.96 g/cm 3 ;

a Mw in a range from about 150,000 to about 750,000 g/mol;

a ratio of Mw/Mn in a range from about 5 to about 40;

a melt index (MI) in a range from 0 to about 0.5 g/10 min; and

a high load melt index (HLMI) in a range from about 0.5 to about 25 g/10 min.

9. The process of claim 1 , wherein:

the polymer composition is in the form of pellets; and

the polymer composition has a PE color number of at least 200 after storage for 8 weeks at a temperature of 80° C.

10. A method for improving the long-term color stability of a polymer composition, the method comprising:

combining an ethylene polymer with an additive system comprising zinc stearate and/or calcium stearate, a phenolic antioxidant, a monophosphite antioxidant, and a diphosphite antioxidant, to form the polymer composition, wherein the polymer composition comprises:

from about 150 ppm to about 350 ppm of zinc stearate and/or calcium stearate;

from about 50 ppm to about 5000 ppm of the phenolic antioxidant;

from about 200 ppm to about 3000 ppm of the monophosphite antioxidant; and

from about 200 ppm to about 3000 ppm of the diphosphite antioxidant;

wherein the polymer composition has a PE color number of at least 200 after storage for 4 weeks at a temperature of 80° C.; and wherein the polymer composition has a Δcolor/Δtime of less than 40.

11. The method of claim 10 , wherein the ethylene polymer is a titanated chromium-based ethylene polymer.

12. The method of claim 11 , wherein the ethylene polymer comprises an ethylene homopolymer, an ethylene/1-butene copolymer, an ethylene/1-hexene copolymer, and/or an ethylene/1-octene copolymer.

13. The method of claim 10 , wherein the polymer composition further comprises at least one other additive selected from an antiblock additive, a slip additive, a colorant, a filler, a UV additive, or any combination thereof.

14. A polymer composition comprising:

(i) a titanated chromium-based ethylene polymer;

(ii) from about 150 ppm to about 350 ppm of zinc stearate and/or calcium stearate;

(iii) from about 50 ppm to about 5000 ppm of a phenolic antioxidant;

(iv) from about 200 ppm to about 3000 ppm of a monophosphite antioxidant; and

(v) from about 200 ppm to about 3000 ppm of a diphosphite antioxidant;

wherein the polymer composition has a PE color number of at least 200 after storage for 4 weeks at a temperature of 80° C.; and wherein the polymer composition has a Δcolor/Δtime of less than 40.

15. The composition of claim 14 , wherein the polymer composition comprises from about a 750 ppm to about 3000 ppm of the monophosphite antioxidant.

16. The composition of claim 14 , wherein:

the polymer composition comprises zinc stearate; and

the ethylene polymer comprises an ethylene homopolymer and/or an ethylene/α-olefin copolymer.

17. The composition of claim 14 , wherein:

the polymer composition comprises calcium stearate; and

the ethylene polymer comprises an ethylene homopolymer, an ethylene/1-butene copolymer, an ethylene/1-hexene copolymer, and/or an ethylene/1-octene copolymer.

18. The composition of claim 14 , wherein the ethylene polymer contains:

less than 0.2 ppm of zirconium;

less than 0.2 ppm of hafnium; and

less than 5 ppm of magnesium.

19. The composition of claim 14 , wherein the polymer composition has a PE color number of at least 200 after storage for 8 weeks at a temperature of 80° C.

20. An article of manufacture comprising the polymer composition of claim 14 .