Additive Systems Containing an Antioxidant and a Glycerol Stearate for Improved Color in Polyethylene Resins

Abstract

Polymer compositions containing an ethylene polymer, 50-1500 ppm by weight of a glycerol stearate, and 250-7500 ppm by weight of an antioxidant selected from a phenolic antioxidant, a phosphite antioxidant, a thioester antioxidant, or any combination thereof, are described. These polymer compositions have improved initial color, improved color after long-term aging, or improved color after multi-pass extrusion processing.

We claim: 
     
         1 . A polymer composition comprising:
 (i) an ethylene polymer;   (ii) from about 50 ppm to about 1500 ppm of a glycerol stearate; and   (iii) from about 250 ppm to about 7500 ppm of an antioxidant comprising a phenolic antioxidant, a phosphite antioxidant, a thioester antioxidant, or any combination thereof.   
     
     
         2 . The composition of  claim 1 , wherein:
 at least about 75 wt. % of the glycerol stearate is glycerol monostearate; and   the ethylene polymer comprises an ethylene homopolymer and/or an ethylene/α-olefin copolymer.   
     
     
         3 . The composition of  claim 1 , wherein the polymer composition comprises:
 from about 100 ppm to about 850 ppm of the glycerol stearate; and   from about 500 ppm to about 5000 ppm of the antioxidant.   
     
     
         4 . The composition of  claim 1 , wherein:
 the polymer composition is in the form of pellets; and   the polymer composition has an initial PE color number of at least 200.   
     
     
         5 . The composition of  claim 1 , wherein the antioxidant comprises:
 the phenolic antioxidant and a monophosphite antioxidant; or   the thioester antioxidant and a monophosphite antioxidant.   
     
     
         6 . The composition of  claim 1 , 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.   
     
     
         7 . An article of manufacture comprising the polymer composition of  claim 6 . 
     
     
         8 . The composition of  claim 1 , wherein:
 the polymer composition has an initial PE color number that is greater than that or within 20 of an otherwise equivalent polymer composition that contains 500 ppm of a diphosphite antioxidant instead of the glycerol stearate;   the polymer composition has an aged PE color number after storage for 4 weeks at temperature of 80° C. that is greater than that or within 20 of an otherwise equivalent polymer composition that contains 500 ppm of a diphosphite antioxidant instead of the glycerol stearate;   the polymer composition has a multi-pass extrusion PE color number after three extrusion passes at an extrusion temperature of 240° C. that is greater than that or within 20 of an otherwise equivalent polymer composition that contains 500 ppm of a diphosphite antioxidant instead of the glycerol stearate; or   any combination thereof.   
     
     
         9 . The composition of  claim 1 , wherein the ethylene polymer is a metallocene-based ethylene polymer, a Ziegler-Natta based ethylene polymer, a chromium-based ethylene polymer, or a titanated chromium-based ethylene polymer. 
     
     
         10 . The composition of  claim 1 , wherein the ethylene polymer is characterized by:
 a density in a range from about 0.88 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.   
     
     
         11 . The composition of  claim 1 , 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, an anti-stat additive, a processing aid, an acid scavenger, a carbon centered radical scavenger, or any combination thereof. 
     
     
         12 . A process for producing a polymer composition with improved color, the process comprising:
 (a) contacting a transition metal-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 a glycerol stearate and an antioxidant to form the polymer composition, wherein the polymer composition contains:   from about 50 ppm to about 1500 ppm of the glycerol stearate; and   from about 250 ppm to about 7500 ppm of the antioxidant, wherein the antioxidant comprises a phenolic antioxidant, a phosphite antioxidant, a thioester antioxidant, or any combination thereof.   
     
     
         13 . The process of  claim 12 , wherein:
 the transition metal-based catalyst system is a metallocene-based catalyst system, a Ziegler-Natta based catalyst system, a chromium-based catalyst system, a titanated chromium-based catalyst system, or a combination thereof;   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, an ethylene/1-butene copolymer, an ethylene/1-hexene copolymer, and/or an ethylene/1-octene copolymer; and   the step of combining comprises blending and/or melt processing.   
     
     
         14 . The process of  claim 12 , wherein:
 the polymer composition comprises from about 500 ppm to about 3500 ppm of the antioxidant;   the antioxidant comprises a monophosphite antioxidant and the phenolic antioxidant or the thioester antioxidant;   the polymer composition has an aged PE color number of at least 200 after storage for 4 weeks at a temperature of 80° C.; and   the polymer composition has a multi-pass extrusion PE color number of at least 200 after three extrusion passes at an extrusion temperature of 240° C.   
     
     
         15 . A method for improving the color of a polymer composition, the method comprising:
 combining an ethylene polymer with an additive system comprising a glycerol stearate and an antioxidant to form the polymer composition, wherein the polymer composition contains:   from about 50 ppm to about 1500 ppm of the glycerol stearate; and   from about 250 ppm to about 7500 ppm of the antioxidant, wherein the antioxidant comprises a phenolic antioxidant, a phosphite antioxidant, a thioester antioxidant, or any combination thereof.   
     
     
         16 . A masterbatch composition comprising:
 (1) a carrier resin; and   (2) an additive mixture comprising a glycerol stearate and an antioxidant comprising a phenolic antioxidant, a phosphite antioxidant, a thioester antioxidant, or any combination thereof, at a weight ratio of glycerol stearate:antioxidant in a range from about 6:1 to about 1:150;   wherein a weight ratio of the carrier resin:additive mixture is in a range from about 95:5 to about 20:80.   
     
     
         17 . The composition of  claim 16 , wherein:
 the carrier resin comprises an ethylene polymer, a propylene polymer, or any combination thereof; and   the weight ratio of glycerol stearate: antioxidant is in a range from about 3:1 to about 1:75.   
     
     
         18 . The composition of  claim 16 , wherein:
 the masterbatch composition is in the form of pellets; and   the masterbatch composition further comprises at least one other additive selected from an antiblock additive, a slip additive, a colorant, a filler, a UV additive, an anti-stat additive, a processing aid, an acid scavenger, a carbon centered radical scavenger, or any combination thereof.   
     
     
         19 . An additive blend composition comprising:
 (1) at least about 95 wt. % of an additive mixture comprising a glycerol stearate and an antioxidant comprising a phenolic antioxidant, a phosphite antioxidant, a thioester antioxidant, or any combination thereof, at a weight ratio of glycerol stearate:antioxidant in a range from about 6:1 to about 1:150; and   (2) less than or equal to about 3 wt. % of an ethylene polymer, a propylene polymer, or any combination thereof.   
     
     
         20 . The composition of  claim 19 , wherein the composition comprises:
 at least about 98 wt. % of the additive mixture; and   less than or equal to about 1 wt. % of the ethylene polymer, propylene polymer, or combination thereof.   
     
     
         21 . The composition of  claim 19 , wherein the weight ratio of glycerol stearate:antioxidant is in a range from about 1:1.5 to about 1:15.