High density polyethylenes with improved processing stability

Abstract

Phillips' process high density polyethylene resin compositions having improved processing stability are obtained using a combination of stabilizers comprised of a hindered phenol and a phosphite compound having three cyclic phosphite moieties linked via a nitrogen atom. More specifically, the phosphite compound is 2, [[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]-dioxaphosphepin-6-yl]oxy]-N,N-bis [2-[[2,4,8,10-tetrakis (1,1-dimethylethyl) dibenzo[d,f][1,3,2]dioxaphosphenpin-6-yl]oxy]-ethyl]ethanamine. With the HDPE compositions stabilized in accordance with the invention, it is possible to eliminate undesirable rapid changes in melt viscosity which can occur when Phillips' process HDPE resins are processed under conditions of high shear.

We claim: 
     
       1. A high density polyethylene resin composition having improved processing stability comprising: (a) an ethylene homopolymer or copolymer of ethylene and C 3-8  -olefin having a density of 0.94 or greater, melt index from 0.01 to 10 and unsaturation content greater than 0.5 vinyl units per 1000 carbon atoms;   (b) from about 50 to 2000 ppm by weight hindered phenol; and   (c) from about 25 to 1000 ppm by weight 2,[[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]-dioxaphosphepin-6-yl]oxy]-N,N-bis[2-[[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2dioxaphosphenpin-6-yl]oxy]-ethyl]ethanamine.   
     
     
       2. The HDPE composition of claim 1 wherein (b) contains one or more substituted aromatic moieties of the formula ##STR5## wherein R' is a C 1-4  alkyl group. 
     
     
       3. The HDPE composition of claim 1 wherein the weight ratio of (b) to (c) is from 10:1 to 1:3. 
     
     
       4. The HDPE composition of claim 3 wherein (a) has a density from 0.945 to 0.965. 
     
     
       5. The HDPE composition of claim 4 wherein (b) is selected from the group consisting of 2,6-di-t-butyl-p-cresol, α-tocopherol, octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate and a compound of formula ##STR6## wherein x is an integer from 2 to 4, R' is a C 1-4  alkyl group and L is a linking group selected from the group consisting of ##STR7## 
     
     
       6. The HDPE composition of claim 5 wherein the ethylene homopolymer or copolymer has an unsaturation content from 0.5 to 2 vinyl units per 1000 carbon atoms. 
     
     
       7. The HDPE composition of claim 6 wherein (a) has a melt index from 0.05 to 5. 
     
     
       8. The HDPE composition of claim 6 wherein (a) is a copolymer of ethylene and butene-1. 
     
     
       9. The HDPE composition of claim 6 wherein (a) is a copolymer of ethylene and hexene-1. 
     
     
       10. The HDPE composition of claim 6 wherein (b) is selected from the group consisting of α-tocopherol, octadecyl 3,5-di-t-butyl-4-hydroxhydrocinnamate, tetrakis[methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane, thiodiethylene bis-(3,5-di-t-butyl-4-hydroxy)hydrocinnamate, and mixtures thereof. 
     
     
       11. The HDPE composition of claim 10 wherein (b) is present in an amount from about 100 to 750 ppm by weight of said ethylene homopolymer or copolymer and (c) is present in an amount from about 50 to 750 ppm by weight of said ethylene homopolymer or copolymer. 
     
     
       12. The HDPE composition of claim 11 wherein the weight ratio of (b) to (c) is from 3:1 to 1:2. 
     
     
       13. The HDPE composition of claim 11 wherein (c) is present in an amount from 100 to 500 ppm by weight of said ethylene homopolymer or copolymer. 
     
     
       14. The HDPE composition of claim 11 wherein (b) is α-tocopherol or mixture of hindered phenols having α-tocopherol as the major component. 
     
     
       15. The HDPE composition of claim 11 wherein (b) is octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate or mixture of hindered phenols having octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate as the major component. 
     
     
       16. The HDPE composition of claim 11 wherein (b) is tetrakis[methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane or mixture of hindered phenols having tetrakis[methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane as the major component. 
     
     
       17. The HDPE composition of claim 10 wherein (b) is thiodiethylene bis-(3,5-di-t-butyl-4-hydroxy)hydrocinnamate or mixture of hindered phenols having thiodiethylene bis-(3,5-di-t-butyl-4-hydroxy)hydrocinnamate as the major component. 
     
     
       18. The HDPE composition of claim 1 wherein the rate of torque change between the torque max  and torque mn  when processed for 40 minutes in air at 225° C. and 60 rpm in a Haake Buchler torque rheometer with counter-rotating sigma style rotors is less than 15 meter-grams per minute. 
     
     
       19. The HDPE composition of claim 18 where the rate of torque change is less than 10 meter-grams per minute.