Stabilizers for thermo plastic materials

Stabilizing mixtures for a plastic material are disclosed. The mixture is comprised of (a) about one part by weight of a tocopherol compound; and (b) from about 1.4 to about 5.0 parts by weight of a secondary antioxidant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to stabilizers for plastic materials, and more 
specifically, to stabilizing mixtures to improve the color and the 
processing of plastic materials. 
2. Background 
Reducing oxidation and stabilizing the color of polymers during processing 
is important to the plastics industry. Polymers, during processing into 
articles, are subjected to high temperatures and pressures which can have 
an adverse affect on the molecular weight and physical properties of the 
polymer and appearance of the finished articles made from these polymers. 
The need exists, therefore, for antioxidant and color stabilizing 
materials which minimize oxidization and color instability. 
U.S. Pat. No. 4,806,580 discloses a stabilizer mixture for stabilizing 
plastic material in processing. The disclosed stabilizer mixture contains 
(a) a chroman derivative and (b) an organic phosphite or phosphonite in a 
ratio (a:b) of 1:5 to 1:14. 
Japanese Patent Publication No. JP62-86036 discloses a composition 
containing per 100 parts by weight of a polyolefin, 0.005 to 0.5 parts by 
weight of a compound selected from the vitamin E group, and 0.01 to 0.5 
parts by weight of a phosphite. 
Japanese Patent Publication No. JP 54-55043 discloses a composition 
containing polyolefin and a compound from the vitamin E group and various 
phosphites. In Examples 1-6 thereof the ratios of vitamin E:phosphite 
evaluated in polypropylene were 1:1.6 and 1:3.3. 
It is an object of the present invention to provide a stabilizer mixture 
for plastic materials which contain polyethylene to improve the processing 
stability and color. 
SUMMARY OF THE INVENTION 
A stabilizing mixture for plastic material containing polyethylene 
comprises: 
(a) about one part by weight of a tocopherol compound; and 
(b) from about 1.4 to about 5 parts by weight of a phosphorus-or 
sulfur-containing secondary antioxidant. 
Examples of such phosphorus-containing secondary antioxidants include 
compounds selected from the group consisting of 
bis(2,4-di-t-butylphenyl)pentaerythrityl diphosphite, 
distearylpentaerythrityl diphosphite, tris(p-nonylphenyl) phosphite, 
2,2'-ethylidene-bis-(4,6-di-t-butylphenyl) fluorophosphonite, 
tetrakis(2,4-di-t-butylphenyl) biphenylenephosphonite, 
tetrakis(2,4-di-butyl(phenyl)-4,4'-biphenyl diphosphonite, and tris 
(2,4-di-t-butylphenyl)phosphonite. 
An example of a sulfur-containing secondary antioxidant is 
distearylthiodipropionate.