New Processing Stabilizers for Polypropylene, 1999 Polymers, Laminations & Coatings Conference Proceedings

Dr. Henry C. Ashton

GE Specialty Chemicals

1000 Industrial Park

Morgantown

West Virginia, 26501

Polyolefin stabilization is essentially the prevention of the effects of thermo-oxidative degradation on polymers and the resultant loss of properties and performance, by chemical means.

New molecules, including one based on a novel chemistry have been developed that address these issues while augmenting existing methods of stabilization. This paper will highlight the key process stabilizing attributes of these unique materials compared to traditional additive packages in use today.

Two classes of materials are discussed - the first is a phosphite with a unique balance of activity, processing performance and ease of handling. The second is a new amine oxide material.

Polyolefins are inherently unstable being susceptible to thermo-oxidative degradation which requires process stabilizers for any melt processing beginning with post-polymerization pelletization and any subsequent melt processing operation during the polymer lifetime. Catalyst residues also require neutralization in the majority of cases and require the use of materials such as hydrotalcites, metal oxides and metal salts such as stearates. Many of these materials or their reaction products can contribute to problems in downstream processing operations.

The classes of materials traditionally and currently used can be classified as primary and secondary antioxidants (see Figures 1 & 2) of which hindered phenolics, aromatic amines, and organo-phosphites, thioesters and thioethers are typical well-known examples of the former and latter respectively (see Figure 2). Neutralizers are often used to boost performance and diminish effects of residual polymer acidity.

It is an accepted fact that the several classes of materials listed above have advantages in particular aspects of polymer protection but in some cases have side effects. For example hindered phenolic additives alone do not provide sufficient melt processing stability and generally chemically synergistic co-stabilizers such as phosphites are used. Although hindered phenolic additives provide long term stability as a consequence of the stabilization mechanism (via scavenging of peroxy radicals - see Figure 1) an unwanted side effect is the generation of color bodies, both during polymer processing and also during aging via formation of quinone methides and quinoid structures. Furthermore on contact with NO x and SO x gases found in exhaust fumes phenolic discoloration leads to color stability deterioration. Again, color stabilizing co-additives such as high activity phosphites are used to alleviate this problem of gas fading color.