Stabilization of Gamma Irradiated High Density Polyethylene, 1999 Polymers, Laminations & Coatings Conference Proceedings

R.E. “Rick” King III; Ciba Specialty Chemicals Corporation

Polyolefins are finding a variety of end use applications, such as medical packaging, hospital garments, surgery utensils, and hygiene related articles where sterilization plays a key role in the life cycle of the plastic product.

Sterilization techniques, such as gamma irradiation, are used to make sure that the article can be used in these type of demanding environments. While this approach of killing unwanted organisms and bacteria is valued by the end users, the stress on the plastic article is quite significant in terms of not only maintaining the original strength and toughness of product, but also it’s aesthetic appearances. Subtle issues, such as color, taste and odor can have an effect on perceptions of product quality, which can be just as important as practical utility.

A study was carried out in a high density polyethylene where a variety of “phenol” and “phenol free” stabilization systems were compared in regard to their ability to maintain physical and aesthetic (discoloration) properties of the polymer upon:

a) repeated extrusions

b) increasing doses of gamma irradiation.

The results of this study demonstrate that the hindered amine based “phenol free” systems were as good or better than the phenol based systems, at least in regard to maintenance of color development as a function of extrusion pass. In regard to gamma irradiation dose, the story is not quite as clear in that selected phenol and phenol free formulations provided excellent results, while others did not. In regard to color maintenance, the phenol free systems have the edge. On the other hand, neither stabilization system completely prevented crosslinking of the polymer during gamma irradiation. The maintenance of aesthetic and physical properties during mildly accelerated oven aging should hopefully help to sort the preferred stabilization system. For the time being, the HA-1/P-1/NOH appears to be the superior system.