Estrogenic chemicals originating in plastics materials are known to play a causative role in many health problems we face such as cancer, infertility, precocious puberty, and obesity. They have been linked to diabetes, asthma, neurological misdevelopment and increased aggression. The estrogenic components may be residual monomers, small oligomers, plasticizers, antioxidants or other additives. Polyethylene and polypropylene, widely used in food packaging, often contain antioxidant additives which are estrogenic. Our goal in this project was to create high-density polyethylene and polypropylene formulations which were non-estrogenic and also had acceptable processing stability and thermal performance. After testing various commercial formulations using multiple extrusions and Melt Index measurements, we tested several new non-estrogenic formulations. One PP formulation containing 0.8% Chimassorb 2020 and 0.2% distearyl pentaerythritol had "acceptable" melt index increases of 31% after one extrusion and 50% after two extrusions. The HDPEs we tested were very stable, with small changes or no change in Melt Index after one extrusion. The addition of Chimassorb 2020 only made a small difference in the processing stability of additive-free HDPE, demonstrating that antioxidant additives may not be necessary for all HDPEs. In the exploratory phase of this work, we searched for correlations between the degradation caused by prolonged MFI residence time and by 1-2 extrusions. Out of nine Its tested, four had equivalent times of ca. 30 min for one extrusion and five had equivalent times of 60 min for two extrusions. The equivalent times of PP were found to vary with the amount of Chimassorb 2020, with short-term stability being impaired and long-term stability being improved. Out of three commercial HDPEs tested, two had equivalent times of 7 min for one extrusion. One commercial HDPE had excellent thermal stability and did not degrade measurably at all. We conclude that some non-estrogenic formulations of polypropylene and HDPE are thermally stable and their use in food packaging would benefit the health of babies and children in first-world countries. Further research and development may result in even more stable non-estrogenic polypropylenes and HDPEs, as well as increased fundamental knowledge of polymer stability and degradation.