An increasing number of biopharmaceuticals — including vaccines, stem cells, and proteins — require cold storage to maintain efficacy before use. However, the ability to maintain container–closure integrity (CCI) during cold storage is not completely understood. Concerns about CCI failure have been raised for storage and shipment of such products in rubber-stoppered vials under cold conditions (e.g., −80 °C or on dry ice). Commonly used butyl stoppers are believed to lose their elastic properties below their glass transition temperature (Tg),…