Polypropylene after thirty years of storage: mechanical proof of heterogeneous aging

Abstract

The effects of 30 years of storage on the mechanical behavior and hierarchical structure of isotactic polypropylene were characterized. In addition, the structure and properties of the aged samples exposed to subsequent annealing were assessed. The long-term storage caused a dramatic loss of ductility, as manifested by the strain-at-break and impact strength data. The embrittlement was accompanied by a distinct increase in the crystallinity of both the alpha and beta phases, as revealed by X-ray diffraction. The dynamic mechanical behavior was influenced, but insignificantly. A comparison of static and dynamic mechanical data suggests the development of structural heterogeneities in the amorphous phase of the semicrystalline material upon aging. Under mechanical stress, these heterogeneities serve as fracture loci. Subsequent annealing of the aged samples partly reversed the loss of ductility. This indicates healing of the hypothetical irregularities by the thermal treatment. In addition, annealing caused complete transformation of the beta crystallites into the alpha phase.