Crystallization and chemi-crystallization of recycled photo-degraded polypropylene

Injection moulded bars have been made from virgin polypropylene and from blends containing recycled photo-degraded polypropylene, then subjected to further ultraviolet (UV) exposure. Crystallinity measurements have been made at different depths from the exposed surface using X-ray diffraction and differential scanning calorimetry. Strong evidence for chemi-crystallization is provided for the photo-degraded samples. The crystallinity results are interpreted in terms of molecular scission and photo-initiated molecular defects. Scission leads to greater crystallizability and accounts for the observed chemi-crystallization. Molecular defects inhibit crystallization and eventually limit chemi-crystallization, as observed with a blend containing 75% virgin material and 25% recycled photo-degraded polymer after further UV exposure.