An Effect of Water Presence on Surface Exfoliation of Polypropylene Film Initiated by Photodegradation

A polypropylene (PP) film photodegradation test was performed in water at 60 °C with a specific photocatalyst under visible light irradiation to identify its fragmentation mechanism. The photodegradation degree reached a maximum at the 144 h photodegradation time and showed the decreases at the 288 h. The surface being uneven was observed at the 288 h in the SEM photograph. These results showed that the surface exfoliation was provoked by the test. The spherulite structure contributed to the surface microcrack location and growth. The microcrack allowed entrance of water into the inner amorphous part. The water caused an internal stress by expanding itself due to radiant heat from the visible light irradiation and induced a microcraze in the interlamellar amorphous region. The microcraze developed into an internal microcrack, and surface microcracks finally coalesced together, producing a planar exfoliation. The major long side length of the exfoliation part was below 100 μm at both the 144 h and 288 h. The weight change ratio and rate of the photodegraded PP film showed multi-stages with the increase of photodegradation time. The increment of positive rate was small, showing that the exfoliation caused suppression of the autoxidation rate. The negative rate was due to the exfoliation behavior and was nonlinear to the photodegradation time.