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Photooxidative degradation and fragmentation behaviors of oriented isotactic polypropylene
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Laboratory simulations showed that highly crystalline and oriented polypropylene films degrade more slowly under UV exposure and fragment into larger microplastic particles, while less crystalline films break down faster into smaller, more numerous fragments. Polymer crystallinity and molecular orientation are key determinants of how quickly plastics generate microplastics and what size distribution those particles take—factors directly relevant to predicting environmental MP loading from different plastic products.
The photooxidative degradation and fragmentation behaviors of isotactic polypropylene (itPP) were simulated in laboratory after various postconditions, such as quenching, gradual cooling and drawing, using an artificial weathering machine and a blender. The crystallinity of the itPP films and orientation of the molecular chains play important roles in the photooxidation of the itPP films. Compared to quenched itPP films with the same ultraviolet (UV) exposure time, highly oriented itPP films and gradually cooled itPP films with higher crystallinity exhibited a lower rate of photooxidative degradation. To clarify the photooxidative degradation mechanism, the surface morphology, chemical structure, and microstructure of the UV-exposed itPP films were investigated using scanning electron microscopy, infrared spectroscopy, differential scanning calorimetry, and wide- and small-angle X-ray scattering. Photooxidative degradation was inhibited as the orientation degree of the itPP film increased. These results indicate that photooxidation likely occurs in the amorphous phase of itPP. Oriented molecular chains effectively slowed the photooxidative degradation of the itPP films. The artificial fragmentation test of UV-exposed itPP films showed that itPP films with lower crystallinity and orientation degrees were crushed into microplastics that were much smaller in size than those with higher crystallinity or orientation degrees. Photooxidation and fragmentation behaviors of itPP were studied. Photooxidation likely occurred in the amorphous regions of itPP due to the higher oxygen diffusion. Surface deterioration was observed on the UV-exposed itPP films. Pressed films exhibited much denser cracks compared to uniaxially oriented itPP films. Notably, cracks in the uniaxially oriented itPP films were formed along the direction of orientation and decreased with increasing draw ratio. The crystalline structure and oriented molecular chains notably inhibited the photooxidative degradation and fragmentation of the itPP films.
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