Color Hierarchy in Plastic Degradation: Why Darker Colorants Fail to Protect Polypropylene Straws from Photodegradation?

The widespread use of colored plastics contributes to global pollution, yet the role of color in governing photodegradation remains unclear. We systematically investigated the photodegradation hierarchy of polypropylene (PP) straws in clear, red, blue, yellow, green, and black colors by quantifying oxidation, optical response, and radical formation. Photodegradation was strongly color-dependent, where weight-loss rates followed clear > red > blue > yellow > green > black. Clear PP underwent the most pronounced surface oxidation, fragmentation, and endogenous additive release, highlighting the susceptibility to photodegradation, whereas the highest resistance occurred on the black counterpart. Analysis revealed that color modulated light-absorbing ability and reactive oxygen species (ROS) generation. Compared to black PP, transparent PP exhibited a higher light absorption capacity and a higher ROS generation capacity (with a 48% increase in •OH yield). Despite the differences, all PP plastics followed a common degradation pathway, where photoexcitation initiates C–H cleavage, followed by oxygen addition, ROOH homolysis, and β-scission yielding low-molecular-weight acids. These findings highlight that plastic color directly influences the transformation and ecological risks by regulating its photoreactivity. Therefore, color should be considered a key factor controlling the degradation, additive release, and product formation of plastic debris in ecological risk assessments.