Photoaging behavior of black mulch films: the dual roles of carbon black

Black low density polyethylene mulch film (CBLDPE), containing carbon black (CB) as a key colorant and UV absorber, is a major source of agricultural microplastic (MP) pollution. However, the complex role of CB in the photoaging of thin mulch films (MFs) remains unclear. This study systematically investigated the photoaging behavior of CBLDPE and transparent LDPE MFs (TLDPE) under irradiation, focusing on the complex role of CB. Results showed CBLDPE exhibited three aging phases of initiation, rapid growth, and dynamic stabilization, whereas TLDPE underwent delayed aging. Their macroscopic mechanical differences are minimal. However, nanoscale crack propagation, hardness, and modulus in the aging process increased more in CBLDPE than in TLDPE, likely due to CB. Initially, CB acted as an antioxidant, but as oxygen content increased, its pro-aging effect was dominated by generating hydroxyl radicals (·OH). Research suggested that CBLDPE had a higher risk of MP release. TLDPE experienced mechanical degradation before visible oxidation or pore formation. Photoaging altered CB's structure and thermal stability, and peeling of the oxidation layer weakened its anti-aging efficiency. This study sheds light on the vital influence and mechanism of CB on the photoaging behavior of MFs, offering new insights into their geochemical behavior and environmental risks.