Toward Greener Agricultural Films: Properties, Environmental Stability, and Enzymatic Degradation of Polylactic Acid (PLA)/Acetyl Tributyl Citrate (ATBC)/Lignin Biocomposites

The intense use of petroleum-based plastics for agricultural purposes exacerbates the “plastic pollution” concern, as synthetic plastics and their additives can remain in the ground for many years. Biobased, biodegradable alternatives with suitable flexibility and UV resistance are needed. Here, we formulate and characterize polylactic acid (PLA) films that contain acetyl tributyl citrate (ATBC) as a biodegradable plasticizer and kraft lignin powder as a bioderived UV absorber. Both thermal and mechanical analyses showed that addition of ATBC successfully plasticized PLA, resulting in a considerably higher elongation at break and lower glass transition temperature. Of the compositions tested (from 0 to 15 wt %), 12% ATBC was found to result in the best properties; 0.5% and 1% lignin had little effect on mechanical and thermal properties of the materials but almost completely blocked UV radiation, indicating its effectiveness as a UV absorber. To mimic the impacts of these materials as agricultural films, they underwent accelerated weathering. Samples containing ATBC were found to be the most susceptible to weathering, showing the largest reduction in mechanical properties. They also had the highest oxygen transmission, suggesting that photo-oxidative degradation was the main degradation mechanism in accelerated weathering. To investigate the degradability of the films by microbes at the end of life, enzymatic degradation tests were performed using proteinase K. Samples containing lignin underwent the highest mass loss and pH change, likely due to the enhanced water absorption of these samples. This study presents an in-depth understanding of the relationship between the physical properties of PLA and additives and the behaviors of their blend and biocomposite films. These findings suggest that further modifications of additives could improve the compatibility between polymer and additives in order to achieve better stability against weathering, while also ensuring high flexibility and biodegradability of films.