Technical Lignin Valorization in Biodegradable Polyester-Based Plastics (BPPs)

Synthetic plastics have been reshaping the planet since the early 20th century. They have brought plenty of conveniences to our lives; meanwhile, plastic wastes discarded into the environment are resistant to biodegradation, accumulating in soil, water, and even in organisms. Biodegradable polyester-based plastics (BPPs), such as polylactic acid (PLA), poly(butylene adipate-co-terephthalate) (PBAT), poly(butylene succinate) (PBS), polycaprolactone (PCL), and poly(β-hydroxybutyric acid) (PHB) are considered to be ecofriendly and sustainable alternatives and have thrived rapidly in the past few decades. However, the applications of BPPs have been limited by their high prices compared to conventional plastics, while blending with low-cost fillers has been deemed to be a facile and efficient solution to the foregoing issues. Technical lignin, a class of biodegradable aromatic polymers, has been extracted/isolated at a large scale as a coproduct in the conventional paper-making and the emerging lignocellulosic biorefinery industries. The economically competitive BPP composites reinforced by coproduct lignin could not only facilitate the lignin valorization but also improve the economic viability of the lignocellulosic biorefinery. At present, the key obstacle lies in the incompatibility between lignin and BPPs resulting from self-aggregation and thermal condensation of lignin due to its polyhydroxy aromatic structures. Herein, we overviewed a series of effective approaches to promote the interaction between lignin and BPPs by adding plasticizers and blocking the hydroxyl groups. Furthermore, lignin can act as functional fillers endowing lignin/BPP composites favorable versatilities and expanding their applications in the fields of flame retardant packaging, food packaging, medical materials, outside packaging, and mulching films. In the end, we review the developments of lignin/BPP blending composites in the future and propose current problems that need to be solved.