Enhancement of Polypropylene Adhesion through Acetylated Kraft Lignin Incorporation

Lignin, one of Earth’s most abundant biopolymers, is rich in phenolic and aliphatic functional groups, offering significant potential for chemical modification. Technical lignin, a byproduct of the kraft process, is produced in large quantities annually and can be used to enhance the properties of polymer matrices such as polypropylene (PP). PP, a widely used nonpolar polymer, suffers from low surface free energy, leading to poor adhesion properties. Combining PP with polar, renewable-source polymers like lignin can improve these properties. This study investigates the direct acetylation of kraft lignin (KL) to improve its dispersion in the PP matrix and enhance wettability and adhesion. The acetylation of KL was confirmed through FTIR and DSC analyses. PP and acetylated KL (AKL) were combined and processed via continuous extrusion. The blends’ thermal and mechanical properties, lignin dispersion, and wettability were evaluated. Additionally, PP and PP–lignin films were bonded to aluminized biaxially oriented polypropylene (BOPP) for peel tests. Results showed increased surface free energy and improved adhesion, particularly in samples with AKL due to better dispersion. This direct acetylation route significantly enhances PP’s surface free energy and adhesion, presenting a sustainable alternative to fossil-based materials and promoting the use of lignin, a renewable and low-cost polymer.