Casein matrix composites reinforced with recycled cellulose and cellulose acetate fibers: formulation and mechanical performance for sustainable applications

Abstract Casein-based composite materials show promise for sustainable applications due to their biodegradability and mechanical properties. With over 1.3 million tons of milk wasted annually, this study investigates repurposing this resource into composites reinforced with 50 wt% recycled cellulose and cellulose acetate fibers from cigarette butts. Mechanical testing revealed that composites with recycled cellulose fibers outperformed those with cellulose acetate, exhibiting a flexural modulus of 3527.7 MPa and a flexural strength of 24.2 MPa, compared to a flexural modulus of 1411 MPa and a flexural strength of 13.4 MPa for cellulose acetate. Shear strength tests also showed superior results for recycled cellulose (5.7 MPa) compared to cellulose acetate (3.3 MPa). Charpy impact tests further confirmed this trend, with impact strengths of 8.1 kJ/m 2 for cellulose and 6.7 kJ/m 2 for cellulose acetate. These findings highlight the potential of cellulose-reinforced casein composites as eco-friendly alternatives to synthetic materials, addressing both milk waste and cigarette pollution. By incorporating waste materials into the production process, this study aligns with the principles of the circular economy, promoting waste reduction, resource efficiency, and sustainability. Future research will focus on scalability, fiber treatments, and applications in construction, packaging, and automotive industries. Graphical abstract