Enhancing Barrier and Mechanical Performance of Recycled HDPE and Renewable Origin PP Composites Using Slate and Bivalve Shell Mineral Fillers for Green Packaging

The development of sustainable polymer composites requires the integration of renewable or waste-derived fillers that improve performance without compromising processability. In this study, Slate Powder (SP) and Bivalve Shells (BS) were evaluated as alternative mineral reinforcements (5, 10, 20, and 30% (w/w)) for partially recycled High-Density Polyethene (HDPE) and renewable-origin Polypropylene (PP) composites used in packaging applications produced through blow molding extrusion. Mechanical analysis showed that BS increased Young’s modulus, while SP preserved flexibility and maintained mechanical properties close to those of fossil-based polymers. No significant influence was observed in rheology (Melt Flow Rate (MFR)). Importantly, both SP and BS enhanced barrier performance by reducing air and water vapour permeability. Ultraviolet (UV) transmittance tests confirmed that SP was highly efficient, achieving near 0% transmittance with only 5% incorporation, while BS required higher loadings (30%) to achieve similar results. When compared to reference materials, all composites exhibited superior barrier properties, underscoring their potential in packaging applications. The approach here described not only supports the circular economy but also offers a viable path to produce high-performance and sustainable polyolefin-based packaging materials.