Sandwich Structures Made of Discarded Bottle Caps Core and Hybrid Glass Fibre Composite Skins

This work investigates a novel concept of sandwich panel composed of discarded polypropylene (PP) bottle caps as circular honeycomb core and skins made with a hybrid glass fibre/silica composite. A full factorial design (DoE) is carried out to identify the effects of the core packing system and the inclusion of silica particles in the composite skins, on the flexural properties of the sandwich panels. Finite element analysis validated by the experimental results is used to investigate and predict the failure mechanisms of these sandwich structures. The DoE reveals that the hexagonal packing of PP bottle caps with the presence of silica in both skins lead to increased mechanical properties, such as core shear stress (0.79 MPa), facing stress (42.56 MPa) and flexural modulus (1589.21 MPa), especially when compared to the orthotropic packing with silica only on the bottom skin which obtained a lower performance of 21.01%, 11.41% and 17.22% for core shear stress, facing stress and flexural modulus, respectively. The findings reveal an alternative upcycling route for discarded polypropylene caps.