Multilayer paper composites interconnected by oblong cuts and sodium silicate adhesive reinforced with cellulose nanofibrils derived from cardboard waste

Multiple-layered composites are produced industrially by pressing paper layers together with a sodium silicate adhesive. They have been utilized in a variety of technological applications, including as formwork for reinforced concrete in civil construction and in the food, textile, and pyrotechnic industries. However, the major limitation is the low strength of the adhesive. To overcome this challenge, we propose the reinforcement of sodium silicate with cellulose nanofibrils (CNFs), as well as the use of paper layers with longitudinal cuts, innovating in a composite with interconnected glue lines. Thus, the objective of this study was to evaluate the mechanical effect of paper layers containing longitudinal cuts and adhesives reinforced with 0.5% CNFs. The composites were produced with 20 layers of recycled Kraft paper and 50 g/m² of nanoreinforced sodium silicate per glue line. The pressing effort applied to the composites by a sliding cylinder was equal to 4.3 MPa. In the composites with paper layers with longitudinal cuts, the modulus of rupture was 55.9 MPa, whereas in the control sample, it was 38.8 MPa. For these same configurations, the modulus of elasticity was 4197 and 3473 MPa. The incorporation of CNFs in the adhesive provided greater wettability on the paper surface, with greater penetration and formation of rigid bonds. With respect to the unreinforced product and the composite with the adhesive reinforcement, the toughness increased from 0.0023 to 0.0045 N mm/mm³. This technique to produce paper composites using nanomaterials has proven to be efficient in obtaining a product with better performance.