Use of Cellulose Fibres from Posidonia oceanica to Obtain Chitosan Biocomposites and Poly(lactic Acid) Laminates

New sustainable materials have been developed to replace conventional plastics obtained from non-renewable sources. In this study, cellulose fibres from Posidonia oceanica (PO) were obtained by applying subcritical water extraction and bleaching with hydrogen peroxide or sodium chlorite. The PO fibres were used to obtain cellulose films, chitosan–cellulose composites, and PLA–cellulose laminates. These films were characterised as to their optical properties, mechanical performance, oxygen and water vapour permeability, thermal stability, and microstructure. The cellulose films exhibited low mechanical resistance, with different colouration depending on the degree of delignification. The composites had lower mechanical strength than pure chitosan films. The PO cellulose fibres had a similar, but attenuated, effect when laminated with PLA layers. The fibres improved the oxygen barrier capacity of chitosan films, although this effect only occurred in PLA laminates for cellulose purified with sodium chlorite. In no case did cellulose improve the water vapour barrier of the films compared to pure polymers. The thermal stability was not notably altered by the blending effect, thus reflecting the absence of significant interactions between the fibres and polymer. However, there is a need to improve the functionality of cellulose fibres from PO waste for their incorporation as fillers or laminates in biodegradable food packaging materials.