Composite materials based on halloysite clay nanotubes and cellulose from Posidonia oceanica sea balls: from films to geopolymers†

Combination of natural clay nanoparticles and sustainable biopolymers represents a powerful tool to obtain composite green materials useful for packaging and building applications. Here, we developed hybrid films based on halloysite nanotubes (HNTs) and cellulose recovered from Posidonia oceanica sea balls (egagropili), which are marine wastes accumulating along the Mediterranean beaches. The halloysite content of the composites was systematically changed from 30 to 80 wt%. Despite the very high amounts of clay nanotubes, the composite films exhibited macroscopic properties comparable to that of pristine cellulose. SEM analyses evidenced that halloysite nanotubes form aggregates within the fibrous structure of cellulose and the microstructural characteristics of the films are affected by the cellulose/HNT composition. Accordingly, we detected that the wettability of the films can be controlled by the halloysite content. The addition of halloysite generated a thermal stabilization of cellulose and a worsening of the tensile properties. Nevertheless, the cellulose/HNT composite films showed competitive performances in terms of elasticity and mechanical resistance, although the contribution of inorganic nanotubes is predominant with respect to that of the cellulosic matrix. Cellulose/HNT films were converted into compact geopolymers by using Na(OH) solution for the alkaline activation of nanoclay, which was proved by thermogravimetry. The geopolymerization process induced significant increase in the rigidity and stress at break for the composite film containing the largest (80 wt%) HNT content. This work represents a starting step to develop composite materials through the valorization of egagropili. The cellulose/HNT films can be promising for packaging applications, while the corresponding geopolymers might be employed as building materials.