Advanced binderless board-like green nanocomposites from undebarked cotton stalks and mechanism of self-bonding

Self-bonding of air-dried undebarked cotton stalks during hot pressing in a closely fitting mold was studied. Advanced board-like green nanocomposites from ground undebarked cotton stalks were introduced for the first time in the present work. The dry forming process was adopted. Moderate molding pressure and temperature were selected and applied in a tight die, thus saving water and energy and avoiding the use of any binders to achieve an environment-friendly green product. Green nanocomposites having densities in the range of 1.27–1.29 g/cm³ and 1.03–1.06 g/cm³ were prepared. Particle size and cell wall morphological structure were found to play a major role in self-bonding. Properties of composites prepared from the fine fraction of cotton stalks were superior to those prepared from the cotton stalk coarse fraction at the same conditions. This is attributed—among other things—to the dominance of pith (parenchymal cells) in the fine fraction. Such cells possess a high lumen-to-cell wall ratio, which renders them more deformable under pressure, leading to more intercellular or interparticle bonding. Advanced binderless green nanocomposites having bending strength as high as 637 kg/cm² and water absorption as low as 12.1 % were obtained from the ground undebarked cotton stalks. The results show clearly that the advanced green nanocomposite obtained by the dry forming process, without the addition of any binders, is superior to hardboard obtained from cotton stalks by the conventional wet web formation process. The mechanism of self-bonding is discussed.