How the chemical nature of Brazilian hardwoods affects nanofibrillation of cellulose fibers and film optical quality

A wide range of alternative cellulose fibers for the development of new green nanomaterials can be obtained from Brazil's natural resources. The objective of the work is to evaluate the influence of the chemical composition of hardwoods on the nanofibrillation process and optical quality of nanofiber films. Wood wastes were selected from three native Amazonian species and from exotic planted Eucalyptus grandis species. Wood sawdust was submitted to chemical alkali and bleaching pretreatments. Nanofibers were produced from the bleached fibers after 10, 20, 30 and 40 passes through a Super Mass Colloider grinder, and films were produced by the casting method. Raw sawdust, alkali-treated fibers and bleached fibers were evaluated by the major chemical components, syringyl/guaiacyl ratio, Fourier transformed infrared spectroscopy, oxygen/carbon ratio and scanning electron microscopy. Morphological characteristics of nanofibers and films were analyzed by transmission and scanning electron microscopies. Optical parameters studied for the films were the opacity, total color difference and b value. The main challenge to delignification was attributed to the low syringyl/guaiacyl ratio. The different chemical natures of Amazonian and eucalyptus hardwoods greatly affected pretreatments and, consequently, the nanofibrillation and optical quality of the films. Consequences observed for highly purified cellulose starting fibers are: (1) lower diameters for individual nanofiber elements; (2) fewer opaque and colored films produced from nanofibers; (3) a tendency to stabilization of the nanofibrillation process after 20 passes through the grinder. For species whose chemical nature hindered cellulose purification, the increased number of passes through the grinder continuously decreased the opacity.