Kinetic evaluation of tobacco stalk waste exposed to alkaline surface treatment under different conditions

Lignocellulosic materials (some natural fibers) have relevant mechanical properties when incorporated in polymeric matrices, being a renewable source to replace synthetic fibers; however, for good adhesion and durability, it is necessary to modify the fibrous surface structure in order to eliminate unwanted compounds. This work aims to evaluate different conditions of alkaline treatment of tobacco stalk waste using 10 and 15 wt% sodium hydroxide (NaOH) during an exposure time of 3 or 5 h. Thermal, chemical, and morphological characterizations as well as kinetic analysis were performed. Through Fourier transform infrared spectroscopy, it was possible to identify the partial removal of compounds such as lignin, hemicellulose, and extractives, increasing the thermal stability, which was proven by thermogravimetric analysis. Furthermore, X-ray diffraction shows a higher crystallinity index from cellulose after the alkaline treatment. Through the kinetic analysis, it is possible to observe that both methods presented similar activation energy values (Ea). In addition, after the alkaline treatment there was a reduction in Ea, the most evident result being observed in the TSW\10\5 sample, which presented an average value of Ea of 223.95 ± 15.79 kJ/mol by the Flynn–Wall–Ozawa method and 218.74 ± 24.69 kJ/mol by the Friedman method. Scanning electron microscopy showed the exposure of the microfibrils and an enhancement of roughness after alkaline treatment. Tobacco stalk wastes has potential application as reinforcing filler in composite materials since surface alkaline modification provides higher fiber/matrix adhesion through the removal of amorphous compounds.