Value-added derivatives of soda lignin from alfa grass (Stipa tenacissima). I. Modification and characterization

Alfa grass (Stipa tenacissima) is the only raw material for manufacturing paper pulp in Algeria, Tunisia, and Morocco. Soda lignin (SL) is obtained from black liquor (BL), a byproduct of manufacturing paper from alfa grass. This work was aimed at modifying SL and using it as a polymer lubricant. Modified SL was obtained in two steps: the precipitation of BL at pH 4 with sulfuric acid (2N) followed by the modification of SL by esterification using stearic anhydride (SA) dissolved in dioxane at 50°C for 8 h. The effects of different contents of SA on the hydrophobic properties and heat stability of modified SL were evaluated with contact angle analysis and thermogravimetric analysis. A commercial stearate used in plastic processing was chosen as the reference. Its thermal properties were ascertained with thermogravimetry, and its behavior was similar to that of the modified lignins. A structural comparison study was performed with Fourier transform infrared (FTIR) spectroscopy and solid-state ¹³C-NMR. The FTIR and ¹³C-NMR results showed that there were significant differences between the main structures of SL samples reacted with 10, 30, or 100% (w/w) SA. The main part of the work consisted of modifying SL, evaluating the properties of the obtained products with respect to their use as lubricants, and comparing them to industrial lubricants.