Experimental and theoretical investigations of lignin-urea-formaldehyde wood adhesive: Density functional theory analysis

In view of its chemical structure, lignin has recently become an interesting candidate for various applications. This work serves to optimize the isolation process of lignin from sugar industry by-product (bagasse can, molasses beet) applying alkali treatment. For this purpose, the effect of alkali and sulfuric acid concentrations have been studied ranging from 10% (w/v) to 20% (w/v) and 1.5 M–3 M, respectively. Moreover, the isolated lignins were characterized by different complementary analysis such as FT-IR and TGA/DTG. The aim behind this work is to reduce formaldehyde emissions and improving mechanical as well as physical properties of wood adhesive. Lignin was added with different ratios, as partial substitution, in urea-formaldehyde (UF) (5:95; 10:90; 13:87 and 15:85, w: w) and compared to commercial control UF (0:100). Furthermore, a theoretical calculation, using the density functional theory (DFT)/B3LYP method validated that the coniferyl alcohol (G), sinapyl alcohol (S) and paracoumaryl alcohol (H) can suck up on the energy of the orbital borders, electronic affinity, electronic chemical potential and thermodynamic properties. The results obtained showed that the efficiency of these molecules is influenced with different degrees by several descriptors. It was also found that the compounds G and S out of H studied were more efficient within the density functional theory (DFT) at the B3LYP/6–31 (d,p) computational level.