Bio-based epoxy resins with low molecular weight kraft lignin and pyrogallol

A low-molecular lignin fraction (L) was extracted for the preparation of bio-based epoxy resins. Various compositions with the “green” 1,3-glycerol diglycidyl ether (1) and the co-component pyrogallol (2) were tested. In a first series of experiments, thermosets consisting of 1 and L, were studied with respect to variable lignin contents between 20 and 50%. The best thermoset has a tensile strength of 37 MPa and a Young’s modulus of 2.2 GPa at 40% lignin input. Secondly, lignin-free compositions of 1 and 2 were prepared. For a molar fraction of the functional groups (nOH nEpoxy -1) of 130%, the tensile strength could be enhanced to 93 MPa and the modulus reached 3.7 GPa. Finally, systems with all three components were examined. The best mechanical performance of the corresponding neat thermosets was reached at nOH nEpoxy -1 of 130%. Tensile strength decreases slightly to 86 MPa and Young’s modulus remains nearly unchanged at 3.2 GPa. Two resin compositions, L+1 and L+1+2, were tested for the preparation of unidirectional composites reinforced with man-made cellulosic fibers (50% by vol.). The bending strength was 208 MPa in combination with a bending modulus of 12.5 GPa.