Influence of lignin type on the mechanical properties of lignin based compounds

Several types of sulfur-free lignins were melt compounded in a Werner & Pfleiferer twin screw extruder (ZSK 25) together with polyamide 11 (PA11), a bio-degradable polyester (Ecoflex®) and a biodegradable poly-esteramide (BAK®). Sulfur-free lignins, precipitated from black liquors of alkaline pulping of annual plants, like sisal and abaca, were compared to a sulfur-free lignin (Alcell TM ) obtained by organosolv wood pulping. Lignin content and lignin type were varied systematically in order to examine stress/strain characteristics and impact behavior. Compounds with lignins from annual plants showed similar or better mechanical properties in comparison to compounds with Alcell TM lignin. A polyamid 11 compound containing 30 wt.-% abaca lignin gave a Young's modulus of 1890 MPa, compared to 1420 MPa for PA11 and a 28% increase of yield stress. The morphology of the lignin compounds was investigated by means of environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM).