Several types of sulfur-free lignins were melt compounded in a Werner & Pfleiderer twin screw extruder (ZSK 25) together with polyamide 11 (PA11), a biodegradable polyester (Ecoflex®) and a biodegradable polyesteramide (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™) 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™ lignin. A polyamid 11 compound containing 30 wt.-% abaca lignin gave a Young's modulus of 1 890 MPa, compared to 1 420 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).
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Nitz et al. (2001) studied this question.