Use of kraft lignin as filler for starch films

Films containing 0-30% of commercial kraft lignin mixed with wheat starch were obtained by extrusion followed by thermal moulding. The influence of lignin content on the him mechanical properties was evaluated by stress-strain tests run at two ambient relative humidity levels. At 58% relative humidity and up to 20% lignin, a very slight increase of elongation and stress at break was observed. At 71% relative humidity and up to 30% lignin, the film resistance to elongation significantly decreased, compared to the control starch film. Dissolution tests and water sorption isotherms showed that lignin actually reduces the overall water affinity of the films. These results are consistent with the hypothesis of a two-phase material consisting of a hydrophilic starch matrix filled with hydrophobic lignin aggregates, as confirmed by microscopic observations. Casting experiments showed that the lower molecular weight fraction of kraft lignin may be responsible for a slight plastification of starch. (C) 1998 Elsevier Science Limited. All rights reserved.