Films containing 0-30% of commercial kraft lignin mixed with wheat sta
rch were obtained by extrusion followed by thermal moulding. The influ
ence 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 over
all 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 mic
roscopic observations. Casting experiments showed that the lower molec
ular weight fraction of kraft lignin may be responsible for a slight p
lastification of starch. (C) 1998 Elsevier Science Limited. All rights
reserved.