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Kraft lignin-containing polyurethane adhesives: the role of hydroxypropylation on thermomechanical properties

ABSTRACT During the last years, several efforts were dedicated to the development of lignin-based polyurethane adhesives, in which phase separation and brittleness were the most significant reported limitations. Hydroxypropylation emerged as a feasible method to overcome their restricted performance, since this method generates lignin with higher reactivity and branched molecular structure, at different extends. However, the effect of lignin hydroxypropylation on properties of lignin-based PU adhesives remains unclear. In this work, mixtures of lignins and castor oil were used as renewable raw material polyol in the PU synthesis. First, a pristine kraft lignin (KL) was used as reference, followed by incorporation of two hydroxypropylated lignins: highT_HPL – obtained under high temperature and pressure (150°C, 18 bar) and lowT_HPL – synthesized under milder conditions (40°C, atm). Temperature modulated optical refractometry (TMOR) revealed a slower curing process for than highT_HPL-containing PU. LowT_HPL lignin improved the PU modulus of elasticity and ultimate tensile stress. KL and lowT_HPL incorporation resulted in higher strength of wooden adhesively bonded joints (single lap shear tests – 4.5 MPa). The results elucidated the main effects of hydropropylation on PU and its application on adhesively bonded joints, supporting the development of further lignin-based adhesives.