The role of bondline thickness on mechanical properties of bio-based polyurethane

Abstract

Bio-based adhesives have been largely applied on several applications in many industrial areas, including
automotive and laminated food packaging. Polyurethanes (PU) occupy a leading position among the most
used bio-based polymers due to their large range of properties and application versatility. Recent works reported
that PU obtained from castor oil and Kraft lignin as polyol developed high mechanical properties and
blend miscibility. In spite of its well-known mechanical behavior, additional investigations might support the
application of this bio-based PU as adhesive. In this context, our work investigated the effect of adhesive
(bondline) thickness variation (size effect) on shear modulus of a bio-based polyurethane under finite deformation
and related shear stress distribution. Polyurethane was obtained using a blend of Kraft lignin and castor
oil as polyol (20 wt% of Kraft lignin) and applied as adhesive for metal bonding, in which adhesive thickness
changed from tens of micrometers up to 220 micrometers. Steel plates were used as substrates and adhesive
shear modulus was evaluated using substrates with large thickness (4.76 mm), which minimized the effect
of substrate deformation on shear modulus measurement. Single lap joints were uniaxial loaded in order
to generate the shear stress on adhesive bondline. Additionally, shear modulus changes were simulated by
finite element analysis (FEA), analytical models and compared to experimental data. Results revealed the
size effect and pointed to high shear modulus of the PU bondline, justifying its use as an adhesive for structural
applications.
Keywords: bio-based polyurethane, Kraft lignin, finite element analysis, adhesive.