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.
Highlights
The dependence of adhesive shear modulus (G) on bondline thickness, known as size effect, was extensively investigated in the last decade [1,4]
Our work investigated the effect of adhesive thickness variation 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
Summary
The dependence of adhesive shear modulus (G) on bondline thickness, known as size effect, was extensively investigated in the last decade [1,4]. Authors demonstrated that changes of adhesive thickness might affect shear modulus, as a consequence of metallic substrate influence on polymeric structure and properties. Shear or effective modulus of these adhesives was usually stabilized for bondline thickness around 200 μm and thicker. In this work we investigated the size effect up 220 μm thickness and shear stress distribution of a bio-based polyurethane adhesive, in which castor oil and Kraft lignin were used as polyol. Kraft lignin is a natural polymer with a complex phenolic structure. It is available at industrial scale as a byproduct of pulp and paper manufacturing [7,9]. Notwithstanding, the powder state of unmodified lignin restricts its application, due to its inherent poor dispersion in many polymer matrices, including polyurethane [14,15]
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