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.

ABSTRACT Polyol structure can be affected by the catalyst used for its synthesis and may affect the curing kinetics and mechanical strength of the final polyurethane (PU). This work aimed to study the influence of catalyst concentration on the structure of bio-based polyols and on the final properties of the polyurethane adhesive obtained from these polyols. Polyols were investigated by Fourier transform infrared (FTIR), gel permeation chromatography (GPC) and hydroxyl index analysis, with the purpose of investigating the polyols chemical state after the synthesis. Results suggest that higher catalyst concentration led to polyols with higher hydroxyl index and other differences in the polyols structure. The influence of the catalysts concentration on the cure kinetics of the polyurethanes was verified using the Fourier transform infrared (FTIR). In addition, peel strength of PU adhesives was obtained by increasing catalyst concentration on polyol synthesis.

Low surface energy and poor adhesion are well-known characteristics of polypropylene (PP). Surface treatments such as plasma, corona, and laser are usually applied to overcome these limitations. However, current studies highlight the incorporation of hydrophilic or amphiphilic polymers into hydrophobic low-surface-energy polymers as an alternative for increasing surface energy and thus improving adhesion. Lignin could be a promising amphiphilic polymer for use in increasing surface energy. In this work, PP/kraft lignin composites were obtained by incorporating up to 5 wt% of kraft lignin (KL) into a PP matrix. Corona treatment was applied to pristine PP and composites surfaces. Contact angle measurements and peeling tests were carried out to investigate the effects of KL incorporation and corona treatment on the surface energy and the mechanical strength of adhesion. Differential scanning calorimetry (DSC) was used to evaluate the PP’s crystallinity index and recrystallization temperature and to dismiss their effects on the surface energy changes. Scanning electron microscopy (SEM) was applied to investigate the lignin dispersion. The results show that KL incorporation has potential as a method to improve the surface energy of PP, improve its poor adhesion, and enhance the effects of corona treatment.

Characterization of polymeric materials mechanical behavior requires some previous knowledge about their structure, which allows the choice of more appropriated models and methods. Polymeric materials, below their glass transition temperature (Tg), may be handled as perfect elastic solids, allowing the use of classic mechanics to characterize their behavior. Polymers above their Tg present a viscous contribution to mechanical behavior, which has to be taken into consideration by modeling it. Adhesively bonded joint, joining of different materials using a polymer as adhesive, adds to the mentioned requirements more parameters, such as surface roughness, adhesive thickness and different types of contributions to adhesively bonded joint strength. This work has the purpose of presenting a mechanical behavior characterization of adhesive bonded joints, concerning their average stress at rupture. A modified Arcan´s device was used to obtain the average stress at rupture under different angles or loading conditions, such as pure shear 0°, pure tensile strength 90° and combined conditions. The experimental results were applied to a theoretical model, which takes into consideration the hydrostatic contribution to the mechanical behavior, called Drucker-Prager Model, which was initially developed to characterize soils.