Search

[Display omitted] • A renewable adhesive was developed from hardwood lignin biomass for joining steel substrates. • Lignin content in the compositions were varied, to alter toughness, stiffness, and surface energy. • Highest adhesive strength of 13.1 MPa was measured for an optimal composition. • This study demonstrates the viability of lignin as a valuable building block for adhesives. Adhesive bonding of metals has become increasingly relevant in recent years due to the demand for reducing weight and improving performance in structural applications such as automobiles and aerospace. We developed renewable thermoplastic adhesives from technical organosolv lignin isolated from hardwood biomass and acrylonitrile butadiene co-polymer rubber (NBR) for joining steel substrates. NBR33, NBR41 and NBR51 with acrylonitrile molar ratios of 33, 41 and 51%, respectively, were blended with lignin to form two-phase thermoplastic adhesives, and their adhesion, viscoelastic and surface properties were measured. Lignin content in the compositions were varied, ranging from 40% to 80% (w/w), to alter toughness, stiffness, and surface energy characteristics of the material. Better interaction or reactivity between the lignin and NBR phases was observed with greater nitrile content in NBR, leading to greater modulus and stiffness of the adhesive. Simultaneously, increasing the proportion of lignin reduced toughness and improved stiffness, with the highest adhesive strength of 13.1 MPa measured in a 60% lignin loading ratio with NBR51. Surface energy measurements revealed that total surface energy (sum of polar and dispersive surface energy) raised with lignin loading, suggesting that both surface energy and matrix strength play a critical role in the adhesive properties of the synthesized materials. A finite element-based cohesive zone model (CZM) was developed and implemented to study the failure strength of the adhesively bonded joint. This study demonstrates the viability of lignin as a valuable building block for adhesives, not only due to its inherent chemical structure and rigidity, but also for its surface energy characteristics. [ABSTRACT FROM AUTHOR]

This work proposes an approach to minimize surface energy deactivation of plasma-treated metal and carbon fiber-reinforced polymer (CFRP) surfaces by vacuum sealing. Plasma treatments enhance adhesive wettability on post-treated surfaces for adhesive joints, but the treated surfaces deactivate quickly in air. The surface energy of aluminum alloy AA6061 and carbon fiber-reinforced polymer-polyamide (CFRP-PA66) optimally treated by a blown air plasma instrument returns to the original surface energy within 1 h. Vacuum sealing of AA6061 and CFRP-PA66 reduced the surface energy deactivation over 7 days by at least 230 times and 970 times compared to in air. Double Cantilever Beam (DCB) tests performed on adhesively-bonded AA6061/CFRP-PA66 joints showed that the total energy release and energy dissipation before failure of plasma-treated and vacuum-sealed materials was up to 60% more than plasma-treated materials without vacuum sealing and up to 125% more than non-plasma-treated materials. • Vacuum sealing significantly slowed down the deactivation of plasma-optimized surface energy of AA6061 and CFRP-PA66. • Vacuum sealing preserved hydrogen-bonded carbonyl groups on the plasma-treated CFRP-PA66 surface. • Vacuum sealing protected over-oxidation of carbon to carbonate on AA6061 surface after the plasma treatment. • In the interface between adherend surfaces and adhesive, hydroxyl groups reacted with dicyandiamide to form amide bonds. • Vacuum sealing maintained the plasma-enhanced Mode I fracture energy of adhesively-bonded AA6061-CFRP-PA66 dissimilar joints. [ABSTRACT FROM AUTHOR]