Impact of the processing method on the properties of castor oil/cellulose acetate polyurethane adhesives for bonding wood

During the last decades, the replacement of the traditionally petro-based raw materials by eco-friendly substances in polyurethane production has become a great challenge. The synthesis of polyurethanes is mainly conducted by means of a selective reaction between active isocyanate and hydroxyl groups, yielding a characteristic segmented chemical structure. In this study, the traditional solvent-based polycondensation reaction for the synthesis of bio-based polyurethane adhesives, involving first cellulose acetate modification with 1,6-hexamethylene diisocyanate and subsequent crosslinking with castor oil, was evaluated and compared with a simpler solvent-free single-step protocol. The impact of the preparation procedure and reagent proportions, on the chemical structure, thermo-rheological behaviour and ultimate adhesion performance were evaluated. The rheological response and adhesion performance of these bio-inspired polyurethanes were also compared to those found in some commercial adhesives. This research highlights the reinforcing and stabilizing effect of cellulose acetate on bio-polyurethane performance. Particularly, the inclusion of cellulose acetate into the adhesive manufacture allowed a reduction in the maximum curing time under room conditions to almost one week, while yielding a higher thermal stability and a fourfold increase in elastic modulus in comparison to the cellulose acetate-free counterpart. Moreover, the proposed solvent-free synthetic route enhanced the microphase mixing of the ensuing bio-based polyurethanes, exhibiting an adhesion performance comparable to the considered commercial benchmarks, but with failure occurring primarily within the substrate material – the preferred locus of failure. ; Funding: This work is part of two research projects (CTQ2014–56038-C3-1R and TEP-1499) sponsored by MINECO-FEDER and Junta de Andalucía programmes, respectively. One of the authors (Adri´an Tenorio-Alfonso) has also received a PhD. Research Grant from “Ministerio de Educaci´on” (FPU13/01114). ...