Development of polyurethane formulations based on cellulose acetate and castor oil

Abstract

Given the emerging environmental concern prevailing in the industrial production, this PhD Thesis has been devoted to the development of sustainable alternatives to the traditional petroleum-based polyurethane adhesives, which are also generally associated with the release of toxic solvents during curing. Therefore, the development of bio-sourced polyurethane adhesives with a remarkable renewable content was proposed in this work by using natural polyols, viz. cellulose acetate and castor oil, which were subjected to the reaction with aromatic or aliphatic diisocyanates -4,4'-methylenediphenyl (MDI) and 1,6-hexamethylene (HMDI) diisocyanates, respectively- through a two-step polycondensation reaction. More specifically, cellulose acetate was first functionalized to a certain degree with the diisocyanates at room temperature and under inert atmosphere, using a reaction solvent (toluene) and a catalyst (triethylamine). Subsequently, the resulting NCO-modified biopolymer was blended with castor oil in specific weight proportions, yielding bio-based polyurethanes which were left for curing under room conditions upon application. Furthermore, the chemical structure and properties of the thus prepared polyurethane adhesives were assessed by means of diverse experimental techniques. First, Fourier transform infrared spectroscopic analysis was conducted to characterize the polyurethanes' chemical structure as well as to monitor their curing process. Thermogravimetric analysis, along with standard and temperature modulated differential scanning calorimetric tests were carried to analyse their thermal transitions and stability. Finally, the rheological behaviour and adhesion performance of the fully cured bio-sourced adhesives were studied by carrying out dynamic oscillatory torsional measurements and standardized mechanical tests to determine shear, peeling and flexural strengths and further compared with those found in commercial benchmarks. By dint of these methods, the influence of the diisocyanate -whether comprising aromatic or aliphatic backbones- and the reagents proportions were studied. In addition, the impact of the modification of the previously described two-step protocol by a simpler and more sustainable solvent free straight-through synthetic pathway on the ultimate polyurethane properties was evaluated. Conventional two-step procedure was successfully applied to synthesize bio-inspired polyurethane adhesives from mixtures of NCO-functionalized cellulose acetate and castor oil. In all cases, thermal and spectroscopic characterizations confirmed the typical microphase segregated chemical structure, exhibiting glass transition temperatures attributed to both soft and hard segment domains. Moreover, a rise in the NCO:OH molar ratio was proven to favour the thermodynamic phase compatibility, leading to higher values of both soft domain glass transition temperatures and viscoelastic functions. Optimum adhesion properties on wood were achieved when using a 1:1 castor oil/HMDI-functionalized cellulose acetate weight ratio, associated to a NCO:OH molar ratio of 1.87, slightly higher than the 1.45 corresponding to their more reactive aromatic diisocyanate-based counterparts. Synthesized bio-based polyurethane developed adhesion strengths comparable or even superior to the studied benchmarks on wood and steel substrates, while failing in bonding non-polar polymeric surfaces, such as polyethylene, or at high temperatures (100 ºC). The implementation of the proposed single-step procedure highlighted not only its feasibility for a more sustainable preparation of these polyurethane adhesives, but also the resulting improvement in their adhesion performance. The reinforcing effect of the cellulose derivative was also confirmed using this straightforward protocol. Finally, the utilization of cellulose acetate revealed an increase of the polyurethane thermal stability, while reducing the time scale of the moisture-induced curing process.