Cellulose based nanofibers as reinforcement of polyurethanes

N. E. MARCOVICH; M. A. MOSIEWICKI; M. I. ARANGUEN; M. L. AUAD; T. RICHARDSON

Santiago, Chile

Conferencia; X International Conference on Frontiers of Polymers and Advanced Materials, ICFPAM 2010; 2009

Polyurethanes (PU) can be produced to display a wide range of properties from those of flexible elastomers (including segmented PU that exhibit shape memory properties) to rigid crosslinked polymers, which can be used to fit in different applications. Being polar polymers, they can be successfully reinforced using polar fibers. Cellulose fibers are between the most promising natural, abundant and high modulus fibers [1], in particular through the use of nanocrystals, whose size would minimize the effect of fiber defects. In this work, the cellulose nano crystals obtained from microcrystalline cellulose (MCC) were incorporated into a polar organic solvent, dimethylformamide (DMF) and ultrasonicated to obtain a stable suspension [2]. The suspension was an effective medium for incorporating this reinforcement into a polyol-isocyanate reactive mixture or into a thermoplastic polyurethane solution, to produce polyurethane composite films by casting. An important concept in the field of nano-composites related with the improvement in properties (mechanical, thermal, rheological, etc.) is the percolation threshold, a characteristic filler concentration at which a dramatic step variation in properties occurs, because of the formation of a 3-D structure of touching nano-fillers. Thus, the experimental data for the equilibrium storage modulus (measured at low frequencies) of filled liquid mixtures (unreacted PU mixture or melted thermoplastic PU) were modeled as a function of the cellulose crystals concentration according to a percolation expression as follows [3]:[1], in particular through the use of nanocrystals, whose size would minimize the effect of fiber defects. In this work, the cellulose nano crystals obtained from microcrystalline cellulose (MCC) were incorporated into a polar organic solvent, dimethylformamide (DMF) and ultrasonicated to obtain a stable suspension [2]. The suspension was an effective medium for incorporating this reinforcement into a polyol-isocyanate reactive mixture or into a thermoplastic polyurethane solution, to produce polyurethane composite films by casting. An important concept in the field of nano-composites related with the improvement in properties (mechanical, thermal, rheological, etc.) is the percolation threshold, a characteristic filler concentration at which a dramatic step variation in properties occurs, because of the formation of a 3-D structure of touching nano-fillers. Thus, the experimental data for the equilibrium storage modulus (measured at low frequencies) of filled liquid mixtures (unreacted PU mixture or melted thermoplastic PU) were modeled as a function of the cellulose crystals concentration according to a percolation expression as follows [3]: ! G " #(m$mcG" )%G "" #(m$mcG" )%G " where m is the weight percent of crystals, mCG is m at the threshold and âG’ is an exponent, and the equation can be applied only near the percolation threshold. The critical concentrations were reached at 0.88 and 1.45 wt% nanocellulose for the reinforced polyolisocyanate mixture and the thermoplastic PU, respectively. Mechanical and thermal properties resulted also reasonably affected by nano-cellulose addition, since covalent bonds or strong physical filler-matrix interactions were developed in the nano-reinforced PU films, crosslinked or thermoplastic, respectively. The percolation effect appeared to be lost in the solid films, because of the reduced celullose-cellulose interactions.m is the weight percent of crystals, mCG is m at the threshold and âG’ is an exponent, and the equation can be applied only near the percolation threshold. The critical concentrations were reached at 0.88 and 1.45 wt% nanocellulose for the reinforced polyolisocyanate mixture and the thermoplastic PU, respectively. Mechanical and thermal properties resulted also reasonably affected by nano-cellulose addition, since covalent bonds or strong physical filler-matrix interactions were developed in the nano-reinforced PU films, crosslinked or thermoplastic, respectively. The percolation effect appeared to be lost in the solid films, because of the reduced celullose-cellulose interactions.