Environmentally friendly polymer composites prepared with cellulose nanocrystals
Use of petroleum-based chemistry for polymer production is increasingly challenged because of its undeniable environmental consequences and the diminishing availability of raw materials. Therefore, we have focused on using more ecofriendly and more widely available renewable resources to obtain commercially competitive polymers. In particular, we are interested in the use of vegetable oils because of their relatively low cost and their potential application as reactants after adequate chemical modification.1
We decided to work with polyurethanes (PUs) because of the broad variety of properties that can be achieved. PUs can be flexible or rigid, solid, and partially or fully foamed, depending on the chemicals employed, initial reactant ratios, and reaction conditions. Several authors have considered replacing at least one of the main components—usually an isocyanate and a polyol—with a bio-based counterpart. In particular, chemical modifications of vegetable oils incorporating hydroxyl groups to obtain a bio-based polyol have been reported.2–4 Additionally, it is well-established that the mechanical properties (especially stiffness) of polymer composites can be improved by incorporating microsized fibers of high rigidity. In the past few years, attention has shifted to use of nanosized particles or fibers in the hope of improving stiffness at much lower reinforcement concentrations. Most studies have focused on use of petroleum-based polymers and synthetic or inorganic fillers, and much less research has been done on bio-based materials.5,6
We obtained a polyalcohol by alcoholysis of castor oil, which we used to produce rigid PUs. We applied the same polyol to prepare a lightly foamed material and a solid (unfoamed) compound. We also incorporated cellulose nanocrystals obtained from acid hydrolysis of microcrystalline cellulose and analyzed their effect on the solid PU properties. Analysis of the rheological behavior of liquid suspensions of nanocellulose in polyol showed that, at concentrations as low as 0.5% by weight (wt%), these suspensions display solid-like responses. We also observed the usual dispersion problems at higher concentrations of nanocrystals.3 The glass-rubber transition moved to higher temperatures upon incorporation of minimal quantities of nanocrystals (as a result of their strong interaction with the matrix). The materials' stiffness increased significantly from 479.5 (unfilled PU) to 682.9MPa (for the 1wt% composite), while the partially foamed material exhibited a lower modulus (292.8MPa) compared to that of unfoamed PU.3
Figures 1 and 2 show surface micrographs corresponding to nonreinforced solid PU and a nanocomposite containing 1wt% of nanocellulose, respectively. In the composite, the roughness of the fracture surface increases as the nanocrystals deviate from the path of the advancing crack. Otherwise, the fracture surface of the unfilled, lightly foamed PU (see Figure 3) shows isolated bubbles. This correlates with the lower density and rigidity of this material.5
Polymer production from renewable resources—as an alternative to synthetic materials—and use of rigid nanofibers to improve composite properties appear very promising. Our results show that we can successfully use chemically modified castor oil as one of the main reactants for preparation of PUs that are stiff enough to be viable in real applications. Incorporation of minimal quantities of nanocellulose in the formulation of PU composites results in improved mechanical performance. Our next step will be to study these new materials as foams or low-density composites to obtain performances comparable to those of present commercial formulations.
- S. N. Khot, J. J. Lascala, E. Can, S. S. Morye, G. I. Williams, G. R. Palmese, S. H. Kusefoglu and R. P. Wool, Development and applications of triglyceride-based polymers and composites, J. Appl. Polym. Sci. 82 (3), pp. 703-723, 2001.
- Y. H. Hu, Y. Gao, D. N. Wang, C. P. Hu, S. Zu, L. Vanoverloop and D. Randall, Rigid polyurethane foam prepared from a rape seed oil based polyol, J. Appl. Polym. Sci. 84, pp. 591-597, 2002.
- V. M. Wik, M. I. Aranguren and M. A. Mosiewicki, Castor oil-based polyurethanes containing cellulose nanocrystals, Polym. Eng. Sci.. In press.
- M. A. Mosiewicki, U. Casado, N. E. Marcovich and M. I. Aranguren, Polyurethanes from tung oil: polymer characterization and composites, Polym. Eng. Sci. 49, pp. 685-692, 2009.
- N. E. Marcovic, M. L. Auad, N. E. Bellesi, S. R. Nutt and M. I. Aranguren, Cellulose micro/nanocrystals reinforced polyurethane, J. Mater. Res. 21, pp. 870-881, 2006.
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