Green barrier materials from cellulose nano fibers
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Renewable, recyclable, and high performing barrier materials were made from cellulose nano fibers. Various strategies to enhance performance in dry, wet and humid conditions were proposed. These methods included thermal treatment to induce hornification, PAE resin based cross linking, and inclusion of high aspect ratio filler materials to form composites. Results indicated that hornification alone, even though effective in enhancing the barrier properties comes at the cost of severe degradation of mechanical properties. In the second case, where a cross linker was used, lower heating temperature limited the degradation of mechanical properties. Moreover, the new bonds included due to cross linking also modified the mechanical properties of the material and cause significant improvement. In the case of inclusion of filler materials, improvement of mechanical properties due to reinforcing effect was observed, and additionally the improvement in barrier properties was observed due to increased tortuosity of the materials. Furthermore, when the composites were made with cross linker, there was a significant improvement in barrier and mechanical properties as compared to the barrier material made from the pure cellulose nano fibers. In all cases the barrier materials were found to be resistant to degradation by water, as measured by water retention value, and surface contact angle. The resistance to water in the first case was as a result of severe hornification of the material. Whereas in the second and third case the cross linking and concomitant limited hornification played a significant role in water resistance. In addition to the three methods to improve barrier properties, the use of nano fibers made from cellulose II was also studied. Different stages of fibrillation of the starting cellulose pulps were studied and the fibers and films made from them were characterized in detail. Results from this study indicated that fibers made from cellulose II pulp are much harder to fibrillate as compared to cellulose I fibers. Moreover, due to fibril aggregation it is harder to form nano fibers from cellulose II. Even though from the perspective of better inter and intra fibril bonding cellulose II might be favorable over cellulose I, significant work in the formation of nano fibers from cellulose II is required before they can be used for making barrier materials.