Development of hierarchical cellulosic reinforcement for polymer composites

Abstract: Cellulose is an environmentally friendly material which is obtainable in vast quantities, since it is present in every plant. Cellulosic fibers are commercially found in two forms: natural (flax, hemp, cotton, sisal, wood, etc.) and regenerated cellulose fibers (RCF). The biodegradability, the morphological and mechanical properties make these fibers a good alternative to the synthetic reinforcement (e.g. glass fibers). However, as all other cellulosic fibers these materials also have similar drawbacks, such as sensitivity to moisture and poor adhesion with polymers. The first part of this work concerned a heterogeneous modification of cellulose nanocrystals (CNC) by using esterification and amidification to attach long aliphatic chains. Long-chain aliphatic acid chlorides and amines were used as grafting reagents. Surface grafting with acyl chains was confirmed by Fourier-transform infrared spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy. It was found that the degree of substitution (DS) of the surface is highly dependent on the method of modification. The contact angle measurement showed that after modification, the surface of CNC was found to be hydrophobic. The second part was devoted to modification of RCF by CNC using Isocyanatopropyl triethoxysilane as coupling agent. Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy and X-ray diffraction analysis were performed to verify the degree of modification. The mechanical properties of the unmodified and modified fibers were analyzed using fiber bundle tensile static and loading–unloading tests. To show the effect of cellulose whiskers grafting on the Cordenka fibers, epoxy based composites were manufactured and tensile tests done on transverse uni-directional specimens. It was found that the mechanical properties were significantly increased by fiber modification and addition of the nano-phase into composite reinforced with micro- sized fibers.