Spectroscopic study of acetylated kraft pulp fibers

Abstract

Abstract The unbleached and bleached kraft pulp samples were partial acetylated by heterogenous acetylation method using acetic anhydride in the presence of sulfuric acid as a catalyst, to modify their physical properties. The overall degree of acetylation varied from 0 to 1.04 and 0.82 by changing the time of reaction up to 24 h. The characterization of acetylated kraft pulp samples, by solid-state CP/MAS 13 C nuclear magnetic resonance, Fourier transform infrared spectroscopy and X-ray diffraction, showed the formation of a heterogeneous structure. All three investigation methods give consistent results in qualitative terms. The NMR results confirm the successful acetylation, indicating the regio-nonselective reactivity of the three kinds of OH groups. This may be due to the characteristic reaction that proceeds in a very thin layer between the acetylated and non-acetylated regions in each microfibril. The FT-IR data further provided a clear evidence for successful acetylation by decreasing of the bands assigned to the O H stretching or deformation vibrations, and increasing of the bands assigned to the C O and C O in acetyl and carboxylic groups. Furthermore, as the acetylation reaction proceeds, was identified a new band at 3032 cm −1 assigned to the CH 3 asymmetric stretching belonging to acetyl groups. The band from 2945 cm −1 , assigned to the >CH 2 asymmetric stretching, was shifted to higher wavenumbers and both bands increased in intensity with increasing acetylation time. These modifications occur because the hydroxyl groups from the anhydroglucose are partially substituted by acetyl groups in the reaction. X-ray diffraction profiles of the acetylated samples showed similar crystalline patterns with that of unmodified cellulose, but the changes of band widths and intensities indicated that acetylation proceeded from the surface of microfibrils towards the core of each microfibril.