A systematic study for the structures and properties of phosphorylated pulp fibers prepared under various conditions

A bleached softwood kraft pulp was phosphorylated with (NH4)2HPO4 and urea at 150 ℃ for 0‒40 min, and the structures and properties of the resulting phosphorylated pulps were systematically investigated for the first time in terms of reaction time and the amount of (NH4)2HPO4 added. The phosphorous and weak acid group contents, and the weight recovery ratio increased with increasing reaction time, and were 1.9 and 4.5 mmol/g, and 114%, respectively, when the reaction time was 20 min. Therefore, numerous phosphate ester and weak acid groups were introduced into the pulp, maintaining its fibrous morphology and cellulose I crystal structure. It was found that almost all the ammonium phosphate groups in the phosphorylated pulp behaved as weak acids. The solid-state carbon-13 nuclear magnetic resonance (13C-NMR) spectrum of the phosphorylated pulp showed that neither carboxy nor carbamate groups were formed in the phosphorylated pulp; only phosphate ester groups were introduced into the pulp under the conditions used in the present study. The X-ray photoelectron spectra of the phosphorylated pulp surfaces suggested that ammonium phosphate groups were introduced into the pulp by phosphorylation. A longer reaction time or a greater amount of (NH4)2HPO4 added during phosphorylation resulted in lower water swelling behavior, indicating that some intrafiber and/or interfiber crosslinking occurred in the pulp under these conditions. The results obtained in the present study show that phosphorylated pulp fibers are suitable for the preparation of new functional cellulosic sheets and materials, in which large amounts of weak acid groups can be used as scaffolds for diverse ion-exchange sites.Graphical abstract