Properties of phosphorylated cellulose nanofiber dispersions under various conditions

Abstract

The effects of pH, inorganic ion concentration, and dephosphorylation by hydrolysis on the transparency and viscosity of phosphorylated cellulose nanofibers (P-CNF) with different phosphate group contents and degrees of fibrillation were studied in order to boost its use as a rheology modifier. From the neutralization titration curve of P-CNF dispersions, acidity values of pK_a1 = 3.1 and pK_a2 = 8.3 were determined. These values were almost constant regardless of the amount of phosphate groups. The P-CNF dispersion maintained high transparency and viscosity in the pH range of 3–11 owing to a wide range of acidity values and the buffer capacity of the phosphate group. The viscosity of P-CNF dispersions was influenced by the amount of phosphate groups and the degree of fibrillation, which also affected the viscosity behavior of the dispersion when inorganic salts were added. The long-term storage stability of phosphate groups introduced on P-CNF was also examined by accelerated (heating) test. The hydrolysis reaction of P-CNFs took several days to reduce the phosphorous content to one-tenth of the initial content at 80 °C. Kinetic analysis showed that dephosphorylation proceeded as a first order reaction, as found for other phosphorylated esters. As dephosphorylation proceeded, the transparency and viscosity of the P-CNF dispersion decreased because P-CNFs tended to aggregate through hydrogen bonding, which also seemed to suppress dephosphorylation. Furthermore, the activation energy of deesterification obtained was 130.0 kJ/mol. The decrease in the ratio of phosphate groups was highest when the degree of anion neutralization of the phosphate groups was 50%.