Evaluation of the distribution of fluorescent cellulose nanofibrils in paper

Abstract

Many researchers have reported that cellulose nanofibrils (CNFs) are an effective paper strength enhancer. However, due to their high viscosity, the dispersibility of CNFs in pulp stock is low unless appropriate measures are taken. Such low dispersibility results in a decreased paper strength due to the uneven distribution of the fibers in the sheet. Therefore, the first step for the effective use of CNFs as strength enhancer is the development of an analytical method to evaluate the distribution of CNFs in paper products. In this study, we prepared fluorescent CNFs (F-CNFs) using a di-sulfonic fluorescent whitening agent (D-FWA). The CNFs were produced from bleached hardwood kraft pulp via refining and micro-grinding processes. Then, F-CNFs were obtained by D-FWA adsorption. The D-FWA dosage was determined by measuring the unadsorbed D-FWA in the supernatant of a CNF slurry. After the adsorption process, handsheets were made by adding the F-CNFs to stock made from both softwood and hardwood bleached kraft pulps. The CIE whiteness, ISO brightness, and fluorescence index of the sheets were measured, and the distribution of the F-CNFs in the sheets was observed using a confocal laser microscope (CLSM). According to the D-FWA adsorption curve, its dosage for manufacturing the F-CNFs was 0.5% using oven-dried CNFs. Then, as the F-CNF dosage increased, the CIE whiteness and ISO brightness of the sheets increased, indicating that the amount of retained F-CNFs in the sheets also increased. Additionally, the distribution of F-CNFs in the sheets was observed using CLSM images obtained under a UV irradiation. Based on the results, we concluded that F-CNFs can be effectively used to analyze the distribution of CNFs in paper products. ? 2019 Korean Technical Assoc. of the Pulp and Paper Industry. All rights reserved.