Effect of recycling number on the fluorescence reduction of fluorescent whitening agents in white wastepapers

Abstract

White wastepaper is a very important resource in the Korean paper industry. However, its use is limited because it contains fluorescent whitening agents (FWAs). To improve the use of recycled white wastepaper, FWAs should be removed. The first step in eliminating FWAs from wastepaper is to analyze the behavior of FWAs from wastepaper during the recycling process. In this study, the effect of recycling on FWA detachment from white wastepaper and the distribution change of FWAs in the sheet was investigated. Disulfo-, tetrasulfo- and hexasulfo-FWAs commonly found in paper were prepared and recycled many times through a laboratory process. After each round of recycling, the optical properties and fluorescence index of the sheet were measured, and the fluorescence reduction was calculated as an indicator, which represents the FWA detachment from wastepaper, using fluorescence indices before and after recycling. The FWA distribution in the paper cross section was observed using a CLSM. Disulfo-FWA exhibited a lower fluorescence reduction and even distribution in the paper cross section; however, surface-treating FWAs, including tetrasulfo-FWA, and hexasulfo-FWA, extensively de-sorbed from the paper during the recycling process. The surface-treating FWAs showed similar distributions in paper cross section after the recycling because of their re-adsorption on the fibers. Therefore, surface-treating FWAs can be detached easily compared with internal FWAs, and some detached FWAs can re-adsorb on cellulosic fibers during the recycling process. ? 2019 Korean Technical Assoc. of the Pulp and Paper Industry. All rights reserved.