Localization of cellulosic fines in paper via fluorescent labeling

Mathias A Hobisch,Wolfgang Bauer,Rene Eckhart,Stefan Spirk,Sylvia M Bardet,Julie Bossu,Daniel Mandlez

doi:10.1007/s10570-019-02556-0

Abstract

Generated in the pulp and paper production process, cellulosic pulp fines are fibrous cellulosic materials capable of passing a 200 mesh screen. Processed pulp fines remain in the pulp and affect pulp and paper sheet properties. Their specific morphology promotes interactions with fibers and alters sheet properties. Nevertheless, the 3D distribution of fines in paper sheets has not been revealed so far. Localizing fines within a matrix of fibers is challenging since both elements have the same chemical composition and fines have small dimensions. It is required to increase the contrast of pulp fines, while avoiding any alteration of initial fines morphology. In this study, fines were first separated and labeled with Rhodamine B isothiocyanate, a fluorescent dye. Labeled fines were blended with pulp at different concentrations and paper sheets were produced from each mixture. Prior to imaging, pulps and handsheets were characterized by standard pulp and paper tests and compared with references, showing no significant differences between sheets containing labeled and untreated fines. Thus, mechanical and physical tests indicated that no, or only minor alteration of fines properties by the labeling process occurred. We then applied two imaging techniques to detect the labeled pulp fines in the paper network, namely confocal laser scanning microscopy and multiphoton microscopy, visualizing the 3D distribution of fluorescent fines within the fiber network. The results obtained also allowed a differentiation between morphologically different fines showing fiber fragments more attached to single fibers whereas more fibrillar fines concentrate in fiber–fiber joints, thereby strengthening bonding.Graphic abstract

Highlights

Natural fibers have a wide field of applications, for instance in paper and board, composites and insulating materials, textiles or as an energy source
Relevant physical and mechanical properties were compared between paper sheets containing no additional, unlabeled (BSK1 and BSK2) and fluorescent fines (FFBSK1 and FFBSK2)
The interactions of pulp fines in paper sheets were investigated by developing a labeling procedure for these small cellulosic constituents in order to visualize them in a coherent matrix investigating their distribution, form and impact within the sheets

Summary

Introduction

Natural fibers have a wide field of applications, for instance in paper and board, composites and insulating materials, textiles or as an energy source. The suspension passes several processing steps depending on the final application of the paper product. Primary fines are produced during pulping, generated under harsh chemical and mechanical conditions (Mark 1984). Primary fines feature lower bonding ability compared to the more fibrillar secondary fines and affect the optical properties of the resulting papers. The following refining process generates up to 40% highly fibrillar secondary fines (Odabas et al 2016). The secondary fines induce strong swelling and have a significant positive effect on mechanical strength by increasing the density of the final product while reducing air permeability. Primary and secondary fines exhibit a dominating morphology, both contain fiber segments or vice versa fibrillar parts to a lower extent (Mayr et al 2017a). Fines in paper affect several paper properties such as shrinkage potential, sheet density and air permeability as well as mechanical properties (Odabas et al 2016)

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