The chromophore release and identification method isolates well-defined chromophoric substances from different cellulosic matrices, such as highly bleached pulps, cotton linters, bacterial cellulose, viscose or lyocell fibers, and cellulose acetates. The chromophores are present only in extremely low (ppm to ppb) concentrations. The concept of primary and secondary chromophores is introduced, with primary chromophores arising only from the polysaccharides inherent to cellulosic materials. Secondary chromophores also include atoms from the chemicals used to process the cellulose. Most primary chromophores belong to one of three compound classes: hydroxy-[1,4]-benzoquinones, hydroxy-[1,4]-naphthoquinones, and hydroxyacetophenones. Among them, three individual compounds dominate: 2,5-dihydroxy-[1,4]-benzoquinone, 5,8-hydroxy-[1,4]-naphthoquinone, and 2,5-dihydroxyacetophenones, amounting to more than 80 % of the total isolated chromophores in most cases. In lignin-free cellulosics, these three compounds can thus be regarded as key chromophores. The prevalence of these molecules is due to both exceptionally strong resonance stabilization, as reflected in delocalized double bonds, and their ready reformation from carbohydrate degradation products by recondensation reactions. The findings that (a) most chromophores in lignin-free cellulosic materials belong to only three compound classes and that (b) three chromophore compounds make up the bulk of the chromophore mixtures are foundational for future bleaching research: Based on this knowledge, specific searches for optimized bleaching conditions can now concentrate on these compounds and still cover the vast majority of chromophores.

The swelling behavior of pulp fibers has a significant influence on process and product properties. The water uptake of fibers is controlled by a different mechanism. While fiber charge is a driving factor for swelling, the swelling is hindered by the solid structure of the fiber wall. In the case of the fines fraction of pulps, this structure is broken to some extent and the fines are able to swell two to three times more compared to fibers. Thus fines are an important factor regarding the swelling behavior and water retention of pulps, although, at least for chemical pulp, their mass fraction is only between 4 and 15%. For this reason, it is of interest to investigate not just the swelling behavior of pulps, but also of the fiber and fines fractions separately. Swelling is often characterized using the water retention value (WRV) based on a centrifugation technique. WRV measurement is a standardized method for the measurement of the amount of water retained in a given pulp sample. For fine cellulosic materials the standardized procedure cannot be performed. Thus, various modifications of the standard method have been applied by different groups for the evaluation of these materials. Due to these modifications the values obtained cannot be related to the standardized method. In this work a novel approach to determining the WRV of the fines fraction in a given pulp based on the standard procedure will be presented. This allows a quantitative investigation of the contribution of the fibers and fines fraction to the WRV of any given pulp sample.

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The dihydroxyacetophenones 2,5-dihydroxyacetophenone (2,5-DHAP) and 2,6-dihydroxyacetophenone (2,6-DHAP) belong to the key chromophores in cellulosic materials. The pulp and paper industry targets these key chromophores in their bleaching sequences to obtain brighter products. 2,5-DHAP and 2,6-DHAP were degraded with hydrogen peroxide in alkaline media, similar to conditions of peroxide bleaching (P stage) in industrial pulp bleaching. Degradation product analyses were performed by GC-MS and NMR. The degradation reaction starts by loss of acetic acid originating from the acetyl moiety of the dihydroxyacetophenones (Baeyer-Villiger rearrangement). Further reaction steps involve introduction of another hydroxyl group at C-1 (previously acetyl bearing), and further oxidation of the resulting trihydroxybenzene to quinone intermediates which are ultimately degraded to a mixture of low-molecular weight carboxylic acids.Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10570-018-1817-0) contains supplementary material, which is available to authorized users.

2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ, 1) is the most prominent representative of cellulosic key chromophores, which occur almost ubiquitously in all types of aged cellulosics. The degradation of DHBQ by chlorine dioxide under conditions of industrial pulp bleaching (''D stage'') was studied, i.e. in moderately acidic medium (pH 3) at temperatures between 50 and 90°C. The degradation in the presence of excess ClO 2 generates rhodizonic acid (RhA, 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone, 2) as a secondary chromophore which is even more stable and more potent as a chromophore than the starting DHBQ, especially in the form of its salts. At least a threefold ClO 2 excess is needed for complete DHBQ consumption. The reaction from DHBQ to RhA involves pentahydroxybenzene (PHB, I) as an intermediate which is either readily further oxidized to RhA by excess ClO 2 or slowly reconverted to DHBQ in the absence of ClO 2. The RhA yield after 30 min reaction time had a maximum of 83% at a DHBQ/ ClO 2 molar ratio of 1:5, and decreased with increasing ClO 2 charge, reaching 38% at a DHBQ/ClO 2 ratio of