The swelling tendencies of pulps have an important impact on sheet consolidation and interfiber bonding. (Chen et al. 2013; Hii et al. 2012; Koskenhely et al. 2005). As swelling is influenced by structural changes and pulp chemistry, the effect of different pulp treatments can be monitored by its characterization and thus swelling yields important information regarding, e.g., dewaterability on the paper machine or strength development after sheet consolidation. The swelling behavior influenced by the pulping process and yield (Andreasson et al. 2003; Forsstrom et al. 2005; Stone and Scallan 1967), by drying and rewetting (Stone et al. 1968; Wang 2006) and by refining (Bäckström and Haimnar 2010; Laivins and Scallan 1996) have been documented. Further, swelling is influenced by enzymatic treatment (Gil et al. 2009; Stock et al. 1995), by chemical modification (Chen et al. 2013; Racz and Borsa 1997; Scallan 1983; Zhao et al. 2016) or by addition of cationic polymers (Aarne et al. 2012; Strom and Kunnas 1991). Swelling depends to a certain extent, also, on charge density. The more carboxylic groups, the higher the swelling potential. The effect of anionic groups can be explained based on the Donnan theory (Rasanen et al. 2001; Scallan 1983). The anionic groups located in the cell are balanced by positively charged counterions. If there are more ions in the fiber wall than in the surrounding liquid, an osmotic pressure difference arises. This pressure difference is balanced by the fiber wall absorbing water and thus `diluting` the charges within the fiber wall. Based on this fundamental effect it is obvious that a change in swelling can result from pH, conductivity of the water phase, the types of counterions of the carboxylic groups and addition of chemical additives (Scallan 1983; Scallan and Grignon 1979; Stone et al. 1968; Strom and Kunnas 1991). The final degree of swelling of a pulp fiber is limited by the structure of the fiber wall, which hinders the unlimited dilatation of the fiber. Stone and Scallan (1967), therefore, defined different regions of a fiber, whose water holding capacity obviously is affected by the fiber morphology. The accessible volume and therefore the water holding capacity of these regions change due to chemical, mechanical or thermal action.
Several methods for the measurement of swelling and related properties are reported in the literature. These include water evaporation (Stone and Scallan 1967), thermoporosity measurement (Wang et al. 2003), nuclear magnetic resonance (NMR) (Forsstrom et al. 2005; Hui et al. 2009; Maloney et al. 1997) and inverse size exclusion chromatography (ISEC) (Berthold and Salmén 1997). The two most common methods, however, are the fiber saturation point (FSP) and the water retention value (WRV).
In FSP measurement, a weighed quantity of wet pulp of known moisture content is immersed in a dilute aqueous solution of a high molecular weight dextran polymer (approximately 1wt% in water). The polymer molecules are larger in size than the pores and therefore cannot enter the cell wall. Thus water contained in the pores will lead to a change in the polymer concentration. The FSP, which can be seen as the amount of water in the cell wall, is then determined based on Stone and Scallan (1967). As swelling also depends on the surrounding liquid, it is advantageous that this measurement is performed in aqueous solution, without any structural changes to the fibers, which are constantly in a wet state. Thus it is considered the most direct measurement of fiber swelling (Maloney et al. 1999). Nevertheless, a high measurement uncertainty compared to other methods has to be accepted, as changes in polymer concentrations are quite small. The accessibility of dextran to the fiber lumen might also depend on the fiber damage. Dextran molecules may have difficulties approaching a fiber when its surface is highly fibrillated and water entrapped between these fibrils and the fiber surface could then be considered as inaccessible (Stone et al. 1968).
WRV measurement has a quite different principle. The WRV is a centrifugation technique, determining the water retained in a fiber pad after the application of centrifugal forces under defined conditions. It is an indirect method influenced by sample mass, centrifugation force and time, and therefore has to be performed under defined conditions specified by the standard (ISO 23714:2014). Nevertheless, a good correlation of FSP and WRV for different types of pulps was observed up to certain values (Scallan and Carles 1972), after which the WRV fell below the FSP. To reach this level of swelling the fibers have to be highly fibrillated. It was assumed that the more swollen a fiber is, the more sensitive it will be against compression, forcing out the water during centrifugation (Scallan and Carles 1972). On the other hand, one might argue that the FSP obtains higher values, as water molecules trapped between highly swollen fibrils and the fiber surface become inaccessible as mentioned before (Stone et al. 1968). In another study it was shown that WRV also differs from FSP for less swollen pulps. It was concluded that water retained between the fibers and water removed from the cell wall in centrifugation are responsible for these differences (Maloney et al. 1999). The influence of drying and rewetting on swelling was found to be higher for WRV than for FSP (Forsstrom et al. 2005), and the correlation of WRV and FSP is affected by different refining strategies for various types of pulps (Hui et al. 2009). It can be concluded that measurement of pulp fiber swelling is not a straightforward task and that the method, as well as the morphology and origin of the pulp sample, have to be considered. Depending on the method used, different aspects of swelling are evaluated.
As mentioned before, both charge and structural properties of fibers affect swelling. For example, carboxymethylation of unbleached hardwood kraft pulp (UBHK) leading to a 280 µeq/g higher total fiber charge resulted in an increased WRV of 1.4 g water per g fiber (Chen et al. 2013), while the fiber morphology remained almost unchanged. Refining, on the other hand, has little effect on the fiber total charge (Horvath and Lindström 2007), but generates fines (particles passing a 200 mesh screen), or more precisely, secondary fines (Krogerus et al. 2002). Primary and secondary fines are usually both present in a refined pulp. Bäckström et al. (2008) removed primary fines prior to refining and found differences in WRV between unbleached kraft secondary fines and the corresponding fibers to be 4.2 g water per g fiber. As secondary fines are parts of the fiber wall torn out by the refining treatment, this change in swelling can be assumed to be mainly caused by structural changes. Several other studies also reported that fines after refining, which include primary and secondary fines, swell two to three times more than fibers, i.e. fines hold up to three times more water than fibers (Laivins and Scallan 1996). Although there are these obvious differences, the impact of the swelling of fines on paper properties was rarely discussed. Fines were often neglected from such investigations, because it is a tedious task using the common methods to separate fines in a high enough quantity to measure swelling. Swelling of fines can be determined by the FSP method directly, but a high amount of fines is required and the FSP method is also quite a time consuming procedure. For WRV measurement, a fiber pad first needs to be formed by filtration, which is not practicable in the case of the high dewatering resistance of fines. Thus several modifications of the method were made to be able to measure the WRV of pulp fines or other fine cellulosic materials. The mass of the sample was reduced in almost all modifications, centrifugation was replaced by other methods to remove excess water, membranes were implemented to be able to hold back the fine material and centrifugation time and force were varied. It was shown that each of these modifications had an influence on the results (Cheng et al. 2010; Scallan and Carles 1972). Due to these modifications the values obtained for the fines fraction cannot be related to the standardized method anymore, and the contribution of fibers and the corresponding fines fraction on the WRV of a given pulp sample cannot be directly determined. An approach to measure the WRV of micro and nanofibrillated fine fractions on the basis of the standard procedure was presented by Rantanen et al. (2015). With this method, however, only the WRV of the fines fraction can be determined, neglecting the WRV of the fiber fraction, which would be important for papermaking pulps. In this work a novel approach to determining the WRV of the fiber and the fines fraction based on the standard procedure is presented.