Cellulose (2016) 23:2249–2272
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In experiments about the effect of freezing on pulp properties, Kibblewhite determined that the IWWS decreases as the drying rate increases (Kibblewhite 1980). Similar to other research work, this study argues that the increased fiber rigidity/hornification leads to a reduction in bonding intensity. Adapting the sample preparation process enables the investigation of fiber surface effects and the hornification, charac- terized by comparing the fiber collapse of conven- tionally dried samples with freeze-dried samples by means of SEM imaging (Belle et al. 2015a). This study shows that in comparison to conventional drying, freeze-drying leads to significantly reduced fiber, fibril and surface hornification. Dependent on dryness, hornification affects the IWWS in several ways. Below 25 % dryness, horni- ficated fibers are stiffer with a lower young-modulus (Scallan and Tigerstro¨m 1992), higher friction coef- ficients and flatter shapes. This leads to denser sheets with better conditions for capillary forces and fric- tional connection. From 25 % dryness onwards a non- hornificated fiber is needed for better IWWS. Higher young-modulus and a flexible fiber results in better conformability and more initial contact points as well as a better entanglement.
Surface roughness of the fibers
The surface roughness of the fibers is given by the type of wood and is modulated to a significant degree by the pulping process (Fengel and Wegener 1989). Addi- tionally, the fiber surface roughness is specifically influenced by pulp beating and the associated gener- ation of fiber fragments and suspended fibrils. The surface roughness of the individual fibers and the associated fiber–fiber adhesion play an important role in regard to the strength in the wet paper web because they affect the capillary forces during dewatering (Alince et al. 2006; Hubbe 2006; Thomson 2007). Fiber pore sizes also play an important role (Erhard et al. 2010; Scallan and Tigerstro¨m 1992; Scallan 1977). However, only a few values for fiber surface structure and roughness were available. Additional values can now be generated by the AFM method published in 2011 describing the scanning of fiber surfaces, including the calculation of the surface roughness and fibril angle of fibers shown in Fig. 7 (Heinemann et al. 2011).
Fig. 7 ‘‘AFM phase images and corresponding fibril angle and surface roughness parameters for each individual image representing the inner (S2) and outer (ML/P) fiber wall layers … The images are 3 l m 9 3 l m’’. [Reprinted with permission of Heinemann et al. (2011)]
S q : RSM (root-mean-squared) roughness; S ku : kurtosis; S sk : skewness; REF: Reference; OX: chem- ical treatment of the sample with buffered oxalic acid
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