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Cellulose (2016) 23:2249–2272
Fiber swelling
Reeves 1991; Weise 1998; Young 1986). The process of hornification has been divided by Weise (1998) into two different phenomena, called wet and dry hornifi- cation. The wet hornification process is defined in a range of 40–70 % dryness and describes the removal of free water from fiber lumen and fiber surface without or with pressing of the wet web. This causes cell wall collapse and pore closure. Thanks to morphological restrains of the fiber cell wall this process stops at a certain point. This kind of hornifi- cation is mostly reversible if the fibers are rewetted. Further drying of the wet web results in dry hornifi- cation (Bawden and Kibblewhite 1997; De Ruvo and Htun 1981; Laivins and Scallan 1993; Weise 1998), the type of hornification this paper refers to. Depend- ing on the drying temperature this hornification is only partly reversible by mechanical energy or chemical use. The reaction of hydroxyl and carboxyl groups to H-bonds and/or lactone bridge formation in closed pores and collapsed fibers is still under discussion (Fernandes Diniz et al. 2004; Lindstro¨m 1986; Lind- stro¨m and Carlsson 1982; Wang 2006). The degree of hornification can be measured as the reduction in the water retention value (WRV) (Jayme 1944, 1958). Jayme suggested to calculate the degree of hornification in Eq. 6: Degree of hornification according to Jayme (1944) Hornification ¼ ð 6 Þ WRV 0 : initial WRV; WRV 1 : WRV of hornificated sample WRV 0 WRV 1 WRV 0 100 % ½ Hornification as a result of drying leads to better dewatering, decreased fiber flexibility and stiffer fibers as well as to a decrease in the dry strength of paper (Lindstro¨m and Carlsson 1982; Ro¨der and Sixta 2004; Weise 1998). Experiments comparing never-dried with dried unbleached, unbeaten spruce sulfite pulp show that hornification leads to a decrease in strength at any dryness level (Brecht and Erfurt 1959b). Own studies with commercial bleached softwood sulfite pulp demonstrate that hornification has a significant nega- tive impact on the strength values at dryness levels greater than 25 % (Belle et al. 2014a). Below 25 % dryness there is a slightly positive effect. The differ- ences in the results can be explained by the higher accuracy of the DIN 54514 method (see also ‘‘Macro level—Measurement techniques’’).
The ‘‘fiber saturation point’’ (FSP) has been used to characterize the water adsorption and swelling prop- erties (Christensen and Giertz 1965; Scallan and Tigerstro¨m 1992; Scallan 1977; Stone and Scallan 1967; Tejado and van de Ven 2010; van de Ven 2008). An additional method of determining the swelling state is to measure the water retention value (Ho¨pner et al. 1955; Thode et al. 1960; Zellcheming 1957). Scallan and Tigerstro¨m used the van’t Hoff equation to calculate the bulk modulus of the fiber wall shown in Eq. 5 supposing ‘‘that the hydrogen form of the charged groups of the fibers can be taken as a reference state’’ (Wa˚gberg and Annergren 1997). Bulk modulus ‘‘K’’ of the fiber wall (Scallan and Tigerstro¨m 1992)
RT n V V V 0 ð Þ V C þ V 0 ð Þ
K ¼
ð 5 Þ
In this equation the RT n V
is the osmotic pressure in
the fiber wall. R: gas constant; T: absolute temperature; n: mole of charged groups per kg oven dried pulp; V: Volume of water in the swollen fiber wall; V 0 : Volume of water in the fiber wall when the charged groups are in their hydrogen form; V C : specific volume of the material in the fiber wall assuming the density to be 1.5 9 10 3 kg/m 3 . The swelling contributes to the fibers’ flexibility, resulting in considerably better entanglement (Barzyk et al. 1997; Brecht 1947; Brecht and Erfurt 1959b; Erhard et al. 2010; Linhart 2005; Lyne and Gallay 1954a, b; Scallan 1983; Weise et al. 1998). The degree of beating enhances or lessens this effect (Brecht and Erfurt 1959b; Kibblewhite 1973; Lindqvist et al. 2011; Salminen 2010). All authors cited above conclude that proper swelling increases the dry strength of the paper. This means that conditions such as beating, alkaline pH and low conductivity contribute to a strong dry paper. There has been done only few research in case of IWWS.
Hornification
Drying and pressing introduces an irreversible change in lignocellulosic fibers resulting in a loss of swelling ability of fibers that is also named hornification (Fernandes Diniz et al. 2004; Po¨nni et al. 2012;
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