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dewatered on wet presses and dried on a paper dryer up to several times, not only are strength properties of the paper, such as tensile and bursting strengths significantly reduced, but also the micro-structure of the pulp fibers is damaged 8 . The recycling process causes morphological changes such as delamination and crack formation in cell wall of the pulp fibers 8,9 . The solute exclusion method has been devised by Stone and Scallan to elucidate the structure of pulp fiber cell walls in the presence of water 10,11 . The solute exclusion method utilizes the phe- nomenon that solute molecules invade pores in the cell wall according to the molecular diameter when the wet pulp is immersed in monosaccharides and dextran aqueous solutions having various molecular diameters. It was found by using the solute exclusion method that the pore volume in cell wall of the pulp fibers decreased with the number of recycling increased 10,12 . Stone and Scallan 13 also noted significant decreases in the specific surface areas of once-dried bleached sulfite pulp by using nitrogen adsorption technique. It was confirmed that the decrease in the specific surface areas of the dried bleached sulfite pulp increased with an increase in drying temperature. The decrease in the potential of recycled pulp fibers with respect to the strength of paper is largely due to changes in the cell wall structure of the fibers themselves. This change affects the swelling potential and conform- ability of the fibers in the papermaking process, and controls the refining characteristics 14,15 . In the pulp fiber wall, the S2 layer repeatedly swells and shrinks due to recycling treatment, and microscopi- cally forms a matrix structure with few fine pores. However, on the other hand, densification causes a twitching phenomenon in the matrix, and macroscopically, layered cracks are generated in the fiber wall, and cracks are also generated in the radial direction. Therefore, the inter-fiber bond formed by recycling is broken, and the strength of the paper sheet is also reduced 8,9 . The so-called “hornification” of pulp fibers caused by recycling is the irreversible loss of fiber swelling, which is determined as water retention value. The irreversible hornification leads to remarkable reductions in fiber–fiber bonding. It occurs strongly in the fiber cell wall matrix of chemical pulp, but not so much in mechanical pulp 6 . Cidir et al. 16 also confirmed that as a result of papermaking using refined bleached sulfite pulp (BSP) and then recycling, the tensile strength, tearing strength and elongation of the paper decreased, but the opacity improved. Furthermore, by measuring and substituting the zero-span tensile strength, which is an index of single fiber strength, into the Page’s equation 17 , it was evaluated that the decrease in paper tensile strength due to recycling depended on the decrease in fiber–fiber bonding strength. Howard et al. 18 showed that there is few changed in both wet and dry zero-span tensile strength during recycling of various pulps. Regarding the strength of paper such as tensile and bursting strengths, hydrogen bonds are considered to be the most important for the bonding strength of cellulose fibers in paper. The ability of cellulose fibers to form fiber–fiber bonds depends on the hydrophilicity of the fiber surface, that is, the ability to form hydrogen bonds based on it 19 . Since fiber–fiber bonds are formed between fibers interacting in water, wet adhesion influenced by the wetting of a fiber–fiber bonding and the strength of fiber network. It is important to gain a better under- standing of the effect of the surface behavior of a single pulp fiber, because the fiber–fiber bonding strength is influenced by both the physicochemical properties of the pulp fiber surface and the contact area. The evaluation of the paper surface wettability contributes to the control of various industrial processes. Young 20 prototyped a contact angle measuring device using Wilhelmy’s principle and determined the wettability of single pulp fibers. That is, after measuring the weight increase and the peripheral length of the fibers generated when the pulp fibers are suspended from a balance and immersed in a liquid, the contact angle is calculated using the following equation, and the wettability W is calculated by F/P.
(1)
F = P γ LV cos θ
where F is the tensile force acting on the solid rod immersed in the liquid, P is the peripheral length of the rod along the boundary line of the three phases, γ LV is the surface tension of the liquid, θ is the contact angle. The contact angle of pulp fibers with water was reported to be 52° for Douglas fir unbleached kraft pulp (UKP) fibers and 43° for Aspen thermomechanical pulp (TMP) fibers. From the dynamic wetting test of wood pulp fiber by Wilhelmy method, unbleached neutral sulfite semi-chemical pulp (NSSCP) fibers have higher wettability than UKP and TMP fibers. The reasons for this were the degree of removal of lignin on the fiber surface, the presence of hemicellulose, other carbohydrates and extract components, and suggested the effect of the sulfone group introduced into lignin 20 . Klungness 21 modified Young’s equipment to improve measurement accuracy and determined the contact angle of water to loblolly pine kraft pulp fibers with different lignin contents. As a result, it was confirmed that the contact angle with water increases due to the hydrophobic effect of lignin as the lignin content in the pulp fiber increases. Jacob et al. 22 measured the contact angle of pulp fibers using liquids with different surface ten- sions and attempted to calculate the critical surface tension from the Zisman technique. Although the results showed significant variations in surface characteristics within a single fiber type, chemi-thermomechanical pulp (CTMP) fibers were more wettable compared with softwood and hardwood UKP fibers. When the critical surface tension was measured by the Zisman technique using a film sheet prepared from each of the three main constituents of wood, it was estimated that a relatively large amount of lignin was distrib- uted on the fiber surface in UKP 23,24 . On the other hand, in the case of TMP fibers, it depends on the temperature condition during the process because of the relationship with the softening temperature of lignin. Therefore, it was concluded that the wettability of pulp fibers depends on the chemical composition and structure of the fiber surface 20 . In general, chemical pulp fibers such as bleached kraft pulp (BKP) and UKP with low lignin content have been found to be more hydrophilic than high yield pulp fibers such as groundwood pulp (GP), refiner mechanical pulp (RMP) and TMP fibers with high lignin content 25 . Berg 26 evaluated the surface free energy, including the contribution of acid–base interactions, from the wet- ting measurements of pulp fibers. As a result, it was shown that the BKP fiber has a slightly larger dispersion force component γ sd and a considerably smaller electron donor (base) parameter γ s —of the surface free energy
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(2022) 12:1560 |
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