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version of the 4-O-methylglucuronic acid side groups (MeGlcA) of xylan to hexenuronic acids (HexA) by elimination of the 4-Omethoxyl group and deprotonation during peeling at alka- line conditions. [47] The carboxyl content of hemp pulp produced by carbonate was similar compared to carbonate hardwood pulp, but lower than the other carbonate pulps. However, the kraft hemp pulp contained almost similar carboxyl content like eucalyptus and hardwood but lower than the bamboo and softwood pulps produced by mild kraft. The lower carboxyl content of the kraft pulp is due to higher alkaline conditions at which the pulping was carried out which decreased reprecipitation of lignin and hemicellulose onto the surface of the fibers. [47] The relatively higher surface charge of the carbonate pulps was due to its high anionic charge density contributed by resins, fatty acids, carboxylic acid groups of the hemicelluloses, and lignin’s phenolic groups. [49] The hydrophilic carboxyl group in the fiber interacts with water and enables fibers to absorb more water to increase swelling capacity, important for the tissue paper industry. [50] The presence of greater numbers of carboxyl groups and surface charge in the fiber facili- tates better entanglement by inducing hydrogen bonding to increase the tensile and burst properties of the fibers. [44,51] The carboxyl groups act as binding sites and also enables controlled chemical loading and release by regulating the interactions between the carboxyl group and chemicals. [34,35] Thus, tissue, towels and wipes with varying surface charge and carboxyl content can be produced by using different fiber types or pulping (Figure 4).
stiffness indices, while it increases tear and fracture tough- ness indices. [46] The curl index gives the maximum elongation of the fiber up to breakage, which indicates the stability of the paper web in the open draw. The deformed fibers contribute uneven dis- tribution of stress along the length of a curled fiber in a frac- ture zone, transferring more stresses to the bonds, which increases the tear strength of the sheets. Thus, fibers produced by carbonate form sheets having lower tensile strength, but higher tear strength. The fiber curl, in addition, increases the bulk and porosity of the pulp sheet but decreases the drainage resistance. [46] Curly fibers cause more scattered reflection of light, which results in a matte appearance and opacity. Thus, fibers produced by carbonate have higher coarseness and lower brightness than kraft. The presence of lower lignin content in kraft also affected the brightness. Thus, carbonate pulps with higher coarseness, curl index and fines could be used to make paper or tissue products with high bulk and tear strength, whereas, kraft pulp generates longer, brighter fibers to give stronger, flexible and brighter paper products.
3.4. Carboxyl Content and Surface Charge
In addition to the above mentioned properties, the charge on fibers is a well-known parameter in papermaking and plays a vital role in their performance through electronic interactions of the charged soluble and particulate fractions. [43] The elec- trical charges at the surfaces of the cellulose fibers are a sig- nificant parameter strongly affecting the swelling ability of cellulose fibers and provides a driving force for adsorption of retention aids, sizing agents, and strength enhancing chemi- cals. [47] Fiber charge is a significant characteristic of cellulose fibers, which strongly affects post-processing such as enzy- matic modification. [48] The carboxyl content and surface charge of different fibers are shown in Figure 4 . The carbonate pulps have relatively higher carboxyl content and surface charge than kraft. The carboxyl content of the fibers is formed due to con-
3.5. Crystallinity Indices
The effect of the carbonate and mild kraft pulping on the crys- talline structure was evaluated further to study the changes in crystallinity index of different fibers. It was found that the crystallinity indices of the kraft fibers were higher than that of carbonate fibers as shown in Figure 5 . The amorphous nature
Figure 4. Carboxyl content and surface charge of different fibers pro- duced using two different methods.
Figure 5. The Crystallinity Index of different fibers produced using two different methods.
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