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of the fiber is imparted by the hemicelluloses which is a short, linear and highly branched chain of sugars and possesses shorter side chains. [52] Thus, they induce a branched network through extensive crosslinking to lignin and cellulose by cin- namate acid ester linkages and hydrogen, respectively. [53] The kraft fibers has higher crystallinity since the process was oper- ated at higher pH which facilitated dissolution of a higher level of amorphous cellulose. pH range at which carbonate ruptured the structural linkages between lignin and other carbohydrate fractions in the lignocellulosic biomass, but could not remove lignin as much as kraft. [54,55] As a result, the carbonate treat- ment of the fibers resulted in a carbohydrate heterogeneous matrix with lower crystallinity compared to kraft and a lower degree of polymerization. [47]
using kraft (Figure 6). The carbonate fibers, though they have less crystallinity, are associated with less HR water. This might be due to the presence of higher lignin content which limits the ability of water molecules to form hydrogen bonds with the hydroxyls of the cellulose. [58] On the contrary, kraft fibers have relatively much less lignin content which allows the cellulosic hydroxyls to interact with the water forming hydrogen bonds and increasing HR water content. Thus, kraft fibers would col- lapse more as shown in Figure 7 because the HR water regu- lates extent of fiber collapsing. The reactivity of the kraft fibers will be negatively affected as well, since HR water hinders the facile diffusion of chemical reagents by forming a hydration barrier which limits accessibility to the hydroxyl groups and reduces the reaction kinetics. [56] Thus, lower HR content of the carbonate fibers indicates that they will suffer less hornification than kraft fibers, supported by the coarseness data of the fibers as shown in Table 3 and SEM images in Figure 7.
3.6. Hard-to-Remove Water
The Hard-to-Remove water is defined as trapped water, freezing bound water, and non-freezing bound water which requires a higher energy for drying. [37] It is an important parameter as it affects the different properties of the paper products like water absorption, tensile, drying energy, and the reactivity cellulose towards chemicals. [37,44,56] Thus, the effect of the different pulping processes on the HR water content of the dif- ferent feedstocks were investigated graphically represented in Figure 6 . It can be observed form Figure 6 that the hemp fiber pro- duced using carbonate and mild kraft contained the highest levels of HR water compared to other fibers, whereas the car- bonate and kraft softwood fibers contained the least amount of HR water. The hemp fiber had higher HR water than the others since it has a much higher hemicellulose content. Hemicellu- lose is amorphous in nature and allows more water penetration which facilitates more fiber-water interaction increasing the HR water content. [52,56] The presence of relatively higher lignin content in softwood reduces the HR water content as lignin being hydrophobic diminishes fiber-water interactions. [57] However, HR water content was higher for fibers produced
3.7. Scanning Electron Microscopic Image Analysis
SEM image analysis was done to visualize the effect of car- bonate and mild kraft on fiber morphology. It can be observed form the images of the different fibers that the handsheets made from kraft suffer greater flattening than carbonate (Figure 7). These results directly relate to the coarseness and HR water of the fibers. Higher bulk of carbonate handsheets can be attributed to higher coarseness of the pulps produced by carbonate. Thus, kraft hemp pulp having the least coarseness (Table 3) yielded sheets with the least thickness (Figure 7B), whereas, the carbonate softwood showed highest thickness owing to its highest coarseness. The kraft fibers exhibited higher HR water and as HR water leaves during drying, more hydroxyls of cellulose become avail- able for bonding. These hydroxyls interact with each other forming hydrogen bonds which leads to irreversible flattening of the fibers resulting in higher flattening of the fibers. [56] Therefore, hemp fibers suffered the highest flattening due to its least coarseness (Table 3) and highest HR water value (Figure 6). 4. Conclusions This study reports production of pulp from several fiber feed- stocks after sodium carbonate and mild kraft pulping pro- cesses. The pulp yield was higher for sodium carbonate, which is a significant economic benefit because fiber is the dominant cost for producing paper products. The high yield of hemp fibers using both pulping processes and its low coarseness, low fines, and high brightness value qualify it an emerging poten- tial resource for paper and tissue paper industries because it improves tensile, burst resistance, and softness of tissue hand- sheets. The lignin content was lower for pulp produced using mild kraft because it removes more lignin, which results in decreased coarseness. Higher coarseness and lower HR water content using carbonate led to less fiber collapsing, very useful for maintaining higher bulk for tissue paper products which has a direct correlation with water absorbency and bulk
Figure 6. Hard to remove water content of different fibers.
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Global Challenges 2021 , 5 , 2000065
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