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Figure 3. Graphical representation of A) Total yield and B) Klason lignin content of different fibers produced using two different methods.
fines showed an opposite trend, that is, decreased when com- pared with the fibers produced by carbonate. The production of longer fiber and lower fines during kraft can be attributed to higher pH. Higher pH resulted in longer fibers, lower coarse- ness, and lower fines by removing more hemicelluloses, lignin, and extractives as reported earlier. [16,43] Studies of fiber properties have shown that the fiber strength increases with increases in fiber length and decrease in coarseness. Longer fibers have more fiber joints and a higher area for bonding and therefore create a stronger net- work compared to shorter fibers. On the other hand, thinner fibers collapse more easily than a coarser fiber with thicker cell walls. The collapsed fibers will create a network with much higher density and lower bulk. The flexibility of the fibers and the number of fiber joints will therefore increase the tensile strength. The uncollapsed fibers will create a net- work with higher porosity and higher bulk compared with the collapsed ones. [44] Thus, fibers produced in kraft will form stronger sheets and possess better tensile properties. [45] However, fiber curliness negatively affects tensile and tensile
amounts of lignin at high pH as shown in Figure 3B and supported by FTIR (Figure 2B). The peaks corresponding to 1730 and 1650 cm − 1 arise from lignin functional groups from carbonate which is less visible in mild kraft because lignin content is relatively low in the latter. Thus, carbonate hydrol- ysis and mild kraft produced pulps with different yields, lignin percentages, and different compositions.
3.3. Fiber Morphology
The fiber structure was further investigated by comparing the fiber morphology of the pulps produced by the two pro- cesses. Fiber morphology is related to fiber structure, which is primarily regulated by feedstock and pulping. Generally, fiber length, coarseness, kink, curl and fines are used to describe fiber morphology. Table 3 provides data on the morphology of fibers. The length of all the fibers produced by kraft pulping increased; but the other properties: coarseness, kink, curl, and
Table 3. Physical Properties of Different fibers.
− 1 ]
Kink index [1 mm − 1 ]
Sample ID
Fiber length L w , [mm]
Curl index
Fines [%]
Brightness ISO
Coarseness [mg m
EU_C
1.06
0.20
0.05
0.67
2.29
10.9
EU_K
1.09
0.08
0.05
0.08
2.12
22.5
HM_C
0.69
0.18
0.07
1.27
7.30
15.7
HM_K
0.77
0.07
0.05
0.08
3.93
23.4
BMB_C
1.22
0.36
0.07
0.80
17.73
10.9
BMB_K
1.41
0.11
0.06
0.62
15.09
23.3
HW_C
0.8
0.20
0.08
0.84
5.81
12.7
HW_K
1.17
0.10
0.05
0.62
2.33
19.7
SW_C
1.94
0.45
0.06
0.65
4.29
12.6
SW_K
2.45
0.27
0.05
0.55
1.25
13.2
2000065 (5 of 10)
Global Challenges 2020 , 2000065
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