GÜLSOY and KILIÇ PEKGÖZLÜ / Turk J Agric For
of both species decreased significantly (P < 0.05) with increasing screen number. The highest tear index values were determined in the R16 fraction at 11.35 mNm 2 /g for European black pine and in the R30 fraction at 3.03 mNm 2 /g for European aspen. Increase in tear index with decreasing screen mesh numbers could be attributed to a positive correlation between fiber length and tear index (Casey, 1961; Horn, 1978; Seth and Page, 1988; Mohlin, 1989; Horn and Setterholm, 1990; Seth, 1990; Scott et al., 1995; Retulainen, 1996; Levlin, 1999; Shin and Stromberg, 2005, Azizi Mossello et al., 2010a), and also higher aspect ratio of longer fiber (Rydholm, 1965; Shakhes et al., 2011) (Table 2). In addition, the increase in fiber flexibility (a higher sheet density and better interfiber bonding) causes a higher tear index (Bronkhorst and Bennett, 2002). On the other hand, the decrease in tear index with increasing screen mesh numbers could be ascribed to increasing vessel elements numbers with decreasing screen mesh numbers. The vessel elements are generally short and thin- walled, with pitting and open ends (Li et al., 2012). The vessel element-rich fractions cause a decrease in the tear index compared to that of vessel element-poor fractions (http://www.eucalyptus.com.br/capitulos/ENG04_vessels. pdf). Abubakr et al. (1995) noted that tear index of long fiber fractions in recycled pulp fractionation was higher than that of short fiber fractions. The relationships between the fiber fractions and burst index of handsheets are presented in Figure 3. Burst index of European black pine handsheets decreased with increasing screen mesh (P < 0.05), while burst index of European aspen handsheets increased with increasing screen mesh (P < 0.05). The lowest and highest burst index values of European black pine samples were determined in R100 and R30 fractions as 1.43 kPa m 2 /g and 1.99 kPa m 2 /g, respectively. This result can be explained by fiber
flexibility differences between R100 and R30 fractions (Table 2). Also, it can be attributed to decreasing fiber length with increasing screen mesh (Table 2). The lowest and highest burst index values of European aspen samples were found in the R30 and R200 fractions at 1.12 kPa m 2 /g and 1.90 kPa m 2 /g, respectively. The high burst index with rich vessel element fractions can be ascribed to improved fiber bonding due to collapsed vessel elements during papermaking. In recycled pulp fractionation, higher burst index of long fiber fractions than short fiber fractions was reported by Abubakr et al. (1995). As can be seen in Figure 4, apparent density of handsheets in both species was positively correlated with increasing screen mesh (P < 0.05) (Figure 4). These results can be explained by short and narrow fibers of higher screen numbered fractions, which give a compact structured paper due to more fibers per area. The lowest apparent density values were determined in the R16 fraction at 470 kg/m 3 for European black pine and in the R30 fraction at 580 kg/m 3 for the European aspen. A positive correlation between apparent density and mesh screen was also reported by Reyier (2008). It is known that the relationship between bulk and apparent density is negatively correlated. Demuner (1999) noted that the fine fractions produced sheets with lower bulk than the coarse fractions. The results indicated that roughness of handsheets increased with increasing screen mesh (P < 0.05) (Figure 5). Roughnesses of R16, R30, R50, and R100 fractions in European black pine kraft pulp were determined as 1566 mL/min, 1228 mL/min, 1053 mL/min, and 826 mL/min, respectively. In European aspen samples, roughnesses of R30, R50, R100, and R200 fractions were found as 1275 mL/min, 891 mL/min, 654 mL/min, and 536 mL/min, respectively. These findings can be explained by shorter
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R16 R100 European aspen Figure 3. Effect of fiber fractionation on the burst index of handsheets. R100 R30 R30 R50 European black pine R50
R200
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