Cellulose (2019) 26:1995–2012
1999
Fig. 1 Sample holder with 500 l m gap used for tensile testing
treated BSKP sheets were dried unrestrained and restrained, whereas HC ? LC BSKP sheets were only dried unrestrained. Additionally, high pressure wet pressing and addi- tion of strength chemicals were applied in order to improve bonding of the fibers. A few untreated BSKP sheets were wet pressed using 670 kPa pressure (instead of 350 kPa) before unrestrained drying. Combination of 1.5% cationized starch, Cationic polyacrylamide (CPAM) 200 g/t and 5% Masuko micro-fibrillated cellulose (CMF) from VTT (E393HWM3) was added into untreated BSKP and the sheets were dried unrestrained and restrained. The starch Classic 135 was a potato-based wet-end starch with a degree of substitution of 0.035 from Chemigate. The CPAM Fennopol K3400R with a molecular weight of 6–7 million g/mol and charge density of 1 meq/g was obtained from Kemira (Finland). The CMF was prepared from never-dried birch kraft pulp by mechanical disintegration. The dispersed pulp (1.7% consistency) was first pre-refined with a grinder (Supermasscolloider MKZA10-15 J, Masuko Sangyo Co., Japan) at 1500 rpm, followed by treatment with a fluidizer (Microfluidics M-7115-30, VTT, Espoo, Finland). The CMF was produced after three passes at an operating pressure of 1800 bar. No chemical modification was applied. The dry cationic starch was diluted in deionized water (conductivity \ 1 l S/m) to a final concentration of 1% and heated for 30 min at 90 C under constant stirring. The dry CPAM was diluted in deionized water at 0.3% concentration. The solution was kept under constant stirring until fully dissolved and then further diluted to 0.01% for actual use. The influence of the addition of a similar CMF to a
BSKP pulp on the shrinkage, extensibility and strength of hand sheets is presented in Ketola et al. (2018). Testing of the dry sheets took place at a temperature of 23 C and at 50% relative humidity, using a strain rate of 10 mm/min (0.167%/s) in a Lloyd tensile tester, in accordance with ISO 5270:2012. The thick- ness and apparent sheet density of the dry paper were measured using ISO 534, and air-dry grammage using ISO536.
Preparation and identification of single fibers
Fibers for individual testing were prepared following the method described by Kappel et al. (2009). Dry pulp was dispersed and allowed to swell in water for at least 12 h. The fibers were disintegrated according to DIN EN ISO 5263-1. A highly dilute suspension with a consistency of 0.01% was prepared. Small drops of the suspension were placed on a piece of Teflon foil (40 mm 9 40 mm) and covered with another piece of Teflon foil. This sandwich was dried in a conventional sheet dryer for 45 min. The drying method used allowed shrinkage of the individual fibers. In order to perform single fiber tensile tests, the individual fibers were glued onto the sample holder (Fig. 1) using a two component epoxy resin (UHU PLUS ENDFEST). The sample holder used for the fiber tensile strength testing is described in Lorbach et al. (2014) and Jajcinovic et al. (2016). The gap in the sample holder was 0.5 mm. The span length is known to influence the strength (Hardacker 1970). After gluing of the indi- vidual fibers (Fig. 1), they were conditioned for at least 24 h prior to identification and testing to ensure the maximum holding strength of the resin and moisture equilibrium. The dry BSKP fibers were
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