Cellulose (2020) 27:7215–7225
7217
The conductometric titration procedure described by Rattaz et al. (2011) and Katz et al. (1984) was used to determine the charge density of the pulp. The hemicellulose content was determined with the TAPPI-T 212 om-02 standard and alkali solubility at 25 C was determined with the TAPPI-T 235 cm-00 standard. The lignin content was determined using TAPPI-T 222 om-02 standard. Moreover, the extrac- tive content was analyzed using SCAN-CM 49:03 standard and ash content was determined using the ISO1762 standard. The chemical properties of the board raw material are shown in Table 1. The average (length-weighted) length and width of the board fibers after washing were determined with a Valmet FS5 Fiber analyzer ultra-high definition (UHD) camera unit (Finland). Three replicates of each sample were measured, and the results were averaged and shown in Table 2.
three different CNF samples: T1, T2, and T3 (Table 3). The stones of the grinder were first carefully brought into close contact, as determined by the low friction sound, and then the pretreated pulp slurry was poured into the grinder at a consistency of 1.5%. First, the pulp was passed through the grinder three times using a zero-grinding stone gap (sample: T0); after which the stones were adjusted to negative gap values in order to start the actual fibrillation. The energy consumption of the fibrillation procedure was recorded with an energy meter (iEM3250 Schneider- Electric, France) attached to the fine friction grinder. The board pulp passed through the grinder a total of 14 times, using negative gap values of 3 times - 20 l m, 1 time - 40 l m, 1 time - 50 l m, 1 time - 60 l m, 3 times - 80 l m, and 5 times - 90 l m to obtain different board CNF (Table 3). The reference CNF samples were obtained from untreated boxboard pulp using a similar procedure. The reference samples are named: U0, U1, U2 and U3.
Production of CNF from the recycled boxboard
DES of choline chloride and urea was used as a pretreatment to produce CNF from the boxboard pulp. The DES solution was produced by heating 1620 g of choline chloride and 1223 g of urea in a large beaker (5 dm 3 ) at 100 C until the mixture melted, after which it was placed into a water bath at 100 Cunder constant stirring for approximately 5 min to obtain a clear and colorless liquid. Then, 25 g (abs) of recycled board material (dry matter content of * 30%) was added to the suspension and mixed for 2 h. Then, the beaker was removed from the water bath and 1000 cm 3 of deionized water was added while mixing (Sirvio¨ et al. 2015). Next, four identical DES-treated batches of boxboard pulp were combined. Then, the treated board was washed with water using a Somerville screen (Lorentzen and Wettre) until clear rinse water was obtained. After completing the washing procedure, the DES- treated pulp was disintegrated with a Masuko super- masscolloider grinder (MKCA6-2J, Japan) using three different grinding times (grinding energy) to obtain
Manufacture and characterization of the investigated materials
Visualization of CNF and the board samples
After being subjected to different grinding energies, the board pulps were visualized using a Valmet FS5 Fiber analyzer UHD camera unit. Field emission scanning electron microscope (FESEM) (Zeiss Ultra Plus, Germany) was used for the samples obtained after a longer grinding time because of their much smaller average particle size and the need for a higher magnification. As a pretreatment, the FESEM samples were filtered using a polycarbonate membrane with a pore size of 0.2 l m (Whatman). Then, the filtration samples were rapidly frozen with liquid nitrogen and freeze-dried in a vacuum overnight. The dried samples were sputter-coated with platinum. An accelerating voltage of 5 kV and a working distance of around 5 mm was used when imaging the samples.
Table 1 Chemical properties of the recycled boxboard pulp
Sample Charge (mmol/g)
Cellulose (wt%)
Hemicel-lulose (wt%)
Lignin (wt%)
Extrac-tives (wt%)
Ash (wt%)
Board
- 0.19
57.5
16.4
13.3
0.8
12.3
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