Cellulose (2020) 27:7215–7225
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Manufacturing and testing of the board handsheets
(i.e., particles which length and width are smaller than 200 l m) were mainly in the two smallest size categories (i.e., 0–40 l m and 40–80 l m) with U3 and T3 samples while in primary board pulp (T0/U0) there is only around 5% fines. Based on the fiber analysis, the fiber properties of the DES-pretreated samples and the samples without chemical treatment (reference samples) were similar. The surface structure of the board handsheets (160 g/m 2 ) was visualized directly from the surface of the prepared sheets with the absence (Fig. 1a) and the presence of additional reinforcement CNF (T1, T2 and T3, Fig. 1b–d). CNF were observed on the surface of the handsheets, and smaller fibrils were visible on the surface of the sheets when the fibrillation level increased (i.e., T3 vs. T1/T2).
Laboratory handsheets were prepared from recycled boxboard pulp using various dosages (0%, 2%, 4%, or 6%) of CNF as a reinforcement. CNF were added directly to the pulp board slurry without any other chemical additives. Board sheets with 160 g/m 2 grammage were prepared in a laboratory sheet-form- ing machine (Lorentzen and Wettre) according to the ISO 5269-1 standard method. The board handsheets were conditioned before testing at 23 C in 50% relative humidity according to the ISO 187 standard. Eight handsheets which means in practice at least eight replicates of each sample were tested using the standard conditions. The grammage and thickness of the handsheets were measured according to ISO 536 and ISO 534 standards. The tensile strength of the board handsheets was measured with a Zwick D0724587 universal material testing machine according to the ISO 1924-2 standard using 15 mm 9 141 mm test strips. The internal bond strength of the paperboard (z-direction tensile strength) was measured with a Zwick D0724587 machine according to the TAPPI T541, 09/2005 standard. The tearing strength was measured with the Lorentzen and Wettre Tearing Tester according to TAPPI T414 om-12.
Energy consumption of the CNF production and the strength properties of the nanopapers
Table 3 presents the grinding energy demand for the production of the CNF reinforcements. The grinding time varied from 0 to 114 min, while the correspond- ing energy consumption was 0–8.8 kWh/kg of prod- uct. The consumed energy used in this research was well in line what have been noticed earlier in many publications related to mechanical fine friction grind- ing (Eriksen et al. 2008; Klemm et al. 2011; Laitinen et al. 2017; A¨ mma¨la¨ et al. 2019). The reinforcement potential of CNF was evaluated from the nanopapers fabricated directly from CNF without board fibers (Table 3). Overall, the strength properties of the CNF obtained from the DES-treated samples were better in comparison to the CNF produced using mechanical grinding without chemical pretreatment, and the strength increased as a function of grinding time. For example, the tensile strength of the nanopapers from the DES-treated CNF increased from 14.0 to 181.75 MPa (T0 and T3), while the maximum tensile strength of the reference CNF was 154.17 MPa (U3). These values are comparable to many of the recently published strength values of lignocellulosic nanopa- pers and films (Spence et al. 2010; Rojo et al. 2015; Visanko et al. 2017; Hietala et al. 2018).
Results and discussion
Characteristics of the board fibers and CNF
The visual appearance of the DES-treated board fibers and CNF obtained from grinding (Fig. S1) were studied with Valmet FS5 Fiber analyzer UHD camera unit (samples T0, T1, T2, and T3). Furthermore, example images (Fig. S2) of the DES-treated CNF (T2 and T3) were obtained with FESEM. Length weighted fiber length and width of each sample are presented in Table 2. The optical resolution of FS5 UHD camera unit is close to 1 l m, which means that the smallest particles are not visible in practice and thus not included in the calculations. Length of the fibers decreased from around 0.8 mm (T0/U0) to 40 l m(T3/ U3), when grinding time increased and finally led to a suspension containing mainly CNF (i.e., U3 and T3). Also fiber width decreased from around 19 l m (T0/ U0) to around 2 l m (T3/U3). The proportion of fines
Mechanical properties of the board handsheets
The CNF produced from recycled boxboard were used as a reinforcement in the board handsheets. During the
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