Nanomaterials 2022 , 12 , 790
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pieces, reducing their impact to cover the pores of the fibers and the ability to interlace the fibers of the matrix reducing the tensile strength. As other researchers have demonstrated, the bursting index (Figure 7d) shows the same trend as the tensile index [56]. OCC blank has a bursting index of 1.87 kPa · m 2 /g. Handsheets with CNFs increase progressively until a gradient velocity of 500 s − 1 ,withan increase of 25% in the bursting strength (2.34 kPa · m 2 /g). However, a higher speed makes an increase in burst strength of only 17%. The same occurs with compression strength, measured with the SCT index in Figure 7e. The higher SCT is obtained when the CNFs are stirred at 500 s − 1 , with an increase of 21% respect to the blank, then the SCT Index is reduced more than the obtained with 100 s − 1 . Finally, the tear index is evaluated in Figure 7f with a blank value of 8.74 mN · m 2 /g. The addition of CNFs stirred at 3 s − 1 decreases the tear index in 8.2%. This reduction is also observed in other studies, when CNFs are applied in bulk to prepared handsheets due to the reduction of the length of the fibers in the matrix making the tearing easier [10,57]. Low stirring speeds do not improve the results of blank until CNFs are stirred at 500 s − 1 , maintaining tear index of blank with a slightly increase of 1.1%. This effect could be due to a good disposition of the CNFs in the OCC matrix, avoiding their aggregation and the tearing weakness. Extreme agitation of CNFs, 3000 s − 1 , produces only a little decrease in the tear index but also maintained the blank value. In summary, looking at all properties together, the optimal conditions to improve mechanical properties of recycled cardboard were CNFs stirred at 500 s − 1 rpm with the best results in all mechanical properties studied and maintaining bulk and the tear of the OCC matrix without CNFs. These conditions coincide with the minimum Ø g of E-CNF (around 500 s − 1 ). Besides the importance of dispersing CNFs adequately to prepare the handsheets, OCC disintegration is also a key factor to analyze. For that, the influence of OCC disintegration and CNF dispersion in the same pulper or separately at different stirring speeds has been studied. Figure 8 shows physical and mechanical properties of handsheets prepared with OCC and CNFs disintegrated and dispersed, respectively, at the same time in the same pulper at 3000 s − 1 for 10 min and 60 min. Blank prepared only with OCC and stirred 10 min is the same as in Figure 7, being able to compare the results of stirred OCC and CNF together or separately. Bendtsen porosity of Figure 8a shows the variation of adding CNF in the pulp matrix in both speeds. A higher stirring speed does not produce a high diminution of porosity, being ineffective a higher time of disintegration. In addition, both samples are more porous than when OCC and CNFs are stirred separately. This fact could be due to in agitation separately the CNFs are placed more superficial whereas in the stirring together both materials mix with each other in a higher extent. In respect to bulk, as in Figure 7, there are not differences in respect to blanks without CNFs. As for the mechanical properties, tensile, bursting, and SCT indexes show the same trend (Figure 8c–e). In the three indexes, there is a decrease in these properties when the blank is stirred for 60 min due to a higher break of the fibers in the disintegrator. As for the sheets with OCC and CNF in both times of stirring, they have an increase in the properties respect to the blanks, even more after 60 min of agitation. Among the results stand out the tensile index of OCC and CNF stirred together for 60 min with 44.6 kN · m/kg. This property has the highest difference with the tensile of OCC (10 min) and CNF (500 s − 1 ) stirred separately with 40.7 kN · m/kg (Figure 7c), although if we compare with the OCC and CNF stirred together but only 10 min (39.1 kN · m/kg) we see that the OCC disintegration and CNFs dispersion separately is still better. As for the bursting and SCT indexes, the separated stirring of OCC and CNFs (500 s − 1 ) shows better properties than the obtained together independently the time selected (Figure 8d,e). This fact indicates that, as in porosity, the mix of CNF with OCC after its disintegration favors the mechanical properties of the sheets, due to a more external situation of CNF whereas a more blended situation reduces the efficiency of CNF that is more mixed in the bulking agent. Finally, the tear index in Figure 8f indicates that this property barely has influence in the handsheets when
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