Nanomaterials 2022 , 12 , 790
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they are prepared with OCC and CNFs at the same time respect to the blanks, obtaining the same maintenance of the properties as stirring OCC and CNFs separated at the optimal speed. Therefore, to sum up among the samples with OCC stirred for 10 min, the agitation separately of OCC and CNFs, these dispersed in the optimal conditions in which the minimumØ g is obtained (500 s − 1 ), producing the best results in tensile, bursting, and SCT indexes, maintaining the tear index, and reducing notably the porosity. Comparing these results with those obtained after 60 min of stirring the OCC and CNFs together, the only property that increases is the tensile index, such that a longer disintegration time would be discarded due to the higher energy consumption. 4. Conclusions Dispersion of CMF/CNF hydrogels has been quantified for the first time based on theØ g methodology and validated by the morphological characterization with TEM. The important effect of this key parameter has been developed to facilitate the optimal use of CMFs/CNFs in applications at industrial scale. The stirring speed required to obtain the maximum expanded CMF/CNF network without clusters and without breaking down of the network depends on the fibrillation degree and the charge of each CMF/CNF hydrogel. Increasing stirring speed produces a decrease in the Ø g that indicates the CMF/CNF structure is more open and spongier. However, too high levels of agitation produce an increase in the Ø g because the nanofibrils are broken and separate from the network by the high mechanical forces from the stirring of the suspensions; the CMF/CNF network collapses, which results in the compaction of the nanofibrils. Based on the results, it is concluded that the optimum stirring speed for a given CMF/CNF sample is the one corresponding to the minimum Ø g , which in turn corresponds to the maximum AR. At the minimum Ø g , the aggregation of nanofibrils is avoided as well as the reduction of their surface area, observed when CMF/CNF hydrogels are not well dispersed, and the shortening of the fibers is minimized. This value can be easily obtained on-site and will allow for the optimization of industrial CMF/CNF applications. However, the optimal dispersion of each CMFs/CNFs must be studied separately, due to depending on several parameters such as the fibrillation, treatments in the production, or composition. This technique has been tested in a real application, as in the case of the reinforcement of the mechanical properties of cardboards. When E-CNF suspensions are dispersed at different stirring speeds and then added to the OCC matrix disintegrated separately, the best mechanical properties were obtained when CNFs were dispersed at the minimum Ø g . At this dispersion degree, there is not an excess break of the fibers nor the presence of clusters or agglomerations that allow the ability to interlace the fibers of the matrix with the CNFs. In addition, this configuration also presents better properties than when the OCC was disintegrated at the same time that the CNFs were dispersed. The separate agitation of OCC and CNFs allows CNFs to be placed more superficially, covering the pores in the OCC network, without an excessive mixture of both components avoiding the CNFs being located more internally, reducing the effect of covering pores and taking part of the own matrix. Author Contributions: Conceptualization, A.B. (Angeles Blanco) and C.N.; formal analysis, J.L.S.-S. and M.C.M.; funding acquisition and project administration, C.N.; investigation, J.L.S.-S. and A.B. (Ana Balea); supervision, M.C.M. and A.B. (Angeles Blanco); writing of the original draft, J.L.S.-S.; writing of review and editing, A.B. (Ana Balea), M.C.M., C.N., and A.B. (Angeles Blanco). All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Science and Innovation Ministry of Spain (PID 2020- 113850RB-C21) and the support of Universidad Complutense de Madrid and Banco de Santander for the grant of J.L. Sanchez-Salvador (CT17/17). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable.
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