**1. Introduction**

In recent years, there has been great growth potential in the tissue paper market. Factors such as the abundance of fibrous raw materials, the high demand for hygiene and health products and the evolution of socio-economic living standards have promoted this global growth. In addition, advances in manufacturing technologies for these materials have made the market increasingly competitive [1]. Tissue paper materials are increasingly used in consumer goods products such as napkins, facial tissues, paper towels, toilet papers, diapers and tissue masks. Tissue papers are produced to meet the desired properties for a specific application in the market such as a low basis weight, softness, strength and absorption [2]. To optimize the process, a great deal of effort been made to improve the properties of the raw materials used in the production of these papers [1–4]. The highest quality tissue products are produced with a blend of hardwood and softwood fiber pulps. The production of tissue papers with 100% eucalyptus fibers generally presents a higher softness and absorption; however, the strength is degraded during the creping and converting processes in the tissue manufacture [3,4]. Softwood fibers are introduced because these

**Citation:** Morais, F.P.; Carta, A.M.M.S.; Amaral, M.E.; Curto, J.M.R. An Innovative Computational Strategy to Optimize Different Furnish Compositions of Tissue Materials Using Micro/ Nanofibrillated Cellulose and Biopolymer as Additives. *Polymers* **2021**, *13*, 2397. https://doi.org/ 10.3390/polym13152397

Academic Editors: José Miguel Ferri, Vicent Fombuena Borràs and Miguel Fernando Aldás Carrasco

Received: 26 May 2021 Accepted: 17 July 2021 Published: 21 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

pulps provide strength to the papers and ensure the paper machine's runnability [5–8]. The fibers can also be subject to modification processes such as enzymatic and mechanical modifications. Refining is an important process in the development of functional tissue properties; however, it is associated with a large energy consumption [9,10]. The application of enzymes enhances not only the development of the tissue properties but also the reduction of energy costs associated with the refining process [11,12].

In addition to the modification of fiber pulps, chemical additives are used to modify properties or improve the production process for different materials. Different chemical additives used in the tissue paper industry are designed and applied according to different needs such as dry strength agents, wet strength agents, softeners and lotions [13,14]. Usually, these additives are cationic as the fibers carry negative charges and they are also more effective in much lower concentrations than anionic polymers. Nevertheless, these cationic additives normally adhere in excess to the paper, negatively affecting the creping process and the product quality [14]. The addition of strength agents can ensure that tissue products remain integrated when applied in wet conditions while the addition of debonding agents can improve the softness and bulk of these products [15–21]. However, reaching a balance between these two properties is a challenge because the conditions that maximize the softness are those that minimize strength properties. This balance can be achieved by selecting fibers, refining treatments and adding chemical polymers. Therefore, the tissue manufacturer must balance these factors in terms of cost and effect. The tissue paper industry provides the design, development and production of many chemical and polymeric additives [22].

The application of innovative and versatile additives such as biopolymers and micro/nanofibrillated cellulose (CMF), replacing synthetic additives currently used, is a sustainable strategy to be considered in order to produce high-quality tissue premium products with innovative features. Biopolymers as additives can be new natural and green approaches to new possibilities and advances in modifying or improving the final end-use tissue properties [23]. The same is true for CMF as an additive that has been extensively investigated for its application in tissue products due to its high specific surface area, excellent mechanical properties, flexibility, biocompatibility, biodegradability and lack of toxicity, among other properties [24–27]. The furnish formation is also affected by the CMF incorporation as an additive. CMF decreases porosity and air permeability when added to the tissue structures. CMF content and structural changes present a proportional behavior. These decreases are caused by the CMF bonding with the cellulose fibers in the 3D network structures, closing the porous structure. Furthermore, this reduction in porosity is also correlated with the increase in the structure's apparent density [28].

The presence of CMF can present an impact on the structure porosity because of the increasing number of hydrogen bonds present in this nanocellulose. The increase of these bonds is related to its increase in the surface/volume ratio due to the nanosized scale [28]. By filling the large pores created by the 3D fiber network, the CMF incorporation contributes to smaller pore dimensions and a narrower distribution of the structures, providing abundant pores with a uniform size and, consequently, a decrease in the properties of water absorption capacity and capillarity penetration [24,25]. The flexibility of CMF can be a filler for large pores in structures [25]. Although the mechanical properties of tissue products are improving, CMF also brings a difficulty to the dewatering of papermaking furnishes. The fibrillation degree influences the dewatering capacity of pulp suspensions. The drainage time increases depending on the CMF fibrillation degree. Particles resulting from fibrillation are more easily incorporated into the fiber network, partially closing the pores between the fibers and consequently limiting the wet network's ability to drain water. This drainage difficulty is also caused by the increased water retention capacity of CMF. However, this dewatering of pulp suspensions containing CMF can be improved with the use of retention aids and also with cationic starch-treated CMF with colloidal silica [28]. In addition to a negative impact on dewatering, the accumulation of CMF particles in the closed circuit of the tissue machine can increase the viscosity of the recirculating water,

thus affecting processability. Therefore, there is a need to study the feasibility of using CMF to reduce the fiber content. Our study [24] confirmed that the use of 1 to 8% of CMF incorporated into tissue paper formulations, along with a reduction of hardwood and softwood fiber content, achieves a trade-off between the final end-use tissue properties and fiber blend costs, with drainability between 26 and 32 ◦SR (Schopper–Riegler degree). In this range of drainability, the furnish formulations are able to produce high-quality tissue papers without difficulties in tissue paper machine runnability and efficiency [2,24].

The application of these additives leads to the optimization of strength properties without compromising the softness and absorbency properties of these types of papers. These strategies can allow a reduction of softwood fibers in the production of tissue paper without affecting the strength performance and decrease the overall tissue manufacturing cost [24,27–30]. The aim of this work was to perform a comparative study between different tissue furnish compositions and the incorporation of two additives, a commercial tissue biopolymer additive (CBA) as a cationic starch substitute and CMF, and their effect on the properties of the final end-use tissue paper. For this purpose, the fiber and suspension drainability characterizations of different investigated samples were performed. Laboratory isotropic structures with a light basis weight of 20 g/m<sup>2</sup> were prepared and the softness, tensile strength and absorbency properties were evaluated.
