**4. Conclusions**

This work presented experimental and computational approaches to evaluate and predict the effects of the incorporation of CBA and CMF with eucalyptus and softwood fibers on the softness, strength and absorbency properties of tissue paper materials. Overall, the addition of both additives enhanced tensile strength properties at the expense of reduced softness and absorbency properties, followed by CMF and CBA. This cationic polymeric solution also improved the inter-fiber bonding and CMF particle retention. With the softwood fiber reduction in the formulations, the incorporation of additives promoted a strength and absorbency increase with a low softness decrease. It was possible to obtain a trade-off between the tissue end-use properties with CBA and CMF additions separately. With this work, it was possible to produce tissue furnish formulations using these additives with a reduction of the softwood fiber percentage in a drainage range of the tissue industrial process. This suggested that these additives presented a good potential to be introduced in the production of high-quality tissue materials such as tissue masks or premium napkins and facial papers. These biocompatible and biodegradable additives can promote the maximization of eucalyptus fibers in industrial tissue production, optimizing the process and associated manufacturing costs.

**Author Contributions:** Conceptualization, F.P.M. and J.M.R.C.; methodology, F.P.M. and J.M.R.C.; software, F.P.M. and J.M.R.C.; validation, F.P.M. and J.M.R.C.; formal analysis, F.P.M. and J.M.R.C.; investigation, F.P.M. and J.M.R.C.; resources, M.E.A., A.M.M.S.C. and J.M.R.C.; data curation, F.P.M. and J.M.R.C.; writing—original draft preparation, F.P.M. and J.M.R.C.; writing—review and editing, F.P.M., M.E.A., A.M.M.S.C. and J.M.R.C.; visualization, F.P.M. and J.M.R.C.; supervision, M.E.A. and J.M.R.C.; project administration, J.M.R.C.; funding acquisition, M.E.A., A.M.M.S.C. and J.M.R.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by Project InPaCTus—Innovative Products and Technologies from eucalyptus, Project No 21 874 funded by Portugal 2020 through European Regional Development Fund (ERDF) in the frame of COMPETE 2020 no 246/AXIS II/2017.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.

**Acknowledgments:** The authors are also very grateful for the support given by the Fiber Materials and Environmental Technologies Research Unit (FibEnTech-UBI) on the extent of the project reference UIDB/00195/2020.

**Conflicts of Interest:** The authors declare no conflict of interest.
