3.2.1. Structural Properties

Bulk and porosity are essential parameters for the quality of tissue products that influence the softness and liquid absorption properties [6,7]. Figure 6a shows, for the different tissue formulations, the behavior of the structural properties. The addition of CBA in formulation 2 promoted a decrease in bulk and porosity by 1% and 0.1%, respectively, compared with formulation 1. The addition of CMF (formulation 3) showed a tissue structure with lower bulk (<4%) and porosity (<0.5%) compared with formulation 1. The combination of these versatile additives (formulation 4) also promoted an increase of hydrogen and inter-fiber bonds, inducing a decrease of 11% in bulk and 1% in porosity of the tissue structures in comparison with formulation 1. This densification effect can be observed in the cross-section of the structures, as shown in Figure 7. With the reduction of the reinforcement fibers by 15% in the formulations, bulk and porosity were increased. This reduction promoted an increase of 20% in bulk and 2% in porosity when comparing formulations 1 and 5. The same trend in formulations 1–4 was observed in formulations 5–8; i.e., the combination of both additives presented a higher influence on the structural properties followed by CMF and CBA. Formulations 6, 7 and 8 promoted a reduction in bulk by 4, 10 and 22%, respectively, and a reduction in porosity by 0.4, 1 and 3%, respectively, compared with the control (formulation 5). The addition of both CBA and CMF in formulations with a softwood fiber reduction (90% eucalyptus fiber and 10% softwood fiber) allowed the obtaining of structures with increased structural properties compared with typical industrial structures (75% of eucalyptus fiber and 25% softwood fiber). The results indicated that formulations 6 and 7 increased the structure bulk by 16% and 9%, respectively, compared with formulation 1 (Figure 8). However, the combination

of these two additives promoted a 6% reduction in bulk compared with this formulation typically used industrially.

**Figure 6.** Properties of bulk and porosity (**a**), softness (**b**), tensile index (**c**) and water absorption capacity per unit of gram (**d**) of all tissue formulations.

**Figure 7.** SEM images of the cross-section of the tissue structures prepared with formulation 1 (**a**) and formulation 4 (**b**).

**Figure 8.** Percentage of variation in the properties of bulk, softness HF, softness TS7, tensile index and water absorption capacity of formulations 6, 7 and 8 compared with formulation 1 obtained with SimTissue.
