Materials 2025 , 18 , 228
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The first method, defined as our reference for all the remaining and subsequential work, was the Tissue Softness Analyzer (TSA) [36]. This laboratory equipment simulates the sensa- tion of touch and measures a series of parameters—“real” softness, roughness/smoothness, and stiffness—that are used to estimate a measure of overall softness for the tested tissue papers. This measure is designated handfeel (HF) and is calculated by the TSA equipment through specific algorithms depending on which samples are being tested. In our particular case, the chosen algorithm was the one denominated TPII [36]. For the second implemented method, a Subjective Evaluation (SUB) of all the 29 finished tissue products was carried out in blinded experiments to avoid bias, by 12 different evaluators that have been previously selected to complete this task, depending on their performances in prior tests. In general terms, to each person of the panel was presented a particular sample of the set that he/she had to compare with 5 references in terms of the 3 parameters: bulk, roughness, and flexibility. Then, the evaluators gave a score for each one of the parameters relative to the references. In the end, a global measure of softness for the 29 tissue product samples was calculated based on the 3 tested parameters, considering the average score of all the 12 evaluators in order to improve the accuracy of the method. The next methodology used by our research team was the Kawabata Evaluation System (KES) [37,38]. This very versatile equipment, composed by different test modules, was used to perform analyses of compression, roughness, and bending, which, similarly for the TSA equipment, allowed us to reach a global value of softness for each one of the 29 tissue products. The fourth method implemented in the current work was an Optical System (OPT), already used recently in related works, to perform the characterization of different industrial base tissue papers [39], and to study the effect of different embossing patterns [40]. This system was then used to reconstruct the 3D maps of the 29 finished tissue products, and, through those maps, obtain their surface topographies, and other values, related with their roughness, thickness, and bulk, which were also used to calculate an overall measure of softness for the tested products. All the data acquired with the 4 described methods (TSA, SUB, KES, and OPT) were collected in several experiments carried out at the facilities of the Forest and Paper Research Institute—RAIZ, and University of Beira Interior—UBI. Finally, in the scheme of Figure 2, one can realize that the Methods section ends with a comparison of the different assessment approaches, which is the core of the whole work. However, it must be pointed out that this comparison of methods was defined from the very beginning with a few considerations to simplify the interpretation of the results. The reason for this was the fact that each method has an enormous amount of data associated with it, especially the KES and the OPT methods. Some of the measured parameters are directly proportional, others are inversely proportional, and some are even represented in different ranges and at different scales, which increases greatly the complexity of the results interpretation, as it is not an easy task to compare them directly. Having these points in mind, a very simple procedure was taken into account in this work in order to make the comparison between methods straight forward, and easy to understand, since all of them will be expressed in the same way. Basically, the idea is to compare the methods among themselves by simply knowing the position of the tissue products on a ranking table, regardless of what values were obtained through each one of the four used methods independently. To better understand this idea, let us suppose that only 3 tissue products, A, B, and C, compose our set. By using the TSA equipment on these 3 products, let us say that a high handfeel of 91 is obtained on A, a low handfeel of 50 is obtained on B, and an intermediate
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