Materials 2025 , 18 , 228
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aspects are also taken into account even more if the person is uncertain between two or three equivalent products to purchase. It is not very hard to understand that, for instance, when buying a kitchen roll or a paper towel, a product with a good absorption and resistance are paramount, since this kind of product was designed precisely to assist people in cleaning tasks. Thus, the product in question should absorb liquids efficiently and it should be resistant to breaking to not falling apart easily during the task [4,5,7]. If the purchased product delivers in the performed task, the client probably will continue buying it. If not, it may most likely be substituted for a new one, more in line with the demands of the user. Now, if we consider products not designed for this kind of task but, instead, to be used primarily for hygiene or cosmetic purposes, in contact with the skin, one of the most important parameters to consider, if not the most important, is softness [1,2,4,7]. Softness along with strength and absorption properties are influenced by the raw materials and paper process operations [8]. Softness is a very complex feature and relies on many aspects, which is why it has motivated many different studies with very interesting findings that have been published over the years [9–33]. For instance, a stylus instrument using a modified gramophone cartridge was implemented by Hollmark [9], a mechanical stylus surface analyzer was used by Rust et al. [10], the FRICTORQ equipment was designed by Lima et al. [12], topographic modelling was explored by Rosen et al. [14], a prediction neural model was developed by Rastogi et al. [15], and acoustic emission analysis was used by Kraljevski et al. [30] to address tissue softness. In addition to the presented studies, the works of Hollmark and Ampulski [34] and Pawlak et al. [35] present very comprehensive review studies regarding softness and its measurement that were and are still widely used by different researchers all over the world. On the aforementioned studies, references to the use of subjective panels, the Tissue Softness Analyzer [36], the Kawabata Evaluation System [37,38], and image analysis and processing techniques are very common. What is also very common regarding this topic is the usage of two different methods to assess softness in order to establish a correlation of the results for comparison purposes. The current work aims to accomplish the same goal. However, instead of using only two methods, it explores the four indicated methods to assess softness from a multifaceted perspective on a set of several tissue paper products belonging to the hygiene category. The results obtained in the conducted experiments were then processed and compared to each other, allowing us to organize the tissue products, determine the differences between approaches, identify cases of interest, and, when one or more of the methods differ in their rankings, understand exactly why that happened. The following sections of this work will address the methodologies that were used in our experiments, the principal results that were obtained, and, finally, the main conclusions achieved.
2. Materials and Methods 2.1. Set of Paper Products
In this work, 29 different paper products of the hygiene tissue category, designated from now on as T01 to T29, were selected to form our set of samples. These products specifically were chosen for our tests due to their market demand, and also because the entire set covers a wide range in terms of softness. As a side note, the initial order of the tissue products in the set are solely based on their arrival at our facilities and is not related with any of their characteristics. The majority of the tissue products, 18, to be more precise, are 2-ply papers, of which 9 are from the industrial line, while the other 9 are from the commercial line. Of the remaining 11 products, 4 of them are 3-ply papers, 4 of them are 4-ply papers, and, finally, 3 of them are 5-ply papers, all being from the commercial lines, as depicted in the diagram of Figure 1.
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