Abstract
The surface of tissue paper is relatively complex compared to other paper grades and consists of several overlapping structures like protruding fibres, crepe and fabric-based patterns at different spatial frequencies. The knowledge of tissue surface characteristics is crucial when it comes to improvement with respect to surface softness and the perceptual handfeel of tissue products. In this work we used the optical based, non-contact measurement principle of focus variation for surface characterization of dry-creped, textured and through air dried (TAD) tissue. Based on the three tissue grades, a procedure which includes the characterization of the whole tissue surface throughout different scales within one setup, was developed. Surprisingly, focus variation was rarely used in tissue-related research, as it provides robust and reliable 3D surface information which can be used for further areal surface analysis. Special attention was given to the preparation and discussion of the raw data up to the final analysis including several spatial filtering steps. Enhanced surface parameters like the developed interfacial area ratio (Sdr) and the power spectral density (PSD) were used to describe the surface adequately. The surface roughness of the three tissue grades was compared, with the textured tissue showing the highest roughness in Sdr and PSD analysis. Although both methods are based on different principles, a high correlation in terms of evaluated roughness is evident. Regular structures like crepe and patterns are obtainable as peaks at the respective frequency with a certain intensity in the PSD evaluation. Apart from topography in terms of structures and roughness, the wide field of view of the focus variation measurement also allows assessment of effects related to flocculation and sheet formation. The developed procedure could also be appropriate for other fibre based materials and/or fabrics, which are similar to tissue with respect to optical properties such as for example nonwovens.Graphic abstract
Highlights
In contrast to other paper grades, tissue paper is often in intensive contact with the human skin, especially facial and toilet tissue
On the other hand the surface roughness is accessible for analysis without the underlying topographical structure that might affect the used roughness parameters
To power spectral density (PSD) evaluation surface roughness can be described with the developed surface area ratio (Sdr)
Summary
In contrast to other paper grades, tissue paper is often in intensive contact with the human skin, especially facial and toilet tissue. The subjective perception of the overall quality and the softness of the tissue plays a major role when it comes to a purchase decision of the costumer. Depends on many physical properties which can be related either to a surface or bulk component (Hollmark 1983). When it comes to bulk softness, paper stiffness, respectively paper flexibility plays a major role and is assessed in various ways (Hollmark and Ampulski 2004; Ko et al 2017; Park et al 2019). The other major component is tissue surface softness which is depending on the structural surface characteristics. Independent from the analytical tool of softness evaluation, the surface structure and topography of the tissue affects the results
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