An Analytical Approach to Assess the Interrelation of Surface Properties and Softness of Tissue Paper

The tissue industry constantly strives for improving the overall quality of tissue paper, as customers pay more attention to special quality features when it comes to a purchase decision between different products. As producers need to optimize their processes and furnish mixtures to keep production costs low, accurate and fast methods are crucial for characterization of important tissue properties. Especially surface roughness and its impact on softness is of great importance, standardized and objective test methods to correlate these properties are, however, still rarely described in literature. Herein, we present a method for the characterization of the tissue surface regarding roughness and describe its relation to the tissue softness properties, based on a sample set of dry-creped bath tissue (DCT) with different amounts of softwood (SW), hardwood (HW) and non-wood pulp (NWP). The surface of tissue is complex and consists of several overlying structural features, thus, the optical non-contact measurement principle of focus variation was used to provide robust and reliable topographical surface information. Based on the obtained 3D data, areal surface analysis was performed to determine the surface roughness of the tissue samples, which is described by the developed interfacial areal ratio (Sdr) and the power spectral density (PSD). To determine the softness properties of the tissue the industrially widely applied tissue softness analyzer (TSA) was used. The surface softness (TS7) and the stiffness (D) parameters of this instrument were considered for surface and structural characterization of tissue softness. The results of the surface roughness (Sdr and PSD) and surface softness TS7 measurements show a good linear correlation, with higher surface roughness implying a higher surface softness. With decreasing stiffness of the tissue network, an increase in surface softness and roughness becomes evident. The non-panel-based evaluation of these aspects of tissue softness allows an objective, fast and accurate assessment of the relevant properties and is also applicable to other hygiene products.