No abstract

For the mechanical properties of paper, tensile testing has been widely used. Among the tensile properties, the tensile stiffness has been used to determine the softness of low-density paper. The lower tensile stiffness, the greater softness of paper. Because the elastic region may not be clearly defined in a load-elongation curve, it is suggested to use the tensile modulus which is defined as the slope between the two points in the curve. The two points which provide the best correlation with subjective softness evaluation should be selected. Low-density paper has a much lower tensile strength, but much larger elongation at the break. It undergoes a continuous structural change during mechanical testing. The degree of the structural change should depend on tensile conditions such as the sample size, the gauge length, and the rate of elongation. For low-density paper, the tensile modulus and the tensile strength should be independent of each other. The structure efficiency factor (SEF) is defined as a ratio of the tensile strength to the tensile modulus and it may be used a guideline in developing superior low-density paper products.

Surface characterization is important and has many applications in the paper industry. Surface characterization requires both surface roughness and surface friction. The relationship between the two has not been fully established for paper and paperboard. It has been a common practice that only the average property and the standard deviation with the coefficient of variation (COV) are reported for surface roughness and friction measurements. This practice, however, provides few information on surface structure and can lead to wrong judgments because two samples having the same average and the COV can have different physical properties. To avoid such mistake, a new surface characterization method has been developed. To this end, surface roughness- and friction-profiles have been obtained using a latest version of Kawabata surface tester (Model: KES-SESRU, Kato Tech, Kyoto Japan). This new version uses the same stylus for both measuring surface roughness and friction under the same operating conditions. It was found that a correlation between the surface roughness and surface friction was very low. This indicates that they should be independent of each other. Therefore, both should be determined for surface characterization.

Surface characterization is important and has many applications in the paper industry. It includes both surface roughness and surface friction. In determining the surface roughness of paper and paperboard, non-contact methods such as air-leak methods In characterizing the surface roughness of paper and paperboard, it has been a common practice that the average roughness with the coefficient of variation (COV) has been conventionally determined. This practice, however, this runs the risk of drawing wrong conclusions since two different surfaces having the same average roughness and COV can exhibit totally different properties. To avoid such mistake, a stylus-type contact method has been developed to determine surface roughness of paper and paperboard such as printing & writing (P&W), kraft and liners. In this method, surface roughness profile has been generated to determine its variability which has been defined as the mean deviation from the roughness average, referred to as MDRA. In determining the MDRA, it is noted that thickness measurement is not required. In this method, stylus shape and size, contact force of the stylus to the surface, scan speed and resolution have been identified as the key parameters of generating stable surface roughness profiles. It has been further identified that the optimal conditions on these parameters should depend on paper grade. It is suggested that a stylus-type contact method should be used to determine surface roughness of paper and paperboard to help determine their practical applications such as printing, coating and embossing.

Strength, softness, and absorbency have been identified as the key attributes for disposable consumer products such as tissue, towel and diapers. These attributes are generally evaluated subjectively by users. Although subjective evaluation may be necessary for developing such consumer products, it is time-consuming and expensive. More importantly, it seldom provides guidance or direction regarding process control or product development. The main reason is because subjective evaluation is performed after the fact (ex post facto), not before the event (not ex ante). Accordingly, it has been desirable to develop physical test methods to predict in-use performance of a consumer product. In developing physical test methods to link with subjective evaluation, reliable and relevant subjective data is necessary. Developing physical test methods for softness of hygiene paper has been extremely challenging. The objective of this paper is to demonstrate a feasibility of developing physical test methods which can be highly related to subjective evaluation. To this end, it is hypothesized that tissue softness mainly consists of bulk softness and surface softness. It is assumed that the bulk softness should be determined from bulk stiffness which is defined as the slope between the two specified points in a load-elongation curve from tensile testing. It is also assumed that the surface softness should be determined from the surface properties (roughness and friction) from a surface tester (e. g., Model: KES-SESRU. Kato Tech, Japan). It is suggested that the principles used here for developing physical test methods for tissue softness should be also applicable to other physical properties such as strength and absorbency.