Relationships between the local apparent density, sheet roughness, and z-direction compressive characteristics

摘要

The z-direction compressive characteristics of paper play a role in printing, calendering, friction, and a number of converting processes. This work measures the z-direction compressive characteristics using a micro-indentation technique. This technique measures the mechanical properties within a spatially localized region, which allows for the characterization of the local compressive properties of the paper sheet as well as the mean compressive properties of the sheet. In this work, the mean compressive tangent modulus is shown to be dependent on the apparent density of the paper sheet, and relatively independent of the densification method (i.e. refining, wet pressing, and calendering) or fiber type used (i.e. Kraft hardwood or softwood). In order to explore relationships between the local properties of the paper sheet, the local grammage (i.e. formation), the local thickness, and the local compressive properties (i.e. tangent modulus) were measured for a set of experimental sheets. Storage phosphor b-radiography was used to measure the local grammage of the samples at a spatial resolution of 100 mm by 100 mm. A double laser profilometer with a nominal inspection area (i.e. spot size) of ～ 10 mm was used to measure the local thickness of the paper sample. Thickness measurements were taken in a grid pattern every 100 mm over the surface of the paper sheet. The local compressive tangent modulus was measured over a 45 mm by 45 mm area with an indentation spacing of 1 mm in each direction (i.e. a total of 2025 data points). The typical contact diameter between the spherical probe and the paper sheet was ～ 500 μm. Relationships between the local grammage, thickness, apparent density, and tangent modulus were explored. It was expected that a similar relationship would be found between local tangent modulus and the local apparent density as was found for the mean tangent modulus and the mean apparent density. However, the correlation between the local apparent density and the local tangent modulus was found to be insignificant. This indicated that the local tangent modulus of the sheet is not dependent on the local apparent density, but is dependent on other factors such as local roughness, local structural variations, and local differences in fiber characteristics. The local thickness variation was taken to be an indication of the local structural variations of the sheet, as the local thickness variation was shown to be sensitive to the local sheet structure. The correlation between this measurement of the local sheet structure and the local tangent modulus was found to be significant, indicating that the local thickness variation, i.e. roughness, is more important for determining the local compressive characteristics of the paper sheet than is the local apparent density.