Investigation of the Pore Structure of Paper by Novel Porosimetric Techniques: Application to Super and Soft-nip Finishing
Authors:   SCA Graphic Research, Kettle John , Matthews Peter ,   University of Plymouth, Ridgway Cathy ,   University of Plymouth, Wagberg Lars ,   SCA Research
Published: March 2024

Over the years, large research efforts have been spent on determining the pore volume distributions of graphic papers . The pore volume distributions determine the absorption properties and light scattering properties of these papers and can be modified by a variety of calendering conditions. In the present work we apply a variety of tests and void space modelling techniques to a series of five paper samples. The tests range from the standard to the novel, and all depend in some way on the void space structure of the samples. Three void space modelling techniques are presented. The first, traditional, method is based on the Laplace equation. The second model, developed by Yamasaki, implicitly assumes unconnected pores of a range of sizes, some of which saturate. The third, ‘Pore-Cor’, assumes a simplified three-dimensional structure. The samples all used the same 51 gM-2 SC grade paper, the first being uncalendered, and the other four involving combinations of soft- and super-nip , with and without the prior application of steam. Advanced imaging techniques such as optical microscopy and Environmental Scanning Electron Microscopy (ESEM) were used to obtain structural details of the paper cross-sections and paper surfaces, and with the ESEM it was also possible to investigate the effect of moisture on sheet structure. Absorption properties of the sheets were determined by using the well-known Bristow equipment and newly-developed equipment for determining liquid absorption by fibrous sheets based on Liquid / Air Displacement Analysis (LADA). The LADA equipment applies liquid in a well-defined way which enables valid comparisons to be made with common absorption theories. Results from the investigation with liquid porosimetry using water and hexadecane probe liquids show that there is considerable sheet expansion when the sheets are exposed to water. Undoubtedly this will lead to a change in the absorption process when the sheets are in contact with moisture. The shape of the absorption curves in both the LADA and the Bristow test equipment also indicate that this expansion will affect the absorption process. A simple mathematical model was also used to take this into account. The mercury porosimetry data and the liquid porosimetry data were combined to yield a complete pore volume distribution curve for the sheet -structure. These data were then used in the Yamasaki absorption model to simulate absorption, and were compared with the measured absorption values. These results show that the absorption can be simulated with a knowledge of the pore volume distribution curve and basic properties of the absorbed liquid. Large pores dominate the liquid absorption at short contact times (<Is), whereas the smaller pores dominate the absorption at larger contact times, as simulated with Yamasaki model. The overall time limit for absorption is naturally dependent on the total pore volume in the paper. The combined porosimetry curves were also simulated using a recently developed three-dimensional void space modelling package, named Pore-Cor. This package generated three-dimensional structures with the same percolation characteristics and porosity as the experimental samples. The absolute gas permeabilities, of these structures showed the same trend as the permeabilities measured by a Gurley Densometer. The simulated structures facilitate the mathematical investigation of other effects, such as the trapping of non-wetting fluids and the effect on permeability of the inclusion of colloids, Matthews (1).