Proceeding Articles

Single fibre flexibility is widely recognised as an important parameter in the papermaking process. A novel flow cell based method for its measurement was developed at the Institute for Paper, Pulp and Fibre Technology at Graz University of Technology. A special flow channel geometry is used to induce high shear forces in a laminar flow regime. The movement of single fibres passing the highly sheared region is recorded by means of a high speed image acquisition system. Based on the reactions of the fibres to the fluid forces a flexibility parameter is determined for each evaluated fibre.

The perceived value of paper products depends not only upon their performance but also upon their visual appeal. The optical properties of paper, including whiteness, brightness, opacity, and gloss, affect its visual perception and appeal. From a practical point of view, it is important to quantify these optical properties by means of reliable and repeatable measurement methods, and furthermore, to relate these measured values to the structure of paper and characteristics of its constituents. This would allow papermakers to design new products with improved quality and reduced cost. In recent years, significant progress has been made in terms of the fundamental understanding of light-paper interaction and its effect on paper’s appearance. The introduction of digital imaging technology has led to the emergence of a new category of optical testing methods and has provided fresh insights into the relationship between paper’s structure and its optical properties. These developments were complemented by advances in the theoretical treatment of light propagation in paper. In particular, wave scattering theories in random media are finding increasing applicability in gaining a better understanding of the optical properties of paper. In this document, a review of these advancements is presented.

The conventional way to evaluate dimensional stability, regardless of end-use purpose, is to measure the change in dimensions when the moisture content is changed by changing the relative humidity. Sorption of moisture from moist air is a relatively slow process and for the evaluation of printing papers this may not be the most appropriate method.

In the present work, data from conventional hygroexpansion measurements has been compared with data from hydroexpansion measurements, i.e. expansions caused by the sorption of liquid water, sprayed onto papers printed with a random speckle pattern, the expansion being monitored by electronic speckle photography.

Sheets made from different pulps, with different fines contents and different modifications were studied at different grammages and water-transfer levels. The effect of drying-mode, i.e. restrained drying or free drying, was also studied. It was concluded that sheets expand less with a given amount of adsorbed water when it is sorbed in liquid form rather than from moist air. Chemical treatments known to increase both the dry and the wet strength, e.g. polyelectrolyte multilayers and cross-linking through periodate oxidation, did not significantly improve the dimensional stability when the papers were exposed to liquid water.

Cockling and curling are unwanted phenomena that occur already in the manufacturing process and continue through the end use of paper due to moisture content changes. This paper compares the effects of different in-plane tensions and throughthickness drying profiles during the drying process on cockling and curling by means of finite element simulations. In this study, paper is treated as a heterogeneous orthotropic elasto-plastic material. The results predict that finding the optimal way to dry paper is not straightforward; it also depends on fiber orientation structure. On the other hand, defects in the fiber orientation structure of paper can be redressed at some level by drying the paper appropriately.

In this work we demonstrate the use of computerized x-ray micro-tomography and numerical simulations in evaluating flow permeability of fibrous porous materials. This ab-initio approach involves solving fluid flow through material samples in the actual pore space obtained by tomographic techniques. The procedure is applied here in three different materials, namely plastic nonwoven felt, newsprint and wet pressing felt. All numerical results presented are compared with experimental data for the same materials. The non-woven felt material, having a relatively simple structure, is first used as a test case for comparing two different numerical schemes, lattice-Boltzmann method and a finitedifference method. Here, values of both transverse and in-plane permeability are obtained. The transverse permeability of newsprint and wet pressing felt under varying degree of compression is then found using lattice-Boltzmann method. Finally, we apply the same approach in estimating permeability in different structural layers of the wet press felt material. These material parameters are laborious or even unfeasible to determine experimentally. The procedure is applicable e.g. in finding the relevant material parameters for macroscopic models describing calandering, drying and wet pressing processes.

The diffusion of moisture in paper is a complex phenomenon with pore diffusion dominating at low moisture contents and diffusion through fibres dominating at high moisture contents. Vapor diffusion through the pore space depends on the topology of the pore connections. Recently available three dimensional digital reconstructions of the pore space using X Ray Micro computed tomography (XμCT) enable us to determine the impact of the pore and fibre tortuosities and connectivity to moisture diffusion in an explicit manner.

In this study, moisture diffusion was simulated through XμCT reconstructions of paper structures using a hybrid random walk algorithm that was developed to allow simultaneous diffusion in both the pore and fibre spaces with differential ‘intrinsic diffusivities’. The algorithm is specifically applied to simulate simultaneous diffusion under low and high relative humidity conditions where diffusion occurs predominantly through one medium i.e. pore space and high humidity conditions where both media (i.e. fibre and pore spaces) are highly conductive. The ‘intrinsic diffusivity’ of moisture through fibres was determined by using numerical simulation and experimental results. This intrinsic diffusivity a fundamental fibre characteristics is found to be independent of refining level but depends only the fibre moisture content under the conditions studied.

The algorithm also allowed the determination of the anisotropy in diffusivity. One interesting result is that the anisotropy in diffusion is most significant at low moisture contents when diffusion through the pore space dominates. At high relative humidities (i.e. at high moisture contents), fibre conduction provides an alternative diffusion path, homogenizing diffusion to a large extent. As a result, diffusion becomes more isotropic with increased moisture contents in paper.

We have developed a simple model of the air permeability of paper and shown that it is in reasonable agreement with experimental results for softwood chemical pulps. The permeability is given as a function of fibre external height and width and fibre volume fraction including the lumen volume if present. The most important conclusion from the model is that the average fibre thickness in the paper is the critical fibre property controlling air permeability. In fact, the theory predicts that the sheet permeability is roughly proportional to the fourth power of the fibre thickness.

This paper is based on a laboratory-scale experimental study of machine direction microstriations (MDM) on board surfaces. We developed a pressing and drying simulator in which we can replicate some of the phenomena which are believed to be the origin of MDM: density stratification in wet pressing as well as in-plane restraining conditions during drying. Our laboratory experiments showed that we could generate surface features, visually similar to those classified as MDM in industrial paper production. In particular we could replicate the elongated appearance, the characteristic wavelength interval (1–4 mm) and the occurrence on one surface only.

The most important parameter in respect to the absolute amount of surface roughness was the in-plane restraining conditions during drying. Biaxial restraining resulted in much lower surface roughness and prevented the occurrence of MDM. MDM started to appear as soon as uniaxial shrinkage was permitted. Interestingly, however, shrinkage perpendicular to the main direction of fibre orientation in an oriented paper sheet caused a less pronounced occurrence of MDM in spite of a larger absolute value of shrinkage. The surface presented topography features parallel to the restraining direction. The press felt surface, in our investigation the coarseness of the batt fibres, influenced the surface roughness of paper, however at a characteristic length that was much smaller than that of MDM.

Web troughs are defined as an out-of-plane instability of a web in a free span between rollers. Troughs may be only an annoyance in some web process machines but are often detrimental to quality in coating or printing operations.

Web wrinkles are defined as an out-of-plane instability of a web crossing a roller. Wrinkles are much more serious than troughs in that they result in creases, fold-overs, and bursts which can shut down a paper-making, coating, or printing operation or result in decreased quality or productivity.

This paper will provide proof that troughs are a necessary precursor to wrinkles. Examples will show that linear buckling analyses can be used to predict web troughs. Further examples will show how nonlinear post buckling analyses can then be used to predict wrinkling.

The present work is concerned with the system dynamics and stability of the open draw sections of paper machines where web breaks occur most frequently. We have applied a novel particle based system dynamics model that allows the investigation of complex interactions between web property fluctuations and system parameters, without any constraints of a particular geometrical web shape or boundary conditions assumed a priori. The result shows that, at a given machine draw and web property parameters, the open draw section maintains its steady-state until it reaches a certain machine speed limit. At this speed the system looses its stability and the web strain starts growing without any limit, and thus leading to a web break. A similar instability can also be triggered when web properties suddenly fluctuate during steady-state operation. The parametric sensitivity studies indicate that, among the web property parameters studied, the elastic modulus of the wet web has the largest impact on the critical machine speed as well as on the detachment point where the web is released from the first roll. Further analysis shows that the decrease in dryness has a (negative) synergistic effect causing an increased risk of system instability. It is, therefore, most important to control, not only average dryness, but also its variations in order to enhance paper machine runnability.