2009 Volume 1

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.

The runnability of a wet web is the sum of many factors, ranging from furnish variables to papermaking running parameters (speed, draws, distance from wet pressing etc). The relative importance of these factors was studied using several different experimental methods. The dynamic stress-strain relationship was determined in situ by measuring it on a wet web winder installed on a pilot paper machine. It was then compared with values obtained by testing the wet rolls on a separate running device. The comparison suggests that tensile strength is a more fundamental characteristic of the stress-strain curve than the dynamic stiffness affected by creep. Tensile strength is dominated by moisture content in a transition region where free water enters the fiber network. Its sensitivity to moisture content weakens as the paper becomes very wet. The location of the transition region depends on the fiber saturation point. This leads to complex changes in ranking when different pulps are compared at different moisture contents. The fines content of the furnish has a significant impact on wet web strength, whereas the fiber stiffness affects the measured dynamic stiffness but not tensile strength.