2013 Volume 1
Cambridgepp 3-39Local Structural Orientation of Towel and Tissue Grades in Two and Three DimensionsAbstractPDF
This paper describes the study of new methods for characterizing the orientation of fiber segments in low density paper towel from two- and three-dimensional X-radiographic data sets. The end use properties of the absorbent hygiene grades such as paper towels and tissues stem from an open porous structure where stochastically distributed fibers are contorted by post forming processes to increase bulk, stretch, flexibility and softness, while maintaining adequate strength. The orientation of free fiber segments that form the network are kinked and curved in three dimensions by processes including creping, through air drying and embossing. Providing a linkage between process conditions and the end use properties through the characterization of the network structure is the overarching goal of this investigation. A method is presented for mapping the 2D, in-plane orientation of fiber segments using soft (6kV) X-radiographs and an algorithm for calculating the image moments for circular sub-regions that surround each point. The eigenvectors form the major and minor axes of the inertial ellipse from which the principal orientation may be extracted. Colorized maps representing the local orientation are used to examine the effects of embossing and creping, as well as comparing different forming processes. A method for characterizing fiber segment orientation in three dimensions uses a similar approach applied to binarized X-ray micro-computed tomographic data sets. The inertial ellipsoid is determined by performing principal component analysis on the covariance matrix of the voxels contained within a spherical region surrounding each solid voxel within the structure. The eigenvectors are used to extract the shape and principal orientation of the ellipsoids which are plotted as colorized representations in 3D space. The 2D and 3D plots demonstrate the sensitivity of the method to orientation of fiber segment mass, while mean fiber orientation plots reveal differences between samples.
Cambridgepp 41-53Revisiting the Random Disk Model: Decomposition of Paper Structure Using Randomly Deposited Disks with Arbitrary Size DistributionsAbstractPDF
In this study, a numerical algorithm was developed to decompose the planar mass structure of paper into a random array of grey disks with a discrete size distribution. The optimum size and the frequency of these disks were determined such that the second order statistics of the corresponding random disk structure resembled that of the paper sample. Using this method, eighty two (82) commercial and laboratory-made samples were analyzed. It was found that; independent of the forming conditions, the average disk size was proportional to the standard deviation of the disk size distribution. The utility of this new tool in analyzing the effect of papermaking conditions on paper formation is illustrated.
Cambridgepp 55-70Information Geometry and Dimensional Reduction for Statistical Structural Features of PaperAbstractPDF
Information geometry provides a metric on spaces of probability density functions. Here we apply it to the space of trivariate Gaussian distributions of joint variation among the areal density variables for pixels and their first and second neighbours, from radiographs and simulations. At a pixel scale of one millimetre these distributions can pick up essential structural features including flocculation intensity and scale. We do this by applying the technique of dimensionality reduction to large mixed data sets of samples and the results show promise for classification, including extraction of groupings that represent different former types. This kind of analysis could be valuable in evaluating trials, comparing different installations of similar formers and for identifying anomalous behaviour.
Cambridgepp 71-100The Effect of Moisture and Structure on Wet Web Strength and Its Variation – A Pilot Scale Approach Using Dry and Rewetted Mill Made PapersAbstractPDF
In spite of extensive research on wet web strength properties and rheology, knowledge of the effect of web structure, e.g. formation and fibre orientation, on wet web strength properties has been limited. Therefore the topic was studied by running the re-wetted mill-made paper reels on a pilot runnability device at low dry solids contents of 56% and 68%. In addition, one trial was conducted by measuring the wet web strength properties in situ on the press section of a pilot Fourdrinier. The bene t of both these approaches is the ability to measure the strength properties in more realistic conditions compared with standard laboratory methods.
In order to differentiate between the effect of formation and fibre orientation on strength properties, the variables should not be correlated. This requirement was met in the main mill trial by suitably selecting the headbox and wire section parameters. Formation was measured using a -radiographic method and local grammage variation was examined as standard deviation in different wavelength bands and size classes. In addition, formation was also measured with Ambertec formation tester. Fibre orientation was determined using layered fibre orientation measurements.
It was shown that formation has an influence on the tensile strength variation and the effect depends on the scale of formation and dry solids content. In contrast to dry strength, the wet strength does not follow the Weibull distribution, but rather the Gaussian one. In addition, the distribution of wet strength is sensitive to centimetre-scale variability in paper structure instead of millimetre-scale in dry paper. When formation is good, as it typically is on modern paper machines, further improvement does not improve average wet strength properties. Only when large scale formation is poor, it has an influence on average wet web tensile strength and tensile stiffness. Presumably this would be the situation on a Fourdrinier type machine. Unlike formation, anisotropy does not affect the strength variation but it has an influence on the average tensile strength and tensile stiffness of wet and dry papers. The anisotropy profile in z-direction has no influence on the mentioned properties.
Cambridgepp 227-259Achieving Maximum Filler Retention by Improving Chemical and Mechanical RetentionAbstractPDF
Conditions for maximizing chemical and mechanical filler retention were studied through a combination of laboratory, semi-pilot and pilot scale experiments. In the first part of this work, we investigated the impact of particle size on the mechanical retention of particles in a fibre network using a modified laboratory hand sheet former. Quartz particles of well-de ned and narrow-size fractions were used to simulate pre-flocculated filler of different sizes. Five different size fractions were studied. The mechanical retention was found to increase linearly with both web fibre grammage for each quartz fraction and with particle size. These results were then validated through pilot- scale production trials where different filler floc sizes were created through pre-flocculation techniques. In the second part of this work, we studied the stability of pre-flocculated filler flocs through a set of semi-pilot scale ow loop trials. Trials were performed by pre-flocculating filler (PCC) with flocculating agents continuously and exposing the filler flocs to controlled levels of hydrodynamic shear created by ow through a partially closed gate valve. Changes in filler floc size were monitored continuously using Focused Beam Reflectance Measurements (FBRM). A clear reduction in the particle size was observed as the pressure drop increased. A major part of the floc degradation occurred at relatively low shear conditions while under the highest shear conditions, the pre-flocculated PCC floc size was reduced close to the unflocculated state. In the third part of this work, we investigated the effect of different forms of process related shear on retention polymer stability and its effect on chemical retention. A set of semi-pilot scale ow loop trials were performed to investigate the effect of elongational shear-strain and shear due to velocity differences created inside and outside a dosage nozzle respectively. We show that the effect of elongational strain created inside the dosage nozzle leads to significant retention polymer degradation while shear created outside the dosage nozzle due to velocity gradients has a smaller effect on polymer degradation. We investigate these results with a series of pilot scale production trials and show that high shear conditions created inside the dosage nozzle leads to significant reductions in chemical filler retention. However, pilot trials indicate that shear created outside the dosage nozzle can also have a significant effect on filler retention, although to a lesser extent.
Pulp screens remove contaminants from pulp suspensions and are critical to the production of high-quality paper products. Screen performance is determined by two internal components: a cylinder with apertures that pass acceptable fibres and block oversize contaminants, and a rotor that clears the apertures of any blockages. Capacity is an essential parameter of screen operation and a necessary consideration in evaluating changes made to enhance debris removal or reduce power consumption. The present study uses a pilot pulp screen to assess capacity limits, and a specialized laboratory screen with a-high-speed video camera to study what happens at a screen aperture.
What results is an understanding of some mechanisms related to the deposition and removal of fibres at an aperture where there is a time- varying ow bifurcation, and which is proposed herein as the essence of screen capacity.
Cambridgepp 301-319Stochastic Analysis of the Critical Stable Velocity of a Moving Paper Web in the Presence of a CrackAbstractPDF
In this study, we present a probabilistic approach for analysing runnability of a moving paper web with random defects. The paper web is modelled in the case of an open draw as an axially moving elastic plate that has an initial crack of random length. We derive a formula for the optimal velocity, at which the probability of fracture is limited. We study edge and central cracks perpendicular to machine direction (mode I cracks) and oblique central cracks (mixed mode). The crack length is modelled with the Weibull distribution. The effect of changing the value of distribution parameters and the probability of fracture on the optimal velocity is illustrated. It is found that the optimal velocity decreases when the expected value and variance of the crack length increase. The results also show the relation between the effect of edge and central cracks on the optimal velocity. The study is fundamental for rigorous analysis of the paper making process.
Cambridgepp 321-357Review: Variability, Non-uniformity and Complexity: From Product to ProcessAbstractPDF
The refining impulse is here de ned as the product of the normal loading force by the time. The refining impulse can be used in a beater for understanding and controlling the refining effects on fibres. The SR-degree and WRV evolutions depend only on the proposed refining impulse. For the shortening evolution of fibres, the normal loading force has to be introduced as a supplementary variable. By analogy with the variables controlling the pressing operation, namely press impulse and maximal applied pressure, the refining impulse alone, or complemented with the normal applied force have been experimentally shown to control the kinetics effects of refining on the fibrous suspension.
Cambridgepp 369-401A Proposed Link between Machine Runnability and Stickies Distribution within the SheetAbstractPDF
In recycled paper processes, stickies are at the origin of many production disturbances, such as machine breaks, defects in paper and converting problems. At the end of the recycling process, the most abundant and disturbing macro contaminants are fragments of pressure sensitive adhesives. These particles adhere to machines clothes, and clog the felts or even cause the break of the running paper web. The contamination is typically evaluated by measuring the total stickies concentration in the pulp after screening. However, industrial experience shows that it is difficult to correlate this stickies concentration with the occurrence of process disturbances. We suggest that only the amount of stickies that is effectively exposed at the surface of the sheet to the machine clothes is disturbing and is at the origin of runnability problems. In this work, we recall the definition of the stickies exposure, and use it to anticipate the effect of geometrical parameters on the fraction of stickies that are exposed at the surface of the sheet. Parameters such as stickies length and thickness, sheet thickness, or stickies orientation in the z-direction, are investigated. A new sensor is developed to characterise stickies in their 3 dimensions (without prior pressing), and discriminate them from other type of contaminants. Improvements compared to classical stickies measurements methods are discussed. The exposure of real stickies populations to machine clothes is measured in handsheets, and compared with results from the modelling.