2009 Volume 2
The literature related to the three-dimensional ﬁbre networks is reviewed since 2000. Indeed, a review was presented in these symposia in 2001 . Therefore, the updated articles are only considered here. The previous review concentrated on theory, whereas the focus here is on modern 3D analysis and comparisons of these with theory. Moreover, the focus is on paper structure. However, the general context of 3D structure is considered in the introduction in order to illustrate the main ideas that may be applied to paper. Then, the experimental methods are presented in order to show the potential of such techniques. In a third part, improvements of the description of the 3D structure will be presented. Namely, the quantitative description that completes the visualisation of the structures will be presented. The main 3D morphological properties will be presented and some examples will illustrate the existing developed tools. The Representative Elementary Volume (REV) dedicated to the structure properties will then be introduced. The theoretical models are brieﬂy presented to prove the necessary development of both experimental tools and dedicated theories. Theoretical studies will be exempliﬁed. Indeed, both theoretical and experimental demarches will enrich each other as will be shown. The inﬂuence of both deformation and humidity modiﬁcations on the 3D structure will illustrate the interest of the knowledge of the 3D structure, to tackle its inﬂuence on physical properties. The main perspectives and challenges related to the structure description will end up this presentation.
We use simulation and analytic modelling to probe the structural similarity reported in the literature for ﬁbre networks with manifestly different degrees of uniformity. From simulations of point processes in the plane to represent random, clustered and disperse ﬁbre centres, we show that the distribution of distances between pairs of centres is very insensitive to the extent of clustering. Further, we quantify the correlation between the lengths of adjacent polygon sides arising from a Poisson line process in the plane as being ρ = 0.616 ± 0.001 and show that this is very insensitive to ﬁbre orientation and only weakly inﬂuenced by clustering. The relevance of this correlation to pore geometry is discussed.
In the ﬁnal part we analyze simulated areal density maps and show that their variance relative to that of a random ﬁbre network of the same constituent ﬁbres, as quantiﬁed by the formation number, depends at small scales on the ﬂocculation intensity only and depends at large scales on the number of ﬁbres per ﬂoc only.
Oxfordpp 693-719Spatial Partitioning of the Structural Properties of Tissue and Towel GradesAbstractPDF
This paper describes an investigation conducted to examine the mesoscale structural properties of single-ply tissue and towel papers by mapping thickness, out-of-plane deformation, formation, and density. The uniformity of each of these properties was studied to a zone size of 0.1 mm in mapped regions of about 10 mm square. The sampling regions were partitioned into subsets that were separately analyzed. Regions of interest included wet pressing marks, embossed patterns and through air drying (TAD) pressed patterns. The relationship between thickness and grammage (basis weight) in the different regions of interest were compared. Differences were attributed to the indentation process that could affect the structure in different ways, depending on the process conditions. Six commercial tissue and towel products were tested.
The regional differences were most apparent in the results for paper towels. Deformation of the structure to form textured patterns compressed regions and increased the local density. There was selected in-plane movement of fibers, in response to the indentation. The towel sample formed by wet pressing did not show the same extent of densification at the indentation site. The structure was deformed out of plane, but not significantly com- pressed. Tissue samples formed by conventional wet pressing showed increased densification at points of indentation. Differences in the out-of-plane response to the pressing process could be distinguished between different tissue samples.
Oxfordpp 721-747Introducing a Concept to Link 3D Paper Structure to 2D Paper PropertiesAbstractPDF
This paper introduces a concept that spatially links local paper properties from a 3D paper dataset to a 2D paper property map of the same sample. The concept exploits the fact that paper is a planar structure. This permits successive conversion of 3D paper datasets first to a 3D surface (the paper center surface) and further to a plane. The flattened 3D paper data can then be linked to the 2D paper property map. Laser micro holes applied to the paper are employed as marks for alignment of the 2D maps. The key step of the procedure is flattening the paper data from a warped 3D surface to a 2D plane.
Some exemplary applications for the introduced method are given. The effect of local coating layer thickness on local bright- ness and local print density is examined for wood free coated paper. Furthermore, the concept can be applied for quantitative comparison of data from different 3D-imaging techniques. We measured local coating layer thickness on the same paper specimen applying first fan-beam X-ray microtomography and then microtome serial sectioning. The results are compared using point-wise correlation.
Oxfordpp 841-875Developing a Deeper Understanding of the Constitutive Behavior of PaperAbstractPDF
A constitutive equation for the mechanical behavior of paper is presented. Since the initial work in the late 1940’s rheology studies of paper seem to come in waves and the results have not always been viewed favorably. With the benefit of fifty years of literature and the relative speed of modern computing, a more robust constitutive model is certainly within reach. The presented model captures the essence of the mechanical response and links the stress-strain curve to relaxation and creep in a reasonable manner. Experimental results are provided to illustrate the important components of the behavior and the model can easily be generalized to include more subtle features.
The notched shear test (NST) will be proposed to measure shear strength of paperboard by utilizing standard in-plane tensile testing equipment. The test is a further development of the double notch shear test specimen; where plastic lamination has been utilized. With the new test setup it becomes possible to measure shear strength profile in the thickness direction of paperboard. As a spin-off of the NST also the strip shear test (SST) was suggested. The SST test can be used as a quick measure of shear strength. It is shown that the SST measurement correlates well with the traditional rigid shear test (RST). In order to verify that the NST specimen failed due to shear stresses, finite element simulations were performed. The simulations showed that the shear zone had a uniform shear stress field at the time of failure. Moreover, with the finite element simulations it was also possible to predict the force-displacement curve.
Oxfordpp 899-930The Influence of Microfibrillated Cellulose, MFC, on Paper Strength and Surface PropertiesAbstractPDF
This study demonstrates recent advances in microfibrillated cellulose (MFC) research. This novel material, which is mostly com- posed of nano-sized cellulose fibrils seems to have large potential within several applications. Three case studies are considered, i) MFC films that may be applied for packaging applications, ii) MFC applied in the paper bulk and iii) MFC applied as a coating layer on paper. It has been demonstrated that MFC forms films with high strength, high density and low permeability. The tensile strength of the TMP sheets assessed in this study have been increased by more than 20% by addition of 4% MFC to the furnish. Application of MFC as a coating layer on paper forms a dense continuous surface film. However, despite a considerable reduction of the surface roughness, the paper gloss was reduced. This contradictory phenomenon is most probably caused by the MFC layer covering the clay particles on the surface of the base paper.
In this study, a new technique for producing almost fully aligned paper sheets was developed, and the resulting sheets were used to test the validity of the theory most commonly used to relate zero-span tensile strength to individual ﬁbre strength. The standard theory predicts that a zero-span test of a sheet with randomly oriented ﬁbres should yield a breaking load equal to 3/8 of the load that would be observed if all the ﬁbres were aligned in the direction of loading. It is widely used, in spite of the fact that the underlying assumption of afﬁne deformation is questionable under true zero-span conditions. The results obtained here suggest that the ﬁbre strength may be overestimated because inclined ﬁbres in a zero-span grip actually contribute more than the theory predicts. However, the results also suggest that this effect may be confounded because other factors lead to a variable contribution of individual ﬁbres to the zero-span strength of the sheet.
This document reviews degradation phenomenology and life modelling of paper, illustrates different approaches, showing their advantages and applicability limits and raises problems relevant to practical applications of degradation indicators and degradation rate models and evaluation of paper permanence and durability. It deals with the fundamentals of paper degradation rate theory and life modelling, seeks to develop the degradation rate equations of paper that will closely track real degradation experimental results over extended period under practical service conditions, shows a collection of degradation rate and life data of different paper and paper insulation materials under a varied of in-service environmental conditions, develops continuous distribution kinetics theory of cellulose and paper degradation to form a united basis in deriving the traditional Ekenstam degradation rate equation and the alternative percentage DP (and TS) loss rate equations recently proposed by the present author and his coworker, and presents effective methodology for combining time temperature superposition method and paper degradation investigation towards a more reliable prediction of rate and life of paper degradation over a long time period under real in-service conditions.