Ulrich Hirn - Profile on Academia.edu (original) (raw)

Papers by Ulrich Hirn

The underlying mechanism of hygroexpansion of paper products' deformation resides on the microsca... more The underlying mechanism of hygroexpansion of paper products' deformation resides on the microscale and is a product of the coupling between hydrophilic fibers and micromechanical details of the fiber network, including the geometry and alignment of fibers and bonds. We present a micromechanical framework suitable for studying hygroexpansion from the fiber-and bond-level. Using the developed model, we show that due to the threedimensionality of the bond, the transverse deformations of the fibers are not transferred effectively to the in-plane deformation of the sheet. At the same time, the longitudinal deformation of the fiber accounts for a large portion of the hygroexpansion even in highly oriented sheets. On the other hand, the out-plane deformation of paper is predominantly controlled by the strain gradient in the bonds which stems from transverse shrinkage or expansion of the fibers in the bond region. Therefore, considering the bonds as three-dimensional entities is vital for the analysis since a two-dimensional approximation overestimates the impact of the transverse deformation of the bonds to the in-plane properties and underestimates moisture-induced out-of-plane deformations of the fiber network.

Tensile tests of individual bleached kraft pulp fibers and paper sheets of industrial origin were... more Tensile tests of individual bleached kraft pulp fibers and paper sheets of industrial origin were conducted in order to investigate the effect of refining and small-scale fiber deformations on the extensibility of fibers and paper. The shape of the tensile curves of most of the fibers was concave upward (i.e., increasing slope) and consisted of two or three phases suggesting that the fibrillar structure and disordered regions in the fiber wall were straightened out during straining. Only a few of the individual BSKP fiber tensile curves were apparently linear. Elongation of the individual kraft fibers varied from 8 to 32% and the average elongation was not increased by high consistency refining. Tensile test results of laboratory sheets made of the same BSKP pulp suggested that the fiber bonding not only governs paper strength, but also is highly relevant for the elongation of fiber networks. The key conclusion related to this investigation and freely dried sheets was that the increased network elongation and strength after refining is mainly due to increased inter-fiber bonding and a higher shrinkage tendency of the fiber network and not due to the increased elongation or strength of individual fibers.

Chitosan is a versatile biopolymer with many interesting functionalities. Its effects on the barr... more Chitosan is a versatile biopolymer with many interesting functionalities. Its effects on the barrier and mechanical properties of single-or double-coated fibre-based packaging papers in dependence on the applied drying regime were successfully tested. Our investigations revealed chitosan to be a highly robust biopolymer, since the different drying regimes did not alter its contribution to the development of strength and barrier properties of the coated packaging papers. These properties showed a stronger influence of the applied coat weights than of the different drying regimes. The effect of chitosan coatings were quantified by measuring tensile strength (TS), burst strength (BS) and tensile energy absorption (TEA). These revealed that TS, BS and TEA of the coated papers increased significantly. Moreover, the chitosan-coated papers were less permeable against water vapor and air. High grease resistance was observed for double-coated papers, irrespective of the drying regimes. The coated paper surface showed a more hydrophilic character, resulting in lower contact angles and higher water absorption properties. In this study, industrially produced chitosan has been proven to be a renewable, robust biopolymer that can be utilized as an additive to increase strength and the barrier properties of fibre-based materials.

Changes in moisture content of single pulp fibers have an immense influence on the behavior of pa... more Changes in moisture content of single pulp fibers have an immense influence on the behavior of paper and paper products. Here, an atomic force microscopy (AFM)-based method is applied to investigate the viscoelastic properties of pulp fibers at varying relative humidity (RH) in the transverse direction. Pulp fibers have not only anisotropic properties, but also a very rough surface due to their hierarchical structure. For this reason, we have developed a specific load schedule for the AFM-based test method to overcome uncertainties and limitations due to surface roughness of the pulp fibers. The evaluation of the experimental data combines contact mechanics and viscoelastic models which consist of springs and dashpots in series or parallel describing elastic and viscous behavior. Here, it will be demonstrated that the so-called Generalized Maxwell (GM) model yields comparable results for single pulp fibers at five different RH values and in water. The moisture changes lead to a decrease in the elastic modulus but increase in the relaxation effects with increasing RH. All the determined parameters for the elastic and viscous behavior exhibit a gradual decrease with increasing RH from 10 to 75% RH. The elastic moduli decrease by a factor of 10 and the viscosities are decreasing by a factor of 10-20. In water, there is an even more pronounced decrease of the elastic moduli by a factor 100, and the viscosities decrease by at least three orders of magnitude compared to 10% RH. This indicates that the mechanical response of pulp fibers in water is significantly different than in humid air. This is also illustrated by the fact that a GM model of order two suffices to describe the material behavior in humid air but a GM model of order three is necessary to fit the material behavior in water. A possible interpretation is an additional relaxation effect of the pulp fiber wall in water.

The degree of molecular contact, i.e. the contact area on the nanometer scale, between paper fibe... more The degree of molecular contact, i.e. the contact area on the nanometer scale, between paper fibers is crucial for the van-der-Waals and hydrogen bond adhesion between the fibers and thus for the fiber-fiber bond strength. We apply Förster resonance energy transfer (FRET) to investigate the degree of contact in the distance range of 1-10 nm between pulp fiber bonds and between thin films. The FRET system with DCCH and FTSC as fluorescence dyes has been validated for spectrophotometry and for local imaging with widefield microscopy, using pHema thin films. Cellulose (2019) 26:7037-7050 https://doi.org/10.1007/s10570-019-02575-x( 0123456789().,-volV) ( 01234567 89().,-volV)

Frontiers in Chemistry, 2019

An approach for themultilayer density analysis of polysaccharide thin films at the example of cel... more An approach for themultilayer density analysis of polysaccharide thin films at the example of cellulose is presented. In detail, amodel was developed for the evaluation of the density in different layers across the thickness direction of the film. The cellulose thin film was split into a so called “roughness layer” present at the surface and a “bulk layer” attached to the substrate surface. For this approach, a combination of multi-parameter surface plasmon resonance spectroscopy (SPR) and atomic force microscopy (AFM) was employed to detect changes in the properties, such as cellulose content and density, thickness and refractive index, of the surface near layer and the bulk layer. The surface region of the films featured a much lower density than the bulk. Further, these results correlate to X-ray reflectivity studies, indicating a similar layered structure with reduced density at the surface near regions. The proposed method provides an approach to analyse density variations in thin films which can be used to study material properties and swelling behavior in different layers of the films. Limitations and challenges of the multilayer model evaluation method of cellulose thin films were discussed. This particularly involves the selection of the starting values for iteration of the layer thickness of the top layer, which was overcome by incorporation of AFM data in this study.

RSC Advances, 2018

In this study the short timescale penetration and spreading of liquids on porous sheets is invest... more In this study the short timescale penetration and spreading of liquids on porous sheets is investigated. Three measurement techniques are evaluated: ultrasonic liquid penetration measurement (ULP), contact angle measurement (CA) and scanning absorptiometry (SA). With each of these techniques liquid penetration as well as surface wetting can be measured. A quantitative comparison between the methods is carried out. For our studies we are using model liquids with tuneable surface tension, viscosity and surface energy which are the governing parameters for pore flow according to the Lucas–Washburn equation. Scanning absorptiometry turns out to be an adequate tool for direct measurement for liquid penetration. Ultrasonic liquid penetration showed a stable correlation (R2 ¼ 0.70) to SA and thus also gives a suitable indication on the liquid penetration behaviour. Absorption of individual microliter drops measured in the CA instrument showed different results than the other two measurements. For characterisation of the wetting behaviour the measurement techniques gave substantially different results. We thus conclude that ULP and SA do not capture the wetting behaviour of liquids on paper in the same way as conventional contact angle measurement, it is unclear if their results are meaningful. Finally we are proposing two parameters indicating a combination of liquid penetration and wetting, the slope of the contact angle over time dq/dt and a contact angle calculated from SA. These two parameters are moderately correlated, supporting the idea that they are indeed capturing a combination of liquid penetration and wetting. While our investigations are restricted to paper, we believe that the methods investigated here are generally applicable to study liquid absorption in thin porous media like microfluidic paper based analytical devices, thin porous storage media, membranes and the like. Our findings are highlighting the importance to have a match in timescale (time for penetration and wetting) and size scale (liquid amount supplied) between the testing method and the actual use case of the material, when analyzing wetting and penetration on porous materials.

Cellulosic materials are highly sensitive towards environmental changes such as temperature and e... more Cellulosic materials are highly sensitive towards environmental changes such as temperature and especially towards humidity. Besides morphological changes like swelling and/or shrinking, the mechanical properties of pulp fibres and fibre to fibre joints change as well. The current study sets to elucidate the changes and the extent to which elevated or decreased relative humidity (RH) influences the load bearing capacity of individual hardwood and softwood fibres and joints. The device used for the measurements is a micro bond tester with a custombuilt humidity chamber in which the humidity can be controlled in a range from 25 to 95% RH. Individual fibres and joints have been tested at 30, 50 and 80% RH after equilibration time of 120 min. The breaking load of individual softwood fibres and joints displayed a maximum breaking load at 50% RH, with the values showing a decreasing trend towards higher or lower RH. In case of hardwoods, no change in the breaking load of either individual fibres or fibre to fibre joints has been observed at different RH.

Trans. of the XVIth Fund. Res. Symp. Oxford, 2017

In this paper we give a literature overview on three different aspects of pulp fiber-fiber bondin... more In this paper we give a literature overview on three different aspects of pulp fiber-fiber bonding. First we are reviewing how the adhesion between the pulp fibers is created by the capillary pressure during drying of a sheet. Second we are discussing the individual mechanisms relevant for fiber-fiber bonding. They can be grouped in three different groups: (a) The area in molecular contact , which also includes interdiffusion. (b) the intermolecular bonding mechanisms hydrogen bonding, van derWaals forces and Coulomb interaction. (c) the mechanical bonding mechanisms which are capillary bridges and mechanical interlocking. The third and last part of the review discusses the failure process of fiber-fiber bonds and related single fiber-fiber bond testing methods. The general emphasis of the paper is set on providing a general understanding of the processes responsible for how bonds between fibers are created, how they work and how they are failing.

Strength of individual hardwood fibres and fibre to fibre joints

Cellulose, 2016

Heat of Sorption: A Comparison Between Isotherm Models and Calorimeter Measurements of Wood Pulp

Drying Technology, 2015

Comprehensive analysis of individual pulp fiber bonds quantifies the mechanisms of fiber bonding in paper

Scientific Reports, 2015

The process of papermaking requires substantial amounts of energy and wood consumption, which con... more The process of papermaking requires substantial amounts of energy and wood consumption, which contributes to larger environmental costs. In order to optimize the production of papermaking to suit its many applications in material science and engineering, a quantitative understanding of bonding forces between the individual pulp fibers is of importance. Here we show the first approach to quantify the bonding energies contributed by the individual bonding mechanisms. We calculated the impact of the following mechanisms necessary for paper formation: mechanical interlocking, interdiffusion, capillary bridges, hydrogen bonding, Van der Waals forces, and Coulomb forces on the bonding energy. Experimental results quantify the area in molecular contact necessary for bonding. Atomic force microscopy experiments derive the impact of mechanical interlocking. Capillary bridges also contribute to the bond. A model based on the crystal structure of cellulose leads to values for the chemical bonds. In contrast to general believe which favors hydrogen bonding Van der Waals bonds play the most important role according to our model. Comparison with experimentally derived bond energies support the presented model. This study characterizes bond formation between pulp fibers leading to insight that could be potentially used to optimize the papermaking process, while reducing energy and wood consumption.

Local fiber orientation and its impact on small scale out-of-planeness

The Austrian Graz Technical University's Institute for Paper, Pulp and Fibre Technology has d... more The Austrian Graz Technical University's Institute for Paper, Pulp and Fibre Technology has developed a new fibre splitting technique by which local fibre orientations can be determined. Paper samples are heat laminated between film layers and split. Each of the two halves is then re-laminated and split again. The process is repeated until the original sample has been divided into approximately 30-100 layers allowing individual fibres to be seen. The fibre orientation in each layer is determined by image analysis. Individual images are then combined electronically to create a three-dimensional fibre orientation model. The technique shows local fibre structures and will detect any local fibre orientation two-sidedness. The method has been used to investigate the structure of woodfree papers exhibiting cockling or ridging. It showed that such flatness defects are caused by localised two-sidedness in fibre orientation. (11 fig, 13 ref)

The Institute of Pulp and Paper Technology, Graz, Austria, has developed an image analysis system... more The Institute of Pulp and Paper Technology, Graz, Austria, has developed an image analysis system to assess the quality of pulp suspension. It consists of a measurement cell providing digital images of fibres and appropriate image analysis software. Recently an analysis of vessel size and number of vessel cells was added to the system. For measurement purposes the pulp suspension is diluted to a consistency of 0.2g/litre and then pumped through the measurement cell where digital images with a resolution of 6.25 micron per pixel are taken. This new tool can be used to evaluate quality of hardwood pulp as well as recycled fibre. Three Eucalyptus pulps with highly different picking propensity were tested for their vessel content. A good correlation between vessel cell count and handsheet pick count was found. Comparing refined and unrefined Eucalyptus pulp, about 15% of the vessels were destroyed during refining, and an additional 20% of the vessels were damaged. For Eucalyptus the va ...

Assessing the refining process via a novel method of evaluating mechanical damage to the outer fibre wall layers

A new method using the swelling behaviour of pulp fibres to study the condition of fibre walls in... more A new method using the swelling behaviour of pulp fibres to study the condition of fibre walls involves treating with a copper II ethylenediamine swelling agent for a specified time to cause intense swelling without dissolution. This causes points in the fibre wall that have been damaged in the refining process to burst, allowing the swollen inner layer to protrude. The protrusions are then examined by microscope. The extent of the protrusions will depend on the stress of processing and gives an indication of the damage caused by individual processes. Test fibres are suspended in a flow cell, with images taken by a CMOS camera fitted to a transmitted light microscope. The extent of swelling in individual fibres is calculated by image analysis. This data provides a measure of the damage sustained by the outer layer of the fibre wall. The method has been used to determine the effect of varying the refining intensity and of different refiner fillings and aggregates on the damage cause ...

Local paper properties and their correlation with local ink transfer (abstract and slides only)

Research has been undertaken to investigate print unevenness of supercalendered paper. High resol... more Research has been undertaken to investigate print unevenness of supercalendered paper. High resolution on exactly the same area of a paper specimen was used to measure five paper properties as well as local print density. Surface maps were generated for local basis weight, local liquid penetration, local brightness, local roughness and local compressibility. The paper was then printed on a laboratory printing press and local print density measured at exactly the same positions the paper properties had previously been measured. The various measurements were then submitted to image analytical registration, which introduces a common coordinate system to the various paper property maps and enabled data for local paper properties in one hand and the resulting print density on the other to be obtained. Data sets for 20%, 40%, 70% and 100% tone value were acquired, from which local print density was modelled using multiple regression analysis. The resulting models were capable of explaini ...

Synthetic images of stochastic structures of defined size have been used to examine three aspects... more Synthetic images of stochastic structures of defined size have been used to examine three aspects of image matching and image correlation. First, it was demonstrated that for correlating images, a minimum area had to be evaluated to prevent accidental correlations, and then the effect of matching error on the image correlation was investigated. Finally, it was determined that similarity maximisation based image matching techniques may also produce misleading correlations between images. In order to have images with exactly defined structure size, synthetic images were generated from Gaussian distributed random numbers. The effect of matching error was studied quantitatively using the stochastic images. It was demonstrated that the effect of matching error can be directly expressed in terms of absolute structure size. Results indicated that the accuracy of image matching must be specified depending on the structure of the images. Random matching of two different images was used to s ...

In offset printing, ink tack forces during film splitting may be strong enough for material to be... more In offset printing, ink tack forces during film splitting may be strong enough for material to become detached from the paper surface. In hardwoods, vessel elements have reduced bonding ability and are more likely to be torn off the surface. This process, known as vessel picking, can be reduced by improving the bonding of vessels to the paper surface. The number and size of vessel cells is significantly influenced by the hardwood type. Eucalyptus, for example, contains numerous large vessel cells, making it prone to vessel picking. However, the size and quantity of vessels varies significantly depending on eucalyptus species and growth conditions. Properties of eucalyptus pulps from eight different suppliers (five in Europe and two in South America) were compared, and revealed marked differences in vessel counts, with the two South American pulps having more and significantly larger vessels than the European pulps. The most effective way to reduce picking is through refining. The i ...