Ulrich Hirn | TU Graz (original) (raw)
Papers by Ulrich Hirn
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.
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.
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.
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 ...
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 ...
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 ...
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.
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.
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.
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 ...
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 ...
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 ...