articles by Antti I . Koponen
When dealing with papermaking fiber suspensions, particle flocculation takes place even before th... more When dealing with papermaking fiber suspensions, particle flocculation takes place even before the paper web is formed. The particle flocculation depends on several aspects, including particle mass concentration (consistency), particle collisions, electrochemical interactions promoted by chemical additives, etc. Due to its importance, fiber suspension flocculation has been studied for a long time in papermaking, and several methods have been developed for this purpose. The traditional techniques include, for example, focused beam reflectance microscopy (FBRM) and high-speed video imaging (HSVI). Recently, a new optical method, optical coherence tomography (OCT), has emerged for flocculation analysis. The advantages of OCT are the possibility to study opaque suspensions, its micron-level resolution, and its high data acquisition speed. The OCT measurements can be combined with rheological (Rheo) measurements, allowing simultaneous measurement of both the time evolution of the floc size and the suspension viscosity. In this work, we used this approach, Rheo-OCT, to study the flocculation of suspensions of various papermaking furnishes. We analyzed the time evolution of the floc size and the fiber suspension viscosity when the studied papermaking suspensions were treated with highly refined furnish (HRF)-a furnish that contained a significant amount of micofibrillated cellulose (MFC)-type fibrils-and/or chemical additives. Such studies can lead to a better understanding of the impact of flocculation on the produced paper web in terms of qualities like formation, drainage potential, and strength behavior. Application: Readers of this research can use the results to improve their understanding of fiber suspension flocculation in papermaking and learn about a new method, Rheo-OCT, that can be used in such studies. In addition, the insights gained in this study regarding the effect of HRF and chemical additives on flocculation can lead to improvements in paper formation, drainage potential, and strength properties.
The FBB filler ply typically contains a significant amount of mechanical pulp such as BTMP, BCTMP... more The FBB filler ply typically contains a significant amount of mechanical pulp such as BTMP, BCTMP, CTMP, etc. This is usually reinforced with either refined broke from the same PM line and converting process, or by utilizing traditional BKP. There has been a trend towards reducing the refining of mechanical pulp compensated with additional refining of the reinforcement stock related to the filler ply. In response to the drive for increased bulk, reduced basis weight, and minimized energy consumption, papermakers have experimented with various options and strategies. In between the common approaches, choices have been made between reducing the refining of the mechanical pulp, increasing the broke refining (more frequently practiced on the production scale), or increasing the BKP refining (a potentially superior choice). The present study focused on a simplified approach to assess the impact on dewatering of three reinforcement stock refining conditions. As the reinforcement stock HWBKP was used at the SR levels of 30-35, 50-55 and 70-75. The reinforcement stock was added to the core stock for the FBB filler plya mechanical pulp that was either BTMP SR35 or BCTMP SR25. The proportions of HWBKP were 20% of 30-35 SR, 10% of HWBKP 50-55 SR and 5% of HWBKP 70-75 SR. The intention behind using varying percentages of reinforcement pulp was to attain a controlled internal bond while enhancing bulk through increased mechanical pulp content. The dewatering of the stock mixture was assessed at three distinct vacuum levels-estimated, 7, 14, and 28 kPawhich are possible values employed in progressive stages within production-scale forming sections. This study provides indicative changes for dewatering responses of basic FBB filler ply stock composition and suggests possible optimal strategy to deal with reinforcement stock.
When dealing with papermaking fibre suspensions, particle aggregation takes place already before ... more When dealing with papermaking fibre suspensions, particle aggregation takes place already before the paper web is formed. The particle aggregation depends on several aspects including particle mass concentration (consistency), particle collisions, electrochemical interactions promoted by chemical additives, etc. Due to its importance, fibre suspension aggregation (or flocculation) has been studied for a long time in papermaking, and several methods have been developed for this purpose. The traditional techniques include e.g. Focused Beam Reflectance Microscopy (FBRM) and High-Speed Video Imaging (HSVI). Recently, a new optical method, Optical Coherence Tomography (OCT) has emerged for flocculation and aggregation analysis. The advantages of OCT are the possibility to study opaque suspensions, its micron-level resolution and its high data acquisition speed. OCT measurements can be combined with rheological measurements, allowing simultaneous measurement of both the time evolution of the aggregate size and the suspension viscosity. In this work, we used this approach to study the flocculation of suspensions of various papermaking furnishes. We analyzed the time evolution of the aggregate size and the fibre suspension viscosity when the studied papermaking suspensions were treated with a Highly Refined Furnish (HRF, this furnish contains a significant amount of MFC type fibrils) together with chemical additives. We show how such studies can lead to a better understanding of the impact of flocculation on the produced paper web on such quantities as formation, drainage potential and strength behavior.
Using respiratory protective equipment is one of the relevant preventive measures for infectious ... more Using respiratory protective equipment is one of the relevant preventive measures for infectious diseases, including COVID-19, and for various occupational respiratory hazards. Because experienced discomfort may result in a decrease in the utilization of respirators, it is important to enhance the material properties to resolve suboptimal usage. We combined several technologies to produce a filtration material that met requirements set by a cross-disciplinary interview study on the usability of protective equipment. Improved breathability, environmental sustainability, and comfort of the material were achieved by electrospinning poly(ethylene oxide) (PEO) nanofibers on a thin foam-formed fabric from regenerated cellulose fibers. The high filtration efficiency of sub-micron-sized diethylhexyl sebacate (DEHS) aerosol particles resulted from the small mean segment length of 0.35 μm of the nanofiber network. For a particle diameter of 0.6 μm, the filtration efficiency of a single PEO layer varied in the range of 80-97 % depending on the coat weight. The corresponding pressure drop had the level of 20-90 Pa for the airflow velocity of 5.3 cm/s. Using a multilayer structure, a very high filtration efficiency of 99.5 % was obtained with only a slightly higher pressure drop. This opens a route toward designing sustainable personal protective media with improved user experience.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Foam application of chemicals to the wet web is currently being developed for the paper and board... more Foam application of chemicals to the wet web is currently being developed for the paper and board industry. An important part of this work is to understand the rheology of the used application foams. Polyvinyl alcohol (PVOH) is widely used as a strength additive in paper and board, and it was the main surfactant in this study. The PVOH foam density varied between 100 kg/m 3 and 300 kg/m 3 and the dosage of PVOH varied between 0.5% and 6%. The foam viscosity and slip flow were determined with a pipe rheometer using three pipe diameters. The slip velocity was quantified by recording the foam motion in the vicinity of the wall of an acrylic pipe with a highspeed video camera. A measurement setup was also built for measuring the slip flow indirectly in opaque pipes. General formulas for the foam viscosity and slip flow, based on several physical quantities describing both the foam and the base liquid, were obtained using dimensional analysis. Specifically, dimensionless shear stress and dimensionless wall shear stress were found to be proportional to certain powers of the capillary number and slip capillary number, respectively. The contribution of the slip flow to the total flow rate was significant, especially with lower flow rates when most of the volumetric flow was due to the slip. In the literature, many papers have suggested that there is no slip flow in steel pipes. Our results suggest that this is due to the high pipe roughness used in those works. In our measurements, the slip velocity of a smooth-walled steel pipe was equal to the slip in an acrylic pipe. The obtained viscosity and slip models form a solid basis for developing and running various industrial processes including foam application processes. For new foam recipes, quite a small number of rheological measurements are needed to determine the model parameters.
11th World Congress of Chemical Engineering, 2023
A rheometric method based on velocity profiling by optical coherence tomography (OCT) was used in... more A rheometric method based on velocity profiling by optical coherence tomography (OCT) was used in the analysis of rheological and boundary layer flow properties of a 0.5% microfibrillated cellulose (MFC) suspension. The suspension showed typical shear thinning behaviour of MFC in the interior part of the tube, but the measured shear viscosities followed interestingly two successive power laws with an identical flow index (exponent) and a different consistency index. This kind of viscous behaviour, which has not been reported earlier for MFC, is likely related to a sudden structural change of the suspension. The near-wall flow showed existence of a slip layer of 2-12 µm thickness depending on the flow rate. Both the velocity profile measurement and the amplitude data obtained with OCT indicated that the slip layer was related to a concentration gradient appearing near the tube wall. Close to the wall the fluid appeared nearly Newtonian with high shear rates, and the viscosity approached almost that of pure water with decreasing distance from the wall. The flow rates given by a simple model that included the measured yield stress, viscous behavior, and slip behavior, was found to give the measured flow rates with a good accuracy.
Utilization of mechanically manufactured lignocellulosic fines (LCNFs) was investigated in making... more Utilization of mechanically manufactured lignocellulosic fines (LCNFs) was investigated in making filaments and films. The LCNFs particles were prepared by using a mechanical grinding method with a w-profile grinding stone that produces mostly fines with dimensions in the micrometer scale. The chemical and elemental composition of the w-stone ground LCNFs particles was investigated. It was found that the mechanically manufactured material exhibited the chemical structure of native wood. The LCNFs particles had an anionic surface charge making them colloidally semi-stable in water. The short length of the fines particles prevents their effective mechanical entanglement, which sets some limitations on preparation of filaments and films. Filament manufacturing required the use of a composite approach with carboxymethyl cellulose (CMC) as a binder polymer. The filament was manufactured by using dry-jet wet spinning with aluminium sulfate crosslinking. The chemical composition, crosslinking mechanism, and mechanical properties of the composite filaments were investigated. The composite approach with CMC was also used to prepare composite films with good mechanical performance. The investigated LCNFs material could be utilized in all-lignocomposite applications with cellulose derivatives, where biodegradability and biobased characteristics are desired properties.
Foam forming has recently attracted increasing interest due to the paper industry's continual eff... more Foam forming has recently attracted increasing interest due to the paper industry's continual efforts to find new possibilities to minimize raw material consumption, and to improve energy and water efficiency. Foam forming is also thought to be a possible solution to the industry's need to widen its product portfolio with novel and more valuable products. In foam forming, foam properties (air content, bubble size and half-life) are obviously key process variables, but there are only a few studies in which their effect on the sheet properties have been studied in pilot conditions. Moreover, all previous studies have used foam generated in stirring tanks, and there are hitherto no studies in which in-line foam generation has been considered. In this paper both these gaps are filled with experiments performed in VTT's pilot foam forming environment. The combination of tank and in-line generation was found to work well in foam forming, providing extra flexibility for foam generation and decreasing surfactant needs. The results show that foam forming generally improves formation, but the foam quality can have a significant effect on sheet properties.
A sub-micron resolution optical coherence tomography device was used together with a pipe rheomet... more A sub-micron resolution optical coherence tomography device was used together with a pipe rheometer to analyze the rheology and flocculation dynamics of a 0.5% microfibrillated cellulose (MFC) suspension. The bulk behavior of the MFC suspension showed typical shear thinning (power-law) behavior. This was reflected in a monotonously decreasing floc size when the shear stress exceeded the yield stress of the suspension. The quantitative viscous behavior of the MFC suspension changed abruptly at the wall shear stress of 10 Pa, which was reflected in a simultaneous abrupt drop of the floc size. The flocs were strongly elongated with low shear stresses. With the highest shear stresses, the flocs were almost spherical, indicating a good level of fluidization of the suspension.
Foam forming is an intricate option to lessen fiber flocculation and to get better energy and wat... more Foam forming is an intricate option to lessen fiber flocculation and to get better energy and water efficiency when making fiber-based products. Developed during the 1970s, this approach has recently received renewed attention, mainly because it also offers possibilities to widen the fiber-based product portfolios with novel and more valuable products. In addition to air content, bubble size is the most important property of foam. Foam quality control is essential for building real-world foam forming processes. In this work, we show how bubble size can be monitored with direct optical imaging in real time in real process conditions, and how such analysis helps adjust foam quality and discover process faults in foam forming.
Abstract: The influence of the physical and colloidal properties of W-stone ground lignocellulose... more Abstract: The influence of the physical and colloidal properties of W-stone ground lignocellulose native fines (LF) on the properties of lignocellulosic composite filaments was investigated. W-stone ground LF is a low-cost material exhibiting a microfibrillar structure with the chemical structure of native wood. The physical properties of manufactured LFs were investigated by utilising SEM imaging, turbidity measurements and image-based particle analysis using a Kajaani fibre analyser. The properties of LFs were varied by adjusting the process energy input that altered the produced material{\textquoteright}s particle size and shape and subsequent fractionation with a wire. The reduction in particle size was observed to increase the colloidal stability of produced LFs, but no significant changes in the chemical profile of the LFs were observed. The effect of the properties of LF on the manufacture of composite filaments with carboxymethyl cellulose (CMC) was studied by using a dry-jet wet spinning approach. The smaller particle size had a positive effect on the mechanical properties of composite filaments (tenacity increased from 5.5 to up to 7.6 cN/tex). The compatibility of different plasticisers with LFâCMC composite filaments was also studied. It was observed that the number of free hydroxyls per a monomer unit of the plasticiser had a positive correlation with the plasticisation effect in the LFâCMC composite filaments. Regenerated cellulose filaments are often rather expensive to be used in many applications such as composites. The investigated filaments could thus be used in low-cost applications requiring a fully biodegradable material profile. Here, the presence of lignin may increase the structural compatibility of the produced matrix. Graphical abstract: [Figure not available: see fulltext.].
Rheological properties and boundary layer flow behaviour of Micro Fibrillated Cellulose (MFC) sus... more Rheological properties and boundary layer flow behaviour of Micro Fibrillated Cellulose (MFC) suspended in water was studied using Ultrasound Velocity Profiling (UVP) and Optical Coherence Tomography (OCT). The high-resolution OCT provided velocity profiles near the transparent tube wall, while UVP yielded corresponding information in the interior parts of the flow. The results from interior part of the flow showed typical power law shear thinning behaviour of MFC suspension. Close to the wall the suspension was found to have strong (apparent) slip flow associated with depletion of MFC particles from the wall. In this thin depletion layer the consistency and the viscosity of the suspension decrease rapidly with decreasing distance from the wall. In the immediate vicinity of the wall the viscosity of the suspension was found to be close to that of pure water.
In addition to carding and airlaid processes, nonwovens are produced from staple fibers by a wet-... more In addition to carding and airlaid processes, nonwovens are produced from staple fibers by a wet-laid process. A drawback of this process is the necessity to use very dilute fiber suspensions to avoid fiber entanglement and consequent poor fiber web uniformity. As a result, flow volumes are very high and process speeds are rather low compared to water forming used in the paper and board industry. A promising option for making nonwovens is foam laying. The bubbles in foam keep the fibers apart until the foam is removed, and much higher fiber weight consistencies can be used compared to traditional wet laying. A key challenge in foam forming of nonwovens is to obtain uniform dispersion of the fibers in the foam. In this work, we studied this with 24-mm staple fibers, and analyzed the homogeneity of the obtained foams by making fibrous sheets from them in a laboratory sheet mold. We found that dispersion was highly dependent on the mixing conditions, such as mixing time, foam air content, and fiber weight consistency. Remarkably, excellent fiber disintegration and uniform sheets were obtained without mechanical pre-treatment of staple fibers, with fiber consistencies as high as 0.3%. By comparison, conventional wet-laid processes typically operate with fiber consistencies lower than 0.05%. Thus, at an industrial scale, foam forming promises production of uniform webs from stable fibers with clearly lower water volumes and possibly also higher machine speeds compared to conventional wet-laid forming.
Lightweight lignocellulosic fibrous materials (LLFMs) offer a sustainable and biodegradable alter... more Lightweight lignocellulosic fibrous materials (LLFMs) offer a sustainable and biodegradable alternative in many applications. Enthusiastic interest in these materials has recently grown together with the newly risen interest in foam forming. Foam bubbles restrain fiber flocculation, and foam formed structures have high uniformity. Moreover, the bubbles support the fibrous structure during manufacturing enabling the formation of highly porous structures. Mechanical pressure cannot be applied in the manufacture of LLFMs as the materials would lose their porous structure. Water is therefore typically removed by a combination of drainage and thermal drying. Thermal drying of porous materials has been studied intensively. However, there are only a few studies on the drainage of fiber-laden foams. Thus, in this work, we conducted a systematic analysis of this topic. Our findings show that after drainage a stationary vertical moisture profile similar to that of pure foams is developed. Raising the initial fiber consistency was found to increase the final fiber consistency of the foam until the drainage ceased. Increasing mold height was found to increase the final consistency considerably. Without vacuum and heating, the shrinkage of samples during drainage was only slightly higher than the volume of the drained water. Drainage rate and final consistency increased clearly with increasing vacuum, but simultaneously sample shrinkage increased considerably. The best compromise was obtained with a vacuum of 0.5Â kPa, which increased the final consistency by 60% without extra shrinkage. Using warm foam and heating the foam during drainage increased the final consistency considerably, but this also led to significant shrinkage of the sample.
The shear rheology of two mechanically manufactured microfibrillated cellulose (MFC) suspensions ... more The shear rheology of two mechanically manufactured microfibrillated cellulose (MFC) suspensions was studied in a consistency range of 0.2â2.0% with a pipe rheometer combined with ultrasound velocity profiling. The MFC suspensions behaved at all consistencies as shear thinning power law fluids. Despite their significantly different particle size, the viscous behavior of the suspensions was quantitatively similar. For both suspensions, the dependence of yield stress and the consistency index on consistency was a power law with an exponent of 2.4, similar to some pulp suspensions. The dependence of flow index on consistency was also a power law, with an exponent of â 0.36. The slip flow was very strong for both MFCs and contributed up to 95% to the flow rate. When wall shear stress exceeded two times the yield stress, slip flow caused drag reduction with consistencies higher than 0.8%. When inspecting the slip velocities of both suspensions as a function of wall shear stress scaled with the yield stress, a good data collapse was obtained. The observed similarities in the shear rheology of both the MFC suspensions and the similar behavior of some pulp fiber suspensions suggests that the shear rheology of MFC suspensions might be more universal than has previously been realized.
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articles by Antti I . Koponen