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Journal articles
Pre-damping effects on water absorption and drying dynamics in flexographic printing, TAPPI Journal November 2025
ABSTRACT: Optimizing flexographic printability can involve the ink and the substrate, as well as the printing process. It has been widely reported in the literature that controlling topography of the substrate and its porosity are vital for good flexographic printability, especially when using water-based inks. This study focuses on how pre-damping a surface impacts liquid absorption and improves wet trapping (ink on ink with no intermediate drying) in flexographic printing. A Prüfbau universal print tester was adapted to analyze flexographic wet-on-wet ink printing and trapping using yellow and magenta inks for contrast. Slow drying of the first ink layer (yellow) leads to mottle when the second layer (magenta) is applied. The study explores the “wet sponge” hypothesis: a pre-damped surface should absorb liquid faster. The Lucas- Washburn equation describes long-term absorption, but it does not capture short-term uptake, which instead follows a linear dependence on time.
Journal articles
Water and grease resistance of paperboard coated with long chain cellulose fatty acid esters using electrostatic powder coating, TAPPI Journal April 2026
ABSTRACT: Fiber-based materials used for foodservice, takeaway, and other packaging applications must typically provide water and grease resistance. Simultaneously, there is growing interest towards bio-based and renewable barrier coatings. In this study we applied thermoplastic long chain fatty acid cellulose esters onto paperboard using electrostatic powder coating to create barrier properties of interest. Electrostatic powder coating provides a water-free process to tackle moisture-induced quality issues and to provide an even coating layer. Cellulose octanoate and palmitate esters were produced in pilot scale. These were ground into fine particles using liquid nitrogen and applied onto paperboard sheets using an electrostatic powder gun. The loosely packed coatings were fixed and fused onto the paperboard by hot pressing. We characterized the coated sheets for coating thickness, contact angles with water, water absorption, grease resistance (KIT), and olive oil barrier. Two commercial polyethylene powders were used as references. Our results indicated that the long chain cellulose fatty acid ester coatings were thermoplastic and hydrophobic with contact angles above 100°. Water absorption was similar to the reference coatings. While grease resistance was lower than with the references, the cellulose ester coatings slowed down diffusion of olive oil through the paperboard. Cellulose octanoate ester with a lower melt viscosity already provided smooth coatings after the initial hot pressing step, while in most cases, the second post hot pressing step further improved the barrier properties.
Journal articles
Point load measurements on paperboard packages and bulging, TAPPI Journal March 2026
ABSTRACT: Paperboard packaging is made by processing board materials into sheets or rolls and shaping them through creasing, cutting, folding, and erecting. The conversion process generates residual moments at the folds that cause panel bulging. This study experimentally investigates how the bulging introduced during the converting processes influence the mechanical response of paperboard packages during point load testing within the elastic deformation range. The study shows that panel bulging may significantly affect packaging performance as-perceived strength and stiffness. Bulging, influenced by the board’s basis weight, can affect the package performance even more than packaging stiffness. Point load tests in the elastic region were performed on empty packages (78 mm × 50 mm × 110 mm) with force applied at specific points along their long sides. The packages evaluated in this study were made of two identically processed materials of different grammages. The heavier material showed more pronounced bulging than the lighter one, leading to overlapping force-displacement curves for the packages, and to that, a lower force and stiffness may be measured at a certain indentation depth for the package of heavier material. This complicates material choice according to functional requirements. The results show that a highly bulged package might resemble one with less bulging of another material. According to the results, it is not certain that a higher grammage package shows a higher indentation force and stiffness than a lower grammage package when measured at a certain indentation. This indicates that optimizing the creasing and folding processes can be a way to enhance performance rather than simply increasing board weight. The study underscores the importance of controlling converting parameters, especially creasing and folding behavior. Well-performed creasing and folding gives a low residual momentum, little bulging, and a high stiffness and compression strength at point loading in the elastic region. Proper optimization can improve packaging performance and manual handling user-friendliness.
Journal articles
Magazine articles
Techno-economic analysis of hydrothermal carbonization of pulp mill biosludge, TAPPI Journal March 2023
ABSTRACT: For many mills, the biosludge from wastewater treatment is difficult to recycle or dispose of. This makes it a challenging side stream and an important issue for chemical pulping. It often ends up being burned in the recovery or biomass boiler, although the moisture and non-process element (NPE) contents make it a problematic fuel. Biosludge has proven resistant to attempts to reduce its moisture. When incinerated in the biomass boiler, the heat from dry matter combustion is often insufficient to yield positive net heat. Mixing the sludge with black liquor in the evaporator plant for incineration in the recovery boiler is more energy efficient, but is still an additional load on the evaporator plant, as well as introducing NPEs to the liquor. In this study, treating the biosludge by hydrother-mal carbonization (HTC), a mild thermochemical conversion technology, is investigated. The HTC process has some notable advantages for biosludge treatment; taking place in water, it is well suited for sludge, and the hydrochar product is much easier to dewater than untreated sludge. In this study, two HTC plant designs are simulated using IPSEpro process simulation software, followed by economic analysis. Low temperature levels are used to minimize investment costs and steam consumption. The results show that if the sludge is incinerated in a biomass boiler, payback periods could be short at likely electricity prices. The HTC treatment before mixing the sludge with black liquor in the evaporator plant is profitable only if the freed evaporator capacity can be used to increase the firing liquor dry solids content.
Journal articles
Magazine articles
Lignin-based resins for kraft paper applications, TAPPI Journal November 2019
ABSTRACT: We investigated miscanthus (MS) and willow (W) lignin-furfural based resins as potential reinforce-ment agents on softwood and hardwood kraft paper. These resins might be sustainable alternatives to the commercial phenolformaldehyde (PF) resins. Phenol is a petrochemical product and formaldehyde has been classified as a carcinogen by the U.S. Environmental Protection Agency. The lignin used in this study was derived from hot water extraction (160ºC, 2 h) of MS and W biomass, and may be considered sulfur-free. These biorefinery lignins were characterized for their chemical composition and inherent properties via wet chemistry and instrumental techniques. The resin blends (MS-resin and W-resin) were characterized for their molecular weight, thermal behavior, and mechanical properties. Mechanical properties were measured by the resin’s ability to reinforce softwood and hard-wood kraft papers. The effect of adding hexamethylenetetramine (HMTA), a curing agent, to the resin was also examined. Mixtures of PF and lignin-based resins were investigated to further explore ways to reduce use of non-renewables, phenol, and carcinogenic formaldehyde. The results show that lignin-based resins have the potential to replace PF resins in kraft paper applications. For softwood paper, the highest strength was achieved using W-resin, without HMTA (2.5 times greater than PF with HMTA). For hardwood paper, MS-resin with HMTA gave the highest strength (2.3 times higher than PF with HMTA). The lignin-based resins, without HMTA, also yielded mechanical properties comparable to PF with HMTA.
Journal articles
Magazine articles
Rheological characteristics of platy kaolin, TAPPI JOURNAL September 2019
ABSTRACT: Platy kaolin can provide significant value in the coating of paper and paperboard. It can be used in multiple applications and can provide benefits such as titanium dioxide (TiO2) extension, smoothness improvement, improved print gloss or ink set rates, calendering intensity reduction, and improved barrier properties. It is not a pigment that can be simply substituted for traditional hydrous kaolin without some adjustment to the coating formulation. These adjustments can be as simple as reducing solids, but may require binder changes as well. The coater setup may need to be adjusted because of the unique rheological behaviors these pigments exhibit.The unique rheological characteristics of platy kaolin are explored here. Measurements of the water retention of platy kaolin containing coatings confirm that water retention is not reduced in comparison to more blocky kaolin pigments, despite the lower coating solids at which they need to be run. This means that the rheological characteristics are the most important in understanding the runnability. An extensive analysis reveals some unique behaviors that need to be understood when utilizing these materials. Viscoelastic measurements indicate that, for this binder system, Tan d is mainly a function of solids. This may explain how weeping is initiated on a blade coater. The degree of shear thinning behaviors is investigated using the Ostwald de-Waele power law. The immobilization point was determined using the Dougherty-Krieger equation and related to the work of Weeks at the University of Maine on blade coater runnability. An indirect measure of particle shape and size synergy is also demonstrated using the Dougherty-Krieger equation parameters.
Journal articles
Magazine articles
Flow characteristics of drag-reducing natural bamboo fiber suspensions with minimal environmental load, TAPPI Journal September 2019
ABSTRACT: The reduction of pipe friction loss by adding drag-reducing agents has attracted attention as an aid to energy conservation. Drag-reducing agents induce drag reduction (DR) effects and should have a minimal environmental load, with natural resource-saving potential. This study demonstrates bamboo fiber as a drag-reducing agent that saves natural resources and has a low environmental load. Using pressure drop measurements, we report DR with suspensions of bamboo fibers with the average diameter of 13.3 µm and aspect ratio of 98.7. The maximum DR obtained in this experiment is 43% at the concentration of 4000 ppm and pipe diameter of 30 mm; DR is affected by the Reynolds number, suspension concentration, and pipe diameter. In addition, the bamboo fibers can be easily removed from the suspensions by filtration. We found that low-environmental-load bamboo fiber has DR effects like those of other fibers; its effects are greater than those of conventional synthetic fibers and wood pulp. Furthermore, it is resistant to mechanical degradation, recoverable, and recyclable. Therefore, DR effects can be selectively obtained by adding the fibers only when DR is needed; the fibers can then be collected when DR is no longer necessary. This method might greatly expand the application range of DR agents. The results demonstrate the usefulness of bamboo fibers as DR additives.
Journal articles
Magazine articles
A novel unit operation to remove hydrophobic contaminants, TAPPI Journal April 2020
ABSTRACT: For mills making paper with recovered fiber, removal of hydrophobic contaminants is essential for trouble-free operation of paper machines. Significant cost savings on paper machine operation can be achieved by reducing deposits, which results in better quality, reduced downtime, increased fiber yield, and reduced energy consumption. Bubble nucleation separation (BNS) is a relatively new process for removing hydrophobic particles. When vacuum is applied to a slurry, dissolved gas bubbles nucleate on hydrophobic particles and drag them to the surface for easy removal. We constructed a 16-L batch unit to evaluate the effect of operating parameters on removal of hydrophobic particles, using statistical design of experiments. These results were used to guide our design of a 16-L continuous unit. We tested this unit on laboratory and mill samples. The removal of 60%•80% of hydrophobic particles was achieved with a low reject rate of < 2%.Following on this success, we built a 200-L pilot unit and tested it in our pilot plant. With promising results there, we installed the pilot unit at a commercial paper recycling mill. Over the course of several mill trials, we showed that it was possible to remove a considerable amount of suspended solids from paper machine white water with less than 2% rejects. Unfortunately, due to the unit only treating 50 L/min and the mill flow being 12000 L/min, we were not able treat a sufficient portion of the white water to know whether a large-scale implementation of BNS would improve paper machine runnability.
Journal articles
Magazine articles
Control of continuous digester kappa number using generalized model predictive control, TAPPI Journal September 2024
ABSTRACT: Kappa number variability at the digester impacts pulp yield, physical strength properties, and lignin content for downstream delignification processing. Regulation of the digester kappa number is therefore of great importance to the pulp and paper industry. In this work, an industrial application of model-based predictive control (MPC), based on generalized prediction control, was developed for kappa number feedback control and applied to a dual vessel continuous digester located in Western Canada. The problem was complicated by the need to apply heat at multiple locations in the cook. In this study, the problem was reduced from a multiple to a single input system by identifying three potential single variable permutations for temperature adjustment. In the end, a coordinated approach to the heaters was adopted. The process was perturbed and modeled as a simple first order plus dead time model and implemented in generalized predictive control (GPC). The GPC was then configured to be equivalent to Dahlin’s controller, which reduced tuning parameterization to a single closed loop time constant. The controller was then tuned based on robustness towards a worst-case dead time mismatch of 50%. The control held the mean value of the kappa number close to the setpoint, and a 40% reduction in the kappa number’s standard deviation was achieved. Different kappa number trials were run, and the average fiberline yield for each period was evaluated. Trial results suggested yield gains of 0.3%•0.5% were possible for each 1 kappa number target increase.
Journal articles
Magazine articles
Effects of hydrodynamic shear during formation of paper sheets with the addition of nanofibrillated cellulose, cationic starch, and cationic retention aid, TAPPI Journal September 2024
ABSTRACT: Laboratory tests were conducted to evaluate effects of hydrodynamic shear levels on papermaking process variables and paper handsheet properties. The furnish was from 100% recycled copy paper, to which was added nanofibrillated cellulose (NFC) at the 5% level following its optional pretreatment with cationic starch. A cationic copolymer of acrylamide (cPAM) was used as the retention aid. Different levels of hydrodynamic shear were applied both after mixing the NFC with the cationic starch (pre-shearing) or after all the furnish components had been combined (final shearing). The presence or absence of pre-shearing was found to have little effect on the measured outcomes. By contrast, increasing final shear hurt filler retention and made the resulting paper more uniform. However, the final shear level did not have a significant effect on the tensile strength of the resulting handsheets. Medium-charge density cationic starch, used in pretreating the NFC, consistently gave greater strength in comparison to a high-charge cationic starch. The significance of these findings is that though the relatively high hydrodynamic shear levels associated with modern paper machines can have some beneficial effects, they do not necessarily overcome all challenges associated with wet-end addition of nanocellulose in combination with other additives.