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Multifunctional barrier coating systems created by multilayer curtain coating, TAPPI Journal November 2023
ABSTRACT: Functional coatings are applied to paper and paperboard substrates to provide resistance, or a barrier, against media such as oil and grease, water, water vapor, and oxygen, for applications such as food packaging, food service, and other non-food packaging. Today, there is increasing interest in developing recyclable and more sustainable approaches for producing these types of packages. This paper focuses on water-based barrier coatings (WBBC) for oil and grease resistance (OGR), water, moisture vapor transmission rate (MVTR), and oxygen barrier performance. The main goal is to create coated systems that can achieve more than one barrier property using multilayer curtain coating (MLCC) in a single application step. One advantage is in optimizing coating material cost with the use of functional chemistry in confined layers where performance is balanced within the coating layered structure. This allows simultaneous application of layers of different polymer types in one step to achieve the appropriate performance needs for a given barrier application. This paper provides working examples of using MLCC to create coating structures with multiple barrier properties in a single application pass. Barrier polymers studied include styrene butadiene, styrene acrylate, starch-containing emulsions, and polyvinyl alcohol. The paper also shows the effect of increasing the pigment volume concentration with platy clay or fine ground calcium carbonate on MVTR and OGR barrier properties.
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Surface energy considerations for offset printing of coated paper and paperboard, TAPPI Journal November 2023
ABSTRACT: Offset printing of coated paper involves the complex interactions of ink with a surface that is characterized by three major properties: roughness, porosity, and related pore network structure and surface chemistry (related to surface free energy [SFE]). The effects of porosity and roughness are relatively well understood and are documented in the literature, whereas the influence of surface chemistry is much less studied and therefore the focus of this paper. The key results shown include: i) Coating porosity has a negligible effect on SFE determination by contact angle using two fluids. ii) The chemistry of the latex polymer in the coating formulation dominates the influence on SFE compared to pigment, with any surface energy differences present in the pigment being almost completely masked by latex. iii) Wetting agent and corona treatment can impact water absorption rate and surface spreading of water, resulting in small differences in printability. Increasing the concentration of the surfactant on a coated surface indicates switching orientation of the surfactant molecules, giving a “step wise” printing result. When looking to improve offset printability by selection of different pigments, the variation in SFE is less important than variation in either surface roughness or porosity.
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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.
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Study on the effect of aluminum diethyl phosphinate in synergy with ammonium polyphosphate on the flame retardancy of cellulose paper, TAPPI Journal April 2025
ABSTRACT: This paper involved the synergistic incorporation of ammonium polyphosphate (APP) and diethyl aluminum phosphinate (AlPi) as flame-retardant fillers for producing flame-retardant paper. The research revealed that APPs were square particles with a smooth surface, and their solubility was 0.29 g/100 mL at 20°C, which increased to 4.12 g/100 mL at 60°C. The surfaces of AlPis were rough and irregular. The solubility of AlPi was 0.023 g/100 mL at 20°C, and the solubility remained stable when the temperature increased. The addition of AlPi had a minor influence on the pulp beating degree. The tensile strength of kraft/APP/AlPi decreased with the increase of the AlPi addition. For a paper with 20 wt% APP and 0 wt% AlPi, the limiting oxygen index (LOI) value was 27.2%, and it burned completely at the eighth second during vertical combustion. When the AlPi additive content increased to 20 wt%, its LOI value increased to 32.2%, and the vertical combustion self-extinguished as soon as the flame was removed. Scanning electron microscopy (SEM) showed that the char residue of the kraft/APP/AlPi had a more complete fiber network structure than that of kraft/APP. The Raman spectroscopy indicated that the area ratio of the D (amorphous phase; disordered graphite vibration) band to the G (crystal phase; graphite carbon vibration) band (ID/ IG) ratio of kraft/APP/AlPi was lower than that of kraft/APP, meaning that the graphitization degree of the char residue of kraft/APP/AlPi was higher than that of kraft/APP, which indicated the kraft/APP/AlPi had better flame retardancy.
Journal articles
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Effect of pressure and time on water absorption of coated paperboard based on a modified Cobb test method, TAPPI Journal April 2024
ABSTRACT: This manuscript presents the study of water absorption by paperboard subjected to water at high hydrostatic pressure based on a modified Cobb tester. The new tester is based on TAPPI Standard Test Method T 441; however, the water column can reach up to 550 mm. The evaluation consisted of measurements of water absorption for coated and uncoated paperboard at different exposure times from 5 s to 45 s and water column heights from 10 mm to 500 mm (corresponding to hydrostatic pressures 98 Pa and 4.9 kPa, respectively). The coatings were formulated as a combination of styrene acrylate (SA; two binder levels) and two types of ground calcium carbonates (differing particle sizes) to form the two pre-coating structures: open and closed. The coating weight was 6 g/m2 applied on 210 g/m2 solid bleached board (SBB). In addition, 210 g/m2 uncoated boards were studied. Characterization of the coatings was performed with scanning electron microscopy (SEM), mercury intrusion, and roughness. It was found that the new device properly mimics the conditions of the current Cobb tester. The characterization of the coating also confirmed the presence of more open/larger pores of open coatings, confirming the desired coating structure. The absorption of boards was mainly driven by exposure pressure by comparing with exposure time. This was already evident after shorter periods of exposure time at 5 s and also 15 s exposure time. Paperboards with open coatings showed slightly higher absorption than other boards.
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Predicting strength characteristics of paper in real time using process parameters, TAPPI Journal March 2022
ABSTRACT: Online paper strength testing methods are currently unavailable, and papermakers have to wait for manufacture of a complete reel to assess quality. The current methodology is to test a very small sample of data (less than 0.005%) of the reel to confirm that the paper meets the specifications. This paper attempts to predict paper properties on a running paper machine so that papermakers can see the test values predicted in real time while changing various process parameters. This study was conducted at a recycled containerboard mill in Chicago using the multivariate analysis method. The program provided by Braincube was used to identify all parameters that affect strength characteristics. Nearly 1600 parameters were analyzed using a regression model to identify the major parameters that can help to predict sheet strength characteristics. The coefficients from the regression model were used with real-time data to predict sheet strength characteristics. Comparing the prediction with test results showed good correlation (95% in some cases). The process parameters identified related well to the papermaking process, thereby validating the model. If this method is used, it may be possible to predict various elastic moduli (E11, E12, E22, etc.) in the future as the next step, rather than the traditional single number “strength” tests used in the containerboard industry, such as ring crush test (RCT), corrugating medium test (CMT), and short-span compression strength test.
Journal articles
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Effects of phosphogypsum whiskers modification with calcium stearate and their impacts on properties of bleached softwood paper sheets, TAPPI Journal September 2021
ABSTRACT: By combining the structural properties and characteristics of phosphogypsum whiskers, a preliminary study on the modification of phosphogypsum whiskers and their application in papermaking was carried out. The effects of reaction temperature, reaction time, and reaction concentration on the solubility and retention of modified phosphogypsum whiskers and the effects of phosphogypsum whiskers on the physical properties of paper under different modified conditions were explored. The research results show that, after the phosphogypsum whiskers are modified with calcium stearate, a coating layer will be formed on the surface of the whiskers, which effectively reduces the solubility of the phosphogypsum whiskers. The best modification conditions are: the amount of calcium stearate relative to the absolute dry mass of the phosphogypsum whisker is 2.00%; the modification time is 30 min, and the modification temperature is 60°C. The use of modified phosphogypsum whiskers for paper filling will slightly reduce the whiteness, folding resistance, burst resistance, and tensile strength of the paper, but the tearing degree and retention of the filler will be increased to some extent.
Journal articles
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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
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Web lateral instability caused by nonuniform paper properties, TAPPI Journal January 2022
ABSTRACT: Lateral or cross-machine direction (CD) web movement in printing or converting can cause problems such as misregistration, wrinkles, breaks, and folder issues. The role of paper properties in this problem was studied by measuring lateral web positions on commercial printing presses and on a pilot-scale roll testing facility (RTF). The findings clearly showed that CD profiles of machine direction (MD) tension were a key factor in web stability. Uneven tension profiles cause the web to move towards the low-tension side. Although extremely nonuniform tension profiles are visible as bagginess, more often, tension profiles must be detected by precision devices such as the RTF. Once detected, the profiles may be analyzed to determine the cause of web offset and weaving problems.Causes of tension profiles can originate from nonuniform paper properties. For example, by means of case studies, we show that an uneven moisture profile entering the dryer section can lead to a nonuniform tension profile and lateral web movement. Time-varying changes in basis weight or stiffness may also lead to oscillations in the web’s lateral position. These problems were corrected by identifying the root cause and making appropriate changes. In addition, we developed a mathematical model of lateral stability that explains the underlying mechanisms and can be used to understand and correct causes of lateral web instability.
Journal articles
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Dynamic out-of-plane compression of paperboard — Influence of impact velocity on the surface, TAPPI Journal February 2024
ABSTRACT: Processes that convert paperboard into finished products include, for example, printing, where the paperboard is subjected to rapid Z-directional (ZD) compression in the print nip. However, measuring and evaluating the relevant properties in the thickness direction of paperboard are not necessarily straightforward or easy. Measuring at relevant, millisecond deformation rates further complicates the problem. The aim of the present work is to elucidate some of the influences on the compressive stiffness. Both the initial material response and the overall compressibility of the paperboard is studied. In this project, the effect on the material response from the surface structure and the millisecond timescale recovery is explored.The method utilized is a machine called the Rapid ZD-tester. The device drops a probe in freefall on the substrate and records the probe position, thus acquiring the deformation of the substrate. The probe is also allowed to bounce several times on the surface for consecutive impacts before being lifted for the next drop. To investigate the time dependent stiffness behavior, the probe is dropped several times at the same XY position on the paperboard from different heights, thus achieving different impact velocities. The material response from drops and bounces combined allows study of the short-term recovery of the material. The material in the study is commercial paperboard. The paperboard samples are compared to material where the surface has been smoothed by grinding it. Our study shows that there is a non-permanent reduction in thickness and a stiffening per bounce of the probe, indicating a compaction that has not recovered in the millisecond timescale. Additionally, a higher impact velocity has an initial stiffening effect on the paperboard, and this is reduced by smoothing the surface.