Two types of cellulose nanofibrils (CNF) were produced to prepare coating formulations: enzymatically pre-treated CNF (CNF-Gen1) and carboxymethylated CNF (CNF-Gen2). Different formulations were prepared varying the solid content percentages and the amounts of carboxymethylated cellulose - CMC. Specifically, CNF-Gen2 at 1.2 wt% and 2 wt%, and CNF-Gen1 at 2 wt% combined with different amounts of CMC (2.5, 5, 10 and 20 wt%) were selected based on their water retention and rheological properties. A coat weight of 5 g/m2 was chosen as optimum to be the minimum coating required to reduce the air permeability to 0.003 μm/Pa.s. Among the formulations, CNFGen1+10wt%CMC and CNF-Gen2 at 1.2 wt% showed the best performance in terms of applicability, rheology, air permeability and oil barrier properties. Both of these formulations were successfully applied on two substrates (paper and board) using a semi-pilot Comma coating unit, enabling multilayer coatings without any web breakage or any expected problems related to high water content in the coatings. However, the paper substrate showed some wrinkles that were partially solved by adding PVA as a pre-coating layer and the board substrate presented a non-homogenous coating with the CNF-Gen1+10%CMC. New coating formulations were prepared by combining both types of nanocelluloses coatings (90%CNF-Gen1+10%CMC and 10% CNF-Gen2) showing an improvement in the processability and coating quality. Air permeability was drastically reduced with the application of all the coatings. Water resistance was kept in the case of the paper substrate and even reduced by 50% in the case of some coatings on the board. Almost all the coating systems passed the oil test. The influence of the pre-coating layer and the number of layers in the coating was also evaluated. Coatings without PVA show comparable results to the ones pre-coated.

The aim of this study is to develop a model for predicting the liquid absorption of coated boards subjected to external pressure. To create a simplified approach, the mathematical formulation was entirely based on existing models from the literature. The model’s predictions were then compared with practical laboratory Cobb measurements taken at various pressures on more porous and on less porous coatings. The results demonstrate that the model effectively captures the key trends in water absorption of coated boards under external pressure, aligning with findings from the laboratory study. However, due to the model’s sensitivity to system inputs, a thorough understanding of both the coating and paperboard characteristics is essential to improve prediction accuracy. Additionally, the original authors of the model (Equation (2)) suggest that the expression is best used for predicting water absorption over short periods.

A jet mixer was designed for nanoparticle production from lignin solutions at high concentrations, i.e. above 10%. Experiments were performed with industrial lignin dissolved in alkali and with HCl or NaHCO<inf>3</inf> as counter solvents. A first set of experiments was performed with dissolved lignin and HCl. These experiments showed that the mixing flow rate was crucial for obtaining lignin nanoparticles. The experiments also showed that the particles obtained were quite unstable. A larger particle size was obtained when solutions were left for 72 hours. Lignin solubility was investigated by measuring the particle size distribution at different pH. Two peaks were observed in the size distribution, one around 10 nm corresponding to dissolved lignin and one around 300 nm corresponding to lignin nanoparticles. The intensity of the second peak decreased with increasing pH, and the lignin could be considered dissolved above pH 13. Aging experiments were performed on lignin precipitations by adding glucuronic acid (GA), lauric acid (LA) or sodium dodecyl sulphate (SDS). There were some problems of dissolving LA and SDS, while GA prevented particle growth. It was also shown that the stability was increased with addition of ultrapure water (MQ), probably due to a lower ionic strength related to the lower pH. When dissolved lignin was mixed with NaHCO<inf>3</inf> as counter solvent, agglomeration tendencies were observed during aging of the particle dispersions. A second set of experiments were conducted in the jet with NaHCO<inf>3</inf> as counter solvent where the lignin particles were collected in an empty beaker or in a beaker with deionized water. Higher concentrations gave more nanoparticles, but these particles agglomerated when collected in a beaker. Agglomeration could be prevented by collecting the particles in deionized water. A batchwise mixing experiment followed by DOSY analysis also showed the lignin particle size growth upon pH reduction, suggesting also the value to apply the NMR characterization in further research.

In the first part of this work, Pickering emulsions were prepared using enzymatically pretreated CNF (CNF-Gen1), carboxymethylated CNF (CNF-Gen2) or cellulose nanocrystals (CNC) as stabilizers. 10:90 oil-in-water (O/W) Pickering model emulsions were prepared with an oil fraction consisting of soybean oil or epoxidized soybean oil. Emulsions were evaluated for optimum coating formulation stability at 0.5, 1.0 and 1.5 wt.% CNF-Gen1, CNFGen2 and CNC. The most promising formulation from the first part of this work was selected for continuation in part two. The soybean oil and epoxidized soybean oil were replaced by a proprietary lignin oil. The lignin ester component in the lignin oil contributes to the formation of a water barrier functionality in the coating layer. The subsequent water barrier formulations could be divided into two categories. In the first category, the O/W ratio was kept constant at 10:90, but the lignin content varied between 5 wt.% and 40 wt.% in the oil fraction. In the second category, the lignin content was kept constant at 40 wt.%, but the O/W ratio varied between 10:90 and 40:60. The aim of this approach was to improve the water barrier by maximizing the lignin content in the Pickering emulsion coating layer, while keeping the formulation viscosity within a favorable process window for industrially relevant future coating needs. At higher O/W ratios, these Pickering emulsions showed a very steep increase in viscosity, making them difficult to coat. The use of pectin in combination with CNF-Gen1 resulted in a co-stabilized Pickering emulsion that was both coatable and provided a water barrier layer with a Cobb60 value of 1-4 g/m2 at a dry coating weight of 6-8 g/m2 on board.

Paper was pre-calendered in a pilot scale configuration with a traditional soft nip calender and a metal belt calender. All calendering strategies reduced surface roughness and permeability of the samples, but different strategies affected the surface roughness and permeability differently. Six test points from the pilot calendered papers were chosen for laboratory coating studies. Uncalendered paper was included as reference samples. The calendered samples and the reference were pre-coated with a regular pigmented coating consisting of a GCC pigment and a SA latex. Both uncoated and pre-coated substates were barrier coated with a PVOH solution in one and two layers. The coating pick up was determined gravimetrically and the barrier properties were evaluated with TAPPI T454 grease resistance tests.

Paper was pre-calendered in a pilot scale configuration with a traditional soft nip calender and a metal belt calender. All calendering strategies reduced surface roughness and permeability of the samples, but dif-ferent strategies affected the surface roughness and permeability differently. The metal belt calender seemed to have a larger effect on the large-scale variations compared to the soft nip calender.Six test points from the pilot calendered papers were chosen for laboratory coating studies. Uncalendered paper was included as reference samples. The calendered samples and the reference were pre-coated with a regular pig-mented coating consisting of a ground calcium carbonate (GCC) pigment and a styrene acrylate (SA) latex. Both uncoated and pre-coated substrates were barrier coated with a polyvinyl alcohol (PVOH) in one and two layers. The coating pickup was determined gravimetrically, and the barrier properties were evaluated with TAPPI Standard Test Method T 454 grease resistance test.All samples needed two PVOH coating layers to form a grease barrier. The uncalendered sheets showed the best results with one coating layer, but this was at the expense of a higher coating pickup compared to the calendered sheets. The barrier coating pickup could be reduced by a combination of high temperature metal belt calendering and pre-coating. The high temperature and long residence time in the nip enabled plasticization of the fibers. This led to an irreversible deformation, even after water application. This meant that the smoothness obtained during cal-endering would be less affected by water-induced roughening during the coating operation.Application: Most works on barrier research have previously focused on either coating formulations or coating processes. This work complements earlier research by demonstrating that the amount of required coating color can be reduced through an appropriate calendering strategy and use of pre-coating.

Paper was pre-calendered in a pilot scale configuration with a traditional soft nip calender and a metal belt calender. All calendering strategies reduced surface roughness and permeability of the samples, but dif-ferent strategies affected the surface roughness and permeability differently. The metal belt calender seemed to have a larger effect on the large-scale variations compared to the soft nip calender.Six test points from the pilot calendered papers were chosen for laboratory coating studies. Uncalendered paper was included as reference samples. The calendered samples and the reference were pre-coated with a regular pig-mented coating consisting of a ground calcium carbonate (GCC) pigment and a styrene acrylate (SA) latex. Both uncoated and pre-coated substrates were barrier coated with a polyvinyl alcohol (PVOH) in one and two layers. The coating pickup was determined gravimetrically, and the barrier properties were evaluated with TAPPI Standard Test Method T 454 grease resistance test.All samples needed two PVOH coating layers to form a grease barrier. The uncalendered sheets showed the best results with one coating layer, but this was at the expense of a higher coating pickup compared to the calendered sheets. The barrier coating pickup could be reduced by a combination of high temperature metal belt calendering and pre-coating. The high temperature and long residence time in the nip enabled plasticization of the fibers. This led to an irreversible deformation, even after water application. This meant that the smoothness obtained during cal-endering would be less affected by water-induced roughening during the coating operation.Application: Most works on barrier research have previously focused on either coating formulations or coating processes. This work complements earlier research by demonstrating that the amount of required coating color can be reduced through an appropriate calendering strategy and use of pre-coating.

One severe weakness of most biopolymers, in terms of their use as packaging materials, is their relatively high solubility in water. The addition of kraft lignin to starch coating formulations has been shown to reduce the water solubility of starch in dry coatings. However, lignin may also migrate into aqueous solutions. For this paper, kraft lignin isolated using the LignoBoost process was used in order to examine the effect of pH level on the solubility of lignin with and without ammonium zirconium carbonate (AZC). Machine-glazed (MG) paper was coated in a pilot coating machine, with the moving substrate at high speed, and laboratory-coated samples were used as a reference when measuring defects (number of pinholes). Kraft lignin became soluble in water at lower pH levels when starch was added to the solution, due to the interactions between starch and lignin. This made it possible to lower the pH of the coating solutions, resulting in increased water stability of the dry samples; that is, the migration of lignin to the model liquids decreased when the pH of the coating solutions was reduced. No significant difference was observed in the water vapor transmission rate (WVTR) between high and low pH for the pilot-coated samples. The addition of AZC to the formulation reduced the migration of lignin from the coatings to the model liquids and led to an increase in the water contact angle, but also increased the number of pinholes in the pilot-coated samples. © 2021 by the authors

Barrier coatings based on starch and starch-PVOH plasticized with glycerol and without plasticizer were applied to two different paperboard substrates, a triple coated board and duplex board, in order to investigate the tendency for cracks to develop in the barrier coating layers during creasing and folding. Tensile properties of films based on the starch and starch-PVOH blend were determined to investigate the relationship between the flexibility of the films and the cracking in the barrier coating layers. Furthermore, the oxygen transmission rate through the barrier-coated paperboard was measured before and after creasing and folding. The oxygen transmission rate through the barrier-coated samples was over the measurable range i. e. OTR > 10000 cm 3 / m 2 day\text{OTR}>10000\hspace{0.1667em}{\text{cm}}^{3}/{\text{m}}^{2}\hspace{0.1667em}\text{day} after creasing and folding, which indicated failure in the barrier coating layers. Optical microscopy revealed small cracks in the barrier coating layers, probably related to an increase in flexibility of the barrier coating layers. It was observed in scanning electron micrographs that cracks in the barrier coating layers seemed to follow the fibers when the barrier coating was applied on the rear side of the duplex board. Scanning electron micrographs and surface profiler images revealed that cracks in the barrier coating layers might have originated from the mineral coating layer when the starch and starch/PVOH coating layers were applied on the mineral-coated side of the triple coated board. An increase in the thickness of the barrier coating layer did not seem to increase the resistance to failure.