Creasing coated carton board or folding coated magazine paper, result in large strains in the surface layer of the paper product and might result in surface cracks, which decrease the quality of the products. A better understanding of the mechanical properties of coated layers increases the knowledge needed to reduce crack formation in coated fiberbased materials. The crack area on a coated board was measured after creasing and folding and the crack area on a coated copy paper was measured after folding. A clay pigment and a Ground Calcium Carbonate (GCC) pigment were used. The binder was either an S/B latex or an S/B latex combined with starch. The type of pigment seemed to have the greatest influence on the crack area during creasing and folding of thick board. Clay based coatings showed a larger crack area than the GCC based coatings and coatings containing starch showed generally a larger crack area than the coatings that contained only S/B latex. After folding of thin copy papers, the crack area showed a slightly different pattern. Here, it seemed that the binder was of greater importance than the type of pigment. Coatings containing only latex binder showed a considerably lower crack area than the coatings containing both latex and starch binders. The clay based coatings also showed similar crack areas as the GCC based coatings. A micromechanical model was used to estimate the in-plane and out-of-plane mechanical properties of clay based- and GCC based- coating layers. The clay-based coatings showed an anisotropic material behavior with considerably lower stiffness and strength during out-of-plane compared to in-plane. GCC based coatings, on the other hand, showed an isotropic material behavior with similar in-plane and out-of-plane properties. The calculated material behavior of coating layers, together with the data from the literature was used to explain the difference in cracking behavior between Clay- and GCC- based coatings during creasing of thick coated board and folding of thin coated copy paper.
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.
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
The mechanical properties and chemical stability in water of self-supporting films made from aqueous solutions of starch and lignin, and the barrier properties of paperboard coated with solutions of these polymers have been studied. The dissolution of starch from the starch-lignin films in contact with the model liquids was decreased significantly when lignin was added to the starch films. The addition of ammonium zirconium carbonate (AZC) to the formulations as a crosslinking agent substantially increased the storage modulus of the starch-lignin films, which indicated that crosslinking had occurred. The addition of AZC to the formulations also led to a decrease in dissolution of both starch and lignin from the starch-lignin films in contact with model liquids. The effect of AZC on the water stability of the films was greater when the pH of the starch-lignin-AZC solution was adjusted with ammonia rather than NaOH. The addition of NH4Cl solution as a presumed catalyst to the recipe when the pH adjustment was performed with NaOH did not improve the effect of AZC on the water stability of the films. The water vapour transmission rate of the coated paperboard decreased slightly when AZC was added to the coating formulation.
A hemicellulose-based barrier formulation was produced from wood chips to pilot coating application in a bladecoater. The development process is described from a project planning point of view with network plan and decisionpoints.Different hemicelluloses were produced and compared in the research laboratory. Oxygen barrier properties fromfilms and rheology of water suspensions were evaluated for different laboratory produced hemicelluloses originatingfrom wood chips or from kraft pulps. The viscosity at low and high shear rates were evaluated. From these results,hemicellulose produced from birch wood chips was chosen.Hemicellulose was then produced at in kg quantities. The hemicellulose was produced employing pre-hydrolysistreatment of birch wood chips. The concentration of hemicellulose pre-hydrolyses was increased from 3.3% to11.5% by membrane filtration. The barrier properties of the hemicellulose coated material were evaluated inlaboratory scale. A pre-coating was also evaluated in laboratory scale.Trials were performed at Iggesund PaperBoard in their pilot scale coater. A pre-coating was applied during the firstday, and the hemicellulose-based coating was applied on the second day. The pilot trials went well without anylarger obstacles. Quick barrier evaluations showed that the hemicellulose-based barrier coating had a good greaseresistance even if the coating consumption during the trials indicated a low application grammage.
A new method to produce SEM cross sections was used to analyze coated and creased samples. It was found that the binder surrounded the smaller particles leaving the large pores around the larger pigment particles. Consequently, cracks would propagate next to the large particles, and at least one crack area also showed a clear pigment surface, indicating an adhesive failure between the binder and the pigment particles. GCC and clay showed different cracking directions. GCC-based coatings showed cracks that had been initiated at the surface of the coating layer and then went through the thickness direction of the coating layer. The clay-based coatings on the other hand showed cracks that could be initiated anywhere in the thickness direction in the coating layer and then continued at an angle in the thickness direction. The cracking behaviour of the clay based coatings was probably due to anisotropic mechanical properties combined with shear stresses or out-of-plane tensile stresses during creasing.
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.