Polyester films were coated with a coating colour based on a ground calcium carbonate with narrow particle size distribution. Four different coat weights were produced. The coated sheets were calendered and the samples were exposed to up to twenty nips in the calender at a line load of 300 kN/m at 50°C. The brightness and the thickness were measured after each calendering nip. Reflectometry was used to measure the refractive index and the microroughness of the samples. Calendering lead to a decrease of the Kubelka-Munk scattering coefficient and an increase of the absorption coefficient of the coated transparent films. Taking into account non-uniform surface reflection at the boundaries between media of different refractive indices, a large part of the reflectance decrease due to calendering could be attributed to an increase of the effective refractive index at the coating surface and a decrease of the surface microroughness.
Calendering of coated paper leads to a brightness decrease. The mechanism for this is not clear, although it has been discussed in the past. One common explanation is that the porosity of the coating layer decreases and hence scattering. By comparing simulated and measured results this paper shows that modifications of the surface properties account for the brightness decrease of GCC coated substrates with calendering. The effect of a deformable cartonboard substrate is investigated here and compared to a less deformable plastic film substrate. From simulations based on a two-layer Kubelka-Munk model, it is shown that the brightness decrease of the cartonboard due to calendering has a negligible contribution to the brightness decrease of the coated cartonboard. The brightness decrease was similar for coated plastic film and coated cartonboard. The thickness of GCC coated plastic films was not affected by calendering irrespectively of the pigment and latex size distribution. Monte Carlo light scattering simulations, taking into account the measured decrease of surface microroughness and increased effective refractive index, showed that surface modifications accounted for most of the observed brightness decrease of the GCC coated substrate, whereas the bulk scattering and absorption coefficients were not affected by calendering. It is also shown that the scattering coefficient is significantly dependent on the coat weight whereas the physical absorption coefficient is not.
Ink transfer and ink penetration into a coated surface, and variations thereof, influences the print density, mottling and dot gain, which affects the achievable print quality and visual appearance. The pressure in the printing nip and the porosity of the substrate are conditions and properties that will regulate the amount of ink that penetrates into a porous coating structure. The purpose of this study was to relate print quality aspects to ink penetration of water-based flexographic ink into calcium carbonate based coatings of differently engineered structures. Pilot-coated paper-boards with different coating porosities were printed in a laboratory flexographic printer. Results indicate that ink transfer distribution is strongly affected by the roughness and the porosity of the substrate. A coating layer of broad pigment particle size distribution resulted in a lower print density, compared to coatings of narrowly distributed particle sizes. A structure characterized by larger pore volume and greater dominating pore radius, showed a higher amount of z-directional ink penetration, which was supported by estimating the penetration using a physical model accounting for both capillary- and pressure driven penetration. A coating with narrow particle size distribution also showed a lower dot gain.
Effective food packaging is a major factor in the current global drive to minimise food waste. Starch is an excellent oxygen barrier for packaging but it is brittle and moisture sensitive. The addition of layered minerals and plasticizers can significantly improve the moisture barrier and flexibility of the resulting composite. Some combinations of starch and plasticizer are incompatible but our results show that the addition of bentonite ensures the formation of coherent starch films with much improved moisture barrier regardless of the starch-plasticizer compatibility. It was clearly demonstrated that improvement of the moisture barrier was critically dependent on the layer charge of the bentonite used. Starch was readily accommodated in the interlayer space of bentonites with a layer charge of <0.4 electrons per formula unit but was not adsorbed if the layer charge was above this value. Starch-bentonite-plasticizer coatings prepared using bentonites with the lower layer charge routinely produced higher barriers to water vapour. The water vapour transmission rate (WVTR) of the base paper was reduced from 780 to 340 ± 20 g m2 day−1 when coated with starch alone. This was further reduced to 48 or 66 g m2 day−1 if glycerol or lower charge bentonite, respectively, was added to the starch. Optimised coatings of starch-lower charge bentonite-plasticizer provided WVTR values of ≤10 g m2 day−1 whereas WVTR values for comparative coatings prepared using the higher charge bentonites were three to four times higher (35 ± 7 g m2 day−1). Scanning electron micrographs provided clear evidence for the presence of 60 nm thick supramolecular layers formed from starch-bentonite-plasticizer in the samples coated on either glass or paper. The WVTR values for these low-eco footprint coatings are competitive with proprietary coatings prepared using petroleum derived resins.
Creating efficient water-borne dispersions based mainly on renewable materials for coating of flexible packaging paper was the aim of this study. The effects of an ethylene modified poly(vinyl alcohol) grade and a standard poly(vinyl alcohol) on the oxygen and water vapor barrier performance of corn starch and potato starch coatings was studied. The results showed that a coating composition with a high fraction of a renewable polymer was effective in keeping the oxygen barrier at a technically and commercially applicable level. An ethylene modified poly(vinyl alcohol) grade was found to provide lower oxygen transmission rates at high relative humidity, as compared to a standard poly(vinyl alcohol) grade. The oxygen barrier properties of blends of starch and poly (vinyl alcohol) were similar to that of the pure modified poly(vinyl alcohol) in the range from 0% starch to 60% starch. This was observed with both hydroxypropylated and octenyl succinate modified starch grades. The drying conditions of the mixed starch:poly(vinyl alcohol) coatings were based on drying trials with pure poly (vinyl alcohol) coatings. Drying at moderate temperatures indicated the possibility to slightly decrease water vapor transmission rate by higher drying temperature. Several secondary effects of increased drying temperature such as coating hold-out and formation of defects may also be of importance.
Starch and poly(vinyl alcohol) based barrier coatings for flexible packaging papers were studied. Both octenyl succinate modified and hydroxypropylated corn and potato starches were blended with regular and ethylene modified poly(vinyl alcohol) to increase the water vapor barrier properties and enhance the flexibility of the starch coatings, in order to accomplish superior barrier performance. Phase separation between starch and poly (vinyl alcohol) was studied in detail, both in the solution and in dry draw-down coatings on paper. The barrier performance of the coated paper was evaluated with respect to water vapor transmission rate. Conditions for the creation of a thin surface layer consisting of only one of the pure polymers were identified and discussed in terms of phase separation in solution migration of poly(vinyl alcohol) to the uppermost surface layer. The phase separation promoted low water vapor transmission rates also with a rather high fraction of starch in the coatings
The function of packaging is to protect the packed food and to maintain its integrity and quality. The package should hinder gain or loss of moisture, prevent microbial contamination and act as a barrier against transfer of oxygen, carbon dioxide and aromatic compounds. Of utmost importance is that the packaging material itself does not promote deteriorative food quality changes or endanger the health of the consumer of the packed food as a consequence of uncontrolled migration of any chemical substances from packaging into food. Recent legislative issues concerning food safety related to migration are summarized. An overview of functional barriers on the market, the sources for and identity of potential migrating substances, and the mechanisms behind migration, as well as of modern analysis techniques, is given. Novel strategies for improvement of functional barriers are discussed. In recent years, increased environmental concern has set the focus on the use of recycled plastics and paper in food packaging, thus enhancing the need for effective functional barriers to prevent contaminants from migrating to food. Future trends in the food packaging market are increased use of biobased polymers, enhanced utilization of nanotechnology for performance improvement and increased use of active or intelligent packaging.
Barrier dispersion coatings are applied to paper or board in order to lower the permeability of the material to liquids and gases. In the present study, the film formation and barrier properties of latices used for coating applications were investigated. The film topography and surface chemical composition of styrene/butyl acrylate latex films were studied by Atomic Force Microscopy, AFM, and Electron Spectroscopy for Chemical Analysis, ESCA, to see the effects of carboxylation and neutralizing agent on polymer interdiffusion. Barrier properties were determined for materials coated on different substrates, particularly the water vapor permeability. Latex dispersions were dialyzed in order to evaluate the effect of purity on film properties. Different three-ply base papers were coated with a pre-metering roll coater on a pilot scale, and the effects of drying conditions and soft calendering on the coating properties were studied. Heat sealability, barrier properties and blocking tendency of the coated material were studied.Surfactant migration in annealed films was confirmed with both AFM and ESCA and was taken to be a measure of polymer interdiffusion. The results showed that interdiffusion was retarded in carboxylated latex films and that this process was further slowed down when sodium hydroxide was used as neutralizing agent. Drying of the films at temperatures above Tg of the rigid outermost shell was required to achieve interdiffusion in carboxylated films. Non-carboxylated dispersions formed smooth films at the temperatures expected from their bulk Tg values. Dialysis removed excess amounts of electrolytes and surfactants and gave more densely packed films with a smoother surface topography. The lowest water vapor permeability was exhibited by carboxylated latex films, presumably because of the formation of a rigid polymer network with a low free volume available for water diffusion, combined with lower surfactant migration. Surfactant enrichment on the coated surface had a negative effect on the heat sealability and gave rise to an increased blocking tendency.The distribution of hydrophobic sizing agent and starch in surface-sized porous materials was studied by microscopy and ESCA. The degree of penetration of starch could be controlled by achieving a weak flocculation between the components through regulation of the pH and salt concentration. The results indicated that the sizing agent remained on the substrate surface, i.e. simultaneous penetration of the two components did not occur.
The oxygen and water vapor permeability at high relative humidity was studied for composite films formed by incorporation of three different bentonites (MMT) into an ethylene-modified, water-soluble poly(vinyl alcohol), EPVOH. The oxygen permeability decreased linearly with an increased addition of hydrophilic MMTs. X-ray diffraction and Fourier transform infrared spectroscopy suggested a homogeneous distribution in the thickness direction with disordered and probably exfoliated structures for hydrophilic MMTs. In contrast, organophilic modified clay showed an intercalated structure with the clay preferentially located at the lower film surface, a combination which was however efficient in reducing the water vapor- and oxygen permeabilities at low addition levels. Composite films of EPVOH and Na+-exchanged MMT resulted in high resistance to dissolution in water, which was ascribed to strong interactions between the components resulting from matching polarities. Annealing the films at 120 degrees C resulted in enhanced resistance to water dissolution and a further reduction in oxygen permeability.
Composite films were formed by incorporating three different bentonites into an ethylene modified, water-soluble poly(vinyl alcohol), EPVOH. The interaction of EPVOH with both hydrophilic and hydrophobic bentonites was investigated. EPVOH provided lower water vapor and oxygen transmission rates compared to a conventional PVOH grade when exposed at high relative humidities (70–90% RH). EPVOH films which exhibited oxygen barrier properties comparable to that of a biaxially oriented PET packaging film at 80% RH were produced. High compatibility between EPVOH and hydrophilic bentonites provided an even distribution of clay platelets in the composites. A strong increase in Young's modulus with increased addition of any of the three bentonites was found. At low addition levels the hydrophobic bentonite proved to be effective in terms of maintaining high elongation at break, high transparency and high gloss