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dispersion coat grammages representing less than 5% of the total packaging weight, that is, maximum non-cellulosic content that might constitute an insignificant part of the packaging unit, hence possibly considered as monomaterial by EU member countries 40 . Therefore, the aim is to assess if and in which cases the use of lightweight DCs (<5% w/w non-cellulosic content) might substitute PET-coated substrates.
numerous coatings that are brittle, fragment in tiny particles during repulping, and detach from fibres. Previous studies focused on fillers 10 – 12 inside coating formula- tions and different latex polymeric nature, generally based on styrene- butadiene and styrene acrylates latexes 10 – 14 . Recent research focused on biobased polymers 15 – 19 ; still, in many cases, they feature lower barrier properties as against synthetic counterparts, higher cost, and are challenging to scale up in their production process for an industrial technology transfer 20,21 . Nowadays, there are many commercially available aqueous dis- persion grades, some of which were already applied and tested on paper substrates 22 . Most grades are based on synthetic latexes because of higher industrial availability and easier production pro- cesses, though academic and industry efforts to shift to bio-based counterparts must be acknowledged 15,16,23 . Besides a complete barrier properties characterization, the pro- cessing of fibre-based substrates might affect coating integrity, hence the final packaging performance 24 – 27 . Therefore, the evaluation of processing effects on the barrier properties, and the actual process- ability of such coatings is of extreme interest. Creasing is a method that creates a groove in the processed mate- rial. It aims to locally decrease bending stiffness to generate preferential folding lines 28(p916) . Different methods were reported to obtain creases, both in the flatbed die-cutting process 29,30 – commonly used in large- scale production – and with creasing wheels mounted on sample makers 30 – 32 to study, for example, both creasing patterns and crease orientation. Additionally, previous studies discussed the effect of crease rule and groove width on the processed material's thickness 29,32 . Heat-sealing, in contrast, is aimed at bonding two surfaces thanks to the action of temperature, pressure and time. Previous literature focused on both flexible 33,34 and rigid 35 packaging. Different seal bar geometries can be used, especially for flexible packaging, whereas rigid packaging commonly adopts a flat bar. A successful heat seal is required to ensure spill-proof packaging as well as sufficient adhesion to ensure unintentional package opening. Aqueous dispersions provide inferior heat-seal strength compared with laminated counterparts 36 . Multiple converting properties might be involved in a single forming process like paperboard 3D forming (referred to as drawing) to achieve a final packaging structure, for example, trays. Former studies explored several parameters affecting output quality, including packaging geome- try, substrate moisture content, blank holding force, creasing depth and creasing width 31,35,37 – 39 . In summary, multiple process parameters must be finely tuned to guarantee that both the substrate and the coating layer do not crack; this would allow for a material-coating configuration that might compete under both economic and environmental perspec- tives against polymeric packaging counterparts, as well as against fibre- based solutions with higher non-cellulosic content. In this work, the authors explored the production and characteriza- tion of both experimental mineral-filled and commercial aqueous disper- sions on paperboard. The authors aimed for a broad characterization, going from barrier properties to converting properties and tray forming to be compared with polyethylene terephthalate (PET)-coated paper- board, currently the leading industrial material. This study involved dry
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| MATERIALS AND METHODS
2.1
Materials
|
2.1.1 | Substrate
StoraEnso (Helsinki, Finland) Trayforma 350 g/m 2 paperboard (UC) (average measured grammage was 348.5 g/m 2 ) was used as the basis substrate. UC is a three-layer 457 ± 4 μ m thick bleached sul- phate pulp paperboard, featuring chemithermomechanical pulp in the middle layer. All the layers are Alkyl Ketene Dimer (AKD) sized. Sizing improves the hydrophobic behaviour of the substrate. Moreover, StoraEnso (Helsinki, Finland) Trayforma PET 40 extrusion-coated substrate (grammage: 350 + 40 g/m 2 , UC + PET, respectively) (PET) was tested as reference commercial material, representing the commonest material used at an industrial scale to produce, for example, paperboard trays. The paperboard substrate of Trayforma PET 40 is UC.
2.1.2 | Experimental aqueous dispersions formulation
Two experimental formulations were produced at a lab scale involving HPH 39 highly crosslinked carboxylated styrene-butadiene latex (T g ffi 0 C, dry solid content 54%) as binder – which was kindly pro- vided by Trinseo (Horgen, Switzerland) – and CamCoat 80 kaolin (63% of the particles <2 μ m in size) from Amberger Kaolinwerke (Hirschau, Germany) as filler:
• H39K 80, containing 80:20 latex: pigment ratio – dry weight ratio; • H39K 60, containing 60:40 latex: pigment ratio – dry weight ratio.
Coating preparation followed the procedure described in previous publications 22,24 . PCC Exol SA (Brzeg Dolny, Poland) kindly provided Exolat C40 sodium polyacrylate, used as a dispersant (0.16% dry weight on dry pigment weight). NaOH 1 M was used to adjust the pH level to 8. At first, a water-based kaolin slurry was produced by adding kaolin powder, dispersant and NaOH inside deionized water. The slurry was continuously stirred with a turbine-type stirrer at 1200 rpm for at least 1 h. The final solid content of the slurry was 63%. Follow- ing, the kaolin slurry was mixed with HPH 39 latex (the amounts were according to the defined dry weight ratio) and stirred at 500 rpm for 30 min with the turbine type stirrer, adjusting the pH to 8 with NaOH. Experimental formulations had 50% solid content.
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