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an area of 25 cm 2 . Water absorptiveness is defined as the weight increase of the sample due to water uptake during the test, divided by the testing area. The result is then converted to g/m 2 . Lower water absorptiveness values indicate higher water barrier properties. WVTR, instead, followed the evaluation procedure defined in BS ISO 2528:2017 46 using cups filled with 35.0 ± 0.1 g of silica gel. Three samples for each coating material were tested; the testing area was 20cm 2 . WVTR is defined as the amount of water vapour absorbed by silica gel at a steady state. It was calculated as the slope of mass increase in time, divided by the testing area. The result is reported in g/(m 2 day). Lower WVTR values show higher water vapour barrier performance. OGR followed a similar methodology to the one defined in BS ISO 16532-1:2008 47 . Evaluation times and setup were the same (50 g weights with a diameter of 30 mm), but the testing temperature was 60 C. This allowed harsher testing conditions for the substrates, simu- lating contact with hot greasy food. Given the temperature, dyed palm kernel oil was liquid, and the amount applied to the samples was 0.1 ml. A total of five samples for each coating were tested and the results were averaged. The outcomes were reported as defined in BS ISO 16532-1:2008 47 .
FIGURE 1
Heat sealing investigation matrix.
The equipment mounted flat 25 mm wide tools; the top one – moved by two pneumatic cylinders – was heated, whereas the bottom one, a flexible tool, was kept at room temperature. A 2 3 full-factorial design of experiments (DoE) including a central point was designed adopting as variables temperature, dwell time and pressure. A schematic representation of such DoE is reported in Figure 1. The number of replicates was five. The reader should con- sider that the pressure in Figure 1 represents the cylinder pressure that allows a sample-specific pressure of 2, 2.5 and 3 MPa, respectively. Heat-sealed samples were subsequently T-peel tested – unsupported peel testing, according to ASTM F88/F88M- 21 49 – using a Shimadzu (Kyoto, Japan) Autograph AGS-X machine with pneumatic clamps. The pre-test speed was 50 mm/min until reaching 0.2 N. Afterwards, the speed was 300 mm/min until the end of the test. T-peel test curves were analysed to determine peak force, average force in the seal area, and peel energy in the seal area. Linear Pareto charts provided heat-sealing relevance for the analysed pro- cessing parameters. Since low dry coat grammages might negatively affect the heat-seal ability of rough substrates, the authors also con- sidered the dry coat grammage of the single specimens as a factor to be included in Pareto charts. Indeed, the aim was to assess if small changes in the coat grammage could affect the heat-sealing performance.
2.2.4 | Creasing
Each aqueous dispersion configuration underwent creasing with a three-rule tool developed at LUT and already presented in a single- rule version in previous work 48 . The tool was mounted on a Shimadzu (Kyoto, Japan) Autograph AGS-X machine. It features three equally spaced 2 pt. (0.706 mm) rules that differ in length and whose corners are rounded — total length is almost 43 mm, excluding rounding. On the bottom, the authors used a 0.5 mm deep and 1.4 mm wide creas- ing matrix, leading to a creasing factor μ of 1.5 (see Tanninen et al. 32 for the equation to calculate μ ). The creasing pre-load was set to 10 N. The speed was constant at 5 mm/min for the pre-test and actual creasing. The authors assessed the effect of two different creasing strokes (0.5 mm and 0.6 mm, respectively) for both MD and CD fibre orientation of every double- coated layer configuration, leading to a crease depth of around 90 – 120 μ m. A Keyence (Osaka, Japan) VR-3200 wide-area 3D measurement system determined the real crease depth. For each sample, 11 multi- line profiles were equally spaced (0.5 mm distant) and averaged for each crease line. Next, possible coating damage was evaluated through an OGR test, as described in Section 2.2.3. Five samples were tested for each creasing condition and each coating configuration. ORG testing of five uncreased samples provided reference resistance time.
2.2.6 | Tray forming
Tray forming behaviour was assessed for H39K 80, H39K 60, and SA- B + SAP-H coated paperboard. For comparison, the PET-coated paperboard was also investigated to ensure that the processing condi- tions corresponded to industrial production ones. Tray blanks were produced using commercial die-cutting equipment with 1/9 foodstuff container – nomenclature according to BS EN 631 – 1:1993 50 – die and matrix. Matrix groove width and depth were 1.4 mm and 0.5 mm, respectively; creasing rules were 2 pt. wide (0.706 mm) and 23 mm high, like for the creasing tests defined in Section 2.2.4. The crease pattern featured creases that were radial toward the rotation axis of the tray corner, as discussed in previous research 32 . The blanks were
2.2.5 | Heat sealing
An RDM Test Equipment (Hertfordshire, England) HSB-1 heat sealer was used to seal 25 by 130 mm coated strips with facing coated sides.
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