160
MARINELLI ET AL .
FIGURE 14
Dynamic friction results
for H39K 80, H39K 60, and SA-B + SAP-H at different moisture contents.
reason lies in the energy contribution, which is not due to coat- to-coat interface separation; instead, the external energy provided by the system is mainly associated with fibre separation. Linear Pareto charts for both processing parameters and dry coat grammage (Figure 12) highlight different statistically significant parameters for each material, given the same DoE. For H39K 80, it seems to be a matter of dry coat grammage. However, taking a closer look at the results for such coating (Figure 11), the change in sealing parameters appeared negligible in the measured outcomes. Indeed, almost every sample showed paperboard delamination (fibre tearing) during the peel test, underlining a coat-to-coat interface bonding that is stronger than the one of bulk paperboard, regardless of tempera- ture, time and pressure applied during heat-sealing 34 . H39K 60 and SAP-H, instead, undoubtedly show the importance of time, plus temperature, notwithstanding the lower magnitude of the latter. Coherently with previous work 24 and despite higher sub- strate grammage (i.e., thickness), similar time range, and higher specific pressures (0.4 to 0.6 MPa versus 2 to 3 MPa of the current work), time was the most crucial parameter to obtain heat-sealed samples. The effect of time was discussed in previous work 33 to be crucial at lower temperatures because of thermal insulation properties of cellu- losic substrates. As of Figure 11, data is coherent with such argument for all the investigated properties at 80 C and 100 C comparing sam- ples sealed at 1 s and 2 s, with a sharper difference at lower sealing pressures. With a good approximation, also pressure was a significant parameter according to Pareto charts. The authors compared the heat-seal ability of dispersion-coated paperboard with the one of PET-coated paperboard. As of Figure 13, maximum seal force clearly shows how aqueous dispersions are capa- ble of sealing at temperatures that are around 100 C lower than PET ones. This represents a crucial advantage, leading to lower energy consumption, hence lowering production costs. Additionally, a major difference in peeling behaviour is that DCs generally fail by paper- board delamination, whereas PET separates at the coat-coat
FIGURE 15 Blank breakage during tray forming. Higher moisture content increases friction, leading to blank adhesion to the male mould.
interface — despite some PET-paperboard interface bonding being damaged at higher temperatures. Higher maximum forces for PET- coated paperboard could be explained by the higher coat grammage – as well as the tough nature of the material – that pro- vided higher Tensile Energy Absorption (Figure 6), that is, it was able to withstand peeling without breaking nor transferring the stress to paperboard fibres which, in turn, resists to a lower extent compared with coat – coat bond at the sealing interface (as observed for disper- sion coated substrates). Recent research 36 showed how dispersion-
Made with FlippingBook Online newsletter maker