162
MARINELLI ET AL .
FIGURE 17
Effect of forming speed on coat defects
for SA-B + SAP-H formed trays (dyed ethanol): a) 45 mm/s; b) 90 mm/s. Other forming parameters were 120 C, 1.014 kN BHF, and 0.6 s dwell time.
BHF is associated with friction, too. Increasing BHF leads to increased normal forces applied on the blank, hence friction. How- ever, BHF is also proportional to the smoothness of the flange — which is, in turn, crucial for trays that need to be heat-sealed 35 . H39K 60 and SA-B + SAP-H could be processed at a BHF of 1.014 kN, whereas H39K 80 failed, badly sticking to the mould. Once again, higher pigment content seemed to help limit friction coefficient and stickiness. The effect of processing temperature correlates to the thermal properties of the coatings. Higher mould temperature leads to increased chain mobility, which, in turn, helps the heat-sealing of the creases and adhesion to other substrates in general. SA-B + SAP-H trays were formed without any issues when mould temperature increased from 100 C to 120 C at both BHF of 0.780 kN and 1.014 kN. Similarly, H39K 80 and H39K 60 could be formed at 120 C at a BHF of 0.780 kN. Unfortunately, despite kaolin content, a BHF of 1.014 kN led to mould sticking for H39K 60 at 120 C. Additionally, both H39K 80 and H39K 60 were tray-formed at a BHF of 0.546 kN to have further data to assess forming speed. From a material perspective, forming speed is primarily associated with deformation rate and latex softening (because of an extended contact time with the heated mould). The former, given other factors to be constant, influences the brittle or plastic behaviour of the coat- ing, with higher speeds that can be associated with more fragile behaviour due to less time for the polymeric chains to flow without breaking. At 45 mm/s forming speed (120 C, BHF of 0.780 kN and 1.014 kN for SA-B + SAP-H, and 0.546 kN and 0.780 kN for H39K 80 and H39K 60), all the materials could be tray formed without stick- ing occurrence. Lower speed seemed to have a negligible effect on the adhesion and blocking effect of the investigated coated paperboard. Despite successful tray forming at the given parameters' range, dyed water and ethanol generally highlighted quite damaged coatings (Figure 16). The main damaged areas were the corners and the flange. Specifically, the flange showed defects in the area exposed to tensile stresses (convex coated side), leaving fewer damages to the concave coated side. Moreover, despite lower sticking behaviour, H39K 60 was more damaged compared with H39K 80. The reason lies within lower latex continuity in highly pigmented formulations that lead to early breakage, similar to filled polymer matrix composites 67 .
On the contrary, the PET sample trays showed no defects or sticking behaviour within the investigated parameter range, highlighting how the processing arrangement corresponded to the ones adopted for industrial production. Nonetheless, it must be noted that the PET coating was much thicker compared with dispersion-coated counter- parts, allowing possible higher tensile resistance. No marked difference was observed because of BHF and temper- ature parameter effects for the investigated ranges. Nevertheless, lower pressing speed showed significantly fewer defects, highlighting how higher speed helped damage the thin DCs (Figure 17). Addition- ally, taking a closer look at the defects, stained areas clearly show transversal cracks to the crease lines, suggesting damages caused by axial deformation.
4
CONCLUSIONS
|
This work provided encouraging results from multiple points of view to sustain a narrow yet possible material substitution to reduce non- cellulosic content in paperboard-based packaging. SA-B aqueous dispersion provided water and moisture barrier properties that were similar to PET, whereas SAP-H for grease barrier. Experimental dispersions, on the contrary, featured intermediate values, apart from water barrier properties. Nevertheless, the results were satisfying because of the lower dry coat grammage involved for dispersions as against PET. The investigated creasing depth range did not affect the coating integrity of both experimental formulations (i.e., H39K 80 and H39K 60) and SAP-H, generally leaving grease permeation values intact. Some damages were observed with SA-B and SA-B + SAP-H, highlighting a more fragile behaviour at room temperature. Aqueous dispersion-coated paperboard could be heat sealed at temperatures that were up to 100 C lower compared with conven- tional PET-coated paperboard, representing a better choice to reduce power consumption. Such behaviour appeared to be proportional to the T g of each polymeric fraction. Despite lower seal strength, such materials may be used in applications where a temperature-sensitive content is involved. Unfortunately, the peel mechanism for strongly sealed samples was paperboard delamination, suggesting early coating breakage, whereas PET separates at the heat-sealed interface.
Made with FlippingBook Online newsletter maker