161
MARINELLI ET AL .
carboxyl groups, leading to reduced T g 61,62 . H39K 80 and H39K 60 gap was explained by higher kaolin content, which reduced the equivalent latex (hence, fewer carboxyl groups) facing the sliding plane, thus limiting friction. The lower friction coefficient achieved by the presence of pigment is also coherent with previous findings 63,64 . Additionally, the friction coefficient was aligned with Bendtsen rough- ness values (Figure 4); indeed, H39K 60 features a rougher surface, hence fewer contact points to stick to the smooth steel surface. Despite friction, coated paperboard trays were successfully achieved. The processing window in terms of MC for the experimental formulations was limited to 4.5 – 5.0%, whereas SA-B + SAP-H could be formed even at 5.6% — female mould: 100 C; BHF: 0.780 kN; pressing speed: 90 mm/s; dwell time: 0.6 s. Each coating material could not form at a lower MC (i.e., 4.0%). Such results differ from typi- cal paperboard forming MC, which previous research reported to be around8.0 – 9.0% 38,65,66 . Indeed, both trials at MC of 4.0% and prelim- inary tray forming at higher moisture content (>6.0%) showed highly adhesive behaviour on the male mould, which led to material break- age, as documented in Figure 15. Therefore, all the following proces- sing was carried out at a blank MC of 4.5 – 5.0% for all the coatings involved.
coated paper could withstand maximum forces that were almost 50% less than PE-coated similar substrates. In this work, the authors expe- rienced similar behaviour, though the seal strength gap was consider- ably less, ranging from 10% to 20%. Although seal strength at a given set of sealing parameters is mainly driven by the polymeric nature, as well as possible pigment amount and fibre tear resistance, it should be considered that coat grammage might play a role in such lower strength, as suggested by Figure 12.c.
3.6
Tray forming
|
Different materials showed different dynamic friction coefficients, as reported in Figure 14. It is clear how moisture plays a crucial role in determining the friction coefficient, with increasing MC leading to increased friction. Friction increased linearly with MC (for H39K 80 and SA-B + SAP-H, R 2 ≥ 0.98) in the investigated moisture con- tent range. Such behaviour might find a similar explanation as in previ- ous research 33 , since water increases paperboard thermal conduction, hence helping coating softening. Additionally, previous works dis- cussed the hydroplasticizing effect of moisture on polymeric coating
FIGURE 16 Dyed water and ethanol defects in the corners for H39K 80, H39K 60, and SA-B + SAP-H coated paperboard trays: a) H39K 80, water; b) H39K 60, water; c) SA-B + SAP-H, water; d) H39K 80, ethanol; e) H39K 60, ethanol; f) SA-B + SAP-H, ethanol. All trays were produced at 100 C, 0.780 kN BHF, 90 mm/s, 0.6 s dwell time.
Made with FlippingBook Online newsletter maker