154
MARINELLI ET AL .
TABLE 3
Average number of pinholes measured for each coating
grade, both commercial and experimental.
Coating
Average number of pinholes
PET
0
H39K80
2
H39K60
0
SA-B
0
SAP-H
1
SA-B + SAP-H
0
FIGURE 5
Dispersion-coated, uncoated, and PET-coated
paperboard measured contact angle.
microbubbles incorporated in the aqueous dispersion during mixing since both experimental formulations feature no defoamers. Bendtsen roughness is reported in Figure 4. UC and PET-coated samples achieved the highest results, whereas aqueous dispersions reached, on average, 200 ml/min roughness, meaning more than 55% reduction. It must be observed how, in general, H39K 60 showed higher Bendtsen roughness compared with H39K 80, which was attributed to increased kaolin content, which is coarser compared with latex particles that film-form 22,51 . Regarding air permeability, UC showed 4.23 ± 0.23 μ m/(Pa s) air permeability, whereas all the DCs and PET achieved results that were almost null ( ≤ 0.002 ± 0.001 μ m/(Pa s)). Contact angle (Figure 5) clearly shows how experimental kaolin-containing styrene-butadiene coatings feature highly hydro- philic surface 52 , without statistically significant difference due to dif- ferent kaolin amounts. On the contrary, uncoated paperboard achieved the highest value (127 ), highlighting the hydrophobic nature due to sizing agents. Interestingly, the SA-B + SAP-H coated paper- board featured a contact angle that was close to the SA-B one; this suggests that the wet-on-wet coating allows the two coating layer particles to interdiffuse before drying.
FIGURE 4 Bendtsen roughness of the coated side of different aqueous dispersion-coated paperboard. Uncoated and PET-coated paperboard are included, too.
of temperature, time, pressure and dry coat grammage processing parameters for heat-sealing.
3
RESULTS
|
3.1
| Sample preparation and preliminary
characterization
3.2
Mechanical properties
|
Coated paperboard was successfully obtained. Average dry coat gram- mages for each material under investigation are reported in Figure 3. Differently from what was expected, dry coat grammage for a double 20 μ m wet thickness coating achieved an average of 12 g/m 2 . This might be attributed to paperboard surface sizing, repelling to some extent the waterborne dispersion spread with the rod coater. Generally, coated substrates showed few or no pinholes, as reported in Table 3. H39K 80 pinholes were explained by
Tensile and bending test results are reported in Figure 6 and Figure 7, respectively. The results differed depending on substrate fibre align- ment, that is, MD or CD. MD alignment provided, as predictable, higher tensile strength, average maximum force, as well as bending resistance and stiffness due to parallel fibre alignment in tensile tests and orthogonal alignment to the bending axis — up to threefold values compared with CD.
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