Coatings 2023 , 13 , 195
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Table3. Experimental coating formulations for the HPH 39 and HPH 40 styrene-butadiene latex.
Δ E* (CIE2000)
Δ E* (CIE2000)
Substrate
Coating
Substrate
Coating
KB KB KB KB KB KB KB KB KB KB KB
UC
-
KP KP KP KP KP KP KP KP KP KP KP
UC
-
SB-B SA-B
1.2 0.9 1.2 1.3 1.4 1.8 1.2 1.4 1.8 1.7
SB-B SA-B
0.7 0.8 1.4 1.6 1.1 2.2 1.1 1.6 2.0 2.7
H39K100 H39K80 H39K60 H39K40 H40K100 H40K80 H40K60 H40K40
H39K100 H39K80 H39K60 H39K40 H40K100 H40K80 H40K60 H40K40
Figure9. 8 ◦ glossiness for each substrate and coating involved in this study. 4. Discussion
Over KB, generally, HPH 39-based formulations performed better compared with HPH 40; however, over KP, it was the opposite. Since the KB substrate is of higher industrial interest due to its lower price, HPH 39 should be preferred as the latex in the developed dispersion coatings. Lower T g implies a softer polymer at a given temperature, hence a higher chain mobility; moreover, lower T g is reported to be beneficial to spread over kaolin [33]. Despite lower T g also meaning a tendency to blocking [19], the addition of kaolin—or pigments, more in general—is reported to be beneficial, reducing such behavior. The water barrier (Cobb 1800) properties proved to be better compared with a previous extensive work [34], especially considering a similar pigment ratio and a despite lower aspect ratio. Unfortunately, the water barrier property is not commonly evaluated in similar works, where WVTR was preferred. Water barrier is quite limited compared with commercial grades (Figure 3); however—as it is the case for this work—the Cobb test time is the most restrictive among the ones reported in the respective standard. Comparing WVTR results with other works [14,35,36], it seems that the mass fraction of kaolin in the coating formulation does not widely affect WVTR, given that kaolin addition in the coating formulation is beneficial to moisture barrier properties. The results are coherent despite a higher kaolin aspect ratio in [14,34], which should be beneficial to obtain better moisture barrier properties; additionally, WVTR values are coherent with other works using a different filler [21]. The grease barrier properties of filled experimental formulations improved compared with unfilled formulations (i.e., H39K 100 and H40K 100), showing the positive effect of kaolin. A previous work [37] showed that kaolin did not influence grease barrier properties; however, it must be considered that polyvinyl alcohol, which was used in the formulation, already showed high grease barrier properties. Although the TAPPI T 559 (kit test) and ISO
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