Coatings 2023 , 13 , 195
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PCC Exol SA (Brzeg Dolny, Poland), was used as a dispersant (0.16% dry on dry pigment). The experimental formulations were developed at a constant dry solid content of 50%. Table1. CamCoat 80 particle size distribution.
Size
% 62 45 10
<2 μ m <1 μ m
<0.2 μ m
For both HPH 39 and HPH 40 latices, four formulations were considered, as reported inTable 2.
Table2. Experimental coating formulations for HPH 39 and HPH 40 styrene-butadiene latex.
Latex: Filler Ratio (%)
Abbreviation 1
Latex
Filler
H39K100 H39K80 H39K60 H39K40 H40K100 H40K80 H40K60 H40K40
HPH39 HPH39 HPH39 HPH39 HPH40 HPH40 HPH40 HPH40
Kaolin Kaolin Kaolin Kaolin Kaolin Kaolin Kaolin Kaolin
100:0 80:20 60:40 40:60 100:0 80:20 60:40 40:60
1 K stands for kaolin.
Before any use, and similar to the commercial grades, the experimental formulations were stirred with a magnetic anchor for at least 30 min at 500 rpm. 2.3. Sample Preparation Paper substrates were coated with a Meyer K-Bar (12 μ m wet film thickness). The coating process was carried out manually at a constant speed of 40 mm/s. After the coating, the wet-coated paper was placed in a frame (open on both the top and bottom) to reduce possible curling yet unaffecting heat access to the surface, and dried in an oven at 120 ◦ C for 90 s. After cooling down at room conditions, squared samples (80 mm per side) were cut and conditioned at 23 ± 1 ◦ Cand50 ± 2% relative humidity for at least 24 h prior to any characterization. A total of three test samples were prepared for each test. Uncoated (UC) samples were used as a reference to detect the effect of the addition of a coating layer. The thickness of each coated sample was calculated from the composition of the formulations—the density of the latex was to ρ =1.05g/cm 3 , and the kaolin one to ρ =2.6g/cm 3 . 2.4. Water Absorption (Cobb Test) The water barrier property was evaluated with a Cobb 1800 test. The test aims to evaluate the water absorptiveness of the sample after a given time under 10 mm deep water, to be calculated as weight difference before and right after the test (Equation (1)):
m 1 − m 0 A
(1)
Water Absorptiveness ( Cobb1800 )=
where m 1 is the final sample mass (in grams), m 0 is the initial mass (in grams), and A is the test area (in m 2 ). As defined in BS EN ISO 535:2014 [28], the samples underwent a 30 min test time at a temperature of 23 ± 1 ◦ C and a relative humidity of 50 ± 2%. Due to the specimen dimension, the test area was set to almost 40 cm 2 .
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