paper dimensions (deformation) and deterioration of mechanical properties, which is important, especially for packaging papers. From the utility point of view, depending on the intended use, the coated paper should therefore have a strictly defined hydrophobicity, and thus the assessment of water absorption by the paper is one of the key methods of assessing the performance of paper. The appearance of the coating and its thickness determine the final appearance of the paper product.Coating grammage primarily determines the functional properties of the coated paper, which undoubtedly include its hydrophobicity, but also strength properties. Therefore, this study investigated how the coating grammage influences the hydrophilicity as well as the surface and strength properties of the coated paper. MATERIALS AND METHODS For the tests, the commercially available Mondi ProVantage Kraftliner paper with a basis weight of 125 g/ m² and the coating mixture in the form of a paraffin -water emulsion was used. Before the coating process, paper samples were cut from the rolls and subjected to air conditioning at 23 °C and 50% relative humidity, according to ISO 187:1990, for a minimum of 24 hours. The coating process was carried out with the use of Mayer rods of different numbers: 4, 5, 6, 7, 8, 9, 12, and 15, to vary the coating grammage. The thickness of the coating increased with the developing number of rods. The photo of one of used rods is shown in the drawing (Fig. 1). After the coating process, the coated paper samples were dried in a thermal research chamber (Wamed KBC-65W) at a temperature of 60° C. Thereafter, the coated paper samples were conditioned at 23 °C and 50% relative humidity according to ISO 187:1990 for a minimum of 24 hours.
Figure 1. Mayer Rod After the samples were conditioned, the following utility properties of the paper were tested: 1. The Degree of hydrophobicity and/or hydrophilicity of paper, according to ISO 535:2014 (water absorption by Cobb 60 method) - Cobb apparatus (Danex, Katowice, Poland); 2. The Air permeability, according to ISO 5636-3:2013 - Bendtsen apparatus (Kontech, Lodz, Poland); 3. The Roughness of the coated paper surface, in accordance with ISO 8791-2:2013) – Bendtsen apparatus (Kontech, Lodz, Poland); 4. The Priority strength properties of the base papers, in accordance with ISO 1924-2:2010 - Zwick 005 ProLine testing machine (Zwick-Roell, Ulm, Germany), coupled with testXpert III software. The tensile paper properties were examined as follows: • I B – breaking length [m]; • V T b – width related force with break [N/m] ; • V T W – force at break index [Nm/g];
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