• H T • W T • W T • E b • E w • E *
– strain at break [%]; – energy absorption [J/m 2 ];
b
W – energy absorption index [J/g]; – tensile stiffness [kN/m]; – tensile stiffness index [Nm/g]; – Young’s modulus [MPa];
• F B – tensile force at break [N]. The detailed statistical analysis was performed for individual research series, determining the basic indicators - arithmetic mean, extended deviation and percentage relative error. RESULTS AND DISCUSSION The obtained results of the hydrophobicity of the tested samples are shown in the drawing (Fig. 2). The initial Cobb 60 water absorption value for the base paper prior to the coating process was 47 g/ m². On the other hand, for the tested samples of papers coated with a paraffin mixture, Cobb's value was in the ra nge of 0.33 ÷ 5.12 g/m² and decreased with the the increasing thickness of the coating applied to the base paper. Consequently applying a paraffin coating to the paper allowed to reduce water absorption by as much as 99 ÷ 89%, depending on the coating weight. Interestingly, changing the rod number by 11 units causes only a 10% difference in the water absorption value. Therefore, regardless of the number used in the Mayer rod research, coating the paper with a mixture in the form of a paraffin-water emulsion reduces the rate of water penetration into the coated substrate. Such a significantly increased hydrophobicity causes a significant development in the usability of the paper, especially in the packaging and printing sectors.
6.0
5.0
4.0
3.0
2.0
1.0
0.0
3 4 5 6 7 8 9 10111213141516 Mayer Rod Number
Figure 2. Influence of coating grammage on the Cobb water absorption Printing papers should also be suitable for printing on the surface. The roughness of the paper (or its smoothness) determines the susceptibility of the surface to printing. The roughness/smoothness significantly affects also many other functional and processing values of the finished product. Table 1 shows the results obtained for the coated surface of the tested papers. The initial roughness value for the base paper before the coating process was 489 ml/min, while for the coated samples it ranged from 149 to 272 ml/min. Thus, the obtained results indicate that the increase in the thickness of the coating, as a result of using the Mayer rod with a higher number, causes the expected surface smoothing of the coated papers by 70 ÷ 44%. It is important in the case of printing because the increase in smoothness allows to make
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