## 2 Materials and Material Production
### Coating Application  
The paper and board industry continually optimise mineral-based coatings to be applied reliably and efficiently at high speed over large web widths and maintain excellence in printability, smoothness, and optical properties. Multilayer coating, for example, is used for the highest quality art printing papers and high-priced goods packaging. Multiple layering enables smoothness and opacity to be achieved by different functioning layers, and the top layer accounts for the printability properties. Scheduling is also used to generate the high gloss needed in some grades, and matte and silk grades are produced via careful choice of mineral particle size. Figure 8: Optical microscope images present the mix of micro nano-fibrilated cellulose (MNFC) and CaCO\({}_{3}\) particles that were used in coatings: (**a**) and (**b**) fibres of recycled newsprint sheet (NP), and (**c**) dispersion of CaCO\({}_{3}\) in the matrix of MNFC\({}_{\text{NP}}\). In this work, much of this knowledge with respect to achieving smoothness and evenness of both substrate and coating layer no longer applies, which may cause discomfort for skilled papermakers. Already optimised criteria must be ignored because the production of a porous, permeable, and rough surface distribution of coating regions to maximise exposure for gas contact, permeation, and diffusion is necessary. The schematic in Figure 9 illustrates the challenge. The concept is primarily to devise a filter that can be used either in a permeable through-flow or, preferably for energy reasons, exposed in a surface flow mode. A high-quality thick coating, as shown at the top of Figure 9, is both relatively impermeable (unsuitable for through-flow) and, due to its smoothness, prone to establishing a static (stagnant) boundary layer of gas, irrespective of the flow rate of gas. Therefore, only slow diffusion can engender contact between the gas and the reactive coating bulk regardless of whether the static surface layer is in contact with any extended regions of coating. In contrast, a thin coating layer, broken up by poor coverage (fibrous exposed areas) allows gas to circulate. However, the transport mechanism at low coat weight also remains mostly by gas diffusion, but the path length for diffusion is reduced. Unfortunately, by reducing coat weight, the capacity of the filter for NO\({}_{x}\) uptake is greatly reduced. Figure 9: Illustration of the need for and methods that can be used to optimise gas-coating contact. The most convenient method to present a greater interactive surface is shown schematically since the pixelated coating in Figure 9 consists of a spatial distribution of coating agglomerates over the surface of the permeable substrate. Such a configuration aids both through-flow and surface flow modes by generating turbulence at the solid-gas interface, which destroys the static boundary layer. The destruction of the static layer provides ready access for the gas to the surface of the agglomerates, which, due to their largely flocculated particle-particle structure, are themselves highly permeable and have open and highly connected pores for pressure-driven ingress and easy diffusion of gas. One possible method to achieve pixelated coating is to use pin/needle coating. A hand-operated apparatus is shown in Figure 10. In this case, the pins were constructed from the polymer. Therefore, a surface treatment of the pins by surfactant was employed. Upscaling would require reel-to-reel methods for efficiency, and printing could be used instead of metered coating technology, e.g., a large cell ceramic Anilox cylinder in flexography, to achieve a pixel structure. In contrast with contact metering methods, such as blade coating, pin coating, spray, or flexographic printing, introduce little to no strain on the substrate, which, because the substrate becomes wet by the water content in the coating, and, thus, even further weakened, is not strong enough to withstand such a dynamic process.