Abstract

The long-term preservation of paper with the same qualities of brightness and color is one of the important requirements for certain paper products. A major problem limiting the wider use of mechanical pulp is the sensitivity of lignin-containing pulps to yellowing induced by ultraviolet (UV) light. One of the yellowing inhibitor systems contains an Ultra Violet Absorber (UVA) and a Radical Scavenger (RS). The application of this inhibitor system to paper made of mechanical pulps requires the addition of two components, UVA and RS. To Properly understand the use of a paper yellowing inhibition system; it is important to reveal the effect of adding the inhibitors on the global properties of the resulting coating mixture. We need to optimize the input factors to achieve the longest in time inhibition effect. It is essential to study the rheological properties of the resulting system and the effects provoked by each of the ingredients. Studying the microstructure of the materials sheds more light on the interaction of the additional components with the original formulation. The coating formulation and the inhibition ingredients undergo mixing and transport processes. A study of the effect of pre-shear adds to the understanding of the system. Numerical simulation of 3D hydrodynamics with non-Newtonian rheology in the vicinity of the coating head creates a powerful tool for investigation of the coating system under these conditions. Among the operating parameters, the total concentration of inhibitors or total charge proved to have the largest effect of brightness stability or longer in time yellowing inhibition. The rheology of RS is Newtonian. The Ultra-violet absorber (tin 1130) is an extremely viscous shear-thinning thixotropic material. Water-retention studies show that inhibitors increase water loss, with more water penetrating the paper web. Zeta potential measurements revealed the repulsive forces between pigment and inhibitor particles. RS adsorbs on ground calcium carbonate GCC and disrupts interactions between kaolinite particles. UVA adsorbs on particles of kaolinite and GCC. The inhibitors increase the viscosity, thixotropy, and elasticity of the original coating formulation. A study of pre-shear reveals decrease in viscosity, while inhibitors concentration under pre-shear increases viscosity and elasticity. Transmission Electron Microscopy shows the microstructure of increasing agglomerations with added inhibitors. Numerical 3D simulation of the flow in the coating nip taking inertia forces into consideration depicts vortex formations on the surface of the coater's rolls.