The purpose of this Master’s thesis is to describe and illustrate the function of lubricants in the pigment coating of paper. The original goal was to show that lubricants might contribute to up-speeding of the pigment coating process. As this appeared to be a very complicated task to prove in reality and required test equipment, which was not available at that time in the Paper Laboratory of Helsinki University of Technology, the experimental part of the thesis was left out and a more comprehensive and versatile literature study of the subject was realized instead. I hope that the advanced level of information served by this literature study could inspire researches interested in this subject to further analyse the function of pigment coating lubricants, and based on this information develop new lubricant products for the pigment coating market.
Pigment coating lubrication can be divided into two categories: lubrication in wet state coating and lubrication in dry (or semi-dry) coating. Lubrication in wet state coating involves hydrodynamic lubrication and boundary lubrication, and occurs when moving pigment surfaces are completely or partially separated by a water layer. The lubricants have now influence on the viscoelastic character of the wet coating color by altering the manner in which various ingredients of a coating color interact with one another. Lubrication in dry coating involves both boundary lubrication, where paper surface and process machinery contact, and elastohydrodynamic lubrication - in connection with supercalendering - where hard roll surfaces are in contact with elastomeric surfaces of coated paper and filled calender rolls. In both cases, absorbed films of lubricants reduce friction between the respective surfaces. Lubricants then influence the manner in which the pigment particles and other materials are aligned with each other after the drying and supercalendering steps. This results in better runnability of the machines and improved optical properties of the finished sheet.
In the synthesis of this Master’s thesis some guidelines are given for evaluating different substances for their lubricating performance in pigment coating: An effective lubricant should be attracted either chemically or physically to the surface and thus decrease the real area of contact, and therefore the adhesion, of the two rubbing surfaces. It should have strong cohesion energy between the molecules. The optimal chain length must be found to ensure insolubility together with low viscosity. Organic Sulfur-compounds could be added to a lubricant in order to ensure lubrication performance under high temperatures and pressures. Finally, novel mechanisms are suggested for lubricant performance, especially in blade coating: Lubricants may turn the balance of a viscoelastic coating color from the elastic side to a slightly viscous side and thus enable the flow of a coating color still in very high coating solids. Together with higher coating solids, higher coating speeds may also be achieved, which was indeed the initial interest of this study. Secondly, there seems to be a link between blade bleeding/extrusion and the slip boundary within the color. One possible mechanism for lubricant function may well be their ability to reduce friction and increase slip at the blade boundary, thus preventing bleeding and dry stalagmite formation at the blade tip from occurring.