A STUDY OF HOW SURFACE WETTING MAY AFFECT THE COATING LAYER WHEN COATING PAPER
Advancing the chemical engineering fundamentals
Interfacial & Colloidal Phenomena - I (T2-6a)
Keywords: drying, porous media, colloidal system, immobilization, sorption
Coated paper is made in industrial coating units. In year 2006, a single industrial coating unit for light weight coated paper (LWC paper) makes 100 000 ton paper/year, or far more, and the paper prize has been around 600 euros/ton paper, and for other coated paper grades much more. Single industrial coating units are considered to be the step that control final quality and hence one of the most essential components when controlling annual turnovers greater than 60 000 000 euro/year. We may now say that the scale of economics, calls for advanced and reliable multi-disciplinary models, to assist high-tech product innovation in the field of paper coating. General opinion is that the final pore structure of the coated layer is developed in the dryer section of the coater. It is therefore considered that drying of the coating layer is the process step that has direct consequences for the final product quality. Advanced drying studies have further shown that chemical additives in the base paper or coating color may reduce or increase energy efficiency, productivity and quality considerably, (e.g. paper ink interaction, print mottle etc.).
In industrial coaters drying of the coating layer may take less than one second. The extremely fast and harsh drying process of coated paper is impossible to measure. The objective of this work will therefore be to look at basic micro level simulations of liquid state and movement already in the initial phase when coating color (i.e. coating solid contents 55 % to 65 %) is brought to the surface of the hygroscopic paper, which includes physicochemical property changes at fiber surfaces and time spans that are less than 100 milliseconds. Drying of coatings are said to have a consolidation phase where the coating is immobilized. This often takes place in the dryer at a coating solid content between 73 % and 85 %. A common way to define consolidation is to set it between the FCC (first critical concentration) point where the pigment matrix starts to form and the SCC (second critical concentration) point where the three dimensional matrix is fixed. Previous research has indirectly shown that the amount of drained water, during initial drying, can affect product quality considerably (print mottle etc.). We will penetrate this issue further in our extended abstract by connecting drained water to the amount of liquid which is left to participate in film formation (or skinning) of the surface of the coating layer. This is done by looking at the basic phenomena that are significantly influencing liquid movement at fiber-coating boundaries and enforcing local changes of the structure of the coating layer before the coated layer is immobilized. The results of this work will be an increased knowledge of the formation of the coating layer.
Since this topic is fundamental, we believe that the conclusions in the text of the article are useful when developing existing and new products for the coating industry. This work may also be useful for other products where quality is dependent on initial surface wetting and fast liquid transfer rates during remoistening (e.g. paper ink interaction, drying of food, etc.).
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 16:00 to 16:20, in session Interfacial & Colloidal Phenomena - I (T2-6a).
