Experimental Investigation and Modeling of a New High Speed Coating Process
Résumé
Health regulations are becoming increasingly restrictive which leads to develop solvent free laminating processes. Hot melt adhesives represent an interesting solution to bond two pre-printed substrates of polymer for packaging applications for example, but their high viscosities require extruding the polymer formulation through important die gaps (between 0.5 mm and 1 mm) in order to limit the extrusion pressure. Thus, delivering a final very thin layer of hot melt (a few μm) between the polymer substrates requires applying very high draw ratios between the die and the contact point on the substrate. These high draw ratios would result in marked thickness and width instabilities (called draw resonance) when using usual stretching distances as in cast film (a few cm) or in coating thermoplastic layers on a metal or paper substrate (more than 10 cm). Surprisingly, reducing drastically the stretching distance (around 1 mm) and applying a differential pressure between the two sides of the melt allow postponing the instability occurrence. Nevertheless the processing window is narrow and the objective of the present paper is to describe the cotformatating process developed at Bostik Company, to characterize the drawing instabilities and to investigate the influence of different processing parameters. In addition, a numerical model explaining these surprising stabilizing conditions is proposed. This model results in the definition of a high speed coating processing window.