“Revoluflex” is an innovative laminating process consisting in bonding two plastic films with a thin layer of hot melt adhesive. The molten adhesive is extruded through a flat die (gap ~ 1 mm). Then, it is stretched into the air at very high draw ratio (Dr> 100) over a very short distance (~ 1 mm) and set down on the primary plastic film. A vacuum pump, located beneath the extruded film, stabilizes the process and prevents air bubble intake between the primary and the adhesive film. A secondary plastic film is then laid on the coated primary film to give a laminate. Many defects can be observed as a function of the process parameters such as wavelike instabilities characterized by periodical sustained oscillations in the hot melt adhesive layer. This instability is similar to the “Draw Resonance” instability encountered with classical processes involving the stretching of a molten polymer. Other defects looking like small bubbles, cracks or rips in the adhesive layer are also observed. These defects hinder commercial deployment of the process. The aim of this study is there to understand their origins in order to remove them or at least delay their onset.These defects have first been characterized and their appearance was quantified according to process parameters and adhesive rheology. Several theoretical models of increasing complexity, in terms of flow kinematics and polymer rheology, have been developed. Newtonian and Viscoelastic constant width membrane models involving a pressure differential between the two sides of the extruded film represent a first theoretical approach that accounts for the vacuum pump effect. The linear stability method was used to investigate the influence of adhesive rheological behavior and process parameters on the onset of periodic instabilities. It was shown that the results were highly dependent on the initial flow conditions at die exit. Since the membrane assumption is not valid for very short stretching distances, we developed two Newtonian 2D models accounting for both extrusion and drawing steps. The first one is a symmetric model that does not account for the vacuum pump effect. It was solved using two different approaches: a front-tracking method coupled with linear stability analysis and a direct numerical simulation with interface capturing method (Level set method). It was shown that both methods lead to the same stationary solution and the same stability results. The second model accounts for the pressure differential and it was solved using only the front-tracking method. This latter 2D model enables to check the validity of the initial flow conditions of the membrane model.These models allow us to explain several experimental phenomena such as the stabilizing effect of the pressure differential and the short stretching distance. In addition, they help explaining experimental features which contradict the classical literature on drawing instabilities such as the stabilizing effect of increasing the draw ratio under certain operating conditions. Finally, they show that the pressure differential induces a high stress at the bottom lip of the extrusion die, which may clarify the cracks and rips observed in the adhesive layer.
展开▼
